Tectono-magmatic evolution of the Gaoligong belt, southeastern margin of the Tibetan plateau: Constraints from granitic gneisses and granitoid intrusions

摘要

The Gaoligong belt is located in the southeastern margin of the Tibetan plateau, and is bound by the Tengchong and Baoshan blocks. This paper presents new data from zircon geochronology, geochemistry, and whole-rock Sr-Nd-Pb-Hf isotopes to evaluate the tectonic evolution of the Gaoligong belt. The major rock types analysed in the present study are granitic gneiss, granodiorite, and granite. They are metaluminous to peraluminous and belong to high-K, calc-alkaline series. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of zircons from nine granitic rocks yielded crystallization ages of 495-487 Ma, 121 Ma, 89 Ma, and 70-63 Ma. The granitoids can be subdivided into the following four groups. (1) Early Paleozoic granitic gneisses with high epsilon(Nd)(t) and epsilon(Hf)(t) values of - 1.06 to - 3.45 and - 1.16 to 2.09, and model ages of 1.16 Ga to 1.33 Ga and 1.47 Ga to 1.63 Ga, respectively. Their variable Sr-87/Sr-86 and Pb values resemble the characteristics of the Early Paleozoic Pinghe granite in the Baoshan block Our data suggest that the rocks were derived from the break-off of the Proto-Tethyan oceanic slab between the outboard continent and the Baoshan block, which induced the partial melting of Mesoproterozoic pelitic sources mixed with depleted mantle materials. (2) Early Cretaceous granodiorites with low epsilon(Nd)(t) and epsilon(Hf)(t) values of - 8.92 and - 4.91 with Nd and Hf model ages of 1.41 Ga and 1.49 Ga, respectively. These rocks have high initial Sr-87/Sr-86 (0.711992) and lower crustal Pb values, suggesting that they were derived from Mesoproterozoic amphibolites with tholeiitic signature, leaving behind granulite residue at the lower crust. (3) Early Late Cretaceous granites with low epsilon(Nd)(t) and epsilon(Hf)(t) values of - 9.58 and - 4.61 with Nd and Hf model ages of 1.43 Ga and 1.57 Ga, respectively. These rocks have high initial Sr-87/Sr-86 (0.713045) and lower crustal Pb isotopic values. These rocks were generated from the partial melting of Mesoproterozoic metapelitic sources resulting from the delamination of thickened lithosphere, following the closure of the Bangong-Nujiang Ocean and collision of the Lhasa-Qiangtang blocks. (4) Late Cretaceous to Paleogene granitic gneisses with low epsilon(Nd)(t) and epsilon(Hf)(t) values of - 4.41 to - 10 and - 5.95 to - 8.71, Nd model ages ranging from 1.08 Ga to 1.43 Ga, and Hf model ages from 1.53 Ga to 1.67 Ga, respectively. These rocks show high initial Sr-87/Sr-86 (0.713201 and 714662) and lower crustal Pb values. The data suggest that these rocks are likely related to the eastward subduction of the Neo-Tethyan Oceanic slab, which induced partial melting of Mesoproterozoic lower crustal metagreywacke. The results presented in this study from the Gaoligong belt offer important insights on the evolution of the Proto-Tethyan, Bangong-Nujiang, and Neo-Tethyan oceans in the southeastern margin of the Tibetan Plateau. (C) 2015 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.