Carboniferous,tectono-magmatic,evolution,of,the,northern,Luliang,arc:evidence,from,geochemistry,and,petrography,of,Carboniferous,volcanic,rocks,in,the,northern,Luliang,Uplift,NW,China

Sijie Han• Shuxun Sang,• Wenfeng Wang• Jinchao Zhang•Guanlong Zhang

Abstract The Northern Junggar Basin experienced extensive subduction and a complex tectono-magmatic evolution during the Late Paleozoic,resulting in a heterogeneous distribution of volcanic rocks in the Junggar Basin.In this study,the Carboniferous tectono-magmatic evolution of the northern Luliang arc was described by exploring the petrography and geochemistry of Carboniferous volcanic rocks collected from well Y-2 and outcrop WW′ in the northern Luliang Uplift.The distribution,types,and formation ages of these volcanic rocks were characterized and the volcanic sequence in well Y-1 was divided into upper and lower parts according to vertical variations in selected geochemical data.Then the petrogenesis and tectonic settings of different volcanic rocks were evaluated and this was used to infer the tectonomagmatic evolution of the northern Luliang arc during the Carboniferous.The results indicate that:(1) Carboniferous high-K calc-alkali andesite–dacite associations are distributed in the west of the northern Luliang Uplift,and Lower Carboniferous calc-alkali basalt–dacite–rhyolite assemblages are preserved in its east.(2)The intermediateacid volcanic rocks in wells Y-1 and Y-2 were derived from calc-alkali basaltic magma through melting of the juvenile lower crust,and geochemical variations indicate increasing addition of slab melting in a subduction-related arc environment.The bimodal volcanic rocks from outcrop WW′ were derived from lithospheric underplating of basaltic magma in an intra-arc extensional setting.(3) The closure of the eastern Keramaili Oceanic basin occurred before the Early Carboniferous,and the tectono-magmatic difference between the east and the west of the northern Luliang Uplift appeared before the Carboniferous period.

Keywords Tectonic setting · Bimodal volcanic rocks ·Carboniferous · Keramaili oceanic basin · Subduction ·Northern Luliang arc

The Junggar Basin and its surrounding areas experienced a complex tectonic evolution and sedimentary filling due to the closure of the Junggar Oceanic basin,which resulted in the production of a variety of tectonic settings,including oceanic island arcs,continental arcs,within-plate rifts,and post-collisional extensional environment (Han et al.2015;Li et al.2015a,b;Wu et al.2009;Su et al.2011).Several scholars have reported on the petrology,geochemistry,geochronology,and geophysical properties of the Carboniferous volcano-related rock associations from different areas within the Junggar Basin(Kou et al.2010;Xiao et al.2011;Yang et al.2014b);moreover,various mantle source signatures and tectonic transitions have been recorded for these volcanic rocks (Liang et al.2017;Su et al.2012;Zhang et al.2016).Significant variations of petrography and geochemistry exist in the Carboniferous volcanic rocks,and those rocks in the Luliang Uplift may have different petrogenesis (Tan et al.2009;Wu et al.2011;Zhang et al.2015a,b).Multiple subductions of different oceanic basins (e.g.,the Keramaili Oceanic basin and the Junggar Oceanic basin) occurred around the Luliang arc(the Luliang Uplift was called Luliang arc during the Late Paleozoic) during the Carboniferous;therefore,the Carboniferous volcanic rocks in various areas also have different petrographic and geochemical characteristics.As a result,previous studies have suggested that different petrogenetic and tectonic settings including an arc environment (Zhang et al.2008b,2018),oceanic ridge subduction(Geng et al.2009,2011),an intra-oceanic arc (Shen et al.2013),and a rift setting (Wang et al.2002) existed concurrently in the Luliang arc during the Carboniferous.

Tectonically,the Luliang Uplift has been broadly characterized as having an ‘‘uplift–graben–uplift’’ profile from north to south (Yang et al.2004),and the Carboniferous volcanic rocks have been shown to have an anisotropic compositional distribution (Zhou et al.2018).There has been much controversy over the tectonic setting and the evolution of the Luliang arc as indicated by the nature of these volcanic rocks.Uplift-type mafic rocks,mainly preserved in the eastern parts of the Luliang Uplift indicate a post-collisional extensional environment,and these high-K calc-alkali basalts show a wide range of eruption ages from 350 ± 6.3 to 308.6 ± 4.7 Ma(Tan et al.2009;Yang et al.2010;Li et al.2015a;Luo et al.2017).The Lower Carboniferous rhyolitic tuffs in well DX-14 of the eastern part of the Luliang Uplift conversely show some crust-derived characteristics,indicating a continental active arc environment (Li et al.2012).Compared to the volcano-related rocks in the eastern part of the Luliang Uplift,significantly different rock associations in the western parts of the Luliang Uplift indicate the difference in their geodynamics as well.Ocean island basalt (OIB)-like alkaline and calcalkaline volcanic rocks (basalt–andesite–dacite assemblages) have been identified in the western parts of the Luliang Uplift and have been interpreted as being derived from the upwelling of an asthenospheric mantle through a slab window during ridge subduction (Geng et al.2009;Yang et al.2016).Conversely,the Carboniferous volcanic rocks in the southern,central,and western parts of the Luliang Uplift are significantly associated with arc-volcanic rocks derived from the thickened crust of a mature arc environment (Zhang et al.2008a;Li et al.2015b).Recently,Zhang et al.(2018) reported arc-type calc-alkali volcanic rocks yielding a Ziron LA-ICP-MS age of 310.8 ± 2.9 Ma in the western parts of the Luliang Uplift,clearly indicating a subduction-related continental arc setting,which is in good agreement with the previous investigation(Han et al.2019b).However,previous studies have mainly focused on the results of subduction of the Junggar Oceanic basin in the southern parts of the Luliang arc(Long et al.2006;Li et al.2014;Xu et al.2015;Zheng et al.2019),and few studies focused on the Carboniferous volcanic rocks that have been derived from the subduction of the Keramaili Oceanic basin.This may be attributed to the following reasons:(1) the volcanic rocks are less accessible,and (2) bidirectional subduction of the Keramaili Oceanic basin has been neglected due to a lack of comprehensive investigations.The closure age of the Keramaili Oceanic basin is thus not well understood,and nor are the different tectono-magmatic evolutionary histories between the eastern and the western parts of the northern Luliang arc during the Carboniferous.

In this study,new volcanic rock samples of the northern Luliang Uplift (seven from the eastern and two from the western part of the northern Luliang Uplift)were collected and studied,and these results together with the published data were used to investigate the petrogenesis and tectonic setting.First,the spatial and temporal distribution of the Carboniferous volcanic rocks of the northern Luliang Uplift were studied and volcanic rock formation age constraints for well Y-1 were obtained.Furthermore,selected geochemical indicators were then used to discern a geochemical transition within well Y-1,and the reason for its occurrence was explored.Third,the petrogenesis of these volcanic rocks,which range from mafic to felsic rock associations,was discussed.Finally,geochemical results and other geological evidence obtained in this study were applied to interpret the tectono-magmatic evolution of the northern margin of the Luliang arc during the Carboniferous.This study aimed to comprehensively explore the geodynamic differences across the northern Luliang arc during the Carboniferous and to accurately specify the differences in the tectono-magmatic evolution resulting from the subduction of the Keramaili Oceanic basin.

The Junggar Basin is located at the southern margin of the Central Asian Orogenic Belt and lies at the junction between Siberia,Russia,and the Tarim Plate (Fig.1a).Late Paleozoic ophiolitic me´lange,granitoids,and subduction-related volcanic rocks widely occur in suture orogenic belts around the Junggar Basin(Fig.1b,e.g.Windley et al.2007;Chen et al.2010;Tang et al.2010),indicating multiple subductions of oceanic crust and microplate collisions.Tectonically,the Junggar Basin is composed of the Wulungu Depression,the Luliang Uplift,the West Uplift,the Central Depression,the East Uplift,and the North Tianshan Thrust Belt.The tectonic framework of the Luliang Uplift after post-Carboniferous tectonic movements has been characterized as ‘‘uplift–graben–uplift’’from south to north(Yang et al.2004).The Shiyingtan high and the Dibei high are the secondary tectonic units,located in the western and the eastern parts of the northern Luliang Uplift.The northern part of the Luliang Uplift is sutured to the Wulungu Depression and records the southward subduction of the Keramaili Oceanic basin.Wells Y-1 and Y-2 are located in the western area,and outcrop WW′lies in the eastern area,as shown in Fig.1c.

Fig.1 a Location of the study area in the Central Asian Orogenic Belt(S¸engör et al.2018);b Simplified geological map of the Junggar basin and surrounding areas(Li et al.2015b);c The locations of wells Y-1,Y-2 and DX-14 and outcrop WW’in the northern Luliang Uplift(Li et al.2012;Han et al.2019b)

What is now termed as the Luliang Uplift was denoted as the Luliang arc in the Late Paleozoic.This arc formed due to the northward subduction of the Junggar Ocean during the Carboniferous.Significant subduction in the Devonian resulted in the emplacement of a large volume of arc-volcanic rock,and the Early Carboniferous tectonic framework was inherited from that of the Devonian.However,from the middle of the Carboniferous,a dramatic change occurred in the tectono-sedimentary environment,and thus different types of volcano-related rocks were emplaced in the eastern and the western parts of the Luliang arc (Yang et al.2012).The tectonic differences indicated by this spatial variability have been attributed to the scissor-like closure of the Keramaili Ocean (Li et al.2012;Han et al.2019a).Furthermore,previous studies have suggested that,though there is still controversy over the related tectonic setting,the basalt–andesite–dacite associations in the western Luliang arc can be interpreted as arc-like (Zhang et al.2008a,2018;Geng et al.2011).Conversely,investigations of mafic rocks in the eastern Luliang arc and the East Junggar area have concluded that their emplacement resulted from the roll-back of the subducted crust of the Junggar Ocean,which caused extension in the Luliang arc and formation of an intra-arc extensional basin(Su et al.2012;Li et al.2015a).During the end of the Late Carboniferous to the Middle Permian,the eastern Luliang Uplift was transformed into a continental magmatic arc system (Xiao et al.2009),and the tectonic evolution of the basin entered an intra-continental stage.At this time,the Luliang arc experienced significant uplift and denudation,resulting in the incomplete preservation of the Carboniferous stratigraphy (Li et al.2015a).

Boreholes and geophysical results have revealed that Mesozoic–Cenozoic sedimentary rocks directly covered the underlying Carboniferous volcanic rock basement in the Luliang Uplift (Yang et al.2014a;Tao et al.2013).However,these Carboniferous volcanic rocks present significant heterogeneity in their distribution and composition.This may be attributed to the following two reasons:1)subduction was of the Keramaili Ocean and Junggar Ocean,respectively,to the north and the south of the Luliang arc during the Late Paleozoic (Li et al.2016),and 2) non-synchronous closure of the Keramaili Ocean contributed to diachronous volcano-sedimentary sequences during the Late Carboniferous (Han et al.2019b).The Lower Carboniferous volcanic sequences,consisting of intermediate volcanic rocks and related pyroclastic rocks,are widely distributed in the Luliang Uplift.Moreover,some post-collisional basalts with characteristics inherited from arc volcanic rocks occur in the eastern Luliang Uplift(Su et al.2010).The volume of the bimodal volcanic rocks exhibits an eastwardly increasing trend extending into the East Junggar terrane (Su et al.2012).The Upper Carboniferous volcanic sequences are only exposed in the eastern Luliang Uplift and Shiyingtan area,and a significant difference is observed in the rock associations and their compositions in these two areas compared to rocks from the Lower Carboniferous.

Seven volcanic rocks from outcrop WW′and two dacite samples from well Y-2 were collected to better understand the dynamic setting in the northern Luliang arc during the Carboniferous.Moreover,whole-rock geochemical data that are available for all 12 volcanic rocks with low loss of ignition(LOI)in well Y-1 were also considered(Han et al.2019b).Data for samples WW’8 (basalt) reported in our previous publication are also referenced (Han et al.2015).Before the geochemical analysis,petrographic observations under plane-polarized and cross-polarized light were carried out to select fresh samples.

