Elsevier

Precambrian Research

Volumes 222–223, December 2012, Pages 265-289
Precambrian Research

Zircon ages and geochemistry of late Neoarchean syenogranites in the North China Craton: A review

https://doi.org/10.1016/j.precamres.2011.05.001Get rights and content

Abstract

At the end of the Neoarchean continental evolution, voluminous syenogranites were emplaced in the North China Craton, together with other magmatic rocks (trondhjemite–tonalite–granodiorite (TTG), monzogranite, diorite, gabbro). Syenogranites are widely distributed in Anshan-Benxi, Qinhuangdao and western Shandong, and also occur in southern Jilin, northern Liaoning, northwestern Hebei and central Henan. Based on geological relationships, degree of metamorphism, deformation and magmatic zircon ages, two phases of syenogranite magmatism are recognized. Rocks produced during the first phase show a gneissic texture and were formed between 2.53 and 2.52 Ga and locally comprise abundant TTG. Rocks of the second phase cut late Neoarchean TTG and supracrustal rocks, display a massive structure, and mainly formed between 2.52 and 2.50 Ga. All syenogranites share the same features in major element compositions, being high in SiO2 and low in CaO, total FeO, MgO, TiO2 and P2O5. However, they are different in trace and REE compositions and can be subdivided into three types. (1) Type 1 shows a large variation in total REE contents, low (La/Yb)n ratios, strong negative Eu*/Eu anomalies and Ba depletion; (2) Type 2 is similar to Type 1 but has higher (La/Yb)n ratios. (3) Type 3 shows a large variation in total REE and (La/Yb)n ratios and significantly do not show strongly negative Eu*/Eu anomalies and Ba depletion. Whole-rock Sm–Nd isotopic compositions show large variations in ɛNd(t) values and tDM(Nd) modal ages, ranging from −9.49 to −4.72 and 3.70 to 3.25 Ga (Type 1), 0.55–1.03 and 2.77–2.71 Ga (Type 2) and −2.35 to 1.23 and 2.93–2.66 Ga (Type 3), respectively. Hf isotopic compositions of zircons from three samples have ɛHf(t) values and tDM1(Hf) ages of 0.7–7.2 and 2.84–2.56 Ga (Type 1), 2.6–7.4 and 2.74–2.56 Ga (Type 2) and 2.1–6.3 and 2.76–2.60 Ga (Type 3). It is concluded that syenogranites were generated by melting of continental crust with different mean crustal residence ages, and most of them were emplaced during the second phase (2.52–2.50 Ga) in an extensional tectonic regime. The formation of these voluminous syenogranites marks a tectono-magmatic event resulting in stabilization of the North China Craton at the end of the Neoarchean.

Highlights

► Late Neoarchean syenogranites are widely distributed in the North China Craton. ► Two phases (2.53–2.52 Ga and 2.52–2.50 Ga) of syenogranite magmatism are recognized. ► There are three types of syenogranites in terms of element and isotope compositions. ► They mark a tectono-magmatic event resulting in stabilization of the craton.

Introduction

Neoarchean crustally-derived granites are widely distributed globally as a result of continental evolution and stabilization. The 3.0 Ga Tiejiashan granite in Anshan is the oldest potassium-rich granite in the North China Craton (NCC), and probably throughout Asia (Wu et al., 1998, Wan et al., 1998, Wan et al., 2007). Similarly, in the eastern Kaapvaal craton, southern Africa, there is the large 3.1 Ga potassium-rich Mpuluzi-Piggs Peak Batholith which separates the 3.53–3.2 Ga Barberton Greenstone Belt from the 3.66.3–2.0 Ga Ancient Gneiss Complex of Swaziland (Kamo and David, 1994). Although granites of the early Neoarchean (2.7–2.6 Ga) occur in many cratons as a result of partial melting of older continental crust, potassium-rich granites of this age only occur locally such as in southeastern Greenland (Nutman and Rosing, 1994), Wyoming (Frost et al., 1998) and southern India (Jayananda et al., 2006). In this paper we use two terms. namely (1) syenogranite, being high in K2O (commonly >4%) and having a K2O/Na2O ratio of >1.3; (2) potassium-rich granite, being high in K2O but not necessarily high in K2O/Na2O ratio. In terms of this subdivision, some syenogranites in the literature are not syenogranites but potassium-rich monzogranites. Both syenogranite and potassium-rich granite can occur in a same area.

