Jean-Michel Bertrand

Petrology and U/Pb geochronology of the Telohat migmatites, Aleksod, Central Hoggar, Algeria

The Aleksod region is composed of metasedimentary rocks and large areas of biotite and hornblende-bearing migmatites. Anatexis associated with the main deformation stages, occurred under high pressure and temperature conditions estimated at 13±2 Kbar and 750±50° C. The bulk mineralogical composition of the Telohat migmatites shows that their protolith was granodioritic. Internal structures of zircons and U-Pb data suggest a polyphased evolution, with a 2131±12 Ma age for the protolith and a 609±17 Ma age for the Pan-African tectono-metamorphic evolution, thus precluding any Kibaran event in the Aleksod area. Leucosomes are richer in Sr and display lower Rb, Zr, Nb, Y, Th, U and REE contents than melanosomes wherein accessory phases are stored. Eu contents are also lower in the leucosomes but in lesser proportion than the other rare earths, leading to a significant positive anomaly. Petrogenetic modelling accounting for accessory mineral phases clearly shows that the trace element contents of leucosomes and melanosomes follow a distribution law consistent neither with equilibrium nor fractional melting. Their trace element patterns are best explained by the model of disequilibrium melting, with mixing of a few residual phases. The present results and previous Sr isotopic data as well raise the question of disequilibrium melting in anatexis of crustal material

Géochronologie U-Pb sur zircon de granitoïdes éburnéens et panafricains dans les boutonnières protérozoïques d'Igherm, du Kerdous et du Bas Drâa (Anti-Atlas occidental, Maroc)

Abstract U-Pb ages obtained from zircons of granitoids from the Proterozoic western Anti-Atlas confirm the existence of Palaeoproterozoic magmatism in the inliers of Igherm (Ait Makhlouf granite: 2 050±6 Ma) and Bas Drâa (Sidi Said granite: 1 987±20 Ma) and neoproterozoic magmatism in the inliers of Bas Drâa (Taourgha granite: 575±4 Ma) and Kerdous (Tarcouate granodiorite: 583±11 Ma, Tarcouate gabbro-diorite: 560±2 Ma). The emplacement of these granitoids is therefore polycyclic: Eburnian and Panafrican.

Géochronologie U-Pb sur zircon et monazite du massif composite de granite à deux micas hercynien de Cabeceiras de Basto (Nord-Portugal)

Abstract The zircon and monazite U-Pb minimum age of the Hercynian peraluminous two-mica composite granite pluton of Cabeceiras de Basto (Northern Portugal) is 311±1 Ma (2 σ). This age, well constrained by a concordant monazite of 311±4 Ma (2 σ), is in good agreement with the age suggested for the synkynematic, syn-D3, Iberian granites of the Hercynian orogeny (315–305; Pinto et al., 1987). The ( 87 Sr/ 86 sr), and ɛ Nd isotopic ratios, calculated for 311 Ma, suggest that the three granite units which constitute the massif may derive from an heterogeneous crustal source dated (upper intercept of the discordia) around 1 200 Ma.