Granulite facies metamorphism in the Mallee Bore area, northern Harts Range: implications for the thermal evolution of the eastern Arunta Inlier, central Australia

The Mallee Bore area in the northern Harts Range of central Australia underwent high-temperature, medium- to high-pressure granulite facies metamorphism. Individual geothermometers and geobarometers and average P–T  calculations using the program Thermocalc suggest that peak metamorphic conditions were 705–810 °C and 8–12 kbar. Partial melting of both metasedimentary and meta-igneous rocks, forming garnet-bearing restites, occurred under peak metamorphic conditions. Comparison with partial melting experiments suggests that vapour-absent melting in metabasic and metapelitic rocks with compositions close to those of rocks in the Mallee Bore area occurs at 800–875 °C and >9–10 kbar. The lower temperatures obtained from geothermometry imply that mineral compositions were reset during cooling. Following the metamorphic peak, the rocks underwent local mylonitization at 680–730 °C and 5.8–7.7 kbar. After mylonitization ceased, garnet retrogressed locally to biotite, which was probably caused by fluids exsolving from crystallizing melts. These three events are interpreted as different stages of a single, continuous, clockwise P–T  path. The metamorphism at Mallee Bore probably occurred during the 1745–1730 Ma Late Strangways Orogeny, and the area escaped significant crustal reworking during the Anmatjira and Alice Springs events that locally reached amphibolite facies conditions elsewhere in the Harts Ranges.     

A sequence of partial melting reactions at Mt Stafford, central Australia

Metasedimentary gneisses show a rapid change in grade in a 10  km wide low- P /high- T  regional aureole at Mt Stafford in the Arunta Block, central Australia. Migmatite occurs in all but the lowermost of five metamorphic zones, which grade from greenschist (Zone 1) through amphibolite (Zones 2–3) to granulite facies (Zones 4–5). The sequence of partial melting reactions inferred for metapelitic rocks is dependant upon protolith, temperature and fluid conditions. The metapelite solidus in Zone 2 reflects vapour-present melting at P ≈3  kbar and T  ≈640  °C, melting having initially been controlled by the congruent breakdown of the assemblage Crd–Kfs–Bt–Qtz. At slightly higher temperature, andalusite in leucosome formed via the reaction Kfs+Qtz+Bt+H₂O→And+melt; And+melt having been stabilized by the presence of boron. Sillimanite coaxially replaces andalusite in the high-grade portion of Zone 2. In Zone 3, large aluminosilicate aggregates in leucosome are armoured by Spl–Crd±Grt symplectites. Garnet partially pseudomorphs biotite, cordierite or spinel in high-grade portions of Zone 3. Zone 4 Grt–Crd–Opx-bearing metapsammite assemblages and garnet-bearing leucosome reflect T  ≈800  °C and P =2.2±0.9  kbar. In the model KFMASH system the principal vapour-absent melting step reflected significant modal changes related to the breakdown of the As–Bt tie-line and the establishment of the Spl–Crd tie-line; the bulk rock geochemistry of migmatite samples straddle the Spl–Crd tie-line. The aluminous bulk-rock composition of the common bedded migmatite restricted its potential to witness garnet-forming and orthopyroxene-forming reactions, minor textural and modal changes in and above Zone 3 reflecting biotite destablization in biotite-poor assemblages.     

Sm–Nd evidence for high-grade Ordovician metamorphism in the Arunta Block, central Australia

Sm–Nd ages from the Harts Range in the south-eastern Arunta Inlier in central Australia indicate that regional metamorphism up to granulite facies occurred in the Early Ordovician (c. 475 Ma). This represents a radical departure from previous tectonic models for the region and identifies a previously unrecognized intraplate event in central Australia. Peak metamorphic assemblages (800 °C and 10.5 kbar) formed at around 476±14 Ma and underwent approximately 4 kbar of near-isothermal decompression at 475±4 Ma. A coarse-grained unfoliated garnet–clinopyroxene-bearing marble inferred to have recrystallized late in the decompressional evolution, gives an age of 469±7 Ma. Two lines of evidence suggest the Early Ordovician tectonism occurred in an extensional setting. First, the timing of the high-grade lower crustal deformation coincides with a period of marine sedimentation in the Amadeus and Georgina basins that was associated with a seaway that developed across central Australia. Second, isothermal decompression of lower crustal rocks was associated with the formation of a regional, sub-horizontal mid-crustal foliation. In the Entia Gneiss Complex, which forms the structurally lowest part of the Harts Range, upper-amphibolite facies metamorphism (c. 700 °C, 8–9 kbar) occurred at 479±15 Ma. There is no evidence that P–T conditions in the Entia Gneiss Complex were as high as in the overlying units. This implies that the extensional system was reworked during a later compressional event. Sm–Nd data from the mid-amphibolite facies (c. 650 °C and 6 kbar) detachment zone that separates the Irindina Supracrustal Assemblage and Entia Gneiss Complex give an age of 449±10 Ma. This age corresponds to the timing of a change in the pattern and style of sedimentation in the Amadeus and Georgina basins, and indicates that the change in basin dynamics was associated with mid-crustal deformation. It also suggests that compressional deformation culminating in the Devonian to Carboniferous (400–300 Ma) Alice Springs Orogeny may have begun as early as c. 450 Ma. At present, the extent of Early Ordovician tectonism in central Australia is unknown. However, granulite facies metamorphism and associated intense deformation imply an event of regional extent. An implication of this work is that high-grade lower crustal metamorphism and intense deformation occurred during the development of a broad, shallow, slowly subsiding intraplate basin.     

