Journal of Petrology Advance Access originally published online on January 17, 2008
Journal of Petrology 2008 49(2):315-351; doi:10.1093/petrology/egm083
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Petrogenesis of an Alkali Syenite–Granite–Rhyolite Suite in the Yanshan Fold and Thrust Belt, Eastern North China Craton: Geochronological, Geochemical and Nd–Sr–Hf Isotopic Evidence for Lithospheric Thinning
1State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Po Box 9825, Beijing 100029, China
2Department of Applied Geology, Curtin University of Technology, Po Box U1987, Perth, WA 6845, Australia
3State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, XI'AN 750069, China
RECEIVED JUNE 3, 2006; ACCEPTED DECEMBER 3, 2007
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ABSTRACT |
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The Yanshan Fold and Thrust Belt in eastern China has been intruded by a series of alkalic igneous rocks, ranging in composition from granite and rhyolite to syenite and trachyte. Laser ablation inductively coupled plasma mass spectrometry U–Pb analyses of zircon from three alkaline suites yield Early Cretaceous ages of 130–117 Ma. Three groups of rocks have been identified based on their mineralogical, geochemical and Sr–Nd–Hf isotope characteristics. The alkali granites and rhyolites are ferroan and have low Al2O3, MgO, CaO, Sr, Ba and Eu concentrations and high SiO2, total Fe2O3, K2O, Nb, Ga, Ta, Th and heavy rare earth element abundances and Ga/Al ratios. Geochemical data and Sr-, Nd- and zircon Hf-isotopic compositions [(87Sr/86Sr)i = 0·7050–0·7164,
Nd(t) = –8·4 to –13·6 and Hf(t) = –5·7 to –16·8] indicate that they were probably generated by shallow dehydration melting of biotite- or hornblende-bearing granitoid crustal source rocks and then mixed with contemporaneous magma from a mantle and/or lower crustal source. Ferroan syenites have distinct geochemical features from those of the alkaline granites and rhyolites, suggesting that they were produced by clinopyroxene and plagioclase fractionation of melt derived from an enriched mantle source, mixed with lower and upper crustal-derived magmas. The magnesian syenites and trachytes have Sr-, Nd- and zircon Hf-isotopic compositions that are distinct from those of the ferroan syenites. They were mainly derived from partial melting of lower crustal materials, mixed with enriched mantle-derived alkali basaltic magma. The emplacement of an alkali syenite–granite–rhyolite suite, coeval with the formation of metamorphic core complexes and pull-apart basins in eastern China, indicates they formed in an extensional setting, possibly as a result of lithospheric thinning. KEY WORDS: alkaline rocks; zircon U–Pb dating; petrogenesis; crustal extension; Yanshan Fold and Thrust Belt; North China Craton
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHYANALYTICAL METHODSGEOCHRONOLOGYGEOCHEMISTRY AND ISOTOPIC...DISCUSSIONCONCLUSIONSREFERENCES |
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Well-exposed alkaline complexes are widely distributed within the Yanshan Fold and Thrust Belt (YFTB), North China Craton (NCC) (e.g. Xu et al., 1994
, 1996, 1999; Yan et al., 2000, 2001; Wei et al., 2002) and contain a suite of alkalic igneous rocks that includes alkali-feldspar granite, alkali amphibole granite, syenite and associated alkali rhyolite and trachyte. Although several studies have previously reported aspects of the geochronology, mineralogy, petrography and geochemistry (Xu et al., 1994, 1996, 1999; Yan et al., 2000, 2001; Davis et al., 2001; Wei et al., 2002), there is still controversy concerning the age and petrogenesis of these alkalic igneous rocks (Xu et al., 1994; Davis et al., 2001; Wei et al., 2002). We present new zircon U–Pb ages, whole-rock geochemistry, and Sr–Nd and zircon Hf isotopic compositions for three of these alkali syenite–granite–rhyolite suites in the YFTB and use these data to trace their sources and petrogenesis to help constrain the tectonic setting in which they evolved. |
GEOLOGICAL SETTING |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHYANALYTICAL METHODSGEOCHRONOLOGYGEOCHEMISTRY AND ISOTOPIC...DISCUSSIONCONCLUSIONSREFERENCES |
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East China is composed of the Central Asian Orogenic Belt (CAOB) in the north, the NCC in the centre and the Yangtze Craton in the south (Fig. 1a; Wang & Mo, 1996
). The YFTB, including Beijing, northern Hebei and western Liaoning Province, is located in the northeastern part of the NCC and is widely considered to be the eastern segment of a major east–west-trending orogenic system of Jurassic–Cretaceous age (Wong, 1928; Davis et al., 1998, 2001; Yang et al., 2006a) (Fig. 1b).
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The YFTB consists of Early Archean to Paleoproterozoic basement rocks overlain by unmetamorphosed Mesoproterozoic to Cenozoic cover successions (Fig. 1b). Early Archean basement rocks, which range in age from 3·85 to 3·2 Ga, have been reported from Eastern Hebei (Liu et al., 1992
). Late Archean basement rocks consist predominantly of 2·6–2·5 Ga tonalite–trondhjemite–granodiorite (TTG) gneisses, 
2·5 Ga syntectonic granites and a variety of supracrustal rocks that underwent greenschist- to granulite-facies regional metamorphism and polyphase deformation at 2·5 Ga (Jahn et al., 1984; Kröner et al., 1998). Paleoproterozoic (2·1–1·9 Ga) rocks overlie or are interleaved with the Late Archean rocks, commonly in tectonic contact (Zhao et al., 2001, 2002, 2005; Wilde et al., 2002). The main deformation and metamorphism of the Precambrian basement occurred at 1·85 Ga and is considered to mark cratonization of the NCC (Zhao et al., 2001, 2005). Thick sequences of predominantly clastic sedimentary rocks were deposited across the YFTB between 1·85 and 1·40 Ga, forming the Changcheng Group (Li et al., 1985). Paleo- to Mesoproterozoic volcanic rocks and granitoid plutons are also present locally in the YFTB (Rämö et al., 1995; Yang et al., 2005a). Early Paleozoic Cambrian–Middle Ordovician strata are dominated by neritic carbonates, whereas a Late Carboniferous to Early Permian alternating marine and terrestrial sequence is characterized by carbonates and coal-bearing rocks, overlain by Late Permian–Triassic red beds and conglomerates.
