To date the formation of ruby deposits and link it to the regional metamorphism associated with Tertiary Himalayan orogenesis, 40Ar–39Ar stepwise heating experiments were performed on single grains of phlogopite syngenetic with ruby, and zircon inclusions in ruby and spinel were dated with the U–Pb method by ion-probe. The Ar–Ar ages of phlogopites associated with ruby are Oligocene (24.7 ± 0.3 Ma) at Jegdalek in Afghanistan; Miocene at Mogok in Myanmar (18.7 ± 0.2 to 17.1 ± 0.2 Ma), at Hunza in Pakistan (10.8 ± 0.3 to 5.4 ± 0.3 Ma), and Chumar in Nepal (5.6 ± 0.4 Ma); and Pliocene (4.6 ± 0.1 Ma) at Ruyil in Nepal. In Vietnam, a zircon included in a ruby from the Quy Chau deposit yielded a 238U–206Pb age of 53.8 ± 4.6 Ma, whereas in the Red River shear zone, ruby formed at around 40–36 Ma during ductile deformation under peak metamorphic conditions. The ages obtained in this study are in agreement with those previously published for the ruby-bearing metamorphic belts and document extensional tectonics that were active from Afghanistan to Vietnam between the Oligocene and the Pliocene. Ruby-bearing marbles define a high-quality gem belt linked to the high-temperature metamorphism of the Himalayan fold belt that developed during the Tertiary collision of the Indian plate with Asia.

Marble-hosted ruby deposits occur in the Lo Gam tectonic zone along the northeastern flank of the Day Nui Con Voi Range in the Red River Shear Zone, in the northern part of Vietnam. Crystals of zircon included in ruby and spinel from Luc Yen and An Phu deposits were dated in situ by the U-Pb method with an ion microprobe. The patchy zoning of the zircon crystals and the wide range of ages (266-38 Ma) provide evidence for a complex metamorphic history, with at least two main thermal events: (1) Zircon included in spinel crystals probably crystallized during the Permian (256.6 +/- 9.4 Ma), with a possible reopening of their U-Pb system in the early Triassic (231.7 +/- 5.6 Ma). (2) Ruby formed at about 38 Ma when the Red River Shear Zone was the site of ductile deformation during the peak of metamorphism. The dating of zircon and phlogopite syngenetic with ruby documents the temporal relationship between high-temperature metamorphism and the cooling history of the Red River metamorphic belt. Ruby deposits hosted by marble sequences, in central and southeastern Asia seem to be an excellent marker; they allow an interpretation of the timing of the activity of Cenozoic structures linked to the collision between Indian and Eurasian continents.

The Nordfjord area, north of the Hornelen Devonian basin in Western Norway, is the southernmost part of the Ultra-High Pressure (UHP) Province, defined by the occurrence of coesite-bearing and diamond-bearing continental rocks. Compilation of structural, petrological, and chronological data from the area leads to a model for the formation of dome structures at the crustal scale and the behavior of the continental crust during its exhumation from mantle depths. The Nordfjord area appears as a 100 x 50 km dome-shaped boudin affected by at least two deformation stages. A stage of E-W stretching and top-to-west shearing produced several envelopes of migmatitic gneisses bounded by narrow high-strain zones over a core preserving the Precambrian granulite protolith. This dome is affected by the west-vergent Nordfjord Mylonitic Shear Zone on its southern limb during late exhumation under the Nordfjord-Sogn Detachment Zone. The first stage of deformation is coeval with reequilibration from maximum pressure conditions around 2.8 GPa, 650 degrees C (THERMOCALC multiequilibrium method) in the coesite stability field to higher temperature and lower pressure conditions (1.8 GPa, 780 degrees C). Subsequent retrogression was recorded in the amphibolite facies (0.7 GPa, 580 degrees C) and in the greenschist facies (0.4 GPa, 420 degrees C). Dates for these stages yield exhumation velocities higher than 2 mm/yr. Ar-40/Ar-39 ages in the area, compared to a spectrum of cooling ages along a 500-km-long N-S profile, show that cooling of the northern part of the Western Gneiss Complex is at least 20 Ma younger than in the south. The Western Gneiss Complex is therefore the result of the late juxtaposition of two complexes, the Northwestern Gneiss Complex, characterized by UHP relics, constrictive stretching, partial melting, and doming during a multi-stage exhumation from the deep parts of the orogen, and the Southwestern Gneiss Complex with Devonian basins, a well-developed detachment system, and distinct high pressure to medium pressure units stacked together during a single and rapid exhumation stage. The two complexes may represent deep subduction channel dynamics ( north) and shallower wedge circulation ( south) in the Caledonian orogen. The Nordfjord Mylonitic Shear Zone appears as a major tectonic in the Western Gneiss Complex. Partial melting in the Northwestern Gneiss Complex may have favored the late exhumation of E-W elongated domes such as the Nordfjord crustal-scale boudin and their juxtaposition to the Southwestern Gneiss Complex during top-to-west shearing.

