Two different types of igneous rock formed during separate Cenozoic magmatic phases in the Varan-Naragh area in the central part of the Urumieh-Dokhtar Magmatic Arc (UDMA) of Iran as a part of the Alpine-Himalayan system. The first phase comprises late Eocene – early Oligocene Naragh gabbroic rocks (Ns), and the second phase is characterized by the emplacement of both volcanic and plutonic rocks of the early Miocene. Both phases display moderate enrichment of large rare earth elements and depletion of high field strength elements coupled with negative Nb, Ti, and P anomalies, indicative of subduction-related magmatic events within an active continental margin. Initial values of 87 Sr/ 86 Sr and εNd T are 0.70684 and +0.15 and 0.70560–0.70654 and +2.55 to +3.49 for Ns and early Miocene intrusive and volcanic rocks, respectively. Comparisons of rare earth element patterns and mantle-like isotopic ratios suggest that Ns mafic and early Miocene magmatic rocks were derived from partial melting of a common subcontinental lithospheric mantle. Geochemical and isotopic ratios of Ns gabbroic rocks, in combination with the data related to other coeval and proximal mafic-intermediate intrusions (such as Nashalj), suggest enrichment of the lithospheric mantle by slab-derived fluids with a minor subducted sediment melt. The low εNd T of Ns gabbroic rocks can reflect involvement of slab-derived components. The geochemical similarity and the close spatial and temporal association of Varan intrusive and volcanic rocks suggest a common petrogenetic relationship. Geochemical, isotopic, and geochronological evidence from the region indicate three major phases of igneous activity in the Kashan magmatic segment of the central UDMA during late Eocene to Miocene, resulting in complex tectonic regime transition from compressional subduction to extensional post-collisional settings. Integrated with published studies, the new results support a model suggesting that subduction-related magmatic activity was still influencing the central UDMA in the early Miocene time and are also consistent with the notion of oblique and diachronous collision along the northeast margin of the Arabia plate.
Abstract The Cenozoic Arabia–Iran continental collision was associated with emplacement of a large variety of magmatic rock types. This aspect is particularly evident in the Bijar-Qorveh area of NW Iran, where Miocene andesitic to rhyolitic rocks and Quaternary basic alkaline rocks crop out. The Miocene intermediate to acid products show radiogenic Sr and Pb isotopic compositions (87Sr/86Sri 0.70531–0.71109, 206Pb/204Pb 18.71–19.01, 207Pb/204Pb 15.66–15.73, 208Pb/204Pb 38.76–39.14), coupled with unradiogenic Nd isotopic ratios (143Nd/144Ndi 0.51223–0.51265). These characteristics, together with primitive mantle-normalised multielemental patterns resembling ‘subduction-related’ geochemical fingerprints, are considered ultimately derived from the Iranian plate mantle wedge, metasomatised during previous NE-directed Neothetyan Ocean subduction. The alkali-rich andesitic and dacitic rocks evidence both closed- and open-system differentiation, as typically observed for collisional settings in general. Both rock types display a high Sr/Y (37–100) and La/Yb (29–74) ‘adakitic’ signature that it is interpreted here with plagioclase (± amphibole) accumulation or melting of local mafic crustal rocks. Open-system processes involve recycling of crustal cumulates for pyroxene-rich andesite and biotite-rich dacite varieties and low-degree partial melting of the local crust for rhyolites. A radical change occurred during the Quaternary, when SiO2-understaturated to SiO2-saturated poorly evolved rocks (basanites, tephrites, alkaline and subalkaline basalts) were emplaced. The complete change of mantle sources suggests a phase of local extensional tectonics related with WNW-ESE right-transcurrent faults. The major oxide, as well as incompatible trace element and Sr-Nd-Pb isotopic fingerprint of these younger rocks is more akin to that of intraplate magmas, but still bearing some evidences for a variable contribution from a ‘subduction-modified’ mantle source. The NW-trend of increasing involvement of this subduction component is indicative of the strong tectonic control on magmatism. Additional lithotypes indicate the presence of open-system differentiation and remelting processes in the youngest phase of magmatic activity.
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