English (pdf)
Article in xml format
Article references
Automatic translation
Send this article by e-mail
Cited by SciELO 
Cited by Google 
Similars in
SciELO 
Similars in Google 
Permalink
Introduction
As one of the oldest cratons in the world, the formation of the North China Craton (NCC) can traced back to approximately 1.8 Ga (Zhao et al., 2001; Wilde et al., 2002). The interior of NCC is generally believed to have been stable from the Mesoproterozoic to Triassic (Robinson et al., 1999; Wilde et al., 2002; Kusky et al., 2007). However, the southward subduction of the Paleo-Asia Ocean (PAO) caused dramatic tectono-magmatic activities during the late Paleozoic in the northern NCC (Davis et al., 2001; Xiao et al., 2003; Zhang et al., 2007a; Ying et al., 2011; Menzies et al., 1993; Wilde et al., 2003; Yang et al., 2008; Wu et al., 2008). Moreover, large numbers of the late Paleozoic-early Mesozoic plutonic rocks are distributed along the northern NCC (Pan et al., 1996; Zhang et al., 2007b).
Similar to the circum-Pacific orogenic belt, the northern NCC margin comprises an island arc, a ridge, an oceanic island, a seamount, an oceanic basin, an accretionary wedge, and a micro plate, forming a complex tectonic framework (Windley et al., 2007). In addition, there still is a great controversy on the PAO closure time in terms of the spatio-temporal distribution of the ophiolites, multi-stage tectonic deformations, and magmatic activities in the northern NCC margin (Zhao et al., 2011). According to the prevailing hypothesis, the PAO closed in the Ordovician-Silurian (Han et al., 1997; Kheraskova et al., 2003), the Devonian-Late Carboniferous (Shi et al., 2013; Zhao et al., 2013), or the Permian-Late Triassic (Jian et al., 2010; Li et al., 2014a; Li et al., 2014b).
In this research, we study the Wuhaolai mafic complex that is located in Xinhure, Inner Mongolia. The Wuhaolai mafic complex is located in the middle segment of the northern NCC, south of the Central Asia Orogenic Belt (CAOB; Fig. 1a). The complex is part of a mafic-ultramafic belt, which is distributed among the Wulatezhongqi, Damaoqi, and Bayan Obo areas (Fig. 1b) (Cao et al., 2002; Zhao, 2008). Studies on this mafic belt indicated that the Huheengeer pluton (242-287 Ma) was formed in an active continental margin (Zhao et al., 2008). Nevertheless, the Beiqigetao pluton (269 Ma ± 8 Ma) and the Tugurige pluton (273.5 Ma ± 1.3 Ma) formed in a post-collisional extension setting (Zhao et al., 2011; Wang et al., 2016). However, previous research mainly focused on the petrology and whole-rock geochemistry, but the mineralogical analyses were not enough documented. In this research, we focused on the characteristics of the rock-forming minerals (e.g., clinopyroxene and hornblende) using electron microprobe analytical data to investigate the evolution of the parental magma and discuss the tectonic setting in the study area.
Figure 1 (a) Geotectonic context of the studied area (after Jahn, 2004); (b) A simplified geological map of central Inner Mongolia (after Cao et al., 2002).
Geological background and petrography
The Wuhaolai mafic complex is located near the Wulatezhongqi area, Inner Mongolia, China. The Solonker ophiolite belt, which is considered as a suture zone between the NCC and the Siberian Craton (SC), is situated in the northern part of this complex (Fig. 1b) (Wu et al., 1998; Xiao et al. 2003; Li et al., 2006; Windley et al. 2007). The late Archean crystalline basement of this area is represented by the East Wufuzi formation, which comprises actinolite-biotite schist, biotite-plagioclase gneiss, and biotite-monzonitic gneiss (Fig. 2a). In addition, the Bayan Obo Group, which comprises conglomeratic sandstone, sandy slate, slate, and sandy limestone, lies uncomfortably over the crystalline basement (Fig. 2b).
