In-situ constraints on timescales of orogenesis: monazite EMPA petrochronology of the Grand Canyon's Upper Granite Gorge
Nicole Aikin, University of Washington
Cailey Condit, University of Washington
Michael Williams, University of Massachussetts, Amherst
Chloe Bonamici, University of Wisconsin, Madison
Karl Karlstrom, University of New Mexico
Abstract
Understanding the timing and tempo of lithospheric evolution during convergent and transpressional tectonics is essential to unravel the complex geological history of a region, yet the evolution of metamorphic and ductile deformation processes often remains relatively unconstrained due to limitations of exposure, preservation and/or an absence of precise in-situ petrochronology that can tightly constrain pressure-temperature-deformation-time (P-T-d-t) relationships. We investigate timescales of orogenic evolution by integrating field observations, structural analyses, geochemical signatures, and in-situ monazite Electron Microprobe chemical age data from Vishnu Schist metapelites in two adjacent tectonometamorphic blocks in the Grand Canyon's Upper Granite Gorge (UGG; Arizona, USA) with the aim of identifying key events in the lithospheric history, their temporal relationships, and the underlying dynamic mechanisms driving these changes. Both blocks experienced prograde metamorphism and deformation (D1) but were variably overprinted and transposed during a second phase of deformation (D2). We contrast new monazite petrochronology from the Clear Creek block (CCb), which preserves D1 structures and reached peak conditions of 500°C and 6 kbar during D2, with new and previously published dates from the Mineral Canyon block (MCb), which is pervasively deformed during D2 and reached peak conditions of 700°C and 6 kbar during anatexis +/- pegmatite emplacement.
Preliminary petrochronologic findings across both blocks suggest that metamorphic and magmatic monazite yield U-Pb ages spanning deposition, prograde and D1 (ca. 1755 Ma – 1735 Ma), a transition from prograde and D1 to peak and D2 (ca. 1735 - 1712 Ma), and peak, D2 to retrograde thermotectonism (ca. 1708-1680 Ma), reflecting the entire duration of crustal accretion during the Yavapai orogeny.
Overall, this study demonstrates that integration of in-situ monazite petrochronology is invaluable for unraveling complex orogenic processes, which contributes to our broader understanding of midcrustal dynamics and their relationships within regional tectonic frameworks.
Session
Convergent and transpressional orogens