Marnie Forster

Earth’s youngest known ultrahigh-temperature granulites discovered on Seram, eastern Indonesia

Episodes of ultrahigh-temperature (UHT, ≥900 °C) granulite metamorphism have been recorded in mountain belts since the Neoarchean. However, evidence for the tectonic mechanisms responsible for the generation of such extreme thermal conditions is rarely preserved. Here we report the discovery of 16 Ma UHT granulites—the youngest identified at the Earth’s surface—from the Kobipoto Mountains of Seram in eastern Indonesia. UHT conditions were produced by a modern tectonic system in which slab rollback–driven lithospheric extension caused core complex–style exhumation of hot subcontinental lithospheric mantle. Overlying continental crust, heated and metamorphosed by exhumed lherzolites, developed spinel + quartz and sapphirine-bearing residual assemblages, shown by phase equilibria modeling to have required temperatures of ∼950 °C at ∼8 kbar pressure. Seram is therefore a possible modern analogue for ancient orogens that incorporate UHT granulites.

The nature and origin of the Barrovian metamorphism, Scotland: 40Ar/39Ar apparent age patterns and the duration of metamorphism in the biotite zone

Abstract: A geochronological traverse across the Barrovian metamorphic series, Scotland, shows 40Ar/39Ar apparent age spectra that reflect the influence of progressive metamorphism during the Grampian orogenic episode. The lowest-grade units of the Barrovian metamorphic series retain pre-Grampian detrital ages as components of their white mica 40Ar/39Ar apparent age spectra. These relict ages are progressively obliterated in the direction of increasing metamorphic grade, with a Grampian-age 40Ar/39Ar step-heating plateau first occurring in the biotite zone. The microstructure at this point shows only limited recrystallization, suggesting loss of argon mainly by diffusion. Forward modelling of argon diffusion from white mica grains was therefore carried out, for various thermal histories and grain sizes, to match 40Ar/39Ar step-heating apparent age spectra patterns preserved within the biotite zone of the Barrovian metamorphic series. The results imply a thermal duration of between 1 and 10 Ma for Barrovian metamorphism in the biotite zone. Such short time scales for metamorphism place a limit on length scales for the heat sources responsible. Mid-crustal extensional ductile shear zones that crop out in the NE of the Grampian Terrane once focused narrow, Grampian-age heat sources (e.g. magmas, hot fluids, shear heating) that drove a brief thermal episode, resulting in the Barrovian metamorphism. Supplementary material: 40Ar/39Ar data tables and plots, including (1) data from analyses on flux monitors and plots used for J-factor determination, and (2) data from analyses on unknowns and 40Ar/39Ar step-heating spectra and inverse isochron plots for each unknown, are available at http://www.geolsoc.org.uk/SUP18442.

40Ar/39Ar geochronology and the diffusion of 39Ar in phengite–muscovite intergrowths during step-heating experiments in vacuo

Abstract Step-heating experiments in vacuo are routine when conducting 40Ar/39Ar geochronology, including for white mica. White mica can break down, due to dehydroxylation and delamination, so experiments involving mica are often conducted in relative haste, and not with the care and precision necessary when intending to apply multi-diffusion-domain theory to model the results. Here we show, however, that carefully managed step-heating experiments appear to allow release of argon through solid-state diffusion processes alone. We analysed phengite-muscovite intergrowths in high-pressure metamorphic rocks exhumed in and beneath extensional ductile shear zones during continental extension. Such materials often yield Arrhenius plots in which there is a distinct steepening of slope mid-way through the step-heating sequence. This steepening appears to correspond with steps in which release of argon from phengite components dominate. We analysed the data using a computer program (eArgon) and numerically simulated mixing of gas released from multiple diffusion domains. The results suggest that diffusion of 39Ar in phengitic white mica involves radically different diffusion parameters in comparison with muscovite. If these results extrapolate to nature then 40Ar/39Ar geochronology may allow direct dating of white mica mineral growth during metamorphism. Supplementary material: Data files A, B and C are available at www.geolsoc.org.uk/SUP18619. Data file A C++ computer code used to infer data for an Arrhenius plot, assuming different diffusion geometries. These methods are excerpted from the eArgon computer program used to analyse these data. Data file B Analytical methods and procedures used in the laboratory for 40Ar/39Ar geochronology performed on the samples reported. Data file C XML formatted data tables for the step-heating experiments reported in this study, in a form that can be read by the eArgon computer program.

