2016, Vol. 32
2. 矿床地球化学国家重点实验室, 中国科学院地球化学研究所, 贵阳 550002;
3. 青海省西宁市武警黄金第六支队, 西宁 810021
2. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China;
3. No.6 Gold Geological Party of CAPF, Xining 810021, China
1 Introduction
Mafic dykes, ranging in size from tens to several hundreds of metres thickness and from several to many hundreds or even thous and s of kilometres in length, have been emplaced in many parts of the world in response to regional-scale lithospheric extension that commonly accompanies the initial stages of the breakup of supercontinents(Halls, 1982; Halls and Fahrig, 1987; Féraud et al., 1987; Tarney and Weaver, 1987; Zhao and McCulloch, 1993; Gudmundsson, 1995; Hou et al., 2006; Liu, 2004; Liu et al., 2004, 2005, 2006, 2008a, b, c, 2009a, b, 2012a, b, 2013a, b, c, d, e; Peng, 2010; Peng et al., 2005, 2007, 2008, 2010, 2011a, b). The compositions of these mafic dykes include basalt, dolerite, gabbro, dolerite-porphyry, picrite, norite and lamprophyre. Examples of continental-scale suites of mafic dykes include the Kennedy and San Rafael mafic dykes; the Mackenzie, Matachewan, Mistassini, Franklin, Grenville and Marathon dyke swarms of Canada(Halls and Hatts, 1990; Ernst et al., 1992, 1995, 2005; Liu et al., 2013c); the Great Dyke of Zimbabwe(Oberthür et al., 2002); the Jimberl and and Widgiemooltha mafic dykes of Australia(Ma et al., 2000); and the southwestern Greenl and mafic dykes(Nisson et al., 2013). Other important occurrences of mafic dykes include swarms from Brazil, India, Italy, Japan, Norway, Scotl and , Siberia, South Africa and China(e.g., the southern China and NCC; Hou et al., 2006; Liu, 2004; Liu et al., 2004, 2005, 2006, 2008a, b, 2009a, b, 2012a, b, 2013a, b, c, d, e; Peng, 2010; Peng et al., 2005, 2007, 2008, 2010, 2011a, b; Chen et al., 2015). And the swarms of mafic dykes have very old ages(>2.5~1.0Ga).
In terms of China, workers to date have focused primarily on the mafic dykes of Proterozoic age in both of the southern China and NCC(e.g., Chen and Shi, 1983, 1994; Chen et al., 1992; Peng et al., 2005, 2007, 2008, 2010, 2011a, b; Hou et al., 2006; Piper et al., 2011; Li et al., 2010; Liu et al., 2012b, 2013b, c; Peng, 2010). However, in addition to these older mafic dykes, many mafic dykes of Mesozoic age occur across China. For example, in southern China(e.g., Zhejiang, Jiangxi, Hunan, Fujian, Guangdong, Guangxi and Hainan provinces), more than 700 dykes have been recorded, while a similar number crop out over the entire NCC(e.g., the Sh and ong, Liaoning, Jilin, Shanxi, Hebei and Inner Mongolia regions). While many studies have been carried out on these Mesozoic dykes, there are controversy regarding their petrogenesis and ages of emplacement(e.g., underplating, magma immiscibility, metathesis, delamination; 150~80Ma; Shao and Zhang, 2002; Zhang and Sun, 2002; Shao et al., 2003; Xie, 2003; Zhai et al., 2003, 2004; Liu, 2004; Xu, 2004; Yang et al., 2004; Zhao, 2004; Liu et al., 2005, 2006, 2008a, b, c, 2009a, b, 2012c, d, 2013a, d, e; Zhang, 2006; Cao, 2007; Wang et al., 2007; Wu et al., 2008; Zhang, 2009; Zhai and Santosh, 2013).
To further refine the ages and petrological models for Mesozoic mafic magmatism in China, a more detailed geochronological, geochemical and isotopic study of a variety of Mesozoic mafic dykes from southern China and the NCC is required. We therefore present here the results of new zircon U-Pb dating using LA-ICP-MS, as well as petrological, whole-rock geochemical, and Sr-Nd-Hf isotopic data for representative samples of mafic dykes from the Eastern Block of the NCC and the Zichuan region in western Sh and ong Province. These new data allow us to constrain the emplacement ages of these dykes and the origin of this mafic magmatism.
