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Polymetamorphic metapelitic rocks in central‐west Tasmania, southern Australia, contain high‐pressure mineral assemblages that formed during Cambrian‐aged subduction and relict garnet with published Lu–Hf ages of c. 1285–1240 Ma. These garnet ages, along with published detrital zircon data from throughout western Tasmania and western North America, have been used to propose the presence of Mesoproterozoic Laurentian crust in western Tasmania. In this study, we combine zircon petrochronology with compositional information from the inclusion assemblages in relict garnet to extract Mesoproterozoic pressure–temperature data from subduction‐overprinted rocks, which effectively constitute an interpreted remnant of Laurentian crust now residing in central‐west Tasmania. The new data suggest Mesoproterozoic metamorphism involved two stages. The first event is recorded by c. 1480–1235 Ma zircon that formed in a garnet‐absent, plagioclase‐present, high‐thermal gradient environment at pressures no greater than ~5–5.5 kbar. The second event recorded by c. 1285–1240 Ma relict garnet was characterized by the development of a moderate‐pressure kyanite–plagioclase–biotite‐bearing mineral assemblage, which formed at ~8.5 kbar and ~590–680°C. These pressure–temperature constraints are attributed to extension within a deep basin system associated with the cryptic East Kootenay Orogeny in North America, which coincides with the final stages of c. 1450–1370 Ma upper Belt‐Purcell Basin sedimentation. Taking into account new detrital zircon U–Pb–Hf isotopic data from central‐west Tasmania in this study and existing zircon provenance data from throughout western Tasmania and the Belt‐Purcell Basin, our results strengthen the hypothesis of a Laurentian footprint that potentially encompasses much of western Tasmania and relates to both Nuna and Rodinian tectonism. [ABSTRACT FROM AUTHOR]

Garnet is a common rock‐forming mineral that occurs in a variety of rock types and over a wide range of pressure (P)–temperature (T) conditions in the Earth's lithosphere. Because garnet is considered a high‐strength mineral stable across an extensive range of conditions (1–25 GPa, <300–2000°C), it is generally accepted that garnets can retain their microstructures and chemical composition during deformation and metamorphism. Therefore, garnet is commonly used as a geothermobarometer and geochronometer to provide P–T and timing constraints on tectonic events. Herein, we study garnet from an eclogite facies mylonite (central Australia) to investigate the mechanisms of element mobility during high‐strain deformation under relatively dry, lower crustal conditions. Electron backscatter diffraction (EBSD) and electron channelling contrast imaging (ECCI) reveal evidence of crystal plasticity associated with brittle deformation in the form of heterogeneous misorientation patterns and low‐angle grain boundaries developed over length scales of 20–50 μm in the rims of garnet porphyroclasts. Atom probe tomography (APT) analysis of a low‐angle grain boundary within a highly strained portion of a clast shows Ca enrichment and Mg depletion along dislocations, whereas APT data along the rim of a mostly undeformed clast reveal a homogeneous distribution of garnet major components in the specimen matrix with the exception of Ca, Fe and Mg enrichment within a healed microfracture. The above‐mentioned results suggest that under relatively dry conditions, crystal plasticity enhances bulk element mobility via pipe diffusion, highlighting the importance of deformation‐induced microstructures on element mobility, with important implications for the robust and reliable use of garnet as a petrological tool. [ABSTRACT FROM AUTHOR]

Since 2007, at the Electron Science Research Institute (ESRI) nano-fabrication laboratories, Edith Cowan University, Australia, we have devoted research efforts to the synthesis and characterization of bismuth-containing ferrite-garnet-type thin-film magneto-optic (MO) materials of different compositions. We report on the growth and characteristics of radio frequency (RF) magnetron sputtered bismuth-substituted iron-garnet thin films. We study the process parameters associated with the RF magnetron sputter deposition technique and investigate the results of optimizing process parameters. To achieve the best MO properties, we employ a few unique techniques, such as co-sputtered nanocomposite films and all-garnet multilayer structures, as well as the application of oxygen plasma treatment to amorphous garnet layers immediately following the deposition process. We demonstrated a remarkable enhancement in the MO properties of Bi-containing ferrite-type garnet thin-film materials, including record-high MO figures of merit and improved conventional and unconventional hysteresis loops of Faraday rotation. Previously unpublished research results on the forward-looking applications of magnetic garnet coatings applied to microparticles of advanced luminescent materials are reported. In the context of developing the next-generation ultra-fast optoelectronic devices, such as light intensity switches and modulators, high-speed flat panel displays, and high-sensitivity sensors, it is important to consider the desirable optical, magnetic, and magneto-optic properties that are found in highly bismuth-substituted iron garnet thin-film materials of various composition types. [ABSTRACT FROM AUTHOR]

Garnet is a fundamental expression of metamorphism and one of the most important minerals used to constrain the thermal conditions of the crust. We used innovative in situ laser-ablation ICP-MS/MS Lu-Hf geochronology to demonstrate that garnet in metapelitic rocks enclosing Cambrian eclogite in southern Australia formed during Laurentian Mesoproterozoic metamorphism. Garnet porphyroblasts in amphibolite-facies metapelitic rocks yielded Lu-Hf ages between 1286 ± 58 Ma and 1241 ± 16 Ma, revealing a record of older metamorphism that was partially obscured by metamorphic overprinting during ca. 510 Ma Cambrian subduction along the East Gondwana margin. Existing detrital zircon age data indicate the protoliths to the southern Australian metapelitic rocks were sourced from western Laurentia. We propose that the metapelitic rocks of southern Australia represent a fragment of western Laurentian crust, which was separated from Laurentia in the Neoproterozoic and incorporated into the East Gondwana subduction system during the Cambrian. The ability to obtain Lu-Hf isotopic data from garnet at acquisition rates comparable to those for U-Pb analysis of detrital zircon means, for the first time, the metamorphic parentage of rocks as expressed by garnet can be efficiently accessed to assist paleogeographic reconstructions. [ABSTRACT FROM AUTHOR]

