Introduction to Middle Tertiary Cordilleran Volcanism: Magma Sources and Relations to Regional Tectonics

Cenozoic magmatism in the North American Cordillera has attracted the attention of volcanologists, structural geologists, and petrologists for more than a century. From horseback reconnaissance in the 1800s, when a hand lens was the analytical instrument of choice [e.g., von Richthofen , 1868; King , 1878] to today's studies, where mass spectrometers, electron microprobes, and computers are essential tools, attention has focused on the broad regional extent, large volumes, and complexly shifting space‐time‐composition patterns of Tertiary magmatism in the Cordillera. Cordilleran igneous rocks provide key information on regional geologic history, primary geologic constraints on many types of ore deposits, and the basis for development of broad concepts applicable to global magmatism and plate tectonics. Studies of Cordilleran igneous rocks have been especially intense in the past few decades, fueled by improved analytical techniques and by societal concerns such as the search for geothermal and mineral resources and evaluation of volcanic hazards.