Physical and chemical fractionation processes in subaerial and subaqueous pyroclastic rocks

Thesis/Dissertation · Sun Jan 01 04:00:00 EST 1989

OSTI ID:6721006

Stix, J

Chemical and physical fractionation in subaerial rocks focuses upon the Cerro Toledo Rhyolite, a series of Pleistocene high silica rhyolite lave domes and tephras the crop out in the Jemez Mountains of northern New Mexico, USA. The Cerro Toledo Rhyolite is situated stratigraphically between the Lower Bandelier Tuff (LBT) and Upper Bandelier Tuff (UBT) and was erupted after the formation of Toledo caldera at 1.45 Ma and before that of Valles caldera at 1.12 Ma. The bulk of the Cerro Toledo Rhyolite tephra sequence exhibit progressive enrichment upsection of trace elements such as Rb, Cs, HREE, Y, Nb, Th, U and decline of Sr and K/Cs. Hf/Cs and La/Cs ratios initially decline, then level off upsection, indicating that crystallization of zicron and allanite/chevkinite was suppressed with time. Physical and chemical fractionation in subaqueous pyroclastic rocks focuses upon the current state of knowledge, lithofacies codes, and an Archean-age example from northwestern Ontario, Canada. A conceptual framework for understanding explosive subaqueous volcanism is presented comprising three essential aspects: (1) environment of the eruption, (2) orientation of the eruption, and (3) composition of the juvenile debris. A set of lithofacies codes for subaqueous volcaniclastic rocks is presented to describe deposits in the field and to make genetic and environmental interpretations. The codes may be used to distinguish between pyroclastic and epiclastic emplacement. Data collected from subaqueous volcaniclastic deposits at Confederation Lake, Ontario, reveal that the rocks are relatively primary products from explosive eruptions, probably from dome fragmentation, but that emplacement may not have occurred directly after eruption.

• Research Organization: Toronto Univ., ON (Canada)
• OSTI ID: 6721006
• Country of Publication: United States
• Language: English

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58 GEOSCIENCES
580000* -- Geosciences
ACTINIDES
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CALDERAS
CANADA
CESIUM
CHEMICAL COMPOSITION
CHEMICAL REACTIONS
CHEMISTRY
CRYSTALLIZATION
ELEMENTS
ENRICHMENT
FEDERAL REGION VI
FRACTIONATION
GEOCHEMISTRY
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IGNEOUS ROCKS
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NIOBIUM
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ONTARIO
PHASE TRANSFORMATIONS
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RHYOLITES
ROCK MECHANICS
ROCKS
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