Pergamon

PII: S 1464-1895(01)00048-5

Phys. Chem. Earth (A), Vol. 26, No. 4-5, pp. 213-223, 2001 © 2001 Elsevier Science Ltd.

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Melt Segregation, Pervasive Melt Migration and Magma Mobility in the Continental Crust: The Structural Record from Pores to Orogens

O. Vanderhaeghe

UMR G2R, Universit6 Henri Poincart, Nancy I; BP 239; 54506 Vandoeuvre-l~s-Nancy

Received 21 June 2000; accepted 14 November 2000

Abstract. Structural analysis of migmatites based on the dis- tribution and proportion of the granitic fraction at the outcrop scale is taken as a guide to decipher the behavior of partially molten rocks during orogenesis. This approach evaluates the various mechanisms that control melt segregation and magma mobility from pores to orogens. During partial melting, the first liquid appears at grain interfaces but such textures are rarely preserved in igneous rocks. Mechanisms of melt move- ment are scale-dependent and it is important to distinguish melt segregation at grain-scale from melt migration which occurs over larger distances. Melt segregation is controlled by melt connectivity and ubiquitous localization of granites in structurally-controlled dilatant sites provides evidence of the efficiency of melt segregation at the outcrop scale, prob- ably achieved by porous flow at the grain scale. Magma mo- bility is controlled by the continuity of the solid framework which controls the rheologic threshold at the transition from solid-dominated metatexites to liquid-dominated diatexites. The presence of laccoliths of homogeneous leucogranite, emplaced at higher structural levels far from their source, in- dicates the efficiency of melt migration beyond the grain scale. The transition zone between diatexites (melt source) and gra- nitic laccoliths (melt sink) is characterized by a network of granitic veins centimeter- to meter- thick. The geometric char- acteristics of this network suggest that, depending on struc- tural level and the competency contrast between liquid and solid, veins propagate by either channeled porous flow, duc- tile deformation or fracturing. The main driving forces for upward melt migration appear to be buoyancy and dilatancy; the characteristics of local and regional deformation control the patterns of the granitic vein networks. Partial melting and redistribution of melt and magma from segregation by perco- lation at the grain scale relayed by pervasive migration through vein networks, is associated with chemical differentiation and generation of new rheological layering of the orogenic crust. © 2001 Elsevier Science Ltd. All fights reserved

Correspondance to : O. Vanderhaeghe

Keywords : partial melting, granite, migmatite, anatexite, melt segregation, pervasive melt migration, magma mobility, orogen .

1. Introduction

The goal of this paper is to discuss field observations of the structural record of melt segregation (separation of a melt from a solid rock) and magma mobility (motion of the melt with part of the solid) and to propose a model for the relationships between migmatites and granites in convergent orogens. Migmatites and granites generated by crustal anatexis form the core of a number of eroded orogenic belts of various age (e.g. Brown, 1993; Mehnert, 1968). The pressure and tem- perature conditions required for melting of a range of crustal protoliths are constrained by numerous experimental data and petrologic studies (Clemens and Vielzeuf, 1987; Gardien et al., 1995; Johannes, 1985; Patino-Douce and Johnston, 1991; Rushmer, 1991; Thompson, 1982; Wolf and Wyllie, 1991; Wyllie, 1977). As a result, it appears that relatively fertile crustal rocks may generate from 20 to 60 % of melt in the range 700-900°C (Patino-Douce, 1994). Following pioneer- ing 1D models of the thermal evolution of orogenic belts (En- gland and Thompson, 1986) more recent 2D numerical mod- els show that it is possible to reach these temperatures owing to radioactive decay in zones of thickened orogenic crust, even during active convergence and subduction of a cold slab (Henry et al., 1997; Huerta et al., 1998). These results show that in a context of plate convergence, the orogenic crust can potentially be affected by significant partial melting. Despite these advances in the fields of petrology and numerical mod- eling, the nature of the link between granites and migmatites is still debated. Addressing this question necessitates an un- derstanding of the behavior of partially molten rocks as well as access to crustal scale sections.

