Slab roll-back triggered back-arc extension south of the Paleo-Asian Ocean: Insights from Devonian MORB-like diabase dykes from the Chinese Altai

Highlights

• The ca. 386 Ma diabase dykes record back-arc extension of PAO in Chinese Altai.

• The dykes originated from a N-MORB-like asthenospheric mantle source.

• Asthenospheric upwelling in response to roll-back of PAO as the trigger of magmatism.

• MORB-like mafic rocks could be an important tectonomagmatic process indicator.

Abstract

To better understand the origin of mid-ocean ridge basalt (MORB)-like mafic magmas and its geodynamic implications for the subduction zone system, we present an integrated study of petrology, mineralogy, geochronology and geochemistry of newly-discovered diabase dykes from the Chinese Altai, southwestern Central Asian Orogenic Belt. The diabases have zircon UPb ages of ~386 Ma, and are mainly composed of clinopyroxene (Wo_43–55En_17–27Fs_26–34) and albite (An_0–1Ab_92–99Or_0–7). Although the clinopyroxenes have consistent Mg# values (up to 75) that appear to be in equilibrium with bulk compositions, they can be divided into two groups based on their differences in other geochemical variables: type ǀ has high SiO₂ and CaO but low TiO₂ and Al₂O₃ contents while type ǁ displays low SiO₂ and CaO but high TiO₂ and Al₂O₃ contents. This indicates that the two types of clinopyroxenes may share a common source but crystallized at different depths under different P-T conditions (e.g., type ǀ: 6.3 kbar (~20 km), 1227 °C; type ǁ: 15.5 kbar (~49 km), 1332 °C). The dykes have low SiO₂ (44.5–48.2 wt%) and K₂O (0.17–0.33 wt%), but high (Fe₂O₃)_T (11.2–13.6 wt%) and MgO (7.16–9.90 wt%) contents, placing them within the low-K tholeiitic series. With both N-MORB- and arc-like geochemical characteristics, including weakly fractionated rare earth element (REE) patterns ((La/Yb)_N 1.43–1.62), enrichment in large-ion lithophile elements (LILE) (e.g., Pb) and slightly depletion of high field strength elements (HFSE) (e.g., Nb, Ta and Ti), and highly depleted SrNd isotopic compositions (⁸⁷Sr/⁸⁶Sr_i 0.7039–0.7042, ɛ_Nd(t) +8.3 to +8.6), the diabase dykes were probably derived by partial melting of a N-MORB-like asthenospheric mantle source metasomatized by subduction-related fluids under spinel-facies conditions. Melting temperatures and pressures for the primary magmas were estimated at 13711394 °C and 2.22.4 GPa, respectively. The dykes underwent insignificant crustal contamination, but trace-element modeling indicates that minor subcontinental lithospheric mantle (SCLM) components could have been involved in their source region. We suggest that the MORB-like magmatism was triggered by asthenospheric upwelling in a back-arc extensional setting, in response to the roll-back subduction of the Paleo-Asian oceanic slab. Combined with the coeval arc magmatism in the study area, we envisage that an active arc–back-arc basin system developed in the Chinese Altai during Devonian time, linked to the northward subduction of the Paleo-Asian Ocean. This study emphasizes that MORB-like mafic rocks in paleo-subduction zones could be an important tectonomagmatic process-indicator (e.g., back-arc extension), which offers a new perspective in reconstructing past tectonic regimes.

Introduction

Subduction zones are the Earth's largest and most complicated recycling system, in which crustal materials return to and re-equilibrate with the mantle and mantle-derived melts ascend to the crust, and thus they play a central role in Earth tectonics, melt generation and continental growth (e.g., Stern, 2002). Back-arc basins, a key component of the subduction zone system, are defined as regions behind the magmatic arc and are characterized by active extension, rifting and seafloor spreading (e.g., Pearce and Stern, 2006). Lavas erupted in back-arc basins are generally referred to as back-arc basin basalts (BABB). Because back-arc spreading systems are remarkably similar to mid-ocean ridges, back-arc basin magmas usually show geochemical variations spanning between mid-ocean ridge basalt (MORB) and island-arc basalt (IAB) (e.g., Gribble et al., 1998; Pearce and Stern, 2006; Shinjo et al., 1999; Stern et al., 1990). Most studies suggest that BABB is fed by decompression melting of upwelling mantle, coupled with fluxing by slab-derived components (e.g., Pearce and Stern, 2006; Xu et al., 2003). Although BABB have been extensively investigated in present-day subduction zones during the last decades (e.g., BABB from the Mariana Trough in Western Pacific, Stern et al., 1990; Gribble et al., 1998), attention to paleo-back-arc basin systems remain scarce. Furthermore, several key issues concerning the origin of BABB are still under debate, such as the nature of the mantle source, the composition of the subduction component as well as the driving mechanism of back-arc basin formation (e.g., Pearce and Stern, 2006).

Situated between the Europe-Siberia and Tarim-North China Cratons, the Central Asian Orogenic Belt (CAOB) is one of the world's largest and longest-lasting accretionary orogens (Xiao et al., 2015; Fig. 1a). Its development includes successive accretion of island arcs, microcontinents, seamounts, ophiolites and accretionary complexes from the early Neoproterozoic to the Mesozoic (ca. 1000250 Ma), accompanied by the opening and closure of the Paleo-Asian Ocean (PAO) (e.g., Jiang et al., 2019; Long et al., 2007; Sengör et al., 1993; Su et al., 2012; Wang et al., 2020; Windley et al., 2002; Xiao et al., 2015; Yuan et al., 2007). Given its long subduction-accretion history, the CAOB represents an ideal natural laboratory to probe subduction zone processes. The Chinese Altai, located between the Siberian block to the north and Junggar terrane to the south, plays a crucial role in reconstructing the geological evolution of CAOB (e.g., Wang et al., 2009; Windley et al., 2002; Xiao et al., 2009). Despite numerous studies, the tectonic setting of the Chinese Altai during the Devonian time has long been debated, with various perspectives proposed such as an island arc (Sengör et al., 1993), a Precambrian microcontinent (Yang et al., 2011), a back-arc basin (Xu et al., 2003) or an active continental margin (Cai et al., 2010; Long et al., 2007). Moreover, geodynamic models proposed for Devonian magmatism and the tectonic evolution of the Chinese Altai also remain controversial, including flat oceanic slab subduction (Wang et al., 2006), slab break-off (Niu et al., 2006) or oceanic ridge subduction (e.g., Cai et al., 2010, Cai et al., 2012; Yu et al., 2017). Previous studies were mainly focused on the widespread granitoid plutons in Chinese Altai (e.g., Cai et al., 2011; Song et al., 2019; Wang et al., 2009; Yuan et al., 2007), but granites are the products of re-melting continental crust and can usually be applied as a proxy of the crust (e.g., Yuan et al., 2007). To better understand the nature of the mantle and deep geodynamic processes, further investigations on mantle-derived mafic rocks are necessary.

In this contribution, we report new mineral chemistry, zircon UPb dating, whole-rock major- and trace-elements as well as SrNd isotopes for Devonian MORB-like diabase dykes from the Chinese Altai. Based on these results, together with published data of other Devonian mafic igneous rocks in the Chinese Altai, we attempt to: (1) discuss the origin and petrogenesis of MORB-like magmas; (2) reveal regional tectonic evolution and geodynamic significance; and (3) decipher the back-arc basin development of Paleo-Asian Ocean.