赣东北中元古界铁沙街组石英角斑岩和流纹岩锆石SHRIMP U-Pb年龄、Hf同位素及地球化学特征   总被引：1，自引：0，他引：1  

张恒  李廷栋  高林志  耿树方  丁孝忠  刘燕学  寇彩化 《地质论评》2015,61(1):65-78

本文首次测得江西弋阳中元古界铁沙街组石英角斑岩SHRIMP锆石U-Pb年龄为1154±5 Ma。测定发现铁沙街组石英角斑岩和流纹岩高SiO_2(73.53%~81.11%),低CaO(0.06%~0.11%)、Na_2O(0.05%~2.77%),K_2O为2.16%~6.43%和Al_2O_3为11.06%~13.96%;在Zr/TiO_2—Nh/Y微量元素分类图解上绝大多数样品落于碱性流纹岩区域;在SiO_2—FeO/MgO图解中绝大多数样品位于钙碱性岩石系列区域;稀土总量(ΣREE)为182.48×10~(-6)~818.82×10~(-6),轻稀土较富集,铕明显负异常(δEu=0.20~O.45);在原始地幔标准化图解中,表现出亏损大离子亲石元素(Ba、Sr、Ti、P)和富集高场强元素(Nb、Ta、La、Ce、Zr、Hf、Sm)。铁沙街组石英角斑岩和流纹岩的锆石的ε_(Hf)(t)值分别为-6.1~5.3和-2.3~4.8,两阶段Hf模式年龄分别为1653~2380 Ma和1675~2132 Ma。锆石Hf同位素特征显示铁沙街组石英角斑岩、流纹岩的成因主要为岩石圈拆沉,深部地幔物质上涌,并由此造成的古元古代晚期下地壳物质的部分熔融。综上认为,铁沙街组石英角斑岩和流纹岩形成于中元古界后造山阶段的板内环境,可能与华夏陆块板内裂谷减薄开裂阶段的地幔物质上涌有关。  相似文献   

Liquidus Equilibria in the System K2O-Na2O-Al2O3-SiO2-F2O-1-H2O to 100 MPa: I. Silicate-Fluoride Liquid Immiscibility in Anhydrous Systems     

DolejS  David; Baker  Don R. 《Journal of Petrology》2007,48(4):785-806

Liquidus relations in the four-component system Na₂O–Al₂O₃–SiO₂–F₂O_–1were studied at 0· 1 and 100 MPa to define the locationof fluoride–silicate liquid immiscibility and outlinedifferentiation paths of fluorine-bearing silicic magmas. Thefluoride–silicate liquid immiscibility spans the silica–albite–cryoliteand silica–topaz–cryolite ternaries and the haplogranite-cryolitebinary at greater than 960°C and 0· 1–100 MPa.With increasing Al₂O₃ in the system and increasing aluminum/alkalication ratio, the two-liquid gap contracts and migrates fromthe silica liquidus to the cryolite liquidus. The gap does notextend to subaluminous and peraluminous melt compositions. Forall alkali feldspar–quartz-bearing systems, the miscibilitygap remains located on the cryolite liquidus and is thus inaccessibleto differentiating granitic and rhyolitic melts. In peralkalinesystems, the magmatic differentiation is terminated at the albite–quartz–cryoliteeutectic at 770°C, 100 MPa, 5 wt % F and cation Al/Na =0· 75. The addition of topaz, however, significantlylowers melting temperatures and allows strong fluorine enrichmentin subaluminous compositions. At 100 MPa, the binary topaz–cryoliteeutectic is located at 770°C, 39 wt % F, cation Al/Na 0·95, and the ternary quartz–topaz–cryolite eutecticis found at 740°C, 32 wt % F, 30 wt % SiO₂ and cation Al/Na 0· 95. Such location of both eutectics enables fractionationpaths of subaluminous quartz-saturated systems to produce fluorine-rich,SiO₂-depleted and nepheline-normative residual liquids. KEY WORDS: silicate melt; granite; rhyolite; fluorine; liquid immiscibility  相似文献   

Crystal Size Distributions (CSD) in Three Dimensions: Insights from the 3D Reconstruction of a Highly Porphyritic Rhyolite   总被引：7，自引：2，他引：5  