Major elements and some trace elements (except for U,Cs,Ta,Sn,and rare earth elements (REEs)) were detected by X-ray fluorescence (XRF,BRUKER S8 TIGER) spectroscopy,with standard deviations within 2%.The concentrations of U,Cs,Sn,Ta,and REEs were obtained by inductively coupled plasma mass spectrometry (ICP-MS)using an X Series-2 spectrometer.The detailed procedures for determining the concentrations of major and trace elements are reported by Gou et al.(2013).The limits of detection for the major and some trace elements(except for U,Cs,Sn,Ta,and REEs) were 0.1% and 1 ppm,respectively.The lowest detection limit for U,Cs,Ta,Sn,and REE was about 0.1 ppm.Furthermore,the USGS standard AGV-1 and the Chinese national rock standard GSR-2 were used as standard reference materials during the analysis to monitor the accuracy of the data (Cheng et al.2015;Gladney et al.1992).The whole-rock geochemical data for these 22 samples are presented in Table 1.

Table 1 Major element (wt.%) and trace elements (ppm) analyses of Carboniferous volcanic rocks from field profile WW’ and well Y-1 and Y-2.The data from Y-1 are cited from Han et al.(2019b),Samples WW’-8 was also cited from our previous published paper(Han et al.2015)

Table 1 continued

Table 1 continued

In general,the volcanic rocks exposed at outcrop WW′exhibit a typical bimodal nature,with rhyolite/dacite occurring initially and significant basalt/basaltic andesite arising later.Their length ratio in the section is approximately 1:4,indicating that mafic rocks dominate rock associations in the east of the northern Luliang Uplift.Andesite,dacite,and minor tuff are found within wells Y-1 and Y-2.Moreover,some basaltic andesites occurred in the upper volcanic sequence of well Y-1,and quartz and alkali feldspar occurred widely in the lower volcanic sequence of well Y-1.

The basalts display a porphyritic texture,with plagioclase (up to 65%) and pyroxene phenocrysts set in a groundmass of plagioclase,clinopyroxene,and minor Fe–Ti oxides (Fig.2a).The plagioclases are subhedral and their sizes range from 0.1 to 0.5 mm,and the pyroxenes are subhedral–euhedral,averaging 0.5 mm in size.The crystal size distributions in the porphyritic lavas and the enrichment in plagioclase relative to pyroxene indicate the accumulation of plagioclase in the magma.The finegrained rhyolite is made up of alkali feldspar (up to 0.4 mm),quartz (0.05 mm),and plagioclase,with minor amounts of hornblende and biotite (Fig.2b).

The andesite shows a porphyritic texture,and its phenocrysts consist of plagioclase (up to 0.5 mm),pyroxene(0.2 mm),and hornblende (0.2 mm) (Fig.2c,d).The plagioclases are slabby or columnar,commonly with corroded margins.In contrast,the pyroxenes form octagonal graininess and consist of two groups of near orthometric cleavage planes.The hornblendes show a rhombic section and darkened margins.The matrix,made up of 0.02 mm crystals,presents a hyalopilitic texture,and the microcrystals are needle-like plagioclase and strip-like hornblende containing minor crypto-crystalline or glassy components.The basaltic andesites are aphyric,and the matrix mainly consists of the 0.2-mm-long strip-and needle-shaped plagioclase (Fig.2e).Granular pyroxenes,characterized by some chloritization,fill the voids between the plagioclase crystals.

Figure 2f exhibits that the phenocrysts in the dacite include columnar plagioclase,slabby alkali feldspar,and columnar hornblende with an average size of 0.2 mm.Fine-grained hornblendes are trapped in alkali-feldspar phenocrysts.The matrix is mainly needle-like microcrystalline plagioclase(0.02 mm)showing a pilotaxitic texture.

Crystal pyroclasts account for 50%–60% of the tuff,which contains orthoclase,quartz,plagioclase,and hornblende pyroclasts.Figure 2g shows that the rhombic orthoclase and angular to sub-rounded quartz are the main crystal pyroclasts therein and have sizes of 0.5–1 mm.Most of the tuffs are cemented with volcanic ash and dust.

The secondary alteration,including sericitization of plagioclase and chloritization of hornblende,are commonly observed (Fig.2h).More importantly,deep diorite and minor granitic xenoliths,comprising mainly of plagioclase,quartz,metamorphic rock debris,and minor hornblende glomerocrysts,are seen throughout the Carboniferous volcanic sequence of well Y-1(Fig.2i)but are rare in well Y-2 and outcrop WW′.

Fig.2 Representative microphotographs of the Carboniferous volcanic rocks from well Y-1,Y-2,and outcrop WW’from the northern Luliang Uplift. a basalt,1950 m in the outcrop WW’; b rhyolite,670 m in the outcrop WW’; c pyroxene andesite,3876 m in well Y-2; d hornblende andesite,3480 m in well Y-2;e basaltic andesite,2611 m in well Y-1;f dacite,2731 m in well Y-1;g dacite tuff,2740 m in well Y-1;h crystal tuff with chloritization of hornblende,2679 m in well Y-1;i Carlsbad bicrystals of plagioclase,2800 m in well Y-1.Pl-plagioclase;Aug-augite;Qtz-quartz;Afs-alkali-feldspar;Hbl-hornblende

5.1 Major elements

The collected samples showed a medium to high LOI(0.42%–6.73%),and all the major oxides were LOI-free normalized before petrogenetic interpretation.In general,high-field-strength elements (HFSEs;e.g.,Ti,Ta,Zr,and heavy REEs (HREEs)) exhibit low mobility.This makes them useful for petrographic classification and petrogenesis(Pearce and Cann 1973;Pearce and Norry 1979).The Carboniferous volcanic rocks in the northern Luliang Uplift considered herein consist of varying contents of SiO2(50.24 wt.%–75.52 wt.%),total alkali (Na2O+K2-O=4.22 wt.%–8.27 wt.%),Al2O3(12.03 wt.%–18.82 wt.%),MgO (0.3 wt.%–4.19 wt.%),CaO (0.36 wt.%–8.36 wt.%),TFe2O3(1.67 wt.%–11.21 wt.%),TiO2(0.12 wt.%–2.63 wt.%),and P2O5(0.03 wt.%–0.83 wt.%).They are plotted in the SiO2–Na2O+K2O diagram,and fall in the basalt,andesite,dacite,and rhyolite fields,and outcrop WW′lacks intermediate volcanic rocks (Fig.3a).Furthermore,in the Nb/Y–Zr/TiO2diagram (Fig.3b),the major rocks in well Y-1 fall in the andesite field,and rocks from outcrop WW′fall in the rhyolite,trachyandesite,basalt,and alkali-basalt fields.Importantly,the volcanic rocks from outcrop WW′indicate an obvious bimodal nature.Moreover,the SiO2–K2O diagram shows that the rocks in well Y-1 plot mainly as calc-alkaline assemblages,but those in well Y-2 correspond to high-K assemblages(Fig.4a).The basaltic volcanic rocks from outcrop WW′are low–high-K assemblages;however,the acid rocks plot in a high-K field.The Ta/Yb–Th/Yb diagram confirms that these rocks originated from calc-alkaline assemblages,excluding one rock in well Y-1 (Fig.4b).In the Harker diagrams,MgO,Al2O3,CaO,TFe2O3,TiO2,and P2O5show negative relationships with SiO2;however,K2O shows a slight positive relationship with SiO2,and there is no significant relationship between Na2O and SiO2(Fig.5).Interestingly,the samples in well Y-1 form a different branch to the main tendency in five diagrams,but they exhibit a high consistency with other samples in TiO2,TFe2O3,and MgO diagrams.Although some branches exist,most samples in well Y-1 fall in the main tendency composed of the samples from well Y-2 and outcrop WW′.

