Hervé Diot

Analogue and numerical modeling of shape fabrics: application to strain and flow determination in magmas

We summarise numerical and analogue models of shape fabrics, and discuss their applicability to the shape preferred orientation of crystals in magmas. Analyses of flow direction and finite strain recorded during the emplacement of partially crystallised magmas often employ the analytical and numerical solutions of the Jefferys model, which describe the movement of non-interacting ellipsoidal particles immersed in a Newtonian fluid. Crystallising magmas however are considered as dynamic fluid systems in which particles nucleate and grow. Crystallisation during magma deformation leads to mechanical interactions between crystals whose shape distribution is not necessarily homogeneous and constant during emplacement deformation. Experiments carried out in both monoparticle and multiparticle systems show that shape fabrics begin to develop early in the deformation history and evolve according to the theoretical models for low-strain regimes. At large strains and increasing crystal content, the heterogeneous size distribution of natural crystals and contact interactions tend to generate steady-state fabrics with a lineation closely parallel to the direction of the magmatic flow. This effect has been observed in all 3D experiments with particles of similar size and for strain regimes of high vorticity. On the other hand, studies of feldspar megacrysts sub-fabrics in porphyritic granites suggest that these ones record a significant part of the strain history. Thus, fabric ellipsoid for megacrysts evolves closer to the strain ellipsoid than for smaller markers. This behaviour results from the fact that the matrix forms of the melt and smaller crystals behave like a continuous medium relative to the megacrysts. Consequently, in absence these markers, and because the fabric intensities of smaller particles such as biotite are stable and lower than predicted by the theory, finite strain remains indeterminate. In that case, strain quantification and geometry of the flow requires the addition of external constrains based on other structural approaches.

Rigid Particles in Simple Shear Flow: Is Their Preferred Orientation Periodic or Steady-State ?

The theory of the rotation of isolated rigid particles within a linearly viscous fluid deforming in progressive simple shear is often invoked in models of Shape Preferred Orientations (SPO) of crystals in igneous rocks. A classical result of the theoretical model is that the SPO should rotate and pulsate with increasing strain, with a periodicity equal to that of the rotation of an individual particle. However, the initial theoretical model makes a large number of assumptions, many of which are unlikely to be satisfied by actual crystalline suspensions in magmatic melts. The purpose of this note is to review three of the reasons why periodicity of rigid particle SPO may not really be expected in igneous rocks: (i) SPO in igneous rocks are generally defined by suspensions of crystals that are concentrated enough to allow mechanical interactions between the crystals; (ii) the porphyroblast/matrix interface may not always be coherent; and (iii) the aspect ratios of the crystals defining the SPO are unlikely to be unique and constant, as assumed in the model. The last point is discussed on the basis of some 2D simple calculations of the development of SPO defined by different types of heterogeneous populations of particles. The combined effects of these deviations from the standard model point to two fundamental conclusions: (i) there is no simple relationship between fabric and finite strain, and (ii) the SPO in magmatic rocks may be considered as good markers of the flow, whatever the significance of the inferred flow pattern in terms of geodynamics and/or emplacement processes.

Magnetic fabric and deformation in charnockitic igneous rocks of the Bjerkreim-Sokndal layered intrusion (Rogaland, Southwest Norway)

The Bjerkreim‐Sokndal intrusion (BKSK) belongs to the Proterozoic Rogaland igneous complex of Southwest Norway. The BKSK displays an anorthosite to mangerite cumulate series, folded into a syncline, in the core of which crop out massive acidic rocks of quartz mangerite to charnockite composition. We present a study which focuses on the acidic rocks, in which we use the low-field anisotropy of magnetic susceptibility (AMS) technique. Systematic microscope examination of the samples collected at 148 AMS stations, and considerations based on mineral intrinsic magnetic susceptibilities reveal magnetic mineralogy and anisotropy controlled by magnetite. Macro- to micro-scale petrofabrics and magnetic fabrics indicate that the acidic rocks, which intruded after deposition of the underlying cumulate series, were also aAected by the syn- to post-magmatic folding event which gave rise to the BKSK syncline. In absence of any recognized plate-scale regional stress field, it is suggested that this folding results from a gravity-induced subsidence linked to the final stage of diapiric emplacement of huge neighbouring anorthosite plutons. This model is mainly supported by the structural pattern in the BKSK, which reveals a convergent flow

