Geochronology and geochemistry of the Mesozoic volcanic rocks in Western Liaoning: Implications for lithospheric thinning of the North China Craton

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1 Available online at Lithos 102 (2008) 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 , 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 ( Ma), the Yixian Formation ( 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 low ɛ Nd (t), moderate 87 Sr/ 86 Sr i, 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 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 The lithospheric mantle of the North China Craton (NCC) has attracted considerable attention over the last Corresponding author. address: lsg@ustc.edu.cn (S. Li). two decades (e.g., Menzies et al., 1993; Deng et al., 1994, 1996; Griffin et al., 1998; Guo et al., 2001; Gao et al., 2002; Zhang et al., 2002, 2003; Chen et al., 2003; Wu et al., 2003; Deng et al., 2004; Xu et al., 2004a,b; Rudnick et al., 2004; Zhang et al., 2004; Zhang, 2005). Studies on diamond-bearing kimberlites and mantle xenoliths indicate a thick ( 200 km) and cold ( 40 mw/m 2 ) /$ - see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.lithos

2 W. Yang, S. Li / Lithos 102 (2008) lithosphere existing in the NCC during the Paleozoic (Fan and Menzies, 1992; Griffin et al., 1992, 1998; Zheng et al., 2003). However, investigations of Cenozoic basaltborne spinel lherzolite xenoliths show that the Cenozoic lithosphere is relatively thinner (b80 km) and hotter ( 60 mw/m 2 ) beneath the eastern NCC (Fan et al., 2000; Zheng et al., 2001). This was also demonstrated by geophysical data (Ma, 1987). Therefore, it is suggested that about 120 km of lithosphere has been removed since the early Paleozoic. In addition, the Paleozoic lithospheric mantle also differs from the Cenozoic one in geochemical characteristics. The former is characterized by EMII isotopic compositions, such as high 206 Pb/ 204 Pb ( 20.2), significant variation of 87 Sr/ 86 Sr, and negative ɛ Nd ( 5) (Zheng and Lu, 1999; Zhang et al., 2002), while the Cenozoic lithospheric mantle shows Sr Nd Pb isotopic compositions similar to the mid-ocean ridge basalt (MORB) and ocean island basalt (OIB) (Peng et al., 1986; Song et al., 1990; Basu et al., 1991). Apparently, the geochemical features of the lithospheric mantle in eastern China have been significantly changed during the evolution from the Paleozoic to the Cenozoic. The reason for the removal and replacement of the Paleozoic lithospheric mantle has not been well understood yet. Possible mechanisms include destabilization of the NCC due to the Indo-Eurasian collision (Menzies et al., 1993), mechanical chemical erosion and replacement by asthenosphere upwelling (Menzies and Xu, 1998; Xu, 2001; Xu et al., 2004a,b), delamination and foundering of thickened lower continental crust (Gao et al., 2004; Wu et al., 2005), destruction of the lithosphere due to the subduction of oceanic crust in the Paleozoic and continental crust in the Mesozoic beneath both the northern and southern margins of the NCC (Zhang et al., 2003), and hydro-weakening of the sub-continental lithospheric mantle (SCLM) due to migratory or slabderived fluids (Niu, 2005). In addition, the mantle sources of the Mesozoic basalts from the NCC are highly heterogeneous, with negative ɛ Nd (up to 20), variable 87 Sr/ 86 Sr i,unradiogenicpb isotope ratios, and typical continental geochemical signatures such as enrichment of large ion lithophile elements (LILE, e.g., Rb and Ba) and depletion of high field strength element (HFSE, e.g., Nb and Ta) (Qiou et al., 1997; Fan et al., 2001; Guo et al., 2001; Qiou et al., 2002; Zhang and Sun, 2002; Zhang et al., 2002, 2003; Guo et al., 2003; Chen and Zhai, 2003; Li and Yang, 2003; Liu et al., 2004a; Xu et al., 2004a,b; Yang et al., 2004; Zhang et al., 2004, 2005b; Zhang, 2005). These features are not consistent with a source in either the Paleozoic or Cenozoic lithospheric mantle (Peng et al., 1986; Song et al., 1990; Basu et al., 1991; Zhang et al., 2002). The petrogenesis of the Mesozoic mantle-derived rocks is still controversial. It has been generally suggested that such continental geochemical signatures were derived from an enriched SCLM or hybridized upwelling asthenosphere and three models have been proposed. The first model considers that the SCLM has an EMI-type composition resulting from subduction-related multiple metasomatism processes in the Archaean and Mesoproterozoic during the accretion of the NCC (e.g., Yang et al., 2004; Ma and Xu, 2006). The second model suggests that the Mesozoic SCLM has been modified by a Si Al enriched melt from partial melting of deeply subducted crustal materials from the South China block (SCB) (Zhang et al., 2002, 2003). The third model proposes that the Mesozoic SCLM was formed by hybridization of the upwelling asthenospheric mantle and SiO 2 -rich melts from partial melting of the foundered mafic lower continental crust (Gao et al., 2004; Lustrino, 2005; Huang et al., 2007a,b). Mesozoic volcanic rocks with variable ages are widely developed in Western Liaoning, the north margin of the NCC (Chen et al., 1997, 1999). Four major periods of volcanism have been identified by stratigraphic studies (Chen et al., 1997; Wang et al., 1989): the early Jurassic (Xinglonggou Formation), the mid-jurassic (Lanqi Formation in Western Liaoning or Tiaojishan Formation in Northern Hebei), the early Cretaceous (Yixian Formation), and the late early Cretaceous (Zhanglaogongtun Formation) (Table 1). Geochronological and geochemical studies of these Jurassic Cretaceous rocks provide an excellent opportunity to probe the evolution of the underlying lithospheric mantle and to give constraints on the NCC lithospheric thinning process. Previous studies have mainly focused on the origin of the Mesozoic volcanic rocks in Western Liaoning (e.g., Chen et al., 1997; Li et al., 2001; Shao et al., 2001; Li et al., 2002; Zhang et al., 2003; Gao et al., 2004; Wang et al., 2005; Zhang and Zhang, 2005; Zhang et al., 2005a; Li, 2006). However, more data are still required, because (1) only the timing of the Yixian Formation ( Ma) has been well dated by both U Pb and Ar Ar methods (Swisher et al., 1999, 2001; Wang et al., 2001a,b; Zhou et al., 2003; Ji et al., 2004; Yang et al., 2007) due to the discovery of the famous Jehol biota in the formation (Hou et al., 1995; Hou, 1996; Chen et al., 1998; Ji et al., 1998), (2) previous geochemical studies mainly focused on the high-sr, low-y andesites, but neglected the basalts (e.g., Chen et al., 1997; Li et al., 2001, 2002; Gao et al., 2004;Wang et al., 2005; Zhang and Zhang, 2005; Zhangetal.,2005a;Li,2006), and (3) the published geochemical data have not been well related to the regional tectonic evolution, which is critical in discussion of the mechanism of the lithospheric thinning.

