Mylonitic Event Research Articles

Hydration of mafic crustal rocks at high temperature during brittle‐viscous deformation along a strike‐slip plate boundary

AbstractThe Poroshiri ophiolite (Hidaka metamorphic belt, Japan) occurs within a crustal‐scale network of high‐temperature, dextral shear zones that accommodated hundreds of kilometres of displacement due to the opening of the Japan Sea in the Neogene. The opholitic rocks comprise ultramafic, mafic and sedimentary protoliths that have been variably hydrated and metamorphosed. This work investigates the mechanisms and timing of fluid influx relative to viscous deformation in metagabbros and amphibolites deformed during exhumation from granulite‐ to amphibolite‐facies conditions. We consider a range of microstructures, from low strain domains and 1–2 mm thick shear bands to mylonites with a thickness of a few meters. Low strain domains of metagabbros exhibit corona textures with symplectites consisting of pargasitic amphibole (Ed0.7) ‐ anorthitic plagioclase (An80–92) ± orthopyroxene ± clinopyroxene forming around olivine and igneous pyroxene and with similar plagioclase–amphibole‐orthopyroxene ± clinopyroxene granoblastic aggregates in micrometric‐thick fractures. These textures result from hydration under low fluid‐rock ratio, with elevated H2O content only occurring locally (H2O > 1–1.2 wt%). Igneous mineral replacement leads to grain size reduction from 1 mm to ~10 μm. Amphibole exhibits a strong core‐rim zonation primarily controlled by the high diffusivity of Fe, Mg and OH and the low diffusivity of Al. Mineral compositional equilibrium is achieved at the scale of 100–200 μm. In mm‐thick localized shear bands and in metric‐scale mylonitic amphibolites, the heterogeneous mineral composition of amphibole (Ed0.2–0.5) and plagioclase (An40−An80) indicates only partial re‐equilibration (at the scale of 200–500 μm) despite higher fluid‐rock ratios and more pervasive fluid percolation than in metagabbros. Plagioclase–amphibole thermometry and equilibrium phase diagrams indicate that the initial fluid infiltration and corona formation occurred at 800–850°C by fracturing and percolation along grain boundaries. This was followed by the main fluid percolation and mylonitization event, which occurred during exhumation and cooling at conditions of 720–580°C, ~4 kbar. Continuous hydration during retrogression was achieved by the influx of dominantly seawater‐derived fluid, as attested by the high chlorine (Cl) contents (>300–400 ppm) of amphiboles in fractures. The heterogeneous distribution of fractures controls the distribution of fluid from the earliest stages of hydration, creating positive feedback where the growth of hydrous minerals as amphibole (up to +300 vol% of amphibole in high strain areas relative to the low‐strain ones) and the formation of fine‐grained mixed domains that led to further localization of viscous strain and mass transfer (variations of ± 30–40% in major elements). The Poroshiri ophiolite therefore represents a good fossil example of a former transpressional plate boundary where coupled metamorphic and deformation processes were triggered by seawater‐derived fluid that percolated to depths of ~15 km.

Assessing the importance of H2O content in the tectono‐metamorphic evolution of shear zones: A case study from the Dora‐Maira Massif (Western Alps)

AbstractMetamorphic reactions are commonly driven to completion within shear zones thanks to fluid circulation, making the re‐equilibration of the mineral assemblage one of the dominant processes. Despite the important role of H2O in such processes, forward thermodynamic modelling calculations commonly assume either H2O‐saturated conditions or only fluid loss during prograde evolution to peak conditions. These assumptions influence the understanding of shear zones during the retrograde evolution. Here, we investigate the P–T–MH2O retrograde evolution of the Mt. Bracco Shear Zone (MBSZ), an Alpine ductile tectonic contact which marks the boundary between two HP units in the Dora‐Maira Massif (Western Alps, Italy). After the eclogite‐facies peak (at 500–520°C and 1.8–2.2 GPa), the subsequent mylonitic event is constrained at amphibolite‐facies conditions, continuing its evolution at decreasing pressure and temperature during rock exhumation, from ~590°C, 1.0 GPa down to ~520°C, 0.7 GPa. The P/T–MH2O forward modelling highlights different behaviour for the two analysed samples. After reaching a minimum H2O content at the transition from eclogite‐ to amphibolite‐facies conditions, a significant fluid gain is modelled for only one of the two analysed samples just before the mylonitic event. The MBSZ then evolves towards H2O‐undersaturated conditions. This work thus underlines the necessity of investigating the H2O evolution within shear zones, as the H2O content is susceptible to change through the P–T path, due to dehydration reactions or fluid infiltration events. Furthermore, lithological heterogeneities influence possible different fluid circulation regimes in shear zones, resulting in externally or internally derived fluid gain.

