REJUVENATION OF K-AR SYSTEMS FOR MINERALS IN THE TAIWAN MOUNTAIN BELT

The late Mesozoic amphibolites and granitic rocks in the basement comp lex of the Taiwan Mountain Belt are overprinted by late Tertiary green schist facies metamorphism. The K-Ar isotope systems of late Mesozoic hornblende, muscovite, biotite and microcline have been reset by this metamorphic event, as reflected in three systematic variations: (1) a regional decrease in Ar-40/Ar-39 dates with increasing metamorphic ove rprinting, (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 betw een the grain-size fractions and their Ar-40/Ar-39 dates (e.g., smalle r grain-size fractions of a given mineral yield younger dates). In the lower greenschist facies area, hornblendes and coarse-grained muscovi tes still retain most of their radiogenic argon and display fairly fla t Ar-40/Ar-39 age spectra with plateau dates of 82-95 Ma which record a rapid cooling during the Nanao Orogeny, a late Mesozoic tectonotherm al event. In contrast, all microclines are completely reset and yield young plateau dates of approximately 1.6 and approximately 1.7 Ma, ref lecting the rapid uplift associated with the Penglai Orogeny. Most of the partially reset minerals (e.g., muscovites from the upper greensch ist facies area and biotites from the lower greenschist facies area) y ield geologically meaningless Ar-40/Ar-39 integrated dates that fall b etween the ages of the two tectonothermal events and exhibit distribut ed age spectra which are caused mainly by mixing of argon released fro m two generations of phases (excess argon and partial argon loss). Aft er isolating possible recrystallization and mixing effects, the region al variation in the Ar-40/Ar-39 dates for the earlier generation phase s could be simulated by a volume diffusion model coupled with a therma l model for the Taiwan Mountain Belt.