Fig.3 Geochemical classification of the Carboniferous volcanic rocks from the northern Luliang Uplift. a SiO2-Na2O+K2O diagram (Le Maitre et al.1989); b Nb/Y-Zr/TiO2 diagram (Winchester and Floyd 1977)

Fig.4 Discrimination diagrams of volcanic magma series of the Carboniferous volcanic rocks from the northern Luliang Uplift. a SiO2-K2O diagram (Ewart 1982); b Ta/Yb-Ce/Yb diagram (Pearce 1982)

Fig.5 Major elements Na2O,K2O,Al2O3,TFe2O3,CaO.MgO,TiO2,P2O5,and MnO versus SiO2 for the Carboniferous volcanic rocks from the northern Luliang Uplift

5.2 Trace elements

The total REE concentrations of the 22 volcanic rock samples vary widely from 73.82 to 244.28 ppm.Samples in outcrop WW′show higher REE concentrations(141.82–244.28 ppm);nonetheless,samples in wells Y-1 and Y-2 show lower REE concentrations (73.82–151.24 ppm).All these rocks exhibit various chondrite-normalized REE patterns that are characterized by slightly right-skewed trends(Fig.6a),namely relative enrichment of LREE (LaN/SmN=0.83–3.40;LaN/YbN=0.71–7.29),slight negative or no Eu anomalies(δEu=0.77–1.07,except for WW′-3 which has a particularly low value of 0.28),and slightly negative or no Ce anomalies(δCe=0.87–1.08).The REE patterns of the rocks in well Y-2 are similar to those in well Y-1,but differ from those in outcrop WW′.The REE patterns of rocks from outcrop WW′present lower LREE/HREE ratios,and the basalts show no significant variation in Eu anomaly.The negative Eu anomalies in some intermediate volcanic rocks of wells Y-1 and Y-2 indicate that plagioclase fractionation played an important role in their evolution.Moreover,all these samples exhibit LaN/YbNratios ranging from 0.71 to 7.29,lower than that of OIB(12.29)and higher than that of normal mid-ocean ridge basalt(N-MORB)(1.0)(Sun andMcDonough 1989).Spider diagrams ofsamples from outcrop WW′show a chaotic distribution,but they can still be divided into two groups according to the depletion in some HFSEs(Nb,P,and Ti).The spider diagrams for samples from wells Y-1 and Y-2 are similar to each other and indicate enrichment in large ion lithophile elements(LILEs,e.g.,Rb,Ba,and K) and depletion in HFSEs (U,Nb,Ta,P,and Ti)(Fig.6b).These features are distinct from those of N-MORB andOIB.However,some ThandSr anomaliescanbe observed in the samples from well Y-1.Furthermore,most basalts from outcrop WW′show the presence of Nb element (Nb=8.9–19.6 ppm) consistent with Nb-enriched basalts(7 ppm ＜Nb ＜20 ppm)affected by melting of the metasomatized mantle(Defant 1992;Martin et al.2005).

Fig.6 Chondrite normalized REE patterns a and primitive mantle normalized trace element spider diagrams b for the Carboniferous volcanic rocks from the northern Luliang Uplift.OIB,N-MORB,E-MORB,chondrite,and primitive mantle normalized values according to Sun and McDonough (1989)

6.1 Spatial and temporal distribution of the Carboniferous volcanic rocks in the northern Luliang uplift

The distribution of the Carboniferous volcanic rocks across the western Luliang Uplift is complex(Zhang et al.2008a).Basic–ultrabasic volcanic rocks occur in the Madong area(Li et al.2015b),granitoid plutonites and bimodal volcanic rocks occur in the Shixi area(Wang et al.2002;Chen et al.2010),and andesite–dacite associations are found in the Shiyingtan area (Zhang et al.2018).The gravity and magnetic anomalies detected in the Shiyingtan High are stronger than those in its northern and southern sides,implying possible arc-like andesite–dacite zone (Fig.2,Han et al.2019b).The Carboniferous lithology preserved in the eastern Luliang Uplift is comparatively simple and mainly consists of basalt,pyroclastic rock,and minor rhyolite emplaced in a post-collisional extensional setting(Wu et al.2009;Su et al.2012).Moreover,the gravity and magnetic anomalies also indicate the possibility of the distribution of the mafic–ultramafic complexes in the eastern Luliang Uplift(Fig.8,Li et al.2015a).Discoveries of paleontological fossils (e.g.sporopollen fossil andSpirifer subgrandisRotai) collected from outcrop WW′indicate an Early Carboniferous formation age (cited from unpublished results from the Geological Scientific Research Institute,Shengli Petroleum Administration).Previous studies on outcrop WW′also indicate that its intermediate-acid volcanic rocks formed in the Early Carboniferous (332–336 Ma),and record some island-arc signatures (Xiao et al.2011).Furthermore,the eastern Luliang arc and East Junggar have been interpreted to have evolved into a post-collisional and then an intraplate extensional setting from the middle of the Early Carboniferous,causing post-collisional volcanic rocks and intraplate volcanic rocks to be emplaced widely (Zhang et al.2015a,b).

