Geosciences

Eclogite-facies rocks along the Paleozoic active margin of Gondwana are rare. They are limited to isolated segments of northern Victoria Land (Antarctica), western Tasmania and south-eastern Australia. New petrological data for mafic rocks and their host garnet-kyanite schists from the Franklin Metamorphic Complex (western Tasmania) permit reconstruction of six main stages of mineral growth for the eclogite.  Stage I and II occurred at greenschist/amphibolite-facies conditions (ca. 500-600 °C; 0.55-0.7 GPa for stage II) before attaining high pressure conditions (at ≈600-650 °C; > 1.5 GPa - stage III). The following stages, IV and V, record the decompression from high pressure conditions to amphibolite facies (ca. 500-600 °C; 0.4-1.0 GPa). Finally, stage VI represents the late greenschist-facies retrogression. However, the pelitic schist, surrounding the eclogite, records only the medium-pressure amphibolite-facies stage. The P-T evolution over time outlines a clockwise path which is quite steep both in the prograde and retrograde segments. The latter shows a nearly isothermal decompression between the eclogite and the high pressure amphibolite-facies stage IV which was achieved at deep crustal levels (≈ 30 km), and a final decrease in both pressure and temperature from deep/intermediate to shallow crustal levels with a typical cooling-unloading path. The lack of a complete re-equilibration during the different stages and the high dP/dT for both the prograde and retrograde paths are indicative of a rapid burial and initially rapid exhumation. The similarity of the mafic whole rock chemical composition including N, T to E – MORB and of the peak metamorphic age (≈ 500 Ma) between the Tasmanian eclogites and the UHP rocks from northern Victoria Land supports the idea that they formed in the context of the same contractional event. However, the different P-T conditions and dP/dT point to different tectono-metamorphic settings for the two sectors of the paleo-Pacific margin of Gondwana during the Ross/Tyennan orogeny.

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 westernmargin of the South Tasman Rise is a fragment of the Ross Orogen and does not correlate directly with Tasmania.

The Toyuk thrust zone is a regional tectonic boundary in the central Brooks Range of northern Alaska that has been interpreted previously as a single large-displacement fault. Just west of the Dalton Highway, the thrust zone is defined by several north-vergent imbricate thrust faults.  These faults have cut detachment folds formed in the competent Kanayut Conglomerate between detachments in the underlying Hunt Fork Shale and overlying Kayak Shale, thereby forming a duplex.  The exposed structural geometry suggests to end-member models of duplex geometry. In model one, each linking thrust has roughtl the same small displacement and the roof thrust is overlain by a normal stratigraphic succession. In model two, the uppermost and hindmost fault has greater displacement than the other faults and so forms a duplex roof above which the stratigraphic section is duplicated.  Discrimination between these models is not possible because the duplex roof has been eroded in the field area.

Apatite fission-track analyses of samples collected with the Toyuk thrust zone record two distinct episodes of cooling interpreted to represent unroofing in response to tectonic deformation. The initial episode occurred at ~100 Ma, while the later episode occurred at ~60 Ma.  These two cooling/denudation events have been documented previously in the region, but the samples from the thrust zone are the first to document displacement on a specific fault during the ~60 Ma event so far south of the front of the central Brooks Range. The results suggest that at least part of the growth of the Toyuk thrust zone accommodated structural thickening within the orogenic wedge coeval with deformation at the range front.

The Toyuk thrust zone is a regional tectonic boundary in the Brooks Range that has been interpreted previously as a large-displacement fault. Just west of the Dalton Highway the thrust zone is defined by the intersection of the erosional surface with several imbricate thrust faults. These faults have cut detachment folds formed between incompetent shale units and define a duplex limited to three units. Apatite fission-track dating indicates cooling in the area at approximately 60 Ma as well as “mixed ages” that reflect both the 60 Ma cooling and earlier cooling at approximately 100 Ma. These ages have been documented previously in the region, but these samples are the first to record the 60 Ma event so far south of the range front. This suggests that the formation and growth of the Toyuk duplex accommodated structural thickening with the orogenic wedge coeval with deformation at the range front.

