The Tanigawadake plutonic complex

Abstract

In the northern Kanto district belonging to the Fossa Magna there are distributed Neogene Tertiary volcanics and sediments of the so-called Green Tuff series, Cretaceous granite, highly metamorphosed Paleozoic to Mesozoic formations, and some masses of Tertiary plutonics. The Tanigawadake plutonic complex, one of these Tertiary plutonics, has been reported to consist of quartz diorite with a large xenolith-like hornfels of probably Paleozoic formation. From the petrological study on this complex, the following conclusions are obtained. 1. Two geological units are recongnized in the northern Kanto district; basement complex of Paleozoic and Mesozoic formations and Cretaceous granite in the north west part and Neogene Tertiary volcanics of basalt-rhyolite series of the so-called Green Tuff formations in the south east part. The Tertiary plutonics are generally distributed at the transitional zone of the above two units. 2. This area is consisted of two geological units of the basement complex and the Tertiary plutonics. The former consists of many kinds of crystalline schist of probably Paleozoic formation and potassh-feldspar porphyritic granites of the Cretaceous igneous activity, and the latter consists of quartz gabbro, diorite porphyrite, and quartz diorite. Between these two units above mentioned, ultramafic rock and diabase and leucocrate are distributed, evidently suffered from contact effect of the quartz gabbro and seems to have intruded into the basement. 3. The quartz gabbro is remarkable of their peculiar mineral assemblages composed of pyroxene, hornblende, plagioclase, biotite, quartz, albite, and potassh-feldspar, all of which can be often observed even in one thin section. It is evident that these constituent minerals have at least two stages of mineralization, i. e., pyroxene and plagioclase in the earlier stage and hornblende, biotite, albite, quartz, and potassh-feldspar in the later stage which may well be called metasomatic stage.