3 90 W. Yang, S. Li / Lithos 102 (2008) Table 1 A summary table showing the strata units in Western Liaoning, volcanic rock types, ages, geochemical characteristics, tectonic settings and interpretations Strata The Sunjiawan Formation The Zhanglaogongtun Formation The Fuxin Formation The Jiufotang Formation The Yixian Formation The Tuchengzi Formation The Lanqi (Tiaojishan) Formation The Haifanggou Formation The Beipiao Formation The Xinglonggou Formation Isotopic age range (Ma) Rock types Geochemical characteristics ca. 106 Basalt Alkaline basalts with MORB-like Sr Nd Pb isotopic ratios Basalt, basaltic andesite, andesite and rhyolite Basalt, basaltic andesite, andesite and rhyolite. Basalts and basaltic andesites show the typical continent geochemical signatures and relative higher Mg, Ni and Cr contents. Andesites are high-mg# adakites with the low crustal Sr Nd Pb isotopic compositions. Basalts and basaltic andesites show the typical continent geochemical signatures but low Mg, Ni and Cr contents. Low Mg andesites with crustal Sr Nd Pb isotopic compositions ca. 177 andesite and dacites High-Mg# adakites with arc-like Sr Nd Pb isotopic compositions Tectonics The strike-slip Tan-Lu fault was transformed into an extensional graben. Large sale strike-slip of the Tan-Lu fault, which caused the lithosphere destruction and pull apart basin. Pre-135 Ma thrust tectonics is marked by coarseclastic deposits of the Tuchengzi formation. Pre-160 Ma thrust tectonics is marked by unconformity beneath the Lanqi formation Interpretations Asthenospheric upwelling caused by the large scale E W extension. Foundering of the mafic lower continental crust A magma underplating event with the AFC process. Partial melting of the subducted oceanic crust In the present study we have selected a suite of volcanic rocks from the region for a detailed geochronological and geochemical investigation. Our primary objectives include (1) to date each Mesozoic magmatic event in the region precisely, (2) to characterize the source composition of basalts with variable ages, and (3) to constrain the source and melt generation processes of the high-sr, low-y andesites with variable ages. The new geochronological and geochemical data provide insights into the lithospheric evolution of the north margin of the NCC. Using the present data, together with previously published geochemical and geological results from this area, we propose a multi-stage model to explain the lithospheric thinning process of the NCC. 2. Geological background and petrography The North China Craton is one of the oldest continental nuclei in the world (Jahn et al., 1987; Liu et al., 1992) and the largest cratonic block in China. It is bounded on the south by the Paleozoic to Triassic Qinling Dabie Sulu orogenic belt (Li et al., 1993; Meng and Zhang, 2000) and on the north by the Central Asian Orogenic Belt (Sengör et al., 1999; Davis et al., 2001). The craton is cut by the Tan-Lu Fault Zone, which is a strike-slip fault from the Jurassic to early Cretaceous (Zhu et al., 2001a, 2005) and was transferred into an extensional graben in the later Cretaceous and Tertiary (Zhu et al., 2001b) (Fig. 1a).

4 W. Yang, S. Li / Lithos 102 (2008) The Qinling Dabie Sulu belt resulted from the continental collision between the NCC and the Yangtze Craton in the Triassic (Li et al., 1993). The northward subduction of the Yangtze slab was proposed to have affected the upper mantle beneath the Dabie orogen and the south margin of the NCC and influenced the Mesozoic basaltic magmatism in the south margin of the NCC, such as the post-collisional mafic ultramafic intrusions (Li et al., 1998; Huang et al., 2007a) and the Fangcheng basalts (Zhang et al., 2002). The Central Asian Orogen was formed to the north of the NCC by a southward subduction and an arc arc collision followed by arc continent collision during the Paleozoic (Robinson et al., 1999; Davis et al., 2001; Buchan et al., 2002). After collision of the NCC with the southern Central Asian Orogen at the Solonker suture at the end of the Permian (Xiao et al., 2003), the NCC and the attached southern Central Asian Orogen collided with the northern Central Asian Orogen in the Jurassic (Tomurtogoo et al., 2005). The Solonker suture and the Xilamulun River Fault form the northern boundary of the NCC. The Yanshan belt may be due to this southward subduction and the subsequent collision (Davis et al., 2001). Western Liaoning lies in the east of the Yanshan belt and to the west of the Tan-Lu fault, which is bounded by the Jinxi Yaolugou fault on the south and the Chifeng Kaiyuan fault on the north (Fig. 1b). Voluminous Jurassic Cretaceous volcanic rocks were erupted into a series of small Mesozoic basins in the area. Fig. 1. (a) Simplified geological map of the North China Craton and its surrounding areas (modified after Huang et al., 2004). The distribution of Cenozoic basalts and Archaean terrains in the NCC is after Liu et al. (1994) and Jahn (1990). (b) Distribution of Mesozoic volcanic rocks in Western Liaoning (modified after Zhang et al., 2003). XL-ML Fault, CF-KY Fault and JX-YL Fault represent Xilamulun River Fault, Chifeng Kaiyuan Fault, and Jinxi Yaolugou Fault, respectively.

