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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 Dave...
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... of kyanite and quartz, assuming water-absent conditions. These results are within error of those obtained via other geothermobarometers for this sample and other samples in this region (Chmielowski 2009). Both of the two highest-grade metamorphic blocks in Tasmania (FMC and PDMC) began their metamorphism at medium P / T ; their initial growth of garnet cores occurred in the vicinity of * 560 8 C, * 0.56 GPa. However, their subsequent pressure–temperature paths are quite different (see the red and blue arrows on Figure 9). For the FMC there is a marked increase in pressure (from 0.56 to 1.4 GPa) with only a slight increase ( * 100 8 C) in temperature. The PDMC, on the other hand, shows only a slight increase in both temperature ( 5 100 8 C) and pressure ( * 0.04 GPa) sufficient to shift these samples from the kyanite field into the sillimanite field. This is consistent with the observations of McNeill (1985), who described samples in the Nye Bay area (e.g. #68331) with sillimanite-rimming kyanite. The metapelites of the FMC are intermediate in their peak recorded pressure and temperature as compared with the Forth Metamorphic Complex to the north of the state and the nearby samples from the Raglan Range (see Figure 1 for locations and Figure 9 for various regional P / T results); they are lower in pressure and higher in temperature than that interpreted for the initial growth of garnet cores of the whiteschist of the FMC; the samples from the nearby Mt Mary region are slightly higher in their peak P – T than the initial conditions of garnet growth of the FMC metapelites; the P – T path interpreted for the FMC eclogites, which are located adjacent to the metapelites discussed here, is very similar to the path of the metapelites, although their peak P – T conditions are intermediate between the peak conditions of the metapelites and the initial garnet growth of the whiteschist (Figure 9; Chmielowski 2009). Each of these regions is fault-bounded, and so each may have undergone slightly different P – T paths. The primary peak metamorphism in Tasmania is known as the Tyennan Orogeny and took place in the Cambrian at ca 510 Ma. It pre-dates the Mount Read Volcanics (Berry et al . 2007), which, within the limits of the current data, started around 506 Ma, or perhaps as late as 504 Ma (Turner & Bottrill 2001). The Tasmanian U–Th–Pb monazite ages by region are shown in Figure 10 and Table 3 (see Chmielowski 2009 for detailed monazite results); these dates are compatible with earlier geochronological studies (e.g. Turner et al. 1998). Most of these regions give results, which are the same within error. However, the Mersey River Metamorphics gives younger than expected results, which may be due to some Pb loss from a more recent local alteration event (around 250–200 Ma) that caused chloritisation in samples from this area. The Tasmanian metapelites studied show little to no evidence of decompression. The zoning patterns in ...
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... has been nearly 50 years since Alan Spry completed his PhD research focusing on the structure and petrology of Tasmania’s Precambrian rocks (Spry 1962). In the intervening years the technology avail- able for metamorphic studies has expanded dramati- cally. There have been a few subsequent studies of the metamorphic history of regions within Tasmania (e.g. Goscombe 1990; Everard 1999; Turner & Bottrill 2001; Holm 2002; Reed et al. 2002) but there had been no attempt to review the metamorphic history of Tasmania as a whole; therefore this study was initiated to review all the existing data, gather new data, and bring them together into a modern tectonic format. Modern geothermobarometry can be used both to constrain the peak metamorphic conditions via conven- tional methods and to determine the conditions under which the earliest growth of zoned phases must have initiated via isochemical section modelling (Tinkham & Ghent 2005; Powell & Holland 2008). The combination of these techniques with geochronological data links Tasmania’s Cambrian metamorphic history to its tectonic history. The Cambrian regional metamorphic rocks of Tasmania are concentrated in the western third of the state in a series of fault-bounded metamorphic complexes, which lie in a belt from Ulverstone on the northwest coast to Port Davey on the southwest coast (Figure 1). Previously published pressure–temperature estimates for Tasmanian metamorphic complexes range from low greenschist faces, through blueschist, epidote–amphibolite, amphibolite and eclogite facies (Figure 2). The largest of these fault-bounded units include the Franklin Metamorphic Complex (FMC), and the Port Davey Metamorphic Complex (PDMC). The timing of the Cambrian metamorphism as determined via chemical U–Th–Pb monazite dating (515–505 Ma) was addressed by Berry et al. (2007) and further refined by Chmielowski (2009). The intensely faulted FMC is located in central Tasmania (Figure 1). It contains metapelites, eclogites and whiteschist (Spry 1962; Ra ̊ heim 1976; Kamperman 1984; Goscombe 1990). These are the highest-grade regional metamorphic rocks in the state, and snowball garnet porphyroblasts from the FMC have been featured in studies of crystallisation and deformation (Spry 1963; Bell et al. 1986). The metapelites of the FMC are a focus of this project. The FMC eclogites, which appear as fault-bounded lenses within the metapelites, were the focus of a companion study to this project; they yielded estimates of * 500–600 8 C; 0.55–0.7 GPa for an early stage of metamorphism (stage II) and peak metamorphic conditions of * 600–650 8 C; 4 1.5 GPa (stage III) (Palmeri et al. 2009). The FMC whiteschist is located in a separate fault-bounded block within the complex, than the metapelites analysed for this study (see Chmielowski 2009 for detailed maps of these blocks). The FMC also includes lower grade metapelitic rocks within other fault-bounded blocks (Gee 1962; Williams 1971). The PDMC is exposed on the southwest coast of Tasmania (Figure 1); it contains pelitic schist, with amphibolite and garnet–amphibolite lenses (Meffre et al. 2000). McNeill (1985) reported amphibolite-facies conditions for the garnet-bearing schists from the Nye Bay region of the PDMC and noted that the Ca and Mn zonation in the garnet crystals imply two different sets of growing conditions. The PDMC medium-grade metapelites are the second focus of this project. In addition to the larger metamorphic complexes mentioned above, smaller fault-bounded packages of metamorphic rocks are also found in Tasmania (e.g. Green 1959; Burns 1964; Gee & Legge 1979; Turner 1989; Calver et al. 1990; Henson 2002). These are generally of a lower grade than the above-mentioned complexes. The best previous estimates for the peak metamorphism in these blocks are shown on Figure 2. These estimates show metamorphism mostly at greenschist and/or marginal blueschist-facies conditions. However, all currently known examples of blueschist-faces assemblages are overprinted by greenschist-facies mineralogy (Everard 1999; Turner & Bottrill 2001; Reed et al. 2002; Holm et al. 2003). Amphibolite-grade assemblages are found in the Forth Metamorphic Complex but have suffered extensive low temperature alteration; it was not possible to improve upon published estimates for their metamorphic history with the samples that have been collected from this complex. The Cameca SX100 electron probe microanalyser at the Central Science Laboratory of the University of Tasmania, equipped with five wavelength dispersive spectro- meters, was used to acquire quantitative analyses of individual mineral grains within selected rock samples. The microprobe was operated at 15 keV, 10 nA for analysis of silicate minerals. Whole-rock chemical analysis was accomplished via X-ray fluorescence (XRF) using a PANalytical (Philips) PW1480 X-ray spectrometer located at the School of Earth Sciences— CODES, University of Tasmania using procedures out- lined by Robinson (2003). Isochemical sections were calculated for samples from both the FMC and PDMC to determine the pressure–temperature conditions, which would have prevailed under the early stages of metamorphism, during the formation of the cores of the zoned garnet. These were calculated using Perple_X version 6.6.6 (Connolly & Petrini 2002, updates downloaded on 12 Dec 2011) based on XRF whole-rock compositions (e.g. Vance & Mahar 1998) in the model system MnO–Na 2 O– CaO–K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 (MnNCF MASHT) with an internally consistent thermodynamic data set (Holland & Powell 1998). The thermodynamic data file used was hp02ver.dat (version downloaded December 2011), and the solution models employed in these calculations were TiBio(HP), Chl(HP), Pheng(HP), St(HP), Ctd(HP), feldspar, GlTr TsPg, Gt(HP), IlGkPy and Omph(HP). These sections were used to estimate the conditions of formation for the early garnet cores based on intersections of isopleths corresponding to the composition of the garnet end- members. Conditions of peak metamorphism were estimated using THERMOCALC average pressure– temperature calculations (Powell et al. 1998) for suites of matrix minerals and porphyroblast rims, which exhibit equilibrium textures and are in close proximity to one another. Diffusive re-equilibration during cooling is rare in these samples and garnet boundaries showing a zoning towards lower Mg/(Mg þ Fe) on the rims were avoided for these calculations. The high-grade metapelites from the FMC commonly contain garnet porphyroblasts (up to 2 cm long), which range from euhedral (generally smaller garnets) to skeletal (larger garnets). Both expressions of garnet porphyroblasts tend to have fairly homogeneous cores and zoned rims, with a decrease in X Alm and an increase in X Prp at the rims (Figure 3). The matrix contains small grains of garnet, with compositions similar to that of the rims of the garnet ...
