TONALITE-TRONDHJEMITE-GRANITE GENESIS BY PARTIAL MELTING OF NEWLY UNDERPLATED BASALTIC CRUST - AN EXAMPLE FROM THE NEOPROTEROZOIC BIRBIR MAGMATIC ARC, WESTERN ETHIOPIA

Partial melting of young (i.e., hot) subducted oceanic crust and of ne wly underplated basaltic crust are regarded as alternative processes c apable of generating high-Al tonalite-trondhjemite-granodiorite suites . High Sr/Y ratio (>40) is the most distinctive trace element characte ristic of such rocks and has been modelled as a product of variable de grees of partial melting of subducted oceanic crust transformed into e clogite or garnet amphibolite. By contrast, other are-related rocks wi th low Sr/Y ratios (<40) are generally thought to be generated at low pressure either by fractionation of mantle-derived magmas or by partia l melting of basaltic rocks. Tonalite-trondhjemite-granite (TTG) rock samples from two bodies with ages of >800 Ma intruding the Neoproteroz oic Birbir magmatic are in western Ethiopia, however, form a curved ar ray on the Sr/Y versus Y plot, which extends from high to low Sr/Y rat ios. In these rocks, the abundance of Sr varies inversely with those o f other incompatible elements, including Rb, Ba and K, but is positive ly correlated with CaO and Al2O3. These compositional variations provi de strong evidence for the formation of the Birbir TTG in a granulitic residue. Experimental studies on basalt melting indicate that the Bir bir TTG may have been generated by water-undersaturated and dehydratio n partial melting of garnet amphibolite in the pressure and temperatur e ranges of 8-12 kbar and 800-1000 degrees C, respectively. The depth range of partial melting suggests that the source of the Birbir TTG wa s newly underplated basaltic crust.