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Mesozoic Adakites in the Lingqiu Basin of the Central North China Craton: Partial Melting of Underplated Basaltic Lower Crust

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Mesozoic Adakites in the Lingqiu Basin of the Central North China Craton: Partial Melting of Underplated Basaltic Lower Crust Geochemical Journal, Vol. 40, pp. 447 to 461, 2006 Mesozoic adakites in the Lingqiu Basin of the central North China Craton: Partial melting of underplated basaltic lower crust XUAN-CE WANG,1,2,3,4* YONG-SHENG LIU2 and XIAO-MING LIU3 1Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China 2State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China 3Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China 4Graduate School of the Chinese Academy of Sciences, Beijing 100039, China (Received June 14, 2005; Accepted February 16, 2006) Intermediate to felsic volcanic rocks of the Baiqi formation from the Lingqiu basin in the central part of North China Craton were studied. Single zircon U-Pb dating indicates that these volcanics formed at 125.8 ± 3.0 Ma. Their Sr and Nd isotopic compositions (143Nd/144Nd = 0.51180–0.51182, 87Sr/86Sr = 0.7062–0.7063) fall in the range of the nearby late- Mesozoic basaltic rocks. These volcanics share geochemical affinities to the adakites formed in the modern arcs, e.g., high Na2O (>4.06%), Al2O3 (>15.4%) and Sr (645–1389 ppm) contents and Sr/Y ratios (55~103), and thus being termed as adakitic rocks. However, the Baiqi adakitic rocks were not temporospatially associated with active subduction. Further- more, their low Cr (2.19–47.4 ppm, with average of 25) and Ni (1.57–20.7 ppm, with average of 12) contents and Mg# (22–47, with average of 32) argue against interaction with the lithospheric mantle. Combined with the geological setting, we suggest that the Baiqi adakitic rocks resulted from partial melting of a thickened lower continental crust associated two episodes of basaltic underplating events. We propose that enormous conductive heating from 80–140 Ma basaltic underplating resulted in partial melting of pre-existing mafic lower crust formed by ~150–160 Ma basaltic underplating. This study provides a case for partial melting of the thickened lower continental crust in association with basaltic underplating events. Keywords: adakitic rocks, basaltic underplating, lower crust, Lingqiu, North China Rapp et al., 1999), and thicken continental crust (Wang, INTRODUCTION Q. et al., 2005). Adakites in subduction zones have been Adakite is characterized by low K, Y, HREE contents widely investigated (Defant and Drummond, 1990; Mar- ≤ ≤ (Y 18, Yb 1.9 ppm), and high Al, Na (Al2O3 >15% at tin, 1999; Martin et al., 2005; Peacock et al., 1994; Stern the 70% SiO2; Na2O/K2O >1), Sr (>400 ppm) contents, and Kilian, 1996), whereas the adakitic rocks within the and high Sr/Y and La/Yb ratios (Defant and Drummond, continent are yet to be fully understood (Gao et al., 2004; 1990). Partial melting of basaltic rocks can produce Liu et al., 2005; Wang, Q. et al., 2005). adakitic melt at pressures equivalent to a crustal thick- Many adakitic rocks in the Eastern China were found ness of >40 km (Rapp and Watson, 1995; Rapp et al., and studied in the last five years (e.g., Gao et al., 2004; 1991). This rock attracts widespread attention due to its Xiao et al., 2004; Xu et al., 2002; Zhang et al., 2001a, b). significance in revealing deep geodynamic processes, e.g., These adakitic rocks are predominately formed in late oceanic crust subduction (Defant and Drummond, 1990; Jurassic to early Cretaceous (160–120 Ma, peak at 137– Martin et al., 2005; Martin, 1999; Peacock et al., 1994; 130 Ma). Zhang et al. (2001b) classified adakitic rocks Stern and Kilian, 1996), basalt underplating (Atherton and into two types: O-type adakites (typical adakites, related Petford, 1993; Petford and Atherton, 1996; Rapp and to the slab subduction) and C-type adakites (produced Watson, 1995; Xiong et al., 2003), recycling of lower within intracontinent). Xiao et al. (2004) classified the continental crust (Gao et al., 2004; Kay and Kay, 1991; C-adakites from North China Craton into type A and type Xu et al., 2002), melt-peridotite reaction (Liu et al., 2005; B. The type A rocks (most of C-type adakites, not all) have lower Mg# number and higher K contents than typi- cal subduction-related adakites, which could be produced *Corresponding author (e-mail: [email protected]) by partial melting of underplated basaltic rocks (Xiao et Copyright © 2006 by The Geochemical Society of Japan. al., 2004; Zhang et al., 2001a, b), whereas the type B 447 N 15km Archean genesis Division of the North China craton 5 Mesozoic volcanics 1 6 Western Block 2 Intrusive rocks Hunyuan 3 4 Sediment rocks Eastern Block Trans-North China Orogen 39 30 (B) 1100 Xiguayuan formation 1000 Dabiegou formation Baiqi adakitic A 900 sample site Zhangjiakou Lingqiu formation Thickness(m) 200 Baiqi formation 114 (C ) (A) Fig. 1. (a) Simplified geological map of the study area. (b) Tectonic division of the North China Craton (After Zhao et al., 2001). Locations of the Mesozoic gabbros and lower crustal xenoliths in the Trans-North China Orogen were also marked. 