Petrochemistry of late miocene peraluminous silicic volcanic rocks from the Morococala field, Bolivia
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Abstract
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 type | Article |
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Publication Subtype | Journal Article |
Title | Petrochemistry of late miocene peraluminous silicic volcanic rocks from the Morococala field, Bolivia |
Series title | Journal of Petrology |
DOI | 10.1093/petroj/39.4.601 |
Volume | 39 |
Issue | 4 |
Year Published | 1998 |
Language | English |
Publisher | Oxford Academic |
Description | 32 p. |
First page | 601 |
Last page | 632 |
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