Deep Crustal Levels Research Articles

A crustal magnetic model of Britain obtained by 3D inversion

The national baseline aeromagnetic survey of Britain allows a uniform assessment of the shallow and deep magnetic properties of the British tectonic terranes. The most significant is that associated with destruction of early Palaeozoic oceanic lithosphere across the Iapetus Suture separating Baltica and Avalonia from the Laurentian terranes. Here a formal 3D inversion of a continuous swathe of the data is considered. The study provides a uniform volumetric whole crust assessment extending for over 1000 km. Normally a 3D inversion of magnetic data is controlled using a variety of constraints however this is not appropriate at the crustal scale due to our increasingly imprecise knowledge of lithology at increasingly greater depths. The main crustal interface encountered occurs at the Curie isotherm depth. We demonstrate the behaviour of introducing different magnetic crustal depths and suggest the crustal ‘magnetic depth’ of our models can be independently constrained using global or regional studies of the deep geotherm. Static magnetic data have no inherent depth resolution. Here an empirical ‘1D depth’ weighting and a more formal ‘3D distance’ weighting are assessed. The inversion procedure is regularised to provide stable models appropriate to the data and their errors. To gain confidence when using such a ‘geologically-unconstrained’ inversion, we compare our 3D inversion results with an existing geologically-constrained 2.5D profile inversion across northern Britain. A surprising agreement in the 3D susceptibility magnitudes is observed. The chosen study area traverses 10 British terranes and images their tectonic fabric by way of non-magnetic zones (i.e. susceptibilities <0.0001 to 0.001 SI) and magnetic zones displaying geological relevance and tectonic significance at deeper crustal levels. Here we discuss the more significant 3D model features which, by virtue of a continuous crustal-scale assessment and fitting the data with a high degree of fidelity, provide additional structural insights.

Tracking C-O-H fluid-rock interactions in reworked UHT granulite: Tectonic evolution from ca. 990 Ma to ca. 500 Ma in orogenic interior of Eastern Ghats Belt, India

A suite of high-grade granulite rocks occurring close to one of the earliest reported ultra-high temperature rocks (“Paderu” rocks) from the central part of the Eastern Ghats Belt, India has been studied. Here we present a detailed pressure-temperature-time-fluid history of the suite of aluminous granulite and associated gneisses from a single exposure, which were injected by granitic aplite veins at a high angle at a ductile depth of the host gneissic rocks. Detailed studies of the sapphirine-spinel-quartz-bearing aluminous granulite reveal a peak metamorphic condition of ~1000 °C at ~8 kbar. Porphyroblastic garnet and quartz in the peak assemblage have mono-phase, high-density, CO2-rich primary fluid inclusions (~1 g/cm3). Orthopyroxene-garnet-sillimanite forming corona textures in the aluminous granulite and the presence of cordierite-K-feldspar-quartz intergrowths imply post-peak cooling and subsequent decompression down to ~6 kbar. Secondary fluid inclusions in quartz grains of the aluminous granulite have the signature of bi-phase CO2-H2O fluids with a comparatively low estimated density (0.8 g/cm3). The CO2 isochore plots suggest a pressure drop from 7 to 8 kbar to 4–5 kbar (simultaneous or after the cooling from 1000 °C to 800 °C). Texture-constrained monazite dating using U-Th-total Pb EPMA technique reveals that there are strong peaks at ~940 Ma, ~900 Ma, and ~ 800 Ma with several smaller and younger peaks. On the other hand, the UPb SHRIMP dating of zircon from the same rock reveals discordant ages with upper and lower intercepts at ~1700 Ma and ~ 990 Ma. Late pegmatite dykes and aplite veins were emplaced cutting across the dominant structural fabric of this litho-assemblage at a high angle. The asymmetric dragging of the foliation of the host leptynite possibly implies a shear-induced intrusion. The fluid inclusion study of the aplite reveals that the quartz grains contain numerous primary and pseudosecondary fluid inclusions (CO2 monophase to biphase). The calculated density is in the range of 0.71–0.82 g/cm3, suggesting the entrapment of carbonic fluid at <3 kbar. Monazite dating yields a strong single peak at 497 ± 4 Ma, one of the rare dates from the orogenic interior of the Eastern Ghats Belt. The studied suite of deep crustal granulites with aplite veins preserves a history of at least two events of fluid-rock interactions during its residence at the extremely hot deep crustal level until its exhumation to shallower levels where a magmatic fluid is trapped, through a time corridor between the Grenvillian and Kuunga orogenies.

Deciphering magma storage and ascent processes of Taranaki, New Zealand, from the complexity of amphibole breakdown textures

Lava flows that constitute the current edifice of Taranaki volcano, New Zealand, contain a complex crystal cargo including plagioclase, pyroxene, amphibole, oxides, and rarely olivine. Amphibole textures and mineral chemistry indicate that crystals are pargasites entrained at various depths by ascending magmas. The crystals record a complex growth history within a range of temperature and pressure conditions (c. 950–1010 °C at mid- to deep crustal levels). Most amphiboles show the development of distinct reaction rims where the mineral is in contact with the ambient melt. Texturally, the rims are identified as detached, symplectic, granular or coarse. True rim thicknesses vary little (±20%, 1σ, on average) within individual thin sections but show a large variation between samples from different lava flows, from <5 to >50 μm. Reaction rim formation on Taranaki amphiboles is attributed to degassing during magma ascent as well as small increases in temperature. Associated with the formation of rims, the amphiboles also show two types of volumetric decomposition, identified as irregular and aligned. The former is indicative of slow reaction of amphibole with melt entrapped in fractures and cleavages during decompression-induced degassing, while the other is indicative of heating-induced breakdown of amphibole triggered by uptake into hot magmas prior to the onset of eruption. The combined evidence indicates that despite the complex and variable growth history of the remobilized crystal cargo, such crystals may be useful in constraining volcanic pre- and syn-eruptive processes and, when appropriate experimental data become available, their rates, which may be of the order of days to weeks.

