Evidence of a Cadomian arc in the northern margin of Gondwana: The Ediacaran-Cambrian Mahabad rhyolitic complex, NW Iran

Abstract

The Mahabad rhyolitic complex, mostly composed of rhyolite but also including granite and granodiorite, is exposed in NW Iran as a part of the Central Iran Block. Porphyritic, hyalo-porphyritic and spheroidal are the main textures of the studied samples of rhyolite. U-Pb zircon chronology on three samples of Mahabad rhyolitic complex yielded Cambrian to Ediacaran ages of 537.6 ± 6.6 Ma, 547.4 ± 6.5 Ma and 556.2 ± 7.1 Ma. Based on geochemical analyses, the original magma was high potassium calc-alkaline to shoshonitic. The rocks are enriched in LREEs relative to HREEs. Trace element patterns of Mahabad rhyolite normalized to chondrites show negative anomalies of high-field-strength elements (Ti, Nb, Ta, Hf, Yb, Y and Zr) and high LREEs and large ion lithophile element contents (Rb, K, Th and Ba). ²⁰⁸Pb/²⁰⁴Pb (36.7219–39.0367), ²⁰⁷Pb/²⁰⁴Pb (15.4963–15.7669) and ²⁰⁶Pb/²⁰⁴Pb (16.9405–19.9567) ratios indicate an EM-II enriched mantle source for the rhyolite magma. Large variation of εHf(t) from −5.2 to + 4.5 points to a mantle source with crustal material contribution in the magma genesis. The rhyolitic magma erupted in an active continental margin. The formation of calc-alkaline high potassium magma was probably related to metasomatism of the mantle because of the north to south subduction of Proto-Tethys oceanic crust beneath the northern margin of Gondwana continental crust.