Archean blocks and their boundaries in the North China Craton: lithological, geochemical, structural and P–T path constraints and tectonic evolution
Introduction
Determinations of Archean terranes (blocks) and their accretion and collision boundaries are important as they suggest that Phanerozoic-style geodynamic processes operated as far back as the Paleoproterozoic and Archean. In most cases, however, the Archean terranes underwent polyphase supracrustal deposition, granitoid emplacement, prograde and retrograde metamorphism and deformation, and thus, they often show complex lithotectonic assemblages, geochemical characteristics, geochronological ages, structural-styles, metamorphic history and tectonic evolution. Therefore, it is difficult to identify their early geological features and original tectonic boundaries. This is the case with the North China Craton, the largest and oldest known cratonic block in China.
The North China Craton is not well-constrained in terms of its tectonic divisions and terrane boundaries. Traditionally, it has been considered to be composed of a uniform Precambrian crystalline basement, overlain by younger cover, and its tectonic history was explained using a pre-plate tectonic geosynclinal-style model (Huang, 1977). Terrane accretion and collisional models have only recently been applied (Li et al., 1990, Wu et al., 1991, Zhai et al., 1992, Wang et al., 1996, Bai and Dai, 1998, Wu and Zhong, 1998, Zhao et al., 1999a), including recognition of a Paleoproterozoic collisional orogen, here defined as the Trans-North China Orogen, which separates the craton into two distinct blocks, called the Eastern and Western Blocks (Zhao et al., 1998, Zhao et al., 1999a). P–T–t determinations reveal that the Archean basement rocks from the Eastern and Western Blocks are characterized by near-isobaric anticlockwise paths, interpreted to reflect underplating and intrusion of mantle-derived mafic magmas, whereas the Archean to Paleoproterozoic basement rocks in the Trans-North China Orogen are characterized by near-isothermal decompressional clockwise paths, reflecting continental collisional environments (Wu and Zhong, 1998, Zhao et al., 1998, Zhao et al., 1999a, Zhao et al., 2000a). A coherent outline of the timing and tectonic processes involved in the Paleoproterozoic amalgamation of the North China Craton has recently emerged and there is also much increased knowledge concerning the pre-amalgamation histories of the Eastern and Western Blocks that were subsequently incorporated into the North China Craton. On this basis, Zhao et al. (1999a) proposed that in the Late Archean, the Eastern and Western Blocks of the North China Craton represented two separate continental blocks that developed through the interaction of mantle plumes with the lithosphere, whereas the Trans-North China Orogen was formed by the collision of these two blocks in the Late Paleoproterozoic (∼1.8 Ga), resulting in the final amalgamation of the North China Craton. This tectonic model for the Late Archean to Paleoproterozoic evolution of the North China Craton is currently being tested by a Chinese–Australian co-operative project of which this paper is a part. The aim of this contribution is to summarize some fundamental constraints on the existence of the two disparate Archean blocks and their Paleoproterozoic collisional boundaries in the North China Craton. These data, in combination with our previous studies, provide an insight to understanding the tectonic evolution of the craton from the Late Archean to Paleoproterozoic.
Section snippets
Regional setting
The South China, North China and Tarim Cratons compose the tectonic framework of China (Fig. 1). The South China Craton consists of two major blocks: the Yangtze Block to the northwest, which has a Late Archean–Paleoproterozoic (>1.7 Ga) nucleus, and the Cathaysia Block to the southeast, which has a Paleoproterozoic nucleus and amalgamated with the Yangtze Block along the Jiangshan–Shaoxing Fault during the Jinning (1.0–0.85 Ga) orogeny (Fig. 1; Shui, 1987, Zhao and Cawood, 1999). The Tarim
Archean blocks and their boundaries
The structural grain of the North China Craton is dominated by north–northeast to northeast-trending fault systems. An integrated examination of lithological, geochronological, structural and thermobarometric data suggests that two of these fault systems define major terrane boundaries, which roughly correspond with the limits of a 100–300 km wide zone called the Trans-North China Orogen that separates the craton into distinct Eastern and Western Blocks (Fig. 2). The blocks and the orogen can
Constraints from lithotectonic assemblages and geochronology
Table 1 lists the major lithologies and their geochronological data for Early Archean to Paleoproterozoic basement of the North China Craton. There are some striking differences in lithotectonic assemblages between the Eastern and Western Blocks and the Trans-North China Orogen. The main differences include the following aspects:
(1) Early Archean lithotectonic assemblages are present in the Eastern Block, but are absent in the Western Block and in the Trans-North China Orogen. In the North
Constraints from geochemistry of granulites
Mafic granulites are widely distributed in both the Eastern and Western Blocks and the Trans-North China Orogen, and thus, their geochemical compositions can provide an insight into possible tectonic environments. The available geochemical data show that all mafic granulites in the North China Craton plot in the tholeiitic or calc-alkali basalt fields (Zhai, 1997). There are, however, some striking systematic compositional differences between the mafic granulites from the Eastern Block and
Structural styles
In both the Eastern and Western Blocks, the structural style of Archean basement is dominated by 2.6–2.5 Ga TTG gneiss domes of different scales (5–60 km long, 2–40 km wide), separated by anastomosing networks or linear belts of open to tight synforms of ∼2.5 Ga supracrustal rocks. These domiform TTG batholiths and intervening linear belts characterize the Archean basement rocks of the Eastern and Western Blocks and distinguish them from those of the Trans-North China Orogen where the main
Constraints from P–T paths
In the past 10 yr, extensive research has been undertaken on the P–T–t evolution of the basement rocks in the North China Craton (Jin, 1989, Cui et al., 1991, Jin et al., 1991, Zhai et al., 1992, Li, 1993, Sun et al., 1993a, Chen et al., 1994, Ge et al., 1994, Mei, 1994, Liu, 1995, Liu, 1996, Zhao et al., 1999b, Zhao, 2000, Zhao et al., 2000a, Zhao et al., 2000b) and a P–T–t data base, largely published in Chinese, is available. The detailed data for metamorphic stages, mineral assemblages, P–T
Tectonic models
In this section, we discuss possible tectonic models for the formation and evolution of the Archean blocks and their boundaries in the North China Craton, based on the available lithological, geochemical, geochronological, structural and metamorphic P–T path data. The discussion focuses on the Late Archean to Paleoproterozoic tectonic evolution of the craton, since the Early and Middle Archean lithotectonic assemblages are only locally present in the Eastern Block and geological constraints
Acknowledgements
We would like to express our thanks to A.R. Cruden and R.M. Easton for organizing the stimulating symposium ‘Precambrian Terrane Boundaries’ at 1999 GAC–MAC Joint Annual Meeting at which a preliminary version of this paper was presented. A. Kröner, J. Myers, L.Z. Lu, K.Y. Wang, M.G. Zhai and S.W. Liu are thanked for thoughtful discussions. The paper benefited substantially from detailed reviews from K.C. Condie, A. Yin and A.R. Cruden. This research was supported in part by an ARC Large Grant
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Present address: Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong.