Chapter 7

Migmatitic Gabbros From a Shallow-Level Metamorphic Contact Aureole, Fuerteventura Basal Complex, Canary Islands: Role of Deformation in Melt Segregation

Alice HOBSON, Fran~ois BUSSY and Jean HERNANDEZ lnstitut de Mineralogie et Phrographie, Universite de Lausanne, BFSH2, CH-I015 Lausanne, Switzerland

Key words: gabbro, partial melting, migmatite, melt segregation, microstructures

Abstract: Partial melting and deformation-assisted melt segregation, resulting in migmatization, has occurred in a shallow-level gabbroic metamorphic contact aureole, in the roots of a volcanic edifice. Melt segregated into small tension gashes and larger conjugate veins leading to unusual "zebra" and "striped" structures. In areas preserved from deformation, melt did not segregate and froze as smallleucocratic pods. Geometry of the network of leucosome veins allows reconstruction of a transtensional stress field at time of melting; which can be related to large-scale regional tectonic activity. Mass transfer processes resulting in the stromatic structures involved small-scale melt segregation, input of allochtonous melts and pervasive circulation of high-temperature fluids. Deformation triggered and enhanced these processes by opening fractures into which the melts could migrate, and by promoting the percolation of fluids in the contact aureole.

1. INTRODUCTION

Migmatites result from the partial melting of a protolith and the subsequent segregation of this melt into dilatant structures [Brown et al., 1995]. A migmatite is the sum of a light-colored component or leucosome, which represents a melt product ("frozen" melt or cumulate product), and of a dark-colored component, or meianosome, which represents the refractory residuum, or restite [see Mehnert, 1968; Brown, 1973 and Ashworth, 1985,

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210 A. HOBSON et al.: Chapter 7

for reviews of the nomenclature of migmatites. Terminology used in this chapter follows Ashworth, 1985]. Segregation of the melt into layers, pods, or veins, gives migmatites their characteristic banded or patchy structure. Commonly, melt segregation is deformation-assisted [e.g., Stevenson, 1989; Sawyer, 1991; 1994] and microstructures often record evidence of shearing.

Interest in migmatites lies in the fact that during the process of migmatization, a melt of quartzo-feldspathic or feldspathic composition is produced. Therefore, these rocks have been regarded as a possible source for dioritic to granitic or trondhjemitic-tonalitic magmas. If sufficient melt is produced and is allowed to segregate from its residuum and escape from the source, then a pluton may form [Sawyer, 1996]. Migmatites are found in various geological settings; favourable conditions for their formation are commonly obtained in the middle to lower crust, in the amphibolite to granulite facies. In most described examples, migmatites were formed by partial melting and deformation of felsic protoliths [e.g., Brown, 1983; Barr, 1985; Tracy, 1985]. Anatexis of mafic rocks is less common; most cases relate to the migmatization of tholeiitic rocks under amphibolite facies conditions in orogenic belts [e.g., Sorensen, 1988; Sawyer, 1991; Williams et at., 1995] or oceanic shear zones [e.g., Flagler and Spray, 1991]. However, a few examples of low pressure migmatization in oceanic crust or related units have been documented [Bedard, 1991; Mevel, 1988].

Fuerteventura offers a unique example of migmatization of gabbroic rocks at low pressure and high temperature in the basement of an ocean­island volcano. The main interest of the Fuerteventura migmatites lies in their structures, which record the initial stages of melt formation, segregation and crystallization, and the stress field in the contact aureole during metamorphism. In other words, they offer a snapshot of the anatexis processes, without subsequent deformation.

These rocks have been described in Hobson et al. [1998]. In this chapter, we shall focus on the different facies of these rocks in the field and on the textural and mineralogical aspects of melt formation and segregation.

