Triassic 40Ar/39Ar ages from the Sakaigawa unit, Kii Peninsula, Japan: implications for possible merger of the Central Asian Orogenic Belt with large-scale tectonic systems of the East Asian margin

International audienceThe 218.4 ± 0.4, 228.8 ± 0.9 and 231.9 ± 0.7 Ma 40Ar/39Ar laser probe pseudo-plateau ages (2σ; 49–63% 39Ar-release) of very low-grade meta-pelitic whole-rocks from the Sakaigawa unit date high-P/T metamorphism. We argue that this event occurred in a subduction–accretion complex, not along the East Asian continental margin, but on the Pacific side of the proto-Japan superterrane. Proto-Japan was a Permian magmatic arc, presently dispersed in the Japanese islands, which also contained older subduction–accretion complexes. The arc system was fringing but not yet part of the Eurasian continent. The Middle to Late Triassic high-P/T tectono-metamorphic event was partly coeval with proto-Japan's collision with proto-Eurasia along the southward extension of the Central Asian Orogenic Belt, causing the main metamorphism in the Hida-Oki terrane. It is possible that this system continued via the Cathaysia block (China) to Indochina. The Late Permian to Middle Triassic Indosinian event might stem from docking of Pacific-derived terranes with Southeast Asia's continental margin. The concept of the proto-Japan superterrane implies that the Qinling-Dabie-Sulu suture zone joined the Central Asian Orogenic Belt to the east of the North China craton and did not continue to Japan, as commonly assumed

Mono Lake or Laschamp geomagnetic event recorded from lava flows in Amsterdam Island (southeastern Indian Ocean)

We report a survey carried out on basalt flows from Amsterdam Island in order to check the presence of intermediate directions interpreted to belong to a geomagnetic field excursion within the Brunhes epoch, completing this paleomagnetic record with paleointensity determinations and radiometric dating. The directional results corroborate the findings by Watkins and Nougier (1973) : normal polarity is found for two units and an intermediate direction, with associated VGPs close to the equator, for the other two units. A notable result is that these volcanic rocks are well suited for absolute paleointensity determinations. Fifty percent of the samples yields reliable intensity values with high quality factors. An original element of this study is that we made use of the PTRM-tail test of Shcherbakova et al. (2000) to help in the interpretation of the paleointensity measurements. Doing thus, only the high temperature intervals, beyond 400 degres C, were retained to obtain the most reliable estimate of the strength of the ancient magnetic field. The normal units yield Virtual Dipole Moments (VDM) of 6.2 and 7.7 10e22 Am2 and the excursional units yield values of 3.7 and 3.4 10e22 Am2. These results are quite consistent with the other Thellier determinations from Brunhes excursion records, all characterized by a decrease of the VDM as VGP latitude decreases. 40Ar/39Ar isotopic age determinations provide an estimate of 26+-15 Kyr and 18+-9 Kyr for the transitional lava flows, which could correspond to the Mono Lake excursion. However, the large error bars associated with these ages do not exclude the hypothesis that this event is the Laschamp

Fast cooling following a Late Triassic metamorphic and magmatic pulse: implications for the tectonic evolution of the Korean collision belt

International audienceWe discuss the evolution of Korea in the context of a relatively short-lived, tectonically induced, magmatic and metamorphic pulse that affected large portions of the crust of the peninsula's southern part during the Late Triassic. Recent 40Ar/39Ar single grain laser step-heating dates imply a prolonged metamorphic recrystallization between 243 and 220 Ma, which occurred in distinct phases that were not coeval throughout the peninsula. We obtained identical plateau ages between 231.4 ± 0.8 and 228.9 ± 0.8 Ma (1σ; 85–95% 39Ar release) on single grains of detrital muscovite from Jurassic sandstones (Gimpo Group). A literature review shows that the ages of detrital muscovites are identical to: (1) concordant 40Ar/39Ar ages of biotite (228 Ma) and amphibole (230 Ma) in amphibolites of the Deokjeongri Gneiss Formation and the Weolhyeonri Complex, pointing to very rapid cooling of 100–150 °C/Ma, and (2) 231–229 Ma muscovite from the low-grade metamorphic mid-Paleozoic turbidites of the Taean Formation. The efficiency of cooling is further underlined by the near-coincidence of these 40Ar/39Ar ages with 243–229 Ma (average: 234.6 Ma) zircon U–Pb ages in the Gyeonggi Massif and the Hongseong belt, in the literature. It is argued that the Late Triassic magmatic and metamorphic pulse is superimposed on an earlier tectono-metamorphic event, possibly related to collision, indicated by: (1) ~ 243–237 Ma muscovite ages, or age components in age spectra, and (2) two generations of folds and associated tectonic foliations truncated by ~ 229.5-Ma-old syenites and earlier mafic dykes. The Late Triassic thermal pulse could have been the result of post-collisional delamination of the lower crust and uppermost mantle, and/or oceanic slab break-off, which is also suggested by almost coeval, widespread mantle-sourced Mg-rich potassic magmatism. Continuing ductile deformation is shown by mylonitization of Late Triassic magmatic rocks; an ~ 220 Ma muscovite age may be related to this