Paleocene to Miocene migmatization and kinematics of the deformation at the northern boundary of the Xolapa Complex: implications for the Chortís Block-southern Mexico connection

ABSTRACT The Xolapa Complex of southern Mexico preserves evidence of its connection with the Chortís Block of Central America. Orthogneisses yielded U-Pb protolith ages from 172 to 169 Ma and paramigmatites are dated at 217 Ma. El Salitre and the Amatlán granitoids show crystallization ages of 65 and 59–57 Ma, respectively. In central Xolapa, diatexites have ages from 38 to 31 Ma, and granites were dated from 30 to 29 Ma. In the southeastern Xolapa, crystallization ages of 45 Ma and 23 to 21 Ma were obtained from migmatites and anatectic granites, respectively. Previous data indicate three diachronous stages of crustal melting along the Xolapa block. Structural data reveal that the Xolapa Complex is tectonically juxtaposed against the Acatlán Complex through ductile zones. Two mylonitic foliations are predominant in the northern boundary of the Xolapa Complex: (i) a sub-vertical NW-SE foliation with subhorizontal or low-plunging stretching lineation and left-lateral kinematics; and (ii) a subhorizontal NE-SW foliation with normal plunging stretching lineation and top-to-NE kinematics. The first mylonitic event related to the sinistral eastward motion of the Chortís Block occurred between 58 and 54–50 Ma. The second event took place in central Xolapa after or synchronous with the 38–31 Ma emplacement of diatexites. This event is related to an extensional phase that uplifts the Xolapa Complex with concerning the Acatlán Complex. In the southeastern region, deformation occurred after or synchronous with the emplacement of anatectic granites dated at 23–22 Ma. Our geochronological and structural data delineated an eastward decreasing age trend of migmatization, whereas the mylonitic deformation reported through the Xolapa Complex is consistent with a diachronous migration and detachment of the Chortís Block relative to the Acatlán Complex from Paleocene to early Miocene times.

Hercynian subduction‐related processes within the metamorphic continental crust in Calabria (southern Italy)

AbstractLinking the deformation history of mylonitized continental rocks to the progress of devolatilization reactions that trigger reaction softening is critical for the understanding of crustal scale processes. We have analysed the field geometries and microstructures of deformed rocks within the southern Hercynian belt in Calabria, as well as modelled the pressure–temperature–deformation (P–T–d) trajectory of a main ductile shear zone that tectonically coupled the deeper crustal Mammola Paragneiss Unit with the upper crustal Stilo–Pazzano Phyllite Unit. P–T modelling of the mylonitic Mammola Paragneiss Unit was performed through calculation of phase equilibrium diagrams with the software thermocalc in the MnNCKFMASHTO model system. The prograde P–T–d trajectory is based on the zoning profiles of garnet porphyroblasts and their mineral inclusions, primarily barroisite and epidote. P–T modelling shows that peak metamorphic conditions of ~0.9 GPa and 585°C were reached during a Dn‐1 under‐thrusting event. The following exhumation during the Dn mylonitic event, and contact metamorphism during Dn+1 and Dn+2 folding events, have also been modelled because they are essential to restore the previous tectono‐metamorphic history. The exhumation trajectory was modelled down to 0.3 GPa with temperatures of 440–460°C, under fluid‐deficient conditions, as well as the final late Carboniferous contact metamorphism up to Tmax of 680–720°C. The prograde path shows clear evidence for thermal buffering during garnet growth at the expense of chlorite, with a heating‐dominated stage after chlorite breakdown. Subsequently, a rheological change associated with epidote breakdown (i.e. reaction softening) occurred, highlighted by a net steepening of the P/T trajectory towards the pressure peak. On the basis of the barroisite inclusions within garnet porphyroblasts as well as the ‘hairpin’ shape of the reconstructed P–T–d path (before contact metamorphism), we infer that the unusual low T/P gradient for the Hercynian crust exposed in the Mammola Paragneiss Unit records its involvement in the Palaeotethys–Gondwana subduction beneath Laurussia during Dn‐1 under‐thrusting. We present a new palaeotectonic interpretation along the southern Hercynian belt in Calabria during the Upper Mississippian–Lower Pennsylvanian, that is consistent with previous geochronology studies.