The eastern NCC underwent crustal extension and lithospheric thinning from the Late Jurassic to Early Cretaceous (e.g. Menzies et al., 1993; Griffin et al., 1998; Xu, 2001). It is characterized by the formation of metamorphic core complexes (MCCs) (e.g. Davis et al., 1998, 2001; Zhang et al., 2003; Yang et al., 2007a) and pull-apart basins (e.g. Meng, 2003), with regional granitic magmatism (Wei et al., 2002; Wu et al., 2005) (Fig. 1a) and large-scale gold mineralization (Yang et al., 2003). Igneous rocks associated with this event include diorite, granodiorite, monzogranite, syenogranite, alkali feldspar granite, alkali amphibole granite and syenite (Fig. 1b), and associated volcanic rocks (e.g. Bai et al., 1999; Zhou et al., 2001; Liu et al., 2002; Wu et al., 2005).
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PETROGRAPHY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHYANALYTICAL METHODSGEOCHRONOLOGYGEOCHEMISTRY AND ISOTOPIC...DISCUSSIONCONCLUSIONSREFERENCES |
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In this study, three representative alkaline complexes in the YFTB were chosen for investigation: the Xiangshan pluton, the Houshihushan ring complex, and the Qiancengbei–Wulingshan complex (Fig. 1). The Xiangshan pluton (
220 km2, Fig. 1c) consists mainly of arfvedsonite granite, with minor alkali-feldspar granite. The Houshihushan ring complex (80 km2) is a high-level intrusion in the eastern YFTB that contains arfvedsonite granite, quartz syenite, alkali rhyolite and trachyte (Fig. 1d). An outer annulus of syenite surrounds a central area of alkali trachyte and rhyolite that were intruded by a plug of granite (Xu et al., 1996; Wei et al., 2002). The Qiancengbei–Wulingshan complex (650 km2) is composed of the Qiancengbei arfvedsonite granite and Wulingshan syenite plutons (Fig. 1e).
Arfvedsonite granite with a medium-grained, hypidiomorphic texture is a common component in all these intrusions. The main minerals are subhedral alkali feldspar (55–60%), quartz (25–30%) and plagioclase (5–6%). Granophyric intergrowths of perthite and quartz are common and albitic plagioclase occurs as a fine-grained interstitial phase. Mafic minerals make up 5% of the Xiangshan pluton and the Houshihushan complex and are chiefly arfvedsonite, with local relict cores of aegirine–augite. In contrast, the mafic minerals arfvedsonite and ferroan biotite make up 5–15% of the Qiancengbei pluton. Accessory minerals in the arfvedsonite granites include magnetite, ilmenite, zircon, apatite, cyrtolite, thorite and allanite.
Alkali-feldspar granite is a minor phase of the Xiangshan pluton. It is orange to red–brown, medium- to coarse-grained, and consists of alkali feldspar (60%), quartz (30%) and plagioclase (5%), with accessory (<1%) zircon, apatite, thorite and allanite. Granophyric intergrowths of perthite and quartz are common and albite occurs as a fine-grained interstitial phase.
Alkali syenite occurs in the Wulingshan pluton and Houshihushan ring complex. It is fine- to medium-grained, is locally porphyritic, and consists of alkali feldspar (60–80%), quartz (3–15%), plagioclase (<5–15%) and arfvedsonite (3–5%) with minor aegirine–augite. Arfvedsonite commonly forms rims around aegirine–augite and the latter locally encloses fayalite. Alkali feldspar occurs locally as phenocrysts from 3 to 7 cm long and some grains show concentric compositional zoning. Granophyric intergrowths of perthite and quartz are common. Accessory minerals include zircon, magnetite, apatite, thorite and allanite. Enclaves are common in the Wulingshan syenite and have similar mineral assemblages to the host syenites, but with more abundant mafic minerals, including arfvedsonite and aegirine–augite; they locally contain fayalite.
Volcanic rocks in the Houshihushan ring complex (Fig. 1d) consist of alkali rhyolite and trachyte. The alkali rhyolite is composed of a grey to white devitrified matrix enclosing phenocrysts of perthite, plagioclase and quartz in variable proportions, with accessory apatite and zircon. Total phenocryst abundance varies from 5 to 20% and in some samples preferred alignment of the feldspar defines a flow structure. Rounded and embayed quartz and perthite phenocryst margins suggest partial resorption prior to emplacement. No mafic phenocrysts have been identified, but locally fine-grained opaque minerals outline the ghosts of stumpy prismatic crystals, suggesting the prior existence of phenocrysts of Fe-olivine, pyroxene or amphibole. The groundmass is devitrified to a fine-grained intergrowth of quartz and K-feldspar, which may be elongated parallel to flow.
Alkali trachyte is leucocratic, aphanitic and porphyritic in texture and consists of a brown devitrified matrix enclosing phenocrysts of hornblende, pyroxene and perthite in variable proportions, with accessory apatite and zircon. Phenocryst abundance varies from 5 to 20% and the phenocrysts are largely composed of alkali feldspar, with minor amounts of mafic minerals. The rounding and embayment of quartz and perthite phenocryst margins again suggests partial resorption prior to emplacement. The groundmass is devitrified to a fine-grained mixture of quartz, K-feldspar and amphibole, with intergrowth of quartz and K-feldspar common.
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ANALYTICAL METHODS |
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Elemental analysis of 42 fresh rock samples was conducted at the Department of Geology, Northwest University in Xi'an. Major elements were determined by X-ray fluorescence (XRF), with analytical uncertainties ranging from 1 to 3%. Trace element (including rare earth element; REE) concentrations were determined by inductively coupled plasma mass spectrometry (ICP-MS) with an Agilent 7500a system, using the techniques described by Gao et al. (2002
).
Samples for Nd–Sr isotopic analysis were dissolved in Teflon bombs after being spiked with 84Sr, 87Rb, 150Nd and 147Sm tracers prior to HF + HNO3 (with a ratio of 2:1) dissolution. Rubidium, Sr, Sm, and Nd were separated using conventional ion exchange procedures as described by Yang et al. (2004) and measured using a Finnigan MAT 262 multi-collector (MC) mass spectrometer at the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing. Procedural blanks were <100 pg for Sm and Nd and <500 pg for Rb and Sr. The 143Nd/144Nd ratio was corrected for mass fractionation by normalization to 146Nd/144Nd = 0·7219, and 87Sr/86Sr ratios were normalized to 86Sr/88Sr = 0·1194. Typical within-run precision (2
) for Sr and Nd was estimated to be ± 0·000015. The measured values for the La Jolla and BCR-1 Nd standards and NBS-607 Sr standard were 143Nd/144Nd = 0·511853 ± 7 (2n, n = 3) and 0·512604 ± 7 (2n, n = 3) and 87Sr/86Sr = 1·20042 ± 2 (2n, n = 12) during the period of data acquisition.