Ongoing Inversion of a Passive Margin: Spatial Variability of Strain Markers Along the Algerian Margin and Basin (Mediterranean Sea) and Seismotectonic Implications

Subduction initiation is an important but still poorly documented process on Earth. Here, we document one of a few cases of ongoing transition between passive and active continental margins by identifying the geometrical and structural signatures that witness the tectonic inversion of the Algerian continental margin and the deep oceanic domain, located at the northern edge of the slow-rate, diffuse plate boundary between Africa and Eurasia. We have analyzed and tied 7900 km of deep seismic reflection post-stacked data over an area of ∼1200 km long and ∼120 km wide. The two-way traveltime lines were converted into depth sections in order to reconstruct and map realistic geometries of seismic horizons and faults from the seafloor down to the acoustic basement. Along the whole length of this young transitional domain, we identify a clear margin segmentation and significant changes in the tectonic signature at the margin toe and in the deep basement. While the central margin depicts a typical thick- and thin-skinned tectonic style with frontal propagation of crustal thrust ramps, the central-eastern margin (Jijel segment) reveals a higher strain focusing at the margin toe together with the largest flexural response of the oceanic lithosphere. Conversely, strain at the margin toe is limited in the western margin but displays a clear buckling of the oceanic crust up to the Spanish margin. We interpret these contrasting, segmented behavior as resulting from inherited heterogeneities in (1) the geometry of the Algerian continental margin from West to East (wrench faulting in the west, stretched margin elsewhere) and (2) the Miocene thermal state related to the diachronous opening of the Algerian basin and to the magmatic imprint of the Tethyan slab tearing at deep crustal levels. The narrow oceanic lithosphere of the Western Algerian basin is assumed to favor buckling against flexure. From the dimension and continuity of the main south-dipping blind thrusts identified at the margin toe, we reassess seismic hazards by defining potential lengths for ruptures zones leading to potential magnitudes up to 8.0 off the central and eastern Algerian margins.

Neoproterozoic geodynamics in NW India – evidence from Erinpura granites in the South Delhi Fold Belt

ABSTRACT Erinpura Granites form the basement for the Neoproterozoic Malani Igneous Suite in NW India. Based on this study, the Erinpura granites can be divided into Erinpura-East (gneissic fabric), a belt parallel to the southern sector of the South Delhi Fold Belt, and Erinpura-West (magmatic fabrics). EPMA dating on monazites gives a time frame of 890 to 860 Ma for crystallization of both types. The geochemically homogeneous peraluminous S-type granites with εNd values of −2.1 to −10.8 are interpreted as melting products of Archaean crust. REE pattern follows the pattern of the average continental crust, but Erinpura-East samples are more fractionated with steeper HREE depletion indicating melting in a deeper crustal level. A thermal pulse between 835 and 820 Ma constrains the timing of deformation in granite-gneisses during uplift along thrust planes, coeval with shear-bound exhumation of high-grade metamorphic rocks and initiated by delamination of the lower crust in this southern sector of the South Delhi Fold Belt. This is in contrast to the northern sector of the SDFB with arrested orogenic development and without considerable delamination or erosion of the lower crust. Latest movement related to the 200 Ma tectono-magmatic history overlaps with initiation of rifting during the Malani igneous event. A change from S-type to A-type granites and shift of isotopic signatures to εNd values of −2.8 to −1.7 indicate substantial contribution of asthenospheric material in the Malani melting process.

Development of a synorogenic composite sill at deep structural levels of a continental arc (Odenwald, Germany). Part 1: Sederholm-type emplacement portrayed by contact melt in shrinkage cracks

We present geochronological, thermobarometric and computertomographic data from a composite sill exposed in the Weschnitz pluton of the Variscan Odenwald. The wall rock consists of quartzmonzodiorite, which intruded at deep structural levels (P = ca. 0.50 GPa, ca. 18 km depth) in a continental arc setting. Zircons and titanites of the quartzmonzodiorite yielded similar UPb ages at 344.3 ± 0.6 and 343.2 ± 2.1 Ma, respectively, reflecting fast initial cooling (≥76 °C/m.y.) until the solidus was attained at ca. 680 °C. Under these conditions, the quartzmonzodiorite was cut by a spessartite sill, which yielded a UPb titanite age at 342.0 ± 1.0 Ma. The short period between wall rock and sill emplacement (≤3.9 m.y.) suggests that both are genetically related.The quartzmonzodiorite situated adjacent to the spessartite sill is enriched in quartz and plagioclase and depleted in hornblende. As geothermobarometric data of these felsic parts yielded T = ca. 710 °C and P = 0.42 GPa (ca. 15 km depth), they are interpreted as contact melt, which invaded into shrinkage cracks of the cooling sill resulting in thin felsic veins. The migration of contact melt from the wall rock into the shrinkage cracks was suction-induced because of volume gain by contact melting in the wall rock and volume loss by cooling of the sill.The formation of such Sederholm-type composite veins seems to be very fast. Titanites of the felsic veins yielded a UPb age at 341.8 ± 1.5 Ma, which is the same like that of the sill. Moreover, results of thermal modelling indicate a maximum period of 4 months between sill emplacement and the time when both the quartzmonzodiorite and the sill had attained the ambient temperature. The modelling further explains, why Sederholm-type veins, described from several places around the world, are restricted to deep crustal levels.

