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Geochemical Composition Variations and Tectonic Implications of the Baoligaomiao Formation Volcanic Rocks from the Uliastai Continental Margin, Southeast Central Asian Orogenic Belt
Lithosphere ( IF 2.4 ) Pub Date : 2023-01-09 , DOI: 10.2113/2023/lithosphere_2023_176 Jianzhou Tang 1, 2, 3 , Zhicheng Zhang 2, 3 , Zejia Ji 2, 3
Lithosphere ( IF 2.4 ) Pub Date : 2023-01-09 , DOI: 10.2113/2023/lithosphere_2023_176 Jianzhou Tang 1, 2, 3 , Zhicheng Zhang 2, 3 , Zejia Ji 2, 3
Affiliation
The Permo-Carboniferous tectonic evolution in the Uliastai continental margin (UCM), north of the southeast central Asian Orogenic Belt, remains controversial. This work examined the geochemical composition of the felsic volcanic rocks from the lower and upper part of the Baoligaomiao Formation in the UCM. Zircon U-Pb ages reveal that the Baoligaomiao Formation has a long-lived eruption duration, from ca. 285 to 328 Ma. The lower part (ca. 328–310 Ma) of the Baoligaomiao Formation is dominated by clastic and pyroclastic rocks with subordinate intermediate-felsic volcanic rocks, whereas the upper part (ca. 307–285 Ma) mainly consists of felsic volcanic rocks and pyroclastic rocks. Calculations reveal that the felsic volcanic rocks from the lower part have low zircon saturation temperatures (TZr = 747℃–795℃), whereas those from the upper part exhibit high TZr (ca. 793℃–930℃). Zircons from the lower part exhibit high εHf(t) values and 176Lu/177Hf ratios, in contrast to the low εHf(t) values and 176Lu/177Hf ratios of zircons from the upper part. Those petrogeological and geochemical shifts might support the tectonic switch model in the UCM at the end of the Carboniferous, providing new constraints on the Late Carboniferous closure of the Hegenshan Ocean.The Central Asian Orogenic Belt is one of the largest fossil accretionary orogenic belt worldwide [1], which is surrounded by Siberian Craton in the north and North China and Tarim Craton in the south (Figure 1(a)). There is a controversy on the Permo-Carboniferous tectonic evolution of the Uliastai continental margin (UCM) in the southeast Central Asian Orogenic Belt (SE CAOB; Figure 1(b)) [2-4]. This dispute might be diffused by the two phases of rift-related volcanism from the Baoligaomiao Formation in the UCM. Some scholars argue that the two phases of rift-related volcanism are formed in different tectonic settings, the former is subduction-related, and the latter is post-accretionary/collision-related [4-7]. Alternatively, others suggest two phases of rift-related volcanism are continuous, indicating a long-lived rifting process from Late Carboniferous to Early Permian [2, 8]. Thus, it is important to examine the differences between the two phases of volcanic rocks, exploring whether there was a tectonic switch during the end of the Carboniferous.The Baoligaomiao Formation volcanism in the UCM has a long-lived eruption duration from Late Carboniferous to Early Permian [6, 9-13]. Rock association and whole-rock geochemistry variations of this formation have been identified and used to delineate the tectonic switch in the UCM [4, 6]. Recent studies indicate that the joint using of zircon U-Pb and Hf isotope data can also infer a regional tectonic evolution [14, 15]. Zircon 176Lu/177Hf ratios were sensitive proxies of magma crystallization pressure; zircon crystallizes deeper have low 176Lu/177Hf ratios, whereas those crystallizes shallower have high 176Lu/177Hf ratios [14]. In addition, zircon εHf(t) values are mainly restrained by the relative input of depleted mantle and crustal components [15, 16]. Thus, zircons from the Baoligaomiao Formation volcanic rocks are potential proxies to explore the Permo-Carboniferous tectonic evolution of the UCM.In this contribution, this work presented and compiled zircon U-Pb and Lu-Hf isotope, and whole-rock elements data of the