Neoarchean-early Paleoproterozoic granitoids, the geothermal gradient and geodynamic evolution in the Hengshan Terrane, North China Craton

Highlights:

• Five tectonothermal events are identified in the Hengshan terrane, North China Craton.

• ~2.5 Ga TTG magmas were derived from partial melting of back-arc basin tholeiitic basalts.

• Dioritic magmas from mantle enriched by fluids and melts from subducted sediments and slabs.

• These magmatisms from a hot subduction zone with geothermal gradient of 17 ± 2 °C/km.

Abstract

The Hengshan Terrane (HST) is a typical Archean terrane in the central North China Craton (NCC) and features Neoarchean-early Paleoproterozoic granitoid rocks and associated supracrustal rocks. Five geological events are identified in the HST, including (1) ~2.71–2.67 Ga tonalites-trondhjemites-granodiorites (TTGs) and monzogranites, (2) ~2.54–2.48 Ga diorites, TTGs and volcanics, (3) ~2.44–2.43 Ga monzogranites, (4) ~2.14–2.04 Ga monzogranites and (5) ~1.92–1.84 Ga granulite- to high-grade amphibolite- facies metamorphism. The ~2.5 Ga diorites exhibit low SiO₂ concentrations (55.67–62.61 wt%), high MgO content of 1.39–4.89 wt% with Mg# values (45–62) and positive ɛHf(t₂) values of +3.2−+7.8, and the magma originated from a mantle source that had been metasomatized by dehydration fluids and melts from subduction-related sediments and slabs. The ~2.5 Ga TTGs are characterized by variable MgO contents (0.56–2.32 wt%) and Mg# values (44–67), (La/Yb)_N values (13.76–98.19) and positive ɛHf(t₂) values of +2.8 to +6.5, and derived from the partial melting of tholeiitic basalts with slightly enriched REE patterns, similar to those of back-arc basin basalts (BABBs). And these granitoid melts were contaminated by mantle materials. The ~2.4 Ga monzogranites show high SiO₂ (69.09–73.46 wt%) and K₂O (3.31–7.52 wt%) contents and low MgO (0.11–1.23 wt%) contents, and their parent magmas originated from the partial melting of metamorphic graywackes.

The mantle material that generated the dioritic magmas was enriched by the important metasomatic agent of sediment-derived melts and fluids prior to magmatism. These observations, in conjunction with the occurrence of widespread coeval calc-alkaline volcanic rocks in the adjacent Wutai region, suggest that a slab subduction was the most likely geodynamic regime leading to the late Neoarchean magmatism in the HST. In view of the BABB sources of the TTGs, a Neoarchean back-arc basin slab subduction model may be appropriate, which is supported by several Phanerozoic analogs. The partial melting P-T conditions of the ~2.5 Ga slab-derived TTG magmas are estimated to have been 1.5 ± 0.2 GPa and 823 ± 32 °C on the basis of thermodynamic and trace element simulations, indicating a geothermal gradient of ~17 ± 2 °C/km and a possible Neoarchean hot subduction process in the HST.

Introduction

When plate tectonic regimes develop and how they operate have become key research issues in recent years (Bédard, 2006; Dhuime et al., 2012, Dhuime et al., 2015; Hamilton, 1998; Hawkesworth et al., 2013; Stern, 2005; Smart et al., 2016; Wyman et al., 2002). In the early Archean, Na-rich tonalities-trondhjemites-granodiorites (TTGs) and komatiitic and tholeiitic volcanic rocks were the chief components of the continental crust, but the abundance of these lithological assemblages decreased significantly from the Meso- to Neoarchean and were replaced by diverse lithological assemblages including TTGs, K-rich granitoids, tholeiites, niobium-enriched basalts, boninites and adakites that are similar to modern arc volcanic rocks (Liu et al., 2021; Brown et al., 2020; Cawood et al., 2018; Condie, 1993; Dhuime et al., 2012; Kusky et al., 2016; Wang et al., 2015). Given the identification of Meso- to Neoarchean eclogites and the decrease in the crustal growth rate, some scholars have proposed that the plate tectonic regime has operated since the Meso- to Neoarchean (Liu et al., 2021; Belousova et al., 2010; Cawood et al., 2018; Martin and Moyen, 2002; O'Neill and Wyman, 2006; Nutman et al., 2011; Smart et al., 2016; Tappe et al., 2011). However, based on the appearance of reliable ophiolites, blueschists and evidence of ultrahigh-pressure metamorphism, which are generally considered indicators of slab subduction, some scholars suggest that the modern style of subduction tectonics started in the Neoproterozoic rather than the Archean (Stern, 2005). Other researchers recognized that the temperature of the Meso- to Neoarchean Moho discontinuity was ~100–200 °C higher than that in modern times (Herzberg et al., 2010), therefore, they proposed diverse high-angle shallow and hot subductions processes related to a high-temperature and weak lithosphere on the basis of numerical experimental results (Brown, 2007; Gerya et al., 2014; Sizova et al., 2010; van Hunen and van den Berg, 2008). An important aspect of this debate is the geothermal gradient, which impacts the lithospheric strength and the types of dominant tectonic regime.

