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1.
The paper presents new U–Pb zircon ages and geochemical data from early Carboniferous volcanic rocks of the Wuerkashier Mountains in the northern West Junggar region, NW China, and of the Char suture–shear zone in East Kazakhstan. The study included analysis of geological setting, major and trace elements, and rock petrogenesis. Both localities host early Carboniferous volcanic units dominated by plagioclase-porphyry andesites and dacites. A West Junggar dacite yielded a 206Pb/238U age of 331 ± 3 Ma. The Junggar volcanic rocks are tholeiitic, and the Char samples are intermediate between tholeiitic and calc-alkaline. Both the Junggar and Char volcanic units are characterized by LREE enriched rare-earth spectra (La/Smn = 1.1–2.4) with Eu negative anomalies (Eu/Eu* = 0.12–1.0) and Nb-Ta minimums (Nb/Thpm = 0.15–0.35; Nb/Lapm = 0.3–0.7) on multi-element spectra. The Junggar andesites and dacites have higher REE and HFSE (Ti, Nb, Zr, Y, and Th) compared with the Char rocks, suggesting their derivation from a different mantle source. The melting modelling in the Nb-Yb system showed that the Junggar volcanic rocks formed by low- to medium- (2–5%) degree melting of depleted mantle harzburgite and spinel lherzolite. The Char volcanic rocks formed by high-degree melting (15–20%) of spinel lherzolite and garnet-bearing peridotite. The regional geology of West Junggar and East Kazakhstan and the geochemical features of the rocks under study (i.e. depletion in Nb, Ta, and Ti and enrichment in Th, and combination of LREE enrichment and HFSE depletion) all suggest a subduction-related origin of both Junggar and Char volcanic rocks. The early Carboniferous volcanic rocks of West Junggar possibly formed by subduction of the Junggar-Balkhash ocean beneath an active margin of the Kazakhstan continent, whereas those of East Kazakhstan formed by subduction of the Irtysh-Zaisan Ocean beneath an intra-oceanic arc at the active margin of the Siberian continent.  相似文献   

2.
Zircon U–Pb geochronological and geochemical analyses are reported for a suite of the middle Silurian volcanic rocks from northern West Junggar (NW China), southern Central Asian Orogenic Belt (CAOB), with the aim to investigate the sources, petrogenesis, and tectonic implications. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb analysis from an andesite yielded a concordant weighted mean 206Pb/238U age of 429 ± 3 Ma, indicating the presence of middle Silurian volcanic rocks in northern West Junggar. The andesite is tholeiite series and characterized by minor variations in compositions (SiO2 = 55.68–59.17 wt.%, Al2O3 = 14.56–17.7 wt.%, TiO2 = 0.55–1.23 wt.%, Na2O + K2O = 3.46–7.16 wt.%, and P2O5 = 0.15–0.37 wt.%), with wider MgO content (2.18–6.48 wt.%) and Mg# (57.4–77.9). All andesitic rocks are enriched in large-ion lithophile elements (LILEs; e.g. Rb, Ba, K, and Th) and light rare earth elements (LREEs), but strongly depleted in some high field strength elements (HFSEs; e.g. Nb, Ta and Ti), with slight negative Eu anomalies (Eu/Eu* = 0.8–1). These features suggest that the andesitic magmas were derived from 2–8% partial melting of a garnet lherzolite depleted mantle source with subducted sediments metasomatized by slab-derived fluids. Combining the current study with those data in existing literature, we conclude that the middle Silurian volcanic rocks formed in an intra-oceanic subduction setting during consumption of the Irtysh–Zaysan Ocean, and further confirm the eastern extension of the early Palaeozoic Boshchekul–Chingiz volcanic arc of East Kazakhstan in China.  相似文献   