2 Geological setting and petrographyThe NCC is located in northern China, covers a wide area(~170×104km2)(Wu et al., 2008; Zhai and Santosh, 2011, 2013; Zheng et al., 2013; Li et al., 2013), and consists of Archaean Eastern and Western blocks which collided at ~1.85Ga along the nearly N-S-trending Palaeoproterozoic Trans-North China Orogen(Zhao et al., 2001, 2005; Wilde et al., 2002; Guo et al., 2005). The Eastern and Western blocks can be further subdivided into microcontinental blocks and active belts(Zhai and Bian, 2000; Zhao, 2009). For example, the Western Block consists of the Yinshan Block in the north and the Ordos Block to the south, separated by the E-W-trending Palaeoproterozoic Khondalite Belt(Xia et al., 2008; Yin et al., 2009, 2011; Li et al., 2011; Wang et al., 2011). The Eastern Block consists of the Longgang(also known as the Yanliao Block) and Langrim blocks, separated by the Palaeoproterozoic Jiao-Liao-Ji Belt(Li et al., 2006; Li and Zhao, 2007; Luo et al., 2008; Zhou et al., 2008; Huang et al., 2009; Wang et al., 2009; Zhao et al., 2010, 2012; Tam et al., 2011, 2012a, b; Wu et al., 2013a, b; Zhao and Zhai, 2013). The NCC is one of the oldest continents(Lin et al., 2008)with a crustal age of >3.8Ga(Liu et al., 1992), and its southern and northern margins are the Indosinian Qingling-Dabie and Hercynian Yinshan-Yanshan orogenic belts, respectively. Traditionally, the NCC has been considered to comprise uniform Precambrian(Archaean-Sinian)crystalline basement, overlain by a variety of younger cover lithologies of Cambrian-Quaternary age.
Sh and ong Province is located in the Eastern Block of the NCC, and more than 200 Mesozoic mafic dykes occur throughout eastern Sh and ong(Jiaodong), western Sh and ong(Luxi) and adjacent areas of Tan-Lu Fault. The study areas for this investigation are in the Zichuan region of western Sh and ong Province(samples SJ1 to SJ8, KJ1 to KJ4 and CN1 to CN4). The sampled mafic dykes from this area intrude Jurassic sedimentary rocks(JzS) and Proterozoic complexes(P)(Fig. 1). Individual dykes are vertical, trend NW-SE, are 15m to 0.8km wide, and 3.0 to 20km long(Fig. 1b). The dykes are typical medium-grained diabases that contain phenocrysts of clinopyroxene(2.0~6.5mm) and plagioclase(2.0~5.0mm)within a matrix(60%~65%)of clinopyroxene(0.05~0.07mm) and plagioclase(0.03~0.06mm)with minor magnetite(ca. 0.02~0.04mm) and chlorite(0.04~0.05mm). Accessory minerals include zircon and apatite.
 | Fig. 1 Sketch map of study area and adjacent areas, China(a) and geological map of the study areas, showing the distribution of the mafic dykes and sampling localities(b) |
 | Fig. 2 Zircon LA-ICP-MS U-Pb concordia diagrams and CL images of zircons separated from the mafic dykes from western Sh and ong Province, China |
Zircon was separated from four samples(SJ01, SJ02, KJ01 and CN01)using conventional heavy liquid and magnetic techniques at the Langfang Regional Geological Survey, Hebei Province, China. After separation and mounting, the internal and external structures of zircon grains were imaged using transmitted and reflected light, and cathodoluminescence at the State Key Laboratory of Continental Dynamics, Northwest University, China. Prior to zircon U-Pb dating, grain mount surfaces were rinsed in dilute HNO3 and pure alcohol to remove any potential lead contamination. Zircon U-Pb ages were determined using LA-ICP-MS(Table 1; Fig. 2)with an Agilent 7500a ICP-MS instrument equipped with a 193nm excimer laser at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geoscience(Wuhan), China. Zircon st and ard(# 91500)was used for quality control, and the NIST 610 st and ard was used for data optimisation. A spot diameter of 24μm was used during analyses, employing the methodology described by Yuan et al.(2004) and Liu et al.(2010). Common Pb correction was undertaken following Andersen(2002), and the resulting data were processed using the GLITTER and ISOPLOT programs(Ludwig, 2003; Table 1; Fig. 2). Uncertainties for individual LA-ICP-MS analyses are quoted at the 95%(2σ)confidence level.
