Late Cambrian – Middle Ordovician extension of the northern Tasmanides: thinned crust that facilitated intense Silurian (Benambran) shortening deformation

Henderson, Robert1, Fergusson, Chris2 and Withnall, Ian3

1Department of Earth and Environmental Sciences, James Cook University, Townsville, Australia, 2School of Earth, Atmospheric and Life Sciences, University of Wollongong, Australia, 3Geological Survey of Queensland, Brisbane, Australia

The Charters Towers and Greenvale Provinces of the Thomson Orogen provide the most extensive exposure of early Paleozoic rocks in the northern Tasmanides. Upper crust represented by the Charters Towers Province developed a thick, Proterozoic passive margin, sedimentary assemblage with a minor contribution from mafic igneous rocks. This assemblage was subsequently overprinted by active margin tectonism commencing in the mid Cambrian (~510 Ma), reflecting a broad scale Tasmanide regime change of that age. The overprinting regime generated thick (>15 km) basinal infill, inclusive of a substantial intermediate to silicic volcanic complement, of the upper Cambrian – Middle Ordovician Seventy Mile Range Group and other upper Cambrian basinal relicts identified for the Charters Towers and Argentine Metamorphics. Magmatic arc plutonism is widely developed as the Ravenswood Batholith and Fat Hen Creek Complex. Regional metamorphism and penetrative fabric development of Early Ordovician age overprinted all of the Proterozoic passive margin assemblage and part of the Cambrian basin fill, with structural analysis favouring its association with extensional strain. Coeval basin formation, plutonism and metamorphism across the province reflects an enduring episode of crustal extension.  

The Greenvale Province consists largely of metavolcanic and metasedimentary tracts with protoliths of Early Ordovician age. Lower Ordovician granites are also represented and in broad aspect, rock units of the province both resemble, and are correlative with, the active margin assemblages of the Charters Towers Province.  Structural analysis shows a deformation history matching that of the Charters Towers Province with the dominant foliation similarly attributed to extensional strain.  Development of the province represents thick basinal infill developed on thinned crust of a continental margin. Contrary to earlier publications, no rocks older than Ordovician are known for the province. Amphibolite and ultramafic units interleaved with metasediments on its eastern (outboard) side suggest it may have developed on oceanic crust.

For both provinces thin crust had developed by the Middle Ordovician, weakening its resistance to subsequent Benambran contraction. Cambro-Ordovician rock assemblages of both provinces were affected by structural/metamorphic overprint with local generation of mylonite in wide shear zones. Contraction is dated as Silurian by Ar–Ar mineral ages, by fabric development in deformed granites of known age and by the age of overlap strata. Tight upright folds and accompanying foliation, generated by shortening, deformed tracts affected by Ordovician extensional strain, steepening previously formed fabrics. Basin fill represented by the Seventy Mile Range Group was inverted. For the Greenvale Province, widespread amphibolite grade metamorphism indicates that much of it has been exhumed from considerable crustal depth (>15km). In contrast exhumation of the Charters Towers Province was heterogeneous, ranging from <5 km (unmetamorphosed Seventy Mile Range Group) to >15 km (amphibolite facies metamorphism and local migmatite). Silurian structural trends, considered by some authors as registering an orocline, are attributed to strain partitioning consequential on oblique convergence.


Based in Townsville, the presenter has actively pursued research interests in the Tasmanides for over 40 years, with a focus in particular on the Mossman, Thomson and New England Orogens which are extensively developed in Queensland. Mush of this activity has involved collaboration with Chris Fergusson and Ian Withnall.

Early Tasmanides evolution: Passive to convergent margin history in New South Wales, Australia

Greenfield, John1; Gilmore, Phil1; Musgrave, Robert1

1Geological Survey of New South Wales, Department of Regional NSW, Maitland, Australia.

