The generation, segregation, ascent and emplacement of granite magma: the migmatite-to-crustally-derived granite connection in thickened orogens

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Earth-Science Reviews 36 (1994) 83-130

The generation, segregation, ascent and emplacement of granite magma: the migmatite-to-crustally-derived granite connection in thickened orogens Michael Brown Department of Geology, University of Maryland at College Park, CollegePark, MD 20742, USA

(Received June 24, 1992; revised and accepted November 12, 1993)

Abstract

Many granites result from anatexis of common crustal rock types and the segregation, aggregation, ascent and emplacement of the resultant magma. What then is the connection between migmatites, rocks which preserve evidence at outcrop-scale for the presence of former melt now frozen as granite, whether in situ or locally displaced with respect to the site of melting, and map-scale bodies of crustally-derived granite, clearly removed from the site of melting? Both water-rich volatile phase-present melting and volatile phase-absent dehydration melting can occur in the middle and lower crust, but dehydration melting that involves the decomposition of mica and amphibole likely is the more important process in the generation of plutonic volumes of magma with sufficient mobility to reach the upper crust. Both volatile phase-present and dehydration melting can occur in each of the two main types of orogenic belt, those that result from thickening before maximum temperatures are achieved (clockwise in P - T space) and those that result from heating prior to or concomitant with thickening (anticlockwise in P - T space). Depending upon the particular tectonic circumstances, the thermal perturbation to provide the heat necessary for crustal anatexis may be caused by internal radiogenic heat production in overthickened crust, intraplating/ underplating of mantle-derived magma, an enhanced flux from the mantle, or some combination of these mechanisms. The tectonic environment to a large extent also controls the segregation, ascent and emplacement of granite magma. For example, at the present time a majority of convergent plate margins exhibit an oblique net displacement vector, and it is likely, therefore, that oblique convergence was important in the past. Retreating subduction boundaries will result in regional deformation of the overriding plate by horizontal extension or transtension in contrast to advancing subduction boundaries that will result in regional deformation of the overriding plate by horizontal shortening or transpression. Transpression can be considered as a zone of transcurrent shear accompanied by horizontal shortening across and vertical lengthening along the shear plane. It plays a vital role in overthickening of the crust, in structurally shuffling and thickening sedimentary basins, in assisting with the segregation of crustaUy-derived melts, and ultimately allowing for the ascent and emplacement of granite magma in extensional segments of the associated strike-slip system. Segregation of granite melt depends upon a number of factors that include how the liquid is distributed within the matrix of a partially melted rock and the viscosity of that liquid. The geometrical structure of partially molten rock is crucial to understanding its behavior. Migration of melt is a complex process that is achieved by compaction and buoyancy segregation, and flow into extensional and shear fractures and other dilatant sites. The driving force for segregation may be chemical or physical or a combination of both. In this review I stress the important role of 0012-8252/94/$26.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0012-8252(93)E0056-N

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deformation in enhancing segregation of melt. Magma buoyancy is a primary driving force for ascent, but diapirism no longer appears to be a viable mechanism. Rather, fracture-controlled mechanisms and deformation-enhanced ascent are considered to be of prime importance. Magma ascends in dykes to feed tabular batholiths that are constructed from hundreds of individual magma pulses due to magma ponding at roughly horizontal discontinuities in the upper crust. I emphasize the role of ductile shear zones and fault systems in the ascent and emplacement of magma. Many granites appear to have been constructed from sheets emplaced in transient dilational sites along transpressional strike-slip fault systems undergoing net contractional deformation. Emplacement is synkinematic, a void or cavity will not exist and filling at a suitable site occurs simultaneously with dilation. Rates of ascent are fast, consistent with a pulsed magma supply. Some examples cited in the literature of diapiric emplacement may be interpreted better as local ballooning by magmatic flow of granite after upward transport along shear zones. Thus, granite contact relationships likely reflect local emplacement mechanisms rather than regional, crustal-scale ascent mechanisms. A general model for granite magma genesis, ascent and emplacement that may apply to other orogenic belts is developed from relationships interpreted from different crustal levels exposed within the late-Precambrian Cadomian orogenic belt of northwest France. Here, thickening of a volcano-sedimentary basin during transpression led to upper amphibolite facies water-rich volatile phase-present anatexis and development of migmatites (St. Malo migmatite belt) and granite melt production (Mancellian granites), with some evidence of dehydration melting in granites emplaced at the highest structural level. Transcurrent shear was regionally focussed within this zone of softened crust. Granite magma was transported to higher crustal levels in megadike bodies (c. 0.5-1 km width), themselves constructed from multiple sheets, located within major ductile shear zones. Magma entered the shear zones at points of local extension and was expelled upwards in zones of compression, a mechanism referred to as strike-slip dilatancy pumping. The shear zones are inferred to have been linked to major brittle fault zones in the upper crust, and extensional jogs within such systems have provided sites for assembly of plutons (tens of km across) from magmas arriving from below. Granite is generated also by decompression melting during uplift of orogenic belts and commonly has an important role to play in exhumation of the high-grade cores of thickened orogens. The Variscan metamorphic belt of western France provides an example of these interactions between tectonic processes and granite in a thickened orogen. Here, the Eo-Variscan to Variscan P - T - t - d evolution required fast uplift and exhumation of the metamorphic belt at c. 330-300 Ma as a coherent block without internal penetrative strain. Coeval granite was produced by decompression melting during the fast uplift. The granite facilitated exhumation by accommodating strain along a major intracontinental transcurrent shear zone and along thrusts, reactivated to allow tectonic unroofing by ductile normal faulting.

1. Introduction To understand the origin and evolution of granite we must understand the process of initial magma generation, the mechanisms of magma escape (segregation, aggregation and ascent), the processes of modification of magma by differentiation a n d / o r contamination during ascent a n d / or emplacement, and the mechanisms of emplacement. Although melting of continental crust is essential to the generation of many granites, our understanding of processes involved between the beginning of anatexis in the middle and lower crust and the emplacement of granite plutons at structurally higher levels in the crust remains poor. Important questions that remain to be an-

swered satisfactorily include: What is the relationship between mid-to-lower crustal regional migmatite belts and mid-to-upper crustal granites? What is the nature of the heat source and is it sufficient enough for long enough? Under what conditions can granite magma escape its source and what is the transport mechanism for granite magma ascent through the crust, diapiric movement or fracture-control? What is the differentiation mechanism within granite magma, restite unmixing or crystal fractionation or some combination of both processes? How important are open system processes, such as magma mixing and crustal assimilation? Finally, how are granite plutons emplaced or "constructed", is there a "space problem"?

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There have been two main approaches in the investigation of granites, in each of which the full range of petrological techniques, field studies, petrography, geochemistry, experiments and modelling has been applied. One method of attack has been through metamorphic studies, since, if metamorphic grade increases into the upper amphibolite and granulite facies, anatexis is the inevitable consequence in many crustal rock types, and granite is a common neighbor of such highgrade metamorphic rocks. This is demonstrated by experimental work on compositions such as pelite (e.g. Thompson, 1982; LeBreton and Thompson, 1988; Vielzeuf and Holloway, 1988; Patifio Douce and Johnston, 1991), greywacke (e.g. Conrad et al., 1988), tonalite (e.g. Rutter and Wyllie, 1988), amphibolite (e.g. Ellis and Thompson, 1986; Beard and Lofgren, 1989; Hacker, 1990; Rushmer, 1991; Wolf and Wyllie, 1993) and interlayered pelite-tonalite (Skjerlie et al., 1993), and supported by myriad field-based examples (e.g. Tracy, 1978; Jones and Brown, 1990). Thus, the metamorphic perspective on

crustal melting has been developed from intensive study of high-grade gneisses and migmatites. At outcrop scale commonly these rocks appear to represent relatively low-melt-fraction systems, even though the leucosome fraction, representing frozen melt, may vary substantially from rock type to rock type within the outcrop and bearing in mind that some liquid may have escaped from the system. Indeed, Ellis and Obata (1992) have argued that leucosome patches in migmatites at Cooma, Australia, do not represent frozen melt segregation but rather liquidus minerals (cumulates) which precipitated from a locally-derived hydrous melt that subsequently migrated out of the system. Of the many views that derive from this metamorphic perspective, one is that important geochemical differences between granites may be related to the presence of different amounts of residual minerals in the magmas. This is especially true for major elements such as Fe, Mg and Ti because of the low solubility of their host mineral phase(s) in granitic melts (Clemens and Wall, 1981; Naney, 1983; Puziwicz and Jo-

Michael Brown has been studying high-grade metamorphic rocks, migmatites and granites for the last 25 years. This has included studies of migmatites and granites in north-west France, in the Cadomian Belt of the Northern Armorican Massif and in the Variscan Belt of the Southern Armorican Massif, work in southern West Greenland on the Q6rqut granite complex, and research on migmatitic granulite facies gneisses of various ages from Timor, India, Canada and Russia. Recently he has been concerned with

the relationship between magmatism and tectonics during the Mesozoic evolution of the Andean Plate Boundary Zone in North Chile, and he has just started working on the low-P metamorphic belts of Japan. Currently, he is Professor of Geology and Chair of the Department of Geology at the University of Maryland at College Park, having moved to the USA from Britain early in 1990. In addition to writing research papers, Prof. Brown is an editor of the Journal of Metamorphic Geology and edited, jointly with John Ashworth, the book High-grade Metamorphism and Crustal Anatexis. He is the Chair of the Organizing Committee for the Third Hutton Symposium on the Origin of Granite and Related Rocks to be held at the University of Maryland at College Park in August 1995 and he was responsible for the establishment of a Commission on Granites within the International Association of Volcanology and Chemistry of the Earth's Interior.

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hannes, 1988), and trace element concentrations (e.g. Rapp and Watson, 1986; Sawka, 1988; Harris and Inger, 1992) and isotopic abundances (e.g. Hogan and Sinha, 1991) that are controlled largely by accessory phases. In partially molten crust, melt that cannot escape at all will remain in the source and may freeze to form a migmatite, whether in place as a stromatic structure (Fig. 1) or displaced as a diktyonitic structure (Fig. 2). Sawyer (1991) presents evidence that such melts commonly have disequilibrium chemical compositions. This feature suggested to Sawyer (1991) that in some migmatites melt-segregation rates are greater than rates of chemical equilibration. High mobility and efficient separation are supported by these data. In the particular case studied by Sawyer,

melt collected in dilational structures and is thought to be driven by pressure gradients developed during syn-anatexis non-coaxial deformation of the anisotropic protolith, i.e. segregation is deformation enhanced. Melt that can aggregate may form a magma that can ascend some distance to be emplaced as a higher-level granite. Chemical evidence presented by Sawyer (1991) indicates that such melts represent the aggregation of melt batches chemically equilibrated with residual crystals. This feature led Sawyer (1991) to infer that rates of chemical equilibration must exceed those of melt-segregation in circumstances where plutonic volumes of granite magma are accumulated and, thus, chemical equilibration is achieved before segregation occurs. He attributed differences in rates of chemical equilibration and

Fig. 1. Stromatic migmatite from South Brittany, France. Note discontinuous (lensoid) leucosome with associated biotite-rich selvages or melanosome set within schistose paleosome. Leucosome in lower part of field of view shows evidence of original melt migration along the foliation from bulk of rock to accumulate in the overthick part of the leucosome. A buoyancy-driven compaction process may not be appropriate in such migmatites, but some kind of deformation-enhanced filter-pressing process may have operated. Diameter of coin c. 25 mm.

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Fig. 2. Fine-scale stromatic migmatite cut by locally discordant homogeneous diatexite to give a diktyonitic structure. In this case, since melt does not appear to have migrated from the stromatic migmatite leucosomes into the discordant veins, the stromatic leucosomes are interpreted to be below the appropriate solidus at the time of invasion by the homogeneous diatexite. St. Malo Migmatite Belt, Brittany, France. Diameter of coin c. 25 mm.

melt-segregation between segregated migmatites and plutonic volumes of granite to a higher temperature of melting in the generation of larger volumes of melt required in the latter case. The alternative line of investigation has been that of the igneous petrologist. Fractional crystallization and crustal anatexis are two end-member igneous processes that may produce granitic magmas. In this review I am concerned with granites derived from essentially crustal sources. Excluded from this review are granitic rocks of arc environments in which sub-crustal sources and mixing between mantle-derived m a g m a and crustally-derived m a g m a may be more important. Plutonic

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bodies of crustally-derived granite represent high-melt-fraction systems (once migrating as an aggregated body of m a g m a the system is a highmelt fraction one even though the initial melts may have been low-melt fraction segregates during the anatectic process, i.e. representing low values of F in the general equations for melting). In crustally-derived granites, the primary controls on melt composition are the intensive parameters and source compositions. Subsequent modification of melt compositions in high melt fraction systems during ascent and emplacement can occur either by fractional crystallization or by restite unmixing or by some combination of both processes. Thus, the igneous petrologist must address the inverse problem and, using petrographic and geochemical data, coupled with information from experiments and modelling, infer source characteristics and P - T - X n u i d conditions (e.g. Chappell and Stephens, 1988; White and Chappell, 1988). Unfortunately, this task is not straightforward and models must be constrained with a variety of data to minimize error and ambiguity. To illustrate the kinds of complexity that may occur, I outline the main factors that may influence the isotopic composition of a granite magma. In the simplest case a source rock undergoes closed-system melting and generates a m a g m a which ascends without interaction with other rocks and thus inherits the isotopic composition of its source. However, it is more probable that ascent of such a m a g m a will be accompanied by assimilation of country rock and fractional crystallization so that the isotopic composition of the resulting pluton will be controlled both by the source and by the assimilant. Of course, the source will not be homogeneous and the experimental results of Skjerlie et al. (1993) suggest that many crustaUy-derived granites most likely contain contributions from two or more different source rocks, which will be reflected in their isotopic and geochemical compositions. The scenario may be more complex yet, with the source undergoing open-system melting as a result of either m a g m a mixing or fluid infiltration. Further, pre-anatectic subsolidus modification of the source as a result of hydrothermal activity accompanying prograde m e t a m o r p h i s m could have oc-

