Totally found 350 items.

  • [期刊] Thermodynamic modelling of phosphate minerals and its implications for the development of P-T-t histories: A case study in garnet - monazite bearing metapelites
    Accessory phosphate minerals, such as monazite and xenotime, are commonly used as geochronometers to provide timing constraints on tectonometamorphic events recorded by the host rocks within which they occur. However, the formation and recrystallization of accessory minerals, and their interaction with major mineral phases, are still poorly understood. As a consequence, linking ages obtained from accessory minerals to the metamorphic pressure-temperature (P-T) paths obtained primarily from major mineral phases, such as garnet, remains challenging. While there have been studies that have advanced our understanding of the behaviour of various accessory minerals through thermodynamic modelling, limited examples are available to test their reliability in natural metamorphic rocks.
  • [期刊] The Miocene Goldfield-Superstition volcanic province, Central Arizona, USA: Geochemically distinct rhyolite sources, 20.5 to 19 Ma
    The Miocene Goldfield-Superstition volcanic province (G-SVP) in central Arizona is composed of silicic pyroclastic and lava flows, and, to a small extent, silicic domes, and mafic and intermediate lavas. Volcanism began similar to 20.5 Ma with small-scale silicic domes, followed by <= 1.5 m.y. of basaltic, andesitic, and dacitic lavas, after which large-scale pyroclastic flows and lavas created the bulk of this large province from similar to 19 to 18.5 Ma (similar to 8000 km(2)).
  • [期刊] The Devonian back-arc basin and Triassic arc-continent collision along the Imjingang belt in the Korean Peninsula and their tectonic meaning
    The Imjingang belt, which is located between the Nangrim and Gyeonggi Massifs, is divided into the Imjin group in the north and the Samgot unit in the south. The southeastern part of the Imjin group, which is located within South Korea, is called the Jingok unit. The Samgot unit consists of amphibolite, psammitic schist, meta-alkali granite and meta-calc-silicate rock. The amphibolite experienced prograde metamorphism from 648-550 degrees C/13-11 kbar to 715 degrees C/11 kbar and was then retrograded to ca. 579-569 degrees C/6.7-6.3 kbar. The psammitic schist was metamorphosed at ca. 710 degrees C/11 kbar and then retrograded to ca. 670-660 degrees C/11-10 kbar. These data indicate that the Samgot unit underwent clockwise prograde metamorphism with peak metamorphism conditions of 715-710 degrees C/11 kbar, which is confirmed by pseudosection analysis. The results of this and previous studies indicate that the metamorphic grade increases southward from the garnet and staurolite zones (630-500 degrees C/9-3 kbar) through the kyanite zone (710-665 degrees C/9 kbar) in the Jingok unit to the Samgot unit (715-710 degrees C/11 kbar) of the Imjingang belt. This metamorphic pattern indicates that the Imjingang belt underwent regional intermediate-P/T metamorphism. The SHRIMP U-Pb age dating of zircons from the amphibolite and meta-alkali granite yields intrusion ages of ca. 371-361 Ma and 373 Ma, respectively. From the psammitic schist, ages of ca. 401-370 Ma are obtained from the youngest detrital zircons and ages of ca. 251-249 Ma are obtained from the metamorphic zircons. In addition, the SHRIMP U-Pb age dating of titanite from the amphibolite yields a retrograde metamorphic age of ca. 229-214 Ma. The whole-rock chemical composition of the amphibolite plots in the back-arc basin basalt field on the tectonic discrimination diagrams. These data indicate that the Imjingang belt, including the Samgot unit, consists of Devonian sedimentary rocks that were deposited in a back-arc basin and intruded by mafic and felsic igneous rocks at ca. 373-361 Ma. Finally, the Imjingang belt underwent intermediate-P/T metamorphism during the Triassic (251-249 Ma) instead of high-P/T metamorphism, indicating that the Imjingang belt is not the extension of the Dabie-Sulu collision belt but a back-arc basin within the North China Craton that underwent intermediate-P/T metamorphism due to the closure of the back-arc basin (arc-continent collision) during the collision between the North and South China Cratons along the Hongseong-Odesan collision belt within the Gyeonggi Massif. (C) 2019 Published by Elsevier B.V.
