Petrography, Geochemistry, Petrogenesis and Tectonic Situation of Volcanic Units in Zaker area (Northwest of Iran)

Mahboubeh Mansouri, Afshin Ashja-Ardalan, Mohammad Lotfi, Jamal Sheikh-Zakariai

Abstract


Zaker volcanic units are located in the western Alborz Zone (northeast of Zanjan). Volcanic rocks of this area are combinations of basalt, andesite and trachy andesite. In these units, pyroxene, plagioclase and rarely olivine crystals are as phenocrysts in a background of fine crystals and glass minerals. Of course, also, some small outcrops of andesite tuff units are observed. These units are metaluminous and are located in ranges from high calcium calc -alkaline to shoshonitic rocks natures. The significant feature of these units is the enrichment of LREE elements relative to HREE. Furthermore, positive anomalies of LILE elements such as Cs, Ba, K and Sr along with negative anomalies of HFSE elements such as Ti, Nb and P are observed in all samples. Negative anomalies of Nb and enrichment of light rare earth elements relative to heavy rare earth elements, represent the subduction zone environments. The formation environments of these units are continental arcs and the primary magma is composed of partial melting of garnet spinel lherzolite with a very small amount of garnet during the melting process. The low ratio of (La / Yb) N (6.79-4.6) in all volcanic units indicates the absence of garnet at the origin or the partial presence of this mineral in the source of the magma. The geochemical characteristics of these units indicate that magma has originated from high horizons of the mantle (probably a lithospheric mantle) and has tolerated some crust contaminations during the ascent.


Keywords


Clinopyroxene; Calc-alkaline; Garnet spinel lherzolite; Subduction; Western Alborz Zone; Zanjan

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