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1.
Experimental solidification of anhydrous latitic and trachytic melts at different cooling rates: The role of nucleation kinetics 总被引：1，自引：0，他引：1
Gianluca Iezzi Silvio Mollo Guido Ventura Andrea Cavallo Claudia Romano 《Chemical Geology》2008,253(3-4):91-101
Two sets of cooling experiments were run at atmospheric conditions for two anhydrous starting latitic and trachytic melts: 1) five cooling rates (25, 12.5, 3, 0.5, and 0.125 °C/min) between 1300° and 800 °C, and 2) a 0.5 °C/min cooling rate from 1300 °C with quench temperatures at 1200°, 1100°, 1000° and 900 °C. Trachytic run-products are invariably glassy. Nucleation is also suppressed in the latitic run-products at the three highest cooling rates. Conversely, in the 0.5 and 0.125 °C/min runs, latites have a crystal content of 90 vol.%. The phases are: plagioclase, clinopyroxene, glass and iron-bearing oxide (in order of abundance). The variable quench temperatures, investigated by coupling experiments with Pt wire and Pt capsule sample containers in set 2, again did not produce crystallization of trachyte, whereas latitic samples are characterized by 10 vol.% of oxides, pyroxenes and plagioclase (in order of appearance), at temperature < 1000 °C. Effects of (preferential) heterogeneous nucleation on sample holders, of superheating degree, and chemical species loss during cooling are absent for both melt compositions. The difference of solidification paths between these two silicate melts can be ascribed only to their small chemical differences. In comparison with calculated equilibrium conditions all the experimental latitic and trachytic run-products revealed strong kinetic effects, interpretable in the light of the nucleation theory. The glass-forming ability (GFA) of trachyte is higher, whereas their critical cooling rate (Rc) is lower (< 0.125 °C/min), in comparison to latitic melts (Rc > 0.5 °C/min). The experimental results carried out in this study can be applied to lava flows and domes; trachytic lavas are able to flow for longer period with respect to latitic ones in a metastable condition. Glass-rich terrestrial lavas, i.e. obsidians, can be the result of sluggish nucleation kinetics due to the relative high polymerisation of evolved silicate melts. 相似文献
2.
the single ignimbrite cooling unit E (average thickness, 28 m; volume, ca. 30 km3) forms the uppermost member of the Miocene Upper Mogán Formation on Gran Canaria. It is strongly chemically zoned from basal, first-erupted comendite (peralkaline rhyolite) to late-erupted trachyte, and, apart from an upper trachytic zone, it is densely welded. E was emplaced onto a surface inclined ca. 2–5° from the source caldera. Detailed mapping of key sections, up to 300 m long, exposed in barranco walls, ca. 10 km from the caldera margin, reveals structures that are interpreted to have been produced by rheomorphic deformation of the ignimbrite along shear zones. The shear zones formed within the lower-viscosity comenditic tuff. Extensional structures include mega-boudinage and decapitated sequences and compression resulted in sequence repitition by overthrusting. Mechanisms traditionally thought to be important during rheomorphic deformation of welded tuffs (compaction, lateral creep, folding, vertical density-driven diapirism) cannot account for these features, which reflect lateral (post-compactional) rheomorphic movement locally in excess of 800 m. We suggest the following sequence of events: emplacement of the several flow units; compaction, with little lateral movement; rheomorphic deformation. During and after compaction, layers of secondary porosity developed within the comenditic tuff, possibly where upward escape of gas was prevented by overlying, relatively impermeable layers of densely compacted ignimbrite. These structurally weak layers of high porosity subsequently acted as shear zones. 相似文献
3.
Application of the QUILF thermobarometer to the peralkaline trachytes and pantellerites of the Eburru volcanic complex, East African Rift, Kenya 总被引：1，自引：0，他引：1
Minghua Ren Peter A. Omenda Elizabeth Y. Anthony John C. White Ray Macdonald D.K. Bailey 《Lithos》2006,91(1-4):109-124
The Quaternary Eburru volcanic complex in the south-central Kenya Rift consists of pantelleritic trachytes and pantellerites. The phenocryst assemblage in the trachytes is sanidine + fayalite + ferrohedenbergite + aenigmatite ± quartz ± ilmenite ± magnetite ± pyrrhotite ± pyrite. In the pantellerites, the assemblage is sanidine + quartz + ferrohedenbergite + fayalite + aenigmatite + ferrorichterite + pyrrhotite ± apatite, although fayalite, ferrohedenbergite and ilmenite are absent from more evolved rocks (e.g. with SiO2 > 71%). QUILF temperature calculations for the trachytes range from 709 to 793 °C and for the pantellerites 668–708 °C, the latter temperatures being among the lowest recorded for peralkaline silicic magmas. The QUILF thermobarometer demonstrates that the Eburru magmas crystallized at relatively low oxidation states (ΔFMQ + 0.5 to − 1.6) for both trachytes and pantellerites. The trachytes and pantellerites evolved along separate liquid lines of descent, the trachytes possibly deriving from a more mafic parent by fractional crystallization and the pantellerites from extreme fractionation of comenditic magmas. 相似文献
4.
