Rf Berry

Proterozoic Australia–Western United States (AUSWUS) fit between Laurentia and Australia

A comparison of the major geological provinces, belts, and lineaments of Proterozoic Laurentia and Australia results in a reconstruction that fits the Mojave terrane of California-Nevada into a reentrant of the Tasman Line of eastern Australia. This reconstruction, to which K. Karlstrom and others gave the acronym AUSWUS (Australia-Western United States), was first proposed by M. Brookfield in 1993 on the basis of matching major lineaments. AUSWUS brings together the remarkably similar Precambrian geology of Mojavia (California-Nevada) and the Broken Hill block (Australia). AUSWUS also provides suitable intercontinental source terranes for zircons in Tasmania, the Belt basin, and Papua New Guinea.

Internally consistent gahnitic spinel-cordierite-garnet equilibria in the FMASHZn system: geothermobarometry and applications

The equilibrium (Mg, Fe, Zn)3Al2Si3O12+2Al2SiO5=3(Mg, Fe, Zn)Al2O4+5SiO2 garnet + sillimanite/kyanitc = spinel + quartz was calibrated in the piston-cylinder apparatus between 11 and 30 kbar, and over the temperature range of 950 to 1200°C. Three experimental mixes of Mg no. [100*MgO/(MgO+FeO)] 40, 47 and 60, in the FeO −MgO−Al2O3−SiO2−ZnO (FMASZn) system were used under low oxygen fugacities and anhydrous conditions. We derive a ternary Fe−Mg−Zn symmetric mixing model for aluminous spinels in equilibrium with garnet, to quantify the increase in gahnitic end-member of spinel with increasing pressure and descreasing temperature. Further experiments in the spinel-cordieritequartz-sillimanite field were combined with garnet-cordierite data from the literature to produce a consistent set of equations describing the exchange reactions in FMASHZn relevant to quartz-sillimanite bearing rocks at granulite facies conditions. As spinel is an important mineral participating in many rocks of aluminous composition at granulite-facies conditions, and as zinc contributes to an enlargement of spinels stability field towards higher pressures and lower temperatures, the thermobarometric calibrations presented here will be most significant in delineating the prograde and retrograde trajectory of P-T paths.

Tectonic implications of Late Proterozoic-Early Palaeozoic igneous rock associations in western Tasmania

Abstract In contrast to the mainland Lachlan Fold Belt, Late Proterozoic basement(?) and Cambrian rocks are exposed over large areas of western Tasmania. They offer the possibility of solving the paradox regarding the continental versus oceanic nature of the basement beneath southern Australia, and better understanding the Late Proterozoic and Cambrian tectonic history of this southern part of the Lachlan Fold Belt. Around 600 Ma, attenuation and eventual rifting of Proterozoic continental crust resulted in formation of a thinned passive continental margin transected by small rift basins in which rift tholeiites transitional to MORB accumulated. Eastward-directed intra-oceanic subduction commenced to the east of the passive margin at some time before the Middle Cambrian, forming an oceanic arc with boninites and low-Ti magnesian quartz tholeiites in the forearc region. Continued subduction of intervening oceanic crust between the arc and the passive margin resulted in an arc-continent collision within the Middle Cambrian. One or more extensive sheets of forearc crust, dominated by low-Ti basalts and boninites and their cumulate complements, were overthrust onto thinned continental crust. Continued compression at the suture initiated what was probably a short-lived episode of westward-directed subduction beneath the newly-emplaced allochthon and passive margin basement, and generated medium- to high-K andesites and more felsic rocks of the Central Volcanic Complex of the Mount Read Volcanics. Rebound backthrusting along the eastern side of the Central Volcanic Complex of the Mount Read Volcanics belt exhumed underthrust Proterozoic crystalline crust which now forms the Tyennan region, and a foreland basin half-graben, the Dundas Trough, formed along the western edge of this basement inlier. The upper Mount Read Volcanics are mainly post-collisional high-K to shoshonitic basalts and andesites generated by delayed partial melting of subduction-modified, underthrust passive margin subcontinental mantle. Extension in this post-collision collage of crustal elements led to limited rifting and emplacement of the tholeiitic Henty Dyke Swarm through at least the Central Volcanic Complex. Lower crustal generation of extensive sheet-like intrusive quartz-feldspar porphyrite bodies and associated Tyndall Group felsic explosive volcanism occurred within the Dundas Trough. Uplift and excavation of Tyennan region Proterozoic crystalline crust provided abundant coarse siliciclastic detritus of the Owen conglomerate and correlates which flooded into and gradually filled the Dundas Trough. The major events invoked in the above model to have generated continental crust (allochthon emplacement, reversal of subduction, post-collisional volcanism and graben formation, rebounding and excavation of underthrust crystalline basement, graben filling) all occurred probably within a 20-Ma period. Late Cambrian deformation is well known in the Adelaide Fold Belt (Delamerian Orogeny) and in the New Zealand and Transantarctic Mountains segments of the once-continuous fold belt along the eastern margin of Early Palaeozoic Gondwanaland. It is not recorded from the mainland section of the Lachlan Fold Belt in Victoria, although given the nature of Cambrian outcrops along major Devonian faults in Victoria, this is hardly surprising.

