A Thermochronometric Approach to Understanding Exhumation in the Southern Colorado Rockies

Abstract:

We use ⁴⁰Ar/³⁹Ar thermochronometry of hornblende, biotite and feldspar, coupled with apatite (U-Th-Sm)/He thermochronometry, to refine emplacement age and exhumation history of the latest Cretaceous Whitehorn Granodiorite, in the SW Front Range east of the Arkansas River Valley. The pluton intrudes Pennsylvanian and Precambrian wall rocks and has a surface exposure of ~8 km (E-W) by ~25 km (N-S). ⁴⁰Ar/³⁹Ar analysis on Whitehorn samples from two locations (2920 m and 2890 m) yield dates of 68.2 Ma and 69.1 Ma (hbl), 68.4 Ma and 68.9 Ma (bt), and 67.1 Ma and 74.2 Ma (kfs), respectively. AHe dates from 4 samples collected in Proterozoic wall rocks northwest of the Whitehorn pluton along a 625 m vertical transect, range from 56.9 Ma to 63.4 Ma, and 3 samples in a 345 m vertical transect east of the pluton range from 67.3Ma to 135.3Ma. Previous thermochronometric, paleomagnetic, and petrologic studies indicate Whitehorn pluton was emplaced at paleodepths of 8-10 km with ambient host rock temperatures of ~200°C, and insignificant tilting since then. We infer that, the biotite and hornblende Ar⁴⁰/Ar³⁹ dates record timing of pluton emplacement, and K-feldspar MDD modeling records post-emplacement cooling to ambient temperatures by 66 Ma. AHe dates record lower temperature (<60°C) cooling of the wall rocks and must reflect post-emplacement exhumation of the pluton and host rocks. The SW Front Range thus appears to have experienced rapid, large magnitude (~6-8 km) exhumation and differential uplift at ca. 60 Ma with respect to the SE Front Range. Presently, the southern Front Range is overlain by horizontal Oligocene volcanic rocks, deposited on a widespread regional erosion surface across the central CO Rockies. The preservation of Paleocene AHe dates below this regional surface suggests that it formed shortly after exhumation, and experienced < 2 km of reburial, or exhumation, from ~55 Ma until deposition of the Wall Mountain tuff at ca. 34 Ma. The Arkansas River cuts through both volcanic rocks (as young as middle Miocene) and Proterozoic basement, generating up to 1.5 km of local topographic relief. Paleocene AHe dates at the valley bottom indicate volcanic rock thicknesses were insufficient to bury and reset AHe ages, and thus that minimal regional erosion and exhumation occurred following volcanism in the southern Front Range.