John Charles White S. Christian Benker Minghua Ren Kevin M. Urbanczyk Donald W. Corrick 《Lithos》2006,91(1-4):74-94
The Pine Canyon caldera is a small (6–7 km diameter) ash-flow caldera that erupted peralkaline quartz trachyte, rhyolite, and high-silica rhyolite lavas and ash-flow tuffs about 33–32 Ma. The Pine Canyon caldera is located in Big Bend National Park, Texas, USA, in the southern part of the Trans-Pecos Magmatic Province (TPMP). The eruptive products of the Pine Canyon caldera are assigned to the South Rim Formation, which represents the silicic end member of a bimodal suite (with a “Daly Gap” between 57 and 62 wt.% SiO2); the mafic end member consists primarily of alkali basalt to mugearite lavas of the 34–30 Ma Bee Mountain Basalt. Approximately 60–70% crystallization of plagioclase, clinopyroxene, olivine, magnetite, and apatite from alkali basalt coupled with assimilation of shale wall rock (Ma/Mc = 0.3–0.4) produced the quartz trachyte magma. Variation within the quartz trachyte–rhyolite suite was the result of 70% fractional crystallization of an assemblage dominated by alkali feldspar with subordinate clinopyroxene, fayalite, ilmenite, and apatite. High-silica rhyolite is not cogenetic with the quartz trachyte–rhyolite suite, and can be best explained as the result of 5% partial melting of a mafic granulite in the deep crust under the fluxing influence of fluorine. Variation within the high-silica rhyolite is most likely due to fractional crystallization of alkali feldspar, quartz, magnetite, biotite, and monazite. Lavas and tuffs of the South Rim Formation form A-type rhyolite suites, and are broadly similar to rock series described in anorogenic settings both in terms of petrology and petrogenesis. The Pine Canyon caldera is interpreted to have developed in a post-orogenic tectonic setting, or an early stage of continental rifting, and represents the earliest evidence for continental extension in the TPMP. 相似文献
5.
三百山盆地的粗面岩主要出现在盆地东部仰天湖破火山口构造中,粗面岩锆石SHRIMP测年的年龄为141.56±0.96 Ma,应归属于早白垩世。盆地粗面岩具有钾含量高,全碱含量高,稀土元素配分曲线图表现为右倾的轻稀土富集型,Eu负异常不明显或表现为正异常的特征,表明三百山盆地的粗面岩属橄榄玄粗岩系列,具有高压型粗面岩的特征,标志着一个加厚的陆壳存在;岩浆物质来源于由上地壳物质俯冲到深部与地幔的混合而形成的再循环富集地幔,岩石形成的构造环境为＂同碰撞＂环境,代表挤压至拉张过渡构造环境。 相似文献
6.
Jian-Lin ChenWen-Xia Zhao Ji-Feng Xu Bao-Di WangZhi-Qiang Kang 《Gondwana Research》2012,21(1):112-122
It is generally believed that Cenozoic potassic and ultrapotassic volcanic rocks of the Tibetan Plateau were generated by partial melting of an enriched mantle region or lower crustal materials. The Miocene Bugasi volcanic rocks (BVR) in the western part of the Lhasa block are composed mainly of trachyandesites and trachytes, both of which belong to the shoshonite series. The trachytes show somewhat transitional compositions between the mantle-derived trachyandesites of the BVR and the crust-derived potassic rocks of Konglongxiang, most evident in their Sr, Ba, and Ni concentrations, Nb/Ta, Rb/Sr, Th/Nb, Zr/Nb, and Ba/Rb trace element ratios, and Sr and Nd isotopic compositions. These features, coupled with the relatively high Cr and Ni concentrations and Mg#, suggest that the trachytes are the product of mixing between mantle-derived and lower crust-derived melts. 相似文献
7.
