Zircon U-Pb age, geochemical, and Sr-Nd isotopic constraints on the origin of Late Carboniferous mafic dykes of the North China Craton, Shanxi Province, China

LIU Shen, FENG CaiXia, JAHN BoMing, HU RuiZhong, ZHAI MingGuo and LAI ShaoCong

1. State Key Laboratory of Continental Dynamics and Department of Geology, Northwest University, Xi’an 710069, China2. Department of Geosciences, National Taiwan University, Taipei, China3. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China1.

1　Introduction

NE-SW and NW-SE striking mafic dykes are widespread in the North China Craton (NCC; Liuetal., 2008a, b, 2009, 2012a, b, 2013), and are the product of lithospheric extension (Hall, 1982; Hall and Fahrig, 1987; Tarney and Weaver, 1987; Zhao and McCulloch, 1993). These rocks provide valuable information on the processes involved in extension, the nature of the mantle beneath this region, and the temporal and spatial evolution of this area, as well as enabling reconstructions of the agglomeration, extension, and rifting apart of continental blocks. Despite this, little research has been undertaken on mafic dykes within the NCC, and the majority of previous research has focused solely on Precambrian and Mesozoic mantle-crust interaction (e.g., Chen and Shi, 1983; Shao and Zhang, 2002; Zhang and Sun, 2002; Shaoetal., 2003; Zhaietal., 2003, 2004; Xu, 2004; Yangetal., 2004; Liuetal., 2005, 2006, 2008a, b, 2009, 2012b, 2013; Peng, 2010; Pengetal., 2005, 2007, 2008, 2010, 2011a, b; Houetal., 2006; Wangetal., 2007; Huetal., 2008; Linetal., 2008; Wuetal., 2008; Zhuetal., 2008; Johnetal., 2010; Lietal., 2010). In contrast, little research has been undertaken on Late Paleozoic (especially Devonian-Permian) mafic dykes of Shanxi Province, located in the northern NCC.

This lack of research means that further studies on the geochronological, geochemical, and isotopic characteristics of Late Paleozoic mafic dykes of the northern NCC are required. Here, we present new laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb geochronological, petrological, major, trace and rare-earth element geochemical, and Sr-Nd isotopic data for representative mafic dykes within the northern NCC. The aim of this work was to constrain the timing of emplacement and the petrogenesis of the magmas that formed these mafic dykes.

2　Geological setting and petrography

The NCC consists of an N-S striking mid-continental Proterozoic orogenic belt and Archean eastern and western blocks (Zhaoetal., 2001; Fig.1a). The study areas in the present paper are located in the Xituanbao and Tatong areas of northern Shanxi Province (samples XTB1 to XTB16), within the northern NCC. Mafic dolerite dykes from this area were sampled during this study (Table 1; Fig.1b). These dykes were intruded into gneisses and granite country rocks of unknown age; the other major country rock in this area is dolomite (Fig.1b). Individual mafic dykes are vertical, and strike NE-SW. These dykes are commonly 0.05～2.4km wide and 2.2～18.0km long (Fig.1b), and representative photomicrographs of mafic dykes in the Xituanbao area (samples XTB-2 and XTB-8) are provided in Fig.2. All of the mafic dykes are dolerites. They have typical doleritic/diabasic textures and consist of medium-grained clinopyroxene (2.5～4.5mm) and lath-shaped plagioclase (1.5～3.0mm) phenocrysts (32%～35% of the rock mass) in a groundmass (65%～68%) of clinopyroxene (0.03～0.05mm), plagioclase (0.04～0.05mm), minor magnetite (～0.05mm), and chlorite.