5 92 W. Yang, S. Li / Lithos 102 (2008) The Xinglonggou lavas occur along a narrow belt in Western Liaoning (Fig. 2a), which consist of high-mg andesites, and dacites, inter-layered with tuffs and sandstones. A rhyolitic dike cutting across the volcano-sedimentary strata has been observed in the Xinglonggou village. Sample XLG-1 is an andesite from the Xinglonggou village, Beipiao City. It is massive, dark gray and contains orthopyroxene phenocrysts. Sample XLG-4 is a light purple tuff with pyroclasts (sodic plagioclase + quartz). The Lanqi lavas, distributed widely in the Beipiao Yixian area (Fig. 2a), mainly consist of basalts, andesites and rhyolites. HFG-13 and HFG-15 are basalts collected from the lower bed of the Lanqi Formation in Haifanggou of Beipiao (N , E ). The samples are massive and dark gray and contain plagioclase and clinopyroxene phenocrysts. Sample LQ- 6, LQ-9, LQ-10, LQ-11, HFG-27, and HFG-29 were collected from the Haifanggou Dalanqi section crossing the Lanqi formation in Beipiao, and samples LQ-15 to LQ-29 were from the Lanqi formation near the Shuiquan village, Beipiao city (Fig. 2a). They consist of basaltic andesite (LQ-6), andesites (LQ-15 to LQ-22), andesite porphyrite (HFG-29), and rhyolites (HFG-27, LQ-9, LQ-10, LQ-11, LQ-27, LQ-28, and LQ-29). The andesites are massive and gray. The rhyolites are red Fig. 2. Geologic map of Beipiao (a), Fuxin. (b) (after LNGMR, 1989) and Daohugou (c) (after Ren et al., 2002). Our samples are from the outcrop of the Xinglonggou Formation to the west of Beipiao, the type section of the Lanqi Formation to the north of Beipiao, the Sihetun type section of the Yixian Formation to the south of Beipiao (a), the Wuhuanchi and Jianguo section of the Zhanglaogongtun Formation to the northeast of Fuxin (b) and the Daohugou section (c), respectively.

6 W. Yang, S. Li / Lithos 102 (2008) brown with flow lines. A tuff sample (DHG-1) and an andesite sample (DHG-2) were collected from the Daohugou section located in the Daohugou area, Ningcheng County, Inner Mongolia (Fig. 2c). The strata in the Daohugou section are correlated with the lower part of the Tiaojishan/Lanqi Formation by means of extensively regional geological survey (Ren et al., 2002; Liu et al., 2004b) and biostratigraphic studies (Shen et al., 2003). The Yixian Formation contains the most voluminous igneous rocks with a thickness of m (Fig. 2a), beginning with basalts and ending with rhyolites (Ji et al., 2004). The samples were collected from the Huangbanjigou section (N , E ), the Sihetun section (N , E ), and the Zhuanchengzi section (N , E ) of the Yixian Formation (Fig. 1). The samples from Huangbanjigou are dark gray basalts containing olivine and plagioclase phenocrysts. Samples SHT-3 and SHT-14 are basalts from Sihetun, dark gray and massive, and they consist of orthopyroxene, plagioclase, and sparse olivine phenocrysts. The andesite samples from Zhuanchengzi are gray but lack of orthopyroxene and plagioclase phenocrysts. The Zhanglaogongtun Formation (Fig. 2b), defined by Wang et al. (1989), consists of basalts and intermediate-acidic volcanic rocks. The basalt samples (JG-1, 2, 3 and WHC-2) of the Zhanglaogongtun Formation were collected from Jianguo (N , E ) and Wuhuanchi (N , E ), respectively. All these rocks are dark gray and massive with well-developed columnar jointing (Zhang et al., 2003). 3. Analytical methods The geochronological study was conducted using the Ar Ar dating method for basalts and the SHRIMP U Pb zircon dating method for andesites and tuffs. For Ar Ar dating, the rocks were crushed and sieved. The rock fractions without olivine phenocrysts (mesh (230 to 380 μm)) were selected by handpicking, and then the selected sample was washed for several times in distilled water in an ultrasonic cleaner. Only fresh groundmass was separated from cleaned fractions. The groundmass samples were irradiated in a fast neutron flux at the Chinese Academy of Atomic Energy. After 3 months, the irradiated samples were incrementally Fig Ar/ 39 Ar age spectrum and isochron plots for sample HFG-13 (a and b) and WHC-2 (c and d).

