Hornblende phenocrysts record a pressure gradient in and contamination of the Taylor Creek Rhyolite magma reservoir
- North Arizona Univ., Flagstaff, AZ (United States)
- Geological Survey, Flagstaff, AZ (United States)
- Geological Survey, Menlo Park, CA (United States)
The Taylor Creek Rhyolite of southwestern New Mexico comprises 20 coeval porphyritic lava domes erupted from a large vertically zoned reservoir of silicic magma. The rhyolite is high-silica, subalkaline, and is nearly constant in major-element composition. Trace elements and [sup 87]Sr/[sup 86]Sr[sub i] (0.705 to 0.713) define vertical zoning that records a downward-decreasing imprint of minor (<1 wt%) partial assimilation of Proterozoic roof rocks. Consistent with the major-element homogeneity, electron-microprobe analyses of hornblende phenocrysts show little or no measurable variation in principal constituents. The hornblende is edenite whose mean composition and standard deviation of 110 analyses are SiO[sub 2], 44.66 [+-] 0.64; TiO[sub g], 1.27 [+-] 0.13; Al[sub 2]O[sub 3], 6.80 [+-] 0.31; FeO, 21.00 [+-] 1.60; MnO, 1.19 [+-] 0.16; MgO, 9.94 [+-] 1.09; CaO, 10.51 [+-] 0.22; Na[sub 2]O, 2.22 [+-] 0.13; K[sup 2]O, 0.98 [+-] 0.08; F, 2.04 [+-] 0.35; Cl, 0.20 [+-] 0.03. Except for FeO, MnO, and MgO, compositional variations are non systematic and mostly within analytical uncertainty. FeO and Mno exhibit strong negative correlation with MgO. Individual hornblende crystals are zoned to relatively MgO-rich and FeO-MnO-poor rims, opposite what might be expected if the Taylor Creek Rhyolite magma reservoir evolved chemically isolated from its surroundings. Hornblende with rims richest in MgO occurs in domes fed from the uppermost part of the reservoir. Calculated pressures based on Al in hornblende range from 1.6 to 2.0 kb, [+-] 0.5 kb. Though the range of calculated P is encompassed within the uncertainty, the lowest hornblende pressure is for a dome fed from, or near, the top of the reservoir, whereas the chemically defined vertical zoning.
- OSTI ID:
- 5095732
- Report Number(s):
- CONF-9305259-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 25:5; Conference: 89. annual meeting of the Cordilleran Section and the 46th annual meeting of the Rocky Mountain Section of the Geological Society of America (GSA), Reno, NV (United States), 19-21 May 1993; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
GEOLOGIC FORMATIONS
GEOBAROMETRY
GEOCHEMISTRY
PETROGENESIS
NEW MEXICO
RHYOLITES
HORNBLENDE
MAGMA
MAGMA SYSTEMS
MAGMATISM
AMPHIBOLE
CHEMISTRY
DEVELOPED COUNTRIES
GEOLOGY
GEOTHERMAL SYSTEMS
IGNEOUS ROCKS
MINERALS
NORTH AMERICA
PETROLOGY
ROCKS
SILICATE MINERALS
USA
VOLCANIC ROCKS
580000* - Geosciences