 | Table 1 LA-ICP-MS U-Pb isotope data for zircon separates from the mafic dykes of western Sh and ong Province |
The whole-rock and Sr-Nd isotope compositions of 16 samples were determined during this study. Prior to whole-rock geochemical analysis, samples were trimmed to remove altered surfaces, cleaned with de-ionised water, and then crushed and powdered in an agate mill. Major element concentrations were determined on fused glass discs using a PANalytical Axios-advance X-ray fluorescence spectrometer at the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, China. These analyses have a precision of <5%, as determined using the GSR-1 and GSR-3 Chinese national st and ards(Table 2). Loss on ignition values was obtained using 1g of powder heated to 1100℃ for one hour(Table 2). Trace element concentrations were determined using ICP-MS at the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, China, following the procedures outlined in Qi et al.(2000). Triplicate analyses were reproducible to within 5% for all elements, and analyses of the OU-6(Potts and Kane, 2005) and GBPG-1(Thompson et al., 2000)international st and ards are agreed with recommended values(Table 3).
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Table 2 Major element concentrations(wt%)for the mafic dykes from western Sh and ong Province |
Sample powders used for Rb-Sr and Sm-Nd isotope analyses were spiked with mixed isotope tracers, dissolved in Teflon capsules with HF and HNO3 acids, and separated by conventional cation-exchange techniques. Isotopic measurements were performed using a Finnigan Triton Ti thermal ionisation mass spectrometer at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences(Wuhan), China. Procedural blanks yielded concentrations of <200pg for Sm and Nd, and <500pg for Rb and Sr. Mass fractionation corrections for Sr and Nd isotopic ratios were based on 86Sr/88Sr=0.1194 and 146Nd/144Nd=0.7219, respectively, and analysis of the NBS987 and La Jolla st and ards yielded values of 87Sr/86Sr=0.710246±16(2sm), and 143Nd/144Nd=0.511863±8(2sm), respectively.
3.4 In situ zircon Hf isotopic analysisIn situ zircon Hf isotopic analyses were conducted using a Neptune multiple collector-ICP-MS, equipped with a 193nm laser, at the Institute of Geology and Geophysics, Chinese Academy of Sciences, China. During the analyses, a laser repetition rate of 10Hz at 100mJ was used, and spot sizes were 32 and 63μm. Details of the analytical technique are described in Wu et al.(2006). During the analyses, the 176Hf/177Hf and 176Lu/177Hf ratios of the st and ard zircon 91500 were 0.282300±15(2sm, n=24) and 0.00030, respectively, similar to the commonly accepted 176Hf/177Hf ratios of 0.282302±8 and 0.282306±8(2sm)measured using the solution method(Goolaerts et al., 2004).
4 Results 4.1 Zircon U-Pb ages
Euhedral zircon grains in samples SJ01, SJ01, KJ01 and CN01 are clean and prismatic, with magmatic oscillatory zoning(Fig. 2). A total of 13 grains provided a weighted mean 206Pb/238U age of 123.2±0.6Ma(2σ)(95% confidence interval)for SJ01, 12 grains gave a weighted mean 206Pb/238U age of 122.0±0.6Ma(2σ)(95% confidence interval)for SJ02, 12 grains gave a weighted mean 206Pb/238U age of 121.9±0.6Ma(2σ)(95% confidence interval)for KJ01(Table 1; Fig. 2a-d) and 13 grains gave a weighted mean 206Pb/238U age of 124.3±0.5Ma(2σ)(95% confidence interval)for CN01. These weighted mean ages are the best estimates for the times of these doleritic dykes crystallised. Some inherited zircons were observed.