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curred. The isotopic signature of a pluton is some combination of the isotopic composition of the source, a n d / o r magma mixing a n d / o r metasomatic melting in the source, a n d / o r AFC processes during ascent. In spite of this complexity, one outcome of the approach from geochemistry is the idea that the different geochemical characteristics of granite suites are related to different source characteristics (Chappell and White, 1974; Chappell, 1984; Chappell et al., 1988). For example, different source regions will have different accessory mineral assemblages with unique chemical and isotopic signatures. This is an intrinsic property of the source. The response of accessory minerals during anatexis can play an important role in determining geochemical and isotopic characteristics of resulting granites (e.g. Rapp and Watson, 1986; Hogan and Sinha, 1991). Let me illustrate this point with respect to Pb isotope systematics. Granites derived from the same source region can have different initial Pb isotopic compositions. However, the initial Pb isotopic composition of these granites will define a coherent field on 2°7pb/2°4pb-2°6pb/2°4pb diagrams, the slope of which represents the weighted average U / P b age of the accessory mineral assemblage of the source (Hogan and Sinha, 1991). In this way granites derived from the same source region (a suite) can be recognized. The contrasting outcomes from the different approaches both represent valuable contributions to our understanding of granites. Furthermore, since the views are complementary rather than contradictory they can be reconciled into one or more coherent model(s) for genesis of crustally-derived granites. The development of one particular model is a main objective of this paper. Most migmatites represent rocks in which the melt has segregated on a local, centimeter-scale, such as in Fig. 1, but has not been extracted from the whole system (e.g. Brown, 1973). If the melt, or a substantial proportion of it, escaped from

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the system, then a depleted, residual granulite would remain. Individual granite segregations in migmatites are some five orders of magnitude smaller in size than typical plutonic granites, although the latter are increasingly recognized as multiple complexes made up of small batches of melt (Brown et al., 1981; Redden et al., 1985; Duke et al., 1988; Lanord and Le Fort, 1988), such as the Q6rqut granite complex of Southwestern Greenland illustrated in Fig. 3. Areas in which this small-scale melting characteristic of migmatites can be related to specific large-scale plutonic granites are rare (for an example, see Brown and D'Lemos, 1991; D'Lemos et al., 1992). Most plutonic granites are isolated from their source rocks, with the residual material, from which the granite magma has been extracted, remaining in the lower crust as medium- to high-P granulite (e.g. Clifford et al., 1981; Brown and Earle, 1983; Clemens, 1990). This has led to contrasting views about migmatites. On the one hand, a view exists that the study of migmatites is unlikely to provide much information about the formation of granites (White and Chappell, 1990; Clemens and Mawer, 1992). Indeed, migmatites in New Hampshire, USA, have been interpreted to be the result of partial melting driven by infiltration of hot magmatic water derived from magmas in transit from deeper sources through the migmatite terrane to emplacement at shallow crustal levels (Allen and Chamberlain, 1992). On the other hand, the view is held that large areas of crustally-derived granite in high-grade metamorphic terranes represent the end product of the ultrametamorphic process (Read, 1957; Mehnert, 1968; Brown, 1973; White and Chappell, 1977; Brown, 1983; Tagiri et al., 1989; Jones and Brown, 1990; Brown and D'Lemos, 1991; Osanai et al., 1991, 1992; Percival, 1991). Current dogma argues that melt volumes below the critical melt fraction (Arzi, 1978; van der Molen and Paterson, 1979; Wickham,

Fig. 3. a. View to show a c. 1 km section through the intermediate zone of the Q6rqut granite complex, southwestern Greenland, with the upper zone in view in the mountain peaks behind, b. Detail of steep face close to the top of the intermediate zone of the Q6rqut granite complex to show the sheeted internal structure of the pluton; pale shades of gray represent biotite granites and dark shades of gray are country-rock enclaves and rafts.

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1987b) can not escape, whereas larger amounts of melt above the critical melt fraction can aggregate and migrate potentially to form plutonic granites. It is this dogma that has given rise to the perception that migmatites represent failed granites. However, since this view is inconsistent with the geochemical evidence, for example from Himalayan leucogranites (e.g. Inger and Harris, 1993), experimental evidence (e.g. Wolf and Wyllie, 1991) and field evidence from magmatic breccias (e.g. BEdard, 1993), clearly something other than critical melt fraction controls what happens to the melt. The question, then, is not whether there is a connection between migmatites and granites, but rather what factors determine whether or not low-melt fractions can segregate and escape to form plutonic bodies of granite. The ability of granite liquid to segregate depends upon a number of factors that include how the liquid is distributed within the matrix of a partially molten rock and the viscosity of that liquid. The geometrical structure of partially molten rock is crucial to understanding its behavior. An important question is whether small melt volumes will disperse along grain edges or form isolated packets. Melting begins at polymineralic grain corners but the relative interfacial energies, between melt and grains and between grains, control the interconnectivity of the melt at low melt fractions (Beere, 1975; Watson, 1982), at least in ideal isotropic materials if not, apparently, in experiments that involve melting solid rock (Mehnert et al., 1973; Dell'Angelo and Tullis, 1988; Wolf and Wyllie, 1991). In principle, melt can flow through interconnected pores and escape, even if the proportion of partial melt is very small, although in practice the viscosity of the melt may prevent this from occurring. In this case, non-hydrostatic stress still may lead to deformation-assisted melt segregation. Thus low-melt fractions are able to escape their sources under suitable circumstances. There is much insight to be gained from the study of amphibolite and granulite facies migmatites concerning processes involved in crustal melting, and the segregation, aggregation and migration of melt. Further, study of regions where migmatites can be related to plutonic granites

may provide insights into middle and lower crustal processes which may be difficult to gain from either study of granites emplaced into the upper crust or study of lower crustal granulites (e.g. D'Lemos et al., 1992). In particular, vital links frequently missing are good information on the state of the magma during ascent and the ascent mechanism. The ascent mechanism is often inferred from data which reflect only the local emplacement mechanism. Such data may not relate to the regional magma ascent mechanism, and may even lead to misinterpretation. As is the case with many problems in geology, evidence concerning the generation, segregation, ascent and emplacement of granites is fragmentary. Some of the hotly debated questions concerning granites often are sustained by comparisons between granites of different ages and origins in different tectonic settings. The current preference for movement of magma through the crust in shear zones and magma-generated fissures does not necessarily preclude diapiric ascent and emplacement during earlier periods of earth history, in particular during the Archean. Another example concerns the relative proportions of crust and mantle involvement in granites. Certainly the proportions vary as a function of tectonic setting from extensional (e.g. Basin and Range Province) to contractional (e.g. Himalayas), and within a particular tectonic setting such as a convergent plate margin, from retreating subduction boundaries in transtension (e.g. Mesozoic Andes) to advancing subduction boundaries in transpression (e.g. Cainozoic Andes). Therefore, arguments about mantle involvement based upon proportion of cognate enclaves (e.g. in calc-alkaline complexes) versus neodymium isotopic compositions (e.g. in bimodal magma provinces characteristic of continental extension) may be comparing apples with oranges. Similar spurious arguments are generated concerning the heat source for granite magmatism. Assuming a substantial crustal component derived by crustal anatexis, then there are only three heat sources that can be enhanced individually or in combination to create the necessary thermal perturbation for the crust to melt: heat generated internally by radioactive decay; heat transfer from mantle to

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crust by advection; and, heat transfer from mantle to crust by conduction. To avoid some of the problems involved in comparisons between different tectonic settings, this review is limited to intracrustally-derived granites characteristic of thickened orogens, that is principally those orogens characterized by clockwise P - T - t paths. This review is divided into five sections as follows: generation, segregation and aggregation, ascent and emplacement, development of a general model applicable to orogenic granites of predominantly crustal derivation and consideration of the wider role of granites in orogenic processes.

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2. Generation 2.

Crustally-derived granites may be generated in a variety of tectonic settings, providing that the geotherm can be enhanced or an intracrustal thermal perturbation can be created in the particular tectonic setting and assuming that fertile crustal lithologies are available to melt. As pointed out by Vielzeuf et al. (1990), orogenic belts may be granitoid-rich ("fertile") or granitoid-poor ("sterile"), a feature which may be interpreted in terms of contrasting fertility of the major rock types in the orogens, which also may reflect whether the crust has been bled previously of its granitic component. Thus, orogens that rework rigid, cratonized, differentiated and "sterile" crust will be granitoid-poor (e.g. the Alpides of Europe), while those involving large amounts of clastic sediments and low-grade metamorphic rocks potentially would produce large quantities of granitoid magmas, because of the combined effects of their fertility, radiogenic heat production and low thermal conductivity (e.g. the Variscides of Europe).

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Fig. 4. P r e s s u r e - t e m p e r a t u r e diagram (after Thompson, 1990; and primary references therein) to show: (1) Some of the initial melting reactions in metapelites. (2) Initial melting reactions for amphibolites, including the H 2 0 - s a t u r a t e d solidus for olivine tholeiite and an approximate H 2 0 - s a t u r a t e d solidus for quartz tholeiite based upon albite + quartz + H 20. (3) P - T - t paths (CW; 50 km and 70 km depth after thickening) for thickening of continental crust from 35 km to 70 km, followed by erosional thinning in 100 Ma, after a post-thickening isobaric metamorphism of 20 Ma, for an initially " h o t " geotherm that certainly reaches granulite facies conditions. (4) P-T-t path (CCW) for heating followed by crustal thickening and near isobaric cooling. C W = clockwise path in P - T space; CCW = counterclockwise path in P - T space; GWS = granite wet solidus; BWS = basalt wet solidus; /AT = island arc tholeiite; BA = alkali basalt. Mineral abbreviations as follows: Ab = albite; Als = aluminum silicate (andalusite, sillimanite, or kyanite as appropriate); Amp = amphibole; Bt (ss) = Biotite solid solution; Crd = cordierite; Jd = jadeite; Ky = kyanite; L = liquid; Ms(ss)= muscovite solid solution; Or = K-feldspar; Pl = plagioclase; Qtz = quartz; Res = residue and Sil= sillimanite. Reproduced from Brown (1993), with permission of the Geological Society, London, UK.

2.1. Tectonic models

For our purposes, orogenesis may be classified into two types (Brown, 1993). One type of orogenesis produces metamorphic belts that are characterized by evolutionary paths in pressure-temperature space which are clockwise (Fig. 4; C W

paths). Orogenic belts of this type are generated by basin inversion or crustal thickening followed by erosional exhumation a n d / o r extensional thinning a n d / o r lithospheric delamination and orogenic collapse (Oxburgh and Turcotte, 1974; Bird et al., 1975; Houseman et al., 1981; see also

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reviews by Thompson, 1981; Thompson and Ridley, 1987). Such an evolutionary path will lead to decompression dehydration-melting of common crustal rock-types (Thompson, 1982, 1990; Brown, 1983; Brown and Earle, 1983; Jones and Brown, 1990). The other type of orogenesis produces metamorphic belts that are characterized by evolutionary paths in pressure-temperature space which have a counterclockwise direction (Fig. 4; C C W path). In such an evolution, heating precedes crustal thickening or the two may go handin-hand. Models to generate such counterclockwise paths include intraplating of mantle-derived magmas (Bohlen, 1987; Bohlen and Mezger, 1989; Bohlen, 1991) and crustal thickening with concomitant mantle lithosphere thinning (Loosveld and Etheridge, 1990; Sandiford and Powell, 1991). Once again, such a process will generate dehydration melting (Thompson, 1990), and Collins and Vernon (1991) have argued that the increased fluid pressure and thermal softening that result will lead to accelerated, melt-enhanced deformation and thickening of the orogenic belt during cooling.

Convergent plate margins

Woodcock (1986) showed that a majority (59%) of contemporary convergent plate boundaries have a relative velocity vector that is markedly oblique (> 22 °) to the boundary normal; further, a significant proportion (14%) have vectors that are nearly (_+ 22°) parallel to the boundary. Thus, it is reasonable to expect that oblique convergence was important in the past. In such circumstances retreating subduction boundaries will result in regional deformation of the overriding plate by horizontal extension or transtension in contrast to advancing subduction boundaries that will result in regional deformation of the overriding plate by horizontal shortening or transpression. Accommodation of the oblique motion usually involves strike-slip faulting, but strike-slip faulting in general is less commonly recognized in ancient orogenic belts than its abundance in present plate-boundary orogens requires. This reflects both poor understanding of strike-slip kinematics and deeper prejudices about the way

in which orogenic belts form, and possibly absence of kinematic analysis in some orogenic belts. Until recently, models of convergent plate boundaries were fundamentally two-dimensional and the relation between plate motion and orogenesis--magmatism, deformation and metamorp h i s m - w a s unclear. Regardless of the relative velocity vector, boundary-parallel contractional belts were thought to be formed by displacements normal to the plate-margin. Plate margin parallel strike-slip fault systems were recognized in some orogenic belts, but displacements were thought to be unrelated to thrust systems. More recently, it has been demonstrated that displacements parallel to the plate margins may be large and may occur simultaneously with plate-margin-normal contraction in fold and thrust belts. Such belts are called transpressional. A model for the strain and structural development in transpressional belts has been derived by Sanderson and Marchini (1984). Some thermal and mechanical consequences of rapid uplift during transpression have been investigated by Koons (1987). However, the metamorphic and magmatic evolution of transpressional orogens is not well understood; in this review I will consider the relationship between transpression and the development of anatectic migmatites and intracrustal granites and will develop a general model from one particular example. 2.2. Heat

What is the heat source for granite genesis and is there a problem? The presence of granites indicates that if there is a problem it is in our failure to understand how heat of sufficient magnitude is maintained for sufficient time to generate the observed outcome! In order for significant melting of crustal rocks to occur, some kind of positive thermal perturbation is required, but it must have the thermal capacity to absorb the necessary heat of fusion as well as to achieve temperatures at which fusion can occur! Simplifying matters, the thermal perturbation may be generated by one of three mechanisms, which may not always operate independently. (1) Over-

