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Lithos 102 (2008) 88–117 www.elsevier.com/locate/lithos

Geochronology and of the Mesozoic volcanic rocks in Western : Implications for lithospheric thinning of the North Craton ⁎ Wei Yang, Shuguang Li

CAS Key laboratory of - Materials and Environments, School of and Space Sciences, University of Science and Technology of China, , 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 . The typical “continental” geochemical signatures of the Lanqi and basaltic 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 . These features, coupled with the low Mg, Ni, and Cr contents may suggest significant olivine fractionation and a magma underplating event, which caused the 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 mantle hybridized by partial melts from the foundered lower . 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 - “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 -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 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 (Yixian Formation), ment by 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, 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 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 , 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. Andesites are high-Mg# and pull apart basin. adakites with the low crustal Sr–Nd–Pb isotopic compositions. The Tuchengzi Pre-135 Ma thrust Formation tectonics is marked by coarseclastic deposits of the Tuchengzi formation. The Lanqi 166–148 Basalt, basaltic Basalts and basaltic andesites A magma (Tiaojishan) Formation andesite, andesite show the typical continent underplating event and rhyolite. geochemical signatures but low with the AFC Mg, Ni and Cr contents. Low Mg process. andesites with crustal Sr–Nd–Pb isotopic compositions The Haifanggou Pre-160 Ma thrust Formation tectonics is marked by unconformity beneath the Lanqi formation The Beipiao Formation The Xinglonggou ca. 177 andesite and dacites High-Mg# adakites with Partial melting of Formation arc-like Sr–Nd–Pb the subducted isotopic compositions oceanic crust

In the present study we have selected a suite of 2. Geological background and petrography volcanic rocks from the region for a detailed geochro- nological and geochemical investigation. Our primary The North China Craton is one of the oldest objectives include (1) to date each Mesozoic magmatic continental nuclei in the world (Jahn et al., 1987; Liu event in the region precisely, (2) to characterize the et al., 1992) and the largest cratonic block in China. It is source composition of basalts with variable ages, and (3) bounded on the south by the Paleozoic to to constrain the source and melt generation processes of Qinling–Dabie–Sulu orogenic belt (Li et al., 1993; the high-Sr, low-Y andesites with variable ages. The Meng and Zhang, 2000) and on the north by the Central new geochronological and geochemical data provide Asian Orogenic Belt (Sengör et al., 1999; Davis et al., insights into the lithospheric evolution of the north 2001). The craton is cut by the Tan-Lu Fault Zone, margin of the NCC. Using the present data, together which is a strike-slip fault from the Jurassic to early with previously published geochemical and geological Cretaceous (Zhu et al., 2001a, 2005) and was transferred results from this area, we propose a multi-stage model to into an extensional graben in the later Cretaceous and explain the lithospheric thinning process of the NCC. Tertiary (Zhu et al., 2001b)(Fig. 1a). W. Yang, S. Li / Lithos 102 (2008) 88–117 91

The Qinling–Dabie–Sulu belt resulted from the southern Central Asian Orogen at the Solonker suture at between the NCC and the Yangtze the end of the (Xiao et al., 2003), the NCC and Craton in the Triassic (Li et al., 1993). The northward the attached southern Central Asian Orogen collided subduction of the Yangtze slab was proposed to have with the northern Central Asian Orogen in the Jurassic affected the upper mantle beneath the Dabie orogen and (Tomurtogoo et al., 2005). The Solonker suture and the the south margin of the NCC and influenced the Xilamulun River Fault form the northern boundary of the Mesozoic basaltic magmatism in the south margin of NCC. The Yanshan belt may be due to this southward the NCC, such as the post-collisional mafic–ultramafic subduction and the subsequent collision (Davis et al., intrusions (Li et al., 1998; Huang et al., 2007a) and the 2001). Fangcheng basalts (Zhang et al., 2002). Western Liaoning lies in the east of the Yanshan belt The Central Asian Orogen was formed to the north of and to the west of the Tan-Lu fault, which is bounded by the NCC by a southward subduction and an arc–arc the Jinxi–Yaolugou fault on the south and the Chifeng– collision followed by arc–continent collision during the Kaiyuan fault on the north (Fig. 1b). Voluminous Paleozoic (Robinson et al., 1999; Davis et al., 2001; Jurassic–Cretaceous volcanic rocks were erupted into Buchan et al., 2002). After collision of the NCC with the 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. 92 W. Yang, S. Li / Lithos 102 (2008) 88–117

The Xinglonggou lavas occur along a narrow belt in from the lower bed of the Lanqi Formation in Western Liaoning (Fig. 2a), which consist of high-Mg Haifanggou of Beipiao (N 41°50.484′, E 120°46.009′). andesites, and dacites, inter-layered with tuffs and The samples are massive and dark gray and contain sandstones. A rhyolitic dike cutting across the volca- plagioclase and clinopyroxene phenocrysts. Sample LQ- no-sedimentary strata has been observed in the Xin- 6, LQ-9, LQ-10, LQ-11, HFG-27, and HFG-29 were glonggou village. Sample XLG-1 is an andesite from the collected from the Haifanggou–Dalanqi section crossing Xinglonggou village, Beipiao City. It is massive, dark the Lanqi formation in Beipiao, and samples LQ-15 to gray and contains orthopyroxene phenocrysts. Sample LQ-29 were from the Lanqi formation near the Shuiquan XLG-4 is a light purple tuff with pyroclasts (sodic village, Beipiao city (Fig. 2a). They consist of basaltic plagioclase+quartz). andesite (LQ-6), andesites (LQ-15 to LQ-22), andesite The Lanqi lavas, distributed widely in the Beipiao– porphyrite (HFG-29), and rhyolites (HFG-27, LQ-9, Yixian area (Fig. 2a), mainly consist of basalts, andesites LQ-10, LQ-11, LQ-27, LQ-28, and LQ-29). The and rhyolites. HFG-13 and HFG-15 are basalts collected 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. W. Yang, S. Li / Lithos 102 (2008) 88–117 93 brown with flow lines. A tuff sample (DHG-1) and an The Zhanglaogongtun Formation (Fig. 2b), defined andesite sample (DHG-2) were collected from the by Wang et al. (1989), consists of basalts and inter- Daohugou section located in the Daohugou area, mediate-acidic volcanic rocks. The basalt samples (JG-1, Ningcheng County, Inner Mongolia (Fig. 2c). The strata 2, 3 and WHC-2) of the Zhanglaogongtun Formation in the Daohugou section are correlated with the lower were collected from Jianguo (N 42°16′42.9″, E 121°54′ part of the Tiaojishan/Lanqi Formation by means of 15.3″) and Wuhuanchi (N 42°18′29.9″, E 121°53′39.8″), extensively regional geological survey (Ren et al., 2002; respectively. All these rocks are dark gray and massive Liu et al., 2004b) and biostratigraphic studies (Shen et with well-developed columnar jointing (Zhang et al., al., 2003). 2003). The Yixian Formation contains the most voluminous igneous rocks with a thickness of 1000–2000 m (Fig. 2a), 3. Analytical methods beginning with basalts and ending with rhyolites (Ji et al., 2004). The samples were collected from the Huangbanji- The geochronological study was conducted using gou section (N 41°36′52.8″, E 120°48′56.8″), the Sihetun the Ar–Ar dating method for basalts and the SHRIMP section (N 41°34′52.6″, E 120°46′23.4″), and the U–Pb zircon dating method for andesites and tuffs. For Zhuanchengzi section (N 41°42′33.7″, E 121°19′11.9″) Ar–Ar dating, the rocks were crushed and sieved. The of the Yixian Formation (Fig. 1). The samples from rock fractions without olivine phenocrysts (mesh 40– Huangbanjigou are dark gray basalts containing olivine 60 (230 to 380 μm)) were selected by handpicking, and and plagioclase phenocrysts. Samples SHT-3 and SHT-14 then the selected sample was washed for several times are basalts from Sihetun, dark gray and massive, and they in distilled water in an ultrasonic cleaner. Only fresh consist of orthopyroxene, plagioclase, and sparse olivine groundmass was separated from cleaned fractions. The phenocrysts. The andesite samples from Zhuanchengzi groundmass samples were irradiated in a fast neutron are gray but lack of orthopyroxene and plagioclase flux at the Chinese Academy of Atomic Energy. After phenocrysts. 3 months, the irradiated samples were incrementally

