Petrogenesis and tectonic implications of the Triassic rhyolites in the East Kunlun Orogenic Belt, northern Tibetan Plateau

The East Kunlun Orogenic Belt(EKOB),which is in the northern part of the Greater Tibetan Plateau,contains voluminous Late Triassic intermediate-felsic volcanic rocks.In the east end of the EKOB,we identified highly differentiated peralkaline-like Xiangride rhyolites(~209 Ma)that differ from the widespread andesitic-rhyolitic Elashan volcanics(~232–225 Ma)in terms of their field occurrences and mineral assemblages.The older,more common calc-alkaline felsic Elashan volcanics may have originated from partial melting of the underthrust Paleo-Tethys oceanic crust under amphibolite facies conditions associated with continental collision.The felsic Elashan volcanics and syn-collisional granitoids of the EKOB are different products of the same magmatic event related to continental collision.The Xiangride rhyolites are characterized by elevated abundances of high field strength elements,especially the very high Nb and Ta contents,the very low Ba,Sr,Eu,P,and Ti contents;and the variably high ^(87)Sr/^(86)Sr ratios(up to 0.96),exhibiting remarkable similarities to the characteristic peralkaline rhyolites.The primitive magmas parental to the Xiangride rhyolites were most likely alkali basaltic magmas that underwent protracted fractional crystallization with continental crust contamination.The rock associations from the early granitoids and calc-alkaline volcanic rocks to the late alkaline basaltic dikes and peralkaline-like rhyolites in the Triassic provide important information about the tectonic evolution of the EKOB from syn-collisional to post-collisional.We infer that the transition from collisional compression to postcollisional extension occurred at about 220 Ma.

Rhyolites in continental mafic Large Igneous Provinces: Petrology,geochemistry and petrogenesis

We present a detailed review of the petrological and geochemical aspects of rhyolite and associated silicic volcanic rocks(up to 20 vol%of all rocks)reported to date from twelve well known Phanerozoic continental mafic Large Igneous Provinces(LIPs).These typically spread over<104 km^2(rarely 105 km^2 for Parana-Etendeka)area and comprise<10~4 km^3 of extrusive silicic rocks,erupted either during or after the main basaltic eruption within<5 Myr,with some eruption(s)continuing for≤30 Myr.These rhyolites and associated silicic volcanic rocks(60-81 wt.%of SiO2)are mostly metaluminous to peraluminous and are formed via(ⅰ)fractional crystallization of parental mafic magma with negligible crustal contamination,and(ⅱ)melting of continental crust or assimilation and fractional crystallization(AFC)of mafic magma with significant crustal contribution.Rhyolites formed by extensive fractional crystallization are characterized by the presence of clinopyroxene phenocrysts,exhibit steep negative slopes in bivariate major oxides plots and weak to no Nb-Ta anomaly;these typically have temperature>900℃.Rhyolites formed by significant crustal contribution are characterized by strong negative Nb-Ta anomalies,absence of clinopyroxene phenocrysts,and are likely to have a magma temperature<900℃.Geochemical signatures suggest rhyolite melt generation in the plagioclase stability field with a minor fraction originating from lower crustal depths.A large part of the compositional variability in rhyolites,particularly the SrNd-Pb-O isotope ratios,suggests a significant role of continental crust(upper crustal melting or AFC)in the evolution of these silicic rocks in the continental mafic LIPs.

Geochemistry,petrogenesis and tectonic implication of Early Cretaceous A-type rhyolites in Hailar Basin,NE China

Early Cretaceous A-type rhyolites of the Shangkuli Formation in the Hailar Basin of NE China exhibit geochemical characteristics of high silicon,alkali,Fe/Mg,Ga/Al,Zr,Pb,HFSEs,and REE contents but low Ca,Ba,Sr and Eu,which meet the criteria of typical reduced A-type granite.The A-type rhyolites are most probably derived from magmatic underplating and partial melting of quartz-feldspathic lower crust,with the lithospheric mantle material involved,due to the extensional deformation of the Erguna-Hulun Fault.Although the A-type rhyolites show A1-type trace elements characteristics,they were formed in a post-orogenic extensional background together with the coeval widespread bimodal volcanic rocks,metamorphic core complexes,volcanic fault basins and metallogenic province in the Sino-Russia-Mongolia border tract.This extension event was related to the collapse of thickened region of the continental crust after the closure of the Mongol-Okhotsk Ocean.

