Thermotectonic evolution of the western margin of the Yilgarn craton, Western Australia: New insights from 40Ar/39Ar analysis of muscovite and biotite

Abstract

The post-cratonisation history of the Yilgarn craton is somewhat fragmentary due to incomplete or absent stratigraphic records for overlying sediments. However, the exposed Archean crystalline rocks can provide important constraints on the ‘missing’ thermotectonic history if appropriate thermochronological methods are used. Previous thermochronology studies from the southwestern Yilgarn craton suggest that it was subjected to late ‘Pan-African’ tectonism and east–west compression, resulting in tectonic loading (thrusting) of sediments onto basement rocks along its western margin. However, this proposed tectonic model is based largely on bulk Rb–Sr biotite analyses with minimal petrological and geochemical control.This study, across a ∼80km E–W transect along the western Yilgarn craton, employed laser 40Ar/39Ar incremental heating experiments on muscovite and single biotite grains from well documented, sample sites broadly comparable to those sampled previously for Rb–Sr biotite analysis. Muscovites record relatively consistent 40Ar/39Ar ages, ranging from ∼2220 to 2450Ma. Coexisting biotite 40Ar/39Ar results however, reveal significant age variations at both intra- and inter-grain scales. Single-grain biotite 40Ar/39Ar dating results show consistent ages of ∼2480–2580Ma across much of the Yilgarn craton, with ages decreasing abruptly to ∼1000–1100Ma over a narrow transitional zone ∼30–40km wide, before decreasing further to ∼620–630Ma along the western margin of the craton. Petrographic observations indicate that most biotite grains, especially in the transitional zone and western margin, are severely chloritized and contain abundant titanite inclusions that may have resulted from fluid-induced recrystallisation under greenschist facies metamorphism. The transitional zone 40Ar/39Ar biotite ages are temporally compatible with metamorphism of the adjacent Northampton and Mullingarra Complexes, suggesting either their juxtaposition along the western margin of the Yilgarn craton during the Mesoproterozoic Pinjarra Orogeny or coincidental partial re-equilibration at this time during younger thermal events. The westernmost biotite recrystallisation ages of ∼620–630Ma support palaeomagnetic indications of oblique collision between Great India and the Australian continent during Gondwana amalgamation.