A(40)Ar/(39) Ar study of oceanic and continental deformation processes during an oblique collision: Taconian orogeny in the Quebec reentrant of the Canadian Appalachians

International audienceTwo phases of penetrative deformation are documented in the Taconian hinterland of the Appalachian orogen in the Gaspé Peninsula. D1 is associated with the obduction of the Mont-Albert ophiolite onto the Paleozoic Laurentian margin, whereas D2 corresponds to later transport of allochthons across the margin. In the metamorphic sole, S1 is a SE-dipping mylonitic fabric with a downdip lineation. In underlying metabasalts, D1 is characterized by NW-overturned and recumbent folds, and a subhorizontal S1 schistosity with an ENE-trending orogen-parallel lineation. D2 is characterized by a S2 steeply dipping penetrative axial-planar crenulation cleavage and NE-trending F2 folds. The intraoceanic thrusting of ophiolite is dated at 465 Ma (early D1) whereas emplacement of ophiolite and subsequent deformation of the margin was recorded by isotopic signatures between 459 and 456 Ma (late D1). D2 is dated at 448 Ma throughout the hinterland. Taconian transpressive deformation is related to an oblique collision within the Quebec reentrant of the Canadian Appalachians during the Ordovician

Geochronology and geochemistry of Eocene-aged volcanic rocks around the Bafra (Samsun, N Turkey) area: Constraints for the interaction of lithospheric mantle and crustal melts

International audience40Ar-39Ar age, whole-rock chemical and Sr-Nd isotope data are presented for the post-collisional, Eocene (51.3-44.1 Ma)-aged volcanic rocks from the Bafra (Samsun) area in the western part of the Eastern Pontides (N Turkey) aiming to unravel their sources and evolutionary history. The studied Eocene volcanic rocks can be divided into two groups: analcime-bearing (tephritic lava flows and dykes) and analcime-free (basaltic to trachytic lava flows and basaltic dykes). The analcime-bearing volcanic rocks have a fine-grained porphyritic texture with clinopyroxene phenocrysts, whereas analcime-free volcanic rocks show a variety of textures including hyalo-microlitic microgranular porphyritic, intersertal, trachytic, fluidal and glomeroporphyritic. The volcanic rocks also show evidence of mineral-melt disequilibrium textures such as sieved, rounded and corroded plagioclases, partially melted and dissolved clinopyroxenes and poikilitic texture. Petrochemically, the parental magmas of the volcanic rocks evolved from alkaline to calc-alkaline lava suites and include high-K and shoshonitic compositions. They display enrichments in light rare earth and large ion lithophile elements such as Sr, K and Rb, as well as depletions in high field strength elements such as Nb, Ta, Zr and Ti, resembling subduction-related magmas. The analcime-bearing and -free volcanic rocks share similar incompatible element ratios and chondrite-normalised rare rearth element patterns, indicating that they originated from similar sources. They also have relatively low to moderate initial 87Sr/86Sr (0.7042 to 0.7051), high positive εNd(t) values (+ 0.20 to + 3.32), and depleted mantle Nd model ages (TDM1 = 0.63-0.93 Ga, TDM2 = 0.58–0.84 Ga). The bulk-rock chemical and Sr-Nd isotope features as well as the high Rb/Y and Th/Zr, but low Nb/Zr and Nb/Y ratios, indicate that the volcanic rocks were derived from a lithospheric mantle source that had been metasomatised by slab-derived fluids. Trace element modelling suggests that the parental magma(s) of the volcanic rocks represent mixtures of melts derived by low-degree (~ 5–10 %) partial melting of spinel-lherzolite (40–85%) and garnet-lherzolite (15–60 %) mantle sources. Sr-Nd isotopic modelling also suggests that a 25–35% lower crustal component was added in the parental magmas; AFC modelling additionally indicates minor upper crustal contamination during the evolution of the volcanic rocks. In conclusion, integration of the geochemical, petrologic and isotopic data with regional geology suggests that the analcime-bearing and -free volcanic rocks evolved from parental magma(s) derived from melts of a subcontinental lithospheric mantle and lower crustal sources

Géochronologie revisitée du dôme du Léon (Massif armoricain, France)

