Petrological Studies of Igneous Rocks
Because of intense alteration of the volcanic rocks associated with massive sulfide mineralization in the Hokuroku district of northern Honshu, Japan, major element concentrations and modal mineralogy fail as classification tools; only some relatively immobile trace and minor elements can be used to classify the rocks and to infer relationships between them. On that basis, as well as the contents of SiO2, a moderately mobile element, the volcanism is interpreted to be bimodal. The mafic volcanics are basalts that are transitional between the tholeiitic and high alumina (calc-alkaline) Quaternary series of Japan. Some of the samples represent relatively primitive, peridotite-derived melts.
The felsic rocks are calc-alkaline but cannot readily be classified either as dacites or as rhyolites. Based on concentrations of trace elements that reflect primary petrogerietic processes rather than alteration, at least two populations of felsic rocks can be distinguished. Isotope data also show that two different Sources have contributed to the formation of the felsic suite. The geochemistry of the rocks is not diagnostic of a specific tectonic environment. Because preore and postore lavas differ very little in their inferred igneous composition, there are no primary geochemical features that distinguish ore-related from barren volcanics.
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This paper consists of three parts. The first is an overview of the geologic history of the Green Tuff region where all Kuroko deposits occur. The second part presents a description of the stratigraphy and an interpretation of the structural and igneous history of the Hokuroku district, the most important Kuroko mining district. The third part is an analysis of the role of submarine calderas in Kuroko genesis.
The sequence and causes of the major geologic events that have occurred in Japan and its vicinity since the Cretaceous are interpreted as follows: (1) an active but shallow-dipping north-northwestward subduction of the Pacific plate under the Asian continent during a period from approximately 130 to 65 m.y. ago resulted in ilmenite series magmatism in the outer zone of Japan, then still a part of mainland Asia; (2) about 65 to 40 m.y. ago, the direction of the subducted Pacific plate changed to westward and the angle of subduction steepened, initiating back-arc spreading in the Japan basin province and migration of Japan away from the Asian mainland until about 30 m.y. ago; (3) during the period 65 to 30 m.y. ago, the basaltic crust created in the Japan basin province was subducted eastward under the Yamato Ridge province, resulting in calc-alkaline and magnetite series igneous activity in the inner zone of Japan; (4) about 25 m.y. ago, the first sea (proto-Japan Sea) was formed in the Japan basin province as a result of the eustatic rise of the sea following cessation of spreading there about 30 m.y. ago; (5) back-arc spreading was active in the Yamato basin province during the period between 25 and 5 m.y. ago, cansing bimodal volcanism and subsidence in the flanking Inner Honshu and Yamato Ridge provinces [the Hokuroku basin (i.e., a Kuroko-bearing basin), Niigata oil field basin, and Akita oil field basin were all fault-bounded, deep (>2,500 m) marine basins created by rapid subsidence of crustal blocks within a few million years around 17 m.y. ago, although Kuroko mineralization and the accumulation of organic matter were not synchronous]; and (6) the dip of the subducted Pacific plate returned to a shallow angle about 5 m.y. ago, causing the cessation of back-arc spreading and the initiation of subsidence of the Yamato basin province and uplift of the flanking Inner Japan and Yamato Ridge provinces. The Green Tuff activity is, therefore, synonymous with the tectonic and igneous activity that accompanied the formation of the Japan Sea and the Japanese islands during the period from ~65 m.y. ago to the present.