1. Introduction
Post-orogenic mafic magmatism is one of the common features of many orogens around the world (Bonin, Reference Bonin2004), and may indicate that the orogen is in the process of collapsing (Dewey, Reference Dewey1988). The origin of such melts is commonly attributed to lithospheric extension by orogenic collapse (Ruppel, Reference Ruppel1995), slab break-off (Davies & von Blanckenburg, Reference Davies and von Blanckenburg1995), convective thinning (England & Houseman, Reference England and Houseman1989), delamination of continental lithosphere (Bird, Reference Bird1979; Kay & Kay, Reference Kay and Kay1993) and magmatic underplating (Furlong & Fountain, Reference Furlong and Fountain1986). Therefore, petrogenetic studies of post-orogenic mafic igneous rocks not only allow the evaluation of its mantle source, but also provide important constraints for understanding the tectonic evolution of the orogenic belts and adjacent regions.
The Faku dome in northern Liaoning occupies a transitional tectonic position that links a northern Phanerozoic orogen, that is, the Xing-Meng orogenic belt, with a southern Precambrian craton, that is, the North China craton. Based on the notion that the migmatites and metamorphic complexes from the Faku area are of a Proterozoic age (Liaoning Bureau of Geology and Mineral Resources, 1989), the Faku dome has long been regarded as a Precambrian terrane along the northern margin of the North China craton. The 1:50000 scale geological mapping (Liaoning Bureau of Geology and Mineral Resources, 1998) and our 40Ar/39Ar geochronological study (Zhang, Wang & Li, Reference Zhang, Wang and Li2005) revealed that these deformed and metamorphosed complexes, with a variety of protoliths of plutonic intrusions and supracrustal volcanic and sedimentary rocks, were genetically related to later Triassic ductile shearing events. Our zircon U–Pb SHRIMP dating further recognized that the previously established Proterozoic migmatites were in fact syntectonic granitic intrusions that were emplaced during Permian times (Zhang, Su & Wang, Reference Zhang, Su and Wang2005).
In this paper, we present zircon U–Pb ages, major and trace element geochemistry, and Sr–Nd–Hf isotopic compositions for a middle Triassic mafic pluton from the Faku dome to: (1) document the geochemical characteristics of these rocks, (2) investigate their mantle sources and petrogenesis and (3) evaluate the nature of the lithospheric mantle beneath the northern Liaoning block and its tectonic affinity.
2. Geological setting
The North China craton is known as one of the world's oldest cratons, as evidenced by the presence of 3.6 Ga crustal remnants exposed at the surface or in the lower crustal xenoliths (Liu et al. Reference Liu, Nutman, Compston, Wu and Shen1992; Zheng et al. Reference Zheng, Griffin, O'Reilly, Lu, Wang, Zhang, Wang and Li2004). It is bounded on the south by the Palaeozoic to Triassic Qinling–Dabie–Sulu orogenic belt (Meng & Zhang, Reference Meng and Zhang2000) and on the north by the Xing-Meng orogenic belt (Davis et al. Reference Davis, Zheng, Wang, Darby, Zhang, Gehrels, Hendrix and Davis2001). The craton consists of two Archaean continental blocks, namely, the Eastern and the Western, separated by a Proterozoic orogenic belt (Fig. 1a; Zhao et al. Reference Zhao, Wilde, Cawood and Sun2001).
Figure 1. (a) Major tectonic divisions of China, where YZ and SC denote the Yangtze craton and South China orogen. Also shown are the subdivisions of the North China craton (Zhao et al. Reference Zhao, Wilde, Cawood and Sun2001), where EB, TNCO and WB denote the Eastern block, Trans-North China orogen and Western block, respectively. (b) Tectonic framework of northeastern China (modified from Gu et al. Reference Gu, Zheng, Tang, Zaw, Delle-Pasque, Wu, Tian, Lu, Ni, Li, Yang and Wang2007). (c) Sketch distribution map of the basins and faults in western Liaoning, northern Liaoning and southern Songliao areas (modified from Xu et al. Reference Xu, Middleton, Xue and Wang2000); F1 – Xilamulunhe fault; F2 – Chifeng–Kaiyuan fault; F3 – Hongshan–Balihan fault; F4 – Yilan–Yitong fault; F5 – Fushun fault. (d) Sketch geological map of the Faku area (modified from Liaoning Bureau of Geology and Mineral Resources, 1998), (e) Sketch geological map of the Xiaofangshen gabbroic stock, with the sample locations shown.