Lajishan Ayishan Formation Rhyolites: Implications for the Closure Time of the Proto-Tethys Ocean

The Qilian orogenic belt,as an important component of the central orogenic system,can be divided into three tectonic units from north to south:the North Qilian tectonic belt,the Central Qilian tectonic belt and the South Qilian tectonic belt.The Lajishan ophiolitic mélange zone is an important part of the South Qilian tectonic belt.The study of the formation age and tectonic attributes of the different rock units in the Lajishan is of great significance for reconstructing the tectonic evolution of the south Qilian tectonics belt and the Proto-Tethys Ocean.The Ayishan formation is an important unit of the Lajishan mélange belt,but its stratigraphic age and formation environment are still not constrained,which limits the reconstruction of the early Palaeozoic tectonic evolution of the South Qilian tectonic belt and the closure time of the Proto-Tethys Ocean.In recent years,a set of rock assemblages consisting of andesite,rhyolite,sandstone and pebbly sandstone have been identified in the Ayishan formation exposed in the Lajishan through detailed geological mapping in the field.They are in fault contact with the underlying ophiolite mélange.Regionally,a volcano-sedimentary succession consisting of andesite,rhyolite,volcanic breccia,and volcaniclastic sandstone nonconformably overlies the Cambrian arc-accretionarycomplex system in the Ayishan area.These rocks were originally assigned to the Ayishan formation and attributed an Early Ordovician age(BGMRQP,1964,1991).Our rhyolite samples from the Ayishan formation yielded a weighted mean 206Pb/238U age of 447 Ma using zircon U-Pb dating,which can be interpreted as the eruption age of the volcanic rocks.These volcanic rock assemblages are distributed in sandstone,pebbled sandstone and conglomerate in the form of interlayers.Therefore,the zircon UPb age of the volcanic rocks can constrain the age of Ayishan formation that it should belong to the late Ordovician rather than early Ordovician.Additionally,we propose that the Ayishan rhyolites were formed in collision-related tectonic setting based on the geochemistry of the rhyolites,which means the initial continental collision between the Central Qilian block and the Qaidam block occurred at least in the Late Ordovician.Ayishan formation are in fault contact with the Cambrian Donggoumeikuang complex in the south.The Donggoumeikuang complex represents a Cambrian introceanic subduction system that formed in response to subduction of the Proto-Tethyan Ocean(Yan et al.,2015;Fu et al.,2018).At 450 to 420 Ma,the Proto-Tethyan Ocean closed and the Qaidam block collided against the central Qilian tectonics belt.Voluminous 450–440 Ma I-type and S-type granites(Yan et al.,2015;Tung et al.,2016)that straddle the Central and South Qilian belts formed a broad Andean-type continental margin(Yan et al.,2019),which indicates that the transition from oceanic subduction to continental subduction occurred in 450–440 Ma.At 440–420 Ma,the syncollisional and postcollisional granitoids extensively developed,accompanied by high-pressure granulite-facies metamorphism and anatexis in the South Qilian belt and the Qaidam block(Yu et al.,2014;Yan et al.,2015;Fu et al.,2018;Li et al.,2018).

