Mantle and crustal controls on the geochemical and isotopic evolution of collisional magmatism: NE Iran

Magmatic activity that accompanied the collision between Arabia and Eurasia at ~27 Ma, provides unique opportunities for understanding the triggers and magma reservoirs for collisional magmatism and its different styles in magmatic front and back-arcs. We present new ages and geochemical-isotopic results for magmatic rocks that formed during the collision between Arabia and Eurasia in NE Iran, which was a back-arc region to the main magmatic arcs of Iran. We also compile an inclusive dataset for igneous rocks that formed before, during and after the collision between Arabia and Eurasia, from both the back-arc and the magmatic front (Urumieh-Dokhtar Magmatic Belt) of Iran.
Our new zircon U-Pb ages indicate that collisional magmatism began at ~24 Ma in the NE Iran back-arc, although magmatism in this area started in Late Cretaceous time and continued until the Pleistocene. The collisional igneous rocks are characteristically bimodal, and basaltic-andesitic and dacitic-rhyolitic components show significant isotopic differences; εNd(t)= +4.4 to +7.4 and εHf(t)= +5.4 to +9.5 for mafic rocks and εNd(t)= +0.2 to +8.4 and εHf(t)= +3.4 to +12.3 for silicic rocks. The isotopic values and modelling suggest that fractional crystallization and assimilation-fractional crystallization play important roles in the genesis of the NE Iran collisional volcanic rocks. Isotopic modelling further emphasises that three components, comprising a depleted mantle, an enriched lithospheric mantle similar to the source of the Main Ethiopian Rift lavas, and the Cadomian continental crust of Iran were the main magmatic triggers. Our results also emphasise the temporal and spatial magmatic variations both in the NE Iran back-arc and between the magmatic front and the NE Iran back-arc.

约在27百万年前，阿拉伯板块与欧亚板块的碰撞事件引发的岩浆活动，为深入探究碰撞岩浆作用的触发机制及岩浆储库，以及岩浆前弧和后弧中不同岩浆风格的演化提供了独特的研究机遇。本研究针对伊朗东北部地区，即伊朗主要岩浆弧带的后弧区域，在阿拉伯板块与欧亚板块碰撞过程中形成的岩浆岩，提供了新的年龄和地球化学同位素数据。我们整理了包括碰撞前、碰撞期间及碰撞后形成的火成岩在内的综合数据集，涵盖了伊朗后弧区域及岩浆前弧（乌尔米耶-多卡特岩浆带）的火成岩。新测定的锆石U-Pb年龄表明，在伊朗东北部后弧区域的碰撞岩浆作用始于约24百万年前，尽管该地区的岩浆活动始于晚白垩纪并持续至更新世。碰撞成因的火成岩具有双峰特征，玄武岩-安山岩和流纹岩成分显示出显著的同位素差异；对于基性岩，εNd(t)值在+4.4至+7.4之间，εHf(t)值在+5.4至+9.5之间；对于酸性岩，εNd(t)值在+0.2至+8.4之间，εHf(t)值在+3.4至+12.3之间。同位素值及其模型分析表明，分异结晶作用和同化-分异结晶作用在东北伊朗碰撞火山岩的形成过程中发挥了重要作用。同位素模型进一步强调，由贫化地幔、富含的地壳地幔（与主要埃塞俄比亚裂谷岩浆的源岩相似）以及伊朗的卡多曼大陆地壳组成的三个组分，是主要的岩浆触发因素。我们的研究结果还强调了在伊朗东北部后弧区域以及岩浆前弧与东北伊朗后弧之间的时空岩浆变化。