Volcanological investigation of the Banco Bonito eruption and subsurface geology of the ring fracture zone, Valles Caldera, New Mexico: Final technical report, September 1, 1985-December 31, 1986
A low-silica rhyolite eruptive sequence of 3 to 4 kmT (dense rock equivalent volume) forms the youngest volcanic event from the ring-fracture of the Valles caldera. STTh/STSTh - STYU/STSTh activity ratios determined for whole rock, glass, and mineral phases indicate that the magma system became closed isotopically approx.150 Ka. Other evidence from age dating supports an age for the eruption between 150 and 130 Ka. This event was of considerable duration with plinian and ignimbrite-forming activity preceding an effusive phase by a lengthy, but indeterminable, interval. The lava flow was emplaced onto a much-dissected surface cut into the E1 Cajete and Battleship Rock pyroclastic deposits. Twenty-four bulk-rock samples from all 3 eruptive stages exhibit little significant compositional variation, and the 3 units are petrographically identical (apart from differences arising from contrasting eruption styles). Nonetheless, phenocrysts and glasses exhibit substantial compositional variations and complex textural relations throughout the sequence. Very few phenocrysts appear to have been in equilibrium with the matrix rhyolitic liquid. The petrologic relationships are interpreted to result from partial melting of a pre-existing crustal igneous rock, probably of dioritic to granodioritic composition, and subsequent eruption of the resulting liquid and ''restite'' crystals. Kinetic arguments suggest a time-scale for magma production and eruption of the order of thousands of years.
- Research Organization:
- Texas Univ., Arlington (USA). Dept. of Geology
- DOE Contract Number:
- FG05-85ER13413
- OSTI ID:
- 6409195
- Report Number(s):
- DOE/ER/13413-2; ON: DE87011192
- Resource Relation:
- Other Information: Portions of this document are illegible in microfiche products
- Country of Publication:
- United States
- Language:
- English
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