The Neoarchean Ophiolite in the North China Craton: Early Precambrian Plate Tectonics and Scientific Debate

INTRODUCTION

Understanding the early history of the Earth is one of the major challenges to the Earth Science community. Early crust formation is represented by massive tonalite-trondhjemite-granodiorite (TTG), and its peak formation time is about 2.7 Ga. The formation of this stage of TTG is generally considered to be related to mantle plumes (e.g., Condie, 1997), although other models suggest that the TTG terranes may have formed from partial melting of shallowly subducted buoyant oceanic slabs (e.g., Tappe et al., 2011; Rapp and Watson, 1995; Rapp et al., 1991). However, some cratons preserve a tectonic framework of high-grade granulite-gneiss and greenstone belts formed in the Early Archean. Greenstone belts consist of low-grade metamorphic volcanic-sedimentary rocks which are typically exposed as linear fold belts around high-grade rocks (e.g., Kusky and Vearncombe, 1997). For the tectonic setting of greenstone belt rocks, there are different opinions including intracontinental rifts, island arcs, back-arc basin—small ocean basin combinations, although these models are not mutually exclusive. These different ideas led to a debate of whether plate tectonics existed in the Archean and when did plate tectonics begin to operate (e.g., Stern, 2007).

Archean ophiolite discrimination is one of the main bases to explore the issues of whether or not plate tectonics existed in the Archean and when did plate tectonics begin to operate, thus many scientists have been dedicated to this study for many years. There are a number of papers related to this aspect published in international journals. "Precambrian Ophiolites and Related Rocks" edited by Kusky (2004) focused on Archean and Proterozoic ophiolites, and also discussed the oceanic crust evolution model which changes with time. The assumed oldest ophiolite is from the Isua supracrustal rocks in West Greenland (Furnes et al., 2009, 2007a, b), with an isotopic age of ~3.8 Ga. The ophiolites that are assumed to be around 3.0–2.7 Ga age include the 3.0 Ga ophiolite of Olondo in the Aldan Shield, East Siberia, 2.8 Ga SSZ-type ophiolite of the North Karelian belt in the NE Baltic Shield, Russia, and 2.7 Ga ophiolites in the Slave craton, Canada, and Zimbabwe (Cocoran et al., 2004; Hofmann and Kusky, 2004; Puctel, 2004; Shchipansky et al., 2004; Kusky, 1998, 1991, 1990, 1989; Kusky and Kidd, 1992), and 2.5 Ga ophiolites in the North China craton (NCC). All above ophiolites are still controversial, mainly because of their differences compared to the rock association, occurrence and geochemistry of modern spreading ridges. Since documentation of Archean ophiolites is a key scientific issue, the debate and further research will continue and its progress will promote the understanding of early continental evolution and the beginning of plate tectonics.

GENERAL CHARACTERISTICS OF GREENSTONE BELTS AND OPHIOLITES Greenstone Belt

Generally, the term greenstone belt refers to a supracrustal rock belt distributed in linear to arcuate zones in Precambrian shields. Greenstone belts typically contain products of several generations of mafic volcanic-sedimentary rocks. The main rocks consist of basalts, komatiites, intermediate-acidic calc-alkaline volcanic rocks and sedimentary rocks, gabbros and diabases, and minor serpentinized ultramafic rocks (e.g., de Wit and Ashwal, 1997). Metamorphic grades range from sub-greenschist to granulite, with greenschist-amphibolite facies being most characteristic. The chlorite, epidote, actinolite and other metamorphic minerals give the rocks their characteristic dark green color. A complete set of strata of greenstone belt rocks is typically comprised of early volcanic rocks and later clastic sedimentary rocks or volcanic clastic sedimentary rocks, which are mainly turbidites. Underlying volcanic/plutonic rocks are mainly ultramafic-mafic rocks also in some cases including komatiites. Overlying volcanic rocks are typically calc-alkaline volcanic rocks. There are generally ultramafic lenses underlying the greenstone belt, which are explained to represent fragments of ancient mantle. Greenstone belts are structurally complex with a complex series of deformation events, yet many exhibit a broad synclinal shape surrounding high-grade gneiss-granulite zones, formed in the late stages of deformation of these belts (e.g., Kusky and Vearncombe, 1997).