Baoligaomiao Formation volcanic rocks to explore whether there was a tectonic switch in the UCM at the end of Carboniferous.Five tectonic units have been divided in the SE CAOB (Figure 1b), from north to south, the UCM, Hegenshan-Erenhot ophiolite belt (EHOB), North accretionary orogen (NAO), Solonker zone (SZ), and Southern accretionary orogen (SAO).Two episodic spreading events have been identified in the SZ, featured by abundant mafic-ultramafic rocks [3, 17-19]. The NAO and SAO have Cambrian to Early Devonian arc magmatic rocks, which recorded the subduction of the Paleo-Asian Ocean along the SZ [18, 19]. Following a ca. 50 Ma magmatic and depositional hiatus, the NAO was reactivated by Late Carboniferous and Early Permian magmatism, whereas the SAO was dominated by carbonate-clastic deposition. The Paleo-Asian Ocean closed at the end of the Permian, implying the final suture of the CAOB [20].The EHOB documented the spreading and subduction of the Hegenshan Ocean, distinguished by the Carboniferous mafic-ultramafic rocks [3, 21]. Those mafic-ultramafic rocks were covered by Permian sedimentary rocks [22]. Silurian and Devonian limestones, mudstone, and siltstone from the UCM indicate a passive continental margin deposition environment [23]. In the Early Carboniferous, the UCM switched into an active continental margin setting caused by the northward subduction of the Hegenshan Ocean [24]. The passive continental margin deposition was covered by the Hongaobao Formation and Baoligaomiao Formation volcano-clastic deposition [2, 23].The Baoligaomiao Formation, or called as Baolige Formation, is mainly distributed along the ChaganObo-Dongwuqi areas in the UCM [9, 11, 12, 25, 26]. This formation is a suite of volcano-clastic deposition (Figure 2(a)–2(c)), consisting of andesite, rhyolite, pyroclastic rock, mudstone, siltstone, and sandstone [13, 26]. The lower part of the Baoligaomiao Formation from the Wudengmaode area has a large volume of pyroclastic rocks and andesite (Figure 2(b)), whereas other areas (e.g., ChaganObo and Dongwuqi) were dominated by clastic deposition (Figure 2(a) and 2(c)). The upper part is mainly composed of rhyolite and pyroclastic rock [9, 10]. In the east of the Dongwuqi area, the Baoligaomiao Formation is sporadically exposed and dominated by clastic deposition [9]. Plant fossils from the Baoligaomiao Formation imply an Upper Carboniferous to Lower Permian depositional age [9]. In accordance with the fossil records, zircon dating results also gave Permo-Carboniferous ages [9, 25, 26].Eighteen volcanic rock samples, including 2 andesitic, 2 dacitic, and 13 rhyolitic volcanic rock samples, were collected from the Baoligaomiao Formation in the ChaganObo area (Figure 2 and Figure 3). The andesitic rocks are brown in color (Figure 4(a)), whereas the rhyolitic and dacitic rocks are yellowish-white (Figure 4(b)). Phenocrysts of rhyolite samples mainly consist of quartz, alkaline-feldspar, and plagioclase (Figure 4(c) and 4(d)). In addition, biotite phenocrysts were observed in rhyolite sample NM18-140 (Figure 4(e)). The phenocryst of andesitic rock samples was mainly composed of plagioclase with subordinate hornblende (Figure 4(f)).Eighteen volcanic rock samples were selected for whole-rock elements geochemistry analysis. Among them, six samples were dated and conducted to zircon Lu-Hf isotope analysis.Zircon U-Pb dating of volcanic rock samples was conducted on the laser ablation-inductively coupled plasma mass spectrometry (LA‒ICP‒MS) with frequency of 10 Hz and a beam of 32 µm at the School of Earth and Space Sciences, Peking University (SESS-PKU). Harvard sample 91,500 (ca. 337 Ma) was used as a reference. Detailed procedures can be found in Sun et al. [27]. In-situ zircon Lu-Hf isotope analysis using MC-ICP-MS (Neptune) equipped with a New Wave UP213 laser ablation system at the SESS-PKU. The spot size of Lu-Hf analysis was ca. 