The genesis of gold deposits in the Hetai goldfield, South China: New constraints from geochronology, fluid inclusion, and multiple isotopic studies

The Hetai goldfield is one of the largest gold mining districts in the South China Block. A complex paragenesis consists of three mineralization stages: syntectonic metamorphic stage, hydrothermal stage, and supergene stage. The syntectonic metamorphic stage is characterized by banded quartz (Q1) and invisible Au in mylonites. The hydrothermal stage, which is the main gold mineralization stage, can be divided into three substages: an early substage characterized by coarse‐grained quartz (Q2) + pyrite + arsenopyrite + pyrrhotite, an intermediate substage characterized by gray fine‐grained quartz (Q3) + electrum + poly‐sulphides + sericite + chlorite and a late substage with predominate white quartz (Q4) + calcite as well as lesser sulphides. During the supergene stage, malachite, covellite, and limonite were formed. Four types of fluid inclusions (FIs) in these types of quartz were identified: aqueous FIs (type 1), CO2‐bearing aqueous FIs (type 2), CO2‐rich FIs (type 3), and solid‐bearing FIs (type 4). Q1 only contains type 1 FIs; Q2 contains types 2 and 4 FIs; Q3 contains types 2 and 3 FIs; and Q4 only contains type 1 FIs. Microthermometric results show that the homogenization temperatures of fluid inclusions range from 350°C to 400°C for Q1, 250°C to 320°C for Q2, 138°C to 245°C for Q3, and 110°C to 207°C for Q4. Salinities of fluid inclusions range from 3.4 to 5.7 wt.%, 2.6 to 12.9 wt.%, 0.5 to 11.5 wt.%, and 0.2 to 7.3 wt.% for Q1, Q2, Q3, and Q4, respectively. Alteration minerals (sericite and chlorite) from the main mineralization stage have δDH2O‐VSMOW values of −62‰ to −98‰ and calculated δ18OH2O‐VSMOW values of 6.3‰ to 4.4‰. The in situ sulphur isotope analysis of pyrite yields a narrow δ34SCDTrange of −1.5‰ to −5.2‰ (average −3.4‰), combined with the Pb isotopic compositions of the sulphides, wall rocks, and Wucun pluton, revealing that the ore‐forming material was mainly derived from magmatic source. The mineralization age (sericite40Ar/39Ar: 159.3 ± 0.8 Ma) is close to the emplacement age of Wucun biotite monzonitic granite pluton (LA‐ICP‐MS zircon U–Pb age: 158.1 ± 1.9 Ma). These geochemical and geochronological data suggest that the main phase of gold mineralization in the Hetai goldfield is genetically related to the granitic activities in the Late Jurassic rather than the mylonitization event in the Late Triassic to Early Jurassic.

Strain rates of the syn-tectonic Symvolon pluton (Southern Rhodope Core Complex, Greece): an integrated approach combining quartz paleopiezometry, flow laws and PT pseudosections

We studied the mylonitic granodiorites belonging to the Symvolon pluton from the Kavala Shear Zone (Rhodope Massif, NE Greece), to determine strain rates and refining the tectonic exhumation model of this area. With this purpose, we combined a quantitative microstructural analysis with an image assisted thermodynamic modelling. In particular, three samples exhibiting increasing stages of deformation were selected to estimate PT conditions of the mylonitic event by means of a quantitative mineral-chemical study realized via statistical data handling of X-ray maps. Obtained PT constraints were then used in conjunction with quartz paleopiezometry combined with flow laws, to estimate the amount of mean strain rate achieved during the shearing evolutionary stage of these rocks. Such multifaceted approach allows us to infer strain partitioning and pluton cooling rateduring the mylonitic evolution, obtaining an average strain rate of 7.5 * 10-12 (s-1).