The mechanisms of earthquake rupture in lower continental crust, below the usual frictional‐viscous transition, remain uncertain. In addressing this problem, the study of pseudotachylyte (quenched frictional melt produced during seismic fault slip) and related structures from deeply exhumed rocks can provide direct observational constraints. A felsic granulite from the Musgrave Ranges (central Australia) exceptionally preserves pristine microstructures spatially related to a pseudotachylyte. This sample remained dry, without introduction of hydrous fluids, during pseudotachylyte development and subsequent exhumation. It was therefore unaffected by alteration and metamorphic re‐equilibration. Fractures in the damage zone developed asymmetrically to either side of the pseudotachylyte and are marked by new, randomly oriented quartz, feldspar, and garnet grains. Pulverization of garnet occurred locally between intersecting fractures with powder injected into dilatant fractures in a quartz inclusion within the garnet host. Injection of pulverized material can explain the growth of new, compositionally different minerals (quartz, feldspar, kyanite, ilmenite, magnetite, and rutile) along dilatant fractures developed in a short‐lived seismic event. The pseudotachylyte contains only clasts of quartz, suggesting an unusually high melting temperature. The sequentially developed microstructures provide a time‐lapse record of thermomechanical processes during a single earthquake event, including initial rupture propagation with associated off‐fault damage and local pulverization during very dynamic fluctuations in the local stress field; frictional heating and eventual melting during fault slip; flow and injection of melt; and rapid solidification (quenching) and crystallization of new minerals. This occurred under lower continental crustal conditions of ca. 650°C and 1.2 GPa about 550 Myrs ago. Plain Language Summary: Some special rocks (pseudotachylytes: quenched melts produced by frictional heating during seismic slip on a fault) exhumed from deep in the Earth's crust provide the opportunity to study old earthquakes that occurred at depths that are not directly observable. The pseudotachylyte we studied from the Musgrave Ranges in central Australia was formed during a single earthquake some 550 million years ago at depths of ca. 40 km and temperatures of ca. 650°C, but remained remarkably well preserved on its path to the Earth's surface. As a result, the sample preserves a complete time‐lapse record of the sequence of microstructures representative of different thermal and mechanical processes, which lasted only seconds to minutes during a seismic event, including initial fracture propagation, off‐fault damage with local pulverization, and development, flow, and final solidification of frictional melt. Key Points: Time sequence of microstructures related to pseudotachylyte formation in the lower continental crust (Musgrave Ranges, central Australia)Dry conditions with no infiltration of water‐rich fluid before, during, or after seismic rupturing and pseudotachylyte developmentAsymmetric fracturing, local garnet pulverization, and high‐temperature frictional melting represent different stages of a single earthquake rupture [ABSTRACT FROM AUTHOR]

Accessory mineral thermometry and thermodynamic modelling are fundamental tools for constraining petrogenetic models of granite magmatism. U–Pb geochronology on zircon and monazite from S-type granites emplaced within a semi-continuous, whole-crust section in the Georgetown Inlier (GTI), NE Australia, indicates synchronous crystallisation at 1550 Ma. Zircon saturation temperature (Tzr) and titanium-in-zircon thermometry (T(Ti–zr)) estimate magma temperatures of ~ 795 ± 41 °C (Tzr) and ~ 845 ± 46 °C (T(Ti-zr)) in the deep crust, ~ 735 ± 30 °C (Tzr) and ~ 785 ± 30 °C (T(Ti-zr)) in the middle crust, and ~ 796 ± 45 °C (Tzr) and ~ 850 ± 40 °C (T(Ti-zr)) in the upper crust. The differing averages reflect ambient temperature conditions (Tzr) within the magma chamber, whereas the higher T(Ti-zr) values represent peak conditions of hotter melt injections. Assuming thermal equilibrium through the crust and adiabatic ascent, shallower magmas contained 4 wt% H2O, whereas deeper melts contained 7 wt% H2O. Using these H2O contents, monazite saturation temperature (Tmz) estimates agree with Tzr values. Thermodynamic modelling indicates that plagioclase, garnet and biotite were restitic phases, and that compositional variation in the GTI suites resulted from entrainment of these minerals in silicic (74–76 wt% SiO2) melts. At inferred emplacement P–T conditions of 5 kbar and 730 °C, additional H2O is required to produce sufficient melt with compositions similar to the GTI granites. Drier and hotter magmas required additional heat to raise adiabatically to upper-crustal levels. S-type granites are low-T mushes of melt and residual phases that stall and equilibrate in the middle crust, suggesting that discussions on the unreliability of zircon-based thermometers should be modulated. [ABSTRACT FROM AUTHOR]