44 µm, and the zircon Plešovice was the reference (176Hf/177Hf = 0.282482 ± 13). Detailed analytical procedures can be found in Zhang, Ireland, Zhang, GaoandSong [28].The whole-rock elemental geochemistry was conducted at the SESS-PKU. Whole-rock major elements were analyzed on X-ray fluorescence spectrometer, and the trace elements were analyzed on Agilent 7500 inductively coupled plasma mass. The analytical procedures are the same as those of Sun et al. [27]. The analytical uncertainty of the major elements was ±1%, whereas that of the trace elements was ±5%.The analyzed zircons are euhedral and long-columnar (Figure 5). They show oscillatory zones and high Th/U ratios (>0.32). These features indicate that they are of magmatic origin. Zircon dating results were documented in online supplementary table S1, and the weighted mean ages reveal the dated Baoligaomiao Formation volcanic rocks formed at ca. 328–289 Ma.Forty zircon grains from rhyolite sample NM18-140 were analyzed, and 38 of them were concordant. Apart from two grains showing young 206Pb/238U ages (305 and 309 Ma), the other 36 grains gave a weighted mean age of 328 ± 2 Ma (Figure 5(a)). The very few young-concordant and discordant ages could be caused by the Pb-loss resulting from the hydrothermal alteration [29].Forty zircon grains from rhyolite sample NM18-118 were analyzed, and 36 of them are concordant. The 35 youngest analyses gave a weighted mean age of 298 ± 2 Ma (Figure 5(b)). The old age of 321 Ma could be from the captured zircon.Thirty-one zircon grains from rhyolite sample NM18-120 were analyzed, and 29 of them were concordant. The 29 analyses gave a weighted mean age of 289 ± 2 Ma (Figure 5(c)).Thirty-nine zircon grains from rhyolite sample NM18-122 were analyzed, and all of them were concordant. The 39 analyses gave a weighted mean age of 301 ± 2 Ma (Figure 5(d)).Thirty zircons from sample NM18-127 were analyzed, and 20 of them were concordant. Most of the analyses (n = 18) exhibited similar 206Pb/238U ages of 295–310 Ma, with a weighted mean age of 302 ± 3 Ma (Figure 5(e)). In addition, one young 206Pb/238U age of 275 Ma and one old 206Pb/238U age of 341 Ma were also obtained. The youngest concordant age (275 Ma) could be related to hydrothermal alteration [29].Forty-one grains from rhyolite sample NM18-137 were analyzed, and 37 of them were concordant. Thirty-six youngest concordant analyses gave a weighted mean age of 303 ± 2 Ma (Figure 5(f)), and the other grain has an old 206Pb/238U age of 351 Ma.The whole-rock geochemistry data can be found in online supplementary table S2. Except for two andesite samples with high loss-on-ignition (3.16 and 3.53 wt.%), other rhyolite and dacite samples show low loss-on-ignition (LOI < 2.52 wt.%).Rhyolite sample NM18-140 from the lower part of the Baoligaomiao Formation exhibits high SiO2 (74.17 wt.%), Al2O3 (12.72 wt.%), Na2O (4.39 wt.%), and K2O (4.07 wt.%) contents and low TFe2O3 (1.08 wt.%), MgO (0.28 wt.%), and TiO2 (0.29 wt.%) contents. This sample plots within the subalkaline field and belongs to medium-K calc-alkaline (Figure 6(a) and 6(b)). In the N-MORB (normal middle ocean ridge basalt) normalized spider diagrams, they show significant Ba-Sr-Eu and Nb-Ta negative anomalies (Figure 7(a)). The negative Eu anomaly can also be observed in the Chondrite-normalized rare earth elements pattern (Figure 7(b)).The volcanic rocks from the upper part of the Baoligaomiao Formation are dominated by rhyolite and dacite with subordinate andesitic rocks (Figure 6(a)). Except for two andesite samples that have low SiO2 contents (57.71 and 58.51 wt.%), other samples show high SiO2 (64.47, 81.95 wt.%) contents. Likewise, the andesitic samples exhibit high TFe2O3 (7.55 and 9.46 wt.%), MgO (2.42 and 2.66 wt.%), and CaO (2.47 and 4.95 wt.%) contents, whereas dacite and rhyolite samples have relatively low contents of TFe2O3 (0.18, 5.40 wt.