[The Journal of Geology, 2004, volume 112, p. 593–605]  2004 by The University of Chicago. All rights reserved. 0022-1376/2004/11205-0006$15.00 593 Contrasting Cenozoic Lithospheric Evolution and Architecture in the Western and Eastern Sino-Korean Craton: Constraints from Geochemistry of Basalts and Mantle Xenoliths Yi-Gang Xu, Sun-Lin Chung, 1 Jinlong Ma, and Lanbin Shi 2 Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 510640 Guangzhou, China (e-mail: yigangxu@gig.ac.cn) ABSTRACT Cenozoic basalts from both sides of the Daxin’anling-Taihang gravity lineament that separates the Sino-Korean craton (SKC) into western and eastern parts have been studied. In the western SKC, magmas evolved from xenolith-bearing alkali basalts of Late Eocene–Oligocene age to coexisting alkali and tholeiitic basalts of Late Miocene–Quaternary age. This change in basalt type is accompanied by a decrease in La/Yb and an increase in Yb content. Sr-Nd isotopic ratios and relative abundances on incompatible elements are consistent with a prevailing asthenospheric origin, although lithospheric mantle may have also contributed to these basalts. This temporal variation in basalt geochem- istry is interpreted as reflecting progressive lithospheric thinning in the western SKC during the Cenozoic. An opposite trend is observed for Cenozoic basalts from the eastern SKC, suggesting lithospheric thickening during this time period. This thickening was probably related to regional thermal decay following peak magmatism in the Late Cretaceous–Early Tertiary. Such contrasting lithospheric processes may reflect diachronous extension in the SKC, with initial extension in the eastern part owing to the Late Mesozoic paleo-Pacific subduction and subsequent extension in the western SKC induced by the Early Tertiary Indian-Eurasian collision. An implication is that the lithospheric mantle in the western SKC is relatively old compared with that beneath the eastern SKC, which may be a mixture of old lithospheric relicts and newly accreted mantle. This predicted lithospheric architecture is con- sistent with Sr-Nd isotopic data and recent Re-Os age determinations from mantle xenoliths included in Cenozoic basalts. Introduction The Sino-Korean craton (SKC) in eastern China is an important natural laboratory for studying tem- poral change in the lithosphere because of the oc- currence of Ordovician diamondiferous kimber- lites, Mesozoic lamprophyre-basalt and Cenozoic basalts, and the presence of deep-seated xenoliths in these magmas (E and Zhao 1987; Liu et al. 1992b; Lu et al. 1995; Zhang et al. 2002). The SKC expe- rienced widespread thermotectonic reactivations during the Late Mesozoic and Cenozoic, which re- sulted in replacement of the old, cold, thick, and depleted lithospheric mantle by young, hot, thin, Manuscript received November 18, 2003; accepted May 15, 2004. 1 Department of Geosciences, National Taiwan University, Taipei, Taiwan. 2 Institute of Geology, State Seismological Bureau, 100029 Beijing, China. and fertile mantle (Menzies et al. 1993; Griffin et al. 1998). Several studies further suggest a partial replace- ment and that the subcontinental lithospheric mantle (SCLM) in this region consists of old lith- osphere in the uppermost part and newly created “oceanic-type” lithosphere in the lower part (Fan et al. 2000; Xu 2001). These studies focused on the region east of the Daxin’anling-Taihang gravity lin- eament (DTGL), hereafter referred to as the eastern SKC (fig. 1). Despite studies of Miocene basalts and included xenoliths from Hannuoba (Song and Frey 1989; Song et al. 1990; Zhi et al. 1990; Basu et al. 1991; Tatsumoto et al. 1992; Gao et al. 2002; Xu 2002), little is known about the Cenozoic litho- spheric evolution in the region west of the DTGL (i.e., the western SKC). In fact, basaltic volcanism is widespread in the western SKC and began in the Late Eocene and Oligocene and continued into the