Abstract Step-heating experiments in vacuo are routine when conducting 40 Ar/ 39 Ar 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 39 Ar in phengitic white mica involves radically different diffusion parameters in comparison with muscovite. If these results extrapolate to nature then 40 Ar/ 39 Ar geochronology may allow direct dating of white mica mineral growth during metamorphism. Supplementary material: Data files A, B and C are available at http://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 40 Ar/ 39 Ar 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.

Abstract This contribution presents a new model for the Grampian-age tectonothermal development of the Buchan Block and Barrovian-type regions to its west, in the Grampian Terrane, Scotland. The model has drawn on evidence gathered from field mapping, microstructural analysis, metamorphic petrology and mafic magma geochemistry to propose that emplacement of the Grampian gabbros and regional metamorphic heating associated with production of Barrovian- and Buchan-type units occurred during syn-orogenic (Grampian-age), lithospheric-scale extension. Extension followed lithospheric thickening associated with the initiation of Grampian orogenesis and was followed by renewed lithospheric thickening and termination of the extensional heating. Mantle melting to produce the Grampian gabbros of the Grampian Terrane was achieved by extensional thinning of the lithosphere and decompression melting of the asthenosphere at depths of less than 70 km. Advection of heat from the mantle with emplacement of the Grampian gabbros augmented elevated heat budgets associated with attenuation of isotherms during extension. Deposition of the uppermost Dalradian (the Whitehills and Boyndie Bay Groups and the Macduff Slates) occurred during Grampian-age lithospheric extension. A gently-dipping, mid-crustal detachment focused metamorphic heat sources and accommodated significant lithospheric-scale strain, allowing independent thermal evolution of units in its hanging wall (the Buchan Block) and footwall (Barrovian-type units).

The Alpine orogen in the western Mediterranean region, consisting of the Rif-Betic belt and the Apennine-Calabrian-Maghrebide belt, is a classic example of an arcuate orogen. It contains fragments of Cretaceous to Oligocene high-pressure/low-temperature (HP/LT) rocks, which were exhumed and dispersed during post-Oligocene extensional deformation and are presently exposed in the soles of metamorphic core complexes. In this paper, we illustrate that the arcuate shape of the orogenic belt was attained during extensional destruction of the earlier HP/LT belt, driven by subduction rollback in a direction oblique or orthogonal to the direction of convergence. Since the Oligocene, sub-duction of Mesozoic oceanic lithosphere, accompanied by rollback of the subducting slab, led to progressive bending and episodic tearing of the slab. This process resulted in the formation of several slab segments presently recognized in tomographic images beneath the Alboran Sea, North Africa and Italy. The remnant slabs can account for nearly all the volume of oceanic domains that existed in the western Mediterranean during the Oligocene. Subduction rollback led to extension in the overriding plate and to the opening of backarc basins. Extensional tectonism affected the original, relatively non-arcuate HP/LT belt. Allochthonous fragments of the original belt (e.g., Alpine Corsica, Calabria, and the Internal Betic) rotated and drifted as independent units until they were accreted in an arcuate fashion into the continental paleomargins of Africa, Iberia, and Adria. Therefore, the present exposures of HP/LT metamorphic rocks in the western Mediterranean region do not represent sites of continental collisions between major large-scale tectonic plates.