The Tyennan Orogeny produced low to medium-grade metamorphic rocks distributed across the western third of Tasmania. Chemical U-Th-Pb monazite dating reveals that the peak episode of metamorphism took place in the Cambrian, with a weighted mean age for all units analysed of 505 ± 1 Ma. However, variations in the results by region range from ~ 511 to ~ 497 Ma. The pelitic schists of the Franklin Metamorphic Complex contain garnet porphyroblasts which record a rapid, nearly isothermal, pressure increase; the garnet cores formed at ~ 600o C, 6,000 bars and the rims at ~ 700o C, 14,000 bars at 511 ± 3 Ma. Likewise, the eclogite from the same region records a change from ~ 550o C, 6,250 bars to ~ 650o C, 19,000 bars. The whiteschist, which was obtained from the opposite side of a major local fault, formed garnet cores at ~ 545o C, 19,600 bars; its garnet rims and matrix minerals formed at 506 ± 5 Ma after an increase in temperature of at least 30-90o C. All of these units show evidence of very rapid isothermal exhumation. Other Franklin Metamorphic Complex fault blocks record P/T for peak conditions at ~ 570o C, 8,600 bars (Mt. Mary), and ~ 700o C, 11,400 bars (Raglan Range). The Forth Metamorphic Complex achieved peak metamorphism at 509 ± 7 Ma, at conditions of 670o C, 16,900 bars, and the nearby Settlers Schist gives results of 513 ± 8 Ma. The garnet porphyroblasts of the Port Davey Metamorphic Complex record a single episode of metamorphism which took place at 505 ± 2 Ma at ~ 550 to 570o C and ~ 6,000 bars during which a dehydration event resulted in both a change of garnet composition and texture. The regional geology indicates metamorphism predated post-collisional extension and associated eruption of the Mount Read volcanics at 506 to 500 Ma. Most of the monazite dating is consistent with this observation. However, the Mersey River Metamorphic Complex gives very consistent results of 497 ± 3 Ma. This is problematical, as it would have been at depth undergoing metamorphism after that extension took place. This could be the result of an unknown analytical problem, but the Mersey River monazite grains are indistinguishable chemically from monazite in the other units, and this sample has undergone repeated analysis. Alternatively, this sample reflects a different metamorphic event than that recorded in all other samples studied.

An important factor in fostering a positive attitude toward science and scientific careers in young women researchers has been providing them with scientific role models who are women. Such stories give them realistic information about career opportunities. Therefore this booklet has been developed to provide young women scientists with a collection of role models who have benefited from mobility in their careers. These are not famous scientists, but rather real examples of people who express all the passion to "explore, dream, and discover" the world of science. It is hoped that reading about successful scientists who have achieved a healthy work-life balance while moving to new locations will be particularly helpful for those individuals considering mobility in their own career.

The Tyennan Orogeny of Tasmania, Australia took place at ~510 Ma. Its history is recorded in the growth of metamorphic monazite as well as in the changes in the garnet zoning patterns in the pelitic schists, whiteschist, and eclogites of the Franklin Metamorphic Complex.  Two stages of metamorphism are recorded; an initial period during which the largely homogeneous cores of the garnet porphyroblasts formed at ~600o C, 6,000 bars and a later, high-pressure stage (at approximately 600-700o C, 14,000-19,600 bars) during which the garnet rims and matrix minerals grew.  The Cambrian metamorphic monazite found in these samples occurs only in the garnet rims and in the matrix.  In addition, there are older generations of monazite occurring in these samples which are found included within garnet cores in addition to in the rims and matrix are less tightly constrained in their ages; the Mesoproterozoic monazite grains are consistently lower in Sr and higher in Y than are the Cambrian monazite, indicating that they pre-date garnet growth and grew at lower pressures than did those from the peak metamorphic episode.