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thickening of sedimentary basins during structural inversion (Treloar and Brown, 1990) or crustal thickening during collisional orogenesis (England and Thompson, 1984, 1986; De Yoreo et al., 1989) followed by erosional exhumation a n d / o r extensional thinning a n d / o r lithospheric delamination and orogenic collapse may lead to high-temperature metamorphism and crustal anatexis. Heat generation by internal radioactive decay during thermal relaxation and uplift consequent upon thickening can generate a significant thermal pulse with a decay period similar to the time-scale of an orogenic cycle (e.g. Chamberlain and Sonder, 1990). (2) An alternative view, which may be complementary in some circumstances, is that the heat required for crustal anatexis is a result of crustal growth through underplating a n d / o r intraplating of mantle-derived magma (Holland and Lambert, 1975; Wells, 1981; Bohlen, 1987; Bergantz, 1989; Bohlen and Mezger, 1989; Fountain et al., 1989; Clemens, 1990; Bohlen, 1991). (3) Finally, an enhanced mantle heat flux could provide the necessary thermal anomaly for crustal anatexis. Detachment of the lower (mantle) lithosphere to be replaced by hotter asthenosphere is an effective way of increasing the mantle heat flux into the lower crust (Bird, 1979; Loosveld and Etheridge, 1990; Kay and Mahlburg-Kay, 1991; Sandiford and Powell, 1991; Ellis, 1992). Other mechanisms sometimes suggested as heat sources, such as heat transfer by overthrusting of hot rocks onto colder rocks or flow of metamorphic fluid, are modifiers rather than the principal cause of the required thermal perturbation [see for example, Jaupart and Provost (1985) for an analysis of heat focussing as a consequence of overthrusting, Swapp and Hollister (1991) for an excellent example of a tectonically transported heat source, and Brady (1988), Chamberlain and Rumble (1988) and Thompson and Connolly (1992) for an analysis of the heat contribution by fluid flow]. Further, evolutionary processes in orogenic belts naturally lead to combinations of mechanisms. An example is illustrated by the Velay anatectic dome, Massif Central, France, where late-stage intrusion of hot mafic magmas within the crust promoted extensive melting and granite magma production

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(Montel et al., 1992); this thermal spike to high T was superimposed on a clockwise P - T evolution. 2.3. Crustal anatexis

Melting of common crustal lithologies may occur in the presence of a water-rich volatile phase at P and T (Tuttle and Bowen, 1958; Luth et al., 1964). Whether such melting will produce mobile granite magma will be determined by the amount of water-rich volatile phase available. Volatile phase-present crustal anatexis has been suggested by Brown (1979) for the St. Malo migmatite belt, northwest France, by Wickham (1987a) for the Trois Seigneurs Massif in the Pyrenees and by Montel et al. (1992) for the first stage of melting in the Velay anatectic dome, Massif Central, France. In all of these examples, melting of metasedimentary rocks occurs at relatively shallow (mid-crustal) levels. The volatile phase in such examples may be convected, chemically evolved meteoric water (Wickham and Oxburgh, 1985; Wickham and Taylor, 1985; Nesbitt and Muehlenbachs, 1989; Percival, 1989). More generally, volatile phase-absent partial melting of common crustal rock-types is indicated (Brown and Fyfe, 1972; Thompson, 1982). Models and experiments indicate that volatile phase-absent dehydration melting of common crustal rocks, at T of 750-900°C, will yield 10-50 vol.% granitic melt over a narrow temperature interval that corresponds to the breakdown of the major hydrous phase (see, for example, Clemens and Vielzeuf, 1987; De Yoreo, 1988; LeBreton and Thompson, 1988; Rutter and Wyllie, 1988; Vielzeuf and Holloway, 1988; Vielzeuf et al., 1990; Patifio Douce et al., 1990; Patifio Douce and Johnston, 1991; Rushmer, 1991a; Skjerlie et al., 1993; Wolf and Wyllie, 1993). The amount of melt will be proportional to the amount of hydrous minerals. However, the experimental results reported by Skjerlie et al. (1993) show that rocks which are poor melt-producers on their own can become much more fertile if they occur in contact with rocks that contain components that destabilize the hydrous phase(s) and facilitate dehydration melting. One implication of these results is that the continental crust may

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have an even greater potential for granitic melt production than previously thought. The rates of melt production, as a function of increasing T, are non-linear and periods of rapid melt production over narrow intervals of T are to be expected (Rutter and Wyllie, 1988; Vielzeuf and Holloway, 1988; Rushmer, 1991a). Given the steep, generally positive, slopes of volatile-phase absent dehydration melting curves in P - T space (Fig. 4) and the positive AV of reaction implied, then melting is accompanied by a positive volume change which will be on the order of 0-20% (Mawer et al., 1988; Clemens and Mawer, 1992). The consequences of this increase in volume for magma segregation, aggregation and ascent may be significant in some circumstances. The concentration of H 2 0 in the melt will be a function of P and T, the saturating mineral assemblage and the mole fraction of the hydroxy component in the hydrous minerals. CO2-promoted deep crustal melting has been supported by the experimental work of Peterson and Newton (1989). However, the role of carbon dioxide in the metamorphism and melting of the crust remains controversial in view of some implications of their experimental work, such as the requirement that CO 2 should be more soluble in the melts than H 2 0 at P and T in the face of considerable experimental results to the contrary. Furthermore, Clemens (1993) has been unable to replicate the results of Peterson and Newton. This lack of corroboration has cast doubt on the general idea of melt fluxing by CO 2. For a recent review of fluid-absent melting and the role of fluids in the lithosphere see Stevens and Clemens (1993). Volatile phase-absent dehydration melting that involves muscovite, muscovite and biotite, biotite, hornblende, hornblende and biotite, and hornblende and orthopyroxene has been reviewed recently by Thompson (1990) and Vielzeuf et al. (1990), so it will not be dealt with in detail herein. A P - T diagram to show selected melting reactions and P - T - t paths for rocks involved in clockwise orogenic belts (CW paths) and counterclockwise orogenic belts (CCW path) is given in Fig. 4 (after Thompson, 1990). The sequence of events outlined below will only be approximate

because the univariant lines shown in Fig. 4 are for simplified chemical systems. In nature, the real reactions are part of a multisystem, with either a mixed volatile phase present or no volatile phase present and with melts of variable composition, and so the exact position of the reaction curves will be modified. In pelitic metasediments, at pressures below about 15 kbar, first muscovite and then biotite undergo decomposition involving dehydration that leads to melting through reactions which involve feldspars + quartz + AlzSiO 5 (e.g. at b); at pressures above about 15 kbar, initial decomposition of phengitic muscovite and dehydration that leads to melting produces biotite (e.g. at k). Fig. 4 illustrates P - T - t paths generated by doubling a 35 kin-thick crust and subsequent exhumation (CW paths). The deeper crust (from 35 to 70 km) would show some dehydration melting of mica. Dehydration melting of amphibolite occurs over a wide temperature range according to the chemistry of the protolith, island arc tholeiite (IAT) versus alkali basalt (BA), reflected in its mineralogy (Rushmer, 1991a). More magmatically evolved compositions ( I A T , Fig. 4) undergo dehydration melting at lower temperatures than more primitive basalt (BA, Fig. 4). This suggests that some amphibolites might begin dehydration melting at temperatures only slightly higher than those required for melting of metapelites caused by dehydration of biotite (see, for example, Wolf and Wyllie, 1993). As an illustration, simplified to assume closed system behavior without segregation of melt, consider what happens along a CW P - T - t path that represents crust initially at 15 km thickened to 50 km (Fig. 4). Assuming that some free H20 is available, then, for clastic sediments of suitable bulk composition, volatile phase-present melting will occur at (a). However, the amount of free H 2 0 that remains in pores or along grain boundaries from earlier prograde dehydration reactions, or that might migrate from less fertile to more fertile layers, will be small and will allow only very limited melting at the H20-saturated solidus (Connolly and Thompson, 1989), due to the high solubility of water in granitic melts at deep crustal pressures (Clemens and Vielzeuf, 1987; Johannes and Holtz, 1991). Muscovite un-

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dergoes decomposition involving dehydration that leads to melting at (b), producing kyanite and K-feldspar through incongruent melting. At deep crustal pressures, but < 15 kbar, this is followed immediately by biotite decomposition involving dehydration leading to melting at (c), which consumes kyanite and produces garnet at this pressure, as shown in Fig. 5. The latter assemblage will show a transition from kyanite to sillimanite (e) and the reaction of garnet+ sillimanite + quartz to produce cordierite, between about 8 to 5 kbar (g), assuming that melt and residual crystals remain in contact, i.e. no significant segregation of melt has occurred. In a closed system, at (h) "rehydration-crystallization" will consume cordierite and crystallize some melt, as indicated by textures such as that in Fig. 6. The remaining melt will most likely undergo complete "rehydration-crystallization" at point (i), and will not exhibit any boiling at the H20-saturated solidus at

Fig. 5. Stromatic migmatite from South Brittany, France. Large garnet crystals c. 50 mm across are set in a minimal volume of leucosome to suggest some melt migration, at least out of the plane of the photograph. Garnet crystals are the product of decomposition of biotite involving dehydration that led to melting. Diameter of lens cap c. 55 mm.

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(j). However, melt segregated from the melanosome will undergo some "hydration crystallization", and significant amounts of "boiling" may occur when the P - T - t path recrosses the H20saturated solidus. Quartz-bearing amphibolite layers will exhibit amphibole decomposition involving dehydration that leads to melting, at (d), with the production of tonalite melt, as shown in Fig. 7 (at higher pressures, where garnet is stable, the melt will be trondhjemitic). At (f), in a closed system, "rehydration-crystallization" of the tonalite segregations within the amphibolite layers will occur. The complete and pervasive "rehydration-crystallization" expected from closedsystem behavior commonly is not observed either in migmatites or in granulites. In addition to loss of water that escapes from the system in migrating melt, this is at least partly due to coronitic overgrowth of reaction products on anhydrous reactants, for example this occurs during rapid adiabatic decompression which is a common element of the exhumation path in orogens characterized by clockwise P - T - t paths. An example of such a clockwise evolution has been presented by Jones and Brown (1990). In contrast, consider what happens along a CCW P - T - t path that represents some thickening during heating, followed by substantial thickening and approximately isobaric cooling at deeper crustal levels, again assuming closed system behavior without segregation of melt (Fig. 4). Muscovite-bearing sediments will undergo subsolidus decomposition involving dehydration at (p). If this fluid migrates away before the rock sequence reaches the H20-saturated solidus at (q), then no melting will occur. However, anatexis could occur in fertile compositions if H20 released by subsolidus dehydration did not migrate out of the system. Biotite decomposition involving dehydration that leads to melting will occur at (s), which consumes sillimanite and produces cordierite. During thickening, cordierite will react to produce garnet + sillimanite + quartz between about 5 to 8 kbar (u), assuming that melt and residual crystals remain in contact, i.e. no significant segregation of melt has occurred. Approximate isobaric cooling will result in "rehydrationcrystallization" at (w) in a closed system, fol-

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lowed by transition from sillimanite to kyanite at (x) and further "rehydration-crystallization" at (y) if free water is still available. Any remaining melt will exhibit "boiling" at the H20-saturated solidus at (z). Quartz-bearing amphibolite layers will exhibit amphibole decomposition involving dehydration that leads to melting, with the production of tonalite melt, at (t). At (v), "rehydration-crystallization" of tonalite segregations within the amphibolite layers will occur in a closed system. Once again, "rehydration-crystallization" is limited, which is partly due to armoring of anhydrous reactants during rapid isobaric cooling. The discussion above has been simplified from Thompson (1990; see also Ellis and Thompson, 1986; Rushmer, 1991a), who points out that in spite of many attempts to understand the complex details of beginning of melting reactions in common lower crustal rock types, many of the relevant reactions in the appropriate petrogenetic

grids are not yet well understood. Production of a large quantity of melt in the temperature range 750-900°C will have the effect of buffering metamorphic temperature in the melting zone (Fyfe et al., 1978; Rice and Ferry, 1982; Zen, 1988; Vielzeuf et al., 1990), and migration of melt will have the effect of dehydrating the lower crust (Fyfe, 1973a; Powell, 1983). Additionally, the presence of melt will lower the strength of the crust and promote failure by fracturing (Clemens and Mawer, 1992). In effect, the complete sequence outlined above for ideal closed system behavior in a model system likely will not occur in nature due to segregation, aggregation and migration of melt. The high temperature of dehydration melting in the crust and relatively low water content of the resultant melts in fact allows them to rise to shallow levels in the crust (Cann, 1970). As granite magma (liquid plus crystals) ascends adiabatically through the crust, assuming closed system behavior, the melt fraction, the

Fig. 6. Photomicrographto show"rehydration-crystallization"of cordierite rim around garnet in Athis granitecomplex,northwest France. Lengthof field of viewc. 14 mm.

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H20 activity and the effective viscosity increase and the temperature decreases slightly with decreasing pressure (Johannes and Holtz, 1991).