  • [期刊] Rare earth element uptake during olivine/water hydrothermal interaction
    Ultramafic-hosted hydrothermal vent systems link the hydrosphere with the peridotitic mantle via serpentinization. Here, the fractionation and mobility of selected trace (Nd, Sm, Gd, Dy, Yb, Sr and Ba) and major cations (Si, Ca, Mg, Fe, Ni) during seawater-peridotite interaction was investigated through a series of experiments where natural olivine grains were reacted with an artificial seawater solution at a range of temperatures (15-90 degrees C) and grain size distributions. No evidence for any significant olivine dissolution or precipitation of carbonate and, Fe-oxy-hydroxide phases was observed in these experiments. Experimental data show a strong decoupling of REE (Nd, Sm, Gd, Dy, and Yb) from Sr and Ba under all experimental conditions, with Sr and Ba remaining quantitatively in solution. The REE were removed from the solution and were adsorbed onto olivine surface with kinetic rate constants (i.e. uptake over time) that increase with increasing temperature and increasing surface area (i.e. decreasing particle size). Dysprosium and Yb (heavy REE; HREE) were removed from solution with a faster rate than Nd and Sm (light REE; LREE). Gadolinium is decoupled from this trend, with a slower kinetic rate constant than Sm. The activation energies (E-a) of REE adsorption on olivine were higher for Nd and Sm than Dy and Yb. This suggests that the adsorbance of LREE is generally more dependent on temperature than the HREE. The E-a correlates well with the summed 1st, 2nd and 3rd ionization energies of REE suggesting a link between kinetic rates of element adsorption and electron configuration of the 4f-orbitals. Gadolinium has higher Ea than the other analyzed REE, consistent with the electron configuration of Gd3+ where all 4f-orbitals are filled with one electron each. These experimental data suggest that REE are adsorbed on the surface of olivine via inner sphere complexes under low-temperature hydrothermal conditions, when alteration processes are limited or extremely slow. Scavenging and fractionation of REE may occur within the recharge zone of peridotite-hosted hydrothermal systems at relatively low temperatures (<100 degrees C), leading to fluids with progressively higher LREE/HREE which could impose seawater-derived LREE enrichments in serpentinized peridotites during high temperature, high pressure water/rock interaction deeper in the oceanic lithosphere. (C) 2019 Elsevier B.V. All rights reserved.
  • [期刊] Late Cretaceous I- and A-type magmas in eastern Turkey: Magmatic response to double-sided subduction of Paleo- and Neo-Tethyan lithospheres
    We provide geochronological (zircon U-Pb) and geochemical data (whole rock major and trace elements, Sr-Nd-Pb isotopes, mineral compositions, zircon trace elements) for Late Cretaceous I- and A-type granitoids and associated gabbros from three areas (Dereli, Sebinkarahisar, Divrigi), along a north-south traverse in eastern Turkey from the northern part of the Eastern Pontides Orogenic Belt to the Anatol ide-Tauride Block, and from two locations (Sokmen and Pimalli) that are situated away from the traverse line. Our aim is to interpret the tectonic environment in which the granitoids formed. Late Cretaceous plutonic activity began similar to 90 Ma and continued uninterruptedly for about 20 my. The Dereli suite and the Asarcik intrusion at Sebinkarahisar are 1-type granitoids; the others are A-type granitoids. The A-type classification of these rocks is ambiguous; they are A-typerelative to other coeval granitoids in eastern Turkey. The Sebinkarahisar, Murmana (Divrigi), and Pirnalli granitoids evolved primarily by closed-system fractional crytallization. At Sebinkarahisar, I- and A-type intrusions are isotopically indistinguishable, and must share the same source. Open system, AFC processes affected the other granitoids. Crustal components contributed to all of the granitoids. The Dereli granitoids have an old component (1.2 < T-DM < 1.75 Ga) that is unusual among Turkish igneous rocks. No mantle component is recognizable in the A-type granitoids, but the low SiO2 content of some granitoids requires a mantle-derived component. Arc-like geochemical features may be inherited from a mid-crustal source component, in addition to contributions from the arc environment active at the time of magmatism. A petrogenetic relation cannot be demonstrated between the Divrigi gabbros and their contemporaneous granitoids. The gabbros require a subcontinental lithospheric mantle source. Trace elements in zircon track fractional crystallization paths within suites, and faithfully map the arc-related petrogenesis of the granitoids, but do not mirror trace element distributions in zircons from other A-type suites. Available data indicate that Late Cretaceous plutonic activity began at about similar to 90 Ma in both the northern part of the Eastern Pontides Orogenic Belt and the southern part of the Anatolide-Tauride Block. Magma compositions began to change at ca. 80 Ma, and transformed successively into high-K I-type and shoshonitic A-type activity, and magmatism continued until similar to 70 Ma. We interpret this 20 my interval to be the subduction-dominated, arc-construction phase in the Eastern Pontides. The distribution of I-type magmas in eastern Turkey is best explained by double-sided, initially shallow-angle subduction of Paleo- and Neo-Tethyan lithospheres, with subsequent steepening of the subducted slabs leading to an extensional regime and the generation of A-type magmas. This hypothesis also explains the origin of Late Cretaceous I- and A-type Central Anatolian granitoids. (C) 2018 Elsevier B.V. All rights reserved.
