Interlayer and Si content of phengite in HP-LT carpholite-bearing metapelites

Phengite occurring along with carpholite +/- lawsonite and/or chloritoid in HP-LT domains shows not only variable Si-(Mg + Fe) contents, but also vari able interlayer contents (IC). To determine whether these chemical variatio ns are coherently related to variation in P-T conditions on a regional scal e, c. 100 rock samples were sampled in metapelites metamorphosed at conditi ons varying from 350 degreesC, 8 to 12 kbar to 450-500 degreesC, 18 to 20 k bar (Schistes Lustres complex, franco-italian Western Alps). Based on micro structural and habit criteria, four types of phengite were differentiated t hat are related either to the rock mineralogy (carpholite vs chloritoid bea ring samples) or correspond to various generations of phengite occurring in the same rock sample or thin section. Microprobe analyses reveal that each type of phengite is characterized by a specific composition and that pheng ite associated with carpholite has a lower interlayer content than phengite associated with chloritoid. The successive generations of retrograde pheng ite overgrowing carpholite point to a large decrease of interlayer content (c. 0.9-0.7 pfu) and (Fe + Mg) content (c. 0.25-0 pfu) with decreasing P-T conditions. This change is best accounted for by a gradual increase of the pyrophyllite component. In contrast, phengite from higher-temperature, chlo ritoid-bearing rock samples shows an almost constant interlayer content (c. 0.9-0.95 pfu) but a larger decrease of (Fe + Mg) content (c. 0.6-0.1 pfu). Hence, (1) the composition of the different phengite generations occurring (metastably) in the same rock sample may be used to retrieve points in P-T loops and (2) the pyrophyllitic substitution in phengite is large at low-t emperature conditions and cannot be ignored. Thermobarometric estimates bas ed on the Si-content alone will therefore result in pressure overestimates. We propose a tentative location of the phengite Si and IC isopleths in P-T space which could allow a direct determination of the P-T conditions in ca rpholite-bearing rocks. Especially in some carpholite-bearing rocks, new th ermodynamic models accounting for tschermak and pyrophyllitic substitution are also required prior to making reliable thermobarometric estimates in HP -LT metapelites.