Linking the Alxa Terrane to the eastern Gondwana during the Early Paleozoic: Constraints from detrital zircon U–Pb ages and Cambrian sedimentary records

Highlights

• The Alxa block was once a peri-Gondwana terrane.

• The Alxa block was not the western part of the NCP during the Early Paleozoic.

• The provenances of the Xiangshan Group are mainly from eastern Gondwana.

Abstract

The detrital zircon ages of the Early Paleozoic Xiangshan Group in the southeastern Alxa Terrane show that the lower age limit for the Xiangshan group is 512 Ma. The Xiangshan Group is older than the Middle Ordovician strata in the study region and is the Middle–Late Cambrian in age, which is the same age as the fossils found in it. The detrital zircon ages record the Grenville and Pan-African Orogenic Events, and the major age peaks are 963 Ma, 1179 Ma, 551 Ma and 2517 Ma. The North China Terrane, the Alxa Terrane and the Qilian Orogenic Belt were not the provenances of the Xiangshan Group. The age spectrum of the Xiangshan Group is similar to terranes associated with eastern Gondwana. The sandstone composition modes of the Xiangshan Group indicate that the source areas were the recycled orogenic belts. The zircons in the Xiangshan Group with an age peak at ca. 963 Ma may come from the Eastern Ghats–Rayner orogenic belt or other terranes located to the north of eastern Gondwana. The zircons with a peak at ca. 1179 Ma may come from the Wilkes–Albany–Fraser orogenic belt. Those with a peak at ca. 551 Ma may come from the Prydz–Darling orogenic belt, and the zircons with a peak at ca. 2517 Ma may be from northern India and the Yilgarn Craton in Western Australia. Similar paleontology, paleomagnetic data and detrital zircon age spectra indicate that the Alxa Terrane was located to the east of the South China Terrane and to the north of the eastern Gondwanan continent (present coordinates).

Introduction

In the reconstruction of supercontinents during the Neoproterozoic and Early Paleozoic, the locations of the main tectonic units of East Asia, such as the North China Terrane, the Yangtze Terrane and the Tarim Terrane, are under considerable debate (Huang et al., 1999a, Huang et al., 1999b, Huang et al., 2000a, Huang et al., 2000b, Huang et al., 2001, Metcalfe, 2006, Li et al., 2008, McKenzie et al., 2011a, Cawood et al., 2013, Cocks and Torsvik, 2013, Metcalfe, 2013). One school of thought puts South China (including the Yangtze Terrane and Cathysian Terrane) in the center of the Rodinian Supercontinent (Li et al., 2008 and references therein). Another school however argues that South China was located to the north of Australia and India in eastern Gondwana (Cawood et al., 2013, Xu et al., 2013). Huang et al. (1999a) put the North China Terrane to the east of Antarctica, but Veevers (2000) suggested it belonged to the north part of Australia, and Zhao et al. (1993) argued that the North China Terrane was situated to the northeast of Australia (present coordinates). Some recent studies showed that the age components of the basement of the Alxa Terrane are similar to those of the North China Terrane (Zhang et al., 2013, Wu et al., in press), but similar age components have also been found in some other cratons (Condie et al., 2009). Moreover, many studies have found that the Alxa Terrane (regarded as the western extension of the North China Terrane) differs from the North China Terrane in basement composition and tectonic evolution but is similar to the Yangtze Terrane (Li et al., 2005, Geng et al., 2010 and references therein; Dan et al., 2012). Li et al. (2012) and Zhang et al., 2011, Zhang et al., 2012 argued that the Alxa Terrane collided with the North China Terrane in the Early Paleozoic. J. Zhang et al. (2013) suggested that it occurred at the end of the Early Paleozoic, but Yuan and Yang, in press-a, Yuan and Yang, 2015b argued that it happened at the end of the Late Paleozoic. Ge et al. (2009) even suggested that the collision between the Alxa Terrane and the North China Terrane finished in the Mesozoic. Therefore, the paleogeographic location and tectonic attributes of the Alxa Terrane during the Early Paleozoic are the key parameters in understanding the relationship between the Yangtze Terrane and the North China Terrane, their locations relative to eastern Gondwana and their respective tectonic evolution.

In the southeastern Alxa Terrane, there is a set of Early Paleozoic deep-water turbidites (the Xiangshan Group), whose age and provenances are unknown or in heated debate. Some authors have argued that the sediments are the Middle Cambrian and sourced from the North China Terrane (BGMRNHAR (Bureau of Geological and Mineral Resources of Ningxia Hui Autonomous Region), 1990, BGMRNHAR (Bureau of Geological and Mineral Resources of Ningxia Hui Autonomous Region), 1996, Zhou and Xiao, 2010, Zhang et al., 2011), but others have insisted on the Middle Ordovician (Zhang, 1989, Li, 1997, Wang and Zheng, 1998). Additionally, Yuan and Yang (2012) argued that Australia was the main provenance of the Xiangshan Group. Using the method of detrital zircon age dating of the Xiangshan Group, the lower limit age for the deposition of the Xiangshan Group can be fixed, and the provenances of the group can be deduced with the addition of paleocurrent and sandstone composition data. Based on these data, the paleogeographic position of the Alxa Terrane when the Xiangshan Group was deposited will be discussed.