Initial development of the Tasmanides in southeastern Australia involved Early Neoproterozoic rifting/break-up of the Rodinian supercontinent, expressed as tholeiitic dyke swarms and continental rifting in the Adelaide Rift Complex of eastern South Australia. This left highly-extended, transitional crust between the Gawler Craton and Curnamona Province, which became the depocentre for extensive platform carbonate and shallow marine clastic sedimentation. By ~700 Ma, a passive margin developed east of the Curnamona Province which saw the initiation of the palaeo-Pacific Ocean. A final NE–SW phase of rifting in the Late Neoproterozoic was associated with shallow marine platform sedimentation and alkaline magmatism (Mount Arrowsmith Volcanics), further attenuating the eastern Curnamona Province crust and presenting an angular continental salient towards the nascent ocean to the east.

This crustal configuration profoundly influenced the palaeogeography and tectonism that followed during the Delamerian Cycle, as passive margin clastic deposition in the early Cambrian gave way to west-facing subduction in the mid Cambrian. Elements of the resulting Cambrian volcanic arcs (Mount Wright and Loch Lily–Kars) are now immediately adjacent to the Curnamona Province margin. However, regional geological mapping in the Koonenberry Belt has shown that the Mount Wright Arc developed in a rift zone within the Curnamona Province that probably initiated during the last phase of Rodinia break-up in the Late Neoproterozoic. In contrast, the strike-equivalent Loch Lily–Kars Arc was developed in an intra-oceanic setting. Mapping, drillhole and geophysical data shows that this arc segment, along with forearc volcanic rocks of the Ponto Group, were oroclinally folded clockwise almost 90°, and thrust against the southeastern Curnamona Province margin during the Late Cambrian Delamerian Orogeny. If the original arc segments were part of a linear belt, it would have extended southeast from the oroclinal hinge at the Grasmere Knee Zone. Recently acquired and modelled AusLAMP magnetotelluric data show a strong lower crustal conductive anomaly aligned along this trend.

The Delamerian Orogeny caused strong ductile deformation of rocks deposited in the Delamerian Cycle. Areas that were highly attenuated during break-up suffered tight upright folding and oroclinal bending, which may also have been affected by clockwise rotation of the Curnamona Province. Proterozoic Olarian Cycle metamorphic rocks along the margins of the Curnamona Province and the eastern edge of the Gawler Craton also suffered upright open to tight folding during the Delamerian, with σ1 perpendicular to the outboard margin. In the Broken Hill Domain, early southwest-directed fold-thrusting switched to northwest-directed fold-thrust and strike-slip deformation at the end of the Delamerian Orogeny in the Early Ordovician.

Clearly the switch to a convergent setting, with arc accretion and terminal deformation in the Delamerian Orogen, caused extensive shortening of a highly attenuated margin that had been in extension for c. 300 Ma. The Curnamona Province, although acting as a salient in the early Delamerian Orogeny, was itself deformed and could not insulate the distal-foreland Flinders Ranges from deformation late in this event. This had cumulative effects on the ensuing cycles of subduction roll-back, extension and contraction that defined the Tasmanides throughout the Palaeozoic.


John leads the Geoscience Acquisition & Synthesis Unit in the Geological Survey of NSW, which collects and interprets geoscientific data from geological mapping, geophysical surveys, and specialist studies in mineral deposits, palaeontology, and petrography.

Disorientation control on trace element segregation in fluid-affected low-angle boundaries in olivine

Tacchetto Tommaso1,2, Reddy Steven1,2, Saxey David2, Fougerouse Denis 1,2, Rickard William2, Clark Chris1

1School of Earth and Planetary Sciences, Curtin University, Perth, Australia, 2Geoscience Atom Probe, Curtin University, Perth, Australia

The interfaces between minerals (grain boundaries s.l.) play a critical role in controlling the rheological behaviour of rocks and the ability of fluids to penetrate them in the deep crust. However, the role of chemical segregation in controlling the behaviour of mineral interfaces remains largely unexplored. In this work, we combined electron backscattered diffraction (EBSD) and atom probe tomography (APT) to assess the relationship between deformation-related low-angle boundaries in naturally-deformed olivine and the degree of trace element segregation to those boundaries. The sample studied comes from the Bergen Arcs (Norway), where high-grade, dry, metamorphic rocks of the lower crust have been overprinted by fluid-present high-pressure metamorphism during the Caledonian tectonic subduction between 430 and 410 Ma. EBSD orientation mapping of deformed olivine is used to characterise the slip systems associated with deformation and the misorientation relationships within different parts of the microstructure. APT has then been used to systematically target grain boundaries of different misorientation angle (up to 8°). 

