Rapakivi texture: An indication of the crystallization of hydrosilicates, II

Earth - Science Reviews, 22 (1985) 1-92

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Elsevier Science Publishers B.V., Amsterdam--Printed in The Netherlands

Rapakivi Texture: An Indication of the Crystallization of Hydrosilicates, II JOHN N. ELLISTON

ABSTRACT Elliston, J.N., 1984. Rapakivi texture: an indication of the crystallization of hydrosilicates, lI. Earth-Sci. Rev., 22: 1-92. Rapakivi granites have puzzled all who have seriously studied theml Typical rapakivi texture is a mixture of variously mantled, non-mantled or partly mantled, concentrically zoned, plastically distorted, fragmented, reaggregated, large and small ovoids. Commonly they are potash feldspar often mantled by, and having a variable content of plagioclase. Some display remarkable sphericity. In form, composition, zoning sequence, and crystallization pattern each ovoid reflects an individual development. Up to five concentric internal plagioclase rims have been observed and some ovoids may be isolated in autoliths and wall-rocks. Anomalies and contradictions arise from any assumption of genesis from a cooling melt. The recorded objective data imply that the "magma" from which rapakivi textures develop had similar diffusive and rheological properties to those of a partly dewatered macromolecular paste or a mixture of gelatinous hydrosilicates. As indicated by deep oil wells this system is found at somewhat elevated temperatures at considerable depths within accumulated sediments. In addition to the very specific diffusive and rheological properties of such partly dewatered sediments, the system has as its major components normal solvated macromolecules of mixed clays, silica gels and hydrous ferromagnesian minerals which are characterised by distinctive particle sizes and geometric shapes (platelets, spheres and rods, respectively). Thixotropic liquefaction and intrusion of such concentrated gelatinous "magma" or sediment paste introduces relative movement between the component macromolecules whereby they can reduce surface energy by interaction to assume a "close-packed" condition and aggregate during laminar flow into macro-accretions comprised essentially of their respective particle shapes. Syneresis of these precursor accretions desorbs ions, including the small montmorillonite particles behaving as a colloidal electrolyte. These diffuse from the illitic cores to form a montmorillonite-rich rim which it is suggested crystallizes together with the illitic cores to form mantled or polymantled feldspar ovoids. Crystallization of the rapakivi massif is associated with strong temperature rise stemming from exothermic crystallization of the close-packed metastable colloids. This follows the development of the characteristic texture. The rounded and rimmed precursor accretions are formed during earlier lower-temperature episodes of thixotropic liquefaction which are isothermal. The fluidity is an earlier event. There is high temperature dependent on the rate of water loss but no molten stage. 0012-8252/85/$32.20

© 1985 Elsevier Science Publishers B.V.

Forty-six typical features of rapakivi texture are described and illustrated, each of which is directly attributable to specific interactions in an alternately dynamic and static colloidal system. Individual correlation between each observed distinctive feature of the rapakivi texture and the well-documented physico-chemicalprocess is complete. For sediment-derived granites, therefore, the rapakivi texture can confidently be assumed to be an indication of the crystallization of their sedimentary hydrosilicate precursors.

INTRODUCTION The rapakivi granites are claimed (Backlund, 1938, p. 340) to be a special problem of Finnish geology. Their texture is certainly correctly described as abnormal when it relates to the typical coarse-grained rocks with the peculiar ovoidal texture. Rapakivi-type granites and similar rocks containing mantled or non-mantled ovoidal feldspars have in fact a world-wide distribution. In some cases John N. Elliston was born in Huonville, Tasmania on 20th May 1924 and graduated in chemistry and geology (lst Hons.) at the University of Tasmania in 1949 and 1950. After brief experience in regional mapping and as a staff geologist to a consulting group, he began his long career with the Peko-Wallsend Ltd. mining group in 1956. Successful work on the mobile sedimentation of the Warramunga geosyncline in central Australia and its significance in ore genesis led to discovery and development of six new mines on this field. He became a director of the company in 1969, and as a senior executive, responsible for a number of additional mining operations, exploration and development of new mineral projects, technology and research. Research associates and directly employed staff, including up to fifty-five geoscientists, have assisted in the development and extension of the significant initial work on mineral deposits and petrogenesis. An award conferring membership of the Order of Australia recognised his contribution to mineral development and the geological sciences in 1981. Present address: Elliston Research Associates Pty. Limited, 10B The Bulwark, Castlecrag. N.S.W. 2068, Australia.

they form distinctive phases or plutons within batholiths but more commonly are gradational to porphyritic granites and granites of more usual texture. Various theories have been proposed to account for the mantles or rapakivi feldspars, but none are entirely adequate (Key and Wright, 1982, p. 124). Backlund (1938, p. 373) in his early work on the Fennoscandian rapakivis recognises the difficulty stemming from the typical textural peculiarities which are markedly at variance with the supposed mode of evolution. Vorma (1971, p. 60) in his more detailed studies points to the diversity of opinion which has arisen on rapakivi texture and claims that this shows that the origin of the typical wiborgitic ovoids in rapakivi has not yet received a generally acceptable explanation. Stull (1978, p. 243) believes that examination of mantled feldspar-bearing rocks from numerous localities in the western United States has revealed many unresolved problems. One of the most intractable of these is referred to by Dawes (1966, p. 571). His observations reaffirm earlier ones, like those of Spencer (1938), who believed, " T h e occurrence of potash-soda porphyroblasts in xenoliths and similar metamorphosed rocks, under certain conditions which preclude an origin by direct crystallization from a magma, [he means "melt" as opposed to the original meaning of " m a g m a " .1] raises the question as to how far it is justifiable to regard holocrystalline rocks, even of typical granitic appearance, as products of direct magmatic [again he means molten] crystallization". In fact of all of the papers so far studied by the writer none claims direct crystallization of the rapakivi ovoids from a melt. Most writers (Sederholm, 1928, p. 90; Backlund, 1938, pp. 365 and 367; Dawes, 1966, p. 570; Elders, 1968, p. 44; Vorma, 1971, p. 10; Marmo, 1971, p. 125; Stull, 1978, p. 247; K e y and Wright, 1982, p. 119) record at least two generations of the main minerals, and a number of extraordinarily complex crystallization histories

*~ In this text "magma" is used to denote the English language meaning of the word magma as defined by the third edition of the Shorter Oxford English Dictionary, p. 1186, namely: Magma (mae'gm[0. ME. [a. L. magma (sense I), Gr. #&y#a, f. root of #tloo¢lv to knead.] tl. The dregs that remain from a semi-liquid substance after the liquid part has been removed by pressure or evaporation--1856. 2. Any crude mixture of mineral or organic matters in the state of a thin paste--1681. 3. Geol. a. One of two or more supposed strata of fluid or semi-fluid matter lying beneath the earth's crust, b. The amorphous basis of certain porphyritic rocks. 1804. Hence Magma'tic a. Where quoted or used without parenthesis, the word magma is intended to have such meaning as the reader may ascribe to it. This has been done to avoid clumsy euphemisms like "semi-fluid mixture of gelatinous mineral matter" and in the belief that after the many peculiar attributes of granites have been considered in the light of the properties of such mixtures, the word "magma" may be retained in its original usage, as defined above.

(Elders, 1968, p. 44; Vorma, 1971, p. 67; Key and Wright, 1982, p. 121) have been suggested. Backlund (1938, p. 367) concludes, " N o routine explanation based on the usual conceptions of magmatic behaviour satisfies all the implications disclosed by close examination of the ovoids, to say nothing of the ovoids containing central mesostasis or of the inclusions of mafic minerals associated with quartz and fluorite". Elders (i968, p. 48) agrees that " T h e data available on these [rapakivi] granites epitomises the granite controversy. Thus we are free to make eclectic interpretations". Virtually from the commencement of detailed studies of rapakivi rocks, researchers (Frosterus, 1896; Iddings, 1909) have noted the similarity of the rapakivi texture and certain orbicular structures. Sederholm (1928, p. 96) writes: " T h e analogy of the rapakivi ovoids with the smaller spheroids of the orbicular granites and other phenomena observed in them is so great that it seems possible that they have a common explanation". The present writer (Elliston, 1984) has detailed the development of concretionary orbicular rim structures on accretionary proto-orbicules and orbicular cores. These compare with rounded feldspar porphyroblasts of accretionary origin studied (Elliston, 1963, (b), (c); Elliston, 1968) in intermittently remobilised "mud-glacier" deposits from Central Australia.

Fig. 1. A rounded wiborgitic alkali feldspar has additional material clearly moulded onto its rim to indicate accretionary build-up. The composite has a narrow plagioclase mantle and carries abundant inclusions of matrix minerals. Langinkoski,Finland.

Van Bemmelen (1940, p. 426) sets out a series of observations relative to the porphyritic biotite granite exposed in the Bukit Timah quarry, Singapore, which he claims must be interpreted as accretive growth of the porphyroblast at the expense of the fine-grained granulitic matrix. Addition of external material or agglomeration of the feldspar megacrysts indicating accretionary growth (Fig. 1) is clearly observed in relation to the rapakivi texture. This mode of origin will be further pursued after some description of the texture and discussion of the problems arising when a melt-derived crystallization is assumed. This paper does not attempt to extend or contribute further to the very detailed observations on record concerning rapakivi granites. The references cited embody many clear indications of the origin of these granites, their several intrusive phases, and crystallization history. It is therefore proposed to concentrate on how the many specific features were derived and offer a simple solution to rapakivi genesis consistent with all recorded observations. It is necessary first to outline the form and characteristics of the rapakivi intrusions and the difficulties encountered in interpreting them as congealed melts. THE RAPAKIVIGRANITES Because of their textural peculiarities, striking appearance and special features rapakivi granites have been closely investigated. Well studied areas include rapakivis from central and northeastern Wisconsin (Elders, 1968; Anderson and Cullers, 1978; Anderson, 1980), the Golden Horn batholith, Washington (Stull, 1978), the Enchanted Rock batholith, central Texas (Hutchinson, 1956; Ragland, 1969) the Deer Isle batholith, Penobscot Bay, Maine (Stewart, 1959), the Dartmoor granite, U.K. (Hawkes, 1967), the Tasiussak area, southern Greenland (Dawes, 1966) and the Gaborone granite in S.E. Botswana (Key and Wright, 1982). However, the classical area for rapakivis is Fennoscandia where earlier works (Wahl, 1925; Sederholm, 1928 and 1967; Backlund, 1938; Eskola, 1963) have been amplified by more recent detailed studies (Vorma, 1971, 1976; Vaasjoki, 1977). There are sixteen rapakivi areas in Finland, the largest near Wiborg being some 1/i~000 km 2 in extent (Sederholm, 1967, p. 501). Typically the rapakivi massifs are variable in composition and texture (Vorma, 1976, p. 10; Stewart, 1959, p. 318; Key and Wright, 1982, p. 118) with distinct zones of the various rapakivi textures grading into granites lacking rapakivi ovoids. Eight types of rapakivi are characterised (Vorma, 1976, p. 6) within the Wiborg massif but some 80% of it is wiborgite having the predominant (av. content 44%) potash feldspars mantled with oligoclase.

The form of intrusions The normal wiborgite or classical rapakivi granite variety is a coarsegrained porphyritic granite (Fig. 2) containing potash feldspar ovoids 3-4 cm in diameter, many of which are rimmed with plagioclase margins from 1 to 3 mm in thickness. There are three, possibly four intrustive granite phases (Vorma, 1971, p. 2) in the Wiborg massif and several suggestions have been made that these sheet-like bodies (Eskola, 1963, p. 253; Backlund, 1938, pp. 339, 353, 364) were derived from pre-existing sediments. Marmo (1971, p. 46) claims that the Finnish porphyroblastic granites had ultimately derived from clay-rich sediments containing abundant potassium, and Backlund (1938, p. 390) suggests that the calcium may not belong to the "immigration" elements but may have been wholly or partly fixed in the pre-existing sediment~,.. He refers (p. 359) to some pre-rapakivi stage of evolution of the rocks which had a horizontal bedded appearance. Dawes (1966, p. 570) refers to the local basin shape of the layering in the Tasiussak rapakivi, southern Greenland. Key and Wright (1982, p. 123) describe the Gaborone rapakivi granite as spread laterally along a flat

Fig. 2. A normal coarse-grained wiborgite with discontinuous, variable and irregular plagioclase mantling of potassic feldspar ovoids. Note additions to the margins of some individuals, tendency to cluster in "clumps" or "chains", and incorporation of rim plagioclase in reaggregated individuals. Ornamental stone, Lienz, Austria.

interface between volcanics and basement. They are of the opinion that faulting and gravity sliding consequent on subsidence of the western margin of the Bushveld basin can be ascribed to the emplacement of the Gaborone granite, which has the sheet-like morphology similar to Fennoscandian and other rapakivis. Backlund (1938, p. 359) believes the sheet-like forms originated by granitisation of earlier Jotian sediments under near-surface conditions and that they have a rheomorphic origin. These flat bodies (p. 377) could be designated as taphrolites infilling comparatively shallow grabens or troughs on the craton. Age dating work on the zircons (Vaasjoki, 1977, pp. 30, 31) indicates that the Wiborg rapakivi assimilated much older rocks, components of which may have been 2,750 Ma old. Some rapakivi granites are definitely intrusive (Marmo, 1971, p. 129; Stewart, 1959, p. 295) and although gradation between varieties is common, sharp internal contacts between differing textural types without any contact phenomena is observed (Backlund, 1938, p. 350; Vorma, 1976, p. 50). In fact a small section of the Laitila massif (Vorma, 1976, p. 42) consists of rapakivi varieties with distinctly different textural features from normal Laitila rapakivi. These granites form bodies of an autolithic nature measuring from some tens of centimetres up to several kilometres across. The blocks represent a cohesive phase of the rapakivi, brecciated by reliquefaction of the same magma. A close association of the rapakivis with basic rocks occurring as large and small blocks, contemporaneous dykes, marginal breccias, and autolith swarms is discussed (Sederholm, 1928, p. 84; 1967, pp. 192-194; Backlund, 1938, pp. 349 and 353-357). Some of the basic inclusions are of enormous size being measurable in square kilometres (Backlund, 1938, p. 356) and the breccias occur not only on the margins of the rapakivi, but centrally in the massif. Many fragments are strongly digested (Backlund, 1938, p. 355; Sederholm, 1967, p. 192) appearing as diffuse patches of green-coloured rocks within the brownish-red rapakivi. Sederholm (1967, figs. 29, 39, 40, p. 192) also draws attention to the intricate interveining, permeation, and development of rapakivi minerals within the basic rocks near the contact. Some breccia fragments in rapakivis have apparently been soft at the time of their inclusion as Dawes (1966, p. 570) observes fragments in the Tasiussak granite which have been aligned and drawn out to elongated spindle shapes and possibly rotated by the "magma current". Rapakivi granites have wide-ranging ages from 2400 Ma for the Gaborone granite (Key and Wright, 1982, p. 109), to 325 Ma for the Deer Isle batholith in Penobscot Bay, Maine. The main phases of the Wiborg massif are 1700-1650 Ma (Vaasjoki, 1977) but in most cases significant anomalies have been noted in results of age determinations. Vaasjoki (1977, p. 21) notes a 43 Ma age discrepancy which cannot be attributed to analytical errors, in

samples from a single outcrop. He also notes (p. 54) that the zircon age results disagree with the intrusive sequence of three rapakivi phases as determined by field relationships. RAPAKIVI CHARACTERISTICS

Mantling " R a p a k i v i " derives from the Finnish word "rapautuvakivi", meaning "disintegrating rock" or "crumbly stone", but in petrology it has come to mean the occurrence of more or less rounded ovoidal potassic feldspar with plagioclase arranged within or upon it in a crudely concentric mantle or mantles (Stewart, 1959, p. 294). The mantling of orthoclase or microcline by oligoclase is therefore the main criterion by which a granite is designated rapakivi. Mantled feldspars develop in the more calcic varieties of the granite (Stull, 1978, p. 244) but their distribution through the massif is somewhat irregular. The mantles have no relation to the size of the orthoclase ovoid they

Fig. 3. A wiborgite with mantles variable in thickness and independent of the size of the ovoids. Some fracture fragments are mantled or partly mantled and the irregular ovoids have a distinctly "battered look". The aggregate in the centre with large inclusions is probably a newly forming macro-accretion. Ornamental stone, Innsbruck, Austria.

surround and vary in width (Fig. 3), usually 1-3 mm (Vorma, 1971, p. 10), in composition (An~7 to An~0 - - Stull, 1978, p. 243), and in colour from glassy, pale milky to green (Sederholm, 1967, p. 174). Other than perhaps for a very restricted rock volume, some ovoids have mantles while others in the same

Fig. 4. A wiborgite with large ovoids (to 6.7 cm diameter) showing some continuous plagioclase rims and further irregular development of potash feldspar outside the rim. Internal inclusions tend to occur in concentric zones and a part ovoid or fracture fragment is visible. Ornamental stone, Innsbruck, Austria.

