Mo-W鉱床生成区と花崗岩岩石区

抄録

Molybdenum and tungsten deposits provide clear examples of ores related to granitic rocks. The writer (ISHIHARA, 1971a) has pointed out that tungsten deposits tend to be associated with less acid intrusions than molybdenum deposits and that scheelite (not wolframite) in non-calcareous wall rocks occurs where the source granitic magma is calcic. He also proposed a new scheme of the Mo-W metallogenic provinces of Japan, the Kitakami district of which shows a reverse pattern in lateral zoning of those of the Inner Zone of Southwest Japan (ISHIHARA, 1971b). Fundamental data and reasons for the above proposals taken from the Kitakami district have not yet been 'published. This paper, as a sequel to the previous two papers, describes zonal distribution of the metals and nature of the granitic rocks of the Kitakami district. Genesis of the metallogenic provinces of Japan is discussed in terms of the plate tectonics model.
The Cretaceous-Paleogene granitic rocks in the Honshu Island show definitely regional variation in composition. It is basic at one side (either continental or oceanic) and becomes acidic at the other side. This is tentatively called "Lateral differentiation trend", and is shown by changes in regional bulk composition and in ratios of Fe⁺³/Fe⁺², NA/K, Th/K, U/K, Rb/K, etc. of the granitic rocks, and distribution of the Mo-W deposits. One suite of these variation is considered as the most fundamental unit of petrogenetic province. Each province consists of subprovinces such as Mo-, W-, and Barren provinces.
The lateral diffentiation trend is continentward in the Inner Zone of Southwest Japan (arrows in Fig. 9), but oceanward in the Abukuma and Kitakami Belts. Hence, the most distinct discontinuity appears some place along the present Tanakura tectonic line. The asymmetrical zoning in the Inner Zone of Southwest Japan (see Table 5) can be explained by a magmatic differentiation model during ascending of granitic magma along a continentward-dipping weak zone in crust or Benioff zone itself. This hypothesis was discussed in this paper in emphasizing role of H₂O and differences on volatilities of Cl- and F-complexes. This hypothesis implies that Northeast Japan belonged to a different plate from that of the Inner Zone of Southwest Japan and also the paleo-Benioff zone or the conduit of the granitic magma inclined steeply oceanward at the Cretaceous time.