A few wells have been drilled in the Shiyingtan area,and only wells Y-1 and Y-2 have been reported.Notably,the volcanic rocks from these two wells show similar petrographic and geochemical characteristics,comprising intermediate-acid basaltic andesite–andesite–dacite assemblages.Interestingly,the rocks in well Y-1 tend to be low-K calc-alkaline assemblages;however,the rocks in well Y-2 are high-K to shoshonitic assemblages(Han et al.2019b).New geochronological results indicate that the andesite–dacite associations in well Y-2 erupted during the Late Carboniferous (303 ± 7.7 Ma,Han et al.2019a;310.6 ± 6.0 Ma,Han et al.2019b;310.8 Ma,Zhang et al.2018).However,ever more investigations suggest that the eruption of the volcanic rocks in well Y-1 occurred during the Early Carboniferous.For example,Mao (2012) reported two eruption ages 353.1 ± 2.7 and 311.7 ± 5.1 Ma,for these volcanic rocks.Noteworthy,a limited number of zircons were analyzed for the Late Carboniferous.Yang and Chen (2016) synthesized the formation ages of Carboniferous volcanic rocks in well Y-1.The results demonstrated the age with the most selective confidence interval in the Early Carboniferous (356 Ma).The occurrence of minor Devonian zircon is a result of the widespread presence of deep xenoliths (Fig.2i).Although wells Y-1 and Y-2 are geographically close to each other,the significantly higher K,Rb,and Ba contents in well Y-2 imply that the formation age of the rocks in well Y-1 is earlier than the age of those in well Y-2.This is attributed to the fact that deeper subduction leads to more melting of continental material,which is then introduced into island arc magma (Zhao et al.2004).Therefore,it is suggested herein that the low-K calc-alkaline volcanic rocks in well Y-1 were emplaced in the Early Carboniferous.Many investigations on Carboniferous volcanic rocks in the eastern Luliang Uplift have been carried out,and they have mainly reported mafic volcanic rocks with within-plate extensional characteristics.The eruption ages of these volcanic rocks range from the late part of the Early Carboniferous to the Late Carboniferous.For example,Li et al.(2015a) reported new high-precision zircon U–Pb ages for basalts of 308.6 ± 4.7 Ma (SIMS) and 337.2 ± 4.1 Ma(SHRIMP) in the eastern Luliang Uplift.

6.2 Geochemical differences among volcanic rocks in well Y-1

Although Han et al.(2019b)suggested the occurrence of a geochemical transition between 2700 and 2750 m in well Y-1,it is significant to note that new results show little or no variation in the lower volcanic rocks (＞2660 m)(Fig.7).This observation can be supported based on the mineral contents:the upper volcanic rocks have more pyroxene without quartz,and the lower volcanic rocks have more quartz and orthoclase.The first observation of quartz and orthoclase was made at a depth of 2651 m.Geochemical indicators can also assist in observing the transition.For example,normal arc intermediate-acid volcanic rocks present low,but coherent TiO2(0.5%–0.83%)and Zr(＜130 ppm) concentrations (Xia 2014;Pearce 1982).These critical values are in accordance with the lower volcanic rocks in well Y-1 (0.39%＜TiO2＜0.82%;76.6 ppm ＜Zr ＜128.9 ppm).The geochemical stratigraphy allows the rocks to be divided into two parts according to the tendency from being upright to inclined as shown in Fig.7.The transition depth of 2660 m can be discerned in this geochemical stratigraphy,which is consistent with the mineral observations.The transition suggested by Han et al.(2019b) cannot be always discerned in the profiles.However,a significant change has occurred by the time the upper volcanic rocks were emplaced:these rocks are similar to basaltic andesite.The higher HFSEs(Ti,Ta,and Zr)and HREEs (LREE/HREE and (La/Yb)N) in the upper volcanic rocks compared to those in the lower volcanic rocks in well Y-1 may suggest melting of the mantle wedge.Moreover,the upper volcanic rocks in well Y-1 show higher La/Sm and Zr/Y ratios (Fig.7),which indicates a higher degree of continental crust involvement(Pearce and Mei 1990;Shellnutt et al.2012).Smaller differences could be observed between the upper and lower volcanic rocks in the profile including Nb/Y and Zr/Hf ratios.However,a decrease of these ratios in the lower volcanic rocks suggests they were more likely derived from the magmatic source with a stronger melting of oceanic crust (Bolge et al.2006;Luo et al.2016).

Fig.7 Selected geochemical data in the Lower Carboniferous volcanic rocks from well Y-1 showing a geochemical transition

During subduction,the overlying mantle wedge was commonly modified by the addition of materials from the subducting slab,including fluids and slab melts.Spider diagrams of volcanic rocks in well Y-1 show a binary distribution.The upper volcanic rocks show some depletion or no change in HFSEs,and the lower volcanic rocks show depletion of Ta,Nb,and Ti but the enrichment of Th,Zr,and Hf.These findings indicate that subducted sediments played a significant role in the formation of these island arc volcanic rocks.Figure 8a demonstrates fluid enrichment of both the upper and lower Y-1 andesites and dacites,and this indicator is more significant for the lower volcanic rocks.However,higher differentiation between LREEs and HREEs in the upper volcanic rocks (based on LREE/HREE and LaN/YbNvalues) and the similarity of their REE content to OIB indicate more input of fluid and/or melt components from the oceanic crust into the magmatic source.This is also supported by the Nb/Y vs.Rb/Y diagram (Fig.8b),which suggests the involvement of both fluid and melt.The finding that the upper rocks in well Y-1 have higher La/Yb ratios (6.18–7.06) than the lower rocks(1.00–5.48),though not as extreme as many so-called adakites,might be interpreted as reflecting the involvement of slab melts (Walowski et al.2015).The higher Nb/Y values of the upper Y-1 rocks are also indicative of the addition of slab melts,playing an increasingly important role in these arc-type calc-alkaline magmas.Furthermore,their geochemical ratios (La/Yb and Nb/Y) are similar to those of Y-2 volcanic rocks,which have been shown to have been significantly affected by the melting of the subducted slab (Han et al.2019a).Therefore,it was proposed herein that the volcanic rocks in well Y-1 may share the arc-related magmatic signal but that slab melts played a larger role in the upper volcanic rocks.