Unlike other Archaean cratons, the North China craton experienced widespread tectonothermal reactivation during and after Palaeozoic times, mainly due to the compound evolutionary history of the circum-cratonic orogenic belts. To the north, the Xing-Meng orogenic belt, also called the Altaid Tectonic Collage (Şengör, Natal'in & Burtman, Reference Şengör, Natal'in and Burtman1993) or Central Asian Orogenic Belt (Jahn, Wu & Chen, Reference Jahn, Wu and Chen2000), is located between the North China and Siberian cratons (Fig. 1b). It is a complex orogenic belt formed through successive accretion of arc complexes, accompanied by emplacement of voluminous subduction zone granitic magmas mainly during Palaeozoic times (Davis et al. Reference Davis, Zheng, Wang, Darby, Zhang, Gehrels, Hendrix and Davis2001; Xiao et al. Reference Xiao, Windley, Hao and Zhai2003). During this period, multiple Mongolian arc terranes were amalgamated to the active margins of the North China craton (Davis et al. Reference Davis, Zheng, Wang, Darby, Zhang, Gehrels, Hendrix and Davis2001). The Solonker suture marks the closure of the palaeo-Asian ocean and the collision between the North China craton and Mongolian composite terranes (e.g. Yin & Nie, Reference Yin, Nie, Yin and Harrison1996; Davis et al. Reference Davis, Zheng, Wang, Darby, Zhang, Gehrels, Hendrix and Davis2001; Xiao et al. Reference Xiao, Windley, Hao and Zhai2003). With the gradual exhaustion of the palaeo-Asian ocean realm, the North China craton and the southern Mongolia terranes were amalgamated and behaved as a combined North China–Mongolian plate (Davis et al. Reference Davis, Zheng, Wang, Darby, Zhang, Gehrels, Hendrix and Davis2001).
Palaeozoic northeast China is the eastern segment of the Xing-Ming orogenic belt, and it is composed of three microcontinental blocks: the Jiamusi in the southeast, Songliao in the middle and the Xing'an in the northwest (Fig. 1b). The Songliao block is composed of the Songliao Basin and the Zhangguangcai Range. The Songliao basin developed in late Mesozoic times and is an important centre for the oil industry in China.
Located to the south of the Songliao block, the northern Liaoning block mainly consists of three tectonic units: the Zhezhong depression in the west, the Faku dome in the middle and the Tieling depression in the east (Fig. 1c). The Faku dome is mainly composed of igneous and sedimentary rocks metamorphosed to greenschist- to upper amphibolite-facies grade. These rocks have been previously regarded as the Precambrian basement of the North China craton (Liaoning Bureau of Geology and Mineral Resources, 1989). The l:50000 scale geological mapping (Liaoning Bureau of Geology and Mineral Resources, 1998) revealed that these so-called basement complexes turned out to be Phanerozoic deformed intrusions and metavolcanic and sedimentary rocks. The latter can be further divided into the lower and upper Palaeozoic formations (Fig. 1d). Our zircon U–Pb SHRIMP dating established that the majority of the granitic intrusions were emplaced during Permian times (Zhang, Su & Wang, Reference Zhang, Su and Wang2005).
Since Mesozoic times, the northern Liaoning block has become part of the eastern China active tectonic belt, which experienced geodynamic transition from the palaeo-Asian to palaeo-Pacific tectonic realms. Subsequently, a series of NE- to NNE-trending strike-slip faults developed. In the Cretaceous period, the area underwent large-scale continental extension, resulting in the development of a number of basins (Xu et al. Reference Xu, Middleton, Xue and Wang2000).
3. Petrography
The Xiaofangshen gabbros, named after Xiaofangshen village, crop out as small stocks and dykes within the Permian Shijianfang granitoid batholith (Fig. 1e). The gabbros are medium- to coarse-grained rocks with intergranular textures, and show clear intrusive relations with the host granitoids. Typical samples mainly consist of plagioclase (25–70%), amphibole (25–40%), pyroxene (5–15%) and biotite (0–5%), with minor amounts of quartz, magnetite, zircon and apatite. Amphibole, the most abundant mafic mineral, mainly occurs as subhedral to euhedral phenocrysts and is locally altered to calcite and chlorite. All amphiboles belong to the calcic group (Ca+Na 1 and Na < 0.5), according to the classification of Leake et al. (Reference Leake, Woolley, Arps, Birch, Gilbert, Grice, Hawthorne, Kato, Kisch, Krivovichev, Linthout, Laird, Mandarino, Maresch, Nickel, Rock, Schumacher, Smith, Stephenson, Ungaretti, Whittaker and Youzhi1997) and can be classified as magnesiohastingsite, magnesiohornblende and actinolite. The plagioclase also occurs as phenocrysts; they are generally subhedral laths, with occasional albite and carlsbad–albite combined twinning and they range in anorthite content from An39 to An64. They are partly altered to sericite, calcite and epidote.