7 94 W. Yang, S. Li / Lithos 102 (2008) heated at the temperatures of C by steps in a high-vacuum argon extraction system at the Laboratory of Ar Ar Dating, Institute of Geology and Geophysics, the Chinese Academy of Sciences (IGGCAS). Then, the purified argon was analyzed on the VG-5400 gas mass spectrometer. The results of 40 Ar/ 39 Ar dating are presented in Supplementary Table 1andFig. 3. Zircons from the tuff and andesite samples were separated using gravitational and magnetic sorting, and then they were handpicked under a binocular microscope. Measurements of U, Th, and Pb of zircons were conducted using the SHRIMP II ion microprobe at the Beijing Center of Ion Microprobe Analysis, China, following the procedure outlined by Williams and Claesson (1987) and Song et al. (2002).The 206 Pb/ 238 U ratios were corrected using the zircon standards of CL13 (572 Ma) from Cililanca and TEM (417 Ma) from Australia. The analytical spot size is 40 μm inaverage diameter during each analytical run. Each spot was rastered over 80 μm for 3 min prior to analysis (5 mass scans) to remove common Pb on the surface or contamination from the gold coating. One spot on the standard zircon (TEM) was analyzed after every three analyses of sample-spots. Squid and Isoplot programs of Ludwig (2001) were used for data processing and age calculation. The final age results are the weighted mean of the 206 Pb/ 238 U ages because 206 Pb/ 238 Uagesare more precise than 207 Pb/ 235 U ages and 207 Pb/ 206 Pb ages for young zircons. The common lead is corrected by assuming 206 Pb/ 238 U 208 Pb/ 232 Th age-concordance. Errors on individual spots are based on counting statistics and are at the 1σ level, but the average weighted ages are quoted at 2σ or 95% confidence. The analytical results are listed in Supplementary Table 2 and plotted on the concordia diagrams in Fig. 4. Major elements compositions of all samples except XLG-4 were determined using the Vista ICP-AES at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIGCAS). The major elements compositions (Supplementary Table 3) of one tuff sample (XLG-4) for zircon U Pb dating were determined using wet chemical techniques at the Hebei Institute of Regional Geology and Mineral Resources. Analytical uncertainties for the majority of major elements were estimated to be less than 1%. Wholerock trace elements were analyzed with the inductively coupled plasma-mass spectrometry (ICP-MS) at the GIGCAS. The whole-rock powders (50 mg) were dissolved in HF+HNO 3 in 15 ml Teflon screw-cap capsules at 100 C for 1 day, dried to expel most of silica and then dissolved with HF+HNO 3 at 100 C again for 7 10 days till completely dissolved. Dissolved samples weredilutedto50mlusing1%hno 3 before ICP-MS analyses. Six standards (GSR-1, GSR-2, GSR-3, MRG- 1, W-2, and AGV-2) were analyzed using the same procedure to monitor the analytical reproducibility. The measured values of the standards are in satisfactory agreement with the reference values (Supplementary Table 4). Analytical procedures and precision have been described in detail in Liu et al. (1996). Themajorand trace elements data are listed in Table 2. About mg whole rock powder was completely decomposed in a mixture of HF-HClO 4 for Sr Nd isotopic analysis and in a mixture of HF- HNO 3 for Pb isotopic analysis. For Rb Sr and Sm Nd isotope analyses, sample powders were spiked with mixed isotope tracers, dissolvedintefloncapsuleswith HF+HNO 3 at 100 C for 7 10 days. For Pb isotope determination, powder was weighed into the Teflon capsules and dissolved in HF+HNO 3 at 100 C for 7 10 days. Sr and rare earth elements (REE) were separated on quartz columns with a 5 ml resin bed of AG 50 W-X12 ( mesh). Nd was separated from other REEs on quartz columns using 1.7 ml Teflon powder as cation exchange medium. Pb was separated on Teflon columns containing 80 μl AG1-X8, meshbyemployingHBr HCl wash and elution procedure. Procedural blanks were b200pgforsrand b50 pg for Pb and Nd. For the measurement of isotopic compositions, Pb was loaded with a mixture of Si-gel and H 3 PO 4 onto a single-re filament and measured at 1300 C; Sr was loaded with a Ta-HF activator on a single W filament; and Nd was loaded as phosphates and measured in a Re-double-filament configuration. 143 Nd/ 144 Nd ratios were normalized to 146 Nd/ 144 Nd= and 87 Sr/ 86 Sr ratios to 86 Sr/ 88 Sr= Pb standard NBS 981 was used to determine thermal fractionation and measured isotopic ratios of samples were corrected with a value of 0.1% per atomic mass unit. Sr Nd Pb isotopic ratios were measured on a Finnigan MAT-262 thermal ionization mass spectrometer (TIMS) in the Laboratory for Radiogenic Isotope Geochemistry, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing. Raw data obtained were calculated using the Isoplot program (Ludwig, 2001), giving 2σ m error. Analyses of standards during the period of analysis are as follows: NBS987 gave 87 Sr/ 86 Sr= ±12 (n =27, 2σ); JMC and AMES gave 143 Nd/ 144 Nd = ± 7 (n =8,2σ) and ±12 (n =15,2σ), respectively. Details of chemical separation and measurement are described in Chen et al. (2000, 2002). TheSr Nd Pb isotopic data are listed in Table 3.

8 W. Yang, S. Li / Lithos 102 (2008) Results Since the Yixian Formation (126 Ma 120 Ma) has been well dated by zircon U Pb and Ar Ar methods as mentioned above (Swisher et al., 1999, 2001; Wang et al., 2001a,b; Ji et al., 2004; Yang et al., 2007), the geochronological study of this paper only focuses on the timing of the Xinglonggou, Lanqi and Zhanglaogongtun Formations. Fig. 4. U Pb zircon concordia diagrams for samples XLG-4 (a), DHG-1 (c), DHG-2 (e), HFG-27 (g) and HFG-29 (h) and cathodoluminescence (CL) images of the zircons in XLG-4 (b), DHG-1 (d) and DHG-2 (f). The excluded spot analyses are shown with shading error ellipses in (a) and (c). The circles in the CL images indicate the positions of the analyzed spots. The zircons in HFG-27 and HFG-29 have very high U and Th contents (Supplementary Table 2) and cannot be displayed in CL images.

