Metasomatized lithosphere–asthenosphere interaction during slab roll-back: Evidence from Late Carboniferous gabbros in the Luotuogou area, Central Tianshan Gong-Jian Tang a, b , Qiang Wang a, ⁎, Derek A. Wyman c , Zheng-Xiang Li d , Yi-Gang Xu a , Zhen-Hua Zhao a a State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, PR China b Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China c School of Geosciences, The University of Sydney, NSW 2006, Australia d ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS) and The Institute for Geoscience Research (TIGeR), Department of Applied Geology, Curtin University, GPO Box U1987, Perth,WA 6845, Australia abstract article info Article history: Received 14 May 2012 Accepted 18 August 2012 Available online xxxx Keywords: Gabbro Clinopyroxene Lithosphere–asthenosphere interaction Slab roll-back Carboniferous Tianshan Orogen Late Carboniferous igneous rocks are widespread in the western Tianshan, but the tectonic settings for these rocks remain controversial. We report a plagioclase 40 Ar/ 39 Ar age, and geochemical, Sr–Nd isotope and LA–ICPMS clinopyroxene trace element data for gabbros in the Luotuogou region. The tholeiitic Luotuogou gabbros give a Late Carboniferous (312 ± 1 Ma) 40 Ar/ 39 Ar age and are characterized by high and variable ε Nd (t) values ranging from + 3.7 to + 7.8. They have geochemical features of both intra-plate and island arc magmatic rocks, i.e., relative- ly high TiO 2 (0.6–2.2 wt.%), Nb (4.2–24 ppm) and Zr (51.4–283 ppm) contents combined with variable and slight- ly high Nb/La ratios (0.24–1.8, mostly>0.7), and negative to positive Nb anomalies. The gabbros contain zoned clinopyroxenes, with Mg- and Cr-rich cores. Their parental magmas, as calculated using trace element data from Cr-rich (>3000 ppm) clinopyroxene cores and clinopyroxene/basaltic liquid partition coefficients, show enrich- ments in incompatible elements, and prominent negative to slightly positive Nb anomalies, indicative of the influ- ence of subduction-related compositions in their mantle source. These features indicate that the Luotuogou gabbros were most likely formed by interactions between asthenospheric and metasomatized lithospheric mantle. They were most plausibly formed by mixing between the asthenospheric mantle-derived and metasomatized lith- osphere mantle-derived melts. Mixing was the result of asthenosphere upwelling triggered by roll-back of the subducted Paleo-Junggar Oceanic Plate rather than mantle plume-related rifting or post-collisional break-off dur- ing the Late Carboniferous. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Continental basaltic magmas record critical information regarding the chemical composition of the sub-continental mantle and regional tectonic evolution. Several distinct mantle components can contribute to continental basalts, such as subcontinental lithospheric mantle, plume-related OIB (oceanic island basalt)-type mantle sources, or de- pleted MORB (middle oceanic ridge basalt)-type asthenosphere mantle (Garfunkel, 2008; Saunders, 2005). Popular tectonic models for conti- nental volcanism include decompressional melting as a result of litho- spheric mantle removal (detachment) and upwelling asthenosphere mantle (Hoernle et al., 2006; Timm et al., 2009), lithosphere extension induced by continental rifting and breakup (McKenzie and Bickle, 1988), high temperature melting of mantle owing to elevated mantle temperature by deep-seated mantle plume head impinging on the lith- osphere (Campbell and Griffiths, 1990), and the roll-back and/or foundering of flat-subducted oceanic plateaus or aseismic ridges (Coney and Reynolds, 1977; Li and Li, 2007). Identifying the primary magmas for continental basaltic magmas thus has the potential of deciphering their petrogenesis and related tectonic processes. However, continental basaltic magmas generally show variable chemical compositions due to contamination by continental crustal components or fractionation during their ascent (Dorais and Tubrett, 2008). One approach that overcomes these problems uses clinopyroxene chemical composition and basaltic liquid partition coefficients to model the most primitive liquids to have been in equilibrium with the clinopyroxenes (Tribuzio et al., 2008, 2009). The results are then com- pared with various mantle-derived basaltic magmas to draw inferences about the primary magma compositions (Chen et al., 2009; Dorais and Tubrett, 2008; Tribuzio et al., 2008, 2009). The Tianshan Orogen, extends from west to east for over 2500 km through Uzbekistan, Tajikistan, Kyrgyzstan, and Kazakhstan to Xinjiang in northwestern China (Fig. 1a). It is a major part of the southwestern Central Asian Orogenic Belt (CAOB) (Jahn et al., 2000; Sengör et al., 1993; Windley et al., 2007; Xiao et al., 2004). It mainly consists of microcontinents, ophiolite and mélange belts, continental island arcs, and remnant seamounts and oceanic plateaus, accreted Lithos xxx (2012) xxx–xxx ⁎ Corresponding author at. Tel.: +86 20 85290277; fax: +86 20 85290130. E-mail address: wqiang@gig.ac.cn (Q. Wang). LITHOS-02815; No of Pages 14 0024-4937/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.lithos.2012.08.015 Contents lists available at SciVerse ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos Please cite this article as: Tang, G.-J., et al., Metasomatized lithosphere–asthenosphere interaction during slab roll-back: Evidence from Late Carboniferous gabbros in the Luotuogou area, Central Tianshan, Lithos (2012), http://dx.doi.org/10.1016/j.lithos.2012.08.015