长江中下游成矿带贵池抛刀岭金矿含矿岩体年代学及地球化学研究 总被引：13，自引：10，他引：3
贵池抛刀岭金矿位于长江中下游成矿带安庆-贵池矿集区,是该地区首次发现的独立玢岩型金矿床,含矿岩体主要为蚀变矿化的英安玢岩.经过近两年的地质勘查,该矿床已接近大型金矿床规模.通过对抛刀岭金矿含矿岩体锆石LA-ICPMS U-Pb测定,其成岩年龄为146.8±2.4Ma和141.3±1.0Ma(2σ),与长江中下游地区中生代大规模铜金成矿事件时间一致.全岩地球化学研究表明,该岩石为过铝质,富集大离子亲石元素(K、Rb、Pb),亏损高场强元素(Nb、Ta、Ti)和轻稀土富集等地球化学特征.较高的锆石Ce4+/Ce3+和EuN/EuN*显示抛刀岭含矿岩体具有较高的氧逸度特征.构造判别图解显示这套含矿岩石属火山弧花岗岩(VAG),与长江中下游铜金矿床含矿岩体形成于同一构造背景下,即太平洋俯冲作用引起的洋壳和地幔楔熔融形成. 相似文献
8.
石溪盆地和三百山盆地粗面岩均具有高钾、高碱、高K2O／Na2O、高铝、低铁、低钛、氧化指数较高的特点,在SiO2-K2O图解中落入橄榄玄粗岩系列岩石区域之内,且富集高场强元素（HFSE）和大离子亲石元素（LILE）,将两盆地粗面岩归为橄榄玄粗岩系列岩石。两盆地粗面岩均为地壳起源岩浆,且后期有地幔物质的加入,构造环境都是碰撞后拉张构造环境。不同的是石溪盆地粗面岩是经历了较强的结晶分异作用的产物。 相似文献
9.
Y. H. Dawood H. H. Abd El-Naby A. A. Sharafeldin 《Journal of Geochemical Exploration》2004,81(1-3):15-27
El Atshan mining area, central Eastern Desert, represents one of the uranium occurrences related to alkaline volcanic rocks in Egypt. Based on the plot of total alkali elements versus silica, these rocks are classified as trachytes. The U and Eu anomalies appear to be derived from trachyte exposed to a long period of alteration and rock–fluid interaction. The trachyte has been subjected to two phases of alteration. The pronounced chemical changes include the mobility of Si, Na, Fe, U, Zn and REE and the immobility of Mg, Th, Hf, Ta and Sc. The late stage hydrothermal solutions caused the breakdown of the feldspars by losing sodium, potassium and partially silica and eventually formation of argillic alteration products, dissolution of iron-bearing sulphides, formation of iron-oxy hydroxides and corrosion of primary uranium minerals forming uranyl oxide hydrates. The acidic water percolating through the fractured trachyte rock leached not only available major or trace elements, but also REE. Eu originally incorporated in feldspars as Eu+2 has been oxidized to Eu+3 and subsequently leached away leaving a negative anomaly in the host rock. The leached U and Eu were then transported most probably as carbonate complexes. The second phase of alteration occurred at the near surface profile when the late stage hydrothermal fluids cool to the temperature of meteoric water and may have mixed with it, the pH of the fluids would shift to more alkaline values and at these conditions U and Eu were precipitated into the fracture system mainly by being adsorbed on the clay minerals and probably coprecipitated with iron oxy-hydroxides. 相似文献
10.
赣杭构造带新路火山盆地粗面斑岩的年代学、岩石成因及其地质意义 总被引：1，自引：1，他引：0
在赣杭构造带上新路盆地中发现有晚期侵入到黄尖组凝灰岩中的粗面斑岩岩脉。本文运用激光等离子质谱(LAICP-MS)对该粗面斑岩中的锆石进行了U-Pb年龄测定,获得了131±1Ma的形成年龄。新路粗面斑岩具有高碱、高钾、高K2O/Na2O,在Si O2-K2O图解中落入橄榄安粗岩(shoshonite)系列岩石的范围之内。新路粗面斑岩还具有贫铁、钛、钙、磷以及富集轻稀土和大离子亲石元素等特征,具有较高含量的高场强元素,亏损Nb、Ta、Sr、P、Ti,以及中等铕负异常(Eu/Eu*=0.32~0.46)和铈负异常(Ce/Ce*=0.64~0.73)的特点。粗面斑岩的全岩εNd(t)值为-5.01~-4.77,锆石的εHf(t)值为-8.3~-3.8(集中在-6~-4之间),两阶段Hf模式年龄集中在1.4~1.6Ga之间。这些地球化学特征表明新路粗面斑岩是幔源岩浆和壳源岩浆混合形成的产物,为赣杭构造带早白垩世幔源岩浆活动提供了重要证据。 相似文献