High-T, low-P metamorphism in the Palaeoproterozoic Halls Creek Orogen, northern Australia: the middle crustal response to a mantle-related transient thermal pulse

High‐T, low‐P metamorphic rocks of the Palaeoproterozoic central Halls Creek Orogen in northern Australia are characterised by low radiogenic heat production, high upper crustal thermal gradients (locally exceeding 40 °C km^?1) sustained for over 30 Myr, and a large number of layered mafic‐ultramafic intrusions with mantle‐related geochemical signatures. In order to account for this combination of geological and thermal characteristics, we model the middle crustal response to a transient mantle‐related heat pulse resulting from a temporary reduction in the thickness of the mantle lithosphere. This mechanism has the potential to raise mid‐crustal temperatures by 150–400 °C within 10–20 Myr following initiation of the mantle temperature anomaly, via conductive dissipation through the crust. The magnitude and timing of maximum temperatures attained depend strongly on the proximity, duration and lateral extent of the thermal anomaly in the mantle lithosphere, and decrease sharply in response to anomalies that are seated deeper than 50–60 km, maintained for <5 Myr in duration and/or have half‐widths <100 km. Maximum temperatures are also intimately linked to the thermal properties of the model crust, primarily due to their influence on the steady‐state (background) thermal gradient. The amplitudes of temperature increases in the crust are principally a function of depth, and are broadly independent of crustal thermal parameters. Mid‐crustal felsic and mafic plutonism is a predictable consequence of perturbed thermal regimes in the mantle and the lowermost crust, and the advection of voluminous magmas has the potential to raise temperatures in the middle crust very quickly. Although pluton‐related thermal signatures significantly dissipate within <10 Myr (even for very large, high‐temperature intrusive bodies), the interaction of pluton‐ and mantle‐related thermal effects has the potential to maintain host rock temperatures in excess of 400–450 °C for up to 30 Myr in some parts of the mid‐crust. The numerical models presented here support the notion that transient mantle‐related heat sources have the capacity to contribute significantly to the thermal budget of metamorphism in high‐T, low‐P metamorphic belts, especially in those characterised by low surface heat flow, very high peak metamorphic geothermal gradients and abundant mafic intrusions.     

Stable isotopic evidence for fluid infiltration during contact metamorphism in a multiply-metamorphosed terrane: the Reynolds Range, Arunta Block, central Australia

The Lander Rock Beds form the local basement of the Reynolds Range in the Arunta Inlier of central Australia. These dominantly quartzose and pelitic lithologies underwent low-grade ( c.   400  °C) regional metamorphism prior to contact metamorphism ( c.   2.5  kbar) around S-type megacrystic granitoids at 1820–1800  Ma. The Lander Rock Beds are overlain by metasediments of the Reynolds Range Group, which were subsequently intruded by granitoids at c. 1780  Ma. Regional metamorphism at 1590–1580  Ma produced grades varying from greenschist (400  °C at 4–5  kbar) to granulite (750–800  °C at 4–5  kbar) from north-west to south-east along the length of the Reynolds Range. Oxygen isotope ratios of the Lander Rock Beds were reset from 13.4±0.8 to as low as 6.7 adjacent to the contacts of the larger plutons, and to 10.3±1.1 around the smaller plutons. Biotite in all the major rock types found in the aureoles has δD values between −52 and −69, probably reflecting resetting by a cooling igneous+metamorphic fluid near the plutons. Sapphirine-bearing and other Mg- and Al-rich rock types have low δ¹⁸O values (4.0±0.7). The precursors to these rocks were probably low-temperature ( c. 200  °C) diagenetic–hydrothermal deposits of Mg-rich chlorite, analogous to those in Proterozoic stratiform precious metal and uranium deposits that form by the infiltration of basin brines or seawater. As in the overlying Reynolds Range Group, regional metamorphism involved little fluid–rock interaction and isotopic resetting.     

