2. Guizhou Institute of Geological Survey, Guiyang 550081, China;
3. Center for Global Tectonics, China University of Geosciences, Wuhan 430074, China
The formation of the Late-Permian Emeishan large igneous province (ELIP) in Southwest China has been generally linked to mantle-plume mechanism (Ali et al., 2005; Xiao et al., 2004; Xu et al., 2004, 2001; Chung et al., 1998; Chung and Jahn, 1995). The ELIP is dominantly composed of different types of volcanic rocks with continental flood basalt affinity (Xu et al., 2001), picritic basalts (Zi et al., 2008; Zhang et al., 2006) and trachyte and rhyolite (Xu et al., 2001), together with minor intrusive rocks of ultramafics, gabbros, diabases and granitoids (Shellnutt, 2014 and references therein). The continental flood basalts were divided into high-Ti (HT) and low-Ti (LT) groups based on Y/Ti values; the high-Ti basalts dominantly occur in the inner zone, and the low-Ti rocks are exposed in both the middle and outer zones (Xu et al., 2001). The mechanisms of origin and tectonic implications of these rocks suites have been evaluated in several studies, and remain debated (Li et al., 2017; Deng et al., 2016; Zhao et al., 2016; Usuki et al., 2015; Shellnutt, 2014; Lai et al., 2012; Zhong et al., 2011, 2009, 2007; Hanski et al., 2010; He et al., 2010; Xu et al., 2010, 2008, 2007, 2004, 2001; Jian et al., 2009; Song et al., 2008, 2005; Zhou et al., 2008; Xiao et al., 2004, 2003; Chung and Jahn, 1995). Among the various rock types of the ELIP, the mafic intrusions in the outer zone have received less attention, which hampers a better understanding of the tectonic evolution of the ELIP.
Texturally and compositionally zoned diabasic sills occur in the outer zone of the ELIP in the southern Guizhou Province (BGMRGZ, 1987). These were once considered as volcanic lavas in some previous studies (Zhang et al., 1999). The sills consist of gabbros and diabases in the inner zone, with local amygdaloidal diabasic rocks occurring sporadically in the upper margins of the chilled zone, representing intrusive-subvolcanic gabbro-diabase- amygdaloidal diabase sills. In this study, we report the geology, petrography, zircon U-Pb chronology and Hf isotopic and major- and trace-element geochemistry of rocks from one of these sills, with a view to understanding the magmatic process and source region, and providing further insights into understanding the petrogenesis of mafic intrusions in the outer zone as well as the tectonic evolution of the ELIP.1 GEOLOGY 1.1 Regional Geology
The ELIP is located in the western margin of the Yangtze Craton, Southwest China (Fig. 1a) (Ali et al., 2005; Xu et al., 2004, 2001; He et al., 2003; Chung and Jahn, 1995). It dominantly forms a massive sequence of volcanic rocks and minor intrusive rocks, covering an area of ~3.0×105 km2 (Shellnutt, 2014 and references therein). The ELIP is divided into three nearly concentric zones, i.e., the inner, middle and outer zones (Fig. 1a), outwards corresponding to progressively decreasing thickness of the crust (Zhou et al., 2005; Xu et al., 2004; He et al., 2003; Zhong et al., 2002). The inner zone developed layered mafic-ultramafic and minor alkaline granitoid plutons, predominantly low-TiO2 basalts and minor high-TiO2 basaltic and alkaline basaltic rocks, and abundant Ni-Cu-(PGE) and PGE deposits and giant orthomagmatic Fe-Ti-V oxide deposits (Zhou et al., 2005). The middle and outer zones developed extensive to sporadic volcanic rocks of dominantly high-TiO2 and minor low-TiO2 basalts, with minor economic to sub- economic Ni-Cu-(PGE) and PGE deposits (Song et al., 2003). SHRIMP zircon U-Pb dating of some of the mafic intrusions, dikes, and volcanic rocks suggested their formation at 257–263 Ma (Shellnutt and Jahn, 2011; Fan et al., 2008; Zhou et al., 2008, 2002; He et al., 2007; Zhong and Zhu, 2006). More precise zircon U-Pb geochronology yielded a narrow range of ages between 257 and 260 Ma for the mafic and silicic intrusive rocks (Shellnutt et al., 2012).