3. Segregation and aggregation Some high-P granulites have geochemical and mineralogical characteristics consistent with an origin as restitic residues of partial fusion and granite separation (e.g. Clifford et al., 1981; Brown and Earle, 1983; Clemens, 1989; Vielzeuf et al., 1990), particularly if a polycyclic evolution is acceptable (Rudnick and Presper, 1990; Rudnick, 1992). Moreover, textures and structures in some restitic granulites suggest that melt must have been evacuated without disruption of mesoscopic lithological layering (Brown and Earle, 1983; Clemens, 1989). Both of these observations indicate efficient segregation of melt. The important question is, how does melt segregate from the residue? In particular, how do small melt fractions segregate from their matrices? Also, if

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the bulk of the melt is segregated, will the part left behind with the residue, equivalent to the permeability threshold (Maal0e, 1982; Sawyer, 1993), be sufficient to explain the geochemistry of some granulites (Rudnick, 1992)? Are there differences between CW and CCW orogens that affect melt escape? Does the rapid rate of heating/ rate of melting in CCW orogens, likely driven by influx of basaltic magma, facilitate melt escape? Is deformation that produces low pressure structural sites, such as extensional fractures and shear zones, necessary to facilitate melt aggregation and escape in CW orogens that exhibit slower rates of heating and melt production? 3.1. The geometry and strength of partially molten rock

The distribution of a small volume of melt within a material comprised of isotropic grains under conditions of chemical and mechanical equilibrium is controlled by the relative surface tension (or interracial energy) of the interfaces of

Fig. 7. Discordant leucosome patch in amphibolite protolith contains large pyroxene crystals and results from amphibole decomposition involving dehydration that led to melting; note that the style of the resultant leucosome patch, located in an extensional fracture, contrasts with the common stromatic structure of pelitic migmatites; South Brittany, France. Diameter of lens cap c. 55 mm.

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melt against solid by comparison with the solidsolid interfaces. These relative interracial energies determine the geometrical relations of the melt pores with the solid grains, and are most simply expressed in terms of the dihedral angle (or wetting angle), which is defined as the angle subtending two solid grains adjacent to a melt pool. For a dihedral angle of 0° a melt, even at small melt fractions of < 1%, will wet all grain surfaces. If the dihedral angle is greater than 0 ° but less than 60°, the melt is dispersed along curved prismatic channels at the junctions between mineral grains and forms a continuously connected phase throughout the rock even if it is present in only vanishingly small amounts. In materials where the dihedral angle is greater than 60°, small melt volumes will form isolated pores at grain corners and edges such that the establishment of continuous connection for the whole melt fraction depends upon increasing the proportion of melt in the system until the pores can coalesce. As the dihedral angle increases above 60° so the amount of melt necessary to establish connectivity (continuous porosity) also is increased (Beere, 1975). Where several solid phases are present, differences in dihedral angle may result in melt distributions that deviate substantially from simple geometric models and connectivity of melt may be established between certain phases but not others. Furthermore, the interfacial energy theory was developed for structurally isotropic materials and requires modification for minerals with low symmetry and significant structural anisotropy, because grain boundary energy depends strongly on the specific crystallographic planes forming the interface. The whole issue of what controls connectivity has been questioned by the dehydration-melting experiments on solid amphibolite by Wolf and WyIlie (1991) in which melt volumes of only 2% generated at 875°C were interconnected. In this case the interconnectivity depended upon the geometries of the liquid pockets. The geometry of these liquid pockets appears to have been influenced more by hornblende-plagioclase locations and the hornblende crystallography than by dihedral angles. The results from these experiments intimate that the phase equilibrium constraints

and structural anisotropy of the minerals dominate over the surface energy effects (Wolf and Wyllie, 1991). Dynamic melting experiments by Mawer (1993) on rock analog materials led him to believe that wetting angles are unstable and approach 0°, with consequent high connectivity of melt. This implies that in zones of high strain even small volumes of melt will form an interconnected network of grain-edge channels that potentially can escape. Experimentally-determined static solid-solidmelt dihedral angles for alkali feldspar-alkali feldspar, alkali feldspar-quartz and quartzquartz solids against felsic melt are 44-59 ° (Jurewicz and Watson, 1985). Recent work reported by Laporte and Provost (1993), based on melting experiments in quartz-albite-potash feldspar-H20 and quartz-anorthite-H20 systems, indicates much lower wetting angles for quartz against hydrous silicic melts, and strong departure from isotropy for feldspar and hydrous silicic melts leading to significant deviation from the ideal model. For mafic minerals in granitic systems, experimentally-determined static solidsolid-melt dihedral angles for biotite-biotite against silicic melt (Laporte, 1988), biotite-biotite against hydrous felsic melt (Watson and Laporte, 1989) and amphibole-amphibole against basaltic andesite melt (Vincenzi et al., 1988) are 30°, 40° and 33 °, respectively. Overall, the range of static wetting angles determined for felsic melts is similar to that for basaltic melts (see tabulation in Harte et al., 1993). These data indicate that during crustal anatexis in appropriate source materials, however low the fraction of melt, there would be a continuous interconnection. This is extremely important because it means that if the viscosity of the melt is low enough then the melt can flow through the interconnected pores and escape, even if the proportion of partial melt is very small. During the experiments of Jurewicz and Watson (1985) some melt aggregated in pools surrounded by more than three grains, which they attributed to melt in excess of the amount that minimizes the interracial energy of the system. One final point on the geometry of melt distribution, melting experiments on solid-rock samples which generate 5-10% melt commonly do not

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develop homogeneous melt distributions (Mehnert et al., 1973; Dell'Angelo and Tullis, 1988; Wolf and Wyllie, 1991) and some regions in the samples have higher concentrations of melt. Knowledge of bulk-system physical properties, particularly effective viscosity, is clearly important to the segregation (and aggregation and ascent) of granite magma. Both the rheological critical melt percentage and the critical melt fraction are terms used to describe the point, expressed as volume of melt, at which partially melted rocks change from granular framework (crystal dominated, high viscosity) to dense suspension (melt dominated, lower viscosity) behavior (see discussion in Wickham, 1987b; refer to Arzi, 1978; and,

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van der Molen and Paterson, 1979). The concept of a critical melt fraction is based on the Einstein-Roscoe equation (Roscoe, 1952) for the viscosity of a Newtonian fluid that contains a concentration of spheres that are incompressible relative to the fluid. In this theoretical case, when the spheres are closely packed the system is effectively solid. However, some liquid (26 vol.% of the suspension) is trapped between the spheres. Thus, the viscosity is a maximum at a solid concentration of 74% or more. An increase in the range of particle sizes results in a decrease in the amount of liquid trapped between particles and hence the packing density increases at high solid fractions (Ward and Whitmore, 1950a). However,

Fig. 8. Metatexite from South Brittany, France. This example shows outcrop-scale textural and structural variation which could be interpreted to support the idea that a critical melt fraction of c. 30% is a valid concept in some anatectic settings, possible volatile phase-present melting and lower-temperature melting rather than volatile phase-absent dehydration melting and high-temperature melting. The left-hand part of the field of view shows leucosome with biotite selvages or melanosome set in schistose paleosome, the leucosome corresponds to c. 30% melt, which is generally below the critical melt fraction, and a stromatic structure was maintained. In the center of the field of view the leucosome corresponds to > 50% melt which is above the critical melt fraction, and in this case clearly resulted in disruption of the stromatic layering and the development of a schlieric migmatite. Diameter of coin c. 25 mm.

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this effect is balanced by the tendency for irregularly shaped particles in the fluid both to trap liquid on uneven surfaces by surface tension (Ward and Whitmore, 1950b), and by the tendency of particles to form aggregates which further trap liquid (Roscoe, 1952). Thus, there is a complex interplay between particle size distribution, rough vs. smooth surfaces, and tendency to aggregation. Further, identical rigid spheres are not good analogues for equant quartz, tabular feldspar or flakey mica and acicular amphibole. The interplay between these factors means that, in rocks, under static conditions, the critical melt fraction likely is within the range 30-50 vol.% melt; within this interval, a huge change in the effective viscosity of a partially molten granitic system may occur, which will have important implications regarding the susceptibility of such partially molten granitic systems to convection

(Wickham, 1987b). An occurrence interpreted as an example of melt vol.% exceeding locally the critical melt fraction is shown in Fig. 8. The stromatic migmatite in the left hand half of the field of view contains c. 30% leucosome on average, whereas in the center of the field of view the leucosome exceeds 50% and disaggregation of the rock structure has produced a schlieric migmatite. Once the melt vol.% exceeds the critical melt fraction more widely, an enclave-bearing schlieric migmatite is produced (Fig. 9), and eventually a schlieric granite or diatexite (Fig. 10). An alternative approach to the question of bulk-system rheological properties is to consider contiguity (German, 1985; Watson, 1987; Miller et al., 1988), which refers to the fraction of surface area of all solid grains that is shared with other solid grains. The contiguity depends mainly

Fig. 9. Inhomogeneous diatexite from the St. Malo migmatite belt, northwest France. Schlieric migmatite contains enclaves of stromatic migmatite, one of which, to the right of the coin, clearly shows the migmatitic banding deflected into asymmetric folds which presumably represent the edges of syn-anatexis shear zones. One interpretation is that the melt vol.% exceeded the critical melt fraction and resulted in disruption of the structure of the protolith. Diameter of coin c. 25 ram.

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Fig. 10. Schlieric migmatite or inhomogeneous diatexite from the St. Malo Migmatite Belt, northwest France. This structure is a consequence of melting substantially in excess of the critical melt fraction resulting in whole-scale migration of melt plus residual crystals as a mobile anatectic magma. Diameter of coin c. 25 mm.

upon melt fraction and wetting angle, and secondarily on grain size distribution. For static wetting angles of 44-59 ° contiguity drops from 1 to c. 0.45 with 10% melting, to c. 0.2 with 50% melting and to c. 0.1 at 90% melting. The continuous, self-supporting skeleton of solid grains breaks down at contiguity of 0.2-0.15. This means that the contiguity may be sufficiently high even at a melt fraction of c. 0.5 to sustain a rigid, continuous skeleton of interconnected grains. While at low strain rates contiguity likely is maintained by creep of the solid skeleton, at high strain rates it will not be, and the critical melt fraction leading to breakdown of the self-supporting skeleton is likely to be lower than 0.5 (Miller et al., 1988). Furthermore, the concept of critical melt fraction may be inappropriate at high strain rates because the strength of the rock (as measured by its effective viscosity) will decrease not because of a shift to a rheology dominated by melt fraction but because the solid skeleton will break down due to

the high differential stress and fracturing will be induced at low melt fractions. Rushmer (1991b, 1992) has investigated experimentally deformation of partially molten amphibolite. The results indicate that the melt fraction at which initial, dramatic weakening will occur probably is low (5-15%), much lower than the theoretical critical melt fraction (c. 30-40%). The initial weakening certainly is dependent on strain rate, but it is possible that in active tectonic regimes where natural strain rates are high (such as collisional orogens), low fraction, granite minim u m melts may be expelled during deformation. Although the foregoing relates to melting in the anatectic environment, the change within a crystallizing m a g m a from melt dominated (lower effective viscosity) to crystal dominated (high effective viscosity) controls the rheological regime and, therefore, the deformation mechanism and type of fabric that might be produced as a result of deformation. The formation of a framework of

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crystals is critical to the change in rheology, so that stresses are sustained by the crystals rather than by the melt. 3.2. The segregation mechanism The processes by which melt separates from associated solids, either during anatexis to form migmatites or granites, or during crystallization to liberate residual liquids, are not understood completely. There is increasing agreement that small volume melt fractions can be extracted from both residual solids during melting (e.g. Wolf and Wyllie, 1991; Clemens and Mawer, 1992; Sawyer, 1993) and cumulate solids during crystallization (Mahood and Cornejo, 1992; Petford, 1993). One is obliged to ask how long can the crust remain subject only to lithostatic stress. Some deviatoric stress should develop in the c. 1-10 m.y. period of a regional metamorphic e#ent. The almost universal structural complexity seen in regional migmatite terranes suggests that this is the case. Thus, it is likely that deformation will occur at some stage during the evolution of a partially melted rock and will contribute to the ability of that melt potentially to escape. The driving force for melt segregation likely is either chemical, related to surface tension, or physical and either related to buoyancy of the melt and driven by gravity or related to structural features and driven by non-hydrostatic stress and pressure gradients. Although, a large number of mechanisms have been suggested to explain the segregation of melt from residue, individually they reflect some combination of these driving forces. In the absence of externally applied stress, a silicate melt can separate from residual crystals at low melt fraction by a compaction process, in which the solid matrix settles to expel the intragranular liquid (Sleep, 1974; Robin, 1979; McKenzie, 1984, 1985; Richter and McKenzie, 1984; Ribe, 1987). In this process, a partially molten region is regarded as a deformable porous medium or "matrix" saturated with melt. Migration of melt relative to the matrix can be driven by either the buoyancy of the melt or non-hydrostatic pressure gradients associated with the deformation of the matrix. Partially molten material

will compact if the melt is interconnected, due to the lower surface energy of solid-solid contacts, and if the density of the melt differs from that of the matrix, conditions generally satisfied during crustal anatexis (average wetting angle for silicate minerals < 60° and lower density of granite melt than protolith or residuum). However, segregation of granitic magma from residual crystals at low melt fractions is strongly dependent on melt viscosity, and melt viscosity is dependent on H 2° content. Although Wickham (1987b) estimated that typical granite melt viscosities would limit melt segregation by compaction, the data of Wolf and Wyllie (1991), Clemens and Mawer (1992) and Petford (1993) suggest that granite melt viscosities may have been over-estimated, particularly for low volume melt fractions which likely were H20-saturated. Indeed, Wolf and Wyllie (1991) conclude that bodies of H20-saturated granite liquid might be generated and segregated from a source of amphibolite experiencing dehydration melting at 875°C on a time scale of c. 1 m.y. Natural observations indicate that melt segregation commonly is syntectonic which implies that an additional driving force to that provided by compaction results from differential stress during regional deformation. The geochemical aspects of permeability-controlled partial melting, termed critical melting, have been considered by Maal0e (1982) and applied to migmatite leucosome segregation mechanisms by Sawyer (1993). A novel diffusion and gravity-driven compaction mechanism has been proposed by Miller et al. (1988). They suggest that material transfer through the liquid may promote growth of larger grains at the expense of smaller ones, with the result that the larger grains ultimately may detach and drop to the bottom of the melt pocket in which they have been growing. The geological efficacy of such a gravity-driven compaction process is uncertain, but if applicable it has the merit of aggregating melt into pockets. The experimental work of Wolf and Wyllie (1991) indicates that such enlarged melt pockets would be interconnected; this would increase the permeability of the system and, therefore, enhance the prospects for segregation of the liquid from the rock matrix. In a closed system, assuming volatile phase-

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present melting, a volume reduction will occur on crossing the granite solidus because the residue plus melt occupies a smaller volume than the original solid protolith and the associated volatile phase, the reason for negative slopes of the solidii. In contrast, during dehydration melting a volume increase will occur, reflected in positive slopes for such reaction curves. Clearly, whether there is a volume reduction or a volume increase will influence the segregation process, unless the volume change can be accommodated by distributed ductile deformation. The rates of melt production by volatile phase-absent partial melting with increasing T are highly nonlinear, and periods of rapid melt production over narrow intervals of T are to be expected, resulting in high pore melt pressures, lowering effective normal stress and promoting hydraulic fracture as a segregation mechanism. An explanation of how segregation operates under nonhydrostatic stress conditions has been given by Dell'Angelo and Tullis (1988), as follows. In many low-melt-fraction rocks, the amount of melt produced will have

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been in excess of that required for a minimum surface energy texture (Jurewicz and Watson, 1985). In such cases, excess melt will migrate to a lower pressure collection site. If melt is confined so that it can not migrate freely, or if the imposed strain rate is higher than the rate at which melt can flow out of the system, then the fluid pressure will increase and the resultant high pore melt pressure should promote brittle deformation and melt migration through hydraulic fractures (Dell'Angelo and Tullis, 1988; Rushmer, 1991b, 1992; Clemens and Mawer, 1992), and an example is provided by Allibone and Norris (1992). In many migmatites stiffer layers in an anisotropic protolith allow dilatant sites, such as boudin necks, to develop and melt migrates to these sites (c.f. melt-induced fluid-pumping of Percival, 1989). Examples of leucosome migration to dilatant sites, such as boudin necks, are shown in Figs. 7, 11 and 12. That the melt appears to have migrated parallel to lithological layering and tectonic foliation is no surprise given the experimental results of Laporte (1988) that indicate for a

Fig. 11. Migmatite from South Brittany, France. Leucosome is preferentially located in boudin neck, interpreted to reflect melt migration parallel to the tectonic foliation to a low pressure site (dilatancy-pumping). Diameter of coin c. 25 mm.

