an unusual assemblage of talc-phengite-chlorite-k-feldspar in quartz schists from the nahavand area, sanandaj-sirjan zone, iran

for the first time, an unusual assemblage of talc-phengite-chlorite-k-feldspar was found in quartz schists from the sanandaj-sirjan zone in the nahavand area in western iran. the talc-bearing quartz schists occur as small bodies or lenses within pelitic schist layers and contain talc, phengite, chlorite, k-feldspar and quartz as major mineral constituents with subordinate amounts of calcite and graphite. textural analysis revealed that talc, phengite, chlorite and k-feldspar are in sharp contact and no reaction rims between them were observed. constructed petrogenetic gird in the k   2o-feo-mgo-al2o3-sio2-h2o (kfmash) model system containing talc, phengite, chlorite, k-feldspar, phlogopite and kyanite with excess quartz and h   2o shows that divariant assemblage of talcphengite- chlorite-k-feldspar is stable over a wide p-t range defined by the following two univariant reactions: phengite + talc + quartz = chlorite + k-feldspar + kyanite + h   2o and chlorite + phlogopite + quartz = talc + phengite + k-feldspar + h   2o. constructed al2o3-kalo2-mgo+feo (akm) compatibility diagrams predict that phengite (x   ph = 0.280, yph = 0.860), chlorite (xchl = 0.570, ychl = 0.640), talc (xtlc = 0.160, ytlc = 0.02) and kfeldspar are stable at p = 11 kbar and t = 400°c. this relatively high-pressure assemblage could be formed during the subduction of the neo-tethys oceanic plate under iranian microcontinent.