Fig. 3. 40Ar/39Ar age spectrum and isochron plots for sample HFG-13 (a and b) and WHC-2 (c and d). 94 W. Yang, S. Li / Lithos 102 (2008) 88–117 heated at the temperatures of 700–1500 °C by 18–20 7–10 days till completely dissolved. Dissolved samples steps in a high-vacuum argon extraction system at the weredilutedto50mlusing1%HNO3 before ICP-MS Laboratory of Ar–Ar Dating, Institute of Geology and analyses. Six standards (GSR-1, GSR-2, GSR-3, MRG- Geophysics, the Chinese Academy of Sciences 1, W-2, and AGV-2) were analyzed using the same (IGGCAS). Then, the purified argon was analyzed on procedure to monitor the analytical reproducibility. The the VG-5400 gas mass spectrometer. The results of measured values of the standards are in satisfactory 40Ar/39Ar dating are presented in Supplementary Table agreement with the reference values (Supplementary 1andFig. 3. Table 4). Analytical procedures and precision have been Zircons from the tuff and andesite samples were described in detail in Liu et al. (1996).Themajorand separated using gravitational and magnetic sorting, and trace elements data are listed in Table 2. then they were handpicked under a binocular micro- About 100–150mgwholerockpowderwas scope. Measurements of U, Th, and Pb of zircons were completely decomposed in a mixture of HF-HClO4 conducted using the SHRIMP II ion microprobe at the for Sr–Nd isotopic analysis and in a mixture of HF- Center of Ion Microprobe Analysis, China, HNO3 for Pb isotopic analysis. For Rb–Sr and Sm–Nd following the procedure outlined by Williams and isotope analyses, sample powders were spiked with Claesson (1987) and Song et al. (2002).The206Pb/238U mixed isotope tracers, dissolvedinTefloncapsuleswith ratios were corrected using the zircon standards of HF+HNO3 at 100 °C for 7–10 days. For Pb isotope CL13 (572 Ma) from Cililanca and TEM (417 Ma) from determination, powder was weighed into the Teflon Australia. The analytical spot size is 40 μminaverage capsules and dissolved in HF+HNO3 at 100 °C for 7– diameter during each analytical run. Each spot was 10 days. Sr and rare earth elements (REE) were rastered over 80 μm for 3 min prior to analysis (5 mass separated on quartz columns with a 5 ml resin bed of scans)toremovecommonPbonthesurfaceor AG 50 W-X12 (200–400 mesh). Nd was separated from contamination from the coating. One spot on the other REEs on quartz columns using 1.7 ml Teflon® standard zircon (TEM) was analyzed after every three powder as cation exchange medium. Pb was separated analyses of sample-spots. Squid and Isoplot programs on Teflon® columns containing ∼80 μl AG1-X8, 100– of Ludwig (2001) were used for data processing and age 200meshbyemployingHBr–HCl wash and elution calculation. The final age results are the weighted mean procedure. Procedural blanks were b200pgforSrand of the 206Pb/238U ages because 206Pb/238Uagesare b50 pg for Pb and Nd. For the measurement of isotopic more precise than 207Pb/235Uagesand207Pb/206Pb compositions, Pb was loaded with a mixture of Si-gel ages for young zircons. The common lead is corrected and H3PO4 onto a single-Re filament and measured at by assuming 206Pb/238U–208Pb/232Th age-concor- 1300 °C; Sr was loaded with a Ta-HF activator on a dance. Errors on individual spots are based on counting single W filament; and Nd was loaded as phosphates statistics and are at the 1σ level, but the average and measured in a Re-double-filament configuration. weighted ages are quoted at 2σ or 95% confidence. The 143Nd/144Nd ratios were normalized to 146Nd/144Nd= analytical results are listed in Supplementary Table 2 0.7219 and 87Sr/86Sr ratios to 86Sr/88Sr=0.1194. Pb and plotted on the concordia diagrams in Fig. 4. standard NBS 981 was used to determine thermal Major elements compositions of all samples except fractionation and measured isotopic ratios of samples XLG-4 were determined using the Vista ICP-AES at the were corrected with a value of 0.1% per atomic mass Institute of Geochemistry, Chinese Acad- unit. Sr–Nd–Pb isotopic ratios were measured on a emy of Sciences (GIGCAS). The major elements Finnigan MAT-262 thermal ionization mass spectrom- compositions (Supplementary Table 3) of one tuff eter (TIMS) in the Laboratory for Radiogenic Isotope sample (XLG-4) for zircon U–Pb dating were deter- Geochemistry, Institute of Geology and Geophysics, mined using wet chemical techniques at the Hebei Chinese Academy of Sciences, Beijing. Raw data Institute of Regional Geology and Mineral Resources. obtained were calculated using the Isoplot program Analytical uncertainties for the majority of major (Ludwig, 2001), giving 2σm error. Analyses of elements were estimated to be less than 1%. Whole- standards during the period of analysis are as follows: rock trace elements were analyzed with the inductively NBS987 gave 87Sr/86Sr=0.710254±12 (n =27, 2σ); coupled plasma-mass spectrometry (ICP-MS) at the JMC and AMES gave 143Nd/144Nd = 0.511987 ± 7 GIGCAS. The whole-rock powders (50 mg) were (n =8,2σ) and 0.512145±12 (n =15,2σ), respectively. dissolved in HF+HNO3 in 15 ml Teflon screw-cap Details of chemical separation and measurement are capsules at 100 °C for 1 day, dried to expel most of silica described in Chen et al. (2000, 2002).TheSr–Nd–Pb and then dissolved with HF+HNO3 at 100 °C again for isotopic data are listed in Table 3. W. Yang, S. Li / Lithos 102 (2008) 88–117 95

4. Results mentioned above (Swisher et al., 1999, 2001; Wang et al., 2001a,b; Ji et al., 2004; Yang et al., 2007), the geochrono- Since the Yixian Formation (126 Ma–120 Ma) has been logical study of this paper only focuses on the timing of the well dated by zircon U–Pb and Ar–Ar methods as 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. Table 2 96 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 41°46′32″ E 120°38′34″ N 41°50.484′ E 120°46.009′

SiO2 63.56 53.24 52.37 54.90 65.26 65.86 64.40 56.24 57.04 55.92 TiO2 0.58 1.09 1.02 1.16 0.50 0.50 0.57 1.37 1.38 1.40 Al2O3 14.94 18.28 16.5 18.51 16.27 16.27 16.76 17.00 17.06 17.05 Fe2O3 4.25 8.83 8.47 5.88 4.73 4.29 4.66 7.45 7.27 6.9 MnO 0.05 0.14 0.19 0.10 0.06 0.06 0.03 0.13 0.13 0.17 MgO 2.85 3.16 1.61 3.90 0.24 0.24 0.42 1.83 1.38 2.30 CaO 3.46 6.86 8.19 4.42 1.67 1.75 1.54 4.84 4.88 5.17 Na2O 3.52 3.99 3.98 4.69 4.55 4.62 4.62 4.45 4.45 4.40

K2O 3.26 1.74 2.69 2.25 4.96 4.96 4.88 3.05 3.32 3.08 88 (2008) 102 Lithos / Li S. Yang, W. P2O5 0.21 0.33 0.26 0.35 0.19 0.18 0.20 0.64 0.64 0.69 LOI 3.00 1.61 4.96 4.02 1.57 1.31 1.84 2.87 2.47 3.14 Total 99.68 99.26 100.23 100.18 100.01 100.04 99.92 99.87 100.02 100.22 Mg# 57 42 28 57 9 10 15 33 28 40 Sc 8.7 17.3 18.2 20.0 6.5 6.6 7.0 13.2 13.2 13.8 Cr 156 2.35 5.33 6.85 0.35 1.19 0.10 ⁎ 0.45 0.03 Ni 82.2 5.30 9.16 10.3 ⁎ 5.13 ⁎ 5.10 ⁎ 9.64 Rb 113 35.1 76.0 36.9 162 163 159 88.4 110 95.2 Sr 461 658 460 444 295 305 355 541 558 576 Y 12.7 20.9 18.2 21.1 36.3 38.5 35.5 42.1 39.6 41.1 Zr 164 163 130 131 440 442 439 311 311 317 Nb 6.0 8.7 8.4 6.0 23.5 23.9 24.0 18.9 19.6 20.0 – Ba 719 621 1201 822 1318 1288 1340 992 1079 1048 117 Hf 3.9 3.9 3.4 3.2 11.7 11.75 11.5 8.46 8.45 8.61 Ta 0.42 0.52 0.48 0.28 1.15 1.17 1.16 0.89 0.94 0.97 Pb 17.5 6.2 6.8 8.0 27.0 24.1 24.3 17.1 15.6 15.4 Th 11.2 1.97 1.58 1.51 10.0 10.37 9.93 6.82 6.78 7.07 U 2.75 0.403 0.343 0.314 2.11 2.33 1.93 1.58 1.61 1.57 La 26.2 23.2 20.2 21.0 65.7 65.6 62.8 60.6 56.9 57.2 Ce 45.9 51.6 43.0 46.0 109 107 104 119 120 99.9 Pr 5.73 6.65 5.67 5.97 15.5 15.5 14.9 15.9 15.3 15.4 Nd 20.5 28.0 23.9 23.9 53.7 54.2 52.1 60.5 57.6 59.0 Sm 3.82 5.40 4.86 5.11 9.58 9.58 9.13 11.6 11.0 11.4 Eu 1.02 1.59 1.48 1.46 1.81 1.80 1.79 2.52 2.43 2.47 Gd 3.26 4.94 4.58 4.58 8.06 8.31 7.59 9.99 9.50 9.72 Tb 0.421 0.75 0.69 0.668 1.10 1.12 1.03 1.34 1.28 1.32 Dy 2.12 4.19 3.83 3.61 5.87 6.09 5.56 7.07 6.77 7.00 Ho 0.42 0.80 0.75 0.765 1.25 1.28 1.18 1.47 1.38 1.43 Er 1.01 2.13 1.89 1.83 3.28 3.35 3.12 3.66 3.45 3.57 Tm 0.152 0.328 0.292 0.279 0.549 0.548 0.524 0.570 0.531 0.558 Yb 0.97 2.04 1.85 1.73 3.70 3.70 3.57 3.68 3.44 3.57 Lu 0.143 0.320 0.300 0.244 0.559 0.547 0.542 0.544 0.501 0.532 Sr/Y 36.3 31.43 25.3 21.0 8.13 7.92 10.0 12.9 14.11 14.0 Ce/Pb 2.62 8.29 6.28 5.76 4.04 4.44 4.28 6.96 7.69 6.49 (La/Yb)N 18.6 7.88 7.58 8.40 12.3 12.3 12.2 11.4 11.5 11.1

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 41°36′52.8″ E 120°48′56.8″

SiO2 56.46 56.16 57.96 56.86 56.92 65.62 66.06 66.14 54.83 53.16 TiO2 1.44 1.35 1.44 1.42 1.43 0.47 0.47 0.48 0.86 0.9 Al2O3 16.95 16.93 17.02 17.41 17.26 16.59 16.57 16.44 15.22 15.71 Fe2O3 7.81 6.93 7.07 7.93 7.69 3.9 3.24 3.46 7.88 8.11

