Ch. Lo et Tc. Onstott, REJUVENATION OF K-AR SYSTEMS FOR MINERALS IN THE TAIWAN MOUNTAIN BELT, Earth and planetary science letters, 131(1-2), 1995, pp. 71-98
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.