Geochronology and geochemical characteristics of the Shangdan pluton in the northern margin of the Alxa Block, Inner Mongolia and its tectonic implications
Date
2018
Authors
Gan, Lisheng
Wu, Tairan
Chen, Yan
Zhang, Wen
Zhang, Zhao Yu
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Kexue Chubanshe = Science Press
Abstract
The Northern Alxa Orogenic Belt is the southernmost segment of the Central Asian Orogenic Belt. The less investigated Shangdan pluton is a key point to understanding its tectonic evolution. The granodiorites from this pluton yield zircon LA-ICP-MS U-Pb ages of 331. 7 +/- 0. 9Ma, which is the earliest age obtained for the Late Paleozoic plutons in the Zongnaishan-Shalazhashan tectonic zone and indicates an Early Carboniferous magmatic event. The granodiorites from the Shangdan pluton have moderate silicon contents (SiO2 =66. 07% similar to 68. 15%) , high Na2O contents (Na2O/K2O > 1. 6) and relatively high CaO contents ( CaO/Na2O > 1 ) , and belong chemically to metaluminous series (A/CNK <1, A/NK > 1) with medium-K and calc-alkaline (sigma =1. 21 similar to 1. 40, K2O = 1. 55% similar to 2. 21% ) characteristics. The hornblendes are also found in the rock slices. Both the geochemistry and petrography show affinity to the I-type granites. The granodiorites have moderate REE contents and moderate to strong LREE to HREE differentiation ( (La/Yb)(N) 5. 30 similar to 6. 62). They are enriched in Rb, Th, K, Zr, Hf and depleted in Ba, Nb, Ta, P, Ti. No negative Eu anomalies have been found in the samples, suggesting that they might be derived from partial melting of middle-lower crust. Meanwhile, two whole-rock samples have slightly negative epsilon(Nd)( t) values of - 1. 02 and - 0. 91 with Nd model age t(DM1) of 1. 30 similar to 1. 28 Ga. One sample yields zircon epsilon(Hf)(t) values between +0. 30 and +5. 27 with Hf model age t(DM2) of 1. 32 similar to 1. 00Ga. Combined with geochemical analysis, the Shangdan pluton was likely recycled from the Meso-Neoproterozoic crust. In conclusion, the Shangdan pluton with arc characteristics has been discriminated to be formed in a subduction-related setting, suggesting that after the termination of the Early-Middle Paleozoic orogenesis in the northern Alxa, the Late Paleozoic subduction occurred at least in the Early Carboniferous.
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Acta Petrologica Sinica (Yanshi Xuebao)
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2037-12-31