The analysed boundaries formed by sub-grain rotation recrystallisation associated with {100}<001> slip system developed during the fluid-catalysed metamorphism. APT data show that olivine trace elements segregated to the low-angle boundaries during this process. Boundaries with < 2° degrees show marked enrichment associated with the presence of multiple, non-parallel dislocation types. However, at increased misorientation (> 2°), the interface becomes more ordered with dislocation geometries defined by linear concentrations of trace elements, and which are consistent with the EBSD data. These boundaries show a systematic correlation of increasing trace element segregation with misorientation angle. In particular, elements are generally more enriched at higher degrees of distortion, where variations are mostly significant for Ca (from 0.07 up to 0.6 at%) and Cl (up to 0.3 at%). Elements that are segregated to the low-angle boundaries (Ca, Al, Ti, P, Mn, Fe, Na, Mg and Co) are here interpreted to be captured and accumulated by dislocations as they migrate to the sub-grain boundary interfaces. However, the exotic trace elements Cl and H, also enriched in the low-angle boundaries, likely reflect a small but significant contribution of an external fluid source during the fluid-related deformation. In particular, since the occurrence of H in olivine is strongly attributed to Ti defects, the segregation of Ti to grain boundaries is consistent with the detected enrichment of hydrogen in the low-angle interfaces.

The observed compositional segregation of trace elements to low-angle boundaries have significant implications for the deformation and transformation of olivine at mantle depth, the interpretation of geophysical data and the redistribution of elements deep in the Earth. Furthermore, the nanoscale correlation between heterogeneous distribution of elements like Ti and the diffusion of H along boundary interfaces within olivine might have the potential to yield important implication for the understanding of the hydrogen distribution in the upper mantle and its consequences for the rheological properties of mantle-rocks during deformation.


Tommaso completed his MSc in Geology at the Univesity of Padova (Italy) in 2017 and awarded in 2017 of a Temporary Research Fellowship. He is now completing his 3rd year of PhD at Curtin University focused on the investigation of metamorphic processes in the precence of fluids at the nanoscale.

Lapstone Structural Complex and uplift of the Blue Mountains, tectonic backdrop to western Sydney, Australia

Fergusson, Chris1 and Hatherly, Peter2

1School of Earth, Atmospheric and Life Sciences, University of Wollongong, Wollongong, Australia, 2Lavender Bay, Sydney, Australia

The eastern margin of the Blue Mountains uplift is well defined by the Lapstone Structural Complex (LSC) with elevations typically of 150 to 200 m but locally up to 600 m at Kurrajong Heights. While it has been suggested that the LSC is related to normal faulting associated with passive margin development, this suggestion is inconsistent with a west-dipping reverse fault with minimal fault damage development (the Bargo Fault) we have found in the southern part of the LSC. We prefer the alternative view that the LSC consists of east-facing monoclines and reverse faults that dip both east and west. The underlying controlling structure could be a west-dipping thrust and the surface expression is due to the formation of a triangle zone. Further support for our interpretation is found in the northern part of the LSC where there are gravels (Rickabys Creek Gravel) and clay (Londonderry Clay) which are draped along the front of the monocline. Their position indicates that they were folded during development of the LSC. Thus the gravels do not occur in a series of terraces as was previously considered. Unfortunately no direct age of the Rickabys Creek Gravel and the Londonderry Clay has been determined but their unconsolidated state is consistent with a Neogene age thereby constraining formation of the LSC to the Neogene. Ongoing seismic activity associated with the LSC indicates neotectonic activity along the structure. The higher parts of the Blue Mountains are at elevations of over 1000 m and overall rock units of the Sydney Basin slope to the east at an angle of ca 1.5°. We consider that the latest phase of uplift in the Blue Mountains was associated with formation of the LSC and other structures including the Mt Tomah Monocline. Given earlier phases of uplift associated with formation of the eastern highlands it is problematic to identify the limits of this younger phase of Blue Mountains uplift particularly in the west where it apparently merges with the Central Tablelands. The enduring debate about the cause of the uplift of the Great Dividing Range of eastern Australia has become increasingly centred on the importance of mantle upwelling. We consider that the younger phase of uplift of the Blue Mountains associated with formation of the LSC was related to Australia’s setting west of the Southwest Pacific convergent margin rather than mantle upwelling.