New structural field data at various scale and 40Ar–39Ar geochronological results, from the basement rocks in the Truong Son belt and Kontum Massif of Vietnam, confirm that ductile deformation and high-temperature metamorphism were caused by the Early Triassic event of the Indosinian Orogeny in the range of 250–240 Ma. A compilation of isotopic data obtained in other countries along the Sibumasu–Indochina boundary broadly indicates same interval of ages. This tectonothermal event is interpreted as the result of a synchronous oblique collision of Indochina with both Sibumasu and South China, inducing dextral and sinistral shearing along E–W to NW–SE and N–S fault zones, respectively. The collision along Song Ma follows the northwards subduction of Indochina beneath South China and the subsequent development of the Song Da zone which in turn was affected by the Late Triassic Indosinian phase of shortening. Within the Indochina plate, internal collisions occurred coevally in the Early Triassic, as along the Poko suture, at the western border of the Kontum Massif.

Post-convergence evolution of the Variscan belt is characterized by the development of intramontane coal-bearing basins containing volcano-sedimentary successions. In the French Massif Central, K––Ar ages on clay particles from fine-grained sediments of the Bosmoreau basin (Limousin area), help pinpoint the evolution of the basin. In the lower part of the sedimentary pile, illite in a siltstone underlying a volcanic layer previously dated at 332±4 Ma by the U––Pb method on zircon, yields a consistent K––Ar age of ca. 340 Ma. Upward in the sedimentary succession, illite yields Stephanian K––Ar ages, which can be combined to provide a mean deposition age of 296.5±3.5 Ma. The Bosmoreau basin, albeit mainly filled with Stephanian deposits, was initiated during the late Visean, i.e. ca. 30 Ma earlier than inferred from biostratigraphical constraints. During the Stephanian, the same structure was reactivated and late Visean deposits were eroded and subsequently blanketed by thick clastic sediments. These results emphasise a two-stage evolution for the Bosmoreau basin, which is closely related to extensional tectonics identified on basement country rocks, and they are used to propose a geodynamic evolution of the studied area.

The West Sudetes (NE margin of the Bohemian Massif) consist of a complex mosaic of several tectonometamorphic units juxtaposed during the Variscan orogeny. The polyphase Variscan tectonothermal development of the West Sudetes was determined by 40 Ar/ 39 Ar ages of single grains and mineral concentrates. Late Famennian (359 Ma) mica ages from the high-grade Góry Sowie Block suggest continuous uplift after a Late Devonian high temperature-low pressure (HT-LP) event contemporaneous with the end of subduction-related high pressure-low temperature (HP-LT) metamorphism in the East Krkonoše Complex. Mid-Late Devonian high pressure events in the Krkonoše-Jizera Terrane and Orlica-Śnieżnik Dome are followed by coeval high temperature events between 345 and 335 Ma (Viséan). The latter are interpreted as consequence of uplift, and decompression during overthrusting of both complexes on their forelands. Subsequent small- to large-scale shear movements dated at around 325–320 Ma (early Namurian) affected the Orlica-Śnieżnik Dome, Krkonoše-Jizera Terrane, including the Intra-Sudetic Fault, and also the eastern Lusatian Granitoid Complex. They were accompanied by contemporaneous emplacement of the Krkonoše-Jizera pluton. The upper limit of the tectonometamorphic and magmatic activity is dated at 314–312 Ma (Namurian/Westphalian boundary). The final juxtaposition of the diversified tectonometamorphic units, which constitute the West Sudetes, took place in early Namurian times.