Lawsonite eclogite and garnet blueschist occur as metre‐scale blocks within serpentinite mélange in the southern New England Orogen (SNEO) in eastern Australia. These high‐P fragments are the products of early Palaeozoic subduction of the palaeo‐Pacific plate beneath East Gondwana. Lu–Hf, Sm–Nd, and U–Pb geochronological data from Port Macquarie show that eclogite mineral assemblages formed between c. 500 and 470 Ma ago and became mixed together within a serpentinite‐filled subduction channel. Age data and P–T modelling indicate lawsonite eclogite formed at ~2.7 GPa and 590°C at c. 490 Ma, whereas peak garnet in blueschist formed at ~2.0 GPa and 550°C at c. 470 Ma. The post‐peak evolution of lawsonite eclogite was associated with the preservation of pristine lawsonite‐bearing assemblages and the formation of glaucophane. By contrast, the garnet blueschist was derived from a precursor garnet–omphacite assemblage. The geochronological data from these different aged high‐P assemblages indicate the high‐P rocks were formed during subduction on the margin of cratonic Australia during the Cambro‐Ordovician. The rocks however now reside in the Devonian–Carboniferous southern SNEO, which forms the youngest and most outboard of the eastern Gondwanan Australian orogenic belts. Geodynamic modelling suggests that over the time‐scales that subduction products accumulated, the high‐P rocks migrated large distances (~>1,000 km) during slab retreat. Consequently, high‐P rocks that are trapped in subduction channels may also migrate large distances prior to exhumation, potentially becoming incorporated into younger orogenic belts whose evolution is not directly related to the formation of the exhumed high‐P rocks. [ABSTRACT FROM AUTHOR]

Garnet is a high-strength mineral compared to other common minerals such as quartz and feldspar in the felsic crust. In felsic mylonites, garnet typically occurs as porphyroclasts that mostly evade crystal plastic deformation, except under relatively high-temperature conditions. The microstructure of granulite facies garnet in felsic lower-crustal rocks of the Musgrave Ranges (Central Australia) records both fracturing and crystal plastic deformation. Granulite facies metamorphism at ∼1200 Ma generally dehydrated the rocks and produced millimetre-sized garnets in peraluminous gneisses. A later ∼550 Ma overprint under sub-eclogitic conditions (600–700 ∘ C, 1.1–1.3 GPa) developed mylonitic shear zones and abundant pseudotachylyte, coeval with the neocrystallization of fine-grained, high-calcium garnet. In the mylonites, granulite facies garnet porphyroclasts are enriched in calcium along rims and fractures. However, these rims are locally narrower than otherwise comparable rims along original grain boundaries, indicating the contemporaneous diffusion and fracturing of garnet. The fractured garnets exhibit internal crystal plastic deformation, which coincides with areas of enhanced diffusion, usually along zones of crystal lattice distortion and dislocation walls associated with subgrain rotation recrystallization. The fracturing of garnet under dry lower-crustal conditions, in an otherwise viscously flowing matrix, requires transient high differential stress, most likely related to seismic rupture, consistent with the coeval development of abundant pseudotachylyte. Highlights. Garnet is deformed by fracturing and crystal plasticity under dry lower-crustal conditions. Ca diffusion profiles indicate multiple generations of fracturing. Diffusion is promoted along zones of higher dislocation density. Fracturing indicates transient high-stress (seismic) events in the lower continental crust. [ABSTRACT FROM AUTHOR]

Garnet is a high-strength mineral stable across a wide range of pressure and temperature conditions and preserves structures that can consequently be used to understand the evolution of stress within the lower crust. Yet, the deformation mechanisms in the brittle-ductile regime of garnet remain ambiguous. Here, we study garnet porphyroclasts from an eclogite facies mylonite (central Australia) to investigate the mechanisms by which garnet is deformed under relatively dry, lower crustal conditions. Electron backscatter diffraction analysis reveals bands of small, relatively strain-free garnet with scattered orientations, outlined by polygonal to lobate high-angle grain boundaries separating the garnet porphyroclasts. Atom probe tomography of a high-angle grain boundary shows Fe enrichment in the form of planar and equally spaced arrays of nanoclusters. Our data demonstrate Fe segregation along grain boundaries of garnet, resulting in the nucleation of Fe-rich nanoclusters that act as barriers for migrating dislocations which could lead to strain-hardening and eventually facilitate mechanical failure. • High-spatial resolution microscopy is used to study garnet deformation mechanisms. • Atom probe tomography reveals Fe-rich nanoclusters at high-angle grain boundaries. • Fe-rich nanoclusters potentially lead to dislocation entanglement and microfracturing. [ABSTRACT FROM AUTHOR]

Multi-mineral petrochronology can effectively track changes in the thermochemical environment experienced by rocks during metamorphism. We demonstrate this concept using garnet–chlorite schists from the Walter-Outalpa Shear Zone of the southern Curnamona Province, South Australia, which reveal a cryptic and protracted (c. 39 Myr) record of high thermal gradient metamorphism. Petrochronological data including in situ monazite U–Pb and garnet Lu–Hf and Sm–Nd dating suggest elevated geotherms were persistent between at least c. 519–480 Ma, throughout the duration of garnet growth. Additional in situ xenotime U–Pb dating implies that partial garnet breakdown occurred between c. 480–440 Ma, likely induced by fluid-rock interaction or exhumation. Although metamorphism temporally overlaps with the timing of the regional Delamerian Orogeny (c. 520–480 Ma), the thermal mechanism to sustain elevated temperatures has remained enigmatic. One-dimensional thermal models are used to appraise the role of radiogenic heat production in driving the observed high thermal gradient metamorphism. The models reveal that with only modest crustal thickening during orogenesis, the endogenous radiogenic heat production hosted within the basement rocks could plausibly provide the thermal impetus for metamorphism. • Protracted metamorphism in the southern Curnamona Province between c. 519–480 Ma. • Fluid-rock interaction or exhumation occurred between c. 480–440 Ma. • Radiogenic heat produced by basement lithologies likely drove metamorphism. [ABSTRACT FROM AUTHOR]

