Eocene break-off of the Neo-Tethyan slab as inferred from intraplate-type mafic dykes in the Gaoligong orogenic belt, eastern Tibet Yi-Gang Xu a, ⁎, Jiang-Bo Lan a,b , Qi-Jun Yang a,c , Xiao-Long Huang a , Hua-Ning Qiu a a Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 510640 Guangzhou, China b Graduate School of Chinese Academy of Sciences, Beijing 100039, China c Guilin Institute of Technology, Guilin 541004, China abstract article info Article history: Received 5 February 2008 Received in revised form 2 July 2008 Accepted 22 July 2008 Editor: B. Bourdon Keywords: Geochemistry Mafic dykes Intraplate type Slab break-off Eocene Gaoligong East Tibet Eocene (40–42 Ma) basaltic dykes in the Gaoligong–Tengliang belt, eastern Tibet, are characterized by high Na 2 O (2–4%), in contrast with the widespread post-collisional potassic and ultrapotassic rocks in the Tibetan plateau. Despite the ubiquitous negative Nb anomalies, these dykes have relatively high Nb and Zr contents, making them distinct significantly from the Gangdese arc magmas. All these, together with the positive Nb anomaly in some samples, indicate an intraplate affinity for the Gaoligong–Tengliang dykes. Specifically, the Gaoligong dykes represent the asthenosphere-derived melts which has been contaminated to various degrees by the lithosphere mantle-derived melts, whereas the Tengliang samples were directly derived from an enriched lithosphere mantle. The thin lithosphere (b 80 km) inferred from basalt geochemistry is unusual in Tibet, thus demanding a peculiar mechanism to thin lithosphere. A slab break-off model is preferred given the geochemical contrast between pre-40 Ma and post-40 Ma magmas in Tibet, and the temporal correlation among this intraplate magmatism, the termination of Gangdese arc magmatism and regional thermally- driven metamorphism. Therefore, the occurrence of intraplate-type magmas in the Gaoligong orogenic belt likely represents magmatic expression of the detachment of subducting Neo-Tethyan slab from the Indian continental plate during the Eocene. In the light of the slab break-off concept and thermo-mechanical modeling, the Eocene slab break-off furthermore suggests the onset of the India–Asia collision between 52 and 57 Ma. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Petrogenetic understanding of magmas emplaced at different tectonic settings provides insights into thermal and physical relation- ships between crust/mantle melting and tectonic evolution (Wilson, 1989). Magmatism is widespread in Tibetan Plateau and has been used to tackle the tectonic evolution of this active collision zone (Coulon et al., 1986; Arnaud et al., 1992; Turner et al., 1993, 1996; Miller et al., 1999; Williams et al., 2001; Chung et al., 2003, 2005; Ding et al., 2003; Hou et al., 2004; Wang et al., 2005). While there is a consensus that magmatism reflects the response of the upper mantle and crust to the complex geodynamic evolution of this area, the relative role of northward subduction of Neo-Tethyan oceanic plate, slab rollback, slab break-off, continent–continent collision and subsequent detach- ment of sub-continental lithosphere in magmatic generation is a matter of hot debate (e.g., Chung et al., 2005). In particular, while the slab break-off must have taken place during the Indo-Asian collision, its timing and magmatic response to this event remain controversial. On the basis of a petrologic study on the Greater Himalayan metamorphic core, Kohn and Parkinson (2002) argued that decou- pling of the oceanic lithosphere took place during the Eocene time. These authors suggested that late Eocene K-rich magmas in south- eastern Tibet may represent magmatic expression of the slab break- off. A similar timing of slab break-off has been inferred by Chung et al. (2005) but on the basis of the termination age of the Gangdese arc magmatism. On the other hand, Mahéo et al. (2002) proposed that the Neogene magmatic and metamorphic evolution of the South Asian margin was controlled by slab break-off of the subducting Indian continental margin starting at about 25 Ma. This version of slab break- off model has been adopted by Hou et al. (2004) to explain mantle- derived ultrapotassic magmatism (17–25 Ma), which in turn triggered partial melting of the thickened lower crust to generate the Miocene (10–18 Ma) potassic adakites from southern Tibet. If the timing of slab break-off reflects the diachronous evolution of the Tibetan plateau, then it is unclear why different magma composi- tions were produced through time across the Tibetan plateau by essentially the same trigger mechanism. The modeling by von Blanckenburg and Davies (1995) suggested that as the subducted oceanic plate breaks off, the underlying asthenosphere rises into the lithosphere break and impinges at the base of the thickened lithosphere of the overlying plate. This process will result in a heat supply that can Chemical Geology 255 (2008) 439–453 ⁎ Corresponding author. Tel.: +86 20 85290109; fax: +86 20 85290261. E-mail address: yigangxu@gig.ac.cn (Y.-G. Xu). 0009-2541/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2008.07.016 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo