Trachyandesite magmas in the Colorado River Trough
- Univ. of Nevada, Las Vegas, NV (United States). Dept. of Geosciences
Extension related magmatism along the Colorado River Trough (CRT) is typified by trachyandesititic suites with a limited compositional range (58--66 wt% SiO[sub 2], 6.5-8wt% Na[sub 2]O + K[sub 2]O). Although, bimodal basalt-rhyolite/dacite suites are predominant in the vicinity of Lake Mead, and along the CRT axis. The trachyandesites are highly porphyritic, having abundant plagioclase phenocrysts plus clinopyroxene, hornblende and minor occurrences of olivine. Olivine crystals show resorption, and plagioclase and clinopyroxene phenocrysts display complex twinning, zoning and reaction textures indicative of a complex crystallization history. Trachyandesite trace element patterns have many of the characteristics of basalts derived from the lithospheric mantle in the CRT. However, relative depletions of the high field strength elements (Ta, Nb, P, Ti) are more pronounced, and Pb, Rb and Th contents are higher relative to Ba in the basalts. Sr isotope ratios are also rather elevated. Trace element modeling suggests that the trachyandesites are not related to typical CRT basalt compositions by crystal fractionation, even if hornblende is modeled as principal fractionating phase. Rather, simple mixing between basaltic and silicic end-member melts is favored, and fractionation and/or AFC processes only play a minor role. Isotopic evidence indicates that crustal material, possibly Mesozoic granite, is a suitable silicic end-member. The relatively narrow range of observed major element compositions may indicate highly efficient mixing processes or that magma chambers were long-lived and often reached equilibrium before being tapped.
- OSTI ID:
- 5422513
- 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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ANDESITES
CHEMICAL COMPOSITION
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GEOLOGIC FORMATIONS
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CRYSTALLIZATION
CRYSTALLOGRAPHY
FELDSPARS
GEOCHEMISTRY
MAGMA
MAGMATISM
MINERALOGY
OLIVINE
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580000* - Geosciences