Anne-Marie Boullier

Dynamic control on serpentine crystallization in veins: Constraints on hydration processes in oceanic peridotites

Deformation and hydration processes are intimately linked in the oceanic lithosphere, but the feedbacks between them are still poorly understood, especially in ultramafic rocks where serpentinization results in a decrease of rock density that implies a volume increase and/or mass transfer. Serpentinization is accompanied by abundant veining marked by different generations of vein-filling serpentines with a high variety of morphologies and textures that correspond to different mechanisms and conditions of formation. We use these veins to constrain the role of deformation and mass transfer processes during hydration of oceanic peridotites at slow-spreading ridges. We have selected a representative set of veins from ocean floor serpentinites of the Mid-Atlantic Ridge near Kane transform fault (23°N) and characterized these in detail for their microstructures and chemistry by coupling optical and electron microscopy (SEM, TEM) with electron microprobe analyses. Four main veining episodes (V1 to V4) accompany the serpentinization. The first episode, identified as vein generation V1, is interpreted as the tectonically controlled penetration of early seawater-dominated fluid within peridotites, enhancing thermal cracking and mesh texture initiation at 3–4 km up to 8 km depth and at T <300–350°C. The two following vein stages (V2 and V3) formed in a closed, diffusive system and accommodate volume expansion required to reach almost 50% serpentinization of the protolith. The cracks exploited by these veins were caused by the progressive unroofing at depths of ∼4 to ∼2 km along a detachment fault. Degree and rate of serpentinization seem to be controlled by the capacity of the system to create space and to drive the mass transfer needed for ongoing serpentinization, and this capacity is in turn linked to the exhumation rate and local tectonics. During this period, water consumed by hydration may prevent the establishment of convective hydrothermal cells. The onset of an open hydrothermal system in the shallow lithosphere (<2 km), where brittle fracturing and advective transfer dominate and enable the completion of serpentinization, is marked by the last vein generation (V4). These results show a complete history of alteration, with the crystallization of different types of serpentine recording different tectonic events, chemical conditions, and modes of hydrothermal alteration of the lithosphere.

Rare earth and trace element mobility in mid-crustal shear zones: insights from the Mont Blanc Massif (Western Alps)

The behaviour of rare earth elements (REE) during fluid–rock interaction in mid-crustal shear zones has received little attention, despite their potential for mass balance calculation and isotopic tracing during deformation. In this study, several cases of large REE mobility during Alpine fluid-driven shear zone development in the pre-Alpine granitic basement of the Mont Blanc Massif are considered. On a regional scale, the undeformed granite compositions range within 5 wt% SiO2 (70.5–75.3 wt%) and magmatic chemical variations are of the order of 10–20%, ascribed to minor effects of crystal fractionation. Major and trace element mobility observed in shear zones largely exceeds these initial variations. Shear zones developed a range of mineral assemblages as a result of shearing at mid-crustal depths (at not, vert, similar0.5 GPa, 400°C). Five main shear zone assemblages involve muscovite, chlorite, epidote, actinolite and calcite, respectively, as major phases. In most cases, selective enrichments of light or heavy REE (and Y, Ta, Hf) are observed. REE mobility is unrelated to deformation style (cataclastic, mylonitic), the intensity of strain, and to the shear zones major metamorphic mineral assemblages. Instead, the changes in REE concentrations are ascribed to the alteration of pre-existing magmatic REE-bearing minerals during deformation-related fluid–rock interaction and to the syntectonic precipitation of metamorphic REE-bearing minerals (mainly monazite, bastnasite, aeschynite and tombarthite). Minor proportions (<2%) of these accessory phases, with grain sizes mostly <20 μm, account for enrichments of up to 5:1 compared to the initial granite whole-rock REE budget. The stability of the REE phases appears to be largely dependent on the altering fluid composition. REE mobility is ascribed to changes in pH and to the availability of CO32−, PO42−, and SO42−ligands in the fluid. Such processes are likely to influence the mobility of REE, Y, Hf and Ta in shear zones.

