Geochemistry of the paleoproterozonic Nanying granitic gneisses in the Fuping complex: implications for the tectonic evolution of the Central Zone, North China Craton
Introduction
The Precambrian Fuping Complex (Fig. 1) of medium- to high-grade metamorphic rocks is located in the Taihangshan (i.e. Tai-Hang mountains), northern China. This complex was originally considered to be one of the continental nuclei involved in the early development of the North China Craton (Bai, 1986, Bai, 1996, Tian, 1991, Tian et al., 1992, Wang et al., 1996). Subsequent Sr–Nd–Pb isotopic studies and recent U–Pb geochronological analyses (Liu et al., 1985, Zhang et al., 1991, Sun et al., 1992, Wang et al., 1996, Wang et al., 2000, Wilde et al., 1997, Guan et al., 2002, Liu et al., 2002b, Zhao et al., 2002, Wilde et al., 1998, Wilde et al., 2002) revealed that the Fuping Complex formed between ∼2.8 and ∼1.8 Ga, which is considerably younger than the Mesoarchean basement rocks in other parts of the North China Craton (e.g. 3.85–3.2 Ga eastern Hebei and Anshan–Benxi complexes—Jahn et al., 1987, Jahn et al., 1988, Kröner et al., 1988, Liu et al., 1992, Song et al., 1996).
On the basis of geochronological data, Zhao and co-workers (Zhao et al., 1999a, Zhao et al., 1999b, Zhao et al., 2002a, Zhao et al., 2000b, Zhao et al., 2001a, Zhao et al., 2001b, Wilde et al., 2002) suggested that the Fuping Complex and the adjacent Wutaishan and Hengshan complexes (Fig. 1) are part of a younger ‘Central Zone’ that separates the North China Craton into Eastern and Western Archean Continental blocks (Fig. 1). They proposed that the Central Zone represents a Neoarchan continental island arc system that records a collisional event at ∼1.80 Ga, which amalgamated the two Archean continental blocks. On the basis of lithological associations, metamorphic/deformation history and geochronological data, Liu et al., 2004, Liu et al., 2002b suggested that the Fuping and Hengshan complexes had a similar origin and evolution, whereas the Wutaishan Complex might have originated and evolved independently.
These different tectonic models are largely based on geochronological data and metamorphic history (Zhao et al., 1999a, Zhao et al., 1999b, Wilde et al., 2002) and discrimination between them requires a better understanding of the major rock types in all lithotectonic domains of the Central Zone. Granitoid intrusions are widely distributed in the Central Zone, and are the dominant lithology in both the Fuping and Hengshan complexes. These intrusions range in age from ∼2.55 to ∼1.80 Ga, spanning a large part of the history of the Central Zone. Geochemical studies of granitoids in the Central zone should provide independent constraints on its tectonic evolution, but few studies have been reported. For example, the Nanying gneisses in the Fuping Complex, originally considered to be metasedimentary rocks, have only recently been recognized to represent granitoids intruding a major ductile shear zone (i.e. the Chengnanzhuang shear zone; Tang and Liu, 1997, Liu and Liang, 1999, Liu et al., 2000, Liu et al., 2002b). SHRIMP zircon U–Pb analyses (Zhao et al., 2002, Guan et al., 2002) showed that the Nanying granitic gneisses were emplaced at ∼2.0 Ga, significantly younger than the formation of TTG gneisses at ∼2.50 Ga, but older than the collisional event at ∼1.80 Ga (Wilde et al., 2002, Zhao et al., 2002).
In this contribution, we document the field relationships of the Nanying granitic gneisses in the Fuping Complex, and present new geochemical and Sr–Pb isotope data for these rocks. The main purposes of this study are to use the geochemical data, together with previously published Nd isotope data (Zhang et al., 1991, Sun et al., 1992, Liu et al., 2002b), to deduce the petrogenesis of this granitoid magmatism in the Fuping Complex and to test evidence for a collisional event before the ∼1.80 Ga collision that juxtaposed the two Archean continental blocks (Wilde et al., 2002, Zhao et al., 2002).
Section snippets
Geology of the Fuping Complex
The Fuping Complex (Fig. 2) was originally considered to represent an Archean high-grade metasedimentary terrain (Wu et al., 1989, Wu and Zhong, 1998), in contrast to the dominantly metavolcanic terrains of the Eastern Continental Block. Most of the fine-grained granitic gneisses in the Fuping Complex, including biotite (±hornblende) plagioclase gneisses, granodioritic gneisses and syenogranitic gneisses, were interpreted to be paragneisses (Wu et al., 1989, Tan et al., 1989). Wang et al., 1991
Analytical procedures
A total of 16 representative samples of the Nanying granitic gneisses were analyzed for major and trace element geochemistry (Table 1, Table 2). Theses samples were collected from 10 localities (Fig. 2) showing relatively low intensities of deformation and were selected to avoid apparent hydrothermal alteration and weathering. Samples were first crushed to minus 60 mesh (250 μm) and then pulverized to minus mesh (75 μm). Major and trace element geochemical analyses were performed in the Key
Major and trace element compositions
The major and trace element compositions of the Nanying granitic gneisses are given in Table 2. These samples are metaluminous to slightly peraluminous, with the alumina-saturation index [A/CNK=molar AI2O3/(CaO+Na2O+K2O)] increasing from granodioritic gneisses (0.85–0.96), through monzogranitic gneisses (0.87–1.18), to granodioritic gneisses (0.94–1.27; Table 2). Representative bivariant plots show that most major elements (e.g. Al2O3 and CaO) correlate negatively with SiO2, except that K2O
Origin and source of the Nanying granitic gneisses
Despite early debated about sedimentary versus igneous origins for the Nanying granitic gneisses, it is now well established, on the basis of field relationships (Fig. 3a) and U–Pb geochronological data (Zhao et al., 2002, Guan et al., 2002), that these gneisses along the Chengnanzhuang shear zone (Fig. 2) represent a major magmatic episode in the Fuping Complex at about ∼2.0 Ga. Using the field relationship and the presence of metasedimentary xenoliths, Liu and Liang (1999) suggested that the
Acknowledgements
We wish to thank K.M. Ansdell, H.W.Day, and M.F. Zhou for critical comments and helpful suggestions, S.A. Wilde and G.C. Zhao for many stimulating discussions, Z.Y. Chu, L.B. Gu, G.S. Qiao, H.X. Shao, and R.H. Zhang for analytical assistance, and the National Nature Science Foundation of China (49832030) for financial support.
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2020, Ore Geology ReviewsCitation Excerpt :The analytical precision for the major oxides (SiO2, Al2O3, MgO, CaO, TFe2O3) is better than 0.1%, and for major oxides (MnO, Na2O, K2O, P2O5, TiO2) better than 0.01%. The detailed analytical procedures have been documented in Liu et al. (2005), and the results are listed in Table.3. The sample powders were pre-treated at the Key Laboratory of Crustal Dynamics, Institute of Crustal Dynamics, China Earthquake Administration.