ALONG ARC GEOCHRONOLOGIC AND GEOCHEMICAL VARIATION OF THE URUMIEH-DOKHTAR BELT, IRAN: IMPLICATIONS FOR MAGMATIC FLARE-UP AND CRUSTAL GROWTH

The Urumieh-Dokhtar magmatic belt in Iran is a paleoarc formed above accreted transitional lithosphere at the northern margin of the Neotethys before the Arabia-Eurasia continental collision. Earlier investigations indicate a Paleogene flare-up affected not only the paleoarc but also regions elsewhere in Iran. Here we present geochronologic and geochemical data for igneous samples from different segments along the paleoarc and a compilation of published data, aiming to better constrain the duration and origin of the arc flare-up. Our results indicate that the magmatic flux probably started to rise from a background flux at ~35 Ma and peaked between 20 Ma and 15 Ma, implying that the flare-up in the Urumieh-Dokhtar belt might have been younger than previously and commonly appreciated. Magmatic products of the paleoarc are characterized by a large range of silica contents without compositional gaps, calc-alkaline compositions and distinctive trace element patterns typical of arc magmas. The pre-flare-up magmas display large ranges of initial ⁸⁷Sr/⁸⁶Sr values (0.7037 to 0.7085), eNd values (−7.0 to +5.7) and zircon eHf values (−8.6 to +15.2), attesting to involvement of MORB mantle and enriched lithospheric component(s) in magma genesis. If eNd of MORB mantle (+12 ± 3) is taken to represent that of the subarc mantle asthenosphere, the less depleted to enriched isotopic compositions (initial ⁸⁷Sr/⁸⁶Sr = 0.7048 to 0.7077, eNd = −3.3 to +2.4) of the flare-up magmas have to indicate a significant input from the upper plate. High Sr/Y observed in magmas formed at the peak of the flare-up is indicative of a deeper level of crustal processing. Enhanced upper plate input during the high-flux event can be explained by thermal relaxation associated with thickening of preexisting crust and mantle lithosphere. Thus, the dominantly positive eHf values of zircon in the flare-up magmas reflect the general lack of old continental basement (e.g. that having Hf modal age >1.5 Ga) in the upper plate. Our findings corroborate the view that continental arc flare-ups are important episodes of crustal distillation towards an intermediate composition and less important sites of crustal addition from the convective mantle.