Recent reviews on the behavior of partially molten rocks emphasize the role of two thresholds during the geometrical evolution of a rock undergoing partial melting (Brown et al.,

214

The solid framework accommodates deformation

Effective viscosity Strength

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O. Vanderhaeghe: Melt Segregation, Pervasive Melt Migration and Magma Mobility

The liquid phase accommodates deformation

¢ Metate~dtes Diate~tes

incipient synmigmatitic " maate~ites ; layering

i ~ ~ magma mobility: i ~ ~ ~ liquid + part of solid

I !

1 '

r~lteNve

: liquid-.solid i ansition i

0 20 40 60 80 100 I . . . . . . I I

melt segregauon settling of solid fragments

Fig. 1 : Behavior of a partially molten rock as a function of the liquid-solid proportions. Compilation of experimental data and theoretical rheological laws from Roscoe (1952) ;Arzi (1978) ;Van Der Molen and Paterson (1979) ; Rutter and Neumann (1995) ; Philpotts et al. (1996). Geometrical thresh- olds corresponding to melt connectivity and continuity of the solid frame- work are indicated on the horizontal axis. Melt connectivity, which con- trois the ability of melt to segregate by percolation through the continuous solid framework, is achieved for a melt fraction of a few %. The continuity of the solid fraction controls (1) the solid-liquid rheologic transition and thus magma mobility, and (2) settling of solid particles in the magma. Metatexites are identified as solid-dominated anatexites, whereas diatexites correspond to liquid-dominated ones. Segregation of a melt from cumulate phases also occurs during magma crystallization when the crystals in the mush form a continuous solid framework (Philpotts and Caroll, 1996). The liquid-solid proportions that correspond to the continuity of the solid frame- work is likely to be different during partial melting and crystallization (Philpotts et al., 1996; Vigneresse et al., 1996)

1995; Sawyer, 1994; Vigneresse et al., 1996; Wickham, 1987), namely (1) the melt connectivity which corresponds to the formation of an interconnected network of liquid through the continuous solid framework, and (2) the loss of continuity of the solid framework (figure 1). Melt connectivity controls melt segregation and experimental data on crustal rocks sug- gest that it is achieved for only a few volume percent melt (Bulau et al., 1979; Jurewicz and Watson, 1984; Laporte, 1994; von Bargen and Waft, 1986; Waft and Bulan, 1979). The loss of continuity of the solid framework is associated with the transition from a solid-dominated rock to a liquid-dominated rock and controls magma mobility. Experimental data show that this transition occurs within a range of about 30 to 50 vol.% melt in the rock (Arzi, 1978; Dell'Angelo and Tullis, 1988; Paquet et al., 1981; Rushmer, 1996; Rutter and Neumann, 1995; Van der Molen and Paterson, 1979). The granitic liquid generated by partial melting is less dense and

viscous than the solid it comes from (Bagdassarov and Dorfman, 1998; Bottinga et al., 1983; Shaw, 1965). Conse- quently, two physical driving forces control melt segregation and magma mobility, (1) the buoyancy of the melt, and (2) the dilatancy related to heterogeneous deformation of me- chanically anisotropic rocks (Brown, 1994; Burg and Vanderhaeghe, 1993; Sawyer, 1994; Sawyer, 1999; Van Der Molen, 1985a; Van Der Molen, 1985b; Vigneresse, 1995).

Integrated studies of orogens emphasize the close relation- ships, in zones of plate convergence, between thermal-me- chanical evolution of the orogenic crust and partial melting as well as the role of deformation on melt movement (Brown, 1994; Brown and Rushmer, 1997; Brown and Solar, 1998; Sawyer et al., 1999; Vanderhaeghe and Teyssier, in press-b; Vigneresse, 1995; Weinberg and Searle, 1998; Wickham, 1987). The relative efficiency of percolation, ductile defor- mation, and fracturing, the three mechanisms that allow mo- tion of melt and magma, varies primarily as a function of competency contrast between the melt or magma and the host rock (Clemens and Mawer, 1992; Petford et al., 1993; Rubin, 1993; Vanderhaeghe, 1999; Weinberg, 1996).