%), MgO (0.13, 1.45 wt.%), and CaO (0.02, 1.82 wt.%). The K2O+Na2O contents of the Baoligaomiao Formation volcanic rock samples range from 5.24 to 9.83 wt.%, and most of them plot within the subalkaline field (Figure 6(a)). The andesitic samples belong to the medium-K calc-alkaline series, whereas the rhyolitic samples are variable in the K2O contents (Figure 6(b)). The dacite and rhyolite samples have similar trace elements and rare earth element patterns, with distinct Nb-Ta and Sr negative anomalies and variable Ba and Eu anomalies (Figure 7(c) and 7(d)). In contrast, the andesite samples show no negative Ba-Sr-Eu anomalies in the N-MORB normalized trace elements pattern, but negative Nb-Ta anomalies can be identified (Figure 7(c) and 7(d)).Five samples were selected for zircon Lu-Hf analysis, and all of them show depleted Hf isotopic composition, see online supplementary table S3.Eight zircons from rhyolite sample NM18-120 were analyzed. Among them, seven grains have high 176Yb/177Hf values (>0.1). All of them show low 176Lu/177Hf ratios (0.002, 0.005) and positive εHf(t) values (+12.67 to +20.14).Six zircons from rhyolite sample NM18-118 exhibit high 176Yb/177Hf values (>0.1) and 176Lu/177Hf ratios (0.004, 0.006), and positive εHf(t) values (+6.23 to +13.36).Ten zircons from rhyolite sample NM18-122 show low 176Lu/177Hf ratios (0.001, 0.003). Among them, eight grains show low 176Yb/177Hf values (<0.1) and positive εHf(t) values (+7.56 to +14.81).Nine zircons from rhyolite sample NM18-137 show low 176Lu/177Hf ratios (0.001, 0.003), and eight of them exhibit low 176Yb/177Hf values (< 0.1). The eight grains exhibit positive εHf(t) values (+8.34 to +16.80).Seven zircons from rhyolite sample NM18-140 have low 176Lu/177Hf ratios (0.002, 0.004), and five of them exhibit low 176Yb/177Hf ratios (< 0.1). The five grains have positive εHf(t) values (+10.22 to +14.99).Except for zircon grains from samples NM18-120 and NM18-118 have high 176Yb/177Hf ratios (>0.1), most zircons from the other three samples have low 176Yb/177Hf ratios. Therefore, it should be cautious when using the εHf(t) values from samples NM18-120 and NM18-118.All rhyolite samples from Baoligaomiao Formation show negative Sr-Ba-Eu anomalies (Figure 7), indicating the fractional crystallization of feldspars. This is consistent with the observations of plagioclase and alkali-feldspars phenocrysts under the microscope.Rhyolite sample NM18-140 from the lower sequence of the Baoligaomiao Formation has a high Ga/Al ratio and Zr+Nb+Ce+Y content, plotting within the A-type field (Figure 6(c)). The A-type rhyolites have also been identified in the south of the ChaganObo area [6]. In addition, calculations suggest that the sample NM18-140 shows high zircon saturation temperatures of 826℃ [30]. The A-type rhyolite might be derived from the melting of underplated mafic rocks or juvenile crust, as indicated by the depleted zircon Hf isotopic composition. However, in the ternary diagram, this sample overlaps with the partial melts of tonalites (Figure 6(d)). Thus, the precursor of sample NM18-140 is the partial melt of the juvenile felsic crust.Except for three samples (NM18-118, NM18-127, and NM18-138) plotting within I, S, and M type field, most rhyolite samples from the upper part also show high Ga/Al ratios and Zr+Nb+Ce+Y contents, implying the affinities of A-type granites (Figure 6(c)). Likewise, the three samples with low Ga/Al ratios show relatively low zircon saturation temperatures (784℃, 809℃, and 816℃), in contrast to the high zircon saturation temperatures (827℃, 914℃) from other samples with high Ga/Al ratios [30]. In the ternary diagram, the three samples exhibit low K2O/Na2O ratios and overlap with the partial melts of mafic rocks or tonalites, while other samples with high Ga/Al ratios overlap with the partial melts of igneous rocks (mafic rocks and tonalities) and meta-sedimentary rocks (Figure 6(d)). Thus, most of the samples belong to the A-type rhyolite with subordinate I-type or I/A-type rhyolite. In any case, these rhyolite samples have depleted zircon Hf isotopic composition, indicative of the juvenile protolith.Andesite samples from the upper part of the Baoligaomiao Formation show negative Nb-Ta anomalies, meaning the partial melts from the metasomatized lithospheric mantle without the breakdown of rutile [31]. Enrichments of large ion lithophile elements and light rare earth elements suggest the involvement of fluids and/or melts from the subducted oceanic slab.The Baoligaomiao Formation has a long-lived eruption duration, mainly from ca. 328 to 285 Ma (Figure 8(a)), which recorded the tectonic evolution of the UCM [5, 6]. Volcanic rocks from the lower part have U-Pb ages of ca. 310–328 Ma, whereas those from the upper part have U-Pb ages of ca. 307–285 Ma.The A-type rhyolites are found in both the lower and upper part of the Baoligaomiao Formation [6]. Therefore, the Baoligaomiao Formation erupted at an extensional tectonic setting. However, there are some differences between the volcanic rock association, which have been linked with the switch of tectonic setting [5, 6]. In the lower part, the volcanic rocks are dominated by andesite, while the andesite is minor in the upper part [6, 9]. Although both the A-type and I-type felsic volcanic rocks occurred both in the Upper Carboniferous and Lower Permian, the Upper Carboniferous were dominated by I-type rocks and the Lower Permian mainly consisted of A-type rocks [4].Our compilation suggests that zircon saturation temperatures (TZr) and Lu-Hf isotopic composition from the felsic rocks show a significant shift at ca. 310–305 Ma (Figure 8(b)–8(d)). Felsic volcanic rocks from the lower part have low average TZr (ca. 747℃–795 ℃), contrasting to the high average TZr (ca. 793℃–930 ℃) of felsic rocks from the upper part. Although the increase of the zircon saturation temperatures (Figure 8(b)) and the volume of A-type felsic rocks can be linked with a long-lived rifting process, the shifts of zircon 176Lu/177Hf ratios and εHf(t) values disagree with this model (Figure 8(c) and 8(d)). Moreira et al. [14] suggest that the zircons with low 176Lu/177Hf ratios could be crystallized at a compressional tectonic setting, whereas those with high 176Lu/177Hf ratios crystallized at an extensional tectonic setting. However, the explanation of Moreira et al. [14] might not be well matched with the scenario of the UCM, as the presence of A-type rhyolite both in the upper and lower part. Instead, the lower zircon 176Lu/177Hf ratios from the upper part of the Baoligaomiao Formation might imply a more thickened crust origin, consistent with the decrease in zircon εHf(t) values, which has been linked with the collision of arc/continent and continent and more involvement of crustal materials [15, 16].Ophiolites (with a U-Pb age of ca. 336 Ma) from Diyanmiao have similar petrology and geochemistry nature to those from Izu-Bonin-Mariana, and northward subduction initiation of the Hegenshan Ocean has been proposed [24]. The northward subduction of the Hegenshan Ocean would construct continental arcs in the UCM along with the slab descending and formation of mantle wedge [24, 32]. Basaltic andesite and andesite (ca. 326–314 Ma) from the lower part of the Baoligaomiao Formation in the south of ChaganObo are enriched in large ion lithophile elements and light rare earth elements and depleted in high field-strength elements, which represent the Early Carboniferous subduction-related, mature, continental arc volcanism [5, 6]. The ChaganObo A-type rhyolite (ca. 326–318 Ma) from the lower part was also subduction-related, indicative of local extension caused by trench retreat [5, 6]. Monzogranites and granite porphyries with age of ca. 336–320 Ma from Wudengmaode belong to the calc-alkaline I-type granites