Plate tectonics: When ancient continents collide

The geological record preserves scant evidence for early plate tectonics. Analysis of eclogites — metamorphic rocks formed in subduction zones — in the Trans-Hudson mountain belt suggests modern-style subduction may have operated 1,800 million years ago

Determining cooling rates from mica ⁴⁰Ar/ ³⁹Ar thermochronology data: effect of cooling path shape

Tectonic models are commonly underpinned by metamorphic cooling rates derived fromd iffusive-loss thermochronology data. Such cooling agesare usually linked to temperature via Dodson’s 1973 closure temperature (TC) formulation, which specifies a 1/time shaped cooling path. Geologists, however,commonly discuss cooling rates as a linear temperature/time shape. We present the results of a series of simple finite-difference diffusion models for Ar diffusion in muscovite and biotite that show that the difference in recorded age between 1/t and linear cooling paths increases significantly with hotter starting temperatures, slower cooling rates and smaller grain sizes. Our results show that it is essential to constrain the cooling path shape in order to make meaningful interpretations of the measured data

Garnet–monazite rare earth element relationships in sub-solidus metapelites: a case study from Bhutan

A key aim of modern metamorphic geochronology is to constrain precise and accurate rates and timescales of tectonic processes. One promising approach in amphibolite and granulite-facies rocks links the geochronological information recorded in zoned accessory phases such as monazite to the pressure–temperature information recorded in zoned major rock-forming minerals such as garnet. Both phases incorporate rare earth elements (REE) as they crystallize and their equilibrium partitioning behaviour potentially provides a useful way of linking time to temperature. We report REE data from sub-solidus amphibolite-facies metapelites from Bhutan, where overlapping ages, inclusion relationships and Gd/Lu ratios suggest that garnet and monazite co-crystallized. The garnet–monazite REE relationships in these samples show a steeper pattern across the heavy (H)REE than previously reported. The difference between our dataset and the previously reported data may be due to a temperature-dependence on the partition coefficients, disequilibrium in either dataset, differences in monazite chemistry or the presence or absence of a third phase that competed for the available REE during growth. We urge caution against using empirically-derived partition coefficients from natural samples as evidence for, or against, equilibrium of REE-bearing phases until monazite–garnet partitioning behaviour is better constrained

Natural fracture patterns at Swift Reservoir anticline, NW Montana : the influence of structural position and lithology from multiple observation scales

Acknowledgements We gratefully acknowledge constructive reviews by Amerigo Corradetti and an anonymous reviewer and thank Stefano Tavani for editorial handling. Adam J. Cawood is grateful to David Ferrill, Kevin Smart, and Paul Gillespie for helpful conversations about fracture patterns, although the data and interpretations shown here are of course the sole responsibility of the authors. This study was carried out as part of a University of Aberdeen doctoral programme supported by the Natural Environment Research Council (NERC) Centre for Doctoral Training in Oil and Gas. Additional funding for fieldwork was provided by the University of Aberdeen Fold–Thrust Research Group. Petroleum Experts (formerly Midland Valley Exploration) is acknowledged for allowing the academic use of Move 2016.1 software. Financial support This research has been supported by the Natural Environment Research Council (grant no. NE/M00578X/1).Peer reviewedPublisher PD

Fracture distribution on the Swift Reservoir Anticline, Montana : implications for structural and lithological controls on fracture intensity

Title of special publication: Folding and Fracturing of Rocks: 50 Years of Research since the Seminal Text Book of J. G. Ramsay This research was funded by Oil Search Ltd, Santos Ltd and InterOil, through the University of Aberdeen Fold-Thrust Research Group. Electron Microscopy was performed in the ACEMAC Facility at the University of Aberdeen with assistance from John Still. Joyce Neilson is thanked for advice on the use of ImageJ software. Midland Valley are thanked for the use of their Move software for field data collection and model building. We thank Alfred Lacazette and Stefano Tavani for reviewing the manuscript and providing constructive comments.Peer reviewedPostprin

Management options for gamba grass (Andropogon gayanus) in conservation areas of Cape York Peninsula Final report

