Abstract

The Cenozoic Garibaldi belt comprises 6 volcanic fields spaced at irregular intervals along an axis extending 240 km north-northwest from the head of Howe Sound to the Bridge River area; 2 additional fields, the Franklin Glacier and Silvert lie 140 and 190 km west of the north end of the main volcanic belt. The volcanoes erupted lavas ranging in composition from augite-olivine basalt, through hypersthene andesite, hornblende andesite, and hornblende-biotite andesite, to biotite rhyodacite. Many of the volcanic complexes are characterized by geomorphic features which indicate complex interactions between volcanism and the Pleistocene ice sheets, but preglacial and postglacial phases are also present.

Whole-rock samples from 18 volcanic complexes have been dated by the K-Ar method. Most of the results are internally consistent with stratigraphic relationships and with limited 14C and paleomaguetic data.They suggest that volcanic activity was episodic; most of the analyzed andesitic and dacitic lavas were erupted in the intervals 2.3 to 1.7 Ma and 1.1 Ma to present in the northern part and in the intervals 1.4 to 1.0 Ma and 0.7 Ma to present in the southern part of the belt. Basaltic volcanism occurred only during the past 0.15 m.y., except in the Salal Glacier area where hawaiite and alkali-olivine basalt, which are perhaps an "edge effect" related to subduction of the Juan de Fuca plate, were erupted as early as 0.97 Ma. The timing of Garibaldi belt volcanism provides information bearing on the distribution of pre-Wisconsin glaciers in southwestern British Columbia and constrains interpretations of late Cenozoic changes in Explorer-Juan de Fuca-North American plate configuration along the continental margin.

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