Deformed and accreted oceanic plateaus

Andrew C Kerr
School of Earth, Ocean and Planetary Sciences
Cardiff University

September 3, 2004

Often all that is left of a pre-Phanerozoic continental flood basalt province is a dyke swarm and intrusive complex. In contrast however, oceanic plateaus lavas, because of the way in which they accrete on to continental margins, are much more likely to be preserved in the geological record than continental flood basalt lavas. The knowledge gained from accreted Cretaceous oceanic plateaus in the past ten years or so (e.g. Kerr et al., 1997; Neal et al., 1997; Frey et al., 2000) has led to an increasing number of oceanic plateau sequences being identified throughout the geological record. Various criteria can be used to assist in the identification of ancient oceanic plateaus, both geochemical and geological and these have been reviewed by several authors (Kerr et al., 2000; Ernst and Buchan, 2003). The preservation potential of oceanic plateaus has resulted in several authors highlighting the importance of oceanic plateau accretion in crustal growth processes (Abbott, 1996; Condie and Abbott, 1999; Puchtel et al., 1998)
 
Recent papers which have been published on accreted/obducted oceanic plateau sequences are reviewed below.
 
Mordechai Stein (2003) has reviewed the role of oceanic plateaus in the formation of the Arabian-Nubian Shield in the late Proterozoic (~900 Ma). Metabasalts from the region possess chemical signatures characteristic of oceanic plateaus, viz. eNd values of +6.0 and 87Sr/86Sr < 0.7030 along with essentially flat chondrite-normalised REE patterns. Younger (880-700 Ma) calc-alkaline batholiths in the region possess similar radiogenic isotope signatures, suggesting that they have been formed by remelting of accreted oceanic plateau material. This implies that a significant portion of the lower crust of the Arabian-Nubian shield is composed of oceanic plateau material.
 
Henriette Lapierre et al. (2003) present an extensive new set of analyses (major and trace elements and radiogenic isotopes) from Cache Creek Terrane in the western Canadian Cordillera, building on the earlier work of Tardy et al. (2001). The terrane consists of massive and pillowed basalts with layered gabbros and dolerite dykes these are interbedded with upper Triassic (~210 Ma) sediments. However, there still appear to be no reliable radiometric dates for the igneous rocks of the Cache Creek Terrane. Based on geochemical and geological evidence Lapierre et al. (2003) interpret the magmatic rocks of the Cache Creek Terrane as the remnants of a late Triassic oceanic plateau, while associated peridotites are proposed to represent the roots of the plateau. They conclude that the accretion of the Cache Creek oceanic plateau and the accretion of more MORB-like rocks of Carboniferous-Permian age contributed significantly to the growth of the North American Cordillera.
 
The oceanic plateau model for the generation of komatiites has gained widespread acceptance since it was first proposed by Storey et al. (1991). Recent work by Valerie Chavagnac (2004) on komatiites from the type locality in the Barberton greenstone belt (~3.5 Ga) further strengthens the link between oceanic plateaus and komatiite formation. The geochemical data presented by Chavagnac (2004) in particular low La/Nb ratios rule out both a subduction related origin for the Barberton komatiites, and the likelihood of any significant crustal contamination having occurred. Flat chondrite-normalised REE patterns combined with the primitive nature of the lavas strongly supports an oceanic plateau origin for these and other komatiites.
 
Can Genc (2004) has noted the occurrence of metamorphosed (greenschist facies) basaltic pillow lavas extending for 1100 km in the Pontides region of northern Turkey. Metamorphic Ar-Ar ages range from 215-203 Ma for associated eclogites and blueschists (Okay et al., 2002) and Can Genc (2004) has suggested that the basalts are Early-Mid Triassic in age. The geochemical data presented by Can Genc (2004) certainly seems to support the proposed origin of these rocks i.e. in an Early Triassic oceanic plateau which accreted to the active Eurasian continental margin in the Late Triassic during the closure of the Palaeo-Tethys Ocean.
 
Franco Pirajno (2004) has presented new geochemical and geological data from the 2.0 Ga Bryah Basin Group in western Australia, which is one of a series of Palaeoproterzoic basins sandwiched between the Pilbara and Yilgarn Cratons. The rocks of this group chiefly consist of metamorphosed high-MgO lavas and basalts in addition to intrusive ultramafic rocks. The geochemical and geological data are consistent with the formation of these rocks in an ancient oceanic plateau.
 
