Geochronology and Geochemistry of the Karadaban Bimodal Volcanic Rocks in the Altyn Area, Xinjiang: Implications for the Tectonic Evolution of the Altyn Ocean
Detailed geochronological, geochemical, and Sr-Nd-Hf isotopic data are presented for early Paleozoic volcanic rocks in the Karadaban area from the northern Altyn region, NW China, with the aim to constrain their petrogenesis and tectonic implications. The Karadaban volcanic rocks show a bimodal distribution in composition, with rhyolite and basalt. The LA-ICP-MS zircon U-Pb age indicates that the volcanic rocks were erupted at 512 Ma. The mafic rocks are calc-alkaline, enriched in light rare earth elements (LREE) and large-ion lithophile elements (LILE; Ba and U) and depleted in high-field strength elements (HFSE; Nb and Ta). These features together with their depleted isotopic signature (initial , to 3.7) suggest that they were likely derived from a depleted mantle source but mixed with crustal components while upwelling. The felsic rocks show an A-type affinity, with high alkalis and Rb/Sr and Ga/Al ratios; enriched in LILE (e.g., Rb, K, Th, U, and REE) and depleted in Ba, Sr, Nb, P, and Ti; and with fractionated REE patterns with strong negative Eu anomalies. The combination of the decoupling of values (−2.5 to −6.3) and values (+5.5 to +14.7) in the setting of subduction indicates that the felsic rocks were generated by partial melting of the juvenile crustal as a result of magma upwelling. The geochemical and Sr-Nd-Hf isotopic characteristics, coupled with regional geology, indicate that the formation of the Karadaban bimodal volcanic rocks involves an extensional regime associated with a subduction-related environment. The rifting of the back arc in response to the retreat of the subducting northern Altyn oceanic lithosphere may account for the Karadaban bimodal volcanic rocks.
Bimodal volcanism typically characterizes an extensional environment, which can occur within various tectonic settings, including continental rifts [1, 2], within-plate extensional settings [3–5], intraoceanic islands , ocean island arcs [7, 8], incipient back-arc depressions , mature island/active continental margins [7–9], and back arcs [9–12]. In each of these modern tectonic environments, the volcanic activity may give rise to specific features, such as lithological assemblage, geochemical signature, and type of associated ore deposits [12, 13]. Moreover, of all these various associations of mafic and felsic rocks, it is perhaps the A-type rhyolites with basalts that are the most enigmatic but may also prove to be the most instructive. Therefore, the recognition of such distinctive bimodal igneous suites in ancient orogenic belts can not only provide diagnostic geodynamic tracers for constraining the tectonic evolution of these belts but also document important information on crustal growth by magmatic underplating in continental interiors.
The Altyn Tagh Belt (ATB) marks the northern margin of the Qinghai-Tibet Plateau , lying between the Tarim block to the north, the Qaidam block, the Qilian orogen, and the Kunlun orogenic belt to the south [15, 16]. As one of the most important tectonic zones in northwestern China, the Altyn Tagh carved and dominated several tectonic units to match the eastern extrusion of the Tibetan Plateau [17–21]. In the last decade, the North Altyn Tagh region was recognized as an ophiolite [14, 22–24], with HP/UHP metamorphic rocks [25–27] and igneous rocks [28–32]. Previous studies show that three stages can be divided about the collisional orogeny in the Altyn area: the oceanic-continent collision stage [31, 32], the oceanic-continent subduction stage [33–35], and the postcollisional extension stage [36, 37]. At present, the mechanisms of the tectonic setting of the last two stages of magmatism are almost clear due to the zircon U-Pb ages from igneous rocks and metamorphic ages from HP/UHP metamorphic rocks [14, 31, 34, 35, 37, 38]. However, the oceanic-continental collision stage of magmatism in the north Altyn region is still unclear, which is subject to few systematic studies. In addition, the oceanic-continental subduction setting is always accompanied by the mixture of crustal and mantle magmatism, but most of the studies are focused on granite only. Recently, there was a set of bimodal volcanic rocks discovered in the NE of the North Altyn Tagh, which is helpful for studying the magmatic events and determining the timing of oceanic-continental subduction. In this paper, we present U-Pb zircon dating and geochemical and Sr-Nd-Hf isotopic composition for Karadaban bimodal volcanic rocks from the north Altyn region. Our aims are (1) to constrain petrogenesis and magma sources of bimodal volcanic rocks in the subduction setting and (2) to discuss the early Paleozoic evolution of the Altyn Tagh region.
