Understanding the formation, reactivation and destruction of cratons — Preface

Understanding the formation, reactivation and destruction of cratons — Preface

Lithos 149 (2012) 1–3 Contents lists available at SciVerse ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos Editorial Underst...

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Lithos 149 (2012) 1–3

Contents lists available at SciVerse ScienceDirect

Lithos journal homepage: www.elsevier.com/locate/lithos

Editorial

Understanding the formation, reactivation and destruction of cratons — Preface

Cratons are important geological features on the Earth, having formed mainly during the Archean (>2.5 Ga) and early Proterozoic. The preservation of the oldest materials (ca 4.4 Ga) and the most complete geological records on the Earth makes cratons of key importance for studying the formation and long-term evolution of continents. Cratons are devoid of recent seismic and volcanic activities and as such are characterized by thick lithosphere (crust ~ 50 km; mantle ~ 150 km), a cold geotherm, low density and high viscosity. However, recent work suggests that cratons may not be always so stable. The cratonic lithosphere in some regions has been severely disturbed or reactivated in post-Archaean times, resulting in significant loss or modification of the lithospheric “root”. Examples come from the North China Craton in East Asia (e.g., Griffin et al., 1998; Menzies et al., 1993; 2007; Zhang et al., 2002), the southwestern part of the Kaapvaal Craton in South Africa (Kobussen et al., 2008), the Wyoming Craton in North America (Carlson et al., 1999), and the Brazil Craton in South America (Beck and Zandt, 2002). The eastern part of the North China Craton is thought to be one of the best examples of wholesale destruction of a cratonic root (e.g., Carlson et al., 2005; Zhu et al., 2011). This region is presently underlain by thin (b80 km), hot (~ 65 mW/m 2) and fertile lithosphere which is in contrast to the thick (> 200 km), cold (~ 40 mW/m 2) and refractory Paleozoic lithosphere sampled by xenolith-bearing diamondiferous kimberlites. The formation, stability and destruction of cratons are topics of wide discussion and debate. Several major research programs in solid earth sciences are focused on a better understanding of the formation, evolution and dynamics of the lithosphere beneath continents. These include “Lithoprobe” (Canada), “Europrobe” (Europe), “COCORP” and “Earthscope” (USA), and the two projects in China: “Destruction of the North China Craton (DNCC)” and “SinoProbe”. In April 2011 an international conference on ‘Craton Formation and Destruction’ (ICCFD) was held at the institute of Geology and Geophysics, Chinese Academy of Sciences (CAS), in Beijing, China. The conference was sponsored by the DNCC project and the CAS. It brought together more than 200 scientists in geology, geochemistry and geophysics from 16 countries, to exchange ideas, integrate different datasets and discuss a wide range of topics related to the formation and evolution of cratons and their relationship to shallow tectonics and deep mantle dynamics. Fifty talks and more than one hundred poster presentations were organized in three symposia: Destruction of Cratons; Formation of Cratons and its Early Evolution; Craton vs. Orogen. The ICCFD provided a basis for exploring the current state of knowledge and communicating the latest relevant research on these topics. It will be followed by a workshop on craton formation and destruction (WCFD) at the University of Johannesburg, South Africa on 21–22 July, 2012. The WCFD will focus more on 0024-4937/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2012.06.019

comparative studies among cratons, with special emphasis on the cratons of Brazil, Russia, India, China and South Africa. Two special issues stem from the ICCFD, one in Lithos (geology and geochemistry) and one in Gondwana Research (geophysics and tectonics). The Lithos special volume contains thirteen peerreviewed contributions, including 10 original research contributions and 3 comprehensive reviews. The papers are organized according to the three symposia of the ICCFD: 1. Formation of craton and its early evolution Craton formation has been debated for some time with proposals in favor of ridge processes above a hotter mantle, deep mantle plumes or accretion above destructive plate margins. This issue remains unresolved with several contributions arguing for diametrically opposed mechanisms. Herzberg and Rudnick integrate the petrology and thermal history of cratons and conclude that neither a plume nor subduction related processes could have been the main mechanisms responsible for craton formation. The former generates a dunite residue (and not harzburgite) and the latter adds too much water (which affects viscosity and buoyancy of the lithosphere). They propose that cratons formed in response to spreading centers in a hot early Earth. In contrast, Aulbach argues for plume involvement in the formation of cratonic lithospheric mantle on the basis of Fe–Mg and Cr–Al relationships in cratonic peridotites which originally formed as garnet-free harzburgites at >3–5 GPa (150 km), and proposes that subduction-related processes have a later role in cratons and best account for the incorporation of “younger” eclogitic facies. She also argues that cratonic crust may have formed by shallow plate interactions and therefore not entirely cogenetic with the underlying lithospheric mantle. Metasomatism is invoked by Baptiste et al. to explain the relatively high water content in granular peridotites at ~160 km beneath the Kaapvaal craton. At greater depths, toward the base of the lithosphere, there are more sheared peridotites which tend to be dry. Indeed the lack of any correlation with refractory indices (mg#) indicates that the water was induced by metasomatism associated re-hydration after the cratonic root-forming partial melting. Based on the predominance of highly annealed microstructures in the cratonic root, they conclude that this metasomatism was not followed by remobilization of the cratonic root. 2. Destruction of cratons Destruction of old, cold and thick cratons is apparent from the presence of on-craton volcanism, high heat flow, a shallow low velocity zone (thin lithosphere), intensive deformation and decoupled crust–mantle architecture. However, the timing, process, dynamic