High‐P tectono‐metamorphic evolution of mylonites from the Variscan basement of the Northern Apennines, Italy

AbstractStrain localization within shear zones may partially erase the rock fabric and the metamorphic assemblage(s) that had developed before the mylonitic event. In poly‐deformed basements, the loss of information on pre‐kinematic phases of mylonites hinders large‐scale correlations based on tectono‐metamorphic data. In this study, devoted to a relict unit of Variscan basement reworked within the nappe stack of the Northern Apennines (Italy), we investigate the possibility to reconstruct a complete pressure (P)–temperature (T)–deformation (D) path of mylonitic micaschist and amphibolite by integrating microstructural analysis, mineral chemistry and thermodynamic modelling. The micaschist is characterized by a mylonitic fabric with fine‐grained K‐white mica and chlorite enveloping mica‐fishes, quartz, and garnet pseudomorphs. Potassic white mica shows Mg‐rich cores and Mg‐poor rims. The amphibolite contains green amphibole+plagioclase+garnet+quartz+ilmenite defining S1 with a superposed mylonitic fabric localized in decimetre‐ to centimetre‐scale shear zones. Garnet is surrounded by an amphibole+plagioclase corona. Phase diagram calculations provide P–T constraints that are linked to the reconstructed metamorphic‐deformational stages. For the first time an early high‐P stage at >11 kbar and 510°C was constrained, followed by a temperature peak at 550–590°C and 9–10 kbar and a retrograde stage (<475°C, <7 kbar), during which ductile shear zones developed. The inferred clockwise P–T–D path was most likely related to crustal thickening by continent‐continent collision during the Variscan orogeny. A comparison of this P–T–D path with those of other Variscan basement occurrences in the Northern Apennines revealed significant differences. Conversely, a correlation between the tectono‐metamorphic evolution of the Variscan basement at Cerreto pass, NE Sardinia and Ligurian Alps was established.

Systematic changes in orientation of linear mylonitic fabrics: An example of strain partitioning during transpressional deformation in north Golpaygan, Sanandaj–Sirjan zone, Iran

Kilometer-scale, shallowly dipping, NW-striking top-to-the NE reverse and dextral strike-slip shear zones occur in metamorphic rocks of north Golpaygan. These metamorphic rocks are exposed at the NE margin of the central part of the Sanandaj–Sirjan zone in the hinterland of the Zagros orogen. NW-striking top-to-the NE normal shear zones were also found in a small part of the study area. Structural evidence of three deformation stages were found. Pre-mylonitization metamorphic mineral growth happened during D1. The main mylonitization event was during the D2 deformational event, following coaxial refolding, synchronous to retrograde metamorphism of amphibolite to greenschist facies in the Late Cretaceous–Paleocene, and before D3 folding and related mylonitization. We documented the systematic changes in the orientations of D2 linear fabrics especially stretching lineations and superimposition relations of structures. It is concluded that the dextral strike-slip and dip-slip shear zones were coeval kinematic domains of partitioned dextral transpression. The shallowly dipping reverse and strike-slip shear zones are compatible with partitioning in a very inclined transpressional model. Fabric relations reflect that the top-to-the NE normal shear zones were not produced during deformation partitioning of inclined dextral transpression. The Late Cretaceous–Paleocene strain partitioning was followed by later N–S shortening and NE-extension in the north Golpaygan area.

Petrographic Evidences for the Origin of Iron in IOCG IronDeposits of Kuh-E-Faryadoon and Kouli-Kosh, Southeast Central Iran

Kuh-E-Faryadoon and Kouli Kosh iron deposits are hosted by regional metamorphic rocks of Sanandaj-Sirjan zone around Deh-Bid area. Plugs and domes of acid rocks protrude these metamorphic rocks. A mylonitization event is super imposed on acid rock and metamorphic complexes. Field, petrological and geochemical studies have shown that (1) Iron mineralization have occurred during mylonitization in N45W shear zones. (2) Mineralization have many characteric pecularities of IOCG hydrothermal deposits. (3) Mylonitization is super imposed on regional metamorphism therefore early paragenesis of regional metamorphism are destabilized and new paragenesis are formed. (4) Iron is released through destabilization of biotite, clinopyroxene, epidote, ilmenite and titanomagnetite of mafic metamorphic rock (mainly greenschists) and acid rocks during mylonitization event. Petrographic evidences and relevant reaction are presented and discussed. (5) Iron is mainly and preferentially released from greenschists. (6) Transport of iron by carbonile complexes are proposed based on direct evidence of Co2 rich solutions during mylonitization.