Aseismic sliding of active faults by pressure solution creep: Evidence from the San Andreas Fault Observatory at Depth

Active faults in the upper crust can either slide steadily by aseismic creep, or abruptly causing earthquakes. Creep relaxes the stress and prevents large earthquakes from occurring. Identifying the mechanisms controlling creep, and their evolution with time and depth, represents a major challenge for predicting the behavior of active faults. Based on microstructural studies of rock samples collected from the San Andreas Fault Observatory at Depth (California), we propose that pressure solution creep, a pervasive deformation mechanism, can account for aseismic creep. Experimental data on minerals such as quartz and calcite are used to demonstrate that such creep mechanism can accommodate the documented 20 mm/yr aseismic displacement rate of the San Andreas fault creeping zone. We show how the interaction between fracturing and sealing controls the pressure solution rate, and discuss how such a stress-driven mass transfer process is localized along some segments of the fault.

Fluid inclusions in pseudotachylytes from the Nojima fault, Japan

Pseudotachylytes (i.e., rocks formed by frictional melting) have been observed in the Nojima fault that was penetrated by the Hirabayashi borehole drilled 1 year after the 1995 Hyogo-ken Nanbu (Kobe) earthquake. These rocks display millimeter-scale banding defined by different pseudotachylyte layers. The nature of unmolten crystal fragments (K-feldspar, albite, calcite, and/or quartz) allows us to infer a minimum melting temperature of 1200°C. The glass has a 8±3% volatile content and a higher CaO content than that of the parent granodiorite, thus suggesting that pseudotachylyte formation occurred in an already altered, calcite-bearing and hydrated fault zone. Fluid inclusions have been observed in the glass and are filled with a dense low-salinity (4.3±1.2 wt % eq. NaCl) CO2-H2O fluid characterized by steep isochoric curves. The intersection between the isochores and the measured 24°C/km geothermal gradient indicates a minimum 15 km depth for the pseudotachylyte formation. Such a depth suggests that an important uplift has brought the studied pseudotachylytes to their present position and that these pseudotachylytes are probably pre-Miocene in age. The calculated thermal evolution of a typical millimeter-scale pseudotachylyte layer indicates that cooling does not last more than a few seconds. This indicates that related seismic processes, such as deceleration of fault movement and healing of the fault, were both very rapid. The geometry and thermal budget of the millimeter-thick pseudotachylytes suggest that seismic events of magnitude 6 to 7 are responsible for their formation.

Linked fluid and tectonic evolution in the High Himalaya mountains (Nepal)

Fluid inclusions were studied in a quartz lens from the structurally highest unit of the Himalaya mountains in Nepal from a textural, geometrical, chemical and isotopic point of view. Six types of fluid inclusions were distinguished. One of these types consists of annular inclusions; this shape is attributed to a confining pressure increase in a non-isotropic stress field. Two successive stress fields were deduced from the orientation of the inclusion planes relative to the schistosity. The bulk composition of the fluid was dominated by CO2 (>84 mol%) and H2O. The composition remained constant during the whole history of the sample indicating that it was buffered by the carbonaceous host rock and/or that one single fluid was reworked in situ by decrepitation. Stable isotope of fluids and minerals indicate (1) that fluids were buffered by surrounding rocks for O and C and (2) that at least two types of water (metamorphic and meteoric) were involved. Finally, a P-T-t-ε-σ path is proposed for the sample, taking into account the southward thrusting along the Main Central Thrust, the northward tectonic denudation of the Himalaya mountains inducing tectonic burying below the Annapurna Range, and lastly, rapid uplift.

A tectonic model for the location of Palaeozoic ring complexes in Aïr (Niger, West Africa)

The fir region in Niger is one of the largest peralkaline granite provinces in the world. In addition to the granites, some plutons are characterized by an abundance of plagioclase-rich cumulates (gabbros, troctolites and anorthosites), with subordinate metaluminous granites and quartz syenites. These anorogenic ring complexes were intruded along a 400 km zone near the 9”E meridian. The province formed during a major magmatic event, newly dated at around 407 * 8 Ma, and comprises 28 plutons, which range from 0.8 to 65 km in diameter and show no correlation between distribution and age. Remote sensing over the whole fir massif demonstrates the existence of two main trends of faults or lineaments: N50”E-N90”E and N120”E-N150”E. Autocorrelation analysis of the ring complexes reveals three large, parallel, high-density strips orientated N20”E. Based on the structural and geological setting, coupled with map analysis, a new tectonic model for the location of the complexes is presented. We propose a dextral N5”E shear zone model, where the emplacement of the ring structures is controlled by N20”E dextral Riedel shear (R) with a secondary N50”E tensional gash (T). This tectonic model implies a control by lithospheric structures for the emplacement of the ring complexes, as well as a relationship between a transtensional tectonic regime and intra-plate alkaline magmatism. The new geochronological data on the AYir massif allow us to derive a Silurian-Devonian palaeomagnetic pole (representative for Gondwana) which fits data for the other continents better than previous estimates.