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Fig. 12. Boudins on several scales within layered amphibolite, Tolstik Peninsula, Karelia, Russia. Anatectic melt represented by leucosome has migrated preferentially parallel to the tectonic foliation to the boudin necks, at each of the scales, possibly by a dilatancy-pumpingmechanism. Diameter of lens cap c. 55 mm.

biotite-rich, partially molten source with a strong preferred orientation, the degree of connectivity of the melt is higher in sections parallel to the foliation than in sections normal to the foliation. For geological strain rates at melt fractions below the critical volume, extraction of excess melt would be a gradual process, and the r e s t i t e - while melt was still present--would deform by dislocation a n d / o r diffusion creep (Dell'Angelo and Tullis, 1988). Many migmatites are characterized by stromatic structure composed of leucosome-melanosome-mesosome layers on a regular scale. This suggests that some common mechanism is re-

sponsible for migration of granite melt to form leucosome layers. It is unlikely in most of these examples that the mechanism reflects upward flow channels, although vertical interconnected leucosome-bearing crenulation zones have been described by Hand and Dirks (1992). Indeed, the migmatitic layering commonly occurs parallel to both the lithological layering and the tectonic foliation, commonly interpreted as a compressionally-induced feature, although in some circumstances the migmatitic layering may be mimetic. It is likely that such a stromatic migmatite structure is a consequence of smallscale melt segregation within crustal rocks undergoing large-scale deformation as a consequence of gradients in mean effective stress between rheologically contrasting layers of rock. Miller and McLellan (1986) have suggested that if anatexis is accompanied by layer-parallel shearing then mica-poor leucosomes should be more competent and more permeable than mica-rich melanosomes. Further, they suggest that at low to moderate melt fractions mechanical equilibrium will be reached most rapidly through porousmedia flow of melt from the less competent melanosome to the more competent leucosome. This mechanism is sometimes referred to as filterpressing. Pre-existing compositional and textural heterogeneities may be amplified by this process, and Johannes (1988) has suggested that inherited compositional differences may control amounts of water between layers and therefore amounts of melt. Stevenson (1989) argued that melt migrates along the direction parallel to the axis of minimum compressive stress and accumulates in melt-rich lenses or layers. Such a mechanism is necessary to explain the preservation of primary compositional layering in stromatic migmatites. Further, he argued that veins eventually may form an interconnected drainage network to allow rapid vertical flushing of melt. In this case, a depleted granulite with preserved primary compositional layering may be the end product. Deformation-enhanced segregation has been invoked by Robin (1979), Paquet and Francois (1980), Paquet et al. (1981), Barr (1985), McLellan (1988, 1989), Sawyer (1991, 1993) and D'Lemos et al. (1992) among others. During exten-

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sional fracturing local pressure gradients promote melt flow towards the fracture, providing a possible segregation mechanism (Shaw, 1980; Sleep, 1988). Dilatancy-driven flow and melt-pumping, in particular in strike-slip systems, has been suggested as a segregation and ascent mechanism (Davies, 1982; D ' L e m o s et al., 1992). Dilatancydriven and deformation-enhanced segregation mechanisms, such as melt migration into shear zones, are attractive because they could operate in nearly any partially molten system. That deformation is effective in promoting segregation of granite melt in low-melt-fraction rocks has been demonstrated by McLellan (1988, 1989). In partially molten rock undergoing shearing dilatancy leads to an increased pore volume in the shear zone and development of a pore fluid pressure gradient to drive segregation. Examples are shown in Figs. 13-17, individual figure captions give further details. Once again, the melt appears to have migrated parallel to lithological layering and tectonic foliation to accumulate in the shear zone.

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Additionally, processes such as melt migration into shear zones represent an effective mechanism to collect melt into sizeable masses that may ascend along the shear zones to collect as plutons in extensional segments of upper crustal brittle fault systems (D'Lemos et al., 1992; Hutton and Reavy, 1992) or by another mechanism (for example, buoyancy-driven m a g m a fracture). The segregation of melt from its residuum during noncoaxial strain is an efficient process capable of yielding leucosomes with little restite contamination. When can melt become mobile? For compaction of structurally isotropic rock, as soon as the melt forms a continuously connected phase through the rock the threshold permeability (Maaloe, 1982) will be exceeded and if the melt viscosity is low enough the liquid will migrate (Wolf and Wyllie, 1991). For melts with higher viscosities, likely those undersaturated with respect to H 2 0 , the rock mass will become mobile once the critical melt fraction has been exceeded,

Fig. 13. Melt, now represented by leucosome, interpreted to have migrated along the tectonic foliation and into a sinistral shear zone within stromatic migmatites, Tolstik Peninsula, Karelia, Russia. One interpretation is that the shear zone is induced by the presence of the partial melt within the rock. Diameter of lens cap c. 55 mm.

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but t h e liquid is unlikely to s e p a r a t e from residual m a t e r i a l by c o m p a c t i o n in geologically reas o n a b l e time scales ( W o l f a n d Wyllie, 1991). N o n e t h e l e s s , m e l t fractions b e t w e e n the t h r e s h old p e r m e a b i l i t y (c. 0 - 5 % ) a n d the critical m e l t fraction (c. 3 0 - 5 0 % ) clearly can b e c o m e mobile. Sawyer (1993) has s u g g e s t e d t h a t d u r i n g noncoaxial d e f o r m a t i o n any v o l u m e of m e l t in excess of the t h r e s h o l d p e r m e a b i l i t y in a ductilely-deform i n g m a t r i x will be s q u e e z e d out providing it can m i g r a t e to n e a r b y l o w - p r e s s u r e sites, such as b o u d i n necks a n d s h e a r b a n d s d e v e l o p i n g d u r i n g melting. T h e c o n t i n u o u s s q u e e z i n g out from the matrix of m e l t v o l u m e in excess of the t h r e s h o l d p e r m e a b i l i t y m e a n s t h a t the site of m e l t i n g n e v e r

n e e d c o n t a i n m o r e t h a n a few p e r c e n t melt, well below t h e critical m e l t fraction, even t h o u g h the c u m u l a t i v e d e g r e e of p a r t i a l m e l t i n g at the e n d of the e x t r a c t i o n p r o c e s s m a y have b e e n g r e a t e r t h a n 3 0 - 5 0 % . In effect, this is a d e f o r m a t i o n - e n h a n c e d version o f the a c c u m u l a t i v e critical melting m o d e l p r o p o s e d by M a a l 0 e (1982).

4. Ascent and emplacement In o r d e r to m o d e l m a g m a t r a n s p o r t , the p r o p erties o f the melt, the p r o p e r t i e s of the i n c l u d e d p h a s e s a n d the e x t e r n a l c o n s t r a i n t s from the c o u n t r y rocks have to be c o n s i d e r e d . T h e dis-

Fig. 14. Stromatic migmatites, Tolstik Peninsula, Karelia, Russia, in which leucosome shows complex inter-relationships interpreted to represent melt migration along the tectonic foliation and melt accumulation within shear zones. That this interpretation is more likely than melt migrating in along the shear zone and moving out along the foliation of the gneiss is demonstrated at the top of the photograph. Here, lensoid leucosome has been slightly folded by the incipient shear zone but the shear displacement at this point is minimal, further down the line of the shear zone melt occurs within the zone as shear displacement increases and melt can migrate from the layers into the zone. The shear zones are inferred to have been induced in the weakened zone of partial melting. Diameter of lens cap c. 55 ram.

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tance that magma may ascend, before solidifying, depends on temperature, water content and restite content of the magma as well as the physical properties of the wall rocks. Hot, dry, restitepoor magmas potentially can move further, whereas less hot, wetter and restite-rich magmas may have more limited mobility (see, for example, Cann, 1970; Brown and Fyfe, 1972; Clemens, 1984; Johannes and Holtz, 1991). Finally, under what circumstances, if any, does diapiric ascent of whole source regions occur, as required by the restite model for granite genesis of Chappell and White, at least for the end-member case (see, for

Fig. 15. Leucosome, interpreted to be former melt, in shear zones parallel to axial surfaces of D 4 folds of stromatic migmatites, South Brittany, France. Melt does not appear to have migrated from the folded leucosomes into the shear zones, suggesting that the stromatic migmatites were below the solidus at the time of D 4 deformation. Diameter of coin c. 25 mm.

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example, Chappell and White, 1974; White and Chappell, 1977; White and Chappell, 1983; Chappell, 1984; Chappell et al., 1987)? Volatile phase-present anatexis at high melt fraction may lead to convective overturn and mixing of melt with residual crystals (Wickham, 1987a, b; Brown and D'Lemos, 1991), but can this occur in the lower crust as a result of dehydration melting? Mechanisms of ascent and emplacement include the following: diapirism, in particular the "hot Stokes model" (Grout, 1932, 1945; Ramberg, 1967, 1970; Fyfe, 1970, 1973b; Spera, 1980; Marsh, 1982; Bateman, 1984; Emerman and Turcotte, 1984; Daly and Raefsky, 1985; Cruden, 1988; Mahon et al., 1988; England, 1990; Weinberg, 1993), and ballooning (Ramsay, 1975, 1981, 1989; Holder, 1978, 1979, 1981; Brun and Pons, 1981; Brunet al., 1990); zone melting (Dickinson, 1958; Pfann, 1959, 1962; Ahern et al., 1979, 1982; Marsh, 1982); self-propagating fluid-filled fractures, in particular dikes (Shaw, 1980; Turcotte, 1982; Emerman et al., 1986; Sleep, 1988; Emerman and Marrett, 1990; Lister and Kerr, 1991; Clemens and Mawer, 1992; Petford et al., 1993), and sheet intrusions (Pollard, 1973) or sill complexes (Brown et al., 1981; Friend et al., 1985; Redden et al., 1985; Halwas and Simony, 1992); buoyancy-driven ascent along faults/shear zones (Strong and Hanmer, 1981; Davis, 1982; Hutton, 1982, 1988a; Castro, 1986); strike-slip dilatancypumping (D'Lemos et al., 1992); stoping (Daly, 1903, 1914, 1933; Marsh, 1982; Furlong and Myers, 1985); ring-dike intrusion/cauldron subsidence (Bailey et al., 1924; Anderson, 1936; Roberts, 1970; Pitcher, 1978, 1979) and domal uplift (Gilbert, 1877; Pollard and Johnson, 1973; Pollard and Muller, 1976; Corry, 1988). It is clear that magma may ascend buoyantly, but the means by which it actually moves and at what rate remain unclear, although recent work suggests magma ascent in dykes is fast (Clemens and Mawer, 1992; Petford et al., 1993). Questions that remain to be resolved include the relative importance of diapirism vs. fracture-controlled mechanisms for ascent (also discussed by Clemens and Mawer, 1992), and the role of deformation in both ascent and emplacement of granite plutons (also discussed by Hutton, 1988a, b; D'Lemos et

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al., 1992; Hutton and Reavy, 1992). Additionally, there is the question why rising magma should stop rather than continue its ascent to the surface of the earth (e.g. Pollard and Holzhausen, 1979; Weertman, 1980). How important is the depth of the level of neutral buoyancy or does the level of the brittle-ductile transition or some other horizontal discontinuity within the crust play a greater role in arresting magma ascent (see discussion in Clemens and Mawer, 1992)? The shape of granite plutons led to the familiar "space problem". It is clear that both forcible intrusion (ballooning), whereby wall-rock is pushed aside, and stoping, whereby spalled roof-rock a n d / o r wall-rock sinks through the magma, may be part of the final, local emplacement mechanism in any particular case. This issue has been considered most recently by Paterson and Fowler (1993a). They refer to the material transfer problems (MTP's) inherent in emplacement by mechanisms such as

ballooning fed by a dike, perhaps accommodated locally by ductile flow and rigid translation along faults, and the likely need for far field material to be transported towards the Earth's surface or towards the region of magma generation (Fig. 18). In certain circumstances, ring-dike intrusion and cauldron subsidence may be the high-level emplacement mechanism, and the possibility of midcrustal subsidence may be significant. However, what is becoming clear is the role that deformation may play in creating the space into which magma may intrude, for example the extensional segments of strike-slip systems (e.g. Davis, 1982; Hutton, 1988a, b; Brown and D'Lemos, 1991; D'Lemos et al., 1992; Hutton and Reavy, 1992; McCaffrey, 1992), although a note of caution has been sounded by Paterson and Fowler (1993b). Fyfe (1970) proposed a buoyancy-driven segregation and accumulation mechanism for small

Fig. 16. Leucosomewithin amphibolite protolith, Tolstik Peninsula, Karelia, Russia, interpreted to show preferential migration of original anatectic melt into extensional shear zones (top down-to-the-left).One interpretation is that shear zone was induced by the presence of anatectic melt in the rock. Diameter of lens cap c. 55 mm.