Polymetamorphic history of a relict Permian hornfels from the central Gran Paradiso Massif (Western Alps, Italy): a microstructural and thermodynamic modelling study

A petrological and thermobarometric study of the Lago Teleccio hornfelses was undertaken to reconstruct the polymetamorphic evolution and constrain the P–T conditions of Permian contact metamorphism. The Lago Teleccio metasedimentary rocks record a Variscan regional metamorphism characterized by amphibolite facies mineral assemblages including quartz, plagioclase, K-feldspar (Kfs 1), biotite, garnet (Grt 1) and staurolite; this was followed by a late-Variscan mylonitization event. Metamorphism of the Variscan metamorphic rocks at the contact with a Permian granitic intrusion produced static recrystallization and/or new growth of quartz, garnet (Grt 2), plagioclase, K-feldspar (Kfs 2), cordierite, green spinel, biotite and prismatic sillimanite (Contact 1). This thermal event, which occurred at a peak pressure of 0.23–0.35 GPa, temperature of 670–700 °C and aH2O of 0.75–1, was followed either during post-contact metamorphism cooling or, more likely, during the early-Alpine metamorphism by the breakdown of cordierite into an anhydrous kyanite + orthopyroxene + quartz assemblage. The poorly developed early-Alpine eclogite facies metamorphism (Alpine 1) was characterized by relatively anhydrous mineral associations and low strain, which locally produced coronitic and pseudomorphous microstructures in metasedimentary rocks, with scanty formation of jadeite, zoisite and a new high-pressure garnet (Grt 3). Greenschist facies retrogression (Alpine 2) was characterized by the local development of a chlorite- and muscovite-bearing mineral association, suggestive of aqueous fluid incursion. In the hornfelses, the limited extent of metamorphic overprinting is suggested by the fine grain size of the Alpine mineral associations, which formed at the expense of the Permian contact metamorphic associations, and was favoured by the anhydrous mineralogy of the hornfelses.

Counterclockwise tectonometamorphic evolution of the Pringles Metamorphic Complex, Sierras Pampeanas of San Luis (Argentina)

The crystalline basement of the Sierra de San Luis, which belongs to the Eastern Sierras Pampeanas in central Argentina, consists of three main units: (1) Conlara, (2) Pringles, and (3) Nogolí metamorphic complexes. In the Pringles Metamorphic Complex, mafic–ultramafic bodies occur as discontinuous lenses along a narrow central belt concordant with the general NNE–SSW structural trend. A metamorphic gradient from granulite to greenschist facies is apparent on both sides of the mafic–ultramafic bodies. This work focuses on the characteristics of the mylonitization overprinted on the mafic–ultramafic intrusives in the Pringles Metamorphic Complex and their gneissic–migmatitic surroundings, both previously metamorphosed within the granulite facies. Petrogenetic grid and geothermobarometry applied to the paragenesis equilibrated during the mylonitic event, together with mineral deformation mechanisms, indicate that mafic and adjacent basement mylonites developed under upper amphibolite transitional to granulite facies metamorphic conditions at intermediate pressures (668–764 °C, 6.3–6.9 kbar, 0.3 < XCO 2 < 0.7). However, the following mylonitic assemblages can be distinguished from the external limits of the Pringles Metamorphic Complex to its center: lower amphibolite facies ⇒ middle amphibolite facies ⇒ upper amphibolite transitional to granulite facies. Geothermobarometry applied to mylonitic assemblages indicate a temperature gradient from 555 °C to 764 °C and pressures of 6–7 kbar for the mylonitic event. This event is considered to have developed on a preexisting temperature gradient attributed to the intrusion of mafic–ultramafic bodies. The concentration of sulfides in mylonitic bands and textural relationships provide evidence of remobilization of primary magmatic sulfides of the mafic–ultramafic rocks (+PGM) during the mylonitic event. A lower-temperature final overprint produced brittle fracturing and localized retrogression on mafic–ultramafic minerals and ores by means of a water-rich fluid phase, which gave rise to a serpentine + magnetite ± actinolite association. Concordantly in the adjacent country rocks, fluids channeled along preexisting mylonitic foliation planes produced local obliteration of the mylonitic texture by a randomly oriented replacement of the mylonite mineralogy by a chlorite + sericite/muscovite + magnetite assemblage. Observed mineral reactions combined with structural data and geothermobarometry suggest a succession of tectonometamorphic events for the evolution of the Pringles Metamorphic Complex of Sierra de San Luis, developed in association with a counterclockwise P– T– d path. The most likely geological setting for this type of evolution is a backarc basin, associated with east-directed Famatinian subduction initiated in Mid-Cambrian times and closed during the collision of the allochthonous Precordillera terrane in Mid-Ordovician times.

