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  • 1
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Science Inc
    Journal of metamorphic geology 18 (2000), S. 0 
    ISSN: 1525-1314
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Basic and ultrabasic blocks within ophiolitic mélanges of the Cycladic Blueschist Unit in southern Evia provide a detailed insight into its ocean floor igneous and hydrothermal evolution, as well as the regional poly-metamorphism occurring during Alpine orogenesis. The upper structural levels (Mt. Ochi exposures) are dominated by metamorphosed wehrlites, gabbros and highly light rare earth element (LREE)-enriched pillow basalts, whereas the underlying Tsaki mélange consists of basic protoliths with much less fractionated REE patterns as well as mantle harzburgites. Most of the metabasites show Nb anomalies, indicative of derivation from a subduction-affected mantle. The igneous bodies were juxtaposed and incorporated into the enclosing sedimentary sequences prior to high-pressure/low-temperature (HP/LT) metamorphism (M1). Glaucophane, epidote, sodic clinopyroxene and high-Si phengite constitute the Eocene M1 assemblage, which is estimated to have formed at 〉11 kbar and 400–450 °C. High δ18O values of M1 minerals in Ochi metagabbros indicate that the formation of the high-pressure assemblage was controlled by infiltration of fluids from the dehydrating host sediments. Cooling during decompression is indicated by an overprinting (M2, Early Miocene) pumpellyite–actinolite facies assemblage in metabasic rocks, calculated to have developed at P〈8 kbar and T 〈350 °C. Possible mechanisms for such cooling include: exhumation from shallower burial levels relative to the eclogites of the NW Cyclades, accretion of colder rocks from below and extensional unroofing by low-angle normal faults and detachments. The occurrence of sodic augite in the M2 assemblage of Tsaki metagabbros indicates that rocks at the base of the Blueschist Unit cooled faster or longer than their higher level Ochi counterparts. This suggests that differential cooling of the blueschists was enhanced by the underthrusting of colder rock units.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Journal of metamorphic geology 2 (1984), S. 0 
    ISSN: 1525-1314
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Abstract Blueschist-facies rocks on the Seward Peninsula constitute a structurally coherent terrane measuring at least 100 × 150 km. Radiometric age data indicate that high-pressure metamorphism probably occurred in Jurassic rather than in Palaeozoic or Precambrian time, as previously suggested. Protolith sediments (Nome Group) are of intracontinental basin or continental margin type, and of lower Palaeozoic and possibly late Precambrian age, thus predating the high pressure metamorphism by more than 200 m.y.Blueschist-facies mineral assemblages were developed in almost all lithologies of the Nome Group, and are best preserved in FeTi-rich metabasites (glaucophane + almandine + epidote) and pelites (glaucophane + chloritoid + phengite). A lawsonite–crossite subfacies was developed in possible Nome Group rocks on the east flank of the Darby Mountains. Albite–epidote–amphibolite facies assemblages characterize Nome Group rocks in the southwestern part of the Peninsula. Metamorphism in the central zone of the terrane passed from early lawsonitic to subsequent epidote–almandine–glaucophane schist subfacies with the local development (east of the Nome River) of eclogitic assemblages.The high pressure metamorphic minerals were synkinematic with the development of mesoscopic-scale intrafolial isoclinal folds and a flattening foliation of consistent orientation. Initiation of uplift probably corresponded to the growth of barroisite rims on earlier sodic and actinolitic amphiboles, and partial post-kinematic greenschist facies replacements record later stages of decompression. Ophiolites and melange are not associated with the Seward Peninsula blueschists. The high-pressure metamorphism was caused by tectonic loading of a continental plate by an allochthon of indeterminate origin. The PT conditions of high pressure metamorphism were approximately 9–11 kbar, 400–450°C, thus falling between the PT paths of the Shuksan and Franciscan terranes.
