A- and I-type metagranites from the North Shahrekord Metamorphic Complex, Iran: Evidence for Early Paleozoic post-collisional magmatism

Abstract

The North Shahrekord Metamorphic Complex (NSMC) of the central Sanandaj-Sirjan Zone (SaSZ) consists of metagranitoid bodies, which were metamorphosed within high pressure-low temperature conditions. Whole rock chemistry shows relatively high amounts of SiO2 (65–77wt%) and Al2O3 (12–15wt%), low amounts of Nb, P, Sr, Ti, a high ratio of Ga/Al (4–9) and a negative Eu anomaly. The chemical compositions of metagranites are reasonably similar to A- and I-type granites. U–Pb zircon ages of three samples of metagranites indicate that crystallization of the granites occurred at 521.6±9.1 to 513.5±8.5Ma, Middle Cambrian. The initial 87Sr/86Sr and 143Nd/144Nd ratios of samples vary from 0.7057–0.7239 and 0.511801–0.511890, respectively. High initial 87Sr/86Sr ratios and low εNd(t) values (−3.39 to −1.07) associated with high ratios of 206Pb/204Pb(t)=17.8557–18.8045, 207Pb/204Pb(t)=15.6721–15.7220, and 208Pb/204Pb(t)=37.7490–38.4468 infer a significant contribution of continental crust in generating the source magma of the metagranite bodies. The results reveal that the metagranites were mainly produced through mixing of basaltic melts with components similar to metasedimentary sources. The new results show that crystallization of the metagranites occurred in Early Paleozoic times and much earlier than break-up and drifting of the SaSZ from the Arabian plate, suggesting that the metagranites were mainly produced in the western Iran after the closure of the Proto-Tethys Ocean. This model is consistent with the previously suggested models for formation of an Early Paleozoic granitoid belt along the northern rim of Gondwana.