Constraining the garnet-talc join for metasedimentary material attaining eclogite-facies conditions

R. Chmielowski, P. Fumagalli, S. Poli
Dipartimento Scienze della Terra, Università degli Studi, Milano;
email: reiac0@utas.edu.au

The subduction of mica-dominated sediments results in a loss of biotite as garnet-talc becomes a stable assemblage due to increasing pressure.  While this transition is observed in natural samples, it has not yet been well constrained experimentally.  Previous experimental investigations into metapelites at the Laboratory of Experimental Petrology, Department of Earth Sciences, University of Milan (reported in Poli and Schmidt 2002) indicated that further work in the range of 600-700o C, 2-3 GPa was required to elucidate this tie-line transition.  The assemblages leading to garnet-talc stability through tie-line flip reactions include biotite-chlorite, biotite-chloritoid, and biotite-kyanite. Furthermore the mutual stability of garnet-chlorite and chloritoid-biotite at relatively high pressure conditions below the garnet-talc field is reevaluated.

We investigate two synthetic compositions (M, P) in the model metapelitic system CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O. Experiments are carried out in a piston cylinder apparatus at pressures and temperatures up to 2.7 GPa and to 740°C. In order to monitor the effect of H2O saturation and fluid speciation, different charges are loaded for each bulk composition at fluid saturated conditions and fO2 buffered by graphite, Ni-NiO or hematite-magnetite. Experiments are characterized by XRD, BSE images and EMPA.

Poli, S. and Schmidt, M. W. (2002). "Petrology of subducted slabs." Annual Review of Earth and Planetary Sciences 30: 207-235.

Natural samples display a loss of biotite as garnet-talc becomes the stable assemblage in response to increasing pressure due to the subduction of mica-dominated sediments.  However, this transition has yet to be well constrained experimentally.  Previous experimental investigations into metapelitic compositions at the University of Milan (Poli and Schmidt 2002, Ferri et al., 2009) indicated that further work in the range of 600-700° C, 2-3 GPa is required to elucidate this tie-line transition.  The assemblages leading to garnet-talc stability through tie-line flip reactions include biotite-chlorite, biotite-chloritoid, and biotite-kyanite. Furthermore the mutual stability of garnet-chlorite and chloritoid-biotite at relatively high pressure conditions below the garnet-talc field is reevaluated.

Investigations on four synthetic compositions (NM, H, NP, and L) in the model metapelitic system CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O are carried out in a piston cylinder apparatus at pressures and temperatures up to 2.7 GPa and to 740°C. Experiments are buffered with graphite, and are generally run under fluid saturated conditions. The NM and H compositions are representative of metapelites and the NP and L compositions are representative of metagreywackes.  Two capsules, one of each compositional type, are included within the pressure chamber for each experiment. Experiments are characterized by XRD, BSE images and EMPA.

The following summary includes both current investigations and revised analysis of the above mentioned previous work for pressures ≥ 22 GPa All assemblages also contain quartz ± fluid.  Some pressures and temperatures have individual phases present for one bulk composition but not the other; in these cases the symbol ± is used to indicate the phases for which this applies. 

biotite-garnet-chloritoid-kyanite ± muscovite ± carbonate 625° C 2.20 GPa
biotite-garnet-muscovite-chloritoid-lawsonite ± talc ± carbonate 625° C 2.65 GPa
garnet-chloritoid-carbonate ± biotite ± talc ± muscovite ± zoesite 600° C 2.50 GPa
garnet-muscovite-chloritoid-carbonate 625° C 2.50 GPa
garnet-muscovite-chloritoid ± biotite ± talc ± carbonate 600° C 2.35 GPa
garnet-muscovite-chloritoid ± biotite ± kyanite ± carbonate 650° C 2.50 GPa

In conjunction with the experiments, isochemical modeling for both compositions were undertaken with Perple_X (Connolly, 2009).  Comparison of these isochemical sections with our experiments (including those down to 1.2 GPa) shows biotite appearing at higher pressures than predicted and carbonates appearing at lower pressures and temperatures than predicted for both composition types. Our experiments wherein garnet was absent occur within the range that Perple_X predicts garnet to be stable.  Both chloritoid and talc are observed in our experiments at higher temperatures than predicted for either composition, and staurolite is observed at higher pressures than predicted for composition NM (staurolite has not yet been observed for experiments run for composition NP, and is not predicted by Perple_X to be stable in this P/T range).

Literature
Connolly, J. A. D., 2009. The geodynamic equation of state: What and how. Geochemistry, Geophysics, Geosystems, 10(10).