2. TECTONIC AND GEOLOGICAL SETTING

Fuerteventura (Canary Islands) is a volcanic ocean-island, which lies 100 km off the north-western coast of Morocco (Figure 1), parallel to the African passive margin. It has been built by magmatic activity between 25 my and historical times [Cantagrel et at., 1993 and references therein], and lies on Jurassic oceanic floor [Roeser, 1982; Roest et at., 1992]. Plutonic rocks and an overturned series of Jurassic to Tertiary (volcano)-sedimentary rocks, all cross-cut by abundant dikes, crop out in the central western part of

7. Migmatitic Gabbros: Role of Deformation in Melt Segregation 211

the island. This association is called the Basal Complex (BC) [Stillman et ai., 1975] (Figure 1). It has been suggested [Le Bas et at., 1986; Javoy et al., 1986; Stillman, 1987; Ancochea et at., 1996] that the plutonic rocks could represent upper parts of magma chambers below the Miocene volcanoes of Fuerteventura, and that the dikes were the conduits to the vents.

The main outcrop of the BC (the Betancuria Massif [Fuster et at., 1968; Gastesi Bascufiana, 1969]) (Figure 1), hosts five separate intrusions [Le Bas et at., 1986], each accompanied by numerous dikes. These intrusions were emplaced in a regional, WNW-ESE extensional setting, which determined the NNE-SSW alignment of volcanic centers, the geometry of the intrusions, the orientation of the dikes and the internal structure of the intrusions [Stillman et at., 1975; Feraud et al., 1985; Stillman, 1987; Hobson et at., 1998]. WNW-ESE extension could have resulted from the Alpine NNE­SSW compression, which affected the eastern central Atlantic and north­westeru Africa (e.g., Hobson et al. [1998] and references therein).

The extensional tectonic setting of Fuerteventura is similar to that observed in mid-ocean ridges. Such environments are characterized by net extension perpendicular to the rift axis; extension is accommodated by purely tensional faults, as well as by normal and strike-slip faulting in a tension-shear regime [Bergerat et ai., 1990; Geoffroy and Angeiier, 1995]. As a consequence, magma intrudes the crust along vertical tension fractures parallel to the rift axis, and along oblique tension-shear fractures.

Migmatites were formed by contact metamorphism of gabbroic host rocks during the emplacement of PX1, a younger gabbro-pyroxenite intrusion. Based on the parageneses of the migmatites and on the oceanic context, we suggest that migmatization took place at pressures of 200-300 MPa and temperatures of up to -1000°C, in the presence of high­temperature, alkaline- and possibly halogen-rich fluids [Hobson et at., 1998].

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Figure 1. Geological map of the central western part of the Betancuria Massif (Fuerteventura Basal Complex). The Tierra Mala Massif (TM) includes locally fenitized gabbros and pyroxenites, syenites, ijolites and carbonatites. The dikes are not shown for clarity as they intrude all other formations. Stereo graphic projections represent the traces of the planes of the conjugate veins of the migmatites, as well as the pole of al.

7. Migmatitic Gabbros: Role of Deformation in Melt Segregation 213

3. THE PXl INTRUSION AND ITS HOST ROCK

PXl is a NNE-SSW-elongated intrusion (Figure 1), formed of alternating, parallel horizons of gabbro and pyroxenite. Magmatic banding results from compositional or granulometric layering, and/or from preferred crystal orientation. Direction of banding is -N20oE throughout the intrusion (Figure 2); dip varies between 700WNW and 700ESE. Horizontal crescumulates in some horizons reflect lateral differentiation, which implies vertical isotherms. PXl was emplaced as multiple, nested, dike-like bodies, which intruded and melted a composite host rock, the Tierra Mala Massif (TM). This massif consists of basic dikes, gabbros, syenites and pyroxenites. Magmatic breccias occur locally. They are formed of angular pyroxenite and gabbro fragments up to 10 em in diameter, in a gabbroic to syenitic matrix. All the lithologies in TM are locally fenitized by small intrusions of carbonatites and ijolites (Figure 1). The main proto lith of the migmatites is coarse-grained gabbro consisting of Ti-augite, An4o-6o plagioclase, olivine, magnetite, ilmenite, apatite, titanite and epidote.

