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Abstract

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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