2. Geological Background
The Altyn Mountain is located at the juncture of the Tarim Basin, the Qaidam Basin, and the Qilian orogen in Western China [19, 39–43], which plays a significant role in accommodating convergence between India and Eurasia [20, 44, 45]. The Altyn Tagh fault system is the first-order tectonic unit that marks the northern margin of the Qinghai-Tibet Plateau and influences the geological evolution of surrounding areas. Pin and Paquette proposed that the sedimentation in the southern Tarim Basin was associated with this large-scale (>1600 km long) intracontinental fault system  (Figure 1(a)).
The tectonic units of the ATB have been divided into four different schemes (Figure 1(b)) [19, 47–49]: the North Altyn Tagh Archean complex (NAAC), the North Altyn Tagh subduction-collision complex (NASC), the Milanhe-Jinyanshan block (MJB), and the South Altyn Tagh subduction-collision complex (SATSC) (Figures 1(b) and 1(c)) [19, 25, 50]. In the NAAC region, the HP argillaceous rocks, glaucophane schist, and eclogite occurred belonging the HP-LT metamorphic belt. Previous studies showed that the phengite in the eclogite and the paragonite in the glaucophane schist have a yield of 39Ar–40Ar isochron ages of and , respectively . These HP-LT metamorphic rocks are an important indicator of the palaeosubduction zone . Moreover, there are many mafic-ultramafic plutons that are developed in the NASC, in which the ophiolite belt is dated at 524–437 Ma, with the features of the ocean island basalt, indicating that the oceanic crust may have developed probably in the Early Cambrian [14, 53, 54]. Researches about the age of felsic rocks in NASC show that they mainly ranged from 500 Ma to 420 Ma, which is associated with the setting of subduction to collision to postcollision [55–57]. The NASC is about 20 km wide, which extends from Hongliugou eastward to Lapeiquan for 150 km, and is controlled by the NEE-Altyn faults. In the Altyn region, the Paleozoic formation of the Zhuo’abulake and Simierbulake volcanic sedimentary rock series is located at the center of the Karadaban fault, which is fault-contacted to the Cenozoic and Archean strata. In addition, there is a suite of Karadaban bimodal volcanic rocks that developed in the north of NASC, which occurred in the Zhuo’abulake Formation (Figure 2). Magmatism peaked at the early Paleozoic and stretched along the NE faults, which produced a series of granodioritic and granitic rocks. Chen et al. reported that the age of Kaladawan felsic volcanic rocks (zircon SHRIMP U-Pb method) was 485–477 Ma, and the characteristic of geochemistry indicated a setting of active continental margin . Cui et al. (2010) verified the geochemistry of basic volcanic rocks in the Karadaban region. These rocks showed the features of tholeiitic basalt and conjectured an ocean island arc setting, which developed at the hanging wall of subduction in the early Paleozoic . However, the timing of the subduction was still disputable.
3. Petrology and Sampling
In this study, 12 samples within the Karadaban bimodal volcanic rocks were collected for geochemical analysis, 8 of them were used for Sr-Nd isotopic analysis, and analyses of Hf isotopes were made on zircon grains from samples KB-26 and KB-81 that were previously dated by laser ablation multiple collector inductively coupled plasma mass spectrometry (LA-ICP-MS). Lithology of these samples are basalt and rhyolite (Figure 3).
