Pliocene-Quaternary Adakite volcanism in the Isfahan area, Central Iranian magmatic belt

摘要

Pliocene-Quaternary calc-alkaline volcanic rocks are exposed in two areas southeast and northwest of the Isfahan area in the Urumieh-Dokhtar magmatic belt. These volcanic rocks consist of basaltic andesites, andesites and dacites, which can be found as lava flows and domes. The rocks have Adakite and medium to high-K calc-alkaline characteristics. They have higher SiO_2 contents, Sr/Y and La/Yb ratios and lower MgO, Y and Yb contents than typical calc-alkaline rocks. In normalized multi-element diagrams they show enrichments in large ion lithophile elements (LILE) and in light rare earth elements (LREE); furthermore, they show negative anomalies of Nb, Ti, Ta and P. Chondrite-normalized REE patterns display a steep decrease from LREE to heavy rare earth elements (HREE) and lack Eu anomalies. These characteristics are consistent with those observed for volcanic rocks from destructive plate-margin settings. These Adakite rocks may have been produced by partial melting of a slab that was probably transformed to eclogite or garnet-amphibolite. The melts evolved during their ascent. The relative depletions in HREE and Y, the enrichment of Sr and absence of negative Eu anomalies indicate that the residual phase in the source was garnet-bearing but pla-gioclase free. Nb and Ti are strongly depleted in the studied samples, which suggests that the source contains a residual Ti bearing phase and thus was most probably garnet-amphibolite or amphibole-eclogite.