4.2 Major and trace element geochemistryGeochemical data for the mafic dykes in the study area are presented in Tables 2 and Table 3. The diabase samples exhibit a narrow range of geochemical compositions, falling into the alkaline field when plotted on the total alkali-silica diagram(Fig. 3a); the rock type is phonotephrite. The mafic dyke samples also display an affinity to shoshonitic compositions in terms of Na2O vs. K2O(Fig. 3b). The dykes display obscure trends of decreasing TiO2, Al2O3, K2O, Na2O and P2O5 with increasing MgO(Fig. 4b, c, f-h), and a positive correlation between Fe2O3 and MgO. They are also characterised by a relative enrichment in light rare earth elements, with a wide range of(La/Yb)N ratios(10.4~13.9) and negligible Eu anomalies(Eu/Eu*=0.9~1.0)(Table 3; Fig. 5a). On primitive-mantle-normalised trace element diagrams, the mafic dykes show enriched in large ion lithophile elements(i.e., Rb, Ba, U, K and Pb) and P, as well as depleted in high field strength elements(i.e., Nb, Ta and Ti)(Fig. 5b).
 | Fig. 3 Classification of the mafic dykes from western Sh and ong Province (a)TAS diagram(after Middlemost, 1994; Le Maitre, 2002), where all major element concentrations are recalculated to 100% volatile-free compositions, and (b)Na2O vs. K2O diagram(after Menzies and Kyle, 1972) |
 | Fig. 4 Variations in major element contents compared with MgO for the mafic dykes of western Sh and ong Province |
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Table 3 Trace element compositions(×10-6)of the mafic dykes from western Sh and ong Province |
 | Fig. 5 Chondrite-normalized rare earth element patterns(a) and primitive-mantle-normalized incompatible element distribution diagrams(b)for the mafic dykes of western Sh and ong Province(normalization values after Sun and McDonough, 1989) |
Also shown is a field delineating the composition of Mesozoic mafic dykes within the Yangtze Craton, NCC, the Sulu Belt and other areas in Sh and ong Province(Li et al., 1998, 2004; Jahn et al., 1999; Chen et al., 2001; Guo et al., 2001; Fan et al., 2004; Liu, 2004; Wang et al., 2005; Liu et al., 2008a, b, 2009a). The mafic dykes analysed during this study plot within the field of an enriched mantle source
4.3 Sr-Nd isotopes Sr-Nd isotopic data were obtained for 10 representative mafic dyke samples(Table 4). The dykes show uniform(87Sr/86Sr)i values(~0.7098) and little variation in εNd (t)values(from -14.7 to -14.5). The Sr-Nd isotopic compositions(Fig. 6)are comparable to those of Mesozoic mafic rocks from the Sulu Belt(Li et al., 1998; Jahn et al., 1999; Fan et al., 2004; Wang et al., 2005) and Sh and ong Province(Liu, 2004; Liu et al., 2008a, b, 2009a). However, they differ from those of Mesozoic mafic dykes in the Yangtze Craton(Chen et al., 2001; Li et al., 2004) and other parts of the North China Craton(Guo et al., 2001).|
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Table 4 Sr-Nd isotopic compositions of the mafic dykes from western Sh and ong Province |
 | Fig. 6 Variations in initial 87Sr/86Sr vs. εNd(t)values for the mafic dykes of western Sh and ong Province |
Four samples of zircon dated by LA-ICP-MS zircon U-Pb dating were also analysed for their Lu-Hf isotopes, and the results are presented in Table 5. Fifteen spot analyses were performed on zircon from sample KJ01. The determined negative εHf(t)values for this dyke vary between -30.4 and -28.6 (Table 5; Fig. 7a). This sample has initial 176Hf/177Hf ratios that vary between 0.281839 and 0.281896. Fifteen spot analyses were made for sample SJ02. The determined negative εHf(t)values for this zircon vary between -29.5 and -28.4(Table 5; Fig. 7b). This sample has initial 176Hf/177Hf ratios that vary between 0.281864 and 0.281898. Fifteen spot analyses were obtained for zircon sample SJ01, yielding variable εHf(t)values between -31.4 and -27.6(Table 5; Fig. 7c), and giving initial 176Hf/177 Hf ratios ranging from 0.281808 to 0.281921. Fifteen spot analyses were obtained for zircon sample SJ01, yielding variable εHf(t)values between -30.3 and -26.7(Table 5; Fig. 7d), and initial 176Hf/177Hf ratios ranging from 0.281842 to 0.281896.
