Petrogenesis of gabbronorite at Yakobi and northwest Chichagof Islands, Alaska
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Abstract
On Yakobi Island and at Mirror Harbor on the northwest coast of Chichagof Island, gabbronorite occurs as irregular bodies, as much as 5.5 km in maximum dimension, mostly within a 40 to 43 m.y. composite pluton consisting largely of tonalite. The gab-bronorites are the host rocks for a magmatic nickel-copper sulfide deposit consisting predominantly of pyrrhotite, pentlandite, and chalcopyrite. The gabbronorites characteristically have more orthopyroxene than augite and have a significant amount of hornblende. Rock types mapped as gabbronorite range from hornblende pyroxenite to hornblende-pyroxene gabbronorite to quartz-bearing norite and gabbronorite. The tonalite pluton is composed of hornblende diorite, biotite-hornblende diorite, hornblende quartz diorite, biotite-hornblende tonalite, and biotite granodiorite. Contacts between types of gabbronorite are generally gradational on a scale of centimetres to metres; contacts between gabbronorite and the tonalite pluton are gradational on a scale of metres to tens of metres. Rock textures, pyroxene-hornblende relations, and rock and mineral chemistry of the gabbronorites show systematic changes as the gabbronorites grade into the tonalites. The field, petrographic, and chemical data, including trace-element abundances, of the gabbronorites and tonalite pluton rocks can best be explained by either (1) crystallization of gabbronorite from a tholeiitic magma with subsequent assimilation by tonalite that was simultaneously undergoing fractional crystallization or (2) fractional crystallization of a quartz diorite parent magma yielding the range of gabbronorites and tonalite pluton rocks.
Publication type | Article |
---|---|
Publication Subtype | Journal Article |
Title | Petrogenesis of gabbronorite at Yakobi and northwest Chichagof Islands, Alaska |
Series title | Geological Society of America Bulletin |
DOI | 10.1130/0016-7606(1987)98<265:POGAYA>2.0.CO;2 |
Volume | 98 |
Issue | 3 |
Year Published | 1987 |
Language | English |
Publisher | Geological Society of America |
Description | 15 p. |
First page | 265 |
Last page | 279 |
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