The motivation behind further exploring structural analysis of migmatites is to find field criteria for melt connectivity, continuity of the solid framework, and to identify the poten- tial mechanisms that control melt segregation and magma mo- bility at various scales in orogens. The structural evolution of rocks affected by partial melting is related to the evolution of their rheology which in turn is the result of a complex inter- play between melt production and melt segregation (Sawyer, 1994; Sawyer, 1999). These processes are evolving in time and space and, unfortunately, rocks accessible at the surface represent an inert end-product that, at best, preserved an in- complete record of their history. The structural analysis pro- posed here is based on the proportion and spatial distribution of the granitic component (leucosome) of migmatites and anatexites from the outcrop to the map scale. The continuity of the gneissic framework is thought to reflect a solid-domi- nated behavior at the time of partial melting; rocks with a d~smembered gneissic framework are considered as former magmas. Unfortunately, this approach is fraught with diffi- culties. One important assumption is that the granitic fraction represents a liquid and that the metamorphic host was domi- nantly a solid. The granitic fraction currently observed in a rock likely represents neither the total melt fraction nor the melt fraction that was present at a given time. Some of the gneissic paleosome or mesosome also might have contained some liquid which did not segregate as leucosomes and mel- anosomes. If we consider that the granitic fraction was in- deed a liquid, it is at best the one that has been trapped during cooling and/or exhumation of the rock. Another difficulty, is that melt connectivity and disruption of the solid framework are related to three-dimensional geometric thresholds and analysis in two dimensions is only valid under specific con- ditions (Handy, 1992). Consequently, structural analysis can only be taken as a guide. An interpretation of the spatial varia- tions of the geometric characteristics of the granitic fraction in terms of temporal evolution of magmatic processes requires

O. Vanderhaeghe: Melt Segregation, Pervasive Melt Migration and Magma Mobility

a careful a s s e ~ that needs to be oonfirmed by petrologic, geochemical and gcochronologic constraints (Nyrmm et al., 1995; Sawyer, 1998; Sawyer et al., 1999).

This paper is mainly based on field work conducted txinci- pally in the Shuswap Metamorphic Core Complex exhumed in the core of the Canadian Cordillera, and the W~stern" Variscan belt exposed in the French Massif Central (Burg and Vanderhazghe, 1993; Vanderhaeghe, 1999; Vanderhaeghe and Teyssier, 1997; Vanderhaeghe et al., 1999). In these examples, partial melting of thickened crust followed continental colli- siou and probably occurred with limited contribution of sub- duction-related mantle-derived magmas. These areas expose the transition zone between migmatites and granites and al-

215

low a discussion of the significance of melt migration and magma mobility at different scales and by various mecha- nisms during the evolution of these orogenic belts. A discus- sion of the particularities of each orogen is beyond the scope of this paper. The emphasis is put on common structural fea- tures interpreted to represent a record of melt and magma movement in these orogenic segments. The synthesis of these observations completed with published work on these orogens leads to a model that can hopefully provide some insights for the understanding of partial melting in orogens in general.

2. Structures in m i ~ a f l t e s and anatexites

pos~l oe a e~c~eic foli~m, b) ~tmtioo of Sm~tc in bo~lin's ~

the ~Ids. ~ : a, b, and c, Shmwap ~mUu~t~ic C~e

216 O. Vanderhaeghe: Melt Segregation, Pervasive Melt Migration and Magma Mobility

2.1 Metatexites and melt segregation

A metatexite is characterized by the presence of granitic veins and patches included in the foliation of a metamorphic host rock (Brown, 1973; Mehnert, 1968). The terminology is non genetic and the source of the granitic fraction is not specified. The continuous gneissic framework is taken as the distinctive characteristic of metatexites and it is interpreted to represent a solid skeleton accommodating deformation irrespective of the finite amount of partial melting. Indeed, metatexites in- clude rocks that have been affected by limited amount of par- tial melting as well as rocks affected by potentially larger fi- nite amount of partial melting but with segregation efficient enough to keep the partially molten rock below the transition to liquid-dominated rocks (Sawyer, 1994). These various types of metatexites can be distinguished on the basis of the char- acteristics of their gneissic framework.