From long-lived batholith construction to giant porphyry copper deposit formation: petrological and zircon chemical evolution of the Quellaveco District, Southern Peru

Porphyry Cu ore deposits are a rare product of arc magmatism that often form spatiotemporal clusters in magmatic arcs. The petrogenetic evolution of igneous rocks that cover the temporal window prior to and during porphyry Cu deposit formation may provide critical insights into magmatic processes that are key in generating these systems. This study documents the magmatic evolution of the Palaeocene–Eocene Yarabamba Batholith, Southern Peru, that was incrementally assembled between ~ 67 and ~ 59 Ma and hosts three, nearly contemporaneous, giant porphyry Cu–Mo deposits that formed at 57–54 Ma (Quellaveco, Toquepala and Cuajone). Whole-rock geochemistry, U–Pb geochronology and zircon trace element chemistry are reported from Yarabamba rocks that span the duration of plutonic activity, and from six porphyry intrusions at Quellaveco that bracket mineralisation. A change in whole-rock chemistry in Yarabamba intrusive rocks to high Sr/Y, high La/Yb and high Eu/Eu* is observed at ~ 60 Ma which is broadly coincident with a change in vector of the converging Nazca plate and the onset of regional compression and crustal thickening during the first stage of the Incaic orogeny. The geochemical changes are interpreted to reflect a deepening of the locus of lower crustal magma evolution in which amphibole ± garnet are stabilised as early and abundant fractionating phases and plagioclase is suppressed. Zircons in these rocks show a marked change towards higher Eu/Eu* (> 0.3) and lower Ti (< 9 ppm) compositions after ~ 60 Ma. Numerical modelling of melt Eu systematics and zircon-melt partitioning indicates that the time series of zircon Eu/Eu* in these rocks can be explained by a transition from shallower, plagioclase-dominated fractionation to high-pressure amphibole-dominated fractionation at deep crustal levels from ~ 60 Ma. Our modelling suggests that any redox effects on zircon Eu/Eu* are subordinate compared to changes in melt composition controlled by the fractionating mineral assemblage. We suggest that growth and intermittent recharge of the lower crustal magma reservoir from ~ 60 Ma produced a significant volume of hydrous and metallogenically fertile residual melt which ascended to the upper crust and eventually generated the three giant porphyry Cu–Mo deposits at Quellaveco, Toquepala and Cuajone from ~ 57 Ma. Our study highlights the importance of high-pressure magma differentiation fostered by strongly compressive tectonic regimes in generating world-class porphyry Cu deposits.

Tectono-metamorphic evolution and significance of shear-zone lithologies in Akebono Rock, Lützow-Holm Complex, East Antarctica

AbstractWe describe a major shear zone exposed at Akebono Rock and discuss its deformation and metamorphic history, with a view to providing a better understanding of the geological history of the Lützow-Holm Complex. Three deformation episodes are recognized: D1 produced open folds (F1), boudinage and a regional ductile foliation, whilst the related metamorphic facies is characterized by stable garnet. F1 folding is dominantly preserved in the eastern part of the study area. During D2, an isoclinal to tight asymmetric F2 folds developed mainly in the west part of the region, accompanied by an S2 shear, under biotite facies retrograde metamorphism. The D3 episode involved the formation of the major shear zone, characterized by mylonite and L-tectonite fabrics, which took place at ~610–660°C and 4–5 kbar. Large, sigmoidal garnet core domains have S-shaped inclusion trails, suggesting that syntectonic garnet growth occurred before the formation of the shear zone. Estimated P-T conditions suggest that the sigmoidal garnet-bearing amphibolite was recrystallized at a deeper crustal level and was brought to a higher level during the formation of the shear zone. Crustal-scale deformation involving syntectonic recrystallization and shearing of Akebono Rock is a key issue for reconsidering the evolution of the Lützow-Holm Complex.

