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Journal of Earth Science, Vol. 28, No. 2, p. 295–304, April 2017 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-017-0723-y Detrital Zircon U-Pb Geochronologyof Sinian–Cambrian Strata in the Eastern Guangxi Area, China Ruxin Ding 1, 2, Heping Zou *1, 2, Kyoungwon Min3, Feng Yin *4, Xiaodong Du1, 2, Xuxuan Ma5, Zhangxin Su1, 2, Wenjie Shen1, 2 1. School of Earth Science and Geological Engineering, Sun Yat-sen University, Guangzhou 510275, China 2. Guangdong Provincial Key Laboratory of Mineral Resources & Geological Processes, Guangzhou 510275, China 3. Department of Geological Sciences, University of Florida, Gainesville, FL 32611, USA 4. Exploration Branch Company, SINOPECE , Chengdu 610041, China 5. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China Ruxin Ding: http://orcid.org/0000-0002-1174-5193; Heping Zou: http://orcid.org/orcid.org/0000-0002-4054-7292 Feng Yin: http://orcid.org/orcid.org/g 0000-0001-5914-5034 ABSTRACT: The Eastern Guangxi area locates in the southwestern part of the transition zone between Yangtze and Cathaysia blocks, which is an important region because the boundary between two blocks probablycrosses there. We determined LA-ICPMS U-Pb ages for detrital zircons extracted from three sandstone samples in the Sinian-Cambrian strata in this region. The resulting ages are in the range of the Archeozoic and Neoproterozoic, with three notable concentrates at 991, 974, and 964 Ma, all of which are coeval to the Grenvillian magmatic activity. The new age distribution is siimilar to the data reported in the Precambrian strata of the adjacent southwestern Cathaysia Block, suggesting that most of our detrital zircons are likely derived from the Cathaysia Block. Combined with others’ research, we are more inclined to accept the opinion that there was not an ocean basin between the two blocks dur- ing the Sinian-Cambrian period in Eastern Guangxi area if the timing of collision is thhe Early Neopro- terzoic. But if the timing of collision is the Early Paleozoic, we conclude that Luzhai uplift (i.e., the up- lift between Guilin-Yongfu faultand Lipu fault) beyond the west of Dayaoshan regoin might be one part of southwestern sedimentation boundary of Cathaysia Block and Yangtze Block. We also get a few of detrital zircons with ages of ~590 Ma which probably sourced from northeastern Gondwana and 13 de- trital zircons with over 3 000 Ma U-Pb ages which record the early formation of the earth. KEYWORDS: Cathaysia Block, Yangtze Block, eastern Guangxi area, detrital zircon, U-Pb dating. 0 INTRODUCTION In this research, we collected Sinian–Cambrian sandstone South China Block, one of major blocks in East Asia, in- samples in the Dayaoshan region of Eastern Guangxi area, and cludes Cathaysia Block in the southeast and Yangtze Block in the conducted LA-ICP-MS U-Pb dating for detrital zircons. We northwest. Although the formation of South China Block is criti- hope to obtain new useful evidence to test whether the Paleo- cal in understanding Precambrian tectonics of China, the tiiming zoic residual oceanic basin existed or not, and further discuss of collision and the boundary location in the southwest of South the southwestern border between Cathaysia and Yangtze China Block, are yet to be disputed (Shu, 2012; Zhao, 2012). blocks. Recently, some scholars present that there was a residual oceanic basin in Early Paleozoic between Cathaysia and Yangtze blocks 1 GEOLOGICAL SETTING in the southwest of South China Block through the study of vol- The study area locates between the southwestern part of canic rocks in eastern Guangxi area (Qin et al., 2015, Liu et al., Cathaysia Block and the southeastern part of Yangtze Block 2015; Peng et al., 2015), others scholars put forward that there (Fig. 1). The Yangtze basement consists of Proterozoic and a was still a residual oceanic basin at the end of the Permian in the small amount of Achaean TTGG rocks with the ages in the southwest of South China Block (He et al., 2014). ranges of 3 200–2 900, 2 700–2 400 and 2 100–1 800 Ma (e.g., Qiu et al., 2000; Gao et al., 1999). There are minor 1 700 Ma *Corresponding author: [email protected] (Greentree and Li, 2008) and 1 100–900 Ma igneous rocks and [email protected] extensively distributed Neoproterozoic (840–740 Ma) magmat- © China University of Geosciences and Springer-Verlag Berlin ism around the margin of the Yangtze Block (e.g. Wang et al., Heidelberg 2017 2010a; Zheng et al., 2007; Li et al., 2003b; Zhou et al., 2002; Li, 1999). The Cathaysia basement has been proposed to be of a Manuscript received October 2, 2015. largely Mid-Paleoproterozoic (1 800–2 000 Ma) origin and is Manuscript accepted April 13, 2016. composed dominantly of gneiss, amphibolite and migmatite (Shu et al., 2011; Yu et al., 2010). There are igneous rocks in