Journal of Earth Science, Vol. 23, No. 3, p. 277–284, June 2012 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-012-0253-6

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

Timothy M Kusky* Three Gorges Research Center for Geo-hazards, Ministry of Education, China University of Geosciences, Wuhan 430074, China; State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China Mingguo Zhai (翟明国) State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

ABSTRACT: Archean greenstone belts and their possible inclusion of fragments of ophiolites is an im- portant research subject, since it is correlated with the nature of early oceanic crust, and can yield in- formation on the nature of early planetary lithospheres, the origin of TTG (tonalite-trondhjemite- granodiorite) continental crust, the formation of early cratons and continents, and is related to when plate tectonics started in the Earth’s evolutionary history. This article briefly reviews the North China craton’s Archean ophiolite argument and proposes further studies aimed at understanding the genera- tion of greenstone belts and Archean ophiolites, and suggests some key scientific questions that remain to be answered. KEY WORDS: Archean, ophiolite, greenstone belt, North China craton.

INTRODUCTION other models suggest that the TTG terranes may have Understanding the early history of the Earth is formed from partial melting of shallowly subducted one of the major challenges to the Earth Science buoyant oceanic slabs (e.g., Tappe et al., 2011; Rapp community. Early crust formation is represented by and Watson, 1995; Rapp et al., 1991). However, some massive tonalite-trondhjemite-granodiorite (TTG), and cratons preserve a tectonic framework of high-grade its peak formation time is about 2.7 Ga. The formation granulite-gneiss and greenstone belts formed in the of this stage of TTG is generally considered to be re- Early Archean. Greenstone belts consist of low-grade lated to mantle plumes (e.g., Condie, 1997), although metamorphic volcanic-sedimentary rocks which are typically exposed as linear fold belts around This study was supported by the National Natural Science high-grade rocks (e.g., Kusky and Vearncombe, 1997). Foundation of China (Nos. 91014002, 40821061), and Ministry For the tectonic setting of greenstone belt rocks, there of Education of China (No. B07039). are different opinions including intracontinental rifts, *Corresponding author: [email protected] island arcs, back-arc basin—small ocean basin com- © China University of Geosciences and Springer-Verlag Berlin binations, although these models are not mutually ex- Heidelberg 2012 clusive. These different ideas led to a debate of whether plate tectonics existed in the Archean and Manuscript received January 12, 2012. when did plate tectonics begin to operate (e.g., Stern, Manuscript accepted March 5, 2012.

