International Geology Review 2011, iFirst article, 1–23 Petrology, geochemistry, and tectonic significance of Mesozoic shoshonitic volcanic rocks, Luzong volcanic basin, eastern China Jianghong Deng a,b , Xiaoyong Yang a,b∗ , Weidong Sun c,a , Yu Huang d , Yueyu Chi e , Liangfan Yu e and Qianming Zhang e a CAS Key Laboratory of Crust-Mantle Materials and Environments, University of Science and Technology of China, Hefei 230026, PR China; b State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, PR China; c CAS Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, PR China; d Department of Geology, University of Maryland, College Park, Maryland 20742, USA; e No. 327 Geological Team, Anhui Bureau of Land and Mineral Resources, Hefei 230001, PR China (Accepted 17 March 2011) We studied the geochemical characteristics of three types of Mesozoic igneous rocks from the Luzong volcanic basin: basaltic trachyandesite at Shuangmiao, pyroxene monzonite at Bajiatan, and quartz-syenite (A-type granite) at Huangmeijian. Based on analyses of whole-rock major elements, all investigated rocks are enriched in K, Na, Ti, Al, but depleted in Ca, rep- resenting a shoshonitic series. Trace element analyses show that these rocks are characterized by enrichments of large-ion lithophile elements and high field strength elements. Positive Nb and Ta anomalies in the chondrite-normalized spider dia- gram indicate that the shoshonitic volcanic rocks share similar features with Nb-enriched basalts, which are different from normal island-arc volcanical rocks (they are typically strongly depleted in Nb and Ta). Bulk-rock chemical compositions and Sr–Nd isotopes indicate that the three types of igneous rocks are geochemically comagmatic, suggesting that the melts were derived from an enriched mantle reservoir. We postulate an extensional tectonic setting for the formation of Luzong volcanic basin, possibly related to subduction of a palaeo-Pacific plate beneath the east Chinese continent during the Yanshanian period (Cretaceous). Therefore, the petrogenetic features of those volcanic rocks as well as A-type granites in the Luzong basin indicate that the regional large-scale Fe–Cu–Au mineralization was associated with oceanic slab melting, but not delamination or recycling of the ancient lower continental crust, as previously proposed. Keywords: tectonic environment; shoshonite; Nb-enriched basalt; Luzong volcanic basin; Pacific plate subduction Introduction From north to south, eastern China can be divided into several tectonic units, that is, the North China craton, the Dabie-Sulu orogenic belt, the Yangtze Block and the Cathaysian block; each of these units is characterized by unique geological-tectonic evolutionary patterns from basements to caprocks. The Middle-Lower Yangtze River area is located near the junction of the Yangtze Block and North China craton. It is an important magmatic- metallogenic belt (Chang et al. 1991; Zhai et al. 1992, 1996; Tang et al. 1998; Pan and Dong 1999; Mao et al. 2006; Yang et al. 2011), where the development of late Mesozoic magmatic rocks from ultramafic to felsic and alkaline series was extensive. Studying the geochemical characteristics of igneous rocks along the metallogenic belt can set significant constraints to our understanding of the late Mesozoic crust–mantle evolution in eastern China. Many researchers have extensively studied the for- mation ages, geochemical characteristics, magmatic pro- cessing, and tectonic environments of the late Mesozoic igneous rocks in the Middle-Lower Yangtze metallogenic *Corresponding author. Email: xyyang555@163.com belt (e.g. Chang et al. 1991; Ren et al. 1991; Zhai et al. 1992; Wang et al. 1996; Pan and Dong 1999; Xing and Xu 1999; Liu et al. 2002; Wu et al. 2003; Zhang et al. 2003; Mao et al. 2006; Wang et al. 2006; Zhou et al. 2007, 2008 Yang et al. 2011). However, controversies related to the formation of these magmatic rocks remain poorly clarified, especially regarding the tectonic settings. Some scholars have proposed a Pacific plate subduction-related genesis (Deng et al. 1992; Wang et al. 2004; Sun et al. 2007; Liu et al. 2010); others have suggested an intraplate extensional rift environment (Xing and Xu 1999); yet other researchers have favoured a transitional tectonic setting between continental margin environment and intraconti- nental block environment (Chen et al. 2001; Chen et al. 2005). Magmatic rocks in the Middle-Lower Yangtze metal- logenic belt are mainly classified as three series: (1) a high-K calc-alkaline series, mainly developed in uplifted areas such as the Tongling and Anqing regions (Chang et al. 1991); (2) a shoshonite series, normally formed in faulted volcanic basins represented by the Luzong and ISSN: 0020-6814 print/ISSN: 1938-2839 online © 2011 Taylor & Francis DOI: 10.1080/00206814.2011.580628 http://www.informaworld.com Downloaded by [University Of Maryland] at 13:42 18 July 2011