The Gharuchah-Sofla intrusion (northwestern Sanandaj-Sirjan zone, Iran): Age, petrology, geochemistry and tectonomagmatic implications

Abstract

The Cretaceous Gharuchah-Sofla intrusion is a composite batholith consisting of a variety of mafic to felsic rocks in the northwestern Sanandaj-Sirjan zone, Iran. Both its age and the relationship between various rock types in the intrusion are unknown. To address these issues, we conducted a detailed age, petrologic, and geochemical study of the intrusion, their country rocks, and mafic microgranular enclaves (MMEs) hosted therein. Plutonic rocks from the main mass of the intrusion, dated at ~110 Ma by in-situ zircon UPb method, are dominated by gabbroic, dioritic, and quartz dioritic with subordinate granodioritic rocks. Magmatic rocks occurring mostly along margins of the intrusion have Late Ediacaran zircon UPb ages (~560 Ma). Both the Early Cretaceous and the Ediacaran magmatic rocks are sub-alkaline; the former has I-type affinity whereas the latter has S-type affinity with a tendency towards I-type. Both sets of rocks are characterized by continental crust-like trace element patterns showing negative anomalies in Nb, Ta, Sr, P, Eu, and Ti of variable intensity. Early Cretaceous gabbroic rocks exhibit unfractionated rare-earth element patterns (La/Yb = 1.9–3.0) and negative ZrHf anomalies. In contrast, other rocks of the Early Cretaceous and Ediacaran magmatic rocks show fractionated rare-earth element patterns (La/Yb = 11.0–16.8) along with the lack of such anomalies. MMEs, which occur within Early Cretaceous plutonic rocks, have major and trace element compositions similar to the gabbroic rocks. Contrasting Nd isotopic ranges are present between the Early Cretaceous plutonic rocks and the MMEs (ɛNd = +2.0 to +4.1) and the Ediacaran magmatic rocks (ɛNd = −5.8 to −0.4). Based on the geochemical and isotopic data obtained, we suggest that (i) the Ediacaran magmatic rocks formed by melting of Cadomian basement rocks below the blocking temperature of UPb exchange between zircon and melt, (ii) the MMEs represent frozen melts (i.e., calc-alkaline basalts) that formed the gabbroic rocks in the Early Cretaceous instruction, and (iii) fractional crystallization with slightly concomitant assimilation of Cadomian basement rocks was a key process forming intermediate and silicic rocks of the intrusion. The ~110 Ma Gharuchah-Sofla intrusion was likely associated with a continental margin arc related to the subduction of the Neotethys, providing solid evidence that this segment of the arc was built directly on the Cadomian basement.