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... has been nearly 50 years since Alan Spry completed his PhD research focusing on the structure and petrology of Tasmania’s Precambrian rocks (Spry 1962). In the intervening years the technology avail- able for metamorphic studies has expanded dramati- cally. There have been a few subsequent studies of the metamorphic history of regions within Tasmania (e.g. Goscombe 1990; Everard 1999; Turner & Bottrill 2001; Holm 2002; Reed et al. 2002) but there had been no attempt to review the metamorphic history of Tasmania as a whole; therefore this study was initiated to review all the existing data, gather new data, and bring them together into a modern tectonic format. Modern geothermobarometry can be used both to constrain the peak metamorphic conditions via conven- tional methods and to determine the conditions under which the earliest growth of zoned phases must have initiated via isochemical section modelling (Tinkham & Ghent 2005; Powell & Holland 2008). The combination of these techniques with geochronological data links Tasmania’s Cambrian metamorphic history to its tectonic history. The Cambrian regional metamorphic rocks of Tasmania are concentrated in the western third of the state in a series of fault-bounded metamorphic complexes, which lie in a belt from Ulverstone on the northwest coast to Port Davey on the southwest coast (Figure 1). Previously published pressure–temperature estimates for Tasmanian metamorphic complexes range from low greenschist faces, through blueschist, epidote–amphibolite, amphibolite and eclogite facies (Figure 2). The largest of these fault-bounded units include the Franklin Metamorphic Complex (FMC), and the Port Davey Metamorphic Complex (PDMC). The timing of the Cambrian metamorphism as determined via chemical U–Th–Pb monazite dating (515–505 Ma) was addressed by Berry et al. (2007) and further refined by Chmielowski (2009). The intensely faulted FMC is located in central Tasmania (Figure 1). It contains metapelites, eclogites and whiteschist (Spry 1962; Ra ̊ heim 1976; Kamperman 1984; Goscombe 1990). These are the highest-grade regional metamorphic rocks in the state, and snowball garnet porphyroblasts from the FMC have been featured in studies of crystallisation and deformation (Spry 1963; Bell et al. 1986). The metapelites of the FMC are a focus of this project. The FMC eclogites, which appear as fault-bounded lenses within the metapelites, were the focus of a companion study to this project; they yielded estimates of * 500–600 8 C; 0.55–0.7 GPa for an early stage of metamorphism (stage II) and peak metamorphic conditions of * 600–650 8 C; 4 1.5 GPa (stage III) (Palmeri et al. 2009). The FMC whiteschist is located in a separate fault-bounded block within the complex, than the metapelites analysed for this study (see Chmielowski 2009 for detailed maps of these blocks). The FMC also includes lower grade metapelitic rocks within other fault-bounded blocks (Gee 1962; Williams 1971). The PDMC is exposed on the southwest coast of Tasmania (Figure 1); it contains pelitic schist, with amphibolite and garnet–amphibolite lenses (Meffre et al. 2000). McNeill (1985) reported amphibolite-facies conditions for the garnet-bearing schists from the Nye Bay region of the PDMC and noted that the Ca and Mn zonation in the garnet crystals imply two different sets of growing conditions. The PDMC medium-grade metapelites are the second focus of this project. In addition to the larger metamorphic complexes mentioned above, smaller fault-bounded packages of metamorphic rocks are also found in Tasmania (e.g. Green 1959; Burns 1964; Gee & Legge 1979; Turner 1989; Calver et al. 1990; Henson 2002). These are generally of a lower grade than the above-mentioned complexes. The best previous estimates for the peak metamorphism in these blocks are shown on Figure 2. These estimates show metamorphism mostly at greenschist and/or marginal blueschist-facies conditions. However, all currently known examples of blueschist-faces assemblages are overprinted by greenschist-facies mineralogy (Everard 1999; Turner & Bottrill 2001; Reed et al. 2002; Holm et al. 2003). Amphibolite-grade assemblages are found in the Forth Metamorphic Complex but have suffered extensive low temperature alteration; it was not possible to improve upon published estimates for their metamorphic history with the samples that have been collected from this complex. The Cameca SX100 electron probe microanalyser at the Central Science Laboratory of the University of Tasmania, equipped with five wavelength dispersive spectro- meters, was used to acquire quantitative analyses of individual mineral grains within selected rock samples. The microprobe was operated at 15 keV, 10 nA for analysis of silicate minerals. Whole-rock chemical analysis was accomplished via X-ray fluorescence (XRF) using a PANalytical (Philips) PW1480 X-ray spectrometer located at the School of Earth Sciences— CODES, University of Tasmania using procedures out- lined by Robinson (2003). Isochemical sections were calculated for samples from both the FMC and PDMC to determine the pressure–temperature conditions, which would have prevailed under the early stages of metamorphism, during the formation of the cores of the zoned garnet. These were calculated using Perple_X version 6.6.6 (Connolly & Petrini 2002, updates downloaded on 12 Dec 2011) based on XRF whole-rock compositions (e.g. Vance & Mahar 1998) in the model system MnO–Na 2 O– CaO–K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 (MnNCF MASHT) with an internally consistent thermodynamic data set (Holland & Powell 1998). The thermodynamic data file used was hp02ver.dat (version downloaded December 2011), and the solution models employed in these calculations were TiBio(HP), Chl(HP), Pheng(HP), St(HP), Ctd(HP), feldspar, GlTr TsPg, Gt(HP), IlGkPy and Omph(HP). These sections were used to estimate the conditions of formation for the early garnet cores based on intersections of isopleths corresponding to the composition of the garnet end- members. Conditions of peak metamorphism were estimated using THERMOCALC average pressure– temperature calculations (Powell et al. 1998) for suites of matrix minerals and porphyroblast rims, which exhibit equilibrium textures and are in close proximity to one another. Diffusive re-equilibration during cooling is rare ...
