【摘要】 青藏高原北部柴北缘发育一套与超高压变质带并行的早古生代岛弧火山岩带,岛弧火山岩以玄武岩类为主,包括一些中酸性岩类,岩石以普遍遭受绿片岩相蚀变为特征,区别于该地区普遍遭受角闪岩相区域变质的元古代的基性火山岩。该早古生代的岛孤火山岩显示三组地球化学特征:①VTG-Ⅰ,岛弧拉斑玄武岩(IAT);②VTG-Ⅱ,高Al次钙碱性-碱性过渡型玄武岩;③VTG-Ⅲ,较N-MORB更亏损的拉斑玄武岩(异常MORB)。研究认为前两组火山岩是成熟岛弧两个发育阶段的特征性产物:洋壳俯冲到陆壳的初用,由俯冲洋壳和地幔楔的部分熔融形成岛弧拉斑玄武岩(IAT),随着俯冲板块的速度加快和岛弧周围地壳的加厚,则形成钙碱性玄武岩(CA)、高Al玄武岩。第三组火山岩形成于弧间盆地,由亏损的地幔楔高度部分熔融形成比N-MORB亏损的的火山岩(异常MORB)。岛弧火山岩的锆石LA-ICP-MS法U-Pb年龄为514.2±8.5 Ma,说明柴北缘在早古生代发生过洋壳向陆壳的俯冲作用。鉴于该地区代表陆-陆俯冲作用的柴北缘超高压变质岩石也是形成于早古生代(494Ma),认为陆-陆俯冲作用发生在洋-陆俯冲作用之后,二者时间和空间相伴随。更多还原

【Abstract】 A suite of Early Paleozoic island arc volcanic rocks, consisting mainly of basaltic lavas and some intermediate-silicic rocks, crops out in the North Qaidam basin, northeastern Tibet Plateau. These rocks have been extensively metamorphosed under greenschist facies conditions, and hence differ from the older Proterozoic intermediate-mafic volcanic rocks, which have generally experienced amphibolite-facies metamorphism. The Early Paleozoic volcanic rocks are divided into three groups based on their geochemical characteristics: (1) VTG-I, consisting of island arc tholeiitic basalts, basaltic andesites and andesites that crop out in Jilusu and Shuangkoushan; (2) VTG-II, which are high-Al, alkaline and subalkaline basalts in Jilusu; and (3) VTG-III, an assemblage of MORB-type basalts, andesites and dacites and mostly in Shuangkoushan, with stronger trace elements depletion than N-MORB. Groups VTG-I and II are considered to be typical of mature island arcs produced in two stages. In the first stage, following the initiation of ocean-continent subduction, the island arc tholeiitic volcanic rocks were generated by partial melting of the subsiding oceanic crust and mantle wedge beneath the island arc. As subduction progressed and accelerated, calc-alkaline and high-Al basalts were formed and the crust beneath the island arc was thickened. The third group of volcanic rocks (VTG-III) consists of highly depleted MORB-type lavas, and is believed to have been generated by partial melting of the depleted mantle wedge in an inter-arc basin. The island arc tholeiites are dated at 514. 2±8. 5 Ma by the U-Pb method on single zircon crystals using LA-ICP-MS. This suggests that an Early Paleozoic ocean-continent subduction took place in the North Qaidam Basin. UHP eclogite in the North Qaidam Belt has been dated at 494 Ma, suggesting that continent-continent collision occurred shortly after the ocean-continent subduction was initiated.更多还原