Zircon U–Pb dating, trace element and Sr–Nd–Hf isotope geochemistry of Paleozoic granites in the Miao’ershan–Yuechengling batholith, South China: Implication for petrogenesis and tectonic–magmatic evolution
Highlights
► The Paleozoic Miao’ershan–Yuechengling granites consist of five phases. ► The ages of the five phases are 435 ± 4, 427 ± 3, 417 ± 6, 404 ± 6 and 382 ± 2 Ma, respectively. ► The granites were formed from partial melting of Paleoproterozoic basement rocks. ► Crustal growth in the studied region took place mainly at ca. 1.84 Ga.
Introduction
The South China Block (SCB) was formed by the amalgamation between the Yangtze Block in the northwest and the Cathaysia Block in the southeast during the Neoproterozoic time (e.g. Li et al., 2009). It underwent three major tectonic–magmatic events after its formation, and abundant igneous rocks, especially granites, formed in response to these events (e.g. Zhou et al., 2006, Li and Li, 2007, Wang et al., 2007a, Wang et al., 2011, Li et al., 2007a, Li et al., 2010, Zhang et al., 2012). The granites in the SCB are commonly regarded as one of the largest granite province worldwide (e.g. Wang and Zhou, 2005). The granites in the SCB fall in four main age groups, namely, the Neoproterozoic, the Early Paleozoic (traditionally referred to as “Caledonian”), the Triassic (“Indosinian”) and the Jurassic–Cretaceous (“Yanshanian”). Origin and evolution of this large granite province is a hot topic of international interest. Despite intensive scientific research, the geological signatures and geodynamic mechanism of these granites remain controversial, and a series of contrasting models have been suggested (e.g. Hsü et al., 1988, Gilder et al., 1991, Jahn et al., 1990, Charvet et al., 1994, Zhou and Li, 2000, Zhou et al., 2006, Li and Li, 2007, Li et al., 2010, Wang et al., 2007a, Wang et al., 2012, Jiang et al., 2005, Jiang et al., 2009, Zhang et al., 2012, Zhao et al., 2012).
The Paleozoic granites are important parts and widespread in the eastern SCB (Fig. 1). They occur as laccoliths and batholiths to the east of the Anhua–Luocheng fault, and are usually of large volume with a total exposure area of over 20,000 km2. Studies on the Paleozoic granites are relatively fewer due to less economic mineralization than the Mesozoic granites in the SCB. Reliable ages and geochemistry for these granites have only become available in recent years (e.g. Wan et al., 2007, Wan et al., 2010, Wang et al., 2007b, Wang et al., 2011, Shen et al., 2008, Xu et al., 2009, Li et al., 2010, Zhang et al., 2009, Zhang et al., 2010, Zhang et al., 2012). Geochronological studies have defined an age-span of ca. 400–462 Ma for these granites (e.g. Li et al., 2010, Wang et al., 2011, Zhang et al., 2012). These granites are suggested to be petrogenetically related to the Kwangsian orogenic event (Wang et al., 2011, Zhang et al., 2012) or the Wuyi–Yunkai orogeny recently renamed by Li et al. (2010). The orogenic event was likely a major intra-plate orogenic event (Shu et al., 2008, Li et al., 2010). However, details about the orogeny still remain poorly defined, especially for the orogenic processes and affected temporal-spatial extents. Systematical geochronological and geochemical studies for all the Paleozoic metamorphic rocks and igneous rocks in the SCB are needed for a better understanding of the SCB tectonic evolution.