SHRIMP II dating of zircons and monazites: reassessing the timing of high-grade metamorphism and fluid flow in the Reynolds Range, northern Arunta Block, Australia

ABSTRACT Key insights into the timing of tectonometamorphic events in a complex high-grade metamorphic terrane can be obtained by combining results from SHRIMP II ion microprobe studies of individual monazite grains with SHRIMP II studies and scanning electron microscope (SEM)-based cathodoluminescence (CL) imaging of zircons. Results from the Reynolds Range region, Arunta Block, Northern Territory, Australia, show that the final episode of regional metamorphism to high-T and low-P granulite facies conditions is most likely to have occurred at c. 1580 Ma, not at 1785–1775 Ma, as previously accepted. The previous interpretation was based on zircon studies of structurally controlled granitoids, without SEM-based CL imaging. Monazites in a 1806± 6 Ma megacrystic granitoid preserve rare cores that are interpreted to be inherited magmatic monazite, but record no evidence of another high-T event prior to 1580 Ma. Most monazites from the region record only a single high-T metamorphic event at c. 1580 Ma. Zircon inheritance is very common. Zircons or narrow overgrowths of zircon dated at c. 1580 Ma have only been found in two types of rocks: rocks produced by metasomatic fluid flow at high temperatures (≤750°C), and rocks that have undergone local partial melting. Previous explanations that attributed these 1580 Ma zircon ages to widespread hydrothermal fluid fluxing associated with post-tectonic pegmatite emplacement at amphibolite facies conditions are not supported by the available evidence including oxygen isotope data. The observed high regional metamorphic temperatures require the involvement of advective heating. However, contrary to a previous tectonic model for the formation of this and other low-P, high-T metamorphic belts, the granites that are exposed at the present structural level do not appear to be the source of that heat, unless some of the granites were emplaced at c. 1580 Ma.     

In-situ migmatite and hybrid diatexite at Mt Stafford, central Australia

Metasedimentary gneisses show a rapid change in grade within a 10-km-wide low- P /high- T  regional aureole at Mt Stafford, Arunta Block, central Australia. Migmatites occur in all but the lowermost of five metamorphic zones, which are characterized by: (1) muscovite–quartz schist; (2) andalusite–cordierite–K-feldspar granofels with small melt segregations; (3) spinel–sillimanite–cordierite–K-feldspar migmatite; (4) garnet–orthopyroxene–cordierite migmatite and minor diatexite; and (5) biotite–cordierite–plagioclase diatexite that shows a transition to granite. A subsolidus unit comprising interbedded sandstone and siltstone is equivalent to bedded migmatite , the main rock type in Zones 2–4. Mesoscopic textures and migmatite classification of this unit vary with grade. In Zone 2, metatexite is developed in siltstone layers that are separated by quartz-rich, unmelted metapsammite layers. Melt segregation was less efficient in Zones 3 and 4, where the dominant migmatite layering is a modified bedding. High proportions of melt were present in Zone 4, in which schlieren migmatite is transitional between bedded migmatite and metapelite-sourced diatexite. The preservation of sedimentary structures and coexistence of melt reactants and products in Zone 4 metapelite imply that melting proceeded in situ without substantial migration of melt. Zone 5 biotite–cordierite–plagioclase diatexite carries rafts of bedded migmatite with strongly resorbed edges, as well as large K-feldspar and quartz augen. This unit of comparatively Ca-rich migmatites is inferred to have been formed by the mixing of locally derived and injected granitic melt.     

Contrasting behaviour of rare earth and major elements during partial melting in granulite facies migmatites, Wuluma Hills, Arunta Block, central Australia

