Zeynab Gharamohammadi; Ali Kananian
Abstract
The Dehe Bala pluton is exposed approximately 45 km south-west of Boein Zahra town, Qazvin province. This Pluton which intruded the Eocene volcano-sedimentary rocks of the Urumieh-Dokhtar ...
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The Dehe Bala pluton is exposed approximately 45 km south-west of Boein Zahra town, Qazvin province. This Pluton which intruded the Eocene volcano-sedimentary rocks of the Urumieh-Dokhtar Magmatic assemblage (UDMA), is mainly composed of granodiorite and produced narrow thermal metamorphic contact aureoles surrounding the intrusion. The body is characterized by SiO2 content ranging from 64.2 to 66.9 wt%, high-k calc-alkaline nature and metaluminous character (A/CNK<1.1). On the primitive mantle-normalized trace element spider diagram, the intrusive rocks have similar trace element patterns with pronounced enrichment in Cs, Th, U, K, Zr and P, Ti, Nb and Ba depletion. Chondrite-normalized rare earth elements (REEs) patterns display moderate negative Eu anomalies, enrichment of LREES relative to HREES and moderate fractionated REES pattern. The granodiorites under discussion, based on geochemical features, belong to I-type granitoids. Their low TiO2 and P2O5 contents and high Th/Ta=6.23-9.35 ratios are characteristic of subduction-related magmatism. The Dehe Bala granodiorites display geochemical characteristics typical of magmatic arc intrusions related to an active continental margin. These criteria include their calc-alkaline nature, pronounced negative Nb anomaly, light-REE-enriched patterns and weak fractionation of MREE and HREE. Enrichment of incompatible elements such as La, Ce, Rb, Th, K and Nd coupled with negative anomalies of Ti, Ba, Eu, Nb and P suggest that the parent magma originated by partial melting of lower crust. The presence of mafic microgranular enclaves (MMES) in granodiorites along with disequilibrium textures in plagioclase phenocrysts and also the attendance of plagioclase phenocrysts between host granodiorites and the MME across theboundary point to mixing of crustal magma with mantle-derived mafic magma.