The rhyolite samples are gray to light yellow in color, massive, and porphyritic and rhyolitic in texture, which are interbedded with the basalt (Figure 3(a)). The felsic samples contain 10–15% plagioclase and quartz phenocrysts in a groundmass consisting of glassy cryptocrystalline material and feldspar and quartz (). The quartz is subhedral-anhedral, some of which has been rounded by dissolution (Figures 3(b) and 3(c)). The plagioclase is subhedral and lath-shaped (up to 0.5 mm) and hypidiomorphic in texture. The groundmass is typically a fine-grained assemblage of plagioclase, quartz, and muscovite. Muscovite is present as microscopic flakes (generally <0.1 mm), preferentially oriented aggregates, and disseminated single crystals with a preferred orientation (Figure 3(d)). The dark greyish-green basalts occur as massive layers with a porphyritic texture. The main phenocrysts are plagioclase and pyroxene (Figures 3(e) and 3(f)). The plagioclase is euhedral, tabular, and up to 0.3–0.4 mm, and the pyroxene occurs as thin tabular crystals up to 0.5–0.6 mm in size. The groundmass includes fine-grained plagioclase, pyroxene, and magnetite, which is characterized by the fasciculate variolitic structure of fibroid pyroxene and plagioclase (Figure 3(g)).
4. Analytical Methods
The Karadaban igneous samples were collected from adits and outcrops (Figure 2), which were selected for petrographic studies under thin sections. Some of the igneous samples were crushed to 200 meshes for major element, trace element, and Sr-Nd isotopic analyses; other samples were chosen for zircon U-Pb dating and Hf isotopic measurements.
4.1. Major and Trace Elements
Major and trace elements were analyzed in the Analytical Laboratory of Research Institute of Uranium Geology, Beijing (BRIUG). Major element compositions were measured by wavelength dispersive X-ray fluorescence spectrometry (XRF) on fused glass beads using a Philips PW2404 spectrometer; analytical uncertainties are less than ±1% for major elements. FeO content was determined by conventional wet chemical titration technique. Both trace element and REE abundance were analyzed using ICP-MS (Finnigan MAT Ltd.); uncertainties are less than ±5% for , ±3% for La and Lu, and ±2% for other REEs. The detailed analytical procedures are given in Wang et al. .
4.2. Zircon U-Pb Dating
Cathodoluminescence (CL) images were obtained by using a scanning electron microscope (SEM, Leo 1450VP, Germany) at the Chinese Academy of Geological Sciences before in situ U-Pb and Hf isotopes analyses. Zircon LA-ICP-MS U-Pb dating was performed using a laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) at the MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences (CAGS), Beijing. The laser spot diameter and frequency were 40 μm and 10 Hz, respectively. The Harvard zircon 91500 was used as an external standard for zircon U-Th-Pb analyses and NIST610 as an external standard to calculate the contents of U, Th, and Pb. The 207Pb/206Pb and 206Pb/238U ratios were calculated using the GLITTER program , and common Pb was corrected using the method of Andersen . Age calculations and concordia plots were done using Isoplot (ver. 3.0) . The details of the analytical techniques were documented by Hou et al. .
4.3. Lu-Hf Isotope Analyses
In situ zircon Hf isotopic analyses were carried out at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, using a Neptune MC-ICP-MS with an ArF excimer laser ablation system. During analyses, the spot sizes of 32 and 63 μm and a laser repetition rate of 10 Hz with 100 mJ were used. During analyses, the 176Hf/177Hf and 176Lu/177Hf ratios of standard zircon (91500) were (2σn, ) and 0.00031, similar to the commonly accepted 176Hf/177Hf ratio of (1σ) measured using the solution method [64, 65].
4.4. Sr-Nd Isotopic Analyses
Samples for isotopic analysis were dissolved in Teflon bombs after being spiked with 84Sr, 87Rb, 150Nd, and147Sm tracers prior to HF+HNO3 (with a ratio of 2 : 1) dissolution. Rubidium, Sr, Sm, and Nd were separated using conventional ion exchange procedures and measured using multicollector mass spectrometer (IsoProbe-T) at the Analytical Laboratory of BRIUG . Procedural blanks were <100 pg for Sm and Nd and <500 pg for Rb and Sr. 143Nd/144Nd were corrected for mass fractionation by normalization to , and87Sr/86Sr ratios were normalized to . Typical within-run precision (2σ) for Sr and Nd was estimated to be ±0.000015. The measured values for the La Jolla and BCR-1 Nd standards and the NBS-607 Sr standard were (2σ) and (2σ), respectively, and the measured value during the period of data acquisition is (2σ).
