Contrib Mineral Petrol (1988) 98:277-292 Contributions to Mineralogy and Petrology 9 Springer-Verlag1988 New carbonatite complexes in the Archaean In'Ouzzal nucleus (Ahaggar, Algeria): mineralogical and geochemical data Khadidja Ouzegane 1, Serge Foureade 2, Jean-Robert Kienast 3, and Marc Javoy 3 1 Institut des Sciences de la Terre U.S.T.H.B., B.P. n~ 9, Dar el Beida, Algeria 2 Centre Armoricain d'Etude Structurale des Socles (CNRS, LP 4661), Universit~ de Rennes t, Campus de Beaulieu, F-35042 Rennes C6dex, France 3 UER Sciences Physiques de la Terre (and CNRS UA 196), 2 Place Jussieu, F-7525t Paris C6dex 05, France Abstract. Several massifs of very old carbonatites have been discovered in the Archaean granulitic block of In'Ouzzal (Western Ahaggar, Algeria). These carbonatites are original since they are associated with Silica - saturated syenitic magmatism and present, in the late stages of evolution, a very uncommon mineralogy, with silicate minerals, especially wollastonite, allanite, and quartz. The mineralo- gy, C and O isotopes and R.E.E. distributions indicate that the late stages of crystallization occurred under high SiO2 activities, and produced the uncommon mineralogy and ex- tremely high R.E.E. concentrations in phosphate minerals apatite and britholite. Interaction with continental crust is a possible mechanism to explain the original features of these carbonatite complexes. Introduction Carbonatite magmatism is volumetrically insignificant on the earth but these rocks are extremely important for several reasons. With only a few exceptions (e.g. Capo Verde, Ca- nary islands) they are restricted to continental cratonic ar- eas subjected to rifting or crustal swelling (Le Bas, 1977). They are generally associated with undersaturated mag- matic provinces. They are emplaced throughout a long peri- od of time since c.a. 2.6 B.y. (Bell et al. 1982; Larsen et al. 1983) up to recent times. Most authors agree that the source of carbonatites is mantle although they may represent end- products of very complex magmatic processes. Because these rocks generally possess extremely high Sr and Nd abundances, alteration of initial Sr and Nd signatures dur- ing evolution and emplacement within the crust is not ex- pected. Therefore, these rocks appear to be excellent clues for tracing the evolution and chemical characteristics of subcontinental mantle. The In'Ouzzal carbonatites have never been recognized in the past and were described as brecciated metamorphic marbles (Giraud 1964; Le Fur 1966). Their emplacement into Archaean terrains of granulite facies grade and prelimi- nary dating show that they constitute one of the oldest carbonatites known. The present paper is devoted to a de- scription of these carbonatite complexes with emphasis on Offprint requests to : S. Fourcade some peculiar characteristics which make them a unique example known so far among this class of rocks. Prelimi- nary isotope data are also given in order to elucidate the internal evolution of the magmatic system but detailed Nd, Sr, Pb isotope study focusing on the source of such a mag- matism will be presented in a subsequent paper (J. Bernard- Griffiths et al. ; in preparation). I Geological framework The Ihouhaouene syenite and carbonatite complexes are located at longitude 3~ and latitute 23~ in the Archaean In'Ouzzal nucleus (north-western Ahaggar). In this section we will describe the successive events which can be inferred from field relationships and available radio- metric studies. L 1 The In'Ouzzal Archaean basement Most of the In'Ouzzal outcrops are composed of Archaean granulites (2.9 3.3 Ga) behaving as a stable craton since the ---2.0 Ga Eburnean event (Ferrara and Gravelle 1966; Allbgre and Caby 1972; Lancelot et al. 1976; Ben Othman et al. 1984). The rocks are charnockites, norites, leptynites which are associated with a metasedimentary unit made of marbles, magnetite-bearing quartzites (metamorphosed B.I.F.) and sapphirine-bearing A1- Mg granulites (Kienast and Ouzegane 1987). Marbles are intercalated as lensoid bodies in the other granulitic rocks. Their occurrences are generally small-sized but locally they can reach a length of several hundreds of meters and a width of tens of meters. Marbles and quartzites are systematically associated in large scale folds which are oriented broadly E-W but turn to the NE-SW direction South of the Oued Tirahart, and to the NW-SE direction North of the Oued Tirahart, in the vicinity of the eastern faulted contact between the In Ouzzal terrains and the central unit of the Pharusian (Pana- frican) orogenic belt (Fig. 1). Marbles are sometimes ob- served in the vicinity of carbonatite complexes (distance < 1 km) but they can easily be distinguished from carbona- tites by their mineralogy and also by their field occurrence: on the outcrop scale, marbles are characterized by a well- marked banded structure outlined by the alternation of layers rich in olivine+ spinel and carbonates, respectively (2-30 cm). Apart from this layering, dark nodules of oliv-