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trigger and mechanism of craton destruction, which are closely related with the lithospheric architecture and deeper mantle processes, are still hotly debated. Most studies in this volume focus on the destruction of the NCC. Zhu et al. summarize the temporal and spatial distributions of Phanerozoic magmatic rocks in the craton. The early stages of magmatism and tectonism from Carboniferous to Jurassic are mostly distributed in the boundary regions of the NCC, and are thought to be associated with the southward subduction of the Paleo-Asian ocean and assembly of the Sino–Korean and Yangtze cratons. The intensive Early Cretaceous magmatism in the eastern NCC, coeval with widespread extensional deformation and gold mineralization, marks the peak of the craton destruction. Zhu et al. attribute this stage of magmatism and the craton destruction mainly to the effects of the Paleo-Pacific subduction underneath the eastern Asian continent. The role of subduction and collision on the evolution and destruction of the NCC is further emphasized by Zheng et al. They stress that the small size of the NCC relative to other cratons made it more susceptible to subduction and collision throughout its lifetime. By synthesizing the age and composition of the lower crust and upper mantle xenoliths from the NCC, they find that the craton experienced complex accretion and reworking processes in its deep lithosphere, accompanied by the aggregation and differentiation of the ancient continental nucleus. The subcontinental lithosphere mantle was generally coupled to the lower crust throughout the Paleozoic, while decoupling occurred in late Mesozoic–Cenozoic time. Zheng et al. believe that the decoupling resulted from the joint influences of the subduction and collision of the Yangtze Craton with the NCC and subduction of the Pacific plate, processes that successively destroyed the cratonic lithosphere. A major role for Pacific subduction in the destruction of the NCC is supported by Li isotopes in mantle rocks in the northeastern part of the NCC. Tang et al. argue that positive correlations between Li abundances and δ 7Li (olivine) in this region indicate mixing between a low-δ 7Li melt (subduction-derived) and normal mantle. They further suggest that repeated modification of the mantle by melts from the subducted Pacific plate happened from within the lithosphere down to the asthenosphere. Whatever the details of the subduction processes affecting the lithospheric architecture and properties of the NCC, it has been widely accepted that water sourced during subduction is of key importance in destabilizing the craton. Zhang J.F. et al. use laboratory experiments on melt-peridotite reaction to provide new evidence for this process. Their experimental results show that water helps to trigger reactions between eclogite-derived melts and peridotite and produces magnesian andesites and pyroxenites. Without water these reactions would be slow and sluggish. Comparing these results with natural observations from the NCC, they favor long-period, small-scale delamination models in which deformation-driven reactions and weakening of the cratonic lithosphere of the NCC are caused by an increase of water concentration in the mantle. The effects of melt-peridotite reaction are important not only for the destruction of the eastern NCC, but also in lithospheric refertilization in the western NCC, as observed by Zhang H.F. et al. They present petrological and geochemical data for spinel facies mantle xenoliths from the western NCC and reveal the presence of a Pre-Cambrian protolith modified by recent melt infiltration and refertilization. Relative low Os abundances and high 187Os/ 188Os ratios suggest dissolution of the sulfides in the mantle during the refertilization through S-undersaturated melt infiltration and redox changes. Griffin et al. provide an example of melt-related refertilization of cratonic lithosphere in NW Spitzbergen. Younger and more fertile mantle is reported from beneath Archaean lower crust (i.e. decoupled) on one side of the Breibogen–Bockfjorden fault (a major terrane boundary in the region), in contrast to coupled Archean lower crust and mantle on the other side. The younger mantle decoupled from the crust is thought to have formed from an Archean lithospheric mantle precursor

