Full length ArticlePetrogenesis and tectonic setting of the early Mesozoic Xitian granitic pluton in the middle Qin-Hang Belt, South China: Constraints from zircon U–Pb ages and bulk-rock trace element and Sr–Nd–Pb isotopic compositions
Introduction
The South China Block is bounded by the Qinling–Dabie Orogenic Belt to the north, the Songpan–Ganzi Block to the northwest, the Indochina Block and Sibumasu Block to the southwest, and the Philippine Sea Plate to the east (Fig. 1a; Zhou et al., 2015). It was formed by the amalgamation of the Cathaysia Block to the southeast and the Yangtze Block to the northwest at ca. 820 Ma, most likely related to the Neoproterozoic assembly of the Rodinia supercontinent (Zhao et al., 2011). The South China Block has undergone the Indosinian and Yanshanian tectonic events during the Mesozoic. These two tectonic events were accompanied by emplacement of various types of igneous rocks in the South China Block, including graniticrocks, rhyolite, and subordinate mafic rocks (Mao et al., 2008, Wang et al., 2013a, Zhang et al., 2013, Qiu et al., 2014). Being part of the Mesozoic circum-Pacific magmatic belt, these widespread igneous rocks are associated with polymetallic mineralization (Mao et al., 2011, Mao et al., 2013, Wang et al., 2007b, Wang et al., 2013b, Chen et al., 2011, Wang et al., 2013a, Wang et al., 2015, Zhang et al., 2013, Li et al., 2014a, Li et al., 2014b, Li et al., 2014c). The Mesozoic granitic rocks in the South China Block were emplaced during the early Mesozoic (Indosinian event) and the late Mesozoic (Yanshanian event). Origin and evolution of these granitic rocks is a hot topic of international interest. However, the majority of previous studies concern mostly with the Yanshanian event, and the Indosinian event has been poorly studied (Qiu et al., 2014, Qiu et al., 2016).
Recent studies have revealed the presence of many early Mesozoic granitic plutons in the South China Block using single-grain dating technologies, such as the Guidong (239 ± 5 Ma; Xu et al., 2003), Fucheng (239 ± 17 Ma; Yu et al., 2007), Longyuanba (240.7 ± 1.4 Ma; He et al., 2010), Baimashan (241 ± 3 Ma; Wang et al., 2007b), Weishan (211 ± 1.6 Ma; Ding et al., 2006), and Guandimiao plutons (239 ± 3 Ma; Wang et al., 2007b). These studies have significantly improved our understanding of the regional Mesozoic tectono-magmatic evolution and geodynamic settings of the South China Block. However, the petrogenesis and tectonic settings of the early Mesozoic granitic plutons remain controversial (Wang et al., 2002, Wang et al., 2007b, Zhou, 2003, Zhou et al., 2006, Li and Li, 2007, Yu et al., 2007). Some authors suggested that they were emplaced in syn- or post-collisional settings related to the collision of the South China and Indochina Blocks (Wang et al., 2002, Wang et al., 2007b, Wang et al., 2013b, Zhou et al., 2006, Yu et al., 2007). Li and Li (2007) proposed an alternative model, and suggested that the early Mesozoic granitic rocks in the South China Block were formed as a result of flat-slab subduction of the Paleo-Pacific Plate. Other authors argued that there is little evidence to support the flat-slab subduction model, as the model cannot explain the planar distribution of the early Mesozoic granitic rocks, and there is a lack of contemporaneous volcanic rocks associated with the subduction (Zhou et al., 2006). Therefore further studies on the tectonic settings of these granitic rocks are required.