Calcic amphibole growth and compositions in calc-alkaline magmas: Evidence from the Motru Dike Swarm (Southern Carpathians, Romania)

Abstract A Late Pan-African calc-alkaline dike swarm (basalt-andesite-dacite-rhyolite) has been investigated in a region of over 2000 km2 in the Alpine Danubian window, South Carpathians (Romania). Amphibole phenocrysts and microphenocrysts have been investigated by wavelength-dispersive microprobe analysis and BSE imaging. The Ca-amphibole population, represented in all the lithologies, displays a large compositional range, interpreted as the result of two processes: (1) magmatic evolution (kaersutite → Ti-pargasite → pargasite → Ti-magnesiohastingsite → magnesiohastingsite → edenite → tschermakite → magnesiohornblende) linked to magmatic differentiation from andesitic basalt to rhyolite; and (2) deuteritic alteration of the primary amphibole related to late-emplacement hydrothermal activity (yielding numerous varieties comprising those cited above). In all rock types, amphibole phenocrysts equilibrated at a nearly constant pressure of about 0.6 ± 0.1 GPa, but their temperatures of crystallization ranged from 1000.900 °C for basaltic andesites to 700.600 °C for dacites. In rhyolites, edenite to magnesiohornblende crystals reflect a continuous range of P-T conditions from 700 °C/0.6 GPa to 600 °C/0.1 GPa, in agreement with their change of habit from euhedral to subhedral. Complex zonations in pargasite-magnesiohastingsite (including resorption) are interpreted in term of self-organization of oscillatory zoning without significant heating and/or magma mixing.

The Tellnes ilmenite deposit (Rogaland, South Norway): magnetic and petrofabric evidence for emplacement of a Ti-enriched noritic crystal mush in a fracture zone

Abstract The Tellnes ilmenite deposit, a world class titanium deposit, occurs in the Ana–Sira anorthosite (Rogaland anorthosite province, South Norway). It is mainly made up of an ilmenite-rich norite that has been previously interpreted as injected in a crystal mush state, in a weakness zone of the enclosing anorthosite. This emplacement mechanism has produced a faint orientation in the ore due to the flow of the mush. The internal flow structure of the orebody is studied here using the low-field anisotropy of magnetic susceptibility (AMS) method. Partial anhysteretic remanent magnetization (pAARM) indicates that coarse magnetite is the main mineral responsible for the magnetic fabric. Parallelism of the magnetic fabric with the shape-preferred orientation of the ore-forming minerals is checked using image analysis (IA) from oriented sections (intercept method). Interpretation of the AMS data verified by pAARM and IA, provides information on the magmatic foliation and lineation. Emplacement flow of the ilmenite norite crystal mush occurred in the direction of the orebody, parallel to its walls, and with an average SE 18° plunge. The feeder zone was likely situated below a network of veins on the SE end of the orebody. The sickle-shaped outcrop of the deposit suggests a transcurrent, dextral opening of a WNW–ESE-striking weakness zone across the anorthosite pluton.

