Abstract
The zonal structures of porphyritic plagioclases in the dacitic rocks from Shiroishi, Miyagi Prefecture, can be divided according to the ranges in size into the three types as follows:
(1) 0.5-4.0mm in size; they are made of anorthite core and mantle of intermediate composition, the later forming even oscillatory zoning,
(2) 0.5mm± in size; they have an even oscillatory type of zoning as a whole, ranging from An65 to An42 in composition,
(3) microphenocryst; they have the simplest zoning, for example, An48_??_44, An42-54, An55_??_44, etc.
The first type is most interesting from genetic point of view. From its volumetric relation and the charcter of zoning, the crystallizing stage of the mantle of this type is inferred to correspond to that of the second type. The author believes that the anorthite core should have crystallized out from the basaltic magma, which changed its composition to dacitic as a result of contamination of granitic materials in the country rocks, and that this process have resulted in abrupt change of composition in the mantle of the larger phenocrysts.
The zonal structures of second and third types can be simply explained as the result of the fractional crystallization in the contaminated dacitic magmas.
Some geological and petrological significance of these zoned plagioclases is dicussed in detail.