The Nangimali ruby deposit in the southern part of the Nanga Parbat Himalaya, has been investigated through field work, geochemistry, stable and radiogenic isotopes. It outcrops in the Shontar valley in a large north-vergent syncline consisting of high-grade metamorphic gneisses capped by a metasedimentary series dominated by marbles and amphibolites. The ore-body is stratiform. Ruby is found within 0.1-2 cm thick shear-veinlets and gash veins cutting dolomitic marbles and carbonate-bearing bands.The marbles of the Nangimali Formation display restricted ranges in delta(18)O (from 23.6 to 27.6parts per thousand relative to SMOW) and in delta(13)C (from -1.9 to 2.6parts per thousand relative to PDB). Fluid infiltration along the shear-zone in the marble has no effect on the isotopic signatures of the carbonates. Fluids are metamorphic and CO2 is derived from the decarbonation of marbles.Mass-balance and geochemical analyses suggest that the mobilisation by the fluids of aluminium and chromium in the marbles is sufficient to enable the formation of ruby in the shear-zone. Rubies have been indirectly dated using a stepwise Ar-40-Ar-39 laser heating technique on syngenetic phlogopites. The Miocene age records a Neogene cooling in the South of the Nanga Parbat massif and a minimum formation age for ruby of 16 Ma. (C) 2002 Elsevier Science Ltd. All rights reserved.

Ruby growth in phlogopite-bearing marbles has been indirectly dated using the Ar-40/Ar-39 laser stepwise heating technique on purified syngenetic phlogopite and other micas from ruby deposits in Yen Bai, Luc Yen and Quy Chan mining districts, in northern Vietnam. The principal results indicate the following. (1) Across the Red River shear zone, the phlogopites from the Yen Bai deposits yielded Miocene cooling ages between 23.2 and 24.4 Ma identical to those previously published using the same dating method on magmatic and metamorphic rocks from the Day Nui Con Voi range. (2) Luc Yen ruby deposits in the Lo Gam zone, on the eastern flank of the Red River shear zone, yielded Oligocene cooling Ar-40/Ar-39 mica ages between 30.8 and 34.0 Ma. Regarding the age of ruby crystallisation itself, the most plausible hypothesis is that all rubies in both zones formed during the period 40 to 35 Ma. Diachronism of cooling in adjacent zones leads to the conclusion that around 35 Ma, the ductile deformation in the Lo Gam zone ended and the ruby-bearing marbles cooled rapidly while the high-temperature deformation remained in the Red River shear zone, resulting in cooling through blocking temperature, some 15 Ma later. (3) The Quy Chau ruby deposit is restricted to the Quy Chau shear zone that bounds the eastern part of the Oligocene-Miocene Bu Khang dome. Phlogopite and biotite samples reveal Miocene cooling ages between 21 and 22.5 Ma which are minimum ages for ruby formation. These ages could be linked with the end of the extension of the Bu Khang dome. Vietnamese ruby formation is linked to Cenozoic tectonics resulting from continental collision between the Asian and Eurasian plates as for other marble-hosted ruby deposits in Central and Southeast Asia. (C) 2002 Elsevier Science B.V. All rights reserved.

In the Western French Massif Central, the Argentat fault is a major structure through which As–Au fluids percolated in the Late Carboniferous along brittle fractures. New petrostructural investigations show that an early ductile normal‐dextral faulting, coeval to leucogranite emplacement took place during the Late Visean syncollisional extension of the belt and was accompanied by a hydrothermal event marked by the growth of muscovites whose 40Ar/39Ar ages cluster around 335 Ma. This early fluid channelling is associated with brittle deformation only in the hangingwall of the Argentat fault, whereas ductile deformation is restricted to the footwall. These results provide new evidence for the upper crust implication during the syncollisional extension in the French Massif Central. This study stresses the interest of a detailed multimethod analysis to characterize hydrothermal processes, especially in basement areas where the tectonic, plutonic and metamorphic evolution is polyphased.