The Wuhaolai mafic complex is a N-W striking stock, comprising from the center to the rim pyroxenite, gabbro and diorite units respectively (Fig. 2b). In this research, two pyroxenite samples, two gabbro samples and one diorite sample were selected to study the petrography section. Pyroxenite displays medium-coarse grain texture and massive structure with a black-green color (Figs. 3a and 3b). Furthermore, pyroxenite mainly contains subhedral, zonal clinopyroxene (80 vol.%; 0.2-1 mm in length), subhedral hornblende (10 vol.%,; 0.5-1 mm in length), subhedral plagioclase (5 vol.% ; ~0.5 mm in length) and rare magnetite (5 vol.%). Gabbro (Fig. 3c and 3d) was grayish white, with a coarse medium-grained texture and massive structure. It contains 40 vol.% of anhedral-granular clinopyroxene, 40 vol.% of coarse -grained subhedral hornblende, and 20 vol.% of subhedral, zonal plagioclase with characteristic polysynthetic twins. Diorite (Figs. 3e and 3f) displays a coarse medium-grained texture and massive structure. It mainly comprises zonal, coarse medium-grained subhedral plagioclase (45 vol.%) and coarse medium-grained subhedral hornblende (30 vol.%), fine grained quarte (15%) and biotite (10 vol.%). From Figs. 3b and 3d, the subhedral hornblende cuts across the clinopyroxene, indicating that clinopyroxene crystallized earlier than hornblende.
Figure 2 (a) Geological map of the Xinhure area (after Hao et al., 2016); (b) Geological map of the Wuhaolai mafic complex (after IMBGMR, 1991).
Figure 3 Selected outcrop photographs and microphotographs of the representative rocks from the Wuhaolai complex. (a) field photo of pyroxenite from the Wuhaolao complex; (b) microphotograph of pyroxenite from the Wuhaolao complex (under the cross-polarized light); (c) field photo of gabbro from the Wuhaolao complex; (d) microphotograph of gabbro from the Wuhaolao complex (under the cross-polarized light); (e) lithofacies lines between gabbro and diorite of the Wuhaolao complex; (f) microphotograph of diorite from the Wuhaolao complex (under the cross-polarized light) Pl: plagioclase, Cpx: clinopyroxene, Hb: hornblende (Whitney and Evans (2010).
Analytical methods and results
The microanalysis of clinopyroxenes and hornblendes crystals in the studied samples of pyroxenite, gabbro and diorite from the Wuhaolai mafic complex was conducted at the Laboratory of Mineralization and Dynamics, Chang'an University. A JEOL JXA-8200 electron microscope with ZAF matrix correction. The analysis was used with an accelerating potential of 15 kV, a sample current of 1 χ 10-8 A, and a spot diameter of 0.5 μηι, and counting time of 10 s. The standards of Na, Si, Fe, K, Al, Mn, Ca, Mg, and Ti were natural minerals, and the standard for Cr was synthetic oxides. GeoKit software (Lu, 2004) was used to calculate the geochemical parameters of clinopyroxenes and hornblende. The results are shown in Tables 1 and 2.
Based on the pyroxene classification standards (Morimoto et al., 1988), the clinopyroxene grains from the Wuhaolai mafic complex belong to the Ca-Mg-Fe species. In the Wo-En-Fs diagram (Fig.4a), clinopyroxene grains from pyroxenite are plotted into the diopside compositional field (Wo = 48.2449.52, En = 39.06-39.93, Fs = 10.17-11.29), which shows a small variation in the composition, with 51.79 wt.%-52.82 wt.% SiO2, 13.91 wt.%-14.43 wt.% MgO, 24.01 wt.%-24.69 wt.% CaO, 5.52 wt.%-6.48 wt.% FeO, 0.001 wt.%-0.132 wt.% TiO2, 0.42 wt.%-1.21 wt.% Al2O3, 0.23 wt.%-0.42 wt.% Na2O, 0.01 wt.%-0.086 wt.% Cr2O3, and Mg# (=100 Mg/(Mg + Fetotal)) in the range 79-82. Additionally, clinopyroxene grains from gabbro also plot into the diopside compositional field (Wo = 47.43-49.42, En = 38.72-40.1, Fs = 10.3411.21) with the geochemical characteristics of 51.22 wt.%-52.97 wt.% SiO2, 13.59 wt.%-14.49 wt.% MgO, 23.75 wt.%-24.32 wt.% CaO, 5.68 wt.%-6.31 wt.% FeO, 0.1 wt.%-0.2 wt.% TiO2, 0.88 wt.%-1.54 wt.%Al2O3, 0.32 wt.%-0.48 wt.% Na2O, 0.03 wt.%-0.11 wt.% Cr2O3, and Mg# in the range 79-81.
