Files for reproducing Figures 3–7. To open these, download Rcrust from https://www.sun.ac.za/english/faculty/science/earthsciences/rcrust and replace the 'Projects' and 'data' folders with those in the zipped folder. Open each project by double clicking the appropriate x.RData file.

10.6084/m9.figshare.7667912

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The geochemistry of Archaean plagioclase-rich granites as a marker of source enrichment and depth of melting

Geological Society of America eBooks (2010)

Jean‐François Moyen D.C. Champion R.H. Smithies

In geochemical diagrams, granitoids define 'trends' that reflect increasing differentiation or melting degree. The position of an individual sample in such a trend, whilst linked to the temperature of equilibration, is difficult to interpret. On the other hand, the positions of the trends within the geochemical space (and not the position of a sample within a trend) carry important genetic information, as they reflect the nature of the source (degree of enrichment) and the depth of melting. This paper discusses the interpretation of geochemical trends, to extract information relating to the sources of granitoid magmas and the depth of melting....

Sample (material)

10.1130/2010.2472(03)

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The Impact of Measurement Scale on the Univariate Statistics of K, Th, and U in the Earth Crust

Earth and Space Science (2021)

D. Baratoux M. Fall P. Y. Meslin Mark Jessell O. Vanderhaeghe

Abstract The univariate statistics of Potassium (K), thorium (Th), and uranium (U) concentrations, in the Earth’s oceanic and continental crust are examined by different techniques. The frequency distributions of the concentrations of these elements in the oceanic crust are derived from a global catalog of mid‐ocean ridge basalts. Their frequency distributions of concentrations in the continental crust are illustrated by the North Pilbara Craton, and the West Africa Craton. For these two cratons, the distributions of K, Th, and U derived from geochemical analyses of several thousand whole rock samples differ significantly from those derived from airborne radiometric surveys. The distributions from airborne surveys tends to be more symmetric with smaller standard deviations than the right‐skewed distributions inferred from whole rock geochemical analyses. Hypothetic causes of these differences include (a) bias in rock sampling or in airborne surveys, (b) the differences between the chemistry of superficial material and rocks, and (c) the differences in scales of measurements. The scale factor, viewed as consequence of the central limit theorem applied to K, Th, and U concentrations, appears to account for most of the observed differences in the distributions of K, Th, and U. It suggests that the three scales of auto‐correlation of K, Th, and U concentrations are of the same order of magnitude as the resolution of the airborne radiometric surveys (50–200 m). Concentrations of K, Th, and U are therefore generally heterogenous at smaller scales.

Radiometric dating

10.1029/2021ea001786

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Granite petrogenesis and the δ44Ca of continental crust

Earth and Planetary Science Letters (2023)

Michael A. Antonelli Chris Yakymchuk Edwin A. Schauble John Foden Vojtěch Janoušek

Stable Ca isotopes are an increasingly useful tool for understanding the sources and processes leading to the formation of magmatic rocks, yet Ca isotope fractionation during genesis of silicic continental crust is still poorly understood. Here, we present Ca, Sr, and Nd isotope, as well as major- and trace-element whole-rock geochemical data for A-, I-, and S-type granites (n = 30) from Australia/Tasmania, Canada, and France (δ44CaBSE of -0.6‰ to +0.2‰) and compare them to phase-equilibrium models for partial-melting (pelite, greywacke, MORB, enriched Archean tholeiite) and crystallization (hydrous arc basalt, A-type granite) that incorporate novel ab-initio predictions for Ca isotope fractionation in epidote and K-feldspar. The ab-initio calculations predict that epidote has similar δ44Ca to anorthite and that K-feldspar is the isotopically lightest known silicate mineral at equilibrium (Δ44Cakspar-melt of -0.4‰ at 1000 K). Our phase-equilibrium model results suggest that δ44Ca variations in all three granite types can be fully explained through magmatic processes, without necessarily requiring addition of isotopically exotic components (e.g., carbonate sediments). Heavy Ca isotope enrichments in A-type granites from the Lachlan Fold Belt, however, require isotopic disequilibrium between plagioclase and melt, which we use to constrain average plagioclase growth rates in these systems. This also serves to illustrate that whole-rock Ca isotope measurements can be used to estimate crystal growth rates, even in the absence of analyzable phenocrysts. In general, low Ca diffusivities and strong isotopic diffusivity ratios (D44/D40) in low-H2O granitic magmas should lead to resolvable isotopic disequilibrium effects in plagioclase, even at relatively slow growth rates (e.g., > 0.03 cm/yr). Combining our data with those from previous studies, we demonstrate that average granitoids and upper continental crust (with newly estimated δ44CaBSE of -0.25 ± 0.02‰, 2SE) have resolvable low δ44Ca compared to basalts and oceanic crust. Given that pressure has a major influence on Ca isotope fractionation across all of our models, this implies that melts feeding upper crustal granitoids dominantly evolve in the lower crust (10-14 kbar, through either partial-melting or fractional crystallization). This observation also suggests that heavier Ca isotopes are preferentially recycled back into the mantle through subduction and/or lower-crustal delamination events, but this is unlikely to have had a significant influence on the δ44Ca evolution of the upper mantle through geologic time.

