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Structure and Petrology of the San Gabriel Anorthosite-Syenite Body, California

Carter, Bruce and Silver, Leon T. (1972) Structure and Petrology of the San Gabriel Anorthosite-Syenite Body, California. In: 24th International Geological Congress. Micropaleontology Press , New York, NY, pp. 303-311.

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The San Gabriel anorthosite "massif" is part of a large layered intrusive originally ≥ 10 km in thickness, part of which covers about 250 km^2 in the western San Gabriel Mountains between the San Gabriel and San Andreas fault zones 30 km north of Los Angeles. Although not subjected to post-emplacement regional metamorphism, the Precambian anorthosite is intruded by granitic rocks of Permo-Triassic and late Cretaceous ages, and is deformed by broad folds of at least two ages (Permo-Triassic (?) and mid-Cenozoic), which have produced several km of structural relief within the body. Complex Cenozoic faulting (normal and strike-slip) of several ages has strongly deformed the anorthosite, which is tectonically floored by a thick zone of mylonitized gneiss along which important post-late Cretaceous (?) thrust movement probably occurred, and at least the western part of which is underlain by a major thrust fault, as shown by the distribution of the main shock and aftershocks of the February 9, 1971 San Fernando earthquake. Abundant textural and structural evidence suggests that bottom crystal accumulation has produced the classic anorthosite-gabbro-syenite differentiation suite (andesine anorthosite-leuconorite-norite-jotunite-mangerite-syenite-quartz syenite) making up this body. All the rocks of this suite appear to be relatively iron-enriched; extensive deuteric uralitization of pyroxene in all but the last intruded rocks suggests that the magma probably was also relatively water-rich. In the lower part of the intrusion, extensive "annealing" recrystallization, especially of anorthosite, has strongly modified the original fabric of the rock. Some rocks near the top of the body (jotunite and syenite) contain small amounts of pigeonite, both in the cores of strongly zoned pigeonite-augite crystals and as uninverted remnants in hypersthene inverted from original pigeonite, suggesting a relatively shallow depth of crystallization.

Item Type:Book Section
Additional Information:© 1972 Micropaleontology Press. The authors have profited from discussions in the field with L. R. Wager, D. H. Lindsley and R. B. Hargraves. This work was supported by National Science Foundation Grant No. GA-15989 and by Project Agreement #7 under Atomic Energy Commission Contract No. AT(04-3)-767, CALT-767P7-72.
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Atomic Energy CommissionAT(04-3)-767
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Caltech Division of Geological and Planetary Sciences 2065
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ID Code:61706
Deposited By: Tony Diaz
Deposited On:29 Oct 2015 17:56
Last Modified:03 Oct 2019 09:10

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