Low‐P granulite facies metapelitic migmatites in the Wuluma Hills, Strangways Metamorphic Complex, Arunta Block, preserve evidence of polyphase deformation and migmatite formation which is of the same age of the c. 1730 Ma Wuluma granite. Mineral equilibria modelling of garnet‐orthoproxene‐cordierite‐bearing assemblages using thermocalc is consistent with peak S3 conditions of 6.0–6.5 kbar and 850–900 °C. The growth of orthopyroxene and garnet was primarily controlled by biotite breakdown during partial melting reactions. Whereas orthopyroxene in the cordierite‐biotite mesosome shows enrichment of heavy‐REE (HREE) relative to medium‐REE (MREE), orthopyroxene in adjacent garnet‐bearing leucosome shows depletion of HREE relative to MREE. There is no appreciable difference in major element contents of minerals common to both the mesosome and leucosome. The REE variations can be satisfactorily explained by decoupling of major element and REE partitioning, in the context of appropriate phase‐equilibria modelling of a prograde path at ～6 kbar. Sparse garnet nucleii formed at ～760 °C, along with concentrated leucosome development and preferentially partitioned HREE. Further heating to ～800 °C at constant or subtly increasing pressure conditions additionally stabilized orthopyroxene and decreased the garnet mode. Orthopyroxene in the leucosome inherited an REE pattern consequent to the partial consumption of garnet, it being distinct from the REE pattern in mesosome orthoproxene that was mostly controlled by biotite breakdown. Such within‐sample variability in the enrichment of heavy REE indicates that caution needs to be exercised in the application of common elemental partitioning coefficients in spatially complex metamorphic rocks.     

Conductively driven,high‐thermal gradient metamorphism in the Anmatjira Range,Arunta region,central Australia

LA–ICP–MS in situ U–Pb monazite geochronology and P–T pseudosections are combined to evaluate the timing and physical conditions of metamorphism in the SE Anmatjira Range in the Aileron Province, central Australia. All samples show age peaks at c. 1580–1555 Ma, with three of five samples showing additional discrete age peaks between c. 1700 and 1630 Ma. P–T phase diagrams calculated for garnet–sillimanite–cordierite–K‐feldspar–ilmenite–melt bearing metapelitic rocks have overlapping peak mineral assemblage stability fields at ~870–920 °C and ~6.5–7.2 kbar. P–T modelling of a fine‐grained spinel–cordierite–garnet–biotite reaction microstructure suggests retrograde P–T conditions evolved down pressure and temperature to ~3–5.5 kbar and ~610–850 °C. The combined geochronological and P–T results indicate the SE Anmatjira Range underwent high‐temperature, low‐pressure metamorphism at c. 1580–1555 Ma, and followed an apparently clockwise retrograde path. The high apparent thermal gradient necessary to produce the estimated P–T conditions does not appear to reflect decompression of high‐P assemblages, nor is there syn‐metamorphic magmatism or structural evidence for extension. Similar to previous workers, we suggest the high‐thermal gradient P–T conditions could have been achieved by heating, largely driven by high heat production from older granites in the region.     

Proterozoic granulite facies metamorphism in the southeastern Reynolds Range, central Australia: geological context, P–T path and overprinting relationships

In the southeastern Reynolds Range, central Australia, a low- P granulite facies metamorphism affected two sedimentary sequences: the Lander Rock Beds and the Reynolds Range Group. In the context of the whole of the Reynolds Range and the adjacent Anmatjira Range, this metamorphism is M3 in a sequence M1–4 that occurred over a period of 250 Ma. In particular, M1 affected the Lander Rock Beds prior to the deposition of the Reynolds Group. M3 has an areally restricted, high-grade area in the southeastern Reynolds Range, affecting both the Reynolds Range Group and the underlying Lander Rock Beds. The effects of M3 are characterized by spinel + quartz-bearing peak metamorphic assemblages in metapelites, which imply peak conditions of ≥750°C and 4.5 ± 1 kbar, and involved isobaric cooling or compression with cooling. It is concluded that one of a series of thermal perturbations caused by thinning of mantle lithosphere contemporaneous with crustal thickening was responsible for M3. In the southeastern Reynolds Range, evidence of both the unconformity between the two rock groups and previous metamorphism/deformation has been completely erased by recrystallization during M3–D3.     

Proterozoic low-pressure/high-temperature metamorphism and an anticlockwise P–T–t path for the Hazeldene area, Mount Isa Inlier, Queensland, Australia