4. Analytical methods
4.a. Zircon U–Pb isotopic dating
Zircon grains, together with standard CZ3, were cast in an epoxy mount, which was then polished to section the crystals in half for analysis. Cathodoluminescence images were obtained for the zircons prior to analysis, using a JXA-8100 microprobe at the Institute of Geology and Geophysics, Chinese Academy of Sciences, to reveal their internal structures. Measurements of U, Th and Pb were conducted using the SHRIMP II ion microprobe at Curtin University of Technology under standard operating conditions (6-scan cycle, 2 nA primary O2− beam, mass resolution ~ 5000), following analytical procedures as described by Williams (Reference Williams, McKibben, Shanks and Ridley1998). Data were processed using the SQUID (1.02) and ISOPLOT (Ludwig, Reference Ludwig2001) programs. Corrections of Pb/U ratios were made by normalization to zircon standard CZ3 (206Pb/238Pb = 0.0914, corresponding to an age of 564 Ma). The data were corrected for common lead using the measured 204Pb. Uncertainties on individual analyses are reported at the 1σ level based on counting statistics, while pooled ages are quoted at the 95% (2σ) level.
4.b. Major and trace element determination
Both major oxides and trace element compositions were measured by a Phillips PW 2400 X-ray fluorescence spectrometer using fused glass discs and a VG-PQII ICP-MS, respectively, at the Institute of Geology and Geophysics. Analytical uncertainty (2σ) is estimated to be about ± 5% for trace elements with abundances 10 ppm, and about ± 10% for those 10 ppm.
4.c. Sr–Nd isotopic analyses
Sr and Nd isotopic compositions were measured on a Finnigan Mat 262 thermal ionization mass spectrometer at the Institute of Geology and Geophysics, following the procedure described in Zhang et al. (Reference Zhang, Zhang, Tang, Wilde and Hu2008c). Procedural blanks were < 100 pg for Sm and Nd and < 500 pg for Rb and Sr. 143Nd/144Nd values were corrected for mass fractionation by normalization to 143Nd/144Nd = 0.7219, and 87Sr/86Sr ratios normalized to 87Sr/86Sr = 0.1194. Typical within-run precisions (2σ) for Sr and Nd were estimated to be 0.00002 and 0.000015, respectively. The measured values for the La Jolla Nd standard and NBS-607 Sr standard were 143Nd/144Nd = 0.5111853 and 87Sr/86Sr = 1.20042 during the period of data acquisition.
4.d. In situ Hf isotopic analyses
In situ zircon Hf isotopic analyses were conducted using the Neptune MC-ICP-MS, equipped with a 193 nm laser at the Institute of Geology and Geophysics. During analyses, the 176Hf/177Hf and 176Lu/177Hf ratios of the standard zircon (91500) were 0.282270 ± 0.000023 (2rn, n = 15) and 0.00028, similar to the commonly accepted 176Hf/177Hf ratio of 0.282284 ± 0.000003 (1r) measured using the solution method (Woodhead et al. Reference Woodhead, Hergt, Shelley, Eggins and Kemp2004).
We have used a decay constant for λLu = 1.867 10−11 year−1 (Soderlund et al. Reference Soderlund, Patchett, Vervoort and Isachsen2004) and the 176Hf/177Hf and 176Lu/177Hf ratios of average chondrite and estimated depleted mantle at the present day are 0.282772 and 0.0332, and 0.28325 and 0.0384, respectively (Blichert-Toft & Albarede, Reference Blichert-Toft and Albarede1997). These THfDM ages represent a minimum age for the source of the host magma of the zircon. We also present a more realistic estimate TDMC of the age of the source rocks for the magmas, derived by projecting the initial 176Hf/177Hf of the zircon back to the depleted mantle model growth curve, assuming a mean crustal value for Lu/Hf (176Lu/177Hf = 0.015: Griffin et al. Reference Griffin, Wang, Jackson, Pearson, O'Reilly, Xu and Zhou2002).