Cathodoluminescence (CL) images of zircons were obtained prior to analysis, using a CAMECA SX-50 electron microprobe at the Institute of Geology and Geophysics, Chinese Academy of Sciences in Beijing, to characterize internal structures and choose potential target sites for U–Pb dating and Hf analyses. Laser ablation (LA)-ICP-MS zircon U–Pb analyses were conducted on an Agilent 7500a ICP-MS system equipped with a 193 nm laser, housed at the Department of Geology, Northwest University in Xi'an. Zircon 91500 was used as the standard and the standard silicate glass NIST 610 was used to optimize the machine: the beam diameter was 30 µm. The detailed analytical technique has been described by Yuan et al. (2004). The common-Pb correction followed the method described by Andersen (2002). Uranium, Th and Pb concentrations were calibrated using 29Si as an internal calibrant and NIST 610 as reference material. 207Pb/206Pb and 206Pb/238U ratios were calculated using the GLITTER program (van Achterbergh et al., 2001). The age calculations and concordia plots were made using ISOPLOT (version 3.0) (Ludwig, 2003).
Zircon Lu–Hf isotope analyses were conducted by MC-ICP-MS using a Neptune system, equipped with a 193 nm laser, at the Institute of Geology and Geophysics, Chinese Academy of Sciences in Beijing. Spot sizes of 32 and 63 µm were used for analysis, with a laser repetition rate of 10 Hz at 100 mJ. The detailed analytical procedure and correction for interferences followed that described by Wu et al. (2006). During analysis, the 176Hf/177Hf and 176Lu/177Hf ratios of the standard zircon (91500) were 0·282294 ± 15 (2n, n = 20) and 0·00031, similar to the commonly accepted 176Hf/177Hf ratio of 0·282306 ± 10 (1), measured using the solution method (Woodhead et al., 2004), and the 176Hf/177Hf ratio of 0·282284 ± 22, measured using the laser ablation method (Griffin et al., 2006).
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GEOCHRONOLOGY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHYANALYTICAL METHODSGEOCHRONOLOGYGEOCHEMISTRY AND ISOTOPIC...DISCUSSIONCONCLUSIONSREFERENCES |
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Previous dating of alkaline rocks in the YFTB was predominantly by whole-rock Rb–Sr or K–Ar methods on K-feldspar, and gave a large age range from 132 to 109 Ma (Xu et al., 1994
; Davis et al., 2001; Wei et al., 2002). These ages are unlikely to record the time of emplacement because of the relatively low closure temperatures of the isotopic systems. To determine the emplacement ages of the alkaline igneous rocks, 10 samples were chosen for zircon U–Pb dating from the Xiangshan pluton, Houshihushan ring complex and Qiancengbei–Wulingshan complex. Unfortunately, zircon obtained from the arfvedsonite granite (sample F04-036) from the Xiangshan pluton was strongly metamict and had high common lead values; this sample is thus excluded from further consideration. Representative CL images of analyzed zircon are shown in Fig. 2 and the U–Pb ages are listed in Table 1 and presented in concordia diagrams in Figs 3–5.
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Xiangshan pluton
Zircon grains from the alkali feldspar granite (F04-033) have a size range of 100–150 µm, a length/width ratio of 1:1 to 2:1, and show fine-scale oscillatory zoning in CL images (Fig. 2), suggesting a magmatic origin. Twenty-two analyses form a coherent group with a weighted mean 206Pb/238U age of 117 ± 1 Ma (2
; MSWD = 0·16) (Fig. 3a). This is interpreted as the best estimate of the time of crystallization of the alkali feldspar granite.
Houshihushan ring complex
Zircon grains from the arfvedsonite granite (F04-073) have a size range of 50–150 µm, a length/width ratio of 2:1 to 3:1, and show fine-scale oscillatory zoning or are homogeneous in CL images (Fig. 2). Twenty-seven analyses yield ages between 123 ± 3 Ma and 113 ± 3 Ma (1; Table 1), with a weighted mean 206Pb/238U age of 118 ± 1 Ma (2; MSWD = 1·11) (Fig. 4a). This is interpreted as the best estimate of the time of crystallization of the arfvedsonite granite.
Zircon grains from the syenite (F04-067) have a size range of 50–100 µm, a length/width ratio of 1:1 to 2:1, and show weak oscillatory zoning with local homogeneous patches in CL images (Fig. 2), suggesting a magmatic origin. Twenty-five analyses give ages ranging between 122 ± 5 Ma and 114 ± 3 Ma (1; Table 1), with a weighted mean 206Pb/238U age of 120 ± 1 Ma (2; MSWD = 0·60) (Fig. 4b), which is interpreted as the emplacement age of the syenite.
Zircon grains from alkali rhyolite sample F04-057 have a size range of 50–100 µm, a length/width ratio of 1:1 to 3:1, and show well-developed oscillatory zoning in CL images (Fig. 2), indicative of a magmatic origin. Thirty-two analyses record ages between 126 ± 4 Ma and 111 ± 3 Ma (1; Table 1), with a weighted mean 206Pb/238U age of 120 ± 1 Ma (2; MSWD = 0·89) (Fig. 4c). Zircon grains from another sample of alkali rhyolite (F04-062) also have a size range of 50–100 µm, with a length/width ratio of 2:1 to 4:1, and show fine-scale oscillatory growth zoning in CL images (Fig. 2), suggesting a magmatic origin. Twenty-nine analyses yield ages between 123 ± 3 Ma and 112 ± 3 Ma (1; Table 1), with a weighted mean 206Pb/238U age of 119 ± 1 Ma (2; MSWD = 0·89) (Fig. 4d), which is interpreted as the eruption age of the alkali rhyolite.
The crystallization ages of all samples are similar, within errors, indicating that the formation age of the Houshihushan granite–syenite–rhyolite alkali ring complex is 119 Ma.
