Petrogenesis of Plio-Quaternary post-collisional ultrapotassic volcanism in NW of Marand, NW Iran

摘要

Post-collisional potassic and ultrapotassic (UP) alkaline magmas were erupted northwest of Marand in northern part of Urumieh-Dokhtar magmatic arc (UDMA) during Plio-Quaternary. The studied rocks display microlithic porphyritic texture with phenocrysts of clinopyroxene, leucite, and plagiodase ± bio-tite ± olivine. The UP volcanic rocks are mostly silica undersaturated with normative nepheline, high Mg#, and high K_2O/Na_2O ratios. They are characterized by significant enrichment in LILE and LREE and depletion in selected high-field-strength elements, such as Nb, Ta and Ti. All of the studied samples display fractionated REE pattern (Ce/Yb)_N = (15.39-18.08) and negative Eu anomalies (Eu/Eu* = 0.71-0.78). Furthermore, they exhibit high Ba/Nb (41-60) and Ba/Ta (682-1139) ratios, which are a typical feature of subduction related-magmas. Rare-earth element modeling, Rb/Sr and Ba/Rb ratios indicate that they can be generated from low degree partial melting of lithospheric mantle with phlogopite-garnet peridotite source. We suggest that post-collisional volcanic activity during late Miocene to Quaternary occurred by rollback and slab breakoff processes after Neo-Tethys subduction. The slab breakoff mechanism would have caused the direct contact of hot asthenospheric mantle with metasomatised sub-continental lithospheric mantle and also has increased the thermal gradient, which weakened the lithosphere and assisted to lithospheric extension and magma ascent.