Abstract The western sub-province of the Paleozoic Lachlan Orogen in Victoria is dominated by thick turbidite sequences overlying Cambrian basement volcanics. The region was subjected to multiple Cambrian to Late Devonian regional deformation events, followed by extensive post-tectonic granitic magmatism. The western Lachlan Orogen is considered a typical 'orogenic' gold province and hosts a large number of goldfields, including the world-class Bendigo–Ballarat goldfields. A variety of geochronological methods (e.g. U–Pb zircon; Re–Os sulfide; 40Ar/39Ar whole-rock, mica) have been used to constrain the timing of gold mineralisation, and the relationship to metamorphism/deformation/magmatism. Regional granitic magmatism is relatively well constrained from U–Pb zircon dating, with the timing of deformation/metamorphism and gold mineralisation reliant largely on 40Ar/39Ar dating results. Owing to inconsistencies in the available 40Ar/39Ar data and recent revisions to 40Ar/39Ar monitor ages and decay constants, we recalculate and re-evaluate all existing 40Ar/39Ar age results. These revisions confirm that the western Lachlan Orogen is characterised by multiple deformation/metamorphism events, with the Stawell structural zone deformed during the ca 500 Ma Delamarian and ca 445 Ma Benambran orogenies, the Bendigo Zone deformed during the Benambran orogeny (with minor Tabberabberan overprinting), and the Melbourne Zone affected by the ca 380–370 Ma Tabberabberan orogeny. Post-tectonic granitic magmatism occurred in two main time intervals, the Early Devonian (ca 400 Ma) and the Late Devonian (ca 380–370 Ma), with the former limited to the Stawell and northwest Bendigo Zones, and the latter distributed throughout the Bendigo and Melbourne Zones and southeast Stawell Zone. Gold mineralisation occurred in two main episodes at ca 445 Ma and ca 380–370 Ma, with another possible (minor) event at ca 410–400 Ma. The ca 445 Ma event is prevalent across the Stawell and Bendigo Zones, with Late Devonian gold mineralisation restricted to the Melbourne and eastern Bendigo Zones. The timing of the two main events is supported by geological constraints, the reproducibility of 40Ar/39Ar results and, in the case of the Bendigo goldfield, coincidence with Re–Os data. Suggestions of a single Devonian age gold mineralisation event are not supported by the available data. The two main gold mineralisation episodes (ca 445 Ma; ca 380–370 Ma) coincide with the waning stages of the Benambran and Tabberabberan orogenies, respectively. Crustal thickening and consequent metamorphic devolatilisation during the Benambran orogeny may have been the main cause of fluid flow related to gold mineralisation at ca 445 Ma. In contrast, crustal anatexis is considered responsible for metamorphic fluid generation and Early Devonian gold mineralisation. Key Words: geochronology 40Ar/39ArU–PbRe–OsgoldLachlan Orogen Acknowledgements Thanks are due to Dennis Arne, Frank Bierlein and David Foster as well as Geoscience Australia for providing previously published U–Pb and 40Ar/39Ar age data. This contribution was supported by funds from GeoScience Victoria 'Gold Under-Cover' and Australian Research Council (LP0882157) grants, with support from Bendigo Mining, Lihir Gold and Northgate Minerals (formerly Perseverance). We acknowledge Ross Cayley, David Moore, Vince Morand, Peter O'Shea, Tim Rawling and Clive Willman for thoughtful comments on an earlier version of the manuscript. The manuscript has also benefited from the thoughtful reviews of Geoff Fraser and an anonymous referee. Additional informationNotes on contributorsD. Phillips †Present address: Research School of Earth Sciences, The Australian National University, Canberra, ACT, Australia‡Present address: School of Geosciences, Monash University, Melbourne, VIC, Australia
Melbourne University is advertising its new Melbourne Model under the inane slogan 'dreamlarge'.The entire Australian university community should be worried about what is involved; but Melbourne's Arts Faculty has greatest cause for concern.