Figure 4 (a) Wo-En-Fs diagram of clinopyroxene in Wuhaolai mafic complex (after Morimoto et al., 1988); (b) Classification of hornblende in the Wuhaolai mafic complex (after Leake et al., 1997). Di: diopside, He: hedenbergite, Au: augite, Pi: pigeonite, En: enstatite/clinoenstatte, C1En: bronzite, C1Fs: ferrohypersthene, Fs: ferrosilite/clinoferrosilite.
The cations of hornblende from pyroxenite are characterized by CaB = 1.90-1.97 and (Na + K)A = 0.28-0.61, and the cations of hornblende from gabbroare characterized by CaB = 1.77-1.93 and (Na + K)A = 0.45-0.54. The hornblende grains from the complex belong to calcic hornblende. Based on the classification scheme of calcic hornblende (Leake et al., 1997), the hornblendes from pyroxenite and gabbro are edenite, which have relatively lower SiO2 (43.61 wt.%-48.97 wt.%), MgO (12.22 wt.%-15.13 wt.%), CaO (12.15 wt.%-12.55 wt.%), and Si/(Si + Ti + Al) in the range 0.28-0.68 as well as higher TiO2 (0.59 wt.%-1.67 wt.%), Al2O3 (6.24 wt.%-11.30 wt.%), and Na2O (0.81 wt.%-1.48 wt.%). The hornblendes from diorite are magnesio-hastingsites with relatively higher SiO2 (46.75 wt.0/0-48.3 wt.%), MgO (14.06 wt.%-14.93 wt.%), CaO (12.23 wt.%-12.86 wt.%), and Si/(Si + Ti + Al) in the range 0.72-0.75 as well as lower TiO2 (0.86 wt.%-1.03 wt.%), Al2O3 (8.21 wt.%-8.60 wt.%), and Na2O (1.01 wt.%-1.28 wt.%) (Fig. 4b).
Discussion
Mineral genesis and magmatic evolution
In the Ti-(Ca + Na) diagram (Fig. 5), the clinopyroxenes from the Wuhaolai mafic complex belong to the alkaline series. Compared with the characteristics of arc basalt (ArcB) and ocean island basalt (OIB) (Condie K.C., 1982), the clinopyroxenes from the Wuhaolai mafic complex have lower Mg#, CaO, SiO2, and Cr2O3 contents and higher Al2O3 and Na2O contents. Additionally, the Mg# of clinopyroxene has a negative correlation with FeO and Al2O3 and a positive correlation with Na2O (Fig. 6a-d). Furthermore, all analyzed clinopyroxene grains plot into or near the continental alkaline basalt (CAB) area (Figs. 6e-h), indicating that the parental magma of this mafic complex has chemical characteristics consistent with the intra-plate alkaline series. The geochemical profile of the clinopyroxene grains (Figs. 7a and 7b) reflects a normal zonal feature with a higher Mg# and lower Al2O3, Na2O, and TiO2 contents in the core than at the rims (Zhao et al., 2011). It is interpreted that clinopyroxenes in pyroxenite and gabbro are crystallized from the same magma (Yu et al., 2011; Zhao et al., 2011). With the crystallization of clinopyroxene, Mg first entered into the crystalline phase, followed by Ca and Al, and finally was Na and K (Bowen, 1928). Thus, the crystallization of clinopyroxene may led to the magma rich in the Si, Na and K and depleted in the Mg, Ca and Al. Thus, the decreasing MgO, FeO and Al2O3 contents of clinopyroxenes from pyroxenite to gabbro were conforming to the magma evolution process. However, compared with the normal crystallization evolution of magma, the CaO content of gabbro was higher than the pyroxenite. Since crustal contamination depends on the compositions of the surrounding rocks (Su et al., 2009; Liu et al., 2012), the crystalline basement in the study area, which comprises biotite schist, limestone, and other rocks, contains a sufficient amount of Ca. Previous research shows that the parental magma of ankaramite in South Margin of the Altay Mountains has not suffered the crustal contamination (Su et al., 2008). The clinopuroxene CaO content of the ankaramite was 18.5%-22.07%, which was obviously lower than the Wuhaolao complex. Thus, the parental magma of the Wuhaolai mafic complex may have suffered the crustal contamination during its evolution. Previous research has shown that the equilibrium state between clinopyroxene and its host rocks can be reflected by the Fe-Mg exchange partition coefficient Kd = (Fe/Mg)Cpx /(Fe/Mg)liquid (Putirka, 1999; Toplis et al, 1995). In this research, the Kdcpx values, which were calculated based on the results of clinopyroxene and whole-rock geochemical (Liu et al., 2017), were between 0.23 and 0.27. Thus, based on Kdcpx and the Mg# of the Wuhaolai clinopyroxenes (77.780.6), the Mg# of the equilibrium melt was found to be in the range 43-53 (Fig.8). This relatively low Mg# number indicated that the parental magma may derive from the lithospheric mantle and could have experienced crystal fractionation.
