Zircon U–Pb and geochemistry of the north Shahrekord metamorphosed felsic rocks: implications for the Ediacaran–Cambrian tectonic setting of Iran

Abstract

The basement felsic igneous rocks associated with the continental tectonic zones of Iran are key elements that contain a well-preserved geological record of the protracted evolutionary history of the North Gondwana margin within the Ediacaran–Cambrian timespan. Two distinct Ediacaran–Cambrian magmatic pulses are recognized in the North Shahrekord Metamorphic Complex (NSMC) as a part of the Sanandaj–Sirjan Zone (SaSZ) in Iran. The NSMC is defined by a Pan-African/Cadomian basement dominated by two felsic suites of granitic gneisses and meta-granitoids, which have experienced mylonitization and high-grade metamorphism. Zircon U–Pb dating displays magmatic crystallization ages of 555 ± 7 Ma and 561 ± 10 Ma corresponding to the Late Neoproterozoic (Ediacaran) for the granitic gneisses. Geochemically, the gneisses are differentiated as I-type granites and subalkaline in composition, and similar to the mylonitic meta-granitoids with characteristics similar to those of both I-type and A-type characters, have an affinity to Cordilleran granites. Considering the enrichment in LILEs (e.g., Rb, Th, and U), depletion in HFSEs (e.g., Nb, Ta, and Ti), high ratios of Th/La and Th/Zr, low Nb/U ratio, Y/Nb ratio > 1.2, and low Mg# reported from both felsic rocks and high initial 87Sr/86Sr ratio (0.71088 to 0.74514), negative ƐNd(555 Ma) value (− 3.7 to − 2.3) and Nd model age (TDM2 = 1.40 to 1.51 Ga) reported from the granitic gneisses, a protolith probably derived from partial melting of a common pre-existing felsic crustal source is plausible. Available data indicate that the source magma of the granitic gneisses may have been generated within the Ediacaran convergent margin environment during the subduction of proto-Tethys under north Gondwana before thinning lithospheric and formation of the Middle Cambrian meta-granitoids, as with other peri-Gondwana terranes.