MOCK  A.; JERRAM  D. A. 《Journal of Petrology》2005,46(8):1525-1541

Growth histories and residence times of crystals in magmaticsystems can be revealed by studying crystal sizes, size distributionsand shapes. In this contribution, serial sectioning has beenemployed on a sample of porphyritic rhyolite from a Permo-Carboniferouslaccolith from the Halle Volcanic Complex, Germany, to reconstructthe distribution of felsic phenocrysts in three dimensions inorder to determine their true shapes, sizes and three-dimensionalsize distributions. A model of all three phenocryst phases (quartz,plagioclase, K-feldspar) with 217 crystals, and a larger modelcontaining 1599 K-feldspar crystals was reconstructed in threedimensions. The first model revealed a non-touching frameworkof crystals in three dimensions, suggesting that individualcrystals grew freely in the melt prior to quenching of the texture.However, crystal shapes are complex and show large variationon a Zingg diagram (intermediate over long axis plotted againstshort over intermediate axis). They often do not resemble thecrystallographic shapes expected for phenocrysts growing unhinderedfrom a melt, indicating complex growth histories. In contrast,the three-dimensional size distribution is a simple straightline with a negative slope. Stereologically corrected size distributionsfrom individual sections compare well with stereologically correctedsize distributions obtained previously from the same sample.However, crystal size distribution (CSD) data from individualsections scatter considerably. It is shown that CSDs can berobustly reproduced with a sampling size of greater than 200crystals. The kind of shape assumed in stereological correctionof CSDs, however, has a large influence on the calculation andestimation of crystal residence times. KEY WORDS: 3D reconstruction; crystal shapes; CSD; porphyritic rhyolite; quantitative petrography  相似文献   

The volcanic history of Volcán Alcedo,Galápagos Archipelago: A case study of rhyolitic oceanic volcanism     

Dennis Geist  Keith A. Howard  A. Mark Jellinek  Scott Rayder 《Bulletin of Volcanology》1994,56(4):243-260

Volcán Alcedo is one of the seven western Galápagos shields and is the only active Galápagos volcano known to have erupted rhyolite as well as basalt. The volcano stands 4 km above the sea floor and has a subaerial volume of 200 km³, nearly all of which is basalt. As Volcán Alcedo grew, it built an elongate domal shield, which was partly truncated during repeated caldera-collapse and partial-filling episodes. An outward-dipping sequence of basalt flows at least 250 m thick forms the steepest (to 33°) flanks of the volcano and is not tilted; thus a constructional origin for the steep upper flanks is favored. About 1 km³ of rhyolite erupted late in the volcano's history from at least three vents and in 2–5 episodes. The most explosive of these produced a tephra blanket that covers the eastern half of the volcano. Homogeneous rhyolitic pumice is overlain by dacite-rhyolite commingled pumice, with no stratigraphic break. The tephra is notable for its low density and coarse grain size. The calculated height of the eruption plume is 23–30 km, and the intensity is estimated to have been 1.2x10⁸ kg/s. Rhyolitic lavas vented from the floor of the caldera and from fissures along the rim overlie the tephra of the plinian phase. The age of the rhyolitic eruptions is 120 ka, on the basis of K-Ar ages. Between ten and 20 basaltic lava flows are younger than the rhyolites. Recent faulting resulted in a moat around part of the caldera floor. Alcedo most resently erupted sometime between 1946 and 1960 from its southern flank. Alcedo maintains an active, transient hydrothermal system. Acoustic and seismic activity in 1991 is attributed to the disruption of the hydrothermal system by a regional-scale earthquake.  相似文献   

西藏多龙矿集区发现早白垩世流纹岩夹层   总被引：2，自引：1，他引：1  

王伟  李才  范建军  吴浩  许伟 《地质通报》2016,35(8):1255-1262

继西藏冈底斯成矿带之后,新近探明的具有超大型远景的典型斑岩型铜金矿区——多龙矿集区地处西藏改则县西北部,班公湖-怒江缝合带北缘多不扎地区。早先西藏1∶25万物玛幅区调工作对多龙矿集区做过详细调查,填补了该缝合带北部研究的空白。近来区域地质调查发现,该区原先厘定的下侏罗统曲色组内出露早白垩世流纹岩夹层。流纹岩LA-ICP-MS锆石U-Pb年龄为118.8±1.4Ma,表明该套夹火山岩沉积地层的形成时代为早白垩世。这一发现补充了南羌塘早白垩世岩浆活动记录,为研究多龙矿集区成矿地质背景和班公湖-怒江洋演化提供了重要线索。初步研究认为,在中生代增生杂岩广泛出露的多龙矿集区,该套地层很可能代表一套早白垩世增生杂岩,同时也可能是同时期区域伸展环境下重沉积的产物。  相似文献   

