The Shangcang pillow lavas from the bottom of the Emeishan Large Igneous Province,southwestern China: Petrogenesis and tectonic implications
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摘要: 云南上沧二叠纪枕状玄武岩位于峨眉山大火成岩省内带底部。该玄武岩全岩SiO2含量为49.36%～51.36%,TiO2含量为1.28%～1.91%,MgO含量为6.08%～10.79%,Mg#值为53～69,Al2O3含量为13.91%～16.31%,Na2O含量为1.59%～3.72%,K2O含量较低,为0.18%～0.76%,并且Na2O/K2O比值较高,为4.27～10.48,属拉斑系列玄武岩。具有右倾斜型稀土元素配分模式,重稀土和部分高场强元素与N-MORB接近,轻稀土和部分大离子亲石元素介于N-MORB和上地壳之间。(La/Sm)N比值大多数低于2.0(1.4～2.2),(Ce/Yb)N比值介于2.2～3.4之间,明显低于OIB,不具有OIB的地球化学特征,在Th/Hf-Ta/Hf构造环境判别图解中位于陆缘裂谷玄武岩区域,推测上沧枕状熔岩是古特提斯洋俯冲到扬子板块边缘形成的初始裂谷扩张的产物。Abstract: The Shangcang Permian pillow basalts in Yunnan Province crop out on the bottom of the inner zone of the Emeishan Large Igneous Province(ELIP).They are belong to tholeiitic series with SiO2 contents between 49.36% and 51.36%,1.28%～1.91% for TiO2,13.91%～16.31% for Al2O3,6.08%～10.79% for MgO,with Mg#[100×Mg/(Mg+Fe)] of 53～69,and the Na2O contents of them between 1.59% and 3.72%,higher than K2O(0.18%～0.76%),corresponding the high Na2O/K2O ratios(4.27 to 10.48).These basalts are enriched in LREE and LILE with(La/Sm)N values of 1.4～2.2,and(Ce/Yb)N values of 2.2～3.4,but lower than those of OIB(2.3 and 9.6),they instead have the N-MORB affinity in HREE and HSFE,while the characteristics of LREE and LILE are considered as the mixing result of N-MORB and the upper crust.In the Th/Hf-Ta/Hf tectonic diagrams,these pillow basalts plot in the fields of continental marginal rift tholeiitic series rocks,thus the Shangcang pillow basalts are considered as the spreading products of the initial continental marginal rift caused by the subduction of Paleo-Tethys ocean under the edge of Yangtze terrane.
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Key words:
- Pillow basalts /
- Shangcang /
- Continental marginal rift /
- Emeishan Large Igneous Province
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