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... has been nearly 50 years since Alan Spry completed his PhD research focusing on the structure and petrology of Tasmania’s Precambrian rocks (Spry 1962). In the intervening years the technology avail- able for metamorphic studies has expanded dramati- cally. There have been a few subsequent studies of the metamorphic history of regions within Tasmania (e.g. Goscombe 1990; Everard 1999; Turner & Bottrill 2001; Holm 2002; Reed et al. 2002) but there had been no attempt to review the metamorphic history of Tasmania as a whole; therefore this study was initiated to review all the existing data, gather new data, and bring them together into a modern tectonic format. Modern geothermobarometry can be used both to constrain the peak metamorphic conditions via conven- tional methods and to determine the conditions under which the earliest growth of zoned phases must have initiated via isochemical section modelling (Tinkham & Ghent 2005; Powell & Holland 2008). The combination of these techniques with geochronological data links Tasmania’s Cambrian metamorphic history to its tectonic history. The Cambrian regional metamorphic rocks of Tasmania are concentrated in the western third of the state in a series of fault-bounded metamorphic complexes, which lie in a belt from Ulverstone on the northwest coast to Port Davey on the southwest coast (Figure 1). Previously published pressure–temperature estimates for Tasmanian metamorphic complexes range from low greenschist faces, through blueschist, epidote–amphibolite, amphibolite and eclogite facies (Figure 2). The largest of these fault-bounded units include the Franklin Metamorphic Complex (FMC), and the Port Davey Metamorphic Complex (PDMC). The timing of the Cambrian metamorphism as determined via chemical U–Th–Pb monazite dating (515–505 Ma) was addressed by Berry et al. (2007) and further refined by Chmielowski (2009). The intensely faulted FMC is located in central Tasmania (Figure 1). It contains metapelites, eclogites and whiteschist (Spry 1962; Ra ̊ heim 1976; Kamperman 1984; Goscombe 1990). These are the highest-grade regional metamorphic rocks in the state, and snowball garnet porphyroblasts from the FMC have been featured in studies of crystallisation and deformation (Spry 1963; Bell et al. 1986). The metapelites of the FMC are a focus of this project. The FMC eclogites, which appear as fault-bounded lenses within the metapelites, were the focus of a companion study to this project; they yielded estimates of * 500–600 8 C; 0.55–0.7 GPa for an early stage of metamorphism (stage II) and peak metamorphic conditions of * 600–650 8 C; 4 1.5 GPa (stage III) (Palmeri et al. 2009). The FMC whiteschist is located in a separate fault-bounded block within the complex, than the metapelites analysed for this study (see Chmielowski 2009 for detailed maps of these blocks). The FMC also includes lower grade metapelitic rocks within other fault-bounded blocks (Gee 1962; Williams 1971). The PDMC is exposed on the southwest coast of Tasmania (Figure 1); it contains pelitic schist, with amphibolite and garnet–amphibolite lenses (Meffre et al. 2000). McNeill (1985) reported amphibolite-facies conditions for the garnet-bearing schists from the Nye Bay region of the PDMC and noted that the Ca and Mn zonation in the garnet crystals imply two different sets of growing conditions. The PDMC medium-grade metapelites are the second focus of this project. In addition to the larger metamorphic complexes mentioned above, smaller fault-bounded packages of metamorphic rocks are also found in Tasmania (e.g. Green 1959; Burns 1964; Gee & Legge 1979; Turner 1989; Calver et al. 1990; Henson 2002). These are generally of a lower grade than the above-mentioned complexes. The best previous estimates for the peak metamorphism in these blocks are shown on Figure 2. These estimates show metamorphism mostly at greenschist and/or marginal blueschist-facies conditions. However, all currently known examples of blueschist-faces assemblages are overprinted by greenschist-facies mineralogy (Everard 1999; Turner & Bottrill 2001; Reed et al. 2002; Holm et al. 2003). Amphibolite-grade assemblages are found in the Forth Metamorphic Complex but have suffered extensive low temperature alteration; it was not possible to improve upon published estimates for their metamorphic history with the samples that have been collected from this complex. The Cameca SX100 electron probe microanalyser at the Central Science Laboratory of the University of Tasmania, equipped with five wavelength dispersive spectro- meters, was used to acquire quantitative analyses of individual mineral grains within selected rock samples. The microprobe was operated at 15 keV, 10 nA for analysis of silicate minerals. Whole-rock chemical analysis was accomplished via X-ray fluorescence (XRF) using a PANalytical (Philips) PW1480 X-ray spectrometer located at the School of Earth Sciences— CODES, University of Tasmania using procedures out- lined by Robinson (2003). Isochemical sections were calculated for samples from both the FMC and PDMC to determine the pressure–temperature conditions, which would have prevailed under the early stages of metamorphism, during the formation of the cores of the zoned garnet. These were calculated using Perple_X version 6.6.6 (Connolly & Petrini 2002, updates downloaded on 12 Dec 2011) based on XRF whole-rock compositions (e.g. Vance & Mahar 1998) in the model system MnO–Na 2 O– CaO–K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 (MnNCF MASHT) with an internally consistent thermodynamic data set (Holland & Powell 1998). The thermodynamic data file used was hp02ver.dat (version downloaded December 2011), and the solution models employed in these calculations were TiBio(HP), Chl(HP), Pheng(HP), St(HP), Ctd(HP), feldspar, GlTr TsPg, Gt(HP), IlGkPy and Omph(HP). These sections were used to estimate the conditions of formation for the early garnet cores based on intersections of isopleths corresponding to the composition of the garnet end- members. Conditions of peak metamorphism were estimated using THERMOCALC average pressure– temperature calculations (Powell et al. 1998) for suites of matrix minerals and porphyroblast rims, which exhibit equilibrium textures and are in close proximity to one another. Diffusive re-equilibration during cooling is rare in these samples and garnet boundaries showing a zoning towards lower Mg/(Mg þ Fe) on the rims were avoided for these calculations. The high-grade metapelites from the FMC commonly contain garnet porphyroblasts (up to 2 cm long), which range from euhedral (generally smaller garnets) to skeletal (larger garnets). Both expressions of garnet porphyroblasts tend to have fairly homogeneous cores and zoned rims, with a decrease in X Alm and an increase in X Prp at the rims (Figure 3). The matrix contains small grains of garnet, with compositions similar to that of the rims of the garnet porphyroblasts, quartz and white mica + plagioclase, biotite, tourmaline and kyanite. Both monazite and rutile are common accessory minerals. Some samples exhibit late-stage chlorite alteration, particularly of the biotite and garnet grains. White mica, which is ubiquitous throughout the FMC metapelites, ranges from 0.02 to 0.26 Na/Na þ K. Biotite is present as a minor phase ( 5 5 vol%) in most, but not all, of the FMC metapelite samples. It ranges in composition from 0.33 to 0.51 Mg/(Mg þ Fe) (Figure 4). Plagioclase is typically a major phase (20–30 vol%) in the FMC metapelites but is occasionally absent or present as a minor phase ( 5 5 vol%). When present as a major phase, the plagioclase sometimes occurs as porphyroblasts with curved inclusion trails. The plagioclase is usually nearly pure albite, irrespective of the physical expres- sion of the grains or whether it is a major or minor phase. Most samples have plagioclase with less than 0.10 X An (commonly less than 0.05 X An ). However, one sample (160717, collected from 406129E, 5335750N) was found to contain ...
