Timing of the final closure of the Proto-Tethys Ocean: Constraints from provenance of early Paleozoic sedimentary rocks in West Kunlun, NW China

Highlights

• Final closure of the Proto-Tethys Ocean occurred at 431–420 Ma in Western Kunlun.

• “Soft” collision between the NKT and the SKT created ca. 40% of juvenile materials.

• Rodinia-related “hard” collision generated ca. 12% of juvenile materials in southern Tarim.

Abstract

Collision can be subdivided into “soft” and “hard” types, with the “soft” collision occurring after double-sided oceanic subduction and the “hard” collision after single-sided oceanic subduction. Although two types of collision involve different geodynamics and generate distinct petrological assemblages, whether they can preserve distinct records of detrital zircons remains unclear. This study confirms “soft” collision between the north Western Kunlun terrane (NKT) and the south Western Kunlun terrane (SKT) after the closure of the Proto-Tethys Ocean. We further compare detrital zircon Hf isotope compositions of the “soft” collision with those of the “hard” collision related to the amalgamation of Rodinia in southern Tarim. Our results show that the NKT is characterized by dominant ca. 800 Ma zircons, whereas the SKT is featured by ca. 244 Ma, ca. 440 Ma, and ca. 620 Ma zircons. As such, sample 17WP53 deposited at 431 Ma in the NKT displays a dominant peak at ca. 500 Ma, indicating minor material exchange between the NKT and the SKT at ca. 431 Ma. Given the 420–405 Ma North Kudi granites displaying geochemical features of within-plate granites formed at a post-orogenic stage, we infer that the final closure of the Proto-Tethys Ocean occurred at 431–420 Ma along Western Kunlun. Moreover, zircon ε_Hf(t) data indicate that the “soft” collision between the NKT and the SKT during the amalgamation of Gondwana produced ca. 40% of juvenile crustal materials, whereas the “hard” collision related to the formation of Rodinia generated ca. 12% of juvenile crustal materials. More juvenile materials generated in the “soft” collision may be attributed to complete detachment and sinking of a oceanic slab.

Introduction

Episodic amalgamation and breakup of supercontinents occurred via oceanic plate subduction followed by (arc-) continent-continent collision (Nance et al., 2014; Pastor-galan et al., 2018; Zhao et al., 2018). Moreover, collision can be subdivided into “soft” and “hard” types that involve distinctly different geodynamics (Soesoo et al., 1997; Zhao, 2015). During “hard” collision, a passive continental margin may subduct as a down-going plate after closure of a oceanic basin and result in (ultra)high-pressure metamorphism and possible collision-related magmatism, but lack oceanic subduction-related magmatic record (Najman et al., 2017; Soesoo et al., 1997; Zhu et al., 2016). Meanwhile, an active continental margin generally develops large-scale arc magmatic rocks and obducts after oceanic closure (Collins et al., 2011; Najman et al., 2017). In contrast, “soft” collision between two blocks does not involve continental subduction and high-pressure metamorphism, but induces complete detachment and sinking of a oceanic slab (Soesoo et al., 1997; Zhao, 2015; Zhu et al., 2016). In addition, two welded blocks generally witness crustal thickening, slab breakoff and lithospheric delamination as demonstrated by widespread post-collisional (ultra)potassic volcanic rocks due to continuous convergence, such as the Cenozoic Himalaya orogenic belt (Collins et al., 2011; Moyen, 2009; Nomade et al., 2004; Najman et al., 2017; Williams et al., 2004). However, unlike the Himalaya orogenic belt, numerous ancient collisional orogenic belts related to the evolution of supercontinents did not preserve complete magmatic records from oceanic subduction to post-collisional (ultra)potassic volcanic rocks related to lithospheric delamination, due to complicated processes such as uplift and erosion.

Although detrital zircons from sedimentary rocks were sourced from various sources and some grains were transported for several thousand kilometers, sedimentary rocks in collisional basins (e.g. foreland basins) incorporated predominately detritus from uplifted nearby sources and have a restricted distributary province. Moreover, the youngest zircons approximate the maximum depositional age of sedimentary rocks deposited in such settings (Cawood et al., 2012; Dickinson and Grels, 2009; Liu et al., 2019). In addition, zircon Hf isotope compositions are primarily controlled by the nature of magma sources which are closely associated with geodynamic settings (Collins et al., 2011; Griffin et al., 2006; Zhang XR et al., 2019). For example, zircons crystallized during advancing subduction or oceanic detachment are likely to have high ε_Hf(t) values, whereas those crystallized during retreating subduction generally have negative ε_Hf(t) values (Collins et al., 2011; Han et al., 2016; Zhu et al., 2016). Thus, detrital zircons in collisional basins can provide complementary information on local geological processes (Collins et al., 2011; Cawood et al., 2012; Dickinson and Grels, 2009; Morag et al., 2011).

The Tarim Craton in northwestern China has been incorporated into the evolution of the Columbia, Rodinia, and Gondwana supercontinents (Wang et al., 2020; Zhao et al., 2018). In southern Tarim, intrusive rocks and strata record the evolution of the Rodinia and Gondwana supercontinents. Particularly, the southern Tarim lacks subduction-related igneous rocks and was a passive margin during amalgamation of Rodinia, confirming a “hard” collisional orogeny along Altyn Tagh (Wang et al., 2013; Wang et al., 2020). Subsequently, breakup of Rodinia resulted in opening of the Proto-Tethys Ocean and drifting of the south Western Kunlun terrane from the north Western Kunlun terrane, and the Proto-Tethys Ocean closed due to amalgamation of Gondwana during early Paleozoic time (Li et al., 2018; Zhao et al., 2018).

Several well-documented subduction-related intrusions in the north and south Western Kulun terranes indicate that the Proto-Tethys Ocean underwent double-sided subduction along Western Kunlun during early Paleozoic time and formed a “soft” collisional orogen (Wang et al., 2013; Xiao et al., 2005; Ye et al., 2008; Zhang QC et al., 2019). However, the timing of the final closure of the Proto-Tethys Ocean along Western Kunlun remains a puzzle (Liu et al., 2014; Li et al., 2018; Yuan et al., 2002; Zhang et al., 2018b; Zhang QC et al., 2019). In detail, the north Kudi within-plate granites in the south Western Kunlun terrene confirm that the final closure of the Proto-Tethys Ocean occurred prior to 421–405 Ma (Liu et al., 2014; Yuan et al., 2002). However, Xiao et al. (2005), based on ages and geochemical features of igneous rocks, proposed that the Western Kunlun terrane witnessed long-lived subduction processes of multiple oceans which lasted to 214 Ma and the Proto-Tethys Ocean closed prior to 470 Ma (Xiao et al., 2005). In contrast, Zhang et al. (2018b) suggested that the southward subduction of the Proto-Tethys Ocean lasted to 440 Ma, similar to the assembly of most of terranes in northern Tibet and eastern Asia to the northern margin of Gondwana (Zhang et al., 2018b). After investigations of several intrusions close to 484 Ma Qimanyute ophiolite, Zhang et al., 2019c, Zhang et al., 2019d proposed that the Proto-Tethys Ocean underwent double-sided subduction and that the initial collision between the south and north Western Kunlun terrenes occurred at 460–450 Ma (Zhang QC et al., 2019).

Combined with magmatic rocks and detrital zircons, this study constrains the final closure of the Proto-Tethys Ocean along Western Kunlun. We then take the southern Tarim as an example to demonstrate different zircon Hf isotope profiles in response to Rodinia-related “hard” and Gondwana-related “soft” collision.