3.
ABSTRACT

Volcanic rocks in the Hala’alate and Aladeyikesai formations, which are composed of basaltic andesite and pyroxene andesite, are widespread in Hala’alate Mountain, West Junggar, Northwest China. These rocks (plagioclase + clinopyroxene/olivine) formed in the late Carboniferous and show a remarkable geochemical affinity with typical sanukitoids with oversaturated SiO2 (52.9–56.9 wt.%) and high MgO (3.47–6.88 wt.%, Mg# >48) contents. They also exhibit a narrow range of Sr-Nd-Pb isotopes within (87Sr/86Sr)i = 0.7037–0.7041, εNd(t) = 4.4–6.2, 206Pb/204Pb = 18.22–18.41, 207Pb/204Pb = 15.48–15.52, 208Pb/204Pb = 37.99–38.30. Hala’alate Formation volcanic rocks are similar to the sanukitoids of Karamay, with high Sr (633.5–970.1 ppm), Ba (268.7–796.3 ppm), and Sr/Y (61.34–84.28), formed by partial melting of the mantle metasomatized by slab-derived adakitic melts. In contrast, Aladeyikesai Formation volcanic rocks show some affinity with sanukitoids of the Hatu area and the Setouchi Volcanic Belt, with low Sr (442.2–508.7 ppm), Ba (199.2–485.1 ppm), and Sr/Y (25.03–30.28), generated by the partial melting of subducting sediments. Identification of late Carboniferous sanukitoids in Hala’alate Mountain provides important constraints on the closing time of the remnant ocean basin in West Junggar, and implies that multi-stage subduction–accretionary orogeny plays a crucial role in the evolution and growth of the continental crust in the Central Asian Orogenic Belt.  相似文献   

4.
《International Geology Review》2012,54(13):1668-1690
The western Junggar Basin is located on the southeastern margin of the West Junggar terrane, Northwest China. Its sedimentary fill, magma petrogenesis, tectonic setting, and formation ages are important for understanding the Carboniferous tectonic evolution and continental growth of the Junggar terrane and the Central Asian Orogenic Belt. This paper documents a set of new zircon secondary ion mass spectrometry U–Pb geochronological and Hf isotopic data and whole-rock elemental and Sr–Nd isotopic analytical results for the Carboniferous strata and associated intrusions obtained from boreholes in the western Junggar Basin. The Carboniferous strata comprise basaltic andesite, andesite, and dacite with minor pyroclastic rocks, intruded by granitic intrusions with zircon secondary ion mass spectrometry U–Pb ages of 327–324 Ma. The volcanic rocks are calc-alkaline and show low high εNd(t) values (5.3–5.6) and initial 87Sr/86Sr (0.703561–0.703931), strong enrichment in LREEs, and some LILEs and depletion in Nb, Ta, and Ti. Furthermore, they also display high (La/Sm)N (1.36–1.63), Zr/Nb, and La/Yb, variable Ba/La and Ba/Th and constant Th/Yb ratios. These geochemical data, together with low Sm/Yb (1.18–1.38) and La/Sm (2.11–2.53) ratios, suggest that these volcanic rocks were derived from a 5–8% partial melting of a mainly spinel Iherzolite-depleted mantle metasomatized by slab-derived fluids and melts of some sediments in an island-arc setting. In contrast, the granitic intrusions represent typical adakite geochemical features of high Sr and low Y and Yb contents, with no significant Eu anomalies, high Mg#, and depleted εNd(t) (5.6–6.4) and εHf(t) (13.7–16.2) isotopic compositions, suggesting their derivation from partial melting of hot subducted oceanic crust. In combination with the previous work, the West Junggar terrane and adjacent western Junggar Basin are interpreted as a Mariana-type arc system driven by northwestward subduction of the Junggar Ocean, possibly with a tectonic transition from normal to ridge subduction commencing ca. at 331–327 Ma.  相似文献   

5.
Well Drilling shows that the volcanic rocks from the Carboniferous Batamayineishan Formation in the Eastern Junggar basin are mainly composed of volcaniclastic rocks (av. 52%) and volcanic lavas (32%), with a small amount of volcanic pyroclastic lavas (av. 11%). The volcanic lavas are basalt‐basaltic andesite‐andesite‐dacite assemblage. The LA‐ICP‐MS zircon U‐Pb dating of the andesite and the dacite yielded 325～321 Ma and 310 Ma ages, respectively, which is of high agreement with the published age (300 Ma) of basalts from this Formation, it is implied that an important volcanic activity occurred in Junggar basin in the late Carboniferous. The lavas have low TiO2 and high Na2O, indicating a calc‐alkaline series. Geochemical data show that they are characterized by LREE‐enriched patterns with slightly negative Eu anomalies. The rocks have high large ion lithophile element (LILE), and low high field strength element (HFSE) concentrations, with strong negative Nb, Ta and Ti anomalies. From basic through intermediate to felsic, the depletions in Sr, Ti and P of the studied volcanic rocks increase gradually. These geochemical characteristics indicate that the volcanic rocks are magmatic evolution products attributed to partial melting of mantle‐derived spinelle lherzolite related to oceanic subduction in an island‐arc setting. In combination with the LA‐ICP‐MS zircon U‐Pb dating, it is inferred that subduction of the Junggar Ocean in eastern Junggar basin lasted to the Late Carboniferous. Consequently, the final closure of the Junggar Ocean occurred most likely after 310 Ma.  相似文献   

