Abstract

The prevalent concept of magmatic evolution in orogenic belts holds that ophiolitic eruptions are characteristic of the geosynclinal phase and that felsic magmas first appear on a large scale during the orogenic phase of emplacement of granitic batholiths. In the Fraser Belt of western North America, however, the geosynclinal Jurassic volcanic rocks are dominantly fragmental augite andesites; subordinate rock types include dacite and rhyolite, as well as basalt. Typical Jurassic andesitic rocks are chemically similar to continental Tertiary andesites of the same region; the more mafic Jurassic rocks are chemically similar to high-alumina basalts. The geosynclinal Permian and Triassic volcanic rocks of the Fraser Belt are also dominantly andesitic and closely resemble the Jurassic rocks.

The chemical similarity of the Late Mesozoic batholithic rocks to the geosynclinal andesites suggests that the granitic magmas possibly formed in part by partial fusion of geosynclinal volcanic rocks. The andesitic character of the geosynclinal volcanic rocks indicates that mobilized sial, either as a contaminant of mantle-derived basaltic magma or as primary andesitic magma, contributed substance to the geosynclinal volcanism. The evidence for a subjacent sialic crust during the geosynclinal phase and chemical similarities among the geosynclinal volcanic rocks, the orogenic granitic plutons, and certain postorogenic volcanic rocks suggest that the fundamental character of the crust beneath the Fraser Belt has not changed since mid-Paleozoic time.

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