In the Hazeldene area, situated in the Mount Isa Inlier, Queensland, the metamorphic grade changes from chlorite zone, through biotite and cordierite zones, to sillimanite/K-feldspar zone.
Microstructural studies of rocks near the sillimanite isograd demonstrate that cordierite grew early during the development of a steep foliation (S2), was replaced by biotite, andalusite and sillimanite at the metamorphic peak late in S2, and in turn by kyanite + chlorite adjacent to localized small post-D2 shear zones. Although the anticlockwise P–T–t path is well defined, the precise P–T conditions are uncertain because of problems with experimental and thermodynamic data. The best estimate for the metamorphic peak for rocks close to the sillimanite isograd is around 600° C at 4 kbar.
The metamorphism has been dated at 1544 Ma, and was synchronous with a major crustal shortening event. Because proposed extensional events occurred more than 60 Ma earlier, their contribution to the peak metamorphic thermal perturbation would have been insignificant. The syn-metamorphic Mica Creek Pegmatites, the abundance of high heat-producing elements in the nearby pre-D2 Sybella Granite, and advective heat by fluids which caused considerable metasomatism in the Hazeldene area, may have each contributed to the thermal budget. However, the metamorphic thermal gradient may be 80°C km^-1 or higher, strongly suggesting a local magmatic control. As none are known in the area, such syn-metamorphic plutons would have to lie beneath the exposed high-grade rocks.     

Spatially-focussed melt formation in aluminous metapelites from Broken Hill, Australia

Large garnet poikiloblasts hosted by leucosome in metapelitic gneiss from Broken Hill reflect complex mineral–melt relationships. The spatial relationship between the leucosomes and the garnet poikiloblasts implies that the growth of garnet was strongly linked to the production of melt. The apparent difficulty of garnet to nucleate a large number of grains during the prograde breakdown of coexisting biotite and sillimanite led to the spatial focussing of melting reactions around the few garnet nuclei that formed. Continued reaction of biotite and sillimanite required diffusion of elements from where minerals were reacting to sites of garnet growth. This diffusion was driven by chemical potential gradients between garnet‐bearing and garnet‐absent parts of the rock. As a consequence, melt and peritectic K‐feldspar also preferentially formed around the garnet. The diffusion of elements led to the chemical partitioning of the rock within an overall context in which equilibrium may have been approached. Thus, the garnet‐bearing leucosomes record in situ melt formation around garnet porphyroblasts rather than centimetre‐scale physical melt migration and segregation. The near complete preservation of the high‐grade assemblages in the mesosome and leucosome is consistent with substantial melt loss. Interconnected networks between garnet‐rich leucosomes provide the most likely pathway for melt migration. Decimetre‐scale, coarse‐grained, garnet‐poor leucosomes may represent areas of melt flux through a large‐scale melt transfer network.     

Contrasting styles of Taconian, Eastern Acadian and Western Acadian metamorphism, central and western New England

The two major Early to Middle Palaeozoic tectonic/metamorphic events in the northern Appalachians were the Taconian (Middle to Late Ordovician) in central to western areas and the Acadian (Late Silurian to early Middle Devonian) in eastern to west-central areas. This paper presents a model for the Acadian orogenic event which separates the Acadian metamorphic realm into eastern and western belts based on distinctively different styles. We propose that the Acadian metamorphism in the east was the delayed consequence of Taconian back-arc lithospheric modification. East of the Taconian island arc, thick accumulations of Late Ordovician and Silurian sediments, coupled with plutons rising along a magmatic arc, produced crustal thermal conditions appropriate for anomalously high-T, low-P metamorphism accompanied by major crustal anatexis. In this zone, upward melt migration was coupled with subsequent E-W crustal shortening (possibly due to outboard collision with the Avalon terrane) to produce mechanical conditions that favoured formation of fold and thrust nappes and resultant tectonic thickening to the west (and probably to the east as well). The basis for the distinction between the Eastern and Western Acadian events lies in the contrasting styles of metamorphism accompanying each. Evidence for contrasting metamorphic styles consists of (1) estimated metamorphic field gradients (MFGs) based on thermobarometric studies, and (2) petrological evidence for contrasting P–T trajectories. West of the Acadian metamorphic front, the Taconian zone has an MFG in which peak temperatures of 400-600° C were reached at pressures of about 4–6 kbar, with both P and T increasing to the east. Near its western edge, the Western Acadian metamorphic overprint has a similar MFG to the Taconian, and is mainly discriminated by ⁴⁰Ar/³⁹Ar dating and microtextural evidence. East of this narrow zone, the Western Acadian overprint is characterized by progressively higher temperatures (600–725° C) and pressures (6.5–10 kbar, or more) to the east, yielding an overall MFG that lies along, or slightly above, the kyanite–sillimanite boundary on a P–T diagram. There is little or no plutonism accompanying Western Acadian metamorphism. In contrast, thermobarometry in the Eastern Acadian, east of the Bronson Hill Belt, yields high-T, intermediate-P conditions for the highest grade rocks known in New England: T= 650–750° C, P= 4.5–6.5 kbar for granulite facies assemblages which apparently formed along an ‘anticlockwise’P–T path. The Bronson Hill Belt lies geographically between the Eastern and Western Acadian zones and shows transitional petrological behaviour: anomalously high temperatures at intermediate pressures, but a ‘clockwise’ path with decompression cooling. Radiometric dating indicates peak Taconian conditions may have been achieved as early as 475 Ma in the Taconian hinterland and as late as 445 Ma in the Taconian foreland (including the Taconic allochthons). Eastern Acadian magmatism may have started as early as 425 Ma, and most nappe-stage deformation and metamorphism in the Eastern Acadian zone appears to have ended by about 410 Ma. Tectonic thickening in the Western Acadian (including the western counterparts of the nappe-stage deformation documented in the Eastern Acadian) must pre-date attainment of peak metamorphic conditions dated at 395–385 Ma. Dome-stage deformation clearly post-dates peak metamorphism and deforms metamorphic isograds. The end of Western Acadian deformation is well constrained by 370-375 Ma radiometric ages of late pegmatites and granitoids which cross-cut all structures.     