6.
ABSTRACT

The subduction of oceanic lithosphere during the Carboniferous Period contributed to the formation of widely distributed subduction-related volcanic rocks within the Junggar basin. These volcanic rock associations contain significant clues for understanding the subduction of the Keramaili oceanic lithosphere and the filling of the remnant oceanic basin. Here, we report regional gravity and magnetic data, petrology, geochemistry, and U–Pb dating for Carboniferous volcanic rocks from the North Junggar basin (NJB). Using samples from well Y-1, we distinguish upper and lower volcanic sequences on the basis of selected geochemical data. An isochronous stratigraphic framework of Carboniferous volcano-sedimentary sequences is then constructed and the petrogenesis of these volcanic rocks is discussed. Finally, we propose an explanation for the genesis of these diachronous Carboniferous volcano-sedimentary sequences. The results show that various volcanic rocks are distributed in different areas of the NJB, and mainly consist of calc-alkaline basalt–andesite–dacite assemblages and alkaline basalt–basaltic andesite–andesite assemblages. The geochemical data also demonstrate a binary nature of the Carboniferous volcanic rocks. In the eastern NJB, the lower and upper volcanic sequences are formed during the early and late Carboniferous, respectively. However, all of these volcano-related sequences in the western of the NJB are formed during the late Carboniferous. These volcano-sedimentary sequences exhibit a ‘ladder-style’ of temporospatial evolution from east to west. The geochemical results also indicate that the upper volcanic rocks include island arc components formed in an extensional setting, whereas the lower volcanic rocks were derived from deep crustal cycling metasomatism by various mantle components in a continental arc environment. Earlier closure of the Keramaili oceanic basin and slab roll-back of the Junggar oceanic lithosphere in eastern versus western Junggar basin led to the formation of these diachronous volcano-sedimentary sequences.  相似文献   

7.
U–Pb zircon geochronological, geochemical, and whole-rock Sr–Nd isotopic analyses are reported for a suite of Karamay A-type granites from the Central Asian Orogenic Belt (CAOB) in the western Junggar region of northern Xinjiang, Northwest China, with the aim of investigating the sources and petrogenesis of A-type granites. The Karamay pluton includes monzogranite and syenogranite. Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating yielded a concordant weighted mean 206Pb/238U age of 304 ± 5 Ma (n = 11), defining a late Carboniferous magmatic event. Geochemically, the rock suite is characterized by high SiO2, FeOt/MgO, total alkalies (K2O + Na2O), Zr, Nb, Y, Ta, Ga/Al, and rare earth elements (REEs) (except for Eu), and low contents of MgO, CaO, and P2O5, with negative Ba, Sr, P, Eu, and Ti anomalies. These features indicate an A-type affinity for the Karamay granitic intrusions. Isotopically, they display consistently depleted Sr–Nd isotopic compositions (initial 87Sr/86Sr = 0.7014–0.7022, ?Nd(t) = +5.6–+7.0). Geochronological, geochemical, and isotopic data suggest that the Karamay A-type granites were derived from remelting juvenile lower crust, followed by fractional crystallization. The Karamay A-type granites as well as widespread late Carboniferous magmatism in the western Junggar region of the southwestern CAOB may have been related to ridge subduction and a resultant slab window. This further demonstrates the importance of the late Palaeozoic granitic magmatism in terms of vertical crustal growth in northern Xinjiang.  相似文献   

