Petrogenesis of Tarom high-potassic granitoids in the Alborz–Azarbaijan belt, Iran: Geochemical, U–Pb zircon and Sr–Nd–Pb isotopic constraints

Abstract

Large-scale Upper Eocene plutons in the Western Alborz–Azarbaijan orogenic belt mostly show calc-alkaline and I-type geochemical features contrasted by the Tarom complex with its high-potassic to shoshonitic affinity. The pluton was emplaced in the Tarom subzone of the orogenic belt and its laser ICP-MS zircon U–Pb age of 41Ma is interpreted as the age of magma crystallization. The Tarom complex is composed of quartz monzodiorite, quartz-monzonite and monzogranite, the SiO2 contents range from 57 to 70wt.%, the K2O+Na2O content is high (5.0–8.9wt.%) and K2O/Na2O ratio ranges from 0.4 to 1.9. All the investigated rocks are enriched in light rare earth elements (LREEs), large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs), and bear a weak Eu anomaly (Eu/Eu*=0.46 to 1.38) in chondrite-normalized trace element patterns. The samples display some variety in initial Sr and Nd isotopic compositions, marked with low ISr=0.704–0.705 and εNd (40Ma)=−4.2 to +3.4 (−5.7 for an enclave) values. The Pb isotopic ratios are (206Pb/204Pb)=18.52–18.86, (207Pb/204Pb)=15.57–15.72 and (208Pb/204Pb)=38.47–39.08. Comparison with experimental studies, together with mantle-like isotopic ratios and comparisons of REE patterns, points to an origin of chemically enriched lithospheric mantle source for the Tarom plutonic complex. Partial melting process involving different partial melting degrees affecting heterogeneously metasomatized mantle is a process that seems likely to have occurred in the studied complex as the major differentiation process. The Tarom monzonitic plutons are considered to be post-orogenic intrusions that were emplaced in an environment of lithospheric extension, causing asthenospheric upwelling. Asthenospheric upwelling induced a thermal anomaly which caused partial melting of metasomatized subcontinental lithospheric mantle in the Tarom area.