  • [期刊] Evidence for rutile-bearing eclogite in the mantle sources of the Cenozoic Zhejiang basalts, eastern China
    Cenozoic intraplate basalts are widely distributed along the southeastern margin of Eurasia. Both carbonated and eclogitic/pyroxenitic components, which are considered related to oceanic crust recycling, have been identified in the mantle source of these basalts. However, the origin of the recycled oceanic crust and the genetic relationship between different source lithologies remain unclear. Here, we assess the source lithologies of late Miocene intraplate basalts from Ninghai, Zhejiang Province, eastern China, using new elemental and Sr-Nd-Hf isotopic compositional data. These data are also compared with those of the nearby Xinchang-Shengzhou flood basalts and other Cenozoic basalts in Zhejiang Province. New Mg isotopic compositions of the Ninghai and Xinchang-Shengzhou basalts are used to evaluate the potential influence of carbonated components during their formation. Nephelinites and basanites from Ninghai and Xinchang-Shengzhou have negative Ti and Hf anomalies, superchondritic Ca/Al ratios, and light Mg isotopic compositions (delta Mg-26 = -0.35 to -0.38 parts per thousand), indicating a carbonated component in the mantle source. By comparison, other Ninghai basalts have positive Ti and Nb anomalies, with Sm/Yb and Lu/Hf ratios that generally correlate with Ti/Ti* values. These observations indicate that rutile-bearing eclogite should also play an important role in the formation of the Ninghai basalts, in addition to mantle peridotite. The terrestrial mantle-likeMg-isotopic compositions of the Ninghai basalts further support their formation by high-degree partial melting of such mixed source. In the plots of Sr-Nd isotopes and Ti/Ti*, the nephelinites and basanites from Ninghai and Xinchang-Shengzhou show different trends compared with other Ninghai basalts, and the carbonated component in the mantle source of the former has similar and depleted Sr-Nd isotopes to the rutile-bearing eclogite source of the latter. Additionally, samples of these basalts show negative Delta epsilon(Hf) values similar to those of Pacific MORB. Subducted Pacific oceanic crust is therefore considered an appropriate source for both the carbonated and rutile-bearing eclogitic components. The Ninghai basalts provide an important insight into oceanic crust recycling in eastern China, with data indicating a genetic relationship between the carbonated component and rutile-bearing eclogite in the mantle sources of intraplate basalts. (C) 2018 Elsevier B.V. All rights reserved.
  • [期刊] Calcium amphibole exsolution lamellae in chromite from the Semail ophiolite: Evidence for a high-pressure origin
    We report here for the first time aluminous Ca-amphibole exsolution lamellae in an anhydrous chromite host of podiform chromitite from the Nakhl massif of the Semail Ophiolite. Transmission electron microscopy observations show that the Ca-amphibole lamellae have a crystallographic to potaxy and d-spacing relationship with the host chromite, i.e., [103](am)//[118](chr,) (020)(am)//(220)(chr) and d(060am) approximate to d(220chr), that indicate an exsolution origin. The presence of Ca-amphibole exsolution implies the incorporation of Si4+ and Ca2+ cations and hydroxyl in the host precursor chromite. The reconstructed precursor chromite had SiO2 concentrations of 033-0.58 wt%, suggesting upwelling from a minimum pressure of 8 GPa (depth of similar to 250 km). Hydroxyl groups occupy octahedral vacancies, which can be introduced in high-pressure chromite by substituting 4 Cr3+ by 3 Si4+ and a vacancy. During upwelling, vacancies decrease when Si4+ cations exsolve stepwise from chromite. It leads to a coupled decrease of OH occupied vacancies and exsolution of Ca-amphibole lamellae at pressures below 3 GPa. Our results provide new evidence for a deep origin of the chromitite-bearing mantle section of the Semail ophiolite. (C) 2019 Elsevier B.V. All rights reserved.
