Petrology, Geochemistry and Tectonomagmatic Setting of Neshveh Intrusion (NW Saveh)

Neshveh intrusion is located in the NW of Saveh City and is a part of Orumieh-Dokhtar magmatic arc. This intrusion consists of quartz monzodiorite, granodiorite and granite that have intruded into the Eocene volcano-sedimentary rocks. This intrusion is high-K calk-alkaline and metaluminous and is classified as I-type granitoids. Field investigations along with petrographic and geochemical studies indicate that all phases of Neshveh intrusion are derived from a common magma source as a result of mineral differentiation. Different phases of this intrusion have low Mg#, Ni, Cr, Co and V which are indicative for higher evolution of magma during the magma ascent and before complete crystallization. All phases of Neshveh granitoid are characterized by LREE-rich patterns with high LREE/HREE ratio and negative Eu anomalies. Similarity of the mentioned patterns suggests a comagmatic source for these rocks and demonstrates the role of magmatic differentiation in their evolution. There are negative anomalies in the Nb and Ti along with positive anomalies of Rb, Ba, K and Pb on the spider diagrams. These anomalies are indicative for a subduction setting for magma source of these rocks. Geochemical studies indicate that the Neshveh granitoid is formed in a volcanic arc and active continental margin. In this base, it is assumed that this intrusion is formed as a result of Neo-Tethys oceanic lithosphere subduction beneath the Central Iran zone which is replaced in the Orumieh-Dokhtar magmatic arc.

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

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 Al2O3, K2O and Na2O) are negatively correlated with SiO2?and K2O, Ba, Rb, Ce, Nb, and Zr are positively correlated with SiO2. Some elements such as Na2O, 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% SiO2. Hornblende took over from 55 wt% SiO2, 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 K2O and Ba vs. SiO2?reflect the absence of significant fractionation of biotite and K-feldspar.