Provenance discrimination of upper Yangtze River basin sediments: New insights from heavy mineral signatures and detrital magnetite geochemistry

Abstract

Magnetite is typomorphic, and its crystal textures and chemical fingerprint form unique signatures that can be used to determine sediment provenance. Determination of the provenance of sediments in the upper Yangtze River is critical for understanding its evolution and the uplift of the Tibetan Plateau. In this study, petrographic analysis and electron-probe microanalysis (EPMA) of 800 detrital magnetite grains were performed to differentiate the provenance of sediments within the mainstream and major tributaries of the upper Yangtze River catchment. Chemical analyses show that the elemental composition of magnetite grains shows pronounced variations among the different sections of the mainstream and tributaries. Based on the observed chemical fingerprints, we infer that the source rocks control the elemental composition of magnetite grains. The magnetite contribution is closely related to the tectonics, climate, and erosion of parent rocks within different drainage basins. Magnetite from the trunk stream of the Jinsha River is found to be a significant contributor to the upper Yangtze River. Magnetite grains of the upper Jinsha River are primarily derived from source areas composed of silicic plutonic rocks and ophiolite lithologies that crop out on the SE Tibetan Plateau. Magnetite grains within the sands of the lower Jinshajiang sediments are characterized by high concentrations of Ti, Mg, Cr, Al, and Mn, and are derived from the widely distributed Emeishan flood basalts and Pan-Xi layered basic–ultrabasic igneous rocks. This source is also a major contributor of magnetite to the upper Yangtze River, and the parent rock distributed in this region is of typomorphic significance to the development of the Yangtze River. The origin of magnetite grains in the tributaries depends on the parent rocks distributed in their catchments. In summary, the presence of detrital magnetite grains provides valuable information for discriminating the provenance of sediments in the upper Yangtze River drainage basins.

Introduction

Large river systems form important links between continental sources and marginal sea sediments, reflecting interactions among tectonics, earth surface processes, and global change (Brookfield, 1998; Clark et al., 2004; Clift et al., 2005; Garzanti et al., 2005; Clift, 2006; Clift and Sun, 2006; Yang et al., 2006; van Hoang et al., 2010; Zheng et al., 2013; Clift et al., 2014; He et al., 2014; Ferrier et al., 2015). For example, the relationships amongst drainage basin evolution, continental margin deposits, Asian monsoon systems (Wehausen and Brumsack, 2002; Wan et al., 2007; Steinke et al., 2010; Clift et al., 2014), and uplift of the Tibetan Plateau have global consequences (Clark et al., 2004; Yan et al., 2012; Robinson et al., 2014; Luirei et al., 2015; Zhang et al., 2019a,b). Large river systems are also of prime importance for understanding “source to sink” systems (Driscoll and Nittrouer, 2000; Saito et al., 2001; Cawood et al., 2003; Clift et al., 2008; Yang et al., 2007, 2008; van Hoang et al., 2009; Armitage et al., 2011; Hu et al., 2011, 2012; Clift et al., 2012; Yan et al., 2012; Pan et al., 2016; Zhang et al., 2019a,b). Clastic sediments provide an essential record of basin responses to changes in regional tectonics, paleogeography, and the evolution of river basins. The Yangtze River is the largest river in Asia; it originates in the eastern Tibetan Plateau and delivers a considerable amount of water, eroded materials, and dissolved elements to the middle–lower reaches of the river and the East China Sea (Yang et al., 2006, 2009). Assessment of the sediment flux is critical for exploring the relationships between the formation and evolution of the East Asian monsoon, as well as uplift of the Tibetan Plateau.

