Identification of Paleoproterozoic Metasedimentary Rocks on Hainan Island and Its Geological Implications

1. Introduction

The present-day South China Block is composed of the Yangtze and Cathaysia blocks, which are widely considered to have amalgamated along the Jiangnan Orogen during the Neoproterozoic, thereby forming a unified continental entity [1,2,3]. The Yangtze and Cathaysia blocks are characterized by markedly different Precambrian crystalline basement compositions [4]. It has been well documented that the Yangtze Block preserves Archean crystalline basement, as exemplified by the Kongling Group (also referred to as the Kongling Complex) [5,6,7,8,9], whereas the oldest crystalline basement so far identified in the Cathaysia Block is of Paleoproterozoic age, which is typified by the Badu Group (also termed the Badu Complex) [10,11,12,13]. Hainan Island is situated at the southernmost margin of the South China Block and is generally regarded as a constituent part of the Cathaysia Block [14]. A number of distinctive geological records are preserved on the island, including the occurrence of ~1.4 Ga granitic rocks and metabasic rocks [15,16,17], voluminous Devonian–Triassic magmatic rocks that are related to the evolution of the Paleo-Tethys [18,19], and large-scale Cenozoic basalts that were erupted in the northern part of the island [20]. Extensive investigations have been conducted on these geological records, and a wealth of results has been obtained [17,19,20,21]. Nevertheless, owing to intense overprinting by subsequent tectono-thermal events and dense vegetation coverage, robust geochronological evidence for the existence of a Paleoproterozoic crystalline basement on Hainan Island is still lacking. A suite of Paleoproterozoic metasedimentary rocks newly recognized in the Jiangbian area, Dongfang City, western Hainan Island, is reported in this study. This recognition implies the presence of a Paleoproterozoic crystalline basement on the island and offers a new window into the material composition, formation age, and tectonic evolution of the Precambrian crystalline basement of Hainan Island.

2. Regional Geological Setting and Sample Description

2.1. Regional Geological Setting

Geotectonically, Hainan Island is situated at the junction of the Pacific, Eurasian, and Indian plates and is controlled by both the Pacific and Tethyan tectonic domains. It has been subjected to multiple phases of tectono-thermal events, including the Mesoproterozoic (~1.4 Ga; [15,16,17]), the Early Paleozoic (~430 Ma; [22]), the Late Paleozoic–Early Mesozoic (353–237 Ma; [18,19,23]), the Late Mesozoic (107–81 Ma; [24,25,26]), and the Cenozoic [20], thereby possessing a complex tectonic evolutionary history. From north to south, the island is transected by four approximately E–W-trending faults, namely the Wangwu–Wenjiao, Changjiang–Qionghai, Jianfeng–Diaoluo, and Jiusuo–Lingshui faults, together with the NE-trending Baisha and Gezhen faults (Figure 1). The dominant structural trends are oriented E–W and NE (Figure 1). Magmatism on Hainan Island is widespread, and voluminous magmatic rocks of various episodes are exposed, covering approximately 64% of the island’s total area [27] (Figure 1).

The oldest strata currently delineated on the island are assigned to the Mesoproterozoic Baoban Group. Investigations of the Baoban Group can be traced back to the 1960s. In 1964, during 1:200,000 regional geological mapping conducted by the Guangdong Regional Geological Survey Party on Hainan Island, a suite of intensely metamorphosed mixed gneiss complex was recognized in areas including Baoban–Yaowen–Chongzuling in western Hainan Island and Huangzhuling in Qiongzhong in the east, and was designated the “Baoban Group”. In 1979, based on the study of this “mixed gneiss complex”, geologists proposed that its metamorphic grade was markedly higher than that of the Upper Ordovician–Lower Devonian Jiucunling Group (currently subdivided into the Ordovician Nanbigou Formation and the Lower Silurian Tuolie Formation) and that it had undergone conspicuous migmatization, thus representing a high-grade metamorphic sequence. The historical subdivision of the age assignment of the Baoban Group is summarized in Table 1.

The Baoban Group crops out over an area of approximately 160 km², with a thickness exceeding 3562 m. Lithologically, it is composed of a lower gneiss series and an upper schist series, which correspond, respectively, to the Gezhencun Formation and the Ewenling Formation as subdivided by Ma et al. (1997) [28] (Table 1). The gneiss series consists predominantly of plagioclase gneiss, granitic gneiss, monzonitic gneiss, and plagioclase amphibolite. The plagioclase amphibolite occurs largely as layered or stratiform bodies within the gneiss, with thicknesses mostly on the order of several meters and locally up to tens of meters. All of these rocks are characterized by intense migmatization [14].

2.2. Sample Description

The metasedimentary rock sample investigated in this study (sample DF1779) was collected from the north of Nawen Village, Jiangbian Township, Dongfang City, Hainan Island (Figure 2). Preliminary field geological survey reveals that the metasedimentary rock is grey to greyish-yellow on the surface, moderately to slightly weathered, intensely deformed, and locally characterized by pronounced fracturing, which imparts a distinct thinly layered appearance, and is relatively rich in quartz (Figure 3a).