Qiancengbei–Wulingshan complex
Zircon grains from the Wulingshan fine- to medium-grained syenite (F04-111) have a size range of 70–130 µm, with a length/width ratio of 2:1 to 4:1, and show fine-scale oscillatory growth zoning or are relatively homogeneous with lighter rims in CL images (Fig. 2), suggesting a magmatic origin for the oscillatory zoned parts of the crystals. Twenty-seven analyses yield ages between 133 ± 3 Ma and 126 ± 3 Ma (1; Table 1) and form a tight cluster on concordia, with a weighted mean 206Pb/238U age of 129 ± 1 Ma (2; MSWD = 0·49) (Fig. 5a), which is interpreted as the best estimate of the time of crystallization of the syenite.
Zircon grains from Wulingshan coarse-grained syenite sample F04-113 range in size from 50 to 150 µm, with a length/width ratio of 1:1 to 3:1, and show well-developed oscillatory zoning in CL images (Fig. 2), suggesting a magmatic origin. One grain gives a 206Pb/238U age of 142 ± 2 Ma, whereas the remaining 27 analyses yield ages between 133 ± 2 Ma and 128 ± 1 Ma (1; Table 1) and form a tight cluster on concordia, with a weighted mean 206Pb/238U age of 130 ± 1 Ma (2; MSWD = 0·41) (Fig. 5b), which is interpreted as the best estimate of the time of crystallization of this syenite.
Zircon grains from the Wulingshan porphyritic syenite (sample F04-114) have a size range of 100–200 µm, a length/width ratio of 1:1 to 2:1, and show fine-scale oscillatory zoning with or without an homogeneous core in CL images (Fig. 2), suggesting a magmatic origin. One grain gives a 206Pb/238U age of 148 ± 2 Ma, which is interpreted as indicating an inherited grain. The remaining 24 analyses yield ages between 134 ± 3 Ma and 128 ± 3 Ma (1; Table 1) and form a tight cluster on concordia with a weighted mean 206Pb/238U age of 130 ± 1 Ma (2; MSWD = 0·62) (Fig. 5c), which is interpreted as the best estimate of the time of crystallization of the porphyritic syenite.
The ages from the Wulingshan pluton are therefore the same, within errors, and are consistent with the previous U–Pb TIMS zircon age of 131·7 ± 1·5 Ma (Davis et al., 2001), which indicates that 130 Ma is the time of crystallization of the Wulingshan syenites.
Zircon grains from the Qiancengbei arfvedsonite granite (F04-106) have a size range of 75–150 µm, a length/width ratio of 2:1 to 3:1, and show either oscillatory zoning or relatively homogeneous structures in CL images (Fig. 2), the former suggesting a magmatic origin. Twenty-six analyses give ages ranging between 132 ± 2 Ma and 125 ± 3 Ma (1; Table 1) and form a tight cluster on concordia, with a weighted mean 206Pb/238U age of 129 ± 1 Ma (2; MSWD = 0·45) (Fig. 5d). This age is consistent with the zircon TIMS U–Pb age of 128·8 ± 1·5 Ma (Davis et al., 2001) and is interpreted as the time of crystallization of the arfvedsonite granite.
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GEOCHEMISTRY AND ISOTOPIC COMPOSITION |
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Geochemistry
Major and trace element data for 42 rock samples from the Xiangshan, Houshihushan and Qiancengbei–Wulingshan alkaline intrusions and associated volcanic rocks are listed in Table 2. The samples can be divided into three groups according to their distinct mineralogical and geochemical features: (1) ferroan alkali granite and rhyolite (including arfvedsonite granite, alkali-feldspar granite and alkali rhyolite); (2) ferroan syenite; (3) magnesian syenite and trachyte.
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Ferroan alkali granite and rhyolite
The alkali granites and rhyolites exhibit anhydrous and hypersolvus mineralogy and are alkaline, with high abundances of SiO2, total alkalis (Na2O + K2O = 8·5–10·5 wt %), Ga (20·7–30·2 ppm) and Zr (140–835 ppm) and high ratios of total FeO/MgO and Ga/Al, and low abundances of CaO, Sr and Ba (Table 2), displaying the characteristics of A-type magmas as defined by Whalen et al. (1987
) and Eby (1990). This is also confirmed by the criteria of Whalen et al. (1987) and Sylvester (1989) using the diagrams of Ga vs Al2O3, (Na2O + Na2O), Zr vs Ga/Al, and (Al2O3 + CaO)/(total FeO + Na2O + K2O) vs 100(MgO + total FeO + TiO2)/SiO2 (not presented), which are effective in discriminating alkali granites from calc-alkaline and strongly aluminous granites. In the total Fe-number [FeO/(total FeO + MgO)] and (Na2O + K2O – CaO) vs SiO2 discrimination diagrams (Fig. 6), all these rocks fall in the ferroan and alkalic to alkali–calcic fields (Frost et al., 2001). Therefore, we conclude that these rocks evolved from ferroan, alkalic magmas. The alkali granites and rhyolites vary from peralkaline to weakly metaluminous with A/CNK [molar Al2O3/(CaO + Na2O + K2O)] = 0·86–1·0 and A/NK [molar Al2O3/(Na2O + K2O)] = 0·90–1·1 (Fig. 7a) and have an agpaitic index (AI = [molar (Na2O + K2O)/Al2O3]) between 0·91 and 1·1, plotting in the peralkaline and alkaline metaluminous fields in the AI vs SiO2 diagram (Fig. 7b).
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Ferroan syenite
The ferroan syenites from the Wulingshan pluton have relatively low SiO2 and high Na2O + K2O (9·8–12 wt %) contents and belong to the alkaline series in the total alkali vs SiO2 diagram (Fig. 7c), although one sample (F04-110) has high SiO2. In the total Fe-number [FeO/(total FeO + MgO)] vs SiO2 discrimination diagram (Fig. 6a), the Wulingshan syenites fall in the ferroan field (Frost et al., 2001
). They are weakly metaluminous to peralkaline (A/CNK = 0·83–1·03, A/NK = 1·01–1·15) (Fig. 7a), although sample F04-110 is again an exception and is peraluminous.