The Arunta Inlier: a complex ensialic mobile belt in central Australia. Part 1: Stratigraphy,correlations and origin

The Arunta Inlier is a 200 000 km² region of mainly Precambrian metamorphosed sedimentary and igneous rock in central Australia. To the N it merges with similar rocks of lower metamorphic grade in the Tennant Creek Inlier, and to the NW it merges with schist and gneiss of The Granites‐Tanami Province. It is characterized by mafic and felsic meta‐igneous rocks, abundant silicic and aluminous metasediments and carbonate, and low‐ to medium‐pressure metamorphism. Hence, the Arunta Inlier is interpreted as a Proterozoic ensialic mobile belt floored by continental crust. The belt evolved over about 1500 Ma, and began with mafic and felsic volcanism and mafic intrusion in a latitudinal rift, followed by shale and limestone deposition, deformation, metamorphism and emergence. Flysch sedimentation and volcanism then continued in geosynclinal troughs flanking the ridge of meta‐igneous rocks, and were followed by platform deposition of thin shallow‐marine sediments, further deformation, and episodes of metamorphism and granite intrusion.     

Ordovician high-grade metamorphism of a newly recognised late Neoproterozoic terrane in the northern Harts Range,central Australia

Granulite facies rocks from the northernmost Harts Range Complex (Arunta Inlier, central Australia) have previously been interpreted as recording a single clockwise cycle of presumed Palaeoproterozoic metamorphism (800–875 °C and >9–10 kbar) and subsequent decompression in a kilometre‐scale, E‐W striking zone of noncoaxial, high‐grade (c. 700–735 °C and 5.8–6.4 kbar) deformation. However, new SHRIMP U‐Pb age determinations of zircon, monazite and titanite from partially melted metabasites and metapelites indicate that granulite facies metamorphism occurred not in the Proterozoic, but in the Ordovician (c. 470 Ma). The youngest metamorphic zircon overgrowths from two metabasites (probably meta‐volcaniclastics) yield ²⁰⁶Pb/²³⁸U ages of 478±4 Ma and 471±7 Ma, whereas those from two metapelites yield ages of 463±5 Ma and 461±4 Ma. Monazite from the two metapelites gave ages equal within error to those from metamorphic zircon rims in the same rock (457±5 Ma and 462±5 Ma, respectively). Zircon, and possibly monazite ages are interpreted as dating precipitation of these minerals from crystallizing melt within leucosomes. In contrast, titanite from the two metabasites yield ²⁰⁶Pb/²³⁸U ages that are much younger (411±5 Ma & 417±7 Ma, respectively) than those of coexisting zircon, which might indicate that the terrane cooled slowly following final melt crystallization. One metabasite has a second titanite population with an age of 384±7 Ma, which reflects titanite growth and/or recrystallization during the 400–300 Ma Alice Springs Orogeny. The c. 380 Ma titanite age is indistinguishable from the age of magmatic zircon from a small, late and weakly deformed plug of biotite granite that intruded the granulites at 387±4 Ma. These data suggest that the northern Harts Range has been subject to at least two periods of reworking (475–460 Ma & 400–300 Ma) during the Palaeozoic. Detrital zircon from the metapelites and metabasites, and inherited zircon from the granite, yield similar ranges of Proterozoic ages, with distinct age clusters at c. 1300–1000 and c. 650 Ma. These data imply that the deposition ages of the protoliths to the Harts Range Complex are late Neoproterozoic or early Palaeozoic, not Palaeoproterozoic as previously assumed.     