MnO 0.08 0.11 0.09 0.07 0.09 0.09 0.12 0.08 0.10 0.11 88 (2008) 102 Lithos / Li S. Yang, W. MgO 1.50 2.34 1.14 0.78 1.59 0.33 0.69 0.69 6.45 6.23 CaO 4.79 5.21 4.17 4.38 4.54 2.17 2.08 2.29 6.73 7.16 Na2O 4.62 4.29 4.55 4.55 4.35 5.10 5.00 5.20 3.79 4.18 K2O 3.30 3.12 3.50 3.62 3.32 3.88 3.70 3.62 1.65 1.92 P2O5 0.71 0.65 0.66 0.67 0.68 0.15 0.17 0.17 0.34 0.36 LOI 2.34 3.12 2.14 2.07 2.41 1.33 1.51 1.44 2.08 2.29 Total 100.00 100.21 99.74 99.76 100.28 99.63 99.61 100.01 99.93 100.13 Mg# 28 40 24 16 29 14 30 29 62 61 Sc 13.4 13.3 13.6 13.8 13.5 3.50 3.61 3.28 22.53 21.62 Cr 0.64 ⁎⁎⁎0.20 0.30 0.18 ⁎ 292.4 295.6 Ni 6.79 6.38 0.81 ⁎ 13.0 1.23 1.66 ⁎ 104.3 105.8

Rb 102 95.2 109 129 112 105 100 89.5 41.89 40.72 – Sr 544 571 547 554 555 415 392 387 848 840 117 Y 39.7 39.6 40.2 41.8 43.0 28.4 27.8 28.0 16.5 16.2 Zr 309 313 318 322 313 283 276 270 154 153 Nb 19.7 19.8 20.2 20.3 19.9 11.5 11.4 11.1 17.5 17.4 Ba 1117 1038 1187 1246 1101 1246 1225 1140 767 752 Hf 8.60 8.56 8.50 8.57 8.63 6.47 6.33 6.17 3.48 3.49 Ta 0.961 0.966 0.958 0.954 0.963 0.533 0.530 0.511 1.02 1.07 Pb 17.4 15.1 17.9 16.9 16.8 15.9 15.0 14.7 9.85 9.67 Th 6.89 6.91 6.94 6.97 6.94 4.85 4.77 4.57 4.19 4.20 U 1.51 1.49 1.59 1.49 1.59 0.730 0.749 0.731 0.909 0.909 La 57.9 56.9 57.5 58.5 61.8 43.4 42.3 41.0 26.4 25.8 Ce 120 119 118 106 123 77.8 80.3 78.9 54.6 54.1 Pr 15.5 15.3 15.3 15.5 16.3 10.3 9.87 9.54 6.71 6.51 Nd 58.9 58.2 58.0 58.9 62.3 37.0 35.1 33.8 25.9 25.5 Sm 11.4 11.2 11.1 11.4 12.01 6.725 6.31 6.04 4.58 4.48

(continued on next page) 97 98 Table 2 (continued ) 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 41°36′52.8″ E 120°48′56.8″ Eu 2.49 2.41 2.46 2.50 2.67 1.63 1.56 1.50 1.31 1.32 Gd 9.66 9.32 9.59 9.72 10.39 5.79 5.48 5.27 3.87 3.84 Tb 1.31 1.28 1.29 1.31 1.41 0.807 0.762 0.734 0.58 0.59 Dy 6.87 6.72 6.77 6.92 7.46 4.44 4.16 4.09 3.18 3.19 Ho 1.42 1.40 1.39 1.42 1.54 0.961 0.907 0.892 0.611 0.606 Er 3.52 3.46 3.44 3.53 3.83 2.46 2.42 2.36 1.62 1.58 Tm 0.545 0.539 0.528 0.550 0.598 0.411 0.403 0.396 0.239 0.240

Yb 3.54 3.52 3.40 3.56 3.87 2.77 2.77 2.65 1.55 1.53 88 (2008) 102 Lithos / Li S. Yang, W. Lu 0.514 0.514 0.494 0.520 0.577 0.414 0.420 0.404 0.241 0.253 Sr/Y 13.7 14.4 13.6 13.2 12.9 14.6 14.1 13.8 51.3 51.7 Ce/Pb 6.89 7.88 6.59 6.27 7.32 4.89 5.35 5.36 5.54 5.59 (La/Yb)N 11.3 11.1 11.7 11.3 11.0 10.8 10.5 10.7 11.7 11.6

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 41°36′52.8″ E 120°48′56.8″ N 41°34′52.6″ E 120°46′23.4″ N 41°42′33.7″ E 121°19′11.9″ N 42°16′42.9″ E 121°54′15.3″

SiO2 52.84 55.78 56.67 60.19 59.84 61.67 62.17 45.18 44.63 45.19 –

TiO2 0.89 1.1 0.74 0.77 0.76 0.74 0.74 2.97 2.96 2.97 117 Al2O3 15.6 15.47 15.04 15.17 14.96 14.51 14.53 14.98 14.94 15 Fe2O3 8.33 7.9 6.26 5.66 5.17 5.25 5.21 11.79 11.64 11.77 MnO 0.09 0.11 0.09 0.09 0.08 0.07 0.07 0.17 0.17 0.17 MgO 7.03 5.21 5.92 3.3 3.36 3.77 3.58 8.4 8.52 8.49 CaO 7.17 5.92 5.36 4.14 4.24 3.82 3.83 10.39 10.27 10.38 Na2O 4.13 4.06 4.26 3.52 3.71 3.65 3.63 2.98 3.06 3 K2O 1.91 2.4 2.66 2.68 3.19 3.09 3.1 1.37 1.37 1.37 P2O5 0.36 0.55 0.46 0.22 0.23 0.21 0.23 0.63 0.63 0.61 LOI 2.34 2.36 2.63 4.05 4.53 3.05 3.68 1.35 1.35 1.42 Total 100.71 100.87 100.07 99.78 100.07 99.83 100.77 100.21 99.54 100.37 Mg# 63 57 65 54 57 59 58 59 59 59 Sc 21.7 14.8 15.0 12.4 11.6 11.9 12.0 25.1 24.5 24.1 Cr 300 193 320 171 174 175 179 207 204 201 Ni 112 117 208 107 95.6 102 107 162 162 162 Rb 41.6 40.2 71.8 93.9 96.1 95.9 97.9 48.9 47.8 49.6 Sr 847 1046 899 560 568 560 574 769 771 727 Y 16.2 15.7 12.6 12.6 12.5 12.0 12.1 26.0 25.6 25.4 Zr 152 223 198 208 212 209 209 251 247 246 Nb 16.6 11.2 9.64 17.2 17.9 16.8 12.0 58.4 58.3 57.2 Ba 771 1062 1078 955 997 984 988 763 754 745 Hf 3.45 4.88 4.52 4.73 4.90 4.81 4.84 5.50 5.57 5.57 Ta 0.96 0.69 0.65 1.04 1.01 0.91 0.74 3.70 4.01 3.98 Pb 10.3 11.5 14.4 15.1 15.5 15.3 15.3 3.12 3.12 3.31 Th 4.26 5.68 6.88 7.47 7.69 7.48 7.49 5.45 5.69 5.72 U 0.90 1.11 1.54 1.48 1.54 1.46 1.48 1.34 1.37 1.35 La 26.3 45.8 34.8 36.3 37.0 36.6 36.4 41.2 40.9 41.1 Ce 54.9 93.8 69.2 70.5 71.4 70.4 71.4 82.7 81.7 82.8 Pr 6.70 11.4 8.12 7.82 8.02 7.97 7.99 10.2 10.0 10.2 Nd 26.1 43.5 30.3 28.8 29.4 28.7 28.8 41.1 40.9 41.6 Sm 4.54 6.72 4.87 4.68 4.65 4.63 4.52 8.11 7.98 8.06 Eu 1.32 1.82 1.29 1.18 1.22 1.19 1.20 2.48 2.51 2.55 Gd 3.85 4.74 3.64 3.59 3.37 3.50 3.38 7.43 7.24 7.38 Tb 0.594 0.692 0.506 0.520 0.516 0.512 0.499 1.10 1.08 1.08 Dy 3.24 3.41 2.67 2.71 2.65 2.62 2.64 5.71 5.65 5.73 88 (2008) 102 Lithos / Li S. Yang, W. Ho 0.62 0.60 0.47 0.47 0.47 0.47 0.47 1.01 1.01 1.04 Er 1.64 1.61 1.22 1.26 1.25 1.23 1.23 2.50 2.53 2.57 Tm 0.23 0.23 0.18 0.18 0.17 0.17 0.18 0.36 0.36 0.35 Yb 1.59 1.44 1.22 1.13 1.13 1.16 1.15 2.19 2.21 2.20 Lu 0.254 0.219 0.189 0.181 0.183 0.187 0.182 0.338 0.335 0.342 Sr/Y 52.2 66.4 70.9 44.3 45.4 46.4 47.3 29.5 30.0 28.6 Ce/Pb 5.32 8.12 4.80 4.64 4.59 4.59 4.64 26.4 26.2 25.0 (La/Yb)N 11.4 21.9 19.7 22.1 22.6 21.8 21.9 13.0 12.7 12.9

Zhanglaogongtun Formation JG-4 JG-5 WHC-1 WHC-2 WHC-3 WHC-4 – 117 Basalt Basalt Basalt Basalt Basalt Basalt Sample site N 42°16′42.9″ E 121°54′15.3″ N 42°18′29.9″ E 121°53′39.8″