8.
The northwestern margin of the Junggar Basin is a transition zone between the Zaire-Hala'alate mountains and the Junggar Basin in West Junggar, which developed a large amount of volcanic rocks in the late Carboniferous. An investigation of the tectonic evolution of this area will be helpful for understanding the accretionary process of West Junggar. Here, we selected well-preserved drill core samples of andesite and andesitic tuff for detailed petrogenesis and geochemical studies, while high-quality seismic and resistivity prospecting cross-sections were also used to reveal the tectonic setting. Zircon U-Pb dating results of three andesite and andesitic tuff samples show that these rocks were erupted at ca. 312 Ma. The results of the geochemical and isotopic analyses of seven samples are characterized by relatively high MgO (1.84–5.52 wt%), Cr (26.19–246.61 ppm), Ni (16.53–82.85 ppm) contents, Ba/La (14.19–218.48) ratios, and high positive ɛHf(t) (+4.8 to +14.2) values, but low TiO2 (0.68–1.25 wt%) contents, FeOT/MgO (1.18–2.81), Sr/Y (5.63–27.40), and Th/Yb (0.19–2.18) ratios, which are similar to the Bieluagaxi sanukitoids of West Junggar. LREEs are enriched in all samples, while Nb and Ta are significantly depleted. All the evidence suggests the volcanics were most likely derived from partial melting of oceanic slab that was contaminated by the overlying mantle wedge during the magma ascent. The seismic and resistivity cross-sections show apparent southeastward-vergent imbricate thrust fault systems, implying northwestward subduction in the late Carboniferous. Together with previous studies and our geochemical and geophysical data, we suggest that the formation of these late Carboniferous volcanic rocks is probably related to a northwestward ridge subduction process.  相似文献   

9.
ABSTRACT

The West Junggar region, located in the Central Asian Orogenic Belt (CAOB), is characterized by extensive Carboniferous magmatism and porphyry Cu (-Au) deposits. The Shiwu porphyry Cu-Au deposit, located in the east of the Barluk Mountains, the West Junggar region, is not only a newly discovered deposit but also a representative porphyry Cu-Au deposit in this area. The volcanic rocks (including andesite and tuff) and intrusive rocks (including diorite, quartz diorite, quartz diorite porphyry, and tonalite porphyry) occurred in the Shiwu area and the mineralization associated with the quartz diorite porphyry. The secondary ion mass spectrometry (SIMS) zircon U–Pb ages of quartz diorite porphyry and tonalite porphyry are 310.4 ± 2.3 Ma and 310.1 ± 2.4 Ma, respectively, indicating that the Shiwu deposit is related to the Late Carboniferous magmatism. Intrusive rocks, which were characterized by the enrichment of large ion lithophile elements (LILEs) and pronounced negative high field strength elements (HFSEs), belong to the calc-alkaline or tholeiitic series. Their (87Sr/86Sr)i, (143Nd/144Nd)I, and εNd(t) values range from 0.703569 to 0.704311, 0.512488 to 0.512512, and 4.9 to 5.3, respectively. Volcanic rocks, which belong to the calc-alkaline series, have similar geochemical features as the intrusive rocks, and their (87Sr/86Sr)i, (143Nd/144Nd)i, and εNd(t) values, respectively, are 0.703704–0.704071, 0.512520–0.512542, and 5.49–5.92. These characters indicate that the igneous rocks in the Shiwu area derived dominantly from the mantle and formed in an island arc setting. These characters also further confirmed that the Barluk Mountains is still in an island arc setting in the Late Carboniferous and the accretionary orogenesis can exist until 310 Ma at least.  相似文献   

10.
西天山乌孙山地区大哈拉军山组由玄武岩、安山岩、英安岩、流纹岩及相应的火山碎屑岩组成,安山岩和流纹岩分布最广。LA-ICP-MS锆石U-Pb定年结果表明,火山活动喷发的安山岩与安山质晶屑凝灰熔岩分别形成于353.9Ma±6.5Ma和356.3Ma±4.4Ma,属于早石炭世早期。通过区域对比,西天山大哈拉军山组的火山岩浆作用显示从伊犁中天山板块南北缘向伊犁盆地内部逐渐变年轻的特点,且火山岩喷发时代差别不大(约40Ma)。岩石地球化学研究表明,火山岩属钙碱性系列,富集轻稀土元素,相对亏损重稀土元素。中性火山岩富集大离子亲石元素(如Cs、Rb、Th、U),而相对亏损高场强元素,具有明显的Nb、Ta、Ti负异常,显示出岛弧火山岩的特征;酸性火山岩相对富集Rb、Th、U、Ta等元素,具有明显的Ba、Sr、P、Eu、Ti等元素的负异常。综合伊犁-中天山板块南缘的构造演化特征,认为大哈拉军山组形成于活动大陆边缘环境,产在板块俯冲-碰撞的最后阶段。  相似文献   