Fig.1　Location of the sampling transect undertaken during this study (a) and map showing the geology, the distribution of the mafic dykes, and sampling locations within the study area (b)

Fig.3　Zircon LA-ICP-MS U-Pb concordia diagrams for zircons separated from the mafic dykes within the study areaInset shows CL images of zircons analyzed during this study

3　Analytical procedures

3.1　LA-ICP-MS U-Pb dating

Zircons were separated from one sample (XTB01) using conventional heavy liquid and magnetic techniques at Langfang Regional Geological Survey, Hebei Province, China. The internal and external structures of zircons were observed using transmitted and reflected light and cathodoluminescence (CL) petrography at State Key Laboratory of Continental Dynamics, Northwest University. Zircon U-Pb dating was perfromed by LA-ICP-MS (Table 1; Fig.3) using an Agilent 7500a ICP-MS instrument, equipped with a 193nm excimer laser at State Key Laboratory of Geological Processes and Mineral Resources, China University of Geoscience, Wuhan, China. A 24(m laser spot diameter was used during analysis, a #91500 zircon standard was used for calibration, and a NIST 610 standard was used for optimization. Grain mount surfaces were washed in dilute HNO3and pure alcohol prior to analysis to remove any potential lead contamination. The analytical methodology followed Yuanetal. (2004) and Liuetal. (2010), and common Pb was corrected following Andersen (2002). The resulting data were processed using the GLITTER and ISOPLOT programs (Ludwig, 2003; Table 1; Fig.3), and uncertainties on individual LA-ICP-MS analyses are quoted at the 95% (1σ) confidence level.

3.2　Whole-rock geochemistry

The whole-rock and Sr-Nd isotope geochemistry of 16 mafic dyke samples was determined. Prior to analysis, samples were trimmed to remove altered surfaces, cleaned with de-ionized water, and crushed and powdered in an agate mill. Major element concentrations were determined using a PANalytical Axios-advance X-ray fluorescence spectrometer (XRF) at State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, China. Major element concentrations were determined on fused glass discs and these analyses have an analytical precision of <5%, as determined using the GSR-1 and GSR-3 Chinese National standards (Table 2). Losses on ignition values (LOI) were determined on 1g of powder that was heated to 1100℃ for 1 hour. Trace element concentrations were determined by ICP-optical emission spectrometry (OES) and ICP-MS at National Research Center of Geo-analysis, Chinese Academy of Geological

Table 1Zircon LA-ICP-MS U-Pb isotopic data for the mafic dykes within the NCC.

XTB01IsotopicratiosAge(Ma)SpotTh(×10-6)U(×10-6)Pb(×10-6)ThU207Pb206Pb1σ207Pb235U1σ206Pb238U1σ207Pb206Pb1σ207Pb235U1σ206Pb238U1σ1.126231819.20.820.05240.00200.33550.01290.04660.0005303682941029432.120225715.70.790.05170.00200.33380.01350.04670.0006273692921029443.163742630.11.500.05350.00170.34000.01110.04610.000535055297829034.115821713.20.730.05690.00270.36230.01660.04680.0006489773141229545.129530519.10.970.05040.00210.31720.01330.04620.0006211742801029146.152450333.21.040.05410.00170.34860.01090.04670.000637647304829447.133130719.81.080.05360.00200.34190.01320.04630.0005353662991029138.133336923.50.900.05340.00190.34850.01220.04720.000534758304929739.124425716.40.950.05610.00230.36030.01420.04720.00064586531211297410.121128917.60.730.05560.00210.35810.01360.04660.00054356631110294311.117528516.80.610.04930.00200.31330.01200.04650.0005162702779293312.127226517.11.020.05340.00210.34310.01310.04680.00063466430010295413.129134321.20.850.048330.00160.310810.01060.046550.0005115602758293314.156957136.41.000.052960.00150.33950.00930.046340.0004327442977292315.11081539.230.710.052560.00280.331760.01700.046620.000731089291132944

Table 2Major element concentrations (wt%) for the mafic dykes from Shanxi Province, northern NCC, China