SiO2 43.89 45.75 43.57 42.71 43.37 43.31 TiO2 2.98 2.96 2.99 2.96 2.97 2.94 Al2O3 15.2 15.06 15.14 14.88 15.12 14.99 Fe2O3 11.85 11.63 12.32 12.22 12.18 11.9 MnO 0.18 0.17 0.18 0.17 0.18 0.17 MgO 8.86 8.36 8.43 8.39 8.54 8.32 CaO 10.46 10.22 9.76 9.67 9.78 9.5 Na2O 3.33 3.04 3.91 3.76 3.95 3.94 K2O 1.46 1.4 0.8 0.7 0.74 0.76 P2O5 0.65 0.62 0.73 0.71 0.71 0.71 LOI 1.57 1.38 2.57 3.19 3.17 2.71 Total 100.42 100.59 100.38 99.35 100.7 99.26 Mg# 60 59 58 58 58 58

(continued on next page) 99 100

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 42°16′42.9″ E 121°54′15.3″ N 42°18′29.9″ E 121°53′39.8″ Sc 24.2 23.5 22.2 22.4 22.0 21.7 Cr 213 200 148 152 158 146 Ni 163 157 130 131 137 130 Rb 50.4 49.0 14.8 10.7 9.86 12.4 Sr 760 776 772 768 734 767

Y 25.6 25.2 26.0 26.1 25.8 26.4 88 (2008) 102 Lithos / Li S. Yang, W. Zr 245 245 249 250 249 254 Nb 59.9 57.8 58.5 58.3 59.0 60.4 Ba 771 752 742 856 795 754 Hf 5.55 5.45 5.51 5.67 5.69 5.60 Ta 3.74 3.77 3.77 3.74 3.73 3.87 Pb 3.07 3.20 3.14 3.36 3.14 3.44 Th 5.71 5.55 5.58 5.73 5.80 5.80 U 1.39 1.37 1.45 1.39 1.40 1.47 La 42.1 40.6 41.7 42.3 42.5 42.8 Ce 83.0 82.2 83.4 85.4 85.6 86.4 Pr 10.2 10.1 10.4 10.5 10.7 10.8

Nd 41.0 41.0 42.8 42.9 43.4 43.7 – 117 Sm 8.04 7.91 8.25 8.52 8.53 8.39 Eu 2.50 2.45 2.59 2.61 2.68 2.65 Gd 7.45 7.22 7.54 7.72 7.69 7.61 Tb 1.07 1.07 1.14 1.14 1.12 1.14 Dy 5.61 5.76 5.93 5.89 6.02 5.88 Ho 1.00 1.03 1.06 1.06 1.06 1.07 Er 2.49 2.47 2.60 2.60 2.58 2.58 Tm 0.35 0.35 0.36 0.36 0.36 0.36 Yb 2.21 2.18 2.29 2.29 2.30 2.34 Lu 0.34 0.33 0.34 0.34 0.36 0.36 Sr/Y 29.6 30.6 29.6 29.3 28.4 28.9 Ce/Pb 26.9 25.6 26.5 25.4 27.2 25.0 (La/Yb)N 13.1 12.8 12.5 12.7 12.7 12.6 LOI = loss on ignition; Mg#=Mg/(Mg+TFeO) atomic ratio. “⁎” stands for non-detective. W. Yang, S. Li / Lithos 102 (2008) 88–117 101

4.1. Geochronology sample DHG-1 and the Ar–Ar ages of 165 to 159 Ma for samples from the same locality (Chen et al., 2004; He 4.1.1. The Xinglonggou Formation et al., 2004). Fig. 4a show the SHRIMP U–Th–Pb analytic results Fig. 3a–b shows the 40Ar/39Ar spectra with plateau on 15 spots of zircons from the tuff sample (XLG-4). The age and isochron plot of basalt sample HFG-13. All major element compositions (Supplementary Table 2) of errors are reported at the 2σ level. This sample yields the the tuff sample (XLG-4) are similar to those of 40Ar/39Ar plateau age of 166.1±0.9 Ma, which is or rhyolites, suggesting that the tuff mainly consists of defined by 92.2% of total released 39Ar. The isochron + volcanic materials, while its high Fe2O3/FeO and HO2 diagram (Fig. 3b) indicates an age of 166.7±2.9 Ma contents indicate its surface deposit environment. The and an initial 40Ar/36Ar ratio of 290.9±16.5, which are analysis of XLG-4-13 gives an apparently higher consistent with its plateau age and the present at- 206Pb/238Pb age (240.6±3.4 Ma) and U (206 ppm), mospheric 40Ar/36Ar ratio (295.5), suggesting that ex- Th (149 ppm) contents than those given by other zir- cess argon is insignificant and the plateau or isochron age cons (Supplementary Table 2), suggesting that this is reliable. zircon grain is different in origin from other zircons. The The analytical results on 11 spots of zircons from cathodoluminescence (CL) images also show the rhyolite sample HFG-27 are shown on Fig. 4g. They deference between zircon grain XLG-4-13 and the yield a weighted mean 206Pb/238Pb age of 160±6 Ma. others, i.e., the former is characterized by oscillatory The analytical results on 16 spots of zircons from zoning with no fan-structure and others are characterized andesite porphyrite HFG-29 (intruded into the Lanqi by oscillatory zoning with well-developed fan-structure Formation) are shown on Fig. 4h. They give a mean age (Fig. 4b). The analysis of XLG-4-16 also gave an of 153±2 Ma, which may indicate the last activity of the apparently higher 206Pb/238Pb age (205.7±7.3 Ma) than Lanqi magmatism. those of other zircons. The age of XLG-4-16 could be in error because of its very low radiogenic Pb content 4.1.3. The Zhanglaogongtun Formation (0.7 ppm) (Supplementary Table 2). Therefore, the Fig. 3c–d shows the 40Ar/39Ar spectra with plateau analyses of XLG4-13 and XLG4-16 are excluded from age and isochron plot of WHC-2. The sample yields the the calculation of the weighted mean age. The remaining 40Ar/39Ar plateau age of 106.1±0.8 Ma, which is defined 13 analyses give a weighted mean 206Pb/238Pb age of by 89.1% of total released 39Ar. The isochron diagram 176.7±3.5 Ma. This age is slightly younger than the (Fig. 3d) indicates an age of 105.1±1.3 Ma and an initial previous reported Ar–Ar ages of 188–194 Ma for the 40Ar/36Ar ratio of 306.6±21.5, consistent with its plateau Xinglonggou andesites (Chen et al., 1997), but signifi- age and the present atmospheric 40Ar/36Ar ratio (295.5). cantly older than the U–Pb SHRIMP age of the This suggests that the excess argon is minimal and the Xinglonggou rhyolites (159±3 Ma) reported by Gao plateau age is reliable. The plateau age is consistent et al. (2004). with the previously reported Ar–Ar and K–Ar ages of ca. 109–93 Ma (Zhang et al., 2003; Zhu et al., 2002). 4.1.2. The Lanqi Formation Five samples (DHG-1, DHG-2, HFG-13, HFG-27, and 4.2. Geochemistry HFG-29) from this formation have been dated. Fig. 4c show the SHRIMP U–Th–Pb analytic results on 10 spots 4.2.1. Xinglonggou andesite of zircons from the tuff sample (DHG-1), which given an Because geochemistry of the Xinglonggou andesite age of 164.1±2.4 with a relative high MSWD (mean has been well studied (Gao et al., 2004; Li, 2006), only square of weighted deviation) value of 3.5. Since the one Xinglonggou andesite sample (XLG-1) was analyzed analysis of DHG1-9 gives an apparently lower for major-trace element and Sr–Nd–Pb isotopic composi- 206 238 Pb/ Pb age than those given by other zircons, when tions. Sample XLG-1 is characterized by high SiO2 # it was excluded from the calculation of the weighted mean (63.56 wt.%) and Al2O3 (14.94 wt.%), high Mg (57), Cr age, the remaining 9 analyses give a weighted mean (156 ppm), and Ni (82.2 ppm) contents (Table 2). It is also 206Pb/238Pb age of 165.0±1.2 Ma with a relative smaller enriched in light rare-earth elements (LREE), LILE, and MSWD value of 1.5. Pb, and depleted in HFSE, heavy rare-earth elements Fig. 4e shows the SHRIMP results on 10 spots of (HREE), and Y. It has high Sr/Y (36.3) and (La/Yb)N zircons from the andesite sample (DHG-2). They yield a (18.6, where subscript N denotes C1-chondrite normal- mean age of 164.3±2.2 Ma. This age is consistent with ization) without negative Eu anomaly (Table 2; Fig. 6). the SHRIMP zircon U–Pb age of 165.0±1.2 Ma for This sample has moderate 87Sr/86Sr (0.70660), slightly 102

Table 3

Sr–Nd–Pb isotopes of the Mesozoic volcanic rocks from Western Liaoning 88 (2008) 102 Lithos / Li S. Yang, W. 87 Rb/86Sr 87 Sr/86 Sr 87Sr/86 Sr 147 Sm/144 Nd 143 Nd/144 Nd 143 Nd/144 Nd ɛNd(t) 206 Pb/204 Pb 207 Pb/204 Pb 208 Pb/204 Pb 206 Pb/204 Pb 207 Pb/204 Pb 208 Pb/204 Pb (t) (t) (t) (t) (t) Xinglonggou Fm. XLG- 0.2767 0.707218 0.706597 0.0971 0.512459 0.512357 −1.5 18.471 15.567 38.520 18.211 15.554 38.176 1 Lanqi Fm. HFG- 0.1374 0.706565 0.706247 0.1228 0.511969 0.511836 −11.5 16.485 15.253 36.619 16.385 15.249 36.459 13 HFG- 0.1393 0.706565 0.706242 0.1210 0.511940 0.511809 −12.0 16.477 15.254 36.616 16.400 15.250 36.499 15 LQ-6 0.0507 0.706707 0.706593 0.1234 0.511970 0.511841 −11.5 16.502 15.271 36.639 16.441 15.268 36.543 LQ-9 2.5478 0.710065 0.704344 0.1068 0.511921 0.51181 −12.1 16.582 15.297 36.792 16.460 15.291 36.602 −