The presence of granitic veins and pockets, superimposed on a transposed inherited compositional layering of sedimen- tary or magmatic origin, is an indication of incipient anatexis (figure 2). Partial melting of crustal protoliths is a grain-scale process, as illustrated by incipient melting in experiments, localized at grain boundaries and in between reactant phases (Mehnert et al.; 1973; Rushmer, 1996). Textures of partial melting at the grain scale in natural samples are only described in a few studies (Bouchez et al., 1992; Rosenberg and Riller, 2000; Watt et al., 2000). The melt origin of leucosomes is inferred from their granitic composition, the presence of restites or xenocrysts and by their magmatic texture includ- ing euhedral crystals, myrmekites and coarse-grained plastic strain free interfmgered minerals (Henkes and Johannes, 1981; Johannes, 1988; Mehnert, 1968; Sawyer, 1999; Vernon and Collins, 1988). Localization of granitic veins in specific struc- tural sites such as the foliation plane, axial planes of folds, pressure shadows around porphyroclasts, shear zones and boudins' necks is ubiquitous (Brown and Rushmer, 1997; Burg and Vanderhaeghe, 1993; Collins and Sawyer, 1996; Hand and Dirks, 1992; Vanderhaeghe and Teyssier, 1997; Vanderhaeghe et al., 1999; Williams et al., 1995). These fea- tures suggest that segregation of the melt occurred through the rock by porous flow from the site of melting (interfaces between reactant phases) to a structurally-controlled nearby dilatant site (figure 2). In a deforming rock the geometry and connectivity of the melt at the grain scale is probably con- trolled by the shape and size distribution of grains and their deformation mechanisms whereas melt migration beyond the grain scale is dominantly controlled by the presence of inher- ited rock anisotropy and heterogeneous ductile deformation

Fig. 3 : Photographs of metatexites with a synmigmatitic layering, a) Metatexite with alternations of leucosome/melanosome developed on a meter-scale transposition foliation, b) leucosomes separated by melanosomes and alternating with mesosomes, c) synmigmatitic layering affected by com- plex folds with folded and cross-crazing leucosomes, d) Synmigmatitic lay- ering with boudinaged amphibolite layer and concentration of the leucosomes in the boudins' neck. Location • Shuswap Metamorphic Core Complex, Canadian Cordillera

O. Vanderhaeghe: Melt Segregation, Pervasive Melt Migration and Magma Mobility 217

(Brown et al., 1999). The heterogeneity in the inherited com- positional layering is associated to variations of fertility and thus to variations in the amount of melting at the outcrop- scale. However, the abundance of leucosome may preferen- tially reflect accumulation of melt in dilatant sites created by heterogeneous deformation of the most competent layers rather than fertifity of the host layer (figure 2).

The development of a synmigmafitic layering, underlined

I 4 : Photographs of synmigmafitic way-u~ ~ a) ~ -

ing granitic pockets with drags of foliation on each sides of a . pocket in~preted as reflecting channelized flow of the melt. b) Right way up cau- liflower ~ in a metapelific meu~xite (Je~a-Plen~ Burg f~ scale), c) Asymmetric vein cluster and branching fractures emmmi in amphilmlite layer. The way-up is to the fight of the photo. ~ : Sbus~ap Meta- morphic Core Complex, C4madlan ~ except for a), Val~c.~m F1ench Massif Central.

by regular alternations of continuous centimeter- to meter- thick granitic and mesosome layers, Substituted to the inher- ited compositional layering, is interpreted as the result of com- bined segregation, intrusion and transpositicm during defor- mation of the pa~daliy m o ~ n rock (figure 3). In this case, the mesosome could represent either a l~tleosame or a neosome in which the solid and liquid remained unsegregated (Brown, 1973; Johannes, 1988, Mehnert, 1968). In extreme cases, ac- tive segregation of melt from a metatexite could lead to the formation of layers of restites characterized by an enrichment of residual minerals (Nyman et al., 1995). At the map scale, the synmigmatitic layering is typically concordant to major lithologic boundaries and is inferred to represent a coherent solid framework accommodating deformation (Burg and Vanderhseghe, 1993; Vanderhse~e and Teyssier, 1997). How- ever, in detail, metatexites show complex local deformation of the synmigmatitic layering. Multiple generations of dis- harmonic folds with curvilinear axes, referred to as viscous

218 O. Vanderhaeghe: Melt Segregation, Pervasive Melt Migration and Magma Mobility

of instabilities localized around heterogeneities in the deform- ing partially molten rock (Burg, 1999). Mutual cross-cutting relationships between structures and granitic veins indicate that deformation occurred in presence of a melt phase and suggest that transient disruption of the synmigmatitic layer- ing may have occurred (figure 3). This complex behavior of metatexites implies that tools of structural analysis should be used with caution, and precludes the determination of dis- tinct phases of deformation based solely on the orientation and style of structures.

