可可西里雄鹰台中新世橄榄安粗质火山岩的成因

Origin of Miocene shoshonitic volcanic rocks from Xiongyingtai, Hoh Xil, North Xizang

可可西里雄鹰台火山熔岩的全岩K-Ar年龄为11.1～12.8Ma,SiO2含量为55.28%～58.65%,K2O含量为3.52%～4.42%,属橄榄安粗岩系列,主要岩类为橄榄安粗岩和安粗岩。富集轻稀土(LREE)和大离子亲石元素(K、Rb、Cs、Sr、Ba、U、Th),Sr同位素比值(87Sr/86Sr)较高,在0.7082～0.7086之间,Nd同位素比值(143Nd/144Nd)较低,在0.51236～0.51243之间,206Pb/204Pb、207Pb/204Pb、208Pb/204Pb的比值大体分别在18.58～18.70、15.60～15.68、37.98～38.33之间变化,微量元素和同位素地球化学示踪研究表明,火山岩源区为富集的地幔类型(EMⅡ),该类型地幔的形成与缝合带的成因有关。火山岩在时空及成因上与昆仑南缘断裂的活动有密切的联系。

The Xiongyingtai volcanic lavas occur in Hoh Xil, North Xizang Autonomous Region and are mainly composed of shoshonite and trachyandesite with K Ar whole rock ages in the range of 11.1～12.8 Ma. The lavas have a constant content of SiO2 (55.28%～58.65%) and K2O (3.52%～4.42%), and are rich in LREE and large ion lithophile elements (K, Rb, Cs, Sr, Ba, U and Th). The high ratios of 87Sr/86Sr (0.7082～0.7086), low 143Nd/144Nd (0.51236～0.51243), and 206Pb/204Pb (18.58～18.70), 207Pb/204Pb (15.60～15.68) and 208Pb/204Pb (37.98～38.33) suggest an EMⅡ(enriched mantle) source under the Xizang Qinghaiplateau. It is proposed that the shoshonitic lavas are probably genetically related to the South Kunlun suture, and a close spatial and temporal relationship to the south marginal fault of Kunlun Mountains.