The Emeishan volcanic sequences unconformably overlie the Middle Permian Maokou Formation and are in turn covered by the Upper Permian strata in the east and west parts, and Upper Triassic or Jurassic sediments in the central part (e.g., He et al., 2003). The unconformity is restricted to the Upper Yangtze Craton and resulted from subaerial chemical and physical weathering and erosion, with relict gravels, basal conglomerates, alluvial fans and karst relief in the inner and middle zones, and by paleoweathering crust and paleosoil in the outer zone (He et al., 2003).1.2 Geology of the Diabasic Sill
The diabase sill in this study is located in the outer zone of the ELIP, in the Luodian County, south of the Guizhou Province (Fig. 1a). The region exposes the Devonian terrigenous clastic, Carboniferous neritic carbonate, Early- to Middle- Permian neritic carbonate Sidazhai Formation and early Late- Permian to Early-Triassic terrigenous clastic Linghao Formation (Fig. 1b). The Sidazhai Formation is 92–239 m thick. It consists of terrigenous clastic silt-bearing sandstones in the bottom, limestone in the middle and gravel limestones on the top, with hiatus of total or partial of gravel limestones elsewhere, suggesting basal scouring erosion locally in the region. In addition, the bottom part of the Linghao Formation shows 1–2 m thick purple claystone in the Luomu area, or 2–3 cm thick Fe- and Mn-rich layer in the Yangliwan area, consistent with the unconformity between the Maokou and Xuanwei formations regionally. Therefore, the Sidazhai and Linghao formations in the study area are considered to be comparable with the Maokou and Xuanwei formations in the regional Upper Yangtze Craton, respectively (e.g., Huang et al., 2018), suggesting an important uplift and erosion. However, in contrary to the regional ELIP, the strata of Upper Triassic, entire Jurassic and Lower Cretaceous are missing (e.g., Huang et al., 2018), indicating another important uplift event in the study area.
The diabase sill is 58–300 m thick and broadly occurs within the Sidazhai Formation, parallel or sub-parallel to the bedding (Figs. 1b, 1c). Field studies on several typical transects reveal that the sill is zoned, with inner zone composed of diabase-gabbro rocks and the chilled zone of very-fined diabasic rocks near the outermost margins, or amygdaloidal diabasic rocks locally within the chilled zone along the upper sill margin (Fig. 2). The diabases and gabbros display massive structure and coarse-grained texture (Figs. 3a, 3b); the chilled zone shows massive structure but displays micro-grained to aphanitic textures (Figs. 3c, 3d). The amygdaloidal diabasic rocks are usually meter-wide, with increasing content and size for the proto-vesicles upwards to the sill margin, and less and small original vesicles downwards in the inner zone (Fig. 3d). The sill was locally intruded by millimeter-, centimeter- to decimeter-wide irregular intermediate to acidic veins, plugs and blebs (Fig. 3d). Han et al. (2009) and Zhu et al. (2018) separately reported an LA-ICP-MS zircon U-Pb age of 255.0±0.62 and 261.2±2.6 Ma for the basic sill in the Luokun area (Fig. 1b), Huang et al. (2017) yielded an LA-ICP-MS zircon age of 255.2±3.1 Ma for one of the felsic veins inside the sill in outcrop LD03, cross section KPM07 in northwestern Luomu (Fig. 2a), suggesting formation of the sill in Later-Permian and the transition between the Middle and Late-Permian. However, recently Zhu et al. (2019) reported a younger age of ~160 Ma for the felsic vein in Luokun. Thus the timing of the felsic magmatism in Luodian, southern Guizhou Province becomes complex and needs further investigation. Intrusion of the diabasic sill led to contact metamorphism of the overlying carbonates of the Sidazhai Formation, producing meters-wide contact aureole of marbles that consist of diopside, wollastonite and tremolite diagnostic metamorphic minerals, and quartzites and felsic hornfels, followed by hydrothermal alteration of both the basic sill and some felsic veins, accompanying nephrite deposits locally inside the aureole (Huang et al., 2018, 2012).2 SAMPLING AND ANALYTICAL METHODS
The diabasic sill shows different degrees of alteration through hydrothermal activity and has undergone highly weathering due to the humid and warm climate. Samples were collected from sections KPM01 and KPM07 on outcrop LD03, sections KPM08 and KMP22 on outcrop LD01 along the road cuts or ground surfaces (e.g., Figs. 1c, 2a). The cross section along a mountain stream bed on outcrop LD08 (Figs. 2b, 3b) is parallel to the section KMP07 in distance of 50 m (Fig. 3a), which exposes relatively fresh samples, with or without low-degree hydrothermal alterations but are free of weathering. The zircon dating sample (sample LD01-7) was collected from the cross section KPM22 (see Fig. 1c).
Zircon U-Pb ages were analyzed using Agilent 7700e ICP-MS, coupled to COMPexPro 102 ArF Excimer laser (wavelength of 193 nm and maximum energy of 200 mJ) with a spot diameter of 32 μm and a laser pulse frequency of 6 Hz at the Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. The instrumental conditions and the analytical procedures were similar to those described in detail by Zong et al. (2017). An Excel-based software ICPMSDataCal was used to perform off-line selection and integration of background and analyzed signals, time-drift correction and quantitative calibration for trace element analysis and U-Pb dating (Liu et al., 2010, 2008). Concordia diagrams and weighted mean calculations were made using Isoplot 3.0 (Ludwig, 2003). The uncertainty for individual analyses is quoted at the 1σ level, and the errors on weighted mean ages are at the 95% confidence level.
Zircon Hf isotopes were analyzed using Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Germany) in combination with a Geolas 2005 excimer ArF laser ablation system with a beam size of 44 μm and a laser pulse frequency of 8 Hz at the Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. Detailed operating conditions for the laser ablation system and the MC-ICP-MS instrument and analytical method are the same as description by Hu et al. (2015). The half-life of Lu is following a recommended value of (1.865±0.015)×10-11 year-1 (Scherer et al., 2001). Present 176Lu/177Hf and 176Hf/177Hf of depleted mantle (DM) values, 0.038 4 and 0.283 25, respectively (Griffin et al., 2000), and intrusion age (260 Ma) of the sill (see the following results) were adopted for single-stage model age T1DM calculation.
