T140 The Dance of Arcs: Causes of Arc Migrations, Flare-Ups, and Magmatic Focusing II

Monday, 23 October 2017
1:30 PM - 5:30 PM
Skagit 4

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

Paper number: T140-13
Program: Technical Sessions
Time: 4:35 PM – 4:50 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-296008

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