Formula omitted] analysis of K-feldspars from Cretaceous granitic rocks in Japan: Significance of perthitization in Ar loss

Abstract

40 Ar 39 Ar spectrum ages have been determined for five K-feldspars from Cretaceous granitic rocks in Japan. Three samples yield flat age spectra, giving plateau ages of 60.1 ± 1.1 Ma (94.3% of integrated 39Ar) for the Naegi granite of the Sanyo Belt, and 101.9 ± 2.5 Ma (95.5%) and 103.3 ± 1.7 Ma (96.1%) for granitic rocks from northeast Japan. The results suggest that these granitic rocks experienced a simple and rapid cooling history. Two feldspars from granitic rocks of the Ryoke Metamorphic Belt in the vicinity of the Median Tectonic Line give two plateau ages each: 61.1 ± 1.0 Ma (46.0%) and 66.0 ± 1.1 Ma (32.4%) for low-temperature steps; and 64.6 ± 1.0 Ma (51.6%) and 71.2 ± 1.1 Ma (65.4%) for high-temperature steps, though plateau ages for high-temperature steps are less well defined. This age pattern may suggest a later thermal event related to the Median Tectonic Line. The cooling rates calculated from KAr ages of coexisting mineral pairs range from 17° to 32°C Ma −1 for granitic rocks from the Sanyo and Ryoke Belts, whereas mineral ages for granitic rocks from northeast Japan are nearly equal, suggesting a very rapid cooling. Diffusion parameters of K-feldspars calculated from the 39Ar release yield activation energy, E, ranging from 22.3 to 53.4 kcal mol −1, and D 0 a 2 ranging from 0.0297 to 1.73·10 5 s −1. The closure temperatures are calculated to be from 115° to 331°C. The activation energy and closure temperature are inversely correlated to the degree of perthitization, which is the decisive factor for Ar loss from K-feldspar.