A North American provenance for Neoproterozoic to Cambrian sandstones in Tasmania

Abstract Tasmania forms an enigmatic province within the Neoproterozoic to Cambrian history of Australia. It lies at the boundary between Australia and North America in most Rodinia reconstructions but no reliable lithostratigraphic correlations have been reported with either mainland Australia or North America. We used detrital zircon age spectra, measured by LAM-ICP-MS, of Neoproterozoic and Cambrian sandstones in Tasmania to search for evidence of correlations with these two continental blocks during the time slice critical to Rodinia breakup. The Tasmanian sandstones are dominated by 1600–1900 Ma and 1200–1500 Ma age zircons. There is little evidence for Grenville (∼1100 Ma) and Ross (∼550 Ma) Orogen sources in these sandstones, in contrast to detrital zircon age spectra of similar age rocks in South Australia. The detrital zircon age spectra of Tasmanian sandstones are different from age spectra reported from British Columbia. They are very similar to age spectra reported from Cambrian sandstones of Nevada, supporting Rodinia reconstructions that place southwestern USA near to Tasmania in the Neoproterozoic.

The tectonic significance of Cambrian allochthonous mafic‐ultramafic complexes in Tasmania

A reassessment of the structural significance of the large Cambrian mafic‐ultramafic complexes in Tasmania indicates that existing tectonic models for the Late Proterozoic and Early Palaeozoic history of Tasmania are inadequate. Pronounced similarities exist between the Cambrian stratigraphy of Tasmania, and the less deformed allochthonous terranes of Oman and western Newfoundland. The mafic‐ultramafic complexes are allochthonous relics of a forearc terrane which collided with, and was thrust over, passive continental margin sediments in the Middle Cambrian. Extensional tectonics in the Late Cambrian, and eruption of the Mount Read Volcanics, may have resulted from post‐obduction flipping of the subduction zone. The present complex distribution of interpreted allochthonous units in Tasmania is the result of strong folding and faulting in the Devonian.

MINSQ - a least squares spreadsheet method for calculating mineral proportions from whole rock major element analyses

MINSQ is a spreadsheet adaptation of the least squares method that utilizes the Solver tool in Microsoft Excel™ to quantitatively estimate the proportions of constituent minerals in rocks from whole rock lithogeochemical data. It is simple to use and easily and interactively adaptable to observed mineral or normative assemblages. The structure facilitates input of actual mineral analyses for individual rock samples. The spreadsheet format and the numerical Solver tool in Microsoft Excel™ put the user in a powerful position to adapt this method to a wide range of numerical problems in geochemistry.

Tectonic implications of the Nan Suture Zone and its relationship to the Sukhothai Fold Belt, Northern Thailand

Abstract The Nan Suture and the Sukhothai Fold Belt reflect the processes associated with the collision between the Shan-Thai and Indochina Terranes in southeast Asia. The Shan-Thai Terrane rifted from Gondwana in the Early Permian. As it drifted north a subduction complex developed along its northern margin. The Nan serpentinitic melange is a thrust slice within the Pha Som Metamorphic Complex and in total this unit is a Late Permian accretionary complex containing offscraped blocks from subducted oceanic crust of Carboniferous and Permian age. The deformational style within the Pha Som Metamorphic Complex supports a west-dipping subduction zone. The Late Permian to Late Triassic fore-arc basin sediments are preserved in the Sukhothai Fold Belt and include a near continuous sedimentary record, at least locally. The whole sequence was folded and complexly thrust in the Late Triassic as a result of the collision. Late syn- to post-kinematic granites place an upper limit of 200 Ma on the time of collision. Post-orogenic sediments prograded across the suture in the Jurassic.

