Yang Tian, Si-fang Huang, Ling-zhan Wang,*, Xiang Li, Guo-gang Xie

a Research Center for Petrogenesis and Mineralization of Granitoid Rocks, China Geological Survey, Wuhan 430205, China

b Wuhan Center of China Geological Survey, Ministry of Natural Resources, Wuhan 430205, China

c State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China

1. Objectives

As one of three major tectonic blocks of China, the South China Block has undergone multiple periods of tectonicmagmatic-metamorphic events. Among these events, the Caledonian orogeny results in the formation of NE-trending Wuyi−Yunkai orogen in South China. This orogen shows complex characteristics, including widespread angular unconformity between Ordovician and Devonian strata, pre-Devonian intense deformation and metamorphism(amphibolite to granulite facies), and extensive crustal remelting (Shu LS et al., 2020). However, the tectonic nature of this orogeny remains controversial, and two main models are present: (1) Intracontinental orogen (Wang YJ et al. 2013)and (2) subduction-accretionary orogenic belt related to the amalgamation between the Yangtze and Cathaysia blocks (Liu SF et al., 2018).

The Chenzhou-Linwu fault, regarded as the suture zone between the Yangtze and Cathaysia blocks (Wang YJ et al.,2003; Tian Y et al., 2020), passes the Yingyangguan district and connects Yunkai domain to the southeast. Therefore, the outcropping Pre-Devonian rocks in the Yingyangguan area are significant agencies in deciphering the tectonic evolution of the South China Block. To address this, the authors present here a typical study, for the first time, on a Caledonian gabbro from the Yingyangguan area. Whole rock geochemistry and zircon U-Pb-Hf isotopes are analyzed on the Yingyangguan gabbro to constrain their petrogenesis and tectonic implication, as well as the nature of the Caledonian orogen in South China.

2. Methods

A fresh gabbro (SJC18-1) was collected near Shuijingchong village, eastern Guangxi (111°59′24″N;24°45′18″E) (Fig. 1). About 8 kg sample was crushed and separated for zircon crystals using standard density and magnetic separation techniques. Zircon grains were handpicked under a binocular microscope and subsequently mounted on epoxy resin. CL images were photographed by Tescan-Mira3 at Nanjing Hongchuang Geological Exploration Technology Service Co. Ltd. Zircon U-Pb dating and Hf isotopic analyses were conducted using LA-ICP-MS at Wuhan Sample Solution Analytical Technology Co., Ltd.Major and trace elements were determined by XRF and ICPMS at State Key Laboratory of Geological Processes and Mineral Resources, China University of Geoscience, Wuhan,with the relative standard derivations of less than 5% and 3%,respectively.

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3. Results

Sample (SJC18-1) exhibits medium-grained gabbroic texture (Fig. 2a), whereas petrography analyses argue that most pyroxene grains have been altered by amphibole (Fig.2b) with metasomatic pseudomorphic texture. Accordingly,the sample is mainly composed of hornblende (65%),plagioclase (35%) and some accessory phases of magnetite and zircon.

Zircon grains from sample SJC18-1 are euhedral to subhedral and 150−250 μm in length with aspect ratios of 2∶1 to 3∶1. Most zircon grains display light to dark luminescence, with faint or no obvious zoning in the CL images (Fig 2c). Twenty analyzed zircon grains have high and variable U (1654×10−6–5294×10−6) and Th (1552×10−6–11998×10−6) contents, with Th/U ratios of 0.89 to 1.64(except spot #19 with 2.27 value) (Table 1), indicating an igneous origin of these zircons. Seventeen of twenty zircon grains yield concordant206Pb/238U ages, ranging from 445 ±3 Ma to 460 ± 4 Ma with a weighted mean age of 450 ± 1.9 Ma(MSWD = 0.97) (Fig. 2d), indicating the emplacement age of the gabbro. Fifteen zircons display consistent ɛHf(t) values between −1.0 and −5.8 (mean = −4.1) with Hf single-stage model ages range from 1103 to 1282 Ma (Table 1).

The gabbroic sample has low SiO2(52.30%), high MgO(12.49%), CaO (9.88%), Cr (906.00×10−6), Ni (315.75×10−6)and Nb (8.84×10−6) contents. The low Nb/Y (0.39) and FeOT/MgO (0.57) ratios indicate its calc-alkaline affinity. In the chondrite normalized rare earth elements (REE) diagram,the gabbro display highly fractionated LREE pattern[(La/Sm)N=2.35] and HREE [(Gd/Yb)N=1.96] with moderate negative Eu anomaly (Eu/Eu*=0.66). In the primitive mantle normalized multi-element spider diagram, the studied gabbro show pronounced negative Nb, Zr and Ti anomalies and enriched LILE (eg., Rb, U and Pb).

4. Conclusion

The studied gabbro was emplaced at ca. 450 Ma,representing the first reported Caledonian mafic intrusion with arc-related geochemical affinity in the Yingyangguan area of South China.

High MgO, Cr and Ni contents and high (Gd/Yb)Nratio(1.96) of the studied sample suggest the parental magma was derived from a garnet peridotite source. Furthermore, the low Ta/Yb (0.52), high Th/Yb (5.25) ratio and negative zircon ɛHf(t) values (−1.1 to −5.8) indicate an enriched material source. Therefore, we suggest that the studied gabbro was derived from a deep mantle source with input of fluid/melt from the subduction slab and/or sediment (Wang YJ, et al.,2013).

In conjunction with other available observations, this study implied that there was a 1.3−1.1 Ga paleosubductionmodified wedge column beneath the South China Block,which might be undisturbed until the Kwangsian intracontinental orogen.

CRediT authorship contribution statement

Fig. 1. Sketch geological map of the Yingyangguan area, eastern Guangxi ((a) after Wang YJ et al., 2013; (b) after Tian Y et al., 2020).1−Yingyangguan Group; 2−Cryogenian; 3−Devonian; 4−Lower Cretaceous; 5−Quaternary; 6−Upper Ordovician gabbro; 7−Lower Silurian quartz diorite; 8−Lower Silurian granodiorite; 9−Upper Silurian monzonite; 10−Upper Jurassic monzonite; 11−Cretaceous granite; 12−microcrystalline quartzite block; 13−marble block; 14−stratigraphic boundary; 15−normal fault; 16−reverse fault; 17−strike-slip fault; 18−unidentified fault; 9−sampling location.

Fig. 2. Outcrop photo (a), microscope photograph (b), CL images of representative zircons (c) and U-Pb concordia diagram (d) of new discovered gabbro from the Yingyangguan area of the eastern Guangxi. Hb−hornblende, Pl−plagioclase.

Table 1. Zircon LA-ICP-MS U-Pb data and Lu-Hf isotopes of gabbro from the Yingyangguan area in the eastern Guangxi.

Yang Tian and Ling-zhan Wang conceived of the presented idea. Ling-zhan Wang, Xiang Li and Guo-gang Xie collected and preparing samples for analysis. Yang Tian and Si-fang Huang carried out the experiments and analyzed of results. All authors provided critical feedback and contributed to the final manuscript.

Declaration of competing interest

The authors declare no conflict of interest.

Acknowledgement

This study was financially supported by Open Fund of the Research Center for Petrogenesis and Mineralization of Granitoid Rocks (PMGR202002) and projects of China Geological Survey (12120113063200, DD20160031,DD20190050).