On the dolerite and allied rocks of Semi, Yamagata Prefecture

Abstract

So called “Semi dolerite” is represented by sills and dykes of seven masses. The writer classified those dolerites geologically and petrologically in two groups, namely Semi dolerite and Nagasawa dolerite. Semi dolerite intrudes in the lower part of Nagao formation and the upper part of Semi formation as sill form, but rarely in Nozoki formation in dyke form. Then, Nagasawa dolerite intrudes in the lower part and extrudes in the middle part of Nagasawa formation. Sills and dykes of Semi dolerite show chilled margins at the lower and upper contact with the intruded sediments. The thickness of the largest sill is 24.4 meters, while its chilled base is 0.8 meters and its upper chilled margin is 0.5 meters thick. When the thickness of sill is over about 20 meters, the petrographyic and chemical compositions vary remarkably from the margins to the central and upper part in one sill, by differentiation “in situ”. The writer found a differentiation sequences in Semi dolerite sill as follows, namely, chilled margins, normal dolerite, felsic dolerite (dioritic laver in the upper part), dolerite pegmatite and granophyric segregation vein. Nagasawa dolerite sills 18.6 meters in thickness, and the grain size is relatively small and uniform throughout the whole body. As a characteristic nature, it includes numerous xenocryst of quartz, skeleton like plagioclase and hornblende like matters. The chemical composition of Nagasawa dolerite, comparing with the felsic differentiates of Semi dolerite, has the next characteristics: SiO2 component of the latter is equall to the one of the former, while Fe2O3+FeO component is poor, and A12O3 and MgO components are richer in the latter than in the former. Judging from the chemical composition as well as the mineralogical compositions, Nagasawa dolerite is believed to have been derived from the original magma of Semi type dolerite not by the differentiation process, but by the assimilation of the felsic materials as probably the granitic materials.