Magmatic Microstructures Research Articles

Baddeleyite microstructural response to shock metamorphism in three enriched shergottites and implications for U–Pb geochronology

Baddeleyite (monoclinic zirconia; m-ZrO2) occurs as a late-stage accessory mineral in shergottites and has been used to determine U–Pb igneous crystallization ages via in-situ secondary ion mass spectrometry (SIMS). During shergottite ejection from the surface of Mars, baddeleyite develops a range of microstructures primarily due to a series of shock-induced transformations to high pressure and temperature polymorphs. It remains poorly constrained to what extent U–Pb systematics in baddeleyite are sensitive to shock conditions experienced by shergottites. To investigate this, we examined baddeleyite in the enriched shergottites Jiddat al Harasis (JaH) 479, Northwest Africa (NWA) 10299, and NWA 12919, which bridge the gap in shock conditions represented in previous microstructural studies. Electron backscatter diffraction (EBSD) analysis reveals that although some baddeleyite grains retain magmatic microstructures (i.e. homogenous crystallographic orientations and twinning of igneous origin), there is widespread phase transformation to high-pressure orthogonal polymorphs (o-ZrO2) followed by reversion. JaH 479 contains more grains with preserved magmatic microstructures than the other two shergottites, suggesting that it experienced lower bulk shock pressures. Nanometer-scale reverted m-ZrO2 in NWA 10299 and NWA 12919 further points to insufficient post-shock temperatures; this contrasts with JaH 479 where greater variation in local temperature conditions enabled the development of µm-scale domains of reverted m-ZrO2. Individual grains that are separated into two distinct microstructural domains may reflect controls on shock propagation due to relative density contrast among the surrounding phases.SIMS U–Pb baddeleyite analysis yields igneous crystallization ages of 210 ± 9 Ma (JaH 479), 196 ± 11 Ma (NWA 10299), and 188 ± 11 Ma (NWA 12919). At the SIMS resolution, we find no clear evidence for significant Pb loss in the surveyed baddeleyite grains, suggesting that temperatures during the formation of both nm-scale and µm-scale reverted m-ZrO2 in the three shergottites were insufficient to cause significant Pb diffusion. Given the robust baddeleyite U–Pb isotope systematics in the majority of shergottites dated by SIMS methods thus far, we argue that shock conditions experienced by the bulk of shergottites were insufficient to introduce significant U–Pb isotopic mobility, which is limited to grains showing microstructural evidence for extensive post-shock heating and recrystallization. Our findings place new constraints on baddeleyite microstructural response to shock conditions of shergottite ejection and demonstrate that microstructural observations are critical when using baddeleyite as a chronometer in shocked planetary materials.

Microstructures in the Baima layered intrusion, SW China: Constraints on the late-stage magmatic processes during solidification of a crystal mush

Magmatic microstructures in layered intrusions record key petrographic evidence for the late-stage magmatic processes that took place during solidification of a crystal mush. Systematic examination of the magmatic microstructures can reveal dynamic evolution of interstitial liquid in crystal mush, which however has been only conducted in a few studies on layered intrusions by far. In order to constrain the late-stage magmatic processes related to the Fe-Ti oxide-bearing layered intrusions, we investigated the magmatic microstructures of the Baima intrusion in the Panxi region (SW China). The intrusion is divided into three major units from the base upwards, including the lower zone (LZ), middle zone (MZ) and upper zone (UZ). Through detailed examination of a series of drill core samples, we found several different types of reactive microstructures, such as replacive symplectites, fish-hook pyroxene/olivine/amphibole and amphibole rim. Replacive symplectites include fine-grained, lamellar intergrowths of An-rich plagioclase + olivine/clinopyroxene/amphibole/phlogopite/biotite that are rooted to Fe-Ti oxides and replacing plagioclase primocrysts (Type 1) and intergrowths of An-rich plagioclase + orthopyroxene/olivine/amphibole that are rooted to olivine and replacing plagioclase primocrysts (Type 2). These microstructures are particularly abundant in the LZ except for the Type 2 symplectites, which are rare throughout the intrusion. An-rich plagioclase in the replacive symplectites and fish-hook textures have An contents up to 96, much higher than those of plagioclase primocrysts. Overall, fish-hook amphibole and amphibole rims contain 0.19−4.24 wt.% TiO2, higher than those of amphibole lamellae in the replacive symplectites that varying from 2.38 wt.% TiO2 to below the detection limit. These microstructures were developed during solidification of the intrusion due to the reaction between plagioclase primocrysts and ambient Fe-rich liquid, accompanying with addition of Fe, Ti and Ca and removal of alkalis and silica in an open system. The abundance of such reactive microstructures in the LZ are therefore attributed to silicate liquid immiscibility within a crystal mush, in which the segregation of the buoyant Si-rich component left behind the dense Fe-rich liquid in the lower part of magma chamber. The common presence of amphibole and phlogopite/biotite in the replacive symplectites indicates that continuous fractionation of the Fe-rich liquid leads to hydration of interstitial liquid during the late-stage solidification.

