Mineralogy of a layered gabbro deformed during magmatic crystallization, western Sierra Nevada foothills, California

Journal Article · · American Mineralogist; (USA)

OSTI ID:6904317

Springer, R K ^[1]

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1. Brandon Univ., Manitoba (Canada)

In the 162 Ma Pine Hill intrusive complex of northern California, a synformal, layered pyroxenite-gabbro-diorite body, the crystallization sequence and compositional variation of cumulus and intercumulus minerals indicate that magmatic crystallization was characterized by a high initial oxygen fugacity, log{sub 10}f{sub O2} in the range of {minus}4 to {minus}9, which allowed the early precipitation of abundant titanomagnetite; the oxygen fugacity decreased less than two log units with crystallization. The magmatic crystallization sequence is ol + aug, ol + aug + mt, ol + aug + mt + pl, ol + aug + mt + pl + opx (inverted from pig) and aut + mt + pl + opx (inverted from pig) + qz. Superimposed upon this sequence are intercumulus minerals, which crystallized from magma trapped within cumulates, and biotite and calcic amphibole, which largely formed from a subsolidus reaction of a residual hydrous fluid phase with primary minerals. Mineral compositional ranges are plagioclase, An{sub 95} to An{sub 32}; olivine, Fo{sub 77} to Fo{sub 40}; augite, Ca{sub 48}Mg{sub 43}Fe{sub 9} to Ca{sub 43}Mg{sub 35}Fe{sub 22}; Ca-poor pyroxene, Ca{sub 1}Mg{sub 74}Fe{sub 25} to Ca{sub 3}Mg{sub 48}Fe{sub 49}; Mg/(Mg + Fe + Mn) of calcic amphibole, 0.74 to 0.48; and Mg/(Mg + Fe + Mn) of biotite, 0.78 to 0.45. Prior to complete solidification (>90% crystallized), deformation of the complex coincided with an abrupt decrease in Mg/(Mg + Fe) values for the mafic silicates. This compositional discontinuity is attributed to a decrease in oxygen fugacity of the crystallizing magma caused by an exchange between the fluid phase of adjacent country rock and the fluid phase of the magma during deformation. Generation and emplacement of an olivine tholeiite, low in alkalies and high in Ca, within an island-arc environment is proposed for the Pine Hill intrusive complex.

• OSTI ID: 6904317
• Journal Information: American Mineralogist; (USA), Journal Name: American Mineralogist; (USA) Vol. 74:1-2; ISSN AMMIA; ISSN 0003-004X
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
ALKALINE EARTH METAL COMPOUNDS
ANORTHOSITES
BIOTITE
CALIFORNIA
CHEMISTRY
CRYSTALLIZATION
DEFORMATION
ELEMENTS
FEDERAL REGION IX
GABBROS
GEOCHEMISTRY
GEOLOGIC DEPOSITS
GEOLOGIC HISTORY
GEOLOGIC MODELS
IGNEOUS ROCKS
IRON COMPOUNDS
IRON SILICATES
MAGMA
MAGNESIUM COMPOUNDS
MAGNESIUM SILICATES
MICA
MINERALOGY
MINERALS
MOUNTAINS
NONMETALS
NORTH AMERICA
OLIVINE
OXYGEN
OXYGEN COMPOUNDS
PHASE TRANSFORMATIONS
PLUTONIC ROCKS
PYROXENES
ROCKS
SIERRA NEVADA COLORADO
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
SOLIDIFICATION
TRANSITION ELEMENT COMPOUNDS
USA