New age and geochemical constraints on the origin of Quaternary adakite-like lavas in the Arabia–Eurasia collision zone

Abstract

Adakite-like lavas from the Iranian segment of the Arabia–Eurasia collision zone have been considered as partial melts derived from a subducted Neotethyan slab, a hypothesis that cannot be further evaluated due to the lack of systematic age and radiogenic isotopic data. Here, we revisit the Anar region, Iran where such lavas were reported by providing these data to examine their probable origin as slab melts. New SIMS zircon U–Pb ages attest to eruption at ca. 1 to 2Ma, postdating the Arabia–Eurasia collision. The lavas are characterized by dacitic to rhyolitic, metaluminous to slightly peraluminous and trondhjemitic compositions, and incompatible element patterns similar to continental crustal rocks. Extreme depletion in Nb, Ta, Y and the heavy REE relative to the light and middle REEs reflects control by garnet and rutile. Except one sample presumably affected by secondary gain of radiogenic Sr, the depleted Sr–Nd–Hf isotopic compositions relative to the Bulk Silicate Earth [(87Sr/86Sr)i=0.7040 to 0.7043; εNd(t)=+3.2 to +5.3; εHf(t)=+10.5 to +11.3] indicate magma genesis from juvenile source rocks. A slab melt origin for the Anar lavas is very unlikely and the reason is twofold. First, the Y and HREE of the source rocks estimated by inversed batch melting models assuming garnet pyroxenite residue are far too low compared with MORB-like lithologies and subducted sediments. Second, the low MgO, Ni and Cr of the lavas point to limited interaction with peridotites in a mantle wedge. Fractionation-related models are also unlikely due to the apparent lack of broadly coeval, mafic precursor magmas without adakitic signature. The low-Y and low-HREE source rocks revealed by the melting models, together with the low MgO, Ni and Cr of the Anar lavas, can be explained by lower crustal melting under eclogite-facies conditions. These source rocks were most likely high-pressure cumulates in the root of the Urumieh–Dokhtar magmatic arc, a Neotethyan arc that was active in the Paleogene. Lower crustal melting might be related to localized destruction of the continental mantle, giving rise to the temporally and spatially associated ultrapotassic magmas. The hypothesis of an ‘adakitic province’ near Anar where slab detachment and melting occurred does not hold in light of available data and observations.