5.1. Major and Trace Elements
Twelve whole-rock major and trace element data are presented in Table 1. The loss on ignition (LOI) for the rocks was in the generally range from 0.74 to 3.68 wt.% (one sample at 6.87 wt.%), which probably reflects the presence of alteration-related hydration. Considering the potential modification from the concentrations of mobile components [67, 68], the effects of alteration and the mobility of the major and trace elements were evaluated prior to the geochemical and petrological investigation. Only immobile elements were used to classify rock samples and discuss their tectonic settings and petrogenesis.
The SiO2 content of the volcanic rocks lie within a range from 44.74 to 76.14 wt.% and with a compositional gap at 46–68 wt.% SiO2, which shows the bimodal feature (Table 1). Combining with the Zr/TiO2-Nb/Y plot, the samples were identified as basalt and rhyolite (Figure 4(a)). Mafic rocks, on the TiO2-Zr/P2O5 diagram (Figure 4(b)), show alkalic features, with relatively low K2O contents (0.18–1.44 wt.%), high concentrations of TFe2O3 (11.49–17.12 wt.%), MgO (4.23–8.74 wt.%), and TiO2 (1.79–3.75 wt.%). On a chondrite-normalized REE diagram, the samples exhibit slightly LREE enrichment (), with weak negative Eu anomalies () (Figure 5(a)). The primitive-mantle-normalized trace-element spider diagram shows slight enrichment in large-ion lithophile elements (LILEs; e.g., Rb, U, and Ba) with moderate to negative anomalies in high-field strength elements (HFSEs; e.g., Nb and Ta) (Figure 5(b)).
The rhyolite exhibits a wide range of TFe2O3 content (2.07–8.30 wt.%), with low TiO2 (0.19–0.65 wt.%) and P2O5 (0.03–0.10 wt.%) content, and high Al2O3 (9.60–14.57 wt.%). The A/NK and A/CNK values are 1.17–6.85 and 1.03–6.03, respectively, which classify the felsic samples as peraluminous. The felsic rock samples show values of 4.30–6.38 and are characterize by LREE enrichment with moderate HREE and LREE fractionation and show strong negative Eu anomalies (). They are also enriched in LILE (e.g., Th, U, and K) and are relatively depleted in HFSE and LREE, with remarkably negative Nd, P, and Ti anomalies (Figure 5(d)).
5.2. Zircon U-Pb Geochronology
Two samples were collected for LA-ICP-MS U-Pb zircon dating. The results are listed in Table 2. Zircons selected from the medium- to coarse-grained (sample KB-81) and medium- to fine-grained (sample KB-26) rhyolites are mostly display euhedral crystals with clear oscillatory zoning, euhedral, and up to 50 μm in length, with length to width ratios of nearly 2 : 1. Their CL images commonly show oscillatory zoning (Figure 6), which indicated a typical feature of magmatic zircons .
For sample KB-26, 206Pb/238U ages determined from 24 analytical spots ranged from 502 to 523 Ma with a weighted mean age of (, ). For sample KB-81, 206Pb/238U ages determined from 26 analytical spots ranged from 502 to 533 Ma with a weighted mean age of (, ). The age of 512 Ma represents the crystallization time of zircon in the rhyolite. According to the relationship between rhyolite and basalt in the outcrop, the two rocks are the contemporary volcanic rocks, which belong to the early Paleozoic.
5.3. In Situ Zircon Hf Isotopes
Zircon Lu-Hf isotopic data are listed in Table 3. Their initial values were calculated using their U-Pb zircon ages.
Fourteen Hf isotopic spot analyses were conducted on zircons from the rhyolite with radiogenic 176Hf/177Hf(t) ratios varying from 0.282584 to 0.282827, with an average of 0.282694. Their and Hf single-stage model ages () range from 4.5 to 13.0 (with an average of 8.5) and 596–949 Ma (with an average of 790 Ma), respectively.
Fourteen in a total of fifteen Hf isotopic spot analyses were conducted on zircons from the rhyolite with radiogenic 176Hf/177Hf(t) ratios varying from 0.282432 to 0.282870, with an average of 0.282728. Their and Hf single-stage model ages () range from 5.5 to 14.7 (with an average of 10.4) and 538–912 Ma (with an average of 710 Ma), respectively.