5. Analytical results
5.a. Zircon U–Pb data
The SHRIMP U–Pb analysis results of the Xiaofangshen gabbro (Sample Fk04–5) are listed in Table 1. Zircons from this sample are mostly clear, euhedral to subhedral, stubby to elongate prisms (Fig. 2a). They are about 80 to 150 μm long, with length-to-width ratios between 2:1 and 4:1. Eight analyses from this sample were conducted on eight grains during a single analytical session. Measured U concentrations vary from 203 to 894 ppm, and Th ranges from 33 to 530 ppm. All analyses have Th/U ratios of 0.19–0.64 and yield a weighted mean 206Pb–238U age of 241 ± 6 Ma with an MSWD of 0.81 (Fig. 2b). We interpret this as the emplacement time of the Xiaofangshen gabbros.
Table 1. SHRIMP U–Pb zircon data for the Xiaofangshen gabbros (sample Fk04-5)
1f 206 = percentage of common 206Pb in the total measured 206Pb.
Figure 2. (a) Cathodoluminescence (CL) images of the dated zircons and (b) U–Pb zircon concordia diagrams for the Xiaofangshen gabbros.
5.b. Major oxides and trace elements
Major and trace element analyses are presented in Table 2. Samples from the Xiaofangshen pluton are mafic in composition (SiO2 46.64–52.73%), with high abundances of total Fe2O3 (7.66–13.82%), Al2O3 (13.24–18.39%) and CaO (6.35–15.28%), low contents of TiO2 (0.63–1.70%) and P2O5 (0.08–0.47%), and various concentrations of MgO (3.64–8.34%) and K2O (0.65–2.18%). In the total alkali v. silica plot (Le Maitre, Reference Le Maitre2002) (not shown), the samples mainly plot in the field of gabbro and occasionally in the field of monzodiorite. They also exhibit a transitional character between low-K tholeiitic and medium-K calc-alkaline.
Table 2. Major and trace element data for the Xiaofangshen gabbros
In terms of trace elements, samples from the Xiaofangshen gabbros have total REE contents ranging from 70.6 ppm to 181 ppm. On the chondrite-normalized REE diagram (Fig. 3a), they display moderate light REE enrichment (LaN/YbN = 4.5 to 8.6) and small negative Eu anomalies (Eu/Eu* = 0.78–0.93; Table 3). On the primitive mantle-normalized spidergram (Fig. 3b), they are enriched in large ion lithophile elements, with positive Ba, Sr, Th and U anomalies, and are depleted in high field strength elements, with pronounced negative Nb, Ta, Zr, Hf and Ti anomalies.
Figure 3. (a) Chondrite-normalized REE patterns and (b) primitive mantle-normalized trace element spidergrams for the Xiaofangshen gabbros. Normalization values are from Sun & McDonough (Reference Sun, McDonough, Saunders and Norry1989). The data for oceanic island basalt (OIB), normal mid-ocean ridge basalt (N-MORB) and enriched mid-ocean ridge basalt (E-MORB) are also from Sun & McDonough (Reference Sun, McDonough, Saunders and Norry1989).
Table 3. Rb–Sr and Sm–Nd isotopic compositions for the Xiaofangshen gabbros
Chondrite Uniform Reservoir (CHUR) values (87Rb/86Sr = 0.0847, 87Sr/86Sr = 0.7045, 147Sm/144Nd = 0.1967, 143Nd/144Nd = 0.512638) are used for the calculation. λRb = 1.42×10−11 year−1 (Steiger & Jäger, Reference Steiger and Jäger1977); λSM = 6.54×10−12 year−1 (Lugmair & Harti, Reference Lugmair and Harti1978).
5.c. Whole rock Sr–Nd isotopes and zircon Hf isotopes
The results of Sr–Nd isotope analyses are given in Table 3. The initial isotopic ratios were calculated based on the age of 241 Ma. As shown in a plot of ϵNd(t) v. (87Sr/86Sr)i (Fig. 4), the samples have a restricted range of isotopic compositions with initial 87Sr/86Sr ratios of 0.7053 to 0.7055, slightly positive ϵNd(t) values of +0.40 to +0.68 and model ages (TDM2) of 956 to 980 Ma.
Figure 4. Plot of initial ϵNd(t) and (87Sr/86Sr)i for the Xiaofangshen gabbros. The field for the Palaeozoic kimberlites and mantle xenoliths from the eastern North China craton are from Wu et al. (Reference Wu, Walker, Yang, Yuan and Yang2006). Data for the hosting Permian granite (our unpub. data), post-collisional asthenospheric melt as represented by Permian basaltic rocks from neighbouring Inner Mongolia (Zhang et al. Reference Zhang, Zhang, Tang, Wilde and Hu2008c), Triassic mafic–ultramafic complexes from NE China (Wu et al. Reference Wu, Wilde, Zhang and Sun2004) and early Triassic adakitic rocks and A-type granites from northern Hebei (Jiang et al. Reference Jiang, Liu, Zhou, Yang and Zhang2007; Zhang et al. Reference Zhang, Zhao, Song, Hu, Liu, Yang, Chen, Liu and Liu2008b) are included for comparison.
