Recognizing juvenile and relict lithospheric mantle beneath the North China Craton: Combined analysis of H 2 O, major and trace elements and Sr–Nd isotope compositions of clinopyroxenes Yantao Hao, Qunke Xia ⁎, Shaochen Liu, Min Feng, Yaping Zhang CAS Key Laboratory of Crust–Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, PR China abstract article info Article history: Received 5 August 2011 Accepted 2 March 2012 Available online 13 March 2012 Keywords: H 2 O contents Sr–Nd isotope composition Peridotite xenolith North China Craton Recognizing juvenile and relict lithospheric mantle is crucial in unraveling the mechanism of lithospheric thinning (delamination vs. thermal/mechanic erosion). The H 2 O contents and Sr–Nd isotopic compositions of juvenile lithospheric mantle are expected to be similar to the MORB source (asthenospheric mantle), whereas those of the relict lithospheric mantle should be of typical cratonic character. Consequently, com- bined analysis of H 2 O contents and Sr–Nd isotope compositions could be an effective way to distinguish the juvenile and relict lithospheric mantle. Among the peridotite minerals, clinopyroxene is the major host for rare earth elements as well as H 2 O contents, making it the most suitable target sample for such analyses. We collected fresh peridotite xenoliths hosted by Cenozoic basalts from Beiyan, Shandong province and Yangyuan, Hebei province to carry out combined analyses of major elements, trace elements, Sr–Nd isotopes and H 2 O contents for clinopyroxene. At both Beiyan and Yangyuan, pyroxene from peridotite xenoliths shows homogenous H 2 O contents within individual grains, and equilibrium distribution of H 2 O contents between clinopyroxene and orthopyroxene has been achieved. There is a positive correlation between H 2 O contents and Al contents in clinopyroxene and orthopyroxene, these features imply that the pyroxenes largely pre- serve the H 2 O contents of their mantle source. The variations of H 2 O contents in clinopyroxene are controlled by partial melting rather than the later episode of mantle metasomatism, because there is a correlation between H 2 O contents of clinopyroxene and degree of partial melting index (Yb content of clinopyroxene and Mg# of olivine). Based on the correlation between H 2 O contents and Sr isotope ratios of clinopyroxene, the estimated H 2 O contents and 87 Sr/ 86 Sr ratios of mantle source of peridotites in Beiyan (450 to less than 600 ppm, and ~ 0.7028 respectively) are similar to the MORB source, thereby implying that the lithospheric mantle beneath Beiyan is juvenile. In contrast, the variation of H 2 O contents and Sr–Nd isotope compositions of clinopyrox- ene from the Yangyuan peridotites is best explained as relict mantle (H 2 O contents less than 300 ppm and EM1-type Sr–Nd isotope ratios) coexisting with juvenile lithospheric mantle (H 2 O contents more than 600 ppm and 87 Sr/ 86 Sr about 0.7030). These conclusions are in agreement with previous studies which have demonstrated that the lithospheric mantle beneath Beiyan is made up of juvenile material accreted from the asthenosphere after the North China Craton had undergone thinning. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The North China Craton (NCC) is one of the major cratons in eastern Eurasia with crustal remnants older than 3.8 Ga (Liu et al., 1992). The eastern NCC experienced widespread lithospheric extension and dramatic changes of mantle characteristics. A thick (∼200 km), cold (∼40 mW/m 2 ) and highly refractory lithospheric mantle was in place until the mid-Ordovician, but was replaced by a hot (60–80 mW/m 2 ), thin (80–60 km) and fertile lithospheric mantle during the late Mesozoic period (Menzies et al., 2007, and references therein). The timing, spatial and temporal variation as well as mechanism for lithospheric thinning are hotly debated. Delineating the juvenile (e.g., newly accreted mate- rials from upwelling asthenosphere after the NCC thinning) versus relict lithospheric mantle is crucial to unravel the mechanism of lithospheric thinning (delamination vs. thermal/mechanic erosion). The H 2 O con- tents and Sr–Nd isotopic compositions of the juvenile lithospheric mantle are expected to be similar to Mid-Ocean Ridge Basalt (MORB) source, whereas the relict lithospheric mantle should be more like typical cratonic mantle. Therefore, combined analyses of H 2 O contents and Sr–Nd isotope compositions of mantle-derived xenoliths could be an effective way to distinguish between the juvenile and relict litho- spheric mantle. Lithos 149 (2012) 136–145 ⁎ Corresponding author at: No. 96, Jinzhai Road, 230026, Hefei, Anhui, PR China. Tel.: +86 551 3607008; fax: +86 551 3607386. E-mail address: qkxia@ustc.edu.cn (Q. Xia). 0024-4937/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2012.03.013 Contents lists available at SciVerse ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos