Detailed mapping and geochemical analysis of Oligocene to early
Pliocene volcanic rocks in the Little Walker volcanic center, Mono
County, California have revealed a complex eruptive history. After
eruption of widespread rhyolitic ash flows of the Valley Springs
Formation in the Oligocene, Miocene to early Pliocene volcanism of
the western Great Basin and northern Sierra Nevada was dominated by
eruption of calc-alkalic, andesitic lavas bearing abundant hydrous
mafic phenocrysts, and, thus, high H���O contents. These kinds of
calc-alkaline magmas are associated with most of the major epithermal
Au-Ag districts of the western Great Basin.
A highly potassic latitic pulse of volcanism occurred at the Little
Walker volcanic center about 9.5 m.y. ago during the ongoing calc-alkalic
activity. The latitic series is unusually enriched in K and
other incompatible elements, as well as Fe compared to the surrounding
calc-alkaline rocks. The latites have mineralogic evidence of
much lower H���O content than the calc-alkaline lavas; yet early latitic
magmas were rich enough in volatiles to produce very large, welded
ash-flow sheets (e.g., the Eureka Valley Tuff). Rapid evacuation of
the magma reservoir beneath the Little Walker center during the
ash-flow activity resulted in formation of the Little Walker caldera.
Intracaldera volcanism culminated with extrusion of viscous,
phenocryst-rich plug domes and coulees of transitionally calc-alkaline,
low-K latite lava of the Lavas of Mahogany Ridge. The low-K latite
series is severely depleted in all incompatible elements relative to
early latitic rocks and has mineralogic, geologic, and trace element
evidence of higher H���O content relative to early latites. Significant
phenocrystic hornblende, association with hydrothermal alteration,
and high Eu����� /Eu����� all suggest significant H���O concentration in the
low-K latite magmas. These rocks probably come from a source
region intermediate between that of the calc-alkaline and latite series.
Trace and major element data favor generation of latitic magmas
from a primitive mantle diapir. The diapir rose into a subduction
zone that was actively generating widespread calc-alkalic lavas
throughout the region from hydrous mantle and, possibly, lower
crustal sources. The latite magmas were drier and hotter than the
calc-alkaline magmas, but were also enriched in volatiles, particularly
CO���, and incompatible elements from their undepleted mantle
source. Rising latitic magmas may have gained additional incompatible elements by wall rock reaction and zone refining of
upper mantle and lower crustal rocks. Extensive qualitative trace
element evidence of crystal fractionation shows that incompatible
elements may have been further concentrated by variable amounts of
crystal settling. High-pressure (plagioclase-poor, pyroxene-rich)
fractionation of the early, dry latitic series produced low-Ca-Mg
latites with high Fe/Mg and A1���0��� but low Si0���. Low-pressure
(plagioclase rich) differentiation of the early latitic magmas produced
quartz latite ash flows with high Si0��� and moderate Fe/Mg, while low-pressure
differentiation of hydrous low-K latite magmas yielded
silicic low-K latite and quartz latite lavas with low Fe/Mg. More
extensive separation of olivine relative to pyroxenes at low pressures
and increased stability of subsilicic hydrous crystals and Fe-Ti oxides
in the hydrous magmas account for changes in differentiation trends
caused by Ptotal and PH���O variations.
Lack of giant welded ash-flow sheets in the hydrous calc-alkaline
series and common eruption of such ash flows from volcanic centers
with rather anhydrous magmas led to the conclusion that H���0/CO��� as
well as total volatile content are critical controls on the likelihood of
large scale, hot ash-flow eruptions. Giant, hot ash-flow sheets and
associated calderas are favored in magmas with low H���0/CO��� and
high total volatile content. Basaltic and latitic volcanism in areas of
thick sialic crust, where crystal fractionation is extensive are,
therefore, the best sources of giant ash-flow sheets.
H���0/CO��� and total volatile content were also critical controls
of the probability of hydrothermal ore deposition. Magmas with high
H���0/CO��� and moderate total volatile contents are most favored for
ore deposition, because such magmas tend to form mesozonal or
epizonal plutons rather than volcanic rocks. Plutonic crystallization
of hydrous magma will yield a fluid phase capable of transferring
incompatible metals and geothermal heat to ground water. If permeable
structures and rocks are present, as in a caldera, widespread
mineralization will be favored, but there may be no genetic relation
between ore-forming magmas and magmas which produce calderas. / Graduation date: 1980 / For master (tiff) digital images of maps contained in this document contact scholarsarchive@oregonstate.edu
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/33360 |
| Date | 29 May 1979 |
| Creators | Priest, George R. |
| Contributors | Taylor, Edward M. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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