The hand specimen displays a greyish-yellow surface with local reddish patches, indicating the presence of ferruginous minerals; the fresh surface is grey to greyish-white, with a granoblastic texture and massive structure, and abundant muscovite is visible. Microscopic observation reveals that the mineralogy is dominated by quartz (~60%) and muscovite (~40%) (Figure 3b, c). Fractures within fragmented quartz crystals are commonly filled by aligned muscovite. Both quartz and muscovite occur as crystalline aggregates. The width of quartz domains is mostly ~60 μm, locally reaching up to 120 μm, whereas that of muscovite domains is markedly narrower, typically ~30 μm. Influenced by tectonic deformation, quartz locally assumes a lenticular shape; muscovite is conspicuously bent, and both minerals exhibit a distinct preferred orientation (Figure 3b, c).

3. Zircon Geochronological Analysis Method

Zircon grains from the metasedimentary rock were separated using conventional heavy liquid and magnetic separation techniques. After separation, zircon grains exhibiting euhedral crystal morphology and relatively large sizes were preferentially handpicked under a binocular microscope and mounted, together with zircon reference standards, in epoxy resin. The preparation of the zircon grain mounts was undertaken by Chongqing Yujin Science and Technology Co., Ltd.

Transmitted-light, reflected-light, and cathodoluminescence (CL) imaging of the zircon grains, together with U-Pb geochronological dating, were all carried out at the Guangxi Key Laboratory of Hidden Metallic Ore Deposits Exploration, Guilin University of Technology.

Zircon U-Pb dating was conducted using laser ablation–inductively coupled plasma mass spectrometry (LA-ICP-MS) with an Agilent 7500cx ICP-MS coupled to an NWR-193 laser ablation system. The laser ablation system has an output wavelength of 193 nm and a spot size range of 2–150 μm; a beam diameter of 32 μm was adopted during the experiments. Data reduction, processing, and the calculation of U-Th-Pb isotopic ratios and ages were performed using the software package ICPMSDataCal 7.2. Zircon U-Pb age concordia diagrams and related plots were generated with the international standard program Isoplot 4.11, and all individual data-point errors are reported at the 1σ level.

4. Results

The zircon U-Pb dating results for the metasedimentary rock sample are presented in Table 2. The zircons selected for dating are transparent and predominantly light brown to brown in color. The majority of the grains are short-prismatic, with lengths typically ranging from 50 to 100 µm. Some zircons exhibit well-developed oscillatory zoning (Figure 4a), which is characteristic of a magmatic origin; others display metamorphic overgrowth rims (e.g., spots 06 and 10 in Figure 4a) or unzoned to weakly zoned internal structures (e.g., spots 03 and 04 in Figure 4a), indicative of a metamorphic origin.

A total of forty-nine spot analyses were conducted on zircons from sample DF1779, yielding Th/U ratios that vary predominantly from 0.10 to 1.33 (Table 2). Given that all the dated spots yield ages older than 1000 Ma, the ²⁰⁷Pb/²⁰⁶Pb ages were used for data analysis and plotting (Figure 4). The forty-nine spot analyses record ²⁰⁷Pb/²⁰⁶Pb ages ranging from 1589 to 2561 Ma (Table 2), with two prominent age peaks at 1736 Ma and 2339 Ma (Figure 4c).

5. Geological Significance

The Baoban Group on Hainan Island represents the oldest known exposed stratigraphic unit on the island, and is distributed primarily along both sides of the Gezhen Fault and the Baisha Fault, cropping out mainly in the Changjiang–Gancheng area of western Hainan Island and the Baoting–Qionghai area of the central-eastern part of the island (Figure 1). With regard to the formation age of the Baoban Group, several geochronological investigations have previously been conducted. For example, Liang (1995) [33] identified a plagioclase amphibolite schist from the Baoban Group in the Tuwai Shan area of Hainan Island that yielded a Sm-Nd isochron age of 1701 ± 20 Ma. Ma et al. (1997) [28] obtained a zircon upper-intercept age of 1824 ± 77 Ma for a migmatitic granite and an Rb-Sr isochron age of 1637 ± 31 Ma for a migmatite vein from the Gezhencun Formation. These data imply that the initial depositional age of the Baoban Group predates the Paleoproterozoic (1824 Ma).

Available research data show that a considerable amount of Mesoproterozoic age information has been obtained from the Baoban Group: Yu et al. (1992) [34] determined a zircon U-Pb crystallization age of 1441 Ma for granite of the Baoban Group; Ding (1995) [35] obtained a zircon Pb-Pb age of 1397 ± 46 Ma for granodiorite of the Baoban Group; zircon U-Pb ages of 1456–1364 Ma were reported for metagranite from the Ewenling Formation [28]; granite intruding the meta-volcaniclastic sedimentary rocks of the Baoban Group in the Gezhen area gave a SHRIMP zircon U-Pb age of 1455 ± 12 Ma [36]; SHRIMP zircon U-Pb dating of metavolcanic rocks from the Baoban Group yielded an age of 1433 ± 6 Ma [15], SIMS zircon U-Pb dating of metarhyolite gave an age of 1420 ± 12 Ma [16], and LA-ICP-MS zircon U-Pb dating of metabasic rocks gave ages of 1441–1424 Ma [17]. These data indicate that the formation ages of magmatic rocks developed within the Baoban Group are predominantly concentrated between 1456 and 1364 Ma, implying that the latest depositional age of the Baoban Group postdates the Mesoproterozoic (1364 Ma). The minimum age peak of 1440 Ma obtained from metasedimentary rocks of the Baoban Group [21] further corroborates this inference.

