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- 저자 : Whattam, S. (검색결과 5 건)
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Whattam, S.A.,Cho, M.,Smith, I.E.M. Universitetsforlaget ; Elsevier Science Ltd 2011 Lithos Vol.127 No.3
The Andong Ultramafic Complex (AUC) mainly comprises peridotites (wehrlites+/-plagioclase or spinel; or plagioclase+spinel) and related serpentinites with subordinate low-Al pyroxenites (clinopyroxenites, orthopyroxenites, and websterites). These rocks are compositionally similar to sub-continental lithospheric mantle peridotites and pyroxenites. Wehrlites formed predominantly by fractional crystallization processes within supra-subduction zone magmas and the pyroxenites are generally consistent with segregation and accumulation in similar magmas. Bulk rock ratios of Al<SUB>2</SUB>O<SUB>3</SUB>/SiO<SUB>2</SUB> (0.01-0.03) and MgO/SiO<SUB>2</SUB> (up to >1) exhibited by the wehrlites and serpentinites indicate crystallization from a refractory source that underwent high degrees of melt extraction. Spinel chemistry confirms this and demonstrates that wehrlite and clinoproxenite protoliths underwent approximately 20-23% and 12-15% partial melting, respectively. Wehrlites and serpentinites also preserve evidence of extensive melt-peridotite interaction manifest as bulk rock SiO<SUB>2</SUB>-depletions and FeO<SUP>t</SUP>-enrichments relative to mantle residua as well as low Mg# (0.39-0.45) spinels with variable Ti contents but constant Cr# (0.42-0.47). These features are identical to those of 'impregnated' plagioclase-peridotites of abyssal and sub-continental environments and compositional trends in spinel space imply reaction between secondary, MORB-like melts saturated in olivine+clinopyroxene or olivine and a harzburgitic protolith. High olivine:pyroxene (∼3:1) and clinopyroxene:orthopyroxene ratios of the wehrlites coupled with chemical data dictate that reactions entailed orthopyroxene dissolution and olivine recrystallization. All AUC rock types exhibit primitive mantle-normalized incompatible element signatures characterized by LILE-enrichments, high fluid-mobile/immobile element ratios (Sr/Nd, Ba/La and Pb/Ce@?1) and prominent HFSE (Nb, Zr, and Ti) depletions indicative of generation in a sub-arc environment within a supra-subduction zone system. A candidate for the associated arc-system is the one responsible for nearby arc-related Jurassic granitoids. Southeast-directed thrusting along the Andong Fault System may account for subsequent emplacement of the AUC into the Gyeongsang Basin.
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Seo, J.,Choi, S.,Park, J.,Whattam, S.,Kim, D.W.,Ryu, I.,Oh, C.W. Universitetsforlaget ; Elsevier Science Ltd 2016 Lithos Vol.262 No.-
<P>The Yonghwa phoscorite-carbonatite complex occurs as an isolated individual body with an inclined pipe shape within the Precambrian Gyeonggi Massif, South Korea. The phoscorite consists mainly of olivine, apatite, magnetite, carbonates, amphibole, and phlogopite, and can be subdivided into two types, olivine-rich and apatite-rich. The carbonatite is composed of calcite, Mg-rich dolomite, Fe-rich dolomite, magnetite, apatite, and/or siderite. Intensive fenitization occurred along the boundary between the complex and the wall rocks of leucocratic banded gneiss and garnet-bearing metabasite. The paragenetic sequences of the phoscorite-carbonatite complex demonstrate that the early crystallization of silicate minerals was followed by the crystallization of carbonates as the carbonatitic melt cooled. Magnetite occurs within the complex, and the carbonatites have Fe contents that are higher than typical ferrocarbonatites, due to the high magnetite contents. The rare earth elements (REEs) in the phoscorites and carbonatites are weakly fractionated and show enrichments of LREEs and Nb relative to HREEs. Furthermore, the apatites reflect the fractionated trends of LREEs relative to HREEs. Phoscorite apatites are enriched in Sr and show substitutions between Ca and Sr. Mica chemistry reflects the evolutionary trend of Fe2+ and Mg2+ in the phoscorite-carbonatite melt without Al substitution. Micas exhibit high values of Mg# in the phoscorite-carbonatite complex, but lower values in fenites. Via thermodynamic analysis, the early stability fields of magnetite-pyrrhotite-graphite-carbonate assemblages indicate that the Yonghwa phoscorite and carbonatite crystallized under conditions of 600 degrees C, 2 kbar, and X-CO2 = 0.2. Afterward, melts underwent an evolution to the late stability fields of magnetite-pyrite-pyrrhotite-ilmenite assemblages. The delta C-13 and 8180 isotopic compositions of carbonates in the Yonghwa phoscorite-carbonatite complex are -8.2 parts per thousand to -3.4 parts per thousand and 6.6 to 11.0 parts per thousand., respectively, and together with the sulfur isotope compositions of the sulfides (delta S-34 values of about 0.2 parts per thousand to 2.2 parts per thousand) indicate a primary mantle source of the magmas. Phlogopites from the fenites yielded K-Ar ages of 193.4 +/- 4.9 and 195.0 +/- 5.1 Ma, which demarcate the timing of the cooling of the phoscorite-carbonatite intrusion, and indicate that the phoscorite-carbonatite may be related to a post-collisional magmatic regime. The discovery of this complex marks the first known occurrence in Korea, of Fe and Nb-REE mineralization related to phoscorite-carbonatite igneous activity. (C) 2016 Elsevier B.V. All rights reserved.</P>
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Plate tectonics on the Earth triggered by plume-induced subduction initiation
Gerya, T. V.,Stern, R. J.,Baes, M.,Sobolev, S. V.,Whattam, S. A. Nature Publishing Group, a division of Macmillan P 2015 Nature Vol.527 No.7577
Scientific theories of how subduction and plate tectonics began on Earth—and what the tectonic structure of Earth was before this—remain enigmatic and contentious. Understanding viable scenarios for the onset of subduction and plate tectonics is hampered by the fact that subduction initiation processes must have been markedly different before the onset of global plate tectonics because most present-day subduction initiation mechanisms require acting plate forces and existing zones of lithospheric weakness, which are both consequences of plate tectonics. However, plume-induced subduction initiation could have started the first subduction zone without the help of plate tectonics. Here, we test this mechanism using high-resolution three-dimensional numerical thermomechanical modelling. We demonstrate that three key physical factors combine to trigger self-sustained subduction: (1) a strong, negatively buoyant oceanic lithosphere; (2) focused magmatic weakening and thinning of lithosphere above the plume; and (3) lubrication of the slab interface by hydrated crust. We also show that plume-induced subduction could only have been feasible in the hotter early Earth for old oceanic plates. In contrast, younger plates favoured episodic lithospheric drips rather than self-sustained subduction and global plate tectonics.
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