Andropogon gayanus (gamba grass) is a high-biomass grass native to tropical and subtropical Africa and introduced into Australia as a pasture grass. Under well-managed grazing conditions, gamba grass has proven a useful and palatable addition to tropical cattle pastures. However, it has also become a significant environmental weed and is considered an ecosystem transformer. In recognition of the significant threat posed by gamba grass, it has been listed (along with 4 other invasive grasses) as a key threatening process under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act). One of the major problems limiting the effective management of gamba grass once established as an environmental weed is the lack of registered herbicides for use in natural systems and conservation areas. Glyphosate is the primary herbicide in use in northern Australia. There are several current and emerging issues which make a reliance on glyphosate for gamba grass control problematic. Application of glyphosate is logistically difficult in wet and remote areas, it has no residual action and largely relies on follow-up treatments, and there are emerging resistance issues. In addition, there is growing concern that glyphosate may be linked to carcinogenicity, genotoxicity and epidemiological disorders. Alternative herbicides are critical to allow long-term, effective and timely control of gamba grass in the environments encountered on Cape York Peninsula and across northern Australia. The goal of this project was to collate existing knowledge related to control and management of gamba grass and test alternative herbicide options for use in natural areas of Cape York Peninsula. Three herbicides were tested alongside glyphosate in field trials and 10 residual pre-emergence herbicides were tested in pot trials. Neither the field nor pot trials identified a clear suitable alternative to glyphosate that selectively controlled gamba grass with low off-target effects in the contexts in which we tested them. However, there are several herbicides that warrant further testing at a range of additional application rates and in a range of environments (flupropanate, clomazone, oxyfluorfen, imazapyr and indaziflam). In particular, the granular form of flupropanate is worthy of further experimentation because of its portability in the field and flexibility in application, and because it showed the most promising results in the field trials. Ultimately, land managers may need to trade-off significant, short-term, off-target effects for longer term, more effective and permanent control of gamba grass with herbicides

Evolution of the melt source during protracted crustal anatexis: An example from the Bhutan Himalaya

The chemical compositions of magmatic zircon growth zones provide powerful insight into evolving magma compositions due to their ability to record both time and the local chemical environment. In situ U-Pb and Hf isotope analyses of zircon rims from Oligocene–Miocene leucogranites of the Bhutan Himalaya reveal, for the first time, an evolution in melt composition between 32 and 12 Ma. The data indicate a uniform melt source from 32 Ma to 17 Ma, and the progressive addition of an older source component to the melt from at least ca. 17 Ma. Age-corrected εHf ratios decrease from between –10 and –15 down to values as low as –23 by 12 Ma. Complementary whole-rock Nd isotope data corroborate the Hf data, with a progressive decrease in εNd(t) from ca. 18 to 12 Ma. Published zircon and whole-rock Nd data from different lithotectonic units in the Himalaya suggest a chemical distinction between the younger Greater Himalayan Series (GHS) and the older Lesser Himalayan Series (LHS). The time-dependent isotopic evolution shown in the leucogranites demonstrates a progressive increase in melt contribution from older lithologies, suggestive of increasing LHS involvement in Himalayan melting over time. The time-resolved data are consistent with LHS material being progressively accreted to the base of the GHS from ca. 17 Ma, facilitated by deformation along the Main Central thrust. From 17 Ma, decompression, which had triggered anatexis in the GHS since the Paleogene, enabled melting in older sources from the accreted LHS, now forming the lowermost hanging wall of the thrust

Recycling Argon through Metamorphic Reactions: the Record in Symplectites

The 40Ar/39Ar ages of metamorphic micas that crystallized at high temperatures are commonly interpreted as cooling ages, with grains considered to have lost 40Ar via thermally-driven diffusion into the grain boundary network. Recently reported laser-ablation data suggest that the spatial distribution of Ar in metamorphic micas does not always conform to the patterns predicted by diffusion theory and that despite high metamorphic temperatures, argon was not removed efficiently from the local system during metamorphic evolution. In the Western Gneiss Region (WGR), Norway, felsic gneisses preserve microtextural evidence for the breakdown of phengite to biotite and plagioclase symplectites during near isothermal decompression from c. 20–25 to c. 8–12 kbar at ~700°C. These samples provide an ideal natural laboratory to assess whether the complete replacement of one K-bearing mineral by another at high temperatures completely ‘resets’ the Ar clock, or whether there is some inheritance of 40Ar in the neo-crystallized phase. The timing of the high-temperature portion of the WGR metamorphic cycle has been well constrained in previous studies. However, the timing of cooling following the overprint is still much debated. In-situ laser ablation spot dating in phengite, biotite-plagioclase symplectites and coarser, texturally later biotite yielded 40Ar/39Ar ages that span much of the metamorphic cycle. Together these data show that despite residence at temperatures of ~700°C, Ar is not completely removed by diffusive loss or during metamorphic recrystallization. Instead, Ar released during phengite breakdown appears to be partially reincorporated into the newly crystallizing biotite and plagioclase (or is trapped in fluid inclusions in those phases) within a close system. Our data show that the microtextural and petrographic evolution of the sample being dated provides a critical framework in which local 40Ar recycling can be tracked, thus potentially allowing 40Ar/39Ar dates to be linked more accurately to metamorphic history