Magnesian syenite and trachyte
The magnesian syenites and trachytes from Houshishushan are alkaline and have low SiO2 contents of 55·7 to 65·6 wt % and high Na2O + K2O contents of 7·4–10·1 wt % (Fig. 7c). They are magnesian in the Fe-number vs SiO2 diagram (Fig. 6a) and are also metaluminous, with A/CNK of 0·82–0·95 and A/NK of 1·11–1·67 (Fig. 7a).
Looking at the suite overall, differences are evident in the chondrite-normalized REE patterns (Fig. 8a–c). The alkali granites and rhyolites show enrichment of light REE (LREE), (La/Yb)N ratios of 16–60 and strongly negative Eu anomalies [(Eu/Eu*)N = 0·01–0·18] (Fig. 8a). The Wulingshan ferroan syenites, although enriched in LREE, have lower (La/Yb)N ratios of 9·6–15·7, and tend to show weaker Eu anomalies from 0·14 to 1·06 (Fig. 8b). Conversely, the Houshihushan magnesian syenites and trachytes have steep LREE-enriched and slightly concave heavy REE (HREE) patterns, with relatively high (La/Yb)N ratios (15·2–17·6) and no negative Eu anomalies, although the syenites show weak positive anomalies (Fig. 8c).
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These contrasts become more obvious in multi-element plots normalized to primitive mantle (PM) (Fig. 8d–f). The alkali granites and rhyolites are enriched in Rb, K, Nd, Zr and Hf and have variable negative Nb and Ta anomalies with extremely strong negative anomalies in Ba, Sr, P, Eu and Ti (Fig. 8d). The Wulingshan ferroan syenites without negative Eu anomalies are enriched in Ba and show weak negative anomalies in Th, U, Nb, Ta, Sr, P, Zr and Hf, whereas those with negative Eu anomalies are depleted in Ba and enriched in Zr and Hf, and show strongly negative anomalies in Sr, P, Eu and Ti (Fig. 8e). The Houshihushan magnesian syenites and trachytes are enriched in Cs, Rb, LREE, Zr and Hf, and show moderate negative anomalies in Th, U, Nb and Ta, and weakly negative anomalies in P and Ti (Fig. 8f). On Harker diagrams (Figs 9 and 10), alkali granites and rhyolites, ferroan syenites and magnesian syenites and trachytes also show distinct trends.
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Sr and Nd isotopic data
The alkali granites and rhyolites have variable 87Rb/86Sr and initial 87Sr/86Sr ratios and
Nd(t) values (Table 3; Fig. 11a). The Xiangshan alkali-feldspar granites have initial 87Sr/86Sr ratios of 0·7059–0·7074 with low 87Rb/86Sr ratios and consistent Nd(t) values of –12·4. Many of the Xiangshan arfvedsonite granites have high 87Rb/86Sr ratios such that the initial 87Sr/86Sr ratios cannot be used in petrogenetic discussion (see Jahn et al., 2000; Wu et al., 2002). However, in relatively low 87Rb/86Sr samples, the initial 87Sr/86Sr ratios are 0·7116–0·7122. Their Nd(t) values of –10·9 to –11·3 are a little higher than those of the alkali-feldspar granites. The Houshihushan arvedsonite granites have a wide range of initial 87Sr/86Sr ratios, ranging from 0·7050 to 0·7164, and variable Nd(t) values of –8·4 to –12·5, although most are –12. The Houshihushan alkali rhyolites have a more restricted range of initial 87Sr/86Sr ratios (0·7070–0·7080), and a large range of Nd(t) values from –9·0 to –13·6. The Qiancengbei arfvedsonite granites show little variation in initial 87Sr/86Sr ratios, ranging from 0·7057–0·7084, and have uniform Nd(t) values mostly –9·0.
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The ferroan syenites from the Wulingshan pluton have initial 87Sr/86Sr ratios of 0·7047–0·7109 and variable 87Rb/86Sr ratios, with uniform
Nd(t) values of –6·1 to –7·5. Conversely, the magnesian syenites and trachytes from the Houshihushan ring complex have more uniform initial 87Sr/86Sr ratios of 0·7057–0·7099, low 87Rb/86Sr ratios and highly negative Nd(t) values of –13·0 to –14·4.
Zircon Hf isotope data
Hf isotopic analyses of zircons from 10 samples are listed in Table 4 and shown in Fig. 12. Alkali granites and rhyolites have variable Hf isotopic compositions (Fig. 12a–c). Similarly, zircons from the alkali granites (F04-033, 036, 073 and 106) also have variable 176Hf/177Hf ratios (0·28229–0·28254) with Hf(t) values ranging from –5·7 to –14·6 (Fig. 12a and b). Their depleted mantle model ages (TDM) range from 1·1 to 1·5 Ga and crustal model ages (TDMC) from 1·5 to 2·1 Ga (Table 4). Zircons from two alkali rhyolites from the Houshihushan complex have distinctly different Hf isotopic compositions. Zircons from sample F04-062 have relatively low and variable 176Hf/177Hf ratios with Hf(t) values ranging from –10·2 to –16·8 (Fig. 12c), depleted model ages (TDM) ranging from 1·2 to 1·5 Ga and crustal model ages (TDMC) of 1· 8–2·2 Ga (Table 4). However, zircons from sample F04-057 have relatively high 176Hf/177Hf ratios with Hf(t) values ranging from –7·5 to –10·8 (Fig. 12c), and slightly younger depleted model ages (TDM) ranging from 1·1 to 1·3 Ga and crustal model ages (TDMC) from 1·7 to 1·9 Ga (Table 4).