海拉尔盆地上库力组两类流纹岩的球粒对比研究     

刘芳 《地质与勘探》2020,56(2):337-345

通过野外观察,结合薄片、铸体薄片等手段研究,发现海拉尔盆地呼伦湖凹陷西界上库力组火山通道相次火山亚相的柱状节理流纹岩和喷溢相上部亚相的球粒流纹岩中发育两种不同结构的球粒,两种球粒均形成于高的过冷却动力学效应下。柱状节理流纹岩中的球粒肉眼不可见,球粒直径小于0. 2 mm,球粒中心发育结晶核,组成球粒的纤维体多为隐晶质。条带状球粒流纹岩中的球粒在野外露头上醒目,圆形、椭圆形特征明显,球粒直径在0. 5～2 mm之间,均匀分布,结晶核不明显,组成球粒的纤维体为霏细质。本文认为导致这两种球粒结构差异性的主要原因是受熔浆过冷度大小的影响,柱状节理流纹岩过冷度较条带状球粒流纹岩大,因此成核速率高,成核密度大,球粒直径小,排列紧密,条带状球粒流纹岩较之相反。同时,流纹质熔浆的均质性程度影响了球粒间孔的充填特征。此外,研究发现,球粒间孔的发育程度和球粒形态直接相关。柱状节理流纹岩基质中的球粒结构紧密排列,球粒受相互生长制约呈不规则状,由于柱状节理本身由十分致密的球粒构成,储集空间不发育,因此不含油。由于条带状球粒流纹岩的球粒排列相对疏松,孔隙大量发育,可能含有丰富的油气,储集空间主要类型为球粒间孔和球粒溶蚀孔。  相似文献   

The emplacement of an obsidian dyke through thin ice: Hrafntinnuhryggur, Krafla Iceland   总被引：2，自引：1，他引：1  

Hugh Tuffen  Jonathan M. Castro   《Journal of Volcanology and Geothermal Research》2009,185(4):352

An eruption along a 2.5 km-long rhyolitic dyke at Krafla volcano, northern Iceland during the last glacial period formed a ridge of obsidian (Hrafntinnuhryggur). The ridge rises up to 80 m above the surrounding land and is composed of a number of small-volume lava bodies with minor fragmental material. The total volume is < 0.05 km³. The lava bodies are flow- or dome-like in morphology and many display columnar-jointed sides typical of magma–ice interaction, quench-fragmented lower margins indicative of interaction with meltwater and pumiceous upper surfaces typical of subaerial obsidian flows. The fragmental material compromises poorly-sorted perlitic quench hyaloclastites and poorly-exposed pumiceous tuffs. Lava bodies on the western ridge flanks are columnar jointed and extensively hydrothermally altered. At the southern end of the ridge the feeder dyke is exposed at an elevation  95 m beneath the ridge crest and flares upwards into a lava body.Using the distribution of lithofacies, we interpret that the eruption melted through ice only 35–55 m thick, which is likely to have been dominated by firn. Hrafntinnuhryggur is therefore the first documented example of a rhyolitic fissure eruption beneath thin ice/firn. The eruption breached the ice, leading to subaerial but ice/firn-contact lava effusion, and only minor explosive activity occurred. The ridge appears to have been well-drained during the eruption, aided by the high permeability of the thin ice/firn, which appears not to have greatly affected the eruption mechanisms. We estimate that the eruption lasted between 2 and 20 months and would not have generated a significant jökulhlaup (< 70 m³ s^− 1).  相似文献   