11.
为加深对中亚造山带西段石炭纪构造背景的认识,对准噶尔盆地西北缘中拐凸起金龙10井区石炭纪安山岩岩心样品开展岩相学、年代学和地球化学研究。安山岩的LA-ICP-MS锆石U-Pb定年结果显示,其结晶年龄为(322.4±1.1) Ma,属中石炭世。地球化学分析结果表明,研究区安山岩属于低钾-中钾钙碱性岩石系列,其稀土元素配分曲线均略向右倾,具微弱的Eu负异常;微量元素原始地幔标准化蛛网图显示整体相对富集Pb和大离子亲石元素Rb、Ba等,亏损高场强元素Nb、Ta、Zr等,具有消减带弧火山岩的特征。安山岩的Th/La、Th/Ce和Lu/Yb值均低于大陆地壳的平均值,而接近幔源岩浆的比值。研究区安山岩属于正常的岛弧火山岩,源于富水地幔楔,是由俯冲板片脱水产生的流体交代地幔楔发生部分熔融所形成的。结合区域地质背景并对比相近时期岩浆岩,综合认为研究区安山岩形成于大陆弧环境,为西准噶尔残余洋盆俯冲消减作用的产物,且残余洋盆在(322.4±1.1) Ma时还未完全关闭。  相似文献   

12.
《International Geology Review》2012,54(12):1479-1503
ABSTRACT

Early Cretaceous volcanic rocks are widely distributed in northeast China and being extensively observed recently. However, petrogenesis and tectonic setting of these volcanic rocks are still on debate. We present zircon U–Pb ages, whole-rock geochemistry and zircon Hf isotope for these volcanic and sub-volcanic rocks surrounding the Erlian Basin including basic-intermediate volcanic rocks, intermediate-felsic volcanic rocks, and dacites and trachyandesite from dikes. The zircon U–Pb dating results indicate that these rocks formed in the Early Cretaceous (146–129 Ma). The basic-intermediate volcanic rocks mainly consist of basaltic andesite, which are featured by low SiO2 concentrations (49.96–58.34 wt. %), high Mg# values (54–37) and Co contents (17.85–25.98 ppm), and positive εHf(t) values (+7.11 to +13.87). Moreover, they show high La/Nb (1.79–2.87) and low La/Ba (0.02–0.08) ratios. Such features indicate that they were derived from partial melting of lithospheric mantle that had been modified by fluids. The intermediate-felsic volcanic rocks consist of trachydacite and andesite, which show medium SiO2 concentrations (58.31–66.44 wt. %), a wide range of Mg# values (28–53) and with A1-type granites affinities. These features, along with slightly positive to negative εHf(t) values (+0.53 to ?17.71), indicate that they originated from mixed magma of melted lower crust and mantle substances. Dacites from dikes are distinguished by high SiO2 concentrations (65.72–67.2 wt. %), negative εHf(t) values (?2.55 to ?6.72) and old zircon Hf TDM2 ages (1453–1653 Ma), suggesting they were generated by melting of Mesoproterozoic and Palaeoproterozoic crustal material. All of the investigated volcanic and sub-volcanic rocks exhibit geochemical signatures of extension setting. In combination with previous studies, we suggest the Early Cretaceous extension in northeast China is related to the collapse of thickened lithosphere after closure of the Mongol–Okhotsk Ocean and to the slab break off of the Mudanjiang Ocean.  相似文献   

13.
《International Geology Review》2012,54(16):1991-2007
A series of Lower Carboniferous volcanic rocks occur in the Hatu, Darbut, and Baogutu areas of Xinjiang Province. Secondary ion mass spectrometry (SIMS) zircon U–Pb isotopic data indicate that two samples of these rocks coevally erupted at 324.0 ± 2.8 Ma and 324.9 ± 3.4 Ma. Three detailed profile measurements show that the volcanics include the Hatu basalt, the Baogutu andesite and dacite, and the Darbut andesite. Whole-rock compositions suggest that the Hatu volcanics are tholeiites and have a mid-ocean ridge basalt (MORB)-like signature with a small negative Nb anomaly, suggesting formation in a back-arc basin. Their isotopic compositions (?Nd(t) = +2.2 to +4.0, (87Sr/86Sr)i = 0.70414 to 0.70517) suggest a mixing origin from depleted to enriched mantle sources. In contrast, the Baogutu and Darbut rocks are andesite and dacite possessing a transitional tholeiite to calc–alkaline character and have E-MORB-like and OIB signatures, with a marked negative Nb anomaly and Th/Yb-enrichment, indicating that they were generated in a subduction zone setting. Isotopically, they display consistently depleted Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.70377–0.70469, ?Nd(t) = 1.0–5.2], suggesting that they were derived from a depleted mantle, and that fluid and sediments were involved in their petrogenesis. These features suggest that an early Carboniferous intra-oceanic arc and back-arc basin system generated the studied volcanic units in the West Junggar.  相似文献   