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The Wulingshan ferroan syenites (F04-111, 113 and 114) have heterogeneous Hf isotopic compositions (Fig. 12d) with zircons having 176Hf/177Hf ratios of 0·28248–0·28265,
Hf(t) values ranging from –1·7 to –7·9, depleted model ages (TDM) ranging with 0·9 to 1·2 Ga and crustal model ages (TDMC) of 1·3–1·7 Ga (Table 4). However, zircons from the Houshihushan magnesian syenite (F04-067) have variable but lower 176Hf/177Hf ratios (0·28215–0·28239), more strongly negative Hf(t) values ranging from –11·1 to –19·5, and older depleted model ages (TDM) of 1·3–1·8 Ga and crustal model ages (TDMC) of 1·9–2·4 Ga (Table 4 and Fig. 12d). |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHYANALYTICAL METHODSGEOCHRONOLOGYGEOCHEMISTRY AND ISOTOPIC...DISCUSSIONCONCLUSIONSREFERENCES |
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Alkaline igneous rocks commonly occur in extensional tectonic settings (Whalen et al., 1987
; Sylvester, 1989; Eby, 1990, 1992; Black & Liegeois, 1993; Nedelec et al., 1995; Pitcher, 1997), and provide significant information on post-collisional or intraplate extensional magmatic processes within the continental lithosphere (Turner et al., 1992, 1996; Mushkin et al., 2003). They can form by: (1) direct fractionation of mantle-derived alkali basalts (e.g. Turner et al., 1992; Frost & Frost, 1997; Litvinovsky et al., 2002; Mushkin et al., 2003; Yang et al., 2005b); (2) partial melting of crustal materials at deep (e.g. Collins et al., 1982; Clemens et al., 1986; Litvinovsky et al., 2000) or shallow (Skjerlie & Johnston, 1992; Beard et al., 1994; Patiño Douce, 1997) crustal levels; (3) hybridization between crustal anatectic granitic and mantle-derived mafic magmas (Kerr & Fryer, 1993; Wickham et al., 1996; Yang et al., 2006b). An important and fundamental question is to determine the proportion of crustal to mantle components involved in the formation of each occurrence of such rocks. The alkaline igneous rocks of the YFTB show a wide range of whole-rock Sr and Nd and zircon Hf isotopic compositions (Figs 11 and 12) and distinct trends on Harker diagrams (Figs 9 and 10), precluding simple crystal–liquid fractionation of a common parental magma to generate the entire suite. The geochemical features and isotopic compositions of the various plutons indicate that they were derived from different sources and/or generated by different petrogenetic processes.
Petrogenesis of syenite and trachyte
Source of ferroan syenites
The Wulingshan ferroan syenites have low initial 87Sr/86Sr ratios (except for one sample), slightly negative Nd(t) values and weakly negative zircon Hf(t) values (Figs 11a and 12d). The low initial 87Sr/86Sr ratios are accompanied by high 87Rb/86Sr ratios that are usually attributed to partial melting involving mica breakdown and/or late plagioclase fractionation. The samples without negative Eu anomalies may represent the parental magmas of the syenites that have not experienced plagioclase fractionation, indicating that their source rocks should have high 87Rb/86Sr (0·40) (Kemp & Hawkesworth, 2003). Therefore, the syenites would be derived from a mica-bearing crustal source with a short residence time (i.e. juvenile crust). Conversely, melting of deep, mafic to intermediate gneisses or granulites by underplating of mantle-derived magmas would not produce syenitic melts (Litvinovsky et al., 2000).
Most of the Wulingshan ferroan syenites have relatively low initial 87Sr/86Sr ratios and relatively high Nd(t) values (Fig. 11a), similar to those of the Early Cretaceous alkali basalts in the YFTB (Zhou et al., 2001; Fig. 11b), suggesting a common source. The geochemistry of these alkali basalts indicates derivation by partial melting of an enriched lithospheric mantle source (Zhou et al., 2001). If the Wulingshan ferroan syenites were formed by differentiation of these melts, they too were mainly derived from an enriched lithospheric mantle source. However, the variable major and trace element and zircon Hf isotopic compositions (Fig. 12d), especially for sample F04-110, which has a high initial 87Sr/86Sr ratio and low Nd(t) value with a negative Eu anomaly and high REE and SiO2 contents, indicate incorporation of an upper crustal component (Yang et al., 2006b). The geochemical and isotopic compositions of the Early Cretaceous ferroan syenites can thus be interpreted as a result of multiple-component mixing between enriched lithospheric mantle and crustal materials, possibly Archean gneisses and granulites that are present in the area (Yang et al., 2006b).
Source of magnesian syenites and trachytes
The Houshihushan magnesian syenites and trachytes show a range of whole-rock initial 87Sr/86Sr, Nd(t) values and zircon Hf(t) values that are distinct from those of the Wulingshan ferroan syenites (Figs 11a and 12d). They plot near the field of lower crustal granulites and gneisses (Fig. 11a) (J.-H. Yang, unpublished data) and with the synchronous calc-alkaline granites (Fig. 11b) that were derived from partial melting of lower crustal sources in the YFTB (Wang & Zhang, 2001). In addition, the Houshihushan magnesian syenites and trachytes have older Nd model ages of 1·77–1·96 Ga, Hf model ages of 1·77–1·96 Ga and crustal model ages of 1·9–2·4 Ga, indicating that an ancient lower crustal component was involved in their genesis.
Experimental results show that partial melting of muscovite granite, with 74 wt % SiO2 (without mafic components) under 15 kbar pressure and with 5 wt % of water, may result in the formation of initial syenitic melts (Huang & Wyllie, 1975; Johannes & Holtz, 1990). However, as discussed by Gao et al. (1998) and Liu et al. (2001), the lower crust beneath the YFTB is mainly composed of intermediate to mafic granulites and gneisses. Deep melting of such rocks at elevated pressures, with their high concentrations of Ca, Mg and Fe, would produce residual clinopyroxene, garnet and Ca-rich plagioclase. Consequently, partial melts would be enriched in SiO2 and depleted in HREE and Eu (negative Eu anomalies), making them granitic rather than syenitic (Litvinovsky et al., 2000). The Houshihushan syenites and trachytes do not consistently show negative Eu anomalies (Fig. 8a) and they also have relatively high HREE abundances, indicating that they were not derived by partial melting of lower crust beneath the YFTB. Instead, the variable geochemical and isotopic compositions suggest that they were derived from a mixed source. The variable negative Hf(t) values preclude an entirely crustal source and indicate that a high Hf(t) component was involved in their genesis. In addition, they have relatively low Nb and Ta concentrations and variable Nb/Ta ratios with increasing SiO2, which cannot be produced by fractionation of titaniferous phases, because this would result in a slight increase in Nb/Ta with increasing SiO2 (Wolff, 1984; Green, 1995). They must therefore reflect different source regions and partial melting conditions (Green, 1995; Horng et al., 1999). The Nb/Ta values for the magnesian syenites and trachytes are between those of mantle-derived melts (17·5 ± 2·0) (Green, 1995; Kamber & Collerson, 2000) and average continental crust (11–12) (Rudnick & Gao, 2003), also implying mixed crust and mantle sources.
