Evolution of the Tugtutôq Central Complex, South Greenland: A high-level, rift-axial, late-Gardar centre

195

Journal of Volcanology and Geothermal Research, 43 (1990) 195-214 Elsevier Science Publishers B.V., Amsterdam

Evolution of the Tugtut6q Central Complex, South Greenland: a high-level, rift-axial, late-Gardar centre B . G . J . U P T O N a, A.R. M A R T I N a a n d D. S T E P H E N S O N

b

a Department of Geology and Geophysics, University of Edinburgh, Edinburgh, EH9 3JW, U.K. b British Geological Survey, Murchison House, Edinburgh, EH9 3 LA, U.K. (Received May 30, 1989; revised and accepted December 12, 1989)

Abstract Upton, B.G.J., Martin, A.R. and Stephenson, D., 1990. Evolution of the Tugtut6q Central Complex, South Greenland: a high-level, rift-axial, late-Gardar centre. J. Volcanol. Geotherm. Res., 43: 195-214. The Tugtut6q Central Complex (TCC) comprises at least six intrusions emplaced by ring-faulting and/or stoping. These intrusions are of syenites, quartz-syenites and granites, with peralkaline affinities, crystallized at relatively shallow depths, probably as a subvolcanic complex. The intrusions transect a major dyke-swarm marking the axial zone of a narrow mid-Proterozoic continental rift. The TCC appears to have formed from a succession of increasingly fractionated magma batches which are inferred to have arisen from a differentiating parent magma body near the base of the crust. A culminating perthosite intrusion (Bla Mane $o Syenite) joined the two separate early intrusive centres to form a composite body elongate along the rift axis. A Rb-Sr whole-rock isochron indicates an age of 1124 + 20 Ma, with a (S7Sr/86Sr)i of 0.70345. The magmas were relatively anhydrous (although probably halogen-rich) producing hypersolvus rocks. Olivines range from FogFassTP3 to Fo0Fa95TP5 and pyroxenes from Di35Hd62Ac 3 to virtually pure aegirine. Relative to members of the preceding dyke swarm (regarded as fast-quenched magmas derived from the same deep crustal site of differentiation) the TCC units are enriched in alkali-feldspar. The TCC rocks are interpreted in part as alkali-feldspar-rich roof-facies cumulates and in part as products of salic magmas selectively depleted in Fe-rich ferromagnesian minerals by gravitative separation. As with the salic members of the dyke swarm, the TCC rocks exhibit marked negative Eu anomalies. The TCC is inferred to be underlain by layered meso- to melanocratic syenite cumulates comparable to those seen in some of the other South Greenland alkaline intrusions.

Introduction T h e T u g t u t S q C e n t r a l C o m p l e x (TCC) is a small ring-complex (c. 4.5 x 2.5 km) composed of syenites, q u a r t z - s y e n i t e s a n d a l k a l i n e g r a n i t e s w i t h i n t h e mid-Proterozoic G a r d a r A l k a l i n e Igneous P r o v i n c e of S o u t h G r e e n l a n d ( E m e l e u s a n d Upton, 1976; U p t o n a n d E m e l e u s , 1987). Compositionally, t h e TCC r e s e m b l e s t h e G a r d a r complexes at Kfingn~t, N u n a r s s u i t , P u k l e n , N a r s s a q a n d K l o k k e n (Fig. 1) and, in size it cor-

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responds closely to the Kfingn~t, P u k l e n a n d K l o k k e n centres ( E m e l e u s and Upton, 1976). T h e m a g m a s w e r e anorogenic, r e l a t i v e l y a n h y d r o u s and t h e rocks have clear affinities to A-type granites. A s s u m i n g a d e p t h of at least 1 k m for the exposed TCC salic rocks, the m i n i m u m v o l u m e of m a g m a involved would h a v e b e e n c. 8 k m 3. T h e complex was originally described by Upton (1962, 1964) w i t h f u r t h e r d a t a provided by U p t o n et al. (1971). This p a p e r p r e s e n t s some

(~) 1990 -- Elsevier Science Publishers B.V.

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Fig. 1. Geological sketch map showing the relationship of the Tugtut6q Central Complex (TCC) to other major Gardar intrusive centres shown in black. The positions of the inferred boundary faults to the TugtutSq rift are shown (dashed lines). Stippled ornament marks outcrop of the early Gardar, Eriksfjord Formation. A = Assorutit and N = Narssaq.

new data from the TCC (viz: whole-rock majorand trace-element analyses, REE analysis, RbSr dating and mineral compositions for pyroxenes and olivines) and relates its evolution more precisely to the remarkable alkaline dyke swarm which preceded it. The current erosion level is thought to lie some 4 - 6 km below the contemporary Gardar land surface. The features seen m a y relate to those pertaining at similar depths beneath some of the Pleistocene-Recent sectors of the East African Rift, e.g. in the vicinity of Menengai and Longonot volcanoes in the K e n y a n rift (Macdonald, 1987) and, perhaps with closer analogy, the Erta' Ale volcanic range of Ethiopia (Barbieri et al., 1970; Barbieri and Varet, 1970) and sectors of the MakkahMadinah-Nafud volcanic lineament of western Saudi Arabia (Camp et al., 1989). Structural e v o l u t i o n o f the TCC The TCC (Fig. 2) lies astride the E N E - W S W -

trending swarm of alkaline dykes (the Tugtut6qIlimaussaq swarm, Macdonald, 1969; Martin, 1985) which can be traced some 150 km from the ocean to the inland-ice. It has been postulated t h a t these dykes occupied an extensional riftzone t h a t developed in late Gardar times (1200-1100 Ma) within an older, granitic, Proterozoic terrain, subparallel to the ProterozoicArchaean boundary c. 70 km to the north (Upton and Blundell, 1978). The TCC, together with the Ilfmaussaq, Igaliko and Klokken intrusive centres t h a t also lie along the swarm or offset from it towards its southern side, may mark the sites of late-stage central volcanoes that lay along the palaeorift at spacings of 2 5 - 3 5 km. In the Tugtut6q sector, the rift magmatism commenced with two consecutive intrusive events in which massive " g i a n t " dykes of up to 800 m width, involving basaltic to hawaiitic magmas of transitionally alkaline character were emplaced. These crystallised to coarsegrained suites of gabbroic rocks and their in-

EVOLUTION OF THE TUGTUTOQ CENTRAL COMPLEX, SOUTH GREENLAND

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termediate to salic differentiates. The Older Giant Dyke Complex (OGDC) has a central facies of silica-undersaturated syenites and foyaites (Upton et al., 1985). The Younger Giant Dyke Complex (YGDC) (Upton and Thomas, 1980; Upton, 1987) also produced a, more localised, axial pod of salic rocks although in this case they are silica-oversaturated (syenites and quartz-syenites grading to alkaligranite). These crop out in eastern TugtutSq at Assorutit, c. 10 km ENE of the TCC ("A" in Fig. 1). The opening phase of rifting is inferred to have been attended by a relatively large-scale mantle melting event: intrusion of the YGDC, was pro-

