Archean blocks and their boundaries in the North China Craton: lithological, geochemical, structural and PT path constraints and tectonic evolution

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Abstract

An examination of lithological, geochemical, geochronological, structural and metamorphic PT path data suggests that the basement of the North China Craton can be divided into Eastern and Western Blocks, separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone, called the Trans-North China Orogen. The Eastern Block consists predominantly of Late Archean domiform tonalitic–trondhjemitic–granodioritic (TTG) batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at ∼2.5 Ga, with anticlockwise PT paths. Some Early to Middle Archean rocks are locally present in the Eastern Block, but their tectonic history is unclear due to reworking by the 2.5 Ga tectonothermal event. The Western Block has a Late Archean assemblage, structural style and metamorphic history similar to that of the Eastern Block, but it differs in the absence of early to middle Archean assemblages and in being overlain by and interleaved with Paleoproterozoic khondalites, which were affected by a ∼1.8 Ga metamorphic event involving clockwise PT paths. A mantle plume model is proposed for the formation and evolution of Late Archean basement rocks in the Eastern and Western Blocks based on a combination of extensive exposure of TTG gneisses, affinities of mafic rocks to continental tholeiitic basalts, presence of voluminous komatiitic rocks, dominant diaprism-related domiform structures, anticlockwise PT paths, and a short time span from the primary emplacement of TTG and ultramafic to mafic rocks until the onset of regional metamorphism. Between the two blocks is the Trans-North China Orogen which is bounded by two major fault systems and is composed of Late Archean to Paleoproterozoic TTG gneisses and granitoids, interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra-arc basin environments. These rocks underwent multiple phases of compressional deformation and peak high-pressure metamorphism followed by rapid exhumation during the Late Paleoproterozoic at ∼1.8 Ga as a result of collision between the Eastern and Western Blocks, resulting in the amalgamation of the North China Craton.

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). PTt 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 PT paths

In the past 10 yr, extensive research has been undertaken on the PTt 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 PTt data base, largely published in Chinese, is available. The detailed data for metamorphic stages, mineral assemblages, PT

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 PT 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.

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