Petrographic significance of zoned allanite in garnet amphibolites from a paleo-subduction zone: Catalina Schist, southern California

Journal Article · · American Mineralogist; (United States)

OSTI ID:5738649

Sorensen, S S ^[1]

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1. Smithsonian Inst., Washington, DC (United States)

Zoned allanite is a minor mineral in highly altered blocks of garnet amphibolite that occur in a metaultramafic melange matrix in the amphibolite unit of the Catalina Schist, southern California. Blocks and matrix alike were affected by a high-T, relatively high-P hydrothermal event, which locally induced partial melting of blocks and apparently was accompanied by considerable trace element metasomatism. In blocks that show evidence for light rare earth element (LREE) enrichment, allanite contains almost all of the LREE in the rock. Allanite from four LREE-enriched samples that have different whole-rock REE abundances and fractionations was analyzed with the electron microprobe. Three samples are migmatites, and one shows no evidence for partial melting. The analyses were obtained for empirically defined zones of uniform brightness in backscattered electron images from the scanning electron microscope (SEM). The zoned allanite displays large ranges of anticorrelated Ca and LREE contents. Variations of these elements and of Al, Cr, Sr, Fe, Mg, and Mn within zoned grains indicate a fairly continuous nature for many substitutions of Mg{sup 2+} and M{sup 3+} cations in allanite. The apparent reverse zoning of allanite in migmatites suggests that LREE were added to the rock volume from an external source during allanite growth. Metamorphic textures and petrologic considerations indicate that at least some of the growth of allanite in migmatites predates the partial melting event. The variations of LREE within grains and between samples may in part reflect local, time-dependent charges of fluid compositions in the subduction-related, hydrothermal and metasomatic system the rocks represent.

• OSTI ID: 5738649
• Journal Information: American Mineralogist; (United States), Vol. 76:3-4; ISSN 0003-004X
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
ALLANITE
CHEMICAL COMPOSITION
CALIFORNIA
SUBDUCTION ZONES
GEOCHEMISTRY
ALUMINIUM
CALCIUM
CHROMIUM
ELECTRON MICROPROBE ANALYSIS
ENRICHMENT
GEOLOGIC HISTORY
IRON
MAGNESIUM
MANGANESE
METAMORPHISM
MINERALOGY
PETROGRAPHY
RARE EARTHS
SCANNING ELECTRON MICROSCOPY
STRONTIUM
TRACE AMOUNTS
ALKALINE EARTH METALS
CHEMICAL ANALYSIS
CHEMISTRY
DEVELOPED COUNTRIES
ELECTRON MICROSCOPY
ELEMENTS
MATERIALS
METALS
MICROANALYSIS
MICROSCOPY
MINERALS
NONDESTRUCTIVE ANALYSIS
NORTH AMERICA
RADIOACTIVE MATERIALS
RADIOACTIVE MINERALS
SILICATE MINERALS
THORIUM MINERALS
TRANSITION ELEMENTS
USA
580000* - Geosciences