10 matrix do not (Elders, 1968, p. 39; Vorma, 1971, p. 7; 1976, p. 5; Stull, 1978, p. 243). Anti-rapakivis, that is plagioclase ovoids rimmed with potassium feldspar mantles, have also been noted (Raguin, 1965, p. 72). Size of ovoids

Wiborgites typically have ovoids 3-4 cm in diameter but commonly they develop to 6 or 7 cm (Fig. 4) and particular types or phases of rapakivis are distinguished by the larger-size ovoids. Rapakivi from the harbour at Wiborg has ovoids up to 12 cm in diameter (Sederholm, 1928, p. 94) and Dawes (1966, p. 570) records megacrysts up to 17 cm long in the Tasiussak rapakivi. An exceptional case of ovoids up to 27 cm in diameter is described by Wahl (1925, p. 47). Miarolitic cavities and fluorine content

The occurrence of miarolitic cavities and widespread fluorite or topaz could also be said to be a characteristic of rapakivi granites. Backlund (1938, p. 363) observes that these granites are nearly all exceedingly rich in fluorine with fluorite and locally topaz being well distributed in the rocks. Vorma (1976, p. 88) also cites fluorine as a trace element highly characteristic of rapakivi granites and his detailed work on the Laitila massif showed its average content or fluorine to be 3800 p p m or 4 to 5 times that of granites in general. Fluorite is recorded in the Gaborone rapakivi (Key and Wright, 1982, p. 118), the rapakivis of Wisconsin (Elders, 1968, p. 39) and the Enchanted Rock batholith (Hutchinson, 1956, p. 785). It has been pointed out (Eitel, 1975, p. 672) that in sediments fluorine tends to substitute for OH groups in the hydrous clays. Most of the fluorine is therefore retained in muscovite, illite and related minerals and would be available to form new minerals if these were reconstituted. In the normal more siliceous varieties of wiborgite fluorite is an essential accessory constituent of the rock (Vorma, 1971, p. 45) and its occurrence in the miarolitic druses and in the scantily present pegmatites is marked. Miarolitic and drusy textures are abundant in all the Golden Horn rocks (Stull, 1978, p. 244), and Sederholm (1928, p. 90) points out that in some varieties of the Aland rapakivi the druses lie in the mesostasis between the bigger orthoclase ovoids. This interstitial development of miarolitic cavities is similar to their occurrence in the inter-orbicular matrix of the orbicular granites (Elliston, 1984, fig. 3) and they developed in a similar manner. In addition to fluorite the miarolitic cavities are infilled with quartz crystals, biotite and occasionally other minerals but there is evidence that

11 this crystal growth took place while the walls or the material surrounding the cavity was still unconsolidated. Backlund (1938, p. 363) records free-grown long prismatic low-temperature crystals of quartz with well-formed end faces which do not grow out from the wall of the miarolitic cavity but transect it, originating from within the adjoining granite itself. Sederholm (1967, p. 179) also mentions that there are signs which seem to indicate that the feldspars have crystallized within a partly solid rock. Vorma assumes that the potash feldspar ovoids only came into existence when they crystallized rather than having first formed as a coherent aggregate of specific precrystalline minerals, but he states (1971, p. 61) that the ovoids of the Wiborg massif were formed before the groundmass was completely solidified.

Layering and foliation Generally the rapakivi granites are remarkably free from layering, streaking, gneissose fabrics, etc. and macrotextures are essentially isotropic. However, Backlund (1938, p. 358) suggests that the decay characteristics of the rock may be related to vaguely indicated horizontal bedding which is seen in the field as layer,; characterised by mineral concentrations differing slightly from those of the general rockmass. He suggests (p. 362) that rows of close-spaced or linked ovoids in parallel arrangements may be reminiscent of fluidal textures and that they may reflect some pre-rapakivi features. However, the parallel and sub-parallel alternations of somewhat differing mineral concentration are suggestive of normal or forset bedding rather than of flow textures. There are a few instances of flow textures found immediately at the contact and Sederholm (1967, p. 571) describes how an .~land rapakivi passes gradually into a marginal quartz porphyry which has a well-developed fluidal texture. The rapakivi of the Enchanted Rock batholith shows a well-developed flow fabric (Hutchinson, 1956, p. 778) and alignment of the megacrysts in the Tigerton rapakivi (Elders, 1968, p. 39) gives the rock a faint foliation indicating a primary flow structure associated with shearing. Similarly, the rapakivis from the Gaborone batholith are foliated with about equal proportions of groundmass to phenocrysts. The dark minerals, biotite and hornblende, which are largely confined to the groundmass occur within the phenocrysts in small amounts where they show a zonal distribution. In the foliated rocks, the femic minerals generally form spindle-shaped or streaky discrete masses. These are locally aggregated into elongate schlieren or rounded bodies like basic xenoliths which are regarded as segregations (Key and Wright, 1982, p. 114).

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Lack of chilled margins Eskola (1963, p. 238) remarks that the general absence of chilled border zones in the rapakivi masses and lack of contact metamorphism in the adjoining rocks suggest that any temperature gradient between the rapakivi and country rock was insignificant. Sederholm (1967, p. 180) also illustrates contacts which show no diminution of grain size and his figures 39 and 40 show the large (3-4 cm) ovoids immediately against the apparently unaffected contact and within small apopheses which intimately penetrate into the schists. Similar boundaries with the coarse rapakivi texture at the contact without chilling or local alteration are observed elsewhere (Stewart, 1959, p. 294).

Impurity of oooids and inclusions It is certainly a characteristic of the rapakivi alkali feldspar ovoids that they are impure (Fig. 5). The reddish colour of the feldspar is due to staining by an iron oxide pigment but they contain plagioclase in irregular dispersed patches, fragments, occasional idiomorphs, perthites and veinlets. Hornblende, biotite, quartz, fluorite and zircon are also commonly found as

Fig. 5. Rimmed wiborgiticfeldspar ovoid having a somewhatcurdy internal texture with large inclusions of matrix minerals. Geopeko polished specimen.

13 inclusions (Vorma, 1971, p. 24). Quartz as inclusions in the rapakivi ovoids is variable but frequently important. It occurs as drop quartz (Vorma, 1971, p. 25) which as roundish grains occasionally comprise parts of the concentric rings of inclusions in the ovoids. Concave quartz inclusions (Vorma, 1971, fig. 5) can form aggregates which extinguish together as a unit crystal and the xenomorphic inclusions against the alkali feldspar are sometimes rimmed with an albite quartz myrmekite. Quite often the quartz inclusions are of such abundance that they may be regarded as a kind of skeletal crystal (Sederholm, 1967, p. 175), graphic or micropegmatitic intergrowth (Fig. 6). These micropegmatitic-type intergrowths are very common in both the ovoidal rapakivis and in the evengrained granites to which they grade. They can comprise the whole ovoid, occur within the core as patches (Fig. 7), or at the junction of the core and surrounding plagioclase mantle. Because the micrographic quartz intergrowths with alkali feldspar occur as inclusions within and comprising some ovoids thought to be primary porphyroblasts, Backlund (1938, p. 368) declares that it cannot represent crystallization from a residuum. Yet identical micrographic intergrowths characterise the groundmass alkali feldspar and Vorma (1971, p. 26) records this texture in about 50% of the thin-sections of normal wiborgite.

Fig. 6. Wiborgite showing irregular and "sticky" margins of the ovoids with frequent conjoining. Abundant "lacy" quartz inclusions characterize this variety and the entire large ovoid in the centre has crystallized as a micropegrnatite.Parola-Valkeala, Finland.

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Fig. 7. Wiborgite with irregular and mantled feldspar ovoids and smaller (black) quartz ovoids. A patch of vermiform micropegmatite can be seen in the central ovoid and "lacy" or micropegmatite-type quartz can also be seen in the matrix. Summa, Finland.

Fig. 8. Rimmed wiborgitic ovoid showing irregular development of potassic feldspar outside the mantle, some alteration and abundant inclusions in the centre. Geopeko polished specimen.

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Concentric zoning The occurrence of internal mantles or concentric zones of plagioclase or single and multiple zones of inclusions in the large alkali feldspar ovoids is a characteristic of the rapakivis which has caused much comment and considerable puzzlement. Multiple concentric zones of inclusions are fairly common but internal plagioclase mantles within the alkali feldspar ovoids are more usually a single zone (Figs. 4 and 8) or a mantle which is covered round much of the periphery with additional alkali feldspar. However, Sederholm (1928, pp. 85 and 90) and Backlund (1938, p. 366) refer to m a n y alternating shells of oligoclase in the bigger rapakivi ovoids. Wahl (1925, pp. 54-60) describes some unusual ovoids which include ovoids with more than one concentric plagioclase shell. He records up to five shells occurring within the one ovoid. From the rapakivis of Wisconsin, Elders (1968, p. 40) notes that m a n y of the larger phenocrysts have several internal zones of plagioclase instead of, or in addition to, being mantled by plagioclase. He illustrates one alkali feldspar with more than six internal zones of plagioclase. The amount, arrangement, and nature of the abundant inclusions within

Fig. 9. Wiborgite with variable thickness of rim development and irregular addition of further alkali feldspar externally. Note the "chaining" of conjoined ovoids and incipient concentric pattern of inclusions. Ornamental stone, Innsbruck, Austria.

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the large ovoids is highly variable but frequently these inclusions, like the plagioclase shells, tend to occur in concentric rings (Figs. 4, 9, 10, 11). The minerals comprising the inclusions are typically those of the matrix and up to ten such concentric rings have been reported (Backlund, 1938, p. 366). Backlund (1938, p. 367) remarks that these rings of inclusions tend to reproduce all the embayments and never indicate crystallographic faces. This could suggest that the overgrowth was not on an idiomorphic crystal, but he believes that if the ovoidal form of the rapakivi megacryst points to resorption, then the manifold inclusion rings must suggest repeated phases of resorption. He points also to the rounded quartz inclusions and idiomorphic plagioclase inclusions which occur within many of the ovoids independently of the inclusion rings. These and the biotite, hornblende and other matrix minerals comprising the inclusions must surely indicate that these minerals existed prior to their incorporation within the ovoids, and it is therefore difficult to suggest that the ovoidal porphyroblasts crystallized first before development of the matrix minerals. Sederholm (1928, pp. 86, 94) refers to internal shells rich in included grains of quartz, and Vorma (1971, p. 25) also describes "drop quartz" which occasionally takes part in building the concentric rings of inclusions. In the pyterlitic rapakivis (Vorma, 1971, p. 28), the large ovoidic alkali feldspar

Fig. 10. Mantled and non-mantled ovoids together in wiborglte with concentric zones ol inclusions and irregular impure part-formed ovoids. Ornamental stone, Innsbruck, Austria.

17 crystals are in many instances surrounded by a ring of idiomorphic quartz crystals. Thus in relation to its random inclusions, the graphic or micropegmatitic intergrowths and in its tendency to form the external and internal concentric rings, the quartz can be seen to have the same habit in relation to the alkali feldspar ovoids as the plagioclase. Neither have crystallized independently nor sequentially. Some rapakivis grade to or develop more euhedral feldspar porphyroblasts and these can also reflect the internal concentric zones of plagioclase or of inclusions. Among the typically ovoid or irregular forms of the mantled perthitic orthoclase feldspars of the Golden Horn batholiths, a few (Stull, 1978, p. 246) were found to have euhedral or subhedral faces. In the Tasiussak rapakivi, Dawes (1966, p. 570) records commonly mantled megacrysts varying from ovoid to euhedral. Both occasionally contain concentric rings of the oligoclase-albite which, when they occur, are in optical continuity with the plagioclase of the perthite and the external mantle. Dawes (1966, p. 570) concludes that the presence of the concentric rings of plagioclase in some of the rounded and in some of the euhedral megacrysts indicates that both extremes of shape persisted throughout crystal growth. UNRESOLVED PROBLEMS OF THE RAPAKIVI GRANITES In addition to the extraordinary crystallization features to be further discussed, the patterns of rounding, mantling, multiple mantling, inclusions, occurrence, shape and association of the rapakivi ovoids have created unresolved difficulties in interpretation.

Rounding of ovoids The sphericity of the rapakivi ovoids is remarkable (Raguin, 1965, p. 72; Fig. 11). Sederholm (1928, p. 91) also considers the ovoidal shape of the phenocrysts to be very mysterious as "every thought of a resorption in so great a measure and with such regularity appears, a priori, untenable". The sphericity, as indicated by the internal concentric shells of plagioclase and the zones of inclusions, is inherent in the ovoid and persists to its core. As Sederholm remarks (1928, p. 92), "the ovoidal shape seems thus to have existed during all the time of the growth of the crystal". He says (1967, p. 177), "If the oval form were due to corrosion, we should expect to find crystals showing the usual Carlsbad twinning which had been afterwards rounded, when the boundary between the twins might occasionally intersect the longer diameter of the ovoids. But when twinning occurs the ovoid is always divided into a number of sectors which radiate from the centre. Obviously in each of these sectors, layer has been deposited on layer during the whole growth of the crystal" (see Fig. 22).

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Fig. 11. A typical mixture of large, small, rimmed and non-rimmed ovoids, their fracture fragments and variously re-aggregated shreds of alkali feldspar in wiborgite. In some the sphericity of the ovoids is remarkable and concentric rings of inclusions indicate that the rounding is not by random external corrosion or attrition. Ornamental stone, Innsbruck, Austria. The later more detailed studies of Vorma (1971, p. 60) recognise the diversity of environments in which the rapakivi ovoids are formed and that the rapakivi texture does not originate by the simple resorption mechanism (reaction of earlier formed crystals with a melt) proposed by earlier workers (Tuttle and Bowen, 1958; Savolahti, 1962). Vorma concludes (1971, p. 61) that the formation of the potash feldspar phenocrysts is metasomatic or autometasomatic, but he is clearly uncertain of their origin or the processes involved. In his two page summary (1971, pp. 67-69) on the factors affecting ordering in rapakivi granite alkali feldspars there are 23 expressions, such as "if one assumes", "presumably", "possibly", "it may be", "evidently", "conceivably", etc., which reflect this uncertainty.

Mantled and non-mantled oooids occurring together As the details of their crystallization and perthite development show, each rapakivi ovoid has its own history and independent characteristics. However the widely observed mantled ovoids mixed in the same matrix with unmantied ones (Figs. 10, 12) is the most obvious indication of non-uniform

19

Fig. 12. Dark small ovoidal and variously fractured quartz individuals set among mantled and unmantled wiborgitic ovoids. An inward tonguing part of a plagioclase mantle, rim pieces and abundant inclusions of matrix minerals can be seen in the newly formed ovoids. Ornamental stone, Lienz, Austria.

crystallization conditions applying to adjoining ovoids. Clearly the magma did not cool, suddenly release pressure, or become rich in sodium and calcium uniformly over any significant volume. The conditions necessary for the alkali feldspar ovoids to form mantles, multiple internal plagioclase zones, or zones of inclusions, operated on the scale of the ovoids themselves and individually for each. The mantling can be discontinuous, variable, and irregular (Key and Wright, 1982, p. 114; Vorma, 1971, p. 22; Fig. 2) but in other cases the peripheral zone of plagioclase is remarkably constant (Elders, 1968, p. 40; Stull, 1978, p. 245) and seems independent of the size of the host feldspar (Figs. 3 and 4).

Soft or plastic oooids Some suggestions of a soft or unconsolidated matrix have already been noted (see p. 11) but an inferred soft stage in the development of the rapakivi ovoids is also recognised. Backlund (1938, p. 366) writes, " T h e ovoids, when more closely packed together mutually deform one another's

20

Fig. 13. Mutual indentation and moulding of the ovoids against each other in wiborgite. A typical small dark quartz ovoid is included near the centre of the large potash feldspar. Ornamental stone, Lienz, Austria.

Fig. 14. Conjoining of five or more rather ill-defined pegmatitic ovoids to make a chained composite with different textural patterns in various parts of the composite. Parola-Valkeala, Finland.

21 boundaries, leaving rounded embayments, just as if they had been soft during their formation". Raguin (1965, p. 70) also records how some ovoids in contact mutually mould each other, as if they had been plastic after their formation and before their final crystallization. Examples of this can be seen in Figs. 1, 13, 15 and 18. In some of the otherwise essentially isotropic rapakivis, rows of ovoids joined or partly joined together in parallel arrangements are observed (Fig. 14). Backlund (1938, p. 362) considers these reminiscent of fluidal textures and suggests that they may reflect some pre-existing, pre-rapakivi features of the rock. Dawes (1966, p. 570) notes that the abundance of megacrysts in some places in the Tasiussak rapakivi is so high that they have coalesced to form a continuous network. There is thus evidence both of adherence between ovoids and of a soft stage during their development.

Fracturing of ovoids On most surfaces of rapakivi granite, examples of fractured ovoids or fragments of the ovoidal feldspars can readily be seen (Figs. 3, 4, 11, 15;

Fig. 15. A wiborgite discloses fracture fragments, reaggregation of ovoids, mutual indentations and former rimming plagioclase incorporated in an ovoid. A very impure part-formed ovoid is emerging as a feldspar-rich patch in the reworked matrix. Ornamental stone, Innsbruck, Austria.

22 Vorma, 1976, his fig. 10, p. 38). Half spheres or segments often with some alignment of megacrysts or a tendency to occur in clusters are noted (Elders, 1968, p. 39, plate 1.F; Key and Wright, 1982, p. 115). In the G a b o r o n e rapakivi, Key and Wright (1982, p. 115) also point out how it has been possible to show that certain fractures intersecting a phenocryst were formed prior to crystallization of the groundmass. In some cases the plagioclase rims develop round the angular fracture fragments or segments of the ovoids in the same manner as for the ovoids themselves (Elders, 1968, plate 1, A and B; Fig. 16). Clearly, the ovoids have been coherent enough to fracture at the time that they were free to move within their mobile matrix.

Aggregation and conjoining of oooids The characteristic re-aggregating, conjoining and merging or welding together of rapakivi ovoids and their fragments is widely recorded (Seder-

m

~ n

m ~m

Fig. 16. A dark quartz ovoid with some development of the syneresis vermiform internal embayments (see also Fig. 46), a rimmed fragment and an inward tonguing segment of plagioclase mantle can be seen in this wiborgite. The crystal cleavage in the large ovoid can be seen to extend to the outer margin of the rimming plagioclase. In most cases the crystal framework is common to both the orthoclase and plagioclase parts of the crystal and it often extends beyond into immediately adjoining or "plastered-over" orthoclase. Geopeko polished wiborgite specimen.