LQ-10 2.2643 0.710041 0.704956 0.1076 0.511930 0.511818 12.0 16.594 15.273 36.740 16.443 15.265 36.521 – LQ-15 0.1390 0.707242 0.70693 0.1035 0.511965 0.511856 −11.2 16.762 15.330 36.978 16.616 15.323 36.773 117 LQ-16 0.1881 0.707431 0.707008 0.1026 0.511947 0.511839 −11.6 16.786 15.334 37.001 16.624 15.326 36.778 LQ-17 0.1450 0.707352 0.707026 0.0924 0.511934 0.511837 −11.6 16.767 15.320 36.960 16.607 15.312 36.725 LQ-18 0.2252 0.707466 0.706961 0.1002 0.511956 0.511851 −11.3 16.744 15.307 36.898 16.608 15.300 36.696 LQ-19 0.1481 0.707304 0.706971 0.0924 0.511967 0.51187 −11.0 16.760 15.310 36.929 16.605 15.303 36.695 LQ-20 0.3219 0.707449 0.706726 0.1050 0.511958 0.511848 −11.4 16.754 15.326 36.954 16.614 15.320 36.756 LQ-21 0.3500 0.707612 0.706826 0.0950 0.511935 0.511835 −11.6 16.745 15.294 36.862 16.607 15.287 36.652 LQ-22 0.2662 0.707441 0.706843 0.0953 0.511981 0.511881 −10.7 16.733 15.285 36.828 16.585 15.277 36.617 LQ-27 1.3529 0.708640 0.705602 0.1172 0.511852 0.511729 −13.7 16.179 15.210 36.791 16.108 15.207 36.636 LQ-28 1.6876 0.708662 0.704872 0.1145 0.511847 0.511727 −13.8 16.183 15.221 36.840 16.106 15.217 36.679 LQ-29 0.7523 0.708556 0.706866 0.0987 0.511847 0.511743 −13.4 16.167 15.194 36.750 16.090 15.191 36.593 Yixian Fm. HBJ4- 0.1330 0.706475 0.706242 0.1115 0.512025 0.511934 −10.6 16.675 15.272 36.851 16.566 15.267 36.686 1 HBJ4- 0.1269 0.706510 0.706287 0.1134 0.511955 0.511862 −12.0 16.669 15.270 36.843 16.558 15.265 36.675 2 HBJ4- 0.1452 0.706399 0.706144 0.1114 0.512070 0.511979 −9.7 16.696 15.284 36.891 16.592 15.279 36.731 3 87Rb/86Sr 87Sr/86Sr 87 Sr/86Sr 147 Sm/144 Nd 143 Nd/144 Nd 143 Nd/144 Nd ɛNd(t) 206 Pb/204 Pb 207 Pb/204 Pb 208 Pb/204 Pb 206 Pb/204 Pb 207 Pb/204 Pb 208 Pb/204 Pb (t) (t) (t) (t) (t) SHT- 0.1054 0.706053 0.705868 0.0970 0.511868 0.511789 −13.4 16.554 15.250 36.789 16.440 15.244 36.599 14 SHT-3 0.2207 0.707022 0.706635 0.1010 0.511951 0.511868 −11.9 16.779 15.292 36.963 16.651 15.286 36.778 ZCZ1- 0.4778 0.707907 0.707069 0.1001 0.511938 0.511856 −12.1 16.331 15.225 36.550 16.216 15.219 36.361 1 ZCZ1- 0.4695 0.707732 0.706908 0.1005 0.511918 0.511836 −12.5 16.358 15.230 36.578 16.242 15.225 36.388 2 ZCZ1- 0.5133 0.707577 0.706677 0.1012 0.511900 0.511817 −12.9 16.370 15.230 36.578 16.258 15.224 36.392 3 ZCZ1- 0.4562 0.707630 0.706830 0.0997 0.511910 0.511828 −12.7 16.338 15.225 36.562 16.225 15.220 36.375 4 Zhanglao-gongtun JG-1 0.1620 0.703754 0.703515 0.1232 0.512831 0.512746 4.75 18.343 15.490 38.331 17.897 15.468 37.736 Fm. JG-2 0.1776 0.703782 0.703520 0.1232 0.512815 0.512730 4.44 18.328 15.490 38.317 17.871 15.468 37.696 JG-3 0.1794 0.703827 0.703563 0.1235 0.512811 0.512726 4.36 18.288 15.471 38.252 17.865 15.451 37.665 JG-4 0.1762 0.703725 0.703465 0.1227 0.512830 0.512746 4.74 18.359 15.487 38.341 17.889 15.465 37.709 JG-5 0.1822 0.703781 0.703513 0.1232 0.512808 0.512723 4.30 18.286 15.481 38.260 17.841 15.460 37.672

WHC- 0.05359 0.703935 0.703856 0.1218 0.512869 0.512785 5.51 18.440 15.484 38.366 17.957 15.461 37.760 88 (2008) 102 Lithos / Li S. Yang, W. 1 WHC- 0.03560 0.703692 0.703640 0.1248 0.512883 0.512797 5.74 18.426 15.482 38.366 17.994 15.462 37.785 2 WHC- 0.03776 0.703533 0.703477 0.1275 0.512824 0.512736 4.56 18.393 15.481 38.346 17.929 15.458 37.718 3 WHC- 0.04183 0.703789 0.703727 0.1223 0.512822 0.512738 4.59 18.427 15.488 38.367 17.981 15.467 37.792 4 87 86 87 86 147 144 143 144 − 11 − 1 Chondrite Uniform Reservoir (CHUR) values ( Rb/ Sr=0.0847, Sr/ Sr =0.7045, Sm/ Nd =0.1967, Nd/ Nd = 0.512638) are used for the calculation. λRb =1.42×10 year , − 12 − 1 − 0 − 1 − 10 − 1 − 11 − 1 λSm =6.54×10 year , λU238 =1.55125×10 year , λU235 =9.8485×10 year , λTh232 =4.9475×10 year (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. – 117 103 104 W. Yang, S. Li / Lithos 102 (2008) 88–117

negative ɛNd(T) (−1.5), and radiogenic Pb isotopic geochemical features are consistent with those of the compositions (206Pb/207Pb=18.211, 207Pb/204Pb= Xinglonggou andesites reported by Gao et al. (2004) and 15.554, and 208Pb/204Pb =38.176) (Table 3). These Li (2006). W. Yang, S. Li / Lithos 102 (2008) 88–117 105

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).

4.2.2. Lanqi lavas concentrations (Table 2). These rocks are enriched in The Lanqi basalts and basaltic andesites have SiO2 of LREE, LILE, and Pb, and depleted in HFSE with negative 52∼53 wt.%, high Al2O3 (N16.5 wt.%) and CaO Sr and Eu anomalies (Fig. 6b solid line). They have 87 86 (N6.5 wt.%), low MgO (b3.5 wt.%), Cr (b6 ppm), and moderate Sr/ Sr (0.704∼0.706), negative ɛNd(T) Ni (b10 ppm) contents (Table 2). They are enriched in (−13∼−10), and unradiogenic Pb (206Pb/207Pbb16.6, LREE and Rb, Ba, Sr, Pb, and depleted in high field 207Pb/204Pbb15.4, and 208Pb/204Pbb36.8) (Table 3). strength element, Th, and U (see dash line in Fig. 6b). The These isotopic features are similar to the Lanqi basalts, two basaltic samples (HFG-13, 15) have moderate but different with the Xinglonggou andesites. The Lanqi 87 86 Sr/ Sr (∼0.706), low ɛNd(t) (∼−12), and unradiogenic andesites and rhyolites have relatively low MgO but high 206 207 207 204 Pb ( Pb/ Pbb16.5, Pb/ Pbb15.3, and Al2O3 contents, distinct from the Xinglonggou and the 208Pb/204Pbb36.5) (Table 3). The isotopic compositions Yixian andesites (Fig. 5). are similar to the EMI-like component (Zindler and Hart, 1986). 4.2.3. Yixian lavas The Lanqi andesites and rhyolites are characterized by The samples from the Yixian Formation exhibit a SiO2 of 56∼66 wt.%, high Al2O3 (N16 wt.%), and low wide compositional range with SiO2 varying from 52 to MgO (b2.5 wt.%), Cr (b1ppm),andNi(b7ppm) 62 wt.%. CaO, Al2O3, TFe2O3, MgO, and P2O5 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 Al2O3 contents relative to the others. 106 W. Yang, S. Li / Lithos 102 (2008) 88–117

87 86 negatively correlated with SiO2 (Fig. 5). They have Sr/ Sr (∼0.706), low ɛNd(t) (∼−10), and unradio- enrichments in LREE, LILE, and Pb, and depletion in genic Pb (206Pb/207Pbb16.6, 207Pb/204Pbb15.3, and HFSE. The geochemical characteristics of the Yixian 208Pb/204Pbb36.8), similar to those of the Lanqi andesites basalts and basaltic andesites are similar to the Lanqi (Fig. 7). basaltic rocks except for the higher Ni (N95 ppm) and Cr (N170 ppm) contents in the Yixian basalts. 4.2.4. Zhanglaogongtun lavas The Yixian andesites have SiO2%N56 wt.%, Al3O2%N The Zhanglaogongtun lavas display a limited com- 15 wt.%, MgOb3 wt.% without negative Eu anomaly. positional range with low SiO2 content between 42 and # The high Mg (N54) and Cr (N200 ppm) and Ni 45 wt.%. They have relatively high alkalis (Na2O+ (N100 ppm), high Sr (N400 ppm) and Sr/Y (N40), low K2ON4 wt.%), MgO, TFeO, CaO, TiO2, MnO, and Yb (b18 ppm) and Y (b1.9 ppm) contents indicate that transitional metal elements (Sc, Cr, Ni) contents (Fig. 5). the Yixian andesites are typical high-Mg# adakites. They They are also enriched in LREE and LILE, but without also show positive Pb and Sr anomalies on the spider depletion of HFSE. The positive Nb, Ta anomalies and diagram (Fig. 6c). The Yixian andesites have moderate negative Pb anomalies on the spider diagram distinguish them from the Lanqi and Yixian lavas. In addition, they 87 86 have low Sr/ Sri (b0.704), high ɛNd(T) (N4), and radiogenic Pb (206Pb/207PbN17.8, 207Pb/204PbN15.4, and 208Pb/204PbN37.6), 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

