Petrochemistry of the Siah-Kuh granitoid stock southwest of Kerman, Iran: Implications for initiation of Neotethys subduction

Abstract

The Upper Triassic Siah-Kuh granitoid stock is part of a series of Triassic-Cretaceous intrusions in the Phanerozoic Sanandaj-Sirjan zone in the Zagros orogenic belt, south-central Iran. The Siah-Kuh stock intrudes Lower Paleozoic amphibolites of the Abshur metamorphic complex and Upper Devonian-Lower Carboniferous greenschists of the Sargaz metamorphic complex. The Siah-Kuh granitoid stock consists mainly of coarse to medium grained leucogranodiorite, leucomonzogranite and alkali granite with subordinate syenite. It also contains a number of mafic enclaves of different sizes and is intruded by alkali granite and granophyre dikes along with monzonitic and a few dioritic dikes. The Siah-Kuh granitoid rocks are characterized by enrichment in large ion lithophile elements (LILE) such as Rb, Ba, K, Ce and depletion in high field strength elements (HFSE) such as Y, Nb and Zr. The chondrite normalized REE patterns are characterized by moderate LREE enrichment [(La/Yb) N = 1.31–4.82] and unfractionated HREE [(Gd/Yb) N = 0.50 to 1.33]. Granodiorites with the least fractionated HREE and a general absence of Eu anomalies suggest the involvement of plagioclase and absence of garnet during the melting processes. Alkali granites with the highest (La/Yb) N values and pronounced negative Eu anomalies are more differentiated products. Nd isotope analyses of the Siah-Kuh granitoids yield an “errorchron” age of 199 ± 30 Ma (MSWD = 1.30), consistent with the field relationships and give Nd model ages ( T DM) from 0.81 to 1.64 Ga and εNd( T) values from +1.81 to +2.45 at 200 Ma. These geochemical data suggest that the Siah-Kuh granitoid stock has characterisitics of metaluminous to slightly peraluminous, calc-alkaline, I-type granite formed in a volcanic arc setting. The onset of subduction of the Neotethys oceanic crust beneath the Central Iranian microcontinent in Triassic time could have accounted for the continental arc volcanism. Dehydration of subducted oceanic crust and partial melting of mantle wedge caused partial melting of subcontinental lithosphere, which resulted in the formation of metasomatised and enriched mafic arc magmas, at variable water fugacity and led to the formation of the Siah-kuh granitoid rocks.