Continental crust variance between post-collisional compressional and extensional domains controlled magmatic activity at Damavand volcano, Northern Iran

The Quaternary Damavand stratovolcano, with an elevation of 5670 m, is the highest mountain in Iran and is the subject of this study. The main rocks are trachyandesite and trachybasalt, and Sr-Nd isotope ratios of whole rock samples show small variations in ɛSr (+3.2 to + 6.4) and ɛNd (−1 to + 1). Overall, rock chemistry and Sr-Nd isotope ratios support a metasomatic asthenospheric mantle with minor pelagic sediments from the oceanic subducted slab and/or carbonatite melts from the deep mantle as sources. Collision of the Arabian and Eurasian plates compressed the Iranian plateau between them, resulting in compressional and extensional domains in different regions. As a result of continuous pressure from the Arabian plate, thrusting and folding in compressional domains increased crustal thickness while thinning occurred in extensional areas. These processes cause thermal gradient perturbations at the boundaries between the two, leading to the melting of the mantle and/or lower crustal mafic rocks and production of alkaline rocks. Damavand volcano is located at the junction of compressional and extensional regimes, and its activity has been influenced by the variation of crustal thickness along the Alborz Mountain in the southern Caspian Sea. Therefore, the variation of crustal thickness can be suggested as a main mechanism for the generation of magmatic activity along the entire Iranian plateau in a post-collision system.

第四纪达马万德层状火山（Quaternary Damavand stratovolcano）海拔5670米，为伊朗境内最高峰，亦是本研究的研究对象。其主要岩石类型为粗安岩（trachyandesite）与粗面玄武岩（trachybasalt）；全岩样品的锶-钕（Sr-Nd）同位素比值显示，ɛSr值变化范围为+3.2至+6.4，ɛNd值变化范围为−1至+1，二者均仅存在小幅波动。整体而言，岩石地球化学特征与锶-钕同位素比值均表明，该火山的源区为经交代改造的软流圈地幔（metasomatic asthenospheric mantle），辅以来自俯冲洋壳板片的少量远洋沉积物（pelagic sediments）和/或来自深地幔的碳酸岩熔体（carbonatite melts）。阿拉伯板块与欧亚板块的碰撞挤压了夹持于二者之间的伊朗高原，使得不同区域分别形成挤压域与伸展域。受阿拉伯板块持续挤压作用影响，挤压域内的逆冲作用与褶皱作用加剧了地壳增厚，而伸展域则出现地壳减薄现象。上述过程在两类构造域的边界处引发热梯度扰动，致使地幔和/或下地壳镁铁质岩石发生熔融，进而生成碱性岩（alkaline rocks）。达马万德火山地处挤压与伸展构造体制的交汇地带，其活动受到里海南部厄尔布尔士山脉（Alborz Mountain）沿线地壳厚度变化的调控。据此可认为，地壳厚度变化是后碰撞构造体系下整个伊朗高原沿线岩浆活动产生的核心机制。