197

bably accompanied by graben formation at the surface, with bounding faults lying along zones now occupied by Bredefjord to the north and Skovfjord to the south (Fig. 3A, B). Crustal dilatation in these initial stages would have exceeded I km and the width of the graben feature was c. 15 km. Crustal dilatation continued as further dyke injection took place (Fig. 3C): the maximum width of dykes tended to diminish with time whereas the degree of differentiation tended to increase. Magma compositions evolved from hawaiite through to trachyte and thence to peralkaline rhyolites (comendites) (or, more rarely, to silica-undersaturated products). There is thus a crude correlation between age, maximum dyke width and composition of the intruded magmas suggesting that relief of extensional stress was principally accommodated during the initial phases of O G D C - Y G D C emplacement and that crustal fissuring diminished progressively thereafter. The magmas which were intruded as dykes in the waning stages of extension, were successive fractional crystallisation residues from a large evolving magma chamber. This originated during the initial high-energy event and subsequently underwent prolonged, essentially closed-system crystallisation. From this chamber, m a g m a batches of serially changing composition were supplied episodically to the developing dyke-swarm while crustal rifting persisted. It was at this stage that evolution of the TCC commenced. It is suggested that a substantial volume (> 8 km 3) of residual salic (trachytic) magma had developed from the hypothesized parental magma chamber deep in the crust and that batches of this low-density magma ascended by piece-meal stoping and/or by large-scale block subsidence at twin foci of crustal weakness defined by pre-existing fractures and dyke intrusions. Activity at the two centres may have started more or less contemporaneously. The westernmost of the two foci saw emplacement of a small plug of porphyritic microsyenite or trachyte, c. 700 m in diameter, at an inflection

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Fig. 3. Simplified evolution of "the Tugtut5q rift". A. Lithospheric thinning with graben development and production of a basaltic magma body at or near the base of the crust. B. Partial congelation of the parent magma body and emplacement of (relatively fractionated) batches as giant-dykes. C. Subsequent emplacement of more fractionated magmas in dykes, following crystallisation of the giant dykes, during progressive crystallisation of the parent magma body. D. Post-extensional phase: ascent of low-density residual (salic) magmas by ring-faulting and stoping. Probable development of rift-axial central-type volcanoes.

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in the O(~DC (Fig. 4). Around the second focal point, c. 2.5 km to the ENE, a larger body of similar porphyritic microsyenite was intruded. Ring-faulting, combined with subsidence of a central mass t h a t disintegrated as it sank, may have permitted emplacement of this microsyenite. The bounding fault on the northern side probably followed the northern contact of the OGDC, whereas t h a t on the southern boundary may have extended beyond the contacts of the northern YGDC branch. Rapid congelation of these microsyenites may have been enhanced by volatile loss through surface eruption, so t h a t at this stage the "Tugtut5q rift" may have seen axial development of small trachytic central-type volcanoes. The western plug appears to have been cored out by a younger intrusive facies of syenite (Unit 1A). The coarser grain-size and greater modal proportion of amphibole and biotite indicates a greater degree of volatile retention t h a n in the preceding intrusion. The microsyenite is dissected into subrounded masses up to 2 m diameter by a curviplanar network of syenitic and quartz-syenitic veins. The veins have diffuse margins (Fig. 5), suggesting t h a t the veining was generated by volatile-rich exhalations from the younger core while the microsyenite was still at high temperature. Similar microsyenite occurs as annular "screens" in the eastern centre. These are seen wrapped on both inner and outer margins by younger quartz-syenite and alkali-granite intrusions. However, in places the microsyenite is in contact with xenolithic masses of the (preGardar) country-rock granites. The attitudes of these contacts are: (a) near vertical; (b) dipping outwards towards the northern contact of the

Ornamentation: horizontal lining = supracrustal lavas and sediments; Crosses, grading to horizontal dashes = crustal section indicating downward change from granites to (?) basic granutites; wavy dashes =mantle peridotites; dashes (random orientation) -: magma - primitive basaltic magma in A, evolving to salic magma in D; black = crystallised (solid) igneous intrusions.

EVOLUTION

OF THE TUGTUTOQ

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CENTRAL COMPLEX, SOUTH GREENLAND

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Fig. 5. Coarse-grained syenite veins ramifying through finer-grained Unit 1 microsyenite. Locality in the extreme W of the TCC. TCC; and (c) approximately horizontal, with country-rock "big feldspar granite" overlying microsyenite, and amygdaloidal veinlets of the latter penetrating the older granite. The microsyenite "screens" of the eastern centre are, like the microsyenite of the western plug, pervaded by light-brown coarser-grained syenite veins which break up the grey microsyenite into ovoid masses. In some outcrops the overall appearance suggests a magma-mixing process, possibly between contrasting volatile-poor and volatile-rich m a g m a batches of otherwise similar bulk composition. The preferred interpretation however, is that, as in the western plug, the veining represents an intimate penetration by late-stage volatile fractions from the enclosing magmas, at temperatures above those at which brittle fractures could develop. Thus there appear to be relics of the original contacts of an initial fast-chilled microsyenitic facies against the country-rock. The microsyenite may have congealed around the roof and walls and may have been engulfed by subsequent accessions of more slowly cooled syenitic rocks. The underlying m a g m a may have been more volatile-rich and correspondingly less dense t h a n the already solidified earlier chilled facies.

200

The eastern centre developed more or less symmetrically across the OGDC and the northern branch of the YGDC (Fig. 4). That the magmas reached high crustal levels is suggested by the profusion of xenoliths of basaltic lavas and quartzites, particularly in the core of the eastern centre. These are undoubtedly derived from the early Gardar Eriksi~ord Formation: this formation is not seen in place on Tugtut6q but overlies the earlier Proterozoic granite "basement" unconformably further to the ENE. It is seen in situ closest to the TCC at Narsaq, c. 15 km away ("N" in Fig. 1). A series of ring-faulting events is envisaged. Collapse along a subcylindrical fault c. 1 km diameter, within the confines of the eastern centre microsyenite brought down masses of country rock granite as well as fragments of the overlying lavas and sediments. Whereas areas characterised by sub-unconformity (granitic) xenoliths and supra-unconformity (volcanic and sedimentary) xenoliths retain integrity, the chaotic association of various basaltic lithologies, together with the presence of quartzite xenoliths, suggests that a relatively turbulent mixing attended the subsidence. This event permitted emplacement of the Unit 2 syenite. The Unit 2 event may have been followed rapidly by further ring-faulting and intrusion (mainly around Units 1 and 2 of the eastern centre) of the Unit 3 quartz-syenite. A sub-porphyritic texture indicates relatively rapid crystallisation. Mafic to ultramafic schlieren and some more continuous layers up to c. 1 m thick occur sporadically in Unit 3, due to concentrations of Fe-rich olivines and clinopyroxenes. Attitudes vary from vertical to shallow inward dipping. Some layers appear to have been notably distorted by folding and ductile faulting (Fig. 6b). These melanocratic facies are regarded as localised side-walt olivineclinopyroxene cumulates, produced during brief episodes when feldspar crystallisation was inhibited. Their deformation may be related to subsequent collapse tectonics related to Unit 4 emplacement while Unit 3 was still hot and ductile.

