High-grade metamorphic rocks associated with S-type granites are recorded in the Central Iberian Zone, Iberian Variscides. Though most of these occur as inferred metamorphic core complexes affiliated with detachment faults, others, such as the Figueira de Castelo Rodrigo-Lumbrales Anatectic Complex, crop out between low-grade metamorphic rocks separated by steeply-dipping strike-slip shear zones, such as the Juzbado-Penalva do Castelo Shear Zone. Our structural analysis has been able to constrain two major deformation stages during the Variscan D 3 : (a) D 3a ductile deformation event, with clear sinistral kinematic criteria; and (b) D 3b thrusting ductile-brittle deformation event. The petrological investigation confirmed the jump in metamorphic grade between the host rocks and the anatectic complex. P-T estimates on calc-silicate rocks interlayered with the metapelites of the anatectic complex provided minimum metamorphic peak conditions of T = 761 ± 50°C and P = 5.0 ± 1.0 kbar. However, petrological modelling results show that P-T conditions must have exceeded T > 800°C. Both structural and geothermobarometric data support a two-step model for the exhumation of the Anatectic Complex, including a 5 -8 km vertical exhumation along a 65 -100 km horizontal displacement due to a simple shear-dominated transpression mechanism during the Variscan D 3 events.Supplementary material: The petrology data, mineral chemistry analyses of the calc-silicate units and EPMA analytical conditions, and the P-T modelling methodology can be found at https://doi.org/10.6084/m9.figshare.c.3785648

The Quadrilátero Ferrífero in the São Francisco Craton, Brazil is known for its world-class gold deposits hosted by the Rio das Velhas greenstone belt (RdV). As in other greenstone belts, gold is also concentrated in late Archean/early Paleoproterozoic supracrustal units; in this case within the Moeda Formation. This unit has been compared to the Witwatersrand basin where there has been a long-term debate over the nature of the gold and whether it is detrital (placer), hydrothermal or hybrid (modified placer). Presently, links between gold in the Moeda Formation and RdV are tenuous. To enlighten this area of ambiguity, a new approach using chemical, isotopic and mineral inclusion data from detrital rutile found in the gold-bearing horizons of the Moeda Formation is presented. Grain textures and mineral inclusions have contributed to establishing a primary hydrothermal origin for the rutile, related to As-Fe-Cu-Sb-Pb-W rich mineralising fluids. Fe, Nb and V indicate that the likely source of the gold-related rutiles is the Archean banded iron formations within the RdV, where most of the lode-gold occurs. Average Nb/Ta values of these rutiles are lower than average continental crust ratios which suggests fractionation driven by fluid precipitation and/or competing mineral phases. U-Pb data yield primary crystallisation ages older than deposition ages (> 2.58 Ga), supporting the detrital nature of these rutiles. Rutiles record

A high-resolution study of a marsh sedimentary sequence from the Minho estuary provides a new palaeoenvironmental reconstruction from NW Iberian based on geological proxies supported by historical and instrumental climatic records. A low-salinity tidal flat, dominated by Trochamminita salsa, Haplophragmoides spp. and Cribrostomoides spp., prevailed from AD 140–1360 (Roman Warm Period, Dark Ages, Medieval Climatic Anomaly). This sheltered environment was affected by high hydrodynamic episodes, marked by the increase in silt/clay ratio, decrease of organic matter, and poor and weakly preserved foraminiferal assemblages, suggesting enhanced river runoff. The establishment of low marsh began at AD 1380. This low-salinity environment, marked by colder and wet conditions, persisted from AD 1410–1770 (Little Ice Age), when foraminiferal density increased significantly. Haplophragmoides manilaensis and Trochamminita salsa mark the transition from low to high marsh at AD 1730. Since AD 1780 the abundances of salt marsh species (Jadammina macrescens, Trochammina inflata) increased, accompanied by a decrease in foraminiferal density, reflecting climate instability, when droughts alternate with severe floods. SW Europe marsh foraminifera respond to the hydrological balance, controlled by climatic variability modes (e.g., NAO) and solar activity, thus contributing to the understanding of NE Atlantic climate dynamics.

Earth is unique in the Solar System as the only planet characterised by an active plate tectonic regime. However, there is still no consensus on whether this was fully operational in the Archaean or if it did not develop until the Proterozoic. The metamorphic record offers valuable insight into this debate, since paired, high and low T/P metamorphic conditions are a hallmark of modern plate tectonics. However, much of the metamorphic rock record has either been eroded or overprinted by subsequent tectonothermal events. To address this issue, we examine the detrital rock record and evaluate the potential of rutile, a mineral that forms during subduction and deep-crustal metamorphic processes, as a proxy to track the existence of low T/P and paired metamorphism. At temperatures of 550 ºC, rutile is mostly stable at pressures higher than ca. 13 kbar. We use this relationship and the Zr-in-rutile thermometer to determine peak metamorphic temperatures of detrital rutile grains to track the temporal distribution of low T/P metamorphism. We have compiled a dataset of detrital metamorphic rutile from Archaean to Permian age to trace the occurrence of low and high T/P metamorphism through time. The earliest evidence of low T/P from detrital rutile is at about 2.1 Ga. This agrees with the earliest evidence of eclogite facies conditions from the rock record and it also implies that these conditions must have been more prevalent than the present rock record seems to indicate. Together, our study confirms that subduction zone low T/P metamorphism has been fully operating since at least the late Palaeoproterozoic.

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Click here to view linked References conditions, insights into the age, geochemistry and petrology of basement sources and can also be used to elucidate tectonic environments. Detrital zircon has been used as a means of analysing the tectonic setting of host sedimentary successions, but with potentially ambiguous results. It is important to find additional ways to discriminate depositional settings, particularly in Precambrian sequences where other proxies are either not available or have been lost. In this contribution we provide a new way to discriminate between different sedimentary tectonic environments using the mineral rutile. We present a large compilation of detrital rutile data to show that the U-Pb age distribution is sensitive to the tectonic setting of the basin in which the host sediments were deposited. We then apply this new approach to two case studies, where the depositional setting and age are well-constrained: siliciclastic units of NW Scotland which were deposited in the Neoproterozoic foreland of the Grenville Orogen and on the Cambrian passive margin of the Iapetus Ocean, and the Brazilian Sabará basin located within the Palaeoproterozoic foreland of the Minas orogen. We compare the detrital rutile and zircon age distributions of these successions, showing that in some cases they are different, and that rutile is most sensitive to the youngest metamorphic events affecting the sources, reinforcing the applicability of this tool. By testing this new method on polyphasally-deformed successions (Sabará basin) we show that detrital rutile can still inform the tectonic setting even at medium grades of metamorphism.

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