5.1.1. The Xinglonggou Formation The age of the Xinglonggou Formation is controver- sial. Chen et al. (1997) reported Ar–Ar ages of 188– 194 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 188–194 Ma (Chen et al., 1997) are not reliable because the sample has experi- enced significant alteration (Loss on Ignition=8.3%) and the age is defined by only 40% of total released 39Ar. 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). Fig. 7. Initial Sr–Nd–Pb isotopic compositions of the Mesozoic Actually, the age of 159±3 Ma for Xinglonggou rhyolite volcanic rocks in Western Liaoning. Data source: lower and upper is consistent with the age of 160±6 Ma for the Lanqi crust is after Jahn et al. (1999); MORB, Hofmann (1997); N-MORB, rhyolite reported in this paper. Furthermore, a recent field Zindler and Hart (1986); marine sediments/upper crust, White investigation demonstrates that the age of 159±3 Ma (2005); NHRL stands for Northern Hemisphere Reference Line 207 204 206 204 reported by Gao et al. (2004) may indicate the intrusion (Hart, 1984). ( Pb/ Pb)NHRLB=0.1084×( Pb/ Pb) +13.491; 208 204 206 204 ( Pb/ Pb)NHRL =1.209×( Pb/ Pb)+15.627; the Xinglonggou time of the rhyolitic dike in the Xinglonggou village, as formation: Gao et al. (2004), Li (2006), and this paper; Lanqi formation, their samples were actually collected from a rhyolitic Li et al. (2004) and this paper; Yixian, Ji et al. (2004) and this paper; dike rather than any representative rocks from the Zhanglaogongtun formation, Zhang et al. (2003) and this paper. Solid Xinglonggou Formation. Therefore, the SHRIMP U–Pb symbols and open symbols represent data from this paper and from the literature, respectively. Initial isotopic ratios were calculated by using zircon age of 176.7±3.5 Ma reported in this paper is a 177 Ma, 160 Ma, 125 Ma, and 106 Ma for the Xinglonggou, Lanqi, more reasonable age for the Xinglonggou Formation Yixian, and Zhanglaogongtun lavas, respectively. than the literature ages. W. Yang, S. Li / Lithos 102 (2008) 88–117 107

5.1.2. 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 Forma- tion. 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 40Ar/39Ar 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 (174– 173 Ma) are apparently higher than the others (166– Fig. 8. Sr/Y versus Y diagram after Defant et al. (2002), where the 148 Ma). Their older ages were obtained from an adakite area is defined by Sr/YN40 (Martin, 1999; Martin et al., 2005). andesite that lies unconformably above the Paleozoic or Most Xinglonggou and Yixian samples have high Sr/Y and La/Yb but Proterozoic strata and they are not surely correlated with low Y contents similar to typical adakites, while the Lanqi andesites the Tiaojishan and Lanqi formations. Collectively, these have relatively higher Y content and lower Sr/Y. Data source: the ages indicate an age range of ca. 166–148 Ma for 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. Tiaojishan and Lanqi Formations. (2005) and this paper; Zhanglaogongtun formation, Zhang et al. (2003) and this paper. Solid symbols and open symbols represent data from 5.1.3. The Zhanglaogongtun Formation this paper and from the literature, respectively. The Ar–Ar plateau age of ca. 106 Ma and the previous reported Ar–Ar and K–Ar ages of ca. 109– 93 Ma (Zhang et al., 2003; Zhu et al., 2002) suggest that the age of this formation is between 109 and 93 Ma. convecting mantle (e.g., Xu et al., 2002a; Chung et al., 2003; Gao et al., 2004). Experimental studies of partial 5.2. Origin of the Xinglonggou andesites melting of basalts at high pressure (N1.2 GPa) show that (not plagioclase) can occur as a liquidus phase or The Xinglonggou Formation (ca. 177 Ma) consists residual mineral in equilibrium with high SiO2 and mainly of andesites with geochemical features similar to Al2O3 adakitic melt, resulting in the high Sr and LREE, those of typical adakites: high SiO2 (N56 wt.%), Al2O3 but low HREE and Y contents which are characteristic (N15 wt.%), and Sr contents (N400 ppm), high Sr/Y features of adakitic magma (e.g., Rapp and Watson, (N40) and La/Yb (N20), and low Yb (b1.9 ppm) and Y 1995; Rapp et al., 1999). contents (b19 ppm) (Defant and Drummond, 1990; Kay The origin of the Xinglonggou andesites is still in and Kay, 1993; Martin, 1999; Martin et al., 2005). As debate. Because of a lack of mafic rocks in the shown in Fig. 8, the Xinglonggou andesites have high Xinglonggou Formation and the high Ni+Cr contents Sr/Y and low Y content similar to other Mesozoic of the Xinglonggou andesites, it is unlikely that the adakites observed in Eastern China, such as Ningzhen Xinglonggou andesites were produced by assimilation (Xu et al., 2002a), – (Xu et al., 2006), and fractional crystallization (AFC) processes involving and low-Mg adakites from the Dabie orogen (Wang basaltic or mixing between basaltic and et al., 2007; Xu et al., 2007). The high Mg# (57) of the magmas (Gao et al., 2004). Gao et al. (2004) suggested Xinglonggou andesite indicates that it is high-Mg# that the Xinglonggou high-Mg# adakites were produced adakite. by partial melting of foundered lower continental crust, Adakites were originally considered to be produced based on the abundant inherited Archaean zircons in these by partial melting of young and hot subducted oceanic rocks. However, the following several lines of evidence slabs (Defant and Drummond, 1990). However, alter- argue against the generation of the Xinglonggou high- native petrogenetic processes could also produce Mg# adakite by partial melting of the foundered lower adakitic rocks, e.g., fractional crystallization of basaltic continental crust. First, the radiogenic Pb isotopic magmas and assimilation of felsic crustal materials composition of the Xinglonggou high-Mg# adakites (Castillo et al., 1999), basaltic and felsic magma mixing reported in Li (2006) and this study may suggest a (e.g., Guo et al., 2007), and partial melting of mafic derivation from partial melting of subducted oceanic continental lower crust that has foundered into the crust, instead of lower continental crust. As shown in 108 W. Yang, S. Li / Lithos 102 (2008) 88–117

