ON THE CAUSES OF O-16-DEPLETION AND O-18 O-16 HOMOGENIZATION DURING REGIONAL METAMORPHISM - THE EAST HUMBOLDT RANGE CORE COMPLEX, NEVADA/

Previous stable isotope studies at Lizzies Basin revealed that metased imentary rocks are O-18-depleted relative to protolith values, particu larly in the lower parts of the section (Lower Zone) where the rocks a re also isotopically homogeneous on a scale of hundreds of meters (qua rtz delta(18)O = + 9.0 to + 9.6 per mil). In contrast, metasedimentary rocks at higher levels at Lizzies Basin (Upper Zone) are less O-18-de pleted and more heterogeneous in delta(18)O. In order to understand mo re fully the isotopic evolution of this terrane, a series of detailed, meter-scale traverses across various metamorphic and igneous litholog ies were completed at Lizzies Basin, and at the structurally higher An gel Lake locality. Traverses in the Lizzies Basin Lower Zone and in th e lower parts of Angel Lake (Angel Lake Lower Sequence) across various silicate lithologies, including abundant granitoids, reveal similar d egrees of homogeneity, although the average delta(18)O values are high er at Angel Lake. In contrast, traverses which include substantial thi cknesses of marble and calc-silicate gneiss and very little granitoid have more heterogeneous quartz delta(18)O values (+ 11.9 to + 13.4 per mil), and also have a higher average delta(18)O( + 12.9 per mil), tha n observed elsewhere. The scale of O-18/O-16 homogeneity in quartz obs erved at Lizzies Basin and Angel Lake (meters to hundreds of meters) r equires fluid-mediated isotope exchange, which accompanied Tertiary me tamorphism. There is a correlation between the degree of O-18-depletio n in metasedimentary rocks, O-18/O-16 homogenization between lithologi es, and the proportion of granitoids (leucogranites in particular) wit hin any part of the section, and a corresponding anticorrelation with the proportion of marble. This points to a causal relationship, whereb y the leucogranites (as well as the Tertiary hornblende diorite and bi otite monzogranite) acted as bath a relatively low-O-18 reservoir and a source of fluids to enhance exchange, while the marbles hindered iso tope depletion and homogenization by acting as relatively high-O-l8 re servoirs and impermeable layers. Material balance calculations help de lineate the plausible mechanisms of exchange between granitoids and me tasediments. Single-pass infiltration of magmatic fluids from the gran itoids is not capable of reproducing all of the observations. Fluid-me diated exchange by convective recirculation of magmatic fluids on a sc ale of meters is the mechanism which explains all of the observations. The generalized model for the isotopic evolution of the East Humboldt Range core complex provides an excellent opportunity to establish the main causes and controlling factors of O-18-depletion and O-18/O-16 h omogenization during regional metamorphism.