Peter Hatherly is a retired geophysicist who has taken an interest in the structural and geomorphological evolution of the Blue Mountains. He has published relevant papers on the results of high resolution seismic reflection surveys, analysis of longitudinal stream profiles and mapping of semi-consolidated gravels above river systems.

Provenance of Late Cambrian-Ordovician sedimentary rocks in western, north-eastern Tasmania and southern Victoria: Constraints from U/Pb dating, zircon geochemistry and εHf isotope

Habib, Umer1, Meffre, Sebastien1, Kultaksayos, Sitthinon1, Berry, Ron1

1Centre of Ore-Deposit Geology and Earth Sciences, University of Tasmania, Hobart, Australia

Email address.

The sedimentary sequences in western, north-eastern Tasmania and southern Victoria preserves an excellent record of distinctive Cambrian to Ordovician deposition environments and sediment provenance.  Here we integrate new and already published U/Pb detrital age data, field observations, zircon geochemistry, and Hf isotope data to establish the sediment source and tectonostratigraphic history of these rocks. Overall, the detrital age spans from 2.6 Ga to 0.476 Ga, which includes some major Precambrian and Cambrian age peaks. The 1.8-1.2 Ga peaks for Western Tasmania are consistent with derivation from the Tyenna region which incorporate sediments derived from granitoids in Laurentia (North America) and Baltica. A small population of detrital ages from 1.5-1.1 Ga from north-eastern Tasmania and southern Victoria is suggested to have Grenville provenance, possibly from central Australia or other parts of the Grenvillian Orogen. The 850-545 Ma detrital ages occur in all sedimentary rocks and are from the widely represented Pacific-Gondwana Phanerozoic sediments of south eastern Australia. These 850-545 Ma detrital ages are rare in the Owen Group, relative to the overlying Middle-Late Ordovician Pioneer Sandstone, implying a sharp shift in provenance in western Tasmania in the Early Ordovician. The Pioneer Sandstone is very similar to Early Ordovician sandstones from Waratah Bay in southern Victoria and Ordovician sandstone from eastern Tasmania. Also present in the samples are 480-520 Ma zircons. However, the proportions of these in the sedimentary rocks are variable. The Th/U ratios from the Cambrian zircons in western Tasmania support a proximal source from the Mt Read Volcanics. The Cambrian zircons in eastern Tasmania and southern Victoria have much lower Th/U ratios, implying a different provenance. The εHf isotope signatures and statistical analysis, along with sedimentological and paleocurrent data from previous studies, support a local derivation from Precambrian and Cambrian detrital sources for the western Tasmanian rock units, indicating deposition in a segmented basin-margin fault system. The eastern Tasmanian and southern Victorian sandstone were deposited in a basin offshore from the Tyennan-Delamerian Orogen during the Ordovician. 

Keywords. Western Tasmania, U-Pb dating, Tectonic configuration, Palaeozoic


Umer has done his Honors and masters degree in geology from Pakistan and worked for MOL from 2013-2015. He joined Codes in 2018 to persue his PhD.

