Geochemistry, petrogenesis and tectonic setting of volcanic rocks North Qazvin (Western Alborz)

Abstract

The magmatic activities in north Qazvin, as a part of the Western Alborz zone, contain a sequences of volcanic lavas associated with pyroclastic rocks with Eocene age. According to petrographical investigations, volcanic lavas are basalt, basaltic andesite, pyroxene andesite, trachy-andesite, dacite and rhyolite. The texture of these rocks is more porphyritic with fine-grained groundmass including plagioclase, clinopyroxene, quartz, alkali feldspar, amphibole and biotite minerals. On the basis of geochemical features, all samples show high-K calc-alkaline to shoshonitic affinities. Also, these rocks are enriched in LREE and LILE, and depleted in HFSE characteristic of calc-alkaline series in subduction-related environment. Petrogenesis indicate that the mafic lavas have been derived from 5 to 10 % partial melting of enriched garnet lherzolite mantle source. Qazvin volcanic lavas have moderate Nb/Yb and Ta/Yb ratios indicative of an enriched mantle source compositions (e.g., E-MORB). Furthermore, these lavas have high values of Th, U and Pb consistent with slab-related sediments and/or crustal contamination in their mantle source.

Keywords

• key words: volcanic lavas
• shoshonite
• garnet lherzolite
• subduction zone
• western alborz
• qazvin