Figure 6 Oxide compound correlation diagrams of clinopyroxene (after Kim et al., 2016), arc basalts (after Debari et al., 1987; Bryant et al., 2007), oceanic island basalts (after Bohrson et al., 1988; Chen et al., 1992; Fodor et al., 1997), and continental alkaline basalts (after Arai et al., 2001; Ho et al., 2000; Kovács et al., 2004).
Figure 7 Backscattered electronic images of clinopyroxene crystals from representative samples of the Wuhaolai mafic complex and their compositional profiles. (a) Pyroxenite sample; (b) Gabbro sample.
In the Al2O3-TiO2 diagram (Fig. 9a), almost all the hornblende grains from the Wuhaolai complex are consistent with the composition of hornblende from mantle source (Jiang et al., 1984). In addition, the Al/Si values of hornblende from pyroxenite, gabbro and diorite are 0.15-0.30, 0.20-0.21, 0.23-0.28, respectively, and their Mg/(Fe3+ + Fe2+ + AlVI) values are 1.28-1.96, 1.68-1.92, and 0.75-0.92, respectively, indicating that hornblende compositions are consistent with those from the intermediate-basic magmatic origin (Fig. 9b). All the above features support that hornblende from the Wuhaolai complex is of magmatic origin and not a product of hydrothermal alteration. Additionally, in the AF-A, AIV-total Al, and AF-AF + Fe3+ + 2Ti + A sites and in the AlIV-Ti diagrams (Fig. 10), the hornblende compositions in the different rock types exhibit a high linear correlation. This suggests that the electric charge imbalance, which is caused by the four-coordinated AP-substitution of Si in the tetrahedral position, may be compensated by the isomorphism replacement of AlIV, Fe3+,and Ti in the octahedral position (Niu et al., 2005). Hornblende in peridotite xenoliths can be divided into two types: S-hl and I-hl. S-hl, which originates from the mantle wedge above the subduction zone, is mainly affected by fluids or melts derived from the subducted slab, whereas I-hl, which is formed in the intra-plate environment, is mainly influenced by the metasomatism of asthenospheric materials (I-Amph) (Coltorti et al., 2007). In the SiO2 vs. Na2O and SiO2 vs. TiO2 diagrams (Fig.11), the analyzed hornblende grains plot into the S-Amph area (Fig. 11), indicating that the magma source may have been modified by the fluids or melts derived from the subducted slab (Xu et al., 2009).
Figure 9 (a) Al2O3-TiO2 diagram for hornblende compositions (after Jiang et al., 1984); (b) Mg/(Fe3+ + Fe2++AlVI)-Al/Si diagram of hornblende (after Xue et al., 1986). C: crustal source, M: mantle source.
Estimation of mineral crystallization temperature and pressure
Although many clinopyroxene geothermometers have been proposed (Davis et al., 1966; Nimis 1995; Nimis et al., 1999; Putirka et al., 2003), herein, we used the clinopyroxene-liquid geothermometer (Putirka et al. 2003) to estimate the crystallization conditions (temperature and pressure) of clinopyroxene in the Wuhaolai complex. The principle of this method is that the jadeite (Jd; NaAlSi2O6) content in clinopyroxene is sensitive to the pressure and temperature in the pyroxene-melt exchange reaction (Putirka et al., 2003). Before calculating the crystallization temperature and the pressure, we should verify whether clinopyroxene and its host rocks have reached an equilibrium state (chen et al., 2015). Experimental results indicated that when clinopyroxene and its host rocks reached the equilibrium state, the Kdcpx values were between 0.2 and 0.4 (Irving et al., 1984; Liotard et al., 1988; Kinzler et al., 1997; Hunter et al., 1997). In this research, the Kd values are in the range 0.23-0.27, indicating that clinopyroxene and the melt have reached the equilibrium state. This is also supported by the normal zonal texture of clinopyroxene. Thus, based on the study conducted by Putirka et al. (2003), the crystallization temperature of clinopyroxene in pyroxenite ranged from 1426°C to1434°C and the crystallization pressure ranged from 1.1 to 1.7 GPa. The crystallization temperature of clinopyroxene in gabbro was 1382°C-1389°C, whereas the crystallization pressure was in the range 0.5-1.3 GPa. Therefore, the crystallization temperature and pressure for pyroxenite were slightly higher than those for gabbro, suggesting that the formation depth of gabbro was slightly shallower than the pyroxenite.