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melt volumes eventually to p o n d into a layer of granite m a g m a from which diapirs might then rise. Buoyancy-driven m a g m a segregation mechanisms and related diapirism require a large ratio of melt to restite and may o p e r a t e only within zones of partially molten rock that are capable of ductile flow. Crustally-derived m a g m a s most likely to segregate and move in this fashion are the products of water-rich volatile phase-present melting, dehydration melting leading to large per-

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cent melt volumes, and, possibly, the products of high-T lower crustal melting. If segregation of melt and residual crystals does not occur and diapiric ascent of the whole source region takes place, a s required by the e n d - m e m b e r restite model of granite genesis, then, if this is a common process t h r o u g h o u t E a r t h history, it is difficult to see how the crust would develop its characteristic differentiation into silicic u p p e r crust and more mafic lower crust (Vielzeuf and Hol-

Fig. 17. Metatexite from the St. Malo migmatite belt, northwest France. Center and right-hand part of photograph shows shear zone, dominated by schlieric migmatite and shear bounded enclaves of stromatic migmatite, which cuts across stromatic migmatite from which melt appears to have been substantially drained. To the left of the coin a small dextral shear zone illustrates the same relationships at a smaller scale. Diameter of coin c. 25 ram.

I l0

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Fig. 18. Near- and far-field material transfer problems encountered during ballooning [from Paterson and Fowler (1993a)]. The diagram is schematic and shows a dike feeding a magma chamber that has expanded with time. The extent of wall-rock before pluton emplacement is outlined by the large bold box. Material outside this box was displaced by near-field material transfer processes such as ductile flow and rigid translation along faults, and shows the need for far-field material transfer process if sufficient ductile strain does not occur near the pluton. Black arrows emphasize the need for the far-field material to be transported towards the Earth's surface or towards the region of magma generation. Reprinted from Journal of Structural Geology, 15, Paterson, S.R. and Fowler, T.K., Re-examining pluton emplacement processes, pp. 191-206, copyright (1993), with kind permission from Pergamon Press Ltd., Headington Hill Hall, Oxford OX3 0BW, UK.

loway, 1988; Vielzeuf et al., 1990). Further, if retention of a small proportion of melt in the residue during melt separation is required to explain the geochemistry of residual granulites (Rudnick, 1992), then wholesale restite entrainment as a common process seems unlikely. Diapiric ascent of granite magma is attractive because it is a thermomechanically efficient mechanism that requires no external stresses on the body of magma other than gravity. In theory the process can operate in a range of tectonic scenarios, horizontal extension, horizontal contraction, or no finite horizontal strain. This mechanism was popularized in the 1970s by spectacular aerial photographs of c. 10 km diameter subcircular granite plutons in the Saudi Arabian crust (Fyfe, 1973b). Simple liquid-liquid models seemed to demonstrate diapirism as a viable

crustal process (Grout, 1945; Price, 1975), and simple analysis by Fyfe (1970) using Stokes' Law produced the correct order of magnitude regarding the velocity of upward movement of a granite body (Price, 1975). However, more recent numerical modelling suggests that granite bodies 1-10 km in diameter, segregated and collected at crustal depths of 25-40 kin, at an initial T of 850°-1000°C, will solidify and stop moving up at depths > 15 km (Mahon et al., 1988). Weinberg (1993) has questioned whether experimental models of diapiric ascent of granite magma were based on appropriately scaled physical parameters. In other words, are the conclusions drawn from these experimental models relevant to nature? Further, traces of the passage of granite diapirs have not been identified in exposed middle crustal sections in orogenic belts (Bateman, 1984). Have they been missed, or do they not exist because diapirism is not the general ascent mechanism? Finally, many of the features of diapiric emplacement, particularly shape and strain patterns, are also characteristic of ballooning plutons (Holder, 1979; Ramsay, 1989), particularly if, as seems likely, expansion of the pluton takes place by magmatic flow rather than by solid-state ballooning (Paterson et al., 1989). In his study of the Northern Arran Granite, Scotland, England (1992) attempts explicitly to distinguish between diapirism and ballooning as the final emplacement mechanism for this pluton. However, even though England asserts that his data support diapiric ascent, his data are consistent with magma ascent along the bounding Goat Fell Fault and preferential ballooning upward and outward away from the fault of one continuous pulse of magma. Geophysical data have been used to support the diapir hypothesis, but once again the evidence is not without ambiguity. Sweeney (1975, 1976) interpreted granite plutons in souti~-central Maine to have a thin, tabular shape, and this has been suggested as a regional feature in northern New England (Neilson et al., 1976; Hodge et al., 1982), although the interpretation is disputed by modelling of more recent geophysical data by Unger et al. (1989). Nonetheless, in spite of the modelled tabular shapes of these plutons,

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Sweeney interpreted their apparent shape in combination with the expected viscosity of granite magma to be consistent with an emplacement mechanism as buoyant diapirs that rose to shallow depths. Similarly, extensive studies by Vigneresse in northwest France (Hanmer and Vigneresse, 1980; Vigneresse and Brun, 1983; Gulllet et al., 1985) were interpreted initially to support the diapir hypothesis, although more recent reviews have been less specific in this respect (Vigneresse, 1988, 1990). Indeed, Vigneresse (1988, 1990) states firmly that many orogenic granites commonly are associated with deformation that strongly controls their shape, although in general many are flat bodies whose lateral extent is greater than its thickness. Further, some of the examples studied by Vigneresse (see also Davies, 1982) could represent magma ballooning by magmatic flow after ascent along major transcurrent crustal shear zones. Finally, if diapiric ascent is arrested either at the level of neutral buoyancy or at the brittle-ductile transition or for some other structural reason and magma is able to spread laterally, then an apparent sill-like form may result! Emerman and Marrett (1990) have shown that sheet-like intrusions are favored for all but the most viscous granitic magmas, which suggests that fracture-related ascent mechanisms may be more important than diapirism. Indeed, Emerman and Marrett (1990) predict that even magmatic bodies migrating by ductile deformation will be sheet-like if they are sufficiently small. Examples of anatectic melt migrating through dikes are provided by the "discordant migmatite leucosomes" of Sawyer (1987) and Sawyer and Barnes (1988), and Figs. 19 and 20. The Q6rqut granite complex of southern West Greenland, which outcrops over an area of approximately 50 x 20 km, is one example of a major granite pluton (c. 1000 km 3) which has been constructed from meter-scale sheets (Brown et al., 1981; Friend et al., 1985), see Fig. 3. Almost as large is the Kinnaird Pluton, British Columbia, a complex constructed of 50-250 mthick sills (Halwas and Simony, 1992). Based on outcrop area, an apparent order of magnitude smaller is the Harney Peak Granite complex of South Dakota, although interpretation of Bouger

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Fig. 19. Discordant dike of garnet-bearing diatexite cutting across fine-scale layering in stromatic migmatite, southern Brittany, France.

gravity data suggests more extensive, low density material at shallow depth (Duke et al., 1990). The Harney Peak Granite is a dike and sill complex rather than a single intrusive body (Redden et al., 1982, 1985; Norton and Redden, 1990), but in this case at least two different (composite) sources are required for the constituent magmas (Krogstad et al., 1993). The model proposed by Nabelek et al. (1992) to explain low-6~80 biotite granites in the core of the complex and high-6~80 tourmaline granites around the periphery of the main complex and as satellite intrusions is consistent with this. However, the schematic emplacement model illustrated in their Fig. 11, comprising multiple km 3 diapirs, is not consistent with the description of the Harney Peak Granite as a dike and sill complex. The stereotype of multiple

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tear-drop diapirs of rising granite may be better replaced by sill complexes fed by dikes from deeper levels for the biotite granites and a more local fracture-related mechanism for segregation and emplacement of the tourmaline granites of the margin. It has long been recognized (Harry and Richey, 1963) that many granite plutons are both composite and multiple, that is to say that they are compositionally zoned and constructed by the repeated flow of magma, either from depth as pulses (Harry and Richey, 1963), or by the differential flow of magmas of different mobilities within the same magma chamber, the surges of Cobbing and Pitcher (1972). An interesting example of the multiple nature of a granite pluton is provided by the Himalayan leucogranite of Manaslu, which is interpreted as a low-melt-fraction granite body (Lanord and LeFort, 1988, and ref-

erences therein). It is composed of many magma batches with different initial 87Sr/86Sr ratios. This implies that large-volume intrusions of low-meltfraction leucogranite can exist, not in the form of large single intrusions, but by the successive expulsion of numerous magma batches from the source, which can collect and ascend into the upper levels of the crust. These magma pulses can be more or less continuous. In a wider Himalayan context, the origin of the postcollisional leucogranites is controversial. Partly this may be due to scattering of data on emplacement ages, perhaps related to generation from older crustal material leading to Pb inheritance in zircon and heterogeneous Sr isotope ratios. Very young ages of 7, 5 and 2.3 Ma, based on zircons, reported by Zeitler and Chamberlain (1991) are interpreted to represent the products of decompression melting during rapid exhumation, and a similar mech-

Fig. 20. Dike of interlayered pegmatitic leucosome and dark melanosome within the Moine succession, northwestern Scotland (cf Barr, 1990). Diameter of lens cap c. 55 mm.

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anism may be responsible for older Himalayan leucogranites such as that at Manaslu. These examples are not dissimilar to the Q6rqut granite complex quoted above (Brown et al., 1981), except that the individual magma pulses are more easily identified in the field in the Greenland case, or to the muscovite-rich, two-mica granites from northern Portugal described by Holtz (1989). Why does rising magma stop? Numerical models suggest that once a crack contains a critical volume of melt it will continue to propagate upwards towards the Earth's surface (e.g. Pollard and Holzhausen, 1979; Weertman, 1980). A number of possible explanations have been suggested over time and these include: attainment of a level of neutral buoyancy (Gilbert, 1877; Corry, 1988; Lister and Kerr, 1991); and arrival at a stress barrier or rheological transition, such as the ductile-to-brittle transition between the middle and upper crust (e.g. Gretener, 1969), or a weaker horizon (e.g. Lagarde et al., 1990), or some other kind of horizontal discontinuity such as the "crack stoppers" of Weertman (1980). This question has been considered also by Clemens and Mawer (1992), who developed the mechanism proposed by Cook and Gordon (1964; see also Pollard, 1973) which leads to a geological model with, as its fundamental element, the ponding of magma along roughly horizontal discontinuities. This model predicts that granitoid plutons commonly should be laccolithic or tabular in shape with horizontal dimensions far in excess of vertical thickness. Examples of granite intrusions potentially consistent with such a model include the Harney Peak Granite and the Q6rqut granite complex, both mentioned earlier, and the Chemehuevi Mountains plutonic suite described by John (1988). Megadikes frozen in ascent along shear zones have been described by D'Lemos et al. (1992) from northwest France. In many cases, granitic sheets occur at a high angle to the inferred direction of maximum principal stress; this may reflect an exploitation geometry as a dike of granite magma propagates along the weak medial plane of shear zones and faults. At higher levels in the crust, where dilatant sites for emplacement are available, such as strike-slip releasing bends and

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extensional settings, granite magma will accumulate as plutonic complexes, probably fed from dikes (for example, Hutton, 1982; Guineberteau et al., 1987; D'Lemos et al., 1992; McCaffrey, 1992; Morand, 1992), or as a sill complex (Halwas and Simony, 1992). In a wider context, compositionally-expanded granitoid suites generated and emplaced within continental arcs often are associated with, and controlled by, arc-parallel strike-slip fault systems (Krohe, 1991; Tikoff and Teyssier, 1992; Brown et al., 1993; Grocott et al., 1993; Tobisch et al., 1993) during transpressional or transtensional tectonics. A relationship between fault zones and melting is long established in the literature (Reitan, 1968a, b; McKenzie and Brune, 1972; Nicolas et al., 1977). A genetic relationship between major transcrustal shear zones and granite genesis and emplacement was inferred by Hanmer and Vigneresse (1980), Strong and Hanmer (1981) and Hanmer et al. (1982). The importance of faults and shear zones in granite ascent and emplacement has been recognized by Berth6 et al. (1979), Davies (1982), Hutton (1982, 1988a, 1988b), Hartmet (1988), Hanmer et al. (1992), Reavy (1989), Gapais and Bal6 (1990), and is further emphasized by Hutton (1988b, 1992), D'Lemos et al. (1992) and Hutton and Reavy (1992) in their development of more general models. Hutton (1982) proposed a crack opening model for the Main Donegal granite of northwest Ireland with successive emplacement of two essentially vertically-oriented tabular bodies of magma. In a study of the Ox Mountains igneous complex of western Ireland, McCaffrey (1992) also proposed a granite emplacement mechanism in which successive sheets of muscovite granite, equigranular and K-feldspar megacrystic biotite granodiorite and tonalite are emplaced into transient, dilational sites developed in response to transpressive deformation along a major strike-slip shear zone. McCaffrey speculates that emplacement into transient dilational cavities may be one way by which granite is emplaced into fault zones undergoing net contractional deformation. Other younger granites were emplaced as individual intrusions in dilationai cavities at releasing bends produced during reactivation along the fault zone.