Petrogenetic processes
Two processes can explain the multiple-component mixing identified in the origin of the ferroan and magnesian syenites and trachytes in the YFTB; either (1) assimilation of Archean gneisses or granulites of the lower crust or upper crustal materials by alkali basaltic magma, or (2) mixing of alkali basaltic magma with felsic magmas derived by partial melting of these crustal rocks. Zircons from the syenites have distinct crystal morphologies and large variations in Hf(t) values, but most record uniform U–Pb ages, thus precluding crustal assimilation and an inherited origin for the low Hf(t) zircons (see Elburg, 1996; Griffin et al., 2002; Belousova et al., 2006; Yang et al., 2006b, 2007b). Furthermore, the syenites and trachytes have higher total trace element contents than either the Early Cretaceous alkali basalts (Zhou et al., 2001) or the NCC crust (Gao et al., 1998; Liu et al., 2001; Yang et al., 2006b). The Archean gneisses and granulites also have relatively high MgO contents (2·7–7· 4 wt %) and Sm/Nd ratios, and low Th and U concentrations and (La/Yb)N ratios (J.-H. Yang, unpublished data). It is therefore evident that assimilation of crustal materials by alkali basaltic magma cannot produce syenitic and trachytic magmas, which are characterized by low MgO and high contents of trace elements. Conversely, felsic melts derived by partial melting of lower or upper crustal materials would have relatively higher trace element contents than their source rocks. Therefore, it seems likely that the Early Cretaceous ferroan and magnesian syenite and trachyte magmas in the YFTB were produced by mixing of alkali basaltic magma with crustal-derived melts. Given the abundance of basalts and mafic intrusions in the YFTB (Wang & Zhang, 2001; Zhou et al., 2001), it is likely that the heat for crustal melting was also provided by the mantle-derived mafic magmas and that this created an environment favorable for magma mixing.
The syenites also plot along the trends of clinopyroxene and feldspar fractionation in the Ni vs Cr, Eu/Eu* and Rb/Sr, and Ba vs Sr diagrams (Fig. 13). This is evidenced by the decrease in CaO and Al2O3 contents with increasing SiO2 contents (Fig. 9b and e), consistent with continuous fractionation of pyroxene and feldspar. Feldspar fractionation is supported by significant depletions in Sr, Ba, and Eu shown in the mantle-normalized trace element patterns (Fig. 9d). The negative Eu anomalies in some Wulingshan ferroan syenites, combined with decreases in Sr (and Ba) (Fig. 13b), indicate that plagioclase and K-feldspar have been removed during magma evolution, which is further supported by the decreasing Ba contents and increasing Rb/Sr ratios with decreasing Sr concentrations (Fig. 13b and c). Furthermore, extensive clinopyroxene and feldspar fractionation would strongly increase the Ca/Al ratio of the melt. Because accessory minerals control much of the REE variation and there is an increase in 
REE with increasing SiO2 contents (Table 3), this suggests enrichment of the residual liquids in these elements. The sharp decrease in P2O5 and increasing negative P anomalies (Fig. 8f) with increasing SiO2 contents are consistent with continuing fractionation of apatite. Accessory allanite (increasing Ce/Sr, not presented) is also probably involved.
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Therefore, geochemical and isotopic data suggest that multiple sources were involved in the origin of the Early Cretaceous syenites in the YFTB (i.e. lower and upper crustal materials and an enriched mantle source). The Wulingshan ferroan syenites were mainly generated by partial melting of an enriched mantle source, mixed with some component of lower and upper crustal-derived magmas, coupled with clinopyroxene and plagioclase fractionation. However, the Houshihushan magnesian syenites were mainly derived from partial melting of lower crustal materials, mixed with enriched mantle-derived alkali basaltic magma.
Petrogenesis of alkali granites and rhyolites
The alkali granites and rhyolites in the YFTB display characteristics of A-type magmas as defined by Whalen et al. (1987), Eby (1990) and Frost et al. (2001), exhibiting anhydrous and hypersolvus mineralogy, with low MgO, Al2O3 and CaO contents, and high FeO*/MgO and (Na2O + K2O)/Al2O3. They are enriched in high field strength elements, such as Zr, Nb and Y, as well as showing elevated Ga/Al, and depletion in Eu and Sr. They have variable Ga, Rb, Nb, Y, HREE, Th, U and total FeO* contents, which all increase with fractionation. A large increase in Rb/Sr and decrease in Al2O3 and CaO with small changes in SiO2 contents (Fig. 9b and e) indicate feldspar-controlled fractionation. This is supported by the Eu/Eu*, Rb/Sr and Ba vs Sr diagrams (Fig. 13a–c), in which they plot on the K-feldspar and plagioclase fractionation trends.
Although extensive fractional crystallization can explain the major and trace element variations of the alkali granites and rhyolites, it cannot explain the large range of isotopic compositions. These rocks have variable whole-rock Sr and Nd and zircon Hf isotopic values (Figs 11 and 12), precluding formation by direct fractionation of mantle-derived alkali basalts, partial melting of lower crust at deep crustal levels, or partial melting of calc-alkaline rocks at shallow crustal levels, because all these processes would produce felsic melts with homogeneous isotopic compositions. Likewise, their isotopic compositions are distinct from those of mantle-derived alkali basalt (Nd(t) = –5·0, Zhou et al., 2001), lower crustal granulites and gneisses, and crustal-derived calc-alkaline granites (Nd(t) < –15) in the YFTB (Wang & Zhang, 2001) (Fig. 11b), supporting the view that some other process must be involved.
