Palaeozoic and Cenozoic lithoprobes and the loss of >120 km of Archaean lithosphere, Sino-Korean craton, China

Abstract

In eastern China Palaeozoic kimberlites and Cenozoic basalts have been erupted through the same Archaean crust, thus providing deep probes of the cratonic lower lithosphere over a period of 400 Ma. While Palaeozoic diamondiferous kimberlites point to the existence of thick, refractory lower lithosphere in the east, Cenozoic basalt-borne xenoliths reveal the presence of hot, thin, less refractory lower lithosphere. Remnants of the Archaean lithosphere may have survived as harzburgites which are chemically similar to those from the Kaapvaal craton but very different from recently accreted lherzolites. In the absence of convincing evidence for supra-subduction or intraplate processes it is believed that the dramatic change of lithosphere architecture in the Phanerozoic was caused by indentor tectonics resulting from the collision of India and Eurasia. Passive reactivation and remobilization of the Archaean lower lithosphere, in particular metasome horizons, contributed to Cenozoic magmatism aligned along major lithospheric faults. Traditionally the oldest Archaean cratonic nuclei are thought of as the most stable, inert parts of the Earth's surface. In the case of South Africa, Canada and Western Australia, Archaean cratons (Liu et al. 1992) lie atop a thick mechanical boundary layer characterized by high velocity anomalies (Anderson et al. 1992). In addition, the occurrence of Archaean P-type diamonds in on-craton kimberlites confirms the presence of an ancient thick lithospheric keel that, in some cases, was stabilized to depths of 200 km in the first billion years of Earth's history (Boyd & Gurney 1986). However, not all cratons have retained their structural integrity. In the case of the GreenlandHebridean craton, elevated mantle temperatures associated with the Iceland plume and tectonic forces related to the opening of the North Atlantic, may have been responsible for erosion of the craton margin. This would account for the existence of thinned Archaean crust (< 30 km) on the eastern Atlantic margin (i.e. Hebridean craton) and the survival of a thick cratonic nucleus in Greenland (Scott-Smith 1987). Similarly a thick cratonic keel does not underlie the SinoKorean Archaean craton, eastern China. Detailed seismic tomography (Chen et al. 1991; Liu 1992) indicates that the 'present-day' lithosphere is < 80 km thick (see Fig. 4) with greatly thinned lithosphere around the Bohai Sea (Ma & Wu 1981). The presence of thin lithosphere with a low velocity structure similar to an ocean ridge is substantiated by heat flow studies in eastern China (Teng et al. 1983) which reveal a region of very high heat flow on the craton in the vicinity of the Bohai Sea and Beijing (Fig. 1). The measured heat flow (1.2-2.53 HFU) corresponds to geotherms observed in tectonically active continents or ocean basins (50-105 mW m-Z). The aim of this paper is to review the temporal evolution of the lower lithosphere beneath the Sino-Korean craton, a crustal province known to contain some of the oldest crustal rocks on Earth (Jahn et al. 1987). In this review we will: (a) present petrological and geochemical evidence for the character of the Palaeozoic and Cenozoic lithosphere; (b) review the available geological and geochemical data on eastern China pertinent to lithosphere evolution, (c) outline a model to explain the temporal changes in lithosphere architecture. Palaeozoic kimberlite-borne xenoliths Palaeozic (400 Ma) kimberlites entrain a variety of xenoliths and megacrysts including diamonds (Lu et al. 1991; Zhang et al. 1991; Chi et al. 1992) (Fig. 1). While the petrology, mineralogy and thermal history of peridotite xenoliths and heavy mineral concentrates have been determined across the Sino-Korean craton, very little geochemical data are available for these xenoFrom Prichard, H. M., Alabaster, T., Harris, N. B. W. & Neary, C. R. (eds), 1993, Magmatic Processes and Plate Tectonics, Geological Society Special Publication No. 76, 71-81. 71 at Royal Holloway University of London on July 10, 2013 http://sp.lyellcollection.org/ Downloaded from