23 holm, 1928, pp. 94, 95; Backlund, 1938, pp. 362, 366, 392; Hutchinson, 1956, p. 773; Raguin, 1965, p. 70; Dawes, 1966, p. 570; Sederholm, 1967, p. 176; Elders, 1968, pp. 38-40; Vorma, 1971, pp. 22, 26; Key and Wright, 1982, p. 115). This is a clear indication of their accretionary origin. The mechanism of accretion and rapakivi ovoid genesis will be discussed in a later section but several examples and different types of re-aggregation are observed. A very clear example of conjoining is described by Sederholm (1928, p. 94) where he says a very big feldspar "obviously has originated by the combination of several smaller feldspars". His illustration is traced here as Fig. 17. Virtually every exposure or photograph of a rapakivi surface shows some examples of the conjoining or re-aggregation of the ovoids. Figs. 1, 14, 15, 18, 19 and 20 are typical. There are also many examples where the ovoids, either rimmed or pyterlitic, are "plastered over" with additional, apparently new, feldspathic material added externally round the rim (Figs. 4, 8). Sederholm (1928, p. 94) illustrates how (his plate XII, fig. 2, traced here as Fig. 21) "one orthoclase crystal is surrounded by a rim of oligoclase around which we find, again, other shells consisting mainly of orthoclase". Sometimes the additional feldspathic material is relatively uniformly spread round the outer periphery of the ovoid but frequently it is uneven or irregular with most of the addition on one side. Figs. 4, 8 and 9 show examples of this. In many cases the big feldspar ovoids consist of rounded aggregations of individual anhedral grains of feldspar often containing some quartz. Sederholm (1928, p. 95) points out that these rounded aggregates may be sur-

~,.

I

~I~ "

•

~

""..

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•

~,

-

-

.:~1~1.,,

,---~.1

-

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~

~

..

~?..'

I

-.-.i~-.,...--.~-I

,-.1._,, .::.y "--::":.-,:.,

Fig. 17. A large accretionary feldspar (7.5 cm by 10.5 cm) combining two smaller ovoids and large segments from the rapakivi granite at Wiburg near the harbour. Redrawn from the illustration of Sederhoim, 1928, plate 12, fig. 3.

24

Fig. 18. A reaggregated mass of accretionary potash feldspar in which are embedded pieces of earlier-formed plagioclase rim and other inclusions. Geopeko polished wiborgite specimen.

Fig. 19. Aggregation of a mixed ovoid incorporating a rimmed fragment of an earlier-formed ovoid, quartz and plagioclase fragments and matrix minerals within the new accretionary feldspar mass. Fragments of rim-type plagioclase are also isolated in the matrix. Geopeko polished wiborgite s/gecimen.

25

Fig. 20. Reaggregation of ovoids in which new conjoining feldspar can be seen "splashed" in an irregular contact against an older ovoid. The smaller ovoid on the left is reaggregated plagioclase with an impure sericite-rich centre. Geopeko polished wiborgite specimen.

~i~i

~

Fig. 21. A spheroidal crystal of 0rthoctase su/'rounded :by a tightly welded heavy mantle of plagioclase around which again are oute~ shells consi.sting mainly of orthoclase. The specimen is from the Wiburg rapakivi granite near,the harbour, redrawn from the illustration of Sederholm, 1928, plate 12, fig. 2.

26 rounded by coatings of orthoclase and plagioclase in which the individual segments or anhedral crystals have the same size and character as those comprising the ovoid core. Elders (1968, p. 40, plates 1D and 2A) also illustrates this phenomenon from the Wisconsin rapakivis. The alkali feldspar ovoids sometimes occur as a single crystal or twin but in most cases (Vorma, 1971, p. 22) are composed of more than two grains sectorially grown. Elders (1968, p. 40 and plate 1E) illustrates six individual twinned microcline crystals radiating from a c o m m o n centre to form a composite circular augen, shaped as indicated by Fig. 22. The aggregates comprising the mantled ovoids often contain mesostasis minerals (Figs. 5, 23) or segments of groundmass granite. Backlund (1938, p. 366) points out that among the mantled ovoids are some which have cores consisting of a granular mixture of groundmass minerals which is then surrounded by an inner mantle of coarse potassium feldspar individuals with the whole being enclosed by an outer plagioclase mantle. Some of the mantled feldspars appear to have come in contact with each other (Elders, 1968, pp. 38, 40) after being rimmed by plagioclase. Amongst the ovoids with somewhat unusual features, Wahl (1925, pp. 54-60) de-

Fig. 22. The ovoid crystallization pattern is often as a single crystal or a crystal in each of the two halves. In most cases feldspar ovoids are composed of more than two grains sectorially grown. Six individual twinned microcline crystals radiating from a common centre form a composite circular augen in this ovoid illustrated by Elders, 1968, plate 1E.

27

Fig. 23. Accretion of fragments illustrating a stage m the reaggregation of the darker grey plagioclase to form a new ovoid. The orthoclase feldspar contains large inclusions of matrix minerals and a smaller spherical quartz accretion (black) can be seen on the left of the aggregating oligoclase. Geopeko polished wiborgite specimen.

scribes one orthoclase ovoid engulfing another and a single plagioclase mantle enveloping two orthoclase ovoids.

Granitic and micropegmatitic ovoids In places the amount of inclusions in the rapakivi ovoids increases to the extent that the centre forms a granitic aggregate (Vorma, 1971, p. 26). In rare cases the whole ovoid inside the mantle consists of a granitic aggregate, and Sederholm (1967, p. 176) observed a few cases where these aggregates of granitic minerals had a major diameter of more than a decimeter having the same ovoidal form and surrounded by borders of orthoclase and plagioclase. Raguin (1965, p. 70) refers to the rapakivi ovoids occupied by granitic aggregates resembling the groundmass and points out that they are thus similar to the granitic cores in some orbicular granites. Wahl (1925, pp. 54-60) describes large granite balls up to 20 cm or even 50 cm in diameter surrounded by plagioclase mantles. Both Backlund (1938, p. 366) and Vorma (1971, p. 26) cite instances where the mantled granite

28 balls contain two or three pyteditic potash feldspar ovoids within the rimmed granite aggregate. Although more usual in the orthoclase of the groundmass, as "patches" within the ovoidal cores (Fig. 7), or at the junction of the orthoclase core with the rimming plagioclase mantle, micrographic intergrowths of quartz and orthoclase occasionally comprise the ovoids (Fig. 14; Sederholm, 1967, p. 570; Key and Wright, 1982, p. 115). The quartz inclusions in rapakivi alkali feldspar ovoids are not always "graphic" or "micrographic" in the sense that the individual parts of the inclusion necessarily develop euhedral crystal outlines against the enclosing feldspar as in the typical graphic pegmatites. Although generally extinguishing together, these inclusions can be isolated or occur in patches of relatively rounded vermiform inclusions. These "drop quartz" inclusions can also occur in the mantling plagioclase and Vorma (1971, p. 25) distinguishes them from another type of quartz inclusion designated "concave quartz" which is always xenomorphic against the alkali feldspar host. Concave quartz can form irregular anastomosing aggregates that extinguish optically as a whole, and in some cases this type of "cuspate crack filling" quartz inclusion is surrounded by an albite-quartz myrmekite rim (see Vorma, 1971, fig. 5).

Inclusion of rimming material within ovoids and re-aggregation of rims Addition of potassium feldspar to the exterior of mantled ovoids to further cover or enclose the plagioclase rims has already been pointed out, but there are examples where the rapakivi ovoids have a rather "battered look" with segments of the rim "ripped off" (Figs. 3, 24, 25). There are pieces of rim plagioclase apparently isolated in the matrix (Fig. 19) and instances where such plagioclase segments seem to be re-aggregating to form a new plagioclase ovoid (Fig. 23). Stull (1978, p. 246) records portions of the oligoclase mantle protruding into the orthoclase ovoid and this type of inward "tonguing" mantle is illustrated by Elders (1968, plate 1C; and Figs. 12 and 16). What are apparently pieces of rimming plagioclase incorporated in new or reconstituted ovoids are seen in Figs. 15, 18, 25, and 26. In Fig. 27 an ovoid contains a folded or double piece of included plagioclase which is almost certainly part of a former rim. From these observations it can be concluded that the rimming plagioclase behaves as a coherent entity prior to the cessation of the mobility which permits the ovoids to fracture, mould against each other, accrete additional external material to their margins and re-aggregate into new ovoids which incorporate pieces (or even entire ovoids) of those previously established.

29

Fig. 24. "Battered" and "churned up" ovoids with irregular and incomplete rims, ovoids of reaggregated plagioclase (white) and patches of irregular inclusions of groundmass minerals. Some of the small dark quartz ovoids are incorporated within the feldspar ovoids. Summa, Finland.

The re-aggregation of the ovoids clearly includes those previously rimmed (Fig. 18) and pieces of rimming material.

Development of ovoids in inclusions and wallrocks The occurrence of mantled ovoids within xenolithic inclusions and wall rocks in circumstances where they could not have crystallized directly from a melt has provoked somewhat speculative and anguished discussion in relation to rapakivi genesis (Stewart, 1959, p. 316; Dawes, 1966, p. 570; Sederholm, 1967, pp. 188-189; Vorma, 1971, p. 60; Didier, 1973, pp. 214-215). It is claimed that such development of ovoids in the xenoliths and either isolated or in "islands" within invading microgranite dykes (Key and Wright, 1982, p. 115) establishes a metasomatic or autometasomatic origin for the potash feldspar phenocrysts. Vorma (1971, p. 60) cites the occurrence of wiborgitic ovoids: (i) Set in the groundmass of fine-grained dyke rocks belonging to the rapakivi suite;

30

Fig. 25. The well-developed concentric zone of inclusions in this ovoid reflects the outline of a former ovoid against which were entrapped pieces of rim plagioclase and mesostasis minerals as the outer material, now alkali feldspar, was "plastered over" it in a subsequent mobilisation. The irregular frayed outer periphery reflects the plastic kneading at this rim with material adhering and shredding off and with indentation and incorporation of some matrix material. It is not the way a crystal forms from a melt. Geopeko polished wiborgite specimen.

(ii) F r o m the effusive quartz p o r p h y r y which is c o m a g m a t i c with the rapakivi granites; (iii) F r o m within xenoliths in rapakivi granite; a n d (iv) In basic rocks and in gneisses penetrated by granitic material. Backlund (1938, p. 351) records the spread of scattered n o n - m a n t l e d ovoids into the pelitic m e t a m o r p h i c wall rocks at the contact of the W i b o r g granite southeast of Borgh. In the Tasiussak rapakivi, Dawes (1966, p. 570) describes feldspar megacrysts present either singly or in irregular clusters, in

31

Fig. 26. Ovoids "sticking" to each other tend to form chains. External fragments can be seen adhering to the ovoid on the right and the ovoid on the left contains a tabular plagioclase inclusion, probably a piece of a former rim. Geopeko polished wiborgite specimen.

Fig. 27. A folded or double piece of plagioclase included in an ovoid is almost certainly a part of a former rim. A more completely rimmed fragment is included in a small ovoid near the bottom of the photograph. Ornamental wiborgite, Innsbruck, Austria.

32

UNRIMMED

RGE THICK IMMED

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Fig. 28. Outline of ovoid rimming patterns sketched from various photographs to illustrate the main types which have been recorded.

33 net-veined basic rocks in situation which excludes all possibility of direct melt crystallization or diffusion of alkalis from the magma. From the Shap granite in Westmoreland, calcareous shale inclusions which contain mantled ovoids, pose a special problem for Stewart (1959, p. 316). No mantled ovoids are reported in the surrounding granite and he therefore suggests isolation from equilibrium or the calcareous environment within the inclusion as being responsible for the development of mantled ovoids within the sedimentary xenoliths. The most common enclaves in which scattered mantled ovoids are found appear to be essentially granitic, similar to the host rapakivi, as in the Yt~3 granite where Vorma (1976, p. 49) interprets the enclaves as autoliths. In the rapakivi massifs there is a great diversity of ovoidal sizes, shapes, rimming patterns, inclusions, fracture features, aggregations and compositions. This, together with the wide variety of rock types and situations in which the ovoids are found, is a clear repudiation of any derivation such as cooling a molten rock through its melting point. The main types of ovoids, rimming patterns, fracture fragments and aggregations are summarised in the sketches of Fig. 28.

Rapakivi disintegrationproblem Within some varieties of ovoidal rapakivi granites there are horizontal streaks or 'schlieren', nearly flat bed-like bodies, or irregular rounded patches several metres in diameter which, although quite fresh, have crumbled into gravel. The ovoids become concentrated within the collapsed matrix which, according to Backlund (1938, p. 358), gives the impression, not of having redissolved, but of never having been there. In these local patches within certain rapakivi granites there is a very sparse or no matrix to support the feldspar ovoids. It is stressed that there is no mineralogical or chemical difference between the crumbled rock (called " m o r o " in Finnish) and its adjoining fresh rocks. Nor is the " m o r o " connected with any form of tectonic deformation, original cooling surface, or exposure to weathering. Backlund (1938, p. 359) suggests that there must have been some prerapakivi stage in the evolution of the rocks where certain "beds" or zones had a somewhat different permeability. The 'schlieren'-like and rounded forms of the " m o r o " bodies point also to differences of permeability within some porous pre-existing rock. The inexplicable disintegration of local patches or zones of matrix in the Finnish mantled-ovoid granites gave these rocks their name. However, the successful and extensive use over many years of rapakivi as an ornamental

34 stone shows that the normal varieties are no more susceptible to weathering than any other coarse-grained or coarse porphyritic rock. The tendency of the " m o r o " zones to disintegrate or crumble appears to be entirely due to a lack of cementing matrix. This constitutes an insurmountable problem if an interpretation of the rock as a crystallized melt is attempted. PROBLEMS OF RAPAKIVI CRYSTALLIZATION

Variability of potash feldspar ouoids In the rapakivi granites each ovoid has its own pattern of crystallization. This is most clearly indicated by the occurrence of various types of rimmed and internal concentrically-zoned ovoids in the same matrix as those which have no rims. The large feldspar ovoids occur as single crystals, Carlsbad twins, or in most cases (Vorma, 1971, p. 22) as aggregates of more than two grains sectorially grown but not related to any twinning operator. One specimen or even a single feldspar crystal can exhibit film perthite, string perthite, and vein perthite in various parts of the perthitic ovoid. The various types of perthitic ovoids occur in the same matrix as non-perthitic ones (Vorma, 1971, pp. 30, 31). In the rapakivi Gaborone granites, Key and Wright (1982, p. 118) report microperthitic microcline with a wide range of textural variations, with the perthitic plagioclase occurring irregularly in patches, braided networks, veins, and streaks. They claim that some of this is demonstrably of replacement origin. Ragland (1969, p. 177) measures obliquities of the potassic phase in the perthites from the rapakivi zone in the Enchanted Rock batholith to find that all obliquities from 0.00 to about 0.85 apparently exist in each specimen. Individual phenocrysts exhibit any one of a number of structural states and a study (Ragland, 1969, p. 179) of variation in 2Vx of the potassic phase over several thin-sections located domains or areas of "orthoclase" with much lower optic angles in untwinned portions of the large microcline perthite phenocrysts. These areas of lower 2Vx seem to be optically continuous with other portions of the host potassic crystals and there is no apparent relationship between the type and abundance of perthite lamellae. This detailed work on rapakivi alkali feldspars by Ragland clearly shows patchy compositional variation within individual crystals which would be very difficult to ascribe to any melt-cooling derivation. Marmo (1971, p. 125) also cites a large variation of triclinicities from spot to spot within the same potassic ovoid crystal from the Wiborg massif, and Elders (1968, p. 44) records some slight variability in the compositions of the

35

alkali feldspars from crystal to crystal and within crystals from the northeastern Wisconsin rapakivi granites. Elders points out (p. 45) that the ratio of alkali feldspar to plagioclase is variable from crystal to crystal and that for the nucleation of oligoclase to be localised, not only on the rims of the ovoids but as internal concentric zones, patchy inclusions, poikilitic skeletal inclusions (plate 2D) and various types of perthitic lamellae, must require earlier inhomogeneity. Backlund (1938, p. 363) draws attention to the variability in quartz content of the ovoids• There is a high variation in the bulk content of SiO 2 but nearly all the rapakivi varieties are characterised by intergrowths between potassium feldspar and quartz which are irregular in their distribution in the ovoids (Fig. 7). Nearly quartz-free varieties of rapakivi also contain such intergrowths. Epitomising the hitherto unexplainable mixed crystals of the rapakivis are some illustrated by Vorma, 1971, in his figs. 13 to 16. His fig. 15 (p. 39, outlined here as Fig. 29) for example, shows an impure twinned plagioclase sharply transitional into anhedrally adjoining alkali feldspar with some faint preservation of the plagioclase twinning planes. In the potash feldspar part of the crystal, sparse plagioclase forms a microperthitic texture in optical continuity with the plagioclase end of the crystal, but most of the potash end of the crystal is a micropegmatite with irregular vermiform graphic quartz patches intergrown as a unit quartz crystal within the potash feldspar. This weird "three in one" crystal would certainly defy all laws of melt derivation!

ORTHOCLASE / \ i / "q~:...~

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Fig. 29. A hitherto inexplicably mixed-up feldspar crystal from an even-grained biotite rapakivi. Micropegmatitic texture is abundant in the matrix of this granite of medium coarseness which contains sparsely scattered potash feldspar ovoids. Sketched from Vorma, 1971, fig. 15.

