18 Tectonic Processes in Cordilleran Arcs

Thursday, 16 May 2019
8:00 AM - 12:00 PM
Room C125-126

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

Paper number: 18-1
Program: Theme Sessions
Time: 8:00 AM – 8:20 AM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 51, No. 4

doi: 10.1130/abs/2019CD-329854

© Copyright 2019 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.