Tsujimori and Liou (2004) have recently suggested that the P-T evolution of Early Palaeozoic amphibolites and melanocratic metagabbros of the Fuko Pass unit in the Oeyama belt of SW Japan, may reflect the partial subduction of an oceanic plateau and its subsequent exhumation.
 
References
Abbott, D.H., 1996. Plumes and hotspots as sources of greenstone belts. Lithos, 37: 113-127.
 
Can Genc, S., 2003. A Triassic large igneous province in the Pontides, northern Turkey: geochemical data for its tectonic setting. Journal of Asian Earth Sciences, 22: 503-516.
 
Chavagnac, V., 2004. A geochemical and Nd isotopic study of Barberton komatiites (South Africa): implication for the Archean mantle. Lithos, 75: 253-281.
 
Condie, K.C. and Abbott, D.H., 1999. Oceanic plateaus and hotspot islands: Identification and role in continental growth. Lithos, 46: 1-4.
 
Ernst, R.E. and Buchan, K.L., 2003. Recognizing mantle plumes in the geological record. Annual Review of Earth and Planetary Sciences, 31: 469-523.
 
Frey, F.A., Coffin, M.F., Wallace, P.J., Weis, D., et al., 2000. Origin and evolution of a submarine large igneous province: the Kerguelen Plateau and Broken Ridge, southern Indian Ocean. Earth and Planetary Science Letters, 176: 73-89.
 
Kerr, A.C., Tarney, J., Marriner, G.F., Nivia, A. and Saunders, A.D., 1997. The Caribbean-Colombian Cretaceous igneous province: The internal anatomy of an oceanic plateau. In: J.J. Mahoney and M. Coffin (Editors), Large Igneous Provinces; Continental, Oceanic and Planetary Flood Volcanism,. American Geophysical Union Monograph, 100: pp. 45-93.
 
Kerr, A.C., White, R.V. and Saunders, A.D., 2000. LIP reading: Recognizing oceanic plateaux in the geological record. Journal of Petrology, 41: 1041-1056.
 
Lapierre, H., Bosch, D., Tardy, M. and Struik, L.C., 2003. Late Paleozoic and Triassic plume-derived magmas in the Canadian Cordillera played a key role in continental crust growth. Chemical Geology, 201: 55-89.
 
Neal, C.R., Mahoney, J.J., Kroenke, L.W., Duncan, R.A. and Petterson, M.G., 1997. The Ontong Java Plateau. In: J.J. Mahoney and M. Coffin (Editors), Large Igneous Provinces; Continental, Oceanic and Planetary Flood Volcanism,. American Geophysical Union Monograph, 100: pp. 183-216.
 
Okay, A.I., Monod, O. and Monie, P., 2002. Triassic blueschists and eclogites from northwest Turkey: vestiges of the Paleo-Tethyan subduction. Lithos, 64: 155-178.
Pirajno, F., 2004. Oceanic plateau accretion onto the northwestern margin of the Yilgarn Craton, Western Australia: implications for a mantle plume event at ca.. 2.0 Ga. Journal of Geodynamics, 37: 205-231.
 
Puchtel, I.S., Hofmann, A.W., Mezger, K., Jochum, K.P., Shchipansky, A.A. and Samsonov, A.V., 1998. Oceanic plateau model for continental crustal growth in the archaean, a case study from the Kostomuksha greenstone belt, NW Baltic Shield. Earth and Planetary Science Letters, 155: 57-74.
 
Stein, M., 2003. Tracing the plume material in the Arabian-Nubian Shield. Precambrian Research, 123: 223-234.
 
Storey, M., Mahoney, J.J., Kroenke, L.W. and Saunders, A.D., 1991. Are oceanic plateaus sites of komatiite formation? Geology, 19: 376-379.
 
Tardy, M., Lapierre, H., Struik, L.C., Bosch, D. and Brunet, P., 2001. The influence of mantle plume in the genesis of the Cache Creek oceanic igneous rocks: implications for the geodynamic evolution of the inner accreted terranes of the Canadian Cordillera. Canadian Journal of Earth Sciences, 38: 515-534.
 
Tsujimori, T. and Liou, J.G., 2004. Metamorphic evolution of kyanite-staurolite-bearing epidote- amphibolite from the Early Palaeozoic Oeyama belt, SW Japan. Journal of Metamorphic Geology, 22: 301-313.