ABSTRACT

In order to meet the challenge of the K-Ar age determination of mafic and ultramafic rocks from ophiolite complexes in orogenic belts. we developed a convenient and precise method of analyzing low and very low potassium in rock and mineral using ordinary flame photometry. The method which uses a newly designed and constructed ultra low blank chemical line makes it possible to analyze potassium as low as 0.001 wt.% in a rock or mineral by flame photometry with about 1% blank contribution to the sample potassium. The method was applied to igneous and metamorphic hornblendes from the Mikabu and Poroshiri ophiolite complexes, and to coexisting hornblende and clinopyroxene from the Cretaceous volcanic rocks in the Tanba and Nohi areas. The results revealed that the conventional K-Ar ages obtained are consistent with the geology of the host rocks. Hornblende (0.487 wt.% of K) and clinopyroxene (0.0563 wt.% of K) from a basaltic andesite in the Tanba area have the same age (ca. 100 Ma), and clinopyroxene (0.0119 wt.% of K) from the Nohi basaltic andesite gives the age of 59 Ma which is completely consistent with the recent chronological data. This strongly suggests that the clinopyroxene in the Cretaceous volcanic rocks had no significant amount of exess argon in primary isotopic systematics.