HORNBLENDE GABBROIC INCLUSIONS IN THE CALC-ALKALINE ANDESITES FROM THE NORTHERN DISTRICT OF NAGANO PREFECTURE, JAPAN (I)

Abstract

There are many hornblende gabbroic inclusions found in the later Miocene to the early Pleistocene lavas developed in the northern Fossa Magna district, central Japan. These lavas are all hypersthenic rock series rich in groundmass orthopyroxene. It is noticed that a few augite-olivine-basalt lavas are often intercalated in the enormous effusives (1000m in the maximum thickness) of the calc-alkaline andesites. For this study 18 new chemical analyses have been made by one of the writers (Õji). In addition to these, 3 analyses already published by Yagi and Yagi (1958) and one analysis reproted by S. Yamada (1934) are cited; of these six are ultramafic hornblende gabbros and three are pyroxene-(±hornblende)-diorites, both as inclusions, five are andesites and the rest are augite-olivine-basalts both occurring in intimate association. They include 12 analyses of the later Miocene or early Pliocene rocks from the Shigarami Formation and 10 analyses of the later Pliocene to Pleistocene rocks.
In MgO-total iron-alkalis diagram the plots of the rocks including hornblende gabboric and dioritic inclusions and their host andesites are arranged along a calc-alkaline trend. The points of the gabbros are closer to MgO-total iron side, while the points of the andesites are plotted on the extension of the gabbro-diorite line twoard the alkali apex. From intimate association of the gabbro-andesite-basalt and their petrochemistry, the writers are led to the conclusion that a calc-alkaline andesite magma rich in mafic components (SiO₂ range 52-56%) has been derived from basaltic magma by subtracting hornblende gabboric crystalline aggregates, without contamination by any granitic or sedimentary materials. It is estimated that a considerable amount of ultramafic hornblende gabboric mass should be remained by this means within the depths, for instance at the geosynclinial bottoms of the Fossa Magna, and that the volume of the gabbro should exceed that of the effusive andesite composing the volcanics. Any geological evidences for such an enormous mass of hornblende gabbro of Pliocene, or even of Miocene age, have not been found in the regions composed mainly of volcanic rocks. On the contrary hornblende gabbros are much more commonly found in granite regions of Japan and they are intimately associated with diorite intrusives which had been under condition of granitization, and hence they have somewhat different characters from the segregated gabbros here cited.