Granulite facies metamorphism and subsolidus fluid-absent reworking, Strangways Range, Arunta Block, central Australia

Orthopyroxene‐rich quartz‐saturated granulites of the Strangways Range, Arunta Block, central Australia, record evidence of two high‐grade metamorphic events. Initial granulite facies metamorphism (M₁, at c. 1.7 Ga) involved partial melting and migmatization culminating in conditions of 8.5 kbar and 850 °C. Preservation of the peak M₁ mineral assemblages from these conditions indicates that most of the generated melt was lost from these rocks at or near peak metamorphic conditions. Subsequent reworking (M₂, at c. 1.65 Ga) is characterized by intense deformation, the absence of partial melting and the development of orthopyroxene–sillimanite ± gedrite‐bearing mineral assemblages. Gedrite is only present in cordierite‐rich lithologies where it preferentially replaces M₁ cordierite porphyroblasts. Pseudosection calculations indicate that M₂ occurred at subsolidus fluid‐absent conditions (a_H2o ～ 0.2) at 6–7.5 kbar and 670–720 °C. The mineral assemblages in the reworked rocks are consistent with closed system behaviour with respect to H₂O subsequent to M₁ melt loss. M₂ reworking was primarily driven by increased temperature from the stable geotherm reached after cooling from M₁ and deformation‐induced recrystallization and re‐equilibration, rather than rehydration from an externally derived fluid. The development of the M₂ assemblages is strongly dependent on the intensity of deformation, not only for promoting equilibration, but also for equalizing the volume changes that result from metamorphic reactions. Calculations suggest that the protoliths of the orthopyroxene‐rich granulites were cordierite–orthoamphibole gneisses, rather than pelites, and that the unusual bulk compositions of these rocks were inherited from the protoliths. Melt loss is insufficient to account for the genesis of these rocks from more typical pelitic compositions. In quartz‐rich gneisses, however, melt loss along the M₁ prograde path was able to modify the bulk rock composition sufficiently to stabilize peak metamorphic assemblages different from those that would have otherwise developed.     

Successive overprinting granulite facies metamorphic events in the Anmatjira Range, central Australia

The Anmatjira Range and adjacent Reynolds Range, central Australia, comprise early Proterozoic metasediments and othogneisses that were affected by three, and possibly four, temporally distinct metamorphic events, M_1–4, and deformation events, D_1–4, in the period 1820–1590 Ma. The north-western portion of the range, around Mt Stafford, preserves the effects of ±1820 Ma M₁-D₁, and shows a spectacular lateral transition from muscovite + quartz-bearing schists to interlayered andalusite-bearing migmatites and two-pyroxene granofelses that reflect extremely low-pressure granulite facies conditions, over a distance of less than 10 km. Orthopyroxene + cordierite + garnet + K-feldspar + quartz-bearing gneisses occur at the highest grade, implying peak conditions of ±750°C and 2.5 ± 0.6 kbar. An anticlockwise P–T path for M₁ is inferred from syn- to late-D₁ sillimanite overprinting andalusite, petrogenetic grid considerations and quantitative estimates of metamorphic conditions for inferred overprinting assemblages. The effects of M₁ have been variably overprinted to the south-east by a c. 1760 Ma M₂–D₂ event. Much of the central Anmatjira Range, around Ingellina Gap, comprises orthogneiss, deformed during D₂, and metapelites that have M₁ andalusite and K-feldspar overprinted by M₂ sillimanite and muscovite. The south-eastern portion of the range, around Mt Weldon, comprises metasediments and orthogneisses that were completely recrystallized during M₂–D₂, with metapelitic gneisses characterized by spinel + sillimanite + K-feldspar + quartz-bearing assemblages that suggest peak M₂ conditions of >750°C and 5.5 ± 1 kbar. Overprinting parageneses in metapelitic gneisses imply that D₂ occurred during essentially isobaric cooling. A third granulite facies event, M₃, affected rocks in the Reynolds Range, immediately to the south of the Anmatjira Range, at c. 1730 Ma. A possible fourth event, M₄, with a minimum age of c. 1590 My affected both Ranges, but resulted in only minor overprinting of M_1–3 assemblages. The superimposed effects of M_1–4, mapped for the entire Anmatjira–Reynolds Range area, indicate that only minor or no dislocation of the regional geology occurred during any of the metamorphic and accompanying folding, events. Although the immediate cause of each of the metamorphic events involved advection, the ultimate causes were external to the metasediments and most probably external to the crust.     

