Paleoproterozoic evolution of the eastern Alxa Block, westernmost North China: Evidence from in situ zircon U–Pb dating and Hf–O isotopes Wei Dan a, b , Xian-Hua Li a, c, ⁎, Jinghui Guo a , Yu Liu a , Xuan-Ce Wang a, d a State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China b Graduate School of Chinese Academy of Sciences, Beijing 100039, China c State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, Hubei 430074, China d The Institute for Geoscience Research, Department of Applied Geology, Curtin University of Technology, Perth, WA 6845, Australia abstract article info Article history: Received 5 April 2011 Received in revised form 4 September 2011 Accepted 11 September 2011 Available online 25 September 2011 Keywords: North China Craton Alxa Block Paleoproterozoic U–Pb zircon age Zircon Hf–O isotopes The Alxa Block is the westernmost part of the North China Craton (NCC). In the past, it has been considered to be part of the Archean NCC. However, formation and evolution of this block remain poorly understood, and this has hampered a broader understanding of the NCC. In this study we analyzed the in situ zircon U–Pb age and Hf–O isotopes for the two oldest rock units in the eastern Alxa Block, namely the Bayanwulashan and Diebusige Complexes. The Bayanwulashan Complex consists mainly of metamorphic rocks with mafic and felsic protoliths. SIMS U–Pb zircon dating results indicate that the primary magmatic ages of the mafic and felsic igneous rocks are ca. 2.34 Ga and ca. 2.32–2.30 Ga, respectively, and both sets of rocks were over- printed by two metamorphic events at ca. 1.89 Ga and ca. 1.79 Ga. Geochemical and zircon Hf–O isotopic data suggest that the ca. 2.34 Ga amphibolites within the Bayanwulashan Complex are characteristically high in TiO 2 (up to 3.2%), Zr (up to 394 ppm) and Ti/V (N 30), resembling the basaltic rocks formed within continen- tal rifts. The ca. 2.32–2.30 Ga felsic rocks were generated by remelting of dominant meta-igneous rocks that have zircon Hf model ages of 2.92–2.81 Ga. The Diebusige Complex is composed of amphibolites, mafic gneisses and paragneisses. It was intruded by granites dated at ca. 1.97–1.98 Ga, and subjected to high grade metamorphic events at ca. 1.89 Ga and ca. 1.79 Ga. Re-examination of U–Pb ages for detrital and meta- morphic zircons indicate that the depositional ages of protoliths of the Diebusige paragneisses may be con- sidered to be between ca. 2.45 and 2.0 Ga. These U–Pb zircon age data do not support the existence of exposed Archean rocks in the eastern Alxa Block, although the possibility of Archean rocks at deeper crustal levels cannot be ruled out entirely. The eastern Alxa Block contrasts to the neighboring Yinshan Block and Khondlite Belt, as well as the Trans- North China Orogen, in terms of the timing and evolution of magmatism and metamorphism. Thus, we con- clude that the Alxa Block is a separated Paleoproterozoic terrane from the Western Block of the NCC, rather than the western extension of the Yinshan Block or part of the Khondalite Belt, as previously proposed. © 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. 1. Introduction In the past decade, understanding of the formation and evolution of the North China Craton (NCC) has been improved by the recogni- tion of two Paleoproterozoic orogenic belts, i.e., the Khondalite Belt (e.g., Zhao et al., 2005; Xia et al., 2006; Yin et al., 2009) and the Trans-North China Orogen (TNCO) (e.g., Zhao et al., 2001). These two belts divide the NCC into three separate blocks, namely the East- ern, Ordos and Yinshan blocks, with the latter two blocks amalgamat- ed to form the Western Block at ca. 1.95 Ga (e.g., Zhao et al., 2005, 2010; Yin et al., 2009, 2011; Santosh, 2010). The unified NCC was formed by a collision between the Eastern and Western blocks along the TNCO at ca. 1.85 Ga, during the assembly of the Columbia supercontinent (e.g., Zhao et al., 2002, 2004; Santosh et al., 2007; San- tosh, 2010). Compared with the Eastern Block, the Western Block has been poorly studied, limiting a broader understanding of the NCC. The ca. 2.5 Ga tonalitic–trondhjemitic–granodioritic (TTG) rocks, known to be the oldest Late Archean basement rocks in the region, are ex- posed in the eastern Yinshan Block (Jian et al., 2005; Zhang et al., 2006a; Fan et al., 2010); whereas, the Ordos Block is entirely covered by Phanerozoic sedimentary rocks. The westernmost NCC remains the least studied area. This region is largely covered by Cenozoic sedimentary deposits, with Precambri- an basement rocks being sporadically exposed in its eastern and southern parts (Fig. 2). While this region was traditionally named the Alxa Block in the Chinese literature (Ren et al., 1987; Wu et al., 1998; Zhai et al., 2000; Geng et al., 2002, 2006a), it has recently Gondwana Research 21 (2012) 838–864 ⁎ Corresponding author at: State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China. Tel.: +86 10 62007395; fax: +86 10 62010846. E-mail address: lixh@gig.ac.cn (X.-H. Li). 1342-937X/$ – see front matter © 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2011.09.004 Contents lists available at SciVerse ScienceDirect Gondwana Research journal homepage: www.elsevier.com/locate/gr