Metamorphism and metasomatic alteration of an eclogite block in the Franciscan Complex near Mt. Hamilton, Diablo Range, California

Conference · Tue Jan 01 04:00:00 EST 1985 · Geol. Soc. Am., Abstr. Programs; (United States)

OSTI ID:6319042

Herrington, K L; Cloos, M

Petrographic, microprobe and whole rock analysis of a rounded, 10 m eclogite that is typical of the rare high-grade blocks found in the Franciscan mud-matrix melange terranes show the block also underwent high-pressure/lower-temperature hydration and metasomatic alteration. Fe-Mg partitioning between garnet and omphacite and experimental phase relations indicate initial recrystallization at 5007 kb. Hydration of the block is evident because towards the margins omphacite is replaced by brown calcic-amphibole + white mica and by blue sodic-amphibolte + white mica. During hydration and rind formation, the block lost SiO/sub 2/, CaO, and Na/sub 2/O and gained K/sub 2/O. Parts of the rind are replaced by tremolite + chlorite. Cavities lined with radiating, euhedral sodic-amphibole + aragonite and pseudomorphic textural relationships show the block was not penetratively deformed after recrystallization. The tectonic history of the eclogite block can be explained as follows: The eclogite is a fragment of the top of the descending plate that was imbricated with hot ultramafics at the base of the overriding plate to form a metamorphic aureole at 20+ km-depth during the initial stages of Franciscan subduction. The overriding plate and imbricated aureole cooled with continued convergence. The blueschist assemblage and rind formed as waters migrated upwards into the aureole from underlying subducting or accreted sediments. Aragonite and the lack of alterations characteristic of greenschist-facies conditions indicates either synsubduction or very rapid post-subduction uplift.

• Research Organization: Univ. of Texas, Austin (USA)
• OSTI ID: 6319042
• Report Number(s): CONF-8510489-
• Conference Information: Journal Name: Geol. Soc. Am., Abstr. Programs; (United States) Journal Volume: 17
• Country of Publication: United States
• Language: English

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58 GEOSCIENCES
580300* -- Mineralogy
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& Rock Mechanics-- (-1989)
580400 -- Geochemistry-- (-1989)
ALKALINE EARTH METAL COMPOUNDS
AMPHIBOLE
ARAGONITE
CALCIUM CARBONATES
CALCIUM COMPOUNDS
CALIFORNIA
CARBON COMPOUNDS
CARBONATE MINERALS
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CHEMICAL REACTIONS
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FEDERAL REGION IX
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GEOLOGIC HISTORY
HYDROTHERMAL ALTERATION
METAMORPHIC ROCKS
METAMORPHISM
MINERALOGY
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OXYGEN COMPOUNDS
PETROGENESIS
PETROGRAPHY
PYROXENES
RECRYSTALLIZATION
ROCKS
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
TECTONICS
USA