The Late Carboniferous–Early Permian Ocean-Continent Transition in the West Junggar,Central Asian Orogenic Belt: Constraints from Columnar Jointed Rhyolite

The West Junggar of the western Central Asian Orogenic Belt is one of the typical regions in the term of ocean subduction, contraction and continental growth in the Late Paleozoic. However, it is still controversial on the exact time of ocean-continent transition so far. This study investigates rhyolites with columnar joint in the West Junggar for the first time.Based on zircon U-Pb dating, we determined that the ages of the newly-discovered rhyolites are between 303.6 and 294.5 Ma, belonging to Late Carboniferous–Early Permian, which is the oldest rhyolite with columnar joint preserved in the world at present. Geochemical results show that the characteristics of the major element compositions include a high content of SiO_2(75.78–79.20 wt%) and a moderate content of Al_2O_3(12.21–13.19 wt%). The total alkali content(K_2O +Na_2O) is 6.14–8.05 wt%, among which K_2O is 2.09–4.72 wt% and the rate of K_2O/Na_2O is 0.38–3.05. Over-based minerals such as Ne, Lc, and Ac do not appear. The contents of TiO_2(0.09–0.24 wt%), CaO(0.15–0.99 wt%) and MgO(0.06–0.18 wt%) are low. A/CNK=0.91–1.68, A/NK=1.06–1.76, and as such, these are associated with the quasi-aluminum-weak peraluminous high potassium calc-alkaline and some calc-alkaline magma series. These rhyolites show a significant negative Eu anomaly with relative enrichment of LREE and LILE(Rb, Ba, Th, U, K) and depletion of Sr, HREE and HFSE(Nb, Ta, Ti, P). These rhyolites also have the characteristics of an A2-type granite, similar to the Miaoergou batholith,which indicates they both were affected by post-orogenic extension. Combining petrological, zircon U-Pb dating and geochemical characteristics of the rhyolites, we conclude that the specific time of ocean-continent transition of the West Junggar is the Late Carboniferous–Early Permian.

Mineral Chemistry Indicates the Petrogenesis of Rhyolite From The Southwestern Okinawa Trough

To reveal the petrogenesis of rhyolite from the southwestern Okinawa Trough, the mineral chemistry of plagioclase, orthopyroxene, amphibole, quartz and Fe-Ti oxide phenocrysts were analyzed using an electron microprobe, and in suit Sr and Ba contents of plagioclase analysed by LA-ICPMS were chosen for fingerprinting plagioclases of different provenances. Results indicate an overall homogeneous composition for each of the mineral phases except for plagioclase phenocrysts which have a wide range of composition(An=39~88). Plagioclase crystals characterized by An contents of >70 are not in equilibrium with their whole-rock compositions, and coarse-sieved plagioclase phenocryst interiors record high An contents(>70) and Sr/Ba ratios(>7), which are similar to the those of plagioclase crystals in basalt. Therefore, these crystals must have been introduced to the rhyolitic magma from a more mafic source. Equilibrium temperatures estimated using orthopyroxene-liquid, iron–titanium oxide, titanium-in-quartz and amphibole geothermometers show consistent values ranging from 792 to 869℃. The equilibrium pressure calculated using amphibole compositions is close to 121 MPa which corresponds to an approximate depth of 4 km. The fO_2 conditions estimated from Fe-Ti oxides and amphiboles plot slightly above the NNO buffer, which indicates that the rock formed under more oxidized conditions. Our results suggests that petrogenesis of the rhyolite due to basaltic magma ascend with the high An and Sr/Ba plaigoclases from deep magma chamber into the shallow chamber where the fractional crystallization and crustal assimilation happened. It also indicates that a two-layer magma chamber structure may occur under the southwestern Okinawa Trough.

Geochemistry and zircon U-Pb geochronology of the rhyolitic tuff on Port Island, Hong Kong： Implications for early Cretaceous tectonic setting

Early Cretaceous rhyolitic tuffs, widely distributed on Port Island, provide insights into the volcanism and tectonic setting of Hong Kong. In this paper we present petrological, geochronological and geochemical data of the rhyolitic tuff to constrain the diagenesis age and petrogenesis of the rocks, tectonic setting and early Cretaceous volcanism of Hong Kong. The first geochronological data show that the zircons in the volcanic rocks have U-Pb age of 141.1-139.5 Ma, which reveals that the rhyolitic tuff on Port Island was formed in the early Cretaceous （K1）. Geochemically, these acid rocks, which are enriched in large ion lithophile elements （LILEs） and light rare earth elements （LREEs）, and depleted in high field strength elements （HFSEs）, belong to the high K calc-alkaline to shoshonite series with strongly-peraluminous characteristic. The geochemical analyses suggest that the volcanic rocks were derived from deep melting in the continental crust caused by basaltic magma underplating. Based on the geochemical analysis and previous studies, we concluded that the rhyolitic tufts on Port Island were formed in a back- arc extension setting in response to the subduction of the Paleo-Pacific Plate beneath the Eurasian Plate.

