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Geoscience ›› 2019, Vol. 33 ›› Issue (05): 990-1002.DOI: 10.19657/j.geoscience.1000-8527.2019.05.06

• Petrology • Previous Articles     Next Articles

Discovery and Significance of Early Jurassic Bimodal Volcanic Rocks in Huoshaoyun, Karakoram

ZHOU Nengwu(), CHEN Bangxue(), DENG Zhongfei, SANG Mingshuai, BAI Quanjin   

  1. No.11 Geological Party, Xinjiang Bureau of Geology and Mineral Exploration and Development, Changji, Xinjiang 831100, China
  • Received:2018-12-23 Revised:2019-05-22 Online:2019-10-26 Published:2019-10-28
  • Contact: CHEN Bangxue

Abstract:

The Karakoram volcanic rocks are close to the Qiao’er Tianshan Fault, and the two are closely space-time related. The SiO2 contents of the basalt and dacite are of 48.84%-52.44% and 62.22%-65.63%, respectively. They have obvious Daly intercepts and are typically bimodal. Basalt has a low content of Al2O3, and high contents of MgO, CaO, TiO2. Some trace elements (Sr, Ba) are strongly depleted, whereas the high field strength elements (HFSEs, e.g.Ta, Nb, Ti, Zr) are not depleted. Dacite has relatively high Al2O3, Na2O and K2O contents, but low MgO, CaO and TiO2 contents, and are enriched in large ion lithophile elements (LILEs, e.g.Rb, Th, U, K), depleted of HFSEs, showing typical island arc geochemical affinity. Both the basalt and dacite are calc-alkaline, co-magmatic and likely formed in back-arc basins. Dacite may have formed from the fractionation of basaltic magma. LA-ICP-MS zircon U-Pb dating of the dacite yielded Early Jurassic age of (195.5±1.1) Ma. Considering the regional tectonic evolution, it is concluded that the Qiao’er Tianshan area may have been a developing backarc basin. We suggest that the lithospheric mantle of the basin during the volcanism had not been broken off and no oceanic crust was formed.

Key words: Karakoram, bimodal volcanism, basaltic rock, dacite

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