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Fig. 6. Layering features in Unit 3 syenites A. Discontinuous, graded mafic layers. B. Discontinuous wispy layering that has probably undergone some degree of high-temperature ("softsediment") deformation. Unit 4, distinguished (often with some dift]culty) from Unit 3 by its field characteristics (less resistant to weathering, paler colour, lack of subporphyritic texture and greater content of quartz), is also thought to have been emplaced as an annular body around the Unit 3. Thus, during the time spanning the Unit 2 to Unit 4 events the complex may have grown by successive collapses within outward dipping (domical) ring-faults, with increase in overall diameter (Fig. 4c). Unit 4 contains several large xenolithic masses of country-rock granite and occasional xenoliths of lavas and quartzites indicating some disintegration of the subsiding blocks within the ringfaults.

EVOI,UTION OF THE TUGTUTOQ CENTRAL COMPLEX, SOUTH GREENLAND

Contacts between Units 3 and 4 are diffuse and the two proposed events are regarded as having followed rapidly one after the other. Contacts of Unit 4 against the country-rock granites dip outwards and are knife-sharp. The somewhat angular plan of the eastern centre (particularly of its southern margin) suggests some control of the ring-faulting by pre-existing fracture patterns. Unit 5 is a narrow, partial ring-dyke of quartzsyenite. It appears to be related to renewed ringfaulting along or adjacent to, the contacts between Units 3 and 4 and the outer "screen" of Unit 1, in the northern sector of the eastern centre. It is of some interest as a rare example of a true salic ring-dyke in the Gardar province. Caldera collapse events may well have accompanied the intrusion of Units 2 - 5 . The final event in the formation of the TCC involved intrusion of Unit 6 (the "Bla Mane So intrusion") as a sub-cylindrical syenite mass, c. 1.5 km across, intersecting both the western and the eastern centres and (again) removing by block subsidence, sectors of the OGDC, YGDC and intervening pre-Gardar granites (Fig. 4D). A diagrammatic impression of the overall geometry of the complex is presented in Figure 7. The Unit 6 syenite differs compositionally and texturally from those of the preceding eastern and western centres. It is highly variable in

Fig. 7. Block-diagram of the TugtutSq Central Complex (random dash ornament), showing relationship to the giant-dykes and outward dips of the syenite and granite margins.

201

grain-size, grading irregularly over a matter of metres, from pegmatitic (up to 15 cm crystals) to granular (< 1 cm). It is miarolitic and is dominantly composed of alkali-feldspar to the extent t h a t it can be described as a perthosite. These characteristics suggest t h a t we see, at the present erosion level, a roofing facies crystallised from a volatile-rich magma. The Unit 6 perthosite may well be composed of feldspar cumulates (of m i n i m u m melting point composition; Upton, 1964) grown in situ as a roofing encrustation. This youngest syenite unit has been selectively weathered to form a depression occupied by a lake (the Bla Mane So). The s y e n i t e - q u a r t z - s y e n i t e - a l k a l i granite association developed in the YGDC at Assorutit has clear petrological affinity to t h a t of the TCC. The close spatial relationship between the TCC and the giant dykes, and the sequential development of its component parts along the giant dykes, prompts speculation t h a t fractional crystallisation of YGDC-related basaltic magma, retained at depth, produced the salic (trachytic) m a g m a parental to those of the TCC (Fig. 2D). It is proposed t h a t this magma was produced at depth in an elongate ( E N E - W S W ) cupola in the same m a n n e r as the salic residues of the OGDC and YGDC were produced. The density of the Tugtut6q trachytes at compositions of c. 60% SiO 2, has been calculated at approximately 2.4 (Martin, 1985). This may have been the critical upper density limit at which the salic m a g m a residue could commence ascent, by ringfaulting and stoping, through the upper crust. Whereas the components of the TCC show sharp contacts with their country rocks, these contacts are often highly irregular in detail with thin veinlets, commonly < 5 cm thick, penetrating up to 20 m from the main body of the intrusion into the country-rocks. This behaviour is well shown for example by the Unit 4 alkaligranites of the eastern centre and the Unit 6 perthosites. These thin veinlets provide further confirmation t h a t the magmas were very fluid. The TCC rocks contain a variety of minor components, including aenigmatite, astrophyllite,

202 white (?Li) mica and fluorite, suggesting that they crystallised from peralkaline, halogen-rich magmas of low viscosity and density that readily penetrated fractures in the surrounding rocks and mechanically separated blocks which subsequently sank. The miarolitic nature of many of the TCC rocks suggest that crystallisation occurred at relatively low pressures. The TCC magmas clearly intruded through the sub-Eriksfjord Formation unconformity and the uppermost parts of the intrusions must have crystallised at depths of no more than a few kilometres. Gabbroic xenoliths, which occur sparingly in the eastern centre, are believed to be derivatives from the YGDC, either from its sub-unconformity dyke portions or from its (inferred) supra-unconformity sill-like portions in, or at the base of, the Eriksfjord Formation. Apart from the NW margins of Unit 4, xenoliths of the OGDC rocks appear to be missing throughout the complex. This observation throws doubt on the hypothesis that the upper termination of the OGDC lies at or near the subEriksfjord Formation unconformity (Upton et al., 1985). If it did, xenoliths of OGDC material would be expected to occur abundantly. Their apparent absence suggests that the upper termination lay at considerable depth beneath the unconformity and that stoped masses of the OGDC in the TCC may lie below the present erosional surface. Intrusion of small (< 1 m) dykes of trachytic composition along ENE trends continued on a very small-scale after the crystallisation of the TCC. A later small-scale basic dyke swarm of camptonitic affinity on the same trend also cuts the complex (Martin, 1985). The age of this swarm is unknown but the swarm may represent a renewed, very subordinate, mantle melting event towards the close of the Gardar period.

BG.J [,m~x' wr AI been dated (whole-rock Rb-Sr isochron) at 1154 ± 16 Ma (Upton et al., 1985). An age of 1143 ± 37 Ma for the TCC was obtained by the same method by Van Breemen and Upton (1972). A redetermination of the age of the TCC has been made (A.R.M.) using 6 whole-rock samples (Fig. 8). Two samples, 50288 and 40599 which plot very close to each other, can control the slope of the regression line by their inclusion or exclusion. If both are included the regression gives an age of 1112 ± 30 Ma (MSWD 36.0); excluding sample 50288", 1109 ± 38 Ma (MSWD 45.7); excluding sample 40599, 1124 ± 20 (MSWD 12.9). The latter value, with the lowest MSWD is chosen as the preferred regression fit. At the 2a confidence level the new ages for the OGDC and TCC bracket the period of principal magmatic activity in the Tugtut6q region within the limits 1170 M a - 1 1 0 4 Ma. The 2a error bars overlap by only 6 million years and it is considered likely that the 30 Ma gap is real. The revised age for the TCC suggests that it may be Age

1124-+20Ma

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Geochronology and Sr-isotope ratio As stated earlier, the TCC cuts the Older and Younger Giant Dyke Complexes. The OGDC has

* Sample numbers relate to collections with the Geological Survey of Greenland.