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... Recent tectonic models for the Cambrian Tyennan Orogeny incorporating structural, petrological, geochemical and metamorphic datasets outline a scenario whereby the eastern margin of the Tasmanian microcontinent was subducted beneath oceanic lithosphere, with the products of highpressure metamorphism exhumed within~5 Myr (e.g. Meffre et al., 2000;Palmeri et al., 2009;Cayley, 2011;Chmielowski and Berry, 2012;Mulder et al., 2016). ...
... The middle Cambrian Tyennan Orogeny involved metamorphism and deformation of the Proterozoic strata in western Tasmania and resulted in the exposure of a series of high-grade metamorphic complexes distributed throughout western Tasmania between the Rocky Cape Group in the north-west and the Tyennan Region in central-west region ( Fig. 1; Meffre et al., 2000;Turner and Bottrill, 2001;Foster et al., 2005;Berry et al., 2007;Chmielowski and Berry, 2012). While many of the complexes record greenschist-to amphibolite-facies metamorphism, local occurrences of blueschist-facies rocks and eclogite-facies rocks are documented in the Arthur Lineament and the Franklin Metamorphic Complex, respectively ( Fig. 1; Spry, 1963;Råheim, 1976;Turner and Bottrill, 2001). ...
... Recent geochronology on the eclogites has yielded zircon SHRIMP U-Pb ages of 502 ± 8 Ma, 511 ± 8 Ma and 504 ± 7 Ma (Black et al., 1997;Turner et al., 1998;Fergusson et al., 2013). Geochronology on the host metapelitic schists has returned similar ages to the eclogites, with a biotite K-Ar age of 515 Ma (Adams et al., 1985), and monazite U-Pb-Th ages of 521 ± 11 Ma and 512 ± 4 Ma (Berry et al., 2007;Chmielowski and Berry, 2012). The suite of ages for the Franklin Metamorphic Complex eclogites and their host metapelites have been used to constrain the duration of Tyennan orogenesis from c. 515-505 Ma, and the similarity in ages has given rise to the suggestion that both the mafic eclogites and the metapelitic schists were subducted to similar P-T conditions. ...
Evidence for high-pressure metamorphism along the Cambro-Ordovician East Gondwana margin is limited to occurrences of eclogite-facies rocks in the central Transantarctic Mountains, northern Victoria Land, western Tasmania, and eastern Australia. The western Tasmanian eclogites in the Franklin Metamorphic Complex are enclosed by high-grade metapelitic lithologies and contain a high-pressure mineral assemblage of garnet + omphacitic clinopyroxene + pargasitic amphibole + phengite + quartz + rutile + clinozoisite. Mineral relationships and compositions in the eclogites, combined with mineral equilibria forward modelling, indicate the eastern margin of Tasmania was subducted along a relatively cold thermal gradient to pressures between 18 and 21.5 kbar at temperatures of 650–700 °C. LA–ICP–MS U-Pb analysis of zircon yields middle Cambrian ages, consistent with previous geochronology, as well as older Mesoproterozoic ages. Rare earth element concentrations in Cambrian-aged zircon suggest they formed at high pressures whereas Mesoproterozoic-aged zircon formed at lower pressures. Garnet preserves elemental concentrations consistent with prograde growth, with middle and heavy rare earth element concentrations resolving a complex growth history involving the consumption of clinozoisite and apatite. In-situ rutile LA–ICP–MS U-Pb ages are early to middle Cambrian and indicative of prograde to peak growth during subduction, with Zr concentrations in matrix rutile yielding upper-amphibolite-facies temperatures. In this contribution, the pressure-temperature (P–T) results for the western Tasmanian eclogites revise existing conventional thermobarometric P–T data. The updated thermal gradient information can be more precisely allied with subduction zone thermal models, which provide predictions of the thermal structure of the subduction geodynamic system. Furthermore, when integrated with the mineral equilibria and P–T data from other eclogites along the margin, these results expand upon the existing interpretations of the geodynamic character of the Cambro-Ordovician East Gondwana margin.