Compositional Evolution of Interstitial Liquid After Onset of Abundant Fe–Ti Oxide Crystallization in Crystal Mush: Insights from Late-Stage Microstructures and Mineral Compositions of the Bijigou Layered Intrusion, Central China

ABSTRACT Compaction is important to the expulsion and compositional modification of interstitial liquid of crystal mush in the solidification of layered intrusions hosting Fe–Ti oxide mineralization. However, the compaction manner and compositional evolution of interstitial liquid after the onset of abundant Fe–Ti oxide crystallization in crystal mush are still enigmatic. In this study, we collected samples from a drill core profile across the major Fe–Ti oxide-rich segment of the Bijigou intrusion, a large and differentiated layered intrusion in Central China, and carried out a detailed study on the late-stage magmatic microstructures of rocks and crystal size distributions (CSDs) and spatial distribution patterns (SDPs) of clinopyroxene and plagioclase to examine the compaction manner of interstitial liquid. The Fe–Ti oxide-rich segment is composed of, from the base upward, a Fe–Ti oxide ore layer (~80 m), a troctolite unit (~105 m), an oxide gabbro unit (~153 m) and an apatite–oxide–gabbronorite unit (~165 m). The CSD curves for both plagioclase and clinopyroxene grains of the samples from the Fe–Ti oxide ore layer display gentle slopes for small crystals and consistent slopes for large crystals. In addition, there is a positive correlation of the SDP R-value (ratio of observed and predicted nearest neighbor distance of plagioclase/clinopyroxene) and mineral mode except plagioclase/clinopyroxene of the samples. The interstitial liquid was likely expelled by intensive pressure solution concomitant with crystallization of abundant Fe–Ti oxides, evidenced by the positive correlation of aspect ratio and alignment factor of plagioclase and the low fraction of trapped liquid (FTL) of 4–6% for the Fe–Ti oxide ore layer. The cumulus plagioclase of the Fe–Ti oxide ore layer was partially dissolved during pressure-solution compaction, leading to ambient Fe-rich interstitial liquid becoming Al-enriched in local places so that abundant amphibole and hercynite are present in the replacive symplectites adjacent to cumulus plagioclase. Above the Fe–Ti oxide ore layer, plagioclase grains in the samples from the troctolite unit show consistent slopes of the CSD curves and negative correlation of SDP R-value and mineral mode in the SDP plot, indicating that the interstitial liquid was mainly expelled by mechanical compaction, with FTL ranging from 7 to 9%. The CSD curves for clinopyroxene grains of the samples from the oxide gabbro unit have been modified by Ostwald ripening, but the expulsion of interstitial liquid in this unit is also attributed to mechanical compaction due to higher compaction rate than accumulation rate. This study demonstrates that the compaction manner of interstitial liquid can change from pressure solution to mechanical compaction with dramatically diminishing crystallization of Fe–Ti oxides. The pressure-solution compaction accompanying with large amounts of Fe–Ti oxides can remarkably modify the composition of interstitial liquid. This study also demonstrates that a combined measurement of CSDs and SDPs for cumulus minerals can be used to quantify the compaction manner of interstitial liquid during solidification of layered intrusions.

The microstructure of layered ultramafic cumulates: Case study of the Bear Creek intrusion, Trinity ophiolite, California, USA

