INHERITED ARGON IN A PLEISTOCENE ANDESITE LAVA - AR-40 AR-39 INCREMENTAL-HEATING AND LASER-FUSION ANALYSES OF PLAGIOCLASE/

By using Ar-40/Ar-39 incremental-heating and laser-fusion techniques, xenocrystic plagioclase was discovered in a late Pleistocene andesitic lava that erupted through the Andean Cordillera. Inherited argon in t he xenocrysts is as much as similar to 450 times older than the host l ava, the age of which is independently known, and is an obstacle to da ting the lava by using a whole-rock sample. The xenocrysts are impossi ble to identify from petrography or chemical parameters such as their K/Ca ratios. Holocrystalline groundmass, carefully separated from plag ioclase and other phenocrysts, gives an accurate Ar-40/Ar-39 age far t he lava. The xenocrysts could not have been degassed far more than sev eral days in the magma and probably were assimilated from Paleozoic ro cks buried under kilometers of Mesozoic and Tertiary are rocks composi ng the Cordillera in central Chile. The required magma ascent velocity , on the order of kilometers/day, is extraordinarily high compared to the 10(-4) km/day minimum implied by the Ra-226 excess in continental are lavas, These data permit magma migration and storage in the mantle and lower crust for as much as thousands of years, followed abruptly by rapid ascent to the surface.