En septembre 1998, un des auteurs (WW) a decouvert un indice important de mineralisation en emeraude dans la region du lac Finlayson, dans le secteur sud-est du Yukon. L'indice de Regal Ridge est situe dans les roches metavolcaniques deformees de facon complexe dans le socle de Yukon-Tanana, pres du contact avec un pluton granitique mis en place au cretace moyen. Les cristaux d'emeraude se sont developpes ou des veines de quartz recoupent des niveaux micaces de l'unite schisteuse tardidevonienne de Fire Lake, mafique et a faible pendage. Au moins huit veines semblables ont ete reperees. Dans la plupart des cas, les veines sont entourees d'une masse de petits cristaux enchevetres de tourmaline foncee. Leurs sont associes localement de petites quantites de scheelite, et, dans les veines elles-memes, des sulfures. Une zone de sulfures epars coinciderait avec la zone a tourmaline, marquee par des produits d'oxydation ochres. Des cristaux de beryl vert atteignent 4 cm en longueur dans les zones a tourmaline et, dans certains cas, les veines de quartz. Certains des plus petits cristaux, et des portions des plus gros, ont une qualite gemme. La teneur en Cr (moyenne 3208 ppm) en fait le chromophore principal. Les donnees sur les inclusions fluides indiquent que la phase fluide responsable de la mineralisation avait une salinite maximale equivalente a 3% de NaCl. La composition isotopique de l'oxygene de l'emeraude est tres variable (entre 12.3 et 14.8‰), mais sans variation comparable dans les valeurs de 8D correspondantes (-57.3 et -59.8‰, respectivement), ce qui suppose une phase fluide isotopiquement homogene qui a amorce un echange isotopique avec l'encaissant sans toutefois atteindre l'equilibre. Les valeurs de δ 1 8 O du quartz et de la tourmaline des veines de quartz indiquent une temperature de formation d'environ 365 et 498°C. A la lumiere des donnees isochores des inclusions fluides, ces temperatures correspondraient a une pression entre 1.0 et 2.5 kbar, et donc une profondeur entre 3 et 7.7 km. La proximite du granite fait penser qu'il etait la source du beryllium, quoique sa teneur en Be est assez faible (entre 12 et 13.2 ppm). La source du Cr est le schiste (520 ppm Cr). Un âge 4 0 Ar/ 3 9 Ar d'un echantillon de mica du schiste, 109 Ma, pourrait temoigner de l'âge d'un rechauffement lie a la mineralisation, ou bien un refroidissement suivant la mise en place du granite, ou les deux.

In most orogenic belts, the age of HP metamorphism and subsequent exhumation events still remain open to debate since geochronology can yield results which appear to conflict with the closure temperature concept [Dodson, M.H., 1973. Contrib. Mineral. Petrol. 40, 259–274], and because the behaviour of daughter radiogenic isotopes under HP to UHP conditions is poorly constrained. To obtain new data on isotope migration under high-pressure conditions, two undeformed HP metagranites with partially preserved magmatic assemblages from the French Variscan belt were investigated using the 40Ar/39Ar laser probe and U–Pb ion probe methods. In both cases, 40Ar/39Ar biotite and U–Pb zircon ages are consistent and could be related to the emplacement of pre-orogenic granites, despite petrological evidence of a strong metamorphic overprint during the Variscan orogeny. Temperatures experienced by the granites during subduction and exhumation events were more than 400 °C above that normally estimated for argon retention in biotite, but failed to cause significant resetting of the mica 40Ar/39Ar chronometer. Only a weak intragrain redistribution of argon is evidenced with the laser probe up to the contact with metamorphic garnet fringing biotite. By contrast, a complete resetting of biotite ages occurs in gneisses enclosing the metagranites. These results suggest that, in these dry undeformed HP metagranites, the thermally activated diffusion was relatively ineffective and that recrystallisation is the main process which controlled isotopic exchanges of Ar and Pb. It is likely that the absence of pervasive deformation and fluid circulation has also exercised some control on the preservation of pre-metamorphic isotopic signature in the studied rocks. The possible influence of several other parameters is also discussed. This example reveals that recovering thermochronological information in high-pressure metamorphic rocks is not straightforward and that great caution must be paid in the use of ages for the reconstruction of P–T–t paths.