High pressure and temperatures during the early stages of tungsten deposition at Panasqueira revealed by fluid inclusions in topaz

The Variscan vein-type Panasqueira W-Sn(Cu) deposit, one of the main tungsten deposits in Western Europe, has a long and complicated geological history. The first vein infillings, which consist of the quartz-wolframite association as well as the first generation of topaz, underwent significant deformation. As a consequence, most fluid inclusions of the earliest hydrothermal event are deformed and destroyed. Two preserved fluid inclusion assemblages are, however, found in the topaz overgrowth band and are dense aqueous-carbonic inclusions as well as dense CO2 dominated fluid inclusions. The P-T conditions of fluid trapping are constrained by using the intersection between isochores, as well as graphite-water equilibrium data and yield the following trapping conditions: 500 ± 20 °C and 250 ± 20 MPa. These P-T conditions are incompatible with fluid unmixing. Fluid chemistry results from water-graphite equilibrium, probably in metapelites, at two distinct temperatures: around 450–500 °C for the predominant aqueous-carbonic fluid, and higher temperatures of maximal 550 °C for the CO2-rich fluid enriched in N2. These P-T estimates are consistent with deep crustal levels around 8–10 km depth and a high geothermal gradient around c. 60 °C/km−1. The ascending non-magmatic fluids, enriched in volatiles, are essential in the ore genesis.The high thermal gradients may be related either to new magma pulse after the formation of the Panasqueira granite intrusion or to anomalous heat flux produced by the hot fluids ascending from migmatitic levels present at greater depth. This hypothesis necessitates to consider the role of a crustal weakness, which is attested both by the successive intrusions of several granitic magmas at the same place, and the presence of inherited quartz filled structures so-called Seixo-Bravo found only in the Panasqueira area.

HT–LP crustal syntectonic anatexis as a source of the Permian magmatism in the Eastern Southern Alps: evidence from xenoliths in the Euganean trachytes (NE Italy)

Oligocene trachytes from the Euganean Hills include various regionally metamorphosed gneissic and granulitic xenoliths. These xenoliths provide the unique opportunity to investigate South Alpine intermediate to deep crustal levels that are not at present exposed in the Eastern Alps. The estimated P – T conditions are in the range of 780–850°C and 0.45–0.55 GPa for a migmatitic gneiss xenolith. Sensitive high-resolution ion microprobe (SHRIMP II) U–Pb analyses on zircon from this xenolith provide concordant ages around 259.7 ± 3.5 Ma, consistent with a proton-induced X-ray emission (PIXE) U–Th–Pb age on monazite of 262 ± 12 Ma. The Sr–Nd–Pb isotopic compositions, and major and trace element data show distinct origins for the different types of xenoliths. Mafic granulite xenoliths have an isotopic signature close to mantle-derived rocks and to Permian gabbroic rocks from the Western Southern Alps. Metapelite xenoliths have high Sr and low Nd initial ratios like those of acid crustal rocks and could possibly represent the source of the crustal component that is dominant in the acid Permian supervolcanoes. The migmatitic xenolith provides the first documented evidence for a Permian thermal event associated with crustal thinning in the Eastern Southern Alps. Here the South Alpine basement escaped most of the Alpine crustal shortening and still preserves most of the original Permian extension under thick Mesozoic cover. Supplementary material: Microprobe analyses of mineralogical phases and Ti-in-biotite geothermometric calculations are available at https://doi.org/10.6084/m9.figshare.c.5032337

Tectonic domains and exhumation history of the Omineca Belt in southeastern British Columbia from 40Ar/39Ar thermochronology

A large geochronological data set comprising 40Ar/39Ar and K–Ar (hornblende, muscovite, biotite, and K-feldspar), Rb–Sr (muscovite), fission track (zircon and apatite) and U–Pb (zircon and monazite) dates has been compiled for the southern Kootenay Arc and western Purcell anticlinorium in the Omineca Belt of the Canadian Cordillera in southeastern British Columbia. New 40Ar/39Ar data for hornblende, muscovite, biotite, and alkali feldspar are presented and combined with data from other studies. We integrate these data with recent advances in the geology of the region to define three partially fault-bounded domains with differing geological and exhumation histories, here termed the western, central, and eastern domains. The western domain is characterized by (1) late synkinematic Jurassic plutons with hornblende, muscovite, and biotite 40Ar/39Ar plateau dates between 170 and 165 Ma, some of which are within error of the U–Pb zircon dates for these plutons, and (2) late Early Cretaceous (118–102 Ma) plutons commonly with concordant mica 40Ar/39Ar plateau dates of a similar age range, indicating rapid cooling following emplacement of both suites. The central domain is bounded by regional-scale normal faults (Gallagher and Midge Creek faults, Blazed Creek/Next Creek faults, and Purcell Trench fault) and contains superposed Early and Late Cretaceous zones of Barrovian metamorphic rocks and several mid- to Late Cretaceous, post-kinematic plutons. The transition from the western domain into the central domain is characterized by 40Ar/39Ar mica age spectra showing a progression of increasing thermal overprinting. Along the north–south length of the central domain, biotite and muscovite yield Paleocene to Eocene K–Ar and 40Ar/39Ar plateau dates between 66 and 40 Ma. The eastern domain consists of (1) a southern portion that occurs in the hanging wall of the Purcell Trench fault, comprising mid-Cretaceous intrusions of the Bayonne magmatic suite emplaced into biotite zone metasedimentary rocks of the Mesoproterozoic Belt-Purcell Supergroup in the western Purcell anticlinorium, and (2) a northern portion that shows a continuous transition with the northern part of the central domain north of the terminus of the Purcell Trench fault. Cretaceous igneous rocks in the southern portion of the eastern and western domains have 40Ar/39Ar mica plateau dates that are &lt;9 Myr younger than U–Pb zircon dates, indicating rapid cooling shortly after emplacement. 40Ar/39Ar step-heating reveals that there was a mid- to Late Cretaceous thermal disturbance in the eastern domain, possibly related to emplacement of younger plutons at deeper crustal levels and the Late Cretaceous Barrovian metamorphic event recorded in rocks of the central domain, such that biotite with dates &lt;ca. 73 Ma yield plateau age spectra but those with older dates are disturbed. The new geochronology, combined with recent mapping and metamorphic studies, leads to the conclusion that the exhumation of the Barrovian metamorphic rocks of the central domain was a multi-stage process. The central domain experienced rapid tectonic decompression and minor pluton emplacement in the Late Cretaceous to early Paleocene (76–61 Ma) when the Cordilleran orogen was under regional contraction during which most of the exhumation occurred. Final exhumation in the footwall of Eocene normal faults was less significant and occurred between 53 and ca. 46 Ma when the Cordilleran orogen had transitioned to regional extension, by which time the three domains had attained a similar crustal level. These episodes of exhumation are similar to those found in other core complexes in the southern Canadian Cordillera and contiguous northern Idaho and Washington. The earlier episode is coincident with regional-scale, Late Cretaceous thrust faulting in the Foreland Belt of the Rocky Mountains. Eocene normal faulting and final exhumation of core complexes in the Omineca Belt mark the end of contraction in the Foreland Belt.