36 The significance is that this "impossible" crystal is from an even-grained biotite rapakivi of medium coarseness. Vorma (1971) details many observations on the albite lamellae in perthites, the interdependence of perthite types, the nature of the potassic phase, the cross-twinning of the microclines, and the frequency of occurrence of these potash feldspar types in different varieties of the rapakivi granites. He claims (p. 37) that the data proves consistent over the series of r a p a k i v i s - tirilite, dark wiborgite, normal wiborgite, pyterlite, porphyritic rapakivi, and even-grained biotite rapakivi. Insofar as the crystallization conditions for rapakivis are peculiar and generate unusual or inexplicable crystals, these conditions apply to the wider range of rapakivi types and not just to the mantled ovoidal wiborgites.

Crystallinity of rimming plagiocloase The surprising variety of form of the mantles is surpassed by an even more unexpected behaviour in their crystallinity. In most rapakivis the rimming plagioclase is frequently observed to be in optical continuity with the various types of perthitic lamellae within the alkali feldspar it surrounds (Dawes, 1966, p. 570; Hawkes, 1967, p. 270; Elders, 1968, p. 43; Vorma, 1971, p. 24; Key and Wright, 1982, p. 119). Optical continuity between perthite lamellae and plagioclase mantles is, however, far from being universal (Elders, 1968, p. 45). The same applies to the euhedral or tabular plagioclase inclusions and the internal concentric zones of the multiple rimmed ovoids, although in many cases the polysynthetically twinned mantle plagioclase shares its twinning habit and orientation with the plagioclase patches within the ovoidal orthoclase (Hawkes, 1967, p. 271; Vorma, 1971, p. 23). This observation of frequent optical continuity between the mantling plagioclase and the perthitic lamellae, internal concentric plagioclase zones, and "patch" or irregular tabular inclusions of plagioclase within the alkali feldspar ovoids is important. Similar observations include: (1) Internal included plagioclase is in some cases gradational to the rimming plagioclase (Popoff, 1928, p. 11). (2) Patchy zoning in the plagioclase is widespread (Stull, 1978, p. 247). (3) Film perthite and string perthite textures coalesce to form veins of plagioclase within the alkali feldspars (Vorma, 1971, p. 37). (4) Anti-rapakivis with plagioclase in the centre of the ovoids and potash feldspars round it have been recorded (Raguin, 1965, p. 72; Elders, 1968, p. 44; Key and Wright, 1982, p. 114). (5) Anti-perthites of potassic feldspar lamellae within the oligoclase of the anti-rapakivi cores or of the mantles is occasionally observed (Backlund, 1938, p. 366; Elders, 1968, p. 44; Stull, 1978, p. 243).

37 (6) The rimming plagioclase on the potassic ovoids is irregular, discontinuous and sometimes absent or comprised of up to 60 or more short prismatic individual crystals arranged round the central ovoid (Backlund, 1938, p. 365; Key and Wright, 1982, p. 114). (7) The plagioclase is zoned with a more soda-rich phase commonly developed on the inner margin of the mantles or extending into perthite lamellae in adjoning potassic feldspars. It is regarded as a late crystallization phenomenon after the essential rapakivi rimmed ovoidal texture had developed (Hawkes, 1967, p. 270; Elders, 1968, p. 44; Vorma, 1971, pp. 34, 38; Stull, 1978, p. 245; Key and Wright, 1982, p. 118). (8) The rapakivi plagioclase exhibits only the low-temperature sodic phase (Stewart, 1959, p. 296; Key and Wright, 1982, p. 119). (9) There is some relationship or agreement in crystallographic orientation of the cores and mantles in many cases (Elders, 1968, p. 39). Popoff (1928, p. 13) notes that the albite twins of the ovoid perimeter tend to develop normal to the 001 crystal faces of the orthoclase ovoid while the pericline twins are best developed on the 010 surface. Stull (1978, p. 245) points out that the orthoclase and rimming plagioclase have common b and c axes; consequently a single Carlsbad twin plane is sometimes found transecting the entire mantled feldspar structure. This diversity of crystallization patterns in the plagioclase also indicates that the conditions of crystallization were highly variable for each mantle and that these variable crystallization conditions operated essentially on the scale of the individual ovoids.

Crystal continuity of oooid and matrix minerals Both the potassic feldspars and included quartz within the ovoidal cores occasionally show crystal continuity with the minerals of the surrounding matrix. Plagioclase mantles in the Golden Horn rapakivi are described by Stull (1978, pp. 243, 245, and fig. 3) as being crosscut by channels filled with abundant perthitic orthoclase. This orthoclase is optically continuous with the interior mantled crystal and the exterior intergranular precipitate. Elders (1968, plate 2C) illustrates a twinned microcline crystal extending beyond its plagioclase rim irregularly into the matrix, and Vorma (1971, pp. 28, 61 and his fig. 7) similarly shows an incompletely mantled wiborgitic alkali feldspar crystal extended by development of micropegmatitic groundmass alkali feldspar. This has developed on the unmantled portion of the ovoid. Vorma bases his conclusion that the ovoid was formed before the groundmass was completely solidified on this observation. He also points out (p. 28) that the micrographic quartz developed in the groundmass marginal to discontinu-

38 ously mantled wiborgitic and pyterlitic ovoids, often extinguishes simultaneously with the adjacent concave quartz within the ovoids. Quartz within the rapakivi ovoids in many cases forms a partial mantle or a series of irregular elongate inclusions round the contact of the core orthoclase with its mantling plagioclase. This interfacial quartz (Stull, 1978, p. 246) is often optically continuous with exterior granophyric quartz, indicating simultaneous crystallization. The perthite lamellae also transgress grain boundaries. Vorma (1971, p. 33, fig. 11) illustrates albite vein lamellae transecting the contact between orthoclase and microcline within a pyterlitic ovoid in wiborgite. He also notes (p. 37) that in fine-grained groundmass alkali feldspar, the precipitation of perthite took place over the grain boundaries. These observations of parts of the same crystal extending between the rapakivi ovoids and the surrounding matrix clearly imply that the crystallization was later or was completed later than the formation of the ovoids. The inclusions of matrix minerals, quartz, and plagioclase, which occur in such frequency and abundance within the rapakivi alkali feldspar ovoids, indicate the contrary if a melt derivation were assumed. This would require that these elements crystallized earlier so that they could be incorporated in the ovoids.

Inconsistencies with melt-cooling theory Attention has already been drawn to some of the difficulties and inconsistencies with melt/crystal equilibrium theory. Others recorded include: (1) The feldspar compositions of most rapakivi granites plot in the field where plagioclase would be the first to crystallize (Stewart, 1959, p. 308; Vorma, 1971, p. 59). This is not consistent with the development of orthoclase ovoids (non-mantled pyterlites) as first-forming porphyroblasts or with a subsequent mantling by plagioclase to form wiborgite ovoids. (2) The textural relations of the feldspar phenocrysts in a granophyre from the Gaborone batholith indicate a melt derivation. However, from a melt of the average composition of the analysed granophyres a feldspar richer in orthoclase would have been expected. Several repeat observations confirmed this discrepancy and no satisfactory explanation is available (Key and Wright, 1982, p. 117). (3) Quartz-feldspar intergrowths (micropegmatitic) are common in ovoidal and even-grained rapakivis. They occur within the cores of ovoids, at the junction of core orthoclase and its mantling plagioclase shell, and within the groundmass (Figs. 7, 30). (4) The large megacrysts are supposed to have derived by crystallization at some depth and to have achieved the ovoidal form by later resorption

39

v w ~ w ~

w uw~wm

• m wh,

Fig. 30. A mixture of rounded and "teased out" potash feldspar and plagioclaseovoids which tend to merge with the matrix. Some micropegmatitictexture is visible in the matrix. Summa, Finland. (presumably repeated resorption in the cases of many rings of inclusions). However inclusions of quartz, idiomorphic plagioclase, and groundmass minerals or "patches" within the central parts of the large ovoids indicate that these minerals had formed prior to their incorporation in the ovoids. No routine explanation based on the usual conceptions of melt behaviour satisfies these implications (Backlund, 1938, p. 367). (5) Localisation of exsolved plagioclase along internal zones as well as perthite lamellae and rims in an originally homogeneous ternary feldspar is difficult to explain as in situ exsolution by conventional melt/crystal equilibrium theory (Elders, 1968, p. 45). (6) Microcline from the cores of rapakivi ovoids, from co-existing phenocrysts without mantles, from the groundmass, and from porphyroblasts in inclusions has nearly the same composition. Plagioclase at the same localities varies from An 3 to An36 (Stewart, 1959, p. 300). This variation is not explained. (7) The petrogenetic conclusions to be drawn from homogenization experiments are limited because of the incomplete homogenization of the alkali feldspar perthites on heating for three hours at 1000°C (Vorma, 1971, p. 58). Perthites derived by exsolution on cooling a melt should homogenize completely on reheating.

40 (8) Clearly the theory of unmixing from homogeneous high orthoclase could not produce a closely conjoined oligoclase rimmed with albite which re-enters the orthoclase as vein perthite in optical continuity with the albite rim round the plagioclase grain. The perthite texture cannot, as a whole, be the result of exsolution (Vorma, 1971, p. 38). (9) "Both the wiborgitic and pyterlitic ovoids of the dark wiborgite are characterised by the same kind of perthite as the groundmass alkali feldspar. The amount and grade of perthite exsolution show the same type of variation as the groundmass. Also the ratio of cross-hatched alkali feldspar to the feldspar without cross-hatching or only with an undulatory extinction is the same as in the groundmass" (Vorma, 1971, p. 32). This would indicate that the ovoid alkali feldspar is the same as the groundmass alkali feldspar. Yet if these crystals did derive from a melt, the porphyritic ovoid should have crystallized first thus changing the melt composition and character of the later crystallizing groundmass. (10) Large granitic ovoids mantled by plagioclase appear totally inconsistent with any theory of molten derivation (Wahl, 1925, pp. 54-60; Backlund, 1938, p. 366; Raguin, 1965, p. 70; Sederholm, 1967, p. 176; Vorma, 1971, p. 26).

Summary of crystallization problems The various crystallization patterns in the alkali feldspar of the ovoids and in the plagioclase of the mantles, internal zones, perthites, and inclusions, preclude any uniformity of crystallization applicable to all the neighbouring crystals within a given volume of rock, as would be expected in a cooling melt. Crystallization conditions varied even within individual megacrysts. Variations in type of perthite developed, triclinicity, ratio of potassium feldspar to plagioclase, and variability in quartz content, all reflect a high degree of variability within individual megacrysts. In a normal geological situation it is not tenable to propose markedly differing "micro-environments", such as would cause the observed variations in crystallization patterns between individual adjoining ovoids. It is therefore necessary to propose that the individual ovoids differed in composition and structure before they crystallized. Their common crystallization history and conditions applied to variably comprised precursor ovoids. Sederholm (1928, p. 95) sets out the concept "according to which a magma should always simultaneously pass through the same stages of development, indicating changes which have occurred all over it at the same time". However, having regard to the rapakivi rocks he is forced to go on: "the great individual differences of many of the feldspar ovoids, show that local differences in the conditions of the mineral formation have existed.

41 Thus it is necessary not to apply the idea of definite stages in the crystallization all over the magma too schematically". If at any stage in rapakivi genesis there had been changes in the physical conditions of a melt which either precipitated or resorbed the crystals "all over it at the same time", then clearly it follows that subsequently a great diversity of "micro" alteration or recrystallization environments were necessary. These must have operated on the scale of the individual ovoids and within the crystals themselves in order to generate the observed diversity among the rapakivi minerals. Thus the assumption of such initial uniformity is highly improbable. The series of observations which indicate that the crystallization is later than the formation of the ovoidal and rimming structures resolves the dilemma of the inclusions within the ovoids of matrix material; of parts of rims; of quartz; and parts of, or even the incorporation of entire additional ovoids. However, a concept of multiple liquefaction or successive stages of fluidity is also necessary to mix the rimmed ovoids with those non-rimmed, to enable the incorporation of parts, or of entire (Fig. 17) earlier-formed ovoids within later ones, or to incorporate groundmass minerals as earlier patches or in successive zones. Fortunately a number of phases of liquefaction and reintrusion for rapakivi batholiths is fully supported by the evidence of differing textural types, sharp internal contacts, contemporaneous dykes, marginal breccias, autolith swarms, digestion of fragments, and examples of cohesive autolithic bodies rebrecciated by reliquefaction of the same magma. As is general in the case of reliquefaction of later plutons in a batholith, there is the problem of the source of the latent heat required if remelting of an earlier congealed melt were proposed. In the case of the rapakivi granites with their complex and specific textural and recrystallization features, the following factors preclude absolutely a simple melt-cooling genesis: (i) the diversity of environments in which the ovoids form (Vorma, 1971, p. 60); (ii) the diversity of ovoid types (Fig. 28) (Vorma, 1971, p. 26); (iii) the diversity of crystallization patterns (Ragland, 1969, pp. 167-189); and (iv) the diversity of opinion as to genesis (Sederholm, 1928, pp. 88-94; Backlund, 1938, pp. 372-376; Vorma, 1971, p. 59; etc.). Virtually every writer on the rapakivi granites has acknowledged the wide diversity of opinion on genesis and many lengthy dissertations have agonised over the inconsistencies with melt-cooling theory. It has, in fact, never been concluded that such rocks derive directly from the cooling of a melt. Multiple mantling and repeated zoning within the ovoids will require the path of crystallization to remain most tortuous if a basically molten genesis continues to be pursued. It is therefore timely that the genesis of rapakivi

42 granites be considered in the context of a wider range of physical chemistry than the restricted melt-to-crystal phase-change theory, with its requirement for latent heat on remobilisation. Comparatively recent developments in understanding the behaviour of concentrated suspensoids and macromolecular systems now provide an adequate physico-chemical system to account for the mobilisation of the pre-granite materials, the development of the rapakivi textures, and their later crystallization and elevation of temperature.

Obseroations indicatioe of the solution to the rapakioi problem The very detailed work done on the rapakivi granites and the many observations recorded have already compelled postulation of essentially correct attributes and the environment for rapakivi development. With commendable perception, earlier work records many clear indications that rapakivi "magmas" were mobilised hydrosilicates which have then crystallized. While there are probably many more such examples, perhaps the main pointers from earlier work which should be acknowledged are the following. (1) Backlund (1938, p. 373) ponders in bewilderment the strange textural, mineralogical and crystallization pecularities rapakivis present. He concludes (pp. 353, 377, 379) that they are flat bodies designated as taphroliths, originating at shallow depths as replacements (p. 383) of different formations of Jotian sediments. He maintains (p. 359) that the rapakivi granites disclose a rheomorphic origin involving the "transformation" (p. 387) and "remodeling" (p. 391) of Jotian molasse sediments. He claims (p. 388) that this form of transformation, or granite genesis, is the only one which is able to resolve the apparent contradictions in texture and crystallization and to offer a solution to the "space problem". (2) Two stages in the formation of rapakivi granites have been recognised. The formation of the ovoids before consolidation of the groundmass is then followed by later crystallization (Sederholm, 1928, pp. 90, 94; Dawes, 1966, p. 569; Sederholm, 1967, p. 179; Elders, 1968, p. 37; Vorma, 1971, p. 61). Vorma (1971, p. 60) goes even further by saying that the evolution of the wiborgitic ovoids is not bound to any single process or to one specific stage in rapakivi magma crystallization. (3) The femic minerals crystallized after a long interval following crystallization of the main feldspars in the rapakivi granites (Sederholm, 1928, p. 94; Backlund, 1938, p. 382). (4) The rounded form of the ovoids has existed during the whole time of their growth (Sederholm, 1967, p. 176), and their nature as aggregates of conjoined fragments has already been noted (see pp. 22, 23). (5) The rapakivi ovoids closely resemble the smaller spheroids of the

43 orbicular granites (Sederholm, 1928, p. 96; Raguin, 1965, p. 70) and it is suggested that they have a similar origin (see Elliston, 1984). (6) A soft or plastic stage in the development of the rapakivi ovoids has been noted (see p. 19). (7) In porphyroblastic varieties of granitic composition, such as rapakivis, problems concerning the mechanism of concretionary enrichment in rocks which have not entirely fused are involved (Marmo, 1971, p. 147). (8) When multiple zoning or mantling occurs, the groundmass minerals seem to be concentrated at the contacts of these zones (Elders, 1968, p. 40). (9) Textural details of the ovoids which may constitute criteria for short periodic changes in the concentration of the material crystallizing, such as inclusion rings and plagioclase rims, are suggestive of diffusion processes (Backlund, 1938, p. 392). (10) Rapakivi rims are considered to have developed by migration of exsolved plagioclase and marginal coalescence (Key and Wright, 1982, p. 119). (11) For the perthitic and rimming plagioclase in the ovoids an internal source of sodium is required (Key and Wright, 1982, p. 119), and Stull (1978, p. 245) suggests that a margin of more sodic plagioclase (An10) which appears continuous with the rim and coats the inner rim zone, represents crystallization after rounding of the ovoids and formation of its plagioclase mantle. Prucha (1946) and R.M. Gates (in Emmons et al., 1953) require a donor and recipient for the sodium, calcium, and aluminium which is envisaged as unmixing from the ovoid, migrating, and precipitating at its rim. (12) The rapakivi ovoids are observed to be fractured (see p. 21) and often show a marked elongation (Key and Wright, 1982, p. 115). The fractures can be rehealed at an early stage (Elders, 1968, p. 39). (13) Vorma (1971, p. 64) claims that the presence of miarolitic cavities in rapakivi granites proves that the consolidating magma contained a volatile phase, and he suggests a liquid at temperatures below 400°C, probably containing more than 30% water. (14) The great individual differences in many of the feldspar ovoids show that local differences in the conditions of crystallization existed (Sederholm, 1928, p. 95). Nucleation of oligoclase is localised in perthite lamellae, in internal concentric zones or patches as well as rims. This requires earlier inhomogeneity (Elders, 1968, p. 45). All these conclusions emerging from earlier work are consistent with the behaviour of a "magma" or thixotropic paste of hydrosilicates. They are consistent with retexturing associated with mobility (Elliston, 1968) and subsequent elevation of temperature on crystallization which is confined to the intrusive or remobilised parts of the system.