Zircons from the Xiaofangshen gabbros show a range of initial 176Hf/177Hf ratios from 0.28276 to 0.28282 and ϵHf(T) values from 5.0 to 7.4 (Table 4, Fig. 5). The Hf model ages (TDM) for these zircons mainly range between 568 to 654 Ma.
Table 4. Hf isotope analyses of zircons from the Xiaofangshen gabbros (sample Fk04-5)
Figure 5. U–Pb age v. Initial epsilon Hf for zircons from the Xiaofangshen gabbros. The fields for the Xing-Meng orogenic belt and North China craton are from Yang et al. (Reference Yang, Wu, Shao, Wilde, Xie and Liu2006). NCC – North China craton; XMOB – Xing-Meng orogenic belt.
6. Discussion
6.a. Petrogenesis
The low silica contents (SiO2 = 46.64–52.73 wt%) and relatively high concentrations of Fe2O3 and MgO (7.66–12.80 wt% and 3.64–8.34 wt%, respectively) of the Xiaofangshen gabbros, and high Cr contents (275–438 ppm) in some samples, suggest that they were derived from a mantle source. Nevertheless, their moderate Mg no. (48.0–67.6) and low Ni concentrations (23–126 ppm) indicate that they do not represent primary magmas, but may have experienced some crystal fractionation, most likely of olivine and clinopyroxene, as reflected by the Cr–Ni fractionation vector plot (Fig. 6a) and the negative relationship between Cr and Y (excluding sample Fk06–12) (Fig. 6b).
Figure 6. (a) Cr v. Ni, (b) Y v. Cr and (c) Ba/La v. Th/Yb plots for the Xiaofangshen gabbros. In (a), direction of fractionation vectors for olivine and clinopyroxene separation from a relatively primitive sample are based on partition coefficients from Rollinson (Reference Rollinson1993). The olivine vector is not quantified due to uncertainties concerning the appropriate olivine–basaltic liquid Kd for Ni. In (c), continuous vector – slab-derived fluids; dashed vector – sediment input in the source as represented and discussed by Woodhead et al. (Reference Woodhead, Hergt, Davidson and Eggins2001).
Furthermore, as is usually the case for the mantle-derived magmas erupted in a continental setting, crustal contamination would have been involved in the genesis of the Xiaofangshen gabbros. However, given their higher Sr abundances (487–772 ppm) than those of continental crust (Sr = 280–348 ppm: Rudnick & Gao, Reference Rudnick, Gao and Rudnick2003) and the hosting Permian granite (Sr = 35.1–114 ppm; our unpub. data: Appendix Table 1, available online as supplementary material at http://www.cambridge.org/journals/geo), and the nearly consistent Sr–Nd isotopic compositions, crustal contamination seems to be insignificant. As such, the elemental and isotopic signatures of the Xiaofangshen gabbros were mainly inherited from those of parental mantle sources, and we thus can use the assumed most primitive samples to probe their mantle sources.
Petrographically, the hornblende-rich character of the Xiaofangshen gabbros is reminiscent of that of the high-level hornblende-rich mafic intrusions from the Mesozoic Sierra Nevada batholith (Sisson, Grove & Coleman, Reference Sisson, Grove and Coleman1996), indicating that their parental magma was rich in H2O and arguably was derived from an arc-modified source mantle. This suggestion is further supported by their trace element systematics. As widely documented (e.g. Stern, Reference Stern2002), the enrichment in large ion lithophile elements and light REE and depletion in high field strength elements (e.g. Nb, Ta, Zr and Ti) are typical of subduction-related magmatism. High La/Nb (2.46–4.01), Ba/Nb(22–73) and Zr/Nb ratios (8.8–21.8) in the Xiaofangshen gabbros bear close resemblance to those of arc volcanic rocks worldwide (Wang et al. Reference Wang, Fan, Peng, Zhang and Guo2005). According to the chromatographic model proposed by Stein, Navon & Kessel (Reference Stein, Navon and Kessel1997) for the transport of trace elements in the mantle wedge, the upper zones in the chromatographic column will be enriched in the incompatible and mobile elements such as Rb, Pb and LREE. The slight enrichment of the Xiaofangshen mafic magmas in Rb/Sr and Nd/Sm, as reflected by their isotopic ratios, is consistent with their derivation from such an enriched part of the lithosphere. This scenario is also consistent with the relatively flat MREE to HREE patterns (Fig. 3a) of the Xiaofangshen gabbros, with YbN values ranging from 9.76 to 23.4, implying their derivation from partial melting of a transitional mantle source between spinel and garnet stability fields, at a depth of 60–80 km (Watson & McKenzie, Reference Watson and McKenzie1991).
