Adakitic Rocks Associated with the Shilu Copper–Molybdenum Deposit in the Yangchun Basin, South China, and Their Tectonic Implications
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Acta Geochim DOI 10.1007/s11631-017-0146-6 ORIGINAL ARTICLE Adakitic rocks associated with the Shilu copper–molybdenum deposit in the Yangchun Basin, South China, and their tectonic implications 1,2 1 3 4 Lipeng Zhang • Yongbin Hu • Jinlong Liang • Trevor Ireland • 3 1 1 5,6 Youliang Chen • Rongqing Zhang • Saijun Sun • Weidong Sun Received: 14 December 2016 / Revised: 15 January 2017 / Accepted: 6 February 2017 Ó The Author(s) 2017. This article is published with open access at Springerlink.com Abstract South China is famous for the extensive magma- (MSWD = 9.4) was determined, which is identical to the tism and polymetallic mineralization that took place there in youngest zircon U–Pb age (103.9 ± 0.5 Ma) within error. the Mesozoic. Shilu is a large porphyry–skarn Cu–Mo deposit The Shilu intrusion has high oxygen fugacity as indicated by 4? 3? in the Yangchun Basin, South China. The lithology of the high zircon Ce /Ce and EuN/EuN* ratios. Considering the Shilu intrusion is granodiorite and quartz diorite, both of geochemical characteristics (high Sr, and low Y and Yb which are high-K calc-alkaline series, with high Sr contents), high oxygen fugacity, and copper mineralization of ([400 ppm) content along with low Y and Yb contents. Most the Shilu intrusion, it was most likely formed by partial of the samples have characteristics of adakite except for a few melting of a subducted young oceanic slab. Whole-rock Sr– samples that have slightly higher Y and Yb contents, which Nd isotope-, zircon Hf isotope-, and whole-rock trace element may be plausibly explained by crustal contamination. Laser analyses show that Shilu adakitic magmas may have inter- Ablation Inductively Coupled Plasma Mass Spectrometry acted with type II enriched mantle and/or crustal materials zircon U–Pb dating revealed ages between 106.6 ± 1.3 and during ascent. South China was affected by the Pacific tec- 103.9 ± 0.5 Ma, with multiple magmatic pulses. Molyb- tonic regime to the east and the Neo-Tethys tectonic regime to denite Re–Os isochron age of 102.2 ± 2.9 Ma the south in the Cretaceous. Based on the Pacific Plate drifting and rotation history, it is hard to explain how the Pacific Plate Electronic supplementary material The online version of this would have subducted and melted, forming adakitic rocks in article (doi:10.1007/s11631-017-0146-6) contains supplementary the Shilu region. Considering the tectonic history of Southeast material, which is available to authorized users. Asia and the South China Sea, the Neo-Tethys trench should & Weidong Sun have been much closer to the South China Block in the Cre- [email protected] taceous, and thus have had a greater impact on the South China Block. Based on the subduction direction, time of subduction, 1 CAS Key Laboratory of Mineralogy and Metallogeny, and distance between the Neo-Tethys subduction zone and the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China Shilu deposit, subduction of the Neo-Tethys ridge is the best 2 mechanism for explaining the Shilu adakitic rocks and Cu– University of Chinese Academy of Sciences, Beijing 100094, Mo mineralization. China 3 Department of Geochemistry, Chengdu University of Keywords Shilu deposit Adakitic rocks Cretaceous Technology, Chengdu 610059, China Á Á Á Neo-Tethys Á South China 4 Research School of Earth Sciences, The Australian National University, Bldg 61 Mills Road Acton ACT, Canberra, ACT 0200, Australia 1 Introduction 5 Center of Deep Sea Research, Institute of Oceanography, Chinese Academy of Sciences, Qingdao 266071, China In the Cretaceous, South China was surrounded by several 6 CAS Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Science, Beijing 100101, continental terranes and oceanic plates, such as the North China China Block to the north, the Songpan-Ganzi Block to the 123 Acta Geochim west, the Indochina Block to the southwest, the Pacific Fig. 1 a Sketch geologic map of South China and surrounding plates.c Plate to the east, and the Neo-Tethys Plate to the south. b Geological map of the Yangchun basin, South China. c Geological map of the Shilu Cu–Mo deposit. The black pentacles are sample sites South China is famous for large-scale magmatism and related mineralization, especially in the Mesozoic. A variety of tectonic models have been proposed to explain of the asthenosphere mantle and extension of overlying these characteristics specifically and the Mesozoic tectonic lithosphere during the Early Jurassic to the Early Creta- evolution of South China more generally (Gilder et al. ceous (Li et al. 2001). 