The NNE-SSW regional compression resulted in the opening of tension fractures as well as of large-scale, conjugate tension-shear faults (Figure 2). The tension fractures are parallel to the main compression stress crl, and are situated in the plane of the acute angle bisector of the conjugate tension­shear faults. PX1, as well as all the pre- and post-PXl NNE-SSW dikes in the BC, were emplaced along the NNE-SSW tensional fractures. In total, more than 30 km of lithosphere has been added [Le Bas et aI., 1986] in this zone of WNW-ESE extension. The conjugate tension-shear fractures determined the geometry of the northern (respectively southern) extremity of PXl, as well as the orientation of PX2, a pyroxenite intrusion emplaced after PXl. Strike-slip movements along these conjugate tension-shear faults probably accommodated strain caused by emplacement of PXl. Late Oligocene to Early Miocene NW-SE and NE-SW shear zones have been described elsewhere in the Fuerteventura BC [Fernandez et ai., 1997]. These zones accommodated strike-slip movements, which are consistent with those inferred for PXl emplacement.

214 A. HOBSON et al.: Chapter 7

t

/ I, ,

Figure 2. (a) Schematic representation of PXl intrusion with orientation of main principal stresses. Extension took place perpendicular to a3 and the large-scale, conjugate, tension­shear fractures controlled geometry of Nand S extremities of the pluton. (b) Strain ellipse with orientation of main principal stresses and conjugate tension-shear fractures. (c) Rose diagram of orientation of the compositional and granulometric banding in the PXl intrusion. All the structures are oriented -N20°E.

3.1 The contact aureole

The migmatized zone at the contact between PXl and TM is characterized by deformed, fine-grained rocks, with abundant leucocratic veins and fractures, in a darker matrix. Therefore, this zone stands out well in the field against the coarse-grained rocks of PXl and TM. Partial melting

7. Migmatitic Gabbros: Role of Deformation in Melt Segregation 215

affected the host lithologies in a zone 200-300 meters wide around PX1, and transition to unmelted TM rocks occurs over a few meters. Thermal metamorphism is recorded up to 1 km beyond the contact aureole [Stillman, 1987]. Deformation seems to have been restricted to the partially molten zone and was heterogeneous. TM rocks with no plagioclase (i.e. pyroxenites) did not melt.

The migmatites have several macroscopically distinct structures which reflect the various rheologies of the partly molten rocks (Figure 3):

3.1.1 The "zebra" migmatites (Figure 3a-b)

This structure is characterized by extremely abundant, small (1-2 mm wide and 5-10 mm long), usually sigmoid veinlets, in a dark, fine-grained matrix. Larger veins (2-5 mm wide and up to tens of centimetres long) can also occur, they usually form conjugate sets with the veinlets running parallel to the acute angle bisector plane of the conjugate veins. Locally, one of the "conjugate" sets of veins can be missing. Veins are usually bordered by dark selvages where plagioclase is absent.

3.1.2 The striped migmatites (Figure 3b, central zone)

In these migmatites, 1-2 mm wide and 5-10 cm lop.g lensoid veins are all parallel and usually densely packed. They are separated by plagioclase-free melanosomes. This type of migmatite occurs as a distinct rock type but also as restricted areas (2-3 cm wide and several tens of cm long) within zebra migmatites (Figure 3b). They are interpreted as facies of enhanced deformation of the zebra migmatites.

Some TM magmatic breccias (see description above) experienced selective migmatization during contact metamorphism (Figure 3c). Anatexis affected the gabbroic matrix and clasts, whereas the more refractory, plagioclase-free pyroxenite clasts, were preserved. Brecciation did not occur during migmatization; the gabbroic matrix was never a mobilized "migma" that disrupted a pyroxenite, but it was migmatized at the same time as the clasts. This is evidenced by 1) the coherent orientation of leucosomes throughout the migmatized matrix, clasts and surrounding migmatites, 2) occurrence of leucosomes which cross over from matrix to gabbro clasts, 3) the perfect preservation of the delicate zebra structure of the matrix, not expected to survive in a mobilized migma, and 4) by the fine and regular leucocratic rim that occurs around the fragments and which indicates small­scale melt segregation.

