Elsevier

Lithos

Volumes 398–399, October 2021, 106324
Lithos

Research Article
Petrogenesis and tectonic significance of Neoarchean (~2.6 Ga) alkaline ultrapotassic granitic gneisses from the southeastern margin of the North China Craton: Constraints from U-Pb dating, Hf isotope and petrogeochemistry

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Highlights

  • ~2.6 Ga alkaline ultrapotassic granitic gneisses in southeastern margin of the NCC

  • The gneisses' precursor was derived from a subduction-modified ultrapotassic magma.

  • The gneisses underwent two episodes of metamorphism at ~2.5 and ~1.85 Ga.

  • The tectonic transition from compression to extension during the late Neoarchean

Abstract

The Wuhe complex (WC), exposed at the southeastern margin of the North China Craton (NCC), is an important constituent of the Eastern Block of the NCC. In order to better understand the Precambrian crustal composition and evolution of this region, a comprehensive investigation on zircon geochronology and Hf isotopes, as well as whole-rock geochemistry and Sr-Nd-Pb isotopes was conducted on alkaline ultrapotassic granitic gneisses from the WC at Mashan. These gneisses are considered as meta-igneous rocks based on geochemical and mineralogical criteria (with special reference to zircon cathodoluminescence images). They are characterized by modally abundant alkali feldspar (more than 60%) and quartz (more than 30%) while plagioclase is rare. Mafic minerals are sodic amphiboles such as arfvedsonite and eckermannite, aegirine and rare biotite. Minor constituents are rutile, muscovite, apatite and barite. Major elements geochemistry shows high SiO2 (69.85%–74.51%) and K2O + Na2O (9.67%–12.17%) contents, high K2O/Na2O (7.41–22.53) ratios, and relatively low MgO (0.37%–0.87%) and CaO (0.10%–0.23%) contents. Trace elements geochemistry shows significant depletions of Nb, Y, Ce, Ga, and REE (rare earth elements) relative to anorogenic granites. These features suggest that the magmatic protoliths of the studied gneisses belong to ultrapotassic silica-saturated alkaline series from an extensional background, perhaps in a subduction-related rifting environment. As concerning isotope geochemistry, their 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios are 16.6297–17.1877, 15.4454–15.5066, and 36.6036–38.0304, respectively, and εNd(t) values vary from −1.7 to +9.1. Two samples yielded zircon 207Pb/206Pb ages of 2615 ± 4 Ma and 2617 ± 5 Ma, respectively, representing their precursor ages. The zircon igneous core domains exhibit oscillatory growth zoning with positive εHf (t) values (+2.5 − +6.6). These data, coupled with chondrite-normalized and primitive-mantle-normalized element patterns, suggest that the precursors of the studied granitic gneisses were mainly derived from a subduction-modified ultrapotassic syenitic parental magma, and may be considered as a particular group of A-type granites, involved in an important crustal growth and reworking event at ~2.6 Ga. These rocks experienced a granulite-facies metamorphic stage accompanied by partial melting, as testified by clinopyroxene + rutile + K-feldspar + spinel + quartz + apatite inclusions in zircon metamorphic domains which were dated at ~2.5 Ga. The occurrence of this metamorphic stage is also supported by the lower ΣREE contents, negative Eu anomalies, and high Ti-in-zircon temperatures (>800 °C) of metamorphic zircon mantles dated at ~2.5 Ga. Eventually, the studied rocks suffered a later ~1.85 Ga metamorphic overprinting, possibly related to the Paleoproterozoic collisional orogeny recorded in the region.

Introduction

Sodium-rich (K2O/Na2O < 1) tonalite–trondhjemite–granodiorite (TTG) gneisses are the dominant lithologies of Archean crust, while K-rich granitoids gradually increased subsequently on a global scale (Laurent et al., 2014; Moyen, 2011). The transition from Na-rich TTG rocks to K-rich granitoids marks the most significant stage of Archean intracrustal differentiation. The TTG rock suite is considered to be derived from the partial melting of hydrous metabasalts at various depths within subducted slabs, oceanic plateau and root zones of volcanic arcs (Condie, 2014; Jahn et al., 1988, Jahn et al., 2008; Laurent et al., 2014; Moyen, 2011), or from the partial melting of hydrous thickened basaltic lihologies in the lowest crust (Champion and Smithies, 2019 and reference therein). The K-rich granitoids were generally emplaced shortly after the massive formation of sodic TTGs and represent the last Archean major magmatic event in most Archean cratons, reflecting the increased maturity of the continental crust (Frost et al., 1998; Jahn et al., 1988; Moyen et al., 2003). Ultrapotassic rocks were initially defined as basic and intermediate rocks with K2O > 3 wt%, MgO > 3 wt% and K2O/Na2O > 2 (Foley et al., 1987). However, it is now widely accepted that the ultrapotassic rocks can also include a series of silicic rocks with MgO content below 3 wt% (Plá Cid et al., 2000). Silica-saturated alkaline series can be sodic and potassic, as well as ultrapotassic (Plá Cid et al., 2000). Ultrapotassic rocks have been described worldwide, particularly in post-collisional settings. Post-collisional potassic to ultrapotassic magmatism has been recognized in Tibet, Italian peninsula, and Yangtze Craton (Conticelli et al., 2002; Miller et al., 1999; Turner et al., 1996; Williams et al., 2004; Xin et al., 2020). Although geographically widespread, these rock suites are volumetrically and temporally limited. However, because of their rarity, they are also of fundamental importance to petrologists understanding of the crustal and mantle evolution in both time and space and can help in formulating regional tectonomagmatic models (Foley et al., 1987; Miller et al., 1999; Plá Cid et al., 2000). Despite more than a century of intense scientific interest, the processes responsible for the genesis and evolution of the ultrapotassic rocks are still debated, especially those related to early Precambrian alkaline ultrapotassic granites.

