Significant uncertainty remains regarding the exact timing and nature of subduction events during the closure of the Tethyan seas in what is now NW Iran. This study thus presents new geochemical compositions and U–Pb ages for a suite of volcanic rocks emplaced during Cenozoic volcanism in the west Alborz Magmatic Assemblage, which is commonly regarded as the back-arc of the Neotethyan magmatism in Central Iran. The subalkali basalts and andesites are dated to 57 ± 1.2 Ma, and are likely derived from a supra-subduction mantle wedge. Later, trachytic A-type rocks erupted from ∼42 to 25 Ma during an anorogenic (extensional) stage triggered by slab retreat and associated asthenospheric mantle influx. A-type melts were at least partly concurrent with lithospheric mantle magmatism implied by eruption of subalkali basalts–andesites around 26–24 Ma. Next, Amp-Bt trachybasaltic volcanism with high-Nb basaltic affinity at ∼19 Ma likely records slab deepening and slab partial melting, which reacted with the mantle wedge to produce the source material for the high-Nb basalts. Sr–Nd isotopic ratios for SE Ahar mafic as well as A-type rocks imply rather enriched mantle source(s). Some crustal contamination is implied by the presence of inherited zircons dominated by those derived from Neoproterozoic–Cambrian basement rocks and Carboniferous magmatism. Rhyolitic rocks with adakitic affinity probably mark the final volcanism in the study area. The adakitic rocks show crustal signatures such as high K and Th, probably formed as a consequence of higher temperature gradients, at crustal levels, imposed by both slab and mantle partial melts.

中文翻译：

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岩石圈地幔、软流圈、板片和地壳对来自西厄尔布尔士岩浆组合的始新世至中新世火山岩的成因贡献，伊朗 SE Ahar

在现在伊朗西北部的特提斯海关闭期间，俯冲事件的确切时间和性质仍然存在很大的不确定性。因此，本研究提出了西厄尔布尔士岩浆组合中新生代火山活动期间侵占的一套火山岩的新地球化学成分和 U-Pb 年龄，该组合通常被认为是伊朗中部新特提亚岩浆活动的后弧。亚碱玄武岩和安山岩的年代为 57 ± 1.2 Ma，很可能来自超俯冲地幔楔。后来，在板片后退和相关的软流圈地幔流入引发的造山（伸展）阶段，粗面质 A 型岩石在 42 至 25 Ma 期间喷发。A型熔体至少部分与岩石圈地幔岩浆作用同时发生，这暗示着26-24 Ma左右亚碱玄武岩-安山岩的喷发。下一个，Amp-Bt 粗面玄武质火山岩在 19 Ma 时具有高 Nb 玄武岩亲和力，可能记录了板片加深和板片部分熔融，这与地幔楔反应产生了高 Nb 玄武岩的源材料。SE Ahar 镁铁质和 A 型岩石的 Sr-Nd 同位素比表明地幔源相当富集。一些地壳污染暗示了以新元古代-寒武纪基底岩和石炭纪岩浆作用为主的继承锆石的存在。具有埃达克质亲和力的流纹岩可能标志着研究区的最终火山活动。埃达克质岩石显示出高 K 和 Th 等地壳特征，这可能是由于在地壳水平上由板片和地幔部分熔融所施加的较高温度梯度的结果而形成的。