ロシア・東部バイカル地域におけるマントル進化

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The genesis of major groups of xenoliths from the Late Cenozoic volcanic rocks of the eastern part of Baikal Rift System is considered on the basis of petrology and mineral chemistry. Reconstructed mantle sections beneath the Vitim field appear to be more complex than beneath the Udokan field due to significant modification by hydrous metasomatic melts. Peridotite xenoliths from the Miocene picrobasalts represent garnet and spinel depth facies. Pyroxenite xenoliths are interpreted as products of three types of the melt. The first forms Cr-diopside pyroxenites, it is an interstitial melt migrating through peridotite, and it is similar to melts described in peridotite massifs as undergoing percolative fractionation. Textural and compositional relationships indicate melt segregation and fractionation under the high-pressure conditions towards Al-rich pyroxenites and - at shallower levels - towards sp-websterites. The second and third melt types form the hydrous veins and megacrystic pyroxenites, respectively. Both were crystallized in larger channels relative to the first melt type, whereby the amphibole- and phlogopite-bearing assemblages may represent smaller channels on a scale of dozen centimeters. Three series of peridotite xenoliths from Pliocene basanites were determined: (1) high-T garnet and spinel lherzolites, (2) low-T spinel lherzolites and harzburgites, (3) low-T titaniferous spinel lherzolites. Protogranular peridotites of the Series 1 represent primitive to moderately depleted mantle from the depths 60-80km at T=1100-1250℃. Trace element patterns in clinopyroxenes are indicative of low degree partial melting of the primitive mantle. Peridotites of the Series 2 correspond to the depths 40-50km at T=800-900℃. Titaniferous peridotites enriched in pyroxenes and spinel were newly detected. They have a mosaic equigranular texture and are suggested to be a rare type of melt/mantle interaction. T-estimations within 750-850℃ projected to a geotherm allow to interprete their correspondence to the uppermost mantle section (40-50km depth). High Ti content (TiO_2=0.55% in the bulk rock) may characterize a metasomatizing melt generated from ilmenite- and/or phlogopite-bearing source. Clinopyroxene REE patterns (La/Yb)_n=0.01-0.08) reveal an evidence for MORB-like composition of coexisting melt. Xenoliths from the Pliocene basanites of Kuas Lake (Udokan field) show variation of unhydrous depletion and enrichment of lower lithosphere. Xenoliths correspond to spinel facies and may be divided into lherzolite, harzburgite and dunite, and websterite groups. Depleted nodules contain clinopyroxene enriched in LREE and depleted in HFSE. This enrichment is explained by reactive percolation of small melt fraction and accompanying melting of peridotites. Harzburgite-dunite veins seem to be located at the lower part of mantle column and have been formed by olivine-producing reaction with increasing of melt fraction. Lherzolite and websterite are located at the middle and upper part of column and have been formed by pyroxene-producing reaction with decreasing of melt fraction.論文Article
東北大学の論文
2000-03-31

ロシア・東部バイカル地域におけるマントル進化

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