Heterogeneous Exhumation of the Mount Isa Orogen in NE Australia After 1.6 Ga Nuna Assembly: New High‐Precision⁴⁰Ar/³⁹Ar Thermochronological Constraints

Abstract

The circa 1.60 Ga Isan Orogeny in NE Australia has been ascribed to the collision of Australia and Laurentia (North America), marking the final assembly of the Proterozoic supercontinent Nuna. However, details regarding the tectonic evolution of the orogen remain poorly constrained. To investigate the late orogenic to postorogenic thermal evolution and exhumation history, 40 Ar/ 39 Ar dating on hornblende, muscovite, and biotite was conducted in the Mount Isa Inlier, NE Australia, where intense crustal imbrication occurred during the Proterozoic continental collision. Published thermochronological results were recalculated using the current decay constant. Petrological examination and calculation of sample-specific 40 Ar/ 39 Ar closure temperatures and cooling rates were used to reconstruct the pressure-temperature evolution of individual structural domains. Diachronous cooling histories are revealed between western, central, and eastern belts through~525-330°C, mainly between 1.53 and 1.48 Ga. Contrasting cooling across postmetamorphic fault zones records the reactivation of inherited normal (i.e., early basinal) and reverse (i.e., orogenic) faults. Estimated exhumation rates are generally low (<~0.5 mm yr −1), pointing to a modest local relief of <~1,000 m which is comparable to modern analogs, and suggest a "soft" collision with limited crust thickening. Exhumation shortly following orogenesis was contemporaneous with felsic magmatism (1.55-1.48 Ga) in the eastern belt. Magmatism transitioning from trondhjemitic to A-type granitoids over this period suggests progressive heating of the orogen base, ascribed to lower crust delamination. Thus, thermochronological data reveal a regionally heterogeneous exhumation history controlled by orogenic collapse-related extensional faulting following the final assembly of the supercontinent Nuna.

Reversal of Ages of the Kalkadoon/Leichhardt Complex and the Magna Lynn Metabasalt, NW Queensland

“…Following Wyborn & Page (1983) and subse quent authors, ages for the units of the Kalkadoon/Leichhardt complex appear to be quite con sis tent and overlap within error at 1865-1852 Ma (Leichhardt complex), 1855-1864 Ma (Kal kadoon Granodiorite) and 1856-1859 Ma (Ewen Granite). The only dating on the Magna Lynn Metabasalt gives a minimum cooling age of 1521 ± 11 Ma (Li et al, 2020). Therefore, considering this unit alone, it could be older than the Kalkadoon/Leichhardt complex.…”