278 Timothy M Kusky and Mingguo Zhai

2007). greenschist-amphibolite facies being most characteris- Archean ophiolite discrimination is one of the tic. The chlorite, epidote, actinolite and other meta- main bases to explore the issues of whether or not morphic minerals give the rocks their characteristic plate tectonics existed in the Archean and when did dark green color. A complete set of strata of green- plate tectonics begin to operate, thus many scientists stone belt rocks is typically comprised of early vol- have been dedicated to this study for many years. canic rocks and later clastic sedimentary rocks or vol- There are a number of papers related to this aspect canic clastic sedimentary rocks, which are mainly tur- published in international journals. “Precambrian bidites. Underlying volcanic/plutonic rocks are mainly Ophiolites and Related Rocks” edited by Kusky (2004) ultramafic-mafic rocks also in some cases including focused on Archean and Proterozoic ophiolites, and komatiites. Overlying volcanic rocks are typically also discussed the oceanic crust evolution model calc-alkaline volcanic rocks. There are generally ul- which changes with time. The assumed oldest ophio- tramafic lenses underlying the greenstone belt, which lite is from the Isua supracrustal rocks in West are explained to represent fragments of ancient mantle. Greenland (Furnes et al., 2009, 2007a, b), with an Greenstone belts are structurally complex with a com- isotopic age of ~3.8 Ga. The ophiolites that are as- plex series of deformation events, yet many exhibit a sumed to be around 3.0–2.7 Ga age include the 3.0 Ga broad synclinal shape surrounding high-grade ophiolite of Olondo in the Aldan Shield, East Siberia, gneiss-granulite zones, formed in the late stages of 2.8 Ga SSZ-type ophiolite of the North Karelian belt deformation of these belts (e.g., Kusky and Vearn- in the NE Baltic Shield, Russia, and 2.7 Ga ophiolites combe, 1997). in the Slave craton, Canada, and Zimbabwe (Cocoran et al., 2004; Hofmann and Kusky, 2004; Puctel, 2004; Ophiolite Shchipansky et al., 2004; Kusky, 1998, 1991, 1990, An ophiolite is a rock suite that consists of ser- 1989; Kusky and Kidd, 1992), and 2.5 Ga ophiolites pentinized ultramafic rocks, a mafic intrusive complex, in the North China craton (NCC). All above ophiolites mafic lavas and marine sediments. The classical are still controversial, mainly because of their differ- “Penrose” (Anonymous, 1972) representative ophioli- ences compared to the rock association, occurrence tic sequence includes, from base upward, peridotites, and geochemistry of modern spreading ridges. Since gabbros, sheeted dikes, mafic lavas and marine sedi- documentation of Archean ophiolites is a key scien- ments, in which peridotites and gabbros can be re- tific issue, the debate and further research will con- peated several times. During deformation and meta- tinue and its progress will promote the understanding morphism, peridotites are generally serpentinized with of early continental evolution and the beginning of a density reduction, and then can be easily uplifted plate tectonics. and undergo plastic deformation and significant structural displacement. Overlying the igneous rocks GENERAL CHARACTERISTICS OF are pelitic and sandy rocks, which may be intercalated GREENSTONE BELTS AND OPHIOLITES with chert and limestone. Many ophiolitic rocks from Greenstone Belt around the world have similar sequences, which can Generally, the term greenstone belt refers to a be compared with sequences of current ocean floors, supracrustal rock belt distributed in linear to arcuate so ophiolites are generally thought to be fragments of zones in Precambrian shields. Greenstone belts typi- oceanic crust attached to the continental margin or is- cally contain products of several generations of mafic land arc. However, the integrity of ophiolitic se- volcanic-sedimentary rocks. The main rocks consist of quences is always damaged because of the subduction basalts, komatiites, intermediate-acidic calc-alkaline of oceanic crust, tectonic emplacement that forms volcanic rocks and sedimentary rocks, gabbros and overthrust nappes, and in most cases just some sec- diabases, and minor serpentinized ultramafic rocks tions of the sequence or mixed rocks from hybrid ac- (e.g., de Wit and Ashwal, 1997). Metamorphic grades cumulation can be observed. The origin of ophiolite is range from sub-greenschist to granulite, with generally interpreted to be generated by the emplace-

The Neoarchean ophiolite in the North China Craton: Early Precambrian Plate Tectonics and Scientific Debate 279 ment of oceanic lithosphere which is formed because anic lithosphere, therefore, research on ophiolite of ocean floor spreading along a mid oceanic ridge, or composition, components and origin is the main way spreading in a fore-arc environment (e.g., Dilek and to understand the structure, change, and dynamics of Furnes, 2011; Robinson et al., 2008). There are close oceanic lithosphere. Recent work (e.g., Dilek and relations between ophiolites and the evolution of oce- Furnes, 2011; Kusky et al., 2011) shows that there is a

