Constraints from geochemistry, zircon U-Pb geochronology and Hf-Nd isotopic compositions on the origin of Cenozoic volcanic rocks from central Urumieh-Dokhtar magmatic arc, Iran

Considerable debate persists regarding the petrogenesis of high-alumina basalts (HAB) which are purported to occur exclusively in subduction zones. Major and trace element, mineral chemistry, whole-rock Sr-Nd-Hf-isotopes and zircon U-Pb age data are reported for the Cenozoic Eshtehard HABs, in order to constrain the nature of mantle beneath the central Urumieh-Dokhtar magmatic arc, and further investigate the mechanism of HABs generation. Eshtehard HABs, chemically akin to those from continental arcs, include basaltic, basaltic andesite, andesitic and dacitic rocks. U-Pb geochronology of zircon yield ages of 47.2 ± 0.6 Ma, 43.9 ± 0.3, and 40.9 ± 0.5 to 39.4 ± 0.9 for basaltic, andesitic and dacitic samples, respectively. Basaltic andesitic dikes intruded into dacitic hosts yield age of ca. 20–18 Ma. εHf(t) values for Eocene zircons display a range variable from −6.4 to +6.5. Miocene zircons have higher εHf(t), ranging between −1.8 and +10.7. The studied rocks are characterized by enrichment in incompatible trace elements and have relatively homogeneous Sr-Nd isotopes. Integrated studies indicate that Eshtehard HABs were derived from the hydrated, dominantly depleted shallow asthenospheric mantle wedge (and possibly also in the lower lithosphere) overlying subducted oceanic lithosphere. Traversing lithospheric mantle and Cadomian crust, and assimilating crustal material while fractionating plagioclase, high-Ca pyroxene, magnetite, and amphibole, the primary melt formed Eshtehard high-alumina, low-Mg# basalts. We hypothesize that ponding of hydrous magma at the base of the crust allowed for further crustal assimilation and fractionation of Ca-pyroxene and magnetite without plagioclase and zircon nucleation; then, ascending magmas through the crust led to crystallization of plagioclase and eventually zircon. We suggest comparatively high water content (rather than high crystallization pressure), up to 4% sediment melt and less than 10% of continental crust materials were involved, as subordinate components, in the petrogenesis of Eshtehard HABs.