ULTRAHIGH TEMPERATURE METAMORPHISM OF ARCHEAN CONTINENTAL CRUST: INSIGHTS FROM THE PETROCHRONOLOGY ARSENAL

Archean granulite terranes are ubiquitous features of Earth’s continental crust. Petrologic studies of these terranes may provide insight into the mechanisms responsible for stabilizing Earth’s early continental lithosphere. In this study, we use field observations, thermobarometry, and a suite of petrochonometers: U-Pb in zircon (zrn) and U-Pb monazite (mnz) via LA-ICPMS and ID-TIMS, and Sm-Nd bulk and zoned garnet (grt) to reconstruct the metamorphic history of the Neoarchean Pikwitonei granulite domain (PGD), a >15000 km² tract of high- to ultrahigh temperature (UHT) granulites on the NW edge of the Superior craton. Rock samples from different locations throughout the PGD yield a range of peak pressure-temperature (P­–T) conditions, with UHT recorded in the western (> 950 °C, 0.8-0.9 GPa) and central (900 °C, 0.7 GPa) PGD, and lower T in the southern and northern PGD (~800 °C, 0.6-0.7 GPa). However, all locations record similar apparent thermal gradients of ~1200 °C/GPa. Thermoabarometry combined with microstructural observations suggest the terrane experienced limited (0.1-0.2 GPa) HT/UHT decompression, followed by cooling at mid- to lower crustal P, and that metamorphism was synchronous with the development of a pervasive steep NW-dipping fabric. Petrochronologic data suggest some parts of the PGD were cooling while others were heating. Prograde mnz growth during heating to UHT in the western PGD occurred from >2700 to ~2675 Ma, followed by slow cooling and zrn growth from 2673 to 2650 Ma. In the cooler southern PGD, prograde mnz growth in restitic pelite occurred at 2670 Ma, with zrn and mnz from adjacent peraluminous granite suggesting melt crystallization by 2650 Ma. The peraluminous granite gives a bulk Sm-Nd grt age of ~2600 Ma, which is similar to the ~2590 Ma age of matrix mnz aligned with the fabric in restitic pelite. Zoned Sm-Nd chronology of a peritectic grt megacryst from the southernmost part of the terrane suggests initiation of grt growth at 2660 Ma, a pulse of rapid growth at ~2640 Ma, and potentially continued grt growth to ~2600 Ma. The ~2600 Ma ages may record slow cooling or possibly a second HT metamorphic event that is cryptically recorded in the relatively dry rocks. Together, the petrochronologic data suggest diachronous thermal histories and sustained HT/UHT across the PGD over 10s to >100 Ma.