Mineral Chemistry and Magmatic Differentiation Evidences in the Neshveh Intrusion (NW Saveh, Central Iran)

Abstract

Neshveh intrusion which is located in the NW of Saveh City is a part of Sahand-Bazman magmatic arc within the Central Iranian zone. This intrusion consists of quartz-monzogabbro, quartz-monzodiorite, granodiorite and granite that have intruded into the Eocene volcano-sedimentary rocks. This intrusion is medium to high-K calc alkaline, metaluminous, and I-type granitoid. All phases of the Neshveh granitoid are characterized by LREE-rich patterns with high LREE/HREE ratio and negative Eu anomalies. Similarity of patterns suggests a comagmatic source for these rocks and demonstrates the role of magmatic differentiation in their evolution. Clinopyroxene classified as calcic type with varying from clinoenstatite-clinofferosillite to diopside and augite from quartz-monzogabbros to quartz-monzodiorite and granodiorite. Plagioclase composition varies from bytownite and labradorite in quartz-monzogabbros to andesine in quartz-monzodiorites and oligoclase in granodiorites and granites. Core of some plagioclases in granodiorites and granites shows the calcic composition which is labradorite and andesine in granodiorite and andesine in granites. Field investigations along with petrographic and geochemical studies indicate that all phases of the Neshveh intrusion derived from a common magma source as a result of mineral differentiation. Geochemical evidences show smooth differentiation trends in which most of major elements (except Al 2 O 3 , K 2 O and Na 2 O) are negatively correlated with SiO 2 and K 2 O, Ba, Rb, Ce, Nb, and Zr are positively correlated with SiO 2 . Some elements such as Na 2 O, Sr, Eu and Y follow curves that reflect crystal fractionation of clinopyroxene, plagioc1ase and hornblende. Furthermore, large volumes of quartz-monzogabbros compared to granites, as well as the lack of mafic enclaves in more evolved rocks, are also indicative of crystal fractionation. Clinopyroxene fractionation was the main control in the evolution of the magmas up to 55 wt% SiO 2 . Hornblende took over from 55 wt% SiO 2 , resulting in decreasing Dy/Yb with increasing silica content in the most siliceous rocks. Fractionation of opaque minerals and apatite throughout the sequence, and the continuous increase in K 2 O and Ba vs. SiO 2 reflect the absence of significant fractionation of biotite and K-feldspar.