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Figure 3. Structures of the Fuerteventura migmatites (see text for definitions). (a) Zebra structure, scale bar I cm. The unconnected sigmoid veinlets and conjugate veins are illustrated. Geometry of the stress field is represented, Ci2 is given by the intersection of the conjugate veins, Cil is parallel to the acute angle bisector of the conjugate veins and is parallel to the small sigmoid veins. (b) Zebra structure with no conjugate veins, diameter of lens cap 50 mm. The more densely veined zone in the center is interpreted as a zone of enhanced deformation, it is characterized by a higher proportion of leucosome, longer veins which form the striped structure and a more ductile deformation style. (c) Migmatized breccia, scale as in (b). Clasts of TM rocks ranging from migmatized to non migmatized (mainly pyroxenites) in a migmatitic matrix. (d) "Patchy" structure, scale as in (b). Partly molten dolerite preserved from deformation, leu co somes are in situ "pods" (lower part of picture) grading to a migmalite in which melt segregated into veins due to deformation (upper part of picture).

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Meter-sized syenitic bodies also crop out in the aureole, and different rheological behaviors can be observed at syenite/migrnatite contacts (Figure 4). Low viscosity contrasts between the syenite and migmatite are illustrated by ductile contacts (Figure 4a). In this case the migrnatite contained enough melt to deform ductilely. High viscosity contrasts are illustrated by the brittle behavior of the migmatite, in which larger tension fractures were opened (Figure 4b). In this case, the migmatite did not contain sufficient melt to deform ductilely. In both cases the syenites stayed ductile longer and contributed to the leucosomes of the migmatites, which were consequently open systems.

3.2 Microstructures

The association of magmatic textures in the leucosomes, and typical metamorphic textures in the melanosomes, is the best evidence that the Fuerteventura migmatites experienced partial melting. Melanosomes consist of recrystallized, fine-grained «lOOllm) augite, with some titanite, magnetite, ilmenite and apatite and variable (0-30 vol. %) amounts of plagioclase (An2()'30)' Microstructures are granoblastic polygonal, with triple-point contacts between grains. Elongate crystals sometimes define a preferred orientation. Large (1-2 mm) porphyroclasts of clinopyroxene are locally present. Grain-size reduction and polygonization are clearly the result of thermal metamorphism.

Leucosomes consist of 60-85 vol. % plagioclase (An2o-30) with some magmatic augite, magnetite, ilmenite and apatite. Plagioclase and augite are large (mm-sized), euhedral, and often form a comb texture (Figure 5). Augite also crystallized as polygonal grains along the vein walls. Kaersutite and/or phlogopite occur with the augite in both the leucosomes and melanosomes of some hydrated parageneses. Replacement patches of Na, K and Ba-rich feldspar are found in plagioclase of the leucosomes and melanosomes. These are interpreted as evidence for the circulation of high­temperature alkaline fluids in the contact aureole [Hobson et al., 1998].

The comb textures were formed by the inwards epitaxial crystallization of plagioclase and clinopyroxene. The large size of crystals in the leucosome compared to those in the melanosome suggest undercooling and low nucleation rates in the melt. Crystallization of vein material started during deformation as is locally evidenced by comb textures oblique to the vein walls. Symmetrical orientation of minerals in conjugate pairs of veins (see inset Figure 3a) confirm that these veins were opened along conjugate shear planes. Deformation ceased before complete solidification of leucosomes, as inferred from the absence of solid-state deformation microstructures.

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Figure 4. Migmatite/syenite contacts illustrating the different rheological behaviors of migmatites. (a) Ductile/ductile relationship with lobate contacts between the migmatite and syenite (b) Fragile/ductile relationship with apophyses of the syenites penetrating the fractures of the migmatites; this illustrates the input of allochtonous melt into the migmatites (see text).

In a given sample, clinopyroxenes have slightly different compositions in the veins and restite, due to small-scale equilibrium (re)crystallization. They are richer in Ca, Mg, Fe3+ in the restite and in AI, Ti, Fe2+, Na in the veins [Hobson et at., 1998]. On the other hand, plagioclase composition is homogeneous. This, and the fact that plagioclase has been entirely removed from some restites, suggests extensive melting of this mineral throughout the migmatites.

