MAJOR AND TRACE-ELEMENT GEOCHEMISTRY AND ISOTOPE (SR, ND, PB, O) SYSTEMATICS OF AN ARCHEAN BASEMENT INVOLVED IN A 2.0 GA VERY HIGH-TEMPERATURE (1000 DEGREES-C) METAMORPHIC EVENT - IN OUZZAL MASSIF, HOGGAR, ALGERIA

The In Ouzzal granulitic massif is composed mainly of various meta-ign eous rocks which, in spite of Rb, U, Th, Cs and some K and Sr mobility , can be dated and generally classified according to their chemical co mposition as follows. Basic and ultrabasic granulites interlayered wit h the metasediments correspond to (1) ultrabasic cumulates from disloc ated tholeiitic bodies, (2) ancient komatiitic to high-Mg tholeiitic b asalts similar to the suites found in Archaean greenstone belts and (3 ) calcalkaline protoliths of high-K andesitic composition. No geochron ological constraints are available apart from the depositional age of some associated sediments which is younger than 2.70 Ga detrital zirco ns, and the Nd model age of the andesitic granulites of c. 3.4 Ga. In spite of the high-grade metamorphism, the acidic magmatic precursors o f the charnockites can be divided in three groups. (1) The most juveni le acid orthogneisses are trondhjemitic or tonalitic in composition, b eing similar to the TTG suites which are classically considered to be formed by partial melting of mantle-derived protoliths. The 3.3-3.2 Ga T-DM indicates a possible age of separation from the mantle reservoir while the plutons may have been emplaced between 3.3 and 2.7 Ga (U-Pb zircon & Nd ages). (2) A group of alkaline granitic gneisses, similar in composition to rift-relaled-granites, were emplaced at 2650 +/- 10 Ma (U-Pb & Rb-Sr ages) in a thick continental crust. (3) Calcalkaline granodioritic and monzogranitic suites derived from the partial melti ng of continental precursors (3.5-3.3 Ga), in lower to middle levels o f the continental crust. They were emplaced close to 2.5 Ga during cru stal thickening. The very high-temperature metamorphism occurred at 20 02 +/- 7 Ma from the age of synfoliation intrusions and was probably r elated to major overthrusting. Retrogressive metamorphism is dated at 1.95 Ga from garnet-Nd ages. In spite of the very high-temperature con ditions, partial melting during granulite facies metamorphism may be r estricted to scarce cordierite-bearing monzogranitic gneisses. The 2.0 Ga VHT metamorphism could be related to overthrusting, extensional or underplating processes.