Petrology and geothermometry of retrograded Hercynian charnockites and host gneisses, Agly Massif, French Pyrenees

Abstract A Hercynian charnockite occurs within high-grade gneisses in the Agly Massif, French Pyrenees. Its thermal history has been evaluated using the Fe-Mg distribution coefticient ( K _D) between garnet and biotite. These minerals have different origins but similar compositions in the charnockites and host gneisses. In the charnockite, the Bi–Ga pairs are the retrograde products of Opx alteration. This Opx reaction with feldspar can be written. Opx + PI + Fluid 1(H₂O + Al + K + Fe + Ti) = Bi + Ga + Q + Fluid 2(H₂O + Na). The garnets are relatively Ca poor (4–2.5% grossular); they are automorphic and zoned in the gneisses and poikiloblastic in the charnockites. Both types show a retrograde rim (of few hundred microns'width) across which Fe and Mn increase as Mg decreases. The biotites show a good correlation between the octahedral cations (Ti⁴⁺+ Fe²⁺) and (Mg²⁺+ Al³⁺_VI); Ti and Fe both increase, whereas Mg and Al_VI decrease. There is an inverse linear correlation between Fe²⁺ and Mg²⁺ and the Fe/Mg ratio increases as Ti increases. The relation between Ti and K ^Ga-Bi_DFe-Mg is less clear: it seems that K _D slightly decreases as Ti increases. The equilibration temperatures of Ga–Bi pairs are discussed: the charnockite Ga-Bi pairs have equilibrated between 550°C and 600°C; whereas those of the gneisses have equilibrated between 550°C and 650°C. Two main thermal steps appear: one in the gneisses between 600-650°C and a second one in both the gneisses and the charnockites between 550°C and 600°C.