1 = Location of the Baiqi adakitic rocks, 2 = gabbro from Laiyuan (Zhang et al., 2004), 3 = gabbro from Hanxing (Zhang et al., 2004), 4 = gabbro from Linxian (Wang, Y. J. et al., 2005), 5 = lower crustal xenolths from the Hannuoba, 6 = Xinglonggou high-Mg adakites From Liaoxi (Gao et al., 2004). (c) Profile of the Mesozoic volcanics in Hunyuan-Guangling-Lingqiu basin. rocks featured by high Mg# were interpreted as foundering Mesozoic magmatism (e.g., Davis et al., 1998; Chen et lower continental crust-derived melt interacted with al., 2002, 2003, 2004; Chen and Zhai, 2003; Zhang et al., peridotites in the mantle (Gao et al., 2004; Xiao et al., 2004). 2004). Despite these investigations, some questions re- The adakitic rocks in this study were collected from main: how were these adakitic rocks related to the the Lingqiu volcanic basin in the northern part of the foundering lower continental crust or basalt underplating? Trans-North China Orogen. The late Mesozoic volcanics And what is the mechanism triggered the surge of the in the Hunyuan-Guangling-Lingqiu basins (Figs. 1A and Mesozoic adakites in the North China Craton? To address C) are composed of intermediate to felsic volcanics with these questions, typical adakitic volcanics from the Trans- interlayers of sandstones and mudstones. These volcanics North China Orogen, central North China Craton were were classified into four formations (Fig. 1C): from bot- studied in this paper. Except for some adakitic intrusive tom to top, Baiqi, Zhangjiakou, Dabeigou and Xiguayuan. rocks, typical adakitic volcanic rocks have not been re- The Baiqi formation was systematically studied in this ported in the central of North China Craton. work. The emplacement age of diorite-granite-rhyolite and tuff lava of the Trans-North China Orogen has been dated at 127–138 Ma using U-Pb zircon and Rb-Sr whole- GEOLOGICAL SETTING rock isochron methods (Cai et al., 2003; Davis et al., The North China Craton is one of the oldest continen- 1998; Peng et al., 2004). Several episodes of Mesozoic tal nuclei in the world, with basement of mainly Archean mafic igneous rocks in the Trans-North China Orogen to Early Proterozoic gneisses (Jahn et al., 1988). Based have also been identified at 150–160 Ma (e.g., on isotopic age, lithological assemblage, tectonic evolu- Yunmengshan gabbro-diorite complex, Davis et al., 1998; tion and P-T-t paths, the North China Craton can be di- Hanxing gabbro, Zhang et al., 2004), 135–145 Ma (e.g., vided into the Eastern Block, the Western Block and the Laiyuan gabbro, Zhang et al., 2004) and 120–130 Ma intervening Trans-North China Orogen (Zhao et al., 2000, (e.g., Linxian gabbro, Wang, Y. J. et al., 2005). Laiyuan 2001) (Fig. 1B). This craton underwent a dramatic change locate in the eastern of the Lingqiu basin with a distance from a Paleozoic cratonic mantle to a Cenozoic “oceanic” of ~80 km, and Linxian in the southeastern with a dis- lithospheric mantle, accompanied by lithospheric thinning tance of 300 km. Lower crustal xenoliths found in the (Gao et al., 2002; Griffin et al., 1998; Menzies et al., Neogene Hannuoba basalts adjacent to the studied area 1993; Rudnick et al., 2004; Wu et al., 2003; Xu, 2001), are dated at two age intervals of ~160–140 Ma and ~140– lower crustal recycling (Gao et al., 2004) and widespread 80 Ma (Liu et al., 2004; Wilde et al., 2003), and they 448 X.-C. Wang et al. were interpreted as products of ~160–140 Ma basaltic TEMORA 1 as an unknown in this LA-ICPMS over a underplating and subsequent ~140–80 Ma granulite-facies period of 16 months yielded a weighted mean 206Pb/238U metamorphism (Liu et al., 2004). The xenoliths with con- age of 415 ± 4 Ma (MSWD = 0.112) (Gao et al., 2004; vex-upward REE patterns had a cumulate origin (gabbro Yuan et al., 2004), which is in good agreement with the or pyroxenite) (Liu et al., 2001). The Mesozoic Hannuoba recommended ID-TIMS age of 416.75 ± 0.24 Ma (Black granulite xenoliths was products of basaltic underplating et al., 2003). has been widely accepted (e.g., Chen et al., 2001; Fan et Fresh chips of whole rock samples were powdered to al., 1998, 2001; Liu et al., 2001, 2004; Zhang et al., 1998; 200 meshes using a tungsten carbide ball mill. Major and Zhou et al., 2002). trace elements were analyzed using XRF (Rikagu RIX The mafic igneous rocks from the Trans-North China 2100) and ICP-MS (PE 6100 DRC), respectively at the Orogen were derived from a special mantle metasomatised Key Laboratory of Continental Dynamics, Northwest by a SiO2-rich melt (Chen et al., 2004; Wang, Y.
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