Multidisciplinary studies of geotransects across the North European Plain and Southern North Sea, and geological reexamination of the Variscides of the North Bohemian Massif, permit a new 3-D reassessment of the relationships between the principal crustal blocks abutting Baltica along the Trans-European Suture Zone (TESZ). Accretion was in three stages: Cambrian accretion of the Bruno–Silesian, Lysogory and Malopolska terranes; end-Ordovician/early Silurian accretion of Avalonia; and early Carboniferous accretion of the Armorican Terrane Assemblage (ATA). Palaeozoic plume-influenced metabasite geochemistry in the Bohemian Massif explains the progressive rifting away of peri-Gondwanan crustal blocks before their accretion to Baltica. Geophysical data, faunal and provenance information from boreholes, and dated small inliers and cores confirm that Avalonian crust extends beyond the Anglo-Brabant Deformation Belt eastwards to northwest Poland. The location and dip of reflectors along the TESZ and beneath the North European Plain suggest that Avalonian crust overrode the Baltica passive margin, marked by a high-velocity lower crustal layer, on shallowly southwest-dipping thrust planes forming the Heligoland–Pomerania Deformation Belt. The “Variscan orocline” of southwest Poland masks two junctions between the Armorican Terrane Assemblage (ATA) and previously accreted crustal blocks. To the east is a dextrally transpressive contact with the Bruno–Silesian and Malopolska blocks, accreted in the Cambrian, while to the north is a thrust contact with easternmost Avalonia, deeply buried beneath younger sedimentary cover. In the northeast Bohemian and Rhenohercynian Massifs Devonian “early Variscide” deformation dominated by WNW and NW-directed thrusting, records closure of Ordovician–Devonian seaways between detached “islands” of the ATA and Avalonia.

North-west Spitsbergen consists of a complex of Caledonian and Grenvillian crystalline rocks, situated at the north-west corner of the Barents Shelf. The aim of this study is to understand the extent of pre-Caledonian basement rocks and their protoliths. Micas and zircon grains from six rocks from north-west Spitsbergen have been dated by the 40Ar/39Ar and single-zircon Pb-evaporation methods. Two grey granites yielded Late Caledonian mica 40Ar/39Ar and zircon ages of ca. 420-430 My, with inherited zircon grains as old as 1725 My. Zircon grains from a gneissose granite xenolith in a grey granites gave crystallization ages of ca. 960 My; some grains from a migmatite neosome show similar ages. Zircon grains yielding Archean and late Palaeoproterozoic ages (1600-1800 My) are interpreted as xenocrysts of detrital origin. The youngest ages obtained from detrital zircon grains from a greenschist facies quartzite of the Signehamna unit are ca. 1800 My. Similar schists are included as xenoliths in the 960 My old gneissose granite; therefore, the sedimentary protoliths of the unit are Mesoproterozoic. The dating results suggest a significant tectonothermal event during Grenvillian time; subsequent Caledonian events had less extensive thermal effects. However, it is still a matter of debate whether Grenvillian or Caledonian metamorphism produced the majority of the migmatites. A large population of zircon grains with Late Palaeoproterozoic ages suggests a wide surface exposure of rocks of this age in the source area, with some Archean zircons.

ABSTRACT Volume diffusion and dislocation creep at T ∼ 800 °C led to high finite strain in granulite and orthogneiss of the Ohře crystalline complex (North Bohemian shear zone). Intragranular creep by volume diffusion is indicated by (i) lobate phase boundaries between feldspar and quartz, and (ii) removal of perthite lamellae and precipitation of tiny, aluminium‐rich needles at the margins of K‐feldspar. The striking diffusional‐creep structures imply high interfacial free energy that has been preserved from equilibration as a result of rapid cooling. U–Pb dating of monazite (342 ± 1 Ma) and 40Ar–39Ar dating of muscovite (341 ± 4 Ma) of Kadaň orthogneiss result in a cooling rate of 50 + 25/−17 °C Myr−1. This high value is attributed to collapse‐related ‘elevator‐style’ movements along the North Bohemian shear zone, resulting in the juxtaposition of upper crustal rocks of the Tepla–Barrandian unit against lower crustal rocks of the Erzgebirge crystalline complex.