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Table 5 Zircon Lu-Hf isotopic compositions of the mafic dykes from Sh and ong Province |
 | Fig. 7 Histograms of zircon εHf(t)values(a-d) and two-stage Hf model ages(e-h)for the mafic dykes in western Sh and ong Province |
The investigated Early Cretaceous mafic dykes are characterised by low SiO2 contents(51.4%~51.5%; Table 2), suggesting they were derived from an ultramafic source(Liu et al., 2008a, b, 2009a, 2013a, b, c, d). Crustal rocks can be ruled out as possible sources, as partial melting of any of the crustal rocks(e.g., Hirajima et al., 1990) and lower crustal intermediate granulites(Gao et al., 1998)in the deep crust would produce liquids with high Si and low Mg contents(i.e., granitoid liquids; Rapp et al., 2003). The high initial 87Sr/86Sr ratios(~0.7098), negative εNd (t)values(-14.7 to -14.5) and zircon εHf (t)values(-31.4 to -26.7)(Tables 4 and Table 5) for the mafic dykes are consistent with derivation from an enriched lithospheric mantle source rather than an asthenospheric mantle source with a depleted Sr-Nd isotopic composition, such as Middle Ocean Ridge Basalt(MORB).
5.2 Crustal contaminationCrustal contamination might cause a significant depletion in Nb-Ta and enriched Sr-Nd isotopic signatures in basaltic rocks(Guo et al., 2004). The dolerites studied here are characterised by negative Nb-Ta anomalies(Table 3; Fig. 5b), and this implies a crustal component in the genesis of the mafic dyke magma. In Fig. 4a, b, e and h, the major elements such as SiO2, CaO, TiO2, and P2O5 and so on actually show obscure linear correlation with MgO. This would suggest that magma mixing or contamination might have played an important role during the magma ascending. The above observation is further supported by inherited zircons, the fact that the dykes have low Ni(18.8×10-6~96.3×10-6)(Table 3), no correlation between Mg#, Ni and initial Sr ratio(not shown), distinctive negative high field strength elements(Nb, Ta, and Ti), positive Pb anomalies(Fig. 5a, b; Zhang et al., 2005), and high Ba/Nb ratios(103~136; Table 3; Jahn et al., 1999).
5.3 Genetic modelAll of the mafic dykes studied here have similar characteristics in terms of their geochemistry and isotopes, implying a similar source region. On a plot of La versus La/Sm(not shown), all the samples are distributed along a trend line for partial melting, indicating the dykes were derived by partial melting of an enriched mantle. In addition, as noted above, the possibility of significant and direct assimilation of crust during the genesis of the mafic magmas occurred. Moreover, in the primitive-mantle-normalised diagram(Fig. 5b), all the dykes show distinctive negative anomalies in Nb, Ta and Ti, and positive anomalies in Pb. HFSE-depletion could indicate the involvement of components from the proto-Tethyan oceanic or ancient continental crust(Zhang et al., 2005). In addition, the higher La/Nb(3.9~4.9) and Ba/Nb ratios(103~135)in these rocks(Table 3)differ from those of most intraplate volcanic rocks, including Ocean Isl and Basalt(OIB), alkali basalt and kimberlite, which have much lower ratios of La/Nb(2.5~0.5) and Ba/Nb(20 to 1)(Jahn et al., 1999). These data suggest that continental materials(granitoids, granulites, sediments etc.)were involved in the petrogenesis of the mantle-derived magmas, which is further supported by the low εNd(t)values from -14.7 to -14.5 and high(87Sr/86Sr)i values(~0.7098)(Table 4; Fig. 6). Therefore, we propose the involvement of crustal components that were already incorporated into the mantle source. Nevertheless, it is necessary to know by which mechanism those crustal materials may have been incorporated.
