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

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Abstract
The Mboutou complex is one of a line of early Tertiary ring complexes which runs from Lake Chad to the Gulf of Guinea, none of which has hitherto been described in detail. The main rock types are layered gabbros and gabbronorites, with minor bodies of quartz-syenodiorite, quartz-syenite and hypersolvus granite. Feldspars form a continuum with exceptional compositional range, from An85Ab13Or2 to around An1Ab46Or55, and form an entirely hypersolvus sequence with very strong zoning in the syenodiorites. Ca-rich clinopyroxenes (salite and calcic augite) and olivines (Fo78–62) have restricted range. Orthopyroxene-bearing leucogabbros and syenodiorites contain minor orthopyroxene (En62Fs35Wo3) and quartz; olivine and orthopyroxene never coexist. In more evolved rocks amphibole (magnesio-hornblende to ferroedenite) and minor biotite, showing progressive Fe-enrichment, are the only mafic silicates. Major-element rock chemistry, minor elements in clinopyroxenes and biotite chemistry show that, notwithstanding its thoroughly anorogenic setting, Mboutou was, at the outset, only very mildly alkaline. Its more evolved members embarked on a line of evolution with some calc-alkaline characteristics, probably because of ingress of water into residual batches of magma, a possibility supported by stable isotope data. This change in behaviour corresponded with the sudden appearance of quartz and orthopyroxene, which was not in equilibrium with clinopyroxene on the two-pyroxene surface. Amphibole then became the main mafic silicate with further increase in . The more evolved rocks are relatively highly altered, but Fe-Ti oxide pairs suggest that was maintained near to and above the QFM buffer and the range of biotite compositions further suggests crystallization under a regime of decreasing . Biotites maintain alkaline characteristics throughout the sequence. Zoning patterns in the ternary feldspars in the syenodiorites, and the hypersolvus character of the final granite, limit maximum values of to < 1 kb, and suggest minimum temperatures for the end of crystallization in the syenodiorites of˜ 850 °C.

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