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

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