Kimberlite and kimberlitic intrusives of southeastern Australia: a review

Fifteen widely separated occurrences of kimberlite and kimberlitic rocks are now known in southeastern Australia. Those that have been satisfactorily dated isotopically give ages ranging from Permian to Late Jurassic. Field relations indicate that all the occurrences post-date the Proterozoic. Location of the kimberlite intrusions appears to be related to a number of general, structural factors such as the presence of folds, faults and lineaments. Most intrusives are located in belts of present-day seismic activity. The kimberlitic intrusions of New South Wales and Victoria are also associated with a broad zone of epeirogenic uplift, and coincide with the mean migration path of a postulated Cainozoic hot-spot. A number of these features are younger than the age of kimberlitic intrusion, and any causal relationships are unclear. The classification of these generally altered rocks as "kimberlitic" is partly based on their mode of emplacement, and particularly on the presence of crust/mantle inclusions. Compared to African kimberlitic magmas, the southeastern Australian examples have lower incompatible element contents. These differences are interpreted as representing slightly greater degrees of partial melting of a four-phase Iherzolite assemblage at shallower depths than typical African kimberlite magma. Projections of chemical data onto planes within the CaO-MgO-Al2O3-SiO2 system, and comparisons with experimental data, suggest that the magmas were generated at about 65 km depth. Observed fractionation trends of the kimberlitic magma compositions are consistent with minor olivine, and possibly orthopyroxene, fractionation. Upper-mantle nodules in some of the intrusives include spinel, garnet-spinel, and garnet Iherzolite; rare griquaite nodules are found in one pipe at Jugiong. Crustal nodules include garnet clinopyroxenite, eclogite, websterite, and felsic granulite. Most of the mafic crustal nodules are of basaltic composition, and may represent slightly altered, high-pressure, metamorphic equivalents of erupted basalt types, as well as cumulates from basalt magma. None of the salic nodules correspond to the type of source or restite hypothetically involved in granite genesis. P-T estimates for the nodule assemblages give a maximum source of ~ 70 km, and temperatures of ~ 1240Â°C. They also indicate unusually high geotherms at the time of pipe generation, and the calculated geotherms all exceed the estimated mean oceanic geotherm. The abnormally high geothermal gradient implied by most of these data intersects the graphite-diamond stability curve at considerably higher temperatures and pressures than those indicated by the nodules. If the P-T conditions indicated by these calculations represent a steady-state rather than a transient geotherm, then it is unlikely that diamondiferous kimberlites of Permian or younger age exist in most of southeastern Australia. An exception is the Eurelia (South Australia) province, where the rocks are considered on other evidence to have been formed at depths &gt; 125 km. The combined results of seismic and kimberlitic nodule studies give a coherent model of the crust-upper mantle structure in southeastern Australia. The presence of basaltic lower crust in granulite facies produces mineralogical phase changes that can account for the gradual change in P-wave velocity across the crust-mantle interface without the development of a Moho.

Tags: ACT, Earth Sciences, GA Publication, Journal, NSW, Published_External, QLD, SA, TAS, VIC
Organization: Geoscience Australia Data