• Dating pre-1600 Ma metamorphism in the Willyama Supergroup has been difficult. • Monazite in biotite-garnet gneiss grew at 1636.8 ± 4.4 Ma, and at 1597.3 ± 3.3 Ma. • Zircon in adjacent mafic granulite grew at or before 1639 ± 8 Ma, and at 1596 ± 8 Ma. • Robust isotopic evidence for a c. 1640 Ma tectonometamorphic event at Broken Hill. • Inferred 1640 Ma extension mirrors coeval tectonic regime in northeastern Australia. In the Willyama Supergroup of the Curnamona Province, the intensity of granulite facies Olarian metamorphism at c. 1600 Ma has hampered efforts to elucidate the timing and nature of preceding lower-grade metamorphic events. Previous dating of pre-peak monazite inclusions in aluminous metapelites has been complicated by potentially incomplete isotopic 'shielding' by their host porphyroblasts, owing to pervasive deformation and recrystallisation of these rocks during the c. 1600 Ma event. Near the Broken Hill Pb-Zn orebody, quartz-feldspar-biotite-garnet ('Potosi-type') gneiss proved more resistant to recrystallization during granulite facies metamorphism: new Sensitive High Resolution Ion MicroProbe (SHRIMP) U-Pb analyses of monazite from this rock delineated a population of relatively Y-rich (median 0.96 wt%) cores at 1636.8 ± 4.4 Ma (95% confidence), overprinted by Y-poor (median 0.10 wt%) rims and domains at 1597.3 ± 3.3 Ma. Garnet is a sink for Y in metamorphic rocks, which suggests that Y-rich c. 1637 Ma monazite growth predated garnet formation (and possibly occurred at relatively low metamorphic grade), whereas the Y-poor nature of the c. 1597 Ma rims and domains implies that dissolution and reprecipitation during Olarian metamorphism postdated porphyroblastic garnet growth. Coexisting low-Th/U zircon rims are dominated by a 1600.7 ± 3.6 Ma population, emphasising that the event responsible for c. 1637 Ma monazite growth was largely invisible to zircon. Interlayered mafic granulite 70 m away yielded partially recrystallised zircons with dark-CL internal domains which establish a minimum age of 1639 ± 8 Ma for the igneous precursor. Recrystallisation was accompanied by actinide depletion, with bright-CL, U-poor and low-Th/U domains dated at 1596 ± 8 Ma. Our new 1640–1635 Ma dates constitute robust isotopic evidence for pre-Olarian mafic magmatism and metamorphism within the lower Willyama Supergroup. We infer an extensional environment in the upper mid-crust for this tectonothermal activity, coeval with ongoing sedimentation and deposition of the upper Paragon Group at the surface. Extension of the eastern Curnamona Province at c. 1640 Ma is coeval with sediment-hosted base-metal mineralisation in the Mount Isa Inlier and the McArthur Basin, and is consistent with tectonic regimes prevailing throughout Proterozoic southern, central and eastern Australia at that time. [ABSTRACT FROM AUTHOR]

Abstract: Sediment-hosted, Paleoproterozoic low-grade Zn–Pb–Ag and Cu–Au mineralization occurs at the Polygonum, Thunderdome, Hunters Dam, and Benagerie Ridge prospects under a thick sedimentary cover in the Mulyungarie Domain (MD) of the southern Curnamona Province (SCP), Australia. Host rocks and mineralization were metamorphosed to lower greenschist facies at Benagerie Ridge, upper greenschist facies at Hunters Dam, and lower to upper amphibolite facies at Polygonum and Thunderdome. The giant Paleoproterozoic Pb–Zn–Ag Broken Hill deposit, which was metamorphosed to the granulite facies, is hosted in the Broken Hill Domain (BHD). The general lithostratigraphy for each prospect is similar and can be subdivided into four main units. Unit 1, at the base of the stratigraphic section, hosts Cu–Au mineralization and is characterized by the presence of oxidized magnetic metapsammopelites. Unit 2 consists of sulfide-rich, pyritic carbonaceous metapelites, calcareous metashales/siltstones, Mn-bearing sideritic metacarbonates, metacalc-silicates, and carbonate and feldspar lenses and layers. Syn-sedimentary sulfide mineralization occurs as laminations of fine-grained pyrite and sphalerite and resembles that at the HYC deposit, Queensland. Differences in δ13C values (δ13CVPDB =−10.2 and −0.8‰) of calcite in unit 2 from the same area suggest variable amounts of original organic C. The δ13C values of carbonates in Broken Hill ore and those obtained here from the Esmeralda deposit near Broken Hill are extremely low (δ13C=−25 to −21‰) and likely resulted from volatilization and high temperature effects during high-grade metamorphism. Barren laminated carbonaceous metapelites in unit 3 constitute a marker unit throughout the SCP. Unit 4 at the top of the stratigraphic sequence consists of psammopelitic and laminated andalusite or chiastolite phyllites that contain garnet and gahnite, laminated garnetite, quartz garnetite, Mn-bearing banded iron formation, amphibolite, and stratabound hydrothermal Pb–Zn mineralization. This unit, which is present at the Polygonum and Thunderdome prospects, is stratigraphically equivalent to the upper Broken Hill Group, which hosts the Broken Hill deposit. Base metals occur in metacarbonates and metapelites and close to redox boundaries between units 1 and 2, and units 2 and 3. Geochemical indicators of stratabound Pb–Zn mineralization in the northern part of the SCP include whole-rock values of >4ppm Tl, >25ppm Cd, and >17ppm Se in units 2 and 4, (Mn+Fe+Mg)/(Mn+Fe+Mg+Ca) ratios>0.9 for carbonates, Mn/(Mn+Fe+Mg) ratios>0.6 for garnet in garnet–quartz rocks, and Mn/(Mn+Ca+Fe) ratios>0.3 for garnet in metacalc-silicate rocks. [Copyright &y& Elsevier]

