New Mexico Geological Society Annual Spring Meeting — Abstracts

K-feldspar argon analyses can fill the existing ca. 1.4 to 0.1 Ga, 300 to 150°C thermochronological void for many of the Proterozoic uplifts in parts of the southwestern United States. Considerable numbers of geochronological and thermochronological studies using U/Pb, argon mica and amphibole, and fission-track systematics have been employed to better understand the ca. 1.7 to 1.4 Ga metamorphism and deformation associated with continental assembly and the low temperature (~100°C, 0.01 to 0.2 Ga) thermal history, but little is known about the thermochronological evolution for the intervening one billion years.

Preliminary K-feldspar argon age spectrum studies from several locations in New Mexico, Arizona and Colorado yield age gradients spanning the Miocene to the Middle Proterozoic. The age spectrum results can be used in conjunction with the argon kinetic parameters determined from the stepheating data to quantitatively track the thermal evolution of this large time span. For instance, K-feldspar studies indicate that Proterozoic rocks in the Grand Canyon region of Arizona have not been deeply buried or reheated since ca. 1 to 1.3 Ga whereas central and southern New Mexico were buried by several km's of post Mississippian sediments which were stripped away during Laramide and mid-Tertiary Rio Grande Rift uplift events. K-feldspars from the Pikes Peak region in Colorado reveal relatively rapid cooling following emplacement of the ca. 1.1 Ga Pikes Peak Batholith and very minor post 1.0 Ga argon loss. Many K-feldspars from central New Mexico to central Colorado reveal a distinct cooling episode at ca. 0.85 to 0.8 Ga. Considering that only a very limited rock record is preserved for this time period (Grand Canyon Supergroup?), any new information regarding the geological evolution from ca. 1.1 to 0.6 Ga is significant. Furthermore, this ca. 0.85-0.8 Ga cooling episode may be cooling after a reheating event associated with an incipient of Rodin ia breakup.

K-feldspar argon thermochronology provides the opportunity to explore many new hypotheses about the Proterozoic crustal evolution in the southwestern United States. K-feldspar studies bring us to the brink of discovery regarding the ca. 1.1 to 0.6 "Great Unconformity", post continental assembly median thermal histories, effects of Phanerozoic orogenic events (Transcontinental Arch, Ancestral Rocky Mt., Laramide, Mid-Tertiary uplifts,) and basin evolution.