2.8–3.0 Ga plutonism and deformation in the SE Amazonian craton: the Archaean granitoids of Marajoara (Carajás Mineral Province, Brazil)

Abstract The granitoids of Marajoara in the Rio Maria terrain (Carajas Mineral Province, Brazil) consist of: (i) a broad unit of 2.96 Ga syntectonic tonalites (Arco Verde Tonalites) displaying a trondhjemitic differentiation trend; (ii) 2.93 Ga syntectonic monzogranites (Guaranta); and (iii) 2.87 Ga post-tectonic monzogranites (Mata Surrao) and granodiorites (Rio Maria), displaying a calc-alkaline differentiation trend. Deformation of the Arco Verde tonalites is heterogeneous with low strain domains (well preserved magmatic banding and textures) and orthogneissic domains displaying an E–W trending mainly subvertical foliation, associated with horizontal lineations, upright folds and subvertical shear zones. Microstructures and phase assemblages suggest that deformation occurred within a large temperature range (i.e. during magma emplacement and cooling), from high-T conditions (synmagmatic shear zones and subsolidus ductile deformation with intense quartz and feldspar recrystallization; Pl+Qtz+Hbl+Bt assemblages) to medium- and low-T conditions (ductile to brittle deformation with weakly recrystallized quartz and undulose extinction in feldspars; Qtz+Pl+Bt+Mu or Chl+Ep+Ab+Qtz assemblages). These data, finite strain analysis and structures reported from the surrounding greenstone belts suggest that deformation did not result from a post-emplacement prograde tectono-metamorphic event as considered previously, but that the Marajoara granitoids are synkinernatic intrusions which were deformed together with the supracrustal rocks during a regional NS horizontal shortening. Although the Rio Maria terrain presents similarities with Archaean domains controlled by diapiric processes (lithologies dominated by thick greenstone sequences and TTG plutons, and forming a dome-and-keel structure), its structural evolution is controlled dominantly by a transpressional event which shaped the granite–greenstone terrains. The Rio Maria area, probably as many Archaean ‘grey gneisses’ domains, represents an intermediate case between terrains controlled by Raleigh–Taylor instabilities in a thermally softened crust with insignificant external forces related to plate convergence (e.g. east Pilbara craton) and those controlled by thrust tectonics related to convergence of rigid plates (e.g. Superior Province). The closest analog to the Rio Maria terrain seems to be the Chilimanzi area in the Zimbabwe craton.

Fluid inclusions: tectonic indicators

Abstract During the first half of the 20th century fluid inclusions have been studied in ore geology in order to determine the chemistry and physical parameters (density) of the mineralising fluids. Apart from a few pioneering works in the middle of the century, fluid inclusions were not used as structural markers before the late 1980s. During the same time, experiments as well as observations of natural samples have shown that the volume, shape and density of fluid inclusions may change when submitted to pressure–temperature conditions different from their trapping isochores. In view of these experiments, and based on natural examples from the Higher Himalaya metamorphic rocks, it is evident how fluid inclusions can be used as thermobarometric, structural and tectonic markers. Points of interest for further studies on fluid inclusions in nature and experiments are also stressed.

