U–Pb age and Hf isotopes of zircon from basaltic andesite and geochemical fingerprinting of the associated picrites in the Emeishan large igneous province, SW China
U–Pb age and Hf isotopes of zircon from basaltic andesite and geochemical fingerprinting of the associated picrites in the Emeishan large igneous province, SW China
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DOI：
10.1007/s00710-014-0349-z
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发表时间：
2015-02
影响因子：
1.8
通讯作者：
Qingyan Tang;Chusi Li;Mingjie Zhang;Yao Lin
中科院分区：
文献类型：
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作者：
Qingyan Tang;Chusi Li;Mingjie Zhang;Yao Lin
峨眉山大火成岩省二叠纪溢流玄武岩中苦橄岩的时代长期存在争议。有些研究者认为这些苦橄岩与溢流玄武岩是同岩浆的，而另一些研究者则认为它们是三叠纪苦橄斑岩或新生代富橄榄石脉岩。本文报道了宾川火山岩系中最年轻的锆石U-Pb年龄为256.2 ± 1.4Ma。该年龄在峨眉山大火成岩省侵入岩和喷出岩的报道值范围内。宾川火山岩系最年轻锆石的ε Hf值（-0.3 ~6.6）与峨眉山大火成岩省侵入岩中同岩浆锆石的ε Hf值（-5 ~10）相似。我们的新资料证实，宾川火山序列是峨眉山大火成岩省的一个组成部分。四川和云南有争议和无争议的苦橄岩具有相似的橄榄石Fo-Ni-Mn组成和全岩不相容的微量元素比值，如Th/Nb和Ti/Dy。这与它们在空间上与峨眉山溢流玄武岩的密切关系一起表明，有争议的苦橄岩也属于峨眉山大火成岩省。区内新生代富橄榄岩脉与峨眉山大火成岩省的苦橄岩可通过不同的橄榄岩Fo-Ni-Mn组成和全岩不相容微量元素Th/Nb、Ti/Dy比值区分开来。这一研究结果证实了以前有争议的苦橄岩作为中国西南地区二叠纪地幔柱活动证据的正确性。
The ages of many picrites associated with Permian flood basalts in the Emeishan large igneous province, SW China have been debated for a long time. Some researchers believe that these picrites are co-magmatic with the flood basalts whereas others believe that they are Triassic picritic porphyrites or Cenozoic olivine-rich dikes. We report a U–Pb age of 256.2 ± 1.4 Ma for the youngest zircon grains from the Binchuan volcanic sequence which contains the disputed picrites. This age is within the range of values reported for the intrusive and extrusive rocks of the Emeishan large igneous province. The εHfvalues (−0.3 to 6.6) of the youngest zircon crystals from the Binchuan volcanic sequence are similar to those (−5 to 10) of co-magmatic zircon crystals from the intrusive rocks of the Emeishan large igneous province. Our new data confirm that the Binchuan volcanic sequence is an integral part of the Emeishan large igneous province. The disputed and undisputed picrites in Sichuan and Yunnan have similar olivine Fo-Ni-Mn compositions and whole-rock incompatible trace element ratios such as Th/Nb and Ti/Dy. This, together with their spatially close association with the Emeishan flood basalts, indicates that the disputed picrites also belong to the Emeishan large igneous province. The confusing Cenozoic olivine-rich dikes in the region can be distinguished from the picrites of the Emeishan large igneous province by their different olivine Fo-Ni-Mn compositions and whole-rock incompatible trace element ratios such as Th/Nb and Ti/Dy. The results from this study vindicate the use of the previously-disputed picrites as evidences for Permian mantle plume activity in SW China.