TRIASSIC-JURASSIC SILICA-UNDERSATURATED AND SILICA-SATURATED ALKALIC INTRUSIONS IN THE CORDILLERA OF BRITISH-COLUMBIA - IMPLICATIONS FOR ARC MAGMATISM

Alkalic igneous rocks of early Mesozoic age are found in both the Ques nel and Stikine terranes in the Canadian Cordillera and include both s ilica-undersaturated and silica-saturated types. The saturated complex es are most abundant in Quesnellia and are multiphase complexes domina ted by monzonite to diorite intrusions. Undersaturated complexes are d istributed through both terranes, are dominated by syenite with lesser monzonite and pyroxenite, and, when present as a single intrusion, ar e characterized by concentric zoning, igneous layering, and planar min eral fabrics. Both types of complex are associated with Cu-Au minerali zation accompanied by potassic and distinctive sodic and calc-potassic alteration assemblages. Although undersaturated and saturated alkalic intrusions are petrographically distinct, a petrogenetic association is suggested by their spatial coincidence in some districts, and simil arities in their tectonic environment and associated alteration. The u ndersaturated complexes represent a distinctive suite of alkalic intru sion with magmatic are affinities, and their emplacement into both Sti kinia and Quesnellia between 210 and 200 Ma suggests that these terran es were either linked at that time or have shared unusual but similar magma-generating tectonic events at identical times.