かんらん岩捕獲岩の地質学的側面 : 日本列島の例

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We discuss about geological aspects of deep-seated xenoliths in volcanics from the Japan arcs. We can compare xenoliths in magmas with gravels in rivers. Dams on the rivers are equivalent to magma reservoirs; the former hinder the gravels from downstream transportation as the latter may prevent the deep-seated xenoliths from uprise by accumulation. Rivers with protected banks where gravels are not caught are compared with fixed magmatic conduits where wall rocks are hardly sampled as xenoliths. The magmas that are originated as deep as possible, move upward as fast as possible, and do not form reservoirs are appropriate for providing deep-seated xenoliths as the rivers with deep source and without dams and protected banks are favorable for providing gravels for prospecting geology upstream. Modes of occurrence of deep-seated xenoliths on the Japan arcs suggest us the following. (1) Xenoliths are sorted in an uprising magmatic column; the magma catches deeper-seated, higher-density rocks earlier than shallower-seated, lower-density ones as xenoliths. The proportion of mantle xenoliths dramatically increases upward in stratigraphy of xenolith-producing monogenetic volcanoes. Inversely crustal xenoliths are predominant in the lower eruptives, especially in basal pyroclastics. (2) Rocks are selectively caught as xenoliths by magma. If young, fragile cumulative dikes formed by preceding magmas, subsequent magmas would trace them and do not catch wall rocks, which may be invisible as xenoliths. (3) The upper mantle to lower crust of the SW Japan arc is heterogeneous in thickness of cumulus mantle as well as in development of younger cumulative rocks. The mantle peridotite is cryptically or patently metasomatized by melts for the young cumulates beneath the monogenetic volcano clusters. (4) The upper mantle to lower crust of the NE Japan arc is heterogeneous in hydration. Aqueous fluids would move selectively along cracked or sheared parts to hydrate.
日本鉱物学会の論文
2005-07-30

かんらん岩捕獲岩の地質学的側面 : 日本列島の例

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