David E. Kelsey

On the application of in situ monazite chemical geochronology to constraining P–T–t histories in high-temperature (>850 °C) polymetamorphic granulites from Prydz Bay, East Antarctica

We present electron microprobe-based in situ (Th + U)–Pb monazite chemical age data from granulite-facies metapelites in Prydz Bay, East Antarctica. The monazite age data define two distinct age populations, Late Neoproterozoic to Early Palaeozoic (c. 570–520 Ma) and Neoproterozoic (c. 950–820 Ma), that confirm the polytectonic nature of Prydz Bay. Our data suggest that similarity in lineation orientation along the Prydz Bay coast is not sufficient to necessarily indicate time-equivalance. The minimum duration of Early Palaeozoic tectonism, spanning at least 60 Ma, is constrained from an Mg–Al-rich metapelite: monazite hosted by coarse-grained orthopyroxene defines the oldest Early Palaeozoic population, whereas cordierite + orthopyroxene symplectites define the youngest Early Palaeozoic population. The spatial distribution of monazites and their ages is correlatable with the inferred mineral assemblage evolution. We are able to characterize the evolution of Early Palaeozoic tectonism in quantitative P–T–t–mineral assemblage space, demonstrating that ultrahigh-temperature tectono-metamorphism in Prydz Bay is of Early Palaeozoic age. The survival of Neoproterozoic inheritance in Fe–Al-rich metapelites has implications for high Pb retentivity in monazite when chemical and kinetic conditions are favourable. The approach and logic applied herein are entirely and directly transferable to the interrogation of any other metamorphic terrane.

High-pressure granulites at the dawn of the Proterozoic

ABSTRACTHigh-pressure metamorphism is uncommon in the ancient geo-logical record. Kanja Malai, in the Salem crustal block (southern India), contains high-pressure kyanite-garnet − bearing felsic granu-lites that equilibrated at 14–16 kbar and ~820–860 °C. Laser abla-tion − inductively coupled plasma − mass spectrometry U-Pb zircon and in situ monazite geochronology indicate that these assemblages grew ca. 2490 Ma. These pressure-temperature-time constraints pro-vide a rare record indicating that thickened crust and low apparent thermal gradient conditions existed during the Archean-Proterozoic transition, a period of Earth history for which the rock record com-monly preserves evidence for comparatively high apparent thermal gradients. The thermal regimes required to generate these metamor-phic conditions are typical of collisional orogenesis, and suggest that the continental lithosphere was capable of supporting crustal thick-ening to ≥ 45–50 km. Such crustal thickening provides supporting evi-dence that tectonic regimes similar to modern Earth–style tectonics were in operation at the Archean-Proterozoic transition.INTRODUCTION

Paleoproterozoic orogenesis in the southeastern Gawler Craton, South Australia∗

Integrated structural, metamorphic and geochronological data demonstrate the existence of a contractional orogen preserved in the ca 1850 Ma Donington Suite batholith along the eastern margin of the Gawler Craton, South Australia. The earliest structures are a pervasive gneissic foliation developed in the Donington Suite and interleaved metasedimentary rocks. This has been overprinted by isoclinal and non-cylindrical folding, and zones of pervasive non-coaxial shear with north-directed transport, suggesting that deformation was the result of orogenic contraction. SHRIMP U – Pb zircon data indicate that a syn-contractional granitic dyke was emplaced at 1846 ± 4 Ma. Overprinting the contractional structures are a series of discrete, migmatitic high-strain zones that show a normal geometry with a component of oblique dextral shear. U – Pb zircon data from a weakly foliated microgranite in one such shear zone give an emplacement age of 1843 ± 5 Ma. Rare aluminous metasedimentary rocks in the belt preserve a granulite-grade assemblage of garnet + biotite + plagioclase + K-feldspar + silicate melt that formed at ∼600 MPa and ∼750°C. Peak metamorphic garnets are partially replaced by biotite + sillimanite+ cordierite assemblages suggesting post-thermal peak cooling and decompression, and are indicative of a clockwise P – T evolution. Chemical U – Th – Pb electron microprobe ages from monazites in retrograde biotite yield a minimum estimate for the timing of retrogression of ca 1830 Ma, indicating that decompression may be linked to the development of the broadly extensional shear zones and that the clockwise P – T path occurred during a single tectonothermal cycle. We define this ca 1850 Ma phase of crustal evolution in the eastern Gawler Craton as the Cornian Orogeny.

