Crustal Anatexis and Granite Petrogenesis During Low-pressure Regional Metamorphism: The Trois Seigneurs Massif, Pyrenees, France

• Published in: Journal of petrology 1987-02, Vol.28 (1), p.127-169
• Main Author: WICKHAM, S. M.
• Format: Article
• Language: English
• ISSN: 0022-3530; 1460-2415
• DOI: 10.1093/petrology/28.1.127

Detailed mapping of Hercynian basement rocks exposed in the Trois Seigneurs Massif, Pyrenees, France, has demonstrated a continuous metamorphic sequence developed in Palaeozoic pelitic sediments, ranging from chlorite-grade phyllites, through andalusite and sillimanite mica schists to a zone of migmatites and ultimately a heterogeneous, peraluminous, biotite- and cordiente-bearing granitoid (ranging in composition from biotite granite to quartz diorite) at the deepest tectonic levels exposed. In addition to this ‘deep’ pluton, a syn-metamorphic leucogranite suite forms pods and sills within the migmatites and mica schists and a post-metamorphic, homogeneous biotite granodiorite intrudes (and superimposes a contact aureole on) the metasediments. Despite post-metamorphic deformations, it is clear that the small (˜ 3 km) separation of low- and high-grade rocks implies the existence of very high temperature gradients (80–100 °C km −1) during Hercynian metamorphism. Extensive melting occurred at ˜ 700 °C at 10–12 km depth, indicated by the metamorphic mineral assemblages and metamorphic reactions occurring in the mica schists. Whole rock XRF analyses of 50 rock samples, including all the main lithologies, indicate that leucogranite compositions are uniform and identical to those of migmatite leucosomes; they are also close to the major-element composition of experimentally generated partial melts of pelitic rocks from the Trois Seigneurs Massif. Taken with field relationships, this implies that all leucogranites were generated by partial fusion of pelitic material (< 40 wt. per cent) from the metamorphic sequence, with rapid removal of the melt by segregation and intrusion to higher structural levels. The deep biotite granite was probably generated by partial melting and homogenisation of the same source material, with the addition of a small magmatic component that was not derived locally from the pelites. The late granodiorite was not generated by anatexis of pelitic material as observed in the metamorphic sequence, and was probably derived by melting of the lower crust at deeper levels than any contemporary exposure of Hercynian basement in the Pyrenees. Petrological analysis of the metamorphic sequence suggests that water activity was externally buffered to high values throughout the ‘high-level’ anatexis observed in the Trois Seigneurs sequence. Evidence for this is provided by metacarbonate and metapelite mineral equilibria, by the sequence of metamorphic