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首页> 外文期刊>Journal of Asian earth sciences >Magma origins and geodynamic implications for the Makran-Chagai arc from geochronology and geochemistry of Bazman volcano, southeastern Iran
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Magma origins and geodynamic implications for the Makran-Chagai arc from geochronology and geochemistry of Bazman volcano, southeastern Iran

机译：伊朗东南部Bazman火山的地质年代和地球化学对Makran-Chagai弧的岩浆起源和地球动力学意义

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摘要

Bazman is an isolated voluminous (3497 m elevation; 300 km(2)) composite volcano that is located within the western segment of the Makran-Chagai arc. This arc obliquely tracks the modern EW-trending trench offshore the Oman sea coast of Iran and Pakistan, with several Quaternary stratovolcanoes (Bazman and Taftan in SE Iran, and Kuh-i-Sultan in western Pakistan) superimposed on a preexisting Neogene arc. High spatial resolution and high sensitivity U-Pb geochronology using Secondary Ionization Mass Spectrometry (SIMS) yielded four clusters of zircon ages for Bazman volcanic rocks which range from ca. 8.6 Ma to 41 ka. Lava and pyroclastic materials erupted between 8.6 and 5.9 Ma forming a thick Neogene volcanic succession (Ngv). These deposits were intruded by hypabyssal andesite-dacite domes at ca. 4.1 Ma. The next younger group of the Quaternary Bazman center comprises several eruptive pulses at 1.37 Ma, 1.13 Ma, 0.943 Ma, 0.853 Ma and 0.635 Ma, which constructed the main edifice around a central vent through alternating eruptions of lava and pyroclastic rocks (Qtv and Qa2). Subsequently, ignimbrite flows also sourced from the main vent erupted at 0.59 and 0.47 Ma, and covered the earlier erupted deposits. The youngest dated eruptive phase (Qa3) comprises the emplacement of dacitic to andesitic lava around the top of volcano. Based on Uranium-series (U-238-Th-230) disequilibrium in Qa3 zircons, model ages of ca. 41 ka define a maximum eruption age of these lavas, which tapped a long-lived magma reservoir with zircon crystallization ages predating eruption by 10's to 100's of ka. Low- medium-K calc-alkaline basaltic-andesitic eruptions formed parasitic cones located on northeastern flanks of the edifice as the youngest unit (Qa4) of Bazman. Both Neogene and Quaternary rocks from Bazman volcano exhibit homogeneous trace element characteristics, including enrichments in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to the high field strength elements (HFSE) and heavy rare earth elements (HREE), respectively. High Th/Yb and negative anomalies of Ta, Nb and Ti are consistent with an enriched mantle source in a continental subduction zone. Trace element modeling for the most primitive basalts erupted at Bazman suggests an origin of parental magmas through similar to 15% partial melting of an enriched spinell-lherzolite mantle, whereas the genesis of Bazman acidic volcanic rocks with SiO2 62% involves partial melting in the middle/lower continental crust which became subsequently overprinted through assimilation - fractional crystallization (AFC) processes. Compositional continuity of Quaternary Bazman volcanic rocks with their Neogene precursors implies a steady magma source above the subducted Oman slab. Consequently, subduction geometry in the Makran-Chagai arc has experienced little modification over the past few million years.
机译：Bazman是一个孤立的巨大火山（海拔3497 m；> 300 km（2）），位于Makran-Chagai弧的西段内。这条弧线倾斜地追踪了伊朗和巴基斯坦阿曼沿海近海的EW趋势海沟，上面叠置了若干第四纪平流层火山（伊朗东南部的Bazman和Taftan，巴基斯坦西部的Kuh-i-Sultan）。使用二次电离质谱法（SIMS）的高空间分辨率和高灵敏度U-Pb地质年代学得出了Bazman火山岩的四个锆石年龄簇，其范围从大约1到2千倍不等。 8.6马至41 ka。熔岩和火山碎屑物质在8.6Ma和5.9Ma之间爆发，形成了厚的新近纪火山演替（Ngv）。这些沉积物是由约旦山下的安息形安山岩-达克特穹顶侵入的。 4.1马下一个第四纪的Bazman中心群包括几个在1.37 Ma，1.13 Ma，0.943 Ma，0.853 Ma和0.635 Ma的喷发脉冲，这些喷发脉冲通过熔岩和火山碎屑岩的交替喷发（Qtv和Qa2 ）。随后，也从主喷口喷出的火成岩流在0.59和0.47 Ma处喷出，并覆盖了较早喷出的沉积物。最年轻的过时喷发期（Qa3）包括围绕火山顶部的大山到安山熔岩的过渡。根据Qa3锆石中铀系列（U-238-Th-230）的不平衡，模型年龄大约为。 41 ka定义了这些熔岩的最大喷发年龄，这些熔岩利用了寿命长的岩浆储集层，锆石的结晶年龄比喷发10到100 ka的火山喷发还要早。低中K钙碱性玄武岩和麻醉岩喷发形成的寄生锥位于大厦东北侧，是Bazman最年轻的单元（Qa4）。巴兹曼火山的新近纪和第四纪岩石均表现出均一的微量元素特征，包括相对于高场强元素（HFSE）和重稀土元素（HREE）的大型离子亲石元素（LILE）和轻稀土元素（LREE）富集， 分别。高的Th / Yb以及Ta，Nb和Ti的负异常与大陆俯冲带中丰富的地幔源一致。巴兹曼火山喷发的最原始玄武岩的痕量元素模拟表明，亲子岩浆的起源是通过富含15％的尖晶石-锂铁矿地幔部分融化而实现的，而SiO2> 62％的巴兹曼酸性火山岩的成因涉及火山岩的部分融化。中/下大陆壳，随后通过同化-分步结晶（AFC）过程被套印。第四纪的巴兹曼火山岩及其新近纪前驱体的组成连续性意味着在俯冲的阿曼板块上方有稳定的岩浆源。因此，在过去的几百万年中，Makran-Chagai弧中的俯冲几何几乎没有发生任何变化。

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