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

Indicator Importance Status in Status in Dongwanzi Conclusion Phanerozoic ophiolites Full Penrose sequence Diagnostic Rare, about 10% Suggested, needs Not In order Documentation conclusive And verification. Podiform chromites w/ Diagnostic About 15% Present Diagnostic nodular textures Full sequence Convincing About 30%–50% Dismembered units Convincing dismembered Present 3 or 4 of 7 main units Typical for accepting About 80% 6 of 7 units known Convincing present Phanerozoic. Ophiolite Dikes still not convincing Uncertain (age) Sheeted dikes Distinctive, nearly diagnostic About 10% Suggested, age needs Not Verification conclusive Mantle tectonites Distinctive About 20%–30% Present Distinctive Cumulates Present, not distinctive About 70% Present Supportive Layered gabbro Typical About 70% Present Supportive Pillow lavas Typical not distinctive About 85% Present Supportive Chert, deep water seds Typical About 85% Present Supportive Co-magmatic dikes and Necessary, rare to observe About 15% Present Distinctive gabbro High-T silicate defm. ins Rare, but distinctive About 10% Present Distinctive inclus. in melt pods Basal thrust fault Necessary (except in rare About 60% Present Supportive cases), not diag. Dynamothermal Distinctive, almost diagnostic About 15% Not determined Inconclusive sole Sea floor metamor Distinctive All Present Supportive Hydrothermal vents Distinctive Rare Present Strongly black smoker type supports Ophiolites are defined on the basis of field relationships and the overall rock sequence. Many workers have added chemical criteria to the ways to recognize and distinguish between different types of ophiolites. Some of the more common traits are MORB chem. Common About 40% Present Distinctive Arc tholeiite chem. Common About 60% Present Distinctive Flat REE Distinctive About 65% Present Distinctive Calc-alkaline chem. Common About 25% Present in some units Inconclusive Boninite chem. Distinctive About 40% Uncertain Inconclusive

280 Timothy M Kusky and Mingguo Zhai much greater variation in young ophiolites and oce- debate is documenting if rocks in this belt have ge- anic lithosphere than proposed by the Penrose defini- netic relationships with each other. The ophiolite belt tion (Anonymous, 1972), and workers in ancient Pre- was later (Li et al., 2002) extended to the south to the cambrian shields need to appreciate the variation in Zunhua area (Fig. 1) to include a group of ophiolite- Phanerozoic ophiolites and modern oceanic litho- like fragments in high grade mélange, including podi- sphere, when interpreting the tectonic setting of mafic/ form chromites, and the belt has since been referred to ultramafic/sedimentary sequences in greenstone belts. as the Dongwanzi-Zunhua ophiolite belt (e.g., Kusky Because ophiolites are mostly dismembered and and Li, 2010). The Dongwanzi ophiolite, in the origi- disordered fragments, there are different criteria and nal reconnaissance maps and definition of Kusky et al. descriptions in formal research for how to determine (2001) included three belts of rocks, namely the whether or not a rock sequence may be an ophiolite. northern, central, and southern zones. In these belts, Kusky (2004) presented a list of criteria for determin- the sequence of rocks was suggested to grade upwards ing whether or not a rock sequence is an ophiolite or from tectonized harzburgites, through lower crustal not. This list is modified and reproduced above (also ultramafic and mafic cumulates, into a thick gabbro after Kusky et al., 2011) where geologists can com- unit, then into a unit of metamorphosed mafic amphi- pare the different indicators of ophiolitic characteris- bolites that locally include remnants of pillow lavas tics of a rock sequence against well-known Phanero- and dike complexes. zoic ophiolites, to determine how well their sequence One of the most contentious issues has been the of rocks compares to established ophiolite sequences. age of the relatively small central belt of the Dong- The main problem is that even if a rock sequence had wanzi ophiolite. Kusky et al. (2001) interpreted this a sea-floor spreading ridge origin, it is typically dis- belt to be part of the main ophiolite preserved in the membered and only partly preserved because of the southeastern belt, but stated that the central belt is in- structural and metamorphic consequences of the em- truded by several generations of younger intrusions, placement process. It has to be asked, how many of and in 2004 (Kusky et al., 2004) dated these younger the characteristics of a full Penrose-style ophiolite are intrusions as circa 300 Ma. Later, Zhao et al. (2007) needed to recognize a rock sequence as having a confirmed that gabbro, leucogabbro and mafic dikes sea-floor spreading origin? In Table 1, the presence or of only the central belt of the Dongwanzi complex are absence of different units in the Archean Dongwanzi- later intrusions, and considered that Kusky et al. re- Zunhua ophiolite belt of the North China craton are garded this rockmass as a part of the Archean Dong- compared to typical Phanerozoic ophiolites, but these wanzi complex by mistake. Based on the new data, columns can be replaced with the rocks present in any there are at least two current possible interpretations to other given rock sequence to see how well it compares explain this discrepancy. One possibility is that Zhao to other recognized ophiolites. et al. (2007) only dated the younger intrusions in the central belt, and missed the older rocks. The other in- PREVIOUS AND FURTHER STUDY OF terpretation is that the zircons that Kusky et al. (2001) ARCHEAN OPHIOLITES IN THE NCC dated may have been old xenocrystic cores caught in As originally reported in Science (Kusky et al., younger intrusions. Therefore, until further work can 2001), a group of circa 2.5 Ga mafic/ultramafic rocks resolve this ambiguity, Kusky et al. abandon the cor- in eastern Hebei, China, was interpreted to represent relation of the central belt with the main southeastern one of the world’s oldest, most complete yet dismem- belt of the Dongwanzi-Zunhua mafic-ultramafic belt. bered and metamorphosed ophiolite sequences. This Yet they emphasize that most of the data and interpre- sequence was named the Dongwanzi ophiolite and has tation of the Dongwanzi-Zuhua belt as ophiolitic been the focus of much scientific debate since the comes from the southeastern belt, and its extensions to original proposal (Kusky and Li, 2010, 2008, 2002; Palaeoarchean terrains along strike which contain ex- Kusky et al., 2007; Zhao et al., 2007; Zhang et al., tensively serpentinized ultramafic rocks from 2003; Zhai et al., 2002). The main focus point of the Qinglong, Zunhua, Zhangjiakou of Hebei, Miyun of

