Paleoproterozoic tectonic evolution of the North China Craton
Introduction
In many Precambrian orogenic belts worldwide, workers are faced with a paucity of temporal constraints on the timing of events and often construct simple tectonic models using only a few widely scattered geochronologic ages. This has led to the impression that many Precambrian orogenic belts have evolved slowly, with tectonic phases lasting hundreds of millions of years. This is in stark contrast to younger orogens, where orders-of-magnitude better age control has led workers to construct tectonic models recognizing individual tectonic phases that lasted several to tens of millions of years.
China's oldest continental fragment, the North China Craton (NCC), is composed of three main Archean elements including the Eastern Block, Western Block, and the intervening Central Orogenic Belt (Zhao et al., 2001a, Kusky et al., 2001, Li et al., 2002). Rock formation ages for the Eastern and Western Blocks cluster around 2.7–2.5 Ga (with small areas of older rocks, up to 3.5–3.8 Ga, in the Eastern Block) and at 2.5 Ga for the Central Orogenic Belt. There is a current disparity between tectonic models for the tectonic evolution of the North China Craton, with a 700 million-year difference in interpretations of when the craton amalgamated from its late Archean component parts. Some models suggest that the craton did not form until 1.85 Ga, when the Eastern and Western Blocks are postulated to have collided in a continent- continent collision (the Luliang Movement of earlier Chinese literature). These models are based primarily on a plethora of petrologic data on the timing of a high-grade metamorphic event, but do not have a solid geometric or kinematic model that explains the petrologic observations (Wu and Zhong, 1998, Zhao et al., 1998, Zhao et al., 1999a, Zhao et al., 1999b, Zhao et al., 2001a, Zhao et al., 2001b, Zhao, 2001, Kroener et al., 2002). These models also do not explain why high-pressure granulite rocks, one of the hallmarks of the postulated 1.85 Ga event, are confined to the northern one third of the orogen, and are elongate perpendicular to the strike of the orogen. They also do not account for data pointing to a 2.5 Ga metamorphic event that correlates between the Eastern and Western Blocks of the NCC. Furthermore, they do not explain how island arc and ophiolitic rocks that formed at 2.55 Ga could avoid being deformed and metamorphosed for 700 million years until 1.8 Ga.
Other models have suggested 2.5 Ga as the time of amalgamation of the component parts of the NCC, but have not attempted to explain the 1.85 Ga metamorphic event (Kusky et al., 2001, Li et al., 2002). These models have been based on regional field based stratigraphic, structural, and geochronological studies. The basic tenet of these models is that many of the rocks in the Central Orogenic Belt represent remnants of arcs, ophiolites, rifted margins, and accreted fragments that formed between 2.75 and 2.5 Ga. These were deformed and metamorphosed during closure of an ocean basin between the Eastern and Western Blocks at 2.5 Ga, then cut by mafic dikes that do not contain the older fabrics. Collision was followed closely at 2.5–2.4 Ga by rifting and associated sedimentation, intrusion of mafic dike swarms, and eruption of flood basalts. However, these models do not accommodate observations that led to the interpretations of a high-grade metamorphic event related to continental collision between the Eastern and Western Blocks at 1.85 Ga.
Much of the discrepancy between the different models for the timing of amalgamation of the Eastern and Western blocks hinges on the interpretation of single and multi-grain zircon populations plus sensitive high resolution ion microprobe (SHRIMP) ages from a granulite facies terrain in the Central Orogenic Belt. The oldest rocks in the Hengshan complex are 2701±5.5 Ma biotite granitoid gneisses, which Kroener et al. (2002) interpret to be part of a 2700–2670 Ma igneous protolith to the metamorphic terrain. Alternatively, Kroener et al. note that these old rocks could be the oldest part of a circa 2700–2500 Ma igneous suite, but consider this unlikely since intermediate ages are currently unknown. Most gneissic rocks in the Hengshan yield U/Pb ages between 2526 and 2455 Ma, similar to the age range in the adjacent Wutai volcanic belt (Kroener et al., 2002). Upper-intercept U–Pb ages fall between 2.70 and 2.50 Ga, whereas lower intercept ages fall between 2.00 and 1.80 Ga, reflecting a major lead loss (metamorphic) event between 2.0 and 1.8 Ga. Additionally, 40Ar/39Ar ages on metamorphic hornblende and biotite, SHRIMP ages on metamorphic zircon rims, and Sm–Nd ages of garnets have led many to suggest that the lower intercept ages (2.00–1.80 Ga) represent the primary metamorphic event, and the upper intercept ages (2.50–2.70 Ga) represent the rock formation ages. We suggest that this is an oversimplification, and does not account for the presence of orogenic belts of several different ages, orientations, and significance in the North China Craton. Here, we present a new model for the Neoarchean–Mesoproterozoic evolution of the North China Craton that explains 2.5–1.7 Ga history of the craton, and is consistent with the petrologic, field, structural, stratigraphic, and geochronological data pointing to an earlier, more significant history of the craton.