In recent years, traditional geomorphological methods for studying river system evolution have been complemented by innovative methods including elemental/isotopic geochemistry (Yang et al., 2007; Bi et al., 2017; Zhang et al., 2016), magnetic properties, including magnetic susceptibility parameters (Zhang et al., 2008; Liu et al., 2010), detrital mineral geochronology (Yang et al., 2012; Zheng et al., 2013; He et al., 2014; Sun et al., 2018), and heavy mineral composition (Yang et al., 2009; Kang et al., 2009; Wei et al., 2020), and these methods have been used to determine the provenance of sediments within the Yangtze River catchment. Opaque Fe–Ti oxides carry unique geochemical fingerprints that can be used to determine sediment provenance (Grigsby, 1990, 1992; Martinez-Monasterio et al., 2000; Duparc et al., 2016). Among these oxides, magnetite is easily separated from bulk minerals, and has typomorphic signatures and mineral chemistry that are diagnostic of sediment source areas (Darby and Tsang, 1987; Grigsby, 1990, 1992; Razjgaeva and Naumova, 1992; Yang et al., 2009). Although weathering, denudation, transport, and diagenesis in fluvial systems may weaken the original provenance signal (Morton and Hallsworth, 1994, 1999), magnetite has long been regarded as relatively stable mineral and is thus considered a reliable indicator for the identification of sediment source areas (Darby and Tsang, 1987; Basu and Molinaroli, 1989; Grigsby, 1990, 1992; Razjgaeva and Naumova, 1992; Monasterio et al., 2000; Dare et al., 2012). However, this technique has been rarely used in provenance discrimination of sediments from the Yangtze River drainage basin (Wang et al., 2007; Yang et al., 2009; Yue et al., 2016). Wang et al. (2007) and Yang et al. (2009) reported the geochemical characteristics of detrital magnetite from Holocene sands in the Yangtze River and discriminated among potential sources of detrital magnetite. These studies suggested that most of the detrital magnetite grains from the sediments of the Yangtze River are derived from felsic plutonic and volcanic, and/or metamorphic parent rocks. Sediments from different tributaries have distinct chemical compositions, and elemental compositions in magnetite vary considerably among the tributaries and the mainstreams. It has been suggested that parent rocks in the source area control the compositions of magnetite grains in the upper Yangtze River mainstream and tributaries, in other words, the parent rocks control the sedimentary provenance signatures in the Yangtze River (Wang et al., 2007; Yang et al., 2009). Yue et al. (2016) presented elemental compositions of detrital magnetite grains collected from the main river channel/tributaries and from a sediment core taken from the Yangtze delta. Their results revealed that detrital magnetite grains with anomalously high Cr₂O₃ occurred exclusively in the upper reaches of the Yangtze River, where the Emeishan large igneous province (ELIP) is located. Based on these date, they inferred that the ELIP is the major source of magnetite grains with high Cr₂O₃ signatures in the upstream section of the Yangtze River (Yue et al., 2016). Although these findings are notable, the observed magnetite geochemistal signatures may not be fully representative of the bulk river sediments because only lesser magnetite grains were selected for chemical analysis in the previous study. Despite their possible limitations, the results of Wang et al. (2007), Yang et al. (2009) and Yue et al. (2016) still provide useful insight and reference data for subsequent research, which is worth pursuing more thoroughly. For example, a greater number of magnetite grains should be measured to allow more precise discrimination of the sedimentary provenance of the Yangtze River. In this study, we have performed accurate determinations of the composition of magnetite grains from different sources within the Yangtze River catchment, including the upper river reaches and significant tributaries that were not studied comprehensively in earlier works.

This study focuses on the upper Yangtze River basin because it has been identified as the critical region for assessment of the evolution of the drainage basin, especially given the close connection amongst earth surface processes in the upper Yangtze River and uplift of the Tibetan Plateau. In this investigation, based on geological investigations and studies of heavy minerals, we report electron-probe chemical analyses of magnetite grains received from the river sediments of the upper Yangtze River catchment, including the major tributaries and trunk stream. Approximately 100 grains from each of eight sediment samples were analyzed. In addition, different sediment source areas were identified, and the reliability of magnetite grains as provenance indicators was assessed.