refertilized by pervasive melt-related metasomatism. The juxtaposed, but distinct, lithosphere on either side of the fault have some parallels in the boundary between the eastern and central NCC, probably suggesting a similar cause for such a lithospheric architecture in the two cratonic regions. 3. Craton vs. orogen Orogens or tectonically mobile belts of various scales are widely developed either bordering cratons or suturing different tectonic units within cratonic interiors. The formation and secular evolution of cratons are closely related to the properties and tectonics of these orogenic or mobile belts. The four papers in this section present detailed geochemical studies of the crustal and/or mantle rocks of such belts within or at marginal areas of the NCC. These provide important information on the evolution and dynamics of the craton during the Phanerozoic time. Hao et al. utilize geochemical data and water contents to distinguish pre-existent “cratonic” mantle from recently accreted “oceanic” mantle beneath the northern part of the Trans-North-China Orogen in the central NCC. They argue that most of the water represents relict “older” mantle (i.e., EM1, b300 ppm water) and as such relates to “older” partial melting events than “recent” metasomatism associated with accretion of younger mantle. Water–isotope correlations reveal that the pre-metasomatic mantle was MORB-like, indicating the presence of recently accreted asthenosphere (i.e., DMM, 450–557 ppm water). Chen et al., Ma et al., and Zhang Z. et al. present case studies on the Triassic igneous rocks along the Yanshanian fold-and-thrust belt in the northern margin of the NCC. An integration of geochronological and geochemical data from igneous rocks reveals complicated magmatic processes derived either from metasomatized mantle, subducted oceanic crust, cratonic lower crust, or from Phanerozoic igneous rocks. Triassic magmatism in the region happened under a post-collisional extensional regime, possibly following the subduction and closure of the Paleo-Asian Ocean and subsequent amalgamation of the NCC with the Central Asian Orogenic Belt. We would like to thank all contributors and reviewers who provided constructive comments on earlier versions of these papers. We also thank the Editor in-chief, Dr. Andrew Kerr, and journal manager, Ms. Rohini Shivaram, for their efforts in the review and publication processes of this special issue. References Beck, S.L., Zandt, G., 2002. The nature of orogenic crust in central Andes. Journal of Geophysical Research 107 (B10), 2230, http://dx.doi.org/10.1029/2000JB000124. Carlson, R.W., Irving, A.J., Hearn Jr., B.C., 1999. Chemical and isotopic systematics of peridotite xenoliths from the Williams kimberlite, Montana: clues to processes of lithosphere formation, modification and destruction. In: Gurney, J.L., Pascoe, M.D., Richardson, S.H. (Eds.), Proceedings of the 7th International Kimberlite Conference, pp. 90–98. Carlson, R.W., Pearson, D.G., James, D.E., 2005. Physical, chemical, and chronological characteristics of continental mantle. Review of Geophysics 43, 2004RG000156. Griffin, W.L., Zhang, A., O'Reilly, S.Y., Ryan, C.G., 1998. Phanerozoic evolution of lithosphere beneath the Sino–Korean Craton. In: Flower, M., Chung, S.L., Lo, C.H., Lee, T.Y. (Eds.), Mantle Dynamics and Plate Interactions in East Asia: American Geophysics Union Geodynamics Series, vol. 27, pp. 107–126. Kobussen, A.F., Griffin, W.L., O'Reilly, S.Y., Shee, S.Y., 2008. Ghosts of lithospheres past: imaging an evolving lithospheric mantle in southern Africa. Geology 36, 515–518. Menzies, M.A., Zhang, M., Weiming, F., 1993. Palaeozoic and Cenozoic lithoprobes and the loss of >120 km of Archaean lithosphere Sino–Korean Craton China. In: Prichard, H.M., Alabaster, T., Harris, N.B.W., Neary, C.R. (Eds.), Magmatic Processes and Plate Tectonics: Geological Society Special Publication, vol. 76, pp. 71–81. Menzies, M., Xu, Y.G., Zhang, H.F., Fan, W.M., 2007. Integration of geology, geophysics and geochemistry: a key to understanding the North China Craton. Lithos 96 (1–2), 1–21. Zhang, H.F., Sun, M., Zhou, X.H., Fan, W.M., Zhai, M.G., Yin, J.F., 2002. Mesozoic lithosphere destruction beneath the North China Craton: evidence from major, trace element, and Sr–Nd–Pb isotope studies of Fangcheng basalts. Contributions to Mineralogy and Petrology 144, 241–253. Zhu, R.X., Chen, L., Wu, F.Y., Liu, J.L., 2011. Timing, scale and mechanism of the destruction of the North China Craton. Science China-Earth Sciences 54 (6), 789–797.

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Ling Chen* Hong-Fu Zhang State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, No. 19 Beitucheng West Road, Beijing 100029, China *Corresponding author. Tel.: +86 10 82998416; fax: +86 10 62010846. E-mail address: [email protected].

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Martin A. Menzies Department of Earth Sciences, Royal Holloway, University of London, Egham, Surrey, TW20 OEX, UK 11 June 2012