In the Trinity ophiolite, California, USA, several mafic-ultramafic plutons intruded a peridotitic host 435 to 405 m.y. ago in a tectonic setting interpreted as an arc-related spreading centre. One of these intrusions, in the Bear Creek area, exposes basal ultramafic cumulates with igneous layering comprising an alternation of uncommonly thin (down to a few mm) layers of dunite, peridotite and pyroxenite that might be specific to this tectonic setting. These layers offer an excellent opportunity to characterise the microstructure of uncommon cumulates from the lower crust using EBSD (Electron Backscatter Diffraction). This “high-frequency” layering rests on underlying lherzolites and grades upward to more massive pyroxenites (i.e. clinopyroxenites with minor olivine-rich layers). Our field observations and data from the Bear Creek cumulates together with the preservation of magmatic features suggest the environment was tectonically stable after the emplacement of the cumulates. A detailed microstructural investigation of all minerals from the Bear Creek cumulates allows us to decipher their magmatic and plastic deformation history. In a structural reference frame defined by the compositional layering and the elongation direction of the surrounding host peridotites, olivine in the cumulates presents a [010]-fibre fabric and rarely a [001](010) fabric. Clinopyroxene shows a concentration of [010] axes normal to the layering plane with [100] and [001] defining girdles. Orthopyroxene mostly has a fabric with [100] and/or [010] subnormal to the layering plane and [001] scattered along a girdle in the plane of layering. All minerals show a strong fabric. We interpret the formation of the developed planar microstructures as a result of magmatic processes, with high contribution of crystal settling. To a lesser extent, compaction could have been operating and may be linked to the rare evidence of plastic deformation. Clusters of axes within the girdles of olivine and pyroxene CPOs preferentially appear close to the direction of elongation of the surrounding peridotites (i.e. N115°). EBSD analysis of the shape-preferred orientation of Bear Creek's minerals revealed a preferential alignment of the olivine and cpx long axis with the N°115 direction. This magmatic lineation and the preferred direction in the CPOs girdles are both consistent with the stretching lineation acquired during solid-state deformation by the mantle peridotite of the Trinity ophiolite. We suggest that a weak magma flux early on and/or an ongoing but limited regional stress could be responsible for these clusters. Although a direct coupling between asthenospheric flow and magmatic flow cannot be invoked in this context of melt intrusion in the lithosphere, this result highlights that the stress field applied on the mantle could have been still active and similar during the formation of Bear Creek intrusion.Our new field and microstructural data, together with previously presented petrological data, fit a scenario for the evolution of the Trinity ophiolite in which a mantle segment was intruded by a single large batch of primitive boninitic-andesitic melt. Our results emphasise the importance of considering the initial magmatic microstructures and the original shape anisotropy when investigating later deformation in ultramafic rocks.

Field observations, magnetic fabrics and microstructures evidences of syn-kinematic emplacement of the Numba granitic pluton (western Cameroon domain)

The N–S elongated Pan-African Numba granitic pluton (NGP), hosted in banded biotite hornblende gneiss and amphibolite country-rocks, belongs to the Western Domain of the Pan-African Belt of Central African in Cameroon. It consists of biotite hornblende granite and fine-grained biotite granite crosscut by gabbroic dykes. The western border of the granite is mylonitized. Structures and microstructures recorded in granite reveal magmatic, submagmatic and high- to moderate- temperature solid-state deformations. Magnetic characterisation of the NGP reveals its ferromagnetic behaviour due to the presence of multi-domain automorphic or xenomorphic pure magnetite grains. Field and AMS measurements testify that the study area recorded three deformation events: (1) a D1 event of ENE-WSW to E-W flattening deformation; (2) a D2 event of sinistral top-towards south sense of shear movement characterised by low to moderate dips (i) NE-SW strike foliation in country-rocks; (ii) NNE-SSW to NE-SW strike of magmatic foliation marked by the preferred orientation of K-feldspar megacrysts in granite and (iii) low plunges NE-SW mineral stretching lineation in biotite hornblende gneiss parallel to the axes of SW vergence of "S" shape F2 folds and (3) a D3 event of dextral top-towards east sense of shear movement that developed narrow NNE-SSW strike mylonite corridor. The NGP emplaced in a fracture within an amphibo-gneissic country-rock and has recorded opposite sinistral top-towards south (D2 Pan-African event) to dextral top-towards east senses of shear movements (D3 Pan-African event). It can be inferred that the emplacement and the deformation of the NGP was initiated during regional D2 deformation (dated at 613-585 Ma) when its magnetic fabric was acquired, and it was completely achieved during regional D3 deformation (dated ca. 585-540 Ma). Tj vs. Pj diagram associated with Flinn diagram suggests an emplacement in dominant flattening strain regime with a considerable shear component. The different data recorded in the NGP imply that its emplacement mechanism points to a ballooning. Indeed, the eastern domain of NGP with elliptical shape represents the inner part of the architecture while its western domain represents the outer part. The moderate to high dips observed in the inner part of the elliptical domain suggest the probable location of a feeder zone, and the lineation pointing away from the centre of the intrusion in the outer part may be developed related to the magmatic flow. This concentricity is easily associated with internal microstructures that show moderate-temperature solid-state to submagmatic and magmatic microstructures from the outside of the pluton towards the centre. The magma was then emplaced by ballooning in a gneissic country-rock that was subjected to sinistral shearing. Afterwards, the deformation characterised by a dextral shearing contributed to a generation of a NNE-SSW mylonitic band at the western border of the pluton.