In general, the Ba/La fractionation can only be reasonably achieved by elemental mobility in hydrous fluids (McCulloch & Gamble, Reference McCulloch and Gamble1991), whereas Th and LREE are thought to be less mobile in aqueous fluids than the large ion lithophile elements (Pearce et al. Reference Pearce, Kempton, Nowell and Noble1999). As a result, these variables can serve as reliable indicators of potential sediment or fluid contributions to magma source regions (Woodhead et al. Reference Woodhead, Hergt, Davidson and Eggins2001). Figure 6c suggests that this contribution mainly comes from subduction-derived fluids.
To quantitatively evaluate the melting conditions, we adopted the standard non-modal batch melting equations of Shaw (Reference Shaw1970) to model the REE patterns of the Xiaofangshen gabbros with KD values from Gorring & Kay (Reference Gorring and Kay2001). Modelling parameters, mantle source composition, melt and source mode, and the degree of partial melting are listed in Table 5. Concentrations of REE in the peridotitic source are assumed to be 1.3 times primitive mantle of Sun & McDonough (Reference Sun, McDonough, Saunders and Norry1989). With their relatively high Mg no. (59.2–67.6) and Cr (291–438 ppm) abundances, the most primitive samples Fk04-5, Fk06-4 and Fk06-12 can be approximated as the primary melt (or minimally modified melt). For samples Fk04-5 and Fk06-4 with YbN values of 9.76 and 10.79, the melting model is based on a starting assemblage of spinel–garnet peridotite. The best-fit REE patterns correspond to a melt fraction of 4.5–6.3% (Table 5, Fig. 7). In the case of sample Fk06–12 with an YbN value of 23.39, the melting model is based on a spinel-bearing peridotite (0.50Ol + 0.23Opx + 0.25Cpx + 0.02Sp) as starting assemblage. The best-fit REE pattern corresponds to a melt fraction of 1.5% (Table 5, Fig. 7).
Figure 7. Chondrite-normalized REE patterns for the determined values and the non-modal batch melting model results for the Xiaofangshen gabbros.
Table 5. Model parameters and results of non-modal batch partial melting calculations (ppm)
Sources: S – 1.3 times primitive mantle of Sun & McDonough (Reference Sun, McDonough, Saunders and Norry1989); source and melt mineralogy are taken arbitrarily, but similar to those used in other partial melting calculations (McKenzie & O'Nions, Reference McKenzie and O'Nions1995; Tang et al. Reference Tang, Zhang and Ying2006).
The Sr, Nd and Hf isotopic data of the Xiaofangshen gabbros may provide further information on the nature of their source region. As shown above, the Xiaofangshen gabbros exhibit moderate initial 87Sr/86Sr ratios (0.7053–0.7055) and slightly positive ϵNd(t) values (+0.40–+0.68). This is similar to those of the late Triassic Hongqiling mafic–ultramafic complexes from the Xing-Meng orogenic belt (Fig. 4; Wu et al. Reference Wu, Wilde, Zhang and Sun2004), but is distinctive from those of Archaean to Palaeoproterozoic subcontinental lithospheric mantle beneath the northern margin of the eastern North China craton during late Palaeozoic to early Mesozoic times (Wu et al. Reference Wu, Walker, Yang, Yuan and Yang2006; Zhang et al. Reference Zhang, Goldstein, Zhou, Sun, Zheng and Cai2008a). Moreover, on the ϵHf(T) v. emplacement age plot (Fig. 5), all zircon points plot into the field of the Xing-Meng orogenic belt, as defined by the igneous zircons extracted from Phanerozoic granites and volcanic rocks in the Xing-Meng orogenic belt (Yang et al. Reference Yang, Wu, Shao, Wilde, Xie and Liu2006). Therefore, both the positive whole-rock ϵNd(t) values and young Nd model ages and the highly positive zircon ϵHf(T) values and young Hf model ages suggest that the parental magma for the Xiaofangshen gabbros likely originated from the juvenile lithospheric mantle that has an affinity with the Xing-Meng orogenic belt.