Major element compositions electron probe microanalysis (EPMA) of clinopyroxene from the sill were carried out on polished carbon-coated thin-sections of representative samples, using a JXA-8230 electron microprobe with wavelength dispersive (WDS) technique at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing. PAP program was used for matrix correction. The natural and synthetic compounds were selected for standards.
Major elements of 12 samples including gabbros, diabases and amygdaloidal diabasic rocks were analyzed. The rock samples were sawn into slabs and the fresh portions were handpicked and then powdered using stainless steel mill. Whole- rock major and trace elements were determined using XRF, ICP-AES and ICP-MS at ALS Chemex (Guangzhou) Co. Ltd. The FeO content was analyzed using conventional wet chemical methods. The analytical procedures were given in the website http://www.alsglobal.net.cn/. Analytical uncertainties are 1 wt.%–2 wt.% for major elements, 5 wt.% for rare-earth elements, and 5 wt.%–10 wt.% for trace elements, respectively.3 PETROGRAPHY
Detailed studies of the outcrops, hand specimen and thin sections reveal that the sill is composed of amygdaloidal diabases, diabases and gabbros. The amygdaloidal diabasic rocks show amygdaloidal structures and vitreous or micro-porphyritic textures (Figs. 4a-4b). The rocks consist of microphenocrysts of 0.3–1.0 mm long plagioclase (3%–5%), with (1%–2%) or without augite, and groundmass of 75%–90% and amygdaloidal globules of 10%–20% which are mainly filled with chlorites and calcites. The groundmass shows tholeiitic texture and is composed of plagioclase microlites (40%–45%), glass (30%–35%) and Ti-Fe oxides (5%–10%); the glass has changed to chlorite, epidote/zoisite and actinolite or tremolite. Towards the inner zone, the groundmass is changed from tholeiitic texture to ophitic texture with euhedral plagioclase microphenocryst.
The diabases occur near the chilled zone. The rocks are composed of plagioclase (45%–50%) and clinopyroxene (35%– 40%), with minor alkaline feldspar (< 2%), Fe-Ti oxides (~5%) and apatite (~2%). Alteration minerals are mainly chlorite. The plagioclase is mostly fine- to medium-grained euhedral to subhedral laths that are randomly distributed, making up interlocking frameworks filled with anhedral or subhedral clinopyroxene crystal (Fig. 4c). These diabases show textural transition with the gabbros. However, the diabases immediately adjacent to the chilled zone show typical ophitic texture with coarse clinopyroxene crystals (2–3 mm) enclosing random-arranged high length to breadth ratio and euhedral plagioclase slabs (Fig. 4d). However, there coarse plagioclase (2–3 mm) occurs together with clinopyroxene (2–5 mm) in the same thin section. The gabbros occur in the central portion of the inner zone. They show nearly equant clinopyroxenes and plagioclases, with grain sizes in range of 4–6 mm, coarser than the diabases and thus display gabbroic texture (Fig. 4e). They contain the same major and minor mineral compositions as the diabases, including clinopyroxene, plagioclase, K-feldspar, Fe-Ti oxides and apatite. However, they also contain retrograde amphibole and chlorite.
It is noted that four of the diabasic- and gabbroic-samples (LD08-4, LD08-5, LD08-7 and KPM07-3B1) contain minor quartz and alkali feldspar. Some of the alkali feldspar occurs as feathery crystals in range from 2% to 5%, presenting feathery texture (Fig. 4f); some of the quartz occurs as interstitial crystals (not shown). Where the quartz and alkali feldspar exists, clinopyroxene shows chloritization (Fig. 4f). These samples are named as quartz-bearing diabasic-gabbroic group, and those without quartzs are named as quartz-free diabasic-gabbroic group. The increase in the content of quartz and alkali feldspar possibly resulted from the injection or intrusion of intermediate-felsic magma as plugs or blebs into the solidifying or solidified diabasic or gabbroic rocks (Fig. 3d).4 ANALYTICAL RESULTS 4.1 Zircon U-Pb Ages and Hf Isotopes
Ages of the zircons from sample LD01-7 are listed in Table S1. The analyzed zircons are mainly broken euhedral, prismatic and stubby subhedral or anhedral crystals (Fig. 5a). The broken euhedral prismatic zircons show no zoning and yielded concordant to discordant 206Pb/238U ages ranging from 240 to 269 Ma, 9 of them range of 255–269 Ma and give a weighted mean age of 260.0±3.4 Ma, with 95% confidence and MSWD=2.5 (Fig. 5b). The stubby zircons usually display oscillatory zoning (not shown) and yielded ages older than 285 Ma, with populations of ~2 260– 2 285, ~650, ~440, and 300 Ma (Table S1), interpreted to be ages for xenocrystic zircons. The age of ~260 Ma is consistent with the LA-ICP-MS zircon U-Pb age of 255.0±0.62 Ma (Han et al., 2009) and 261.2±2.6 Ma (Zhu et al., 2018) reported for the basic sills in the Luokun area (Fig. 1b), as well as those ages reported for basalts in western Guangxi (e.g., Lai et al., 2012; Fan et al., 2008), within error and is considered to be the best estimated age for the diabasic rocks. Our new results and previous data suggest that the diabasic sill was formed in the late Middle-Permian to transition between the Middle and Late- Permian.
