| 概要 |
About three hundred rock specimens of peridotitic and gabbroic rocks enclosed in basalts and limburgites of Chugoku and north Kyushu, Southwest Japan, have been examined petrographically. The constitu...ent minerals separated from these inclusions, clinopyroxene in particular, were studied in detail by chemical and X-ray methods and packing indices of the clinopyroxenes were calculated to compare with those from other sources. The inclusions examined differ in their rock types among different places, different host rocks, and no longer stable in their ho8t rocks showing disintegration and reaction. They are generally characteristic of strongly deformed structure due to translation, or of wavy extinction, in olivine and pyroxenes. Of these characteristics, complicated exsolution patterns in clinopyroxenes and orthopyroxenes are of great petrogenic significance. Clinopyroxenes in these inclusions are highly remarkable for a large proportion of Al replacing Si in the structure and for their smaller cell volume than those in other igneous rocks. The highest packing index of clinopyroxene from ultrabasic inclusion is noted. These evidences are well represented in terms of packing index to the number of 2-Si atoms, which indicate the possibility that the packing index may be used as a key of geological barometer. The noticeable similarity in mineralogy and petrographic character between ultrabasic inclusions and ultrabasic intrusive rocks has led some writers to adhere to the current view that the upper parts of the earth’s mantle are made up of peridotites which are found in the accessible crust. However, packing index of clinopyroxene and other crystal-chemical features indicate that most ultrabasic inclusions are derived from greater depths, comparatively deeper than the source of peridotite intrusives, where a layered structure is developed, different layers being composed of different varieties of ultrabasic rocks which are strongly deformed. The differences in layer of the mantle where partial remelting could occur are probably responsible for variation in composition of basaltic rocks.続きを見る
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Mendeley出力
15_01_p163