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.