Evidence for 930 Ma metamorphism in the Shetland Islands, Scottish Caledonides: Implications for Neoproterozoic tectonics in the Laurentia-Baltica sector of Rodinia

Abstract: Zircon and monazite laser-ablation inductively coupled plasma mass spectrometry U–Pb geochronological data for two metasediment samples from the Westing Group, northern Shetland Islands, Scottish Caledonides yield ages between 938 ± 8 and 925 ± 10 Ma (Tonian) for upper amphibolites-facies metamorphism. Texturally early metamorphism is recorded by a migmatitic garnet + sillimanite + plagioclase + muscovite + biotite assemblage, which formed at c. 650–700 °C and 7 kbar. Subsequent reworking resulted in the growth of a secondary garnet + kyanite + plagioclase + muscovite + biotite assemblage at c. 650 °C and 8–9 kbar. In situ electron probe microanalysis (EPMA) U–Th–Pb chemical dating of monazite hosted within garnet grains and the matrix of one sample also give Tonian ages, apparently indicating that all the metamorphism occurred during the Neoproterozoic. However, the dominant structural fabrics appear to have formed during the Ordovician–Silurian Caledonian orogeny, suggesting that the reworking was substantially younger despite the apparent absence of Caledonian monazite or zircon ages. Detrital zircons are consistent with Laurentia–Baltica provenance. Deposition of the Westing Group is constrained to between c. 1030 and 930 Ma. The timing of Tonian metamorphism suggests possible correlations with sequences elsewhere in the northern Caledonides, including the Krummedal Succession of East Greenland and Laurentian-derived successions in Svalbard and northern Norway. Supplementary material: U–Pb LA-ICPMS and EPMA data are available at http://www.geolsoc.org/SUP18379.

P – T constraints and timing of Barrovian metamorphism in the Shetland Islands, Scottish Caledonides: implications for the structural setting of the Unst ophiolite

Abstract: An integrated in situ monazite laser-ablation inductively coupled plasma mass spectrometry and metamorphic equilibria study is used to establish the P–T conditions and timing of Barrovian metamorphism in the Shetland Islands, Scottish Caledonides. The results have implications for the structural setting of the Unst ophiolite, which was obducted onto metasedimentary rocks of the Dalradian Supergroup. Metapelites in the footwall of the ophiolite yield U–Pb ages between 462 and 451 Ma with P–T conditions varying from c. 7.5 kbar and 550 °C directly below the ophiolite to c. 10 kbar and 775 °C at structurally deeper levels. The timing of peak metamorphism corresponds closely to that of Grampian (c. 450–470 Ma) metamorphism in mainland Scotland and Ireland, and Taconic (c. 450–460 Ma) metamorphism in the Appalachians, thus confirming the near-synchroneity of this important arc accretion event along the Laurentian margin. There is a significant metamorphic contrast between the low-grade rocks associated with the Unst ophiolite and the P–T conditions recorded in its footwall. If published K–Ar ages of c. 470 Ma broadly record obduction of the ophiolite, its present basal contact is probably a younger tectonic break that was associated with the excision of at least c. 10 km of crustal section. Supplementary material: Electron microporobe chemical analyses of garnet grains are available at www.geolsoc.org.uk/SUP18494

Orogenic versus extensional settings for regional metamorphism: Knoydartian events in the Moine Supergroup revisited

Abstract: Considerable debate exists over the tectonic regimes associated with mid-Neoproterozoic metamorphism of the Moine Supergroup, NW Scotland. Published pressure conditions imply burial to 35–40 km, a potential doubling of crustal thickness, and hence a substantial collisional event. Re-evaluation using updated thermodynamic software suggests more modest peak pressures of c. 7.5 kbar implying burial to c. 21 km. The revised P–T path has a comparatively flat clockwise evolution from early high geothermal gradient conditions. The revised P–T conditions suggest that c. 800 Ma crustal thickening within the Moine Supergroup was less significant than previously envisaged and possibly preceded by extension.

Linking crustal reworking to terrane accretion

The Strangways Metamorphic Complex in central Australia is a key terrane in models for the evolution of the Australian continent. The Arunta Complex preserves a long-lived (c. 1700–320 Ma) record of crustal reworking, the drivers of which are the subject of considerable debate. Pressure–temperature data and in situ monazite geochronology constrain the reworking to be coincident with the c.1645 Ma Liebig Orogeny. This suggests that reworking in the Strangways Metamorphic Complex is related to terrane accretion rather than other factors such as anomalous enrichment in high-heat-producing elements or within-plate processes.

Protolith heterogeneity as a factor controlling the feedback between deformation, metamorphism and melting in a granulite-hosted gold deposit

Abstract: Structures and alteration associated with mineralizing, pre-anatectic hydrothermal alteration at the Challenger gold mine in South Australia strongly influenced and preferentially focused subsequent metamorphism, partial melting and deformation. This event resulted in efficient deformation-enhanced melt extraction from the host rocks immediately adjacent to mineralization, leaving restitic garnet, cordierite, K-feldspar rich mineral assemblages. Progressive reintegration of silicate melt into the highly restitic proximal gneiss bulk composition, using mineral equilibrium modelling in the system NCKFMASHTO, indicates that hydrothermal alteration stabilized a higher proportion of hydrous minerals leading to increased fertility and thus melting in the proximal gneiss. Further modelling at lower temperatures provides an insight into mineral assemblages that may have been associated with mineralization. Temperature–composition modelling provides evidence that composite gold–sulphide melts were armoured by quartz and as such were immobile during peak metamorphism. At Challenger, melt permeability of the mineralized zones was enhanced via the formation of a structural fabric, folding, boudinage, fracturing and the formation of shear zones. Giving consideration to the relationships and orientations of these structures, a model for preferential deformation and melt migration within the mineralized shear zone under transpression is developed, where melt is essentially pumped out of the system parallel to the maximum principal elongation direction.