Tectonic evolution of the Tombel graben basement, southwestern Cameroon

Planar structures (foliations and fractures) around the Tombel graben (southwestern end of the Central African Shear zone system) have been investigated and analyzed with the aim of unraveling the tectonic evolution of the basement. The foliations show two major trends, an older N-S-trending gneissose layering of uncertain agereworked by a later Pan-African (600 + 50 Ma) NE-SW ductile trend that is contemporaneous with sinistral shearing and mylonitization. The brittle phase characterized by NW-SE-trending open and partially filled fractures is younger than the mylonitization event and although it has not been dated, it is suggested that the origin of these fractures is linked to the onset of volcanism along the Cameroon volcanic line-31 m.y. ago.The mylonitic foliation is recognized for the first time and supports a tectonic evolution model for the Tombel graben in which ductile non-coaxial deformation was succeeded by brittle failure.

Tertiary S-C mylonites from the Bajánsenye-B-M-I borehole, western Hungary

Ultramylonites and margarite-bearing quartz-feldspar S-C mylonites, containing amphibolite lenses with symplectitic texture, were encountered in a borehole (Bajánsenye-B-M-I) close to the west of the Transdanubian Central Range Unit. These rocks demonstrate a ductile, horizontal extensional shear zone attaining a thickness of 300 m. Microstructural data, mineral parageneses and mineral chemistry of these rocks indicate a multistage metamorphic evolution, which is consistent with that of the Koralm-Pohorje basement. The youngest mylonitic event (Early Tertiary) took place in the Bajánsenye mylonites at 430-450oC (greenschist-facies); it rejuvenated coarse-grained muscovite crystals of eo-Alpine age (Early-Middle Cretaceous). The radiometric data presented in this paper demonstrate for the first time an important Early Tertiary tectonic zone in this area.

Kinematics of peridotite emplacement during North Atlantic continental rifting, Galicia, northwestern Spain

The western part of the Galicia passive continental margin (western edge of Spain) is bordered by serpentinized peridotites over a N-S distance of about 125 km. These peridotites were emplaced at the end of continental rifting and/or at the very beginning of oceanic accretion. They have been observed in situ and sampled in five sites with the Nautile submersible during the Galinaute cruise (1986). Some crustal continental rocks were also collected at two sites on the tilted blocks adjacent to the peridotite ridge. Most of the escarpments covered by the Nautile on the peridotite ridge have yielded only ultramafic rocks, except at Dive 10 where the peridotites are overlain by a sheared chlonte-bearing schist, and at Dive 14 where the ultramafic rocks are covered by basalts. All the peridotites are plagioclase-bearing harzburgites and Iherzolites, locally cross-cut by rare plagioclase-rich veins and dioritic intrusives. Some pyroxenites and gabbros are associated with the peridotites at Dive 14. The plagioclase-bearing peridotites are similar in composition to those drilled during ODP Leg 103. Primary structures have not been found, except in the northern part of the studied zone where a few samples exhibit magmatic textures. Elsewhere, the peridotites show evidence of a strong mylonitization event which has overprinted a former primary high-temperature fabric. At these localities, boudinage structures and ultramylonitic bands, referred to as shear bands, occur and the rocks have a disrupted mylonitic texture. A few dioritic dykes have also suffered this mylonitization. Shearing occurred in a rotational regime, at high and decreasing temperature (1000-850 °C) and under a high deviatoric stress (> 10 2 MPa) i.e. under lithospheric conditions. The structural pattern of the peridotite ridge, deduced from measurements on oriented samples, is complex. The trend of the mylonitic foliation appears nearly constant from south to north (N125-N105), but its dip increases (20–45°) northward. From south to north, the stretching lineation pitch varies from subvertical to subhorizontal. Stretching has occurred along a shear zone acting as a normal fault in a northeasterly direction in the southern part of the studied area. To the north, it occurred in a northwesterly direction with a strike-slip component. Stretching in the chlorite-bearing schist and in the gneissose rocks of the continental basement seems to be unrelated to the peridotite deformation. This study confirms the uplift of the Galicia peridotites during the continental rifting of the North Atlantic Ocean in Cretaceous times. During their ascent, the peridotites underwent some melting and high-temperature-low-stress ductile deformation, followed by intense mylonitization at decreasing temperature and increasing stress that has overprinted most of the primary textures. The geometry of the mylonitic structures is related both to the ascent of the peridotite dome and to the extensive stress-strain field prevailing in the continental lithosphere during the rifting episode. It is consistent with that expected in the upper part of a dome locally offset by transverse faults in its northern part due to the vicinity of a triple junction point. The kinematics of emplacement agree well with an E–W opening of the North Atlantic Ocean.