SampleSiO2TiO2Al2O3Fe2O3MnOMgOCaONa2OK2OP2O5LOITotalMg#XTB-151.352.2814.6612.580.165.238.133.751.120.320.61100.1948XTB-251.282.3114.5912.640.145.258.153.781.090.340.54100.1148XTB-351.322.2614.6712.560.145.238.123.631.040.310.4399.7146XTB-451.252.2914.5712.620.145.228.133.621.070.350.6799.9348XTB-551.162.2514.7512.650.135.318.253.731.080.330.56100.2147XTB-651.302.2115.0812.660.155.338.133.561.130.260.52100.3348XTB-751.312.2314.9712.650.135.358.093.581.080.240.53100.1648XTB-850.932.1914.9412.570.145.338.053.571.050.320.6999.7848XTB-951.242.3114.5812.610.165.238.143.641.060.360.6599.9848XTB-1050.782.1614.8712.580.145.338.023.551.030.310.7899.5547XTB-1151.312.2614.6512.530.145.218.113.571.140.310.5699.7948XTB-1251.232.3214.5812.550.155.198.123.581.060.350.6499.7748XTB-1351.332.2414.6312.510.135.188.083.581.120.290.5599.6448XTB-1451.252.3114.5612.530.145.168.133.541.030.330.6399.6148XTB-1551.232.3214.5312.480.135.148.123.521.050.350.6499.5148XTB-1650.882.1614.7112.420.155.187.933.521.010.250.3898.5944GSR-3(RV*)44.642.3713.8313.40.177.778.813.382.320.952.2499.88GSR-3(MV*)44.752.3614.1413.350.167.748.823.182.30.972.1299.89GSR-1(RV*)72.830.2913.42.140.060.421.553.135.010.090.799.62GSR-1(MV*)72.650.2913.522.180.060.461.563.155.030.110.6999.71

Note: LOI=loss on ignition; Mg#=100(Mg/(Mg+Fe) atomic ratio; RV=recommended values; MV=measured values

Sciences, Beijing, China, following Qietal. (2000). Triplicate analyses yielded a reproducibility of <5% for all elements, and analyses of OU-6 and GBPG-1 international standards were in agreement with recommended values(Table 3).

3.3　Sr-Nd isotopic analyses

Rb-Sr and Sm-Nd isotope analysis used sample powders spiked with mixed isotope tracers before dissolution in Teflon capsules with HF+HNO3acids, and separation using conventional cation-exchange techniques. Isotopic measurements were undertaken using a Finnigan Triton Ti thermal ionization mass spectrometer at State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, China. Procedural blanks were <200pg for Sm and Nd, and <500pg for Rb and Sr. Mass fractionation corrections for Sr and Nd isotopic ratios used86Sr/88Sr and146Nd/144Nd values of 0.1194 and 0.7219, respectively, and analysis of the NBS987 and La Jolla standards yielded values of87Sr/86Sr=0.710246±16 (2σ) and143Nd/144Nd=0.511863±8 (2σ), respectively; the results of these analyses are given in Table 4.

Table 3Trace element compositions (×10-6) of the mafic dyke from Shanxi Province, northern NCC

SampleXTB-1XTB-2XTB-3XTB-4XTB-5XTB-6XTB-7XTB-8XTB-9XTB-10XTB-11V232236241246226233242235228243239Cr113116121125107116124119128126119Ni118121129131116121131118127133132Rb23.622.724.324.522.322.923.922.622.523.522.9Sr465458471475455469473453453475456Y26.425.627.427.625.326.327.625.325.427.325.4Zr197187209211178198206185175208189Nb29.428.531.531.627.629.631.628.627.431.328.3Ba298287306309276302308289278305283La23.923.524.324.223.224.124.223.223.124.123.2Ce47.346.948.548.646.446.748.645.446.248.247.3Pr5.855.825.935.965.755.845.975.855.735.945.85Nd25.625.226.326.224.325.326.525.324.225.225.3Sm6.166.136.276.255.966.156.286.165.956.136.16Eu2.172.182.262.192.142.192.252.192.162.182.19Gd5.535.485.635.715.515.555.615.465.535.565.51Tb0.980.960.970.950.930.960.950.960.940.970.95Dy5.145.165.235.194.985.175.215.174.975.485.12Ho1.131.151.251.191.091.161.221.161.061.191.13Er2.422.372.532.522.332.362.512.222.322.532.43Tm0.350.330.340.330.310.320.350.310.320.350.32Yb1.831.861.971.881.781.851.981.871.761.971.85Lu0.260.260.250.260.250.250.330.250.260.260.25Hf4.684.634.764.814.614.644.654.644.634.744.65Ta1.651.621.721.751.631.651.661.651.651.651.65Pb3.593.483.653.723.513.493.583.433.343.623.46Th2.622.652.752.762.562.642.732.642.552.742.63U0.750.760.830.860.750.760.760.760.760.750.75(La/Yb)N9.49.18.89.29.39.38.88.99.48.89δEu1.111.131.141.11.121.121.141.131.131.121.13SampleXTB-12XTB-13XTB-14XTB-15XTB-16OU-6(RV*)OU-6(MV*)GBPG-1(RV*)GBPG-1(MV*)V23522724625325612913196.5103Cr11410512913213570.873.5181187Ni11711813514113839.842.559.660.6Rb22.622.423.624.324.212012256.261.4Sr466457472483479131136364377Y26.125.127.528.228.527.426.21817.2Zr194175212223218174183232224Nb29.826.931.232.332.614.815.39.938.74Ba305278303312314477486908921La23.624.123.724.225.133335351Ce45.146.248.449.548.774.478103105Pr5.835.745.936.165.967.88.111.511.6Nd25.224.225.526.325.72930.643.342.4Sm6.125.936.126.286.175.925.996.796.63Eu2.182.132.152.252.231.361.351.791.69Gd5.585.545.615.725.665.275.54.744.47Tb0.960.940.980.960.950.850.830.60.59Dy5.154.955.525.615.564.995.063.263.17Ho1.131.061.221.331.321.011.020.690.66Er2.312.322.522.642.582.983.072.012.02Tm0.330.280.340.350.340.440.450.30.29Yb1.761.751.952.122.063.033.092.032.03Lu0.240.250.260.260.240.450.470.310.31Hf4.634.624.754.874.754.74.866.075.93Ta1.641.631.721.841.821.061.020.40.46Pb3.453.483.583.633.6228.232.714.114.5Th2.622.542.732.762.7511.513.911.211.4U0.730.760.730.780.761.962.190.910.99(La/Yb)N9.69.98.78.28.7δEu1.121.121.11.131.13

