Rejuvenation of K [sbnd]Ar systems for minerals in the Taiwan Mountain Belt

Abstract

The late Mesozoic amphibolites and granitic rocks in the basement complex of the Taiwan Mountain Belt are overprinted by late Tertiary greenschist facies metamorphism. The K Ar isotope systems of late Mesozoic horn-blende, muscovite, biotite and microcline have been reset by this metamorphic event, as reflected in three systematic variations: (1) a regional decrease in 40Ar/ 39Ar dates with increasing metamorphic overprinting, (2) a unique sequence of decreasing dates among minerals (i.e., hornblende > muscovite > biotite > microcline) in the same sample or within closely spaced samples, and (3) a positive correlation between the grain-size fractions and their 40Ar/ 39Ar dates (e.g., smaller grain-size fractions of a given mineral yield younger dates). In the lower greenschist facies area, hornblendes and coarse-grained muscovites still retain most of their radiogenic argon and display fairly flat 40Ar/ 39Ar age spectra with plateau dates of 82–95 Ma which record a rapid cooling during the Nanao Orogeny, a late Mesozoic tectonothermal event. In contrast, all microclines are completely reset and yield young plateau dates of ∼ 1.6 and ∼ 1.7 Ma, reflecting the rapid uplift associated with the Penglai Orogeny. Most of the partially reset minerals (e.g., muscovites from the upper greenschist facies area and biotites from the lower greenschist facies area) yield geologically meaningless 40Ar/ 39Ar integrated dates that fall between the ages of the two tectonothermal events and exhibit disturbed age spectra which are caused mainly by mixing of argon released from two generations of phases (excess argon and partial argon loss). After isolating possible recrystallization and mixing effects, the regional variation in the 40Ar/ 39Ar dates for the earlier generation phases could be simulated by a volume diffusion model coupled with a thermal model for the Taiwan Mountain Belt.