Datation U–Pb des deux faciès du granite de Soultz (Fossé rhénan, France)

International audiencePrésenté par Zdenek Johan Résumé Les sondages profonds réalisés dans le socle du fossé rhénan à Soultz-sous-Forêts ont mis en évidence un monzogranite très largement représenté. Le sondage GPK-2 atteignant 5090 m montre, en outre, un faciès plus leucocrate à grain fin et deux micas, présent localement à partir de 4860 m et de manière continue entre 5047 et 5090 m de profondeur. Grâce aux éléments en traces, et plus particulièrement grâce aux terres rares, on montre que le faciès leucocrate à grain fin serait une expression différenciée du magma potassique ayant conduit au pluton monzogranitique. Ce modèle est en accord avec les données isotopiques Sr-Nd, même si une légère contribution du socle préexistant doit aussi être considérée. L'utilisation de la méthode U-Pb, après dissolution, sur zircons du monzogranite (EPS-1) a permis d'en préciser l'âge de mise en place à 334,0 + 3,8/−3,5 Ma (2 σ). Les datations ponctuelles (SHRIMP II) sur les zircons du faciès à grain fin ont montré que cette mise en place se fait dans un socle très hétérogène en âge : depuis le Protérozoïque inférieur jusqu'au Silurien. L'âge estimé de cristallisation de ce dernier granite est de 327 ± 7 Ma (2 σ), c'est-à-dire légèrement postérieur, voire sub-contemporain, à la mise en place du faciès monzogranitique commun, en accord avec les contraintes structurales

Geochronological evolution of HP metamorphic rocks of the Adula nappe, Central Alps, in pre-Alpine and Alpine subduction cycles

<p>SHRIMP dating of zircon from eclogites, their country rock gneisses and quartz veins from the northern and middle Adula nappe, Central Alps, accompanied by cathodoluminescence imaging and REE geochemistry of these dated zircon crystals revealed that the Adula nappe has been affected by two pre-Alpine high-pressure (HP) metamorphic events (northern: <em>c</em>. 330–340 Ma; middle: <em>c</em>. 370 Ma). In the northern Adula nappe, loss of radiogenic Pb of the <em>c</em>. 330–340 Ma zircon domains in combination with previous geochronological data argue for an overprinting Alpine HP event. In the middle Adula nappe, metamorphic zircon domains record in addition a 33–32 Ma event, interpreted to reflect HT metamorphism, already identified in the southern part of the nappe, where such SHRIMP ages have been related to HT (Lepontine) metamorphism. The Alpine HP event indicated by previous SHRIMP data in the southern Adula nappe at <em>c</em>. 35 Ma is not recorded in the zircons of the rocks dated herein. The absence in this dataset is probably because either no HP (<em>c</em>. 35 Ma) zircon domains were formed or such domains were completely reset during the 33–32 Ma HT event. Protolith ages of orthogneisses and eclogites are 470–457 Ma and 595–587 Ma, respectively, and maximum sedimentation ages of paragneisses 500–460 Ma. The lack of Hercynian protolith zircon ages implies pre-Hercynian formation for the precursors of the dated HP rocks. Our data suggest that the Adula nappe contains various Hercynian basement slivers that underwent subduction to HP conditions at <em>c</em>. 330–340 Ma and <em>c</em>. 370 Ma prior to the Alpine orogeny, similar to many other parts of the Hercynian basement in Central and Southern Europe. </p

A mid-Cretaceous age for the Palmer Land event, Antarctic Peninsula: implications for terrane accretion timing and Gondwana palaeolatitudes

New structural and age data suggest that West Gondwana may have been at lower palaeolatitudes than previously interpreted from Albian sequences in Gondwana marginal suspect terranes. The Palmer Land event, which juxtaposed Mesozoic terranes on the Gondwana margin, deformed granitoids in the southern Antarctic Peninsula. U–Pb SHRIMP dating of zircons from a microgranite dyke yields a crystallization age of 106.9± 1.1 Ma. This result and re-interpretation of the structural position of another granite pluton date the Palmer Land event, and probable terrane collision, as late Early Cretaceous, and not latest Jurassic as formerly interpreted

Provenance of late Palaeozoic metasediments of the SW South American Gondwana margin: a combined U–Pb and Hf-isotope study of single detrital zircons

<p>Combined U–Pb and Lu–Hf isotope measurements of single detrital zircon grains in Carboniferous metasediments from Patagonia delineate the source areas of the sediments. The detritus, represented by four metasandstone samples, was deposited prior to onset of subduction in Late Carboniferous time along the south Patagonian proto-Pacific Gondwana margin. A broad series of detrital zircon age peaks (0.35–0.7 Ga, 0.9–1.5 Ga) and a large spread (0.3–3.5 Ga) in the age spectra require numerous sources. A fifth metasediment was deposited after the onset of subduction. This syncollisional sample shows two distinct U–Pb age peaks at <em>c</em>. 290 Ma and 305 Ma. This points to a few sources only (Patagonia, West Antarctica). Initial Hf-isotope compositions of selected U–Pb dated zircons from the Carboniferous metasediments reveal zircon protoliths originating from both recycled crust and juvenile sources (εHf_{(T=0.4–3.5Ga)}=−14 to +12). A comparison with crustal compositions of possible source areas indicates that the detritus mainly originated from the interior of Gondwana (Extra-Andean Patagonia, the Argentine Sierra de la Ventana, southernmost Africa, East Antarctica), as well as northern Chile and northwestern Argentina. The sediment transportation paths are consistent with an autochthonous palaeogeographical position of Patagonia with respect to Gondwana in Carboniferous time. </p