A curious case of agreement between conventional thermobarometry and phase equilibria modelling in granulites: New constraints on P–T estimates in the Antarctica segment of the Musgrave–Albany–Fraser–Wilkes Orogen

The Windmill Islands region in Wilkes Land, east Antarctica, preserves granulite facies metamorphic mineral assemblages that yield seemingly comparable P–T estimates from conventional thermobarometry and mineral equilibria modelling. This is uncommon in granulite facies terranes, where conventional thermobarometry and phase equilibria modelling generally produce conflicting P–T estimates because peak mineral compositions tend to be modified by retrograde diffusion processes. In situ U–Pb monazite geochronology and calculated metamorphic phase diagrams show that the Windmill Islands experienced two phases of high thermal gradient metamorphism during the Mesoproterozoic. The first phase of metamorphism is recorded by monazite ages in two widely separated samples, and occurred at c. 1305 Ma. This event was regional in extent, involved crustally-derived magmatism and reached conditions of ~3.2–5 kbar and 690–770 °C. corresponding to very high thermal gradients of >150 °C/kbar. The elevated thermal regime is interpreted to reflect a period of extension or increased extension in a back-arc setting that existed prior to c. 1330 Ma. The first metamorphic event was overprinted by granulite facies metamorphism at c. 1180 Ma that was coeval with the intrusion of charnockite. This event involved peak temperatures of ~840–850 °C and pressures of ~4–5 kbar. A phase of granitic magmatism at c. 1250–1210 Ma, prior to the intrusion of the charnockite, is interpreted to reflect a phase of compression within an overall back-arc setting. Existing conventional thermobarometry suggests conditions of ~4 kbar and 750 °C for M1, and 4–7 kbar and 750–900 °C for M2. The apparent similarities between the phase equilibria modelling and existing conventional thermobarometry may suggest either that the terrane cooled relatively quickly, or that the P–T ranges obtained from conventional thermobarometry are sufficiently imprecise that they cover the range of P–T conditions obtained in this study. However, without phase equilibria modelling, the veracity of existing conventional P–T estimates cannot be evaluated. The calculated phase diagrams from this study allow the direct comparison of P–T conditions in the Windmill Islands with phase equilibria models from other regions in the Musgrave–Albany–Fraser–Wilkes Orogen. This shows that the metamorphic evolution of the Wilkes Land region is very similar to that of the eastern Albany–Fraser Orogen and Musgrave Province in Australia, and further demonstrates the remarkable consistency in the timing of metamorphism and the thermal gradients along the ~5000 km strike length of this system. This article is protected by copyright. All rights reserved.

Significance of post-peak metamorphic reaction microstructures in the ultrahigh temperature Eastern Ghats Province, India

Ultrahigh temperature (UHT) granulites in the Eastern Ghats Province (EGP) have a complex P–T–t history. We review the P–T histories of UHT metamorphism in the EGP and use that as a framework for investigating the P–T–t history of Mg–Al-rich granulites from Anakapalle, with the express purpose of trying to reconcile the down pressure-dominated P–T path with other UHT localities in the EGP. Mafic granulite that is host to Mg–Al-rich metasedimentary granulites at Anakapalle has a protolith age of c. 1580 Ma. Mg–Al-rich metasedimentary granulites within the mafic granulite at Anakapalle were metamorphosed at UHT conditions during tectonism at 960–875 Ma, meaning that the UHT metamorphism was not the result of contact metamorphism from emplacement of the host mafic rock. Reworking occurred during the Pan-African (c. 600–500 Ma) event, and is interpreted to have produced hydrous assemblages that overprint the post-peak high-temperature retrograde assemblages. In contrast to rocks elsewhere in the EGP that developed post-peak cordierite, the metasedimentary granulites at Anakapalle developed post-peak, generation ‘2′ reaction products that are cordierite-absent and nominally anhydrous. Therefore, rocks at Anakapalle offer the unique opportunity to quantify the pressure drop that occurred during so-called M2 that affected the EGP. We argue that M2 is either a continuation of M1 and that the overall P–T path shape is a complex counter-clockwise loop, or that M1 is an up-temperature counter-clockwise deviation superimposed on the M2 path. Therefore, rather than the rocks at Anakapalle having a metamorphic history that is apparently anomalous from the rest of the EGP, we interpret that other previously studied localities in the EGP record a different part of the same P–T path history as Anakapalle, but do not preserve a significant record of pressure decrease. This is due either to the inability of refractory rocks to extensively react to produce a rich mineralogical record of pressure decrease, or because the earlier high-P part of the rocks history was erased by the M1 loop. Irrespective of the specific scenario, models for the tectonic evolution of the EGP must take the substantial pressure decrease during M2 into account, as it is probable the P–T record at Anakapalle is a reflection of tectonics affecting the entire province. This article is protected by copyright. All rights reserved.