Fig. 5 : Photographs of granitic vein network, a) Network of diffuse con- cordant and discordant veins in a granodioritic gneiss, b) Dike cross-cut- ting a metapsammite and composed by a restite and enclave-rich granite. Location : Shuswap Metamorphic Core Complex, Canadian Cordillera.

folds (McLellan, 1984), are common. They are attributed to complex fold propagation and amplification in a multilay- ered sequence with considerable competence contrast. Het- erogeneous deformation and local occurrence of superim- posed structures are interpreted to represent the development

2. 2 Pervasive melt migration, synmigmatitic way-up crite- ria, and formation of granitic veins networks

In many metatexites, leucosomes form a connected network of veins dominantly concordant but also in part discordant with respect to the synmigmatitic foliation (figure 4) (Vanderhaeghe, 1999; Weinberg, 1999). This type of feature may reflect intrusion of granite following, preferentially, the weak foliation plane of the rock. Textural continuity between concordant leucosomes and discordant veins suggests that the melt migrates from the synmigrnatitic foliation to dilatant sites. Discordant veins may form by melt migration after the dis- ruption of the synmigmatitic layering by boudinage or by development of shear zones.

Alternatively, other types of discordant veins show contacts with the host migmatite with gradual variations in the align- ment of mafic minerals, such as biotite or hornblende, from an orientation concordant to the synmigmatitic foliation to orientation parallel to the vein contacts (figure 4). This fea- ture is consistent with neither boudinage nor simple shear- ing. A possible interpretation is that the mafic minerals are reoriented and dragged according to the motion of the melt. This structure constitutes a synmigmatitic way-up criterion, assuming that the less viscous and less dense melt fraction migrates generally upward toward the Earth surface. In this case, the melt migrates by porous flow and localization of this flow leads to the development of a conduit with a vein or pipe geometry. Several other asymmetric structures, inter- preted as gravitational instabilities developed at the time liq- uid and solid coexisted in the rock (Burg, 1991), have been successfully applied as a structural tool in migmatitic terrains (Burg, 1991; Burg and Vanderhaeghe, 1993; Vanderhaeghe and Teyssier, 1997; Vanderhaeghe et al., 1999). Asynunetric vein clusters are interpreted as accumulation of melt under- neath an impermeable more competent layer. Cauliflower structures, characterized by granitic veins with a concordant bottom interface while the top interface is convoluted are in- terpreted as incipient diapiric features (figure 4). These are observed at the centimeter- to decameter-scale and indicate that melt segregation and migration can be locally controlled by the development of gravitational instabilities in response to melt and magma buoyancy.

Granite are commonly also tilling fractures from the centi- meter scale (fractured garnet porphyroclast) to the scale of a few hundred meters (granitic dikes cross-cutting lithologic

O. Vanderhaeghe: Melt Segregation, Pervasive Melt Migration and Magma Mobility 219

f

~i~iiiii ii ~ ~i -

~iii~ii!~., ~ ............... '~ i i~ ' -~ ""

220

Fig. 6 (previous page): Photographs of diatexites, a) Biotite schlieren ma- fic enclaves and granitic veins underlining synmigmatitic layering or mag- marie foliation, b) Biotite sehlieren marie enclaves and granitic veins un- derlining synmigmatitic layering or magnmtic foliation, c) Shear zone at a metatexite/diatexite contact, d) Alignment of cordierite and biotite sehiieren underlining synmigmatitic layering or magrnatic foliation, e) Dexameter- scale pocket of homogeneous granite in a diatexite. Locations : Shuswap Metamorphic Core Complex, Canadian Cordillera except for b) and d) Varisean, French Massif Central.

o. Vanderhaeghe: Melt Segregation, Pervasive Melt Migration and Magma Mobility

Vanderhaeghe et al., 1999).

contacts and regional fabric). Dikes are classically filled by pegmatitic or aplitic granite, but in some instances, they are filled with heterogeneous granite, rich in enclaves that are not comparable to the host rock, suggesting that an enclave- rich magma could migrate through the dike (figure 5). These various types of veins concordant and discordant to the re- gional foliation form networks structurally connect ing diatexites to leucogranites and are thus interpreted as drain- age networks for melt at the scale of the orogenic crust (Brown and Solar, 1998; Scalllet et al., 1995; Vanderhaeghe, 1999; Weinberg and Searle, 1998).