Historical Reenactment Societies have grown in popularity over the years; many members within such organizations are also scientists. These researchers take skills honed in the laboratory or field and apply them in their spare time to learning how our ancestors lived, balancing time carefully between doing scientific research with hobby-based historical research; one week presenting a science talk, the following weekend teaching a workshop on embroidery techniques to students interested in creating wall-hangings or embellishing costumes to enhance the beauty and authenticity of Medieval-themed events.
In contrast to such hobbyists, some scientists are called upon to use their skills to assist historians directly. I present case studies of geologists whose research provides keys to determining which specific trade routes have operated historically. One study investigates the strontium isotopes present in wool fibers from the archaeological record and uses that to determine the locations in which the sheep were grazing. Others use the chemical composition of the garnets in jewelry from the Merovingian period as a key to determining the mines from which the stones were obtained.

My current project title is more than a mouthful, so I have broken it down into questions and answers, starting with "Why 3D Modeling?" and explaining each aspect of the title and how it relates to my research.  This talk will be presented at a meeting for the entire department, which includes many other branches of science than just Geology.

The metamorphic complexes of Tasmania formed during the Cambrian (ca 510 Ma) as a result of rapid compression in a subduction zone setting followed by rapid exhumation, which brought various fault-bounded metamorphic complexes back to the surface in less than 5 Ma. The two highest grade complexes, the Franklin Metamorphic Complex, and the Port Davey Metamorphic Complex, experienced initial growth of metamorphic garnets at *5608C, *0.56 GPa. However, their subsequent metamorphic histories diverge, with the FMC displaying a marked increase in pressure (to 1.4 GPa at peak P/T), while the PDMC shows only a slight increase in pressure (to *0.7 GPa). Both complexes show only a minor increase in temperature (*1008C) between initial garnet growth and peak metamorphic conditions. Rapid exhumation of these complexes can be accounted for by a slab-breakoff model. However, the difference in peak pressure between these complexes requires either continued subduction of the FMC while the PDMC had already begun its return towards the surface or that the subduction zone geometry resulted in significantly different pressures occurring contemporaneously within portions of the channel, which are not far removed from one another.

A 3D geochemical model of the Kristineberg area of the Skellefte District, Sweden, is currently under construction, utilizing data from more than 1600 regionally distributed whole-rock lithogeochemical samples. The model will improve our understanding of the formation the VHMS deposits in this area. The model is built by mapping geochemical variations in 3D, and using this as a basis for modelling hydrothermal alteration in the unsampled portions of the rock column. A better understanding of the geometry, intensity, vectors of transport, and zonation of the hydrothermal zones in 3D will aid deep exploration for massive sulphide deposits in the Kristineberg area, and may potentially lead to new discoveries.

This contribution presents a 3D assessment of metamorphosed and deformed, hydrothermally altered volcanic rocks, hosting the massive sulphide deposits of the Kristineberg area in the 1.9 Ga Skellefte mining district in northern Sweden, using six calculated alteration parameters: the Ishikawa alteration index, the chlorite–carbonate–pyrite index and calculated net mass changes in MgO, SiO2, Na2O and Ba. The results, which are also available as film clips in the Supplementary data, confirm inferences from geological mapping; namely that the sericite- and chlorite-rich alteration zones have complex and cross-cutting geometries and that most of these zones are semi-regional in extent and range continuously from surface to over a kilometre deep. The major known massive sulphide deposits occur proximal to zones characterised by coincidence of high values for the alteration index and chlorite–carbonate–pyrite index and large MgO gains, which corresponds to zones rich in magnesian silicates. These zones are interpreted as the original chlorite-rich, proximal parts the alteration systems, and form anomalies extending up to 400 m away from the sulphide lenses. In addition, the stratigraphically highest VHMS are hosted by rocks rich in tremolite, talc, chlorite and dolomite with lesser clinozoisite, which have high chlorite–carbonate–pyrite index and low–medium alteration index values, reflecting a greater importance of some chlorite-carbonate alteration at this stratigraphic level. Vectoring towards massive sulphide deposits in this area can be improved by combining the AI and CCPI indexes with calculated mass changes for key mobile elements. Of the ones modelled in this study, MgO and SiO2 appear to be the most useful.