Middle Eocene volcanic shoshonites from western margin of Central-East Iranian Microcontinent (CEIM), a mark of previously subducted CEIM-confining oceanic crust

Abstract

In western margin of the CEIM (Ashin area), Middle Eocene volcanic shoshonites present very good exposures. This shoshonitic association comprises of all shoshonite group members from basic to acidic. Major minerals of basic members (absarokite and shoshonite) are olivine, clinopyroxene (augite), plagioclase (labradorite), K-feldspar (sanidine and anorthoclase), analcime, calcite, apatite, ilmenite and magnetite. Secondary minerals include chlorite, calcite, epidote and zeolite (mesolite). The Ashin shoshonitic rocks consist of SiO2 undersaturated (absarokite or phonotephrite) to SiO2 oversaturated (toscanite or rhyolite) units. The studied rocks are characterized by wide range of SiO2 (48 to 70 wt %), low content of TiO2 (0.35 to 1.17 wt %), and high values of Alkalis [(Na2O + K2O) = 7.03 to 11.4 wt %]. Other main geochemical characteristics of Ashin shoshonites are potassic to ultra-potassic nature (K2O/Na2O = 1.04 to 5.06), high ratios of LREE/HREE (e.g. La/Yb up to 19.16), and enrichment in LILEs. Primitive mantle normalized spidergram of the studied rocks shows positive anomalies of Cs, U, K, Pb, Hf and negative spikes of Nb, Ta, Sm, Ti and Y. All analyzed samples display markedly negative Nb, Ta, and Ti anomalies, typical features of orogenic magmas. These geochemical signatures point out to the subduction-related nature of Ashin shoshonites and their similarity to potassic volcanic rocks of continental arcs or convergent margins. The parental magma of these shoshonites is an alkalibasalt (absarokite) which produced by low-degree of partial melting of a metasomatized enriched mantle source. Petrographical evidences together with geochemical characteristics (e.g., high values of Pb and U) of the studied rocks conclude crustal contamination of magma during ascending throughout the continental crust. The former subduction of CEIM confining ocean from Triassic to Eocene is too enough for volatile enrichment of the mantle and shoshonitic magmatism in middle Eocene of Ashin area. © 2011 Pleiades Publishing, Ltd.