207 204 Fig. 7b, the Pb/ Pbi of the Xinglonggou high-Mg# LREE, and depletion of HFSE, and positive Pb anomaly adakites are higher than those of both the lower (Fig. 6). These trace element signatures are similar to arc continental crust and N-MORB, but they are similar to magmas or continental crust. It has been suggested that that of the upper continental crust and the marine such trace element signatures of arc magmas could reflect sediments. Thus, reactions between melt derived from the mantle source metasomatized by slab-derived fluids. the lower continental crust and the mantle cannot produce Because LREE, LILE, and Pb are more fluid-soluble than such radiogenic Pb isotopic compositions of the Xin- HREE and HFSE (e.g., Brenan et al., 1995; Keppler 1996; glonggou high-Mg# adakites. Second, the ɛNd(T) values Kogiso et al., 1997; Peate et al., 2001; Manning 2004), and initial 87Sr/86Sr ratios of the Xinglonggou adakites are fluids derived from subducted oceanic crust and sediments unlikely to be produced by reactions of partial melts from usually have high LREE/HREE and LILE/HFSE ratios the foundered eclogitic lower continental crust with and positive Pb anomaly as well as high 87Sr/86Sr. The mantle , either. Gao et al. (2004) suggested mantle wedge metasomatized by the subducting slab- that the Sr–Nd isotopic compositions of the Xinglonggou derived fluids can account for the trace element signatures adakites could be interpreted as a result of mixing of the of arc lavas, and also increase the Rb/Sr ratios and result in Xuhuai and whole peridotite of the upper the EMII isotopic features. However, as shown on Fig. 7, 87 86 mantle. However, experimental studies show that, during the Lanqi basalts have moderate Sr/ Sr, low ɛNd(t), and melt-peridotite interaction, because olivine is under- unradiogenic Pb isotopic ratios. Therefore, it is unlikely saturated in SiO2, the SiO2-rich melt may only assimilate that the Lanqi basalts derived from fluid-metasomatized olivine (Rapp et al., 1999, 2005). Considering the mantle because of their EMI-type isotopic features. olivine/melt −5 extremely low DNd (e.g., 7×10 in Suhr et al. The known terrestrial reservoirs able to evolve such (1998)) and thus low Nd content in olivine, assimilation of low 206Pb/204Pb are the ancient (Archaean/Proterozoic) olivine may not significantly change the Nd isotopic continental lower crust and EMI-type sub-continental composition of adakitic melts. Experimental studies also lithopheric mantle (e.g., Smoky Butte lamproites, Fraser show that while the reaction between melt and olivine can et al., 1985). The Lanqi basalts are also unlikely to be increase the incompatible element concentrations, but it derived from partial melting of an EMI-type sub- does not significantly modify the ratios between those continental lithospheric mantle, because this suggestion incompatible trace elements (Rapp et al., 1999, 2005). requires that the lithospheric mantle not only preserves a Therefore, assimilation of mantle olivine by adakitic melts low-μ signature for a long time, but also have higher Ce/ cannot change either incompatible element ratios (e.g., Pb ratio (N40). For example, the EMI-type mantle LILE/HFSE) or Sr–Nd–Pb isotopic compositions, while xenoliths discovered in the Tertiary alkaline basalt inform the high Mg, Cr, and Ni contents of the high-Mg# adakites the Taihang region have ɛNd(T) values ranging from −6.9 might result from interaction between silicic melts and to −10.6 and their Ce/Pb range from 41.5 to 72.0 (Ma and mantle peridotite. Xu, 2006), similar to the Ce/Pb (N40) of the Smoky Butte In summary, the Sr–Nd–Pb isotopic compositions lamproites with negative Pb anomalies (Fraser et al., and high MgO, Cr, and Ni contents suggest that the 1985). But these features are inconsistent with the Lanqi Xinglonggou high-Mg# adakites are more likely to be basalts with positive Pb anomalies and low Ce/Pb ratios produced by reaction between partial melts from (b10). In addition, the unradiogenic Pb isotopes are not subducted oceanic crust and mantle peridotite. The consistent with the observation that the mantle xenoliths abundant inherited Archaean zircons may come from hosted in the Paleozoic kimberlites from Shandong and terrigenous sediments carried by subducted oceanic eastern Liaoning have EMII-type Sr–Nd isotopic features crust. Such terrigenous sediments, derived from the with 206Pb/207Pb varying between 18 and 20 (Zheng and 206 204 upper continental crust of the NCC, could contain Lu, 1999). Indeed, a Pb/ Pbi as low as 16.2 in abundant Archaean zircons, and have evolved Sr–Nd– continental lower crust is most likely related to the time Pb isotopic compositions. effect of U–Th depletion resulting from granulite-facies (Rollinson, 1993; Rudnick and Fountain, 5.3. Origin of the Lanqi lavas 1995). Thus, the lower continental crust of the North China Craton may be important in generating the 5.3.1. The Lanqi basalts geochemical features of the Lanqi basalts. The Lanqi basalts have SiO2 of 52–53 wt.%, high The contribution of the lower continental crust to the Al2O3 (N16.5 wt.%), CaO (N6.8 wt.%), low MgO source of mantle-derived basaltic igneous rocks has been (b3.5 wt.%), Cr (b6ppm),Ni(b10 ppm), Th (b2ppm), recognized in the genesis of the Plio-Pleistocene tholeiitic and U (b0.5 ppm) contents, enrichment of LILE and and alkaline volcanic rocks in Sardinia (Italy) (Lustrino W. Yang, S. Li / Lithos 102 (2008) 88–117 109 et al., 2000). In this case, the lavas have low Nb/U, Ce/Pb, guish them from the Xinglonggou and the Yixian and unradiogenic Pb isotopes. Consequently, the negative andesites. Enrichment of LREE relative to HREE, high Nb anomaly, low Ce/Pb, and low 206Pb/204Pb of the Lanqi LILE/HFSE, positive Pb anomalies, low Ce/Pb ratios, basalts may also suggest involvement of the lower crust and EM-I type isotopic compositions of the Lanqi materials in their formation process. andesites indicate a derivation from partial melting of The Lanqi basalts have extremely low Ni and Cr the continental lower crust. In addition, the negative Sr contents and lower Th, U contents than the Yixian lavas, and Eu anomalies suggest plagioclase as a residual or suggesting that the Lanqi basalts might have experi- crystallizing phase. Thus, the Lanqi andesites could be enced significant olivine fractional crystallization from formed by partial melting of the lower crust during primary melts. Fig. 9 shows Ni–MgO relationships for basaltic magma underplating. partial melting and fractional crystallization models (Hart and Davis, 1978). Ni content decreases signifi- 5.4. Origin of the Yixian lavas cantly with olivine fractional crystallization. If primary magmas have MgO contents of 12%, N20% olivine 5.4.1. The Yixian basalts crystallization of these primary melts could produce the Basalts from the Yixian Formation are similar to the residual liquids with such low Ni (b16 ppm) and low Lanqi basalts in trace elements and isotopic composi- MgO (b3%) contents. However, olivine fractional tions except for their higher Cr and Ni contents crystallization alone will increase Th, U contents of (Supplementary Tables 2 and 4 and Fig. 2). Enrichment the melt, which is not the case for the Lanqi basalts. of LREE relative to HREE, high LILE/HFSE, positive Therefore, we propose that the basalts underwent AFC Pb anomalies, EMI-type isotopic compositions, and low process at the crust–mantle boundary so that assimila- Nb/U and Ce/Pb ratios strongly indicate the “continental tion of plagioclase from the mafic granulites can lower crust” signatures for the basalts from the Yixian produce very low Th, U, Cr, and Ni, but high Al2O3 Formation. Thus, the continental lower crust might also and CaO contents in the Lanqi basalts. play an important role in generating the Yixian lavas. Because the basalts from the Yixian Formation have 5.3.2. The Lanqi andesites and rhyolites higher MgO, Cr, Ni, Th, U contents and lower Al2O3, The very low ɛNd(T) values and negative Sr and Eu CaO contents than those from the Lanqi Formation, the anomalies of the Lanqi andesites and rhyolites distin- model of magmatic underplating with AFC process is not suitable for the Yixian basalts. The Yixian basalts could be derived from a newly enriched lithosphere mantle formed by ascending asthenosphere hybridized by melt produced by partial melting of the foundering mafic lower continental crust (see below) (Lustrino, 2005). In addition, relatively high ɛNd(T) values (0∼−2) for the basalts with K–Ar age of 133 Ma developed on the bottom of the Yixian Formation in the Xinkailing village, Yixian county have been reported in the literature (Ji et al., 2004; Shao et al., 2006) (see Fig. 11). These olivine basalts with high ɛNd(T) values may suggest an earlier partial melting of the upwelling mantle before hybridization by partial melts from the foundered mafic lower continental crust.

– Fig. 9. Ni MgO relationships for partial melting-fractional crystalli- 5.4.2. The Yixian high-Mg# adakites zation models (after Hart and Davis, 1978). Model mantle peridotite composition and Ni partition coefficients are after Hart and Davis The Yixian high-Mg# adakites show similar geo- (1978). Partial melting curves are shown for batch partial melts of 5% chemical characteristics to the Xinglonggou high-Mg# and 20%. Fractional crystallization curves are shown for liquids adakites, such as SiO2%N56 wt.%, Al3O2%N15 wt.%, starting on the 5% melting line with MgO contents of 8, 12, 16 and MgOb3 wt.%, Yb18 ppm, Ybb1.9 ppm, SrN400 ppm, 20%. Numbers at cross-tics are the amount of olivine crystallized. no negative Eu anomaly, Mg#N54, CrN200 ppm, Solid squares represent the Lanqi basaltic samples. This diagram N shows that the Lanqi basalts with low MgO and Ni contents could be and Ni 100 ppm. However, the Yixian adakites are produced by 20% olivine fractional crystallization of a primary basaltic different from the Xinglonggou high-Mg# adakites in magma with MgO=12%. the Sr–Nd–Pb isotopic compositions. The Yixian 110 W. Yang, S. Li / Lithos 102 (2008) 88–117

206 207 adakites have unradiogenic Pb ( Pb/ Pbb16.7, been metasomatized by a SiO2-rich melt before the late 207Pb/204Pbb15.3, and 208Pb/204Pbb36.7), moderate early Cretaceous to reconcile the contradiction between 87 86 Sr/ Sri (∼0.706), and low ɛNd(T) (b−9.7). These Sr– the depleted Nd isotopic and LREE enriched trace Nd–Pb isotopic compositions and positive Ba anomaly element signatures. The relatively high Nb and Ta relative to Rb and Th are similar to those of the lower contents of the Zhanglaogongtun basalts suggest that the continental crust (Rudnick and Gao, 2003). Therefore, SiO2-rich melt for mantle metasomatism may be the Yixian high-Mg# adakites are more likely to be produced by partial melting of the dehydrated subducted derived by partial melting of a foundered mafic lower oceanic crust (Sajona et al., 1996). continental crust instead of subducted oceanic crust. Large scale E–W extension in Eastern China In addition, although the Yixian lavas are evolved in occurred during the period of the Zhanglaogongtun terms of major elements and compatible trace element, basalt eruption (109∼94 Ma), responding to the all the Yixian lavas can be divided into two groups as subduction of the western Pacific oceanic plate beneath shown by the two negative correlations in the Nb/Ta– the eastern Asiatic continental margin (Zhao et al., SiO2 diagram (Fig. 10): the basaltic series and the 2004). Geochronologic studies indicate that the timing adakitic series. Therefore, it is unlikely that the Yixian for the strike-skip deformation of the Tan-Lu fault zone adakites were produced via AFC processes involving was ca. 143∼114 Ma, and then the Tan-Lu fault basalt and magma mixing. transferred into extension in the late early Cretaceous and the Tertiary (Zhu et al., 2001a,b, 2005). The 5.5. Origin of the Zhanglaogongtun basalts extensional activities resulted in development of a series of basins. The Fuxin Formation, discomformably The geochemical and isotopic compositions of the underlying the Zhanglaogongtun Formation, is cut by Zhanglaogongtun basalts are similar to those of the the fault-bounded depressions (Cheng et al., 1999). This Cenozoic Kuandian and Hannuoba basalts (Zhou and observation indicates that the Zhanglaogongtun basalts Armstrong, 1982; Peng et al., 1986; Song et al., 1990; erupted during the large-scale extension. The extension Basu et al., 1991; Liu et al., 1995a,b; Barry and Kent, induced upwelling and de-compressional partial melting 1998) and Mesozoic Jianguo basalts (Zhang et al., of asthenosphere. Lavas derived from the upwelling 2003). Because Zhanglaogongtun basalts show low asthenosphere accordingly have the isotopic signatures 87 86 Sr/ Sri (b0.704), high ɛNd(T) (N4), and radiogenic of depleted mantle. Pb, sharing some common features of modern MORB The Nd isotope data and ages of the Mesozoic basalts and OIB, they are interpreted to derive from the in western Liaoning are summarized in Fig. 11,showing asthenospheric mantle. However, the enriched LREE that the basalts with depleted Nd isotopic compositions and LILE as well as the positive Nb and Ta anomalies erupted twice in Western Liaoning at about 133 Ma and suggest that the upwelling asthenosphere must have 109–94 Ma. This indicates two tectonic extensions and mantle upwelling events. The first upwelling of the asthenospheric mantle is of relatively short duration, which could be due to the foundering of the mafic lower continental crust, as indicated by the Yixian high-Mg# adakites. The second upwelling of the asthenospheric mantle is significant and related to the large scale E–W extension mentioned above, which occurred earlier than that (75 Ma) in Shandong Province (Xu et al., 2004a and references therein). This observation indicates that the late Mesozoic extensional activities in Eastern China occurred earlier in the north than in the south, which is consistent with the clockwise rotation of the Korea Peninsula relative to the Eurasia plate since Early Cretaceous (Zhu et al., 2002). Fig. 10. Nb/Ta vs. SiO2 diagrams for the Yixian lavas showing two negative correlation trends: basaltic series and andesite series, 5.6. A lithospheric evolution model indicating two independent magma evolution trends. Data source: Ji et al. (2004), Wang et al. (2005) and this paper. Solid symbols and open symbols represent data from this paper and from the literature, The mechanism for the lithospheric thinning of the respectively. NCC is still controversial. There are tree representative W. Yang, S. Li / Lithos 102 (2008) 88–117 111

