Diatexite Migmatites Research Articles

Formation of Diatexite Migmatite and Granite Magma during Anatexis of Semi-pelitic Metasedimentary Rocks: an Example from St. Malo, France

Abstract Petrological and geochemical variations are used to investigate the formation of granite magma from diatexite migmatites derived from metasedimentary rocks of pelitic to greywacke composition at St. Malo, France. Anatexis occurred at relatively low temperatures and pressures (<800°C, 4–7 kbar), principally through muscovite dehydration melting. Biotite remained stable and serves as a tracer for the solid fraction during melt segregation. The degree of partial melting, calculated from modal mineralogy and reaction stoichiometry, was <40 vol. %. There is a continuous variation in texture, mineralogy and chemical composition in the diatexite migmatites. Mesocratic diatexite formed when metasedimentary rocks melted sufficiently to undergo bulk flow or magma flow, but did not experience significant melt–residuum separation. Mesocratic diatexite that underwent melt segregation during flow generated (1) melanocratic diatexites at the places where the melt fraction was removed, leaving behind a biotite and plagioclase residuum (enriched in TiO2, FeOT, MgO, CaO, Sc, Ni, Cr, V, Zr, Hf, Th, U and REE), and (2) a complementary leucocratic diatexite (enriched in SiO2, K2O and Rb) where the melt fraction accumulated. Leucocratic diatexite still contained 5–15 vol. % residual biotite (mg-number 40–44) and 10–20 vol. % residual plagioclase (An22). Anatectic granite magma developed from the leucodiatexite, first by further melt–residuum separation, then through fractional crystallization. Most biotite in the anatectic granite is magmatic (mg-number 18–22).

Criteria for the recognition of partial melting

Partial melting changes rocks from single phase (solid) to two phase (solid+melt) systems. The bulk viscosity decreases as the melt fraction increases and this effect raises the rate of deformation and heat transfer, as well as causing crustal differentiation. Therefore, it is important to be able to recognise which rocks have partially melted.Macroscopic textures provide the simplest criteria for recognising partial melting. Melting and deformation are generally synchronous, and when the melt fraction retained is low (<20%) metatexite migmatites are formed. Typically, these are morphologically complex because the melt fraction is squeezed out of the deforming matrix and collects in whatever dilatant sites are present. The presence of melanosome layers and patches provides the best evidence of where the melt formed, and the leucosomes where it collected. Diatexite migmatites can be easily recognised by the presence of a flow foliation, schlieren, enclaves and vein like leucosomes, and are evidence of a high melt fraction and pervasive partial melting. For the unusual case of melting without synchronous deformation, rounded neosome patches containing both the melt and solid fractions of the melt-producing reaction develop and, as the degree of melting increases these enlarge, to form diatexite migmatites. In both cases the characteristic feature is an increased grainsize and loss of pre-migmatization structures. Migmatite textures are robust, they survive later deformation well.Microscopic textures such as: (1) thin films of quartz, plagioclase and K-feldspar along brain boundaries that represent crystallized melt and, (2) melt-solid reaction textures, also provide good criteria for identifying partially melted rocks. However, these textures are fragile and easily destroyed by deformation. The identification of mineral assemblages from which melt-forming reactions can be inferred is another reliable critera for recognising partial melting, but post-migmatization rehydration in granulite terranes can destroy this evidence.Whole rock geochemistry can be used to model the partial melting process, but problems in identifying the palaeosome and an unmodified melt compositions can restrict its application. However, whole rock geochemistry coupled with good field based control, can be used to deduce what processes have occurred in a terrane where the rocks have partially melted.Variations in field appearance, texture and composition are, in large part a consequence of whether, or not, and when, the melt-fraction separated from the solid fraction.