The KBS Tuff Controversy fifty years on: New ultra-precise ages for the KBS tuff and correlates, Omo-Turkana Basin, Kenya

Phillips, Professor David1, Matchan, Dr Erin1

1School of Earth Sciences, The University of Melbourne, Parkville, d9cec488-596f-4927-97dc-c72b3e7f8ea7

Archaeological expeditions by the National Museum of Kenya to east Lake Turkana (formerly Lake Rudolf) in 1968 and 1969 led to the discovery of remarkable stone artefacts and hominin fossils, associated with volcanic tuffs, including the famous KBS (Kay Behrensmeyer Site) tuff.

Initial attempts to date the KBS tuff proved unsuccessful due to fluvial deposition of most tuffs and contamination by older material. Early 40Ar/39Ar dating of pumice feldspar grains from the KBS tuff yielded a reported age of 2.61 ± 0.26 Ma, which was soon disputed on the basis of faunal correlations, thereby precipitating a controversy that played out in Nature publications for the next decade. In 1975, the Berkeley geochronology laboratory published K-Ar ages of 1.60 and 1.8 Ma for two KBS localities, with the former age later attributed to laboratory error. The Cambridge geochronology laboratory then reported a range of 40Ar/39Ar ages (0.52 – 2.61 Ma) and revised the age of the KBS tuff to 2.42 ± 0.02 Ma, in accord with preliminary zircon fission track ages. Subsequent geochemical correlations with the H2 (=KBS) tuff in Ethiopia, dated at ca. 1.8 Ma by the K-Ar method, heightened the controversy. Later K-Ar and 40Ar/39Ar dating analyses by Ian McDougall at the Australian National University largely resolved the debate, with reported ages of 1.89 ± 0.01 Ma and 1.88 ± 0.02 Ma, coincident with a revised fission track age of 1.87 ± 0.04 Ma. More recent analyses of single feldspar crystals from KBS pumice clasts produced a weighted mean age of 1868 ± 14 ka – but with significant scatter, suggesting the presence of inherited grains or extraneous argon.

New ultra-precise 40Ar/39Ar data obtained for single feldspar pumice crystals from several tuff localities across the Omo-Turkana Basin, including the KBS tuff, show variably complex age distribution patterns even within single pumice clasts. Based on co-irradiated A1T and FCT sanidine aliquots and a Bayesian statistical analysis approach, we calculate astronomically calibrated ages for several tuff horizons at precision levels approaching <<0.1%. The KBS and correlated H2 tuff give an astronomically calibrated, weighted mean age of 1879.2 ± 1.3 ka (0.069% 95%CI). The stratigraphically younger Malbe (=H4) tuff, which was originally misidentified as the KBS tuff, gives a Bayesian eruption age of 1837.75 ± 0.86 (0.047%; 95%) ka. The These results enable effective stratigraphic correlations across the Basin, and reveal paleoclimate and paleo-environmental variability at millennial timescale resolution.

This study is dedicated to the memory of Ian McDougall and Frank Brown, who worked tirelessly to unravel the magmatic and geological history of the Omo-Turkana Basin.


Professor David Phillips is Head of the School of Earth Sciences and Director of the 40Ar/39Ar Laboratory at the University of Melbourne. He is internationally recognised for his research on ultra-precise 40Ar/39Ar dating methods and their application volcanic rocks, including tuffs related to hominin localities in the Turkana Basin, Kenya.

Evaluation of the 40Ar/39Ar technique for kimberlite geochronology: Three case studies from Finland

Dalton, Hayden1, Giuliani, Andrea 1,2, Hergt, Janet1, Maas, Roland1, Matchan, Erin1, O’Brien, Hugh3, Phillips, Professor David1, Woodhead, Jon1

1School of Earth Sciences, The University of Melbourne, Parkville, Australia, 2Institute of Geochemistry and Petrology, Department of Earth Sciences, Zurich, Switzerland, 3Geological Survey of Finland, Espoo, Finland

Kimberlites are enigmatic, volcanic rocks of both great economic and scientific importance due to acting as the primary host-rock to diamonds and being the deepest-derived continental magmas on Earth. Despite this significance, there remains debate concerning the sources of kimberlites and what triggers mantle melting to form these rocks. Robust determination of the timing of kimberlite eruption is a crucial prerequisite if we are to unravel the presence of any spatiotemporal relationships between kimberlite emplacement and large-scale tectonic processes, super-continental cycles or mantle plumes.