9 Table 2 Major oxides (wt.%) and trace elements (ppm) of the Mesozoic volcanic lavas from Western Liaoning Xinglonggou Formation Lanqi Formation XLG-1 HFG-13 HFG-15 LQ-6 LQ-9 LQ-10 LQ-11 LQ-15 LQ-16 LQ-17 Andesite Basalt Basalt Basaltic andesite Rhyolite Rhyolite Rhyolite Andesite Andesite Andesite Sample site N E N E SiO TiO Al 2 O Fe 2 O MnO MgO CaO Na 2 O K 2 O P 2 O LOI Total Mg# Sc Cr Ni Rb Sr Y Zr Nb Ba Hf Ta Pb Th U La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb W. Yang, S. Li / Lithos 102 (2008)

10 Lu Sr/Y Ce/Pb (La/Yb) N Lanqi Formation Yixian Formation LQ-18 LQ-19 LQ-20 LQ-21 LQ-22 LQ-27 LQ-28 LQ-29 HBJ4-1 HBJ4-2 Andesite Andesite Andesite Andesite Andesite Rhyolite Rhyolite Rhyolite basalt basalt Sample site N E SiO TiO Al 2 O Fe 2 O MnO MgO CaO Na 2 O K 2 O P 2 O LOI Total Mg# Sc Cr Ni Rb Sr Y Zr Nb Ba Hf Ta Pb Th U La Ce Pr Nd Sm W. Yang, S. Li / Lithos 102 (2008) (continued on next page) 97

11 Table 2 (continued ) Sample site Lanqi Formation Yixian Formation LQ-18 LQ-19 LQ-20 LQ-21 LQ-22 LQ-27 LQ-28 LQ-29 HBJ4-1 HBJ4-2 Andesite Andesite Andesite Andesite Andesite Rhyolite Rhyolite Rhyolite basalt basalt N E Eu Gd Tb Dy Ho Er Tm Yb Lu Sr/Y Ce/Pb (La/Yb) N Yixian Formation Zhanglaogongtun Formation HBJ4-3 SHT-14 SHT-3 ZCZ1-1 ZCZ1-2 ZCZ1-3 ZCZ1-4 JG-1 JG-2 JG-3 Basalt Basaltic andesite Basaltic andesite Andesite Andesite Andesite Andesite Basalt Basalt Basalt Sample site N E N E N E N E SiO TiO Al 2 O Fe 2 O MnO MgO CaO Na 2 O K 2 O P 2 O LOI Total Mg# Sc Cr Ni Rb Sr Y W. Yang, S. Li / Lithos 102 (2008)

12 Zr Nb Ba Hf Ta Pb Th U La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sr/Y Ce/Pb (La/Yb) N Zhanglaogongtun Formation JG-4 JG-5 WHC-1 WHC-2 WHC-3 WHC-4 Basalt Basalt Basalt Basalt Basalt Basalt Sample site N E N E SiO TiO Al 2 O Fe 2 O MnO MgO CaO Na 2 O K 2 O P 2 O LOI Total Mg# (continued on next page) W. Yang, S. Li / Lithos 102 (2008)

13 Table 2 (continued ) Zhanglaogongtun Formation JG-4 JG-5 WHC-1 WHC-2 WHC-3 WHC-4 Basalt Basalt Basalt Basalt Basalt Basalt Sample N E N E Sc Cr Ni Rb Sr Y Zr Nb Ba Hf Ta Pb Th U La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sr/Y Ce/Pb (La/Yb) N LOI = loss on ignition; Mg# =Mg/(Mg+TFeO) atomic ratio. stands for non-detective. 100 W. Yang, S. Li / Lithos 102 (2008)

14 W. Yang, S. Li / Lithos 102 (2008) Geochronology The Xinglonggou Formation Fig. 4a show the SHRIMP U Th Pb analytic results on 15 spots of zircons from the tuff sample (XLG-4). The major element compositions (Supplementary Table 2) of the tuff sample (XLG-4) are similar to those of granites or rhyolites, suggesting that the tuff mainly consists of volcanic materials, while its high Fe 2 O 3 /FeO and HO 2 + contents indicate its surface deposit environment. The analysis of XLG-4-13 gives an apparently higher 206 Pb/ 238 Pb age (240.6 ±3.4 Ma) and U (206 ppm), Th (149 ppm) contents than those given by other zircons (Supplementary Table 2), suggesting that this zircon grain is different in origin from other zircons. The cathodoluminescence (CL) images also show the deference between zircon grain XLG-4-13 and the others, i.e., the former is characterized by oscillatory zoning with no fan-structure and others are characterized by oscillatory zoning with well-developed fan-structure (Fig. 4b). The analysis of XLG-4-16 also gave an apparently higher 206 Pb/ 238 Pb age (205.7 ±7.3 Ma) than those of other zircons. The age of XLG-4-16 could be in error because of its very low radiogenic Pb content (0.7 ppm) (Supplementary Table 2). Therefore, the analyses of XLG4-13 and XLG4-16 are excluded from the calculation of the weighted mean age. The remaining 13 analyses give a weighted mean 206 Pb/ 238 Pb age of 176.7±3.5 Ma. This age is slightly younger than the previous reported Ar Ar ages of Ma for the Xinglonggou andesites (Chen et al., 1997), but significantly older than the U Pb SHRIMP age of the Xinglonggou rhyolites (159 ± 3 Ma) reported by Gao et al. (2004) The Lanqi Formation Five samples (DHG-1, DHG-2, HFG-13, HFG-27, and HFG-29) from this formation have been dated. Fig. 4c show the SHRIMP U Th Pb analytic results on 10 spots of zircons from the tuff sample (DHG-1), which given an age of 164.1±2.4 with a relative high MSWD (mean square of weighted deviation) value of 3.5. Since the analysis of DHG1-9 gives an apparently lower 206 Pb/ 238 Pb age than those given by other zircons, when it was excluded from the calculation of the weighted mean age, the remaining 9 analyses give a weighted mean 206 Pb/ 238 Pb age of 165.0±1.2 Ma with a relative smaller MSWD value of 1.5. Fig. 4e shows the SHRIMP results on 10 spots of zircons from the andesite sample (DHG-2). They yield a mean age of 164.3±2.2 Ma. This age is consistent with the SHRIMP zircon U Pb age of 165.0±1.2 Ma for sample DHG-1 and the Ar Ar ages of 165 to 159 Ma for samples from the same locality (Chen et al., 2004; He et al., 2004). Fig. 3a b shows the 40 Ar/ 39 Ar spectra with plateau age and isochron plot of basalt sample HFG-13. All errors are reported at the 2σ level. This sample yields the 40 Ar/ 39 Ar plateau age of 166.1±0.9 Ma, which is defined by 92.2% of total released 39 Ar. The isochron diagram (Fig. 3b) indicates an age of 166.7±2.9 Ma and an initial 40 Ar/ 36 Ar ratio of 290.9±16.5, which are consistent with its plateau age and the present atmospheric 40 Ar/ 36 Ar ratio (295.5), suggesting that excess argon is insignificant and the plateau or isochron age is reliable. The analytical results on 11 spots of zircons from rhyolite sample HFG-27 are shown on Fig. 4g. They yield a weighted mean 206 Pb/ 238 Pb age of 160±6 Ma. The analytical results on 16 spots of zircons from andesite porphyrite HFG-29 (intruded into the Lanqi Formation) are shown on Fig. 4h. They give a mean age of 153±2 Ma, which may indicate the last activity of the Lanqi magmatism The Zhanglaogongtun Formation Fig. 3c d shows the 40 Ar/ 39 Ar spectra with plateau age and isochron plot of WHC-2. The sample yields the 40 Ar/ 39 Ar plateau age of 106.1±0.8 Ma, which is defined by 89.1% of total released 39 Ar. The isochron diagram (Fig. 3d) indicates an age of ± 1.3 Ma and an initial 40 Ar/ 36 Ar ratio of 306.6±21.5, consistent with its plateau age and the present atmospheric 40 Ar/ 36 Ar ratio (295.5). This suggests that the excess argon is minimal and the plateau age is reliable. The plateau age is consistent with the previously reported Ar Ar and K Ar ages of ca Ma (Zhang et al., 2003; Zhu et al., 2002) Geochemistry Xinglonggou andesite Because geochemistry of the Xinglonggou andesite has been well studied (Gao et al., 2004; Li, 2006), only one Xinglonggou andesite sample (XLG-1) was analyzed for major-trace element and Sr Nd Pb isotopic compositions. Sample XLG-1 is characterized by high SiO 2 (63.56 wt.%) and Al 2 O 3 (14.94 wt.%), high Mg # (57), Cr (156 ppm), and Ni (82.2 ppm) contents (Table 2). It is also enriched in light rare-earth elements (LREE), LILE, and Pb, and depleted in HFSE, heavy rare-earth elements (HREE), and Y. It has high Sr/Y (36.3) and (La/Yb) N (18.6, where subscript N denotes C1-chondrite normalization) without negative Eu anomaly (Table 2; Fig. 6). This sample has moderate 87 Sr/ 86 Sr ( ), slightly