44

PHYSICAL AND CHEMICAL PROPERTIES OF A "MAGMA" FOR CRYSTALLIZATION OF RAPAKIVI TEXTURE The physico-chemical system for the aggregation of ovoids, the development of internal "shells" and rims richer in sodium and calcium, inclusions of matrix minerals and parts of the rimming material, together with the complex features developed on rapakivi crystallization, must meet numerous and quite specific requirements. If there is no pre-supposed genetic hypothesis, the observations would compel the following properties to be attributed to a rapakivi "magma". (1) Fluidity to enable the material to intrude or to flow as a flat sheet-like sill into shallow grabens. (2) Multiple mobilisation of the same material to enable it to reintrude, break up earlier phases as autoliths, and to incorporate earlier-formed ovoids or parts thereof, together with groundmass minerals, in new ovoidal aggregates and composites. (3) The formation of spherical aggregates or nodules of alumino-silicate material within the fluid "magma" which are transported by it and which as entities can be separated from it to be incorporated in wall rocks, enclaves, associated intrusions, etc. (4) There must be a soft stage in the formation of the rapakivi ovoids to enable them to mould against each other and "weld" together into new aggregates. (5) The ovoids at the time of mobility of the matrix must be cohesive enough to reaggregate and to fracture. (6) Between successive mobilisations some ovoids must be able to form margins richer in sodium and calcium. An internal source of sodium and calcium is required with differing situations as to sodium calcium content and extent of rim development applying on the scale of each ovoid. (7) If sodium and calcium from an internal source are to form various mantling rims a diffusion property might be inferred for the ovoids. (8) Further aggregation, overgrowth and addition of alumino-silicate material to the periphery must be possible in successive mobilisations, and in some cases this new material must be "plastered" over existing rims and then subsequently develop a further mantle on the new outer margin. (9) The successive remobilisation and reaggregation of the ovoids must be able to incorporate matrix material, pieces of earlier ovoids, former rims, etc. (10) Rims must be able to develop on fragments of ovoids, on composites and even on "granite balls" or aggregates of essentially matrix material. (11) Ovoids must be a non-homogeneous admixture of "patches" of alumino-silicate to give the differing areas of triclinicity within the developing crystal. From ovoid to ovoid there must be varying proportions of

45 potassium to sodium and calcium in order to derive the various perthitic unmixing textures on crystallization. (12) Crystallization of the constituent minerals of the rapakivis must be directly from precursor substances which reflect the specific composition and structure of the various parts of the ovoids. (13) Skeletal crystals and intergrowths must be able to develop between isolated inclusions, perthitic lamellae and rims. Intergrowths must be able to extend over grain boundaries. (14) Rapakivi "magmas" must be highly hydrous to enable development of miarolitic cavities, patches of "moro" (see p. 33) and residual hydrous minerals such as chlorite, sericite, biotite, epidote, grunerite, iddingsite, saussurite, leucoxene and muscovite. (15) Minerals such as calcite, iron oxides, chlorite and pale-green epidote, are incongruent with melting or fusion in the presence of excess quartz. However, they are found in rapakivi granites and sometimes within the ovoids. These must reflect non-reactive precursor substances at the time of "magma" fluidity. Because the rapakivi textures and crystallization features are clearly documented and quite specific, it follows that the physico-chemical system which will meet the exacting observational requirements is equally unique and specific. The problems of rapakivi genesis stem entirely from the incorrect assumption that crystallization of the minerals comprising the rapakivi granites was achieved by an anisothermal phase-change on cooling a melt. Even when subsequent extensive alteration and recrystallization is envisaged, a basic melt genesis is clearly inadequate. The plasticity of the rapakivi ovoids (see p. 19) and their similarity to orbicular cores (see p. 4) has already been noted. The writer (Elliston, 1984) has already detailed the much greater range of textures, diffusive properties, and rheological properties of disordered solvated silicates. If we modify the concept of rapakivi magma from "melt with water dissolved in" to "water combined with silicates at some elevation of temperature and at the time of fluidity", an adequate physico-chemical system, identical with that shown to be required for orbicular granites (Elliston, 1984), is available to account for rapakivi occurrence, texture and crystallization behaviour. Understanding the behaviour of concentrated suspensoids and macromolecular systems (Hauser and Le Beau, 1941; Frisch and Simha, 1957, Mielenz and King, 1955; Mysels, 1959; Boswell, 1961; Elliston, 1963(c) and 1968; Van Olphen, 1963) is a comparatively recent development. This simple physico-chemical system, involving hydrosilicates already present in deep geosynclinal sediment accumulations, offers a completely adequate explanation for some 30 puzzling or difficult phenomena relating to granites (Elliston, 1974).

46 In an average-sized sedimentary basin or filled geosynclinal graben there are some 1035 particles of colloidal size. If we generalise, these natural sediment colloids or macromolecules are basically of three completely different geometric shapes. The hydrous alumino-silicates (clays) are platelets (Fig. 31), the hydrous ferromagnesian minerals (glauconites, chamosites, chlorites, attapulgites, etc.) are rods or tubules (Fig. 32) and the hydrous silica polymers (balled chains and opal-type nodules) are spheres (Fig. 33). If a "degree of freedom" is introduced into such concentrated (partly dewatered) colloidal systems by thixotropic reliquefaction (earthquake shock) or pasty flow (gravity sliding), the particles are able to assume "close packed" aggregates which are characterised by their respective shapes. This effects the simple separation into "mottled" green-and-red retextured clays. On subsequent dehydration these aggregates crystallize to the common intrusive rock-forming minerals quite distinct in crystallinity and texture but similar in bulk composition to the enclosing non-reliquified rocks (Elliston, 1963(a)-(c), 1968, 1974, 1975). This is essentially the genesis of the rapakivi texture which must be one of the best examples of macromolecular accretion and subsequent crystallization in natural hydrosilicate pastes or "magmas".

Fig. 31. Electron micrograph of iilite (from Grim, 1953, p. 121, fig. 40) showing the comparatively large platelets, some 1000 nm to 1500 nm. Desorption and diffusion of electrolytes and smaller charged particles (say 20 nm to 50 nm) occur within meshworksof such platelets.

47

Fig. 32. Electron micrograph of attapulgite (from Van Olphen, 1963, p. 7, fig. 4) showing rod-like form of typical hydrous ferromagnesian minerals.

Fig. 33. Electron micrograph of polymer microspheres which are similar to the "balled chains" or opal-type microspheres of amorphous silica showing long-range ordering and the close-packing arrangement of the clusters. Photograph by courtesy of Professor T.W. Healy. (Source: A Kose and H. Hachisu, Inst. Applied Optics, Sci. Univ. Tokyo, Japan.)

48 The rheological and diffusive properties of hydrosilicate systems have already been detailed by the writer (Elliston, 1984) but the features of particular relevance to the development of rapakivi texture are set out in the following section. FLUID, DIFFUSIVE AND ACCRETIVE PROPERTIES OF HYDROSILICATES The " m a g m a " or semi-fluid mixture of mineral matter from which the rapakivi textures derive must be a gelatinous system of mixed hydrosilicates in order to encompass the wider range of fluid, diffusive and accretive properties required by the observations. No other physico-chemical system meets the very specific requirements to account for all the observations recorded. Hydrolysis of silica and silicates is generally enhanced at the somewhat elevated temperatures and pressures due to burial depth, which could be assumed for rapakivi "magmas". It is envisaged that in such buried semi-fluid mixtures of mineral matter the water (or water in the vapour phase if the temperature is sufficiently elevated) has had time to reach chemical equilibrium with the silicates with which it will be fully reacted to form a gelatinous paste of disordered solvated hydrosilicates. Such hydrosilicate systems must have the physico-chemical properties of a macromolecular system where the very large surface-to-volume ratio of the disordered solvated particles is such that the surface energy becomes an important component of the total energy of the system. It will be similar in its rheological, diffusive, and accretive properties to a concentrated siliceous ooze or mud but one in which the water is confined by higher temperatures and pressures. The colloidal particles in such systems range in size over three orders of magnitude, from approximately 1 nm to 1 micron (1,000 nm). Smaller colloidal species (such as hydrous silica and sodium montmorillonite) disperse readily in the interstices between a "meshwork" of much larger and variously aggregated particles (such as clay platelets) comprising the general matrix of the system. The properties of this hydrated mineral matter which must be mobilised to give rise to the rapakivi texture are as follows. 1. Gel cohesion

A gel is the general description for a higher viscosity, essentially rigid, network formed by the partial cross-linking or intermeshing of chains of macromolecules, flocs or other particle aggregates. In deeply buried situations this three-dimensional network is fully infilled with solvent and the gel density is generally dependent on how much water is constrained by the

49 prevailing pressures to be retained within the gel. Gels vary from weak "watery" gels with sparse tenuous and easily disrupted cross-linkages through denser tough "rubbery" gels having a conchoidal fracture, to compact glassy substances, like glue or desiccator silica gel. Denser gels have very much larger numbers of cross-linkages per unit volume and when the network is sufficiently dense (concentrated), such that the interparticle distances are small, greater numbers of particles are in siafficient proximity to be strongly attractive to each other under the influence of van der Waals forces. At particular water contents these gels are "self-densifying" or synerectic. 2. Accretion in thixotropic macromolecular systems (Frisch and Simha, 1957; Elliston, 1963(b) and (c), 1968, 1984; Van Olphen, 1963; Healy, 1975; Yariv and Cross, 1979.) Accretion in colloidal systems is critically dependent on the concentration or the proximity of the particles to each other in the dispersing medium. If, on disruption intrusion and flowage of a gel system there is sufficient pore water, the particles will redisperse to a fluid or "creamy" paste in which the interstitial particles are normally stabilised in dispersion by, for example, the coulombic repulsion between their similar charges. If, however, the same system is remobilised after further dewatering, the concentration or proximity of the particles in the interstitial suspension may be such that van der Waals attractive (short range) forces between the particles exceeds the repulsion due to their similar charges. Accretion in such concentrated colloidal systems is the spontaneous formation in these sheared slimes, pastes or muds of macroscopic spherically symmetric nodules ("clots") of higher density and particle concentration than the suspending fluid or dispersion (Fig. 34). Under conditions of flowage in concentrated suspensions, particles are given a "degree of freedom" by the liquefaction which disrupts most of the gel linkages. They immediately adhere to each other to form aggregates. These aggregates "snowball" to close-packed macroscopic nodules or clusters of denser more "rubbery" fracturable gel. The accretions build up in the mobilised suspending fluid, breaking and reaggregating to an aggregate size distribution which is dependent on the rate of shear. The behaviour of macromolecular particles in an aqueous pasty system is discussed by Van Olphen (1963, pp. 37-43), Elliston (1968, pp. 88-92), Healy (1975, pp. 51-55), and Yariv and Cross (1979, pp. 336-349). The more detailed theoretical treatments have been summarised by the writer in Part 1 of this series on granitic textures and observations which indicate the crystallization of hydrosilicates (Elliston, 1984, pp. 308-323).

50

r

Fig. 34. Accretions formed as gelatinous clots in a settled sludge of flocculated clay which had accumulated in the bottom of a large hilltop water supply tank. The accretions formed when the mud was discharged from the empty tank during cleaning operations.

W h e t h e r d i s o r d e r e d solvated particles in a remobilised fine-grained hyd r o u s paste f o r m accretions or redisperse is essentially d e p e n d e n t on the sum o f the forces of a t t r a c t i o n / r e p u l s i o n acting b e t w e e n t h e m a n d o n their net i n t e r a c t i o n energy. S h o w n in Fig. 35 is a typical total e n e r g y of i n t e r a c t i o n

51

Critical at which Close Potentiall Criti.cal Interparticle Interporticle Separation Sel Energy ~rystallisation/Packing Sets In.

,,

I

"t '.~yneresls;#// Stoble FDrecipitote

VR

I

', s i

Floeeulation

Ii Sta~e I Dispersion I

II

II

. . E n e r g y Barrier Primary M a x i m u m

-O

Secondary

Minimum

VA

F)rimary M i n i m u m [

Interparticle

Separation

h

Fig. 35. The general form of the curve of potential energy plotted as a function of interparticle separation. For discussion of particle interaction as a function of particle separation and similar curves, see van Olphen, 1963, p. 41; Elliston, 1968, p. 89; Healy, 1975, p. 53; and Yariv and Cross, 1979, p. 342. VR and VA are repulsive and attractive energies respectively. curve which is the summation of van der Waals attraction and surface electrostatic repulsion. For simplicity, only the major interaction energies of electrostatic repulsion and van der Waals attraction leading to the net interaction energy curve of Fig. 35 are summarised. In recent years colloid scientists have identified a range of subtle attraction and repulsion energies, usually operative at very close particle separations, that further assist in determining the final architecture of aggregated assemblages of colloidal particles and macromolecules. The net interaction is dependent on temperature, electrolyte concentration, interparticle distance (concentration of the suspension) and the size and

52 shape of the particle which governs dipole orientation, induction, and dispersion forces. The repulsive forces between similarly charged macromolecules or particles are due to their similar charges. Such repulsion normally keeps the particles in stable dispersion by tending to prevent approach at close range. When two particles approach each other their diffuse counter-ion atmospheres interpenetrate and repel. The force of repulsion is particularly dependent on electrolyte concentration because the double layer is compressed as salinity increases and the range of repulsion is considerably reduced (Yariv and Cross, 1979, p. 340). Van der Waals attractive forces are short range and, consequently, largely dependent on the proximity (concentration) of the particles interacting. The attraction between macromolecular particles is much stronger than that between, say, gaseous atoms and molecules, and it increases with increasing size of the particle. Van der Waals forces are therefore highly relevant in concentrated macromolecular pastes or "magmas". The conditions of deep burial, increased pore fluid salinity, higher concentration (partial dewatering), higher temperatures and pressures (higher activity of water and hydrolysis of the silicates), and large particle size (such as clay platelets at 500-800 nm) all strongly favour the formation of accretions. The "degree of freedom" or liquefaction by thixotropic disruption of the gel fabric is essential to enable the hydrosilicate paste to flow and the constituent particles to assume the "close packed" condition. A very well-known example of colloidal accretion is butter-making in a cream churn. The cream first assumes a whipped consistency, then further shear separates out macro-accretions of round yellow butter-fat globules. Accretions in partly dewatered gelatinous natural muds can be produced readily to demonstrate the phenomenon. The clay-rich sediment from a fresh water dam illustrated in Figs. 36 and 37, was first passed through an 80 mesh sieve to ensure that all macro-aggregates or lumps were broken up or removed. This partly dewatered mud at about "toothpaste" consistency, when further stirred and turned out into a shallow pan, had developed large gelatinous accretions which stand out as lumps in the material. In a rather stiffer more silty sample of the same sediment and after similar sieving, the cake would not flow out under its own weight to reveal the accretions but they can be seen by smoothing the surface with a wet spatula (Fig. 38). After allowing this silty mud cake to dry out completely and breaking it in half, the clay accretions are seen on the fractured surface to be rather smaller but some have a definite radiating fibrous structure of the clay round the precipitating nucleus (Fig. 39). This rather common structure in colloidal systems resembles coronas or kelephytes, but in this instance it serves to demonstrate the "built up" or accretionary origin of the aggregate

53

Fig. 36. Dense thixotropic mud in which gelatinous accretions are visible as lumps on the surface. The partly dewatered mud from a freshwater dam was first sieved (-80 mesh) and then poured out as a stiff paste tapping the dish to induce flowage. a f t e r the m a t e r i a l h a d p a s s e d t h r o u g h a sieve m e s h a b o u t o n e - t e n t h its size. T h e i n t e r p a r t i c l e force of r e p u l s i o n increases a p p r o x i m a t e l y e x p o n e n t i a l l y ( r o u g h l y p r o p o r t i o n a l to the inverse o f the s q u a r e of particle s e p a r a t i o n distance) as the particles a p p r o a c h e a c h o t h e r a n d , for thick clay platelets,

Fig. 37. The same sieved mud as Fig. 36 with the rounded gelatinous accretions standing up on the bottom and lip of the pan from which most of the mud has been poured.

54

Fig. 38. A much stiffer silty sieved mud (-80 mesh) from a freshwater dam. This material would not flow out to expose the gelatinous accretions but they are revealed by smoothing the surface of the mudcake with a wet spatula.

Fig. 39. The same contracted dry clay dried mudcake. The pattern confirm the

sieved mudcake as Fig. 38 was allowed to dry out completely. The accretions can be seen on the surface of a vertical fracture through the size (about 10 times that of the sieve mesh) and the radial syneresis crack accretionary origin.