The First Discovery of Rhyolite Interlayer in the Mugagangri Group in the Bangonghu-Nujiang Suture Zone, Tibet and the U-Pb Dating

Objective The Mugagangri group （MP）, named by Wen Shixuan （1979） in the southeast Mugegebori Mountain which is the main peak of Mugagangri, is a flysch formation with deep semi-deep marine sedimentary features in the Bangonghu -Nujiang suture zone （BNSZ）. The lithology is mainly composed of sandstones, graywackes, siltstones and silty mudstones with low-grade metamorphism. The previously defined Mugagangri group continues from east to west for nearly 2000 km in BNSZ as a matrix part associated with ophiolitic melange blocks, and is considered to be part of the deep marine sedimentary cover of ophiolites.

LA-ICP-MS zircon U-Pb age of rhyolitic lithic-crystal tuffs in Erdaohezi lead-zinc deposit,Inner Mongolia

Erdaohezi lead-zinc deposit belongs to the Derbugan metallogenic belt lying on the northwestern Hailaer-Genhe Mesozoic volcanic basin, located on the western slope of the Da Hinggan Mountains. The deposit is considered as one of the hypabyssal low-temprature hydrothermal lead-zinc deposits associated with volca- nism. In order to lay the foundation on studying its diagenesis and mineralization ages, the detailed studies were carried out by dating the host rocks （i. e. rhyolitic lithic-crystal tufts） using zircon LA-ICP-MS U-Pb method. The dating results show three groups ot! ages. The first group is the captured zircons （the weighted mean ^206pb/238U age as 175.6± 2.3 Ma, MSWD = 0.70, n = 3）. The second group can be regarded as the rock- forming age （the weighted mean ^206pb/238U age as 165.3± 1.9 Ma, MSWD = 2.40, n = 14）. The third group should represent the late stage of the magmatic evolution （the weighted mean ^206pb/238U age as 161.0 ± 3.1 Ma, MSWD = 0.86, n = 4）. According to the ages and the crystal form or CL image characteristics of zircons, it is determined that the diagenesis occurred in the late Middle Jurassic. Based on the regional geology and geo- chronological research, the acidic pyroclastic rocks are space accompaniment and time connection with the Tamu- langou Formation intermediate-mafic volcanic rocks. Both of them constitute the host rocks of the deposit together. The rock combination also provides favorable conditions for large-scale silver, lead and zinc mineralization in this area.

A Paleoproterozoic A-type Rhyolite

The rhyolites in the upper Lüliang Group of Shanxi, China, are Paleoproterozoic weakly alkaline volcanic rocks. They are characterized by high SiO\-2, Na\-2O+K\-2O, Zr, Nb, Ga, Y and REE contents and large FeO\+*/MgO, Rb/Sr and Ga/Al ratios, and low CaO, Sr and Eu contents, and share much in common with the A type granitic rocks. They erupted in the rift setting at the continental margin. Chemical features and isotope data, as well as high Nd and low initial Sr ratios, suggest that the original granitic magma was derived from partial melting of Late Archean metamorphic rocks in the lower crust due to the influence of basaltic magma and hot fluid in response to rifting. The A type rhyolites were finally formed after the fractional crystallization of the dominant mineral feldspar.

