Advances in Earth Science ›› 2015, Vol. 30 ›› Issue (9): 1034-1049. doi: 10.11867/j.issn.1001-8166.2015.09.1034

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Geochemical Characteristics and Tectonic Significance of the Granotoids in the Western Section of the Mid-Qilian

Rongna Hou 1( ), Shuhua Wang 1, Xiang Zhang 2, Kexuan Hou 1, Cheng Zhang 3, Jinrong Wang 1( )   

  1. 1.Key Laboratory of Mineral Resources in Western China (Gansu Province), School of Earth Sciences, Lanzhou University, Lanzhou 730000, China
    2. Geological Survey of Gansu Province, Lanzhou 730000, China
    3. Second Institute of Geological and Mineral Exploration of Gansu Provincial Bureau of Geology and Mineral Resources, Lanzhou 730000, China
  • Received:2015-07-15 Revised:2015-08-27 Online:2015-09-20 Published:2015-09-20

Rongna Hou, Shuhua Wang, Xiang Zhang, Kexuan Hou, Cheng Zhang, Jinrong Wang. Geochemical Characteristics and Tectonic Significance of the Granotoids in the Western Section of the Mid-Qilian[J]. Advances in Earth Science, 2015, 30(9): 1034-1049.

Granotoids, mainly gabbro, diorite and granite, are greatly rich in the northern and southern parts of Shibandun area, western section of the Mid-Qilian. LA-ICP-MS zircon and U-Pb dating methods indicate that the age of the rock from the northern belt were(469.3±2.8)Ma,(461.2±3.3)Ma and (470.0±2.5) Ma, respectively. The SiO2 content of the northern rock mass ranges from 53.2% to 66.1% with high content of the Al, Ti, Mg, Fe, Ca, K, Na in the rock mass and A/CNK<1.1. The rock analysis shows enrichment of large ion lithophile elements (LILE) (e.g., Rb, K) and Pb, and depletion of high field strength elements (HFSE) (e.g., Nb, Ta, Ce, Sr, Hf and Ti). The results also indicate LREE enriched pattern with high ∑REE abundance, flat HREE and moderate negative Eu anomaly (δEu=0.6). Tectonic discrimination diagrams showed that the samples are located in the arc environment. The age of the rock from the southern belt are respectively (470.9±2.8) Ma and (472.3±4.2) Ma based on LA-ICP-MS zircon U-Pb dating methods. The southern rock mass contains high amount of the Al, Mg, Fe, Ca and is rich in Na and poor in K (A/CNK<1.1). Rock analysis results show the enrichment of large ion lithophile elements (LILE) (e.g., Rb, K and Sr) and depletion of high field strength elements (HFSE) (e.g., Nb, Ta, Ti and P), and also indicate a LREE enriched pattern with rather low ∑REE, depleted HREE and obvious differentiation in Eu, which is from slight negative to positive anomaly (δEu=0.74~1.18). Our study shows that the northern belt was formed by partial melting of mantle wedge in an island-arc environment, while the southern belt is adakites generated by partial melting of Adakites, and is also the product from the magmatic process by the North Qilian southward subduction. The western part of the Mid-Qilian is an island arc hyperplasia complex created on a remained microcontinental in early Paleozoic.

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