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  • 3
    Electronic Resource
    Electronic Resource
    Palo Alto, Calif. : Annual Reviews
    Annual Review of Earth and Planetary Sciences 5 (1977), S. 397-447 
    ISSN: 0084-6597
    Source: Annual Reviews Electronic Back Volume Collection 1932-2001ff
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 211 (1966), S. 619-619 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Table l. ANALYTICAL DATA FOR TWO SAMPLES OF ZINC MICA 1 SiO2 31-9 A12O3 13-6 "FeaO3 4-9 TiO2 0-35 MnO 12-5 ZnO 19-8 MgO 2-7 Li2O 004 Na2O K2O 018-6 CaO 002 BaO 0-3 H20+,F 3-95 H2O- 0-95 Si [ 50 Fe3+ 0-53 Ti 14-9 Mn 16-9 Zn - Mg 004 Li - Na - K ___ Ca __ Ba (OH,F) 4-1 ...
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  • 5
    Publication Date: 2015-05-29
    Print ISSN: 0008-4476
    Topics: Geosciences
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  • 6
    Publication Date: 2016-02-03
    Description: Despite claims to the contrary, the compositions of magnetite and ilmenite in the Bishop Tuff correctly record the changing conditions of T and f O 2 in the magma reservoir. In relatively reduced (NNO 〈 1) siliceous magmas (e.g., Bishop Tuff, Taupo units), Ti behaves compatibly (D Ti 2–3.5), leading to a decrease in TiO 2 activity in the melt with cooling and fractionation. In contrast, FeTi-oxides are poorer in TiO 2 in more oxidized magmas (NNO 〉 1, e.g., Fish Canyon Tuff, Pinatubo), and the d ( a TiO 2 )/ dT slope can be negative. Biotite, FeTi-oxides, liquid, and possibly plagioclase largely maintained equilibrium in the Bishop Tuff magma (unlike the pyroxenes, and cores of quartz, sanidine, and zircon) prior to and during a mixing event triggered by a deeper recharge, which, based on elemental diffusion profiles in minerals, took place at least several decades before eruption. Equilibrating phases and pumice compositions show evolving chemical variations that correlate well with mutually consistent temperatures based on the FeTi-oxides, sanidine-plagioclase, and 18 O quartz-magnetite pairs. Early Bishop Tuff (EBT) temperatures are lower (700 to ~780 °C) than temperatures (780 to 〉820 °C) registered in Late Bishop Tuff (LBT), the latter defined here not strictly stratigraphically, but by the presence of orthopyroxene and reverse-zoned rims on quartz and sanidine. The claimed similarity in compositions, Zr-saturation temperatures and thermodynamically calculated temperatures (730–740 °C) between EBT and less evolved LBT reflect the use of glass inclusions in quartz cores in LBT that were inherited from the low-temperature rhyolitic part of the reservoir characteristic of the EBT. LBT temperatures as high as 820 °C, the preservation of orthopyroxene, and the presence of reverse-zoned minerals (quartz, sanidine, zircons) are consistent with magma recharge at the base of the zoned reservoir, heating the cooler rhyolitic melt, partly remelting cumulate mush, and introducing enough CO 2 (0.4–1.4 wt%, mostly contained in the exsolved fluid phase) to significantly lower H 2 O-activity in the system.