Garnet-rich rocks occur throughout the Proterozoic southern Curnamona Province, Australia, where they are, in places, spatially related to Broken Hill-type Pb-Zn-Ag deposits. Fine-scale bedding in these rocks, their conformable relationship with enclosing metasedimentary rocks, and their enrichment in Mn and Fe suggest that they are metamorphosed chemical precipitates. They formed on the floor of a 1.69 Ga continental rift basin from hydrothermal fluids mixed with seawater and detritus. Garnet in garnet-quartz and garnet-amphibole rocks is generally light rare earth element (LREE) depleted, and has flat heavy REE (HREE) enriched chondrite-normalized REE patterns, and negative Eu anomalies (Eu/Eu* < 1). Garnet in garnet-rich rocks from the giant Broken Hill deposit has similar REE patterns and either positive (Eu/Eu* > 1) or negative Eu anomalies. Manganese- and Mn-Ca-rich, Fe-poor garnets in garnetite, garnet-hedenbergite, and garnet-cummingtonite rocks at Broken Hill have Eu/Eu* > 1, whereas garnet in Mn-poor, Fe-rich quartz garnetite and quartz-garnet-gahnite rocks from Broken Hill, and quartz garnetite from other locations have Eu/Eu* < 1. The REE patterns of garnet and its host rock and interelement correlations among REEs and major element contents in garnet and its host rock indicate that the Eu anomaly in garnet reflects that of its host rock and is related to the major element composition of garnet and its host rock. The value of Eu/Eu* in garnet is related to its Mn, Fe, and Ca content and that of its host rock, and the distribution of REEs among garnet and accessory phases (e.g., feldspar). Positive Eu anomalies reflect high amounts of Eu that was preferentially incorporated into Mn- and Mn-Ca-rich oxides and carbonates in the protolith. In contrast, Eu/Eu* < 1 indicates the preferential discrimination against Eu by Fe-rich, Mn-poor precursor minerals. Precursors to Mn-rich garnets at Broken Hill formed by precipitation from cooler and more oxidized hydrothermal fluids compared to those that formed precursors to Mn-poor, Fe-rich garnet at Broken Hill and the other locations. Garnet from the Broken Hill deposit is enriched in Zn (> 400 ppm), Cr (> 140 ppm), and Eu (up to 6 ppm and positive Eu anomalies), and depleted in Co, Ti, and Y compared to garnet in garnet-rich rocks from other localities. These values, as well as MnO contents > 15 wt. % and Eu/Eu* > 1 are only found at the Broken Hill deposit and are good indicators of the presence of Broken Hill-type mineralization. [ABSTRACT FROM AUTHOR]

The sequential growth of biotite, garnet, staurolite, kyanite, andalusite, cordierite and fibrolitic sillimanite, their microstructural relationships, foliation intersection axes preserved in porphyroblasts (FIAs), geochronology, P–T pseudosection (MnNCKFMASH system) modelling and geothermobarometry provide evidence for a P–T–t–D path that changes from clockwise to anticlockwise with time for the Balcooma Metamorphic Group. Growth of garnet at ∼530 °C and 4.6 kbar during the N–S-shortening event that formed FIA 1 was followed by staurolite, plagioclase and kyanite growth. The inclusions of garnet in staurolite porphyroblasts that formed during the development of FIAs 2 and 3 plus kyanite growth during FIA 3 reflect continuous crustal thickening from c. 443 to 425 Ma during an Early Silurian Benambran Orogenic event. The temperature and pressure increased during this time from ∼530 °C and 4.6 kbar to ∼630 °C and 6.2 kbar. The overprinting of garnet-, staurolite- and kyanite-bearing mineral assemblages by low-pressure andalusite and cordierite assemblages implies ∼4-kbar decompression during Early Devonian exhumation of the Greenvale Province. [ABSTRACT FROM AUTHOR]

Garnet-rich rocks are locally associated with the giant Paleoproterozoic Broken Hill Pb-Zn-Ag deposit and hundreds of smaller Broken Hill-type occurrences in the southern Curnamona Province, Australia; however, others are present unrelated to sulfides. Mn-bearing garnet-rich rocks are conformable to bedding in meta-sedimentary rocks of the Paleoproterozoic Willyama Supergroup and, together with sulfide-bearing zones, are spatially related to amphibolite and/or felsic volcanic rocks or shallow granite intrusions. The stratiform garnet-rich rocks can be subdivided into two groups: Mn-rich varieties typical of the stratigraphic hanging wall of the Broken Hill deposit and Mn-poor and Fe-rich varieties that are typical of the footwall and are regionally widespread. These garnet-rich rocks formed on the floor of a rifted basin as a mixture of chemical precipitates and detrital sediments and were subsequently metamorphosed to the amphibolite or granulite facies during the Olarian orogeny. A plot of Fe/Ti versus Al/(Al + Fe + Mn) ratios of these rocks suggests equal proportions of hydrothermal and detrital components in the precursor phases. Low Co + Cu + Ni contents are consistent with the input of a small hydrogenous component. Quartz garnetite and garnet-gahnite rocks in the stratigraphic footwall of the Broken Hill deposit (Zn-rich, Cu-poor B and C lodes), and garnet-rich rocks elsewhere in the southern Curnamona Province that are Mn poor and Fe rich, exhibit chondrite-normalized rare earth element (REE) patterns with negative Eu anomalies, which indicate that the precursor minerals formed from hydrothermal fluids at <300°C and under relatively low fo2 conditions. Garnetite and hedenbergite-, bustamite-, and rhodonite-bearing garnet-rich rocks from the stratigraphic hanging wall of the Broken Hill deposit (Pb-rich 2 and 3 lenses and the A lode) are Mn and Ca rich, Fe poor, and have positive Eu anomalies. Precursors to these Mn-rich rocks formed from relatively cool hydrothermal fluids (∼250°C) at fo2 conditions higher than those that formed Mn-poor, Fe-rich garnet- and gahnite-rich rocks at Broken Hill. Correlations among various elements show that the presence of positive or negative En anomalies in garnet-rich rocks depends on the composition of the precipitate (Fe, Mn, and trace metal content) as well as the chemical, crystallographic, or sorption control that the precursor Fe-Mn oxyhydroxides/oxides/carbonates had upon the incorporation of Eu. Garnet-rich rocks from the Broken Hill deposit have higher average Mn, K, Rb, Eu, Ga, Cs, Cu, Pb, Zn, As, Cd, Sb, Ag, W, and Au contents than those from other locations in the southern Curnamona Province, and these are more enriched in Fe, Mg, Ni, P, V, Co, Sc, Sr, Y, and U, which indicates a larger hydrogenetic component to their precursor minerals. Enrichments in Mn, S, Ga, Eu, Cu, Pb, Zn, As, Cd, Sb, Ag, and Au, and a positive En anomaly in garnetite can be used as exploration guides in the search for Broken Hill-type deposits by indicating close proximity to mineralization. [ABSTRACT FROM AUTHOR]

