Crustal Anatexis and Granite Petrogenesis During Low-pressure Regional Metamorphism: The Trois Seigneurs Massif, Pyrenees, France |
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| Authors: | WICKHAM STEPHEN M |
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| Institution: | Department of Earth Sciences, University of Cambridge Cambridge CB2 3EQ |
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| Abstract: | Detailed mapping of Hercynian basement rocks exposed in theTrois Seigneurs Massif, Pyrenees, France, has demonstrated acontinuous metamorphic sequence developed in Palaeozoic peliticsediments, ranging from chlorite-grade phyllites, through andalusiteand sillimanite mica schists to a zone of migmatites and ultimatelya heterogeneous, peraluminous, biotite- and cordiente-bearinggranitoid (ranging in composition from biotite granite to quartzdiorite) at the deepest tectonic levels exposed. In additionto this ‘deep’ pluton, a syn-metamorphic leucogranitesuite forms pods and sills within the migmatites and mica schistsand a post-metamorphic, homogeneous biotite granodiorite intrudes(and superimposes a contact aureole on) the metasediments. Despitepost-metamorphic deformations, it is clear that the small ({smalltilde} 3 km) separation of low- and high-grade rocks impliesthe existence of very high temperature gradients (80–100?C km –1) during Hercynian metamorphism. Extensive meltingoccurred at {small tilde} 700 ?C at 10–12 km depth, indicatedby the metamorphic mineral assemblages and metamorphic reactionsoccurring in the mica schists. Whole rock XRF analyses of 50 rock samples, including all themain lithologies, indicate that leucogranite compositions areuniform and identical to those of migmatite leucosomes; theyare also close to the major-element composition of experimentallygenerated partial melts of pelitic rocks from the Trois SeigneursMassif. Taken with field relationships, this implies that allleucogranites were generated by partial fusion of pelitic material(< 40 wt. per cent) from the metamorphic sequence, with rapidremoval of the melt by segregation and intrusion to higher structurallevels. The deep biotite granite was probably generated by partialmelting and homogenisation of the same source material, withthe addition of a small magmatic component that was not derivedlocally from the pelites. The late granodiorite was not generatedby anatexis of pelitic material as observed in the metamorphicsequence, and was probably derived by melting of the lower crustat deeper levels than any contemporary exposure of Hercynianbasement in the Pyrenees. Petrological analysis of the metamorphic sequence suggests thatwater activity was externally buffered to high values throughoutthe ‘high-level’ anatexis observed in the TroisSeigneurs sequence. Evidence for this is provided by metacarbonateand metapelite mineral equilibria, by the sequence of metamorphicisograds and by their sharp definition. Moreover, ‘wet’melting conditions are required in order to generate the observedlarge quantities(> 40 wt. per cent) of granitic melt frompelitic material over the small (< 30 ?C) temperature increaseimplied by the section through the migmatite zone. Anatexisof pelitic metasediment was thus promoted by an influx of hydrousfluid into the melting zone. Stable-isotope studies suggestthat this influx was derived from the ground surface, allowingmelts to be continuously saturated as they were generated, andimplying that groundwater infiltration was primarily responsiblefor large-scale anatexis of metasediment at such shallow depths. |
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