帕米尔高原Rushan-Pshart古特提斯洋或中特提斯洋争论的岩石地球化学证据

Geochemical evidence for Paleotethys or Mesotethys argument of the Rushan-Pshart suture in eastern Pamir

帕米尔高原上广泛分布的加里东期火山岩,印支期火山岩与燕山期火山岩被认为是块体依次向北俯冲拼贴到欧亚板块上的产物。特别是燕山期火山岩被认为是Rushan-Pshart中特提斯洋闭合的产物。但近年来地层古生物、火山岩证据不断表明Rushan-Pshart洋闭合时间在晚三叠一早侏罗世,由此限定Rushan-Pshart古特提斯洋性质,而帕米尔高原上广泛分布的燕山期火山岩是更南部的Shyok中特提斯洋闭合的产物。目前,中国境内Rushan-Pshart缝合带属性的研究工作展开较晚,研究程度较低。我们对塔什库尔干明铁盖沟一线燕山期火山岩带展开的工作发现零星分布的印支期花岗岩。印支期花岗岩错石U・Pb年龄显示岩体侵位时间在201 Ma左右。全岩主量元素特征表明岩石为I型高钾钙碱性闪长花岗岩;稀土元素在球粒陨石标准化图解中呈轻稀土元素相对富集,重稀土元素相对亏损的右倾海鸥型。明铁盖岩体的微量元素显示大离子亲石元素明显富集,而高场强元素明显亏损。稀土微量元素特征倾向花岗岩属性为岛弧型花岗岩,Sr-Nd-Hf同位素比值特征显示岩体形成于下地壳部分熔融环境。岩体的地化特征表明岩体形成的构造环境为板块汇聚的洋壳俯冲阶段,结合区域地质特征,我们将花岗岩体归为Rushan-Pshart古特提斯洋壳俯冲消减的产物。Rushan-Pshart缝合带传统上认为是中特提斯带,近年的研究进展认为其为古特提斯缝合带,本文的工作支持这种观点。Rushan-Pshart古特提斯缝合带的确立对帕米尔高原与青藏高原主体的块体对比提供了可信的对比方案,并对青藏高原新生代陆内变形方式的争论提供了可靠的证据。

Magmatic rocks are widespread in the eastern Pamir,with a wide range of ages from the time of Rodinia to the Himalayan Orogen.Three magmatic belts are developed in eastern Pamir,which is referred to as the Cambro-Ordovician,Permo-Triassic,and J—K magmatic belts from north to south,the three magmatic belts observed in the Pamir were generated by northward subduction and sequential accretion of blocks onto the southern margin of Eurasia during the closure of the Proto-,Paleo-,and Meso-Tethyan oceans.Magmatic,paleontological and stratigraphic studies indicate that the Cimmerian block was accreted to central Pamir along the Rushan-Pshart suture between the Late Triassic and Early Jurassic,Thus,some studies argued that the Rushano-Pshart suture should be regarded as a Paleo-Tethyan suture rather than Meso-Tethyan suture as traditionally believed.The Indosinian granitic plutons are dated with 206~201 Ma.Samples contain 67%SiO2,yielding low A/CNK values and negative Eu anomalies they represent metaluminous,Ⅰ-type granitoids.They also exhibit high LILE/HFSE ratios,pronounced negative Nb,Ta,P,and Ti anomalies.In tectonic classification diagrams,the granitic rocks fall into the volcanic arc granite field.Based on geochemical and Sr-Nd-Hf isotopic data we interpret that the 201 MaⅠ-type arc-like granites to be related to subduction of Rushan-Pshart Paleo-Tethyan oceanic slab.Correlation of the Rushan-Pshart suture to the Longmu Co-Shuanghu suture and the Shyok suture to the Bangong Co-Nujiang suture will benefit the blocks correlation between Pamir and Tibetan Plateau.