Raman spectroscopy is used to identify minerals in isolated sealed inclusion chambers in some New South Wales (Australia) diamonds and to determine the remnant internal pressure on each inclusion with values of each: 3.03, 3.31 and 3.54 - 3.62 GPa for coesite, 0.42, 1.36 and 2.3 GPa for diopsidic omphacite, and 0.8 GPa for grossular garnet, each determination for a different uncut stone. The diamond stress due to remnant pressure was found to be proportional to the maximum strain birefringence around each inclusion, as measured by the Berek compensator, with the constant (K = 1000 / 5.8, R2 = 0.93) independent of the inclusion mineral, size, orientation and shape within the limits of error. Thus, simple birefringence measurements of diamond adjacent to inclusions can be used to estimate remnant pressures on inclusions. The calibration shows that one of the stressed diamonds has an anomalously low strain birefringence. [ABSTRACT FROM AUTHOR]

Garnet occurs in a variety of rock types spatially related to the giant stratiform Paleoproterozoic Broken Hill lead-zinc-silver deposit, Australia. The deposit was metamorphosed to granulite facies conditions and subjected to at least five periods of deformation (D1-D5). Sulfides at Broken Hill are commonly associated with quartz garnetite, garnetite, and blue quartz gahnite-garnet rock. The origin of quartz garnetite and garnetite is controversial. Proposed models include: metamorphism of manganiferous sediments formed by submarine hydrothermal processes that mixed with aluminous pelagic sediments; metamorphism of an original detrital sediment; metasomatic mobilization of Mn between the sulfide lenses and the wall rocks either synpeak (D1-D2) or postpeak metamorphism (D3); and reaction of Mn derived from partially melted sulfide orebodies with the surrounding pelitic gneisses. REE patterns of whole-rock samples of garnetite and quartz garnetite show high light rare earth element/heavy rare earth element (LREE/HREE) ratios. Chondrite-normalized REE patterns, based on analyses of garnet in garnetite and quartz-garnetite and some samples of blue quartz gahnite-garnet rock, using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) techniques, generally show very low LREE/HREE ratios, flat HREE signatures, and low overall REE contents (<100 ppm). HREE are incorporated in garnet, whereas LREE likely occur in intergranular material between garnet grains and in monazite, zircon, and apatite inclusions in garnet or along grain boundaries. Garnetite and fine-grained quartz garnetite show positive and negative Eu anomalies, respectively. Coarse metasomatic garnet surrounding D3 quartz veins, and metasomatic garnet in quartz garnetite halos around lead-rich orebodies, remobilized quartz garnetite and some blue quartz gahnite-garnet rocks, show positive and negative Eu anomalies, variable HREE, low to moderate LREE/HREE ratios, and arcuate patterns. The REE signature of garnet in garnetite and quartz garnetite is characteristic of metamorphic garnet with Eu anomalies reflecting relative inputs of detrital to hydrothermal components in a premetamorphic protolith, fo2 conditions, and temperature, whereas arcuate HREE signatures are characteristic of metasomatic garnet that formed postpeak metamorphism. The REE chemistry of garnet is consistent with mineralogical, petrological, and structural studies that demonstrate garnetite and quartz garnetite are metamorphosed sediments that formed by submarine hydrothermal processes. [ABSTRACT FROM AUTHOR]

Monazite electron microprobe U–Th–Pb and garnet Sm–Nd isotopic data from metapelitic assemblages in the Willyama Supergroup in the southern Curnamona Province, south-central Australia, indicate that the terrain underwent regional greenschist to amphibolite-grade metamorphism during the c. 500 Ma Delamerian Orogeny. The Delamerian-aged mineral assemblages include prograde garnet–staurolite and kyanite-bearing associations that overprint andalusite- and sillimanite-bearing assemblages that are interpreted to have developed during the c. 1600 Ma Olarian Orogeny. Importantly, the development of secondary kyanite-bearing assemblages in the southern Curnamona Province has been used previously to suggest that the Olarian Orogeny followed an anticlockwise P– T evolution. If such assemblages are the product of c. 500 Ma metamorphism, then the anticlockwise P– T path is an apparent path, due to the overprint of a distinct metamorphic cycle c. 1100 Ma later. Making such distinctions is therefore extremely important when using the textural and metamorphic evolution of polycyclic terrains to model the thermal behaviour of the crust during orogeny. This study highlights the utility of in situ geochronology, linking age data to petrologically important phases and assemblages. [ABSTRACT FROM AUTHOR]

