Geology and geochemistry of the Little Walker Volcanic Center, Mono County, California Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/bg257j516

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  • 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 H₂O 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 H₂O 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 H₂O content relative to early latites. Significant phenocrystic hornblende, association with hydrothermal alteration, and high Eu⁺³ /Eu⁺² all suggest significant H₂O 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 CO₂, 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 A1₂0₃ but low Si0₂. Low-pressure (plagioclase rich) differentiation of the early latitic magmas produced quartz latite ash flows with high Si0₂ 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 PH₂O 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 H₂0/CO₂ 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 H₂0/CO₂ 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. H₂0/CO₂ and total volatile content were also critical controls of the probability of hydrothermal ore deposition. Magmas with high H₂0/CO₂ 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.
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