Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 18-1
Presentation Time: 8:00 AM


MILLER, Jonathan, Department of Geology, San Jose State University, San Jose, CA 95192-0102, LACKEY, Jade Star, Geology Department, Pomona College, Claremont, CA 91711 and SHEA, Erin K., Bishop, CA 93514

The batholiths of the North American Cordillera are now widely recognized as having formed during discrete periods of high magmatic flux (i.e. ‘flare-ups’), although the timing of peak flare-up magmatism varies among individual Cordilleran batholiths. The causes of flare-ups and mechanisms leading to their demise are debated, but to a first order must reflect the large-scale geodynamics of the arc. Geochronologic constraints are key to sorting out plausible models for arc flare-ups, particularly the connection between crustal thickening and flare-up magmatism. Recent work in the Cretaceous Sierra Nevada batholith and in the Cretaceous North Cascades arc offer important insights into the relationship between arc deformation and thickening, and flare-up magmatism.

Key zircon geochemical indicators (zircon REE patterns and ratios, Ti-in-zircon model temperatures) linked to geochronology indicate in both of these batholiths that a pronounced change occurred from overall hotter conditions of magma formation and/or storage (zircon undersaturation) during the peak of the Cretaceous flare-up to markedly cooler conditions of magma formation and storage (at or near zircon saturation) after the peak of flare-up magmatism. Additionally, Sr/Y and La/Yb for plutonic rocks from both arcs shift with time reaching their highest values at the termination of the flare-ups, coeval with the shift to cooler magmatic conditions.

These temporally constrained shifts indicate a greater role for garnet in the melt source and are interpreted to reflect: (1) a progressive increase in crustal thickness that accompanies arc magmatism and/or (2) a shift in the melt source from depths above garnet stability to depths below garnet stability in thickened crust. In general, the data are consistent with geologic studies indicating intra-arc shortening and crustal thickening is intimately linked to flare-up magmatism, and that the thickest crust extinguishes flare-ups.