The thermochronological evolution of the northern Gawler Craton and northern Adelaide Rift Complex

Abstract

Cratons preserve information about the deformation history of the surrounding terrains and the thermal history of mineral deposits. This thesis aims to constrain the effects on the Gawler Craton of various deformation events from surrounding terrains and the thermal evolution of mineralization within the Craton. Multi-method thermochronology was applied to samples throughout the northern Gawler Craton and Willouran Ranges of the Adelaide Rift Complex to unravel the thermal evolution of the northern Gawler Craton and Willouran Ranges. The subsequent thermal histories are compared and the differences between them highlighted. Apatite U-Pb data reveal distinct spatial patterns within the high temperature cooling history of the Gawler Craton. These spatial patterns correspond to the thermal response of a region to different magmatic and metamorphic events. The eastern Gawler Craton cooled to mid-crustal temperatures from high temperatures following magmatism of the ~1850 Ma Donington Suite and ~1590 Ma Hiltaba Suite while the central Gawler Craton cooling following the ~2500 Ma Sleaford and ~1700 Ma Kimban orogenies. The north-western Gawler Craton preserved evidence for deformation that post-dates cooling relating to magmatism with ages relating to the amalgamation of western and southern Australia at ~1300 Ma. In addition to the apatite U-Pb data, muscovite 39Ar/40Ar preserve post magmatic cooling within the Olympic Domain of the Hiltaba Suite at ~1530 Ma within the Olympic Domain. A second hydrothermally altered muscovite 39Ar/40Ar age from the same sample is recorded at ~1380 Ma. Potassium feldspar 39Ar/40Ar data from numerous samples all preserve altered spectra during the Neoproterozoic which corresponds to the deposition of the Adelaide Rift Complex. The region first cooled to low temperatures (<200 °C) at ~1000 Ma before minor reheating during the Neoproterozoic. The main low temperature regional cooling period is preserved at ~430 – 350 Ma and is interpreted to be caused by deformation relating to the 450 – 300 Ma Alice Springs Orogeny. Younger apatite fission track ages are only preserved in close proximity to major Iron-Oxide-Copper-Gold (IOCG) deposits and form a young thermal corridor linking the IOCG deposits. Apatite fission track data along the Karari Shear Zone of the northern Gawler Craton preserves evidence for regional Neoproterozoic cooling and a deformation event during the Carboniferous. The Nawa Domain preserves Carboniferous apatite fission track ages, whereas the Christie Domain preserves older Neoproterozoic – Cambrian ages. This is interpreted to be resultant from southward compression from the Alice Springs Orogeny and caused the Nawa Domain, to the north of the Karari Shear Zone, to thrust over the Christie Domain to the south. Additionally, this thrusting is interpreted to have ceased deposition of the Officer Basin sediments, which overlies the rocks to the north of the Karari Shear Zone. The deposition of the ~300 Ma Arckaringa Basin sediments indicates that once the compressional event stopped, deposition recommenced in the region. East of the Torrens Hinge Zone, apatite fission track analysis from sedimentary samples within the Willouran Ranges revealed cooling ranging from 550 to 100 Ma. U-Pb data records either detrital Proterozoic ages or ages at ~250 Ma. This younger age is interpreted to be caused by hydrothermal activity located proximally to the major fault zones in the region. Samples which preserve this younger U-Pb age also preserve apatite fission track ages of ~100 Ma that is interpreted to be cooling. This hydrothermal activity correlates well with hydrothermal activity in the Mt. Painter Inlier, a region of established high heat flow ~150 km to the east. This hints toward a broader zone of hydrothermal activity related cooling during this time. Regional deformation events are consistently present throughout the study region, yet local thermal histories contain inconsistencies. These inconsistencies can be accounted for by differing key geological factors which are unique to each region. The thickness of any overlying sedimentary packages and the geothermal gradient are greatly important at any location as they considerably influence the local thermal histories. Major structures around the rim of the craton tend to control the thermal histories at both high and low temperatures, however, their orientation within the stress regime is important. The prior thermal history of a region can be reset and overprinted by hydrothermal activity as has been recorded in this study for regions within the Olympic Domain and the northern Adelaide Rift Complex.