5.4. Sr-Nd Isotopic Compositions
The results of Sr-Nd analysis for the rhyolite samples are presented in Table 4. The whole-rock initial 87Sr/86Sr ratios and values have been calculated at on the basis of the zircon U-Pb ages of this study (Figure 6). The initial 87Sr/86Sr ratios for the rhyolite vary from 0.71140 to 0.73287 with 143Nd/144Nd ratios ranging between 0.51207 and 0.51231; they have values of 0.70895–0.71307, values of −6.3 to −2.5, and of 1.94–1.63 Ga. The initial 87Sr/86Sr ratios for basalt vary from 0.70467 to 0.70864 with 143Nd/144Nd ratios ranging between 0.51269 and 0.51277; they have values of 0.70413–0.70817, values of 2.7 to 3.7, and of 1.79–1.59 Ga.
6.1. Geochronological Framework
The North Altyn Tagh subduction-collision complex is one of the most important tectonic units in Western China. The study on Karadaban bimodal volcanic rocks provides new evidences for the early Paleozoic tectonic evolution. In this study, we obtained the zircon LA-ICP-MS U-Pb age conducted on the rhyolite in the northern Altyn region, and the age of volcanic rocks was and , which belongs to early Cambrian. It is an important stage for researchers to study the evolution of the northern Altyn region.
Previous studies show that the magmatic activity in the Altyn area generated a series of magma evolution from basic to felsic rocks, which mainly occurred from 520~480 Ma [29, 30, 56, 76, 77]. The early Paleozoic tectonic belt of Hongliugou-Lapeiquan was recognized, and the results showed that the basic volcanic rocks are mainly with the features of MORB and OIB . In this belt, the LA-ICP-MS zircon U-Pb age of the eutectic gabbro is , which can be classified as a subduction zone ophiolite, and mainly developed in the oceanic subduction belt . Liu et al. (1998) reported that the whole-rock isochron age of the pillow basalt in Hongliuquan area is , which probably represented the forming age of ocean island basalt. Gao et al. (2011) obtained a LA-ICP-MS zircon U-Pb age of from the plagiogranite, which is intrusive in the ophiolite mélange. Chen et al. (2016) obtained the SHRIMP zircon U-Pb age of 482~477 Ma from the felsic volcanic rocks in the Kaladawan area, which indicated that the characteristic of magmatism continued from 520 Ma to 477 Ma. In addition, there are high pressure-ultrahigh pressure (HP/LP) metamorphic zones in the tectonic belt, and the 39Ar–40Ar isochron age of from muscovite in the blueschist showed that the HP/LP was effected by the oceanic subduction. In this paper, we verified that the age of felsic volcanic rocks in Kaladaban is 512 Ma, corresponding with the setting of the north Altyn region. Combining with previous studies, we inferred that there was an ocean basin that existed at the north Altyn region in the late Neoproterozoic and the subduction activities probably continued until the late Ordovician. Since the oceanic crust subduction and collision, a series of subduction-related volcanic rocks were developed in the north Altyn region . The magmatic activity in the northern Altyn region is multistage and probably successive from Neo-Proterozoic to Late Paleozoic.
6.2. Petrogenesis of Bimodal Volcanic Rocks
Bimodal volcanic rocks occur within a variety of tectonic settings, such as continental rifts, oceanic islands, zones of continental break-up, back-arc basins, orogenic belts, intraoceanic island arcs, and mature island arcs/active margins [9, 81, 82]. Recent studies have proposed two models of the genesis of bimodal volcanic rocks, depending on the cognate of the rhyolites and basalts (e.g., ). Since the mafic and felsic magmas originated from different sources, the trace element contents and the Sr and Nd isotopic compositions should differ between the basalts and the rhyolites [84, 85], thereby providing a reliable indicator for the genesis of bimodal volcanic rocks.
Samples from the northern Altyn region have experienced less alteration. The loss of ignition (LOI) values for the mafic and felsic volcanic rocks in the study area are 1.92–6.87 and 0.74–3.68 wt.%, respectively. This alteration is a result of low-temperature metamorphism related to intense deformation and fluid infiltration, whose effects are principally evident in the various types of abundance of the mobile elements such as K2O, Rb, Ba, and Cs. However, the Fe, Al, Ca, Mg, REE, and HFSE elements are considered to be relatively immobile during the low-temperature alteration [65, 85–87]. Accordingly, the following discussion of the magma geochemical composition will focus on the immobile element and their ratios, as well as isotopic compositions.