55 the van der Waals force of attraction increases roughly proportional to the inverse of the cube of the separation distance (Yariv and Cross, 1979, pp. 340-341). In all such colloidal systems, therefore, there is a critical concentration at which the particles "lock on" to each other when freed at the appropriate concentration from their structural position within a gel. Frisch and Simha (1957, p. 595) were possibly the first to recognise the importance of the critical nature of concentration in mobilised sand-clay Water mixtures. They wrote: "At the concentration at which close packing sets in, we would expect a strong and abrupt increase in the viscosity of the suspension, the process resembling in its critical nature superficially, a first-order phase transition, such as the condensation of a vapour to a liquid." The close packing arrangement of the particles achieves a further reduction in surface energy. The important feature of the curve showing net interaction energy as a function of particle separation (Fig. 35) is the potential well or primary minimum at a very close interparticle spacing. This deep potential well with strong forces of attraction at very small interparticle distances (high concentration in partly dewatered hydrosilicate systems) characterises net interaction energies and predominates in almost all systems. In most cases the denser, more coherent gel properties of the accretions, which are held together by the strong forces of attraction in the primary minimum, can only be achieved by supplying energy to the system. In the case of concentrated hydrosilicate systems (thick pastes or muds), it usually involves thixotropic remobilisation induced by mechanical shock and the shear involved during flowage or intrusion. The general behaviour of macromolecular systems is related in large measure to interparticle separation and the regions for crystallization, syneresis, accretion, flocculation and dispersion are indicated on Fig. 35. Reliquefaction and accretion of the particles into dense close-packed macromolecular aggregates has the effect of separating the constituent particle clusters in accordance with their ability to form cohesive aggregates. The coherence of the aggregate in the viscous fluid shear regime is dependent on the geometric particle shape. This is important as the macromolecular particle shape is often related to the chemical composition of the hydrosilicates forming the particle. The particles are "pushed" into the proximity at which they "lock on" to each other due to van der Waals attraction (at the net interaction energy minimum). In the shear regime of viscous flow the breaking and reforming accretions are very sensitive to purity (their cohesive property) and to the size of the aggregate. Larger accretions are subject to greater overall shear on the particle

56 cluster. A uniform size distribution will be achieved which is dependent on the rate of shear. Large accretions are indicative of slower rates of fluid flow. The cohesion of the accretions depends upon their purity or freedom from particles of ill-fitting shape incorporated in the cluster. A bundle comprising a heterogeneous mixture of rods, spheres and plates is much less cohesive than a "wheatsheaf"-type bundle of rods, a " p a c k of cards"-type stack of plates or a " p a c k e d case of apples"-type aggregate of spheres. The " p u r i t y " or degree of refinement into accretions comprising a particular macromolecular shape is therefore dependent on the a m o u n t of shear. This is indicated schematically by Healy (1975, p. 76), redrawn here as Fig. 40. Liquefaction and initial intrusion would be sufficient to generate the macroscopic spherically symmetric clay accretions which are the precursors a.

~ J _ / [~'~ ~

~l]

Dispersed

and/or

and/or TIME

Coagulated

HEAT

Accretion

Crystanisation

b.

v O f

O r HEAT

Dispersed

Coagulated

Accretion

Crystallisation

Fig. 40. (a). A coagulated colloidal system will consist of thermodynamically unstable aggregates for which a further reduction in energy can be achieved by reduction of internal surface area, elimination of pore space and finally by crystallization. This condensation is indicated schematically for dispersed clay platelets, through coagulated clay, an aggregate of "close-packed" books of platelets forming an accretion to crystallization by reaction with SiO2 and Na ÷. (b). In a multi-component colloidal system of mixed aggregates a further effect of shear is "refinement" of the aggregates by breaking and reforming into separate single component aggregates. Specific shape factors effect this process which is illustrated schematically. Accretions in natural sediments are rarely "pure" and retained impurities give rise to the inclusions in natural crystals.

57 of the rapakivi feldspar ovoids. Further episodes of thixotropic reliquefaction of the rapakivi " m a g m a " prior to its crystallization and water loss enables such a semi-fluid mixture of gelatinous mineral matter to reintrude; to develop marginal or internal phases (without contact effects); to rebreak and reinvade surrounding materials to form xenoliths and apophyses (into which preformed ovoids can be introduced); and to form autoliths by reinvading earlier phases. The aluminosilicate (clay) and siliceous (silica gel) accretions in such remobilised pastes are inherently more cohesive, synerectic and self-densifying than the hydrous ferromagnesian constituents which tend to form weaker (less closely-linked) "watery" gels of a rather more "oily" consistency. When well formed and more cohesive aggregates of hydrous aluminosilicates and silica gels have developed, the more fluid and highly hydrated ferromagnesian minerals tend to be left as residual material in the matrix. On successive remobilisations, such lighter and more fluid material can work its way out as dykes or associated "comagmatic" basic bodies carrying more or less (even none) leucocratic ovoids. Individual ovoids or "clouds" of ovoids in associated basic bodies (matrix segregations or "basic fronts") are consistent with properties of the respective hydrosilicates. In circumstances where wall rocks and xenoliths of precrystalline rapakivi " m a g m a " ar,~ similarly hydrated, plastic (Malashin, 1968, p. 1406), and capable of both fracturing as rafted blocks or digestion to form part of the matrix, it is apparent that the preformed accretions can be incorporated locally into these materials. The autoliths represent previously mobilised and retextured material included as cohesive blocks in the successive remobilisation. In this case, initially developed accretionary ovoids would be preserved within the autolith as readily as possibly new ones could be marginally mixed into it. An ovoid situated across the margin of an autolith or impressed into its rim is possible and is occasionally observed (Fig. 41). On the scale of the accretions themselves, remobilisation enables the earlier-formed ovoids to fracture and reaggregate; to form composites; to accrete additional peripheral material; to break off mantling rims or parts thereof (to give the battered appearance); to reincorporate parts of mantling rims (Fig. 27) and matrix within new ovoids (Fig. 12); to "plaster" peripheral material over already mantled ovoids to form an internal zone or mantle (Fig. 25); to reaggregate the mantle material itself forming an ovoid of this composition; and to "replaster" such ovoids to form anti-rapakivis. Details of the formation of rimming material richer in sodium and calcium than that constituting the potassic ovoid cores are given in a later section (see pp. 65, 66), but it is clear that the mantling rims are formed by diffusion under static or non-mobile conditions. Such periods occur between successive reliquefactions of the rapakivi matrix. In the case of a number of

58

Fig. 41. A large potash feldspar ovoid is impressed into the margin of a biotite-rich fine-grained xenolith in the Station Hill granite, Tennant Creek. Beneath the feldspar ovoid is a "squashed" elongate quartz ovoid shaped round the feldspar ovoid as is typical of the rather wispy fluidal quartz accretions in this mainly pyterlitic rapakivi (see also Fig. 47). The coin is 28 mm in diameter.

successive remobilisations, it is therefore possible for an ovoid which has developed a rim to accrete a further marginal layer of potassic material in the following liquefaction, to develop on this new material a further sodacalcic mantle during ensuing static conditions, and so on, until the number of internal rims or zones reflects the successive reliquefactions that the individual has survived. It is clear from evidence of several reintrusions of the rapakivi " m a g m a s " and from such multiple zoning of individual ovoids, that reliquefaction occurred on a number of occasions. In some cases it was at least five or six times. Obviously, only very occasional ovoids reflect multiple reliquefaction to any extent. The typical rapakivi texture is a mixture of variously mantled and non-mantled, part mantled, multiple mantled, fragmented, reaggregated, large, small, and variously comprised ovoids (Fig. 11). Each reflects an individual history of development through one or more of the previous episodes of flowage and intrusion. Each ovoid and part of the ovoid has an individual genetic history within the context of the successively reliquefied hydrosilicate " m a g m a " .

59 3. Concretion

The process of concretion is characterised by the diffusion of macromolecules or colloidal particles from the surrounding disperse phase, particle by particle, to accumulate about a central nucleus. Such concretion produces spherically or elliptically symmetric accumulations of higher particle density and compaction than the medium from which it formed. The process is 'static' in the sense that the gel phases are not thixotropicaUy remobilised or the gel meshwork disrupted. In most systems the disperse particles diffuse down a concentration gradient in the interstices of the pervasive gel of the system. This gradient is created by the removal of particles from dispersion as they precipitate at the nucleus or surface on which the concretion is developing. When Brownian motion moves the charged particles in the direction of the precipitating surface, they "see" the deficiency or absence of similar charge and are thus less repelled in this direction. Diffusion acts to keep the concentration constant and thus is established the gradient along which a continuing supply of particles move to the precipitating surface. The mechanism of precipitation at the surface of the developing concretion is generally dependent on the nature of this surface and its charge but in all cases it results in a reduction of the total surface and of surface energy. Some large orbicular rimming structures are spectacular examples of concretion. Their formation is by precipitation at an internal boundary being the margin of a dense gel nucleus of higher interstitial salinity with the less dense gel matrix of lower salinity. This has been described in detail by the writer (Elliston, 1984, pp. 317, 318). The cores of the rapakivi ovoids are accretions formed in the same way as many of the nuclei of the granitic orbicules. However, the plagioclase mantles are not concretions. Rapakivi ovoid rims differ from characteristic concretionary overgrowths in the following respects. (1) The mantling tends to be discontinuous, variable and irregular, displaying a variety of forms. There are commonly sharp variations in thickness round the ovoid (see pp. 19 and 34). (2) The rims extend into the potassic feldspar core as perthites, often in optical continuity with the rimming plagioclase (p. 36). (3) The rims consist of one type of material with no oscillatory concentric banding characteristic of concretions. (4) Drop quartz inclusions occur in the mantling plagioclases (p. 28). (5) The oligoclase occurs not only as mantling rims on the ovoids but as internal concentric zones, patchy inclusions, poikilitic skeletal inclusions and various types of perthitic lamellae. (6) Gradations of internal plagioclase included within the potassic core, to the external rimming plagioclase have been observed (p. 36).

60 (7) The rimming plagioclase displays patchy zoning, sometimes with albite coating the inner margin of the rim (p. 36). (8) Anti-perthites have been recorded in the mantles (p. 36). (9) The plagioclase rims develop round "granite balls", composite ovoids and their fracture fragments. The migration of the desorbed plagioclase precursor mineral from the ovoid core and its coalescence at the margin (Key and Wright, 1982, p. 119) is in fact rather similar to the internal desorption of electrolytes which induces concretionary precipitation at the margin of the accretion. This phenomenon has been referred to as diffusophoresis. It involves the motion of particles in an electrolytic concentration gradient and is used in modern paint technology to deposit coherent films of binder and pigment on surfaces (Prieve, 1977). However, these phenomena are also related to syneresis and are further discussed under that heading.

4. Thixotropy (Mysels, 1959, p. 270; Boswell, 1961; Van Olphen, 1963, p. 139; Healy, 1975, pp. 55, 73; Yariv and Cross, 1979, p. 381.) Thixotropic reliquefaction allows the colloidal particles of a system at the same fluid content to revert to a dispersed sol or more fluid gel. This reliquefaction of a gel, or coagulated sol, is isothermal and reversible and in natural colloidal systems is usually due to mechanical shock (earthquake) or shear (gravity sliding) which disrupts the interparticle linkages. The thixotropic gel-to-sol or more-fluid gel transformation can be repeated isothermally for the same system a number of times and it is this property of gelatinous mixtures of hydrated mineral matter which has effected the observed multiple reintrusion of the rapakivi granites. The precursor hydrosilicates of the rapakivis were almost certainly derived from the sediments which they incorporate and for which they substitute. These must have been hydrated and "gelatinised" to a dense, partly dewatered, plastic condition by the increasing temperature at depth and either by retention of originally contained water or introduction of additional hydrothermal water. Such geothermal water is not rare in geosynclinal situations where it may originate from subcrustal sources or from rocks dehydrating and crystallizing in the adjoining lithosphere. The initial thixotropic mobility and flowage of the rapakivi "magma" would have induced some accretion and segregation of the constituent colloids into their respective close-packed aggregates. This partial separation into mixed lumps or macro-aggregates in fact preconditions the bulk material for subsequent thixotropic liquefaction as it reduces the Bingham yield value

61 of the matrix. Each accretion comprises a close-packed aggregate of the respective colloid particle shapes which have achieved a greater packing density at the expense of the matrix gel density. Further, within each accretion the particles drawing together under the influence of van der Waals interparticle forces of attraction, exude water. This "lubricates" or weakens the matrix gel against the next mechanical shock or shear which could cause its disruption and thixotropic reliquefaction. This is the reason why the rapakivi "magmas" tend to reinvade themselves as successive "plutons" or isothermal phases of the original "magma". When reliquefied, the main lithostatic load on the rapakivi rock-forming mass can only be carried by the hydraulic pressure. Because of this, only relatively small differential forces due to viscous fluid flow are imposed on the developing accretions. Nevertheless during flowage the ovoids may variously break and reaggregate, remould together, incorporate matrix, and mix within the reaggregated individuals various pieces and types of the constituent hydrosilicates. In any mobilisation where the lighter more fluid ferromagnesian-rich matrix is able to "differentiate" in bulk way from the quartzo-feldspathic accretions (like the fine liquid cement slurry which rises to the top of an agitated cement pour), the accretions are depleted in matrix. They are then left largely in contact or closer contact, and intrusion or movement at this stage mixes them to the even-grained granitic texture observed in some parts of the rapakivi areas. The more fluid ferromagnesian-rich hydrosilicates separate as dykes and associated basic bodies which also subsequently dehydrate and crystallize. The wiborgites derive from rather hydrous mixtures of gelatinous hydrosilicates where the more fluid ferromagnesian precursors have been largely retained in the matrix. The high water content of the precursor matrix hydrosilicates is indicated by the abundant miarolitic cavities and the patches or zones of "moro". In these local pockets of rapakivi granite where the matrix "gives the appearance of never having been there", it would be fully in accord with the properties of the precursor hydrosilicates to suggest that these local inter-ovoid zones, like the miarolitic cavities, contained excess water. On eventual condensation and crystallization of the sparse ferromagnesian hydrosilicates of the " m o r o " zone, they were insufficient to infill the matrix, leaving inter-ovoid cavities or voids. Collapse of the rounded feldspars into these zones when they are exposed is how these rocks came to be called "rapakivis".

5. Rheopexy (Mysels, 1959, p. 271; Boswell, 1961, p. 43) Rheopexy is the accelerated resetting under shear conditions of a flowing

62 colloidal dispersion to a gel or higher-viscosity condition. As movement due to fluid flow declines to a critical rate, flocs and particles within the flowing mass are physically brought in contact in the laminar flow regime so that interparticle linkages can be re-established and held. The effect is an instant 'refreezing', shear thickening, or gelation throughout the flowing mass such that flow textures can be preserved. As already discussed, the inter-ovoid matrices of the rapakivi "magmas" are relatively abundant and fluid. Chains of ovoids and horizontal layers differing slightly in mineral concentrations from those of the general rock mass (p. 11) are therefore relatively rare. However, the typical wiborgitic texture of large ovoids more or less sparsely set in the darker ferromagnesian-rich matrix is preserved by the rapid rheopectic resetting of the hydrosilicate paste as each episode of intrusion and flowage is completed and approaches the rest condition.

6. Syneresis (Pettijohn, 1957, p. 208; Mysels, 1959, p. 292; Elliston, 1963(c), p. Q3; 1984, p. 323). Syneresis is the spontaneous contraction of a gel meshwork within itself by the establishment of a greater density of cross-linkages and exudation of water. The particles or particle chains, drawn together by van der Waals forces of attraction, achieve greater coordination, the total surface energy is lowered, and the internal surface of the gel and its adsorptive capacity are reduced. The accretions which are the precursors of the rapakivi ovoids are close packed macro-aggregates with the colloidal particles strongly adherent and locked on to each other at the net interaction-energy minimum. Syneretic condensation within these accretions begins from the moment of their formation and it is the establishment of greater numbers of cross-linkages per unit volume (tougher, plastic, fracturable gels) which enables them to survive as more coherent entities during subsequent mobilisations. The spontaneous internal contraction of the denser gel aggregates is responsible for shrinkage features observed in the rapakivi ovoids. Examples are: the cuspate contraction cavities which give rise to the concave quartz inclusions; the bordering myrmekites; the 'spongy' or vermiform hole patterns in the potassic feldspars which are infilled with quartz and in some cases give rise to the micropegmatites; and the contraction of the potassic core of many of the ovoids from the rimming plagioclase to give a zone, or almost shell-like layer, of irregular quartz inclusions just inside the plagio~'~ase mantle.