Despite the benefit of the remarkably high precision achieved with modern 40Ar/39Ar analytical techniques, and the presence of K-bearing groundmass phlogopite in many kimberlites, this technique has seldom been applied to kimberlites and related rocks. Early utilisation of this method revealed issues related to the presence of extraneous argon in mica macrocrysts and phenocrysts which yields anomalously old or maximum emplacement ages. Nonetheless, apparently reliable age results have been obtained on magmatic mica from kimberlites and related rocks. In this study we compare new, precise 40Ar/39Ar ages with other independent age constraints (e.g., Rb/Sr, U/Pb) on three clusters of kimberlites and related rocks from Finland to rigorously assess the instances where 40Ar/39Ar dating produces older apparent ages.

Our results indicate that sample selection and groundmass mica (phlogopite or kinoshitalite) separation needs to be extremely judicious prior to analysis. Where fresh mica phenocrysts are available for 40Ar/39Ar analyses we recommend that plateau results are interpreted with caution. Age spectra which are entirely flat, such that an age plateau includes 100% of the gas are likely the most accurate and precise reflection of the emplacement age of a kimberlite. In contrast, aliquots that yield younger apparent ages for heating steps preceding the plateau may reflect argon recoil redistribution resulting in anomalously older high-temperature/plateau ages when compared with independent age constraints. In cases where such discordance exists, we recommend that total-gas ages give a better approximation of the emplacement age and one which agrees more closely with ages from other geochronometers.


Hayden Dalton is presenting on behalf of the greater AuScope Geochemistry Laboratory Network. Hayden is a PhD researcher in the School of Earth Sciences at the University of Melbourne, his research focuses on the geochronology and geochemistry of kimberlites.

Hydrous polymetamorphic crustal rocks in an eclogite-bearing terrane record post-peak recrystallisation during arc-continent collision

Brown, Dillon1, Hand, Martin1, Morrissey, Laura2,1

1Department of Earth Sciences, University of Adelaide, Adelaide, SA, Australia. 2Future Industries Institute, University of South Australia, Mawson Lakes, SA, Australia

Ultrahigh- and high-pressure terranes are identified based on the occurrence of mafic eclogite-facies mineral assemblages, which effectively record burial and subduction metamorphic conditions. In such terranes however, mafic mineral assemblages are volumetrically minor compared to the continental rocks that host them, which typically preserve anhydrous quartzofeldspathic amphibolite-facies mineral assemblages. Continued debate centres around two hypotheses accounting for the petrology of the continental rocks: (1) the protoliths to the continental rocks did not respond to burial and subduction, and (2) the continental rocks developed high-pressure mineral assemblages which were subsequently overprinted during exhumation. However, less is known about continental rocks that preserve hydrous amphibolite-facies assemblages and schistose fabrics, which are also documented in high-pressure rock systems. In western Tasmania, south-east Australia, eclogite-facies mafic boudins with previously constrained subduction metamorphic conditions of 18–21.5 kbar and 650–700 °C are hosted by weakly foliated to mylonitic metapelitic continental rocks preserving hydrous amphibolite-facies assemblages dominated by siliceous muscovite, quartz, and garnet. Weakly foliated metapelites preserve relict kyanite and two possible textural generations of garnet, moderately foliated metapelites record evidence of partial melting, and mylonitic metapelites contain sillimanite within the rock fabric. Monazite LA–ICP–MS U–Pb geochronology documents two instances of monazite growth in the metapelites: a possible Mesoproterozoic growth event at c. 1385 Ma and a younger, yet poorly constrained growth event in the Cambrian. Rutile LA–ICP–MS U–Pb geochronology more precisely constrains Cambrian-aged metamorphism in the metapelites between 520–505 Ma. Monazite-garnet petrochronology reveals that Mesoproterozoic-aged monazite formed in a system with little or no influence of garnet whereas Cambrian-aged monazite formed in the presence of garnet during subduction. Metamorphic mineral equilibrium modelling of Cambrian subduction indicates that the weakly foliated metapelites best approximate peak metamorphism, recording metamorphic conditions of 13–17 kbar and 600–720 °C. Migmatitic metapelites record lower pressure conditions of 8–13 kbar and 660–740 °C and mylonitic metapelites equilibrated at 3.5–7 kbar and 590–680 °C. We infer that the amphibolite-facies metapelites record different stages of Cambrian-aged exhumation and, unlike their mafic counterparts, do not record burial or subduction. We attribute the inferred eradication of peak subduction mineral assemblages in the metapelites to the influence of fluid and localised deformation.