Tectonic significance
Due to the subduction of the PAO, various types of late Paleozoic plutons were widely distributed in the northern margin of the NCC. However, Wulan granodiorites (323.4 ± 3.4 Ma) (Zhang et al., 2011), Wengeng gabbro (280 Ma) (Zhao et al., 2008), Shihahe biotite monzonitic granite (275.0 ± 0.7 Ma) (Hao et al., 2016), and Ondor Sum-Tieshagai intermediate-felsic intrusions (271 ± 2.4 Ma) (Wang et al., 2013) were formed under an active continental margin tectonic setting. The Kebu pluton (291 ± 4 Ma) (Luo et al., 2007), the Beiqigetao mafic pluton (269 ± 8 Ma) (Zhao et al., 2011), and the Tugurige hornblendite (273.5 ± 1.3 Ma) (Wang et al., 2016) exhibited the characteristics of the post-collisional extension environment. Thus, the PAO finally closed at the early-middle Permian. This was further supported by the paleontological results of the Cathaysian flora of the early-middle Permian stratum in the southern margin of the Siberian plate (Zhou et al., 2010). Besides, in the TiO2-MnO-Na2O diagram of Nisbet et al. (1977) (Fig. 12), clinopyroxenes from pyroxenite and gabbro of the Wuhaolai mafic complex mainly belong to the within-plate alkali basalt (WPA) field. Therefore, combining the geochemical characteristics of the unpublished zircon U-Pb data (265 Ma), the Wuhaolai complex was formed in the within-plate tectonic setting in the middle Permian. Combining all these results with the chemical characteristics of the studied clinopyroxene and hornblende crystals in pyroxenite and gabbro samples from the Wuhaolai mafic complex, it can be drawn that the magma source of this complex was modified by fluids or melts derived from the subducted slab in the course of the PAO subduction under the northern NCC margin (Zhao et al., 2011; Tang et al., 2014., Wang et al., 2016). Therefore, it can be conjectured that the parental magma of the Wuhaolai complex may derived from partial melting of an enriched lithospheric mantle source after the PAO closure (Zhao et al., 2011, Hou et al., 2014, Zhang et al., 2011). The clinopyroxene and the hornblende in the studied rocks were produced along with the fractional crystallization of the parental magma of the Wuhaolai complex.
Figure 12 TiO2-MnO-Na2O diagram for clinopyroxene composition of pyroxenite and gabbro samples from the Wuhaolai mafic complex (after Nisbet et al., 1977). WPT: within-plate tholeiitic; WPA: within-plate alkali basalt; VAB: volcanic arc basalt; OFB: ocean floor basalt.
Conclusions
The conclusions of our study are summarized as follows:
The parental magma of the Wuhaolai mafic complex belongs to the intra-plate alkaline series. The feature of the normal chemical zoning and the other chemical characteristics documented for clinopyroxene in the studied samples of pyroxenite and gabbro from the Wuhaolai mafic complex are attributed to the crystallization process of the same parental magma. Additionally, the chemical characteristics of hornblende (S-hl) in the studied samples suggest that the magma source may have been modified by the fluid or melt derived from the subducted slab.
The Fe-Mg exchange partition coefficient Kdcpx calculated for the Wuhaolai mafic complex range from 0.23 to 0.27. Furthermore, the crystallization temperature and pressure calculated for the gabbro unit are lower than those of pyroxenite, suggesting that the formation depth of gabbro was slightly shallower than that of pyroxenite.
Based on the characteristics of the rock-forming minerals, the Wuhaolai mafic complex is interpreted as formed in a within-plate tectonic setting, where clinopyroxene with normal zoning feature and hornblende were crystallized resembling some chemical imprints related with subduction fluids.
