مطالعات پترولوژی، ژئوشیمی و جایگاه زمین ساختی مجموعه ماگمایی مولد کانسار اسکارن آهن تخت، شمال شرق همدان

Introduction The Urumieh-Dokhtar Magmatic Assemblage (UDMA) forms a distinct NW-SE linear intrusive–extrusive complex Magmatism of the UDMA that occurred from Eocene to Quaternary, although the maximum activity was in the middle Eocene (Berberian and King, 1981; Ghasemi and Talbot, 2006). Collision of Arabian and Iranian plates led to termination of Neo-Tethys crust subduction and magmatism activity was abated in the UDMA, although there is no common agreement on collision timing. The Takht magmatic complex is located in the north of the Hamedan province (west Iran), and it belongs to ‎the UDMA. The assemblage of volcano-plutonic rocks is present in the study area. The volcanic rocks include dacite, rhyodacite and trachyandesite with some tuff and agglomerated and the plutonic rocks are mostly occupied by granodiorite and diorite (containing mafic micro-granular enclaves) with some gabbro. These bodies are mostly intruded in Jurassic schists and are in contact with Cretaceous limestone leading to the formation of a skarn iron-ore deposit. The detailed geochemical and isotopic data is lacking and the age of the Takht granodiorite has not been determined. In the present study, the authors mainly have focused on the geochemistry and Sr-Nd isotopic ratios of the Takht magmatic complex to clarify questions regarding pterogenesis and its geodynamic evolution. We also reported U–Pb zircon ages for Takht granodiorite to study the relationship between its genesis and geological evolution history of the UDMA. Materials and methods A total of about 80 samples from the Takht plutonic-volcanic rocks were collected. 16 plutonic-volcanic samples were selected for whole-rock chemical analysis. Major element oxides were analyzed by the X-ray fluorescence spectrometry (XRF) method using an Optima 7300DV XRF instrument in the Lab West laboratory, Australia. Trace elements were also analyzed in this laboratory with the inductively-coupled plasma mass spectrometry (ICP-MS) method using a NeXION 300 ICPMS instrument. Three chip samples with equal weight (4.5 kg) were collected from the Takht granodiorite. Then upon mixing, average samples were obtained for U–Pb dating of zircon. Hand-picked zircon crystals were supplied to the ALC (Arizona Laser Chron Center) in Arizona University. The 14 selected samples for Nd-Sm and Rb-Sr isotope analysis were crushed to less than 60µm. All isotope analyses were performed on a Nu Instruments Nu Plasma HR in the MC-‎ICP-MS facility, in the University ‎of Cape Town, Rondebosch, South Africa. Results The plutonic rocks have metaluminous nature and are of calc-alkaline affinity. The Sr/Nd, Nb/La and Th/U ratios of the granodiorite show that its magma was formed mainly by melting of continental crust, and that its enclaves were formed from a mantle derived mafic magma. The samples have negative anomalies in Nb, Sr, Ti, P and Eu and positive anomalies in Th, K, Zr, Yb and Rb thus indicating contribution of mantle and crustal materials in their generation. The Takht granodiorite has geochemical features of I and A-type granites and also shows properties of both volcanic arc and within plate magmatism association granitoids (high levels of LILEs and HFSEs). In order to obtain better results, all the data were plotted on a common 206Pb/238U versus 207Pb/235U diagram. The results show an age of 16.8 ± 0.24 Ma (Middle Miocene) for the Takht granodiorite. Based on the results, the Takht granodiorite was generated in Miocene. In the Takht magmatic complex initial 87Sr/86Sr range from 0.70678 to 0.70778 and εNd also changes from -0.79398 to -5.83370. Nd-Sm isotopic contents and trace element ratios indicate that the Takht magmatic complex has originated from oceanic slab break-off with continental crust mingling in the post-collision stage. The εNd (16.8 Ma) vs. initial 87Sr/86Sr ratios diagram reveals, the role of continental crust materials in the generation of the granodiorite samples, while where the enclaves lie are plotted in the mantle evolution array field. Discussion The Takht magmatic complex has geochemical properties of arc related igneous rocks such as Ba, Nb, Sr, P, Ti and Y negative anomalies and for Rb, Th, U, K, Nd and Zr positive anomalies. Most of the Takht area samples are plotted in the triple junction of volcanic arc granites (VAG), within plate granites (WPG) and syn-collision (syn-COLG) on Y versus Nb and the Y+Nb versus Rb diagrams (Pearce et al., 1984). These data suggest post-collisional tectonic setting for the Takht magmatic complex. Field, microscopic and geochemical evidences indicate that simple fractional crystallization of a mafic magma was not the only processes involved in the generation of the studied rocks. On this basis, continental crust material had extensive contribution in the generation of the granodiorites whereas the enclaves are from mantle derived magmas. Relatively high fractionated REE patterns of the granodiorite samples with high LREE/HREE indicate an amphibole-bearing, garnet-free source for the samples while small to moderate negative Eu anomalies require residual plagioclase in the source. The granodiorite samples basically have geochemical properties of I-type granites and it is confirmed by their Nd and Sr isotopic ratios. However, relatively high HFSE contents make them similar to A-type granites. Melting of a former continental arc crust and contamination with mantle derived magmas led to both volcanic arc and within plate geochemical properties of the granodiorites that make them similar to I-type and A-type granitoids. The age of 16.8 ± 0.24 Ma (Middle Miocene) of the Takht granodiorite is consistent with the other post-collisional igneous rocks of the area and regarding its post-collisional geochemical properties the age of collision and related orogeny must be considered at least before Miocene. References Berberian, M. and King, G.C.P., 1981. Towards a paleogeography and tectonic evolution of Iran. Canadian Journal of Earth Science, 18(2): 210–265. Ghasemi, A. and Talbot, C.J., 2006. A new tectonic scenario for the Sanandaj–Sirjan Zone (Iran). Journal of Asian Earth Sciences, 26(6): 683–693. Pearce, J.A., Harris, N.B.W. and Tindle, A.G., 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal of Petrology, 25(4): 956–983.

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