Zircon U-Pb-Hf isotopes, bulk-rock geochemistry and Sr-Nd-Pb isotopes from late Neoproterozoic basement in the Mahneshan area, NW Iran: Implications for Ediacaran active continental margin along the northern Gondwana and constraints on the late Oligocene crustal anatexis

Abstract

The Mahneshan granite intrusions and granodiorite-gneiss bodies in northwest Iran are part of the Takab magmatic–metamorphic complex, which includes numerous Precambrian continental crust that are now embedded in the Alpine-Himalayan orogenic system. The Mahneshan magmatic assemblage comprises of late Ediacaran-Cambrian gneiss and granites (531–576 Ma) and late Oligocene biotite (25.41 ± 0.4 Ma) granite intrusion. The late Oligocene granite has intruded into the Neoproterozoic sequences in this area. The bulk-rock geochemistry shows that the Ediacaran-Cambrian granitoids and gneisses are calc-alkaline and preserve arc-related signatures. The zircon Hf and bulk rock Sr-Nd-Pb isotopic data suggest that the Ediacaran-Cambrian granitoid plutons were generated via partial melting of continental crust. The late Oligocene granite is formed by the anatexis of Neoproterozoic igneous rocks due to crustal thickening. The whole rock Sr-Nd and zircon Hf isotopic values, Mesoproterozoic Nd and Hf model ages and the existence of inherited zircons suggest the involvement of older crustal components in the genesis of the Mahneshan Ediacaran-Cambrian granitoids. The late Neoproterozoic rock units in the Takab area together with other Ediacaran-Cambrian magmatic segments in Iran were part of the Cadomian active continental margin that were situated along the northern Gondwana margin during the Neoproterozoic. The geochemical trace elements reported from the upper Neoproterozoic-Cambrian plutons along the Cadomian fragments in Iran toward Turkey and Iberia show that most of these granitoid melts were generated in pressures less than garnet stability field suggesting the continental arc thickness less than those reported from the Andes. Furthermore, the Hf and Nd model ages from the Ediacaran-Cambrian Mahneshan intrusions suggest the existence of hidden Paleoproterozoic-Mesoproterozoic rocks in the lower continental crust of Iran.