The Neoarchean ophiolite in the North China Craton: Early Precambrian Plate Tectonics and Scientific Debate 281

Beijing to some areas of Shanxi-Henan (Fig. 1, Polat hence to explain the Neoarchean tectonic evolution of and Kusky, 2007). They consider that there once ex- the North China craton. isted a relatively large scale ophiolite belt of ~2.5 Ga,

90o 110o 120o 130oE

Changchun

North Hebei orogenic belt Bayan Obo Duolun

N 3 o 1 40 2 Jiayuguan * * Beijing Jiaoliao *** * * 4 belt * * * ** 5 Tanlu fault Imjingang Taiyuan belt Western **

Datong-Wuqi block * * o Central China orogen ** 6 fault Eastern Qingdao 35 block Ogcheon belt Xi’an * *** * * * COB Songpan Sulu belt Qinling-Dabie belt 1.8 Ga granulites Paleoproterozoic Xinyang orogenic belt Proterozoic granite Archean orogenic Shanghai o

(1.9 Ga) belt 30 Khondalites and Wuhan * S-type granite Thrust boundary Yellow Sea (2.2- 1.9 Ga) 2.5 Ga ophiolitic Fault fragments 0 400 km City 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. Dong- wanzi; 2. Zunhua; 3. West Liaoning; 4. North Taihang; 5. Wutaishan; 6. South Taihang.

Since the initial report in 2001 of the possible search is very complicated when compared to the Archean ophiolite in North China, it has led to wide- Phanerozoic, so far, there still exist many differences spread concern of scholars at home and abroad. In this in interpretations. period, Li and Kusky (2003) have led two interna- To these different views, there are three main as- tional field trips to the Dongwanzi-Zunhua belt. The pects which need to be emphasized. The first one is problems concerned are also about the interpretations detailed geological research, including making formal of mantle peridotite, gabbro, sheeted dike complex, detailed geological maps and geochronology research pillow lava, podiform chromite, and geochemistry of on some terrains, and to map, date, and reject later in- igneous rocks besides the ages of mafic-ultramafic trusions as not being part of the Archean complex. At rocks in the Dongwanzi area. Respective evidence has the same time, we suggest that different researchers been presented for different opinions (Zhang et al., can have the chance to do the field investigation to- 2003; Kusky et al., 2010, 2004; Zhao et al., 2007; gether and discuss problems on the spot, to avoid the Polat et al., 2006; Huang et al., 2004; Li et al., 2002). variance in the object of study and sample collection. However, because these Precambrian rocks experi- The second one is understanding and discussion enced complicated metamorphism and deformation, to the concept of ophiolites. For example, the origin of and were overprinted by later magmatism, the re- podiform chromites; so far podiform chromites are