The Esmolfe-Matança granite (Penalva do Castelo, central Portugal): A keystone to understand the ascent and emplacement of magmas under low tectonic stress

The Esmolfe–Matança ilmenite-type granite is analyzed using petrography, microstructures, anisotropy of magnetic susceptibility and microfracturing. The granite occurs in two separated bodies outcropping in distinct villages, Esmolfe and Matança, which are located in the termination of the Juzbado-Penalva do Castelo (JPCSZ) and Dúrico-Beirão (DBSZ) shear zones. The petrography shows a progressive textural overprint from magmatic, through high-, medium- and low-T solid-state deformation microstructures. The presence of dominantly magmatic microstructures and the absence of deformation patterns at macroscopic scale, may indicate an emplacement under low tectonic stress. The directional magnetic data indicates a magma ascending and emplacement conditioned by the JPCSZ and DBSZ. The magma passive intruded upwards, along the confluence zone between the JPCSZ and DBSZ, and it spread laterally forming an asymmetric laccolithic body. The DBSZ worked as physical barrier in Esmolfe, avoiding the spreading of the magma to the north, leading to the rotation of the magnetic lineations and foliations, drawing onion-type trajectories. In Matança the magnetic lineations and foliations are dominantly NW-SE oriented in the western side, and ENE-WSW oriented in the eastern side, parallel to the JPCSZ regional trend. The fluid inclusions planes demonstrated three distinct brittle deformational events linked to the rotation of σ1.

Internal fabrics of the Idaho batholith, USA

The Idaho batholith of the North American Cordillera is a large and long-lived silicic intrusive center. We studied fabrics at the regional scale within the different intrusive suites of the Idaho batholith, using microstructural characterization, anisotropy of magnetic susceptibility measurements, and shape preferred orientation analyses. Each studied outcrop was collocated with existing U-Pb zircon geochronology and ongoing (U-Th)/He zircon thermochronology results. Three new 40 Ar/ 39 Ar biotite ages, collocated with the existing U-Pb zircon ages, constrain the cooling rates within the batholith. The presence of dominantly magmatic microstructures allows us to interpret the results relative to the U-Pb zircon ages and observe spatial and temporal patterns of fabric development. The early (pre–80 Ma) intrusive suites exhibit solid-state microstructures that show a consistent orientation only in the western part of the batholith. Fabrics in the 83–67 Ma Atlanta peraluminous suite are weak and inconsistent in orientation, despite emplacement during regional contraction. We hypothesize that the lack of consistently oriented fabrics in the Atlanta lobe results from either: (1) topographic effects that caused local extensional/neutral strain environments in the upper parts of a crustal plateau; or (2) emplacement in thin, horizontal magmatic sheets. In contrast, fabrics within the 66–53 Ma Bitterroot peraluminous suite are well developed and consistently oriented (NW-striking; NE-dipping), recording localized contraction during magmatism.

Deformation Evidences and Shear Zone in Marziyan Granitic Pluton, Azna, Sanandaj-Sirjan Zone

Marziyan granitic pluton is one of the numerous intrusive bodies in Sanandaj-Sirjan structural zone. The pluton is composed of locogranite, granite and granodiorite and its contact metamorphic rocks are consisting of cordierite and andalusite schist. Tourmaline and garnet bearing locogranite, aplite, pegmatite, mylonite and injected migmatite are exposed in the metamorphic halo. The major minerals include quartz, plagioclase, K-feldspar, biotite as well as muscovite, tourmaline and garnet. Structural studies indicate the presence of shear zones, especially in the southeastern parts with NNW-SSE trend. Four types of microstructures are present in the region; 1) magmatic microstructures, 2) high temperature solid state microstructure, 3) low temperature solid state microstructure and 4) sub-mylonitic and mylonitic microstructure. All rocks of the region have been subjected to shear deformation which led the formation of S-C foliations and other sense of shear markers. The presence of asymmetric porphyroclasts and porphyroblast, biotite and feldspar kink bands, mica-fish mica and andalusite and domino and boudinaged structures of feldspar and tourmaline are evidences of ductile deformation in a contractional and dextral shear regime. All evidences indicate that high temperature solid state zones in greenschist facies to lower amphibolite conditions (300-500°C) have formed simultaneously with emplacement of granitic bodies. Low temperature solid state, sub-mylonitic and mylonitic zones developed following the emplacement of granitic bodies.

