W.L. BROWN Directeur de Recherche 2 Co-animateur M. PICHAVANT Chargé de Recherche 1 Co-animateur P. BARBEY Maître de Conférence, Univ. Nancy 1
J.M. STUSSI Chargé de Recherche 1 A. ROUILLIER Ingénieur d'Etudes 2
Chercheurs associés et post-doc
ASHANO E.C. University of Jos - Nigeria DALL'AGNOL R. Université de Belem - Brésil
Thésards: BROUAND M. GOGHROD H. MORTAJI A. MOTTET B. OHNENSTETTER D. PAILLAT 0. SCAILLET B. DEA JABBORI J. Stagiaires FARGES F. Paris VI
Liste et Résumés
des Publications
In: Granulites and crustal differentiation,D Vielzeuf (Ed.), P Vidal (Ed.), Klüwer Académie Publ., NATO - ASI series, 1990, 111-132.
THE GRANULITE BELT OF LAPLAND
P. BARBEY' and M. RAITH2
lLaboratoire de Pétrologie, Université de Nancy I, BP 239, 54506 Vandoeuvre les Nancy cedex , France and
CRPG-CNRS, BP 20,
54501 Vandoeuvre les Nancy Cedex, France
2Mineralogisch-Petrologisches Institut, Universitàt Bonn, Poppelsdorfer Schloss, D-5300 Bonn, FRG
ABSTRACT. The granulite belt of Lapland is an intensely deformed and metamorphosed upthrust séquence of early Proterozoic pelitic to psammitic sédiments and predominantly synmetamorphic basic to mter- mediate igneous rocks mainly of calc-alkaline character. Compressional déformation and metamorphism culminated 1.95 Ga ago in a synkinemaùc granulite-facies event «850'C, 8kb, XCO2�0.8). The hot granulite belt together with the Inari craton were thrust westward over the cooler, amphibolite-grade, Tana belt and South Lapland craton. Décompression and cooling resulted in widespread anatexis of the metasediments and were accompanied by gravity tectonics. The existence of regional P-T-Xflaid gradients suggests pro- gression of rétrograde équilibration from the lower part of the belt (830'C, 7.2 kb, XC02 - 0.7) toward its intensely migmatized upper part (760"C, 6.2kb, Xco2- 0.7). The granulites of Lapland are likely to be the result of a full Wilson cycle (protoliths depositcd during plate divergence, granulite-facies metamorphism and related nappe tectonics developed through subduction and collision).
Contributions to Mineralogy and Petrology, 1989, 101, 207-219.
Petrology and U/Pb geochronology
of the Telohat migmatites, Aleksod, Central Hoggar, Algeria *
Pierre Barbey ' 1. 2, Jean-Michel Bertrand Serge Angoua ' 2, and Danielle Dautel
1 Laboratoire de Pétrologie. Université de Nancy I, B.P.239, F-54506 Vandoeuvre-lès-Nancy cedex. France 2 Centre de Recherches Pétrographiques et Géochimiques, B P. 20. F-54501 Vandoeuvre-lès-Nancy cedex. France
Abstract. The Aleksod région is composed of metasediment- âge of the migmatitic unit through zircon U/Pbchronology, ary rocks and large areas of biotite and hornblende-bearing as récent data preclude a Kibaran imprint in the neighbour- migmatites. Anatexis associated with the main déformation ing régions (Bertrand étal. 1986), and (2) détermination stages, occurred under high pressure and température con- of the petrogenesis of the migmatites by use of combined ditions estimated at 13 + Kbar and 750 + 50° C. The bulk petrographical observations and major and trace élément mineralogical composition of the Telohat migmatites shows data.
that their protolith was granodioritic. Internai 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 âge for the Pan-African tectono-metamorphic évolution, 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 earth's, leading to a significant positive anomaly. Pe- trogenetic modelling accounting for accessory minerai phases clearly shows that the trace élément contents of leu- cosomes and melanosomes follow a distribution law consis- tent 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 présent results and previous Sr isotopic data as well raise the question of disequilibrium melting in ana- texis of crustal material
Journal of Petrology, 1990, 31, 2, 401-427.
High-Pressure Dehydration Melting of Metapelites: Evidence from the Migmatites of Yaoundé
(Cameroon)*
by P BARBEY,' ' J. MACAUDIERE2 AND J P. NZENTI4 1
Laboratoire de Pétrologie, Université de Nancy I, BP 239, 54506 Vandoeuvre-les-Nancy Cedex, France
2 Centre de Recherches Pétrographiques et Géochimiques, BP 20, 54501 Vandoeuvre-les- Nancy Cedex, France
3
Ecole Nationale Supérieure de Géologie, BP 452, 54001 Nancy Cedex, France *
Centre de Recherche Géologique et Minière, BP 333, Garoua, Cameroun (Received 26 January 1989; revised typescript acceptée 21 August 1989)
ABSTRACT
The migmatites of Yaoundé consist essentially of anatectic metapelitic kyanite-garnet gneisses characterized by granulite-facies minerai assemblages. Several types of migmatitic rocks hâve been recognized: (1) leucosomes associated with garnet-rich melanosomes, conformable with the régional metamorphic layering; some leucosomes are granitic in composition whereas some others are granodioritic and characterized by low K and Rb and by the lack of HREE fractionation; (2) quartzo- feldspathic differentiations without the related melanosomes, occurring as veins conformable with or cross-cutting the regional metamorphic layering or along shear-zones, and corresponding mineralogi- cally to granitic or quartz-rich veins; (3) garnet-rocks mainly composed of garnet with abundant accessories, occurring as intrusive bodies within the migmatitic séries.
Structural and pétrographie data suggest that the migmatites are not derived from the surrounding granulite-facies gneisses but that both types of rock result from a single dehydration melting event. The formation of migmatites or gneisses, interpreted in terms either of absence of melt extraction or of shear-induced melt ségrégation, is ascribed to variations in strain distribution within the metamorphic pile.
The chemical characteristics of the rocks and petrogenetic modelling suggest that the migmatites of Yaoundé arose from the superimposition of the following events: (!) subsolidus differentiation of biotite-gneisses; (2) dehydration melting of biotite-gneisses at températures around 800 °C (P= 10-12 kb), leading to low amounts of melt (F �0,2), which was either tectonically segregated (migmatites) or not (granulite-facies gneisses); (3) injection of anatectic material comprising both partial melts and garnet-rich residues, corresponding to high melt fractions (F �0-5) and probably formed at higher températures (850°C) and at deeper structural levels. The REE signature of equilibrium partial melts (9 3 � CeN/Ybm �78; 1.2 � Yb, � 5-4) indicates that granitic magmas cannot be derived from dehydration melting of biotite-bearing metapelites only. Several other possibilities are discussed.
Journal of Volcanology and Geothermal Research, 1990, 44, 143-161.