14.
We report zircon U–Pb geochronologic and geochemical data for the post-collisional volcanic rocks from the Batamayineishan (BS) Formation in the Shuangjingzi area, northwestern China. The zircon U–Pb ages of seven volcanic samples from the BS Formation show that the magmatic activity in the study area occurred during 342–304 Ma in the Carboniferous. The ages also indicate that the Palaeo-Karamaili Ocean had already closed by 342 Ma. Moreover, the volcanic rocks also contained 10 inherited zircons with ages ranging from 565 to 2626 Ma, indicating that Precambrian continental crust or microcontinents with accretionary arcs are two possible interpretations for the basement underlying the East Junggar terrane. The sampled mafic-intermediate rocks belong to the medium-K to high-K calc-alkaline and shoshonitic series, and the formation of these rocks involved fractional crystallization with little crustal contamination. These Carboniferous mafic-intermediate rocks show depletions in Nb and Ta and enrichments in large ion lithophile elements (e.g. Rb, Ba, U, and Th) and light rare earth elements. The low initial 87Sr/86Sr values (0.7034–0.7042) and positive εNd(t) values (+2.63 to +6.46) of these rocks suggest that they formed from depleted mantle material. The mafic-intermediate rocks were most likely generated by 5–10% partial melting of a mantle source composed primarily of spinel lherzolite with minor garnet lherzolite that had been metasomatized by slab-derived fluids and minor slab melts. In contrast, the felsic rocks in the BS Formation are A-type rhyolites with positive εNd(t) values and young model ages. These rocks are interpreted to be derived from the partial melting of juvenile basaltic lower crustal material. Taken together, the mafic-intermediate rocks formed in a post-collisional extensional setting generated by slap breakoff in the early Carboniferous (342–330 Ma) and the A-type rhyolites formed in a post-collisional extensional setting triggered by the upwelling asthenosphere in the late Carboniferous (330–304 Ma).  相似文献   

15.
The West Junggar lies in the southwest part of the Central Asian Orogenic Belt (CAOB) and consists of Palaeozoic ophiolitic mélanges, island arcs, and accretionary complexes. The Barleik ophiolitic mélange comprises several serpentinite-matrix strips along a NE-striking fault at Barleik Mountain in the southern West Junggar. Several small late Cambrian (509–503 Ma) diorite-trondhjemite plutons cross-cut the ophiolitic mélange. These igneous bodies are deformed and display island arc calc-alkaline affinities. Both the mélange and island arc plutons are uncomfortably covered by Devonian shallow-marine and terrestrial volcano-sedimentary rocks and Carboniferous volcano-sedimentary rocks. Detrital zircons (n = 104) from the Devonian sandstone yield a single age population of 452–517 million years, with a peak age of 474 million years. The Devonian–Carboniferous strata are invaded by an early Carboniferous (327 Ma) granodiorite, late Carboniferous (315–311 Ma) granodiorites, and an early Permian (277 Ma) K-feldspar granite. The early Carboniferous pluton is coeval with subduction-related volcano-sedimentary strata in the central West Junggar, whereas the late Carboniferous–early Permian intrusives are contemporary with widespread post-collisional magmatism in the West Junggar and adjacent regions. They are typically undeformed or only slightly deformed.

Our data reveal that island arc calc-alkaline magmatism occurred at least from middle Cambrian to Late Ordovician time as constrained by igneous and detrital zircon ages. After accretion to another tectonic unit to the south, the ophiolitic mélange and island arc were exposed, eroded, and uncomfortably overlain by the Devonian shallow-marine and terrestrial volcano-sedimentary strata. The early Carboniferous arc-related magmatism might reflect subduction of the Junggar Ocean in the central Junggar. Before the late Carboniferous, the oceanic basins apparently closed in this area. These different tectonic units were stitched together by widespread post-collisional plutons in the West Junggar during the late Carboniferous–Permian. Our data from the southern West Junggar and those from the central and northern West Junggar and surroundings consistently indicate that the southwest part of the CAOB was finally amalgamated before the Permian.  相似文献   

