Early-Cretaceous highly fractionated I-type granites from the northern Tengchong block, western Yunnan, SW China: Petrogenesis and tectonic implications

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• 责任者：Ren-Zhi Zhu ; Shao-Cong Lai ; ; Jiang-Feng Qin ; Shao-Wei Zhao
• 主题词：Early Cretaceous ; Highly fractionated granites ; Zircon U–Pb dating ; Sr–Nd–Pb isotopes ; Tengchong block
• 年：2015
• 作者单位：State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China
• 刊：State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China
• 期：100

摘要

Western Yunnan, an important constituent of the southeastern segment of the East Tethyan tectonic domain, lies along a transformed orientation from the NWW trending Himalayan–Tethyan segment to the northerly trending Southeast Asian segment. However, the geodynamical setting of the Early Cretaceous tectonothermal magmatism along the Bangong–Nujiang–Lushui–Luxi–Ruili belt as the Tethyan branch in western Yunnan (SW China) remains controversial. The Donghe granitoid, which is located between the Gaoligong and Tengliang belts in the northern Tengchong block, reveals its petrogenesis and its tectonics, both of which play a vital role in resolving previous disputes. Our zircon laser ablation inductively coupled plasma mass spectrometry U–Pb dating of granites from the Donghe batholith yields ages of 119.9 ± 0.9–130.6 ± 2.5 Ma. These granites display features typical of highly fractionated I-type granites: high SiO2 contents (>71 wt.%), high K contents (K2O = 3.88–5.66 wt.%), calc-alkaline character, slight peraluminosity (A/CNK = 1.02–1.16), and a highly differentiated index ranging from 83.6 to 95.6. In addition, as SiO2 contents increase, the rare earth element (REE) abundances, especially heavy REE abundances, and REE pattern slopes change gradually, but the negative Eu anomalies increase sharply, while the degree of enrichment in Rb, Th, U, and Pb and depletion in Ba, Nb, Sr, P, and Ti are enhanced. These features indicate that K-feldspar, ±plagioclase, ±biotite, ±amphibole, ±apatite, ±sphene/garnet, and ±Fe–Ti oxides such as ilmenite play the major role in the fractional crystallization process. The high initial 87Sr/86Sr (0.7067 and 0.7079) and negative εNd(t) values (−8.6 to −10.1), with T2DM ranging from 1.39 to 1.49 Ga, indicate that the sources were mainly derived from the mature ancient middle to lower crust and minor mantle-derived materials. The initial 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 18.462–18.646, 15.717–15.735, and 38.699–39.007, respectively, signify that some subduction-related material such as ocean island volcanic rocks and mature arc primitive rocks may be involved as sources. Based on an analysis of similar zircon saturated temperature and geochemical characteristics of typical highly fractionated I-type granites in SE and SW China, and consideration of the regional geological setting, we suggest that the parent magma may be derived from the ancient middle to lower continental crust and mantle-derived basaltic magma. These were generated in the setting of a westward subducted Lushui–Luxi–Ruili (LLR) Tethyan oceanic slab, where mantle-wedge-derived sources provided enough heat and material to melt the middle to lower ancient crust. Taking into account the temporal–spatial distributions of Early Cretaceous magmatic rocks in the region, we further suggest the existence of an Andean-type active continental margin from the Lhasa block to the Tengchong block along the Bangong–Nujiang–LLR Tethys Ocean during the Early Cretaceous.