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Geochronology and Geochemistry of the Mesozoic Volcanic Rocks In Available online at www.sciencedirect.com Lithos 102 (2008) 88–117 www.elsevier.com/locate/lithos Geochronology and geochemistry of the Mesozoic volcanic rocks in Western Liaoning: Implications for lithospheric thinning of the North China Craton ⁎ Wei Yang, Shuguang Li CAS Key laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China Received 17 October 2006; accepted 24 September 2007 Available online 11 October 2007 Abstract Determining the age and petrogenesis of the voluminous Mesozoic magmatic rocks from the North China Craton (NCC) provides critical data for deducing the process and timing of lithospheric thinning. Four Mesozoic magmatic events in the northeast of the craton (Western Liaoning) are delineated by Ar–Ar and U–Pb zircon dating, i.e. the Xinglonggou Formation (177 Ma), the Lanqi Formation (166–153 Ma), the Yixian Formation (126–120 Ma), and the Zhanglaogongtun Formation (∼106 Ma), respectively. The Xinglonggou lavas are high-Mg# adakites with arc-like Sr–Nd–Pb isotopic compositions, suggesting that they originated from the subducted Palaeoasian oceanic crust. The typical “continental” geochemical signatures of the Lanqi basalts and basaltic andesites as well as their 87 86 low ɛNd(t), moderate Sr/ Sri, and extremely unradiogenic Pb isotopes indicate significant involvement of lower crust materials in their magma. These features, coupled with the low Mg, Ni, and Cr contents may suggest significant olivine fractionation and a magma underplating event, which caused the partial melting of the low-middle crust to produce the voluminous low-Mg andesites and acidic volcanic rocks overlying the Lanqi basalts. The Yixian high-Mg adakitic rocks with the lower-crustal Sr–Nd–Pb isotopic compositions suggest foundering of the mafic lower crust into the underlying convecting mantle. The Yixian basalts show similar geochemical characteristics to the Lanqi basalts except the relatively higher Mg, Ni and Cr contents, which could be derived from a newly enriched lithosphere mantle hybridized by partial melts from the foundered lower continental crust. The Zhanglaogongtun lavas are alkaline basalts with MORB-like Sr–Nd–Pb isotopic compositions, suggesting derivation from a depleted mantle. Based on the new data, a multi-stage lithospheric thinning model is proposed. © 2007 Elsevier B.V. All rights reserved. Keywords: North China Craton; Western Liaoning; Lithospheric thinning; Geochronology and geochemistry of volcanic rocks; Magma underplating; Foundering of mafic lower crust 1. Introduction two decades (e.g., Menzies et al., 1993; Deng et al., 1994, 1996; Griffin et al., 1998; Guo et al., 2001; Gao et al., The lithospheric mantle of the North China Craton 2002; Zhang et al., 2002, 2003; Chen et al., 2003; Wu (NCC) has attracted considerable attention over the last et al., 2003; Deng et al., 2004; Xu et al., 2004a,b; Rudnick et al., 2004; Zhang et al., 2004; Zhang, 2005). Studies on ⁎ Corresponding author. diamond-bearing kimberlites and mantle xenoliths indi- 2 E-mail address: [email protected] (S. Li). cate a thick (∼200 km) and cold (∼40 mW/m ) 0024-4937/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2007.09.018 W. Yang, S. Li / Lithos 102 (2008) 88–117 89 lithosphere existing in the NCC during the Paleozoic (Fan petrogenesis of the Mesozoic mantle-derived rocks is still and Menzies, 1992; Griffin et al., 1992, 1998; Zheng controversial. It has been generally suggested that such et al., 2003). However, investigations of Cenozoic basalt- “continental” geochemical signatures were derived from borne spinel lherzolite xenoliths show that the Cenozoic an enriched SCLM or hybridized upwelling astheno- lithosphere is relatively thinner (b80 km) and hotter sphere and three models have been proposed. The first (∼60 mW/m2) beneath the eastern NCC (Fan et al., 2000; model considers that the SCLM has an EMI-type Zheng et al., 2001). This was also demonstrated by composition resulting from subduction-related multiple geophysical data (Ma, 1987). Therefore, it is suggested metasomatism processes in the Archaean and Mesopro- that about 120 km of lithosphere has been removed since terozoic during the accretion of the NCC (e.g., Yang et al., the early Paleozoic. In addition, the Paleozoic lithospheric 2004; Ma and Xu, 2006). The second model suggests that mantle also differs from the Cenozoic one in geochemical the Mesozoic SCLM has been modified by a Si–Al characteristics. The former is characterized by EMII enriched melt from partial melting of deeply subducted isotopic compositions, such as high 206Pb/204Pb (∼20.2), crustal materials from the South China block (SCB) 87 86 significant variation of Sr/ Sr, and negative ɛNd (−5) (Zhang et al., 2002, 2003). The third model proposes that (Zheng and Lu, 1999; Zhang et al., 2002), while the the Mesozoic SCLM was formed by hybridization of the Cenozoic lithospheric