Petrogenesis of post-collisional Plio-Quaternary adakitic rocks in south of Tabriz


استان آذربایجان شرقی- شهرشبستر- خیابان شیخ محمود شبستری- کوی باهنر- ساختمان بهسامان 5- واحد 7- منزل دکتر فرهاد پیرمحمدی


In the south of Tabriz northern part of Uromieh-Dokhtar magmatic zone a set of volcanic rocks crop out with composition of andesite, dacite, rhyodacite and rhyolite with Upper Miocene to Plio-Quaternary in age. This paper reports the dacitic, rhyodacitic and rhyolitic lavas erupted within the pyroclastic rocks. The most abundant minerals in these rocks are plagioclase and amphibole with various textures such as felsitic porphyric, microlitic porphyric, sieve, flow and glomeroporphyric. Based on geochemical study, these rocks are characterized by SiO2 ranging from 58.2 to 69.7 wt%, high Al2O3 (15.1-17.5 wt%) and Sr (391-804 ppm), high Sr/Y (30.7-84.46), La/Yb (13.27-67.36) ratios, and low Y (3.6-20.8ppmindicating the adakitic characteristic of the parent magmas. The parent magma of these rocks has calc-alkaline and per-aluminous nature which is characteristics of the arc volcanics. These rocks fall into the field of high-silica adakites (HAS) in classification diagrams. Enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and depletion in heavy rare earth elements (HREEs) are observed in their MORB, primitive mantle and chondrite normalized spider diagrams. Strong depletion in HFSEs including Nb, Ti and P which is signature of arc volcanic, can be observed in these rocks. Intensive fractionated pattern of REEs and low quantities of HREEs and Y may prove existence of garnet or amphibole in the residua of melt. High Sr and negative anomalies of Ta, Nb, and Ti may be resulted from lack of plagioclase and having iron and titanium oxides in the residua phase. According to the petrogenetic diagrams, the parental magma of the rocks studied generated by partial melting of an eclogitic-garnet amphibolitic source derived from metamorphism of the Neotethyan subducted oceanic slab underneath the Cetral Iranian zone. Also, the magma has been subjected to assimilation, fractional crystallization (AFC) and crustal contamination during ascend to higher levels and emplacement. The presence of the corrosion and chemical disequilibrium of the phenocrysts and their sieve textures in these rocks support the occurrence of these processes.