Magmatic perspective on subduction of Paleo-Pacific plate and initiation of big mantle wedge in East Asia

Abstract

Eastern China provides a precious opportunity to explore how subduction drives evolution of the overlying continental lithosphere and to understand the fate of subducted plates. In this study, a synthesis of geochronological, whole-rock geochemical and zircon Hf isotopic data is used to examine temporal and spatial variations in distribution, composition and generation of Mesozoic magmas in the northern North China Craton. A compilation of age data reveals over 1000 km of inland-ward migration of a magmatic belt during 185– 145 Ma and then back again after 145– 140 Ma, coincident with the transition from contractional to extensional deformation regime in the very early Cretaceous. Distinct trends in lithologies, geochemistry and NdHf isotopes as a function of age and location are also observed in these magmas. The Mesozoic magmatism and deformation, as well as the lithospheric destruction, across the northern North China Craton is interpreted as the consequence of a change in subduction geodynamic regime of the Paleo-Pacific slab and its interaction with overlying continental lithosphere, which involves an active continental arc at Korean and Liaodong Peninsulas in the early-middle Jurassic, progressive shallowing of the subducting Paleo-Pacific plate in the middle-late Jurassic, and subsequent slab rollback in the early Cretaceous. Considering that trench retreat and slab-roll back are demonstrated as the pre-request of slab stagnation in the mantle transition zone, we further propose that the big mantle wedge structure in East Asia was probably initiated at 145– 140 Ma and was likely fully developed by ~120 Ma. Such a peculiar deep mantle structure governed the post-Cretaceous evolution of the Asian continental lithosphere by mediating the chemical and physical properties of upper mantle.

Introduction

Subduction of Pacific and Paleo-Pacific (i.e. Izanagi) plates at eastern margin of Asian continent governs numerous geological processes, such as magmatism, tectonism, earthquakes, topography and even mantle composition (Zhao et al., 2004; Li and Li, 2007; Xu et al., 2018; Wu et al., 2019). A fascinating aspect of evolution of the overlying Asian continent is that the lithosphere was strongly disrupted in the Mesozoic, as typified by the destruction of North China Craton, the Chinese portion of the Sino-Korean Craton (Griffin et al., 1998; Xu, 2001; Zheng et al., 2007; Yang et al., 2008; Liu et al., 2019; Ma et al., 2019; Wu et al., 2019; Zhu and Xu, 2019). The regional lithospheric evolution is widely thought to be driven by subduction of Paleo-Pacific and west Pacific plates (e.g., Xu, 2001; Niu, 2005; Zhu et al., 2012; Zheng et al., 2018; Liu et al., 2019; Wu et al., 2019; Zhu and Xu, 2019), yet our knowledge of this history is very limited. In general, the modified continental lithosphere at convergent plate boundaries is restricted to the spatial scale within the mantle wedge system. In the case of Sino-Korean craton, the disrupted lithosphere, however, extends to more than 1000 km away from the trench of contemporary subduction zone, where is traditionally considered as an intraplate setting. The plausible interpretation, to which most scholars subscribe, is that the subduction dynamics of Paleo-Pacific and Pacific plates beneath East Asian continent is unusual and remains poorly understood. Indeed, seismic tomography reveals that the Pacific slab, and perhaps older Paleo-Pacific slabs are stagnant in the mantle transition zone between 410 and 660 km (MTZ) beneath East Asian continent (Van der Hilst and Engdahl, 1991; Fukao et al., 1992, Fukao et al., 2009) and the overlying upper-mantle region forms a “big mantle wedge” (BMW) that extends >1800 km inland from the trench (Huang and Zhao, 2006). This has led to a fascinating model that the continental lithosphere of eastern China evolved chemically and mechanically in response to subduction dynamics of the BMW (Li et al., 2017; Xu et al., 2018; Liu et al., 2019; Zhu and Xu, 2019). However, the duration of this BMW structure remains largely unknown, with speculated initiation time ranging from >140 Ma to <50 Ma (Kusky et al., 2014; Li et al., 2016; Liu et al., 2017). Numerical simulation studies revealed that the visible flat slab stayed in the MTZ is perhaps no more than 60 Ma (Li et al., 2016; Liu et al., 2017). Nevertheless, considering stagnation duration of the slab visible in the MTZ could be less than age of the BMW because the initial stagnated slab can sunk into the lower mantle (King et al., 2015; Agrusta et al., 2017; Liu et al., 2017), many scholars believed that the BMW formed much earlier (e.g. Kusky et al., 2014; Liu et al., 2017; Ma and Xu, 2017; Xu et al., 2018; Liu et al., 2019; Wu et al., 2019; Zhu and Xu, 2019). Indeed, geochemistry studies on Mesozoic-Cenozoic basalts in eastern China suggest that the regional upper mantle has been repeatedly metasomatized by melts released from the stagnated slab in the MTZ since early Cretaceous (Xu, 2014; Li et al., 2017; Xu et al., 2018).

Subduction magmatism is not unchanging in time and space. It could migrate laterally relative to the continental edge (Coney and Reynolds, 1977; Copeland et al., 2017; Ardill et al., 2018), accompanied by changes in geochemical compositions and isotopes of igneous rocks. Magmatism in eastern margin of Asian continent provides a unique temporal and spatial sampling of melting process within the regional upper mantle and associated continental crust, and thus can be used to explore whether and how subduction of (Paleo-)Pacific plate drives evolution of the overlying continental lithosphere and to understand formation of the BMW structure.

This study synthesizes and evaluates the previously published geochronological, petrological and geochemical data of Mesozoic igneous rocks from the northern Sino-Korean craton (Fig. 1, Fig. 2) in the area above the stagnant slab, extending for more than 1000 km from the Korean peninsula to northern Taihang in the interior of the North China Craton. The available data confirm causality between subduction of (Paleo-)Pacific plate and destruction of the root of the overlying craton. Importantly, we highlight a trend of magma migration that swept inland in the Jurassic and then swept back to the trench in the Cretaceous. This behavior, together with the spatiotemporal variations in petrology, chemistry and isotopes of the igneous rocks, the crustal-deformation patterns and the migration of basins, collectively suggests a geodynamic transition at ca 145–140 Ma from forward-flat-subduction to rollback of the Paleo-Pacific slab. We propose that this subduction geodynamic transition may have resulted in formation of the BMW in East Asia in the Early Cretaceous.