The Xitian pluton in Hunan Province, southeast China (Fig. 1b) is dominated by Indosinian granitoids, with minor Yanshanian granitoids, and is located near a suture zone (the Qin-Hang Belt) between the Yangtze and Cathaysia Blocks. Previous studies in the Xitian area have focused on the abundant W–Sn polymetallic mineralization and the Yanshanian granitoids of the Xitian pluton. Little attention has been paid to the Indosinian granitoids of the pluton. A lack of systematic geochronological, geochemical, and isotopic analysis of the Indosinian granitoids of the Xitian pluton means that their petrogenesis and tectonic setting remain unclear. In this paper we present new data from laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U–Pb dating, and analysis of trace element contents, bulk-rock geochemistry, and Sr–Nd–Pb isotopes of the samples collected from the Indosinian granitoids of the Xitian pluton, with the aims of (1) outlining the petrogenesis of the pluton; (2) constraining the source and origin of the granitic magmas; and (3) discussing the tectonic setting of the pluton.
Section snippets
Geological background
The Xitian pluton occupies an area of ∼240 km2, and consists of about 40 bands of intrusive rocks that are intruded into Paleozoic rocks dominated by Devonian and Carboniferous carbonate and sandstone (Fig. 2). Previous studies indicate that this pluton has recorded three stages of magma emplacement during the Indosinian (230–220 Ma; Chen et al., 2013, Zhou et al., 2013, Niu et al., 2015), early Yanshanian (160–147 Ma; Chen et al., 2013, Zhou et al., 2013, Zhou et al., 2015, Niu et al., 2015), and
Samples and analytical techniques
Five samples that were collected from the Indosinian granitoids of the Xitian pluton and analyzed in this study were all taken from underground mines (Fig. 2b). Sample 17-1s9 was collected from the coarse-grained biotite adamellite with K-feldspar megacrysts, and samples 17-3s4 and 15-11s1 were collected from the medium- to coarse-grained biotite adamellite. Samples Ls22 and Ls21a were collected from the fine-grained biotite granite that is similar in texture to the Yanshanian granitoids in the
Zircon U–Pb dating results
Zircon crystals from sample 17-1s9 are typically euhedral, with lengths ranging from 100 μm to 300 μm, and length-to-width ratios ranging from 1:1 to 3:1. CL imaging indicates that most of the zircon grains have internal oscillatory zoning that is typical of magmatic origin (Hoskin and Schaltegger, 2003; Fig. 4a). The 206Pb/238U ages of 21 zircon grainsfrom this sample range from 223.4 Ma to 234.7 Ma (Table 1), and are plotted on or close to the concordia curve, with a weighted mean 206Pb/238U age
Is the Xitian pluton composed of S-, I-, or A-type granites?
Granitic rocks can be subdivided into I-, S-, and A-types, based on their geochemical characteristics, protoliths, and tectonic settings (Chappell and White, 1974, Chappell and White, 1992, Loiselle and Wones, 1979, Collins et al., 1982, Whalen et al., 1987, Chappell, 1999). The Indosinian granitoids in the Xitian pluton have relatively high A/CNK ratios (average 1.24), and high P2O5 contents (average 0.15%), which indicates that they are most likely S-type granites (cf. Sylvester, 1998,
Conclusions
- (1)
Zircon U–Pb dating yielded precise crystallization ages of 229–223 Ma for the Indosinian granitoids of the Xitian pluton in the South China Block.
- (2)
The Indosinian granitoids of the Xitian pluton are high-K, weakly to strongly peraluminous, and highly fractionated S-type granites that were emplaced under conditions of high temperatures and low oxygen fugacity. They have high ratios (87Sr/86Sr)i, low εNd(t) values, and old Nd model ages. These features are indicative of derivation from magma
Acknowledgments
We thank Fukun Chen from University of Science and Technology of China for his kind help in Sr–Nd–Pb isotopic analysis. The manuscript has been greatly benefited from constructive comments by Editors-in-Chief Mei-Fu Zhou and Bor-Ming Jahn, reviewer Stefan Jung and other two anonymous reviewers. It has also been significantly improved in both English and science by Dr Shefa Chen of the Geological Survey of Western Australia. Editors Irene Yao and Diane Chung are also appreciated for their
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