U Pb Geochronology

... 628–625 Ma) formed in a continental arc. In the Macururé Domain minor conglomerates and greywackes of the Juá Formation (Fig. 2) have undergone minor deformation and metamorphism, but are not intruded by granites. Menezes Filho et al. ...

Geological, geochemical and geochronological data suggest that the basement exposed in the Occidental terrane of the Neoproterozoic-Ordovician Ribeira belt represents the continuation of the São Francisco paleocontinent, which was amalgamated at the end of the Rhyacian ( ...

ABSTRACT Glacial erratics collected on Melville Island, western Canadian Arctic Archipelago, were analyzed to determine their mainland provenance, thereby constraining their long-distance transport by the Laurentide Ice Sheet. These erratics can be broadly subdivided into three main lithologies: granite (n = 15), quartzite (n = 7), and diabase-diorite (n = 3). The granite erratics are most distinctive from a provenance perspective and can be further subdivided into three geochemical groups based on their potassium content: (1) a high-K2O group (K2O> 4.0 wt.%), (2) an intermediate-K2O group (K2O between 2.0 and 4.0 wt.%), and (3) a low-K2O group (K2O< 2.0 wt.%). In situ thin section laser ablation inductively coupled plasma mass spectrometer U-Pb zircon ages obtained for eight granite erratics yielded both Archean (2575 Ma) and a range of Paleoproterozoic (2472-1778 Ma) crystallization ages. In addition, three overprint ages were identified at 1.90, 1.84, and ~1.0 Ga. The most compelling constraint for a northward regional ice flow originating on the mainland are two high-precision conventional U-Pb zircon ages of 1969.5± 1.0 and 2472.3± 0.5 Ma, indicating that these granite erratics must have been derived from the 2.0-1.9 Ga Taltson-Thelon Orogen and the nearby 2.5-2.4 Ga Queen Maud Block, respectively. These granite-dominated terranes are located 600km due south and southeast of the collection area on Melville Island. Although it is unknown whether the final deposition of these erratics on Melville Island involved transport by one or more glaciations, it is apparent that this ice flow cannot be accommodated by the proposed north-south axis of the M'Clintock Ice Divide, the primary topographic feature of the northwestern Laurentide Ice Sheet during the last glacial maximum. The transport of erratics reported here would have required a former ice divide oriented east-west over the mainland, close to that proposed for the Ancestral Keewatin Divide. An east-west ice divide in this region is consistent with previously reported ice-flow indicators that document northward flow from the mainland and recent thermomechanically coupled ice-sheet numerical modeling that indicates former maximum ice thickness on the mainland immediately south of Melville Island.Des blocs erratiques recueillis sur l'île Melville, dans l'archipel arctique canadien, ont été analysés afin de déterminer leur provenance des régions continentales, encadrant ainsi leur transport sur de longues distances par l'Inlandsis laurentidien. Ces blocs erratiques peuvent être subdivisés selon trois principales lithologies : granite (n = 15), quartzite (n= 7) et diabase-diorite (n = 3). Les erratiques de granite sont les plus distinctifs du point de vue de la provenance et ils peuvent être subdivisés de nouveau en trois groupes géochimiques basés sur leur contenu en potassium : (1) un groupe à teneur élevée en K2O (K2O> 4,0 % poids), (2) un groupe à teneur intermédiaire en K2O (K2O entre 4,0 et 2,0 % poids) et (3) un groupe à faible teneur en K2O (K2O< 2,0 % poids). Des âges U-Pb in situ déterminés sur des zircons en lame mince par analyse à couplage inductif et spectrométrie de masse avec ablation au laser (LA-ICP-MS) de huit blocs erratiques de granite ont donné des âges de cristallisation archéens (2575 Ma) et paléoprotérozoïques (2472-1778 Ma). De plus, trois âges de surimpression ont été identifiés à 1,90, 1,84, et ~1,0 Ga. La contrainte la plus convaincante pour un écoulement régional de la glace vers le nord à partir du continent a été la détermination de deux âges conventionnels U-Pb de grande précision sur des zircons, soit 1969,5± 1,0 et 2472,3± 0,5 Ma, indiquant que ces blocs erratiques de granite doivent provenir respectivement de l'orogène Taltson-Thelon (2,0-1,9 Ga) et du bloc Queen Maud (2,5-2,4 Ga) avoisinant. Ces terrains dominés par des granites sont situés à 600km franc sud et sud-est du secteur de collecte sur l'île Melville.

The West Amazon Craton consists of rocks of the Sunsás Orogen and the Rondônia-Juruena Province. The Sunsás Orogen comprises the western part of the Amazon Craton in South America and is best exposed in eastern Bolivia and western Rondônia and Mato Grosso states of Brazil. The integration of available maps and isotopic data together with new U–Pb and Sm–Nd analyses