Use of the 40Ar- 39Ar stepwise heating method for dating mylonite zones: An example from the St. Barthe´le´my massif (Northern Pyrenees, France)

The timing of tectonic events is necessary for the knowledge of the evolution of mountain ranges. The dating of shear zones is especially useful to constrain the successive thermo-tectonic events which affect terranes. The development of mylonitic zones of regional extent is one of the most striking structural features of the Pyrenees. In the Northern Pyrenean Massif of St. Barthe´le´my, shear zones which deform the Variscan high-grade metamorphic series were recently studied. In order to investigate the behavior of the 40Ar- 39Ar isotopic system during a significant shearing episode and to determine the age of these shear zones, 40Ar- 39Ar isotopic analyses were carried out on the sheared gneisses of the St. Barthe´le´my Massif.The biotites from sheared migmatites and granulites of the gneiss core all yield well-defined plateau ages which are bracketed between 110 and 100 Ma. The muscovites of the same samples yield low-temperature ages which agree with these values. We conclude that a major mylonitic event occurred in the St. Barthe´le´my Massif at ∼110-100 Ma. The motion of the Main Mylonitic Band lasted up to — or started again from— ∼90 Ma, as attested by the biotite age spectrum of a mylonite. The motion of these shear zones is contemporaneous with the crustal thinning and the occurrence of a high thermal gradient during Mid-Cretaceous time in the North Pyrenean Zone. Later disturbance is recorded by some of the analyzed minerals at ∼55–60 Ma. Unsheared gneisses from the Trois Seigneurs Massif close to the studied area give an older age of 240 Ma, which is intermediate between Variscan and Pyrenean ages. The lack of Hercynian ages in the mylonites of the St. Barthe´le´my Massif is not conflicting with possible previous Variscan mylonitic events.

The eclogites of the Lato hills, south Togo, West Africa: Relics from the early tectonometamorphic evolution of the Pan-African orogeny

The eclogites of the Lato hills outcrop within the Pan-African orogenic belt of south Togoland (Dahomeyides) are located within the western border of the Bénin plain structural unit (ex Dahomeyen). They are closely associated with epi- to mesozonal metasediments, which are similar to the Pan-African cover formations of the Atacora unit. Two groups of eclogites have been distinguished according to field relations and to the nature of the country rocks; but they display a similar metamorphic history. The mineralogical and textural transformations of eclogites indicate: (1) a progressive increase of the P— T regime; (2) a mylonitization event close to the culmination of eclogitic metamorphism; and (3) a retrograde evolution towards amphibolite-facies conditions. Likewise, quartzites, closely associated with eclogites, exhibit high- P parageneses compatible with that of the eclogite formation. These observations allow us to consider the eclogitisation processes as a crustal transformation of mafic igneous rocks, and to assign a Pan-African age to these recrystallizations. Therefore, the Mount Lato eclogites represent rare witnesses from the early tectonometamorphic evolution of the Dahomeyides range.