• The Kalakashi Group formed during 900–850 Ma and did not record any Grenvillian orogenic event. • The southwest Tarim Craton was probably in a long-term rifting environment in the Neoproterozoic. • The Tarim Craton was likely located at the peripheral, rather than the interior, of the Rodinia Supercontinent. The Rodinia Supercontinent assembled through global-scale Grenvillian (1300–900 Ma) orogeny, so identification and correlation of Grenvillian orogens is fundamental to the geological reconstruction of Rodinia. The Tarim Craton, NW China, has been considered as a building block of Rodinia, but its Neoproterozoic tectonic evolution and relationship with Rodinia remain enigmatic. It was proposed that a Grenvillian orogenic event occurred in the southwestern Tarim, possibly related to its amalgamation with northern Australia. Key evidence for this model came from the ~1.0 Ga 40Ar/39Ar ages of the Kalakashi Group in the Tiekelike belt, southwestern Tarim. In this contribution, we report zircon U-Pb ages and Lu-Hf isotopic data, as well as whole-rock geochemical data for the Kalakashi Group from the Buqiong section where a ~1.0 Ga 40Ar/39Ar age was reported. Our field and petrographic observations indicate that the primary rock assemblage of the Kalakashi Group is orthogneiss, garnet-bearing mica schist, marble and minor amphibolite. Zircon U-Pb dating of five orthogneisses and an amphibolite yielded crystallization ages of 900–850 Ma and a thermal/fluid overprinting event at ~420 Ma. Lu-Hf isotopic analyses of the dated zircon grains from two orthogneisses yielded ε Hf(t) values from −4 to +7, indicating a mixture of mantle- and crust-derived materials. Geochemical data show that the orthogneisses have relatively high FeOT/MgO and 10,000*Ga/Al ratios, elevated contents of high-field-strength elements (HFSE) and negative Ba, Sr and Eu anomalies, characteristic of A 1 -type granite, indicating that the parental magma erupted in an intra-plate rifting setting. Our results indicate that the Kalakashi Group is a volcanic-sedimentary succession that deposited in the Neoproterozoic, rather than the Paleoproterozoic or Mesoproterozoic as previously thought. We found no evidence for a Grenvillian orogenic event from the Kalakashi Group in the Buqiong section. Instead, the age and geochemistry of the studied orthogneisses are remarkably similar to those of the Sailajiazitage Group, although the latter was affected by weaker metamorphism and deformation. Reliable geochronological data from igneous and metamorphic rocks, as well as detrital zircons from Neoproterozoic strata, indicate that no typical record of Grenvillian orogeny occurred in either the north or the south of the Tarim Craton. Instead, we suggest that the northern Tarim was probably dominated by a long-term subduction-accretion, whereas the southern Tarim was likely in a long-term rifting environment in the early Neoproterozoic. [ABSTRACT FROM AUTHOR]

Felsic material rheologically controls crustal flow that ultimately produces gneiss domes. This felsic material is easily re-equilibrated during the later stages of dome emplacement in the shallow crust, at low-pressure, where it crystallizes as gneiss/migmatite at high-temperature (LP-HT) conditions. The early part of dome history is typically obliterated in the felsic material; however, less volumetrically abundant refractory rocks have potential to preserve an early part of the metamorphic history from deeper crustal levels. In this study, we use anisotropy of magnetic susceptibility (AMS) of refractory mafic rocks associated with the dominant gneiss to investigate the record of processes by which rocks emplaced in gneiss domes deform during dome evolution. The Entia dome, central Australia, is an ideal field site to investigate these questions: it is a relatively equant dome with two well-defined sub-domes cored by granitic bodies and separated by a median high-strain zone defined by sub-vertical to vertical foliation trajectories and shallowly plunging lineation. The Entia dome contains exposed refractory lithologies ranging from mafic granulite pods or layers to amphibolite (± garnet, ± clinopyroxene) layers. Entia mafic rocks experienced varying degrees of strain and preserve a differential record of pressure history throughout the dome, evidenced in a range of P-sensitive mineral assemblages and reaction textures such as garnet-stable assemblages, garnet breakdown reaction textures, and the presence of Ti-bearing phases such as rutile, ilmenite, and titanite at different structural locations throughout the dome. We sampled mafic material across key structural sites and traverses, with comprehensive geographic coverage of the domal structure. The AMS of oriented cores was analyzed using both low- and high-field methods to measure the magnetic fabric preserved in representative samples throughout the dome. Principal susceptibility (K1 > K2 > K3) orientations and magnitudes, as well as anisotropy, were measured and used to calculate a shape parameter describing the AMS ellipsoid (oblate to prolate) for each core. In order to better understand the relationship between the orientation and strength of mineral alignment and the AMS ellipsoid, we performed modeling of expected total AMS based on the modal abundances, compositions, and crystallographic preferred orientation (CPO) of carrier phases measured by electron backscatter diffraction (EBSD). Low-field AMS can be controlled by, and therefore informs, the preferred alignment of all magnetic carrier phases (paramagnetic amphiboles, pyroxenes or ferromagnetic iron oxides) in mafic rocks. High-field methods show that in most of our samples, the AMS is dominated by paramagnetic carriers. For a subset of samples, we determined 3-D grain shapes and orientations to characterize the shape preferred orientation (SPO) of carrier phases, and compared these measurements with AMS results to investigate potential correlations between measurable mineral characteristics and total magnetic anisotropy, and how the AMS ellipsoid may relate to strain (e.g. constriction vs. flattening). We compared the record of deformation and metamorphism preserved in rocks exposed at key structural sites (high-strain zone, dome margins) on a horizontal section through the dome, as they represent particles having experienced deformation through different P–T–t–d paths during crustal flow associated with dome evolution. [ABSTRACT FROM AUTHOR]