青藏高原东北缘伯阳地区第三系流纹岩地球化学及岩石成因   总被引：9，自引：2，他引：7  

赖绍聪  张国伟  秦江锋  李永飞  刘鑫 《地学前缘》2006,13(4):212-220

伯阳第三系流纹岩出露于青藏高原东北缘特殊的构造部位,位于青藏、华北和扬子三大构造域的交接转换区域。岩石的w(Si O2)介于68%~76%,w(K2O)>w(Na2O),w(K2O)/w(Na2O)平均值1·25,为一套典型的壳源流纹岩岩石系列。岩石微量及稀土元素具有典型的板内火山岩特征,K、Th、Rb等元素呈较明显的富集状态,而岩石显著的低Sr特征((19~120)×10-6)表明其并非源自加厚的下地壳,而是起源于斜长石稳定的正常下地壳。正是由于新生代期间青藏高原东北缘强烈的造山环境,加之渭河断裂的发育为下地壳物质提供了减压熔融的有利条件,从而诱发下地壳的局部熔融,形成伯阳酸性火山岩的原始岩浆。该岩浆体系沿区域断裂构造体系上升,并经历了较强的结晶分异和演化,最终形成伯阳第三系流纹岩系列。  相似文献   

The age and tectonic environment of the rhyolitic rocks on the western side of Wuyi Mountain,South China   总被引：5，自引：1，他引：4  

LiangShu Shu  Ping Deng  JinHai Yu  YanBin Wang  ShaoYong Jiang 《中国科学D辑(英文版)》2008,51(8):1053-1063

During the geological survey of the metamorphic rocks in Xingning-Wuhua region on the western side of Wuyi Mountain, South China, we discovered the Neoproterozoic rhyolite and rhyolitic greywacke for the first time that outcrop in the Proterozoic metamorphic rocks near Jingnan Town of Xingning County, eastern Guangdong Province. A systematic research on petrology, geochemistry and geochronology of rhyolitic rocks was conducted to understand their tectonic setting and formation age. The Jingnan rhyolite is interbedded with a coeval greywacke, with a total thickness of 60 m; both rhyolite and greywacke display a similar folding and metamorphic pattern. Meta-rhyolite consists of groundmass and phenocrystals including sanidine, orthoclase, and quartz with distinct undulose extinction; the groundmass has been recrystallized into fine-grain feldspar, quartz and sericite aggregation. Meta-greywacke is composed of crystallinoclastic grains (sanidine, orthoclase, quartz and oligoclase) and clay groundmass. Zircon grains used for the SHRIMP U-Pb analysis are light brown-colored and euhedral or subeuhedral. Dating data suggest two age groups; eight grains of magmatype zircon with an idiomorphic form yield an age of 972±8 Ma, and the other seven weakly corroded grains of zircon with euhedral to subeuhedral shape construct an average age of 1097±11 Ma, which were captured from older rocks by an uplifting magma, implying that a late Mesoproterozoic basement exists in the Nanling region. In addition, one Paleoproterozoic age, 2035±11 Ma, is obtained from a rounded detrital zircon, indicating that a Paleoproterozoic thermal event took place in the South China. Geochemically, the Jingnan rhyolitic rocks are characterized by high K2O content, intermediate Al2O3 content, with the ACNK value 0.98―1.11, and belonging to high-K alkaline series. They are rich in ΣREE, Rb, Th and Ce, depleted in Ba, Sr, Eu, Ti, P and Nb-Ta, and with moderate negative Eu and Sr anomalies. These features indicate that the Jingnan volcanic rocks have an affinity of continental arc that is similar to those of acid volcanic rocks in the SE-China Coastal Region, in other words, a Neoproterozoic tectonomagma event might have taken place in the western Wuyi region, leading to an eruption of high-K calc-alkaline granitic magma.  相似文献   

浙江缙云中生代火山岩丝光沸石矿物学特征及其成因浅析   总被引：5，自引：0，他引：5  

张术根  丁俊  刘小胡  王海平  徐宏峰 《矿物学报》2006,26(2):159-164

运用X射线衍射、红外吸收光谱、差热-热重分析、扫描电镜形貌研究及微区化学成分分析等手段,研究了浙江省缙云县产于球泡流纹岩球泡腔体内和产于角砾凝灰岩、凝灰熔岩型沸石矿体内的丝光沸石的矿物学特征及其成因机制。研究表明,球泡腔体内的丝光沸石与矿体内丝光沸石的矿物学特征存在明显差别:前者仅与石英紧密共生,而后者则与斜发沸石、石英以及蒙脱石等紧密共生;前者12元环通道半径收缩,而后者则8元环通道半径收缩。同时,前者具有更高的K2O/CaO、SiO2/Al2O3以及K2O/Na2O比值,而后者则阳离子电价过剩程度更高。研究认为,球泡内丝光沸石是球泡捕获的火山热液结晶产物,而矿体内丝光沸石是火山岩成岩期间隙溶液活动产物。  相似文献