6.3 Petrogenesis

6.3.1 Post-magmatic alteration

These volcanic rocks have relatively variable LOI values(0.42%–4.97%),suggesting that the samples might have suffered varying degrees of low-temperature alteration.The volcanic rocks in wells Y-1 and Y-2 and at outcrop WW′show low to moderate values of LOI (0.42%–4.97%).Petrographic observations also revealed some alteration of volcanic rocks in well Y-1.To detect whether elements were mobilized at the whole-rock scale,bivariate plots of Zr vs.representative trace elements were applied herein (Fig.9).For well Y-1,selected HSFEs(Hf and Ta),mobile elements(Ba and K),and REEs (La and Y) displayed good correlations with Zr,indicating only slight or no secondary alteration(He et al.2008;John et al.1982).Furthermore,Nb also exhibited a strong correlation with Th,U,Ba,Sr,Ta,and La.However,these elements display chaotic distributions,in particular,the mobile elements from outcrop WW′.Therefore,it is suggested that post-magmatic alteration did not have a significant effect on the composition of the rocks from well Y-1,but some low-temperature alteration occurred in the bimodal volcanic rocks from outcrop WW′,which may be associated with chemical weathering.

Fig.9 Bivariate plots of Zr vs.representative HFSE(Hf,Ta,and Nb),LILE(Ba,K,and Rb),and REE(La,Y,and Yb)for volcanic rocks in the northern Luliang Uplift

6.3.2 Magma sources

6.3.2.1 (1) BasaltThe basalts from outcrop WW′have a low SiO2(50.28%–53.97%) content,a low to moderate Mg# (0.37–0.46)value.Moreover,their Cr(33–135.1 ppm) and Ni (27.1–83.1 ppm) concentrations are lower than those of the primary mantle-derived magmas (Cr=300–500 ppm,Ni=300–400 ppm,Frey et al.1978),indicating that the magma experienced significant crystal fractionation after originating from the mantle.The obvious negative relationships among MgO,CaO,and TFe2O3and SiO2are compatible with pyroxene fractionation (Fig.5).The high Al2O3(15%–17.21%) content,together with the negative Eu and Sr anomalies suggests some fractionation of plagioclase.Moreover,the significant negative correlation between Dy/Yb and SiO2indicates amphibole fractionation,as they result from preferential incorporation of Dy over Yb (Garrison and Davidson 2003).The presence of positive P–Hf anomalies,but slightly negative Nb–Ta anomalies within the spider diagrams for the basalts suggests the assimilation of some crustal material in these magmas during ascent.Their high Th/Ce(0.2 and 0.23)and Th/La(0.43 and 0.54)ratios also indicate significant crustal contamination during the generation of the magmas.This is ascribed to the fact that the continental crust has relatively high Th/Ce(＜0.15,Taylor and McLennan 1995) and Th/La (＜0.30,Plank 2005)ratios,while mantle-derived magmas have low Th/Ce(0.02–0.05) and Th/La (＜0.12) ratios (Sun and McDonough 1989).

These basalts show no significant LILE-enrichment or HFSE-depletion,suggesting their derivation from various mantle sources,which is inconsistent with Late Carboniferous basalts in the same areas (Li et al.2015a).This is also supported by (Ta/La)PMvs.(Hf/Sm)PM(Fig.10a) and Zr vs.Nb(Fig.10b)diagrams.This evidence indicates that the magmatic sources of the basalts were complex,originating from depleted mantle to enriched mantle blended with fluids or melts from a subducted slab.Their trace elements are inconsistently distributed,and opposing LILE and HFSE anomalies can be discerned in the multi-element diagrams (Fig.6),which also indicates the existence of various mantle sources.Asthenosphere-derived mafic rocks commonly show low La/Nb (＜1.5) and La/Ta (＜22)ratios,whereas those of rocks derived from the lithosphere are higher (Fitton et al.1988).The basalts from outcrop WW′show various La/Nb (0.66–2.46) and La/Ta(14.61–32.96) ratios,also indicating an important role for lithospheric materials in their magmatic source.Their flat chondrite-normalized HREE patterns and relatively low HREE and Y content might reflect the influence of residual garnet during partial melting in the lower crust (Genc¸ and Tüysüz 2010).The basalt samples exhibit low Sm/Yb and La/Yb ratios,which is consistent with a spinel–garnetbearing mantle source that has undergone 10% partial melting to generate a primary basaltic magma (Fig.10c).

Fig.10 Diagrams of a (La/Sm)PM—(Hf/Sm)PM (La Fle`che et al.1998), b Zr/Y– Nb/Y (Condie 2005) and c Sm/Yb—Sm (Aldanmaz et al.2000)indicate the magma source of basalts in the outcrop WW’.The Late Carboniferous basalts in the eastern Luliang Uplift are derived from Li et al.(2015a)

6.3.2.2 (2) Intermediate-acid volcanic rocksThe rhyolites and dacites from outcrop WW′are all characterized by high SiO2and Na2O+K2O contents,low TFe2O3and MgO contents,strong or slightly negative Eu anomalies(0.28–0.87),and relatively high Ga/Al ratios (1.0–2.0) and Zr (164.9–227.2 ppm),Nb (22.3–46.1 ppm),Ce(50.67–69.24 ppm),and Y (30.63–52.67 ppm) concentrations compared to those in wells Y-1 and Y-2.These characteristics indicate that the samples in outcrop WW′share affinities with A-type granites (Whalen et al.1987).Furthermore,major element Harker variation diagrams and trace element distribution patterns show significant differences between the intermediate-acid volcanic rocks from outcrop WW′and from wells Y-1 and Y-2.This is probably attributed to the difference in their magma sources.For felsic rocks in bimodal volcanic rock assemblages,the generation of silicic magmas can be attributed to the following two mechanisms:(1) fractionation of coeval basaltic magma and(e.g.,Tian et al.2010)and(2)melting of the felsic crust (e.g.,Brewer et al.2004).The different distributions of trace elements between mafic and felsic rocks in the bimodal volcanic rocks and no existence of intermediate volcanic rocks argue against the differentiation of coeval basalt parent magmas.