7. Migmatitic Gabbros: Role of Deformation in Melt Segregation 219

3.3 Stress regime in the contact aureole

Deformation in the PXl contact aureole can be directly related to the regional NNE-SSW compressional setting. Geometry of leucosomes in the migmatites reflect the orientation of the related stress field at the outcrop scale (Figure 1 and 3a). The small sigmoid veins, parallel to the PXI-TM contact, represent the tensional fractures, parallel to crl. The larger conjugate veins are the same tension-shear fractures that are found at the extremities of PXl. Direction of the intermediate compressional stress, cr2, is given by the intersection of the planes of the conjugate veins. crl and cr2 define a sub-vertical plane, parallel to the PXlITM contact. Dip of crl varies between horizontal and -400 (Figure 1). The tensional stress, cr3, is always horizontal, and perpendicular to the contact between PXl and TM (Figure 1 and 3a). This stress field geometry is characteristic of a context of vertical dike emplacement in a transtensional stress regime [e.g., Geoffroy and Angelier, 1995], which is in agreement with the multiple dike-like intrusion of PXl. Local variations of crl dip are controlled by magma injection (processes) and reflect shearing along the PXlITM contact. At the northern (respectively southern) extremity of PXl, orientation of the leucosomes reflect strike-slip movements along the tension-shear faults. Along such faults one of the directions of shear (dextral or sinistral) is expected to dominate and it is in these areas that migmatites with only one set of "conjugate" veins have been observed.

4. PARTIAL MELTING AND MECHANISMS OF MELT SEGREGATION

Anatexis of gabbros in Fuerteventura resulted from the unusual conjunction of several parameters. The shallow-level intrusion of a gabbro is not expected to supply sufficient heat to trigger partial melting in a country rock of similar composition. But in Fuerteventura, the geological setting of focused and almost continuous magmatic activity over -5 my is probably the key factor which allowed partial melting of gabbros. High heat flow due to repeated mafic intrusions kept the TM rocks near their solidus temperatures. The extra heat provided by emplacement of PXI, associated with high­temperature fluids percolating through the contact aureole, were then sufficient to initiate partial melting [Hobson et al., 1998]. The presence of up to 40 vol.% neo-formed ferromagnesian minerals in the leucosome implies a substantial degree of partial melting of the proto lith. Melting occurred in the whole contact aureole, deformation on the other hand was partitioned and certain zones were completely shielded. This is evidenced by

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undeformed rocks with particular patchy textures representing in situ melts (Figure 3d). These rocks are characterized by neo-formed equigranular plagioclase that form small pods between a preserved framework of elongated, partially resorbed, primary clinopyroxene. They represent zones shielded from deformation, where the melts froze in situ. Transition to a typical stromatic migmatite, representing a zone of enhanced deformation, takes place over 10-15 cm. In these zones, the melt coalesced and migrated into tension veinlets, driven by pressure gradients related to the local stress field. Deformation and anatexis occurred more or less at the same time. At first sight, it may be surprising that the ascending PXl magma, which triggered partial melting, did not entirely accommodate deformation. This is probably related to the mechanism of PXl intrusion as multiple dikes. The earlier PXl dikes to ascend through the lithospheric fracture must have initiated partial melting of the country rocks. Due to their high solidus temperature relative to the host rock, they quickly behaved as solid, brittle bodies compared to the partially molten contact aureole, which then preferentially accommodated the ongoing regional deformation. Local exceptions are brittle deformation in PXl, at the contact between adjacent dikes. Therefore, although anatexis and deformation were simultaneous, they occurred independently. Deformation did not trigger anatexis but was the driving force for melt segregation and formation of migmatitic structures. Increasing pressure of the melt could also have played a role in the fracturing or widening of veins in the migmatites.

Mass transfer can take place by diffusion through a static fluid or by fluid (melt or metamorphic volatile phase) advection [Brown et ai., 1995]. Three mechanisms are invoked in Fuerteventura, (a) in situ small-scale melt segregation, (b) input of allochtonous melts and (c) mass-transfer through high-temperature fluid circulation.

4.1.1 In situ small-scale melt segregation

The small sigmoid veinlets which form the zebra structure (Figure 3a-b) are separated by thin (2-3 mm) melanosomes. They were low-pressure sites into which melt migrated. Segregation was local (at a mm-scale) and often incomplete as varying amounts of plagioclase often remain in the melanosome. According to Stevenson [1989], melt migration parallel to a3 will occur naturally because of instabilities of the small-scale melt redistribution. Dark selvages around the veinlets represent zones of complete plagioclase extraction. This mechanism of in situ small-scale melt segregation was not very efficient at extracting the melt. It is only with increasing deformation that more veins were opened and consequently more melt was extracted, ultimately leading to plagioclase-free melanosomes. In

7. Migmatitic Gabbros: Role of Deformation in Melt Segregation 221

other words, the higher the strain experienced by migmatites, the more complete the melt segregation.