The Hf isotope results for 11 zircon grains of three age groups of 260, 275, and 285 Ma are listed in Table S2. It is shown that most of the 175Yb/177Hf is higher than the crust value (0.15). Only those data with 175Yb/177Hf < 0.15 can be considered here. Two of the seven zircons with the 260 Ma age population have initial 176Hf/177Hf ratios of 0.282 662–0.282 724, positive εHf(t) values of 1.8–4.0 and the T1DM model ages of 0.77–0.87 Ga. One of the two zircons of the 275 Ma age population shows initial 176Hf/177Hf ratio of 0.282 721, with positive εHf(t) value of 3.8 and T1DM model age of 0.78 Ga. One of the two zircons of the 285 Ma population display initial 176Hf/177Hf ratio of 0.282 686, with εHf(t) values of 2.6, corresponding to the T1DM model ages of 0.84 Ga.4.2 EPMA Data for Clinopyroxene
Analytical data for clinopyroxenes in the quartz-free diabases are listed in Table S3. The results show that the clinopyroxenes have high CaO (19.8 wt.%–21.1 wt.%), TFeO2 (9.15 wt.%–16.3 wt.%) and MgO (9.97 wt.%–15.0 wt.%), but low Na2O (0.23 wt.%–0.44 wt.%), Al2O3 (0.96 wt.%–3.92 wt.%) and TiO2 (0.21 wt.%–1.03 wt.%). The calculated Fe3+ (0.00–0.09) are evidently lower than Fe2+ (0.19–0.47). The end- member components of wollastonite (Wo), enstatite (En) and ferrosilite (Fs) are 41.6%–45.5%, 30.4%–44.3% and 10.4%– 24.9%, respectively, indicating diopside and augite compositions of the Ca-Fe-Mg pyroxene group (Fig. 6) (Morimoto et al., 1988). The results show that clinopyroxenes from both the quartz-free and quartz-bearing diabasic rocks have the same compositions (e.g., Table S2 and Fig. 6).4.3 Major and Trace Elements
Analytical results of major and trace elements are presented in Table S4. The data were recalculated to 100% without LOI.
The quartz-bearing gabbroic group samples have higher SiO2 (49.8 wt.%–51.7 wt.%), K2O (1.21 wt.%–1.72 wt.%) and P2O5 (1.2 wt.%–1.56 wt.%), but lower MgO (4.23 wt.%–4.74 wt.%) and Mg# ((MgO/40.31)/(MgO/40.31+TFeO/71.85)) (33.9–36.0) than the samples of the quartz-free gabbro-diabase group, with SiO2=45.7 wt.%–49.5 wt.%, K2O=0.30 wt.%–1.39 wt.%, P2O5=0.55 wt.%–0.84 wt.% and MgO=5.66 wt.%–7.88 wt.% and Mg#=40.1–54.4. Both groups show low Al2O3 (11.0 wt.%–15.0 wt.%), excluding the highest MgO value (8.39 wt.%) of the amygdaloidal diabase (sample LD08-9), due to its high alteration as evidenced by high LOI value (9.86 wt.%) and high contents of amygdaloidal chlorite and carbonate minerals (e.g., Figs. 5a, 5b). All samples of the two groups have high TiO2 of 2.54 wt.%–3.65 wt.% (except for sample KPM07-1B1 that shows lower content of 2.40%), high TFeO of 13.5 wt.%–17.2 wt.% as well as higher Fe2O3 (4.02 wt.%–7.76 wt.%), except for the strongly altered sample LD08-9 that have the lowest TFeO (12.5 wt.%) and Fe2O3 (3.49 wt.%), corresponding to abundant Ti-magnetite observed in the rocks. The total alkalis (Na2O+K2O) ranges from 3.10 wt.% to 5.21 wt.%, with Na2O > K2O. In the TAS diagram, most of the samples are plotted in the alkaline gabbro and monzo-gabbro with minor in sub-alkaline domains (Fig. 7a). A similar classification is also shown by plots in the Zr/TiO2×0.000 1 vs. Nb/Y diagram (Fig. 7b). However, the EPMA yields Ca- rather than alkaline- clinopyroxene compositions for the four samples (Table S3). Therefore, the studied rocks are broadly considered to be sub-alkaline rather than alkaline mafic rocks. Further geochemical classification indicates tholeiite affinity of the rocks (Figs. 7c, 7d). These data are consistent with discrimination results in TiO2/Yb vs. Nb/Yb plot in the discussion section.