63 Because the rapakivi ovoids are now observed as aluminosilicates of mixed and variable composition, there is no doubt that the corresponding precursor hydrosilicates were clays. There is equally a clear indication that the precursor clay of the mantles had a composition differing from the clay comprising the potassic cores in that it contained the appropriate sodium and calcium to crystallize as plagioclase. Variations in the proportions of potassium to sodium and calcium in adjoining ovoids, variations in triclinicity, in perthite development and types of lamellae and even in colour (hematite content) indicate a mixture of differing precursor clays. It would be difficult to identify precise precursor clay mineralogy for all variations in the ovoids. However, direct measurements on marine muds and of drill cores from deeply buried basin sediments (Weaver and Beck, 1971, pp. 1, 9) show a predominance of illite and montmorillonite. The deepest samples (7.3 km) have illite-montmorillonite ratios in the range of 7:3 to 8:2. Illite and montmorillonite would therefore be the most common clays in deeply buried sedimentary materials, such as those from which rapakivi granites could derive. They would therefore almost certainly pedominate as precursors of the ovoids. It is the syneresis of the precursor accretions which gives rise to the rimming plagioclase mantles. As has been indicated (Elliston, 1963, p. Q3; Healy, 1975, p. 55), reduction of internal surface reduces adsorptive capacity. Not only does this release surface-adsorbed ions to increase the interparticle fluid salinity (Elliston, 1984, p. 317), but colloidal electrolytes or adsorbed charged particles are released as well. It is the exudation of these desorbed charged particles of colloidal dimensions from the condensing syneretic macro-accretion which generates the 'cutan' or rim of differing composition. Initial formation of the accretion obviously involved a mixture of clays. In the natural mixed hydrosilicate system all the clays are characterised by the platelet shape of the phyllosilicate but in the close-packed aggregated condition they are to various degrees structured and are no longer true chaotic-type hydrogels. Different parts of the accretionary aggregate may contain somewhat variable proportions of illite and montmorillonite, together with lesser amounts of mixed-layer illite-montmorillonite, amorphous silica, chlorite, etc. Adsorbed ions (Na ÷, K ÷, Mg 2÷, Ca 2÷, Ca(OH) +, etc.) will initially vary in accord with the recent history and degree of packing (kneading by movement in the flow shear regime) of the particular segment of the ovoid they occupy. During syneresis with increasing electrolyte concentration within the interparticle meshwork, the ions compete for available adsorption sites and equilibrate. They also equilibrate with internally adsorbed charged particles. Of these

64 the montmorillonite itself is the most important and abundant. Eitel (1964, p. 337) points out that smaller particles of clay minerals behave as colloidal electrolytes and Marshall (1936, pp. 23-31) classified the montmorillonites as true colloidal electrolytes, their suspension being like a dissociated electrolyte where the large ions are within the lower end of the colloidal size range. Although cutans (Brewer, 1960, pp. 280-292), rimming structures, and various types of surface coatings are very common and in fact characterise many colloidal systems (Fig. 42), it seems that little direct experimental work has yet been done at the pore fluid salinities, temperatures and pressures which might be assumed for the genesis of the rapakivi ovoids. Gaudette et al. (1966, pp. 1649-1656) in studying the location of the adsorption sites for large cations on the crystallites of clay minerals of the illite family, did develop the concept of core-rind formation which they described as a structurally coherent silicate core of platelets surrounded by a less coherent

Fig. 42. Rims of potash feldspar in this akerite granite from the Cloncurry area, Australia, reflect the development of cutans by the hydrous precursor minerals. Ovoidal and rounded shapes, conjoining and fragmentation can be seen in the dark quartz phenocrysts, reflecting the accretionary origin. Cutans and the rather well-developed euhedral faces on some of the large plagioclase phenocrysts probably reflect prolonged gelatinous conditions to enable the idiomorphic crystal growth and the "plating out" of the rimming alkali feldspar on available surfaces. The rock still contains small jasper fragments.

65 silicate rind area with a skeletal framework reminiscent of the more coherent portion. This concept closely parallels the envisaged precursor accretion of a mantled rapakivi. It has also been shown that clays take ions by sorption reactions from water suspensions of sparingly soluble minerals and that these reactions probably take place in the presence of relatively little water in suspensions of high concentration (Grim, 1953, p. 141). Temperature also is claimed (Grim, 1953, p. 139) to generally have small effect on cation exchange, and Yariv and Cross (1979. p. 210) state that the pH has only minimal effect on cation sorption by clay minerals but that it is influential on sorption of the non-clay fraction. The preferential adsorption or 'fixation' of the potassium ion on the illites is well known (Grim, 1953, pp. 150, 152, 153; Van Olphen, 1963, p. 70; Eitel, 1975, p. 342; Yariv and Cross, 1979, pp. 298, 409). Potassium is the commonest ion that is fixed to a considerable degree. It has an ionic diameter of 2.66 A which corresponds to the diameter of the basal oxygen layer in the illites. It has been thought (Page and Baver, 1939, pp. 150-155) to fit into these cavities making it relatively difficult to displace by normal cation exchange. Illite can be regarded as if the anhydrous K ions are specifically adsorbed onto the inner Helmholtz layer of the flat oxygen sheet (Yariv and Cross, 1979, p. 293). However, Yariv and Cross (p. 298) suggest the selective adsorption of K + by dioctahedral minerals may also be due to the inclined orientation of the hydroxyl in dioctahedral minerals, such that the positive pole of the OH group is further from the K ÷, causing less repulsion than the vertically oriented hydroxyls of the octahedral sheets in trioctahedral minerals. They also suggest K - O distances are shorter in the dioctahedral than in the trioctahedral minerals. Bolt et al. (1963, pp. 294-299) found that the selectivity of hydrated and expanded frayed edges of illite for the sorption of K ions was about 500 times greater than that of calcium. It is quite clear from experimental work (Grim, 1953, p. 150) that K + in competition with equivalent concentrations of Ca 2+ is preferentially taken up by the micaceous (illite) clay minerals. Weaver (1965, p. 604) indicates the average potassium content of Paleozoic illites as 8.75% K20. In the rapakivi precursor accretions we may certainly assume a high degree of hydration and frayed edges for the reworked illitic clays. They would strongly adsorb and preferentially retain available potassium. The reverse situation applies to the montmorillonites which preferentially take up sodium and calcium (Grim, 1953, p. 150). At the pH that might be assumed for deeply buried hydrous sedimentary environments the hydrolysable calcium ion is probably adsorbed as Ca(OH) + (Healy, 1969, p. 52). The exchangeable cations predominantly determine the water sorption in smec-

66 tites. The cations are sorbed on montmoriUonite chiefly on octahedral layers and a strong hydration of the cations is made possible. In fact, Eitel (1964, p. 245) reports the formation of quite definite montmorillonite hydrates which are complexes of stoichiometrically constant multiples of the ratios between the silicate network and the water interlayers in their structures. One kind of these complexes has two water molecules in the layer adsorbed on the inner surface, a second kind only one water molecule in the layer, and a third no water at al. The double-water-layer particles contain Ca 2+ as exchangeable cations, the single-water-layer particles contain Na + and the zero-water-layer montmorillonites are probably electrically neutral. Sodium montmorillonites consist of very small particles (Yariv and Cross, 1979, p. 201). They break down relatively easily to flakes approaching unit-cell thickness (Grim, 1953, p. 116) and in many situations comprise single unit layers which are shown under the electron microscope to be irregular fluffy masses of extremely small particles (Grim, 1953, p. 117). Some of the individual particles appear to be about 2 nm thick and possibly 20 nm or more across the flake. Calcium montmorillonite appears as irregular aggregates and, although the unit flakes may be approximately the same size, it tends to form tactoids usually of about 3 to 9 clay unit layers (Yariv and Cross, 1979, pp. 200, 386). Nevertheless, the montmorillonites are quite small relative to the illite flakes (Fig. 31) at about 1000 to 1500 nm. Acting as colloidal electrolytes desorbed by syneresis of the mixed illite/ montmorillonite accretion, montmorillonite particles have room to diffuse through the illite gel meshwork. Syneretic exudation of montmorillonite from rapakivi precursor accretions and its coalescence at the margins generates the mixed s o d i u m / c a l c i u m montmorillonite rim. This subsequently crystallizes to rapakivi plagioclase mantles. The s o d a / c a l c i u m montmorillonite exudation from the illite cores is not complete. As crystallinity develops in the core potassic-feldspar (usually initially skeletal with infilling as the crystal develops), residual plagioclase is unmixed as crystal-controlled perthitic unmixing patterns. Variable syneretic exudation from the illite precursor accretions also accounts for: (1) Irregular and discontinuous rims with some steps or "coves" and embayments in either core margin or rimming material. (2) Variable thickness or absence of rims and lack of relationship between ovoid size and rim thickness. (3) Film and string perthites coalescing to vein perthites. (4) Mixed crystals of potassic feldspar with and without perthites and micropegmatites. (5) Perthitic plagioclase continuous into adjoining clear plagioclase grains.

67

(6) Gradations of orthoclase containing increasing plagioclase until it becomes part of the plagioclase rim. (7) Some crystal correlation between rimming and core material such as a single Carlsbad twin plane transecting the entire mantled feldspar structure and uniform cleavage over the entire ovoid core and rim. (8) Frequent but not invariable optical continuity between the plagioclase perthite lamellae and the plagioclase mantles. (9) Anti-perthites in the rimming plagioclase. (10) Albite-rich zones on the inner margins of the rim plagioclase and extending into the optically continuous perthite lamellae (later meshwork fluid within the condensing ovoid becomes increasingly soda rich). (11) Incomplete homogenisation on heating the perthites. The proportions of plagioclase to orthoclase are variable and not necessarily the proportion which can be accommodated in the heated orthoclase lattice. All these observed details match and are consistent with the syneresis of accretions following repeated mobilisation of a gelatinous hydrosilicate paste. They are not consistent with the assumption that rapakivi granite magmas crystallized by cooling through a melt/crystal equilibrium. COMPOSITION OF THE HYDROLYSATES

The development of various types of quartz, feldspar and ferromagnesian porphyroblasts from remobilised siliceous greywackes, chloritic and hematitic shales is described in earlier work of the writer (Elliston, 1963, 1966, 1968). The composition of the more deeply buried higher-temperature precursor hydrosilicates from which the orbicular granites crystallized is discussed in Part 1 of this series (Elliston, 1984, pp. 323-328). Details of the mixed illite/montmorillonite precursors of the rapakivi ovoids have been given (see pp. 63-66), but it should be emphasised that the reactions for formation of the three main rock-forming minerals apply generally. These are: CLAY + METAL ION + H Y D R A T E D SILICA ~ ' HYDROUS FERROMAGNESIAN ~ PRECURSOR

~

"- FELDSPAR + WATER

LESS H Y D R O U S F E R R O M A G N E S I A N + WATER ANHYDROUS FERROMAGNESIAN

HYDROUS SILICA SILICIC ACID POLYMER ~

QUARTZ + WATER

However, since the rapakivi granites derive from large volumes of mixed sedimentary silicates and carbonates in which hydrolysis has been further enhanced at higher pressures and temperatures ('steam pressure cooker' environment at depth), it is not possible to define the exact composition of

68 the starting species. In fact many such hydrolysates do not have a fixed composition but comprise a transitional series where hydroxyl content depends on the degree of internal condensation and the adsorbed surface water depends on the variable surface area. Polymeric silicic acid (silica gel), hydrated smectites (see p. 66) and the variable hydrated chlorites are examples. From the wide range and variety of the mixed sedimentary clays, ferromagnesian minerals, and amorphous hydrated silica, there is obviously the opportunity for partial reactions and the generation of further precursor and intermediate hydrosilicates. Residual hydrous minerals, even embedded within the fully crystalline tectosilicates would be expected. Chlorite, sericite, muscovite, saussurite, epidote, grunerite, iddingsite, and leucoxene are some of those which have been observed. The 'impurity' of rapakivi feldspars and inclusions of hydrous ferromagnesian minerals is widely observed. Bluish milky quartz recorded by Vorma (1976, p. 43) in the Lellainen rapakivi possibly contains dispersed rutile or leucoxene as do the rounded "blue" accretionary quartz megacrysts from the remobilised sediments in Tennant Creek. "Dark smoky" and "milky blue" quartz reflect the impurity of the precursor silica gel. Shadowy extinction reflects lattice distortion by entrapment of hydroxyls. The hydrated gelatinous forms of the precursor silicates are metastable (Weaver and Beck, 1971, p. 14) and relatively short-lived, and unless water is maintained in the system under high pressure, they tend to crystallize. Although the general reactions for the formation of the main rapakivi granite rock-forming minerals are quite clear, a number of intermediate and end products can be derived from differing initial hydrolysates. An indication of the main hydrosilicate components and the dehydration reactions by which they crystallize can be proposed.

Feldspars Set out more fully the reaction for the formation of feldspar could be written: ILLITE MONTMORILLONITE KAOLINITE HECTORITE + DICKITE HALLOYS1TE BEIDELLITE

Na ÷ K÷ + HYDROUS ~ FELDSPAR + H20

Ca2 ÷ Ca(OH) +

SILICA

The accretions of platelets from which the rapakivi feldspars crystallize is a complex mixture of sediment-derived clays. Illite (Fig. 31) and montmoril-

69

lonite would be expected to predominate as they do in normal sediments, but diagenesis, burial depth, higher temperatures, and mechanical shear of the hydrolised materials during laminar flow would ensure partial or complete degradation of much of the material. Mixed-layer clays, chlorites, brucite, gibbsite, goethite, polymeric silicic acids, and similar degradation products would be available for reaction. Especially important for the formation of feldspars are the alkali metal ions in the pore fluid and the short-chain silicic acid polymers or Si(OH)4. As ions and very small particles these originate within the condensing gel meshworks, but as the skeletal crystal framework develops within the densifying and crystallizing accretion they can difuse to maintain equilibrium and supply the necessary balancing reactants as the tectosilicate lattices develop. Mobile diffusible hydrous silica is obviously present in some excess as it forms later infillings of miarolitic cavities, concave quartz inclusions, embayment infillings and micropegmatitic textures. A wide range of more complexly hydrated precursors, subordinate constituents (like fluorine substituting for hydroxyls on montmorillonites), and intermediate products are clearly indicated for the natural hydrosilicates from which rapakivi granites crystallize. HYDROMICA MONTMORILLONITE

~ ILLITE ~ ORTHOCLASE

HYDRATE

KAOLINITE

---, M O N T M O R I L L O N I T E

~ DIOCTAHEDRAL

~ Z E O L I T E ---, P L A G 1 O C L A S E

CHLORITE

K A O L I N I T E ---, M I X E D L A Y E R I L L I T E - M O N T M O R I L L O N I T E ALLOPHANE MIXED

~ H A L L O Y S I T E ---, K A O L I N I T E

LAYER ILLITE-DIOCTAHEDRAL

C H L O R I T E --* M U S C O V I T E / C H L O R I T E

An example of the wide range of hydrosilicate types can be found in the chlorites alone. The numerous homeotype substitutions which can occur in their structure give rise to a great variety of particle shapes and properties. The important point, however, in the case of intruded or remobilised hydrosilicate pastes is the effective separation into close-packed aggregates of the colloidal size platelets, rods, and spheres which characterise the precursors of the feldspars, ferromagnesian minerals and quartz, respectively.

Ferromagnesian minerals Chlorites and glauconites probably comprise the main "rod-forming constituents of natural hydrosilicate intrusions. Others include chamosite, sepiolite, attapulgite (Fig. 32), palygorskite, nontronite, halloysite and ferric hydroxide (akaganeite). Chlorite-to-biotite reactions are recorded and also reactions to inter-

70 mediates and byproducts such as muscovite, sericite, hydrobiotite, epidote and zoisite. However, the most direct dehydration reaction for the formation of biotite is (Carozzi, 1960, p. 50; Stanton, 1982, p. 20): GLAUCONITE ~ BIOTITE + WATER

Quartz Among the rapakivi granite precursors, dispersed hydrous silica species are in equilibrium with longer-chain polymers (Eitel, 1964, p. 320) and the disordered solvated surfaces of other silica and silicates in the system. The silicic acid polymers are long, branched chain macromolecules or, if they are derived directly from the degradation of clay minerals, they are 'net' shaped macromolecules reflecting the residual tetrahedral layers of the original clay structure. Hydration reactions with clay-type minerals 'swell' the layered structure preferentially hydrolysing the octahedral layers first (to interlayer brucite, gibbsite, goethite, etc.). The tetrahedral layer 'net' of residual silica gel is then free to "roll like a carpet" in laminar flow if this occurs. It may aggregate to augen, "knots" and various types of schistose quartz "ribbons". The chains and branched chains of the silicic acid polymers in the mobile hydrosilicate pastes tend to ball into opaline spheres (Fig. 33). This bailing or inward convolution of the chains reduces surface energy and enables condensation by crosslinking with some dehydration. Precious opal is a regular array of uniform spheres of amorphous silica some 170 to 350 nm in diameter which are in themselves concentric aggregates of smaller opaline spheres of amorphous silica about 40 nm in diameter (Eitel, 1975, p. 553). The smaller opaline or amorphous silica "balls" are found in the natural gel minerals, sea-bottom sediments, sands and sandstone, chalcedony, and cherts (Eitel, 1975, pp. 556, 557, 559, 560). Allophane, certain organic materials and possibly titanium hydrosilicates may also form spherical aggregates in hydromagmas, but by far the most abundant and important are the smaller opaline spheres of "balled" silicic acid polymer. These spherical particles are approximately the same size as many of the clay platelets and ferromagnesian rods. Under the appropriate conditions of concentration and flowage they aggregate to form separate close-packed macro-accretions or clusters which crystallize as quartz in the rapakivi granites. Quartz crystallization from its hydrosilicate precursors is very widely documented. Some experimental confirmation has been published by Von Chrustchoff (1887) and Raleigh (1965). POLYMERIC SILICIC ACID SILICA GEL

CHERT CHALCEDONY ~

AGATE OPAL

..---~

QUARTZ + WATER CRYSTOBALITE + WATER

71

Olivine

The occurrence of minor olivine is recorded in some of the Finnish rapakivi granites (Vorma, 1971, pp. 6, 10; 1976, p. 12). Sederholm (1982, p. 94) describes it as "a strange constituent of granite" and Backlund (1938, p. 364) says "its genesis is obscure" as he records the fayalite grains as allotriomorphic against quartz. An ultrabasic mineral grain against quartz is indeed strange and obscure if they were supposed to crystallize together from a melt. Even stranger in this case would be the occurrence of magnetite and hematite (Elders, 1968, pp. 39, 42; Vorma, 1971, p. 9; 1976, p. 36; Stull, 1978, pp. 246, 248; Key and Wright, 1982, p. 115) and of calcite (Vorma, 1971, p. 9; 1976, pp. 36, 46; Key and Wright, 1982, p. 115) in such melts containing excess quartz. Such reactants fuse directly to the respective iron and calcium silicates. Simple olivine melts from which serpentine intrusives are supposed to have been derived are subject to question (Bowen, 1947, p. 271; Wahlstrom, 1958, p. 293; Turner and Verhoogen, 1960, p. 313; Barth, 1962, p. 219). Similar to a number of pyroxenes in crystalline limestones and dolomites and to forsterite found in limestone of the Kaiserstuhl, West Germany (Dana, 1947, pp. 560, 599; Enlows and Oles, 1966, p. 1918), the olivine would appear to be authigenic. Fayalite has been prepared synthetically by heating a ferric oxide silica gel mixture (Taylor, 1969, p. 7) so that the normal precursor hydrolysates from which fayalite in the rapakivi granites have crystallized would appear to be: SERPENTINE + GOETHITE---, FAYALITE + WATER SERPENTINE + BRUCITE ---, FORSTERITE + WATER

2(3MgO.2SiO2.2H 20 ) ANTIGORITE

CRYSTALLIZATION

+

reduction 2Fe(OH)3 , 4(Mg.Fe)2SiO 4 + 7H20 ( + 0 ) FERRIC (ferrous iron OLIVINE WATER HYDROXIDE H 2, etc.)