New structural observations of Oligo‐Miocene deformation in north Vietnam along the Red River Shear Zone suggest that left‐lateral strike‐slip shear was restricted to the upper and middle crust above a horizontal shear zone. Left‐lateral shear deformation is associated with low‐pressure‐low‐temperature parageneses. High‐temperature deformation is restricted to zones where the foliation has a low dip in the core of the Dai Nui Con Voi antiformal dome. These observations complete those made earlier in the Bu Khang extensional dome farther south. Above the horizontal shear zone left‐lateral transpression was active during the first stage and changed to transtension some 33 Myr ago. Purely extensional metamorphic domes (Bu Khang) or transtensional domes (Dai Nui Con Voi) were exhumed during this younger stage. Our observations plead for caution when interpreting strike‐slip structures at lithospheric scale.

The Song Chay Massif is the northeasternmost metamorphic complex in Vietnam, to the east of the Red River Shear Zone. It shows a large antiformal structure involving orthogneisses and migmatites overlain, on its northern flank, by muscovite bearing marbles. An E–W striking fault bounds the dome to the South. Kinematic indicators along a S–N section reveal top-to-the-N shear sense along the interface between the orthogneissic core and the overlying metasediments. Radiometric ages were obtained by the 40Ar–39Ar method using purified mica separates. Across the dome ages range from 236Ma at the southern edge to 160Ma in the core, attesting to a strong imprint in the Early Triassic time. A clear difference is seen between these Mesozoic ages and the Eocene to Miocene ages (from 40 to 24Ma) that obtained in the nearby Red River Shear Zone using the same method. These data show that the Song Chay Massif was already high in the crust when the high temperature deformation of the Red River Shear Zone took place. The final exhumation of the Song Chay orthogneiss constrained by fission-track analysis on samples along the same transect occurred during the Early Miocene and could be interpreted as the consequence of a first normal sense of motion along the fault which bounds the massif to the south. Timing is similar to that of exhumation in the Red River Shear zone.

SynopsisAn attempt has been made at correlation between the Lower Palaeozoic Iglesiente domain of southwest Sardinia and the southern Cevennes-Montagne Noire Variscan domain of France, with particular regard to the Cambrian-hosted economic lead-zinc concentrations and the spatially associated precious-metal (Au) occurrences. Geological, lithogeochemical and lead isotope investigations led to the following conclusions. (1) The major lead-zinc deposits of Iglesiente and southern Montagne Noire have a Mississippi Valley-type origin, which involved remobilization of pre-existing Cambrian syngenetic mineralization during the Ordovician distensive 'Sardic phase'. Lead isotope evidence suggests mixed crustal sources for the Iglesiente lead. (2) The southern Cevennes stratiform Minerai Zero of the Malines district is not syngenetic with the Cambrian host rocks. For this ore type a synto late-tectonic Variscan formation model is proposed, similar to the model indicated for the genesis of the Salsigne gold deposit in the Montagne Noire. A re-evaluation of the Vigan gold occurrences, regionally associated with the Mineral Zero economic lead-zinc bodies, is therefore suggested. (3) The newly discovered Tertiary epithermal gold province of Sardinia is genetically quite distinct from the Palaeozoic lead-zinc province of Iglesiente.