Wagnerite in a cordierite-gedrite gneiss: Witness of long-term fluid-rock interaction in the continental crust (Ile d’Yeu, Armorican Massif, France)

Abstract We describe the first occurrence in the Variscan Belt of Western Europe of the relatively rare phosphate wagnerite, ideally Mg2PO4F. It occurs in albite-rich, cordierite-gedrite-bearing gneisses on the island of Ile d’Yeu, southern Armorican Massif, France. These gneisses are associated with a network of shear zones that crosscut granitoid orthogneisses of calc-alkaline affinity. Wagnerite is zoned and displays a rimward decrease of Fe/(Fe + Mg) from 0.16 to 0.08 and a concomitant increase in F. The F content ranges 0.46-1.05 apfu, but critically depends on the choice of the analytical standard. Based on phase diagrams calculated with THERMOCALC, we infer that the wagnerite-bearing orthoamphibole + cordierite + biotite + chlorite paragenesis equilibrated at ca. 550 °C, and pressures lower than 4 kbar. The presence of staurolite relics requires similar temperatures, but pressures higher than 4 kbar, implying an evolution dominated by decompression. On the basis of whole-rock chemistry and stable isotopes, we suggest that superimposed periods of metasomatic alteration throughout the metamorphic history led to the prograde stabilization of the cordierite-gedrite gneiss at the expense of the orthogneiss. This alteration involved aqueous fluids in isotopic equilibrium with local rocks and caused significant loss of Ca, K, and Si, and gain of Mg and Na. We argue that the Na-enrichment is the most significant difference between wagnerite-bearing and wagnerite-free Mg-rich, Ca-poor rocks on Ile d’Yeu. This emphasizes the possible importance of Na metasomatism for the formation of wagnerite. In light of comparisons with other wagnerite occurrences, we conclude that a long-term fluid-rock interaction, typically associated with shear-zones, may be the rule rather than the exception for the formation of wagnerite in metamorphic rocks unaffected by anatexis.

Deformation-driven Differentiation during in situ Crystallization of the 2·7 Ga Iguilid Mafic Intrusion (West African Craton, Mauritania)

The 2 7 Ga Iguilid mafic intrusion is a post-metamorphic plutonic body made of cumulate gabbronorite and norite characterized by limited variations in modal proportions but significant disparities in trace element contents. The cumulates display microtextural evidence for syn-magmatic deformation, leading to reorientation of cumulus minerals, indented contacts at plagioclase grain boundaries, local bending and fracturing of euhedral cumulus plagioclase and interstitial melt migration/ segregation structures. All these features suggest that the Iguilid intrusion formed after in situ crystallization and syn-magmatic deformation of an unconsolidated mush within solidification fronts. Quantitative investigations of syn-magmatic fabrics together with geochemical estimations of the trapped melt fraction reveal a strong inverse correlation between the anisotropy degree P and the volume of trapped liquid retained in the cumulates. We interpret the dominantly vertical planar and linear fabrics in the Iguilid rocks as a result of horizontal shortening of the cumulates induced by filter pressing of a partly solidified mush against the vertical walls of the intrusion. Flattening of the cumulates occurred in response either to a syn-magmatic rotation of the Iguilid body relative to the tectonic main stress or to stages of overand under-pressurization owing to cycles of magma recharge or discharge. Deformation of the unconsolidated cumulates led to migration of the silicic interstitial melt and controlled their bulk content in incompatible elements. This migrating liquid had similar trace element and isotopic compositions to synchronous 2 7 Ga dioritic to granodioritic plutons, forming a bimodal, post-metamorphic, magmatic suite with the Iguilid body. The interstitial melt extracted from the Iguilid mafic intrusion could have fed synchronous metaluminous intermediate to felsic plutons.

Anisotropie de la susceptibilité magnétique dans une intrusion composite de la suite charnockitique: l'apophyse du massif stratiforme de Bjerkreim-Sokndal (Rogaland, Norvège méridionale)

Deformation in the Bjerkreim-Sokndal layered intrusion apophysis was developed continuously from magmatic to solid state. The tectonic setting dictating these deformations was dominated by anorthosite diapirism. An anisotropy of magnetic susceptibility (AMS) study has been performed in the northern part of this apophysis. It shows that titanomagnetite orientation is governing the magnetic fabrics. It confirms the foliation structures visible in the field and, moreover, clarifies the linear structures. The AMS scalar parameters specify the deformation symmetry and put forward a correlation between the intensity of the deformation which is observed in the field and that revealed by AMS.