Geographic proximity of Yangtze and Cathaysia blocks during the late Neoproterozoic demonstrated by detrital zircon evidence

Highlights

• Provenance analysis of Neoproterozoic siliciclastics in Xiang-Gui Basin, South China.

• Upper Cryogenian tillites of Yangtze Block provenance yield zircon ages of ~774 Ma.

• Lower Ediacaran strata of Cathaysia Block provenance yield ages of ~959–1112 Ma.

• Xiang-Gui Basin was a failed intracratonic rift related to ‘top-down’ breakup of Rodinia.

• No evidence for existence of a late Neoproterozoic South China Ocean Basin (SCOB).

Abstract

The late Neoproterozoic (~720–541 Ma) tectonic evolution of South China may have included the development of a paleo-ocean basin (the South China Ocean Basin, SCOB) between the Yangtze and Cathaysia blocks. The Niuhe section in the Yongfu area of northern Guangxi Zhuang Autonomous Region, owing to its location on the southeastern margin of the Yangtze Block and its preservation of an alternating succession typical of both Yangtze- and Cathaysia-type deposits, provides an important window to test if this oceanic basin existed during the late Neoproterozoic. Cryogenian-Ediacaran siliciclastics in the northern Guangxi region were deposited in the intracontinental Xiang-Gui Basin. Detrital zircon assemblages from the Cryogenian Lijiapo Formation at Niuhe yield a prominent UPb age peak of ~774 Ma, with a minor peak of ~891 Ma, indicative of provenance from the Yangtze Block to the northwest. In contrast, zircon samples from the Cryogenian Zhengyuanling Formation of the Laoshan area (Guangxi Zhuang Autonomous Region), which is located closer to the Cathaysia Block than Niuhe, yield a prominent age peak of 959–1112 Ma, consistent with a Cathaysian provenance. Furthermore, three samples of Ediacaran sandstones from Niuhe yield an age peak of ~900–1100 Ma, also demonstrating provenance from the southeast (i.e., Cathaysia). The Cathaysian provenance of Ediacaran sandstones on the southeastern margin of the Yangtze Block does not support the scenario of a broad SCOB between the Yangtze and Cathaysia blocks during the Ediacaran, and the Xiang-Gui Basin is best regarded as an immature intracontinental rift (aulacogen). Uplift of the Cathaysian Block through slab retreat on its external (southeastern) margin (i.e., the ‘top-down’ model of Rodinian breakup) may have been responsible for generation of a massive influx of siliciclastics into the Xiang-Gui Basin during the Ediacaran.

Introduction

The North China Craton, Tarim Craton, and South China Craton (SCC) are three small Precambrian microplates in China separated by orogenic belts, whose early geological histories were independent (Zhao and Cawood, 2012; Zhai, 2015; Fig. 1a). The North China Craton is the oldest of the three cratons, with a complex tectonic history dating back to the Meso-Archean (Zhao and Zheng, 2007; Jiang et al., 2018). The Tarim Craton has a two-layered structure consisting of Precambrian crystalline basement and Neoproterozoic to Paleozoic sedimentary cover, including three Neoproterozoic till layers (Zhai, 2015; Zhang et al., 2019). The SCC consists of the Yangtze and Cathaysia blocks and has a geological structure similar to that of the Tarim Craton, although its basement rock ages and glacial periods are different (Zhao and Cawood, 2012; Zhai, 2015). The tectonic evolution of the SCC during the late Neoproterozoic is the focus of the present study, which focuses on sedimentation along the boundary of the Yangtze and Cathaysia blocks to explore their amalgamation history and its relationship to the tectonic evolution of the SCC.

The tectonic evolution of the SCC during the late Neoproterozoic (here defined as the Nanhuan and Sinian periods of South China, corresponding to the Cryogenian and Ediacaran, ~720–541 Ma) has long been controversial. The SCC was surrounded by the Paleo-Asiatic, Tethyan, and paleo-Pacific (or Panthalassic) oceans (Li et al., 2016), and it consisted of the Yangtze Block to the northwest and the Cathaysia Block to the southeast (modern coordinates), which are separated by a northeast-southwest-trending fault belt (Fig. 1b). These two blocks are variously proposed to have been conjoined throughout the Neoproterozoic or to have been separated by an ocean called the “South China Ocean Basin” (SCOB) until collision and suturing at some point during the Early Paleozoic (Shui, 1987; Greentree et al., 2006; Shu, 2012; Zhang et al., 2013; Faure et al., 2017).