Dillon is a PhD student from the University of Adelaide working under the themes of metamorphic petrology, geochemistry, and petrochronology. His research focuses on understanding the geodynamic character and tectonism associated with the Cambro-Ordovician East Gondwana margin.

Dating the timing of motion in major ductile shear zones

Forster, Marnie1, Lister, Gordon2

1Research School of Earth Sciences, Australian National University, Canberra, Australia; 2The Virtual Explorer

There is confusion in the argon geochronology world as to what allows movement in a ductile shear zone to be dated. Some assert that all that is necessary is to data mica and to obtain a ‘plateau‘. But this is not at all sufficient to make the argument. 40Ar/39Ar geochronology (like all radiometric dating techniques) does not have the ability to date movement. It is the microstructural modification of existing grains that must be dated, e.g., growth or regrowth during movement. Otherwise it may be that the fabric forming minerals are remnant: namely, relicts of an earlier formed mineralogy, and the ages obtained not at all relevant to the timing of movement in a later shear zone.

An ideal circumstance would be clear and unambiguous demonstration that growth of a particular mica had taken place immediately prior to or during the operation of a ductile shear zone, and that mineral separation (or laser spots) had focussed on those volumes during measurement. An example would be sudden growth of mica porphyroblasts that were then rotated and aligned in a developing shear zone fabric, as occurs in retrograde shear zones in the Cycladic Eclogite-Blueschist Belt. Movement has not been dated, but an estimate has been obtained as to the timing of growth during or immediately preceding movement. Another optimal circumstance would be if the operation of the ductile shear zone had shredded mica, progressively reducing its diffusion dimension. This behaviour leads to staircase spectra characteristic of fractal diffusion. Such age spectra appear to be able to allow the distinction of the start and the end of shear zone operation. In some cases, the age of relict mica microstructures is also evident, e.g., in mica from the Main Central Thrust of the Himalaya.

A more difficult circumstance occurs when age spectra from fabric forming minerals appear to be unrelated to the timing of movement, e.g., for mica from greenschist facies ductile shear zones in the Cap de Creus, Spain. We inverted data from 40Ar/39Ar geochronology step-heating experiments, using potassium feldspar, after conjoint inversion of data from simultaneous ultra-high-vacuum (UHV) 39Ar diffusion experiments. The resultant temperature-time curves imply that these mylonites formed in Eocene to Oligocene time, and therefore that they are not Variscan or Jurassic, as previously argued. Their tectonic significance is likely to be as right-lateral strike-slip shear zones formed in transfer faults accommodating roll-back of the Tethyan subduction zone as it dragged Sardinia and Corsica away from the Palaeo-European margin during opening of the Gulf of Lyon.