OF THE HYDROLYSATES

The essential point which has been made relative to the rapakivi granites is that they do not crystallize from a melt or any kind of fluid. Fluidity is an earlier event. It is not contemporaneous with crystallization and attainment of highest temperatures. The mobility separates and close-packs the particle constituents of the hydrosilicate paste into aggregates which are then particularly well preconditioned for subsequent slow crystallization to a coarse idiomorphic texture. Growth within and from the gelatinous components enhances crystal development and idiomorphic texture (Henish, 1970, pp. 56, 57).

72 Prior to crystallization the fluidity merely develops a "mottled clay" texture of essentially separate accretions of red variegated clays with greenish clays or hydrous ferromagnesian minerals which usually comprise a mesostasis. Such retexturing, whether developed on a large or small scale, at depth or in a shallow sedimentary shelf environment, is dependent on the material containing a high proportion of concentrated gelatinous mineral matter. Clearly the accumulation of hydrous muds and clays in actively deepening geosynclines can be remobilised to develop accretions and various types of retextured sediment. These form a gradation between the "bricky" retextured quickstones, the porphyroids and granitic textures (Elliston, 1963, 1966, 1968; Figs. 43-47). The crystallization of mixed hydrolysates is usually incomplete and residual hydrous minerals are to be expected. For the rapakivi rockmass itself a concept of hydrothermal alteration is totally unnecessary. Marginal alteration of enclosing wallrocks or a metamorphic aureole might be expected due to water loss and attainment of higher temperatures during crystallization. Pitcher (1979, p. 636) has also suggested granite plutons may be pseudoplastic behaving as Bingham bodies with a definite yield strength. He refers to the repeated flow and points out (p. 628) that remelting and metasomatic processes require such considerable energy input that it is most unlikely that granites in any quantity can be produced in situ out of harmony in time and place with their environment. The timing, the repetition and the immense quantities of heat required for melting and remelting enormous volumes of granite is an unresolved traditional problem. In many concumstances, such as the survival of small xenoliths of similar composition or the remobilisation of the cores of orbicules (Elliston, 1984, p. 283), the application of heat for melting would need to be incredibly selective if this were the cause of fluidity.

Fig. 43. Late diagenetic remobilisation of clay-rich tubidite sediments to form the intrusive Olive Wood Quickstone lens (Elliston, 1963, (b), p. L30) has generated a flow foliated chloritic shale containing small clay-sericite rich accretions of poorly crystallized feldspar.

73

Fig. 44. A flow-foliated remobilised chloritic shale has small accretions of milky-blue ovoid quartz, jaspoidal quartz and fragmented drawn out and poorly crystallized feldspars. Explorer 7 prospect, Tennant Creek, Australia (Elliston, 1963, (b), p. L26 and fig. 46).

Here it is suggested that the thixotropic yield is highly selective, being extremely sensitive to gel density and cohesion. The subsequent heat stems from crystallization o f the material itself after preconditioning by liquefaction.

Fig. 45. Distorted and elongate quartz accretions with some curdy flocculent feldspar to the upper right, are set in an essentially chloritic porphyroid. Great Western rheopelite (Elliston, 1963, (b), p. L16 and figs. 17, 24).

74

Fig. 46. A Tennant Creek rheopelite with large quartz ovoids and their fracture fragments shows the deeply embayed and tubular vermiform syneresis shrinkage pattern. Large illitic clay accretions which merge with the argillic matrix and have crystallized essentially to sericite with only minor flecks of feldspar.

Fig. 47. Foliated granite with large pyterlitic alkali feldspar ovoids set in a biotitic groundmass containing plagioclase and pale blue to glassy sinuous quartz wisps which respond to and follow the foliation round the large ovoids and their fracture fragments. The large ovoids contain numerous vermicular quartz inclusions and some concentric zones of inclusions of plagioclase. Some develop rapakivi mantling. Station Hill Granite, Tennant Creek, Australia.

75 Clearly the rapakivi granite precursors liquefied repeatedly by thixotropy. But the energy release or temperature elevation on crystallization is difficult to estimate. For the actual temperatures generated by rapakivi granites on crystallization, there would appear to be no reason to doubt the data from fluid inclusions. For granites generally, these range from 82°C to 530°C with a preponderance of measurements falling between 100°C and 300°C. Since inclusions would be more abundant in rocks formed at lower temperatures and therefore more-fluid inclusion data would be expected from such rocks, it might be more accurate to suggest most sedimentary-type granites achieved temperatures of, say 400-500°C. At depths of 5-7.3 km, Weaver and Beck (1971, p. 18) record temperatures of 120°-170°C in undisturbed partly-dewatered plastic muds. This higher-pressure preheating of sedimentary materials due to burial depth enhances hydrolysis and "gelation". The materials are pre-conditioned for anatexis where this type of melting may be likened to the reliquefaction of a previously set jelly by returning it to the stove. In reality the gel strength of heated sediment is reduced by thermal agitation of the macromolecular interparticle linkages. The Bingham yield point is lowered and the material is thixotropically more "sensitive". While the structure of the sediment is dependent on gel cohesion it does not have any definite anisothermal fusion point. Anatexis, in so far as this means the liquefaction of sedimentary materials to intrude as sedimentarytype granites, is a progressive reduction in the number of inter-particle gelatinous linkages as temperature a n d / o r water content increases. Ultimately this more-fluid-gel condition yields thixotropically (by mechanical shock or shear) to a fluid flow condition. Further heating of the retextured sediments by subsequent crystallization depends on the reaction rate. For sediment-derived granites this is determined by the rate at which water or steam can escape from the system. While perhaps 400°-500°C or even higher may be expected for granites newly intruded to higher levels (with associated pressure reduction), clearly slow-crystallizing granites could form at very much lower temperatures. Colloidal hydrosilicates are metastable and, like authigenic feldspar development in marine muds, the fact that one mineral assemblage changes to another shows that the new mineral assemblage has a lower free energy than the old. The dehydration reactions by which the hydrosilicates crystallize--silica gel to quartz, clays to feldspar, chamosite/glauconite to biotite/ hornblende, etc.--are generally strongly exothermic. For example: Si(OH)4 ~ SiO 2 + 2H20 + 49.8 kJ mo1-1 In sediments silica becomes a balancing species in all forms of dispersion from ionic s o l u t i o n short chain p o l y m e r s polymeric disper-

76 sion - - surface solvation and disordering - - to crystalline silica/silicates. If one form is removed or fixed by reaction, equilibria between the hydrated silica species restore the balance. The natural systems are further complicated by a wide range of variously hydrated clays, carbonates, organic materials and hydrous ferromagnesian minerals. The potential for heat generation on crystallization is also influenced by the following factors. (1) The surface energy which predominates in colloidal particle systems is a significant part of the total energy. (2) The degree of solvation, disordering and hydrolysis are highly variable and dependent on temperature and pressure. The degree of solvation governs surface area. (3) Temperature and pressure in turn depend on the position of the material in the gravitational field (burial depth). (4) The rate of reaction (progression towards crystallinity) and therefore the elevation of temperature are controlled by the rate at which water can escape from the system. (5) Water-rich hydrolysed mineral matter (being lighter because of the water content) intrudes or domes upward into overlying and enclosing sediments. This intrusive movement induces accretion, and the lower pressure at higher levels permits a more rapid water loss (particularly surface water within condensing accretions). Higher temperatures accompany or follow comparatively rapidly after intrusion to higher levels. (6) In an intruded or remobilised hydrosilicate system, part of the surface energy effects separation into accretions of characteristic particle shape (and composition). This close packing of the material in the macro-aggregates facilitates spontaneous exudation of water and crystallization. (7) Differing rates of water loss from various parts of the intrusive rock mass can generate non-uniform temperature maxima. Temperature achieved during crystallization does not depend on composition. It does not appear practical to quantify heat generated following mobilisation or intrusion of rapakivi granite. However, an empiric concept of the total energy latent in a geosyncline full of muddy sediments can be gauged by consideration of the energy applied to the material in filling the geosyncline. Weathering and erosion supply the chemical and physical energy to reduce initially solid rock to muds. Have regard to the negative relief when considering a high mountain chain. From the much larger valleys between ranges, the cavernous side valleys, cirques and passes carved out by the comparatively puny looking little streams on the valley floors, a concept of the vast volume of material removed by erosion impresses itself. If these immense "holes" are equated to our largest man-made open-cuts, like Bingham Canyon, Bougainville, etc., where one is so acutely aware of the enormous amounts of explosive, diesel

77 power and ball mill power draft required to reduce such comparatively minuscule volumes to a powder 10 to 100 times coarser than colloidal materials, then some concept of the magnitude of the energy applied cumulatively during erosion and weathering can be envisaged. It is quite clear that the colloidal finely divided condition is metastable and embodies the high-energy state of sedimentary materials. In view of the higher intrinsic energies, all very fine-grained sediments might be expected to crystallize on lithification to coarsely crystalline granitic or metamorphic-type rocks. There are exceptions but in normal circumstances without reliquefaction, intrusion, or induced laminar flow (shear), they do not do so. Lithification proceeds to normal siltstones, shales, mudstones and limestones. Two energy barriers usually impede sediment crystallization. These can be seen in Fig. 35 firstly as the energy barrier to be overcome in getting the flocculated sediments into the "close-packed" condition. Generally this barrier must be overcome by supplying the mechanical energy of dense laminar flow. With the sediment gel structure disrupted by liquefaction, the sediment constituents can form the close-packed accretions which are then further self-densifying (syneretic) under the strong van der Waals interparticle attractive forces. The second impediment to crystallization is a complex series of variable but strongly net repulsive forces at extremely close interparticle separations. The strongly adsorbed water monolayer has to be desorbed and the Born repulsive force operates as the actual lattice structures begin to impinge. Without the liquefaction necessary to enable the mixed macromolecular sediment particles to separate as close-packed partly ordered accretions, the macromolecular inhomogeneity of shales and muds impedes their crystallization. This can be seen in examples such as agates, cherts, ironstones, dolomites, phosphorites, jaspers, and porcellanites. The respective impurities impede crystallization but the ultimate condensation and development of crystallites indicates that these originally gelatinous sediments revert to the lower-energy crystalline state. If the original sediments were of an appropriately "pure" composition, they may crystallize directly. Feldspathic shales are an example. A welllaminated, fine-grained, graphitic quartz-feldspar-mica rock, called the marker argillites in the Purcell Mountains near the Sullivan Mine in southeastern British Columbia, is also crystalline. It has the texture and mineralogy of a micro-granite. This sequence of laminated crystalline rocks set in unmetamorphosed turbidite sediments can be correlated over very long distances (170 km). There is no doubt that these laminated rocks were originally precipitated as fine-grained sediments. They constitute a very clear example of a granitic rock crystallizing spontaneously from a gel. The crystalline state is indeed the lower energy state of the material (Barth, 1962, p. 381).

78 CONCLUSIONS The main features of a macromolecular system in which surface energy and particle interactions are predominant in the bulk behaviour of the material, are: 1. 2. 3. 4. 5. 6.

gel plasticity gel cohesion and fracturability gel diffusibility gel enhancement of crystal growth reversible hydrolysis concretion

7. 8. 9. 10. 11. 12.

thixotropy accretion rheopexy syneresis adsorption desorption

These very specific attributes uniquely characterise such a system. All the detailed observations which have been recorded relative to rapakivi granite textures are fully in accord with these unique properties and the rheological behaviour of an alternately dynamic and static macromolecular system. Forty-six commonly recorded observations relating to rapakivi granites are correlated with the properties of a colloidal hydrosilicate system as set out in Table I. Such an extraordinarily high correlation between the complex rapakivi features and the unique system which can account for all the rapakivi observations, leaves very little room for any alternative interpretation. The primary conclusion from the present review must therefore be that rapakivi texture results from the crystallization of hydrosilicate precursors. A number of important further conclusions then follow as a consequence: (1) Rapakivi granites do not crystallize from a melt or any kind of fluid. Fluidity is an earlier event. (2) Dense or concentrated but still wet and deeply buried sediments are subject to conditions favouring hydrolysis and respond as a gelatinous hydrosilicate system. These conditions are: (a) initial finely divided materials; (b) some elevation of temperature commensurate with burial depth; (c) increased pressure causing complete "swelling" of clays; (d) increase pore fluid salinity reduces interparticle forces of repulsion due to similar charges; (e) large macromolecules and high concentration favour the operation of van der Waals forces of attraction between particles. (3) Fluidity is not dependent on heat but is induced repeatedly and isothermaly by earthquake shock or gravitational instability. (4) Thixotropic liquefaction and laminar flow develop accretions in the concentrated mobile pastes. (5) Accretions subject to laminar flow in a mixed system survive according to a packing density, which depends on particle shape. Cohesive macro-

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88 aggregates therefore reflect particular macromolecular shapes and, to the extent that particle shapes characterise distinct minerals, chemical compositions. (6) The formation of accretions and their syneretic exudation of water "weakens" the gel fabric of the matrix facilitating repetitive thixotropic yield. (7) Intrusion or flow and formation of accretions retextures the hydrosilicates to "mottled" green and red clays which are close-packed and preconditioned for subsequent crystallinity (as distinct from non-remobilised country rocks where macromolecular inhomogeneity impedes crystallization). (8) The dehydration reactions by which the hydrosilicates crystallize are exothermic. Reaction rates and attainment of highest temperatures are controlled by the rates at which water can escape from the crystallizing rock mass. (9) Minerals not consistent with fusion of granitic components (calcite and iron oxides) and hydrous minerals (even enclosed in feldspar or quartz) are to be expected as residuals. A concept of hydrothermal alteration throughout the rapakivi massif itself is unnecessary. (10) Gel cohesion and syneretic properties of the hydrous precursors of quartz and feldspar differ from those of the weaker hydrous ferromagnesian minerals. In repeated mobilisations the more fluid hydrous ferromagnesian precursors tend to separate from the semi-fluid mixture of mineral matter as dykes and associated basic bodies. Successive rapakivi plutons are increasingly leucocratic and residual autoliths more basic. This conclusion that rapakivi granites congeal isothermally and do not crystallize by the temperature decline of a molten rock mass could be relevant to the genesis of other magma types. A number of indications of the crystallization of hydrosilicates in a wider variety of granites have been listed previously (Elliston, 1974, table 3). Such indications include: The theology of granites and their substitution for sediments. Features of autoliths, skialiths and xenoliths. Enclave plasticity and enclave inclusions. Ductless granite bodies. Lit-par-lit structures and sheet granites. The separation of basic fronts and basic dyke structures in granites. Sedimentary dykes in granites. 'Ghosted' or residual sedimentary structures in granites. Fossils, biogenic amines, coaly plant fragments and carbonaceous matter in granites. Porphyritic textures, mineral and textural variations in granites. Gel textures in granites. Ammonium feldspars and ammonium biotites in granites.

89 Myrmekites. Oscillatory zoning. Miarolitic cavities in granites. Stylolites transecting granite minerals. Plastic deformation of phenocrysts and deformation lamellae transecting grain boundaries. Skeletal crystals and cuniform graphic intergrowths. Incongruent melt minerals and hydrous minerals in granites. Ptygmatic veins and dyke features. Irregular foliation, augen structures, embrechites and akerites. Superficially many of these recorded features relating to granitic rocks would appear to support the crystallization of hydrosilicates as the origin of a much wider range of granitic and porphyritic rocks. Like the rapakivi textures, the subject of this paper, and the genesis of orbicules (Elliston, 1984), these aspects will be examined in the further papers of this series. ACKNOWLEDGEMENTS Mud is usually looked down on and regarded as a material to be avoided, but very special thanks must be accorded to Professor T.W. Healy of the University of Melbourne, who over many years, has patiently taught the writer and his colleagues to recognise its inherent energy and exciting properties as a dynamic and static colloidal system. The writer is indebted to Dr. Kai Hyt6nen, Mr. Kalevi Korsman and Dr. Atso Vorma for an informative tour of the rapakivi granites in southeast Finland and for the excellent quality of the work in bulletins of the Geological Survey of Finland, Nos. 246 and 285 by Dr. Vorma, from which the writer has been able to draw reliable information for this review. The substantial interest and support of Sir John S. Proud, former Chairman and Chief Executive of Peko-Wallsend Ltd. is gratefully acknowledged. His introduction of outstandingly talented specialists over the many years proved invaluable in testing and evaluating the solutions sought to our problems in ore deposition and petrogenesis. The writer is indebted to his fellow workers for their gift on retirement from Geopeko of a large number of excellent polished specimens of wiborgite. The encouragement and example of Emeritus Professor S.W. Carey in the pursuit of alternative hypotheses and avenues of investigation when observations and data present difficulties or defy logical interpretation is deeply appreciated. It has proven to be the most valuable lesson learned as his student and lifelong friend. The writer thanks Mr. K. Wright and Dr. Karl H. Wolf for helpful suggestions regarding the manuscript and Mrs. Robyn Harbour for her untiring efforts and meticulous attention to detail in its preparation.

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