The timing of metamorphic processes is important in understanding the evolution of the earth. Absolute dating can be done by e.g. U-Pb laser ablation analysis of accessory minerals such as zircon and monazite. However, relying solely on geochronological analyses of these important geochronometers would provide little more than timing of crystallisation or timing of recrystallisation of these minerals. Much more powerful is the interpretation of ages and/or trace element data acquired from zircon and monazite in relation to the microstructures in which they are observed. Combining petrography, mineral compositions, accessory mineral occurrence and internal texture with geochronology and trace element concentrations provides a powerful strategy to resolve temporal processes within metamorphic rocks.Here we apply this strategy on monazite from high-temperature metamorphic metasedimentary rocks from the Reynolds Range, central Australia. The metasedimentary sequence here comprises cordierite–garnet–biotite gneisses that contain orthopyroxene-bearing and orthopyroxene-free quartzofeldspathic leucosomes. First order observations led us to believe that partial melt was present and reacted with the host rock. Analysis of monazite is used to provide the chronological context of these events. Using solely U–Pb ages did not solve the order of metamorphic events these rocks experienced. Rather the interpretation of in-situ analysed monazite within the rocks microstructural context provided the temporal resolution of events. We argue that analyses of various internal monazite textures, their age, composition and overprinting relations, allows us to identify the absolute chronology of events following the metamorphic peak. Here we link retrograde reactions involving garnet, orthopyroxene and K-feldspar to melt-rock interaction that is subsequent to partial melting and crystallisation of the partial melt. Retrogression is reflected in the development of compositionally distinct monazite textural domains. This allows us to tie metamorphic processes to specific times in the evolution of these rocks within the Reynolds Range, central Australia. [ABSTRACT FROM AUTHOR]

The final assembly of the Mesoproterozoic supercontinent Nuna was marked by the collision of Laurentia (North America) and Australia between c. 1.6 and 1.53 Ga. The final suture of Nuna has been identified in NE Australia, where the Proterozoic Georgetown Inlier represents the accreted leading edge of Laurentia. However, the structural and metamorphic evolution of the Georgetown Inlier remains uncertain. Here we investigate the orogenic evolution of this final Nuna collisional event using multi-scale petrostructural analysis, thermodynamic modelling, and geochronology. The Georgetown Inlier is characterised by c. 1.7 to 1.65 Ga metasedimentary and metabasic rocks that have been multiply deformed and metamorphosed, and intruded by c. 1.55 Ga S-type granites. Two pervasive fabrics (S1 and S2) are preserved within low strain domains of retrograde greenschist-facies foliations (S3 and S4). S1 is defined by greenschist-facies assemblages in the west, and by amphibolite-facies assemblages in the central Inlier, while S2 is defined by low-pressure and high-temperature mineral assemblages in the central and eastern inlier. Microstructural analysis combined with mineral chemistry and thermodynamic modelling in the central Inlier suggest prograde syn-S1 garnet growth in staurolite–garnet metapelites occurred at 530–550 ̊C at 5.4–6.4 kbar (core) to 620–660 ̊C at 8.6–9.8 kbar (rim). Garnet Lu–Hf geochronology dates this MP–MT metamorphism at c. 1.6 Ga. During the development of S2, staurolite was replaced by andalusite (M2a), which was pseudomorphed by sillimanite (M2b) in the eastern region, where granites are abundant. Calculations suggest 600–680 ̊C and 4–6 kbar for the sillimanite stage, and syn-S2 monazite yields c. 1.55–1.53 Ga U–Pb ages. Garnet in sillimanite-bearing metapelites is a MP–MT relict as its composition is reproduced in the staurolite stability field and it has a Lu–Hf age of c. 1.6 Ga. Field relationships suggest that the low–pressure stage was part of the regional thermal event associated with granite emplacement between c. 1.56–1.54 Ga. Some S-type granites record retrograde greenschist-facies foliations, and were emplaced late- to post-S2. In the eastern migmatitic complex, partial melting of paragneiss and amphibolite occurred early-, syn-, to post-S2. Peak conditions were estimated at 700–820 ̊C and 7.8–8.4 kbar in paragneiss, and at 750–890 ̊C and 7.0–8.4 kbar in garnet-free amphibolite.The P–T–d–t paths reconstructed for the Georgetown Inlier thus suggest an early deformation phase (D1) under MP–MT conditions at 1.6 Ga and a LP–HT metamorphic overprint during D2 between c. 1.56 and 1.53 Ga. These results support that the Georgetown Inlier records collisional orogenesis related to the final assembly of Nuna at 1.6 Ga, whereas the subsequent LP–HT overprint is interpreted as orogenic collapse. [ABSTRACT FROM AUTHOR]

The article provides information on Ireland's Quarry Broken Hill, one of the classic Australian mineral localities, situated in New South Wales. Broken Hill is one of the world's largest lead-zinc-silver orebodies, and it is Australia's premier classic mineral locality because of both the wide range and the high quality of mineral specimens collected there during the past 120 years. The quarry is named after its discoverer, Allan Ireland, and has operated sporadically since the late 1950s to produce garnet specimens.