EVOLUTION OF THE TUGTUTOQ CENTRAL COMPLEX, SOUTH GREENLAND

among the youngest intrusive complexes in the Gardar Province. The value of (S7Sr/S6Sr)i, obtained from the errorchron is 0.70345 + 0.00020, close to the value of 0.70326 +_ 0.00026 obtained from the OGDC (Upton et al., 1985) given the 2a uncertainties in the regresssions. Both intrusions m a y have been derived from a source region of similar, if not identical, Sr isotopic character. Whereas the TCC magmas must have experienced some degree of crustal contamination on the evidence of the observed xenoliths, this appears to have had little effect on the strontium isotope ratios.

Mineralogy Previous studies of Gardar complexes, both oversaturated and undersaturated, have shown that the most useful indices of the fractionation state of the trachytic magmas are the olivine and clinopyroxene compositions (e.g., Stephenson, 1972; Larsen, 1976; Parsons, 1979; Stephenson and Upton, 1982). These two phases are complementary in that whereas olivine shows the widest variation in the less fractionated rocks, (but may be lost by reaction in the more fractionated) the pyroxenes show their greatest range in the more fractionated rocks. In complexes such as the TCC where the units exhibit relatively little post-emplacement fractionation, these phases are particularly useful guides to magmatic evolution in the postulated deeper, parental magma chambers. Accordingly, the olivines and pyroxenes were analysed from representative samples of the principal units of the Complex. Analyses were made on a Cambridge Scientific Instruments Microscan 5 electron-probe microanalyser at Edinburgh University, using crystal spectrometry and a 15-kV accelerating voltage. The standards used were of pure elements, oxides or simple silicate compositions and corrections were made for counter dead-time, atomic number, absorption and fluorescence, using computer programs based on the methods of S w e a t m a n and Long (1969). Representative

203

analyses are shown in Table 1. Olivines (14 analyses): In the early intrusions of Units i and 1A, fayalite is commonly present as small anhedral crystals in the groundmass, often pseudomorphed by iddingsite and commonly rimmed by blue-green amphibole. In the younger intrusions of the eastern centre, fresh fayalite is restricted to the mafic facies of Units 2 and 3, where crystals up to 4 mm in diameter occur with clinopyroxene in layers, schlieren and irregular masses. Some show cumulate textures in which the mafic minerals are enclosed by feldspars. In such mafic facies, olivine alteration is usually to magnetite, but in the more abundant feldspathic rocks, fayalite is almost invariably pseudomorphed by orange-yellow iddingsite and rimmed by blue-green amphibole. Whereas rare pseudomorphs after fayalite occur in Unit 4, no trace of olivine has been found in Units 5 or 6. Owing to the alteration, analyses were only possible of fayalites from Units 1, 1A and the mafic facies of Units 2 and 3 (Table 1, Fig. 2a). All compositions fall within a restricted range of Fo9FassTP3 to Fo0Fa95TP5 reflecting a high degree of fractionation in the magmas prior to emplacement to give a high Fe/Mg ratio. The fayalites show a moderate enrichment in Mn, comparable with that in most other Gardar suites (e.g. Larsen, 1976; Parsons, 1979; Stephenson andUpton, 1982; Upton et al., 1985), but not as high as in the Igaliko Centres (Stephenson, 1974; Jones, 1984). CaO values of 0.11 to 0.26 wt.% are comparable with those of other Gardar Si-oversaturated suites but not as high as in the Si-undersaturated suites (Stephenson and Upton, 1982). Pyroxenes (36 analyses): In the Unit 1 microsyenites, clinopyroxene occurs as idiomorphic crystals in the groundmass and rarely as phenocrysts. The crystals are commonly zoned from colourless to pale green and are rimmed by amphibole. Deeper green, pleochroic pyroxenes occur in the Unit 1A syenites. In the eastern centre, pale brownish to pale greenish ferrosalites occur with fayalite in the mafic facies of Units 2

204

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a n d 3. I n t h e m o r e l e u c o c r a t i c rocks, d a r k e r g r e e n , zoned, p l e o c h r o i c h e d e n b e r g i t i c p y r o x enes are always rimmed by alkali-amphibole. More strongly pleochroic aegirine-hedenbergi t e s occur as s c a r c e r e l i c t cores to a b u n d a n t

p o i k i l i t i c a m p h i b o l e in U n i t s 4 a n d 5. A l a t e r generation ofacmitic pyroxene, which crystallised a f t e r t h e a m p h i b o l e , is f o u n d in some s a m p l e s f r o m U n i t 3 a n d in m a r g i n a l v a r i a n t s of U n i t 4. In U n i t 6 p e r t h o s i t e s t h e d o m i n a n t m a f i c rain-

TABLE 1 Representative olivine and clinopyroxene analyses from the Tugtut6q Central Complex 1 SiO 2 TiO 2 A1203 Fe203* FeO MnO MgO CaO Na20 Total

30.36 0.05 0.03 . . 63.95 2.02 3.64 0.16 . . 100.21

2 29.45 0.08 0.03 . . 65.38 2.94 2.04 0.18 . .

3

4

29.13 0.11 0.01 . 66.97 2.95 0.56 0.26 .

29.52 0.05 65.72 3.62 0.11 0.16

5

6

7

8

9

49.48 0.65 1.22 1.00 19.71 0.77 6.44 20.47 0.39

47.74 0.74 0.92 1.19 26.50 0.94 1.46 20.78 0.46

50.18 0.30 0.31 13.19 16.27 0.77 0.34 14.20 5.12

51.47 1.12 0.27 27.00 4.23 0.59 0.08 5.81 10.48

51.84 0.20 0.85 31.98 0,17 0.03 1.77 12.95

100.10

99.99

99.18

100213

100.73

100.68

101.05

99.79

0.986 0.002 0.001 1.830 0.083 0.102 0.006 . 4

0.987 0.003 1.897 0.085 0.028 0.009

1.005

4

4

1.943 0.023 0.044 0.036 0.902 0.032 0.089 0.906 0.036 6

1.991 0.009 0.015 0.394 0.540 0.026 0.020 0.604 0.394 6

1.978 0.032 0,012 0.781 0.136 0.019 0.005 0.239 0.781 6

1.991 0.006 0.038 0.924

1.871 0.104 0.006 0.006

1.949 0.019 0.057 0.030 0.649 0.026 0.378 0.864 0.030 6

0.006 0.002 0.073 0.965 6

1.41 94.38 4.21

0.28 94.45 5.27

2.75 34.92 62.33

3.43 8.36 88.22

40.20 2.05 57.75

83.02 0.49 16.49

99.22 0.18 0.59

Atomic proportions Si Ti A1 Fe'" Fe" Mn Mg Ca Na O

0.998 0.001 0.001 1.758 0.056 0.178 0.006 . . 4

0.002

.

