Abstract

Mapping in southeastern Alaska along the British Columbia border has shown the presence of hundreds of aligned Tertiary Lamprophyre dikes. Most dikes occur in northeast-trending swarms that cut across northwest-trending structures in the bedrock. More than 60% of the dikes dip within 10° of vertical and strike between N35°E and N80°E. Major topographic features such as fjords and streams have the same trend; differential erosion along the dike swarms and parallel joint sets, not faulting, is responsible for this northeast topographic grain.The dikes show various degrees of deuteric alteration. In relatively unaltered rocks, phenocrysts are subcalcic augite and rarely plagioclase, altered olivine, or amphibole. Deuterically altered dikes have a texture of interlocking milky plagioclase and black amphibole needles.The dikes form an undifferentiated homogeneous suite of alkali olivine basalt composition with an average SiO2 content of 48.8% and a total alkali content of 5.3%. Their chemistry closely resembles Quaternary volcanic rocks in British Columbia and Alaska.Structural setting, extent, and chemistry of the dikes suggest a mantle source and rapid intrusion without time for differentiation.

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