Accordingly, a genetic model is required to decipher the origin of these dykes. At present, at least two competing mechanisms can be envisaged(Liu et al., 2008a, b, c, 2009a, 2013b):(1)contributions from the subducting Yangtze Block, and (2)the action of the subducted ancient Pacific Plate(Cai et al., 2013; Tang et al., 2013). However, it is generally believed that the final collision between the NCC and the Yangtze Block occurred during the Triassic(Meng and Zhang, 1999; Zhang et al., 2005), and there was no westwards subduction of an ancient Pacific Plate below the NCC before the Early Cretaceous(Xu et al., 1993). Furthermore, no evidence has yet been presented that a contribution of the Palaeo-Pacific Plate to Mesozoic magmatism in the eastern NCC(Zhang et al., 2005). Thus, it is unlikely that the petrogenesis of these magmas relates to either a subducting Yangtze lithosphere or ancient Pacific Plate. An alternative model, therefore, is required to account for how the mafic dykes were formed.
Since the direct assimilation of crustal material has been shown to be negligible in the genesis of the mafic dykes in the Zichuan area, and if the role of subducting lithosphere(either the Yangtze or Palaeo-Pacific plates)can been discounted, it is necessary to know by which reasonable mechanism crustal materials may have been incorporated into the underlying lithospheric mantle. The foundering of the lower continental crust has been suggested as a possible genetic model for the origin of the Mesozoic mafic dykes in Sh and ong Province(Liu et al., 2008a, b, 2009a, b), and because of its higher density than lithospheric mantle(Anderson, 2006), eclogite can be recycled into the mantle(Kay and Mahlburg-Kay, 1991; Jull and Kelemen, 2001; Gao et al., 2004). Moreover, eclogites have lower melting temperatures than mantle peridotites(Yaxley, 2000; Kogiso et al., 2003), and so foundered. Silica-saturated eclogite may partially melt to produce silicic melts(tonalite to trondhjemite)that may be hybridised with the overlying mantle peridotite. Such hybridisation could produce an olivine-free pyroxenite, which, if subsequently melted, would generate basaltic melt(Kogiso et al., 2003; Herzberg et al., 2007; Gao et al., 2008). In the eastern NCC, this model is further supported by observations of intensive lithospheric thinning(Liu et al., 2008a, b), voluminous coeval magmatism(130~120Ma)(Wang et al., 1998; Guo et al., 2001; Yang et al., 2003, 2004; Liu et al., 2004, 2008a, b, c; Zhang et al., 2004; Li et al., 2013), large-scale mineralization(Wang et al., 1998; Yang et al., 2003, 2004) and the presence of adakitic rocks(Gao et al., 2004; Liu et al., 2008c, 2009b), all of which could be produced during a process of lithospheric foundering.
Nevertheless, if delamination of eclogitic lower crust occurred, this would lead to rapid uplift of the study area(Menzies et al., 2007). Evidence for this uplift is lacking, however. At the same time, lithospheric delamination would induce asthenospheric upwelling, leading to decompression melting and the formation and eruption of basalt with similar geochemical features to that of MORB or OIB. The absence of such asthenospherically-derived magma contemporaneous with the studied dykes thus argues against a delamination of the lithosphere. Moreover, the study dykes are characterized by depletion in Th and U, and enrichment in K; they all exclude the possibility of delamination of lower crust. Thus, what is the cause of the enriched mantle source to the mafic magmas, and where do the hybridized materials derive? These key issues will be discussed next.
By contrast with the exposed Archaean-Proterozoic metamorphic complexes and typical marine sediment as a possible explanation, Guo et al.(2014)sought help from ‘cold’ subduction of the Yangtze Craton to explain the origin of the mafic dykes(115Ma)from Jiaodong peninsula. Their interpretation has provided credible evidence, based on which, we propose that a sedimentary component derived from Yangtze Craton, continental crust to be the cause of mantle enrichment beneath the study area. The interaction of sedimentary melt with overlying mantle lithologies helped to generate fertile mantle pyroxenite. Partial melting of this modified, and olivine-poor, pyroxenite can produce Mg-rich magmas with low Nb-Ta-Ti concentrations. In the Guo et al.(2014)study, it was indeed suggested that fertile pyroxenite, formed through sediment melt-peridotite interaction, to be the source of mafic dykes emplaced along the western Sh and ong Province(Guo et al., 2014). In accordance with geophysical observations(Engebretson et al., 1985), oblique subduction of the paleo-Pacific Plate towards the NCC occurred during the Early Cretaceous. This subduction exerted a driving force to induce the extensive collapse of the southeastern margin of the NCC, triggering extensive melting of the metasomatized mantle responsible for mafic magmatism across the NCC(Wu et al., 2005).