How Much Water in Basaltic Melts Parental to Porphyry Copper Deposits?

Porphyry copper deposits are formed by aqueous fluids exsolved by differentiated, mantle-derived magmas variably mixed with crustal melts. Water is essential to form porphyry Cu mineralization, and this explains why these deposits are found only at convergent margin settings, where subduction has enriched the mantle source of magmas with slab-derived H2O. Intuitively, the more water occurs in the parental magmas of porphyry deposits, the more fertile the latter should be. Indeed, several studies have proposed that anomalously high H2O contents in the source basalt, resulting, for instance, from subduction of large-scale serpentinized fracture zones of the oceanic slab, could increase the fertility of magmas. However, no studies have ever quantified the effects of variable H2O contents on the fertility of parental basalts to form porphyry deposits. Here, using petrological modeling with a Monte Carlo approach, I show that the optimum amount of slab-derived H2O in fertile parental basalts is ∼2– 6 wt%, which coincides with the measured range of H2O content in arc basalts. Lower and higher amounts of H2O in the parental basalt lead to less porphyry-fertile magmas. The lower fertility of H2O-poor parent basalt (i.e., 6 wt%) predicted by the model is counterintuitive. The reason for the decreased fertility of parental basalts with >6 wt% H2O is that such H2O-rich magmas undergo fluid saturation, losing their fluid and metal cargo at deep crustal levels. Additionally, water saturation-induced crystallization of amphibole at these deep levels prevents such H2O-rich magmas from ascending to shallower crustal levels, where they can form porphyry deposits. The conclusion that arc basalts with normal H2O contents (∼2–6 wt%) are the most porphyry-fertile adds evidence to the hypothesis that intermediate-felsic magmas associated with porphyry Cu deposits are parented by arc basalts formed through normal subduction-related processes and that intracrustal and tectonic processes play the most relevant role in the modulation of Cu endowments of these deposits.

Charnockites from Carajás Province, SE Amazonian Craton (Brazil): Petrogenetic constraints and intensive crystallization parameters

The gabbronorite-charnockite-granite series rocks in the Carajás Province consist of three granitoid varieties: orthopyroxene granodiorite, clinopyroxene monzogranite and amphibole monzogranite; gabbronorites are spatially associated. They occur as a NE-SW-elongated pluton with minor E-W-oriented lenses. In general, the planar fabrics are defined by a NE-SW to E-W trend and steep dip (70–85°) restricted to shear zones. The main mafic phases are pyroxene and amphibole. Biotite is annite with magmatic and subalkaline signature. Plagioclase includes labradorite and andesine. Amphibole thermometers estimates vary from 713 to 800 °C and 765–809 °C in the granitoids and gabbronorite, respectively. Pyroxenes gave higher temperatures of 1194–1573 °C. The pressures were 190–310 MPa during emplacement. The presence of primary magnetite together with geochemical and amphibole-biotite chemical data argues that the studied rocks have crystallized under oxidized conditions (NNO ± 0.5 and NNO +1). The H2O contents are estimated as higher than 4%. The gabbronorite-charnockite-granite series has magnesian affinity, calc-alkaline to high-K calcic-alkaline trends and metaluminous character. The magnesian and oxidized character is inherited from the source, which was likely extracted from a depleted mantle (low HFSE contents). Geochemical modeling shows that the orthopyroxene granodiorite originated by fractional crystallization from a magma akin to the gabbronorite. Water activity was high in the monzogranites, and hydration had an important role in the magmatic evolution of the gabbronorite-charnockite-granite series. The emplacement of these rocks was controlled by a translithospheric-scale shear zone acting as a preexisting conduit for the migration of mantle-derived magmas to epizonal levels (7.0–10.5 km) in a sinistral transpressive regime in a post-collisional setting. Deep crustal levels favor crystallization of orthopyroxene-bearing rocks (poor water conditions), while at shallower crustal levels fluids tend to be dominant in the magma, leading to the generation of amphibole-bearing granite.