Syn-orogenic to late orogenic melting in the southeastern Wuyi–Yunkai orogeny has been constrained to occurring in the interval of ca. 450–420 Ma (Li et al., 2010). The Miao’ershan–Yuechengling batholith (simplified as MYB) lies in the northwestern part of the Wuyi–Yunkai orogeny (Fig. 1). The MYB is one of the largest granitic batholiths in South China (Fig. 1), with a total outcrop area of ca. 3400 km2. Early in 1950s, the MYB was suggested to be composed of the Devonian granites based on field observation that the granites intruded the Silurian strata but overlain by middle Devonian sandstone (Zhao and Zhang, 1958). The pioneering works gave variable ages of 364–422 Ma by whole-rock Rb–Sr dating method for these Devonian granites (Xu et al., 1994, Li and Zhou, 2002). However, due to the lack of systematical studies, the geochronology, petrogenesis and tectonic nature to these granites remain unclear. In this paper, we present systematic SHRIMP and LA–ICP–MS zircon U–Pb ages, geochemistry and Sr–Nd–Hf isotopic compositions for these granites in the MYB. The results are used to constraint their ages, petrogenesis, magma sources and tectonic environments. This study will also provide constraints on the temporal and spatial extents of orogenic magmatism in the northwestern part of the Wuyi–Yunkai orogeny.
Section snippets
Geological setting and petrography
The Yangtze and Cathaysia Blocks have distinctive crustal ages and tectonic evolution histories, and their amalgamation during the Neoproterozoic Jinningian orogeny led to the formation of the SCB. The Yangtze Block was mainly built upon a stable Archean–Proterozoic basement, consisting of Archean rocks up to 3.3 Ga (Jiao et al., 2009, Gao et al., 2011), with an average age of 2.7–2.8 Ga (Gao et al., 1991, Qiu et al., 2000). The Cathaysia Block appears to be much younger and consists
Analytical methods
Granite samples used in this study were collected from all the five phases. Zircon grains were extracted from granite samples by conventional heavy liquids and magnetic techniques, and then purified by hand picking under binocular microscope. Then zircon grains were mounted in epoxy resin and polished to approximately half their thickness. Examination of internal structures was performed using cathodoluminescence (CL) imaging technique with an Electron Microprobe at the Scanning Electron
Phase 1 (sample YCL-14A, N25o49′10.2″, E110o40′55.7″)
Zircon crystals from Sample YCL-14A show euhedral, elongated shape, with lengths ranging from 150 to 250 μm and widths ranging from 50 to 100 μm. Most length/width ratios vary from 2:1 to 3:1. In CL images, most zircon crystals display obviously euhedral concentric zoning (Fig. 4), implying their magmatic origin (e.g. Hoskin and Schaltegger, 2003, Wu and Zheng, 2004). A total of twelve spot analyses were carried out on ten zircon grains and the results are listed in Table 1. U contents vary from
Geochronological framework of Paleozoic granites in the MYB
Zhang et al. (2012) reported LA–ICP–MS zircon U–Pb ages of two samples from the Miao’ershan pluton. Sample 06HG84 from the northern part of the Miao’ershan pluton gave a weighted mean age of 400 ± 4 Ma. The sample location is near to the location of our sample MES-02A (404 ± 6 Ma in this study). Sample 06HG95 from the southern part of the pluton gave a weighted mean age of 415 ± 2 Ma. This age is similar to that of the Phase 3 (417 ± 6 Ma) in this study.
The new SHRIMP and LA–ICP–MS zircon U–Pb ages, in
Conclusions
- (1)
The Paleozoic granites in the Miao’ershan–Yuechengling batholith (MYB) in South China are at least consist of five phases, with the emplacement ages of 435 ± 4 Ma, 427 ± 3 Ma, 417 ± 6 Ma, 404 ± 6 Ma and 382 ± 2 Ma, respectively. The MYB was incrementally emplaced from the southeast to the northwest lasting from early Silurian to late Devonian.
- (2)
Most of these granites are metaluminous to weakly peraluminous, and contain low P2O5 contents (<0.15%). They show relatively high (87Sr/86Sr)i ratios (>0.715), low εNd(t
Acknowledgements
We are grateful to Prof. Dun-Yi Liu and Dr. Zhi-Qin Yang from Beijing SHRIMP Center for helping with zircon U–Pb analyses, and to Prof. Fu-Yuan Wu and Dr. Lie-Wen Xie from Institute of Geology and Geophysics, Chinese Academy of Sciences for helping with Hf isotope analyses. This work was supported by funding from a Major State Basic Research Program (973 Project, 2012CB416706), a project from the China National Science Foundation (No. 41173001), a key project from the Chinese Ministry of
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