Role of magma pressure, tectonic stress and crystallization progress in the emplacement of syntectonic granites. The A-type Estrela Granite Complex (Carajas Mineral Province, Brazil)

The Archaean, syntectonic, A-type Estrela Granite Complex (Carajas Mineral Province, Brazil) consists of three plutons emplaced in a greenstone sequence under low-pressure conditions (180<P<310 MPa). It is composed mainly of annite-, ferropargasite (±hedenbergite)- and ilmenite-bearing monzogranites. The contact aureole is affected by a subvertical penetrative schistosity conformable with the limits of the plutons. Meso- to microstructures and mineral reactions in the granites indicate that deformation occurred in a continuum from above-solidus to low-T subsolidus conditions. Two distinct planar structures are observed: (i) a concentrical primary foliation (S0) corresponding to rhythmic, isomodal, phase layering associated with a faint grain shape fabric; it is horizontal in the centre and vertical towards the edges of the plutons; and (ii) a steep to subvertical foliation (S1) associated with the deformation of S0 and accompanied with emplacement of synplutonic dykes and veins of leucocratic granites and pegmatites. Emplacement, differentiation and consolidation of the Estrela Granite Complex are considered to result from a continuous evolution under decreasing temperatures in a single-stage strained crust (transpression), with two main periods. (1) The first period is controlled by body forces, and it corresponds to inflation with magma ponding. As long as the rheology is melt dominated, magma pressure is the critical parameter and almost no strain is recorded. With decreasing T, magmas crystallize and differentiate leading to a concentrical magmatic phase layering. The growing magmatic bodies are mechanically decoupled from the country rocks and their evolution depends on internal magma chamber processes. (2) For higher amount of crystallization (residual melt fraction F<0.5), the role of magma pressure becomes insignificant. Establishment of a continuous crystal framework leads to the coupling of plutons with their surroundings, and deformation in response to tectonic stress. Most of the strain is recorded during this period which starts from the rigid percolation threshold, and extends to subsolidus low-grade conditions. This leads to deformation of the partially crystallized volume and redistribution of fluid-enriched differentiated melts. The amount of crystallization through the rheological thresholds appears as the critical parameter determining the transition from magma-controlled processes (inflation and differentiation of the magma chamber, with development of a phase layering) to tectonic-controlled processes (deformation of the phase layering and redistribution of residual melts). This accounts for the fact that syntectonic plutons commonly display intermingled, boudinaged layers with distinct modal compositions and in some cases well-preserved rhythmic layering.

40 AR/ 39 AR dating of synkinematic white mica; insights from fluid-rock reaction in low-grade shear zones (Mont Blanc Massif) and constraints on timing of deformation in the NW external Alps

Abstract This paper highlights the use of synkinematic white mica, biotite and phlogopite for the dating of deformation in ductile shear zones within crystalline rocks under low-grade metamorphic conditions. The Mont Blanc shear zones range from 1 mm to 50 m in width and have localized intense fluid flow, resulting in substantial differences in mineralogy and whole-rock geochemistry. On the basis of their synkinematic alteration assemblages and geographic distribution within the Mont Blanc Massif, three main metamorphic zones are distinguished within the network of shear zones. These are: (i) epidote±white mica-bearing assemblages; (ii) chlorite–phlogopite-bearing assemblages; and (iii) white mica±biotite±calcite±actinolite±epidote- bearing assemblages. 40Ar/39Ar age spectra of biotite and phlogopite are complex, and reflect significant variations in chemical composition. In biotite, this is partly due to inheritance from precursor Variscan magmatic biotite. In contrast, new white mica grew at the expense of feldspar during Alpine deformation and its Ar spectra do not show any excess 40Ar. On the SE side of Mont Blanc, ages of shear zone phengites have a narrow range of 15.8–16.0±0.2 Ma, which is in the same age range as 40Ar/39Ar ages of minerals from kinematically related veins. The top-to-SE sense of shear is consistent with initiation of a Mont Blanc flower-structure within a dextral transpressional system by 16 Ma. On the NW side, mini-plateaux ages of 14.5±0.3 and 23.4±0.4 Ma are preserved in the same sample, suggesting the possibility of two phases of deformation. This is also supported by partly preserved ages of 18–36.6 Ma in biotites and phlogopites. Ages between 36 and 18 Ma might reflect ongoing top-to-NW thrusting, following Penninic Front activation, in a context of nappe stacking and crustal thickening. NW-directed thrusting on the NW side of Mont Blanc continued after 18 Ma, synchronous with SE-directed thrusting on the SE side of the massif. These divergent movements produced the overall pop-up geometry of the Mont Blanc Massif, which may correspond to a positive flower structure developed within a zone of regional dextral transpression extending SW from the Rhone valley into the Mont Blanc area.