Abstract In the Late Archean north-trending Closepet pluton, trains of euhedral K-feldspar phenocrysts and matrix-supported idiomorphic K-feldspar crystals in the central part of the pluton define oblique-to-pluton margin steep-dipping east/ENE-trending magmatic fabrics. The magmatic fabric is defined by phenocryst-rich and phenocryst-poor layers, with the euhedral porphyries continuous across the layers. The fabrics are near-orthogonal to the gently-dipping gneissic layers in the host gneisses. The fabrics curve adjacent to locally-developed north/NNE-trending melt-hosted dislocations parallel to the axial planes of horizontal/gently-plunging north-trending upright folds in the host gneisses. In the pluton interior, both fabrics in the intrusives formed at supra-solidus conditions, although the volume fraction of melts diminished drastically due to cooling/melt expulsion. At the pluton margin, the north-trending fabric is penetrative and post-dates magma solidification. Within the pluton, the major element oxides, rare earth elements, anorthite contents in plagioclase, and (Mg/Fe + Mg) ratios in biotite decrease with increasing SiO 2 from phenocryst-rich (up to 75% by volume) granodiorite to phenocryst-poor (<15 vol%) granite that broadly correspond to minimum melt composition. The chemical-mineralogical variations in the pluton is attributed to deformation-driven ascent of magma with heterogeneous crystal content, ascending at variable velocities (highest in crystal-poor magma) along oblique-to-pluton margin east/ENE-trending extensional fractures induced by dextral shearing.

Abstract Ponding conditions of basalts erupted from fissure zones at São Miguel Island (Azores) were investigated through microthermometry of fluid inclusions, hosted in olivines and clinopyroxenes, and whole-rock and mineral chemistry. The Região dos Picos and Achada das Furnas fissure zones formed between central volcanoes and erupted geochemically similar magmas for the last 30 kyr. Hydrocarbonic fluid inclusions that survived to diffusion and re-equilibration recorded a maximum density value of 1000 kg m −3 (29.3 km) at both fissure zones, at the intersection with the feeding systems of older central volcanoes. The maximum density decreases to 875 kg m −3 (23.5 km) towards the central segment of Região dos Picos. Further trapping of low-density fluids is occasional and limited to a few samples. While the deepest event is interpreted to mark the vertical variation of the Moho at each location, all other events are only representative of short-time ponding with fluid re-equilibration. The deepening of the Moho towards the central volcanoes might be the effect of long-lasting underplating. However, the periodic stretching of the lithosphere, in response to the differential stress field acting at central volcanoes, would have been responsible for the thinning of the crust at the central segment of the fissure zone. Supplementary material: Whole-rock geochemistry with international standard used, and representative analyses of selected mineral phases from the two segments of the Região dos Picos fissure zone is available at www.geolsoc.org.uk/SUP18818

The Northern Marginal Zone (NMZ) of the Limpopo Belt, southern Africa, is a high-grade gneiss belt dominated by magmatic granulites of the charnoenderbite suite, which intruded minor mafic-ultramafic and metasedimentary rocks between 2.74 and 2.57 Ga. The intrusive rocks have crustal and mantle components, and occur as elliptical bodies interpreted as diapirs. Peak metamorphism (P ≤800 MPa, T = 800–850 °C) occurred at ca. 2.59 Ga. The highly radiogenic nature of the rocks in the NMZ, supplemented by heat from mantle melts, led to heating and diapirism, culminating in the intrusion of distinctive porphyritic charnockites and granites. Horizontal shortening and steep extrusion of the NMZ, during which crustal thickening was limited by high geothermal gradients, contrast with overthickening and gravitational collapse observed particularly in more recent orogens. The granulites were exhumed by the end of the Archean. The pervasive late Archean shortening over the whole of the NMZ contrasts with limited deformation on the Zimbabwe Craton, possibly owing to the strengthening effect of early crust in the craton. In the southeast of the NMZ, strike-slip kinematic indicators occur within the Transition Zone and the Triangle Shear Zone, where dextral shearing reworked the Archean crust at ca. 1.97 Ga.