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Formation of anorthosite-Gabbro rhythmic phase layering: an example at North Arm Mountain, Bay of Isands ophiolite

Journal of Petrology

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Abstract

Rhythmically layered anorthosite and gabbro are exposed in a 4-10-m thick interval at the base of the layered gabbro unit on North Arm Mountain, one of four massifs that compose the Bay of Islands ophiolite, Newfoundland. The rhythmically layered interval is sandwiched between thick layers of adcumulate to orthocumulate uniform gabbro. Calculated fractional crystallization paths and correlated cryptic variation patterns suggest that uniform and rhythmically layered gabbros represent 20-30% in situ crystallization of two distinct magma batches, one more evolved and the other more primitive. When the more primitive magma entered the crystallization site of the NA300-301 gabbros, it is estimated to have been ~40??C hotter than the resident evolved magma, and may have been chilled by contact with a magma chamber margin composed of uniform gabbro. In this model, chilling caused the liquid to become supercooled with respect to plagioclase nucleation temperatures, resulting in crystallization of gabbro deficient in plagioclase relative to equilibrium cotectic proportions. Subtraction of a plagioclase-poor melagabbro enriched the liquid in normative plagioclase, which in turn led to crystallization of an anorthosite layer. -from Authors

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Formation of anorthosite-Gabbro rhythmic phase layering: an example at North Arm Mountain, Bay of Isands ophiolite
Series title:
Journal of Petrology
Volume
31
Issue:
1
Year Published:
1990
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Journal of Petrology
First page:
1
Last page:
50
Number of Pages:
50