Major and trace elements of abyssal peridotites: evidence for melt refertilization beneath the ultraslow-spreading Southwest Indian Ridge (53° E segment)

This study examines the geochemistry of major and trace elements of abyssal peridotites from the Southwest Indian Ridge (SWIR) (53° E amagmatic segment), to determine the influence of mafic melts on mantle peridotites during melt extraction. The results show a great geochemical variability in the ~90 km-long ridge segment, with a degree of mantle melting ranging from 4% to 24%. An ancient melting event may explain the presence of highly depleted peridotites at the ultraslow-spreading ridge. The 53° E segment peridotites show enrichment of light rare earth elements (LREEs) (average LaN/SmN = 1.87) and significant positive anomaly of U and Pb normalized to primitive mantle (PM). The positive correlations between LREEs (La, Ce, Pr, Nd) and high field strength elements (HFSEs; e.g. Nb and Zr) suggest that the enrichment of LREEs is caused by melt refertilization, which is also supported by prevalent magmatic microstructures in the peridotites. The melt refertilization model shows that the addition of 0.02–2.7% basaltic melts to peridotites can be responsible for the LREE enrichment. We suggest that the positive anomaly of U is probably attributed to fluid alteration whereas the enrichment of Pb is probably attributed to both melt refertilization and fluid alteration. Melt refertilization in the 53° E segment peridotites may be a result of melt–rock reaction and crystallization of melts trapped in peridotites. These processes may be enhanced by increased melt permeability in the mantle owing to the refractory peridotites produced by ancient melting and the decreasing efficiency of melt extraction in the cold and thick lithosphere at the 53° E ridge segment. The presence of melt refertilization implies that melt extraction is incomplete in the ridge mantle, which may be one of the reasons for the extremely thin and irregular variation of the crustal thickness at ultraslow-spreading ridges.

A database of plagioclase crystal preferred orientations (CPO) and microstructures – implications for CPO origin, strength, symmetry and seismic anisotropy in gabbroic rocks

Abstract. This study presents a unique database of 172 plagioclase Crystallographic Preferred Orientations (CPO) of variously deformed gabbroic rocks. The CPO characteristics as a function of the deformation regime (magmatic or crystal-plastic) are outlined and discussed. The studied samples are dominantly from slow- and fast-spread present-day ocean crust, as well as from the Oman ophiolite. Plagioclase is the dominant mineral phase in the studied samples. Plagioclase CPOs are grouped into three main categories: Axial-B, a strong point alignment of (010) with a girdle distribution of [100]; Axial-A, a strong point maximum concentration of [100] with parallel girdle distributions of (010) and (001); and P-type, point maxima of [100], (010), and (001). A majority of CPO patterns are Axial-B and P-type, in samples showing either magmatic or crystal-plastic deformation textures. Axial-A CPOs are less common; they represent 21% of the samples deformed by crystal-plastic flow. Although fabric strength (ODF J index) does not show any consistent variation as a function of the CPO patterns, there is a significant difference in the relationship between the ODF and pole figures J indices; the magmatic type microstructures have high (010) pole figures J indices, which increase linearly with ODF J index, whereas the high [100] pole figures J indices of plastically deformed samples vary in a more scattered manner with ODF J index. The multistage nature of plastic deformation superposed on a magmatic structure compared with magmatic flow, and the large number of possible slip-systems in plagioclase probably account for these differences. Calculated seismic properties (P wave and S wave velocities and anisotropies) of plagioclase aggregates show that anisotropy (up to 12% for P wave and 14% for S wave) tends to increase as a function of ODF J index. In comparison with the olivine 1998 CPO database, the magnitude of P wave anisotropy for a given J index is much less than olivine, whereas it is similar for S wave anisotropy. Despite a large variation of fabric patterns and geodynamic setting, seismic properties of plagioclase-rich rocks have similar magnitudes of anisotropy. There is a small difference in the aggregate elastic symmetry, with magmatic microstructures having higher orthorhombic and hexagonal components, whereas plastic deformation microstructures have a slightly higher monoclinic component, possibly correlated with predominant monoclinic simple shear flow in plastically deformed samples. Overall, plots for CPO strength (ODF J index), pole figure strength, CPO symmetry and seismic anisotropy show significant scattering. This could be related to sampling statistics, although our database is a factor of ten higher than the olivine database of 1998, or it could be related to the low symmetry (triclinic) structure of plagioclase resulting in the addition of degrees of freedom in the processes creating the CPOs.

The Pan-African quartz-syenite of Guider (north-Cameroon): Magnetic fabric and U–Pb dating of a late-orogenic emplacement