282 Timothy M Kusky and Mingguo Zhai only known from ophiolites, but deep mantle rocks might be the oldest, most integral yet dismembered from other environments are rarely preserved so we and metamorphosed Archean oceanic crust and mantle can not be sure if they can form in different tectonic fragment in the world. Mingguo Zhai (e.g., Zhai et al., environments. The significance of pillow lava, and 2005) emphasizes more about the differences of the geochemical indication, identification and formation Archean oceanic crust and the oceanic crust after that, mechanism of sheeted dikes as well as the relationship considering the genetic relationship between the for- between the rate of extension and the rate of magma mation of the huge amount of TTG rocks and the ul- supply, and the change of geochemical characteristics tramafic rocks. The North China craton is one of the of rocks in metamorphic processes all need to be oldest cratons in the world, with a variety of rock carefully assessed as to whether they are unique to types, colorful geological phenomenon, and a complex ophiolites, or if ophiolites may have different charac- geological record of the events. The NCC is the ideal teristics between older and younger ages. place to study early Precambrian geology. This paper The last one is the recognition to Archean oce- calls for researchers to give more study on Precam- anic crust, for example, if old oceanic crust was brian North China, especially on Archean ophiolites, thicker and hotter than that of the Mesoproterozoic Paleoproterozoic high pressure-high temperature and and younger times. If there are disparities in physics ultra-high temperature granulites, Precambrian min- and geochemical characteristics, what are the similari- eral deposits and other key scientific issues, so as to ties and differences of the formation environment of make innovative contributions to earth science. Archean greenstone belts with arcs or oceanic basins, and what is the relationship between the formation of ACKNOWLEDGMENTS TTG and possible old oceanic crust? Study of possible Funds were provided by the National Natural Archean ophiolites will yield clues about if the amal- Science Foundation of China (Nos. 91014002, gamation mechanism of micro-continental blocks in 40821061) and Ministry of Education of China (No. the Palaeo-Mesoarchean NCC is the same or similar to B07039). plate tectonics. REFERENCES CITED ENDING REMARKS Anonymous, 1972. Ophiolites. Geotimes, 17: 24–15 In short, the Early Precambrian ophiolite is a Cocoran, P. L., Mueller, W. U., Kusky, T. M., 2004. Inferred very active scientific issue, its meaning is the charac- Ophiolites in the Archean Slave Craton. In: Kusky, T. M., ter and recognition of the Early Precambrian oceanic ed., Precambrian Ophiolites and Related Rocks. Develop- crust, when did plate tectonics begin to work in the ments in Precambrian Geology, 13: 363–404 evolution of Earth’s history, and are there any differ- Condie, K. C., 1997. Plate Tectonics and Crustal Evolution (4th ences of evolutionary mechanism between the conti- Edition). Elsevier, Burlington nent and the ocean. The problem of the possible Ar- de Wit, M. J., Ashwal, L. D., 1997. Greenstone Belts. Oxford chean ophiolite within the North China craton had Science Publications, Clarendon Press, Oxford. 809 been discussed very early, and the article of Kusky et Dilek, Y., Furnes, H., 2011. Ophiolite Genesis and Global al. (2001) on the Dongwanzi ophiolite has triggered a Tectonics: Geochemical and Tectonic Fingerprinting of great interest among domestic and foreign scholars. Ancient Oceanic Lithosphere. Geological Society of Different views of the controversy impetus the Pre- America Bulletin, 123(3–4): 387–411, doi:10.1130/ cambrian research of North China to a certain extent, B30446.1 showing the importance of this research topic. There Furnes, H., de Wit, M. J., Staudigel, H., et al., 2007a. A Vestige are also different views from the two authors of this of Earth’s Oldest Ophiolite. Science, 315(5819): paper. Timothy M Kusky emphasizes that this belt 1704–1707, doi:10.1126/science.1139170 contains most of the ingredients that a typical ophio- Furnes, H., de Wit, M. J., Staudigel, H., et al., 2007b. Response lite should include, although it is controversial. The to Comments on “A Vestige of Earth’s Oldest Ophiolite”. existing data still supports this interpretation that it Science, 318(5851), doi: 10.1126/science.1144231

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