15 Yixian Fm. ɛnd(t) 206 Pb/ 204 Pb 207 Pb/ 204 Pb 208 Pb/ 204 Pb 206 Pb/ 204 Pb (t) 207 Pb/ 204 Pb (t) Pb/ 204 Pb (t) Table 3 Sr Nd Pb isotopes of the Mesozoic volcanic rocks from Western Liaoning 87 Rb/ 86 Sr 87 Sr/ 86 Sr 87 Sr/ 86 Sr (t) 147 Sm/ 144 Nd 143 Nd/ 144 Nd 143 Nd/ 144 Nd (t) Xinglonggou Fm. XLG- 1 Lanqi Fm. HFG- 13 HFG- 15 LQ LQ LQ LQ LQ LQ LQ LQ LQ LQ LQ LQ LQ LQ HBJ HBJ HBJ W. Yang, S. Li / Lithos 102 (2008)

16 Zhanglao-gongtun Fm. 87 Rb/ 86 Sr 87 Sr/ 86 Sr 87 Sr/ 86 Sr (t) 147 Sm/ 144 Nd 143 Nd/ 144 Nd 143 Nd/ 144 Nd (t) ɛnd(t) 206 Pb/ 204 Pb 207 Pb/ 204 Pb 208 Pb/ 204 Pb 206 Pb/ 204 Pb (t) 207 Pb/ 204 Pb (t) 208 Pb/ 204 Pb (t) SHT SHT ZCZ ZCZ ZCZ ZCZ JG JG JG JG JG WHC WHC WHC WHC Chondrite Uniform Reservoir (CHUR) values ( 87 Rb/ 86 Sr =0.0847, 87 Sr/ 86 Sr = , 147 Sm/ 144 Nd = , 143 Nd/ 144 Nd = ) are used for the calculation. λ Rb = year 1, λ Sm = year 1, λ U238 = year 1, λ U235 = year 1, λ Th232 = year 1 (Steiger and Jager, 1977; Lugmair and Marti, 1978). Initial isotopic ratios were calculated by using 177 Ma, 160 Ma, 125 Ma and 106 Ma for the Xinglonggou, Lanqi, Yixian and Zhanglaogongtun lavas, respectively. W. Yang, S. Li / Lithos 102 (2008)

17 104 W. Yang, S. Li / Lithos 102 (2008) negative ɛ Nd (T) ( 1.5), and radiogenic Pb isotopic compositions ( 206 Pb/ 207 Pb=18.211, 207 Pb/ 204 Pb= , and 208 Pb/ 204 Pb =38.176) (Table 3). These geochemical features are consistent with those of the Xinglonggou andesites reported by Gao et al. (2004) and Li (2006).