Tracking the sources and the evolution of the late Neoproterozoic granitic intrusions in the Brusque Group, Dom Feliciano Belt, South Brazil: LA-ICP-MS and SHRIMP geochronology coupled to Hf isotopic analysis

The central Domain of the Neoproterozoic Dom Feliciano Belt in the Brazilian state of Santa Catarina is mainly composed of a metavolcano-sedimentary sequence, the Brusque Group, and its crystalline basement, such as the Camboriú Complex. This association was intruded in its late-deformational stage by voluminous granitic magmatism, related to the metamorphic climax of the area. The intrusions share a distinct crustal affinity and are divided into three suites, from oldest to youngest, as indicated by field criteria: São João Batista, Valsungana and Nova Trento. We present seven new LA-ICP-MS and two SHRIMP U-Pb zircon ages, coupled with in-situ analysis of Hf isotopes, and use them to discuss the magmatic sources and evolution of the intrusions, and propose an evolutionary model for this plutonic association. Intrusion mostly took place at 615–585 Ma, but the field stratigraphy is not necessarily reflected by U-Pb ages, which establish considerable overlaps between intrusions from different suites. Inherited zircon crystals are common, and reflect a long geological history that matches the geochronological record of the Brusque Group and the Camboriú Complex. Hf ratios are remarkably widespread, but strengthen the characteristic crustal signature of the magmatism, and partially overlap with whole-rock isotopic data for the same rocks. We propose that the intrusions were caused by a regional heating event within the central domain of the Dom Feliciano Belt in Santa Catarina, with limited direct mantellic contribution and more significant indirect thermal contribution through the voluminous magmatism of the recently juxtaposed Florianópolis Batholith. This triggered large-scale partial melting of the local continental crustal and formed the studied granites. The long-lasting magmatic event promoted the protracted generation and emplacement of numerous distinct intrusions synchronically, leading to contrasts between the field and U-Pb stratigraphy. The suites’ varied geochemical and isotopic characteristics probably reflect a combination of the heterogeneities of their source rocks with the impact of different magmatic processes during granitogenesis and emplacement. While the Valsungana and Nova Trento suites were probably generated within a deeper crustal level and underwent some degree of magmatic interaction, the peraluminous São João Batista Suite has an upper-crust signature that indicates the assimilation of metasedimentary rocks.

Structure of the Paleoproterozoic Kédougou-Kéniéba Inlier (Senegal-Mali) deduced from gravity and aeromagnetic data

The Kédougou-Kéniéba Inlier (KKI) corresponds to a window through Paleoproterozoic terranes of the West African Craton (WAC). This study presents, first of all, an interpretation of a regional Bouguer gravity map (14°W-9°W, 11°N-16°N) of the KKI and then introduces a new litho-structural map of the Malian part of the KKI using aeromagnetic data (12°0′W-10°80′W, 12°0′N-14°50′N). The KKI is limited to the west by a negative gravity anomaly forming a 30–100 km wide corridor, oriented N–S to NNE-SSW, and correlated to the Variscan Mauritanides orogenic belt. West of this belt, the Mesozoic to Cenozoic sedimentary deposits are marked by the highest positive anomaly of the region, attributed either to the presence of mafic rocks at intermediate to deep crustal levels or to a shallower Moho depth. Within the KKI, moderately positive to negative anomalies are correlated with exposed plutonic and mafic to intermediate metavolcanic rocks. Gravity data also reveal (i) two north-west trending lineaments attributed to crustal-scale shear zones, (ii) three north-east trending lineaments marked by gravity anomalies sub-parallel to the known Bissau–Kidira–Kayes Fault Zone, and named Kayes, Kédougou-Kéniéba, and South Shear Zones, respectively, (iii) and the extension to the south of the known Mauritanides Belt Thrust.The local aeromagnetic map of the Malian part of the KKI discriminates the Mako metavolcanic belt with granitic intrusions in the west characterized by heterogeneous anomalies, from the Kofi metasedimentary series delineated by magnetic lows in the east. ESE-WNW to ENE-WSW-trending alternation of magnetic anomalies in the metavolcanic and metasedimentary rocks are attributed to pervasive structures associated with ductile deformation. The Senegalo-Malian Shear Zone is marked at the regional scale by deflection of the geophysical lineaments but is not expressed by any offset of identified structures or lithological contacts.

The Archean granulite-enderbite complex of the northern Carajás province, Amazonian craton (Brazil): Origin and implications for crustal growth and cratonization