End-members Na Mg. Fe n Mn

8.95 88.23 2.82

5.05 90.81 4.14

* Fe203 in pyroxenes calculated from Fe'" = Na, FeO by difference 1. olivine (50306) unit 1 2. olivine (50340) unit 3 (mafic facies) 3. olivine (50282) unit 2 (mafic facies) 4. olivine (30699) unit 1A 5. clinopyroxene (50306) unit 1 6. clinopyroxene (50282) unit 2 (mafic facies) 7. ctinopyroxene (30699) unit 1A 8. clinopyroxene (50334) unit 3 9. clinopyroxene (50337) late stage vein in unit 3 Sample numbers refer to the Greenland Geological Survey Collection.

b:VOLUTION OF THE TUGTUTOQ CENTRAL COMPLEX, SOUTH GREENLANI)

erals are amphibole and biotite, but in some marginal facies, rare relics of strongly pleochroic aegirine (deep blue-green-green-yellowgreen) occur together with later crystallising riebeckite-arfvedsonite. Early crystallising pyroxenes fall within the range Di35Hd62Ac3 to Di2Hd58Ac40 on a trend, comparable to those of other Gardar Sioversaturated suites, in which Fe-enrichment proceeds to Hd95 before any Na-enrichment occurs (Fig. 9). The amount of subsequent Naenrichment is far greater than that observed in the Kfingnfit Complex (Stephenson and Upton, 1982) and Klokken intrusion (Parsons, 1979) and approaches that of the Nunarssuit-Alangors-

(a)

,%

(b)

Ac

// sa

_

Qmphibole

--- "

~ s Fo

\

3o

9s

\

Fa

~

, ~\

/

/

%,

A_

,

\

,

Mol.%

Di x

~

unit 1 (50306)

o unit 2 (50282)

unit 1A (30699)

• unit 3 :-

Hd •

unit 6 (5024.2) & late vein {5033?)

mafic band (5034.0) tare pyroxene (50334.) • unit 5 (50272)

Fig. 9. Major-element variation in olivines and clinopyroxenes from the Tugtut6q Central Complex. (a) Olivines: variation of Mg (forsterite Fo), Fe" (fayalite Fa) and Mn (tephroite Tp). K = Fe-rich end of overall olivine trend for the Kfingn&t Complex (Stephenson and Upton, 1982), typical of trends for most Gardar suites. SQ = Fe+Mn-rich end of overall olivine trend for the South Q6roq Centre (Stephenson, 1974), typical of Mn-rich trends for the Igaliko Complex. (b) Clinopyroxenes: variation of Mg (diopside), Fe"+Mn (hedenbergite) and Na (acmite + minor jadeite). K = overall pyroxene trend for the Kfingnfit Complex, typical of Gardar Si-oversaturated suites.

205

suaq intrusions (Anderson, 1974). In each case the crystallisation of pyroxene is terminated by a reaction relationship in which its place is taken by alkali-amphibole in response to falling temperature and f% and rising PH20 and/or PF in the magma. The precise stage at which this reaction occurs varies considerably between units of the TCC, but occurs latest in Units 1A and 5, which consequently contain strongly zoned aegirine-hedenbergite. In parts of some units, notably 3 and 4, and in late-stage veins, pyroxene again becomes stable in place of alkali-amphibole in the lower temperature late stages of crystallisation, when the residual liquids become highly peralkaline. Such pyroxenes are aegirines within the range DilHd16Acs2 to Acl00 and may be followed by crystallisation of riebeckitic amphibole. In the Unit 6 perthosites, almost pure aegirine is the only pyroxene present. Recent studies ofmetasomatism in rocks of the Gardar Province (A.D. Chambers and A. Finch, pers. commun., 1988) have drawn attention to the possible sub-solidus enrichment of Na in pyroxenes and other minerals by late-stage, alkali-rich, aqueous fluid phases. Such effects were also documented by Stephenson (1972, 1976). These compositional changes can confuse any interpretation of magmatic evolution based upon alkali pyroxenes and hence demand consideration. The analysed early-crystallising (pre-amphibole) pyroxenes from the TCC exhibit regular, concentric zoning with none of the features normally attributable to sub-solidus alteration, such as poikilitic textures, overgrowths, patchy zoning and replacement along fractures. Furthermore, in all but Unit 5, they coexist with fayalite which elsewhere is seen to react out in the presence of late fluids (e.g. Stephenson, 1976). High Fe"/Mg ratios in the Unit 5 pyroxenes are an independent (of Na) indication of a highly evolved magma. The latecrystallising (post-amphibole) aegirines probably crystallised from a peralkaline liquid with a considerable aqueous component and hence, although much of their growth may have been

206

B(;J ~;~,'mxm' At.

sub-solidus, there was little scope for compositional changes. Values of A1 are moderately high in the most magnesian pyroxenes (max. 1.31% A1203 = 0.06 atoms per 6 oxygens) but fall rapidly to uniformly low values (0.24% A1203 = 0.01 atoms) in more evolved compositions, as Si increases and eventually fills all the tetrahedral sites (Fig. 10). Slightly increased A1 values in pure aegirines from peralkaline late-stage veins (= 0.04 atoms) are due to octahedral A1vi combining with excess Na to form small amounts of the jadeite molecule (NaA1Si206). Ti shows a similar behaviour to A1 with highest values (1.38% TiO 2 = 0.04 atoms) in late-stage aegirines, probably as complex N a - T i end-members. It has already been suggested that successive intrusive units represent pulses of more and more strongly fractionated magma from an underlying differentiating magma chamber (Upton, 1964; Upton et al., 1971). The limited olivine and clinopyroxene analyses (Fig. 9) confirm that the Unit I microsyenite represents the least evolved magmas, although even it was highly fractionated in absolute terms. A mafic layer from Unit 3 yielded the next most evolved compositions with more evolved clinopyroxenes (sodic hedenbergites) ap-

c

/~

×

!

•

x

*

~

~,,~.~,

~

.o8i

~

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.ooi1 i

D

• x

oo

s

•

~

.02~ -,~ . . . .- . 3

r

__ ^~ <2

-11

0

.1

,2

.3

.~

(No-Ng) atoms --,

,5

.6

.7

.8

.9

] u~

1.0

-~

Fig. 10. A1 and Ti variation (atoms per formula unit) in clinopyroxenes from the TugtutSq Central Complex, relative to the alkali pyroxene fractionation index (Na minus Mg, R.C.O. Gill, personal communication). Symbols as in Fig. 9.

pearing in a mafic syenite from Unit 2. Units 2 and 3 were intruded within a very short time interval and probably involved very similar magmas. From this point on, fayatite was unstable and is pseudomorphed by iddingsite, and pyroxenes are increasingly rimmed by amphibole. Thus no mineral analyses are available from Unit 4, but analyses of strongly zoned aegirine-hedenbergite from Unit 5 show that the magma was, by this stage, significantly more evolved. In the Unit 6 perthosites the mafic minerals are predominantly hydrous, but latecrystallised aegirine indicates the highly evolved, peralkaline nature of the residual liquids in this intrusion and also in marginal modifications and late-stage veins of the eastern centre (e.g. in Unit 3). The original evolutionary sequence inferred by Upton (1964) is thus confirmed, with the notable exception of Unit 1A. This intrusion contains relics of oligoclase in places and had been thought to be relatively basic, or at least as basic as Unit 1. However, the fayalite analyses are the most evolved of the complex and the clinopyroxenes are aegirine-hedenbergites, comparable with those of Unit 5. It is unusual in the Gardar Province to find fayalite stable with such a sodic pyroxene; fayalite normally reacts out as soon as the pyroxene trend changes from dominantly Fe"- to dominantly Na-enrichment (Stephenson and Upton, 1982). It may be that a suppression of volatile build-up, which allowed the pyroxene to evolve to aegirine-hedenbergite before eventually being replaced by alkali-amphibole, also resulted in the prolonged fayalite stability. It has already been suggested that degassing may have occurred in Unit 1 and possibly other units of the complex, due to a connection with surface volcanic activity (Upton, 1964). Such degassing is one mechanism which may have slowed down the build-up of volatile pressure, which allowed the development of relatively sodic pyroxenes compared with, for example, the Kfingn&t Complex in which crystallisation occurred in an essentially closed system at deeper crustal levels (Upton et al., 1971).

