ON THE ORIGIN OF LOW-O-18 MAGMAS - EVIDENCE FROM THE CASTE PLUTON, IDAHO

The Eocene Caste pluton (Idaho) is a newly identified low-O-18 igneous rock, the first intrusive body related to caldera eruptions found to have this character. This pluton crystallized from a magma with a 6.7 parts per thousand range of oxygen isotope ratios. These values vary g radationally across the pluton. The Caste pluton is a high-silica gran ite within the extensional trans-Challis fault zone. It intruded into the basal parts of the Thunder Mountain and Van Horn Peak cauldron com plexes after they collapsed, and its rocks are intrusive equivalents o f the eruptive rocks of the cauldrons. Unaltered Caste samples have de lta(18)O values that range from 2.7 parts per thousand to 9.4 parts pe r thousand. Mineral-mineral fractionations in these rocks exhibit typi cal magmatic values: quartz orthoclase averages 2,2 parts per thousand and quartz-biotite averages 5.1 parts per thousand. Thus, parts of th e pluton crystallized from a low-O-18 magma. The low-O-18 values are t hought to have resulted by assimilation of up to 55% hydrothermally al tered wall rocks. The low values are concentrated in the central part of the pluton, along the southern extension of the Thunder Mountain ca uldron complex. Similar to other Tertiary low-O-18 igneous rocks in th e American Cordillera, the Caste pluton crystallized from a high-silic a magma emplaced within an extensional tectonic environment, and it is intimately associated with ash-flow eruptions and complex caldera col lapse structures.