A key unresolved issue is the nature of the boundary between the Yangtze and Cathaysia blocks. Some studies have inferred the existence of one (or more) suture zones between them, e.g., the Jiangshan-Shaoxing Fault Belt on the east (Charvet et al., 1996; Li et al., 2009; Faure et al., 2017) or the Pingxiang-Chenzou Fault (Li et al., 2018) or Pingxiang-Guilin-Luodian Fault on the west (Du et al., 2013) (Fig. 1). Other studies have inferred that no suture zone is present, and that the basin between these blocks (i.e., the Nanhua Basin) is a failed intracratonic rift (Li et al., 2019a, Li et al., 2019b; Wang et al., 2016; Zhang et al., 2013; Yu et al., 2018b). Despite extensive field and geophysical study of these potential suture zones, there is no agreement regarding their nature to date.

For those studies favoring a collision zone between the Yangtze and Cathaysia blocks, there is disagreement regarding the timing of suturing. A range of ages has been proposed, e.g., 1.0–0.9 Ga (Li et al., 2007; Wang and Li, 2003; Yang et al., 2015b; Wu et al., 2018), ~820 Ma (Goodge et al., 2008; Cawood et al., 2013, Cawood et al., 2018; Wang et al., 2019b), and 458–443 Ma (He et al., 2014; Zhang et al., 2016). Ages of 1.0–0.9 Ga are based on a suturing event during the Grenville-age Jinning Movement, as part of the overall amalgamation of Rodinia. Ages of ~820 Ma are based on a widespread angular unconformity between the Lengjiaxi Group and the overlying Banxi Group that reflects a major tectonic event younger than Grenville age (Li et al., 2019a, Li et al., 2019b; Wang et al., 2016; Zhang et al., 2013; Yu et al., 2018a, Yu et al., 2018b). Both of these models require the absence of a wide SCOB during the late Neoproterozoic. However, younger collision ages (e.g., 458–443 Ma) imply that the two blocks were tectonically independent and possibly widely geographically separated during the Neoproterozoic, providing a basis for hypotheses concerning the existence of a SCOB (Yin et al., 1999; Yang et al., 2009; Greentree et al., 2006).

A second issue is the location of the Yangtze and Cathaysia blocks relative to the Rodinia Supercontinent during the Neoproterozoic. Some studies have inferred that these blocks were tectonically independent of Rodinia (Ren and Li, 2016). More commonly, these blocks are inferred to have been located on the margin of Rodinia (e.g., Wang and Zhou, 2012; Zhao et al., 2017, Zhao et al., 2018; Liu et al., 2018; Xue et al., 2019), although a few studies have inferred a more central position in the interior of this supercontinent (Li et al., 1999, Li et al., 2003). Thus, the location, timing of suturing (if any), and nature of the boundary between the Yangtze and Cathaysia blocks remains controversial. Yet it is crucial for understanding the assembly and dispersal of the Rodinia Supercontinent and contemporaneous events such as Snowball Earth episodes and early metazoan evolution (Hoffman et al., 1998; Torsvik, 2003; Corsetti et al., 2006; Kheraskova et al., 2010; Cawood et al., 2013, Cawood et al., 2016, Cawood et al., 2018; Meredith et al., 2017).

Testing the competing hypotheses above will require information about the nature of the inferred collision zone between them. Particularly needed is an investigation of lateral variations in stratigraphic architecture of the Nanhua Basin across the two putative boundaries between the Yangtze and Cathaysia blocks, i.e., the Pingxiang-Guilian-Luodian Fault (Du et al., 2013) and the Pingxiang-Chenzhou-Fangcheng Fault (Li et al., 2018) (Fig. 1). The northern Guangxi (NG) region is located between these two fault zones in an area of uncertain tectonic affinity (Fig. 2). It has been interpreted as having basement rocks composed of Yangtze Block continental crust (Li et al., 2018; Yan et al., 2019), transitional (continental-oceanic) crust (Li et al., 2019a, Li et al., 2019b), or oceanic crust (Shui, 1987; Yin et al., 1999; Xu et al., 2012). The northern Guangxi region therefore is crucial to an understanding of the tectonic evolution of the SCC and an evaluation of the possible existence of a SCOB during the Neoproterozoic.

In this study, the Niuhe section, which is located between the Pingxiang-Guilian-Luodian and Pingxiang-Chenzhou-Fangcheng faults (Fig. 1, Fig. 2a-b) near Guilin city in the northern Guangxi Zhuang Autonomous Region, was studied to determine the geochemical composition and provenance of its sedimentary succession. Our specific goals were to (1) delineate the Upper Cryogenian-Upper Ediacaran stratigraphic architecture of the Xiang-Gui transitional area between the Yangtze and Cathaysia blocks; (2) identify the provenance of detrital sediments to the Xiang-Gui area; and (3) develop a regional tectonic model consistent with our observations.