Figure 5. Photomicrograph of a migmatite, field of view 5 mm, (a) plain polarized light, (b) crossed polars. Plagioclase forms a comb-like texture in the vein with some clinopyroxene, kaersutite and interstitial Fe-Ti oxides. Dark selvages can be seen on both sides of the vein; they consist of Ti-augite, kaersutite and Fe-Ti oxides. Increasing amounts of plagioclase are found in the melanosome, with distance away from the selvages.

222 A. HOBSON etal.: Chapter 7

4.1.2 Input of allochtonous melts

The volume of leucosome in the migmatites is often higher than expected for partial melting of a gabbro in a closed system, as illustrated by some of the large leucocratic veins. This, in addition to the geochemical composition of the migmatites [see Hobson et ai., 1998] evidences that melt external to the system was mobilized and introduced into the migmatites. Based on geochemical evidence [Hobson et at., 1998], the syenites and other alkaline rocks that were present in the contact aureole during metamorphism are the most likely source for these melts.

4.1.3 Mass-transfer through high-temperature fluid circulation

The pervasive chemical transformations of the country rock within the contact aureole [Hobson et al., 1998], leading to specific mineralogical and geochemical characteristics of the migmatites (e.g., the Ba- , Na- and K-rich replacement patches in the plagioclase), imply the circulation of alkali- and possibly halogen-rich fluids [Giere and Williams, 1992]. These fluids percolated through the contact aureole, possibly during the early stages of thermal metamorphism, favoring partial melting. They were definitely present during crystallization of the melts, as evidenced by enrichment in AI, Na, P, Sr, Ba, Nb, Y, and the REE of the migmatites compared to the TM gabbros, and by the composition of phlogopites. Presence of Ba-rich replacement patches in the feldspars imply that fluid circulation lasted until the late to post-solidus stages of contact metamorphism.

Origin of these fluids could be: (1) late magmatic fluids related to the carbonatites; (2) meteoric water heated by PX 1; (3) fluids released by the country rock during thermal metamorphism.

5. RHEOLOGY OF PARTLY MOLTEN ROCKS

The rheological behavior of rocks during contact metamorphism was dependent on the amount of melt present. Migmatites with the "zebra" structure" (Figure 3a-b) behaved brittlely. They are characterized by essentially unconnected tension veinlets and a few associated conjugate veins. They were cohesive enough to break and the mode of melt segregation was fracture-dominated [Brown and Rushmer, 1997]

With higher melt percentages, or in zones of enhanced deformation, migmatites behaved more ductilely. In such areas, the mode of melt segregation could have been shear-enhanced, where the melt is squeezed out of a deforming matrix (see Brown and Rushmer [1997] and references

7. Migmatitic Gabbros: Role of Deformation in Melt Segregation 223

therein). Plagioclase-free restites would result, as illustrated in the striped migmatites (Figure 3b).

6. DISCUSSION

Most documented cases of migmatization of mafic rocks relate to tholeiites metamorphosed under amphibolite facies conditions in orogenic belts or oceanic shear zones. Common features of such migmatites are quartz-bearing leucosomes of tonalitic to trondhjemitic composition and garnet-bearing residues produced under relatively high confining pressures above 800 MPa and temperatures around 750°C. Fluids often playa major role in lowering solidus temperatures and as means for mass transfer, especially for LIL-elements. Deformation is often essential to melt segregation [Sawyer, 1994; Vigneresse et al., 1991] and is thought to facilitate fluid circulation if present and, correlatively, partial melting.

Anatexis of gabbros in Fuerteventura took place under very different conditions in terms of initial rock composition and lithostatic pressure. In fact, these migmatites represent a rare example of low-pressure (200-300 MPa) partial melting of quartz-free rocks which led to unusual mineral assemblages with quartz-free melts and garnet- and orthopyroxene-free residua.