    Print ISSN: 0003-004X
    Electronic ISSN: 1945-3027
    Topics: Geosciences
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  • 7
    Publication Date: 2014-10-22
    Description: We describe a sample of Proterozoic banded iron-formation from the inner contact aureole of the Duluth complex near Babbitt, Minnesota, that contains a compositionally unique Fe-rich monoclinic ferromagnesian amphibole. Its content of T Al (1.3 atoms per 23 anhydrous oxygens) and A Na (0.55) places it in a compositional field (with the label clinoferrogedrite) outside those recognized in the IMA nomenclature for amphiboles. With declining Al, it grades into Al-rich grunerite, and finally to Al-poor grunerite at the crystal margins. While spot-to-spot compositional trends define basic gedrite substitutions, they differ systematically from the well-established anthophyllite-gedrite trends found in amphibolites. They resemble instead two instances of sodic gedrite described in the literature that are enriched in A Na apfu and depleted in octahedral Al. These unusual trends may be the result of very high temperatures of formation, or a high activity ratio of Fe + Mg to Si. Associated minerals in our sample include orthopyroxene Fs 74–76 , olivine Fa 85–91 , almandine, quartz, plagioclase, and accessories graphite, pyrrhotite, ilmenite, biotite, apatite, zircon, monazite, and löllingite. Conditions are estimated to have been ≥800 °C and 1.3 kbar, with f O 2 close to FMQ-2 log 10 units. Clinogedrite should be considered extremely rare, but not non-existent. In the monoclinic structure the symmetry of the double Si–O chains limits the uptake of gedritic components to amounts lower than found in the orthorhombic structure, where Al is distributed across three of the four distinct tetrahedral sites. We also present electron microprobe data for a highly aluminous cummingtonite grading compositionally into clinogedrite ( A Na = 0.3, M 2 Al + Fe 3+ + 2Ti = 1.0, T Al = 1.1) that was grown from fused and S-augmented Pinatubo dacite in the laboratory at 780 °C, 2.2 and 3.9 kbar. Growth of this MgFe-clinoamphibole was enabled by the capture of Ca into anhydrite away from potential hornblende.
    Print ISSN: 0008-4476
    Topics: Geosciences
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  • 8
    Publication Date: 2012-12-22
    Description: Eruption of the Bishop Tuff magma preserved equilibrium of exchange components and element concentrations among magnetite, ilmenite, biotite, apatite, zircon, and liquid. Orthopyroxene and clinopyroxene were not in exchange equilibrium with the other MgFe-bearing phases, but they appear to have been in equilibrium among themselves. Internally consistent temperatures recorded by the FeTi-oxide, Ti-in-quartz, and 18 O quartz-magnetite thermometers, coupled with evidence for magmatic corrosion of quartz and sanidine, indicate that an initially low- T (700 °C), near-H 2 O-saturated, high-SiO 2 rhyolite magma was heated up to ≥800 °C and its crystal cargo partially melted by recharge of hotter melt from below. Oxygen fugacity and compositions of biotite, ilmenite, magnetite, and silicate liquid initially adjusted by internal rearrangement of components and conservation of oxygen. Partial melting of feldspars liberated Sr and Ba back into the melt. Mixing during recharge eventually re-introduced compatible elements (e.g., Mg, Ba, Sr) as well as foreign crystals of euhedral ortho- and clinopyroxene, which evidently never totally re-equilibrated with the rhyolite liquid. Introduction of CO 2 and accompanying reduction in the a H 2 O during recharge raised crystallization temperatures of quartz and sanidine in the rhyolite sufficient to allow marginal regrowth of these phases with enhanced contents Ti, Ba, and Sr.
    Print ISSN: 0003-004X
    Electronic ISSN: 1945-3027
    Topics: Geosciences
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  • 9
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    Unknown
    Mineralogical Society of America
    In: Elements
    Publication Date: 2013-04-16
    Description: Rock-forming serpentine minerals form flat, cylindrical, and corrugated crystal microstructures, which reflect energetically efficient layering of alternate tetrahedral and octahedral sheets. Serpentinization of peridotite involves internal buffering of the pore fluid, reduction of oxygen fugacity, and partial oxidation of Fe 2+ to Fe 3+ . Sluggish MgFe diffusion in olivine causes precipitation of magnetite and release of H 2 . The tectonic environment of the serpentinization process dictates the abundance of fluid-mobile elements in serpentinites. Similar enrichment patterns of fluid-mobile elements in mantle-wedge serpentinites and arc magmas suggest a linkage between the dehydration of serpentinite and arc magmatism.
    Print ISSN: 1811-5209
    Electronic ISSN: 1811-5217
    Topics: Geosciences
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  • 10
    Publication Date: 1977-05-01
    Print ISSN: 0084-6597
    Electronic ISSN: 1545-4495
    Topics: Geosciences , Physics
    Published by Annual Reviews
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