西藏冈底斯北缘尼玛地区帮勒村一带寒武纪火山岩LA-ICP-MS锆石U-Pb年龄及其地球化学特征

青藏高原南部中国境内是否存在寒武纪地层一直是地质界关注的焦点。研究表明,冈底斯北缘西藏尼玛县控错南帮勒村一带,原划的前震旦纪念青唐古拉岩群为一套浅变质的拉斑系列双峰式火山岩。通过高精度LA-ICP-MS(激光剥蚀等离子体质谱)锆石微区原位U-Pb同位素测年,获得其中变流纹岩206Pb/238U年龄加权平均值为(536.4±3.6)Ma,确定该套火山岩形成于早寒武世。地球化学分析表明,变玄武岩富集LREE、LILE、部分HFSE,Nb、Ta、Zr、Hf等亏损,Zr/Nb比值(2.53~3.61)低;变流纹岩明显富集LREE、LILE、Th、HFSE,具有Eu负异常和Sr、Nb、Ta、Ti负异常。同时,变玄武岩的εNd(t)为负值(-2.80~-4.56),源区为富集地幔端元EMⅡ,反映其源区为岩石圈和软流圈相互作用的产物,并遭受了地壳物质的混染。综合分析认为该套火山岩形成于陆缘裂谷环境,为冈底斯北缘存在寒武纪裂解作用提供了佐证。

Geochemical characteristics of the Cambrian volcanic rocks in Banglecun area on the northern margin of Gangdise,Nyima County,Tibet

Geologists show much concern over the problem whether there exist Cambrian strata in southern Tibetan Plateau.The"Nyainqentanglha Rock Group" originally classified as pre-Sinian strata consists of epimetamorphic tholeiitic series bimodal volcanic rocks in Banglecun area,Nyima County,on the northern margin of Gangdise.The 206Pb/238U weighted average age of(536.4±3.6) Ma was obtained by in-situ LA-ICP-MS zircon dating of the meta-rhyolite,which proves that this volcanic rock was formed in early Cambrian.The results of geochemical analysis show that the meta-basalt is enriched in LREE,LILE and some HFSE and depleted in Nb,Ta,Zr,Hf,and yields low Zr/Nb ratios(2.53~3.61).However,the meta-rhyolite is obviously enriched in LREE,LILE,Th,HFSE,and depleted in Eu,Sr,Nb,Ta and Ti.Meanwhile,the meta-basalt yields a negative εNd(t)(-2.80~-4.56),and the magma was enriched in mantle source EMⅡ,which may suggest that the magma resulted from the interaction between the lithosphere and the asthenosphere and experienced crustal contamination.Based on a comprehensive analysis,the authors hold that the volcanic rocks were formed in a tectonic setting of continental margin rift,which offered evidence of the breakup on the northern margin of Gangdise in Cambrian.