These examples suggest caution needs to be exerted in the dating of movement in ductile shear zones. Laser-step heating (or laser spot analysis) is not suited for this purpose, since this method provides no information in respect to Arrhenius data. In consequence the retentivity of relevant minerals in respect to argon diffusion cannot be assessed. In addition, laser methods do not produce consistent detail in age spectra. In contrast, robotic methods applied to resistance-furnace step-heating experiments offer a cheap, efficient, and reliable way to obtain the detailed age spectra that are necessary (in conjunction with Arrhenius data) to characterise the pattern of argon release.


Gordon Lister and Marnie Forster are structural geologists with a particular interest in the theory and practice of argon geochronology, in particular in the study of the dynamics of the evolution of orogenic architecture and its impact on metallogenesis.

The implications of muscovite sub-spectra in phengitic white mica on the theory and practice of argon geochronology

Forster, Marnie1, Lister, Prof. Gordon1

1Research School of Earth Sciences, Australian National University, Canberra, Australia

This study illustrates a new method for the quantitative determination of the timing of movement in ductile shear bands formed in mylonites, or in strongly stretched metamorphic tectonites. The method is of particular use where phengitic white mica is involved, since interlaying in this mineral is usually so fine as to preclude the application of laser methods. In any case, laser methods as they are currently applied, do not have the capability of extracting exact and detailed age-temperature spectra. Laser methods also fail to achieve the multitudinous steps of the age spectrum evident from our high-definition UHV diffusion experiments. Laser methods also lose all information in terms of the low-volume early release of argon that is essential for the recognition of sub-spectra. Computer modelling and simulation shows that such detail in the age spectrum is essential in terms of being able to accurately infer the timing and duration of metamorphic events,

Here we show that high-definition ultra-high-vacuum (UHV) 39Ar diffusion experiments using phengitic white mica are routinely able to extract muscovite sub-spectra in the first 10-30% of 39Ar gas release during 40Ar/39Ar geochronology. A critical factor is that the recognition of muscovite sub-spectra requires Arrhenius data in order to recognise the steps dominated by release of 39Ar from muscovite. In turn this requires precise measurement of temperature during each heating step. The muscovite sub-spectrum is distinct and separate to the main spectrum, which is itself dominated by mixing of gas released from phengite as well as muscovite. The muscovite sub-spectra allow consistent estimates of the timing of the formation of microstructural shear bands in various mylonites, as well as allowing quantitative estimates of temperature variation with time during the tectonic history of shear zones.

Our new data reveals hitherto unsuspected variation in the timing of exhumation of individual slices of the eclogite-blueschist belt, caused by Eocene and Miocene detachment-related shear zones. With excellent outcrop, the eclogite-blueschist belt exposed in the Cycladic archipelago in the Aegean Sea, Greece, offers a spectacular natural laboratory in which to decipher the structural geology of a highly extended orogenic belt and to ascertain the history of the different fabrics and microstructures that can be observed. Using phengitic white mica we demonstrate a robust correlation of age with microstructure, once again dispelling the myth that 40Ar/39Ar geochronology using this mineral, produces cooling ages alone. Previous work in the Cycladic eclogite-blueschist belt has incorrectly assumed that the diffusion parameters for phengitic white mica were the same as for muscovite. Arrhenius data suggest this is not the case, and that phengitic white mica is considerably more retentive of argon than muscovite. Previous workers have also erred in dismissing microstructural variation in age as an artefact, supposedly as the result of the incorporation of excess argon. This has led to inconsistencies in interpretation, because phengite is able to retain argon at temperatures that exceed those estimated using metamorphic mineral parageneses.

The argon system has been treated as a thermochronometer. However, we demonstrate a robust correlation between microstructure and age, down to the detail present in complex tectonic sequence diagrams produced during fabric and microstructural analysis of individual thin-sections. This points to new strategies being required in terms of the theory and practice of argon geochronology.


Dr Marnie Forster is following in the footsteps of Prof Ian McDougall running the ANU Argon Laboratory  with the assistance of Davood Vasegh and Agnes the argon robot.


About the GSA

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