Although the Earth has been gradually cooling since ~3.5 Ga, the Meso- to Neoarchean geothermal gradients and the degree of cooling is still unclear due to the lack of effective research means (Herzberg et al., 2010; Korenaga, 2006; Labrosse and Jaupart, 2007; Sleep, 2007). Reconstructing pressure-temperature-time (P-T-t) paths of metamorphic rocks may be the most widely used method for calculating ancient geothermal gradients (Brown, 2006; Brown and Johnson, 2019a, Brown and Johnson, 2019b; Mints et al., 2010, Mints et al., 2014; Tappe et al., 2011; Zhao et al., 2001). In recent years, our group has developed a new method to estimate the thickness and geothermal gradient of the Archean continental crust by calculating the partial melting P-T conditions of crust-derived granitoid rocks (Sun et al., 2019c); therefore, Archean granitoid rocks may provide crucial clues to investigating this global issue because modern geophysical data are generally invalid for studies on the Archean thermal state.

Archean granitoid rocks are the predominant components of cratons globally, and they consist chiefly of Na-rich TTGs and K-rich granitoids with different petrogeneses (Keller and Schoene, 2012; Laurent et al., 2014; Moyen and Martin, 2012). The magmatic precursors of TTGs have been proposed to be derived from the partial melting of juvenile crust (Condie, 2005; Martin et al., 2005; Smithies, 2000; Moyen, 2011) or fractional crystallization of mafic magmas derived from partial melting of mantle materials (Arth et al., 1978; Bai et al., 2016; Fu et al., 2017; Laurent et al., 2020; Liou and Guo, 2019; Macpherson et al., 2006). The K-rich granitoids consist chiefly of sanukitoids and granodiorites-monzogranites-syenogranites. Sanukitoid magma generally originate from the partial melting of a metasomatized mantle wedge (Fu et al., 2019; Heilimo et al., 2010; Laurent et al., 2013; Sun et al., 2020), and the other crust-sourced granitoids may develop from the partial melting of K-rich mafic rocks, the reworking of ancient crust or magmatic mixing between mafic and felsic melts (Fu et al., 2019; Gao et al., 2018; Hollings et al., 1999; Laurent et al., 2013; Sun et al., 2019b; Smithies and Champion, 1999; Whalen et al., 2004). Therefore, diverse granitoids might provide significant information on the paleogeothermal gradient, thermodynamic regime and evolution of the continental crust.

The North China Craton (NCC) is an ancient craton that records significant Neoarchean crustal growth (Fu et al., 2019; Gao et al., 2018, Gao et al., 2019; Hu et al., 2019a, Hu et al., 2019b; Liu et al., 2002, Liu et al., 2004, Liu et al., 2018a; Wan et al., 2011; Wang et al., 2015). In the NCC, the ~2.7–2.5 Ga granitoids are widely distributed (Bai et al., 2016; Fu et al., 2019; Liu et al., 2018a, Liu et al., 2018b; Wan et al., 2011; Wang et al., 2015), and provide an opportunity to investigate the Neoarchean evolution and thermodynamic regime of the continental crust using the petrogenesis and some geochemical parameters of these granitoids. The Hengshan Terrane (HST) is a typical Archean terrane in the central NCC and features distributed Neoarchean-early Paleoproterozoic granitoids and associated supracrustal rocks (Miao, 2001; Sun et al., 2019a; Tian et al., 1996; Wilde, 2002; Zhao et al., 2001, Zhao et al., 2007; Zhao, 2012). Some ~2.7 Ga granitoids including TTGs and biotite granites have been identified among the widespread ~2.5 Ga diorites and TTGs (Kröner et al., 2005a, Kröner et al., 2005b), and minor ~2.4 Ga and ~ 2.1 Ga K-rich granitoids and ~ 1.8 Ga high-pressure granulites are well preserved in the HST (Miao, 2001; Zhao et al., 2001; Zhao, 2012). Accordingly, these geological units make it possible to investigate the Archean to Paleoproterozoic thermal state and geodynamic evolution of the NCC.

In this contribution, we report new petrological, whole-rock geochemical and zircon U–Pb–Lu–Hf isotope data for the Neoarchean-early Paleoproterozoic granitoids in the HST in the NCC, and integrate these data with published data, aiming to determine the petrogenesis of these granitoids, to estimate the P-T conditions of the TTG magma generations and the associated geothermal gradients, and to further decipher the Neoarchean geodynamic regime in the northern NCC.