Determining the tectonic origins of the Paleoproterozoic Poudre Basin in Northern Colorado via detrital zircon analysis: trials and tribulations of calculating maximum depositional ages

Session: Evolution of Orogenic Belts Through Time: Insights from Sedimentation, Deformation, Magmatism, and Metamorphism, Part I

Presenting Author:

Graham Baird

Authors:

Baird, Graham B.¹, Kotowski, Daniel², MacMillan, Keaton Cargill³, Campuzano, Narassa⁴, Mueller, Ethan⁵, Miller, Shandon⁶, Müller, Simone⁷

(1) Department of Earth & Atmospheric Sciences, University of Northern Colorado, Greeley, CO, USA, (2) Department of Earth & Atmospheric Sciences, University of Northern Colorado, Greeley, Colorado, USA, (3) Department of Earth & Atmospheric Sciences, University of Northern Colorado, Greeley, CO, USA, (4) Department of Earth & Atmospheric Sciences, University of Northern Colorado, Greeley, CO, USA, (5) Department of Earth & Atmospheric Sciences, University of Northern Colorado, Greeley, CO, USA, (6) Department of Earth & Atmospheric Sciences, University of Northern Colorado, Greeley, CO, USA, (7) Department of Earth & Atmospheric Sciences, University of Northern Colorado, Greeley, CO, USA,

Abstract:

The Paleoproterozoic Poudre Basin spans portions of northern Colorado and likely formed from slab rollback driven arc rifting. Accuracy of this model can be tested by U-Pb analysis of detrital zircon where maximum depositional ages (MDAs) for all basin rocks should be the same age or younger than initial magmatism in the arcs that flank the basin. However, if some of the MDAs are older than initial arc magmatism, it indicates that the basin existed prior to the arcs and the basin did not originate by slab rollback and arc rifting.

 

Previously reported Poudre Basin MDAs calculated from the youngest cluster of geologically feasible and near concordant analyses provide ages in the range of c. 1762-1754 Ma. Given active arc magmatism throughout the region at this time, the MDAs should closely mark the true depositional age of the basin’s sediment. Two newly analyzed samples using similar procedures provide MDAs of 1755+/-18 Ma and 1762+/-17 Ma. Given that first arc magmatism in the area is c. 1780 Ma, the MDAs are consistent with the slab rollback driven arc rifting model.

 

However, use of the Maximum Likelihood Age (MLA) method for determining MDAs is considered a statistically more robust method compared to using the youngest cluster of near concordant, geologically feasible dates. For Poudre Basin samples, the MLA method produces MDAs that are at least 30 million years older. If accurate, it indicates that the Poudre Basin existed before subduction initiation and arc formation. However, testing of the MLA method with simulated detrital zircon datasets with a known MDA reveals that the MLA method closely provides the known MDA for approximately 20% of repeated samplings of the simulated detrital zircon dataset. When incorrect, the MLA methods always overestimate the true MDA. The youngest cluster of near concordant analyses provides the correct MDA for better than 80% of repeated samplings. Given these insights, the best estimate for the Poudre Basin’s MDA is given by the youngest cluster of near concordant dates. Sedimentation at c. 1760 Ma not only supports the slab rollback and arc rifting model, but is also consistent with other independent datas regarding the basin’s history.