Archive of SID IJST (2014) 38A3: 243-252 Iranian Journal of Science & Technology http://ijsts.shirazu.ac.ir An unusual assemblage of talc-phengite-chlorite-K-feldspar in quartz schists from the Nahavand area, Sanandaj-Sirjan zone, Iran J. Izadyar*, S. Mojab, O. Kuroshi and M. Zare Department of Geology, University of Zanjan, University Blvd., Zanjan, Iran E-mail:
[email protected] Abstract 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 K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH) model system containing talc, phengite, chlorite, K-feldspar, phlogopite and kyanite with excess quartz and H2O shows that divariant assemblage of talc- phengite-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 + H2O and chlorite + phlogopite + quartz = talc + phengite + K-feldspar + H2O. Constructed Al2O3-KAlO2-MgO+FeO (AKM) compatibility diagrams predict that phengite (XPh = 0.280, YPh = 0.860), chlorite (XChl = 0.570, YChl = 0.640), talc (XTlc = 0.160, YTlc = 0.02) and K- feldspar are stable at P = 11 kbar and T = 400°C.