16.
Review Section     
ABSTRACT

The petrology, geochronology, and geochemistry of the early Permian volcanic rocks from Houtoumiao area, south Xiwuqi County in central Inner Mongolia of China, are studied to elucidate the early Permian tectonic setting of the region. The volcanic rocks, which are interbedded with sandstone, feature both mafic and felsic compositions and show a bimodal nature. Zircon U–Pb dating reveals that the volcanic rocks formed at 274–278 Ma, similar to the ages of bimodal magmatism in neighbouring areas. The mafic rocks are composed of tholeiitic basalt, basaltic andesite, basaltic trachyandesite, and trachyandesite. They are rich in Th, U, and LILEs, depleted in HFSEs Nb, Ta, and Ti, and have positive εNd(t) values (+3.6 to +7.9). Geochemical analyses indicate that the mafic rocks originated from metasomatized lithospheric mantle. The felsic volcanic rocks are mainly rhyolite, with minor trachyte and dacite. They have different evolutionary tendencies of major elements, chondrite-normalized REE patterns, and isotopic compositions from the mafic volcanic rocks, which preclude formation by fractional crystallization of mafic melts. The εNd(t) values of the felsic rocks are similar to those of the Carboniferous Baolidao arc rocks in the region. It is suggested that Permian felsic melts originated from the partial melting of Carboniferous juvenile arc-related rocks. By comparison with typical Cenozoic bimodal volcanism associated with several tectonic settings, including rift, post-collisional setting, back-arc basin, and the Basin and Range, USA, the bimodal volcanic rocks in central Inner Mongolia display similar petrological and geochemical characteristics to the rocks from back-arc basin and the Basin and Range, USA. Based on the analysis of regional geological data, it is inferred that the early Permian bimodal volcanic rocks in the study area formed on an extensional continental margin of the Siberian palaeoplate after late Carboniferous subduction–accretion.  相似文献   

17.
Mineral chemistry, major and trace elements, 40Ar/39Ar age and Sr–Nd–Pb isotopic data are presented for the Late Cretaceous Hamsilos volcanic rocks in the Central Pontides, Turkey. The Hamsilos volcanic rocks mainly consist of basalt, andesite and associated pyroclastics (volcanic breccia, vitric tuff and crystal tuff). They display shoshonitic and high-K calc-alkaline affinities. The shoshonitic rocks contain plagioclase, clinopyroxene, alkali feldspar, phlogopite, analcime, sanidine, olivine, apatite and titanomagnetite, whereas the high-K calc-alkaline rocks contain plagioclase, clinopyroxene, orthopyroxene, magnetite / titanomagnetite in microgranular porphyritic, hyalo-microlitic porphyritic and glomeroporphyritic matrix. Mineral chemistry data reveal that the pressure condition of the clinopyroxene crystallisation for the shoshonitic rocks are between 1.4 and 6.3 kbar corresponds to 6–18-km depth and the high-K calc-alkaline rocks are between 5 and 12 km. 40Ar/39Ar age data changing between 72 ± .5 Ma and 79.0 ± .3 Ma (Campanian) were determined for the Late Cretaceous Hamsilos volcanic rocks, contemporaneous with the subduction of the Neo-Tethyan Ocean beneath the Pontides. The studied volcanic rocks were enriched in the large-ion lithophile and light rare earth element contents, with pronounced depletion in the contents of high-field-strength elements. Chondrite-normalised rare earth element patterns (LaN/LuN = 6–17) show low to medium enrichment, indicating similar sources of the rock suite. Initial 87Sr/86Sr values vary between .70615 and .70796, whereas initial 143Nd/144Nd values change between .51228 and .51249. Initial 206Pb/204Pb values vary between 18.001 and 18.349, 207Pb/204Pb values between 15.611 and 15.629 and 208Pb/204Pb values between 37.839 and 38.427. The main solidification processes involved in the evolution of the volcanic rocks consist of fractional crystallisation, with minor amounts of crustal contamination ± magma mixing. According to geochemical evidence, the shoshonitic melts in the Hamsilos volcanic rocks were possibly derived from the low degree of partial melting of a subcontinental lithospheric mantle (SCLM), while the high-K calc-alkaline melts were derived from relatively high degree of partial melting of SCLM that was enriched by fluids and/or sediments from a subduction of oceanic crust.  相似文献   

