Miocene magmatic rocks are exposed as lava flows (OKV group), subvolcanic rocks (SRG group), and as lavas interbedded with shallow-basin sedimentary layers (TVN group) in the Takab area, NW Iran. Zircon U–Pb dating yields ages of 18 to 15 Ma. Whole-rock chemistry shows that most of the Early Miocene magmatic rocks are andesite with subordinate dacite. The magmatic rocks have low contents of MgO, Ni, Cr, Ti, Nb and Ta, and high concentrations of Li, large ion lithophile elements such as Rb, K and Ba, and light rare earth elements. The OKV and SRG groups have similar initial 87Sr/86Sr ratios (0·70557–0·70768) and εNd(t) values (+1·0 to +2·2). The TVN group show larger variations of 87Sr/86Sr(i) ratios from 0·70628 to 0·71033 and εNd(t) values from –3·8 to +1·6. This implies a greater role of involvement of supra-crustal domains in the evolution of the TVN group relative to the SRG and OKV groups. Early Miocene magmatic rocks in the Takab area are situated between the Sanandaj–Saqqez Cretaceous calc-alkaline andesite in the SW and the Late Eocene–Oligocene (35–27 Ma) ocean island basalt-like Mianeh–Hashtrood magmatic belt of possible back-arc affinity in the NE. In addition, Late Eocene (40–37 Ma) syn-collision granites in the Baneh–Marivan area along the Zagros suture zone along the west side of the Sanandaj–Saqqez Cretaceous calc-alkaline andesite body indicate that collision of the Arabian Plate and the NW Iran Block occurred in the Late Eocene. These observations support the idea that Early Miocene andesites (18–15 Ma) in the Takab area were generated after collision, which was also associated with doubling of the thickness of the continental crust in the Zagros suture zone, thinning of continental crust far from the Zagros suture zone, and development of shallow-basin sedimentary rocks in NW Iran. Partial melting of mafic calc-alkaline bodies at depth or highly metasomatized fossil mantle owing to thinning of continental crust and asthenospheric upwelling may represent possible sources for the Late Miocene andesite. We conclude that andesitic rocks, even with typical arc signatures, are not always generated in an active margin and that some were probably generated in a post-collision tectonic regime. Misinterpretation of the arc signature can result in erroneous assumptions as to the geodynamic regime, and in the particular case of NW Iran, the timing of collision of the Arabian and Iranian plates.

中文翻译：

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伊朗西北部塔卡布地区早中新世碰撞后安山岩

在伊朗西北部的塔卡布地区，中新世岩浆岩以熔岩流（OKV 组）、次火山岩（SRG 组）和与浅盆地沉积层（TVN 组）互层的形式暴露。锆石 U-Pb 测年得出的年龄为 18 至 15 Ma。全岩化学表明，早中新世岩浆岩大部分为安山岩，次要英安岩。岩浆岩MgO、Ni、Cr、Ti、Nb、Ta含量低，Li、Rb、K、Ba等大离子亲石元素和轻稀土元素含量高。OKV 和 SRG 组具有相似的初始 87Sr/86Sr 比率 (0·70557–0·70768) 和 εNd(t) 值 (+1·0 到 +2·2)。TVN 组显示 87Sr/86Sr(i) 比率从 0·70628 到 0·71033 和 εNd(t) 值从 –3·8 到 +1·6 的变化较大。这意味着相对于 SRG 和 OKV 组，地壳上域的参与在 TVN 组的演变中发挥了更大的作用。Takab地区早中新世岩浆岩位于SW的Sanandaj-Saqqez白垩纪钙碱性安山岩与可能弧后的晚始新世-渐新世（35-27 Ma）洋岛状玄武岩Mianeh-Hashtrood岩浆带之间NE中的亲和力。此外，Sanandaj-Saqqez 白垩纪钙碱性安山岩体西侧 Zagros 缝合带 Baneh-Marivan 地区晚始新世（40-37 Ma）同向碰撞花岗岩表明阿拉伯板块与NW 伊朗地块发生在晚始新世。这些观察结果支持了Takab地区早中新世安山岩（18-15 Ma）是在碰撞后产生的观点，这也与扎格罗斯缝合带的大陆地壳厚度加倍、远离扎格罗斯缝合带的大陆地壳变薄以及伊朗西北部浅盆地沉积岩的发育有关。由于大陆地壳变薄和软流圈上涌导致的深部镁铁质钙碱性体的部分熔融或高度交代的化石地幔可能是晚中新世安山岩的可能来源。我们得出结论，即使具有典型的弧形特征，安山岩并不总是在活动边缘产生，有些可能是在碰撞后的构造状态下产生的。对弧形特征的误解可能导致对地球动力学机制的错误假设，在伊朗西北部的特殊情况下，可能会导致阿拉伯板块和伊朗板块碰撞的时间。