The small Guider pluton (70km2) in-between the cities of Maroua and Garoua (north Cameroon) is one of the quartz-syenite bodies that intrude Pan-African orthogneisses, and that are aligned along the western side of the NNE-directed Poli-Léré volcano-sedimentary corridor. The Guider syenite is here studied for its internal structures, using field and anisotropy of magnetic susceptibility measurements, and for its emplacement age of 593±4 Ma (U–Pb zircon), ∼20Ma younger than the beginning of gneissification of the country-rocks, mainly made of deformed granodiorites and diorites. In these orthogneisses, constantly NNE-trending vertical foliations, variously plunging stretching lineations and fold axes, as well as kinematic markers attest to a transpressive and possibly dextral regime. The Guider syenite, rich in K-feldspar and magnetite, displays typical magmatic microstructures with incipent solid-state features such as chess-board subgrains in quartz. Its magnetic fabric displays a well-defined feeding-zone in its centre and western side, attesting to its unrooted nature. However, N- to NE-trending lineations at its northern and southern ends, parallel to the overall linear structure of the country-rocks, point to its late-orogenic emplacement. Our results reveal that the NE-trending and dextral transpressive shear zone that acted in the Guider area ended after the emplacement of the syenite body; they specify the age of collage of the NW-Cameroon domain with the Mayo-Kébbi domain. They also show that the tectonic evolution of Guider area is closer to the E-Nigerian domain, to the west, than to its eastern neighbouring Mayo-Kébbi and Adamawa-Yadé domains. The connection of our sector with E-Brazil is briefly discussed.

Charnockite microstructures: From magmatic to metamorphic

Charnockites sensu lato (charnockite-enderbite series) are lower crustal felsic rocks typically characterised by the presence of anhydrous minerals including orthopyroxene and garnet. They either represent dry (H2O-poor) felsic magmas that are emplaced in the lower crust or granitic intrusions that have been dehydrated during a subsequent granulite facies metamorphic event. In the first case, post-magmatic high-temperature recrystallisation may result in widespread metamorphic granulite microstructures, superimposed or replacing the magmatic microstructures. Despite recrystallisation, magmatic remnants may still be found, notably in the form of melt-related microstructures such as melt inclusions. For both magmatic charnockites and dehydrated granites, subsequent fluid-mineral interaction at intergrain boundaries during retrogradation are documented by microstructures including K-feldspar microveins and myrmekites. They indicate that a large quantity of low-H2O activity salt-rich brines, were present (together with CO2 under immiscible conditions) in the lower crust.

Microstructures and rheology of hydrous synthetic magmatic suspensions deformed in torsion at high pressure

The relationship between magma rheology and characteristic magmatic microstructures was investigated by performing high‐temperature high‐pressure deformation experiments on hydrous synthetic magmatic suspensions in the range of 0% to 76% solid fraction (alumina grains). Torsion experiments were conducted at 300 MPa confining pressure, temperatures ranging from 475°C to 1000°C and shear strain rates ranging from 2.0 × 10−5 to 2.1 × 10−3 s−1 up to total strains of 21.3. Flow is Newtonian for a solid fraction of φs = 0.0–0.16 with a log dynamic viscosity η = 10.3 Pa s (T = 500°C). A deviation from Newtonian behavior is observed for φs > 0.16 with an increase in apparent viscosity of about 1 order of magnitude between φs = 0.16 and 0.54. The shape fabric of the solid phase is characterized by a unimodal orientation that is almost stable and nearly parallel to the shear direction. Both shape fabric and deviation from Newtonian behavior originate from the increase in the number of particle clusters in the suspension. The apparent viscosity increases by 1.5 orders of magnitude between φs = 0.54 and 0.65, and extrapolation of the data suggests a very sharp increase in apparent viscosity for φs ≥ 0.65. At T ≥ 550°C and φs = 0.54 the solid phase forms an almost entirely connected network composed of two alternating orientation domains. At T ≤ 550°C and φs = 0.65, intragranular fracturing and tensile fractures result from high local stresses at contact points between neighboring particles. The resulting bulk extensional fabric is almost parallel to the shortening direction.

Structure and emplacement of granite plutons in the Paleoproterozoic crust of Eastern Burkina Faso: rheological implications

The Fada N′Gourma area in Burkina Faso is underlain by Paleoproterozoic rocks that make the northeastern West-African Craton. This region is composed of NE-trending volcano-sedimentary belts and foliated tonalites, affected by several shear zones. A generation of younger, ∼2100 Ma-old, non-foliated biotite-bearing granites intrudes the former rock units. We have investigated the younger granite pluton of Kouare that was previously considered as forming a single body with the pluton of Satenga to the west, a pluton which likely belongs to the ∼20 Ma more recent Tenkodogo-Yamba batholith. Magnetic fabric measurements have been combined with microstructural observations and the analysis of field and aeromagnetic data. The granite encloses angular enclaves of the host tonalites. Magmatic microstructures are preserved inside the pluton and solid-state, high-temperature deformation features are ubiquitous at its periphery. The presence of steeply plunging lineations in the pluton of Kouare and its adjacent host-rocks suggests that large volumes of granitic magmas became crystallized while they were ascending through the crust that was softened and steepened close to the contact. Around Kouare, the foliation in the host tonalites conforms with a map-scale, Z-shaped fold in between NNE-trending shear zones, implying a bulk clockwise rotation of the material contained in-between the shear zones, including the emplacing pluton. Regionally, the Fada N′Gourma area is concluded to result from NW-shortening associated with transcurrent shearing and vertical transfer of granitic magmas. This study concludes that the ∼2200 Myears old juvenile crust of Burkina Faso was brittle before the intrusion of the biotite-granites, became softened close to them and that gravity-driven and regional scale wrench tectonics were active together.

