An unusual assemblage of talc-phengite-chlorite-K-feldspar in quartz schists from the Nahavand area, Sanandaj-Sirjan zone, Iran

Document Type: Regular Paper

Authors

Department of Geology, University of Zanjan, University Blvd., Zanjan, Iran

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 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.

Keywords