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The similarity to extensional vein systems developed by successive opening and collapse events caused by seismic rupturing along shear zones is striking (for example, Boullier and Robert, 1992), at least in a geometric sense. In both of these examples, successive sheets of granite are sufficiently distinct one from the other to be identified in the field during mapping. In circumstances where successive batches of magma are more similar it may be impossible to distinguish individual sheets during field mapping or even on the basis of extensive whole rock geochemistry. An example is provided by the Lucerne Pluton, Maine, which could be considered to be homogeneous except that recent work by Hogan and Sinha (1991) has identified variability in its initial Pb isotopic composition. This variability requires that the Lucerne Pluton consists of discrete domains that have not equilibrated. If this is true, then the mineralogic, chemical and isotopic variability in the Lucerne Pluton and others like it can not be explained by differentiation of a single magma but rather reflect chemical heterogeneities of the source coupled with differentiation processes restricted in scope to individual

magma batches (Hogan and Sinha, 1991). Unfortunately, most petrologic studies of granite plutons begin by tacitly assuming geographically dispersed samples represent parts of a single former magma chamber! Strike-slip dilatancy pumping has been suggested as an ascent mechanism by D'Lemos et al. (1992). In many respects, this mechanism for granite magma segregation, aggregation and migration along shear zones is similar to the seismic pumping mechanism for hydrothermal fluid transport at shallower crustal levels (Sibson et al., 1975). Sibson (Sibson et al., 1988; Sibson, 1990, 1992) has refined this mechanism into the faultvalve model to link shear stress variations and fluid pressure fluctuations with time. In a magmatic context, such a model may provide a delivery mechanism for pulses of variably fractionated magma emplaced as sheets in the construction of granite complexes at higher crustal levels within transpressional shear zones. This leads to a general model of granite magma migration towards shear zones, often along a foliation with a subhorizontal sheet-dip in overthickened orogenic belts, aggregation and transport upwards within the

North Armorica

~

A

S

Z

~dr~C)rie

a

\

'd~C

Fig. 21. Generalized geological m a p of the northeastern part of the Armorican massif. Heavy lines represent Cadomian shear belts, a majority of which show sinistral strike-slip displacement, or younger brittle faults, thought to lie along Cadomian shear belts. FSZ = Fresnaye shear zone, which separates arc-related terranes to the northwest from behind-arc terranes to the southeast. Other abbreviations: L H = La Hague; T = Tr~gor; NASZ = North Armorican shear zone; SASZ = South Armorican shear zone; SC = St. Cast; R F = Rance Valley; PB = Port Briac; B = Bonnemain granite complex; VC = Vire-Carolles granite complex; A = Athis granite complex; ADEI = Alexain-Deux-Evailles-Iz6 granite complex. Reproduced from D ' L e m o s et al. (1992), with permission of the Geological Society, London, UK.

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shear zone. D'Lemos et al. (1992) infer that major strike-slip shear zones are located in thermally-weakened crust. Once failure has occurred, the shear zone then facilitates upward migration of granite magma. In contrast to many of the examples quoted above, at very high crustal levels brittle failure of host rock controls final emplacement mechanisms. Stoping and subsidence of blocks inside bell-jar shape intrusions are displayed in spectacular style in the mountain peaks of the Coastal Batholith of Peru (Pitcher, 1978). Cauldron-subsidence is clearly the dominant process by which late-stage emplacement of this batholith occurred. However, as noted by Roberts (1970) such features are produced only at very high levels in the crust.

5. Development of a general model applicable to orogenic granites of predominantly crustal derivation

5.1. The St. Malo migmatite belt and the Mancellian granites, northwest France Regional geology The Cadomian orogenic belt of northwest France records late Precambrian subduction-related magmatism and accretionary tectonism, and is interpreted in terms of amalgamation of calcalkaline magmatic-arc complexes and marginal basin successions along an active margin of Gondwanaland. The resultant North Armorican Composite Terrane comprises four terranes separated, and internally deformed by steeply-dipping ductile shear zones and brittle faults. From north to south these are (Fig. 21) arc-related units in the Tr6gor-La Hague and St. Brieuc terranes, north-west of the Fresnaye shear zone, and behind-arc marginal basin complexes of the St. Malo and Mancellian terranes (for a discussion of the evidence and different views on the interpretation, see Brun, 1992 and Strachan et al., 1992, and the references therein). 4°Ar/39Ar and U-Pb mineral ages imply a complex, polyphase Cadomian tectonothermal history (for a brief review see Brown et al., 1991, and references therein).

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Southward younging of major tectonothermal events within the Cadomian belt is inferred to reflect regionally diachronous terrane accretion at an obliquely convergent plate margin. Docking of the already deformed Tr6gor-La Hague terrane-St. Brieuc terrane composite with the behind-arc marginal basins likely effected basin inversion and structural thickening within the St. Malo and Mancellian terranes, leading first to regional deformation and then to intracrustal granite production at c. 540 Ma (D'Lemos et al., 1992). The St. Malo migmatite belt comprises a syntectonic suite of migmatites (metatexites and diatexites, Brown, 1973) derived through partial melting of Neoproterozoic Brioverian succession (Brown, 1979; Martin, 1979). The Brioverian succession comprises mainly psammites, semi-pelites and pelites of probable turbiditic origin. In the western half of the belt, metatexites are dominant (metatexites are stromatic migmatites produced by low to moderate degrees of partial melting (Brown, 1973), with the generation of up to c. 30 vol.% melt, which generally is below the critical melt fraction so that wholesale disruption of the rock structure does not occur). They are cut by syn-anatexis small-scale, millimetric to metric, shear zones into which melt has been segregated preferentially. Further, they contain decametric to kilometric, elongate diapiric or buoyantly-eraplaced masses of diatexite/anatectic granite, commonly associated with meter-scale pods or thin sheets of tourmaline pegmatite. In the eastern half of the belt, at a higher structural level, several tens of cubic kilometers of homogeneous diatexite/anatectic granite of the core was emplaced syn-kinematically into a Cadomian strikeslip shear zone (Brown, 1979; Strachan et al., 1989; D'Lemos et al., 1992) (diatexites are schlieric to nebulitic migmatites produced by moderate to high degrees of partial melting (Brown, 1973), with the generation of greater than 30-50 vol.% melt, which exceeds the critical melt fraction and results in the disruption of the stromatic structure characteristic of the metatexites). The peak of regional metamorphism and the culmination of anatexis are dated at c. 540 Ma using information from U-Pb on zircon and

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monazite, and Rb-Sr whole-rock isochrons (Peucat, 1986), which is also inferred to be the age of concomitant strike-slip displacement on the major crustal shear zones within and at the margin of the terrane. The Mancellian granites, to the southeast of the St. Malo migmatites, also were emplaced at c. 540 Ma (Dallmeyer et al., 1993) and are overlain unconformably by Cambrian sediments. The rocks which form the bulk of the exposed level of the Mancellian granites vary between granodiorite (the majority) and granite; there is no evidence from either regional gravity or magnetic maps for the existence of substantial uncompensated masses of basic rock at depth. In comparison with the St. Malo migmatites, the Mancellian granites represent farther-travelled products of intracrustal melting emplaced at higher levels in the crust (Brown and D'Lemos, 1991). On the basis of contrasting metamorphic grade, structural complexity, lithologic associations and relative timing of magmatism and deformation, Strachan et al. (1989) proposed that a St. Malo terrane (comprising apparently syn-tectonic migmatites and moderate-grade Brioverian metasedimentary rocks) can be separated from a Mancellian terrane (comprising post-tectonic granites and low-grade Brioverian metasedimentary rocks). Brown and D'Lemos (1991) provided petrographic and geochemical evidence on the basis of which they argued that granites of the Mancellian region might represent the farther travelled, homogenized product of anatectic melting expressed by the St. Malo migmatites. D'Lemos et al. (1992) suggest that the contrasting syn-tectonic nature of magmatism shown by the St. Malo migmatites and apparent post-tectonic character of the Mancellian granites is simply a function of differing crustal level, which is consistent with the kinematic evolution suggested by shallow eastnortheast-plunging lineations, assuming no subsequent crustal tilting. The two terranes of Strachan et al. (1989) thus may represent contrasting levels of essentially the same crustal block, divided by a shear zone, the present juxtaposition being the result of sinistral transpression with a north side up component. This raises the interesting question of whether or not a linking mecha-

nism can be identified between deeper structural levels represented on a crustal scale by the essentially in situ anatexis demonstrated by the migmatites and the clearly intrusive granites that have been emplaced into the upper crust. As D'Lemos et al. (1992) have shown, such a linking mechanism is provided by the transcurrent shear zones that cut the St. Malo-Mancellian terrane. The continuum of transcurrent shear deformation and granite magma migration can be established from relationships within the shear zones. Several c. 1 kin-wide SW-NE trending variably foliated granite bodies ("megadykes" of D'Lemos et al., 1992) are interleaved with heterogeneously deformed migmatitic and metasedimentary host rocks. Pre-, syn- and post-kinematic cordierite porphyroblasts are present in mylonitic metasediments adjacent to and for several hundreds of meters away from granite contacts. These "contact-aureoles" are wider than might be expected from conductive heating adjacent to a megadyke of static granite cooling from c. 750°C, consistent with heating by continuous passage of new magma. Relatively undeformed granite within parts of some shear zones is taken to show that either magma emplacement outlasted ductile shearing, or the magma contained too great a melt fraction at the time of shearing to record deformation. Furthermore, the granites are located in straight segments of shear zones which suggests that they were frozen while on their way somewhere, rather than simply ponded in dilatant sites at jogs along the shear zone. Commonly, the deformed granites display homogeneous grainscale C-S fabrics which contrast with heterogeneous decimetric shear zones developed in adjacent country rocks, features consistent with syntectonic intrusions (Gapais and Bal~, 1990). Thus, shearing was broadly synchronous with the passage of anatectic granite, some of which froze within the shear zones, to suggest that the shear zones are indeed upward conduits for granite migration. Geochemistry

Overall geochemistry and a variety of specific geochemical parameters show that the Mancellian granites and diatexites and anatectic granites

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of the St. Malo migmatite belt are similar (Brown and D'Lemos, 1991), although on a statistical basis this has been questioned by Power (1993). At SiO 2 < 70% the aluminum saturation index of the St. Malo diatexites increases with decreasing SiO2, which suggests control by restite unmixing, principally of plagioclase + biotite + sillimanite, although crystal fractionation controls chemical variations at SiO 2 > 70% (Brown, 1979; Brown and D'Lemos, 1991). Chemical variation within the Mancellian granites is consistent with control by fractional crystallization (Brown and D'Lemos, 1991). Both suites of rocks are peraluminous, they have similar distributions of Rb, Sr and Ba, they have K / R b ratios around 225, and they plot largely within the volcanic arc granite field using trace element discrimination diagrams for the interpretation of tectonic setting (which, in this case, reflect the recently eroded juvenile arc source of the Brioverian sedimentary succession). Samples of Brioverian succession sandstones, St. Malo migmatites and Mancellian granites all exhibit similar primordial mantle-normalized element patterns (Brown and D'Lemos, 1991).

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D'Lemos and Brown (1993) present results of Nd and Sr isotope studies. Three samples of anatectic granite/migmatite from the St. Malo migmatite belt have eNd values in the range -5.8 to -7.3, eSr of +12 to +40, Nd model ages of 1.5 to 1.7 Ga (TDM) and Sr model ages of 550 to 600 Ma (bulk Earth). Three samples of Mancellian granite have eNd values in the range -4.0 to -6.7, eSr of - 3 to +42, Nd model ages of 1.5 to 1.7 Ga (TDM) and Sr model ages of 520 to 580 Ma (bulk Earth). Further, the St. Malo anatectic granites and the Mancellian granites exhibit significantly older Nd model ages than the majority of granitic rocks in the arc-related outboard terranes (ToM of 1.0 to 1.4 Ga). D'Lemos and Brown (1993) argue that the marked similarity in isotopic characteristics of the anatectic components of the St. Malo migmatites and the

Fig. 22. Generalized model for crustal anatexis and the ascent and emplacement of granite magma within middle- to uppercrustal levels of a transpressional orogen by the proposed strike-slip dilatancy pumping mechanism (SZ = shear zone; F = fault). Model is kilometer-scale, but is dependent upon a specific crustal thickness and geotherm. Magma moves up from below in a shear zone. It is expelled up and along from a contractional jog (Tp = zone of transpression) into an overlying extensional jog (Te = zone of transtension) with progressive displacement along the shear zone; upward movement of magma is by a combination of buoyant ascent and dilatancy pumping. A = intracrustal anatexis and zone of transcurrent shear focussed in weakened crust (m = metatexite, stromatic migmatite with melt vol.% below critical melt fraction; d = diatexite, schlieric-to-nebulitic migmatite with melt vol.% above critical melt fraction; black = anatectic granite, may still exhibit relict nebulitic and even schlieric structure). B = generally a zone of diapiric rise of magma through ductilely deforming migmatite, but at right hand side one example is shown of local brittle behavior due to melting (cf. Rushmer, 1991b, 1992). C = zone of magma transported in megadikes within shear zones (black = anatectic granite; c = contact metamorphism); erosion to this paleodepth would reveal apparently "syn-tectonic" granite emplacement with pre-, synand post-tectonic porphyroblastesis in aureole mylonites. D = zone of emplacement within brittlely-deforming upper crust, homogeneous granite magma emplaced passively into extensional jogs with local ballooning by magma flow and stoping; erosion to this paleodepth would reveal apparently "posttectonic" granite emplacement, with porphyroblastesis in aureole hornfelses post-tectonic with respect to regional cleavage (although porphyroblastesis could be syn-tectonic in the zone of contact strain around a ballooning pluton).