Note: values for GBPG-1 are from Thompsonetal. (2000), and values for OU-6 are from Potts and Kane (2005)

Fig.4　Classification of the mafic dykes within the NCC(a) TAS (all major element concentrations were recalculated to 100% anhydrous compositions; Middlemost, 1994; Le Maitre, 2002); (b) K2O vs. Na2O diagrams

4　Results

4.1　Zircon U-Pb dating

Euhedral zircons separated from sample XTB01 are clean and prismatic, and contain magmatic oscillatory zoning (Fig.3). A total of 15 zircons from this sample yielded a weighted mean206Pb/238U age of 293.4±1.7Ma (2σ; 95% confidence interval; Table 1; Fig.3). This age is the best estimate of the crystallization ages of mafic dykes in the Xinfangzi and Shangxigou areas; no inherited zircon cores were identified during this study.

4.2　Whole-rock geochemistry

The whole-rock geochemical compositions of mafic dykes analyzed during this study area are given in Table 2 and Table 3.

The mafic dykes have a narrow range of chemical compositions (SiO2=50.78%～51.35%, TiO2=2.16%～2.32%, Al2O3=14.53%～15.08%, Fe2O3=12.42%～12.66%, MnO=0.13%～0.16%, MgO=5.14%～5.35%, CaO=7.93%～8.25%, Na2O=3.52%～3.78%, K2O=1.01%～1.14%, and P2O5=0.24%～0.36%). All of these mafic dykes plot along the alkaline-sub-alkaline boundary in a total alkali-silica (TAS) diagram (Fig.4a); the dykes also plot within the calc-alkaline field in a Na2O vs. K2O diagram (oxide %; Fig.4b). The major element concentrations of the mafic dykes analyzed during this study do not correlate well with MgO concentrations(Fig.5). The mafic dykes are also characterized

Fig.5　Variations in major element concentrations vs. MgO (%) for the mafic dykes within the study area, Shanxi Province, northern NCC, China

by LREE enrichments and HREE depletions, with a wide range in (La/Yb)N(8.2～9.9) andδEu (1.10～1.13) values (Table 3; Fig.6a). Dykes within the study area are LILE (i.e., Ba, K and Sr) and Nb, Ta, and Zr enriched, and Th, Pb, Nd, P, and Ti depleted in primitive mantle-normalized trace element diagrams (Fig.6b).

