Crustal growth in south-eastern Australia—evidence from lower crustal eclogitic and granulitic xenoliths

Mafic and ultramafic xenoliths in a basaltic cone at The Anakies in south-eastern Australia are geochemically equivalent to continental basaltic magmas and cumulates. The xenolith microstructures range from recognizably meta-igneous for intrusive rocks to granoblastic for garnet pyroxenites. Contact relationships between different rock types within some xenoliths suggest a complex petrogenesis of multiple intrusive, metamorphic and metasomatic events at the crust/mantle boundary during the evolution of south-eastern Australia. Unaltered spinel lher-zolite, typical of the uppermost eastern Australian mantle, is interleaved with or veined by the metamorphosed intrusive rocks of basaltic composition. Geothermobarometry calculations by a variety of methods show a concordance of equilibration temperatures ranging from 880°C to 980°C and pressures of 12 to 18 kbar (1200-1800 mPa). These physical conditions span the gabbro to granulite to eclogite transition boundaries. The water-vapour pressure during equilibration is estimated to be about 0.5% of the load pressure, using amphibole breakdown data. Large fluid inclusions of pure CO₂ are abundant in the mineral phases in the xenoliths, and it is suggested that flux of CO₂ from the mantle has been an important heat source and fluid medium during metamorphism of the mafic and ultramafic protoliths at the lower crust/upper mantle boundary. The calculated pressures and temperatures suggest that the south-eastern Australian crust has sustained a high geothermal gradient. In addition, the nature of the mineral assemblages and the contact relationships of granulitic rock with spinel lherzolite, characteristic of mantle material, suggest that the Moho is not a discrete feature in this region, but is represented by a transition zone approximately 20 km thick. These inferences are in agreement with geophysical data (including seismic, heat-flow and electrical resistivity data) determined for south-eastern Australia. Underplating at the crust/mantle boundary by continental basaltic magmas may be an important alternative or additional mechanism to the conventional andesite model for crustal accretion.     

P–T conditions for the Arendal granulites, southern Norway: implications for the roles of P, T and CO2 in deep crustal LILE-depletion

Abstract The orthopyroxene-clinopyroxene, garnet-orthopyroxene and garnet-clinopyroxene geothermometers, and the garnet-orthopyroxene-plagioclase, garnet-clinopyroxene-plagioclase and anorthite-ferrosilite-grossular-almandine-quartz geobarometers are applied to metabasites and the garnetplagioclase-sillimanite-quartz geobarometer is applied to a metapelite from the Proterozoic Arendal granulite terrain, Bamble sector, Norway. P–T conditions of metamorphism were 7.3 ± 0.5 kbar and 800 ± 60°C.
This terrain shows a regional gradation from the amphibolite facies, into normal LILE content granulite facies rocks and finally strongly LILE deficient granulite facies gneisses. Neither P nor T vary significantly across the entire transition zone. The change in 'grade'parallels the increasing dominance of CO₂ over H₂O in the fluid phase.
LILE-depletion is not a pre-condition of granulite facies metamorphism: granulites may have either 'depleted'or 'normal'chemistries. The results presented herein show that LILE-deficiency in granulite facies orthogneisses is not necessarily related to variations in either P or T . The important mechanisms in the Arendal terrain were (a) direct synmetamorphic crystallization from magma, with primary LILE-poor mineralogies imposed by the prevailing fluid regime, and (b) metamorphic depletion, involving scavenging of LILEs during flushing by mantle-derived CO₂-rich fluids. The latter process is constrained by U–Pb and Rb–Sr isotopic work to have occurred no later than 50 Ma after intrusion of the acid-intermediate gneisses, and was probably associated with contemporary basic magmatism in a tectonic environment similar to a present day cordilleran continental margin.     

The K-band Hubble diagram of submillimetre galaxies and hyperluminous galaxies

We present the K -band Hubble diagrams ( K – z relations) of submillimetre-selected galaxies and hyperluminous galaxies (HLIRGs). We report the discovery of a remarkably tight K – z relation of HLIRGs, indistinguishable from that of the most luminous radio galaxies. Like radio galaxies, the HLIRG K – z relation at   z ≲ 3  is consistent with a passively evolving ∼3 L _* instantaneous starburst starting from a redshift of   z ∼ 10  . In contrast, many submillimetre-selected galaxies are ≳2 mag fainter, and the population has a much larger dispersion. We argue that dust obscuration and/or a larger mass range may be responsible for this scatter. The galaxies so far proved to be hyperluminous may have been biased towards higher AGN bolometric contributions than submillimetre-selected galaxies due to the 60-μm selection of some, so the location on the K – z relation may be related to the presence of the most massive active galactic nucleus. Alternatively, a particular host galaxy mass range may be responsible for both extreme star formation and the most massive active nuclei.