Tom Raimondo

Ediacaran intracontinental channel flow

Migmatitic lower-crustal rocks in the Ediacaran intracontinental Petermann orogen, central Australia, are bounded by regional-scale, low-angle shear fabrics that record opposing shear senses. New sensitive high-resolution ion microprobe (SHRIMP) titanite geochronology suggests that the contrasting shear systems developed synchronously. We argue that the macro scopic structural and metamorphic architecture satisfies the diagnostic criteria outlined by recently proposed channel flow models, and thus the remarkably well-preserved Petermann orogen is potentially an example of ancient intracontinental channel flow.

A simple mechanism for mid-crustal shear zones to record surface-derived fluid signatures

Ion microprobe analyses of garnet porphyroblasts from three separate splays of the mid-crustal Walter-Outalpa shear zone, Curnamona Province, South Australia, indicate homogeneous δ 18 O values of 18 O values in deeply exhumed shear zones may therefore be indicative of fault structures that have a prior history of surface exposure, weathering, burial and re-exposure.

Trace element mapping by LA-ICP-MS: assessing geochemical mobility in garnet

A persistent problem in the study of garnet geochemistry is that the consideration of major elements alone excludes a wealth of information preserved by trace elements, particularly the rare-earth elements (REEs). This is despite the fact that trace elements are generally less vulnerable to diffusive resetting, and are sensitive to a broader spectrum of geochemical interactions involving the entire mineral assemblage, including the growth and/or dissolution of accessory minerals. We outline a technique for the routine acquisition of high-resolution 2D trace element maps by LA-ICP-MS, and introduce an extension of the software package XMapTools for rapid processing of LA-ICP-MS data to visualise and interpret compositional zoning patterns. These methods form the basis for investigating the mechanisms controlling geochemical mobility in garnet, which are argued to be largely dependent on the interplay between element fractionation, mineral reactions and partitioning, and the length scales of intergranular transport. Samples from the Peaked Hill shear zone, Reynolds Range, central Australia, exhibit contrasting trace element distributions that can be linked to a detailed sequence of growth and dissolution events. Trace element mapping is thus employed to place garnet evolution in a specific paragenetic context and derive absolute age information by integration with existing U–Pb monazite and Sm–Nd garnet geochronology. Ultimately, the remarkable preservation of original growth zoning and its subtle modification by subsequent re-equilibration is used to ‘see through’ multiple superimposed events, thereby revealing a previously obscure petrological and temporal record of metamorphism, metasomatism, and deformation.

Evidence for 1808–1770 Ma bimodal magmatism, sedimentation, high-temperature deformation and metamorphism in the Aileron Province, central Australia

Field relationships and LA-ICP-MS U–Pb geochronology from the Yundurbungu Hills (Aileron Province, central Australia) reveal a record of 1808–1770 Ma bimodal magmatism, sedimentation, high-temperature deformation and metamorphism. Specifically, the data presented here provide the first unequivocal evidence for ca 1774 Ma high-temperature deformation and metamorphism during the 1790–1770 Ma Yambah Event in the southern part of the North Australian Craton. Granitic lithologies were synkinematically emplaced between 1808 and 1770 Ma, with early phases recording D1 deformation and the youngest phase postdating D1 deformation. The protolith to a D1 deformed metasedimentary unit was deposited between 1792 and 1774 Ma, followed by the intrusion and deformation of a composite mafic–felsic magmatic association at ca 1774 Ma. An S1 migmatitic fabric in the composite mafic–felsic gneiss is truncated by the youngest (ca 1770 Ma) phase of granitic magmatism, constraining the timing of S1 deformation. A second period of sedimentation appears to post-date D1 deformation, with deposition occurring sometime after ca 1774 Ma. Subsequent overprinting during the 1590–1550 Ma Chewings Event is recorded by the growth of metamorphic monazite and zircon. This event deformed the ca 1774 Ma S1 gneissic fabric, producing a composite S1/S2 gneissic fabric in early metasedimentary and magmatic lithologies and a simple S2-only fabric in lithologies that were intruded or deposited after ca 1774 Ma. Consistent with previous work, we suggest that localised high-temperature deformation and bimodal magmatism at ca 1774 Ma in the Yundurbungu Hills is consistent with a back-arc setting linked to prolonged north-directed subduction.

Further evidence for two metamorphical events in the Mawson Continent

Abstract In this study, in situ and erratic samples from George V Coast (East Antarctica) and southern Eyre Peninsula (Australia) have been used to characterize the microstructural, pressure–temperature and geochronological record of upper amphibolite and granulite facies polymetamorphism in the Mawson Continent to provide insight into the spatial distribution of reworking and the subice geology of the Mawson Continent. Monazite U-Pb data shows that in situ samples from the George V Coast record exclusively 2450–2400 Ma ages, whereas most erratic samples from glacial moraines at Cape Denison and the Red Banks Charnockite record only 1720–1690 Ma ages, consistent with known ages of the Sleaford and Kimban events, respectively. Phase equilibria forward modelling reveals considerable overlap of the thermal character of these two events. Samples with unimodal 1720–1690 Ma Kimban ages reflect either formation after the Sleaford event or complete metamorphic overprinting. Rocks recording only 2450–2400 Ma ages were unaffected by the younger Kimban event, perhaps as a result of unreactive rock compositions inherited from the Sleaford event. Our results suggest the subice geology of the Mawson Continent is a pre-Sleaford-aged terrane with a cover sequence reworked during the Kimban event.