... K16). The area also comprises the eastern zone of the Arthur Metamorphic Complex, which consists of various Proterozoic protoliths metamorphosed during the Cambrian Tyennan Orogeny [63]. ...
... 23). No direct geological indications of this intrusion have yet been identified, nor are they necessarily expected (its depth being no less than 3 km); thus, its age is undetermined but possibly The novel interpretation of an ultramafic complex linking the Heazlewood and Mount Stewart ultramafic complexes in the southwest beneath sedimentary rocks associated with the Bell Syncline implies a greater volume of ultramafic material in the Cambrian successions and points to a larger obducted component than previously thought [63] (p. K23). ...
The Heazlewood-Luina-Waratah area is a prospective region for minerals in northwest Tasmania, Australia, associated with historically important ore deposits related to the emplacement of granite intrusions and/or ultramafic complexes. The geology of the area is poorly understood due to the difficult terrain and dense vegetation. We construct an initial high-resolution 3D geological model of this area using constraints from geological maps, and geological and geophysical cross sections. This initial model is improved upon by integrating results from 3D geometry and physical property inversion of potential field (gravity and magnetic) data, petrophysical measurements, and updated field mapping. Geometry inversion reveals that the Devonian granites in the south are thicker than previously thought, possibly connecting to deep sources of mineralization. In addition, we identified gravity anomalies to the northeast that could be caused by near-surface granite cupolas. A newly discovered ultramafic complex linking the Heazlewood and Mount Stewart Ultramafic Complexes in the southwest has also been modeled. This implies a greater volume of ultramafic material in the Cambrian successions and points to a larger obducted component than previously thought. The newly inferred granite cupolas and ultramafic complexes are targets for future mineral exploration. Petrophysical property inversion reveals a high degree of variation in these properties within the ultramafic complexes indicating a variable degree of serpentinization. Sensitivity tests suggest maximum depths of 2-3 km for the contact aureole that surrounds major granitic intrusions in the southeast, while the Heazlewood River complex is likely to have a deeper source up to 4 km. Our case study illustrates the value of adding geological and petrophysical constraints to 3D modeling for the purpose of guiding mineral exploration. This is particularly important for the refinement of geological structure in tectonically complex areas that have lithology units with contrasting magnetic and density characteristics.
... The most contentious issue is the relationship between the Neoproterozoic rifted margins of western Laurentia and eastern Australia-Antarctica. Although the rifted margins of Laurentia and Australia-Antarctica are of comparable length, record a similar history of punctuated Neoproterozoic rifting, and have both faced the Pacific Ocean since the earliest Paleozoic ( Bond et al., 1984Bond et al., , 1985Young, 1992;Preiss, 2000;Cawood, 2005;Dickinson, 2004), there is no consensus on whether they were direct conjugates (Moores, 1991;Dalziel, 1991;Young, 1992), were significantly offset ( Wingate et al., 2002;Eyster et al., 2019), or were separated by additional continental blocks ( Li et al., 1995;Sears and Price, 2003;Colpron et al. 2002;Li and Evans, 2011; Wen et of a subducted continental margin (Turner and Bottrill, 2001;Berry et al., 2007;Chmielowski and Berry, 2012;Mulder et al., 2015b). ...
The Tonian to Ediacaran geology of Tasmania, Australia preserves an extensive record of continental rifting related to the Neoproterozoic opening of the Pacific Ocean. We integrate new and previously published structural, stratigraphic, sedimentary provenance, age, and geochemical data to establish four tectonostratigraphic stages in Tasmania that formed during three episodes of Neoproterozoic rifting. Rift Event 1 initiated with Tonian (780—750 Ma) intraplate magmatism and clastic sedimentation and was followed by deposition of an eastward thickening and deepening succession of ≤ 780 Ma—730 Ma locally sourced siliciclastic strata and carbonate (tectonostratigraphic stage 1). Cryogenian glaciogenic strata and thick successions of mafic volcaniclastic turbidites (tectonostratigraphic stage 2) contain unimodal 670—640 Ma detrital zircon age populations and may record a second rift event (Rift Event 2). Final rifting (Rift Event 3) involved voluminous ca. 580 Ma basaltic volcanism and active extensional faulting (tectonostratigraphic stage 3) and was followed by latest Ediacaran to early Cambrian sag-phase deposition (tectonostratigraphic stage 4). Neoproterozoic rifting in Tasmania is broadly contemporaneous with punctuated Tonian to Ediacaran rifting along the paleo-margins of the Pacific Ocean in southeast Australia, East Antarctica, and western Laurentia. Rift Event 1 in Tasmania may record the rifting of Australia-Antarctica from western Laurentia to form the nascent Pacific Ocean. Geological correlations are consistent with models in which Tasmania remained attached to either eastern Australia-Antarctica or western Laurentia during the late Tonian and Cryogenian before being isolated as a microcontinent in the late Ediacaran during Rift Event 3.
... In Tasmania, the velocity 'inversion' between the shallow and deep responses of eastern and western Tasmania may be explained by their composition. The Rocky Cape, Burnie and Tyennan Group rocks of the west Tasmania terrane are thought to be underlain by metasedimentary rocks (Black et al. 2004;Black et al. 2010;Chmielowski & Berry 2012;Moore et al. 2016) whereas the east Tasmania terrane may be underlain with basaltic volcanic rocks (Moore et al. 2016). Moreover, basaltic volcanic rocks are present in the Smithton Basin, the upper sequence of western Tasmania, in the area that corresponds to the highest velocities in the west (see Fig. 8h). ...