6.b. Tectonic implications
As outlined in the introduction, the tectonic affinity of the Faku dome has been a controversial but important issue, given its critical locality between a Phanerozoic accretionary orogen and a Precambrian craton. Our previous zircon U–Pb and other mineral 40Ar/39Ar dating on the deformed felsic intrusive rocks, which were once regarded as the Precambrian crystalline basement, reveals that they actually were crustal level records of the Permian to early Triassic magmatic and tectonic events (Zhang, Wang & Li, Reference Zhang, Wang and Li2005; Zhang, Su & Wang, Reference Zhang, Su and Wang2005). These late Palaeozoic to early Mesozoic ages lead us to suggest that no large-scale Precambrian crystalline basement existed in the Faku dome. This echoes the similar suggestions for the basement nature in regions such as the Xing'an block (Miao et al. Reference Miao, Fan, Zhang, Liu, Jian, Shi, Tao and Shi2003), the Songliao Basin (Wu et al. Reference Wu, Wilde, Zhang and Sun2004; Pei et al. Reference Pei, Xu, Yang, Zhao, Liu and Hu2007) and the Jiamusi Block (Wilde, Wu & Zhang, Reference Wilde, Wu and Zhang2003).
The occurrence of the Xiaofangshen mafic rocks suggests that a juvenile lithospheric mantle with an affinity with the Xing-Meng orogenic belt existed in the Faku area during early Mesozoic times. This is consistent with the contrast in lithospheric structure revealed by systematic geological–geophysical sections: Western Liaoning is characterized by a thick lithosphere with high rigidity and strength, whereas Songliao and northern Liaoning are characterized by relatively thin lithosphere with low rigidity and strength (Xu et al. Reference Xu, Middleton, Xue and Wang2000).
Such mantle- and crustal-level coherence implies that the northern Liaoning block has a tectonic affinity with the Phanerozoic accretionary orogenic belt. This revelation provides an important constraint on the suggestion that the surface suture between the North China Craton and the Xing-Meng orogenic belt in northern Liaoning is located along the Chifeng–Kaiyuan fault, but not the Xilamulunhe fault as previously advocated.
As widely documented, the Solonker zone has been regarded as the site of final closure of the Palaeo-Asian ocean (e.g. Tang, Reference Tang1990; Şengör, Natal'in & Burtman, Reference Şengör, Natal'in and Burtman1993; Xiao et al. Reference Xiao, Windley, Hao and Zhai2003). There exists much controversy concerning the timing of its suturing. Some authors propose that the suturing took place during Permian to early Triassic times (Şengör, Natal'in & Burtman, Reference Şengör, Natal'in and Burtman1993; Chen et al. Reference Chen, Jahn, Wilde and Xu2000; Xiao et al. Reference Xiao, Windley, Hao and Zhai2003), whereas some authors prefer suturing during either the middle Devonian epoch (Tang, Reference Tang1990) or late Devonian to early Carboniferous times (Shao, Reference Shao1991; Hong et al. Reference Hong, Huang, Xiao, Xu and Jin1995); still others advocate a middle Mesozoic suturing time based on a controversial amphibole K–Ar age (Nozaka & Liu, Reference Nozaka and Liu2002). However, our recent documentation of the early Permian post-collisional bimodal volcanism along central Inner Mongolia suggests that the North China craton and Mongolian micro-continents amalgamated by early Permian times, and this resulted in the Mesozoic North China–Mongolian Plate (Zhang et al. Reference Zhang, Zhang, Tang, Wilde and Hu2008c). The widespread occurrence of the Late Permian–Middle Triassic post-orogenic intrusive suites along the western segment of the northern margin of the North China craton echoes this suggestion (Zhang et al. Reference Zhang, Zhao, Song, Hu, Liu, Yang, Chen, Liu and Liu2008b).
As reviewed by various authors (e.g. Liegeois, Reference Liégeois1998; Vanderhaeghe & Teyssier, Reference Vanderhaeghe and Teyssier2001; Bonin, Reference Bonin2004), an orogenic cycle typically features a pre-collisional period characterized by subduction, leading to oceanic basin closure and terrane docking, a period of arc–continent or continent–continent collision accommodated by crustal thickening, post-collisional, post-orogenic and within-plate anorogenic episodes. The corresponding four stages of the mantle unrooting process are identified as orogenic growth, initiation of gravitational instability until lithospheric failure, sinking of the detached lithosphere and relaxation of the system (Marotta, Fernandez & Sabadini, Reference Marotta, Fernandez and Sabadini1998).