Migmatization resulted from the conjunction of high heat flow, fluid circulation and deformation according to the following scenario. Emplacement of the earlier PXI gabbro-pyroxenite dikes within the partly fenitized TM gabbros, resulted in partial melting of the host rock. Deformation related to a regional NNE-SSW compressional regime was absorbed by these partly molten rocks and resulted in the opening of tension veins and in the segregation of the melt into these veins. The estimated temperatures of 900-1000°C [Hobson et al., 1998] would probably not have been sufficient to induce a 50 vol. % melting of nepheline-normative gabbros with complete dissolution of the plagioclase phase. High temperature alkaline and probably halogen-rich fluids, present in the contact aureole at the beginning of PXI intrusion, were most probably a decisive factor in lowering the overall solidus temperature of the host rocks and in giving the migmatites their specific geochemical signature. In partly molten rocks which were shielded from deformation, the melts were not segregated but remained as in situ pods. Melt segregation was therefore deformation­induced. The regional stress field controlled the geometry of the leucosomes, most of these are parallel to the PXlITM contact. Discrete tension veinlets and minor associated conjugate veins were opened, as recorded in the zebra structures of the migmatites. Veins were low-pressure

224 A. HOBSON et al.: Chapter 7

sinks to which the melts migrated. In zones of enhanced deformation, melt segregation is facilitated and the proportion of leucosome increases; veins are connected and generally parallel, leading to the striped structures. Deformation continued during crystallization of leucosomes but ended before complete solidification. The role of deformation in melt segregation is clearly demonstrated. On the other hand its role in enhancing melt production is less clear. In the presence of fluids, deformation indirectly promotes partial melting by enhancing their circulation. Pressure gradients in anisotropic stress fields could also promote melting.

The volumetrically dominant small tension veins usually consist of leucosomes with up to 40 vol. % Fe-Mg minerals, as is expected from experimental work. The wider conjugate veins are generally much more feldspathic with as little as 5 vol. % Fe-Mg minerals. These veins are not considered to reflect the initial composition of partial melts, but that of fractionated liquids with a contribution from TM syenitic melts. Neo­crystallization of clinopyroxene from the partial melt is expected to start prior to liquid segregation, which would increase the molar proportion of feldspar in the residual liquid. After segregation of the latter, clinopyroxene saturation and crystallization along vein walls could further fractionate the plagioclase component in the vein.

The particular conjunction of factors which led to the low-pressure partial melting of nepheline-normative gabbros is not expected to occur frequently, nor at a large scale. Similar conditions could be imagined in other high heat flow contexts such as in mid-ocean ridges experiencing intra-oceanic deformation in the presence of fluids or, possibly, in some Large Igneous Provinces (LIPs) like oceanic basaltic plateaus, in which very high production rates of basalts might supply enough heat to melt rocks of similar composition.

7. CONCLUSION

The low pressure migmatization of nepheline-normative gabbros at Fuerteventura was possible because of the particular context of ocean-island magmatism, the presence of metasomatic fluids and of a regional deformation. The structures of the migmatites are exceptional in that they record the stress field which controlled emplacement of PXl, deformation, and the different stages of anatexis and melt segregation. Quartz-free leucosomes have preserved magmatic microstructures, which evidence their anatectic nature and epitaxial crystal growth. The fusion mechanisms of basic material at low pressure illustrated here, may potentially exist in other

7. Migmatitic Gabbros: Role of Deformation in Melt Segregation 225

areas, where the above-cited conditions are satisfied, but the preservation of magmatic textures as observed in Fuerteventura seems to be unusual.

ACKNOWLEDGMENTS

We would like to thank M. Brown for a thorough and stimulating review of this paper. We express our warmest thanks to B. Azambre, J.-L. Epard, R-P. Menot and C. Rosenberg for fruitful discussions. Field-work for A.H. and F.B. was supported by grants from the Swiss Academy of Natural Sciences and from the Friedlander Foundation (Stiftung Vulkaninstitut Immanuel Friedlander). Financial support for J.H. and F.B. by the Swiss FNRS (Fonds National pour la Recherche Scientifique) (grants n° 21-31098.91 and 21-45650.95) is greatly acknowledged.

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