Regional geological data indicate that Neoarchean (2.8–2.7 Ga) TTG suites are pervasively developed in lithotectonic assemblages such as the Yudongzi Group and the Zhongxiang Complex along the northern margin of the Yangtze Block [37,38]. Among these, the Yudongzi Group experienced high-pressure metamorphism, and its TTG zircons display depleted Hf isotopic signatures with dominantly positive ε_Hf(t) values, suggesting that their parental magmas were derived from a depleted mantle source [37,39,40,41]. In contrast, the Zhongxiang Complex is dominated by low-pressure metamorphism, and its TTG zircons exhibit both positive and negative ε_Hf(t) values, implying derivation from both juvenile crustal and ancient crustal materials [42,43,44]. Furthermore, Paleoproterozoic magmatic and metamorphic records are also widely preserved in the Houhe Complex, Huangtuling Complex, and Dongling Complex along the northern margin of the Yangtze Block, as well as in the Cuoke, Dahongshan, Hekou, and Phan Si Pan (Vietnam) complexes along the southwestern margin of the Yangtze Block (Figure 5). These magmatic and metamorphic events are widely considered to have been associated with geological processes such as subduction and collisional orogenesis during the early crustal evolution [40,41,42,43,45,46,47].

The Badu Group and Mayuan Group in the Cathaysia Block preserve the oldest strata identified so far in this block. Li (1997) [54] identified a plagioclase amphibolite dated at 1766 ± 19 Ma in the Tianjingping Formation in Jianning, Fujian. Yu et al. (2009) [10] and Xia et al. (2012) [61] recognized S-type granites of 1888–1875 Ma and A-type granites of 1877–1849 Ma in the Badu Group. Li et al. (2011) [11] reported ~1850 Ma S-type granites from the Mayuan Group, with ε_Hf(t) values ranging from −8.8 to +3.7, and these granites contain clinopyroxenite xenoliths displaying arc-like geochemical signatures, suggesting that their formation was linked to mantle-derived magmatism. Liu et al. (2014) [12] identified I-type granites of 1912–1887 Ma in the Badu Group, and Zhao et al. (2014) [13] also reported a granite with an age of 1929 ± 15 Ma and a metamorphic age of 1872 ± 34 Ma from the Badu Group. The ~1.8 Ga interval marks a critical period in the assembly of the Columbia/Nuna supercontinent [62]. The Paleoproterozoic I-type, S-type, and A-type granites documented in the Cathaysia Block display an overall progression from older to younger ages, implying that the Cathaysia Block may have preserved petrological records corresponding to different stages of the amalgamation of Columbia/Nuna (e.g., syn-collisional, post-collisional, and intraplate settings).

In this study, High-precision LA-ICP-MS zircon geochronological studies indicate that Paleoproterozoic metasedimentary rocks (~1736 Ma; Figure 4) were identified from strata previously assigned to the Silurian in the Jiangbian area, Dongfang City, western Hainan Island, thereby providing critical evidence for the existence of a Paleoproterozoic crystalline basement on the island. This age is broadly consistent within error with that of the migmatitic granite (1824 ± 77 Ma) reported previously from the Baoban Group [28], implying that these migmatitic granites may have served as a provenance for the metasedimentary rocks investigated in this study and further supporting the inference that the protolith of the Baoban Group was, at least in part, formed during the Paleoproterozoic. Consequently, the strata originally mapped as Silurian in this area should be dismembered, and at least a portion of them ought to be reclassified as Paleoproterozoic. Their lithological characteristics, marker horizons, and distribution remain to be constrained by more detailed future field geological surveys and laboratory analytical identifications.

Collectively, the data presented above indicate that Hainan Island, akin to the Cathaysia and Yangtze blocks, also hosts a Paleoproterozoic crystalline basement. Within this basement, well-preserved records of Paleoproterozoic magmatic and metamorphic events are retained, indicating that Hainan Island likewise constitutes an ancient continental block that has recorded the assembly of the Columbia/Nuna supercontinent.

6. Conclusion

A suite of Paleoproterozoic metasedimentary rocks was identified in the Jiangbian area, western Hainan Island, with a minimum zircon U-Pb age peak of ~1736 Ma. This finding provides critical evidence for the existence of a Paleoproterozoic crystalline basement on Hainan Island, indicating that the island is an ancient continental block that has preserved geological records of the assembly of the Columbia/Nuna supercontinent.