asthenosphere (Niu, 2005). Obviously, hydro-weaken- ing of the SCLM is very important for lithospheric thinning or destruction, however, weakening alone can not explain the Mesozoic high-Mg# adakites in the NCC. Perhaps the lithospheric thinning of the NCC is a complex and multi-stage process and was not caused by any single event. The reported age and geochemical data for the Mesozoic volcanic rocks in the Western Liaoning are very useful in understanding the lithospheric evolution. Based on the geochronological, geochemical data of the Mesozoic volcanic rocks, we propose a geodynamic model for the lithospheric evolution of the North margin ɛ Fig. 11. Variation of Nd(T) with time of the mafic rocks from Western of the NCC. Liaoning. Data source: Lanqi formation, this paper; Yixian, Ji et al. (2004), Shao et al. (2006), and this paper; Zhanglaogongtun formation, Zhang et al. (2003) and this paper. 5.6.1. Partial melting of the Palaeo-oceanic crust The Palaeo-oceanic lithosphere subducted beneath the northern margin of the NCC along the Solonker models, i.e., the thermal mechanical and chemical suture at 300–250 Ma (Xiao et al., 2003). The erosion model (Menzies and Xu, 1998; Xu, 2001; Xu Xinglonggou adakite is likely to be produced by partial et al., 2004a,b), the delamination model (Gao et al., melting of this subducted oceanic crust. We suggest that 2004; Wu et al., 2005), and hydro-weakening of the the Palaeo-oceanic lithosphere was underplating be- subcontinental lithospheric mantle due to subduction neath the north margin of the NCC after subduction, and dehydration (Niu, 2005). The first model proposes that then it was foundered in the Jurassic, which was caused the lithosphere was gradually removed by thermal and by the collision between the NCC with the attached chemical erosion from upwelling asthenosphere, which southern Central Asian Orogen and the northern Central is supported by their complied data showing continuous Asian Orogen in the early middle Jurassic (Tomurtogoo magmatism in the eastern China during the Mesozoic et al., 2005). The foundered oceanic crust was partially from 180 to 90 Ma (e.g. Xu, 2001). However, this model melted at 177 Ma. is not consistent with our new dating results showing four Mesozoic magmatic episodes in the Western 5.6.2. Basaltic magma underplating Liaoning instead of a continuous magmatism from The geochemical features of the Lanqi basalts 180 Ma to 90 Ma. The second model proposes that the indicate a magma underplating event which caused lithospheric thinning resulted from foundering of the partial melting of the low-middle crust to produce the mafic lower continental crust (together with the voluminous low-Mg andesites and acidic volcanic rocks underlying lithospheric mantle). This is supported by overlying the Lanqi basalts. Foundering of the Palaeo- the discovery of the Xu-Huai (Xu et al., 2002b), oceanic crust may trigger mantle upwelling and its which suggests the existence of thickened continental partial melting, which resulted in the magma under- crust of the NCC during the Mesozoic. It is generally plating event at ca. 166 Ma. Accordingly, the mafic suggested that following the lithospheric delamination, lower continental crust in the western Liaoning area was de-compressional melting of the ascending astheno- thickened. The newly formed mafic lower crust had spheric mantle will take place, creating basaltic melt with typical geochemical features of the lower continental depleted isotopic signature. Thus the geochemical crust of the NCC, which was overprinted by the AFC signatures of the mantle-derived rocks should change process as mentioned above. abruptly following delamination. Therefore, this model was inconsistent with foundering of the thickened lower 5.6.3. Pre-135 Ma thrust tectonics continental crust at 159 Ma, as suggested by Gao et al. The Pre-135 Ma thrust tectonics in the Yanshan belt (2004), because the basalts with depleted isotopic is marked by coarseclastic deposits of the Tuchengzi signatures in the NCC are not been observed until formation (Davis et al., 2001; Zhao et al., 2004). This 134 Ma (Shao et al., 2006). The third model suggested process may further thicken the crust, resulting in that hydro-weakening of the SCLM due to subduction eclogitic facies transformation in the mafic lower dehydration may transfer the lithosphere mantle to crust. 112 W. Yang, S. Li / Lithos 102 (2008) 88–117

5.6.4. Lower crustal foundering depleted in HFSE. They also have low Ce/Pb, The large-scale strike-skip of the Tan-Lu fault in the Nb/U ratios, low ɛNd(t) (− 14∼−9), high early Cretaceous caused regional extension and the 87Sr/86Sr (0.705∼0.707), and unradiogenic Pb destruction of the lithosphere underneath the northern isotopes. These features suggest the involvement margin of the NCC. This triggered foundering of the of lower continental crustal materials in their thickened mafic lower continental crust. We emphasize magma source followed by olivine fractional that this process was unlikely to have been caused by crystallization. Thus, magmatic underplating any single event. Both convective removal of the with an AFC process at the crust–mantle mantle lithosphere from the bottom, and foundering boundary is a reasonable model for the origin of the mafic lower crust could have significantly of the Lanqi lavas, which could provide the heat thinned the lithosphere. Reaction of partial melts required for partial melting of the low-middle from the foundered eclogitic lower continental crust crust to produce the voluminous andesites and with mantle peridotite produced the Yixian high- rhyolites overlying above the basalts in the Lanqi Mg# adakites. Partial melting of the upwelling as- Formation. thenospheric mantle in response to foundering of lower (4) The Yixian high-Sr, low-Y andesites are high- continental crust produced the Yixian high ɛNd(T) Mg# adakitic rocks. Their Sr–Nd–Pb isotope basalts, and then the ascending asthenosphere hybrid- compositions show typical “lower continental ized by partial melts from the foundered mafic lower crust” features. This suggests that the Yixian continental crust formed a newly enriched lithospheric high-Mg# adakitic rocks are likely to be derived mantle which could be the source of the Yixian low ɛNd from partial melting of foundered lower conti- (T) basalts. nental crust, which resulted in the initial litho- sphere thinning in the early Cretaceous. The 5.6.5. Large-scale extension Yixian basalts and basaltic andesites show similar Large-scale late Cretaceous extension in eastern geochemical characteristics to the Lanqi basalts China, responding to the subduction of the western except the lower Mg, Ni, and Cr contents of the Pacific plate beneath the eastern Asiatic continental latter. The Yixian basalts could be derived from a margin, resulted in further thinning of the lithosphere. newly enriched lithospheric mantle formed by The Zhanglaogongtun basaltic lavas were derived from ascending asthenosphere hybridized by partial upwelling asthenosphere and accordingly show astheno- melts from foundered lower mafic continental sphere isotopic signatures. crust. (5) The geochemical and isotopic compositions of 6. Conclusions the Zhanglaogongtun basalts are similar to those of the Cenozoic alkali basalts in Eastern China. Integrated geochronologic, major and trace elemental They could be derived from the long-term and Sr–Nd–Pb isotope studies of the Mesozoic lavas in depleted mantle enriched by metasomatism of Western Liaoning allow us to reach the following SiO2-rich melt derived from subducted oceanic conclusions: crust of Western Pacific plate in the Cretaceous. This depleted mantle upwelling event occurred in (1) Geochronologic studies suggest that there were four response to large-scale late Cretaceous extension episodes of Mesozoic volcanism in Western Liaon- in Eastern China and resulted in further thinning ing, corresponding to the Xinglonggou Formation of the lithosphere. (ca. 177 Ma), the Lanqi Formation (166–148 Ma), the Yixian Formation (126–120 Ma), and the Acknowledgements Zhanglaogongtun Formation (∼ 106 Ma), respectively. This work was funded by the Natural Science (2) The Xinglonggou high Mg andesites with typical Foundation of China (Grant. No. 40573010) and island arc-type Sr–Nd–Pb isotopic features were Chinese Academy of Sciences (CX0767). We thank produced by partial melting of the subducted Z.W. Lu, J.G. Sha, S. Gao, J.A. Shao, H.F. Zhang for field oceanic crust at ca. 177 Ma. support, and X.L. Tu, H. Qian, A.L. Zheng, Fei Wang, (3) The basalts from the Lanqi Formation are high in Fang Wang, P. 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