1991, 1996;Li2000; Zhou and Li 2000; Zhou et al. 2006; In this study, we present LA-ICP-MS zircon U–Pb ages, Li et al. 2007, 2012; Sun et al. 2007a, 2012b; Ling et al. molybdenite Re–Os ages, zircon Hf isotopes, and whole- 2009; Mao et al. 2011; Wang et al. 2011). The models can rock major and trace elements. These data provide firmer be classified into three groups: active continental margin constraints on the timing and geochemical characteristics related to subduction of the Pacific Plate; continental rifting of the Shilu intrusion and deposit, with implications for the and extension; and mantle plume event. More recently, it tectonic setting of Late Cretaceous magmas in this region. has been proposed that the Neo-Tethys Plate subducted beneath the South China Block in the Cretaceous (Sun et al. 2016b; Sun 2016). 2 Geological setting In addition to the giant W, Sn, Sb, Nb, Ta, U, and rare- earth element (REE) deposit belts, a number of copper 2.1 Regional geology deposits occur in South China as well. Porphyry or skarn copper deposits are the most important deposit types, The Shilu Cu–Mo deposit is located in the Yangchun accounting for *70%–80% of the world’s total copper Basin, South China (Fig. 1). The Yangchun Basin is a NE– reserves (Sillitoe 2010; Sun et al. 2015a), and mainly form SW-trending fault-bounded basin formed from an Indosi- along convergent belts; for example: the Andean region in nian synclinorium, southeast of the Wuchuan-Sihui deep South America. Copper deposits in the northeast part of the fault (Li et al. 2000; Zheng et al. 2015). The area has South China Block have been well-studied. In contrast, the complex geological structure and has experienced intensive genesis and tectonic setting of copper deposits in the magmatic activity. NE–SW-trending faults define the tec- southern part of the South China Block remain obscure. tonic framework. E–W-trending faults can also be found, Shilu is a porphyry–skarn Cu–Mo deposit in the Yang- e.g., the Taishan-Yangchun tectonic belt (Zhang 2008). chun Basin, in the south of the South China Block. Pre- The outcropping strata in this basin include Cambrian, vious studies on this deposit have considered geologic Devonian, Carboniferous, Jurassic, and Quaternary strata features as well as geochronology, geochemistry, and (Fig. 1;Luo1988; Zheng et al. 2013). The basement is com- mineral trace element composition. A range of ages have posed of Cambrian low-grade metamorphic flysch and sandy been obtained through different techniques, including a shales distributed on both sides of the basin, which are covered zircon U–Pb evaporation age of 125 Ma, a mineral Rb–Sr by Upper Paleozoic carbonate rocks, neritic clastic rocks, and age of 122 Ma, an 40Ar–39Ar age of *100 Ma, and ore- sandy shales. Jurassic terrestrial clastic rocks unconformably bearing and ore-barren granodiorite Laser Ablation overlie the Paleozoic strata (Zheng et al. 2013). Inductively Coupled Plasma Mass Spectrometry (LA-ICP- Abundant granitic intrusions of different rock types with MS) zircon U–Pb ages of *107 and *104–107 Ma, varying mineralization have been recognized. These intru- respectively (Zhao et al. 1985; Yu et al. 1998; Duan et al. sions vary from intermediate to acidic in composition, 2013; Zheng et al. 2015). Therefore, the formation age of including monzonite, monzonitic granite, granodiorite, and the Shilu intrusion remains controversial. For the ore- granite (Fig. 1). The ages of these rocks range from Cale- forming age, a molybdenite Re–Os isochron age of donian to Yanshanian, with most magmatic activity having 104.1 ± 1.3 Ma has been obtained (Zhao et al. 2012). The occurred in the Yanshanian. Several polymetallic deposits genesis and tectonic setting of the deposit are also closely related to granitic intrusions have been found, such ambiguous. It is located close to the transition zone as the Shilu porphyry–skarn Cu–Mo deposit, the Bengkeng- between the eastern Tethys and the Pacific tectonic realms. Shiwu Cu–Pb–Zn deposit, the Xishan Sn–W deposit, and the The effect of the two major tectonic realms on this region Yingwuling W–Sn deposit (Chen 1988;WuandZeng2011; remains unclear. Some previous work has proposed that the Duan et al. 2013; Mei et al. 2013; Zheng et al. 2015). Shilu intrusion formed in an extensional environment related to subduction of the Pacific Plate (Duan et al. 2013; 2.2 Ore deposit geology Zheng et al. 2015). In addition, the variation of magma compositions has been used to argue that the region Shilu is a porphyry–skarn Cu–Mo deposit with average belonged to an intraplate setting and experienced upwelling grades of 0.9% for Cu and 0.21% for Mo (Zheng et al. 123 Acta Geochim 123 Acta Geochim 2015). NE-trending faults are the dominant tectonic in a smelting furnace. The whole-rock major elements were framework in this mine, in keeping with the regional fault then measured using XRF. To analyze trace elements, structure. Ore bodies can be divided into two ore types: fused samples were progressively dissolved in nitric acid, oxidized ore and primary ore.