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The Hwacheon granulite complex (HGC), occupying the northeasternmargin of the Gyeonggi massif, consists mainly of garnetiferousleucocratic gneiss and leucogranite together with minor kyanite–garnetgneiss, aluminous gneiss, mafic granulite and garnet amphibolite.Mineral assemblages and reaction textures in various rock typesof the HGC document five distinct metamorphic stages: pre- (M1)and peak (M2) granulite-facies metamorphism; lower temperature,high (M3) and low (M4) pressure upper amphibolite-facies metamorphism;and local retrogression (M5) producing andalusite-bearing assemblages.Each metamorphic stage can be integrated to give a compositeP–T path consisting of two distinct trajectories, characterizedby clockwise P–T loops at relatively high and low temperatures,respectively. The first P–T trajectory (M1–M3) correspondsto a Palaeoproterozoic tectonometamorphic event responsiblefor the formation of the granulite complex at 相似文献
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Tectonometamorphic evolution of the Chuncheon amphibolite, central Gyeonggi massif, South Korea 总被引：9，自引：0，他引：9
Abstract The Chuncheon amphibolite, part of the Gubongsan Group which overlies the Yongduri gneiss complex, is interlayered with calc-silicate rock, marble, quartzite, biotite schist and quartzofeldspathic gneiss in the central Gyeonggi massif, South Korea. Metamorphic pressures and temperatures estimated from the amphibolite are 5.5–10.6 kbar and 615–714°C. These P—T conditions are close to those defined by the reaction curve between kyanite and sillimanite, and suggest medium-pressure-type metamorphism of the Chuncheon amphibolite. For two metapelites intercalated with the amphibolite, temperatures are estimated to be 607–699° C, consistent with those obtained from the amphibolite. On the other hand, pressures estimated from these metapelites are significantly different, 4–6 kbar and 9–13 kbar, when rim and core compositions of garnet are, respectively, used. These P—T estimates obtained from the amphibolite and metapelite suggest a nearly isothermal decompression of 3–7 kbar during denudation. Rapid decompression is likely on the basis of the results of mineral chemistry, phase equilibria and geothermobarometer. Moreover, in conjunction with the occurrence of kyanite in the adjacent Gyeonggi gneiss complex, P—T estimates of the Chuncheon amphibolite and metapelite suggest a clockwise P—T—t path. This evolutionary path may be related to the amalgamation of continents during the late Proterozoic event which corresponds to the Jinningian orogeny in the Qinling belt of China. 相似文献
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Transition from the Zeolite to Prehnite-Pumpellyite Facies in the Karmutsen Metabasites, Vancouver Island, British Columbia 总被引：1，自引：0，他引：1
The upper Triassic Karmutsen metabasites from northeast VancouverIsland, B.C., are thermally metamorphosed by the intrusion ofthe Coast Range Batholith. The amygdaloidal metabasites developedin the outer portion of the contact aureole show a progressivemetamorphism from zeolite to prehnite-pumpellyite facies. Thesize of an equilibrium domain is extremely small for these metabasites,and the individual amygdule assemblages are assumed to be inequilibrium. Two major calcite-free assemblages (+chlorite+quartz)are characteristic: (i) laumontite+pumpellyite+epidote in thezeolite facies and (ii) prehnite+pumpellyite+epidote in theprehnite-pumpellyite facies. The assemblages and compositionsof Ca-Al silicates are chemographically and theoretically interpretedon the basis of the predicted P-T grid for the model basalticsystem, CaO-MgO-A12O3-Fe2O3-SiO2-H2O. The results indicate:(1) local equilibrium has been approached in mineral assemblagesand compositions; (2) the XFe3+ values in the coexisting Ca-Alsilicates decrease from epidote, through pumpellyite to prehnite;(3) with increasing metamorphic grade, the Fe3+ contents ofepidotes in reaction assemblages decrease in the zeolite facies,then increase in the prehnite-pumpellyite facies rocks. Suchvariations in the assemblages and mineral compositions are controlledby a sequence of continuous and discontinuous reactions, andallow delineation of T-XFe3+ relations at constant pressure.The transition from the zeolite to prehnite-pumpellyite faciesof the Karmutsen metabasites is defined by a discontinuous reaction:0·18 laumontite+pumpellyite+0·15 quartz = 1·31prehnite+ 0·78 epidote+0·2 chlorite+ 1·72H2O, where the XFe3+ values of prehnite, pumpellyite and epidoteare 0·03, 0·10 and 0·18, respectively.These values together with available thermodynamic data andour preliminary experimental data are used to calculate theP-T condition for the discontinuous reaction as P = 1·1±0·5 kb and T = 190±30°C. The effectsof pressure on the upper stability of the zeolite facies assemblagesare discussed utilizing T-XFe3+ diagrams. The stability of thelaumontite-bearing assemblages for the zeolite facies metamorphismof basaltic rocks may be defined by either continuous or discontinuousreactions depending on the imposed metamorphic field gradient.Hence, the zeolite and prehnite-pumpellyite facies transitionboundary is multivariant. 相似文献
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Mineral paragenescs in the prehnite-pumpellyite to greenschistfades transition of the Karmutsen metabasites are markedly differentbetween amygdule and matrix, indicating that the size of equilibriumdomain is very small. Characteristic amygdule assemblages (+chlorite + quartz) vary from: (1) prehnite + pumpeUyite + epidote,prehnite + pumpellyite + calcite, and pumpellyite + epidote+ calcite for the prehnite-pumpellyite facies; through (2) calcite+ epidote + prehnite or pumpellyite for the transition zone;to (3) actinolite + epidote + calrite for the greenschist facies.Actinolite first appears in the matrix of the transition zone.Na-rich wairakites containing rare analcime inclusions coexistwith epidote or Al-rich pumpellyite in one prehnite-pumpellyitefacies sample. Phase relations and compositions of these wairakite-bearingassemblages further suggest that pumpellyite may have a compositionalgap between 0.10 and 0.15 XFe?. Although the facies boundaries are gradational due to the multi-varianceof the assemblages, several transition equilibria are establishedin the amygdule assemblages. At low Xco2, pumpellyite disappearsprior to prehnite by a discontinuous-type reaction, pumpellyite+ quartz + CO2 = prehnite + epidote + calcite + chlorite + H2O,whereas prehnite disappears by a continuous-type reaction, prehnite+ CO2 = calcite + epidote + quartz-l-H2O. On the other hand,at higher XCO2 a prehnite-out reaction, prehnite + chlorite+ H2O + CO2 = calcite + pumpellyite + quartz, precedes a pumpellyiteoutreaction, pumpellyite + CO2 = calcite + epidote + chlorite +quartz + H2O. The first appearance of the greenschist faciesassemblages is defined at both low and high XCOj by a reaction,calcite + chlorite + quartz = epidote + actinolite+ H2O + CO2.Thus, these transition equilibria are highly dependent on bothXFe3+ + of Ca-Al silicates and XH20 of the fluid phase. Phaseequilibria together with the compositional data of Ca-Al silicatesindicate that the prehnite-pumpellyite to greenschist faciestransition for the Karmutsen metabasites occurred at approximately1.7 kb and 300?C, and at very low Xco2, probably far less than0.1. 相似文献
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