GEMOC ARC National Key Centre

LITHOSPHERIC MANTLE TYPES IN EASTERN CHINA

W.L. Griffin, S.Y. O'Reilly, GEMOC Macquarie and X. Xu (Earth Sciences, Univ. of Nanjing, Nanjing, China).

Mantle-derived xenoliths and garnet xenocrysts in volcanic rocks have been used to investigate the composition and thermal state of the subcontinental lithospheric mantle (SCLM) beneath different tectonic domains in eastern China.  Paleozoic kimberlites in Liaoning and Shandong provinces, within the Sino-Korean craton, sample typical Archean mantle:  180-220 km thick, low geotherm (35-40 mW/m2), and abundant clinopyroxene-free harzburgites.  Both sections are significantly more depleted on average than Kaapvaal SCLM.  The two areas differ markedly from one another in bulk compostion and rock-type stratigraphy, suggesting that they represent separate microcontinents, each with their own SCLM, that were accreted into the Sino-Korean craton.  Lamproites on the Proterozoic Yangtze craton sample a distinctly different, typically Proterozoic lithosphere:  thinner (¾150 km), hotter (45 mW/m2 geotherm), and dominated by moderately depleted lherzolite, with essentially no cpx-free harzburgite.  The Proterozoic crust of the Cathaysia block has been extensively intruded by the Mesozoic Yanshanian granitoids (with Proterozoic Nd model ages), whose generation commonly is regarded as a result of subduction processes.  The known SCLM beneath this province consists largely of fertile (¾5% melt extraction) lherzolites.  This material is distinctly less depleted than either Proterozoic SCLM, or peridotites generated in oceanic or convergent-margin settings, and it is similar to fertile lithosphere found beneath other young extensional areas, including the Red Sea (Zabargad Is.).  The present SCLM beneath Cathaysia may have been generated by asthenospheric upwelling related to post-subduction extension.  This upwelling provided the heat source for generation of the Yanshanian granitoids, and required displacement, or rifting and dispersal, of any pre-existing Proterozoic lithosphere, and the delamination of any "oceanic" or sub-arc SCLM developed during the convergent phase.  This model may also explain the rarity of typical "oceanic" SCLM in xenolith suites from Phanerozoic mobile belts in eastern Australia and central Asia.