The samples in outcrop WW′and well Y-1 consist of low Sr (51–382 ppm,average:222.36 ppm) and Na2O (K2O/Na2O=0.19–1.42,average:0.5) concentrations and Sr/Y(0.97–25.38,average:10.19) and (La/Yb)N(2.45–7.29,average:3.64)ratios and high Y(14.11–52.67 ppm,average:27.93 ppm) and Yb (1.62–7.48 ppm,average:3.55 ppm)concentrations,similar to normal calc-alkaline granitoids derived from pre-existing juvenile crustal materials (Yang et al.2016).Furthermore,the Nb/Yb-Th/Yb diagram also indicates that these intermediate–acid volcanic rocks originated from a subduction-related crustal anataxis and share some arc signatures (Fig.11a).The samples obtained from wells Y-2 and Y-1 share increasingly significant affinities with arc-type calc-alkali volcanic rocks,reflected by their LILE-enrichment and HFSE-depletion.This is also supported bytheY–Sr/Y diagram,in which they show significant similarity to island arc-type volcanic rocks derived from crustal fractionation (Fig.11b).The higher Nb/Yb(3.56–12.19)and lower Yb/Ta(2.37–4.62)ratios of the felsic rocks exhibit similarities to crust that plots between OIB and IAB,whereas those in wells Y-1(Nb/Yb:2.02–3.91;Yb/Ta:5.05–38.58) show stronger associations with arc-type volcanic rocks (Eby 1992;Pearce 2008).Therefore,the Carboniferous intermediate-acid volcanic rocks in the northern Luliang Uplift showed a strong association with arc-type calc-alkali basaltic magma and were probably generated by the partial melting of the juvenile basaltic lower crust,associated with mantle-derived melts.However,these volcanic rocks in outcrop WW′have a low Mg# value (0.17–0.3),suggesting weak involvement of mantle materials.

Fig.11 Diagrams of a Nb/Yb-Th/Yb(Pearce 2008);b Y—Sr/Yb(Defant and Drummond 1990)indicate the magma source of intermediate-acid volcanic rocks in the northern Luliang Uplift

6.4 Implications for the Carboniferous tectonomagmatic evolution of the northern Luliang Uplift

6.4.1 East–west differences in the tectonic environment of the northern Luliang Uplift during the Carboniferous

The different petrographies of volcanic rocks across the northern Luliang Uplift indicate the possibility of their formation in different tectonic settings.The typical bimodal volcanism in the eastern part of the northern Luliang Uplift suggests a continental extensional setting;in contrast,the arc-type volcanic rocks in the Shiyingtan area in the west imply ongoing subduction.The geochemical character of the latter Carboniferous high-K calc-alkali volcanic rocks is also consistent with that of the arc volcanic rocks,as they show LILE-enrichment and depletion in Nb,Ta,P,and Ti,which is indicative of the formation of subduction-related rocks in an island arc environment(Grove et al.2003).Tectonic setting discrimination diagrams were used for basaltic rocks and acid rocks,respectively,to further interpret their tectonic environments.

On an Nb × 2–Zr/4–Y diagram,three samples fall in the field of within-plate basalt,and one sample falls in a possible volcanic arc basalt field (Fig.12a).However,the association of the one samples in the C field with the Late Carboniferous basaltic rocks investigated by Li et al.(2015a)indicates that they were the products of extensional geodynamics.This is also supported by the Zr–TiO2diagram (Fig.12b).Moreover,these basalts have low Al2O3concentrations (15.00%–17.21%) and low Ba/La(11.47–12.74) ratios and high TiO2(1.50%–2.63%) concentrations and Ta/Hf(0.13–0.20)and Ti/V(44–86)ratios,similar to those of within-plate basalts rather than typical arc basalts (Pearce 1982;Condie 1989;Ajaji et al.1998;Xia 2014).On the Zr/Y versus Zr diagram,the basaltic rocks plot in the within-plate basalt field (Fig.12c),indicating that they share a geochemical association with within-plate basalts and were likely formed in an extensional setting.

Fig.12 Tectonic setting discrimination diagram for the basalt. a Nb × 3-Zr/4-Y diagram (Meschede 1986); b Zr-TiO2 diagram (Pearce and Cann 1973);c Zr-Zr/Y diagram(Pearce and Norry 1979).The Late Carboniferous basalts(the red areas)in the eastern Luliang Uplift are derived from Li et al.(2015a)

The intermediate-acid volcanic rocks exhibit significant differences in their specific tectonic setting discrimination diagrams,related to their binary nature.This difference is associated with the geographical location and eruption age,as they indicate contrary dynamics.The most acidic volcanic rocks from outcrop WW′fall in the within-plate basalt field,consistent with the conclusions regarding basalts(Figs.12a,b)and suggesting that they erupted in an extensional setting.One dacite shares some association with arc volcanic rock,probably resulting from the inheritance of arc volcanic rocks before the collision(Mao et al.2010;Wu et al.2009).However,the intermediate volcanic rocks in the Shiyingtan area (wells Y-1 and Y-2) universally plot in the volcanic arc granite field (Figs.13a,b),suggesting that their tectonic environment was a typical volcanic arc during subduction.Although the lower volcanic rocks in well Y-1 fall in the oceanic arc field(Fig.13c),their high Nb (9.9–11.9 ppm) concentration,and the Th/Yb (2.11–3.03) and Th/Y (0.26–0.37) ratios,which differ from those of an oceanic arc,indicate that they were formed in a continental arc and that their mantle source was metasomatized by melting of subducted oceanic crust (Pearce 1983).This is also in agreement with the above-mentioned results regarding vertical geochemical changes in well Y-1,which indicates a higher melting of the mantle in the lower volcanic rocks.

Fig.13 Tectonic setting discrimination diagram for the intermediate-acid volcanic rocks.a Y-Nb diagram(Pearce et al.1984);b Yb-Ta diagram(Pearce et al.1984); c Yb-Th/Ta diagram (Gorton and Schandl 2000)