Ophiolite

An ophiolite is a rock suite that consists of serpentinized ultramafic rocks, a mafic intrusive complex, mafic lavas and marine sediments. The classical "Penrose" (Anonymous, 1972) representative ophiolitic sequence includes, from base upward, peridotites, gabbros, sheeted dikes, mafic lavas and marine sediments, in which peridotites and gabbros can be repeated several times. During deformation and metamorphism, peridotites are generally serpentinized with a density reduction, and then can be easily uplifted and undergo plastic deformation and significant structural displacement. Overlying the igneous rocks are pelitic and sandy rocks, which may be intercalated with chert and limestone. Many ophiolitic rocks from around the world have similar sequences, which can be compared with sequences of current ocean floors, so ophiolites are generally thought to be fragments of oceanic crust attached to the continental margin or island arc. However, the integrity of ophiolitic sequences is always damaged because of the subduction of oceanic crust, tectonic emplacement that forms overthrust nappes, and in most cases just some sections of the sequence or mixed rocks from hybrid accumulation can be observed. The origin of ophiolite is generally interpreted to be generated by the emplace ment of oceanic lithosphere which is formed because of ocean floor spreading along a mid oceanic ridge, or spreading in a fore-arc environment (e.g., Dilek and Furnes, 2011; Robinson et al., 2008). There are close relations between ophiolites and the evolution of oceanic lithosphere, therefore, research on ophiolite composition, components and origin is the main way to understand the structure, change, and dynamics of oceanic lithosphere. Recent work (e.g., Dilek and Furnes, 2011; Kusky et al., 2011) shows that there is a much greater variation in young ophiolites and oceanic lithosphere than proposed by the Penrose definition (Anonymous, 1972), and workers in ancient Precambrian shields need to appreciate the variation in Phanerozoic ophiolites and modern oceanic lithosphere, when interpreting the tectonic setting of mafic/ultramafic/sedimentary sequences in greenstone belts.

Because ophiolites are mostly dismembered and disordered fragments, there are different criteria and descriptions in formal research for how to determine whether or not a rock sequence may be an ophiolite. Kusky (2004) presented a list of criteria for determining whether or not a rock sequence is an ophiolite or not. This list is modified and reproduced above (also after Kusky et al., 2011) where geologists can compare the different indicators of ophiolitic characteristics of a rock sequence against well-known Phanerozoic ophiolites, to determine how well their sequence of rocks compares to established ophiolite sequences. The main problem is that even if a rock sequence had a sea-floor spreading ridge origin, it is typically dismembered and only partly preserved because of the structural and metamorphic consequences of the emplacement process. It has to be asked, how many of the characteristics of a full Penrose-style ophiolite are needed to recognize a rock sequence as having a sea-floor spreading origin? In Table 1, the presence or absence of different units in the Archean DongwanziZunhua ophiolite belt of the North China craton are compared to typical Phanerozoic ophiolites, but these columns can be replaced with the rocks present in any other given rock sequence to see how well it compares to other recognized ophiolites.

Table  1.  Criteria for Recognition of a Rock Sequence as an Ophiolite

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PREVIOUS AND FURTHER STUDY OF ARCHEAN OPHIOLITES IN THE NCC

As originally reported in Science (Kusky et al., 2001), a group of circa 2.5 Ga mafic/ultramafic rocks in eastern Hebei, China, was interpreted to represent one of the world's oldest, most complete yet dismembered and metamorphosed ophiolite sequences. This sequence was named the Dongwanzi ophiolite and has been the focus of much scientific debate since the original proposal (Kusky and Li, 2010, 2008, 2002; Kusky et al., 2007; Zhao et al., 2007; Zhang et al., 2003; Zhai et al., 2002). The main focus point of the debate is documenting if rocks in this belt have genetic relationships with each other. The ophiolite belt was later (Li et al., 2002) extended to the south to the Zunhua area (Fig. 1) to include a group of ophiolitelike fragments in high grade mélange, including podiform chromites, and the belt has since been referred to as the Dongwanzi-Zunhua ophiolite belt (e.g., Kusky and Li, 2010). The Dongwanzi ophiolite, in the original reconnaissance maps and definition of Kusky et al. (2001) included three belts of rocks, namely the northern, central, and southern zones. In these belts, the sequence of rocks was suggested to grade upwards from tectonized harzburgites, through lower crustal ultramafic and mafic cumulates, into a thick gabbro unit, then into a unit of metamorphosed mafic amphibolites that locally include remnants of pillow lavas and dike complexes.

Figure  1.  Tectonic sketch map of the North China craton (modified after Kusky et al., 2007) showing the eastern and western blocks separated by the central orogenic belt (COB). Note the location of suggested Archean ophiolitic fragments in the central orogenic belt. Proposed ophiolitic fragments include 1. Dongwanzi; 2. Zunhua; 3. West Liaoning; 4. North Taihang; 5. Wutaishan; 6. South Taihang.