Debate is ongoing as to which tectonic model is most consistent with the known geology of southeast Australia, formerly part of the eastern margin of Gondwana. In particular, numerous tectonic models have been proposed to explain the enigmatic geological relationship between Tasmania and the mainland, which is separated by Bass Strait. One of the primary reasons for the lack of certainty is the limited exposure of basement rocks, which are masked by the sea and thick Mesozoic-Cenozoic sedimentary and volcanic cover sequences. We use ambient noise tomography recorded across Bass Strait to generate a new shear wave velocity model in order to investigate crustal structure. Fundamental mode Rayleigh wave phase velocity dispersion data extracted from long-term cross-correlation of ambient noise data are inverted using a transdimensional, hierarchical, Bayesian inversion scheme to produce phase velocity maps in the period range 2-30 s. Subsequent inversion for depth-dependent shear wave velocity structure across a dense grid of points allows a composite 3-D shear wave velocity model to be produced. Benefits of the transdimensional scheme include a data-driven parametrization that allows the number and distribution of velocity unknowns to vary, and the data noise to also be treated as an unknown in the inversion. The new shear wave velocity model clearly reveals the primary sedimentary basins in Bass Strait as slow shear velocity zones which extend down to 14 km in depth. These failed rift basins, which formed during the early stages of Australia-Antarctica break-up, appear to be overlying thinned crust, where high velocities of 3.8-4.0 km s ⁻¹ occur at depths greater than 20 km. Along the northern margin of Bass Strait, our new model is consistent with major tectonic boundaries mapped at the surface. In particular, we identify an east dipping velocity transition zone in the vicinity of the Moyston Fault, a major tectonic boundary between the Lachlan and Delamerian orogens, which are part of the Phanerozoic accretionary terrane that makes up eastern Australia. A pronounced lineament of high shear wave velocities (∼3.7-3.8 km s ⁻¹ ) in the lower crust of our new model may represent the signature of relict intrusive magmatism from failed rifting in the early stages of Australia-Antarctica break-up along a crustal scale discontinuity in the Selwyn Block microcontinent which joins Tasmania and Victoria. © The Author(s) 2019. Published by Oxford University Press on behalf of The Royal Astronomical Society.
... Ma mafic volcaniclastic sandstone, shale and rift-related basalt (Crimson Creek Formation). Cambrian mafic-ultramafic rocks (Vicary 2004) and Upper Cambrian Mount Read Volcanics (Mortensen et al. 2015) often overlie the Burnie Zone; (4 and 5) the Pedder Zone and Tyennan Zone, which both comprise similar geology to the Rocky Cape Zone, but are separated by eclogite facies metamorphic rocks (Chmielowski and Berry 2012). Both are overlain by minor amounts of Mount Read Volcanics; (6) the Sorell-Badger Head Zone, which comprises turbidites similar to those in the Burnie Zone and, in the west, ?Ediacaran melange, chert, volcaniclastic sandstone, black shale, rift-related dolerite and dolostone (Calver and Reed 2001); and (7) the Glomar Zone, which is The box over southern Australia shows the location of Fig.1 a submarine plateau to the south of Tasmania, and includes ca. ...
Using airborne magnetic and marine gravity data, the geological subdivisions of western Tasmania have been interpreted north across Bass Strait into Victoria. The three westernmost Tasmanian zones, the King Island, Rocky Cape and Burnie zones, are inferred to form the largely concealed Selwyn Block in Victoria. The Eastern Tasmania Zone correlates with the Victorian Tabberabbera Zone. Thus the Tasmanian Tamar Fracture Zone corresponds with the Victorian Governor Fault. The Victorian Ceres Gabbro is correlated with magnetic rocks west of King Island that are tentatively considered to be Neoproterozoic. Most of the Cambrian felsic volcanic rocks of the Tasmanian Mount Read Volcanics lie above the Burnie Zone, as do the similar rocks exposed in the Jamieson, Licola and Glen Creek windows in central Victoria. Reinterpretation of a Victorian deep seismic reflection line indicates Burnie Zone equivalent rocks were thrust south-west over Rocky Cape Zone equivalents.
... During the early stages of the Tyennan Orogeny one or more extensive slices of maficultramafic crust and mantle comprising the western Tasmanian ophiolite and mélanges containing allochthonous deep water sedimentary sequences and basaltic rocks, were emplaced over the passive margin. Synchronous with ophiolite emplacement, subduction of the leading edge of the Tasmanian microcontinent resulted in highpressure, greenschist-eclogite facies metamorphism at ca. 510 Ma (Berry et al., 2007;Chmielowski and Berry, 2012). These highpressure, medium-grade metamorphic rocks are now distributed throughout the structurally complex Tyennan Region (Fig. 1). ...
... The closing stages of the Tyennan Orogeny are characterised by rapid exhumation of the high-pressure metamorphic rocks at ca. 505 Ma (Turner and Bottrill, 2001;Foster et al., 2005) as recorded by structures with dominantly east-verging transport vectors and post-collision volcanism, possibly in response to slab-break off (Chmielowski and Berry, 2012). A final phase of east-verging, upright folding may record the accretion of the Tasmanian microcontinent onto the margin of East Gondwana at ca. 495 Ma (Holm and Berry, 2002;Berry et al., 2007;Cayley, 2011). ...
