AN ASSESSMENT OF AR-40 AR-39 DATING FOR THE WHOLE-ROCK VOLCANIC SAMPLES FROM THE LUZON ARC NEAR TAIWAN/

From electron microprobe, petrographic and image analyses of samples f rom the northern tip of the Luzon Arc, the effects of various alterati on reactions, phenocryst and groundmass assemblages on the whole-rock Ar-40/Ar-39 step-heating spectra have been delineated. The alteration of primary hornblende to an assemblage of zoisite + chlorite + sphene, the palagonitization of glass and the crystallization of phillipsite tends to cause discordant Ar-40/Ar-39 dates in the low-temperature ste ps of the age spectra. Discordant Ar-40/Ar-39 dates in the high-temper ature steps appear to result from degassing of inherited or trapped Ar -40 from coarse-grained phenocrysts. Samples in which most of the K re sides in groundmass plagioclase tend to yield meaningful plateau dates , whereas samples in which most of the K resides primarily in palagoni tized glass tend to yield meaningless plateau and isochron dates. Subs tantial loss of Ar-39 (13-40%) occurred from irradiated samples prior to Ar-40/Ar-39 step-heating analysis, which appears to be correlated w ith the abundance of groundmass and the degree of alteration. This arg on loss is attributed to thermally-induced argon diffusion from the fi ne-grained groundmass plagioclase, glass and alteration phases during the pre-step-heating bakeout.