Xenolith evidence for polybaric melting and strati®cation of the upper mantle beneath South China Y.G. Xu a,b, * , M. Sun c , W. Yan d , Y. Liu a , X.L. Huang a , X.M. Chen b a Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, P.O. Box 1131, Wushan, Guangzhou 510640, People's Republic of China b State Key Laboratory of Mineral Deposit Research, Department of Earth Sciences, Nanjing University, Nanjing 210093, People's Republic of China c Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, People's Republic of China d Institute of South China Oceanography, Chinese Academy of Sciences, Guangzhou 510240, People's Republic of China Received 4 May 2001; accepted 24 November 2001 Abstract Mantle xenoliths from Hainan and Qilin, South China have been studied to constrain the nature of the upper mantle and mantle processes beneath a continental margin. The extremely low Ti (160±245 ppm) contents in clinopyroxenes from some spinel lherzolites, indicative of highdegreesofpartialmeltingareinconsistentwiththerelativelyhighclinopyroxenemodes(7.4±13%)inthesesamples.Thisinconsistency could be due to polybaric melting that started in the garnet stability ®eld, then, after the breakdown of garnet to pyroxene and spinel, continued in the spinel stability ®eld. Polybaric melting, due to adiabatic decompression of upwelling mantle, would leave a residual mantle in which the degree of depletion decreases with depth. The predicted strati®ed lithospheric mantle is evidenced by the negative correlation between the forsterite content in olivine and the equilibration temperature, proportional to the depth in the lithosphere from which the xenolith was derived. The lower part of the lithospheric mantle beneath South China consists predominantly of fertile and moderately depleted peridotites, which are either devoid of LREE enrichment, or show the trace element signature of incipient metasomatism, and plot within the Phanerozoic mantle domain. In contrast, the upper part of the mantle contains harzburgite and cpx-poor lherzolite, which are strongly affected by metasomatism of melt/¯uid of highly variable composition. The anomalously high orthopyroxene mode (up to 47%) makes some of these refractory samples compositionally similar to the Proterozoic/Archean mantle. Their low equilibrium temperature (800±900 8C) points to the presence of old lithospheric relicts in the uppermost mantle beneath South China. Such lithosphere architecture may have resulted from partial replacement of Archean±Proterozoic lithosphere by asthenosphere that rose adiabatically subsequent to lithospheric thinning during the Cenozoic. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Metasomatism; Lithospheric thinning; mantle x-enoliths; polybaric melting; South China 1. Introduction While there still exists controversy on the Mesozoic geodynamic environment of Southeast China (Li, 2000; Zhou and Li, 2000), it is widely believed that in late Ceno- zoic time the South China continental margin evolved in an intraplateextensionalsetting(Chungetal.,1997).Cenozoic basalts,whicharewidespreadinSouthChina,arethevolca- nicexpressionoflithosphericextensionthatmigratedsouth- wards and eventually led to the opening of the South China sea at ,30±16 Ma (Chung et al., 1997). The nature and evolution of the lithosphere in a given region can be constrained by studying peridotite xenoliths in alkali basalts, because lithosphere formed in different tectonic settings may show distinct petrological and geochemical ®ngerprints (Boyd, 1989; Maury et al., 1992; McDonough and Frey, 1989; McDonough, 1990; Menzies, 1990). A number of petrological and geochemical investigations have been undertaken on mantle xenoliths from Cenozoic basalts in SE China (Zhao, 1990; Qi et al., 1995; Xu et al., 1996, 2000; Fan et al., 2000). The main results of these studies are: (1) Most of the peridotites from SE China, whether spinel or garnet facies, are fertile in terms of their Al 2 O 3 and CaO content and represent residues of partial melting of less than 5% (Qi et al., 1995; Xu et al., 2000). AnotableexceptionisspinelperidotitefromMingxi(Fujian Province) and Qilin (Guangdong Province), which experi- enced signi®cantly higher degrees of fractional melting (up to 25%) within the spinel stability ®eld. (2) The calculated thermal gradient in the lithosphere was high through the Cenozoic, consistent with regional mantle upwelling, Journal of Asian Earth Sciences 20 (2002) 937±954 1367-9120/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII:S1367-9120(01)00087-6 www.elsevier.com/locate/jseaes * Corresponding author. Tel.: 186-20-8529-0109; fax: 186-20-8529- 0130. E-mail address: yigangxu@gig.ac.cn (Y.G. Xu).