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

Conference · Thu Apr 01 00:00:00 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-; CODEN: GAAPBC
• Journal Information: Geological Society of America, Abstracts with Programs; (United States), Vol. 25:5; Conference: 89. annual meeting of the Cordilleran Section and the 46th annual meeting of the Rocky Mountain Section of the Geological Society of America (GSA), Reno, NV (United States), 19-21 May 1993; ISSN 0016-7592
• Country of Publication: United States
• Language: English

Related Subjects

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