The isotopic data for the alkali granites and rhyolites (Fig. 11) indicate that an upper crustal source, with high initial 87Sr/86Sr ratios, was also involved in their formation, through either crustal assimilation or magma mixing. The total FeO/MgO ratios and Rb and Nb concentrations are higher than those of the Early Cretaceous alkali basalts (Zhou et al., 2001) and Wulingshan ferroan syenites (Fig. 14), indicating that the upper crustal materials should have high total FeO/MgO ratios and Rb and Nb abundances. SiO2, CaO and Eu/Eu* show relatively constant values with decreasing Nd(t) (Fig. 14a–c), possibly indicating that the high SiO2 and low CaO abundances, with strongly negative Eu anomalies, were not the result of multiple component mixing, but reflect instead the characteristics of the crustal materials. The SiO2, total FeO/MgO, Rb and Nb values of the upper crust in the YFTB are relatively higher, whereas the CaO content is lower, than those of the reference upper crust (Rudnick & Gao, 2003). We therefore propose that a felsic melt with high SiO2, total FeO, Rb and Nb abundances, low MgO and CaO (Al2O3) concentrations, and negative Eu anomalies was derived by partial melting of upper crustal materials. Such features are consistent with the experimental melts produced by dehydration melting of calc-alkaline granitoids with a plagioclase-rich residual assemblage in the shallow crust (P 
4 kbar) (Skjerlie & Johnston, 1992; Beard et al., 1994; Patiño Douce, 1997). This is supported by the Hf isotopic data of zircons from the alkali granites and rhyolites, where the large variations in Hf(t) values (–16·8 to –5·8) with uniform Early Cretaceous ages, preclude crustal assimilation, but support magma mixing (Elburg, 1996; Griffin et al., 2002; Belousova et al., 2006; Yang et al., 2006b, 2007b).
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Therefore, we propose that the alkali granites and rhyolites in the YFTB were produced by mixing of felsic melts, derived from partial melting of calc-alkaline granitoids at shallow crustal levels, with mantle-derived alkali basaltic and lower crust-derived felsic magmas, coupled with crystal fractionation, as suggested by previous studies (Kerr & Fryer, 1993
; Wickham et al., 1996; Yang et al., 2006b).
An integrated model
Zircon U–Pb ages of alkaline rocks in the YFTB are in the range 130–117 Ma, in temporal and spatial association with the development of Early Cretaceous (133–116 Ma) metamorphic core complexes (Davis et al., 2001) and pull-apart basins (Meng, 2003) along the YFTB, suggesting that they occurred in an extensional tectonic setting, as proposed by Sylvester (1989), Eby (1992) and Nedelec et al. (1995).
However, the generation of alkaline igneous rocks requires a high melting temperature (Clemens et al., 1986), commonly realized by the emplacement of mantle-derived mafic magma or upwelling of asthenosphere at the base of the crust. The variable whole-rock Nd (Nd(t) = –8·4 to –13·6) and zircon Hf (Hf(t) = –6·7 to –16·8) isotopic compositions of the alkaline rocks in the YFTB indicate that a mafic magma with high Nd and Hf isotopic ratios was involved in their genesis, corresponding to a magma derived from an enriched lithospheric mantle source. The driving force was the emplacement of mafic magma, or its evolved products, into the crust, resulting in the production of crustal melts, which then interacted with the mafic component. This might be achieved by delamination of thickened lithosphere, where the mantle lithosphere is convectively removed from below, or by crustal extension.
Studies of mantle xenoliths from the North China Craton indicate that at least 120 km of continental lithosphere has been removed (Menzies et al., 1993; Griffin et al., 1998; Xu, 2001) in the Early Cretaceous (Wilde et al., 2003; Yang et al., 2003). Several models have been proposed to explain this, including delamination (Gao et al., 2004), possibly induced by subduction of the Pacific Plate beneath eastern China (Yang et al., 2003; Wu et al., 2005), Triassic collision between the Yangtze and North China cratons (e.g. Yang et al., 2007c, 2007d), chemical and thermal erosion by upwelling asthenosphere (e.g. Griffin et al., 1998; Zhang et al., 2004; Zhang, 2005) or by impingement of mantle plumes during continental dispersal (e.g. Wilde et al., 2003). The replacement of lithosphere by ascending asthenosphere would lead to elevation of the geotherm in an extensional setting, inducing partial melting of the lithospheric mantle and production of mafic magmas, including alkali basalts. Underplating by mafic magmas would provide heat for crustal melting, resulting in the production of granitic magmas at different crustal levels. Mixing of mafic magmas, and/or their fractionated products, with crustal-derived felsic melts would produce hybrid magmas and, as in the present scenario, these would rise to various levels in the crust and undergo convective fractionation to produce the felsic differentiates, or even more simply, could form plutonic or volcanic rocks with little or no differentiation.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHYANALYTICAL METHODSGEOCHRONOLOGYGEOCHEMISTRY AND ISOTOPIC...DISCUSSIONCONCLUSIONSREFERENCES |
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The various alkaline igneous rocks in the YFTB have distinct geochemical and Sr–Nd–Hf isotopic compositions, indicating that they were derived from different sources by a range of petrogenetic processes. Three groups of rocks have been identified based on their mineralogical, geochemical and Sr–Nd–Hf isotope characteristics.
The alkali-feldspar granite, arfvedsonite granite and alkali rhyolite have the geochemical characteristics of A-type magmas. Geochemical and Sr, Nd and zircon Hf isotopic compositions rule out simple crystal–liquid fractionation of mantle- or crustal-derived magma. Instead, magma mixing of shallow crustal-derived magmas with mantle- and lower crustal-derived magmas, coupled with crystal fractionation, is compatible with the data.
The ferroan syenites were the result of crystal fractionation of a melt derived from partial melting of an enriched lithospheric mantle source, mixed with crustal-derived magmas. However, the magnesian syenites and trachytes were mainly derived from partial melting of lower crustal materials, mixed with some component of enriched mantle-derived alkali basaltic magma.
The alkaline rocks in the Yanshan Fold and Thrust belt evolved between 130 and 117 Ma and were coeval with the formation of metamorphic core complexes and pull-apart basins, features now equated with Early Cretaceous lithospheric thinning in the eastern North China Craton. Crustal extension or delamination of thickened lithosphere may have been the driving force for uprise and emplacement of mafic magma or its differentiates. This induced partial melting of crustal materials at different levels to produce felsic magmas, which then mixed with the mafic magma and its evolved products to form the alkaline rocks in the YFTB.
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ACKNOWLEDGEMENTS |
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We thank Chao-Feng Li for assistance with the Sr and Nd isotope analyses, and Qian Mao and Yu-Guang Ma for help with zircon CL imaging. We also thank Nelson Eby, Michael Dorais, Jean Bedard, Robert Wiebe, Gregor Markl and Ron Frost for comments that helped us to substantially improve this paper. This study was supported by the National Science Foundation of China (Grants 40672055 and 40325006). This is The Institute for Geoscience Research (TIGeR) publication number 76.
*Corresponding author. Telephone: +86-10-82998510 (O). Fax: +86-10-62010846. E-mail: jinhui{at}mail.igcas.ac.cn
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