The quartz-free and quartz-bearing groups of rocks have Ti/Y values of 536–747 and 399–419, respectively, and are further referred to high-Ti/Y group or high-Ti (HT) and low- Ti/Y or low-Ti (LH) diabasic group that are proposed by Xu et al. (2001) and Xiao et al. (2004), respectively (e.g., Fig. 8). The high-Ti group has total REE (124 ppm–198 ppm) contents, lower than those for the low-Ti group (294 ppm–329 ppm), but both groups show similar chondrite-normalized REE patterns, with moderate fractionation of light REE in relation to heavy REE (La/Yb)cn=7.92–10.2, (Gd/Yb)cn=2.35–2.48 and positive Eu anomaly (Eu/Eu*=1.05–1.27) (e.g., Table S4, Fig. 9a). The low-Ti group has values of Zr, Hf, Ta and Y nearly double as those of the high-Ti group (Table S4). The primitive mantle normalized incompatible element spidergrams show negative Nb-Ta and Zr-Hf anomalies, positive P, and both negative and positive Sr anomalies and enrichment of large ion lithophile elements (LILE) and high field strong elements (HFSE), with (Nb/La)n of 0.59–0.76 and (Hf/Sm)n of 0.62–0.79, respectively for the high-Ti group samples, and display negative Ti, Zr-Hf, Sr, Nb and Pb and positive La-Ce anomalies except for broad enrichment of large ion lithophile elements (LILE) and high field strong elements (HFSE), with (Nb/La)n of 0.62–0.69 and (Hf/Sm)n of 0.72–0.82, respectively for the low-Ti group samples (Fig. 9b). Several other important parameters are: Nb/U=28.4–33.3 and Th/La=0.09–0.10 for the high-Ti group samples, and Nb/U=24.4–30.1 and Th/La= 0.10–0.1 for the low-Ti group samples.
The high-Ti group samples show chondrite-normalized REE patterns and primitive mantle-normalized spidergrams subparallel to, or comparable with those of the Emeishan high-Ti basalts (Fig. 9) (Zhou et al., 2006; Xiao et al., 2004, 2003; Xu et al., 2001). They are also geochemically similar to those of the ~255–590 Ma high-Ti basalts (Figs. 7-9) in Tianlin, the western Guangxi Province, about 100 km south to the Luodian (Fig. 1a) (Lai et al., 2012; Fan et al., 2008). The low-Ti group rocks show similar REE patterns and primitive mantle-normalized spidergrams to, but have higher REE, LILE and HFSE contents than those of both the Emeishan and Tianlin high-Ti basalts, as shown in Fig. 9 (Lai et al., 2012; Fan et al., 2008; Zhou et al., 2006; Xiao et al., 2004, 2003; Xu et al., 2001). Both quartz-free and quartz-bearing groups of rocks have low Y of 22 ppm–54 ppm and high Zr/Y values of 4.4–5.8, showing precursor magmas with within-plate basalt affinity (Fig. 10).5 DISCUSSION 5.1 Mobility of the Elements for the Studied Samples
Discussion on petrogenesis of altered or metamorphosed rocks by using geochemical data should be approached with caution due to mobility of some elements. Previous investigations suggest that most high field strength elements (HFSE) such as Zr, Hf, Nb, Ta, Th, Y, P, Ti and most rare earth elements (REEs) remain immobile during metamorphism or alteration, as indicated by good preservation of REE patterns of upper amphibolite facies, or even granulite facies metamorphic rocks for the original signature of their protolith (Santosh et al., 2017; Wang et al., 2013; Grauch, 1989; Pearce, 1975), while large ion lithophile elements (LILE) such as Rb, Cs, Sr, Ba become strongly mobile (Wang et al., 2013; Wood et al., 1979; Humphris and Thompson, 1978; Pearce, 1975). The whole rock isotopes of Sr and Pb, except for Nd, are not thought to be suitably employed for discussing petrogenesis of our rocks, including those rocks in the basaltic rocks in western Guangxi due to alteration.5.2 Fractional Crystallization and Crustal Contamination
The high-Ti diabasic rocks from the Luodian show obviously low MgO (5.66 wt.%–7.88 wt.%) and Mg# (40.1–46.6) as well as very low content of Cr (1.00 ppm–100 ppm) (Table S4) compared to those of the primary basaltic magmas (MgO≥9 wt.%, Mg#=68–75, Cr > 1 000 ppm; e.g., Gill, 2010), suggesting evolved magmas. The variations of La/Sm vs. La show both partial melting and fractional crystallization trends as those for the pillow basaltic lavas from Tianlin in western Guangxi (Fig. 11a).
