藏南措美残余大火成岩省的西延及意义

The westward extension of Comei fragmented large igneous province in southern Tibet and its implications

在西藏南东部和澳大利亚南西部新识别出的早白垩世Comei-Bunbury大火成岩省的范围还未能得到很好约束。为探讨这一问题,本文报道了西藏南东部浪卡子-洛扎以西、江孜、康马地区广泛分布的近东西向基性岩墙和少量玄武岩的岩石学和地球化学数据。这些以辉绿岩、辉长岩为主的镁铁质岩石属于碱性-亚碱性玄武岩系列,在地球化学上主要包括高Ti的洋岛玄武岩型和低Ti的大洋中脊玄武岩型,两者的εNd(t)值分别为(+0.9～+2.0)和(+4.6～+5.8)。这两种类型岩石的地球化学特征均与措美残余大火成岩省内的对应类型可比,暗示它们与措美残余大火成岩省一样,均形成于由地幔柱作用于大陆岩石圈引起的伸展背景,代表该残余大火成岩省的西延。本文研究结果使得现今保留在特提斯喜马拉雅带上的Comei-Bunbury大火成岩省的残余面积接近50000km2。该大火成岩省的大规模岩浆活动可能是引发研究区甚至全球早白垩世大洋缺氧事件的一个重要因素。

The spatial extent of the newly identified Early Cretaceous Comei-Bunbury large igneous province(LIP)in southeastern Tibet and southwestern Australia is not well constrained yet.To probe into this issue,here we report the petrological and geochemical data of the extensive E-W-trending mafic dykes and minor occurrence of basalt distributed in Gyangze,Kangma,and western Nagarze-Lhozhag area in southern Tibet.These mafic rocks,consisting mainly of diabase and gabbro,are alkali to sub-alkali,and can be divided into two groups according to their geochemical characteristics,i.e.,high-Ti type that is similar to OIB,and low-Ti type that resembles MORB,with εNd(t)values of +0.9^+2.0 and +4.6^+5.8,respectively,both of which are geochemically comparable to the corresponding groups in the Comei fragmented LIP to the east,and are inferred to be formed within an extensional setting caused by mantle plume-lithosphere interaction.It follows that the mafic rocks reported here could be the westward extension of the province.Our results indicate that the areal extent of the fragmented Comei-Bunbury LIP preserved in the present-day Tethyan Himalaya is close to 50,000 km2.The extensive magmatism of the Comei-Bunbury LIP is likely to be responsible for the Early Cretaceous oceanic anoxic event in the studied area or even in a global context.