The timing of sedimentation and Buchan metamorphism in the Grampian Terrane in Scotland from 40Ar/39Ar apparent age spectra

New 40Ar/39Ar geochronological data from the Buchan and Barrovian metamorphic sequences on the NE coast of Scotland have been obtained from in vacuo step-heating experiments conducted on white mica extracted from seven samples. The results confirm the synchronous formation of the Barrovian and Buchan metamorphic sequences, with the 40Ar/39Ar apparent age spectra obtained for samples of andalusite grade and higher recording a range of ages between 470.4 ± 1.8 and 464.6 ± 1.3 Ma. These ages are close to the accepted age for peak metamorphism during the Grampian Orogeny, c. 470 Ma, whereas 40Ar/39Ar apparent age spectra of white mica from the lower grade cordierite zone near Banff preserve pre-Grampian detrital ages. In these spectra an age of c. 560 Ma is recorded. This indicates that a tectonothermal event occurred at c. 560 Ma and may provide further insight into the timing of sedimentation. These samples also preserve evidence of ages older than 750 Ma. Supplementary material: 40Ar/39Ar step-heating data tables for all samples, data from analyses of flux monitors, determination of correction factors and mass discrimination used for data reduction and 40Ar/39Ar step-heating spectra and inverse isochron plots from each sample are available at www.geolsoc.org.uk/SUP18723.

Dating movement in shear zones: The example of the South Cyclades Shear Zone, Ios, Aegean Sea, Greece

The South Cyclades Shear Zone (SCSZ) is a ~1 km thick bowed-up ductile shear zone that defines the carapace of a deeply eroded gneiss dome that outcrops on the island of Ios in the Cycladic archipelago, Aegean Sea, Greece. The discovery of the SCSZ played a pivotal role in the recognition of the Aegean metamorphic core complexes, and this supposedly Miocene shear zone has been studied in detail by several authors. This contribution illustrates the difficulties involved in systematically determining the age of deformation fabrics using 40Ar/39Ar geochronology, and provides a case study that shows the value of carefully designed step-heating experiments on microstructurally controlled samples. In samples such as these, from complex metamorphic tectonites, laser fusion, or step heating experiments with relatively few steps, would have led to erroneous conclusions.

Porphyroblast rotation versus non-rotation: Conflict resolution: COMMENT

[Fay et al. (2008][1], p. 307) state that: “no porphyroblast rotation occurs during ductile deformation relative to spatial coordinates” and that “porphyroblasts can now be routinely used to access lengthy structural and/or metamorphic histories destroyed in the matrix by reactivation such as

Lineaments and earthquake ruptures on the East Japan megathrust

Additional support derived from Australian Research Council Discovery Project DP120103554 and Linkage Project LP130100134. EJR acknowledges Australian Laureate Fellowship FL120100050

Comment on “Dextral transpression and late-Eocene magmatism in the trans-Himalayan Ladakh Batholith (North India): implications for tectono-magmatic evolution of the Indo-Eurasian collisional arc”

We would like to congratulate Sen and Collins (2012) for producing a detailed study of the structural history of the Ladakh Batholith according to new LA-ICPMS U/Pb analyses of zircon and AMS measurements. This work provides some significant findings to the evolution of this part of the Himalaya. However, we wanted to draw the author’s attention to the omission of a discussion of other geochronological results from the region (White et al. 2012). This is because the results are particularly important for understanding the India–Asia collision. We also raise concerns about some of the conclusions that were drawn from AMS measurement in regard to postcrystallization deformation of the Ladakh Batholith granitoids. Switches between Iand S-type granites during the Himalayan orogeny