藏北尼木冬勒钾长花岗岩地球化学特征、物质源区及构造环境分析

Analysis on Geochemical Characteristics,Material Source Area and Tectonic Environment of the Nimudongle Moyite in Northern Tibet

藏北尼木冬勒钾长花岗岩大地构造位置为冈底斯火山-岩浆岩带,岩体呈不规则状侵位于古近纪帕那组、新近纪雄巴组火山岩地层中,与各地层均呈角度不整合接触关系。尼木冬勒钾长花岗岩主微量元素地球化学总体特征表明区内早白垩世钾长花岗岩主要为准铝质-过铝质高钾钙碱性岩石系列。与原始地幔相比,花岗岩相对富集不相容元素Rb、Th、U、La、Ce、Nd、Zr、Hf等,而Ba、Nb、Sr、P、Ti呈现低谷,富集轻稀土和大离子亲石元素,强烈亏损Nb、Ta、P和Ti等高场强元素。通过对尼木冬勒钾长花岗岩体地球化学特征研究,进而讨论该地区钾长花岗岩的物质源区及构造环境。研究表明该区钾长花岗岩岩浆源区遭受了俯冲板片沉积物熔体的交代作用,具有岛弧岩浆岩特征,为典型的活动大陆边缘构造环境。

The Nimudongle moyite in northern Tibet is located in gangdise volcanic-magmatic belt.The rock mass is irregularly emplaced in volcanic strata of Pana formation in Paleogene and Xiongba Formation in Neogene,and it is in angular unconformable contact with all layers.The geochemical characteristics of the main and trace elements of the Nimudongle moyite show that the early Cretaceous potassic feldspar granites are mainly peraluminous high-k calc-alkaline rocks.Compared with the original mantle,the potassic feldspar granites are enriched in incompatible elements Rb,Th,U,La,Ce,Nd,Zr,Hf,etc.,while Ba,Nb,Sr,P,Ti show low trough,enriched in LREE and large ion lithophile elements,and depleted of Nb,Ta,P,Ti and other high field strength elements.Based on the study of the geochemical characteristics of the Nimudongle moyite,the material source area and tectonic environment of the moyite in this area are discussed.It shows that the magma source area of the moyite in this area was metasomatic by subduction plate sediment melt,and has the characteristics of island arc magmatic rock,which is a typical active continental margin tectonic environment.