Precambrian granitoids are volumetrically abundant at the southeastern margin of the North China Craton (NCC) (Wang et al., 2017; Liu et al., 2019 and references therein), but among them alkaline ultrapotassic granites are extremely rare. In particular, the occurrence of late-Archean alkaline ultrapotassic granitic gneisses in the Wuhe complex at the southeastern margin of the NCC has not been reported yet. In this paper, major and trace elements, Sr-Nd-Pb-Hf isotopes and SHRIMP zircon U-Pb ages are presented for the first time to infer the petrogenesis of ~2.6 Ga alkaline ultrapotassic granitic gneisses outcropping in the region. In addition, our results provide new insights on the ages and tectonic setting of late-Neoarchean and Palaeoproterozoic high-grade metamorphism. These data are also compared with previously published data in order to place these new findings within a global geodynamic context.

Section snippets

Regional geological setting

The NCC refers to the Chinese part of the Sino–Korean Craton, which is one of the oldest cratons in the world and preserves ancient crustal remnants as old as 3.8 Ga (Liu et al., 1992; Wu et al., 2005; Zhai and Santosh, 2011). The NCC covers an area of 1,500,000 km2 in central and northern China and is the oldest known craton in China. It is bounded by the Central China orogen (including the Qinling–Dabie–Sulu belts) to the south west, and the Inner Mongolia–Daxinganling orogenic belt (the

Samples and petrography

Five samples (1410MS6, 1504MS1, 1504MS2, 1504MS3 and 18MS2) of alkaline ultrapotassic granitic gneiss and two samples (1410MS1 and 18MS1) of rutile-bearing leucocratic veins were collected from the Mashan area. Fig. 1b gives the location of all the studied samples. Field relationships show that the alkaline ultrapotassic granitic gneisses share irregular contacts with the leucocratic veins, the main fabric being commonly roughly parallel to the regional foliation as a result of the strong

Analytical methods

Seven samples from the Mashan area were selected for geochemical analyses: two samples from the rutile-bearing leucocratic veins (1410MS1 and 18MS1) and five samples from the alkaline ultrapotassic granitic gneisses (1410MS6, 1504MS1, 1504MS2, 1504MS3 and 18MS2). All samples were subjected to whole-rock major and trace element analyses (1410MS1, 18MS1, 1410MS6, 1504MS1, 1504MS2, 1504MS3 and 18MS2), while six samples were selected for electron microprobe mineral analyses (1410MS1, 18MS1,

Mineral chemistry

The composition of sodic amphibole, clinopyroxene, K-feldspar, biotite, muscovite, rutile, apatite and barite from the leucocratic veins and alkaline ultrapotassic granitic gneisses is reported in Table S1 and Fig. 4.

Mineral compositions in Table S1 indicate that the sodic amphiboles in the ultrapotassic granitic gneisses are rich in Fe, Mg and Na, while Ti, Al, and Mg are relatively low. Sodic amphiboles show XMg = Mg/(Mg + Fe2+) = 0.08–0.76 and plot in the fields of arfvedsonite and

~2.6 Ga magmatism

The present study provides the first report of ~2.6 Ga igneous ages for alkaline ultrapotassic granitic gneisses from the southeastern margin of the NCC (Fig. 8). The zircon cores domains are characterized by oscillatory zoning and exhibit high ΣREE (590–2900 ppm) and Th/U ratios of 0.35–0.98, suggesting a magmatic origin. They also exhibit pronounced positive Ce anomalies and negative Eu anomalies, further indicative of magmatic origin (Fig. S2; Hoskin and Schaltegger, 2003). The magmatic

Conclusions

Integrated studies on zircon U-Pb dating and Hf isotopic compositions, and whole-rock element and Sr − Nd − Pb isotope analysis of alkaline ultrapotassic granitic gneisses from the southeastern margin of the NCC led to the following conclusions.

  • (1)

    Ca. 2.6 Ga alkaline ultrapotassic granitic gneisses are reported for the first time in the southeastern margin of the NCC, and are characterized by an affinity with ultrapotassic A-type granites. The gneisses' precursors were derived from ultrapotassic

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was supported by funds from the National Natural Science Foundation of China (41773020 and 42072059). We thank B. Song and C. Yang for help in SHRIMP U-Pb dating on zircon, F.-K. Chen for the Raman and Sr-Nd-Pb isotopic analysis, P. Sun and T. Yang for zircon Hf-isotope analysis, and Y.-H. Shi and J. Wang for the electron microprobe analysis. The authors would like to thank Prof. Xianhua Li for editorial handling and comments, and two anonymous reviewers for their constructive

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