Skip to main content
U.S. flag

An official website of the United States government

Petrochemistry of late miocene peraluminous silicic volcanic rocks from the Morococala field, Bolivia

January 1, 1998

Late Miocene peraluminous volcanic rocks of the Morococala field, Bolivia, define a layered stratigraphy of basal andalusite-, biotite- (± muscovite)-bearing rhyolite tuffs (AR), overlain by cordierite-, biotite-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 differentiation 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 derivation of all three rock types from a single stratified magma reservoir, but age and spatial relations between the three units make this unlikely. Genesis of the QL involved biotite-dehydration melting of an aluminous source at T > 750°C and P ≥ = 4–6 kbar. If not co-magmatic with QL, the other units were generated primarily by muscovite-dehydration melting at T = 730–750°C and P ≥ = 3.5–4.5 kbar for CR, and T ≤ = 750°C for AR with pre-eruptive residence at low pressure (1.5–3.0 kbar). Low hematite contents (XHem ≤ = 0.06) of ilmenite grains in AR, CR, and early grains (as inclusions in plagioclase and sanidine cores) in QL indicate reduced conditions imposed by a graphite-bearing source. Compositional variability among texturally later oxides (ilmenite with XHem = 0.06–0.50, primary magnetite), however, apparently records progressive increases in pre-eruptive f(O2 ) in QL. Plagioclase-melt equilibria and electron microprobe analysis difference for quartz-hosted glass inclusions suggest pre-eruptive melt H2 O contents ≥ = 5–7 wt % for the AR, ∼4–6 wt % for the CR, and ∼3–5 wt % for the QL.

Publication Year 1998
Title Petrochemistry of late miocene peraluminous silicic volcanic rocks from the Morococala field, Bolivia
DOI 10.1093/petroj/39.4.601
Authors VI G.B. Morgan, D. London, R. G. Luedke
Publication Type Article
Publication Subtype Journal Article
Series Title Journal of Petrology
Index ID 70021074
Record Source USGS Publications Warehouse