40Ar/39Ar and conventional K/Ar data on mafic dykes and a felsic dyke, together with previous data have been used to constrain the ages of the dyke magmatism in the south Indian granulite terrain. The age of the mainly ENE–WSW-trending Agali–Anaikatti swarm is estimated to be 1980±25 Ma, with a very limited occurrence of upper Cretaceous (80–90 Ma) dykes in the area. The NW–SE Dharmapuri swarm is dated as at least 1800 Ma (possibly 1855±9 Ma). Both the NW–SE and the NE–SW dyke swarms of Tiruvannamalai are dated at 1650±10 Ma. Only a few dykes in north Kerala are of middle Proterozoic (ca. 1700–1650 Ma) age. Furthermore, the 40Ar/39Ar results of Dharmapuri dykes exhibit age spectrum characteristic of partial degassing; they provide strong evidence of latest Proterozoic overprinting by a thermal event probably related to the Pan-African orogeny. Most of the Phanerozoic dykes are confined to the coastal region in Kerala province. The dolerites of central Kerala and the majority of north Kerala dolerites intruded during the 70–65 Ma period; whereas a large gabbro and a felsic dyke in central and north Kerala, respectively, are of upper Cretaceous age (ca. 85 Ma) and the mean 81±1.5 Ma age from these dykes may represent cooling age. The oldest Phanerozoic (Upper Jurassic; 144±6 Ma) dyke intrusions have a restricted occurrence in south Kerala, very close to the southern tip of India. The Proterozoic dykes are interpreted as being lateral intrusions from under-plating igneous bodies beneath the eastern shield region. The geological record of the terrain does not show indications of significant pre-magmatic extension and rifting, but dyke magmatism may have led to extension, rifting and the consequent development of the India–Antarctica–Sri Lanka Proterozoic high-grade terrain. The data are also interpreted to suggest that the south Indian granulite terrain is an early Proterozoic tectono-metamorphic terrane with strong thermal imprints of Neoproterozoic/early Palaeozoic age. It is difficult to reconcile its extensions into Proterozoic Sri Lanka or Antarctica where tectono-metamorphism is dated to be of Neoproterozoic age. The late Phanerozoic dykes are predominantly related to the Deccan trap event, and are associated with mantle plume decompressional tectonics that gave rise to ocean floor spreading between the Seychelles and the Indian continent. The few older (ca. 145 Ma) dykes in south Kerala may be related to the rift tectonics that preceded the separation of Australia–Antarctica and India.

In western Nevada, the Black Dyke Formation includes volcanic rocks overlain conformably by volcaniclastic sediments. At the base, hornblende-phyric basalts with cognate hornblende-bearing gabbroic cumulates are interbedded with tuffs and pyroclastic breccia. Amphiboles give Ar-40/Ar-39 ages of 276 Ma. Clinopyroxene-phyric pillow basalts and plagioclase-phyric andesitic lava flows are present higher in the section. Facies changes between exposures reflect development near volcanic centers.According to our investigations, the Black Dyke Formation is involved in east-west-trending folds overturned toward the south, and overlain unconformably by the Mesozoic Dunlap Formation, which unconformably overlies the Mississippian-Permian Mina Formation. Interpreted until now as tectonic slices within the Luning allochthon, we suggest that the Black Dyke Formation is part of the Sonoma allochthon associated with the Mina Formation. The Sonoma records closure of the Havallah basin (Golconda allochthon), and collision of an arc-trench system with the North American margin.The Black Dyke Formation exhibits similarities with the Permian are sequence of the northern Sierra Nevada. Both sequences are characterized by amphibole-bearing breccias, clinopyroxene-phyric pillow-basalts, plagioclase-phyric andesites and overlying volcaniclastic sediments. These sequences developed in the same geodynamic environment tan island-arc). (C) Elsevier, Paris.

The Mogok metamorphic belt, exposed along the N‐S trending Shan scarp and Sagaing fault, in the eastern part of Myanmar, has been regarded as Paleozoic to Precambrian for a long time. New observations in the Shan scarp area, close to the Sagaing fault, from Thaton in the south to Mandalay in the north, allowed us to collect samples of high grade metamorphic and intrusive rocks that have been analyzed by 40Ar/39Ar step heating method. The 13 samples we analyzed provide Oligocene to Lower Miocene ages for this metamorphism. Oriented thin sections and field observations suggest that this metamorphism was caused by a NNW‐SSE to N‐S ductile extension. Therefore, we suggest that this metamorphism is not directly related to the Sagaing fault, but could be instead related to the northward migration of the eastern Himalayan syntaxis, characterized by crustal thinning, resulting from the India‐Asia oblique collision.