16: Some quantitative aspects of orogenic volcanism in the Oregon Cascades
Craig M. White, Alexander R. McBirney, 1978. "16: Some quantitative aspects of orogenic volcanism in the Oregon Cascades", Cenozoic Tectonics and Regional Geophysics of the Western Cordillera, Robert B. Smith, Gordon P. Eaton
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Quantitative data on the rates of production and compositional variations of igneous rocks are needed before any of the various models for magmatic activity at convergent plate boundaries can be realistically evaluated. The Cascade Range provides an excellent opportunity to obtain this information, because it contains a record of igneous activity in various structural settings and through most of late Cenozoic time.
When analytical data are combined with volumetric estimates and are grouped according to the measured age relations of the principal eruptive episodes, they show significant compositional trends, both in space and time. The most notable changes since late Oligocene time have been a general decline in the volumes and average SiO2 contents of erupted rocks. These changes have been accompanied by a marked drop in the degree of Fe enrichment, a steady increase in Na, Sr, and K /Rb and a simultaneous decrease in the Rb contents of rocks at the same stage of differentiation. Sr-isotope ratios remained essentially constant with time and show no relation to SiO2 or Rb contents. The abundance of Rb in Quaternary rocks is inversely related to the volume of Tertiary rocks erupted the same region.
No single mechanism can account for all the observed variations. At least three distinct stages of crystal-liquid equilibration seem to be required, one in the mantle source region, another near the base of a steadily thickening lithosphere, and a third in the shallow reservoirs of mature volcanoes. There is no evidence that the magma is generated in a subducted plate.