Characteristics and petrogenesis of Alaskan-type ultramafic-gabbro intrusions, southeastern Alaska

Conference · Thu Apr 01 04:00:00 EST 1993 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5022143

Loney, R A ^[1]; Himmelberg, G R ^[2]

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1. Geological Survey, Menlo Park, CA (United States)
2. Geological Survey, Columbia, MO (United States) Univ. of Missouri, Columbia, MO (United States)

Alaskan-type ultramafic-gabbro intrusions occur along a belt that extends from Duke Island to Klukwan in southeastern Alaska and fall into two age groups, 400 to 440 Ma and 100 to 110 Ma. Most of the smaller bodies are magnetite-bearing hornblende clinopyroxenite; the larger ones consist of dunite, wehrlite, olivine clinopyroxenite, with some gabbro, in addition to hornblende clinopyroxenite and hornblendite. Textural, mineralogical, and chemical characteristics of the Alaskan-type ultramafic bodies indicate that they originated by fractional crystallization of a basaltic magma and accumulation in a crustal magma chamber. The Al[sub 2]O[sub 3] content of clinopyroxene shows a marked enrichment with differentiation, suggesting crystallization from progressively more hydrous melts like those characteristics of arc magmas. REE abundance levels and patterns are markedly similar for given rock units in all the bodies studied suggesting that all the bodies were derived by differentiation of closely similar parent magmas under near identical conditions. The exact composition of the primary melt is uncertain but the authors' preferred interpretation is that the parental magma of most Alaskan-type bodies was a subalkaline hydrous basalt. The striking similarity between the REE abundance levels and patterns of the Alaskan-type clinopyroxenites and gabbros, and the clinopyroxenite xenoliths and plutonic gabbros associated with Aleutian Island Arc volcanism, further suggests that the primary magma was probably a hydrous olivine basalt similar to the primary magma proposed for the Aleutian arc lavas. The mineral chemistry and phase equilibria of the ultramafic bodies suggest that they crystallized in magma chambers at depths greater than about 9 km. Except for the Duke Island body, which has sedimentary structures and shows evidence of ubiquitous current activity, most of the other bodies appear to have accumulated under static conditions.

• OSTI ID: 5022143
• Report Number(s): CONF-9305259--
• Conference Information: Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 25:5
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
AGE ESTIMATION
ALASKA
AMPHIBOLE
CHEMISTRY
CRYSTALLIZATION
DEVELOPED COUNTRIES
ELEMENTS
GABBROS
GEOCHEMISTRY
GEOLOGIC DEPOSITS
GEOLOGIC HISTORY
GEOLOGY
HORNBLENDE
IGNEOUS ROCKS
MAGMA
MAGMATISM
METALS
MINERALOGY
MINERALS
NORTH AMERICA
OLIVINE
PETROGENESIS
PETROLOGY
PHASE TRANSFORMATIONS
PLUTONIC ROCKS
RARE EARTHS
ROCKS
SILICATE MINERALS
USA