Crustal exhumation in the Georgetown Inlier of NE Australia after 1.6 Ga Nuna assembly: New insights from Ar thermochronology

Characterization of the timing and magnitude of vertical and lateral motions of rocks within continental terranes is critical for understanding crustal evolution and orogenic development. In the Georgetown Inlier of NE Australia, the ca. 1.6 Ga crustal thickening/metamorphic event has been ascribed to the collision between NW Laurentia and NE Australia during the final assembly of Nuna, but little is known about the post-collisional crustal evolution. Here we evaluate the late- to post-collisional orogenic evolution by establishing the cooling history of each crustal domain using high-precision 40Ar/39Ar thermochronology of hornblende, muscovite, and biotite of 28 analyses on 19 samples. The Croydon Domain to the westernmost recorded fast magmatic cooling only. The weakly metamorphosed Western Domain recorded the slowest cooling of 0.7−0.7+1.2 °C Ma−1 between ca. 1.6(?) Ga and 1.53 Ga. The Central Domain recorded a cooling from ca. 1.55 Ga low-pressure−high-temperature metamorphism to hornblende closing temperature at ca. 1.53 Ga at a rate of 3.3−3.0+4.1 °C Ma−1, followed by cooling to biotite closing temperature at ca. 1.50 Ga at a rate of 4.0−2.2+1.7 °C Ma−1. A diachronous cooling occurred in the Eastern Domain from 730−770 °C at ca. 1.55 Ga to hornblende closing temperature at ca. 1.49 Ga at a rate of 3.4−1.5+4.3 °C Ma−1. Post-1.55 Ga regional cooling is interpreted to represent a post-collisional extension process that was contemporaneous with orogenic collapse reported in the neighboring Mount Isa Inlier. In combination with previously reported pressure-temperature paths and crustal melting events, we propose a regional tectonic model that features differential exhumation during orogenic collapse across the suture zone between NW Laurentia and NE Australia, ∼50−100 m.y. after the final assembly of Nuna.