Petrology and Mineral Chemistry of Gabbroic Rocks (Ne Qazvin) and Relationship to Surrounding Monzogabbros

Abstract

The Pargeh gabbroic rocks were emplaced in the Cenozoic and probably Late Eocene in Central Alborz, Iran. The pluton is composed of silica under-saturated rocks including olivine gabbro and monzogabbro. In this area, the ultramafic cumulate olivine gabbro is surrounded by monzo gabbroic rocks. The main rock-forming minerals are moderately magnesium olivine (Fo 56-63), clinopyroxene (Mg# = 53-74), plagioclase (An 30-88), and magnesium biotite (Mg # = 53-63) with accessory magnetite and ilmenite. The crystallization order for these rocks is olivine, clinopyroxene, plagioclase, and biotite. Based on lithological and geochemical observations, fractional crystallization of ascending magma was an important process in the evolution of these rocks. The mineral chemistry as well as the major and trace element geochemistry shows that the Pargeh gabbroic magma was alkaline in nature, characterized by enrichment in large ion lithophile elements (LILE) and depletion of high field strength elements (HFSE); these patterns have characteristics of subduction-related arc (slightly negative Nb-Ta anomalies) and intraplate alkaline magmatic rocks (enrichment in Sr, Ba, U and Pb). The most likely tectonic setting was a back-arc basin. The pyroxene thermobarometer gives the crystallization temperature of 1100-1250 °C and a pressure of 2-5 kbar, which indicates the emplacement of mantle-derived magmas into upper crustal depths. Initial 87Sr/86Sr vales and εNd(t) range from 0.7049 to 0.7053 and 1.5 to 1.9, respectively. The 143Nd/144Nd(i) values for the Pargeh rocks range from 0.51269 to 0.51271. The geochemical and isotopic evidence indicates that these rocks were generated from a mantle-derived magma modified by fractional crystallization. There is little evidence in the trace element and isotopic compositions for crustal assimilation.

Keywords: Olivine gabbro, Monzogabbro, Mineral Chemistry, Sr-Nd isotope, Thermobarometry, Central Alborz, Iran.