207

E V O L U T I O N OF T H E T U G T U T O Q C E N T R A L C O M P L E X , S O U T H G R E E N L A N D

lege, London, on four representative samples, using inductively coupled plasma spectrometry techniques on concentrated lanthanide solutions separated by cation exchange techniques (modified after Walsh et al., 1981). The bulk compositions and inter-element ratios confirm the overall affinity between the TCC magmas and those involved in the pre-TCC dyke swarm. The dyke compositions, being from rapidly congealed hypabyssal intrusions, may be taken, as a first approximation, as representing, if not a "line of liquid descent", a bundle of closely related "liquid lines of descent" (Macdonald, 1969). On an alkali-silica plot (Fig. 11), the TCC compositions are displaced from those of the dyke rocks with > 62% SiO 2, having higher total alkalis at comparable silica values. The TCC compositions are also more aluminous (Fig. 12). The TCC compositions are displaced from those of the trachytic dykes towards the compositions of alkali-feldspars. At similar silica content the

Whole-rock compositions Thirty-three new whole-rock analyses for major and trace elements were made for TCC samples. These supplement 15 analyses (major elements only) presented by Upton et al. (1971). Four of the 48 analysed samples are from mafic to ultramafic cumulates. The rock analyses were made at Edinburgh University using a Philips PW1450 XRF system. Major elements were determined on fused glass discs (Norrish and Hutton, 1969) with corrections applied for inter-element mass absorption effects. Trace elements were determined using pressed powder discs and corrected for mass ab+ sorption effects with coefficients calculated from the major element analyses. USGS and CRPG rock standards (Abbey, 1980) were used in the calibration of both major and trace elements. The precision and accuracy of the methods used have been described by Thirlwall (1979). REE determinations were made at King's Col-

%

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i

i

Na20 +

i

1

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I

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+ ; ".'oa:o._ • ++-+P®+° ". • ooo°o+ +° +,.,1's ? o. "+

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I

5'0

'

5

4

'

' 58

[

I 62

'

6

;

I

' 70

I

' 74

]

% SiO 2

Fig. 11. Alkali-silica diagram. Open circles = fast-cooled dyke-rocks from the Tugtut6q-Ilimaussaq "Main Swarm" dykes (Martin, 1985). Data from the Tugtut6q Central Complex in black. Black triangles = microsyenitic Unit I rocks; black circles - syenites and granites from Units 2-5; black diamonds = Blfi Mfine Se (Unit 6). Star approximates minimum melting composition for alkali-feldspar.

208

B,G J, [ I'IY)N ET A I ,

TCC s a m p l e s are also r e l a t i v e l y poor in Mg, Fe, Mn a n d Ti. The r e l a t i o n s h i p b e t w e e n silica a n d total iron is s h o w n in F i g u r e 12. The TCC rocks are c o n s e q u e n t l y inferred to h a v e been e n r i c h e d in alkali-feldspar r e l a t i v e to the dykes, this effect b e i n g at its most e x t r e m e with respect to the U n i t 6 perthosites. I n the case of U n i t 6 it is possible, as outlined above, t h a t this developed as a feldspar c u m u l a t e t h r o u g h p r e f e r e n t i a l g r o w t h of alkali-feldspar (sanidine) a g a i n s t a roof. S u c h a h y p o t h e s i s does not seem supportable, however, for the U n i t s 2, 3, 4 a n d 5 rocks of the e a s t e r n centre. F e l d s p a r e n r i c h m e n t

m a y h a v e occurred t h r o u g h flotation or, more likely, the high-level m a g m a s m a y simply have been relatively enriched by loss of f e r r o m a g n e sian m i n e r a l s , p r i n c i p a l l y Fe-rich p y r o x e n e a n d olivine. Despite the fact t h a t the full R E E a n a l y s e s fox' t h r e e of the four TCC s a m p l e s show p a t t e r n s closely r e s e m b l i n g those of the t r a c h y t e (and b e n m o r e i t e ) d y k e s (Fig. 13), X R F d a t a for La, Ce a n d Y show t h a t the La/Y a n d Ce/Y ratios tend to be h i g h e r in the TCC t h a n in the dyke suite (Fig. 14). It is s u g g e s t e d t h a t this m a y h a v e been b r o u g h t a b o u t by f r a c t i o n a t i o n of Y in some

TABLE 2 Representative whole-rock analyses from the Tugtut6q Central Complex 1

2

3

4

5

6

7

8

9

10

11

12

SiO 2 TiO 2 A1203 Fe203* MnO MgO CaO Na20 K20 P205

59.98 0.66 16.48 7.93 0.17 0.34 2.33 6.34 5.37 0.13

57.24 0.94 16.22 8.28 0.18 0.70 3.21 6.11 4.73 0.30

62.70 0.34 16.45 6.85 0.07 0.22 0.49 7.44 5.06 0.04

44.28 1.37 8.31 35.20 0.96 1.79 2.90 2.78 3.24 0.31

63.90 0.39 15.93 5.86 0.19 0.05 1.58 6.53 5.52 0.03

64.52 0.33 16.62 4.88 0.13 0.14 0.78 6.44 5.80 0.02

41.60 1.06 5.71 43.55 1.42 0.61 3.06 2.35 1.70 0.23

67.74 0.32 14.86 4.47 0.10 0.04 0.86 5.95 5.46 0.01

67.43 0.32 14.83 4.47 0.12 0.03 0.79 6.01 5.24 0.01

66.95 0.25 15.67 3.81 0.09 0.20 0.81 6.21 5.59 0.01

64.98 0.20 17.68 3.49 0.07 0.22 0.47 7.71 5.23 0.01

65.96 0.05 [7.74 2.75 0.06 0.11 0.17 8.16 4.82 0.00

Total

99.72

97.90

99.67

99.24

99.97

99.66

98.59

99.81

99.23

99.59

100.06

99.84

2 4 8 6 176 92 137 543 118 1041 13 91 221 105 52

3 5 8 5 177 326 103 485 111 1585 10 93 212 102 62

2 3 95 6 192 1280 174 93 8 17 188 395 165 104

l 19 8 14 437 60 94 160 54 420 9 64 125 62 46

3 4 .... .... 210 27 154 1016 120 139 22 8 166 353 148 81

wt.%

ppm Ni Cr Sc Cu Zn Sr Rb Zr Nb Ba Pb Th La Ce Nd Y

2 4

138 15 171 794 93 109 33 7 93 218 88 52

5 7 35 925 9 43 124 66 41 15 102 206 100 59

3 2 3 2 4 5 3 3 . . . . . . . . . . . . 169 162 190 98 17 11 24 8 219 174 178 171 631 819 905 310 124 145 81 53 76 29 117 29 26 16 23 9 16 15 10 3 220 371 205 178 445 632 420 396 153 246 162 169 83 97 74 51