The central portion of the Canaã dos Carajás domain (Ouro Verde area) exhibits a peculiar scenario involving two units of orthopyroxene-bearing sodic rocks: (i) the Mesoarchean Ouro Verde felsic granulite with protolith crystallization and metamorphism ages of 3.05–2.93 Ga and 2.89–2.84 Ga, respectively, and (ii) the Neoarchean Café enderbite, composed of tonalites, trondhjemites and scarce quartz diorites that crystallized at 2.75–2.73 Ga and show syntectonic character. While the orthopyroxene in the felsic granulites was formed by breakdown of primary biotite during metamorphism, in the enderbitic rocks, this mineral was formed by crystallization of a hot water-poor magma at deep crustal levels. This terrane evolved in three important geodynamic stages: (1) at 3.05–2.93 Ga, TTG crust was generated by partial melting of LILE-enriched basalts in a N–S subduction setting; (2) from 2.89 to 2.84 Ga, in a collisional setting, large volumes of anatectic granites formed and contributed to crustal thickening, which triggered granulite-facies ultrahigh-temperature metamorphism in the TTG crust, forming the Ouro Verde felsic granulite; and (3) in the Neoarchean, at 2.75–2.73 Ga, the lowermost mafic crust was delaminated during crustal thickening, causing crustal underplating followed by partial melting of Mesoarchean mafic granulites, which was responsible for the origin of the enderbitic magma. This area was subjected to a transpressional tectonic regime, leading to the development of plutons (E–W-trending) amalgamated linearly and emplaced along ancient suture zones (ductile-brittle transition). Lu-Hf isotope data of magmatic zircon cores from the Mesoarchean felsic granulite show Hf-TDM2 values of 3.44–3.15 Ga and εHf(t) values between −1.7 and 3.0, which suggest juvenile sources. The Neoarchean enderbites present Hf-TDM2 ages of 3.46–3.29 Ga and lower εHf(t) values (between −4.8 and −1.9), which indicate a longer crustal residence time for these rocks. All these findings lead us to consider the central portion of the Canaã dos Carajás domain as exhumed lower crust, and the tectonic juxtaposition of anatectic granites, enderbites and high-grade metamorphic rocks suggests that the Ouro Verde area represents middle and deep exposure levels of the same crust.

Umicore gains Hazari pre-catalysts

The Jiangnan Orogen, located between the Yangtze and Cathaysia Blocks in southern China, hosts significant gold and antimony resources. Long-standing controversies over the number and precise timing of gold, gold-antimony and antimony mineralization event(s), and the genesis of these deposits, limit the understanding and exploration of this giant gold and antimony system. Together with geological evidence, a critical review of the published geochronological data of these deposits suggest that there were two gold events in the Triassic (~235 Ma) and Early Cretaceous (~142–130 Ma), two gold-antimony events in the Early Devonian (~402 Ma) and Late Triassic-Middle Jurassic (~224–163 Ma: possibly equivalent to the two gold events), and one antimony event in the Early Cretaceous (~130 Ma). There are other possible gold events in the Neoproterozoic or at an even older age, and Ordovician to Early Devonian, which are constrained only by limited geological evidence and a few non-robust isotopic ages. The regional distribution of the gold, gold-antimony and antimony districts, and deposits therein, reveal a first-order control on the mineralization by crustal scale faults which acted as ore-forming fluid pathways connected to deep fluid and metal source areas. Second- and third-order faults that are situated along the jogs of the first-order faults, especially fault corridors defined by NNE-NE-trending second-order faults and third-order NW to E-W-trending discontinuous oblique faults, provided favorable locations for Pre-Cretaceous gold-(antimony) mineralization. In addition, NE-trending open anticlines, linked to deep crustal levels by first-order faults, host significant antimony and minor gold mineralization, and the curvilinear Fanjingshan detachment Fault hosts some gold and antimony deposits. Locally, at deposit to orebody-scales, pre-ore barren quartz veins and magmatic dikes were the loci for mineralization in some gold-antimony deposits. The structural geometry of the pre-Cretaceous gold and gold-antimony deposits suggests an Early Devonian orogenic gold-antimony mineralization event during a transpressional tectonic regime related to coeval intracontinental orogeny between the Yangtze and Cathaysian Blocks. In contrast, Triassic orogenic gold and Triassic-Middle Jurassic orogenic gold-antimony events are interpreted to relate to distal effects of the collision between the North and South China blocks. The structurally-contrasting major Early Cretaceous epizonal antimony mineralization event, together with contemporaneous hydrothermal gold mineralization, is interpreted to have been controlled by normal faults and pre-ore open folds in an extensional tectonic regime related to the distal effects of rollback of the paleo-Pacific Plate after ~135 Ma. For future geoscience-based exploration in the Orogen, documentation and interpretation of critical structural geometries of gold, gold-antimony and antimony deposits is a vital step towards successful target generation.

Active tectonic structures in northeastern Egypt: a geospatial analysis using structural, remote sensing, and seismic data

The seismotectonic activity in Northeastern Egypt is intimately related to the active tectonic structures (mainly faults) which deform the region and induce intraplate seismicity. The present contribution is a new attempt to integrate structural, remote sensing, and seismic data, and implement a reliable geospatial analysis to investigate and assess the nature of the relation between tectonic structures and seismotectonic activity in the Cairo–Suez district which is an active seismic source zone in Northeastern Egypt. The achieved remote sensing and GIS-based geospatial analysis introduces valuable information on the frequency, orientation, and density of investigated tectonic fault trends. The spatial distribution of earthquakes epicenters which recorded during the period (1997–2016) with magnitude (ML ≥ 3.0) is evidently interrelated with the detected tectonic trends, and normal faulting with subordinate shear component is the most focal mechanisms indicated from the analyzed seismic events. The depth analysis of the studied earthquakes suggests a “hard linkage” between the major rift-boundary faults in the northern Gulf of Suez rift and the tectonic trends in the southern domain of the Cairo–Suez district, and thus indicates seismic activity on relatively deeper crustal levels. Most of the studied earthquakes have occurred on the WNW–ESE to NW–SE oriented faults in predominantly extensional to transtensional stress regime, which indicating the WNW to NW tectonic trend as an active fault trend. The intraplate seismotectonic activity in Northeastern Egypt is highly controlled by two active tectonic boundaries, the Red Sea–Gulf of Suez rift and Aqaba–Dead Sea transform (to the east) and the Pelusium Shear System (PSS, to the west). Any reliable hypothetical model explaining the seismotectonic setting in such region would consider the Cairo–Suez district as a large transfer zone transmitting the “far-field” stresses northwestward to the PSS with an effective mechanism keeping the within-plate WNW–ESE, NW–SE, and E–W tectonic trends highly prone to rejuvenation and seismic rupture.

