Evidence for continental rifting from two episodes of mid-Neoproterozoic silicic magmatism in the northeastern Yangtze Block, China

Highlights

• Two episodes Neoproterozoic magmatic rocks were identified in the Zhangbaling Uplift.

• All these rocks were characterized by the A-type granites.

• Their variable geochemical signatures reveal complete tectonic process of extension.

• These magmatic rocks in the northeast Yangtze consistent with a continental rifting.

Abstract

Mid-Neoproterozoic (850–720 Ma) magmatism around the periphery of the Yangtze Block records an intense extensional event related to the breakup of Rodinia. However, the detailed geodynamic mechanisms of this extensional tectonism are still poorly constrained. To address this issue, we undertook a whole-rock geochemical and Sr–Nd isotopic, and zircon U–Pb–Hf–O isotopic study of mid-Neoproterozoic silicic meta-igneous rocks from the Feidong Complex and Zhangbaling Group in the northeastern Yangtze Block. These rocks can be classified into two groups: Group A (granitoids) and Group B (volcanic rocks). Zircon U–Pb dating revealed that Group A (808–786 Ma) pre-dates Group B (762–750 Ma). All these rocks are characterized by A-type granite features, with high heavy REE, Zr, and Hf contents, (K₂O + Na₂O)/CaO ratios, Fe index values, and zircon saturation temperatures, and formed during a mid-Neoproterozoic extensional event. However, the extremely enriched whole-rock Sr–Nd and zircon Hf isotopic compositions (I_Sr = 0.7024–0.7068, ε_Nd(t) = −19.3 to −14.1, and ε_Hf(t) = −25.7 to −11.8) of Group A suggest derivation by reworking of ancient crustal basement. Group B is characterized by relatively depleted Sr–Nd–Hf isotopic compositions (I_Sr = 0.7015–0.7063, ε_Nd(t) = −1.6 to +3.6, and ε_Hf(t) = −4.8 to +4.1), indicating a significant contribution from juvenile-crust-derived material. Group A contains zircons with δ¹⁸O = 4.1‰–7.1‰, whereas Group B contains zircons with δ¹⁸O = 1.6‰–3.1‰. These chemical and isotopic data, along with the gradual change in residual mineralogy from garnet to plagioclase (Group A to B), indicate that magmatism transitioned from an ancient lower crustal source to a juvenile middle–upper crustal source that had experienced intense high-temperature meteoric water–rock interactions. Our results provide new insights into the tectonic evolution from early extension to peak rifting driven by upwelling asthenosphere, due to rifting of the Yangtze Block from Rodinia during the mid-Neoproterozoic.

Introduction

Growth of juvenile crust and reworking of ancient crust are the final consequences of intense tectonism, including oceanic subduction, collisional orogeny, and continental rifting, which have been well documented at divergent and convergent plate boundaries (e.g., Smedley, 1986, Hart et al., 1989, Courtillot et al., 1999, Peccerillo et al., 2003, Kelemen et al., 2003, Chung et al., 2005). However, the identification of paleo-continental margins and reconstruction of their evolution is challenging. One controversial issue is the distinction between different extensional processes, such as post-orogenic extension, back-arc extension, and within-plate rifting (e.g., Smedley, 1986, White and McKenzie, 1989, Peccerillo et al., 2003, Zheng et al., 2006). These extensional processes are accompanied by characteristic magmatism, which records recycling of deep crustal materials, partial melting, and magma transport. The evolution of magmatism is a reliable indicator of the history of a plate margin. However, because interpretations of geochemical signatures of igneous rocks are not always unambiguous, it is necessary to combine whole-rock and zircon analyses to constrain the tectonic setting.

Sedimentary and igneous rocks are widespread around the periphery of the Yangtze Block in China (e.g., Wang and Li, 2003, Li et al., 2009a, Zhao and Asimow, 2018), and provide an opportunity to constrain the evolution of the block. Late Mesoproterozoic–early Neoproterozoic ophiolites and arc-like mafic igneous rocks are the products of oceanic subduction along the margin of the Yangtze Block (e.g., Li et al., 1999, Peng et al., 2012, Wu et al., 2019). Mid-Neoproterozoic intrusive rocks and volcanic–sedimentary sequences (e.g., Nanhua Basin and Kangdian Rift) are interpreted to be the product of intense extensional tectonism (e.g., Ling et al., 2003, Li et al., 2008a, Li et al., 2008b, Zhao et al., 2011, Zhao et al., 2018, Yang et al., 2016). However, the dynamic mechanism responsible for the mid-Neoproterozoic extension is still unclear, although three main models have been proposed. The plume model hypothesizes that these igneous rocks were the product of Rodinia breakup triggered by a mantle plume during the mid-Neoproterozoic, and that the Yangtze Block was located in the interior of Rodinia (Li et al., 1999, Li et al., 2003a, Li et al., 2003b, Li et al., 2008a, Li et al., 2008b). The slab–arc model advocates that the mid-Neoproterozoic bimodal magmatism and associated sedimentation resulted from back-arc extension induced by long-lived oceanic subduction (ca. 1000–720 Ma), and that the Yangtze Block was located at the margin of Rodinia or did not participate in the convergence and breakup of Rodinia (Zhou et al., 2002a, Zhou et al., 2002b, Zhao et al., 2011, Wang et al., 2017, Armistead et al., 2019). The plate rifting model (Zheng et al., 2004, Zheng et al., 2008, Zhang et al., 2015) proposes that the Yangtze Block underwent a transition from oceanic subduction during the early Neoproterozoic to arc–continent collision and continental rifting in response to Rodinia breakup during the mid-Neoproterozoic, without the involvement of a mantle plume.

Numerous studies have been conducted on Neoproterozoic magmatism (780–740 Ma) in the Qinling–Tongbai–Hong’an–Dabie–Sulu orogenic belt (Zheng et al., 2003, Zheng et al., 2004, Zhang et al., 2016, He et al., 2018). Lu–Hf isotope data for coeval igneous rocks can be divided into two groups, with positive ε_Hf(t) values of 1.1 ± 0.6 to 10.1 ± 0.6 and negative ε_Hf(t) values of −9.1 ± 1.1 to −2.7 ± 0.6, corresponding to two periods of crustal growth events (1.13 ± 0.14 and 1.98 ± 0.22 Ga), respectively (Zheng et al., 2009). These Lu–Hf isotope data suggest the northeastern Yangtze Block experienced two episodes of crustal growth. In particular, δ¹⁸O-depletion has been identified in ultra-high-pressure (UHP) eclogite-facies metamorphic rocks in the Dabie–Sulu orogenic belt (Yui et al., 1995, Zheng et al., 1996, Zheng et al., 2004, Rumble et al., 2002, Fu et al., 2013, Zhang and Zheng, 2013), which reflects interactions between these rocks and meteoric/surface waters during extensional tectonism in the mid-Neoproterozoic (Zheng et al., 2003, Zheng et al., 2006, Bindeman, 2011). However, the lack of detailed Hf–O isotope data hinders our understanding of the mechanisms and nature of the tectonic processes involved from early to peak extension in the northeastern Yangtze Block.

As such, we present in this paper in situ zircon Hf–O isotope data for Neoproterozoic meta-igneous rocks (808–750 Ma) from the Zhangbaling Uplift in the northeastern Yangtze Block. These data and whole-rock geochemical and Sr–Nd isotope data provide important insights into the magma sources and geodynamic setting of the northeastern margin of the Yangtze Block during the mid-Neoproterozoic.