... Berry et al., 2008;Cayley, 2011;Halpin et al., 2014;Mulder et al., 2015a). Highpressure metamorphism recorded in the medium-grade metamorphic complexes of Tasmania at 510 Ma (Fig. 1, Berry et al., 2007) likely reflects burial of the Tasmanian microcontinent as it was dragged into the east-dipping subduction zone (Fig. 10c, Chmielowski and Berry, 2012;Mulder et al., 2015b). Blocking of the east-dipping subduction zone may have caused convergence to be taken up west of Tasmania in the form of either a west-dipping subduction zone or a fold-andthrust belt (Fig. 10c). ...
The Cambrian Ross–Delamerian Orogeny records the first phase of accretional tectonics along the eastern margin of Gondwana following breakup of the supercontinent Rodinia. Western Tasmania represents a key area for understanding the Cambrian tectonic setting of the eastern margin of Gondwana as it is one of the few places where a Tethyan-type ophiolite is preserved and contains the only known exposures of a sub-ophiolitic metamorphic sole associated with the Ross–Delamerian Orogen. This paper presents an integrated study of the field, petrographic, geochemical, and metamorphic characteristics of the metamorphic sole to the western Tasmanian ophiolite. The structurally highest levels of the metamorphic sole consist of granulite–upper amphibolite facies metacumulates and metagabbros. A transition to amphibolite and epidote–amphibolite facies conditions is recorded by metadolerites and metabasalts towards the base of the metamorphic sole. Kinematic indicators in mylonitic amphibolites suggest the metamorphic sole formed in an east-dipping subduction zone located to the east of the Proterozoic continental crust of Tasmania. Major and trace element whole rock and relict igneous spinel geochemistry indicates that the protoliths to the metamorphic sole formed at a back arc basin spreading centre. Our new data supports a model in which east-dipping subduction in Tasmania was driven by collapse of a back arc basin developed above an earlier west-dipping subduction zone outboard of the eastern margin of Gondwana. The proposed model may help to resolve a controversy related to apparent along-strike variations in subduction zone polarity during the Ross-Delamerian Orogeny and suggests a complex geodynamic setting had developed along the eastern margin of Gondwana by the Middle Cambrian. This study highlights the importance of considering the role of multiple subduction zones in generating metamorphic soles and emplacing ophiolites, which are key events associated with the construction of many orogenic belts worldwide.
... As presently understood, the medium-grade rocks typically occur in fault-bound metamorphic complexes that appear to be concentrated along the western and northern margins of the Tyennan Region (Figure 1; Meffre et al. 2000;Chmielowski & Berry 2012). This observation led Moore et al. (2013Moore et al. ( , 2015 to suggest that the medium-grade complexes mark discrete suture zones between the Tyennan Region and adjacent blocks that were assembled during the Cambrian. ...
... The alternative view is that the medium-grade rocks are randomly distributed, fault-bound slices representing the exhumed remnants of a passive margin that was deeply subducted during an arcÀcontinent collision in the Cambrian (e.g. Meffre et al. 2000;Chmielowski & Berry 2012). ...
... Cawood 2005;Boger 2011). The Tyennan Orogeny involved medium-grade, high-pressure metamorphism of the passive margin, emplacement of allochthonous sequences, including supraÀsubduction zone ophiolites, and post-collisional volcanism (Berry & Crawford 1988;Crawford & Berry 1992;Turner et al. 1998;Meffre et al. 2000;Chmielowski & Berry 2012). The Tyennan Orogeny has generally been interpreted in the context of an arcÀcontinent collision during which the Neoproterozoic passive margin was deeply buried in an east-dipping subduction zone beneath an oceanic island arc (e.g. ...
This study presents new data on the deformational and metamorphic history of previously unstudied Cambrian high-pressure metamorphic rocks exposed on the remote south coast of Tasmania. The Red Point Metamorphic Complex consists of two blocks of high-pressure, medium-grade metamorphic rocks including pelitic schist and minor garnet-bearing amphibolite, which are faulted against a sequence of low-grade phyllite and quartzite. The Red Point Metamorphic Complex records five phases of deformation, all of which except the first are expressed at a mesoscopic scale in both the medium- and low-grade rocks. Peak metamorphic conditions in the high-pressure epidote–amphibolite facies is recorded by medium-grade schist and amphibolite and was synchronous with the second major deformation event, which produced a pervasive schistosity and mesoscale isoclinal folds. The juxtaposition of the low- and medium-grade rocks is interpreted to have first occurred prior to the development of upright, opening folding associated with the third deformation. However, the present contacts between the two contrasting metamorphic sequences formed during widespread faulting and ductile-shear zone development associated with the fourth and fifth deformation events. The new data from the Red Point Metamorphic Complex provide insights into the structural and metamorphic history experienced by the medium-grade rocks of Tasmania during the Cambrian Tyennan Orogeny. This study demonstrates that Cambrian medium-grade metamorphic rocks are more widespread throughout Tasmania than previously realised, which represents an important step towards understanding the large-scale architecture of the Tyennan Orogen.