4.3　Sr-Nd isotopes

The Sr-Nd isotopic compositions of eight representative mafic dykes were analyzed (Table 4), yielding uniform (87Sr/86Sr)ivalues (0.70422～0.70423) andεNd(t) values (5.8～6.1), suggesting that they formed from magmas derived from a depleted mantle source (Fig.7).

5　Genesis of the mafic dyke magmas

5.1　Mantle source

Fig.6　Chondrite-normalized rare earth element diagram (a) and primitive mantle-normalized incompatible element distribution diagram (b) for the mafic dykes analyzed during this study (normalization values after Sun and McDonough, 1989)

Fig.7　Initial 87Sr/86Sr vs. εNd(t) diagram for the mafic dykes from Shanxi Province, northern NCC, ChinaThe dykes plot within the depleted mantle source field

Mafic dykes in the study area contain low SiO2concentrations (50.78%～51.35%) (Table 2), suggesting derivation from an ultramafic (i.e., mantle) source, and not from melting of crustal material. This hypothesis is supported by the relatively high concentrations of MgO (5.14%～5.35%), Ni (117×10-6～141×10-6), and Cr (105×10-6～135×10-6), and the elevated Mg#values (44～48) of the mafic dykes. Crustal rocks can be excluded as a potential source of the magmas that formed these dykes, as partial melting of any crustal rocks (e.g., Hirajimaetal., 1990; Zhangetal., 1995a; Katoetal., 1997) or lower crustal intermediate granulites within the deep crust (Gaoetal., 1998a, b) would produce high-Si, low-Mg melts (i.e., of granitoid composition). In addition, the mafic dykes have low initial87Sr/86Sr ratios (0.70422～0.70423) and uniformly positive uniformεNd(t) values (5.8～6.1; Table 4), consistent with derivation from a depleted lithospheric mantle source or from the asthenospheric mantle. It is generally accepted that the lithospheric mantle has enriched initial87Sr/86Sr ratios, and generally has lowεNd(t) values (Zhangetal., 2005), whereas asthenospheric mantle magma is likely to be isotopically depleted, with low (87Sr/86Sr)iand highεNd(t) values (Saundersetal., 1992). These data suggest that the magmas that formed the mafic dykes of the NCC studied here were sourced from the asthenospheric mantle.

5.2　Crustal contamination

Crustal contamination can cause significant enrichment in the Sr-Nd isotopic composition of basaltic rocks. The mafic dykes analyzed during this study have depleted Sr isotopic compositions (0.70422～0.70423) and positiveεNd(t) values (5.8～6.1), suggesting that the magmas that formed these dykes assimilated little or no crustal material prior to emplacement. Furthermore, crustal assimilation would cause significant variation in the Sr-Nd isotope composition of a magma, and would also result in a positive correlation between MgO andεNd(t) values (5.8～6.1), and a negative correlation between MgO and (87Sr/86Sr)iratios (0.70422～0.70423), yet these features are not observed in the dolerite samples analyzed here (figure not shown).

Finally, the lack of inherited zircons in these dykes indicates that the magmas that formed these dykes underwent negligible crustal contamination. In summary, the geochemical and isotopic compositions of the dolerites analyzed during this study support their formation from magmas derived from a depleted asthenospheric mantle source that underwent little to no crustal contamination.

5.3　Fractional crystallization

Mafic dykes within the Xituanbao area have high Mg#values (44～48; Table 2), inconsistent with formation from magmas that underwent significant crystal fractionation. This lack of fractionation is further supported by the lack of correlation between MgO and other major elements (SiO2, TiO2, Fe2O3, Na2O+K2O, MnO, and P2O5(Fig.5). Nevertheless, it is generally thought that mafic magmas undergo fractionation of olivine, pyroxene, and Ti-bearing phases (rutile, ilmenite, titanite, etc.; Liuetal., 2005, 2006, 2008a, b, 2009, 2012b, 2013), as illustrated by the fact that the mafic dykes analyzed during this study plot along a visible fractionation trend on a La vs. La/Sm diagram (Fig.8). This is further supported by the low MgO (Mg#) and Ni contents (Table 2 and Table 3), as well as the Ti depletion (Fig.6b). However, the magmas that formed these dykes underwent some separation of plagioclase, and the presence of small number of feldspar cumulates, as evidenced by the presence of weak positive Eu anomalies in chondrite-normalized REE patterns (Fig.6a).