6.2.1. Petrogenesis of the Mafic Rocks
The low silica content () and relatively high concentrations of TFe2O3 (11.49–17.12 wt.%), Cr (175–215 ppm), and Ni (94–118 ppm) content of the mafic rocks suggest that a mantle component played a prominent role in their genesis . The slight lack of negative Eu anomalies on the chondrite-normalized REE diagrams indicates an absence of plagioclase fractionation during the process of magmatic evolution (Figures 5(a) and 5(b)). Meanwhile, the basalt displays a calc-alkaline nature and is enriched in LREEs and is also depleted in HFSEs (e.g., Nb and Ta). These features are similar to the subduction-related magmas . However, the basaltic samples show MgO, Cr, Ni, V, and Co are negatively correlated with SiO2, indicating that the basaltic magma is not primitive and possibly experienced some fractional crystallization of pyroxene. This is consistent with the presence of pyroxene as the dominant phenocryst in the basalt. The Karadaban basalt is enriched in Th (4.08–11.90 ppm) and positively correlated the plots of Nb/La and Nb/Th (Figure 7(a)). So we conjectured the involvement of Th-enriched component in its petrogenesis, which is due to the crustal contamination . The continental crust is typically depleted in Nb and Ta , and the upper continental crust is enriched in La and Th, while the lower continental crust is not always enriched in Th . Combining with the (Th/Ta)PM vs. (La/Ta)PM plot, the basalt samples define an UCC trend (Figure 7(b); ), implying that the contaminator is the highly evolved upper continental crust. It is noted that mafic magmas derived from the asthenosphere typically have La/Nb ratios of <1.5 and La/Ta ratios of <22, whereas those rocks from lithosphere have and [93, 94]. High La/Nb ratios (2.15–3.36), La/Ta ratios (36.57–53.49), and positive values (+2.7 to +3.7) indicate that the basalt was derived from a depleted mantle source but mixed with crustal components while upwelling. Mantle-derived magmas generally assimilate crustal materials during their ascent or their storage in a crustal magma chamber . The Nb/U values (2.16–9.13) are lower than those of the source mantle (34), MORB (), or OIB (), further indicating that the basalt was affected by the crustal contamination . Moreover, the mafic rocks from the Karadaban area are Nb-Ta depleted but enriched in Zr, Hf, and Ti (Figure 5(b)), yielding patterns similar to those expected for crustal material which suggests that these magmas assimilated crustal material during ascent .
Excluding the garnet residue in the source, the Nb/Yb and Ta/Yb ratios are effective in constraining the relative depletion of the mantle source when the degree of partial melting is even lower [70, 98]. Meanwhile, the Th/Yb ratio is a sensitive indicator of the inclusion of subduction zone materials in magma [87, 91]. The Th/Yb vs. Nb/Yb diagram (Figure 8) indicates that the basalt samples in the present study developed in an active continental margin, mainly close to the boundary between the E-MORB and OIB arrays. In summary, the geochemical and Sr-Nd isotope data demonstrate that the basalt of the Karadaban bimodal volcanic rocks was derived from the depleted mantle source and experienced fractional crystallization, which was dominated by the fractionation of pyroxene and crustal contamination possibly by upper crustal materials.
