Geochemistry and Thermo-barometry of Mafic Igneous Rocks of the Almabulagh Complex (NW Iran)

Abstract

The Almabulagh complex is located in the Sanandaj-Sirjan structural zone, NW of Hamadan. This complex consists of low-grade metamorphic rocks and two intrusions of felsic and mafic rocks. In this research, we studied the mafic intrusion. The intrusion has ophitic, subophitic and inter-granular textures and is mainly composed of plagioclase, clinopyroxene, amphibole and minor amounts of K-feldspar, quartz and opaque minerals. Secondary minerals include epidote, chlorite and amphiboles formed in the rim of clinopyroxene. Clinopyroxene is of augite type; amphibole is tschermakite; plagioclase is in the range of andesine-labradorite; K-feldspar is anorthoclase; and the opaque mineral is ilmenite. According to the bulk rock geochemistry and mineral chemistry of clinopyroxene, the mafic intrusion of the Almabulagh complex has been crystallized from a calc-alkaline magma and is related to subduction zone. In spider diagrams Nb displays a distinctive negative anomalies compared to the primitive mantle, which suggest a possible subduction-related magmatism. Emplacement temperature and pressure of mafic rocks measured by pyroxene-amphibole thermometer and amphibole barometer indicate temperature and pressure ranges of 750 to 900 °C and 6 to 7 kbar, respectively. Based on Sm/Yb and La/Sm ratios, the parental melt of the mafic rocks of the Almabulagh complex derived from partial melting of garnet–spinel lherzolite. According to the bulk rock geochemistry and mineral chemistry of the mafic rocks, they formed in a subduction area during Late Jurassic-Early Cretaceous before the Neo-Tethys closure in Sanandaj-Sirjan zone.