and are generated by the partial melting of juvenile crust in a north-dipping subduction-related environment [4]. The lower volcanic rocks from the Baoligaomiao Formation show relatively uniform zircon Hf isotopic compositions, which could mean minor involvement of crustal materials during the nascent subduction [32]. The low TZr (ca. 747℃–795°C) from the lower part of the Baoligaomiao Formation is compatible with the continental arc environment [6]. Those observations might indicate the lower part of the Baoligaomiao Formation erupted in a continental arc environment.Although there are andesites in the upper part of the Baoligaomiao Formation, which are minor and have been explained as the partial melting of the metasomatized lithosphere mantle during the post-collision or post-accretionary extensional setting [6]. Field investigations show that the mafic-ultramafic rocks are unconformably covered by the Lower Permian Gegenaobao Formation in the Hegenshan area, and a Late Carboniferous emplacement timing of the ophiolites from the EHOB was proposed [22]. Provenance analyses from the Lower Permian sandstones reveal that the north of the Sonid Zuoqi area received multiple detritus from Mongolia arcs, NAO, and microcontinents [23]. The magmatic gap at ca. 314–307 Ma in the EHOB has been linked with the closure of the Hegenshan Ocean [4, 7]. An undeformed dioritic dike with the age of ca. 314 Ma intruded the highly deformed Carboniferous strata in the south of ChaganObo [21]. In addition, the hybridization of the cold-water flora and warm-water flora was identified in the upper part of the Baoligaomiao Formation, implying the Hegenshan Ocean was not a significant geographical barrier at that time [9]. Early Permian rift basins in the UCM and EHOB were regarded as the products of post-collision/accretionary extension [2, 23]. The high TZr (ca. 793℃–930℃) from the upper part of Baoligaomiao Formation is also consistent with an extensional setting. Hence, it is reliable that the upper part of Baoligaomiao Formation was generated in a post-accretionary extensional setting.In summary, the petrogeology and geochemistry differences between the lower and upper part of the Baoligaomiao Formation volcanic rocks could be consistent with the previous inferences [4, 6, 7], supporting a tectonic switch model, from the Carboniferous subduction to the Early Permian post-accretionary extension (Figure 9).Whole-rock geochemistry and zircon U-Pb and Lu-Hf isotope data from the Baoligaomiao Formation felsic volcanic rocks provide new insights into the Permo-Carboniferous tectonic switch in the UCM. A compiled zircon U-Pb age dataset suggests that the Baoligaomiao Formation volcanism has a long-lived eruption duration from ca. 328 to 285 Ma. The differences in zircon Hf isotope and whole-rock geochemistry between the lower and upper part of the Baoligaomiao Formation felsic volcanic rocks support a tectonic switch model from the Carboniferous subduction to the Early Permian post-accretionary extension.The data of this study is available in the supplementary material.The authors declare that they have no conflicts of interest.We thank two anonymous reviewers for their constructive comments, which led to significant improvements in the quality of our paper. This work was financially supported by the National Key Research and Development Project of China (2017YFC0601302), SINOPEC Petroleum Exploration and Production Research Institute (8410102109) and the National Natural Science Foundation of China (42102128).Supplementary table 1: Zircon U-Pb dating results from the Baoligaomiao Formation volcanic rocks. Supplementary table 2: Major and trace elements contents from the Baoligaomiao Formation volcanic rocks. Supplementary table 3. Zircon Hf isotopic composition of the Baoligaomiao Formation volcanic rocks.
更新日期：2023-01-09
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