2 4

1.74 12 202 772 239 32 27 10 109 227 79 60

209

EVOLUTIONOF THE TUGTUTOQCENTRALCOMPLEX,SOUTHGREENLAND TABLE 2 (continued)

1

2

3

4

5

6

7

8

9

10

11

12

0.15 0.25 3.68 0.02 0.35 0.34 28.49 64.43

CIPW norms

. 4.76 . . . . . . 0.37 0.75 4.30 4.29 4.21 4.65 0.34 0.70 4.40 4.43 31,73 27.93

0.06 0.97 0.92 0.35 6.62 29.88

an ab

0.64 45.95

2.89 42.87

mt il ap ac ns

1.36 1.25 0.29 81.85 81.85

1.42 1.78 0.68 75.57 75.57

q ne I en h y fe I en di fs wo ol [ f ° Lfa or

D.I. F.I.

.

.

.

0.47 4.95 4.90 2.80 32.74 19.15

0.86 1.12 . . . 0.07 0.26 4.69 4.99 0.05 0.08 3.55 1.66 3.18 1.56 32.62 34.28

. 0.31 12.17 0.16 6.27 5.71 0,74 32.14 10.05

10.33 10.27 . . 0.07 0.05 4.22 4.40 0.03 0.02 1.94 1.79 1.74 1.60 32.27 30.96

0.34 3.65 0.16 1.66 1,65 33.00

0.50 0.11 0.95 0.96 0.31 3.06 30.88

56.08

0.63 22.48

51.20

53.20

0.01 19.88

. . . 46.03 47.11

. 49.52

. 60.93

0.65 0.08 2.35 0.89 86.17 89.40

6.05 2.60 0.72 42.19 42.19

0.74 0.08 2.01 0.41 84.68 87.10

0.27 0.62 0.05 1.14 88.59 89.74

7.48 2.01 0.53 29.93 29.93

4.17

0.21

0.56

. .

-

.

.

0.61 0.03 1.53 0.61 88.63 90.76

. 0.60 0.02 1.53 0.47 88.34 90,34

7.11

. 0.47 0.02 1.31 0.36 89.63 91.30

. 0.37 0.02 1.20 0.47 92.32 93.99

0.10 0.01 0.94 0.82 93.07 94.84

F e 2 0 3 * . T o t a l F e O + F e 2 0 3 e x p r e s s e d as Fe203. S a m p l e n u m b e r s : 1 = 50231; 2 = 86141; 3 = 40599; 4 = 50235; 5 = 50244; 6 = 50279; 7 = 50280; 8 = 50260; 9 = 86112; 10 = 50327; 11 = 50228; 12 = 50337.

AI203% 15

i

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Fe203%

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52

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£ ak, llb, o_oh I 64

I

oO

~I 68

°o

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I 72

o

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l 76 %SiO 2

F i g . 12. A120 a a n d F e 2 0 a (total iron) vs SiO 2 for T u g t u t 6 q C e n t r a l C o m p l e x r o c k s a n d ( p a r t of) t h e " M a i n S w a r m " ( M a r t i n , 1985). S y m b o l s a s for F i g . 11.

210

t~ (;.,~ [TPTON ET AJ,

600

100 c~ > ~3 Q (n

E

'"o..._

© c (1)

.E

10

\

~3 C ©

"(9 La

Ce

Pr

Nd

(Pro)

Sm

Eu

Gd

(Tb)

Dy

Ho

Er

(Tm)

Yb

Lo

Fig. 13. Chrondrite-normalised REE diagram for 4 samples from the TugtutSq Central Complex. The lowest curve is for a Unit 6 perthosite. Shaded area, range of plots for benmoreite and trachyte dykes in the TugtutSq Main Swarm (Martin, 1985).

minor crystalline phase, as yet unindentified. Whereas Zr and Nb values are generally similar in the TCC and the dyke-suite trachytes, the Zr/Nb ratios are slightly lower in the TCC suggesting that zircon fractionation may also have been significant. The TCC samples, like the more evolved dyke compositions, show the pronounced negative Eu anomalies (Eu/Eu* 0.2-0.4) that would be expected for relatively reduced magmas whose evolution has entailed extended plagioctase (and anorthoclase) fractionation (Fig. 13). It may be inferred that preferential loss of Fepyroxene and Fe-olivine was related to formation of a series of mesocratic to melanocratic syenite cumulates beneath the TCC and t h a t a deeper level of erosion would reveal layered syenites comparable to those of the Kfingnfit Complex (Upton, 1960). Crystal fractionation may have resulted not so much by gravitational settling of individual phenocrysts as by masses of dense sidewall cumulates like those seen in

Unit 3, becoming detached and slumping down to the m a g m a chamber floors. Compositional stratification was probably commonplace among the Gardar magma bodies (Upton and Emeleus, 1987; Larsen and Sorensen, 1987). No clear evidence however, for such a process can be adduced from the TCC. The sequence of intrusions tends, if anything, to show progression to increasingly evolved compositions rather t h a n the reverse, which may have been expected if they had arisen from a deeper-level stratified parent magma. It may be t h a t the sequence from the least evolved (Unit 1 ) to the more evolved later input, merely reflects cooling and increasing fractionation of the parent body at depth. Conclusions and discussion (1) The late Gardar magmatic history of Tugtut5q illustrates a change in intrusive style from dyke formation to focussed ascent of low-

EVOLUTION OF TIlE TUGTUTOQ CENTRAL COMPLEX, SOUTH GREENLAND

density salic magmas as extensional stress waned. (2) Formation of the TCC was one of the youngest intrusive complexes in the Gardar province and was probably crowned by a trachytic volcano at least in its early (Unit 1) stages. (3) The TCC provides evidence for the genesis of relatively large volumes of peralkaline salic magma. The geochemistry, including Sr isotopic data, and relationship to the Tugtut6q dyke swarm suggests that the origin was primarily by crystal fractionation from a greater volume of basaltic magma arrested at deeper crustal or subcrustal levels. The evidence comes (a) from the massive dykeing events at the onset of activity and (b) from the extensive rift-axial positive gravity anomaly beneath TugtutSq (Blundell, 1978; Upton and Blundell, 1978). (4) The inferred high fluidity of the TCC magmas is believed to be related to their peralkaline character and to concentration of fluorine. Evidence for the role of fluorine is largely petrographic, fluorite being a common accessory mineral throughout the TCC. The low viscosities conferred ability to penetrate fractures in surrounding rocks and to react with them. Pervasive metasomatism of stoped blocks of basalt, gabbro, quartzite and granite is widespread. (5) Mafic and ultramafic cumulates, sporadically present in Units 2 and 3 are likely to be representative of more extensive fayaliteferrohedenbergite-rich cumulates at depth. The disppsition of these is assumed to be analogous to that revealed in the layered syenitic sequences of the similar-sized but more deeply eroded Kfingn~t complex. (6) Bulk compositions of the coarse-grained TCC rocks are systematically displaced from those of the (presumed) parental magmas by loss of dense ferromagnesian phases (fayalite, ferrohedenbergite and magnetite) and gain in alkali-feldspar. The feldspar enrichment in Unit 6 may have developed through selective crystallisation of alkalbfeldspar (sanidine) against a magma chamber roof.