Fractional crystallization (geology)

Petrogenesis

Silicic

10.1016/j.epsl.2023.118080

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Instabilities development in partially molten rocks

Bollettino Della Societa Geologica Italiana (2008)

Jean‐Louis Vigneresse Jean‐Pierre Burg Jean‐François Moyen

Partially molten rocks (PMR) are characterized by specific and contrasting behaviours. For instance, large-scale and smaller scale structures are consistently oriented in a migmatitic body with those of the surroundings, indicating that the migmatites were deformed as a whole. By contrast, ubiquitous strain partitioning and melt distribution are widely present in the same migmatitic body, reflecting highly heterogeneous strain and intrinsic rheological instabilities. A continuous transition from a liquid-like to a solid-like rheology, as many averaging processes implicitly assume, cannot explain this two-fold information. We develop a full analysis, considering the stress and strain rate, and the relative proportion of melt and solid phases. Temperature varies from Tsolidus to Tliquidus in a PMR. We also assume that the transition to melting is not dual to crystallization. However, we prefer using the viscosity rather than the stress, since the former is better constrained from experiments. The viscosity of the matrix, which deforms according to a power law, shows shear thinning, whereas that of the melt remains constant. The viscosity contrast between the two phases thus varies with strain rate. The lower the strain rate, the higher is the viscosity contrast, hence instabilities development is controlled by the rheology. The path followed during a transition also controls the intermediate state, and may lead to instabilities, resulting from mechanical reasons or from the respective amount in each phase. In the last case, the concentration in one phase induces instabilities. A surface describing viscosity in a 3D diagram (strain rate-amount of phase-viscosity) is constructed, that presents a cusp shape for low strain rates. The diagram depicts two types of behaviour and a critical state. At high strain, the viscosity contrast between melt and matrix is lowest. The rock behaves as a near-homogeneous body and a continuous description of its rheology may be estimated. Instabilities lead to fabric development resulting from crystals alignment. At low strain rate, three domains are separated by a critical state. When the proportion of one phase is very small, the material behaves as the other end-member. For intermediate proportions, the cusp indicates three possible viscosity values. Two are metastable, whereas the third is virtual. Hence, the viscosity of the mixture jumps back and forth from the viscosity of one phase to that of the other. A similar process occurs for temperature, since the cusp in the viscosity profile has also implications in a diagram linking temperature and stress. Different behaviours result, depending on whether the deformation takes place under a fixed content in each phase, a common stress, a common strain rate or common temperature. We list several implications for partially molten rocks that may explain fabric development, contact melting between crystals, strain localisation, mineral banding, shear heating, welding, stick-slip-like melt extraction, magma fragmentation or formation of strong or fragile glass. A phase diagram that incorporates temperature, stress and concentration is constructed for PMR that bears much similitude with those issued for other soft materials.

Strain partitioning

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Flow of the partiallymolten Variscan orogenic crust fromsyn-orogenic exhumation of subductedcontinental crust to gravitational collapsealong a convergent plate boundary markedby slab retreat

Olivier Vanderhaeghe Véronique Gardien Jean‐François Moyen Oscar Laurent Simon Couzinié

The Variscan belt ofWestern Europe exposed in the French Massif Central is a perfect example of a collision zone characterized by protracted syntectonic magmatism and partial melting (from 380 to 280 Ma) with a wide range of petrologic and geochemical signatures (calc-alkaline, high-K, Mg-K, peraluminous) that have been inferred to fingerprint lithospheric subduction, mantle upwelling and/or partial melting of the orogenic wedge. The nappe pile encompasses an upper gneiss unit (UGU) and a lower gneiss unit (LGU) that are separated by an association of maficultramafic rocks designated as the Leptynite-Amphibolite Group (LAG) and has been interpreted as representing remnants of former small immature oceanic basins. Both the UGU and the LGU are made of migmatites but are distinguished on the basis of their structural position with respect to the LAG and of their metamorphic record. The UGU has preserved relics of high-pressure metamorphism whereas the LGU has only recorded a high-temperature metamorphism. We present a synthesis of structural, petrologic, geochemical and geochronological data from the various lithologic-tectonic units exposed along a transect across the Variscan belt of Western Europe from the French Massif Central to the Pyrenees. In particular, the new geochronological and geochemical dataset presented by Couzinie et al. (this conference) suggests the contribution of mantle and crustal derived magmas with a southward younging of syntectonic emplacement. These data provide a basis to elaborate a model for the structure of the Laurussia- Gondwana plate boundary at the onset of convergence and for the generation and flow of migmatites during orogenic evolution from the early stage of subduction of the continental crust to gravitational collapse of the orogenic belt in a context of a convergent plate boundary marked by southward slab retreat.

Massif

Underplating

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