As such, a special model can explain the formation of the mafic dykes within the sutdy area. Chemenda et al.(2000)proposed a two-dimensional thermo-mechanical, laboratory model for continental subduction, and used this to interpret the evolutionary history of the India-Asia collisional system. They suggested that subducted continental crust or sediments would be detached from underlying lithosphere mantle if the subduction was sufficiently rapid or if the subducted lithosphere had a thick lower crust(Zhang and Sun, 2002). This model may be suitable for the Triassic, Dabie collisional zone because the Yangtze Craton is an old Craton and should have had a thickened lower crust. Thus, we adopt this explanation to help reconstruct possible scenarios for the Dabie collision and for the formation of the Mesozoic lithosphere adjacent to the Dabie Orogen.
At ~240Ma, collision between the Yangtze Craton and the NCC occurred along with northward subduction of the paleo-Tethys oceanic lithosphere(Zhang and Sun, 2002). The Yangtze lithosphere was dragged down into mantle by the denser oceanic lithosphere it comprised. Subsequently, the upper/middle Yangtze crust and sedimentary drape reached a depth of ~200km(Ye et al., 2000; Zhang and Sun, 2002) and subsequently rapidly moved upward between the NCC and Yangtze Craton in response to slab break-off of the subducting oceanic lithosphere(Davies and Von Blanckenburgh, 1995; O’Brien, 2001). At ~220Ma, Yangtze subduction switched to a highly compressional mode(Chemenda et al., 2000), which resulted in the detachment of the Yangtze crust and sediments from the mantle. The crust would be then tectonically underplated beneath the base of the NCC lithosphere because of its buoyancy relative to the surrounding mantle. This process would lead to a thickened continental root and an isostatic uplift of the southeastern NCC. The thickened continental root was then probably eclogitized(Leech, 2001)or melted(Skjerlie and Douge, 2002)by underlying asthenosphere.
Subsequently, silicic melts produced by melting of these crustal materials migrated through the overriding continental lithosphere and interacted with mantle peridotite. Extensive interaction would have completely destroyed the old lithosphere regime, finally generating the Sr-Nd isotopic enriched(hybridized)Mesozoic lithosphere that was the source for the Cretaceous dykes’ intrusion. As a result, decompression melting of this enriched mantle at 125~120Ma produced primary basaltic melts that evolved to form the mafic magmas that were emplaced as swarms of dykes across the western Sh and ong Province of the southeastern NCC.
6 ConclusionsBased on new geochronological, geochemical and Sr-Nd and Hf isotopic studies of mafic dykes from the Zichuan area of western Sh and ong Province, the following conclusions can be drawn:
(1)U-Pb zircon dating indicates that the mafic dykes in western Sh and ong Province were emplaced between 121.9±0.6Ma and 124.3±0.5Ma.
(2)The mafic dykes in the study area are alkaline and shoshonitic, have high light rare earth element concentrations with slight negative Eu anomalies((Eu=0.9~1.0), and have positive Ba, K, Pb and P, and negative Nb, Ta, and Ti anomalies. These dykes were derived from partial melting of an enriched mantle source((87Sr/86Sr)i values=~0.7098, εNd(t)values=-14.7 to -14.5), and formed from parental magmas that were generated during lithospheric extension-related partial melting of an enriched region of the lithospheric mantle beneath the southeastern NCC. In addition, there underwent significant contamination of these magmas during emplacement.
(3)The mafic dykes in the study area formed in an extensional setting following collision between the NCC and the Yangtze Craton. The magmas that formed these dykes were sourced from a hybridized source caused by subduction of Yangtze crustal sedimentary material beneath southeastern NCC before the Late Mesozoic.
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