Formation and Evolution of Granite Magmas During Crustal Reworking: the Significance of Diatexites

(Taylor & McLennan, 1985), the physical processes inA continuous section through reworked Archaean crust records the volved, particularly how granite magma is formed, remain generation of granitic magma and its subsequent development in the controversial (Miller et al., 1988; White & Chappell, Opatica subprovince in the Canadian shield. There, the transition 1990). A partially melted source can form a granitic from palaeosome to granite was a closed-system process through magma only if large volumes of melt are separated from intermediate stages of patch migmatite and diatexite. The average most of their residuum. Understanding this process is degree of partial melting was less than 30%, but the melt crucial to determining how crustal recycling and differfraction was redistributed within individual diatexite layers during entiation occurs. For example, melt–residuum separation deformation. Regions that lost melt became residual diatexites could be a nearly perfect, single-stage process at the enriched in TiO2, FeOT, MgO, CaO, Sc, Cr, Co, Sr, rare earth site of partial melting, as described for leucosomes in elements (REE) and high field strength elements (HSFE). Melt metatexite migmatites (Wickham, 1987a; Sawyer, 1991, accumulated to create diatexite magmas enriched in large ion 1994; Brown et al., 1995). Alternatively, the separation lithophile elements (LILE), but contaminated with residuum macould be an imperfect, multi-stage process that yields a terial. Such diatexite magmas are parental to granites found at magma with a large residual component, as is observed higher crustal levels in the terrane. Flow of the diatexite magma in in diatexite migmatites (Bea, 1991; Greenfield et al., response to deformation separated some of its residuum into schlieren. 1996; Sawyer, 1996). The metatexite model necessarily Parautochthonous plutons were created where ascending granitic produces leucocratic, residuum-free granites, such as magma locally ponded below impermeable layers and structures. those described by Le Fort (1981) and Montel et al. Magma left the anatectic region in dykes and lost its remaining residuum as it crystallized. Consequently, the allochthonous granite (1991), and maximizes the geochemical signature of magmas that rose through 20 km of crust to feed the highest level crustal differentiation. In contrast, the diatexite model plutons in this region are highly fractionated and essentially free of can produce residuum-rich granites, and so reduce the residuum. geochemical effect of crustal differentiation, as Chappell (1996) noted. How anatectic magmas formed in the deeper crust evolve to the granitic magmas emplaced in the upper crust is also poorly known, because few crustal sections

Rapid Variscan exhumation and the role of magma in core complex formation: southern Brittany metamorphic belt, France

ABSTRACT The high‐grade migmatitic core to the southern Brittany metamorphic belt has mineralogical and textural features that suggest high‐temperature decompression. The chronology of this decompression and subsequent cooling history have been constrained with 40Ar/39 Ar ages determined for multigrain concentrates of hornblende and muscovite prepared from amphibolite and late‐orogenic granite sheets within the migmatitic core, and from amphibolite of the structurally overlying unit. Three hornblende concentrates yield plateau isotope correlation ages of c. 303–298 Ma. Two muscovite concentrates record well‐defined plateau ages of c. 306–305 Ma. These ages are geologically significant and date the last cooling through temperatures required for intracrystalline retention of radiogenic argon. The concordancy of the hornblende and muscovite ages suggest rapid post‐metamorphic cooling. Extant geochronology and the new 40Ar/39Ar data suggest a minimum time‐integrated average cooling rate between c. 725 °C and c. 125 °C of c. 14 ± 4°C Ma‐1, although below 600 °C the data permit an infinitely fast rate of cooling. Mineral assemblages and reaction textures in diatexite migmatites suggest c. 4 kbar decompression at 800–750 °C. This must have pre‐dated the rapid cooling.Emplacement of two‐mica granites into the metamorphic belt occurred between 345 and 300 Ma. The youngest plutons were emplaced synkinematically along shallow‐dipping normal faults interpreted to be reactivated Eo‐Variscan thrusts. A penetrative, west‐plunging stretching lineation developed in these granites suggests that extension was orogen‐parallel. Extension was probably related to regional uplift and gravitational collapse of thermally weakened crust during constrictional (escape) tectonics in this narrow part of the Variscan orogen. This followed slab breakoff during the terminal stages of convergence between Gondwana and Laurasia; detachment may have been consequent upon a change in kinematics leading to dextral displacement within the orogen. Dextral ductile strike‐slip displacement was concentrated in granites emplaced synkinematically along the South Armorican Shear Zone. Rapid cooling is interpreted to have resulted from tectonic unroofing with emplacement of granite along decollement surfaces. The high‐grade migmatitic core of the southern Brittany metamorphic belt represents a type of metamorphic core complex formed during orogen‐parallel extensional unroofing and regional‐scale ductile flow.