DEMISE OF GRAND CANYON’S VISHNU MOUNTAINS AND FORMATION OF THE GREAT UNCONFORMITY; DATA FROM 40AR/39AR AND U-TH-PB XENOTIME DATING (Invited Presentation)

Vishnu basement rocks of Grand Canyon formed during 1.84 to 1.65 Ga suturing of arc terranes. The term “Vishnu Mountains” is used for the now-eroded Andean-scale ~4- to 5-km- high inferred topography of this mountain belt. Grand Canyon offers a 400-km-long cross-strike transect of the middle crustal core of this belt. Geochemical proxies (La/Yb; La/0.1Y) suggest crustal thickness reached 65 ± 10 km at 1.70 Ga and was ~55 km at 1.4 Ga. Basement rocks reached variable peak metamorphic temperatures of 500-700 °C at 1.71-1.69 Ga at near-isobaric 0.6-0.8 GPa conditions, hence the transect records a 20- to 25-km–depth-slice. Hot blocks (~700 °C) are juxtaposed with cooler (~500 °C) blocks across subvertical shear zones at near constant depth due to variable heating by 1.68-1.66 Ga granite/pegmatite swarms late during contractional assembly. ⁴⁰Ar-³⁹Ar thermochronology for 20 hornblende (Tc~ 550-500 °C), 75 muscovite (Tc~ 400-350 °C) and biotite (Tc~ 300 °C), and 20 K-spar (Tc ~ 275-175 °C) track post-orogenic cooling of basement rocks. ⁴⁰Ar-³⁹Ar hornblende shows 1.70-1.60 Ga cooling through ~500 °C (~20-km-depth); muscovite and biotite record differential cooling through 400 to 300 °C (~16-12 km depths), with muscovite yielding 1.45-1.40 Ga cooling ages east of the Crystal/96 Mile shear zones and ~1.65-1.60 Ga to the west. Xenotime data preserve very slow cooling (<1 °C/Ma) east of the shear zones and rapid cooling west of it. This is interpreted to be due to shear zone movement during distant plate margin convergence of the 1.45-1.37 Ga Picuris orogeny perhaps during erosion of Vishnu Mountains to more subdued (Appalachian-scale) mountains. K-spar dates vary from 1.40-1.10 Ga across the entire transect and MDD modeling indicates differential cooling through 250-150 °C (10-6 km depths). The first Great Unconformity formed 1.35-1.25 Ga before 1.25 Ga deposition of the Unkar Group and additional differential basement exhumation took place in uplifting intracratonic fault blocks due to distant plate margin tectonism at 1.25-1.10 Ga, 0.77-0.73 Ga, and 0.6-0.5 Ga. Continued P-T-t-D studies offer insights for understanding the evolving rheologic state of continental crust during mountain building, orogenic demise, and continental stability.