THE PETROGENESIS OF THE EDOUGH AMPHILBOLITES, ANNABA, NE ALGERIA - 2 UNRELATED BASIC MAGMAS AND THE LHERZOLITE-HARZBURGITE RESIDUE OF A POSSIBLE MAGMA SOURCE

The conditions of metamorphism and petrogenesis of Kef Lakhal amphibol ites and associated amphibolite and olivine-rich lenses are discussed. The Kef Lakhal amphibolites contain plagioclase ranging from An(85) a nd An(12) and Ca-poor normally zoned garnet (< 9 wt% CaO) and were las t metamorphosed to upper amphibolite facies. The amphibolite lenses in variably contain calcic plagioclase (An(97 - 78)) and Ca-rich (up to 1 4 wt% CaO) doubly zoned garnets. Based on garnet zoning, it is found t hat the lenses experienced the three metamorphisms recorded elsewhere in the massif whereas the main amphibolites suffered only the last two metamorphisms, the last of which reached conditions of about T = 700 +/- 80 degrees C and P = 8 - 10 Kb. This agrees with two unrelated ign eous suites and is supported by the chemistry of the rocks and quantit ative modelling. The Kef Lakhal amphibolites were formed by olivine-do minated fractional crystallisation processes from melts derived by par tial melting of a homogeneous source and preserve MORE affinities. The amphibolite lenses were also evolved by fractional processes dominate d by olivine but from a less homogeneous source and have island are ch aracteristics. The associated olivine-rich lenses are Iherzolite-harzb urgite mantle residua which suffered up to 35% melting. Bearing in min d the lack of isotopic and structural information, the data presented in this study indicate that fractional crystallisation of liquids prod uced through non model melting of these residua does not reproduce the composition of either of the amphibolites. Fractionation of melts sim ilar to those derived from the Ronda peridotites yield compositions si milar to the Edough amphibolites.