The zircon intergrown with xenotime in pegmatite from Piława Górna (the Góry Sowie Block) was studied with respect to complex texture, and geochronological record in zircon. The zircon (ca. 2000 μm in size) demonstrates ca. 1100 μm wide, patchy zoned core and porous rim. The porosity also developed in the intergrown xenotime and mimics continuity along the zircon rim. The zircon core reflects homogeneous patches or patches with numerous up to several microns-sized inclusions of U, Th-rich phases and up to ca. 50 μm-sized inclusions of uraninite. The micro-Raman analyzes revealed increasing broadening of the ν 3 (SiO 4) band at ca. 1000 cm-1 that suggests increasing metamictization from little disordered outermost rim towards amorphous core. The EPMA results indicate high variations of UO 2 (0.28-0.95 wt.% in rim; 2.84-9.62 wt.% in core) and ThO 2 (<0.22 wt.% in rim; 0.33-0.84 wt.% in core) in the zircon. All these results indicate that the zircon domains were affected by a diffusion-reaction process in the core and coupled dissolution-reprecipitation process in the rim. The U-Pb LA-ICP-MS dating of zircon yielded discordant data with upper intercept age of 2086±35 Ma and lower intercept age of 373±18 Ma (n=65; MSWD=5.2). Concerning the youngest ages characterized by <10% discordance, three ages (0.1-2.9% discordant) in the rim yielded concordia age of 382±14 Ma (MSWD=1.4) interpreted as reflecting the timing of complete re-equilibration of the isotopic system due to fluid-mediated coupled dissolution-reprecipitation within the rim. The high radioactivity related to high U-Th concentration resulted in further Pb-loss in the core. The geochronological results strongly contrast with field observations. Although anatectic pegmatite dikes generally discordantly cut the tectonized migmatites and amphibolites related to ca. 380 Ma HT/MT-MP metamorphic event, the morphology, textural features and high-Th,U composition of the zircon suggest its origin related to crystallization in pegmatite at 2086±35 Ma, whereas 373±18 Ma age is related to re-equilibration of the whole system (i.e. Pb-loss and annealing).

The northern border of the Variscan Moroccan Meseta is characterized by the presence of Cambrian rocks affected by pre-Variscan deformation (the Sehoul block) thrust to the south onto Ordovician, Silurian and Devonian sediments and volcanics of the Bou Regreg corridor. A correct appraisal of the significance of this tectonic contact in the framework of the Variscan orogen needs new data on the age, geochemical features and field relationships of granitoids cropping out in the Sehoul block (Rabat granitoids) and the Bou Regreg corridor (Tiflet granitoids). Accordingly, we provide here with new geochemical data and accurate U-Pb geochronology of these granitoids, which lead us to conclude that: (i) the Rabat granitoids are Late Devonian in age (367 Ma); (ii) the Tiflet granitoids are Late Proterozoic in age (605-609 Ma) and display Andean-arc features, thus being genetically linked to the Cadomian/Pan-African subduction and forming part of the Variscan basement during the Palaeozoic; (iii) the previously accepted 430 Ma Rb-Sr age, common for the Rabat and Tiflet granitoids, which would imply a trivial Carboniferous displacement of the Sehoul thrust, must be rejected; and (iv) the Sehoul thrust might be a major Variscan boundary, perhaps hiding the Rheic Ocean suture.

... c Département de Géologie, Faculté des sciences Ben Msik-Sidi Othmane, BP 7955, Casablanca, Morocco. ... area showing, from north to south, the Sehoul block, the Sehoul thrust, the Bou Regreg corridor and the Sidi Bettache Carboniferous basin (from El Hassani (1991 ...

The Semail ophiolite in Oman was emplaced from NE to SW at least 200 km over the Arabian passive margin, probably over 450 km in total, during the late Cretaceous (95-70 Ma). The first phase of obduction involved NE-directed subduction of Triassic-Jurassic basalt at least 45-50 km beneath the ophiolite, whilst the crustal sequence was forming (U-Pb zircons from plagiogranites,

Although isotope-dilution, themal-ioniaztion mass spectrometry (ID-TIMS) is the most precise method available for U-Th-Pb geochronology of accessory minerals, the technique is traditionally not applied to samples youger than ca. 10 Ma due to low concentrations of radiogenic Pb. However, recent analytical advances allow the routine measurement of less than 5-10 picograms of radiogenic Pb, and the reduction of laboratory blanks

The Ribeira belt in southeastern Brazil was generated during the Brazilian Orogeny, the South American equivalent of the Pan-African Orogeny (∼ 700-450 Ma). The central sector of the belt is characterised by a series of northwest-vergent crustal-scale thrust stacks ...

Isotopic dating has established that the middle Paleozoic turbidites of the Taean Formation on Anmyeondo in the West Sea were affected by metamorphism the Late Triassic. We obtained a 206Pb/238U lower intercept age of 232.5 ± 3.0 Ma (95 % confidence, MSWD = 1.2) of metamorphic titanite from a calc-silicate rock by Multi Collector Sensitive High-Resolution Secondary Ion Mass Spectrometry and 40Ar/39Ar laser probe pseudo-plateau ages of 230.7 ± 1.0 Ma and 228.8 ± 1.0 Ma (1σ) for two single grains of metamorphic muscovite. We estimate that the metamorphic temperature was below the closure temperatures of titanite and muscovite. Consequently, the concordant U-Pb and 40Ar/39Ar mineral ages are not cooling ages but demonstrate that the metamorphism of the Taean Formation on the island occurred in the earliest Late Triassic (Carnian), using the most recent international chronostratigraphic chart. The dated muscovite occurs as undeformed grains that cross-cut the main tectono-metamorphic fa...

Abstract Continuous magmatic activity occurred in the western Chinese Tianshan, Central Asia, from the Car-boniferous to the Permian, ie before and after the Late Carboniferous amalgamation of Junggar and the Yili Blocks. Zircon U–Pb LA-ICPMS and Ar–Ar data reveal a ...