Zircon grains from sample LD01-7 have multiple-group ages with some older than the formation age of the host rocks (e.g., Fig. 5) and the primitive mantle-normalized incompatible element spidergram shows negative Nb-Ta anomaly (Fig. 9b). The incompatible element ratio such as Zr/Nb=6.5–8.0 for the high-Ti group samples is higher than that of the EM-OIB (Zr/Nb=4.2–11.5; Weaver, 1991), also seemingly suggesting minor crustal contamination. However, these Zr/Nb values are remarkably lower than that of the continental crust (Zr/Nb=16.2; Hofmann et al., 1986), and their Th/Ta values are mostly in the range of 2.0–2.6, broadly lower than that of the continental crustal value (Th/Ta≥2.7, e.g., Sobolev et al., 2007, 2005). In addition, the high-Ti samples contain high TiO2 contents and Nb/La values that do not support crustal contamination or AFC processes (e.g., DePaolo, 1981). The lack of negative variations of Nb/La vs. SiO2 also does not support AFC (Fig. 11b). Therefore precursor magma of the high-Ti diabasic rocks has been undergone but minor crustal contamination.5.3 Origin of High-Ti and Low-Ti Rocks of the Diabasic Sill
Xu et al. (2001) and Xiao et al. (2004) used Ti/Y value of 500 ppm as the boundary to classify the basaltic rocks of ELIP into high-Ti (HT) and low-Ti (LT) types, with further HT1, HT2, LT1 and LT2 subtypes of basalts, considering that the Ti/Y values do not obviously vary, but the TiO2 contents show increase during fractionated crystallization of the basaltic magmas (Peate et al., 1992). Origin of the high-Ti basalts in the ELIP remains controversial. One school of thought suggested that the high-Ti basaltic magmas were generated by low degree garnet stability field, while the low-Ti basaltic magmas were derived from either partial melting of sub-continental lithosphere (< 8%) partial melting of the Emeishan mantle plume in the mantle (SCLM) or fractionated crystallization of picritic magma that assimilated upper crust (e.g., He et al., 2010; Song et al., 2008, 2001; Xu et al., 2004). In contrast, Xu et al. (2007) proposed that the low-Ti basalts might have originated from a mantle plume reservoir, based on their having the highest Os concentration with γOs(t) values of +6.5, whereas the high-Ti basalts were most likely generated from a sub-continental lithospheric mantle (SCLM), according to their relative lower Os contents with γOs(t) values of -0.8 to -1.4. The third school of thought argued that both the high- and low-Ti basaltic magmas were likely derived from the same garnet-bearing source via different degrees of partial melting, 1.2%–1.5% based on trace elemental modeling and experimental works, with or without crustal assimilation (Shellnutt and Jahn, 2011). Fan et al. (2008) proposed that the high-Ti basaltic magmas from the western Guangxi in the eastern outer zone were derived from a source involving binary mixing of HIMU- and EM1-components, corresponding to plume- lithosphere interaction at the periphery of the plume. However, Lai et al. (2012) argued that the Tianlin high-Ti basaltic magmas from the western Guangxi were possibly derived from higher degrees (10%–15%) of partial melting of the sub-continental lithospheric mantle in garnet stability field and undergone minor crustal contamination while ascent.
The ratios of highly incompatible elements that possess similar bulk distribution coefficients (e.g. Th/La, Zr/Nb and Nb/U) are little affected by degrees of partial melting or crystal fractionation and can be used to distinguish multiple sources from different partial melting degrees and fractional crystallization for a group of samples (e.g., Fan et al., 2008). Fan et al. (2008) suggested that the high-Ti basaltic rocks in western Guangxi were derived from a mantle source involvement of multiple components because of remarkable variations of their Th/La, Zr/Nb and Nb/U values, and further estimated them as HIMU- OIB and subcontinental lithosphere mantle components. The deduced HIMU-member is based on the geochemical evidences: (1) The ratios of Nb/U and Th/La are within or near the ranges for OIB (Weaver, 1991; Sun and McDonough, 1989; Hofmann et al., 1986), the Nb/Th values close to that estimated for OIB (Sun and McDonough, 1989), and the incompatible trace element patterns similar to the estimated OIB, with the exception of lower Nb and Ta contents and negative Zr-Hf anomalies. (2) The alkalis and highly incompatible trace elements are enrichment and Nd isotopic ratios are moderately depleted. The potential EM1- member was suggested according to the higher La/Nb value and the lower contents of Nb-Ta and Zr-Hf for their samples than the estimated OIB-type source. Such negative Nb-Ta and Zr-Hf anomalies in primitive mantle-normalized incompatible element patterns might indicate arc basalts affinity, suggesting involvement of a subcontinental lithospheric mantle component (e.g., Fan et al., 2008). An EM1 member was further deduced based on both the trace elements and Nd and Pb isotopic compositions for these high-Ti basaltic rocks.
Lai et al. (2012) considered that the Tianlin basalts in western Guangxi might have originated from a subcontinental lithosphere mantle that was previously metasomatized according to their high La/Nb values (1.0–2.1), because it is commonly argued that La and Nb behave in a similar way during melt generation, and that their partition coefficients and abundances in the mantle are similar, with La/Nb value of 0.53 for metasomatic melts from the primitive mantle source and 1.02 for those derived from a mid-ocean ridge basalt (MORB) source (McKenzie and O'Nions, 1995). Lai et al. (2012) further suggested EM2 affinity for the subcontinental lithosphere mantle based on the Pb isotopic compositions, which was inconsistent with the EM1 proposed by Fan et al. (2008).
