藏南特提斯喜马拉雅带晚二叠纪基性岩浆作用及其构造地质意义

野外地质调查和SHRIMP锆石U-Pb地质年代学研究在藏南雅拉香波地区厘定了一规模较大,形成于273.0±2.2Ma的辉绿岩体。该辉绿岩体侵入到由页岩和细砂岩组成的特提斯沉积岩中,表明这些沉积岩形成时间早于晚二叠纪,而不是晚三叠纪地层。该岩体具有以下地球化学特征:(1)富集LREE,亏损HREE;(2)高场强元素含量较高;(3)较高的Sr(87Sr/86Sr(t)=0.7063～0.7078)和Nd(εNd(t)=-1.1～-2.4)同位素组成,与西部同时代溢流玄武岩相当;及(4)较高的εHf(t)(+2.5～+3.9)。本研究及已有数据表明:沿新特提斯带发育一系列晚二叠纪基性岩浆岩,是冈瓦纳大陆北缘裂解和新特提斯洋初始张开时深部岩浆作用的产物。

Late Permian mafic magmatism along the Tethyan Himalayan Belt, southern Tibet and tectonic implications

We report a new occurrence of Late Permian mafic magmatic rock near the Yardoi gneiss dome within the Tethyan Himalayan Belt,southern Tibet. This suite of diabase intruded into the fine-grained sandstone and shale of the Tethyan sedimentary sequence and formed at 273. 0 ± 2. 2 Ma ( SHRIMP zircon U-Pb age) ,which indicates that the sedimentary rocks intruded by the diabase formed before Late Permian and not the Late Triassic as assigned by previous studies. This suite of diabase is characterized by (1) enrichment in LREE and depletion in HREE; ( 2) high contents of HFSE; ( 3) relatively high Sr ( 87 Sr / 86 Sr( t) = 0. 7063 ～ 0. 7078) and Nd( ε Nd ( t) = -1. 1 ～ -2. 4) isotope compositions,similar to those in the contemporaneous continental flood basalts and diabases in the western Tethyan Himalaya; and ( 4) positive ε Hf ( t) ( +2. 5 ～ +3. 9) values. Combined with literature data,out new results suggest that the Late Permian mafic magmatism,though distributed sporadically along the Tethyan Himalaya,is more widespread and signifies the rifting along the northern margin of the Gondwana continent and the initial opening of the Neo-Tethyan Ocean.