Cambrian metamorphic complexes in Tasmania: tectonic implications

Cambrian metamorphic complexes containing amphibolite‐ to eclogite‐grade rocks are present throughout western and northwestern Tasmania. These complexes contain mostly quartz‐albite‐biotite schists, garnet‐quartz‐albite‐biotite schists and mafic amphibolite lenses (up to 1 km long). The chemistry of these rocks is similar to unmetamorphosed, Late Neoproterozoic tholeiitic basalts and continental‐derived siliciclastics. A few rocks in the metamorphic complexes have compositions that are transitional between the amphibolites and the schists, representing metamorphosed volcaniclastic rocks formed by mixing between mafic and siliciclastic sources. The rocks in these complexes were probably located on the edge of a thin Late Neoproterozoic passive margin that was partially subducted during a Cambrian arc‐continent collision and uplifted during post‐collisional crustal re‐equilibration. The metamorphic condition, age and chemistry of both the schists and the amphibolites resemble those of metamorphic complexes in North Victoria Land in Antarctica. However, the structural setting of these complexes differs from those in Tasmania. Comparisons of the Tasmanian and North Victoria Land Cambrian structures and lithologies with those from more recent arc‐continent collisions worldwide show that both are compatible with a model involving east‐dipping subduction of a passive margin beneath an intraoceanic island arc. The differences between the two areas probably arise from differences in the geometry of the margins and the thickness of the passive‐margin sediments.

In situ location and U‐Pb dating of small zircon grains in igneous rocks using laser ablation–inductively coupled plasma–quadrupole mass spectrometry

A new U‐Pb zircon dating protocol for small (10–50 mm) zircons has been developed using an automated searching method to locate zircon grains in a polished rock mount. The scanning electron microscopeenergy‐ dispersive X ray spectrum‐based automated searching method can routinely find in situ zircon grains larger than 5 mm across. A selection of these grains was ablated using a 10 mm laser spot and analyzed in an inductively coupled plasma‐quadrupole mass spectrometer (ICP‐QMS). The technique has lower precision (∼6% uncertainty at 95% confidence on individual spot analyses) than typical laser ablation ICP‐MS (∼2%), secondary ion mass spectrometry (<1%), and isotope dilution‐thermal ionization mass spectrometry (∼0.4%) methods. However, it is accurate and has been used successfully on fine‐grained lithologies, including mafic rocks from island arcs, ocean basins, and ophiolites, which have traditionally been considered devoid of dateable zircons. This technique is particularly well suited for medium‐ to fine‐grained mafic volcanic rocks where zircon separation is challenging and can also be used to date rocks where only small amounts of sample are available (clasts, xenoliths, dredge rocks). The most significant problem with dating small in situ zircon grains is Pb loss. In our study, many of the small zircons analyzed have high U contents, and the isotopic compositions of these grains are consistent with Pb loss resulting from internal a radiation damage. This problem is not significant in very young rocks and can be minimized in older rocks by avoiding high‐ U zircon grains.

Chemical U – Th – Pb monazite dating of the Cambrian Tyennan Orogeny, Tasmania

Chemical U – Th – Pb dating of monazite from 12 schists throughout western and central Tasmania define a peak metamorphic age of ca 510 Ma. This age is very close to the age of arc – continent collision and ophiolite emplacement, implying very rapid uplift and cooling. To the south, along the western margin of the South Tasman Rise, metamorphism occurred later at 495 Ma, which correlates with a late stage of the Ross Orogeny, Antarctica. The Tyennan Orogeny in Tasmania has a three-stage history similar in age to the Delamerian Orogeny in South Australia. However, the Tyennan Orogeny only produced metamorphic rocks during the early stage associated with ophiolite obduction at 515 – 505 Ma. The intense compressional event recognised in the Delamerian and Ross Orogeny at 500 – 495 Ma is correlated with a mild basin inversion in Tasmania, and no metamorphism on mainland Tasmania has been recognised associated with this event. The western margin of the South Tasman Rise is a fragment of the Ross Orogen and does not correlate directly with Tasmania.