Mantle derived crystal-poor rhyolitic ignimbrites:Eruptive mechanism from geochemical and geochronological data of the Piedra Parada caldera,Southern Argentina

Trace elements, isotopic modeling and U-Th-Pb SHRIMP zircon age constraints are used to reconstruct the eruption history and magmatic processes of the Piedra Parada Caldera. In the early Eocene, the crystal-poor Barda Colorada ignimbrite(BCI), having >>15% micro-porphyritic crystals with respect to magmatic components, erupted a volume estimated in more than 300 km^3. The Piedra Parada caldera is located in the Patagonian Andes foreland, at the southern end of the calderas field of the Pilcaniyeu Volcanic Belt(PVB). This belt is related to an extensional tectonic setting as a result of the collision of the Farallon-Aluk ridge with South America, which enabled the development of a transform ocean/continental plate margin followed by the detachment of the Aluk plate and the opening of a slab window. The BCI extra-caldera Plateau is a >100 m thick deposit, having a lower unit with high silica(Si O_2> 76 wt.%),potassium poor rhyolitic composition(trondhjemitic like magma), and an upper unit with normal to high potassium rhyolitic composition(granitic like magma). A trace elements modeling of the BCI units shows that the BCI lower and upper units did not evolve from fractionation or immiscibility in the shallow magma reservoir. The BCI also have a primitive isotopic signature(initial87 Sr/86 Sr =0.7031-0.7049 and ε_(Nd)= +3.4 to +3.65). Thus, tectonic, compositional and isotopic constraints suggest the fast ascent of high silica magmas to a shallow reservoir, and point to an upper mantle origin for these rhyolitic magmas in a transitional(Orogenic-Anorogenic) tectono-magmatic setting. U-Th-Pb SHRIMP zircon crystallization ages of the Syn-caldera stage BCI units(56 -51.5 Ma) show a protracted life of 5 Ma for this caldera reservoir. The age of 52.9 ± 0.3 Ma is considered the best fit for the possible maximum age for the caldera collapse. The Late-caldera magmatism has trachyandesitic and rhyolitic compositions.The trace element modeling suggests that these rhyolites evolve from the trachyandesites and do not evolve from the BCI residual magma. The trachyandesites have U-Th-Pb SHRIMP zircon crystallization ages of 52 ± 1 Ma, suggesting that the caldera eruption was triggered by the arrival of the trachyandesitic magma.

Late Cretaceous K-rich rhyolitic crystal tuffs from the Chuduoqu area in Eastern Qiangtang subterrane:evidence for crustal thickening of the central Tibetan Plateau prior to India–Asia collision

In order to constrain whether the Lhasa–Qiangtang collision contributed to an early crustal thickening of the central Tibetan Plateau prior to the India–Asia collision,we present zircon LA–ICP–MS U–Pb ages,wholerock geochemistry,and zircon Hf isotopic compositions of the newly discovered rhyolitic crystal tuffs from the Chuduoqu area in the eastern Qiangtang subterrane,central Tibet.Zircon U–Pb dating suggests that the Chuduoqu rhyolitic crystal tuffs were emplaced at ca.68 Ma.The Chuoduoqu rhyolitic crystal tuffs display high SiO_(2) and K2 O,and low MgO,Cr,and Ni.Combined with their zircon Hf isotopic data,we suggest that they were derived from partial melting of the juvenile lower crust,and the magma underwent fractional crystallization and limited upper continental crustal assimilation during its evolution prior to eruption.They should be formed in a post-collisional environment related to lithospheric mantle delamination.The Chuduoqu rhyolitic crystal tuffs could provide important constraints on the Late Cretaceous crustal thickening of the central Tibetan Plateau caused by the Lhasa–Qiangtang collision.