The Quérigut Complex (Pyrenees, France) revisited by the AMS technique : a syntectonic pluton of the Variscan dextral transpression

Abstract The Variscan Querigut Pluton (eastern Axial Zone, Pyrenees), recently dated at 307 ± 2 Ma, is a classical example for the structural study of granitoids. We present a new structural analysis of this pluton using the powerful technique of magnetic susceptibility anisotropy (AMS). A model of pluton emplacement is proposed on the basis of complementary microstructural analyses allowing the determination of the temperatures of fabric acquisition in the magmatic units, and of the shear sense criteria in the surrounding country rocks. This pluton is constituted by two main units that have intruded metasedimentary rocks where regional metamorphic conditions decrease from southwest to northeast. A well-foliated southern granodioritic unit, rich in Devonian marble xenoliths, is bounded to the south by Cambro-Ordovician metapelites. A weakly foliated northern monzogranitic unit, bounded to the north by Devonian marbles, comprises two sub-types : an outer biotite-monzogranite and an inner biotite-muscovite leucomonzogranite. Abundant basic stocks of variable sizes and lithologies outcrop in the granodioritic unit and in the southern part of the monzogranitic unit. Mean magnetic susceptibility and magnetic foliation maps show a very good agreement with the previous compiled petrographic and structural maps, strengthening the validity of the AMS technique. The northern monzogranitic units display two unevenly distributed structural patterns : (a) a NE-SW-trending pattern of weakly to steeply dipping foliations, dominant in the outer biotite monzogranite, is associated to subhorizontal NE-SW lineations ; and (b) a NW-SE-trending pattern of steeply dipping foliations, dominant in the inner biotite-muscovite monzogranite, is concentrated in NW-SE elongated corridors, associated to subhorizontal NW-SE lineations. In the southern granodioritic unit, foliation patterns follow roughly both the main regional foliation pattern and the pluton boundary, with foliation dips increasing to the south. Subhorizontal NW-SE trending magnetic lineations in the inner parts of this unit, are progressively verticalized toward the southern pluton boundary. A progressive increase in total magnetic anisotropy is observed toward the border of the pluton, correlated with both an increase in solid-state deformation and a decrease of the final temperature of fabric acquisition. These features result from a pluri-kilometric shear zone localized in the western half of the granodioritic unit, decreasing in thickness in its eastern half and along N060oE trending contacts with the country rocks. In the northern monzogranitic unit, one can roughly correlate the magmatic microstructures to the NE-SW trending fabric, and the superimposed subsolidus microstructures to the NW-SE-trending corridors, where rather low-temperature (< 300 oC) fluid-assisted cataclastic microstructures may also appear. The country-rocks, half kilometer away from the pluton border, display the D2 regional Variscan pattern, with subvertical and N110oE-striking foliations and subhorizontal and E-W-trending stretching lineations associated to a dextral shear. Closer to the pluton, the country-rocks are subjected to the pluton influence, particularly along the southern border where a strong flattening is associated to subvertical lineations related to local thrusting of the pluton onto its country rocks. An emplacement model is proposed through the injection of three principal magma batches (granodiorite, biotite-monzogranite and biotite-muscovite monzogranite) that successively and progressively built up the pluton while the whole region was subjected to a dextral and compressive deformation regime, in agreement with AMS results obtained from several other plutons of the Pyrenees.

Application of magnetic fabrics to the emplacement and tectonic history of Devonian granitoids in central Argentina

Abstract Regional considerations on the tectonic regime during the emplacement of the Early Devonian magmatic units in the Sierra de San Luis are inferred from combined field, petrographic and AMS observations. Devonian granitoids of the Sierra de San Luis, in central Argentina, constitute elliptical composite batholiths and make up the most voluminous magmatism that appears in the Sierra. Detailed fabric studies have been carried out on the La Totora batholith (33°09′S, 65°42′W), which complement previous studies on two of the largest plutons in the Sierra de San Luis: The Renca and Las Chacras-Potrerillos batholiths. The studies comprised systematic field surveys, petrographic observations and anisotropy of magnetic susceptibility (AMS) measurements. Microstructural studies indicate that the batholith rocks are mainly characterized by magmatic microstructures with limited sub-magmatic to high-temperature solid-state deformation. All three batholiths possess concentric foliation patterns. The average magnetic foliation patterns in the studied plutons agree well with the macroscopic fabrics measured in the field indicating that the AMS-data can be used to study the orientation of fabric elements. Bulk susceptibility indicates a predominance of ferromagnetic contributions, although some paramagnetic sub-units are also present. Most foliations and lineations reflect magmatic flow and their attitude is linked to the interference between regional deformation and batholith inflation, i.e. fabrics may be due to regional strain in combination with the internal dynamics of the magma bodies. Rock fabrics are mainly described by oblate magmetic fabric ellipsoids. Magnetic lineations generally show a NNE-SSW trend that is interpreted to be controlled by the opening transtensional pull-apart structures during batholith inflation. It turns out that the Devonian batholiths intruded the basement syn-kinematically with respect to the Achalian deformational cycle.