6.2.2. Petrogenesis of the Felsic Rocks
Felsic volcanic rocks are characterized by high SiO2, K2O+Na2O, and Fe2O3T content; low Al2O3, CaO, and MgO content; depletion in HFSEs (e.g., Nb, Ta, P, Ti, and Hf); and strong negative Eu anomalies. These compositional features suggest that the samples in the present study originated from a source that lacked garnet but contained plagioclase in the residue, which indicates partial melting under low-pressure conditions [100, 101]. The absence of Ba, Nb, Ta, Sr, P, and Ti, as shown in the primitive-mantle-normalized trace-element spider diagram (Figure 5(d)), indicates similar trace element composition of A-type granites , and the felsic rocks generally display similar patterns to the basalts. However, the Nb/Ta and Th/Ta ratios of the felsic rocks are low, suggesting that the rhyolite is contaminated by upwelling basaltic magma. Furthermore, the Karadaban felsic samples fall into the A-type granite area on a Ga/Al-Zr plot (Figure 9(a)), and the Yb/Ta and Y/Nb ratios of the felsic samples are higher than those of OIB and close to IAB (Figure 9(b)). This indicated that the felsic rocks are A-type granites and exhibit similarities to the crust between OIB and IAB. Based on the subdivision of A-type granites , the Karadaban felsic samples are type A2 (Figure 10); however, the other granites (e.g., Highland 4337 and Hongliugou) show I-type features. It coincides with the amount of island arc granitoid developed in the north of Karadaban volcanic rocks [32, 57]. Hence, we suggest the Karadaban felsic rocks probably generated in an extensional environment.
Two types of process have been proposed for the origin of the silicic end-member in the bimodal magmatic suite, including (1) extensive fractional crystallization from a common mantle-derived magma parental to the mafic end-member, coupled with crustal contamination , and (2) crustal anatexis caused by mantle-derived mafic magma with distinct isotopic compositions [104–106]. In the case of the felsic rocks, the distinct difference documented by the whole-rock and zircon values between the basalt and felsic rocks indicated that the basalt could not have been produced by fractional crystallization of the coeval basalt. However, the negative (−6.3 to −2.5) and positive (4.5 to 14.7) of Karadaban felsic rocks showed Nd-Hf isotopic decoupling. The decoupling was probably effected by fluid/melt metasomatism in the subduction zone; the solubility of Nd was higher than that of Hf and led to the higher Nd/Hf ratios in the fluid/melt derived from the subducted plate [107, 108]. Coincidentally, from Cambrian to Ordovician, the whole north Altyn region was in a subduction setting, and a great deal of crustal growth occurred through arc-related magmatism (Table 5). For the Nd-Hf isotopic decoupling, the Hf isotope composition of zircon is more realistic than Nd. The positive zircon (4.5 to 14.7) and ages of 551 to 1186 Ma suggest a juvenile source rather than an old basement rocks. In addition, the wide range of indicated the mixture of the end-member during the magma evolution . When the subduction metasomatized mantle-derived felsic crustal materials were rejuvenated, the newly formed magmas would inherit the decoupled Nd-Hf characteristics such as Karadaban rhyolite in accordance with A2-type granites representing magmas derived from the continental crust which has been through a cycle of island-arc magmatism (Figures 11 and 12; ). As presented above, the continued extension of the crust and repeated underplating of subsequent mafic magma partial melting of the juvenile crust to generate felsic melts will ascend and erupt on the surface forming the Karadaban rhyolite.
Based on the discussion above, we conclude that the felsic volcanic rocks produced mainly by partial melting of the juvenile crustal and the origin melt source itself likely resulted from mantle-derived magmatic upwelling in an extensional setting, probably near the subduction-related active continental margin.
6.3. Tectonic Implications
The north Altyn region is a part of the Proto-Tethys tectonic domain. The Cambrian-Ordovician basic rocks and calc-alkaline granite in this region are interpreted to be island arc volcanic rocks [34, 35]. SHRIMP zircon ages of for a granodiorite and for arc granite indicate that the Cambrian to early Ordovician age is related to arc magmatism [30, 31]. In addition, bimodal volcanic rocks indicate an extensional setting in which they form, and several types of the extensional setting can account for the generation of bimodal volcanic suite, such as within-plate (continental rifting), passive continental marginal rifting, and incipient back-arc or continental active margins . Hence, we propose that the Karadaban bimodal volcanism could be regarded as a significant indication of arc-related extensional tectonics.