Pattern and origin of spherulites in a rhyolite dike swarm, northeastern Cheongsong, Korea

Several NNW-trending Cretaceous rhyolite dikes in the northeastern Cheongsong area of southern Korea contain spherulitic fabrics and are locally quarried as an ornamental stone. The dikes, part of the Cheongsong dike swarm, contain a variety of spherulites which are characterized by radial array of acicular intergrowths from fibrous quartz and alkali feldspars. They are classified as some simple, multiple and compound spherulite types from flower-like patterns, and the types are different each other in spatial distribution within the dikes. The various radial fabrics suggest that they crystallized under the different cooling mechanisms and processes in rapidly cooling dikes. Spheroids lacking a flower-like pattern may be precipitated after compositional segregation in a supercooled magma. Simple acicular spherulites could be crystallized by devitrification of true glass which cooled rapidly below the transformation temperature, whereas layered multiple spherulites could result from a crystallization of supercooled magma at relatively rapid cooling rates above the transformation temperature. Acicular multiple spherulites could be produced from a combination of the above three mechanisms operating at slower cooling rates.

Petrology of Metaluminous A-Type Rhyolite Discovered from Hadjer el Hamis Volcanoes (Lake Chad Basin)

Metaluminous (P.I. > 1) rhyolite from Hadjer el Hamis consisted of quartz, alkali feldspar, clinopyroxene (hedenbergite), amphibole (F-arfvedsonite) and oxides-hydroxides (ilmenite, magnetite, limonite) phenocrysts is characterized by the negative Eu, Ba, P, Sr and Ti anomalies. This metaluminous rhyolite and the early discovered peralkaline rhyolites in Hadjer el Hamis volcanoes derive likely from the same source, according to their coexistence on the same sector and their similar Zr/Nb ratios. The causes of magma heterogeneity are likely linked to varying amounts of extraction of an earlier melt phase or tectonic juxtaposition or a sudden increasing of fO2 in silicic magmas, triggered from a hydrothermal process, associated with F- and alkali-bearing fluids influx, which promoted the enrichment of Na in the hedenbergite rims and the crystallization of arfvedsonite.

松辽盆地西缘突泉地区晚侏罗世过铝质流纹岩和英云闪长玢岩的发现:从蒙古-鄂霍茨克洋闭合到陆陆碰撞的地质记录

蒙古-鄂霍茨克洋南向俯冲、大洋闭合和陆-陆碰撞是东北亚地区晚中生代的重要区域构造事件,它与本区的岩浆活动、变质作用、成盆和造山作用都密切相关。准确界定洋壳俯冲、大洋闭合和陆-陆碰撞这三个相互关联地质过程的时空范围是客观认识区域构造演化的前提。然而,这项研究通常难以进行,因为很难找到与之相关的合适的地质记录。作者在松辽盆地西缘突泉地区发现从大洋闭合到陆-陆碰撞相关的过铝质流纹岩和英云闪长玢岩,锆石LA-ICP-MS U-Pb定年结果显示其结晶年龄分别为156±1Ma和155±1Ma,系晚侏罗世岩浆事件产物。流纹岩和英云闪长玢岩为钙碱性过铝质岩石,具有较高的铝饱和指数A/CNK(1.32~2.13),较低的MgO+FeO T含量(0.96%~3.37%)和FeO T/MgO比值(2.84~5.02),岩矿鉴定表明它们含绢云母等原生和次生高铝矿物,在CIPW标准矿物计算中出现刚玉分子(3.77%~9.65%),结合流纹岩和英云闪长玢岩在花岗岩的S-I-M-A型分类方案相关地球化学图解投影结果,综合表明它们具有过铝质S型花岗岩特征。流纹岩和英云闪长玢岩具有较低的Rb/Sr(0.35~0.55)、Rb/Ba(0.08~0.26)和Al_(2)O_(3)/TiO_(2)(38.41~61.36)比值,表明其原始岩浆源于杂砂岩部分熔融。锆石饱和温度计算表明这两类岩石的岩浆形成温度在837~876℃之间,低于A型花岗岩岩浆形成温度(900℃)。两类岩石富集Rb、Ba、K等大离子亲石元素,亏损Nb、Ta、P、Ti等高场强元素,具有较低的Y(5.29×10^(-6)~19.75×10^(-6))、Nb(7.44×10^(-6)~8.50×10^(-6))、Sr(60.6×10^(-6)~154.9×10^(-6))和Yb(0.53×10^(-6)~2.40×10^(-6))含量,具有弧岩浆属性。在Y-Nb图解中流纹岩和英云闪长玢岩投点在火山弧和同碰撞型花岗岩区域内;在Rb/10-Hf-Ta×3图解中投点在火山弧和碰撞型花岗岩区域内;在R1-R2构造判别图解中样品投点在同碰撞和造山期区域内;在Sr-Yb图解中样品投点在地壳加厚区域内。结合全球典型俯冲岛弧和同碰撞酸性岩地球化学特征和同时期本区域岩浆岩成因,表明流纹岩和英云闪长玢岩形成于洋壳俯冲和陆-陆碰撞转换背景,暗示其形成与蒙古-鄂霍茨克茨洋闭合和陆-陆碰撞作用有关。流纹岩(La/Yb)N值为6.62~8.77,指示源区起源深度为40~46km,英云闪长玢岩(La/Yb)N值为7.93~13.39,指示源区起源深度为44~55km,表明在156±1Ma~155±1Ma地壳处于持续加厚过程。结合区域地质资料,构建了蒙古-鄂霍茨克洋从俯冲闭合到陆-陆碰撞的构造-岩浆演化模型。本文认为蒙古-鄂霍茨克构造体系的影响范围到达了松辽盆地西缘突泉地区,156±1Ma~155±1Ma研究区处于大洋闭合到陆-陆碰撞转换阶段。

