ARC MAGMATISM, CRUSTAL THICKENING, AND THE EXTINCTION OF CORDILLERAN FLARE-UPS

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.