211

(7) Assimilation of early volatile-poor microsyenitic wall and roof chilled-facies occurred during the successive arrivals of volatile-rich magmas. The TCC offers considerable scope for further research. With small size, excellent outcrop and rocks virtually unaltered since the midProterozoic it provides an attractive natural laboratory. Much remains to be learned of the precise intrusive mechanisms through study of the distribution of the xenoliths. Much could also be learned concerning the nature of the original cover and of the upper terminations of the giant dykes. On the assumption that a proportion of the late-Gardar intrusions of the Tugtut6q rift supplied surface volcanoes, an extrusive sequence may be inferred which commenced with an elongate, fissure-fed, lava field of transitional b a s a l t i c - h a w a i i t i c lavas. As productivity declined, fissure volcanoes erupted increasingly fractionated products with trachyte, and possibly even comendite, lavas being produced. Activity is thought to have culminated in formation of a t r a c h y t e - c o m e n d i t e central volcano with caldera formation. On the basis of data provided by Macdonald and Smith (1988), relating the area of calderas (or in this case, supposed calderas) to the volume of effusive product it is likely that the TCC eruptives would have totalled < 6 km 3. Close modern analogues to this scenario may be found in Ethiopia and Saudi Arabia. One such analogue is the Erta 'Ale volcanic range of the Danakil Depression, Ethiopia (Barbieri et al., 1970; Barbieri and Varet, 1970). The Erta 'Ale range comprises an elliptical structure, c. 100 km long and 2 0 - 3 0 km broad in a region of rapid crustal extension along the median axis of the Depression. It shows evolution from simple fissural eruptions to more complex central volcanoes, with a remarkably regular volumetric decrease in time from early transitional-type basalts, through Fe-rich intermediate compositions, to highly differentiated products (trach y t e - a l k a l i rhyolite). The latter tend to be re-

B . G . J ( ; P ' I ' O N hiT A L

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I

Ce ppm 800

600

O

400

200

L~ ppm

I

I 50

I

l 100

i

11

]

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I 2 0 0 Y :)pm i

i

400

300 o

• ooo

200 o

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1 50

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I 150

I 200 Yppm

Fig. 14. Ce- Y and L a - Y plots from Tugtut6q Central Complex and "Main Swarm" dykes (Martin, 1985). Symbols as for Fig. 11.

stricted to the late central volcanoes. Where silicic lavas were erupted as fissure-flows, the fissures are located close to the central volcano. 87Sr/86Sr ratios remain low throughout the entire sequence and the authors attribute its origin to low-pressure crystal fractionation of basaltic parent magmas. Field characteristics of the "dark trachyte" lavas of Erta 'Ale indicate high fluidity. The low viscosity of these lavas led Barbieri et al. (1970) to conclude that crystal fractionation could readily have proceeded to yield oversaturated trachytes and rhyolites. The general analogy between the Erta 'Ale situation

and that envisaged for the Tugtut6q lineament in the mid-Proterozoic is close but with important qualifications. Thus Barbieri et al. (1970) and Barbieri and Varet (1970) regard Erta 'Ale as forming in a zone where continental crust is thin or non-existent and the magmas to be uncontaminated by sialic crust. Such was clearly not the case in the TugtutSq situation. A further close parallel to the Tugtut6q rift (or~ more broadly, the T u g t u t S q - I l i m a u s s a q - n u n a tak lineament, Upton and Emeleus, 1987) appears to be provided by the M a k k a h - M a d i n a h Nafud (MMN) volcanic line of western Saudi Arabia (Camp et al., 1989). The MMN line extends for some 600 km and, like the T u g t u t 6 q I l i m a u s s a q - n u n a t a k lineament, is segmented into smaller en-echelon linear vent systems. Volcanism has occurred from the Miocene to Recent, over c. 10 Ma along the MMN line. The volcanic fields associated with the vent systems all show generally similar evolution, commencing with extensive transitional olivine-basalt flows, passing up into less voluminous sequences of transitional olivine-basalt, alkali olivinebasalt and hawaiite. The most recent activity has produced flows of minor transitional olivinebasalt, abundant alkali olivine-basalt and hawaiite, plus some mugearite, benmoreite and trachyte. In the high central vent area of Harrat Khaybar, the latest eruptions saw extrusion of comendite. Camp et al. (1989) proposed that primary mantle melts accumulated and evolved at the c r u s t - m a n t l e boundary, to the stage when transitional olivine-basalts arose quickly through the crust, to be extruded copiously along the whole MMN line. Some magma however, trapped in crustal reservoirs, evolved further and, in at least one case (Harrat Khaybar) produced a high-level zoned chamber in which comenditic magma formed at the top through a combination of trachyte fractionation, crustal assimilation and volatile fluxing. We suggest that uplift and deep erosion of, e.g. Erta 'Ale and Harrat Khaybar, would reveal intrusive structures, lithologies and sequences

EVOLUTION OF THE TUGTUT()Q CENTRAL COMPLEX, SOUTH GREENLAND

broadly comparable to those exposed on TugtutSq. The in tr apl at e alkalic m a g m a t i s m displayed in Ethiopia and Saudi Arabia occurred contemporaneously with MORB eruption, attending the Red Sea opening, a few h u n d r e d kilometers away. The Tugtut6q rift is roughly contemporaneous with the great Mid-continent Rift System of No r th America, whose proximity in space and time to the Grenville Province suggested to Van Schmus and Hinze (1985) the likelihood of a genetic relationship. It is open to speculation t h a t activity along the Tugtut6q rift (and its associated structures) m a y have accompanied opening of a proto-Grenvillian ocean off w h a t is now the southern tip of Greenland.

Acknowledgements The original mapping of the complex was conducted for the Geological Survey of Greenland by B.G.J.U. A re-investigation (B.G.J.U.) of part of the TCC was made possible by a "grant-in-aid" from the Royal Society. XRF and ICPS analyses of the Tugtut6q samples and d e t e r m i n a t i o n of the Rb-Sr isochron were carried out by A.R.M. during t e n u r e of a NERC studentship. Mineral analyses (DS) were supported by the University College of Swansea and t h a n k s are due to P.G. Hill for assistance. T h a n k s are also extended to L. Begg, Y. Cooper and D. Baty for preparation of the manuscript and figures. We are grateful to C.H. Emeleus, R. Macdonald, I. Parsons, R.I. Tilling and an anonymous referee for constructive criticism.

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