3. Melt segregation, pervasive melt migration and magma mobility from pores to orogens : a model

The generic model proposed here depicts the impact o f par- tial melting at different stages of orogenic evolution (Figure 7). It emphasizes the main structural observations reported above gathered from studies conducted by the author in oro- genic segments such as the western Variscan belt exposed in the French Massif Central and the hinterland of the Canadian Cordillera (Burg and Vanderhaeghe, 1993; Vanderhaeghe, 1999; Vanderhaeghe and Teyssier, 1997; Vanderhaeghe et al., 1999). It also takes into account data described in the fitera- ture on similar types o f large convergent orogens (e.g. Barbey et al., 1996; Brown and Solar, 1998; Sawyer, 1998; Sawyer et al., 1999; Scaillet et al., 1995; Weinberg and Searle, 1998).

The continental wedge amalgamated during orogenic evo- lution is typically highly heterogeneous and composed of a

2. 3 Diatexites and magma mobility 1. Early stage of crustal thickening

In contrast to metatexites, diatexites are dominated by the gra- nitic matrix (Brown, 1973; Burg and Vanderhaeghe, 1993; Mehnert, 1968; Sawyer, 1994) and contain a variety of en- claves including marie selvages representing disrupted lay- ers of melanosomes, resisters which correspond to refractory layers, and metatexites (Mehnert, 1968). The transition from gneiss-dominated metatexites to granite-dominated diatexites is correlated to the transition from solid to liquid marked by the disruption of the solid framework (figure 6). The orienta- tion o f discontinuous layers o f resisters and metatectic gneisses, selvages o f marie minerals, all enclosed in the gra- nitic matrix delineate a magmatic fabric together with mag- matic phenocrysts (Brown, 1973; Burg and Vanderhaeghe, 1993; Sawyer, 1998). In detail, complex deformation is es- sentially localized in the vicinity of enclaves that behave as rigid inclusions. Mutual cross-cutting relationships between granitic veins and structures suggest local mingling of coeval crystal mushes during deformation. The presence in the gra- nitic matrix of solid particles, from the size o f a crystal to few hundred meter large enclaves, indicates that sealing was not totally efficient. On the other hand, the presence of outcrop scale bodies of homogeneous granites within heterogeneous diatexites suggests local intrusion and accumulation of melt and indicate that some segregation occured between the melt and the sofid within diatexites (Sawyer, 1994).

The regional pattern of the magmatic fabric of diatexites is relatively simple and generally delineates the metatexite- d ia tex i te t rans i t ion ( B u r g and Vande rhaeghe , 1993; Vanderhaeghe et al., 1999). Diatexites are preferentially ex- posed in the core of mainly immature domes, suggesting that they behaved as a magma that rose by buoyant upwelling and deformation of the surrounding metatexites. At the map scale, diatexites commonly contain a large proportion of more ho- mogeneous granite bodies (Burg and Vanderhaeghe, 1993;

2. Prolonged crustal thickening

3. Orogenic col lapse leucogranltes

Fig. 7 : Model of partial melting during orogenic evolution following a period of crustal thickening. Isotherms 400 °C and 750 °C indicate the approximate location of the brittle-ductile transition and the solidus of crustal rocks, respectively. The large black arrows on the right side depicts subduc- tion which is responsible for downward advection of isotherms. 1 ) The thick- ened orogenic cruist is affected by incipient melting, increase in tempera- ture in the zone of thickened crust is assumed to be related to radioactive decay. 2) Partial melting of the orogenic crust is leading to the generation of an anatectic layer and to pervasive melt migration through a network of granitic veins connecting diatexites to granites emplaced at a higher sm¢* turai level. Arrows pointing upward indicate the sense of melt migration and arrows pointing downward indicate the sense of settling particles, d) Exhumation and crystalliT~tion of a partially molten crust occurs in many examples during orogenic collapse accommodated by normal faulting and extension of the upper crust and by ductile flow of the lower crust.

