RESOLVING MID- TO UPPER-CRUSTAL EXHUMATION THROUGH APATITE PETROCHRONOLOGY

Double-dating using U-Pb and fission-track systems is becoming increasingly widespread. However, these thermochronometers constrain temperature windows that are often difficult to link through geological time. This is in part due to the large difference in temperature window (often > 250 °C) and our limited understanding of U-Pb systematics.

In this study, we apply apatite U-Pb, fission-track, and apatite and whole rock geochemistry to fourteen samples from four tectonic systems common in cordilleran orogenic scenarios: (1) basement cored-uplifts, (2) Laramide-age plutons intruded through a thick crustal column, (3) metamorphic core complex and associated detachment faults, and (4) rapid, extrusive volcanic cooling, in order to provide a link between in-situ geochemical signatures and cooling mechanisms. Comparisons in trace element fractionation between apatite and whole rock geochemistry provide insights into initial rock-forming processes. Apatite trace element geochemistry and the La/Lu ratio provides a mechanism to determine if a rock is primary and representative of its parent melt or if it has undergone secondary thermal perturbation/metamorphism post-crystallization. Further, we demonstrate that using a combined apatite U-Pb, FT, trace element, and whole rock geochemistry approach it is possible to determine if a rock has undergone monotonic cooling since crystallization, protracted residence in the middle crust, and provide unique structural history such as detachment faulting. Finally, insights provided herein offer new applications for apatite petrochronology.