When evaluated within this general context of thermal and mechanical evolution of the continental crust during orogenesis, the northern margin of the newly amalgamated North China–Mongolian Plate was tectonically dominated by post-orogenic to within-plate anorogenic extensional regimes during early–middle Triassic times, possibly corresponding to a transitional regime from the third to fourth stage, that is, lithosphere delamination and subsequent relaxation, in terms of the mantle unrooting process (Marotta, Fernandez & Sabadini, Reference Marotta, Fernandez and Sabadini1998).
During this pivotal period, repetitive generation of water-bearing magmas resulted in an increasingly depleted and dehydrated continental lithosphere (Bonin, Reference Bonin2004). This led to the thermal and mechanical instability of the thickened lithosphere keel and, coupled with the weak link with the crust (Meissner & Mooney, Reference Meissner and Mooney1998), induced delamination of the subcontinental lithospheric mantle and subsequent crustal extension (Marotta, Fernandez & Sabadini, Reference Marotta, Fernandez and Sabadini1998). This extensional tectonic regime enables rapid upwelling of asthenosphere, and triggers concomitant decompressional partial melting of the mantle and the magmatic underplating at the crust–mantle boundary. Upon the complete amalgamation of continental terranes, the juvenile within-plate subcontinental lithosphere grows with time by cooling and by continued underplating of deeper materials (Bonin, Reference Bonin2004).
It was such a favourable scenario that led to the formation of the middle Triassic Xiaofangshen gabbros, the early Triassic adakitic rocks (Jiang et al. Reference Jiang, Liu, Zhou, Yang and Zhang2007) and A-type granites (Zhang et al. Reference Zhang, Zhao, Song, Hu, Liu, Yang, Chen, Liu and Liu2008b) from the northern Hebei area, the Triassic Fanshan ultramafic complex dating from 218 to 243 Ma (Mu et al. Reference Mu, Shao, Chu, Yan and Qiao2001; Jiang et al. Reference Jiang, Chu, Mizuta, Ishiyama and Mi2004) and the Triassic cumulate and granulite xenoliths dating from 220 to 251 Ma from Chifeng of the southern Inner Mongolia (Shao et al. Reference Shao, Han, Zhang and Mu1999; Shao, Han & Li, Reference Shao, Han and Li2000). Dehydration of the thinning lithosphere resulted ultimately in the shift in a few million years from calc-alkaline to alkaline magmatic suites, as indicated by the Triassic alkaline intrusions dating from 205 to 250 Ma within the continental interior of the newly amalgamated North China–Mongolian Plate (Shao, Mu & Zhang, Reference Shao, Mu and Zhang2000; Yan et al. Reference Yan, Mu, Xu, He, Tan, Zhao and He2000).
7. Conclusions
(1) SHRIMP U–Pb zircon dating and geochemical analyses document an episode of middle Triassic mafic magmatism in the Faku dome of the northern Liaoning area at the northern margin of the North China–Mongolian plate, as represented by the Xiaofangshen gabbros. Their hornblende-rich character and typical geochemical signatures argue for an origin that is consistent with a small amount of partial melting of a subduction metasomatized lithospheric mantle.
(2) The juvenile character of both the lithospheric mantle and crustal levels suggests that the northern Liaoning block has a tectonic affinity with the Phanerozoic accretionary orogenic belt. This revelation indicates that the Chifeng–Kaiyuan fault likely represents the Mesozoic lithospheric boundary between the North China Craton and the Xing-Meng orogenic belt in the northern Liaoning area.
(3) The Xiaofangshen gabbros, together with the Triassic cumulate and granulite xenoliths and the Triassic alkaline intrusions, constitute an important post-orogenic to within-plate anorogenic magmatic province within the continental interior of the newly amalgamated, Mesozoic North China–Mongolian Plate.
Acknowledgements
This study was financially supported by the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant no. KZCX2-YW-103), the Major State Basic Research Program of the People's Republic of China (Grant no. 2006CB403504) and the National Natural Science Foundation of China (Grant nos 40534022 and 40873026). The authors thank Mr H. Li and Ms X. D. Jin in major- and trace-element analysis, and Dr C. F. Li in Sr–Nd isotope analyses. We are also grateful to Dr D. Pyle, Dr B. Chen and one anonymous review for their constructive suggestions and Mrs J. Holland for editorial handling. This is The Institute for Geoscience Research (TIGeR) publication no. 120.