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Mancellian granites supports the conclusion of Brown and D'Lemos (1991) that the two are probably genetically related, in the sense that they were derived by anatexis of the same supracrustal sequence within the same orogenic cycle and were emplaced essentially during the same interval of time. Since the St. Malo migmatites are demonstrably derived by partial melting of the Brioverian sedimentary succession, a similar source is invoked for the Mancellian granites. 5.2. A general model related to transpression On the basis of observations within the Cadomian orogenic belt of northwest France, D'Lemos et al. (1992) proposed an integrated model to account for the generation, segregation, aggregation, ascent and final emplacement of granite magma within a transpressional orogen, and to explain the co-location of granites and shear zones (Fig. 22). In their model, oblique collision of arc-related terranes at a late Precambrian continental margin resulted in transpressive thickening of a juvenile supracrustal sequence in a behind-arc terrane. A combination of structural inversion of a sedimentary basin with higher than average heat flow and radiogenic self-heating of the overthickened supracrustal sequence, generated the necessary P - T conditions for water-rich volatile phase-present anatexis. A crustal-scale linked strike-slip shear zone/fault system, developed synchronously with anatexis, provided both mechanism and opportunity for granite migration and ascent. Continuous displacement along the strike-slip system meant that zones of extension progressively became zones of contraction, and vice versa, such that granite magma was forced through the ductile shear zones and, with the added impetus of its own buoyancy, generally upward through the crust (Fig. 22), a mechanism which I call strike-slip dilatancy pumping. Ductile movement within the middle crust is inferred to have been accommodated at upper crustal levels by large-scale fault systems situated directly above zones of magma ascent. Emplacement of magma at extensional jogs within this system is synkinematic, a void or cavity will not exist and filling by granite magma from below occurs simul-

taneously with dilation. Stoping and ballooning effects locally modify granite contacts as the magma spreads laterally. Granite contacts alone can not provide direct information concerning ascent mechanisms because the local interaction between country rock, stress and final emplacement mechanism determine the specific features of local granite contacts. One feature of the model is that apparently post-tectonic intrusions in the upper crust (contact metamorphic porphyroblasts post-date regional cleavage) occur contemporaneously with apparently syn-tectonic magmatism in the middle crust (contact metamorphic porphyroblasts are pre-, syn- and post-tectonic in relation to the mylonite fabric within enclosing shear zones). D'Lemos et al. (1992) suggested that the close temporal relationship between the peak of anatexis and regional strike-slip displacements reflected initiation of strike-slip deformation along zones within the middle crust that had been softened thermally as a result of anatexis. Thus, regional deformation and granite generation and ascent will be genetically-linked processes, with granite melts continuing to influence crustal behavior during their ascent within actively deforming shear zone/fault systems. The common co-location of granites and shear zones is to be expected. Indeed, as D'Lemos et al. (1992) point out, progressive strike-slip displacement may result in complete attenuation and closure of magma conduits, such that evidence or the existence of passage of granite melt may be cryptic in ancient orogens. On a regional scale, granites generated by water-rich fluid-present crustal anatexis will be limited in their ascent capability, but granite magma generated by fluid-absent dehydration melting at higher temperatures, which may reflect deeper levels within more substantially overthickened areas of the inverted sedimentary basin, can reach higher crustal levels. This seems to have occurred at the eastern side of the belt of Mancellian granites where garnet surrounded by cordierite occurs within the Athis granite complex (Fig. 6). The general model outlined above relates specifically to structural inversion of sedimentary basins a n d / o r crustal thickening orogens, both of

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which are commonly transpressional since subduction is generally oblique rather than orthogonal at convergent plate margins. In this model, heat generation by internal radioactive decay during thermal relaxation and uplift consequent upon thickening generated a significant thermal pulse which led to anatexis at c. 540 Ma. The model contrasts with that of Clemens (1990) in which cooling and crystallization of nearly anhydrous basaltic magma provides the thermal energy required for partial fusion of fertile crustal rocks. In this model, granitoid magmatism thus is the consequence of under- and intraplating of man-

tie-derived magma. Restitic granulites, left behind by fluid-absent partial fusion of supracrustal rocks, should surround the under- and intraplated mantle-derived mafic rocks. The melt fraction produced will have been transported to higher levels in the crust via a system of feeder dikes, to produce a high-level granitoid batholith. The two models are not mutually exclusive alternatives, rather they are complementary endmembers for particular, but different, tectonic scenarios. Indeed, the Clemens model is more likely to relate to subduction tectonics at continental margins.

2ow

BA

BA Q L G Qui B1 V M Qu SASZ St.N C

119

48°N+

= = = = = = = = = = = =

Baie d'Audi~me -~ Quimper Lorient r:_'~:;~ lie de Groix Quiberon Qu Belle Ile Vannes Golfe de M o r b i h a n Questembert South Armorican Shear Zone St. Nazaire Eclogite locality, Champtoceaux Complex N = Nozay Na = Nantes BC = Bois de C6n6 E = Eclogite locality, Essarts Complex St.G = St. Gilles-de-Vie LSO = Les Sables d'Olonne N u m b e r e d boxes, sample localities Black represents Variscan granites Lines represent tectonic contacts, two major normal faults identified

#

3ow

47°N+

50km LSO

Mesozoic

Fig. 23. Map to show the distribution of granites, in black, within the South Brittany metamorphic belt, western France; major tectonic units forming the country rock are not distinguished.

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6. Consideration of the wider role of granites in orogenic processes Unambiguous crustally-derived granite emplaced in the middle-upper crust implies at least a partly residual lower crust. Furthermore, the mineralogical and chemical composition of at least some metamorphic rocks in high-T metamorphic belts, such as highly aluminous garnet-cordierite-aluminosilicate gneisses and sapphirinecordierite granulites, requires the removal of a melt of granite composition at some point during their evolution. Clearly, these two observations imply a complementary petrogenetic relationship. One aim of petrology is to relate the physicalchemical history of metamorphic rocks and the generation of crustally-derived granites to the tectonic evolution of orogenic belts. This is achieved through the elucidation and interpretation of the P - T history from petrologic and mineral equilibrium data, the T-t history from thermochronologic data and the burial ( P - t ) and deformation (d) history from structural data, and integration of these histories into one P - T - t - d path. As an example I take the Variscan metamorphic belt of west France (Fig. 23). A well-constrained "clockwise" P - T path has been defined for the amphibolite-to-granulite facies migmatitic core of the belt (Jones and Brown, 1990). The prograde path is characterized by large d T / d P and achieved peak T of c. 780-800°C at c. 9 kbar. Erosion-controlled exhumation is interrupted at high T by c. 3 kbar near-isothermal decompression and was followed by near-isobaric cooling. The early P - T evolution was the result of thrust thickening of an active continental-margin sequence. This resulted in moderate overthickening and radiogenic self-heating during thermal relaxation and erosional exhumation, leading to the development of anatectic migmatites (Jones and Brown, 1990; Treloar and Brown, 1990). The near-isothermal decompression is interpreted to reflect tectonically-enhanced exhumation and unroofing, that generated coeval granite by decompression melting (the distribution of granites is shown in Fig. 23). Peak metamorphism has been dated by the U-Pb method at c. 400 Ma (Peucat,

1983) while the cooling history is constrained by ages on monazite (c. 320 Ma, Peucat, 1983), 4°Ar/39Ar hornblende and muscovite ages (c. 305-300 Ma, Dallmeyer and Brown, 1992), Rb-Sr ages on biotite (c. 300 Ma, Peucat, 1983), and fission track ages on apatite (c. 300 Ma, Carpena et al., 1979). A T-t plot for the metamorphic belt yields a cooling path characterized by a segment with a slow cooling rate of < l°C/m.y, and a segment with a cooling rate of c. 60°C/m.y., the transition occurring at c. 330-320 Ma (Brown, 1993). The data require that penetrative deformation and initial high-T metamorphism were Eo-Variscan (c. 400 Ma) and that post-peak cooling during erosion-controlled exhumation was interrupted by rapid Variscan uplift and subsequent tectonic exhumation (c. 320-300 Ma). Such rapid uplift requires a fundamental tectonic control, which I suggest may be slab detachment or detachment of the thickened orogenic root coeval with a transition from sinistral to dextral displacement on the South Armorican Shear Zone (SASZ) during intracontinental Variscan deformation. The ages of the granites vary systematically across the metamorphic belt. To the north of the SASZ the granites are c. 340-330 Ma in age, within the SASZ the granites are c. 330-320 Ma in age, and south of the SASZ the granites are 305-300 Ma in age (Rb-Sr whole-rock isochron data, from Bernard-Griffiths et al., 1985). The decompression generated by the tectonic detachment also may have been responsible for initiating melting within the crust and the generation of the granites. Granite melt was transported upward through the crust along the SASZ, the melt likely having facilitated the large displacement inferred to have occurred along this zone (e.g. the Questenbert granite, J6gouzo, 1980). At higher crustal levels, the granites were emplaced into major subhorizontal to S-dipping normal ductile fault zones, commonly reactivated thrusts, that facilitated fast unroofing of the metamorphic belt (e.g. the Quiberon granite, Gapais et aI., 1993). In this particular case, migmatites within the metamorphic belt demonstrate one process by which deeply buried crustal rocks may become depleted in a granitic component, while the rapid

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uplift and subsequent exhumation of the belt generated and utilized a granite extracted during a younger cycle of decompression melting. These granites imply that a partly residual lower crust must underlie this sector of the Variscan metamorphic belt. This example emphasizes the intimate relationship between granite and tectonics. The rapid tectonic uplift promoted decompression melting in the crust to generate granite, and the granite was able to utilize a major transcurrent fault system for ascent and to reactivate existing thrust faults as normal faults to facilitate unroofing. Granite magma in crustal shear zones is able to accommodate large amounts of strain due to the low strength of the melt compared to the wall rocks (Davidson et al., 1992; D'Lemos et al., 1992; see also Hollister and Crawford, 1986). The role of melt in focussing deformation has been emphasized by Hollister and Crawford (1986) and Davidson et al. (1992). Carr (1992) has suggested that decompression melting may provide in situ magma which can play an important role in the nucleation of extensional shear zones. Subsequently, the extensional regime can facilitate the emplacement of larger plutonic complexes. Indeed, Lister and Baldwin (1993) have related the origin of metamorphic core complexes to plutonic activity during episodes of continental extension. This raises the general question of cause and effect, given that the evidence for the association of granites with shear zones is syntectonic intrusion and deformation fabrics. Are granite magmas attracted to shear zones because they provide a low pressure sink for magma aggregation and a conduit for magma ascent or is shear strain focussed in the high-grade anatectic core of the hinterland because it is weak? Experiments on partially molten rock samples indicate that before melting is initiated the rock deforms ductilely and it is only later when melt formation begins that fractures are able to nucleate (Dell'Angelo and Tullis, 1988; Rushmer, 1991b, 1992). However, once periodically-spaced oriented fractures have become established, segregation and migration of additional melt into the fractures will occur by dilatancy pumping (Mawer et al., 1988). At the scale of an orogenic belt, Collins and Vernon

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(1991) and Sandiford et al. (1992) have suggested that granite generation and emplacement is a causative factor in localizing deformation, and this may be a general feature of CCW orogenic belts in which heating occurs prior to or concomitant with thickening.

7. Final s t a t e m e n t

Felsic magma generation may occur in the middle a n d / o r lower crust in sources of appropriate composition providing that heat is generated or transferred and either fluid is present or fluid-absent dehydration melting can occur. The only permeable pathways through large volumes of the continental crust must be fractures and shear zones. These two statements may be linked by taking a holistic approach to understanding orogenesis. Orogenic belts characterized by CW metamorphism are driven by thickening and the increased heat flux is due to enhanced heat generation by internal radioactive decay during thermal relaxation and uplift. Deformation accompanies both thickening and uplift. This will facilitate deformation-enhanced melt segregation mechanisms, and ascent and emplacement mechanisms commonly are related to major fault systems that may be either strike-slip in transpressional belts or extensional in strongly overthickened orogens undergoing collapse, or the strain may be partitioned if required by the overall kinematic framework. In contrast, orogenic belts characterized by CCW metamorphism probably are driven by emplacement of mantle-derived magma. This may lead to rapid heating, overstepping of reaction boundaries, rapid generation of melt and fracture-related melt segregation and ascent mechanisms. In both cases tabular sub-horizontal plutons are constructed from sequential pulses of magma. Although in either case magma may migrate to an emplacement site in a dike or along a shear zone, there are sufficient common elements within plutons and differences between plutons in CW and CCW orogenic belts to suggest a fundamental difference in the processes of magma ascent and emplacement. In CW orogenic belts many syn-tectonic plutons either are constructed

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from sub-vertical sheets emplaced in extensional jogs in strike-slip fault systems, or are constructed from steeply-inclined sheets emplaced in dilational jogs along the active breakaway zone of extensional fault systems with ramp-flat geometry. In contrast, in CCW orogenic belts many plutons that are pre- or early syn-tectonic are constructed from horizontal sheets of magma fed by dikes at a roughly horizontal discontinuity in the crust. Further, in CW orogenic belts granite is generated during the middle and late stages of the cycle and may facilitate unroofing of such belts during emplacement along late orogenic extensional fault systems. This contrasts with CCW orogenic belts in which granite emplacement is an early feature that softens the crust to localize deformation within the orogenic belt which then leads to thickening of the orogen and cooling. Finally, post-tectonic plutons likely are fed by dikes.

Acknowledgements In this paper I have attempted to show how some of my own research contributes to a better understanding of crustal anatexis, segregation and aggregation of magma, and ascent and emplacement of granite. Of course, much of this research is collaborative with others and many of the ideas that I present, like good wine, have evolved through complex interactions and matured with time; I acknowledge everyone who has contributed to my continuing education in granite petrology. I thank the following for critical reviews, involving stimulating ideas as well as corrections, at various stages in the preparation of this manuscript: Philip Candela; Bill Collins; Louise Corriveau; John Hogan; John Percival; Wally Pitcher; Tracy Rushmer; Ed Sawyer; and, E-an Zen. However, I take responsibility for those infelicities that remain. I thank John Clemens, Scott Paterson, Alberto Patifio Douce, Nick Petford and Ed Sawyer for supplying preprints of articles in press. Finally, the word processing support of Jeanne Martin is greatly appreciated.

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