As mentioned previously, the Luodian high-Ti diabasic rocks occur in the outer zone of the eastern Emeishan LIP. These rocks were formed coevally with and have similar geochemical characteristics to the pillow basalts in western Guangxi that are in turn cogenetic to the Emeishan basaltic rocks. The Luodian high-Ti diabasic samples have narrow ranges of Th/La (0.09–0.10) but wide variations of Nb/U (28.4–33.3) and Nb/Th (6.49–8.43), suggesting that their precursor magmas were most likely derived from a heterogeneous mantle source. The rocks show both REE patterns and primitive mantle-normalized incompatible element spidergram broadly comparable with those for the estimated OIB (Fig. 9). Plots of samples on Th/Yb vs. Ta/Yb diagram suggest mixing of the OIB source with enriched or normal mantles (Fig. 12a). The high-Ti rocks have high La/Nb values of 1.27–1.64, higher than those of the MORB, suggesting that they might have originated from an enriched mantle source having been metasomatized previously. Their Nb (15.0 ppm–23.2 ppm) and Zr (97 ppm–173 ppm) contents are consistent with those for the Tianlin basalts in western Guangxi, but higher than those of the N-MORB (Nb=2.33 ppm, Zr=74 ppm) and lower than those of the OIB (Nb=48 ppm, Zr=280 ppm), further suggesting an enriched mantle source (Sun and McDonough, 1989). The variations of Th/Yb vs. Nb/Yb and TiO2/Yb vs. Nb/Yb (Figs. 12b, 12c), the negative anomalies for the Nb-Ta and Zr-Hf elements on primitive mantle-normalized incompatible element spidergram (Fig. 9) and the positive εHf(t) values (1.8–4.0) and T1DM model ages (0.77–0.87 Ga) of the zircons (Table S2) probably suggest that the precursor magma of the Luodian high-Ti diabasic rocks was derived from a source with mixtures of plume-mantle (OIB) component with both E-mantle and depleted mantle components of the Yangtze Craton. Though we have got information about subduction source (EM2) of SCLM and a degree of 10%–12% partial melting of magma from the garnet stable field (not shown), we don't think it appropriate to further distinguish EM1 from EM2 and figure out partial melting degree of the precursor magma from the SCLM for our high-Ti diabasic rocks because these rocks have undergone low-grade hydrothermal alteration that can result in strong mobility for the large ion lithophile elements (LILE) such as Rb, Cs, Sr, Ba (Santosh et al., 2017; Wang et al., 2013; Wood et al., 1979; Humphris and Thompson, 1978; Pearce, 1975) and whole rock Sr-Pb isotopes.
The low-Ti group of rocks shows narrow variation trend for Ti/Y and no obvious correlation of Sm/Yb with Ti/Y (Fig. 8a) and Mg# with Ti/Y (Fig. 8b), possibly suggesting weak correlation of Ti/Y with fractional crystallization of the basaltic magmas. They have a very limited variation of Sm/Yb, Ti/Y and Mg# values and are clearly distinguished from the high-Ti group of rocks. When two groups of rocks are taken into consideration, there is a Ti/Y gap of more than 100 ppm between the high- and low-Ti group samples (Fig. 8). The low-Ti is associated and occurs with the high-Ti groups of rocks within the same sill (Fig. 2), has similar major mineral compositions of clinopyroxene and plagioclase, except for containing alkali feldspar and quartz. Some alkali feldspars appear as feathery intergrowth, with or without quartz, and some quartz occurs as irregular interstitial crystal in the low-Ti rocks (Fig. 4f). These alkali feldspar and quartz are considered to be those minerals formed in a fluid-rich magma intruding or injecting into the solidifying or solidified diabasic or gabbroic rocks as veins, plugs or blebs (Fig. 3d). Therefore, we suggest that the mingling of the basaltic rocks with the felsic magmatic intrusions which ascended as diapirs (Fig. 3d), interruptedly decreasing the Ti/Y values (Fig. 8; Table S4) and P2O5 content (Table S4), probably played a key role in changing the original high-Ti to low-Ti basaltic magmas in the diabasic sill in Luodian. It supports the viewpoint that low-Ti basaltic magma can be generated by contamination of crustal materials to the high-Ti basaltic magma (e.g., Lai et al., 2012; Shellnutt and Jahn, 2011).5.4 Formation of the Amygdaloidal Diabasic Rocks
Controversy on the mechanism of formation of the amygdaloidal diabasic rocks of the diabasic sills in the Luodian region remains. The rocks have been considered as intrusive by some investigations because of the presence of diabases and gabbros (BGMRGZ, 1987), whereas they were suggested to represent eruption of lavas by others (Zhang et al., 1999), to interpret the vesicular structure for the amygdaloidal structure observed nowadays. Zhang et al. (1999) also compared the same diabasic sill in our study with the pillow basalts interlayered in deep-water strata in western Guangxi from Wu et al.(1997, 1993). The amygdaloidal diabases in the studied sill are considered as intrusive rocks according to the following evidences. (1) The sill is dominantly composed of gabbros and diabases, and the amygdaloidal diabasic rocks are minor and sporadically and locally distributed in the upper chilled zone of the sill, with clear transition to the diabasic or gabbroic rocks (Fig. 2). (2) The host sill strictly occurs within the Sidazhai Formation (Figs. 13a, 13b) generally parallel or sub-parallel to the layer. (3) A few to tens meters wide prograde contact aureole has been observed with tremolite, wollastonite and diopside towards the upper margin of the sill (Huang et al., 2018). The transition of the amygdaloidal diabases with the associated diabases or gabbros within a uniform diabasic sill, together with development of prograde contact aureole rather than hydrothermal alteration suggest that the amygdaloidal diabases are subvolcanic rocks formed inside a relatively dry strata.
