Lower crustal xenoliths from Junan, Shandong province and their bearing on the nature of the lower crust beneath the North China Craton Ji-Feng Ying ⁎, Hong-Fu Zhang, Yan-Jie Tang State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China abstract article info Article history: Received 14 January 2010 Accepted 20 July 2010 Available online 24 July 2010 Keywords: Granulite xenoliths Lower crust Petrology Geochemistry North China Craton Geochronological, petrological and geochemical studies were performed on the granulite xenoliths from a Late Cretaceous basaltic breccia dike in Junan, Shandong province, eastern China. These xenoliths show close similarities to the Nushan granulite xenoliths from the southern margin of the North China Craton (NCC) and the Archean granulite terrains in terms of mineralogy and bulk rock compositions, but are quite different from the Hanuoba mafic granulite xenoliths from the northern NCC. In-situ zircon U–Pb age and Hf isotopic analyses, together with geochemical data reveal that the protolith of these xenoliths was formed around 2.3 Ga ago, through assimilation–fractional crystallization of a mafic magma. P–T conditions of these xenoliths suggest that the lower crust beneath the Junan region reaches to a depth of 35 km, which agree well with the result deduced from various geophysical methods. The consistent petrological and seismic Moho depths, the observed velocity structure and calculated velocity of these xenoliths imply the absence of underplating induced crust–mantle transition zone, which was well formed in the northern NCC. Compared to 40–50 km depth of the lower crust in Early Jurassic, the lower crust beneath Junan extended to a depth of 30 km in Late Cretaceous, suggesting that the lower crust of NCC was significantly thinned during Late Mesozoic. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Apart from various geophysical methods, there are other two approaches through which the information of the lower crust can be obtained. One is granulite terrain, which was tectonically uplifted and covered areas of hundreds to thousands of square kilometers, the other is granulite xenolith which was brought to the surface by basaltic or kimberlitic magmas (Griffin and O'Reilly, 1987a; Rudnick, 1992; Rudnick and Foutain, 1995). Because significant differences in composition, age and P–T conditions exist between the granulites from exposed terrains and xenoliths (Bohlen and Mezger, 1989; Harley, 1992; Rudnick and Foutain, 1995), different opinions still remain as to which one better represents the lower crust. However, as few exposed terrains retain evidence for having formed near the base of the crust, and they may have been modified by deformation and recrystallization during exhumation, so it is generally considered that the xenoliths could provide more direct evidence of the nature of the lower crust beneath the volcanic regions, and consequently were employed as windows to understand the composition of the deep crust and the geological processes operating in it. The occurrence of outcroppings of Archean granulite terrains as old as 3.8 Ga suggests that the North China Craton (NCC) is one of the world's oldest cratons (Liu et al., 1992). Though the comprehensive, multi-disciplinary studies of the granulite terrains have provided a wealth of data on the formation and evolution of the lower crust beneath the craton, the granulite xenoliths entrained in Mesozoic and Cenozoic basalts on the NCC enable us to investigate the lower crust more directly and to understand what the lower crust beneath the NCC looks like more accurately. Three granulite xenolith-bearing basalt fields have been reported on the NCC so far, namely Cenozoic Hannuoba basalt in the northern margin of the NCC, Cenozoic Nushan basalt and Mesozoic Xinyang volcanic breccia in the southern margin of the NCC (Fig. 1). Previous studies have revealed that the lower crust beneath the NCC is highly heterogeneous in terms of age, composition and architecture. For instance, in Hannuoba area, severe basaltic underplating and crust–mantle interaction occurred at the base of the crust during the Late Mesozoic, which extended the seismic Moho to a depth of 42 km and formed a 10-km thick crust–mantle transition zone between the seismic Moho and the shallower crustal–mantle boundary (CMB) (Fan and Liu, 1996; Chen et al., 2001; Liu et al., 2001; Wilde et al., 2003). In contrast, the lower crust under the Nushan region is mainly composed of the Late Archean crystalline basement with minor underplated materials and no high velocity layer was observed in the lowermost crust. The consistent CMB and seismic Moho depth of 30 km is also much shallower than that in Hannuoba (Chen et al., 2001; Huang et al., 2004). The data of Mesozoic Xinyang granulite xenoliths revealed that the lower crust which extended up to 56 km depth, is much thicker than the present, the zircon ages Lithos 119 (2010) 363–376 ⁎ Corresponding author. Tel.: +86 10 82998532; fax: +86 10 62010846. E-mail address: jfying@mail.igcas.ac.cn (J.-F. Ying). 0024-4937/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2010.07.015 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos