Metasomatic Stages and Scapolitization Effects on Chemical Composition of Pasveh Pluton, NW Iran

Pasveh gabbros are mafic component of a plutonic complex in the northwest Sanandaj-Sirjan Zone. These cumulative rocks are composed of plagioclase and calcic clinopyroxene (Cpx), which yield unusually high CaO (> 19 wt.%) in whole-rock chemistry. Petrographical and geochemical data suggest that Pasveh gabbros can be divided into two groups: free scapolite and scapolite-bearing gabbros. The second group has higher Na₂O, K₂O, and P₂O₅ relative to free scapolite ones and is enriched in LIL (large ion lithophile) and HFS (high field strength) elements. Two stages of metasomatism affected the primary composition of mafic rocks. Firstly, high temperature reaction caused to invert primary high Ti clinopyroxene to low Ti clinopyroxene+high Ti amphibole. This reaction was extensive and included all gabbroic samples. Hydrothermal fluids involved in this process can be derived from dehydration reactions of country rocks or from other magmas incorporated in the formation of Pasveh complex pluton. The second metasomatic stage relates to scapolitization of limited parts of gabbroic rocks. An external saline fluid, which is composed of major NaCl and minor KCl and P₂O₅ components, impacted locally on Pasveh gabbros and formed the second metasomatic stage. Possible sources of Na and Cl are primary evaporites or brines, which were present in the host sediments of the gabbros. The carbonate-free nature of these hydrothermal fluids suggests that hydrothermal fluids responsible for the formation of scapolite in Pasveh gabbros are derived from marine evaporitic parentage.