Geochronology, geochemistry and petrogenesis of Neoproterozoic basalts from Sugetbrak, northwest Tarim block, China: Implications for the onset of Rodinia supercontinent breakup
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Magmatic origin of large (up to 5 cm) phenocrysts of K-feldspar is indicated by mineralogical, chemical, and roentgenometric-structural analysis of the crystals. Perthites are found to be metasomatic products of replacement of plagioclase by K-feldspar. This reaction is accompanied by albitization of plagioclase and then by growth of albite after feldspar, in cleavage planes and elsewhere, or by spot-segregations of albite in places formerly occupied by plagioclase. — V.P. Sokoloff
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Fassaitic augite was found in the Kajishi-yama basanite from Tsuyama Basin, Okayama Prefecture, Southwest Japan. The fassaitic augite occures as follows; (1) An anhedral complex, zoned core, separated from the rim by a optically sharp boundary, (2) An outer most rim of clinopyroxene phenocryst. Both types of fassaitic augite are rich in TiO2 (1.65-5.06 wt%) and Al2O3 (10.44-14.98 wt%). The fassaitic augite core has lower mg value (100 Mg/(Mg+Fe2+) =52.4-67.2) and higher Na2O contents (1.54-2.09 wt%) than those of clinopyroxene phenocryst rim. The fassaitic augite core is considered to have crystallized from an evolved, strongly alkaline basaltic magma in a deep-seated magma chamber, and subsequently the magma might be mixed with the Kajishi-yama basanite magma from which the fassaitic augite rim was crystallized.
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Cooling history of a tholeiitic basalt from the Kurohana area in Funagata volcano is inferred from the microstructures of clinopyroxene and plagioclase. The sample contains pigeonite, orthopyroxene, olivine and plagioclase as phenocrysts. The phenocrysts are surrounded by the overgrown rims. From the compositional difference between phenocrysts and the overgrown rims, the crystallization sequence has been divided into two stages. At the first stage, the phenocrysts of pyroxene and plagioclase crystallized at above 1473K in the magma chamber. They were kept in the chamber less than half a year, which was determined on a basis of the coarsening kinetics of pigeonite-augite intergrowth. Then, at the second stage, the rims of phenocrysts overgrew and groundmass minerals crystallized during the rapid cooling after the eruption. The cooling duration was estimated as about 4-5 years from the size of antiphase domains in plagioclase.
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I. Introduction. The writer has recently made a study of a large number of slices of the Lower Carboniferous porphyritic basalt lavas of North Ayrshire and adjoining areas (MacGregor, 1930,1 pp. 89-99). In these rocks phenocrysts of augite, olivine and plagioclase felspar may often be seen to have sub-rounded outlines, or to have embayments and enclosures of groundmass. One rock-slice in particular, because it contains an exceptionally good example of a honeycombed augite phenocryst, deserves a more detailed description than could be accorded to it in the North Ayrshire Memoir (MacGregor, 1930, p. 97). The rock from which the slice (20,196)2 was cut was collected by Mr J. E. Richey near Craig of Neilston, Renfrewshire, in ground surveyed by him for the Geological Survey. It is an olivine-basalt of Dunsapie type (MacGregor, 1930, p. 91), and contains large phenocrysts of olivine, augite and felspar. II. Description of the Rock. The chief feature of the slice is a large phenocryst of pale fawn-brown augite about 1 cm. across (Plate XLVII., Fig. 1). The phenocryst is traversed by ramifying passages that locally pinch out almost entirely. The material in these cavities has crystallised mainly as felspar, granules of augite, and iron-ore, and closely resembles the groundmass. This material enclosed in the augite is in fact seen to be in communication with the groundmass at the right-hand side of the photograph (Plate, XLVII., Fig. 1). There is a decided tendency for little felspar laths (traversed by numerous delicate apatite needles, and accompanied by
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The dacite of the Pebble Creek Formation (Mount Meager, BC, Canada) is an extraordinary occurrence of lavas containing coarse, sieve-textured plagioclase phenocrysts that appear to have reacted extensively with the melt. We record this unusual occurrence of a naturally reacted crystal/melt pair utilizing Nomarski Differential Interference Contrast (NDIC) imaging and electron microprobe data. Hieroglyphic textures are common in plagioclase of this study, and appear to represent older plagioclase remnants in sharp contact with new plagioclase forming a palimpsest texture outlining the occurrence of previous pools of melt. Throughout the processes affecting the phenocrysts, capillaries of liquid remained open to the external melt.
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