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One of the most contentious issues has been the age of the relatively small central belt of the Dongwanzi ophiolite. Kusky et al. (2001) interpreted this belt to be part of the main ophiolite preserved in the southeastern belt, but stated that the central belt is intruded by several generations of younger intrusions, and in 2004 (Kusky et al., 2004) dated these younger intrusions as circa 300 Ma. Later, Zhao et al. (2007) confirmed that gabbro, leucogabbro and mafic dikes of only the central belt of the Dongwanzi complex are later intrusions, and considered that Kusky et al. regarded this rockmass as a part of the Archean Dongwanzi complex by mistake. Based on the new data, there are at least two current possible interpretations to explain this discrepancy. One possibility is that Zhao et al. (2007) only dated the younger intrusions in the central belt, and missed the older rocks. The other interpretation is that the zircons that Kusky et al. (2001) dated may have been old xenocrystic cores caught in younger intrusions. Therefore, until further work can resolve this ambiguity, Kusky et al. abandon the correlation of the central belt with the main southeastern belt of the Dongwanzi-Zunhua mafic-ultramafic belt. Yet they emphasize that most of the data and interpretation of the Dongwanzi-Zuhua belt as ophiolitic comes from the southeastern belt, and its extensions to Palaeoarchean terrains along strike which contain extensively serpentinized ultramafic rocks from Qinglong, Zunhua, Zhangjiakou of Hebei, Miyun of Beijing to some areas of Shanxi-Henan (Fig. 1, Polat and Kusky, 2007). They consider that there once existed a relatively large scale ophiolite belt of ~2.5 Ga, hence to explain the Neoarchean tectonic evolution of the North China craton.

Since the initial report in 2001 of the possible Archean ophiolite in North China, it has led to widespread concern of scholars at home and abroad. In this period, Li and Kusky (2003) have led two international field trips to the Dongwanzi-Zunhua belt. The problems concerned are also about the interpretations of mantle peridotite, gabbro, sheeted dike complex, pillow lava, podiform chromite, and geochemistry of igneous rocks besides the ages of mafic-ultramafic rocks in the Dongwanzi area. Respective evidence has been presented for different opinions (Zhang et al., 2003; Kusky et al., 2010, 2004; Zhao et al., 2007; Polat et al., 2006; Huang et al., 2004; Li et al., 2002). However, because these Precambrian rocks experienced complicated metamorphism and deformation, and were overprinted by later magmatism, the research is very complicated when compared to the Phanerozoic, so far, there still exist many differences in interpretations.

To these different views, there are three main aspects which need to be emphasized. The first one is detailed geological research, including making formal detailed geological maps and geochronology research on some terrains, and to map, date, and reject later intrusions as not being part of the Archean complex. At the same time, we suggest that different researchers can have the chance to do the field investigation together and discuss problems on the spot, to avoid the variance in the object of study and sample collection.

The second one is understanding and discussion to the concept of ophiolites. For example, the origin of podiform chromites; so far podiform chromites are only known from ophiolites, but deep mantle rocks from other environments are rarely preserved so we can not be sure if they can form in different tectonic environments. The significance of pillow lava, and geochemical indication, identification and formation mechanism of sheeted dikes as well as the relationship between the rate of extension and the rate of magma supply, and the change of geochemical characteristics of rocks in metamorphic processes all need to be carefully assessed as to whether they are unique to ophiolites, or if ophiolites may have different characteristics between older and younger ages.

The last one is the recognition to Archean oceanic crust, for example, if old oceanic crust was thicker and hotter than that of the Mesoproterozoic and younger times. If there are disparities in physics and geochemical characteristics, what are the similarities and differences of the formation environment of Archean greenstone belts with arcs or oceanic basins, and what is the relationship between the formation of TTG and possible old oceanic crust? Study of possible Archean ophiolites will yield clues about if the amalgamation mechanism of micro-continental blocks in the Palaeo-Mesoarchean NCC is the same or similar to plate tectonics.

ENDING REMARKS

In short, the Early Precambrian ophiolite is a very active scientific issue, its meaning is the character and recognition of the Early Precambrian oceanic crust, when did plate tectonics begin to work in the evolution of Earth's history, and are there any differences of evolutionary mechanism between the continent and the ocean. The problem of the possible Archean ophiolite within the North China craton had been discussed very early, and the article of Kusky et al. (2001) on the Dongwanzi ophiolite has triggered a great interest among domestic and foreign scholars. Different views of the controversy impetus the Precambrian research of North China to a certain extent, showing the importance of this research topic. There are also different views from the two authors of this paper. Timothy M Kusky emphasizes that this belt contains most of the ingredients that a typical ophiolite should include, although it is controversial. The existing data still supports this interpretation that it might be the oldest, most integral yet dismembered and metamorphosed Archean oceanic crust and mantle fragment in the world. Mingguo Zhai (e.g., Zhai et al., 2005) emphasizes more about the differences of the Archean oceanic crust and the oceanic crust after that, considering the genetic relationship between the formation of the huge amount of TTG rocks and the ultramafic rocks. The North China craton is one of the oldest cratons in the world, with a variety of rock types, colorful geological phenomenon, and a complex geological record of the events. The NCC is the ideal place to study early Precambrian geology. This paper calls for researchers to give more study on Precambrian North China, especially on Archean ophiolites, Paleoproterozoic high pressure-high temperature and ultra-high temperature granulites, Precambrian mineral deposits and other key scientific issues, so as to make innovative contributions to earth science.