内蒙古东乌珠穆沁旗宝力高庙组流纹岩年代学、地球化学特征及地质意义

兴安-蒙古造山带是古亚洲洋东段洋盆俯冲、闭合形成的增生造山带,但对该地区古亚洲洋俯冲形式、闭合时限及兴蒙造山带构造背景仍存在较大争议。东乌珠穆沁旗巴彦呼热地区位于兴蒙造山带中部,区内出露宝力高庙组为一套陆相中酸性火山碎屑岩-正常沉积碎屑岩建造。通过对该组流纹岩主、微量元素地球化学分析和LA-ICP-MS锆石U-Pb定年,精确厘定宝力高庙组火山岩形成时代,分析岩石地球化学特征。宝力高庙组流纹岩富集Rb,Th,Sr,亏损Nb,Ta,Ti元素。符合典型高分异“A”型花岗岩特点,显示后碰撞花岗岩特征,形成于造山后地壳减薄阶段。宝力高庙组流纹岩锆石U-Pb定年显示^(206)Pb/^(238)U加权平均年龄为(299.6±2.5)Ma(MSWD=1.5),时代为晚石炭—早二叠世。流纹岩SiO2含量70.80%~79.81%,MgO含量0.05%~0.50%,TiO2含量0.05%~0.23%,K2O含量3.25%~9.33%;微量元素Rb,La,Nd,Zr,Sm富集,相对亏损Sr,Ba,强烈亏损P,Ti;明显负铕异常及高锆石饱和温度(708.5℃~899.3℃),表明宝力高庙组流纹岩为形成于造山后地壳减薄阶段地壳部分熔融的产物。综合岩石地球化学、年代学及构造背景,认为兴蒙造山带中段东乌珠穆沁旗巴彦呼热地区在晚石炭—早二叠世处于后碰撞伸展环境,这为兴蒙造山带构造-岩浆演化提供新的证据。

内蒙古西乌旗地区寿山沟组流纹质砾石锆石U-Pb年龄及其意义

为研究兴蒙造山带中部早二叠世构造背景,探讨古亚洲洋闭合时限,本文以内蒙古西乌旗浩热哈达地区寿山沟组流纹质砾岩及大石寨组流纹岩为研究对象,开展了沉积地质学分析及锆石U-Pb同位素研究。结果显示,寿山沟组流纹质砾石锆石U-Pb年龄为(280.0±1.9)Ma,大石寨组流纹岩锆石U-Pb年龄为(276.0±1.9)Ma,指示西乌旗浩热哈达地区寿山沟组流纹质砾岩沉积时限介于280~276 Ma,属早二叠世Kungurian期;寿山沟组流纹质砾石年龄、流纹质砾岩沉积时限与研究区南部大石寨组流纹岩同位素年龄高度一致,表明南部大石寨组火山岩是寿山沟组碎屑岩的物源之一,二者属同期异相。岩石组合及沉积构造特征指示寿山沟组流纹质砾岩为浊流成因;根据寿山沟组源区岩石时空分布特征,综合前人资料,认为寿山沟组为弧后盆地沉积产物,证实古亚洲洋东段在早二叠世尚未闭合。