mantle shows Sr–Nd–Pb isotopic upwelling asthenospheric mantle and SiO2-rich melts compositions similar to the mid-ocean ridge basalt from partial melting of the foundered mafic lower (MORB) and ocean island basalt (OIB) (Peng et al., continental crust (Gao et al., 2004; Lustrino, 2005; 1986; Song et al., 1990; Basu et al., 1991). Apparently, the Huang et al., 2007a,b). geochemical features of the lithospheric mantle in eastern Mesozoic volcanic rocks with variable ages are widely China have been significantly changed during the developed in Western Liaoning, the north margin of the evolution from the Paleozoic to the Cenozoic. NCC (Chen et al., 1997, 1999). Four major periods of The reason for the removal and replacement of the volcanism have been identified by stratigraphic studies Paleozoic lithospheric mantle has not been well under- (Chen et al., 1997; Wang et al., 1989): the early Jurassic stood yet. Possible mechanisms include destabilization of (Xinglonggou Formation), the mid-Jurassic (Lanqi For- the NCC due to the Indo-Eurasian collision (Menzies mation in Western Liaoning or Tiaojishan Formation in et al., 1993), mechanical–chemical erosion and replace- Northern Hebei), the early Cretaceous (Yixian Formation), ment by asthenosphere upwelling (Menzies and Xu, and the late early Cretaceous (Zhanglaogongtun Forma- 1998; Xu, 2001; Xu et al., 2004a,b), delamination and tion) (Table 1). Geochronological and geochemical studies foundering of thickened lower continental crust (Gao of these Jurassic–Cretaceous rocks provide an excellent et al., 2004; Wu et al., 2005), destruction of the lithosphere opportunity to probe the evolution of the underlying due to the subduction of oceanic crust in the Paleozoic and lithospheric mantle and to give constraints on the NCC continental crust in the Mesozoic beneath both the lithospheric thinning process. Previous studies have northern and southern margins of the NCC (Zhang mainly focused on the origin of the Mesozoic volcanic et al., 2003), and hydro-weakening of the sub-continental rocks in Western Liaoning (e.g., Chen et al., 1997; Li et al., lithospheric mantle (SCLM) due to migratory or slab- 2001; Shao et al., 2001; Li et al., 2002; Zhang et al., 2003; derived fluids (Niu, 2005). Gao et al., 2004; Wang et al., 2005; Zhang and Zhang, In addition, the mantle sources of the Mesozoic basalts 2005; Zhang et al., 2005a; Li, 2006). However, more data from the NCC are highly heterogeneous, with negative are still required, because (1) only the timing of the Yixian 87 86 ɛNd (up to −20), variable Sr/ Sri,unradiogenicPb Formation (126–120 Ma) has been well dated by both U– isotope ratios, and typical “continental” geochemical Pb and Ar–Ar methods (Swisher et al., 1999, 2001; Wang signatures such as enrichment of large ion lithophile et al., 2001a,b; Zhou et al., 2003; Ji et al., 2004; Yang et al., elements (LILE, e.g., Rb and Ba) and depletion of high 2007) due to the discovery of the famous Jehol biota in the field strength element (HFSE, e.g., Nb and Ta) (Qiou formation (Hou et al., 1995; Hou, 1996; Chen et al., 1998; et al., 1997; Fan et al., 2001; Guo et al., 2001; Qiou et al., Ji et al., 1998), (2) previous geochemical studies mainly 2002; Zhang and Sun, 2002; Zhang et al., 2002, 2003; focused on the high-Sr, low-Yandesites, but neglected the Guo et al., 2003; Chen and Zhai, 2003; Li and Yang, 2003; basalts (e.g., Chen et al., 1997; Li et al., 2001, 2002; Gao Liu et al., 2004a; Xu et al., 2004a,b; Yang et al., 2004; et al., 2004;Wang et al., 2005; Zhang and Zhang, 2005; Zhang et al., 2004, 2005b; Zhang, 2005). These features Zhangetal.,2005a;Li,2006), and (3) the published are not consistent with a source in either the Paleozoic or geochemical data have not been well related to the regional Cenozoic lithospheric mantle (Peng et al., 1986; Song tectonic evolution, which is critical in discussion of the et al., 1990; Basu et al., 1991; Zhang et al., 2002). The mechanism of the lithospheric thinning. 90 W. Yang, S. Li / Lithos 102 (2008) 88–117 Table 1 A summary table showing the strata units in Western Liaoning, volcanic rock types, ages, geochemical characteristics, tectonic settings and interpretations Strata Isotopic age Rock types Geochemical Tectonics Interpretations range (Ma) characteristics The Sunjiawan Formation The Zhanglaogongtun ca. 106 Basalt Alkaline basalts Asthenospheric Formation with MORB-like upwelling caused by Sr–Nd–Pb isotopic ratios. the large scale E–W extension. The Fuxin Formation The strike-slip Tan-Lu fault was transformed into an extensional graben. The Jiufotang Formation The Yixian 126–120 Basalt, Basalts and basaltic andesites Large sale strike-slip Foundering of the Formation basaltic andesite, show the typical continent of the Tan-Lu fault, mafic lower andesite and rhyolite. geochemical signatures and which caused the continental crust relative higher Mg, Ni and Cr lithosphere destruction contents.
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