Ding, R. X., Zou, H. P., Min, K., et al., 2017. Detrital Zircon U-Pb Geochronology of Sinian–Cambrian Strata in the Eastern Guangxi Area, China. Journal of Earth Science, 28(2): 295–304. doi:10.1007/s12583-017-0723-y. http://en.earth-science.net 296 Ruxin Ding, Heping Zou, Kyoungwon Min, Feng Yin, Xiaodong Du, Xuxuan Ma, Zhangxin Su and Wenjie Shen Figure 1. Geological sketch map of the study area and sample locations map. (a) The geological map and sample locations (all of white regions stand for strata of post Cambrian); (b) the locations of the researched area between Yangtze Block and Cathaysia Block. the range 1 900–1 700 Ma, 1 400 Ma and 1 000–700 Ma and tact, the Devonian and Carboniferous strata present conforma- are mainly distributed in the northeastern (Wuyishan area) and ble contact, the Carboniferous and Permian strata present con- southwestern (Yunkai area) parts of the Cathaysia Block. After formable or parallel unconformable contact (Yin, 1997). The the period at ca. 825 Ma, rift basins were formed in South Chi- Jurassic, Cretaceous and Paleogene strata, corresponding to na Block (Feng et al., 2016; Shu, 2006; Li et al., 2003a, b). The terrestrial facies, are sporadically scattered in the study area. basement rocks of the Yangtze and Cathaysia blocks are un- Most of the granitic plutons in the study area were emplaced conformably overlain in turn by the Upper Neoproterozoic– during Yanshanian periods and few of them were emplaced Lower Paleozoic, Devonian–Lower Triassic and Upper during Indosinian or Caledonian. The Sinian–Cambrian and Triassic–Lower Jurassic strata (e.g., Wang et al., 2014; Shu et Devonian–Permian strata occurred regional folding. In the al., 2011; Wan et al., 2010; Yu et al., 2010). During the Later researched area, the fault strike is mainly NNE-NE with partial Neoproterozoic (Ediacaran or Sinian)–Cambrian, most of NW or close to SN. The fault experienced the multiphased Yangtze Block was covered by carbonate platform system (Xu overprinting. et al, 2012; Jiang et al., 2011); meanwhile the Cathaysia Block was broken up into three sub-blocks, namely, the Wuyi, South 2 SAMPLING Jiangxi-Nanling and Yunkai, which were separated from one Figure 1 shows sampling locations. Figure 2 shows another by intracontinental rift zones (Yao et al., 2011; Shu, field pictures of sample locations and photomicrographs of 2006). The intracontinental rift basins in the Cathaysia Block samples. Sample 712-10 (N24°54.487′, E112°01.292′) is col- were mainly covered by clastic rocks (Zhou et al., 2016; Shu, lected from a meta-siltstone layer in the Sinian stratum in 2006; Guangdong BGMR, 1988; Hunan BGMR, 1988; Gua- Mashi Town, Jianghua County, Hunan Province. Sample ngxi BGMR, 1985; Jiangxi BGMR, 1985). 802-1 (N23°45.257′, E110°39.170′) from the Cambrian sand- The exposed strata of this study area are Sinian stone is taken from near the national highway 321 in –Cambrian, Devonian–Permian, Jurassic, Cretaceous and Pa- Dongrong Town, Teng County, Guangxi Province. Sample leogene sequences. The Sinian–Cambrian strata are conforma- 802-6 (N24°06.195′, E110°35.490′) is a Cambrian sandstone ble in sequence and composed mainly of clastic rocks. The near the national highway 321, ~500 m in the north of Hua- pre-Devonian and Devonian strata present unconformable con- ngcun town, Mengshan County, Guangxi Province. Detrital Zircon U-Pb Geochronologyof Sinian–Cambrian Strata in the Eastern Guangxi Area, China 297 Figure 2. Field pictures of sample locations (left) and photomicrographs of samples (right). 3 ANALYTICAL PROCEDURES were performed at the State Key Laboratory of Geological We followed a standard mineral separation procedure in- Processes and Mineral Resources, China University of Geos- cluding crushing, hand washing, magnetic separation, sepa- ciences (Wuhan). rated by alcohol or heavy liquid, selection under microscope, then random selection of zircon grains under binocular, stick- 4 RESULTS ing grains into target and fixing it by epoxy resin, polishing it In this study, we examined internal structures of 110 by polish plate. The selected zircon grains were examined zircons for Sample 712-10, 106 zircons for Sample 802-1 and with JXA-8100 EPMA and each grain’s CL (cathodolumi- 113 zircons for Sample 802-6. Most of these zircons are hy- nescence) images were taken. In-situ U-Pb dating was con- pidiomorphic to idiomorphic with linear dimensions of ducted using Agilent 7500a LA-ICPMS with a laser ablation 50–260 μm (sample 712-10), 100–300 μm (802-1), and system of GeoLas 2005. The diameter of the laser beam spot is 100–400 μm (802-6), respectively. Zircon grains are colorless 32 μm. The external standard sample of element content adopts to light pink and are euhedral tosubhedral crystals or crystal NIST SRM 610 and the internal standard adopts 29Si.

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