18.
Palaeogene and Neogene volcanic rocks are widespread on the Biga Peninsula of Northwest Turkey. These rocks were formed during the Eocene, Oligocene–Miocene, and late Miocene, and the early Miocene ?apç? volcanic rocks in the Bal?kesir area consist of andesitic lava flows and associated pyroclastics. Temperatures, pressures, and oxygen fugacities calculated for the hornblendes in these andesitic rocks are 903–930°C, 3.3–4.8 kbar, and –9.91 to –11.88, respectively, and for the biotites they are 755–788°C, 1.30–1.74 kbar, and –14.88 to –13.98, respectively. SHRIMP U–Pb dating of zircons from three andesite samples gave ages of 22.72 ± 0.19, 22.97 ± 0.23, and 18.72 ± 0.17 Ma (early Miocene), and these are regarded as crystallization ages. Geochemical analyses show that the volcanic rocks are mainly high-K and calc-alkaline, and have high contents of large-ion lithophile elements and low contents of high-field strength elements, revealing that they evolved from parental magmas that were derived from an enriched subcontinental lithospheric mantle source. The chondrite-normalized rare earth element patterns of the rocks are concave upwards with LaCN/LuCN = 11.9–21.2 and EuCN/Eu* = 0.84–0.92, implying significant fractional crystallization of hornblende during their evolution. According to the petrological data with regional geology, Neogene magmatic activity on the Biga Peninsula has a post-collisional feature, and was closely related to slab break-off geodynamic model after collision of Tauride–Anatolide Block and Sakarya continent.  相似文献   

19.
Two representative geologic sections of the late Mesozoic ‘Nanyuan Formation’ volcanic rocks in eastern Fujian Province have been dated by SHRIMP zircon U–Pb techniques. The results identified two active volcanic stages at 162–150 Ma and 143–130 Ma. The rock association of the earlier period is composed of andesite‐dacitic crystal tuff‐rhyolitic ignimbrite. The rocks are enriched in alkali elements, Al2O3, large ion lithophile element (LILE) and light rare earth element (LREE), depleted in MgO, high field strength element (HFSE) and siderophile elements, and have similar ?Nd (t) values. These features indicate that the rocks were derived from a mantle source metasomatized by subduction‐related fluids, and their chemical variations indicate fractional crystalization during magmatic evolution. In contrast, the rock association of the later period consists of tholeiitic basalt‐rhyolitic crystal ignimbrites. The tholeiitic basalts have negative ?Nd (t) values (?3.4 to ?2.6) and exhibit fractionated REE pattern with weak negative Eu anomalies. These rocks are enriched in LREE and depleted in HFSE with ratios of Ce/Pb = 6.9–11.1, Th/U = 2.7–4.0, La/Nb = 2.2–3.0, suggesting that they were also subduction‐related and experienced proportional fractionation of olivine and clinoproxene along with significant crustal contamination. The rhyolitic crystal ignimbrites are generally characterized by rather high K, Rb, Th and relatively low Nb, Sr, Ti, P, with relatively low ?Nd (t) values (?5.3 to ?6.0), impling that they were derived from a different source from the tholeiitic basalt. Judging by extensive overthrust structures, the occurrence of the earlier rock association is considered to have been generated in a compressional tectonic environment during the early stage (>150 Ma) of Pacific plate subduction, In contrast, the later bimodal association was generated in an extensional tectonic setting during a later stage (<143 Ma ) of subduction.  相似文献   

20.
《地质科学》2018,53(3):1136-1156
准噶尔盆地北三台地区钻井岩心获得的巴塔玛依内山组火山岩主要由玄武岩、玄武安山岩、安山岩和英安岩及部分火山碎屑岩组成。地球化学研究结果显示,火山岩为低TiO2、高Na2O的钙碱性系列,具低到中度轻、重稀土分馏的谱型,相对富集大离子亲石元素,贫高场强元素,明显亏损Nb和Ta、富集Pb;由基性到中酸性火山岩。Sr、Ti和P的亏损逐渐增大,证明是与大洋俯冲相关的岛弧环境同源岩浆演化的产物。LA-ICP-MS锆石U-Pb定年分别获得安山岩和英安岩325～321Ma和310 Ma的形成年龄,结合前人同层位获得基性火山岩300 Ma的形成年龄认为,准噶尔洋在盆地东部的俯冲作用一直延续至晚石炭世,大洋闭合可能发生在晚石炭世末期。  相似文献   

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