Abstract : In August 1981 the Cultural Resource Management Division of the Arizona State Museum carried out a 640-acre archaeological survey at Painted Rock Dam in southwestern Arizona. The study assessed the effects of inundation on rock alignments and other remains. Intensive survey revealed 82 finds in or near the study area, ranging from isolated artifacts to 'Rock City,' a complex of trails, rock alignments, and artifacts. The remains range in age from probably Preceramic to Recent, but in many cases the actual age and cultural affiliation of finds were ambiguous. This report attempts to distinguish ancient from recent alignments and assesses the potential significance of the remains. Ability to assess the effects of inundation was limited by the lack of pre-inundation data; nonetheless, some conclusions were reached. Inundation damage was largely due to wave action and was most pronounced on slopes and on top of ridges or knolls. Wave action could untimately destroy all sites within the reservoir area, but the rate of destruction is unknown and will vary according to the physical setting of sites. The report ends with recommendations for a program of site monitoring and testing and excavation of sites in immediate danger.
Table S5 40Ar/39Ar laser step-heating and laser fusion analytical results for KBS (=H2), Malbe (=H4), Chari (=L), Gele and FCTs feldspars, including all corrections.
The 40Ar/39Ar dating method is among the most versatile of geochronometers, having the potential to date a broad variety of K-bearing materials spanning from the time of Earth’s formation into the historical realm. Measurements using modern noble-gas mass spectrometers are now producing 40Ar/39Ar dates with analytical uncertainties of ~0.1%, thereby providing precise time constraints for a wide range of geologic and extraterrestrial processes. Analyses of increasingly smaller subsamples have revealed age dispersion in many materials, including some minerals used as neutron fluence monitors. Accordingly, interpretive strategies are evolving to address observed dispersion in dates from a single sample. Moreover, inferring a geologically meaningful “age” from a measured “date” or set of dates is dependent on the geological problem being addressed and the salient assumptions associated with each set of data. We highlight requirements for collateral information that will better constrain the interpretation of 40Ar/39Ar data sets, including those associated with single-crystal fusion analyses, incremental heating experiments, and in situ analyses of microsampled domains. To ensure the utility and viability of published results, we emphasize previous recommendations for reporting 40Ar/39Ar data and the related essential metadata, with the amendment that data conform to evolving standards of being findable, accessible, interoperable, and reusable (FAIR) by both humans and computers. Our examples provide guidance for the presentation and interpretation of 40Ar/39Ar dates to maximize their interdisciplinary usage, reproducibility, and longevity.
Quantifying the compositional evolution of mantle-derived melts from source to surface is fundamental for constraining the nature of primary melts and deep Earth composition. Despite abundant evidence for interaction between carbonate-rich melts, including diamondiferous kimberlites, and mantle wall rocks en route to surface, the effects of this interaction on melt compositions are poorly constrained. Here, we demonstrate a robust linear correlation between the Mg/Si ratios of kimberlites and their entrained mantle components and between Mg/Fe ratios of mantle-derived olivine cores and magmatic olivine rims in kimberlites worldwide. Combined with numerical modeling, these findings indicate that kimberlite melts with highly variable composition were broadly similar before lithosphere assimilation. This implies that kimberlites worldwide originated by partial melting of compositionally similar convective mantle sources under comparable physical conditions. We conclude that mantle assimilation markedly alters the major element composition of carbonate-rich melts and is a major process in the evolution of mantle-derived magmas.
'/%3e%3c/defs%3e%3cpath fill='%23012169' d='M0 0h10080v5040H0z'/%3e%3cpath stroke='%23fff' stroke-width='.6' d='m0 0 6 3m0-3L0 3' clip-path='url(%23b)' transform='scale(840)'/%3e%3cpath stroke='%23e4002b' stroke-width='.4' d='m0 0 6 3m0-3L0 3' clip-path='url(%23c)' transform='scale(840)'/%3e%3cpath stroke='%23fff' stroke-width='840' d='M2520 0v2520M0 1260h5040'/%3e%3cpath stroke='%23e4002b' stroke-width='504' d='M2520 0v2520M0 1260h5040'/%3e%3cg fill='%23fff'%3e%3cuse xlink:href='%23d' x='2520' y='3780'/%3e%3cuse xlink:href='%23a' x='7560' y='4200'/%3e%3cuse xlink:href='%23a' x='6300' y='2205'/%3e%3cuse xlink:href='%23a' x='7560' y='840'/%3e%3cuse xlink:href='%23a' x='8680' y='1869'/%3e%3cuse xlink:href='%23e' x='8064' y='2730'/%3e%3c/g%3e%3c/svg%3e)
