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ZusammenfassungFiir eine radiometrische Gesamtgesteinsanalyse yon etwa vierzig Proben des Rieserferner-Massivs, hat man sich der Rb/Sr Methode bedient.Die Proben wurden in ftinf Gruppen zusammengestellt. Zwischen diesen Oligoz~insehmelzen und der Tauernmetamorphose wird eine genetisehe Bindung vorgeschlagen. AbstractThe Rb/Sr whole rock method was used to carry out radiometric analyses on about forty samples from the Vedrette di Ries massif. The samples were arranged in five sample groups. The age of this massif was found to be Middle Oligocene, and the best-defined isochron (80 -8 m. y.) was obtained with a group of nine samples from the Upper Anterselva Valley. The values of the initial Sr isotopic ratio range between 0.709 and 0.727. This fact suggests that these melts were the result of the anatexis of different crustal materials, with some contributions from components with low (87Sr/86Sr)i values. A genetic link is proposed between these Oligocene melts and the "Tauern metamorphism". R6sum4Pour analyser radiom6triquement environ quarante 6ehantillons du massif de Riesenferner (Vedrette di Ries) on a employ6 la m4thode Rb/Sr sur roche totale. Les 6ehantillons ont 6t6 r6unis en cinq groupes. I1 en r6sulte que I'gtge de ce massif remonte A l'Oligoc6ne moyen. L'isochrone la meilleure (80 ---8 m.y.) a 6t6 obtenue avee un groupe de neuf 6ehantillons provenant de la haute vall6e d'Antholz (Anterselva). Les valeurs des rapports initiaux (s7Sr/S6Sr) se plaeent entre 0,709 et 0,727: Cela sugg6re que ees liquides ont 6t6 le r6sultat d'une anatexie portant sur diff6rents mat6riaux de la erofite, avee contribution de eomposants ~ basses valeurs (87Sr/S6Sr). On propose un lien g6n6tique entre ees magmas de l'Oligoe6ne et le m6tamorphisme des >Tauerm. Ilpo6bi pacnpe~enH~H na n~rb rpyan. ~aHHMe aHaJiHaa yKa3blBalOT na TO, qTO BoapacT ~larlnoro MacclIBa --cpe~IHl4fl oJll4roIIen.Hanayqmyro n3oxpo8y (30 -T-3 MnaanOnOB ~eT) noayqn~H Ha rpynrte n39-TrI rtpo6, OTo6pen-HSLX Ha BepxHefi.3Haqeltlt~ HHrIKHaJIBHOrO COOTHOIIIeHH~I H3OTOHOB SI'87/S1 "8 b Ko~e6YIIOTCS OT 0,709 ~O 0,727. ~TO pa3pemaeT Ilpe~noJiaraTb, qTO ~aHHble pacn21aBbl ~IB!/IIl!/ICb pe3yJIt, TaTOM aHaTeKcHca pa3anqn~tx nopo~ KOpbl I4 KOMHOHeHTOB C 6oaee HH3KHMtt 3HaqeHH~tMI4 COOTHOUleHH~I HBOTOIIB cTponana.Hpe~noaaramT reseTn,~ecKy~o CBa3S Me~,~Iy ~TnMn pacnaaBaMl~ BO BpeM~ oanro~eHa n MeTaMopqbn3MOM B pafione Tayep~a.

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Tectonics, volcanism and petrology of the northern Afar (or Danakil) Depression have been investigated during two recent successive expeditions. The tectonics is characterized by numerous NNW trending normal faults. Two important active volcanic chains, namely Erta’Ale Range and Alayta Range, are disposed in en échelon structure. They both display differentiated lavas of subcrustal origin, ranging from abundant basalts to scarce soda-rhyolites with interm ediate products, such as dark trachytes as well as oversaturated ones. 87 Sr/ 86 Sr isotopic ratios are very low and approximately constant through the whole series. This fact, and other petrological considerations, suggest that this series has not been contaminated by any crustal sialic material. In addition to these two well-defined volcanic ranges, the Depression is covered by extensive basaltic fields and significant quantities of rhyolites. The variegated petrography of these silicic lavas and ignimbrites, as well as the 87 Sr/ 85 Sr ratios, on the contrary strongly suggest that these rocks were formed by interaction between the subcrustal m agm a and the sialic crust. It can be tentatively concluded that the sialic crust is missing beneath the northern p art of the Danakil Depression only below the two en echelon elongated volcanic ranges m entioned above.