18 W. Yang, S. Li / Lithos 102 (2008) Fig. 6. Primitive mantle-normalized trace element diagrams for the Mesozoic volcanic rocks. Dash lines represent basalts and basaltic andesites, while solid lines represent andesites in figure b and c. Normalization values for primitive mantle are from Sun and McDonough (1989) Lanqi lavas The Lanqi basalts and basaltic andesites have SiO 2 of wt.%, high Al 2 O 3 (N16.5 wt.%) and CaO (N6.5 wt.%), low MgO (b3.5 wt.%), Cr (b6 ppm), and Ni (b10 ppm) contents (Table 2). They are enriched in LREE and Rb, Ba, Sr, Pb, and depleted in high field strength element, Th, and U (see dash line in Fig. 6b). The two basaltic samples (HFG-13, 15) have moderate 87 Sr/ 86 Sr ( 0.706), low ɛ Nd (t) ( 12), and unradiogenic Pb ( 206 Pb/ 207 Pbb16.5, 207 Pb/ 204 Pbb15.3, and 208 Pb/ 204 Pbb 36.5) (Table 3). The isotopic compositions are similar to the EMI-like component (Zindler and Hart, 1986). The Lanqi andesites and rhyolites are characterized by SiO 2 of wt.%, high Al 2 O 3 (N16 wt.%), and low MgO (b2.5 wt.%), Cr (b1 ppm), and Ni (b7 ppm) concentrations (Table 2). These rocks are enriched in LREE, LILE, and Pb, and depleted in HFSE with negative Sr and Eu anomalies (Fig. 6b solid line). They have moderate 87 Sr/ 86 Sr ( ), negative ɛ Nd (T) ( 13 10), and unradiogenic Pb ( 206 Pb/ 207 Pbb16.6, 207 Pb/ 204 Pbb15.4, and 208 Pb/ 204 Pbb36.8) (Table 3). These isotopic features are similar to the Lanqi basalts, but different with the Xinglonggou andesites. The Lanqi andesites and rhyolites have relatively low MgO but high Al 2 O 3 contents, distinct from the Xinglonggou and the Yixian andesites (Fig. 5) Yixian lavas The samples from the Yixian Formation exhibit a wide compositional range with SiO 2 varying from 52 to 62 wt.%. CaO, Al 2 O 3, TFe 2 O 3, MgO, and P 2 O 5 are Fig. 5. Major oxide variations in the Mesozoic volcanic rocks in Western Liaoning. Data source: the Xinglonggou formation, Gao et al. (2004), Li (2006) and this paper; Lanqi formation, Li et al. (2004) and this paper; Yixian, Ji et al. (2004), Wang et al. (2005), and this paper; Zhanglaogongtun formation, Zhang et al. (2003) and this paper. Solid symbols and open symbols represent data from this paper and from the literature, respectively. Classification of volcanic rocks is based on the total alkali silica diagram of Le Maitre et al. (1989). The Mesozoic volcanic rocks are alkaline and consist of trachy basalt, basaltic andesite, andesite and trachyte except some samples from the Xinglonggou formation. The Lanqi lavas have low MgO and high Al 2 O 3 contents relative to the others.

19 106 W. Yang, S. Li / Lithos 102 (2008) negatively correlated with SiO 2 (Fig. 5). They have enrichments in LREE, LILE, and Pb, and depletion in HFSE. The geochemical characteristics of the Yixian basalts and basaltic andesites are similar to the Lanqi basaltic rocks except for the higher Ni (N95 ppm) and Cr (N170 ppm) contents in the Yixian basalts. The Yixian andesites have SiO 2 %N56 wt.%, Al 3 O 2 %N 15 wt.%, MgOb3 wt.% without negative Eu anomaly. The high Mg # (N54) and Cr (N200 ppm) and Ni (N100 ppm), high Sr (N400 ppm) and Sr/Y (N40), low Yb (b18 ppm) and Y (b1.9 ppm) contents indicate that the Yixian andesites are typical high-mg# adakites. They also show positive Pb and Sr anomalies on the spider diagram (Fig. 6c). The Yixian andesites have moderate 87 Sr/ 86 Sr ( 0.706), low ɛ Nd (t) ( 10), and unradiogenic Pb ( 206 Pb/ 207 Pbb16.6, 207 Pb/ 204 Pbb15.3, and 208 Pb/ 204 Pbb36.8), similar to those of the Lanqi andesites (Fig. 7) Zhanglaogongtun lavas The Zhanglaogongtun lavas display a limited compositional range with low SiO 2 content between 42 and 45 wt.%. They have relatively high alkalis (Na 2 O+ K 2 ON4 wt.%), MgO, TFeO, CaO, TiO 2, MnO, and transitional metal elements (Sc, Cr, Ni) contents (Fig. 5). They are also enriched in LREE and LILE, but without depletion of HFSE. The positive Nb, Ta anomalies and negative Pb anomalies on the spider diagram distinguish them from the Lanqi and Yixian lavas. In addition, they have low 87 Sr/ 86 Sr i (b0.704), high ɛ Nd (T) (N4), and radiogenic Pb ( 206 Pb/ 207 PbN17.8, 207 Pb/ 204 Pb N15.4, and 208 Pb/ 204 PbN37.6), which are similar to the Cenozoic basalts from the NCC and oceanic island basalts (Zhou and Armstrong, 1982; Peng et al., 1986; Basu et al., 1991). 5. Discussion 5.1. Geochronology of the volcanic rocks Fig. 7. Initial Sr Nd Pb isotopic compositions of the Mesozoic volcanic rocks in Western Liaoning. Data source: lower and upper crust is after Jahn et al. (1999); MORB, Hofmann (1997); N-MORB, Zindler and Hart (1986); marine sediments/upper crust, White (2005); NHRL stands for Northern Hemisphere Reference Line (Hart, 1984). ( 207 Pb/ 204 Pb) NHRL B= ( 206 Pb/ 204 Pb) ; ( 208 Pb/ 204 Pb) NHRL =1.209 ( 206 Pb/ 204 Pb) ; the Xinglonggou formation: Gao et al. (2004), Li (2006), and this paper; Lanqi formation, Li et al. (2004) and this paper; Yixian, Ji et al. (2004) and this paper; Zhanglaogongtun formation, Zhang et al. (2003) and this paper. Solid symbols and open symbols represent data from this paper and from the literature, respectively. Initial isotopic ratios were calculated by using 177 Ma, 160 Ma, 125 Ma, and 106 Ma for the Xinglonggou, Lanqi, Yixian, and Zhanglaogongtun lavas, respectively The Xinglonggou Formation The age of the Xinglonggou Formation is controversial. Chen et al. (1997) reported Ar Ar ages of Ma for the Xinglonggou andesites, while Gao et al. (2004) reported a SHRIMP U Pb zircon age of 159± 3 Ma for a rhyolite sample from the Xinglonggou village. The Ar Ar plateau ages of Ma (Chen et al., 1997) are not reliable because the sample has experienced significant alteration (Loss on Ignition = 8.3%) and the age is defined by only 40% of total released 39 Ar. The age of 159 Ma reported by Gao et al. (2004) contradict with the plant fossils recovered from the Beipiao Formation, which overlies the Xinglonggou Formation and points to an early Jurassic age (Li, 2006). Actually, the age of 159±3 Ma for Xinglonggou rhyolite is consistent with the age of 160±6 Ma for the Lanqi rhyolite reported in this paper. Furthermore, a recent field investigation demonstrates that the age of 159±3 Ma reported by Gao et al. (2004) may indicate the intrusion time of the rhyolitic dike in the Xinglonggou village, as their samples were actually collected from a rhyolitic dike rather than any representative rocks from the Xinglonggou Formation. Therefore, the SHRIMP U Pb zircon age of 176.7±3.5 Ma reported in this paper is a more reasonable age for the Xinglonggou Formation than the literature ages.