大兴安岭中生代两类流纹岩成因的地球化学研究

大兴安岭地区晚侏罗世—早白垩世流纹岩类广泛分布 .根据岩石学和微量元素地球化学特征将其划分为两类 ,分别称其为Ⅰ型流纹岩和Ⅱ型流纹岩 .稀土和微量元素分析结果显示 :(1)Ⅰ型流纹岩呈右倾的稀土分布曲线 ,不相容元素以强烈富集Rb ,Ba,Th ,K和亏损Sr ,Ti,P ,Nb为特征 ,其形成与钙碱性系列玄武岩浆的结晶分异作用有关 ;(2 )Ⅱ型流纹岩具有与大陆裂谷流纹岩一致的稀土和不相容元素分布模式 ,以Ba ,Sr的强烈亏损与I型流纹岩相区别 ,与碱性系列玄武岩类构成双峰式火山岩组合 ,其成因与地壳岩石的非理想熔融作用相联系 .两类流纹岩的形成与地幔柱上涌导致上覆岩石圈伸展作用有关 .

东天山博格达造山带石炭纪火山岩及其形成地质环境

东天山博格达造山带早、中石炭世海相火山岩具有双峰式特征 ,主要岩性为富钠的玄武岩和流纹岩 ,其次是英安岩 ,安山质岩石极少出现。玄武岩的特征是 :少数岩石含有实际矿物石英 ,个别岩石含橄榄石斑晶 ;辉石主要是透辉石和次透辉石 ,其成分富含铝 (Al2 O3=4.17～ 5 .99)和钛 (Ti O2 =2 .80～ 4.78) ;基质中的长石主要是钠 -更长石 ,斑晶中有相当数量的中长石和拉长石 ;全岩化学成分 CIPW计算结果绝大部分含 Di、Ol和 Hy,少数含 Qz和 Ne,在全碱 - Si O2 图中主要投影于亚碱性区 ,并接近碱性区 ;10 0 Mg/ (Mg+ΣFe) <6 5 ;ΣREE=38.73～ 134 .48,(L a/ L u) N=2 .75～ 4.0 3,δEu=0 .93～ 1.15 ;强不相容元素适度富集 ,Nb和 Ti适度亏损 ,Ni、Cr含量低 ,εNd和 εSr均为正值。这些特征表明 ,本区的玄武岩总体上属于橄榄拉斑玄武岩 ,并具有向碱性玄武岩和石英拉斑玄武岩两个方向演化的趋势。它们是来自亏损地幔的橄榄拉斑质玄武质或苦橄质岩浆经过分异并混染少量地壳物质的产物。

呼伦湖早白垩世碱性流纹岩的地球化学特征及其意义

大兴安岭呼伦湖一带的上库力组第 3段流纹岩具有高硅 (Si O2 =75.41 %～77.32 % )、富碱 (Na2 O+K2 O=7.98%～ 8.74% )、富 Y、Nb、Zr及低 Al、Mg、Ca、Ni、Cr、Ti和高 Ga/Al比值等特点 ,类似于 A型花岗岩 ,而与高度分异的 I型和 S型花岗岩有明显差别。该流纹岩的地球化学特征类似于 A1型花岗岩 ,且与该区 A型花岗岩同时 ,很可能是 A型花岗岩浆喷出相的产物。流纹岩的全岩 Rb- Sr等时线年龄 1 2 7± 5Ma,与伊列克得组玄武岩(1 2 5± 2 Ma)时代一致 ,两者构成了双峰式火山岩组合 ,形成于早白垩世岩石圈拉张环境。