Section snippets
Regional geology of the North China Craton
The North China Craton (Fig. 1) occupies about 1.7 million square kilometers in northeastern China, Inner Mongolia, the Yellow Sea, and Korea. It is bounded to the south by the Qinling-Dabie Shan orogen, the Yinshan-Yanshan orogen to the north, the Longshoushan belt to the west, and the Jiao-Liao belts to the east (Bai and Dai, 1996, Bai and Dai, 1998). The North China Craton includes a large area of intermittently-exposed Archean crust, including circa 3.8–2.5 Ga gneiss, TTG, granite,
Tectonic division of the North China Craton
We divide the North China Craton into two major blocks (Fig. 2) separated by the Neoarchean Central Orogenic Belt in which virtually all U–Pb zircon ages (upper intercepts) fall between 2.55 and 2.50 Ga (Kroener et al., 1998, Kroener et al., 2002, Li et al., 2000b, Wilde et al., 1998, Zhao, 2001, Kusky et al., 2001). The Western Block, also known as the Ordos Block (Bai and Dai, 1998, Li et al., 1998c), is a stable craton with a thick mantle root, no earthquakes, low heat flow, and a lack of
Evidence for 2.75–2.5 Ga events
Nearly all of the volcanic, and most of the mafic plutonic rocks in the Central Orogenic Belt have igneous crystallization ages ranging between 2.75 and 2.50 Ga (Wilde et al., 1997, Wilde et al., 1998, Zhao et al., 2001a, Zhao et al., 2001b, Kusky et al., 2001, Kusky et al.). Many of these rock sequences have been interpreted as parts of island arcs or ophiolites, and are associated with deformed continental margin sedimentary rocks (Kusky et al., 2001, Li et al., 2002). We do not know of any
Evidence for 2.5–2.4 Ga rifting
Several N–S trending rifts formed in the central North China Craton between 2.50 and 2.40 Ga (Fig. 6), reflecting post-orogenic extension. A large area of mafic to ultramafic dikes, sheets and layered complexes has recently been identified in the Hengshan–Wutai–Taihang area. The Hengshan Mafic Igneous Province is mainly located in the granulite to upper amphibolite-facies terrain, in the northern part of the craton. It can be traced within lower-grade terrains to the central to southern part of
2.4–2.3 Ga events
The Inner Mongolia–North Hebei Paleoproterozoic orogenic belt (IMNHO: Fig. 2) marks the northern margin of the North China Craton. This belt includes the low- to intermediate-grade Guyang and Chifeng metamorphic terranes, 2.49–2.45 Ga tonalitic–granitic gneiss, 2.48–2.40 Ga diorite-gabbro, scattered ultramafic rocks, 2393±3 Ma trondhjemite, and several 2.45–2.33 Ga supracrustal sequences (BIF, turbidites, and biotite-hornblende gneiss), all intruded by 2.44–2.38 Ga granites (Li et al., 1998a, Li et
2.20–1.85 Ga events
The North China Craton experienced additional regional deformation, possible accretion of exotic terranes, and Andean-style arc magmatism between 2.20 and 1.85 Ga. At least two major late Paleoproterozoic orogens (2.1–1.90 Ga) have been identified in the North China Craton. The Inner Mongolia–North Hebei Orogenic Belt along the northern margin of the craton was intruded by a belt of plutonic rocks (gabbro, diorite, and granite) upon which volcanic-sedimentary sequences were deposited. Deformation
1.85–1.40 Ga: deposition of the Changcheng (Great Wall) Series
Thick sequences of predominantly clastic sedimentary rocks were deposited across the northern part of the North China Craton between 1.85 and 1.40 Ga, forming the Changcheng Series (Fig. 6). The sedimentary sequence is largely undeformed, but some sections show shallow-level concentric folds and thrusts (Fig. 7) similar to those found in foreland thrust belts and basins worldwide. The contact between the Changcheng Series and underlying basement is interpreted to be an unconformity in most
1.80–1.40 Ga aulacogens and continental rifts
The North China Craton became dominated by extension by 1.8–1.7 Ga (Fig. 6), with the intrusion of anorogenic rapakivi granite, anorthosite, and mafic dike swarms. Rift and graben systems propagated across the craton. In the south, the Xionger Group shows characteristics of a bimodal volcanic sequence possibly related to the opening of the Qingling Ocean (Sun et al., 1985). Throughout the late Paleoproterozoic to early Mesoproterozoic (1.85–1.40 Ga), the North China Craton was characterized by
Reconciliation of data with a new model for the Paleoproterozoic evolution of the NCC
Major deformation and metamorphism of the northern part of the North China Craton occurred between 1.9 and 1.85 Ga (Li et al., 1998). Many recent metamorphic studies have been carried out in the Hengshan, where exposures are abundant and fresh (Wang, 1991, Zhai et al., 1992, Zhai et al., 1995, Zhang and Cong, 1982, Zhang et al., 1991, Zhang et al., 1994, Li et al., 1998b, Zhao, 2001, Zhao et al., 1998, Zhao et al., 1999a, Zhao et al., 1999b, Zhao et al., 2000, Kroener et al., 2002). The
Acknowledgements
This work was supported by the US National Science Foundation (Grants no. 02-07886, and 01-25925 awarded to T. Kusky), China National Science Foundation (no. 49832030 awarded to J.H. Li), Peking University Project 985, and St Louis University. This represents contribution ## to IGCP 453. We thank Alfred Kroener, Li Sanzhong, Sheila Seaman, and an anonymous Journal of Asian Earth Sciences reviewer for comments that helped to improve the manuscript.
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