Impact-induced melting of Archean granulites in the Vredefort Dome, South Africa. I: anatexis of metapelitic granulites

Mid‐crustal Archean pelitic granulites in the Vredefort Dome experienced a static, low‐P granulite facies overprint associated with the formation of the dome by meteorite impact at 2.02 Ga. Heating and exhumation were virtually instantaneous, with the main source of heat being provided by energy released from nonadiabatic decay of the impact shock wave. Maximum temperatures within a radius of a few kilometres of the centre of the structure exceeded 900 °C and locally even exceeded 1350 °C. This led to comprehensive melting of the precursor Archean granulite assemblages (Grt + Bt + Qtz + Pl + Ksp ± Crd ± Opx ± Sil) followed by peritectic crystallization of aluminous alkali feldspar+Crd + Spl ± Crn ± Sil parageneses and the segregation of small, evolved, biotite leucogranite bodies. However, at a distance of c. 6 km from the centre pre‐impact rock features are largely preserved, although partial replacement of garnet by symplectitic coronas of Crd + Opx ± Spl ± Pl and biotite by orthopyroxene indicate that peak temperatures approached 775 ± 50 °C. Thin interstitial moats of K‐feldspar are closely associated with the orthopyroxene coronas; they are interpreted as the remnants of low‐proportion partial melts generated by biotite breakdown. Both the textures and mineral compositional data support reduced equilibration volumes in these rocks, which reflect rapid isobaric cooling following shock heating and exhumation. The high temperatures and strong lateral thermal gradient are consistent with the modelled impact‐induced isotherm pattern for a 200–300 km diameter impact crater.     

Long‐lived high‐T,low‐P granulite facies metamorphism in the Arunta Region,central Australia

In situ LA–ICP–MS U–Pb monazite geochronology from the Boothby Hills in the Aileron Province, central Australia, indicates that the region records more than 80 Ma of high‐T, low‐P (HTLP) anatectic conditions during the Early Mesoproterozoic. Monazite ages from granulite facies rocks and leucosomes span the interval 1576–1542 Ma. Pegmatites that overprint the regional gneissic fabric and are interpreted to record the last vestiges of melt crystallization give ages between 1523 and 1513 Ma. Calculated P–T pseudosections suggest peak metamorphic conditions in excess of 850 °C at 0.65–0.75 GPa. The retrograde evolution was characterized by a P–T path that involved minor decompression and then cooling, culminating with the development of andalusite. Integration of the geochronological data set with the inferred P–T path trajectory suggests that suprasolidus cooling must have been slow, in the order of 2.5–4 °C Ma^?1. In addition, the retrograde P–T path trajectory suggests that HTLP conditions were generated within crust of relatively normal thickness. Despite the long duration over which anatectic conditions occurred, there is no evidence for external magmatic inputs or evidence that HTLP conditions were associated with long‐lived extension. Instead, it seems probable that the long‐lived HTLP metamorphism was driven to a significant extent by long‐lived conductive heating provided by high crustal heat production in voluminous pre‐metamorphic granitic rocks.     

Low-P–high-T metamorphism and the role of heat transport by melt migration in the Higo Metamorphic Complex, Kyushu, Japan

This paper characterizes the metamorphic thermal structure of the Higo Metamorphic Complex (HMC) and presents the results of a numerical simulation of a geotherm with melt migration and solidification. Reconstruction of the geological and metamorphic structure shows that the HMC initially had a simple thermal structure where metamorphic temperatures and pressures increased towards apparent lower structural levels. Subsequently, this initial thermal structure has been collapsed by E–W and NNE–SSW trending high‐angle faults. Pressure and temperature conditions using the analysis of mineral assemblages and thermobarometry define a metamorphic field P–T array that may be divided into two segments: the array at apparent higher structural levels has a low‐dP/dT slope, whereas that at apparent lower structural levels has a high‐dP/dT slope. This composite array cannot be explained by heat conduction in subsolidus rocks alone. Migmatite is exposed pervasively at apparent lower structural levels, but large syn‐metamorphic plutons are absent at the levels exposed in the HMC. Transport and solidification of melt within migmatite is a potential mechanism to generate the composite array. Thermal modelling of a geotherm with melt migration and solidification shows that the composite thermal structure may be formed by a change of the dominant heat transfer from an advective regime to a conduction regime with decreasing depth. The model also predicts that strata beneath the crossing point will consist of high‐grade solid metamorphic rocks and solidified melt products, such as migmatite. This prediction is consistent with the observation that migmatite was associated with the very high‐dP/dT slope. The melt migration model is able to generate the very high‐dP/dT segment due to the high rate of heat transfer by advection.