6.4.2 Implications for the tectono-magmatic evolution of the northern Luliang Uplift

As discussed above,the volcanic rocks in the eastern and the western parts of the northern Luliang arc exhibit evidence for different magmatic origins in settings with opposing geodynamics.Lower Carboniferous bimodal volcanic rocks in outcrop WW′,along with observations of A-type granites (Han et al.2006),indicate that after the closure of the eastern part of the Keramaili oceanic basin,the eastern part of the Luliang arc became an intra-continental rift setting during the Early Carboniferous(Fig.14a).Furthermore,crustal extension and thinning resulted in the upwelling of lithospheric mantle metasomatized to different degrees by arc-type magma,causing magma underplating.The upwelling of these hot calc-alkali basaltic magmas further resulted in the partial melting of the lower crust.Consequently,the calc-alkali basalt experienced some crustal contamination,and intermediate–felsic eruptions accompanied the basalts.During the Late Carboniferous,slab rollback of the Junggar Oceanic crust contributed to stronger extension in the Luliang arc,causing higher degrees of partial melting of metasomatized wedge mantle by hydrous fluids and melts from subducted sediments (Fig.14b,Li et al.2015a).The Carboniferous low-K to high-K volcanic rocks preserved in the Shiyingtan area indicate subduction and a continental arc environment in the western part of the northern Luliang arc during the Carboniferous that resulted from the southward subduction of the Keramaili Oceanic basin.The intermediate-acid volcanic rocks in the Shiyingtan area display typical I-type granite associations and were most likely generated by the melting of the juvenile basaltic lower crust in a convergent setting.Noteworthy,the western part of the Luliang arc gradually evolved into a mature island arc,as reflected by evidence for an increased crustal component in the melt from the Early Carboniferous (low-K calc-alkali associations in well Y-1) to Late Carboniferous (high-K or shoshonitic associations in well Y-2) (Jakesˇand White 1972).During the Early Carboniferous,the oceanic slab moved down with the continuation of subduction,resulting in a higher temperature in the crust-mantle reaction zone.Therefore,increased subducted slab melt was introduced into the magmatic source,as supported by the geochemical evidence from the upper volcanic rocks in well Y-1(Fig.14c).During the Late Carboniferous,more mantlederived magmas metasomatized due to increasing addition of the subducted slab melts to the lower crust,resulting in the eruption of arc-type high-K and shoshonitic volcanic rocks in well Y-2 (Fig.14d).The appearance of molasse formations and A-type granitoids around the western Junggar area indicates the end of oceanic plate subduction after the Late Carboniferous (Yin et al.2011).

Fig.14 The different tectonic settings and magmatic geodynamics between eastern and western parts of the Luliang arc during the Carboniferous

It is widely believed that the northern Junggar area experienced a major sequence of subduction–accretion–collision and developed extensive volcano-related rocks during the Late Paleozoic,resulting from the amalgamation of multiple plates(Xu et al.2012;Zhang et al.2011,2017).The Carboniferous is the critical epoch for the evolution of the Keramaili Oceanic basin and collision occurred between the Wulungu arc and the Luliang arc until the post-Carboniferous (Long et al.2006;Li et al.2016).The northern part of the Luliang Uplift records the southward subduction of the Keramaili Oceanic basin during the Carboniferous;however,the compositions of the associated volcanic rocks display significant spatial variation(Hui et al.2009;Su et al.2010;Zhang et al.2018).Our data and previous studies both indicate a significant tectonic transition from convergence to extension between the eastern and the western parts of the Luliang Uplift(Li et al.2012;Han et al.2019b).Nevertheless,the new results in this study demonstrate that this tectonic difference may have developed earlier than the Late Carboniferous.According to the eruption ages of the volcanic rocks mentioned above,it was inferred herein that the closure of the Keramaili Oceanic basin in the western part of the northern Luliang arc lagged behind that in the east by at least 34 Ma.The earliest within-plate basalt in the east occurred in the age of 337.2 ± 4.1 Ma (Li et al.2015a)and the latest arc volcanic rocks in the west appeared in the ages of 303 ± 7.7 Ma (Han et al.2019a).This tectonic difference existed between the eastern and the western parts of the northern Luliang arc until the emplacement of bimodal volcanic rocks and A-type granite in the western Junggar Basin at the end of the Late Carboniferous,which indicates the existence of a within-plate rift environment(Liu et al.2016;Yang et al.2014c;Zhou et al.2008).Therefore,the Later Carboniferous subduction-related arctype low-K calc-alkali andesite–dacite assemblages in well Y-1 and the Early Carboniferous bimodal volcanic rocks from outcrop WW′bring the time of formation of this tectonic difference forward to the Early Carboniferous.

1.Carboniferous low-K to high-K calc-alkali andesite–dacite assemblages occur in the western part of the northern Luliang Uplift,whereas Lower Carboniferous bimodal volcanic rocks are preserved in its eastern part.Although arc-type volcanic rocks are distributed in wells Y-1 and Y-2,the geopolarity of the island arc between these two wells indicated that the formation age of volcanic rocks in well Y-1 was earlier than those in well Y-2.

2.The Carboniferous volcanic rocks in well Y-1 show the arc-related magmatic signal,but the upper volcanic rocks consist of more additions from slab melting.Moreover,the limited occurrence of quartz,more mafic minerals,higher REEs,and HSFEs concentrations in the upper volcanic rocks in well Y-1 indicate a stronger partial melting of lower continental crust.The binary feature of volcanic rocks in well Y-1 indicates an evolutionary subduction-related magmatic activity in the western part of northern Luliang Uplift during the Early Carboniferous.

3.In outcrop WW′,the basalts were obtained as the product of the melting of lithospheric mantle that was contaminated by crustal materials during magma upwelling,and their symbiotic dacite–rhyolite associations were derived from partial crustal melting due to underplating.The intermediate-acid volcanic rocks in wells Y-1 and Y-2,with typical arc volcanic rock characteristics,originated from arc-type calc-alkali basaltic magma in the juvenile lower crust.These results indicate a significantly different petrogenesis and magmatism between the eastern and the western parts of the northern Luliang Uplift.

4.Opposing geodynamic settings co-existed in the northern Luliang arc during the Carboniferous.The western part of the northern Luliang arc was in a continental marginal environment during the Carboniferous,whereas its eastern part was in an intra-arc extensional setting during the Early Carboniferous.The eastern part of the Keramaili Oceanic basin had been closed before the Early Carboniferous.This tectono-magmatic difference between the eastern and the western parts of the north Luliang Uplift appeared before the Carboniferous period.

AcknowledgementsWe thank Dr.Zhuangfu Li and Mr.Peiming Zhou for their assistance in lithological analyses.We would also like to extend my gratitude to the two anonymous reviewers for their valuable comments.This research was financially supported by the National Natural Science Foundation of China (Nos.41802182 and 42072192),Open fund of Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process of the Ministry of Education,China University of Mining and Technology (No.2018-004),A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institution (PAPD) and China Scholarship Council.