Hf–Zr anomalies in clinopyroxene from mantle xenoliths from France and Poland: implications for Lu–Hf dating of spinel peridotite lithospheric mantle

Clinopyroxenes in some fresh anhydrous spinel peridotite mantle xenoliths from the northern Massif Central (France) and Lower Silesia (Poland), analysed for a range of incompatible trace elements by laser ablation inductively coupled plasma mass spectrometry, show unusually strong negative anomalies in Hf and Zr relative to adjacent elements Sm and Nd, on primitive mantle-normalised diagrams. Similar Zr–Hf anomalies have only rarely been reported from clinopyroxene in spinel peridotite mantle xenoliths worldwide, and most are not as strong as the examples reported here. Low Hf contents give rise to a wide range of Lu/Hf ratios, which over geological time would result in highly radiogenic εHf values, decoupling them from εNd ratios. The high 176Lu/177Hf could in theory produce an isochronous relationship with 176Hf/177Hf over time; an errorchron is shown by clinopyroxene from mantle xenoliths from the northern Massif Central. However, in a review of the literature, we show that most mantle spinel peridotites do not show such high Lu/Hf ratios in their constituent clinopyroxenes, because they lack the distinctive Zr–Hf anomaly, and this limits the usefulness of the application of the Lu–Hf system of dating to garnet-free mantle rocks. Nevertheless, some mantle xenoliths from Poland or the Czech Republic may be amenable to Hf-isotope dating in the future

Notice de la Carte géologique de France au 1/50000 : feuille de Fontenay le Comte (Vendée)

Service de la Carte Géologique du BRG

Notice de la Carte géologique de France au 1/50000 : feuille de Fontenay le Comte (Vendée)

carte géologique - publisher: Service de la Carte Géologique du BRGMNotice de la Carte géologique de France au 1/50000 : feuille de Fontenay le Comte (Vendée

Contribution of the Corbu Mylonitic Zone to the emplacement and cooling of the Carboniferous Cherbelezu granitoid (Almaj Mountains, Romania)

International audienc

Asymmetrical to symmetrical magnetic fabric of dikes : Paleo-flow orientations and Paleo-stresses recorded on feeder-bodies from the Motru Dike Swarm (romania)

International audienc

Flowage differentiation in an andesitic dyke of th Motru dyke swarm (southern Carpathians, Romania) : Evidence from AMS, CDS and geochemical investigations

Poste

Asymmetrical to symmetrical magnetic fabric of dikes: Paleo-flow orientations and Paleo-stresses recorded on feeder-bodies from the Motru Dike Swarm (Romania)

The fabric in a dike is representative of the magmatic flow, considered as Newtonian. The anisotropy of magnetic susceptibility of the rocks gives a good representation of the shape-preferred orientation which, in turn, is a marker of the magmatic flow. Generally, a symmetrical pattern of the fabric across the dike is in agreement with a flow of magma within a channel: the flow direction is then reliable with this imbrication. An asymmetrical fabric is dependent on the flow and displacement of the wall. We present the case of both symmetrical and asymmetrical dike fabrics recording different emplacements. From a Pan-African calc–alkaline dike swarm (of basaltic–andesitic– dacitic–rhyolitic composition) of the Alpine Danubian window from South Carpathians (Romania), two populations of dikes have been described: thick (1–30 m) N–S-trending dikes and thin (,1 m) E–W dikes. The first display asymmetrical fabric and record the regional sinistral movement of the walls. In contrast, the thin dikes are symmetrical and frequently display an arteritic morphology that limits the dike length, with no cartographic extension. We propose to relate the two types of dikes to the same regional stress field in a continuum of emplacement during a regional brittle event