Early Cretaceous low-Mg adakitic granites from the Dabie orogen, eastern China: Petrogenesis and implications for destruction of the over-thickened lower continental crust Haijin Xu a, b, ⁎, Changqian Ma a, b , Junfeng Zhang a, b , Kai Ye c a Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China b State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China c State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, P.O. Box 9825, China abstract article info Article history: Received 15 August 2011 Received in revised form 28 November 2011 Accepted 11 December 2011 Available online 5 January 2012 Keywords: Low-Mg adakitic granites Lower continental crust Partial melting Foundering of the eclogitized residues Dabie orogen It is generally accepted that the low-Mg adakitic rocks were derived from the partial melting of metabasalts/ eclogites. In this study, we demonstrate that the early Cretaceous low-Mg adakitic granites in the North Dabie Complex (NDC) were generated by the partial melting of the NDC orthogneisses. Here we present in-situ U–Pb and Lu–Hf isotopes in zircon with whole-rock geochemical and Sr–Nd isotopic compositions were carried out for the Tiantangzhai porphyritic monzogranites from the Dabie orogen, eastern China. The monzogranites are characterized by high Sr (576–988 ppm), low Y (7.3–19.0 ppm), and depletion in HREE (Yb: 0.50–1.78 ppm) (thus resulting in high Sr/Y (34.3–135.2) and (La/Yb) N (17.0–105.2) ratios) without a negative Eu anomaly. They also exhibit high SiO 2 (66.5–73.5 wt.%) and K 2 O (2.7–4.7 wt.%), and low MgO (0.4–1.6 wt.%) or Mg # (28.2–45.3, mostly b 40) values. Whole-rock geochemical compositions suggest that the monzogranites represent low- Mg adakitic rock with high-Si and rich-K features equilibrated with residues rich in garnet. Sr–Nd isotopic compositions (ε Nd (t) =-16.2 to -20.3, ( 87 Sr/ 86 Sr) i = 0.707798–0.708804, t DM2 (Nd) = 2.3–2.6 Ga) of the monzogranites are distinct from that of the eclogites and amphibolites in the Dabie orogen, but similar to that of the Neoproterozoic (700–800 Ma) gneisses in the NDC. U–Pb dating of zircons gives a consistent age of 130.0 ± 3.4 Ma with discordia upper intercept age of 716 ± 34 Ma for inherited cores identified by CL imaging. Correspondingly, in-situ Lu–Hf analyses of early Cretaceous young age-spots from zircons yield initial 176 Hf/ 177 Hf ratios from 0.281898 to 0.282361, εHf(t) values from -28.1 to -17.6 and two-stage “crust” Hf model ages (t DM2 ) from 2293±89 to 2949±108 Ma, which are generally in agreement with values of 0.281891 to 0.282218, -28.2 to -11.7 and 1927±87 to 2963±92 Ma for the pre-Mesozoic inherited cores, respectively. As for individual core-rim pairs in zircon, Th/U ratios increase from the inherited cores to the young growth rims possibly due to variable degrees of partial melting, whereas 176 Lu/ 177 Hf ratios greatly decrease because of the garnet effect in residues. Thus, we suggest that the early Cretaceous low-Mg adakitic granites were derived from the partial melting of the NDC Neoproterozoic (700–800 Ma) gneisses, and the foundering of the garnet-bearing residues could have caused the destruction of the over-thickened lower continental crust. © 2012 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. 1. Introduction Orogenic evolution generally comprises a period of crustal thickening followed by extension and thinning of the previously thickened crust (Brown, 2001; Leech, 2001; Vanderhaeghe and Teyssier, 2001). Crustal thinning is commonly accompanied by concomitant granitic magmatism and migmatitization formed by partial melting of the lower continental crust (Foster et al., 2001; Keay et al., 2001). Crustal melting is a significant process in the evolution of collisional orogens because it strongly influences the thermal and rheological behavior of orogenic crust (Andersson et al., 2002; Whitney et al., 2003), and may result in eventual destruction of the over-thickened continental crust (Rey et al., 2001; Vanderhaeghe and Teyssier, 2001). The Dabie orogen is illustrated as an example, which was formed by Triassic continent– continent collision between the Yangtze Craton and the North China Craton (e.g., Li et al., 1993, 2000), whose collision resulted not only in the formation of ultra-high pressure (UHP) metamorphic terranes (e.g., Wang et al., 1989; Xu et al., 1992) but also in the formation of an over-thickened continental crust (e.g., Wang et al., 2007; Xu et al., 2007; Huang et al., 2008), and then underwent extensional tectonic collapse in the early Cretaceous (e.g., Hacker et al., 2000). Gondwana Research 23 (2012) 190–207 ⁎ Corresponding author at: Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China. Tel.: +86 15327190076; fax: +86 27 67883002. E-mail address: xuhaijin@cug.edu.cn (H. Xu). 1342-937X/$ – see front matter © 2012 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2011.12.009 Contents lists available at SciVerse ScienceDirect Gondwana Research journal homepage: www.elsevier.com/locate/gr