20 W. Yang, S. Li / Lithos 102 (2008) The Lanqi Formation Our results indicate that the Lanqi lavas erupted from 166 to 153 Ma. These ages are consistent with the previous reported ages of the Lanqi/Tiaojishan Formation. Zhao et al. (2004) have concluded that the main volcanic section of the Tiaojishan and Lanqi Formations ranges in age from 165 to 156 Ma based on the published SHRIMP U Pb and Ar Ar ages. Davis (2005) suggested that the range of recently published 40 Ar/ 39 Ar ages is considerably wider and concluded that the age of the Lanqi/Tiaojishan Formation is between 175 and 148 Ma. Two of their published ages ( Ma) are apparently higher than the others ( Ma). Their older ages were obtained from an andesite that lies unconformably above the Paleozoic or Proterozoic strata and they are not surely correlated with the Tiaojishan and Lanqi formations. Collectively, these ages indicate an age range of ca Ma for the Tiaojishan and Lanqi Formations The Zhanglaogongtun Formation The Ar Ar plateau age of ca. 106 Ma and the previous reported Ar Ar and K Ar ages of ca Ma (Zhang et al., 2003; Zhu et al., 2002) suggest that the age of this formation is between 109 and 93 Ma Origin of the Xinglonggou andesites The Xinglonggou Formation (ca. 177 Ma) consists mainly of andesites with geochemical features similar to those of typical adakites: high SiO 2 (N56 wt.%), Al 2 O 3 (N15 wt.%), and Sr contents (N400 ppm), high Sr/Y (N40) and La/Yb (N20), and low Yb (b1.9 ppm) and Y contents (b19 ppm) (Defant and Drummond, 1990; Kay and Kay, 1993; Martin, 1999; Martin et al., 2005). As shown in Fig. 8, the Xinglonggou andesites have high Sr/Y and low Y content similar to other Mesozoic adakites observed in Eastern China, such as Ningzhen (Xu et al., 2002a), Xuzhou Suzhou (Xu et al., 2006), and low-mg adakites from the Dabie orogen (Wang et al., 2007; Xu et al., 2007). The high Mg# (57) of the Xinglonggou andesite indicates that it is high-mg# adakite. Adakites were originally considered to be produced by partial melting of young and hot subducted oceanic slabs (Defant and Drummond, 1990). However, alternative petrogenetic processes could also produce adakitic rocks, e.g., fractional crystallization of basaltic magmas and assimilation of felsic crustal materials (Castillo et al., 1999), basaltic and felsic magma mixing (e.g., Guo et al., 2007), and partial melting of mafic continental lower crust that has foundered into the Fig. 8. Sr/Y versus Y diagram after Defant et al. (2002), where the adakite area is defined by Sr/YN40 (Martin, 1999; Martin et al., 2005). Most Xinglonggou and Yixian samples have high Sr/Y and La/Yb but low Y contents similar to typical adakites, while the Lanqi andesites have relatively higher Y content and lower Sr/Y. Data source: the Xinglonggou formation: Gao et al. (2004), Li (2006), and this paper; Lanqi formation, this paper; Yixian, Ji et al. (2004) and Wang et al. (2005) and this paper; Zhanglaogongtun formation, Zhang et al. (2003) and this paper. Solid symbols and open symbols represent data from this paper and from the literature, respectively. convecting mantle (e.g., Xu et al., 2002a; Chung et al., 2003; Gao et al., 2004). Experimental studies of partial melting of basalts at high pressure (N1.2 GPa) show that garnet (not plagioclase) can occur as a liquidus phase or residual mineral in equilibrium with high SiO 2 and Al 2 O 3 adakitic melt, resulting in the high Sr and LREE, but low HREE and Y contents which are characteristic features of adakitic magma (e.g., Rapp and Watson, 1995; Rapp et al., 1999). The origin of the Xinglonggou andesites is still in debate. Because of a lack of mafic rocks in the Xinglonggou Formation and the high Ni + Cr contents of the Xinglonggou andesites, it is unlikely that the Xinglonggou andesites were produced by assimilation and fractional crystallization (AFC) processes involving basaltic magmas or mixing between basaltic and felsic magmas (Gao et al., 2004). Gao et al. (2004) suggested that the Xinglonggou high-mg# adakites were produced by partial melting of foundered lower continental crust, based on the abundant inherited Archaean zircons in these rocks. However, the following several lines of evidence argue against the generation of the Xinglonggou high- Mg# adakite by partial melting of the foundered lower continental crust. First, the radiogenic Pb isotopic composition of the Xinglonggou high-mg# adakites reported in Li (2006) and this study may suggest a derivation from partial melting of subducted oceanic crust, instead of lower continental crust. As shown in