PETROCHEMISTRY OF LATE MIOCENE PERALUMINOUS SILICIC VOLCANIC-ROCKS FROM THE MOROCOCALA FIELD, BOLIVIA

Late Miocene peraluminous volcanic rocks of the Morococala field, Boli via, define a layered stratigraphy of basal andalusite-, biotite- (+/- muscovite)-bearing rhyolite tuffs (AR), overlain by cordierite-, biot ite-bearing rhyolite tuffs (CR), and capped by biotite-bearing quartz latite tuffs, lavas, and late domal flows (QL). Mineral and whole-rock compositions become more evolved from top to bottom, with differentia tion reflected by decreasing Ca, Ba, Mg, Fe, and rare earth elements ( REE) versus increasing F, Na/K, and aluminosity from QL to AR. Mineral , whole-rock, and glass inclusion compositions are consistent with dev iation of all three rock types from a single stratified magma reservoi r, but age and spatial relations between the three units make this unl ikely. Genesis of the QL involved biotite-dehydration melting of an al uminous source at T > 750 degrees C and P greater than or equal to 4-6 kbar. If not co-magmatic with QL, the other units were generated prim arily by muscovite-dehydration melting at T = 730-750 degrees C and P greater than or equal to 3.5-4.5 kbar for CR, and T less than or equal to 750 degrees C for AR with pre-eruptive residence at low pressure ( 1.5-3.0 kbar). Low hematite contents (X-Hcm less than or equal to 0.06 ) of ilmenite grains in AR, CR, and early grains (as inclusions in pla gioclase and sanidine cores) in QL indicate reduced conditions imposed by a graphite-bearing source. Compositional variability among textura lly later oxides (ilmenite with X-Hcm = 0.06-0.50, primary magnetite), however, apparently records progressive increases in pre-eruptive f(O -2) in QL. Plagioclase-melt equilibria and electron microprobe analysi s difference for quartz-hosted glass inclusions suggest pre-eruptive m elt H2O contents greater than or equal to 5-7 wt % for the AR, similar to 4-6 wt % for the CR, and similar to 3-5 wt % for the QL.