National audienceDe nouvelles datations réalisées sur les principaux ensembles granitiques et métamorphiques du Léon permettent de préciser l'histoire de ce domaine de la chaîne hercynienne. Dans les domaines métamorphiques, les trois âges obtenus sur des zircons des orthogneiss granodioritiques de Brest s'échelonnent de 529 à 504 Ma, confirmant le caractère polyphasé de ce complexe orthodérivé cambrien. Dans l'unité autochtone, l'âge dévonien inférieur (385 - 391 Ma) des orthogneiss de Plounévez-Lochrist est confirmé par une nouvelle datation U-Pb sur zircon réalisée dans l'ensemble oriental. Dans les unités allochtones du Nord, les monazites du complexe migmatitique de Plouguerneau livrent un âge de 331 ± 7 Ma, interprété comme celui de la migmatitisation et conforme à celui proposé récemment par Faure et al. (335 - 327 Ma). L'âge des événements magmatiques hercyniens a également été précisé. Le granite porphyroïde de Kersaint se confirme comme le plus ancien du Léon avec un âge sur monazite de 331 ± 4 Ma. Cependant, les nouvelles datations réalisées montrent que les granites léonards sont essentiellement stéphaniens. Le granite porphyroïde rose de Guissény, satellite du granite de l'Aber-Ildut, est ainsi daté par U-Pb sur zircons à 301,4 ± 4,3 Ma. La monzodiorite de Plounévez-Lochrist, satellite mafique du grand massif de Brignogan-Plouescat, donne un âge de 296 ± 3 Ma, cohérent avec celui de 292 ± 15 Ma obtenu par Rb/Sr sur le monzogranite de Brignogan-Plouescat. Le leucogranite à tourmaline de Sainte-Catherine, considéré comme tardif par son allure en dykes sécants, est en fait également stéphanien ; le faciès le plus répandu est daté à 302,7 ± 0,3 Ma, le faciès sécant plus tardif riche en tourmaline à 301,5 ± 0,3 Ma (Ar-Ar sur muscovites). L'âge obtenu sur la mylonite formée aux dépens du leucogranite de Kernilis (292,5 ± 2,7 Ma par Ar-Ar sur muscovite) correspond vraisemblablement au jeu de l'accident mylonitique de Porspoder. Les granitisations du Léon débutent donc immédiatement après le pic métamorphique hercynien, comme dans le Massif central. La principale période de granitisation du Léon s'effectue cependant au Stéphanien, c'est-à-dire de la fin de l'extension syn-collisionnelle (300-310 Ma) à la fin de l'extension post-collisionnelle (post-300 Ma)

Draa Sfar, Morocco: A Visean (331 Ma) pyrrhotite-rich, polymetallic volcanogenic massive sulphide deposit in a Hercynian sediment-dominant terrane

International audienceDraa Sfar is a Visean, stratabound, volcanogenic massive sulphide ore deposit hosted by a Hercynian carbonaceous, black shale-rich succession of the Jebilet terrane, Morocco. The ore deposit contains 10 Mt grading 5.3 wt.% Zn, 2 wt.% Pb, and 0.3 wt.% Cu within two main massive sulphides orebodies, Tazakourt (Zn-rich) and Sidi M'Barek (Zn–Cu rich). Pyrrhotite is by far the dominant sulphide (70 to 95% of total sulphides), sphalerite is fairly abundant, chalcopyrite and galena are accessory, pyrite, arsenopyrite and bismuth minerals are rare. Pyrrhotite is monoclinic and mineralogical criteria indicate that it is of primary origin and not formed during metamorphism. Its composition is very homogeneous, close to Fe7S8, and its absolute magnetic susceptibility is 2.10− 3 SI/g. Ar–Ar dating of hydrothermal sericites from a coherent rhyolite flow or dome within the immediate deposit footwall indicates an age of 331.7 ± 7.9 Ma for the Draa Sfar deposit and rhyolite volcanism. The Draa Sfar deposit has undergone a low-grade regional metamorphic event that caused pervasive recrystallization, followed by a ductile–brittle deformation event that has locally imparted a mylonitic texture to the sulphides and, in part, is responsible for the elongated and sheet-like morphology of the sulphide orebodies. Lead isotope data fall into two compositional end-members. The least radiogenic end-member, (206Pb/204Pb = 18.28), is characteristic of the Tazakourt orebody, whereas the more radiogenic end-member (206Pb/204Pb not, vert, similar 18.80) is associated with the Sidi M'Barek orebody, giving a mixing trend between the two end-members. Lead isotope compositions at Draa Sfar testify to a significant continental crust source for the base metals, but are different than those of the Hajar and South Iberian Pyrite Belt VMS deposits. The abundance of pyrrhotite versus pyrite in the orebodies is attributed to low fO2 conditions and neither a high temperature nor a low aH2S (below 10− 3) is required. The highly anoxic conditions required to stabilize pyrrhotite over pyrite are consistent with formation of the deposit within a restricted, sediment-starved, anoxic basin characterized by the deposition of carbonaceous, pelagic sediments along the flank of a rhyolitic flow-dome complex that was buried by pelitic sediments. Deposition of sulphides likely occurred at and below the seafloor within anoxic and carbonaceous muds. Draa Sfar and other Moroccan volcanogenic massive sulphide deposits occur in an epicontinental volcanic domain within the outer zone of the Hercynian belt and formed within a sedimentary environment that has a high pelagic component. In spite of the diachronous emplacement between the IPB deposits (late Devonian to Visean) and Moroccan deposits (Dinantian), all were formed around 340 ± 10 Ma following a major phase of the Devonian compression