Argon behaviour in an inverted Barrovian sequence, Sikkim Himalaya: the consequences of temperature and timescale on ⁴⁰Ar/³⁹Ar mica geochronology

40Ar/39Ar dating of metamorphic rocks sometimes yields complicated datasets which are difficult to interpret in terms of timescales of the metamorphic cycle. Single-grain fusion and step-heating data were obtained for rocks sampled through a major thrust-sense shear zone (the Main Central Thrust) and the associated inverted metamorphic zone in the Sikkim region of the eastern Himalaya. This transect provides a natural laboratory to explore factors influencing apparent 40Ar/39Ar ages in similar lithologies at a variety of metamorphic pressure and temperature (P–T) conditions. The 40Ar/39Ar dataset records progressively younger apparent age populations and a decrease in within-sample dispersion with increasing temperature through the sequence. The white mica populations span ~ 2–9 Ma within each sample in the structurally lower levels (garnet grade) but only ~ 0–3 Ma at structurally higher levels (kyanite-sillimanite grade). Mean white mica single-grain fusion population ages vary from 16.2 ± 3.9 Ma (2σ) to 13.2 ± 1.3 Ma (2σ) from lowest to highest levels. White mica step-heating data from the same samples yields plateau ages from 14.27 ± 0.13 Ma to 12.96 ± 0.05 Ma. Biotite yield older apparent age populations with mean single-grain fusion dates varying from 74.7 ± 11.8 Ma (2σ) at the lowest structural levels to 18.6 ± 4.7 Ma (2σ) at the highest structural levels; the step-heating plateaux are commonly disturbed. Temperatures > 600 °C at pressures of 0.4–0.8 GPa sustained over > 5 Ma, appear to be required for white mica and biotite ages to be consistent with diffusive, open-system cooling. At lower temperatures, and/or over shorter metamorphic timescales, more 40Ar is retained than results from simple diffusion models suggest. Diffusion modelling of Ar in white mica from the highest structural levels suggests that the high-temperature rocks cooled at a rate of ~ 50–80 °C Ma− 1, consistent with rapid thrusting, extrusion and exhumation along the Main Central Thrust during the mid-Miocene

Timing and conditions of peak metamorphism and cooling across the Zimithang Thrust, Arunachal Pradesh, India

The Zimithang Thrust juxtaposes two lithotectonic units of the Greater Himalayan Sequence in Arunachal Pradesh, NE India. Monazite U–Pb, muscovite 40Ar/39Ar and thermobarometric data from rocks in the hanging and footwall constrain the timing and conditions of their juxtaposition across the structure, and their subsequent cooling. Monazite grains in biotite–sillimanite gneiss in the hanging wall yield LA-ICP-MS U–Pb ages of 16 ± 0.2 to 12.7 ± 0.4 Ma. A schistose gneiss within the high strain zone yields overlapping-to-younger monazite ages of 14.9 ± 0.3 to 11.5 ± 0.3 Ma. Garnet–staurolite–mica schists in the immediate footwall yield older monazite ages of 27.3 ± 0.6 to 17.1 ± 0.2 Ma. Temperature estimates from Ti-in-biotite and garnet–biotite thermometry suggest similar peak temperatures were achieved in the hanging and footwalls (~ 525–650 °C). Elevated temperatures of ~ 700 °C appear to have been reached in the high strain zone itself and in the footwall further from the thrust. Single grain fusion 40Ar/39Ar muscovite data from samples either side of the thrust yield ages of ~ 7 Ma, suggesting that movement along the thrust juxtaposed the two units by the time the closure temperature of Ar diffusion in muscovite had been reached. These data confirm previous suggestions that major orogen-parallel out-of-sequence structures disrupt the Greater Himalayan Sequence at different times during Himalayan evolution, and highlight an eastwards-younging trend in 40Ar/39Ar muscovite cooling ages at equivalent structural levels along Himalayan strike