Fig.8　La vs. La/Sm diagram for the mafic dykes analyzed during this study

5.4　Genetic model and NCC destruction

Mafic dykes in China are thought to have formed from magmas derived from partial melting of either the lithospheric or asthenospheric mantle (Liuetal., 2005, 2006, 2008a, b, 2009, 2012b, 2013). The data presented here suggest that the magmas that formed the mafic dykes within the study area were derived from partial melting of a depleted region of the asthenospheric mantle. In addition, the fact that the mafic dykes are LREE-enriched and HREE-depleted suggests that these magmas were generated during partial melting of a region of the mantle that contained residual garnet.

However, a dynamic model is required to help further decipher the origin of these rocks; most importantly, we need to determine whether subduction of either the ancient Pacific Plate or the Yangtze lithosphere contributed in any way to the formation of these dykes, especially as these dykes provide key constraints on the petrogenesis of magmatism within both the NCC and eastern China. The timing and direction of collisional tectonics within the NCC (Engebretsonetal., 1985; Xuetal., 1993; Zhangetal., 1995b; Xu and Chen, 1997; Meng and Zhang, 1999; Huetal., 2004; Liuetal., 2005; Zhangetal., 2005) means that we can exclude the possibility of any contributions from these two plates.

The tectonic evolution of the northern NCC, including the location and timing of collision between the northern NCC and the Siberian Block, is a controversial and important issue (Tang, 1990; Shao, 1991; Hongetal., 1995; Zhangetal., 2007; Zhangetal., 2008; Luoetal., 2009). However, it is generally agreed that collision took place before the Early Permian (i.e., Silurian or Devonian; Zhangetal., 2008). The study area underwent relaxation and extension after this collision, resulting in crustal thinning and decompression partial melting of the asthenospheric mantle, processes that ultimately resulted in the emplacement of mafic dykes within the study area. Nevertheless, the two plates were separated from each other; they could not collide in Carboniferous evidenced by plate reconstruction. As such, an alternative model that accounts for the formation of these mafic dykes is needed, and is presented below.

Prior to carboniferous, the subduction of Paleo-Asian Ocean and the collision of Mongolia China Block occurred (Shao, 1991; Chenetal., 2000, 2001; Yanetal., 2000). Consequently, NCC lithosphere extension appeared. We therefore propose the following genetic model to account for the presence of mafic dykes within the northern NCC: (a) prior to subduction and collision, the NCC, Paleo-Asian Ocean and Mongolia China plates were three independent blocks; (b) subduction or collision between these blocks occurred before the Carboniferous, resulting in many slab windows and slab breakoff; and (c) lithosphere extension and some tectonic weak zone (e.g., slab window and breakoff) occurred. In this case, the extension led to partial melting of asthenospheric mantles beneath the NCC. These partial melts were the parental mafic magmas of the mafic dykes within the study area. These magmas underwent fractionation, but no crustal contamination, during ascent and emplacement of the mafic dykes within the study area.

For NCC destruction, the carbonatites were derived from partial melting of asthenospheric mantle based on the above interpretation and discussion, implying the NCC destruction might occur in Carboniferous, which is important for the evolution of the NCC.

6　Conclusions

The geochronological, geochemical, and Sr-Nd isotopic data presented here have allowed the following conclusions to be drawn:

(1) Zircon LA-ICP-MS U-Pb dating of the mafic dykes in Shanxi Province, China, indicates a Late Carboniferous (293.4±1.7Ma) age of crystallization.

(2) These mafic dykes were derived from partial melting of a depleted asthenospheric mantle source, and the parental magmas of these dykes underwent fractionation of olivine, pyroxene, and Ti-bearing phases (rutile, ilmenite, and titanite) during ascent and emplacement. Emplacement of the dykes was associated with negligible crustal contamination.

(3) The generation and emplacement of the mafic magmas in Shanxi province, the northern NCC can be attributed to post-subduction and collision (e.g., Paleo-Asian Ocean, Mongolia China Block) lithosphere extension.

AcknowledgementsThe authors thank Lian Zhou, Yongsheng Liu and Zhaochu Hu for assistance during zircon U-Pb dating, Sr-Nd isotope, and Hf isotopic analyses.

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