On the one hand, according to the feature of trace element, the Karadaban volcanic suite displays some geochemical features of subduction-related melts, such as enrichment of LILEs but depletion in HFSEs and Sr-Nd-Hf isotope compositions of magmas. This suggests that the mixing of juvenile crust and mantle-derived magma, as well as the Karadaban bimodal volcanic suite, was formed in a subduction-related environment. The conclusion is further supported by a series of tectonomagmatic discrimination diagrams. Based on the Hf-Th-Nb and Zr-Ti-Y ternary diagrams, the Karadaban rocks are classified as volcanic arc calc-alkaline basalts (Figure 13), which support the hypothesis that magmatic traces existed in the study area . The Karadaban basalts exhibit both arc-like and within-plate basalt affinities, implying that they are probably formed within a back-arc tectonic setting. Generally, the Kaladaban bimodal volcanic suite in the north Altyn region is likely an analogue of the Dzungaria Ocean in the southern Central Asian Orogenic Belt during the late Paleozoic . The Karadaban bimodal volcanic rocks resemble those of the back-arc side of the Dzungaria arc, which are believed to result from the initiation opening of the Dzungaria Ocean [70, 112]. On the other hand, regional geological data suggest that the final closure of the Altyn Ocean did not occur until the Early Ordovician and even until the later Ordovician in the north Altyn region [24, 35]. This demonstrates that the Kaladaban bimodal volcanism was produced in a back-arc environment (Lapeiquan back-arc basin), rather than in the postorogenic or intracontinental rifting setting.
The Altyn region is of great significance to realize the evolution of the Tibetan Plateau and the tectonic pattern of Western China. It developed as a unified back-arc basin system between the northern Altyn Complex and MJB before the formation of the Altyn Tagh, which is the northern boundary of the Proto-Tethys in China . Three stages can be identified by previous studies: north Altyn Oceanic rifting (~750 Ma) [47, 114], north Altyn oceanic subduction (520~460 Ma) [34, 76, 77, 115], and oceanic consumption followed by continental collision (440~420 Ma) [29, 109, 116, 117]. This paper mainly focuses on the study of the bimodal volcanic rocks developed during the subduction of the northern Altyn Ocean. Combined with previous studies, the subduction is presumed to have occurred around 520~460 Ma (Table 5) [19, 77, 117] from the plentiful research of regional magmatic sequences to the mafic to intermediate to felsic rocks [31, 46, 109]. Based on our new data and previous studies, we propose a model for the tectonic evolutions of the north Altyn region during the Early Cambrian. This model emphasizes a southward subduction system of the oceanic lithosphere (Altyn Ocean) between the NASC and MJB. Beneath the MJB, its consolidated NASC margin during the early Paleozoic period and an arc and back-arc system developed, resembling the present western Pacific continental margin, appearing during the early Cambrian [24, 35, 46]. The Karadaban bimodal volcanic rocks formed in the incipient stage of the Cambrian back-arc basin/extension. The system might have lasted into the Late Ordovician, during which the back-arc basin gradually became mature and finally closed during the Early Silurian. In conclusion, the north Altyn oceanic crust subducted towards the MJB and transmitted the latter into the active continental margin. The magma, which derived from the upwelling and mixing caused by subduction, eventually emplaced into the crust as Kaladaban bimodal volcanic rocks.
The following conclusions can be drawn from this study. (1)New LA-ICP-MS zircon U-Pb dating indicates that the newly discovered bimodal volcanic rocks in the northern Altyn Tagh area formed at ca. 512 Ma(2)Whole-rock geochemical and Sr-Nd-Hf isotopic compositions suggest that the mafic rocks were derived mainly from a depleted mantle source but mixed with crustal components. The felsic rocks were probably generated by partial melting of the juvenile crustal as a result of basic magma upwelling(3)The generation of the Karadaban bimodal volcanic rocks was not only related to an active continental margin setting but also associated with the subduction-related back-arc extension environment
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This study was financially supported by the Development and Research Centre of the China Geological Survey, Beijing (Grant No. 12120113090000).
Supplementary Table 1: major (%) and trace element (ppm) data for the volcanic rocks in the northern Altyn Tagh region. Supplementary Table 2: LA-ICP-MS zircon U-Pb dating data for Karadaban rhyolite (KB-26, KB-81) from the northern Altyn Tagh region. Supplementary Table 3: zircon Lu-Hf isotopic data for rhyolite (KB-26, KB-81) from the northern Altyn Tagh region. Supplementary Table 4: Rb-Sr and Sm-Nd isotopic compositions for the volcanic rocks in the northern Altyn Tagh region. (Supplementary Materials)
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