The Neoarchean is an important period of continent-formation with two periods at ∼2.7 Ga and ∼2.5 Ga (Condie, 2000, Condie et al., 2009). Tectono-thermal events at ∼2.7 Ga occurred widely in many cratons worldwide. This event is of global significance and resulted in the formation of Archean continental crust on a large scale during a short period. However, the rocks formed during this period are commonly immature and mainly include supracrustal rocks (metabasalts, ultramafic rocks, intermediate-felsic volcanic rocks and immature metasediments with banded iron formations (BIF) of chemical origin) and tonalite–trondhejmite–granodiorite (TTG). The ∼2.5 Ga tectono-thermal events apparently occurred on a smaller scale and have only been identified in a few cratons such as southwestern Greenland, Antarctica, southern India and the NCC (Jayananda et al., 2000, Shen et al., 2005, Nutman et al., 2007, Condie et al., 2009, Wan et al., 2011a). Many more syenogranites were formed during the ∼2.5 Ga event compared to the ∼2.7 Ga event. Based on geological, geochronological and geochemical studies, this paper focuses on the ∼2.5 Ga syenogranites of the NCC in order to better understand cratonization at the end of the Neoarchean.

Section snippets

Geological background

The NCC is located in eastern Asia and experienced a long geological evolution with the oldest rocks being >3.8 Ga in age (Liu et al., 1992, Liu et al., 2007, Liu et al., 2008, Song et al., 1996, Wan et al., 2005a, Wan et al., 2009a). The Neoarchean was an important period when tectono-thermal events occurred widely, resulting in the formation and stabilization of continental crust (Fig. 1) (Zhao et al., 2005, Liu et al., 2008, Zhai and Santosh, 2011). However, being different from most other

Analytical techniques

Whole-rock chemical analyses were conducted at the National Research Center of Geoanalysis, Chinese Academy of Geological Sciences (CAGS), Beijing. Major and trace elements were determined by XRF and ICP-MS respectively. Uncertainties depend upon the concentration in the sample, but generally for XRF and ICP-MS are estimated at ca. 3–5% and ca. 3–8%, respectively. Sm and Nd isotopic compositions were determined by isotope dilution at the Key Laboratory of Isotope Geology, Ministry of Land and

Qidashan syenogranite A0713 in the Anshan–Benxi area

This sample was taken from the Qidashan area (Fig. 3). The zircons are stubby or elongate in shape and show banded or oscillatory zoning (Fig. 8A). Twelve analyses, except 3.1 which shows strong lead loss, are concordant and yielded a weighted mean 207Pb/206Pb age of 2503 ± 10 Ma (MSWD = 0.45) (Table 1, Fig. 8B). This is interpreted as the time of formation of the Qidashan pluton.

Qinhuangdao syenogranite FW04-54 and J0817 in eastern Hebei

Magmatic, inherited and metamorphic zircons were identified in sample FW04-54 which shows a gneissic structure and was

Geochemistry

All syenogranite s are similar in major element compositions, being high in SiO2 and low in CaO, total Fe as FeO, MgO, TiO2 and P2O5 (Table 2). SiO2 contents are commonly higher than 70%. Potassium-rich monzonite sample S0615 from the Shihaishan pluton is only 65.50% in SiO2 content. These rocks plot in the granite field in an An–Ab–Or diagram (Fig. 12). There is a negative relationship between SiO2 and Al2O3 (Fig. 13A), a reflection of variations in quartz and feldspar contents. K2O and Na2O

Compositional features of zircons

Magmatic zircons of the syenogranites commonly show oscillatory zoning. However, some zircons show banded zoning or homogenous structures which are common for zircons from high-temperature granites and quartz diorite (Corfu et al., 2003, Wan et al., in press). This probably suggests that some syenogranite magmas experienced high-temperatures, consistent with the existence of perthite and antiperthite feldspars. It is also common that oscillatory zoning is better developed in magmatic rims than

Conclusions

  • (1)

    Two phases of syenogranite magmatism have been recognized in terms of zircon ages and deformational features. The first phase rocks (2.53–2.52 Ga) exhibit an ubiquitous foliation, whereas the second phase rocks (2.52–2.50 Ga) are massive, or are only weakly foliated.

  • (2)

    Based on geochemical data, three types of syenogranite have been defined. Types 1 and 2 show strong negative Eu*/Eu anomalies and Ba depletion and are different in their (La/Yb)n ratios. Type 3 does not show strong negative Eu*/Eu

Acknowledgements

We are grateful to Yuhai Zhang and Ziqing Yang for SHRIMP measurements, Fukun Chen for whole-rock Nd isotopic analysis, and Hua Tiao and Liqing Zhou for mount making and zircon CL imaging. We thank Simon Wilde, Allen Nutman, M Santosh, Guochun Zhao, Ziran Zhao, Yuansheng Geng, Shuwen Liu, Jinghui Yang, Jinghui Guo, Mingguo Zhai, Huafeng Zhang, Peng Peng, Songnian Lu, Huicu Wang, Huaikun Li and Chengdong Li for discussions and help in this study. We thank Allen Nutman, an anonymous reviewer and

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