O. Vanderhaegbe: Melt Segregation, Pervasive Melt Migration and Magma Mobility

variety of lithologies. The compositional layering of these rocks resulting from transposition of inherited bedding or mag- matic banding probably acts as a guide for partial melting, some layers being more fertile with respect to partial melting than others. Radioactive decay is a potential cause for the increase in temperature within the zone of thickened orogenic crust (Henry et al., 1997; Huena et al., 1998). During conver- gence, incipient melting is thus potentially controlled by the fertility and the radioactive content of the orogenic wedge (figure 7a).

Further melting of the orogenic wedge (Figure To) leads to the generation of a mid-crustal layer of diatexites character- ized by the development of immature domes (Brown and So- lar, 1998; Burg and Vanderhaeghe, 1993; Vauderhaeghe et al., 1999). The presence of a large volume of enclaves in diatexites suggests that settling of solid particles is not totally efficient. The emplacement of homogeneous granitic rocks at a higher structural level than heterogeneous diatexites re- flects melt migration towards the surface (figure To). This migration appears to be achieved by the formation of a net- work of granitic veins permeating the transition zone between diatexites and granites (Sawyer, 1998; Scaillet et al., 1995; Vanderhaeghe, 1999; Weinberg and Searle, 1998). The geo- metric characteristics of the granitic network are controlled by regional deformation and by competency contrast between the melt and host rock. Granitic veins are dominantly local- ized within the foliation plane which acts as a mechanical guide. Accumulation of melt in these veins leads to a local build up of buoyancy which may lead to formation of throngh- going fractures, or diapir/c instabilities. Motion of the less dense and less viscous melt and magma are thus primarily driven by the pressure gradient resulting from the interplay between the gravity field and local dilation related to hetero- geneous deformation (Brown et al., 1995; Burg and Vanderhaeghe, 1993; Sawyer, 1994; Wickham, 1987).

Efficient melt segregation from an anatectic source with a low-melt fraction potentially leads to the generation of ho- mogeneous granites identified in orogenic crust as residuum- free leucogranites characterized by their peraluminous com- position (Barbarin, 1990; Barbey et al., 1996; Chappell and White, 1974; Inger and Harris, 1993; Sawyer, 1991; Sawyer, 1998). Diatexites or heterogeneous auatectic granites rich in enclaves and restites are probably generated by large amount of melting without successful segregation (Sawyer, 1994).

Data from crustal xenofiths of the Variscan lowermost crust (beneath the anatectic middle crust) indicate a refractory com- position (Costa and Rey, 1995; Leyreloup et al., 1977; Vielzeuf et al., 1990). This feature is intepreted as the result of crustal scale chemical differentiation (Clemens, 1990; Costa and Rey, 1995; Rudnick and Fountain, 1995; Sawyer, 1994; Wyllie, 1988) and is potentially related to redistribution of melt and magma. On the other hand, redistribution of melt and magma from the grain to the crustal scale with ultimate generation of a mid-crustal layer dominated by diatexites and concentra- tion of laccoliths of leucogranites at higher structural level probably leads to a new theological layering of the crust (Vanderhaeghe and Teyssier, in press-a). This theologic lay-

221

ering probably influences the behavior of the continental crust during orogenic collapse accommodated by brittle extension of the upper crust and ductile flow of the mid- to lower crust (figure 7c).

Aknowledgesmmts. This work is a synthesis of projects conducted in colla- boration with a number of colleagues including among others Patrick Ledru, Jean-Pierre Burg, Christian Teyssier, Patrice Rey to quote only a few. The manuscript benefited from the conmmnts of Jean-Louis Vigneresse, Pierre Barhey, Michel Cuney and Joseph Bmraud. The acute reviews of Tracy Rushmer and Roberto Weinherg, helped to considerably reduce the length and complexity of this manuscript, hopefiflly making this paper easier to absorb. Mike Brown is also thanked for a receptive editorial handling.

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