It is known that vesicles in a magmatic rock are formed by entrapment of a volatile fluid bubble as gas expansion and escape occurs during pressure decrease and solidification of the melt (Sang and Ma, 2012; Best, 2003). The volume percentage of vesicles in subaqueous basalts depends on the depth of water, and generally decreases as the depth increases (e.g., Sang and Ma, 2012), with volumes of 10%–40% at ~500 m depth, < 5% at 1 000 m depth and no vesicles at depth > 3 000 m, implying that the hydrostatic pressure of water is an important factor controlling the generation of the volatiles in and escape from the pillow basaltic lava. It is not possible to generate and release volatiles in a magma system closely sealed by thick strata. In contrast, the volatiles can be vented out through local micro-cracks across the strata to the surface. The studied amygdaloidal diabasic rocks have 10%–20% vesicles (Figs. 3c, 4a, 4b), suggesting ~500 m intrusion depth for the sill below the sea level. The amygdaloidal diabases sporadically and locally developed on the upper margins of the sill, and are not evenly distributed as described by Zhang et al. (1999), suggesting that micro-cracks developed locally and sporadically at that time (Fig. 13b).5.5 Potential Tectonic Implications for the Feathery Alkali Feldspar
The feathery alkali feldspars formed in the quartz-bearing or high-Ti group rocks in Luodian region (Fig. 4f) have potentially important implications for tectonic significance. Experimental studies indicate that crystal shape is closely related to undercooling (ΔT) that refers to the difference between equilibrium temperature (Te) and the real crystallization temperature (Tc) of a magma system. Experiments on crystal growth (e.g., Swanson and Fenn, 1986; Lofgren, 1983, 1980) have demonstrated that as ΔT increases, crystals increasingly depart from an equilibrium habit of characteristic crystal faces. Laboratory experiments reveal that the plagioclase crystals will present in euhedral tabular shape at ΔT≤50 ℃, skeletal shape at ΔT≈100 ℃, dendritic, branching and feathery forms at ΔT≈200 ℃ and spherulites at ΔT≈430 ℃ (e.g., Lofgren, 1980). Assuming that both alkali and plagioclase feldspars have the same growth behavior with regard to the high undercooling, the development of the feathery texture of the alkali feldspar in our studied rocks suggests that the undercooling (ΔT) of the remaining magma system suddenly increased from ≤50 to ~200 ℃ or even higher while crystallization was on going.
The undercooling of ~200 ℃ for the crystallization magma system implies a fast cooling process to occur in the outer zone of the ELIP. However, further geochronological work should be carried out for correctly timing of the injection of the intermediate-acidic melts into the diabasic sills in the Luodian region, before definite tectonic implications are constrained, since there are two distinct ages for the felsic veins, one is ~255 Ma reported by Huang et al. (2017) and the other is ~160 Ma which is reported by Zhu et al. (2019).6 CONCLUSIONS
(1) The diabasic sill in the outer zone in the southern part of the Emeishan large igneous province in the Luodian region, Guizhou Province, Southwest China formed as a composite sub-volcanic intrusion. The sill developed inner zone with gabbroic and diabasic rocks and chilled zone with intersertal or vitreous diabases, locally precursor vesicular but now amygdaloidal diabases in the upper margins, constituting an intrusive- subvolcanic gabbro-diabase-amygdaloidal diabasic sill. Formation of the primary vesicular diabase is attributed to local development of micro-cracks in the overlying Sidazhai Formation.
(2) The diabasic and gabbroic rocks are composed of high-Ti and low-Ti groups of rocks. The high-Ti group rocks were formed mainly via fractional crystallization of evolved basaltic magma, minor contamination of crustal materials. The precursor magmas of the high-Ti group rocks might have originated from a mantle source with components of HIUM-OIB and subcontinental lithosphere mantle. The low-Ti group of rocks was produced by mingling of the high-Ti diabasic rocks with minor diapirs of the intermediate and acidic magmatic plugs or blebs. Therefore, mingling with felsic magmas is probably one of the key mechanisms to change the high-Ti basaltic rocks to low-Ti basaltic rocks in the outer zone of the ELIP.
(3) The low-Ti group of rocks developed feathery texture, which signifies rapid cooling event occurring to the felsic rocks within the diabasic sill. However, their definite tectonic implications of this event remain unclear and need further geochronological research.ACKNOWLEDGMENTS
We thank Profs. Yuping Su and Qiang Wang whose critical and constructive reviews greatly improved the manuscript. We thank Prof. M. Santosh who made correction and English edition for the early manuscript. This work was supported by the Guizhou Scientific and Technology Planning Project (Nos. QKHZDZX 6003, QKHPTRC5626 and QKHPTRC5401). We are grateful to Mr. Long Bai for assistance with the fieldwork. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1241-x.
Electronic Supplementary Materials: Supplementary materials (Tables S1–S4) are available in the online version of this article at https://doi.org/10.1007/s12583-019-1241-x.
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