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