Emplacement in an extensional setting of the Mont Lozère–Borne granitic complex (SE France) inferred from comprehensive AMS, structural and gravity studies

The emplacement mode and setting of the Late Hercynian Mont Lozère–Borne granitic complex (French Massif Central), which consists of several plutons, is investigated. Structural and anisotropy of magnetic susceptibility (AMS) studies have been carried out to characterize the internal fabrics of the granitic plutons. Throughout the Pont-de-Montvert–Borne pluton, an E–W-trending magnetic lineation is well developed. In the host rock and the thermal aureole, a conspicuous E–W-trending lineation is interpreted as an evidence of a late-orogenic extensional event. To the east of the pluton, the AMS fabric is characterized by values of anisotropy degree ( P′) around 4–5% with a prolate ellipsoid and subsolidus structures, whereas, to the west, the P′ parameter is weaker with an oblate ellipsoid and purely magmatic microstructures. A gravity investigation allows determination of the 3D shape of the pluton. The western part of the granitic complex is thicker than the eastern one and is interpreted as the feeder zone. This suggests an eastward spreading of the magma. The consistency between regional stretching and directions of AMS lineations in the pluton and the shape of the complex deduced from gravity strongly argues that the emplacement mode of the complex was influenced by the regional extensional tectonic setting during the collapse of the Hercynian belt.

Variscan dextral transpression in the French Pyrenees: new data from the Pic des Trois-Seigneurs granodiorite and its country rocks

New structural data from both the Trois-Seigneurs granodiorite and its country rock lead to new conclusions about the Variscan tectonic history of this part of the Pyrenees. Sedimentary rocks of the massif, that increase in metamorphic grade from north to south, are imprinted by a single major deformation responsible for both the tight, upright, E-W-trending folds with vertical to steeply N-dipping foliations and the gently W-plunging stretching lineations. Syntectonic emplacement of the Pic des Trois-Seigneurs granodiorite is demonstrated by the parallelism between structures of the pluton, as determined by magnetic fabric measurements, and the foliations and lineations imprinted in the country rocks. Large deformation of the granodiorite magma during emplacement is indicated by strong anisotropy and ubiquitous magmatic microstructures. The high thermal gradients of regional metamorphism, which is also syntectonic, are attributed to a large thermal aureole provided by deep-seated magma reservoirs. Shear-sense criteria in XZ thin sections of the metasedimentary country rocks indicate a dextral sense of the non-coaxial component of strain throughout the massif and regardless of metamorphic grade. The overall structure of the massif is concluded to have derived from a major shearing event that is equated with the D 2 main phase observed in the entire Variscan Pyrenees. As the E-W-oriented stretch and dextral shear cannot be separated from compressive structures generally considered to be characteristic of D 2, this phase is concluded as representing regional transpression.

Internal fabric and strike‐slip emplacement of the Pan‐African granite of Solli Hills, northern Nigeria

The Solli Hills pluton (250 km2) in northern Nigeria is an example of a Pan‐African granite emplaced circa 580 Ma ago in the Jos ‐ Adamawa block. Its micro structures and internal fabrics have been studied in order to help constrain the kinematics of the Pan‐African orogeny in this block. We used field observations and measurements, petrographic examinations, and measurements of the magnetic susceptibility and anisotropy of magnetic susceptibility of 124 regularly spaced stations. Most samples are ferromagnetic, i.e., belong to the magnetite‐series granites, and present predominantly magmatic microstructures. Particularly remarkable are the asymmetrical zoning of the petrographic types, the susceptibility magnitudes, and the planar ‐ linear magnetic fabrics. The magnetic foliations and lineations are very consistent in the pluton, around N 20°E, 60°W and N 205°E, 10°, respectively. Field and microscopic observations reveal that deformation occurred everywhere in the pluton and immediate country‐rock envelope during a dextral strike‐slip synplutonic episode. It is suggested that the Solli Hills pluton was emplaced in a late Pan‐African dextral tear‐fault system. The striking fabric asymmetry of the pluton strongly suggests a shear zone termination emplacement model.