The lower crust of the Gangdese magmatic arc, southern Tibet, implication for the growth of continental crust

Magmatic arcs are thought to be the primary sites of modern-day continental crustal growth, and arc crustal sections provide an exceptional opportunity to directly observe the geological processes that occur there, yet few deeply exposed arc sections are available for direct study. The Gangdese magmatic arc, southern Tibet, formed during the Mesozoic subduction of Neo-Tethyan oceanic lithosphere and Cenozoic collision between the Indian and Asian continents, and represent juvenile continental crust. However, the petrological components and compositions of the lower crust of the Gangdese arc remain unknown. Based on detailed geological mapping, we conducted a systemic geochemical, geochronological and zircon Hf isotopic study of well-exposed high-grade metamorphic and migmatitic rocks from the lower crust of the eastern Gangdese arc. The results obtained show that Late Cretaceous garnet amphibolites, dioritic and granitic gneisses, and Paleocene–Eocene garnet amphibolites and granitic gneisses are the main components of the Gangdese lower arc crust. These meta-intrusive rocks witnessed a long period of magmatic, and metamorphic and anatectic processes from the Middle Jurassic to the Late Eocene, and have chemical compositions that range from ultramafic to felsic, with an average SiO2 content of 57.61 wt% and Mg# value of 0.49. These new data indicate firstly that the Gangdese lower arc crust has an overall intermediate composition and typical feature of juvenile crusts, and therefore supports the recent proposition that continental lower crusts are relatively felsic in composition, instead of mafic. We consider that the downward transport of felsic intrusives and associated sedimentary rocks into the deep crustal levels and subsequent partial melting resulted in componential and compositional changes of the Gangdese arc lower crust over time. This is a potential key mechanism in transforming primary lower arc crust to mature continental lower crust for the magmatic arcs with a complete growth history.

Crustal architecture beneath the southern Midcontinent (USA) and controls on Mesoproterozoic iron-oxide mineralization from 3D geophysical models

Several types of critical mineral-bearing ore deposits in the southern Midcontinent region of the U.S. are hosted in Mesoproterozoic igneous rocks largely concealed beneath Paleozoic cover. Discerning the architecture of igneous intrusions and volcanic centers in the crust is fundamental to understanding the geologic evolution of this vast region and its mineral resources. To advance the understanding of the geologic framework beneath the southeast Missouri iron metallogenic province, we invert continental-scale magnetic and gravity anomaly data to three-dimensional (3D) physical property models. The regional models image altered and mineralized igneous rocks near the Proterozoic basement surface and underlying intrusive complexes that extend down to the Moho. At shallow crustal levels, our models confirm that iron oxide-apatite ± rare earth element (IOA ± REE) deposits and iron oxide-copper-gold ± cobalt (IOCG) deposits occur within or near the edges of large low density/low susceptibility early Mesoproterozoic (ca 1.4 Ga) silicic calderas and (ca 1.3 Ga) granitic plutons. Previous isotopic and geochemical studies conclude that the iron deposits and their volcanic host rocks originated from mantle-derived and crustal melts that erupted during regional extension. Extension was associated with thermal event(s) that produced the large-scale silicic magmatism related to the ca 1.45 Ga Eastern Granite Rhyolite Province (EGRP) and the 1.35 Ga Southern Granite Rhyolite Province (SGRP). We postulate that early in the evolution of the EGRP, several trans-crustal magmatic plumbing systems developed that are evident in the 3D models. The southeast Missouri metallogenic province is underlain by one such magmatic system that is expressed as a northwest-trending ∼50 km-wide by 200 km-long elongate track of high susceptibility at deep crustal levels. The high susceptibility corridor splays upward through the crust to the Proterozoic surface where the iron deposits are the epigenetic manifestation of this magmatic event. Our findings confirm that the iron deposits, with no distinct connection at the surface, are connected to one large magmatic system at depth. We propose that other similar susceptibility tracks, which are present along the top of the mantle, mark additional feeder zones that allowed magma to ascend to the main eruptive centers that produced the Granite Rhyolite Provinces. The early Mesoproterozoic extensional tectonic framework established crustal-scale pathways that controlled the distribution of subsequent magmatic activity, including the ca 1.4 Ga calderas and underlying intrusions, ca 1.3 Ga silicic plutons and Phanerozoic alkaline intrusions. If these interpretations are correct, our study has identified large areas that are prospective for critical mineral-bearing ore deposits and, importantly, suggests that the Mesoproterozoic architecture may have influenced subsequent magmatism and hydrothermal activity in the southern Midcontinent of the U.S.