  • [期刊] Feedback of mantle metasomatism on olivine micro-fabric and seismic properties of the deep lithosphere
    The interaction of hydrous fluids and melts with dry rocks of the lithospheric mantle inevitably modifies their viscoelastic and chemical properties due to the formation of compositionally distinct secondary phases. In addition, melt percolation and the associated metasomatic alteration of mantle rocks may also facilitate modification of the pre-existing rock texture and olivine crystallographic preferred orientation (CPO) and thus seismic properties. Here we explore the relationship between mantle metasomatism, deformation and seismic anisotropy using subduction-related mantle xenoliths from the Penghu Islands, western Taiwan. The investigated xenoliths have equilibrated at upper lithospheric mantle conditions (879 degrees C to 1127 degrees C) based on pyroxene geothermometry and show distinct variations in clinopyroxene chemical composition, texture and olivine CPO allowing for the classification of two distinct groups. Group 1 xenoliths contain rare earth element (REE) depleted clinopyroxene, show a porphyroclastic texture and olivine grains are mostly characterized by [100]-axial pattern symmetries. In contrast, REE-enriched clinopyroxene from Group 2 xenoliths occur in a fine-grained equigranular texture and coexisting olivine frequently displays [010]-axial pattern symmetries. The clinopyroxene compositions are indicative of cryptic and modal to stealth metasomatic alteration of Group 1 and Group 2 xenoliths, respectively. Furthermore, the observed olivine [100]-axial pattern of Group 1 xenoliths reflects deformation by dislocation creep at high temperature, low pressure and dry conditions, whereas olivine [010]-axial patterns of Group 2 xenoliths imply activation of olivine [001] glide planes along preferentially wet (010) grain boundaries. This correlation indicates that the variation in olivine CPO symmetry from [100]-to [010]-axial pattern in Penghu xenoliths results from deformation and intra-crystalline recovery by subgrain rotation during metasomatic alteration induced by melt percolation. The microstructural observations and olivine CPO combined with petrological and geochemical data suggest that Group 1 xenoliths preserve microstructural and chemical characteristics of an old, probably Proterozoic lithosphere, while Group 2 xenoliths record localized Miocene deformation associated with wall-rock heating and metasomatism related to melt circulation. Furthermore, the observed transition of olivine CPO from [100]-axial pattern to [010]-axial pattern by deformation in the presence of variable melt fractions and associated metasomatic alteration can be inferred to modify the physical properties of mantle rocks. (C) 2019 Elsevier B.V. All rights reserved.
  • [期刊] Compositional zoning in garnet and its modification by diffusion during pressure and temperature changes in metamorphic rocks; an approach and software
    We present an approach and software for modelling the Compositional Zoning in Garnet and its Modification by diffusion (CZGM), which combines pseudosection modelling using Perple_X and multicomponent diffusion in garnet during metamorphism. CZGM uses the initial compositional profile, obtained by Perple_X with application of garnet fractionation, to generate a population of garnets of prescribed size-distribution and to apply multicomponent diffusion along a selected P-T path during prograde metamorphism and subsequent decompression and/or cooling. As an example, CZGM was applied on average pelitic rocks subjected to metamorphism at different temperatures (600 to 800 degrees C) and time duration of 1-10 Ma. CZGM provides an easy-to-handle tool for petrologists to examine hypothetical scenarios of garnet development along prograde and retrograde paths and to constrain the P-T path of the whole metamorphic event. By treatment of laboratory measured garnet-bearing metamorphic rocks, it allows to improve the accuracy of pressure and temperature conditions, usually estimated only from compositional isopleths of garnet. (C) 2019 Elsevier B.V. All rights reserved.
  • [期刊] Phase relations in the system Na2CO3-CaCO3 -MgCO3 at 3 GPa with implications for carbonatite genesis and evolution
    The phase relations in the system Na2CO3-CaCO3-MgCO3 have been studied at 3 GPa and 700-1285 degrees C using a Kawai-type multianvil press. At 700 degrees C, the system has five intermediate compounds: dolomite, Mg-bearing Na2Ca4(CO3)(5) burbankite, Na2Ca3(CO3)(4), Na4Ca(CO3)(3), and eitelite. As temperature increases to 800 degrees C, the system is complicated by an appearance of Ca-dolomite and Mg-bearing shortite, while Na2Ca4(CO3)(5) disappears. At 850 degrees C, Na4Ca(CO3)(3) decomposes to produce Na carbonate and nyerereite. The latter melts incongruently at 875 +/- 25 degrees C to form Na2Ca3(CO3)(4). Incongruent melting of eitelite to magnesite and liquid, occurs at 925 +/- 25 degrees C. Mg-bearing shortite melts incongruently at 950 +/- 50 degrees C, producing Na2Ca3(CO3)4 and liquid. Na2Ca3(CO3)(4) disappears at 1000 degrees C via incongruent melting to calcite and liquid. The liquidus projection of the studied ternary system has seven primary solidification phase regions for magnesite, dolomite-calcite solid solutions, Na2Ca3(CO3)(4), Mg-bearing shortite, nyerereite, eitelite, and Na carbonate. The primary solidification regions are separated by five peritectic and three cotectic monovariant lines. The system has six ternary peritectic points and one minimum on the liquidus at 850 degrees C and 52Na(2)CO(3).48(Ca0.62Mg0.38)CO3.