GEOCHEMICAL DISCRIMINATION OF DIFFERENTIA TED MAGMATIC ROCKS ATTESTING FOR THE VARIABLE ORIGIN AND TECTONIC SETTING OF CALC-ALKALINE MAGMAS

We show that a Zr vs (Nb/Zr)N diagram allows the differentiated (i.e. SiO2 greater-than-or-equal-to 55%) lavas and plutonic rocks from: 1) w ithin-plate alkaline provinces (continental rifts and oceanic islands) , 2) continent/continent collision zones and 3) subduction zones to be discriminated. The locations of the different groups are consistent w ith the following origins: 1) fractional crystallization of an alkalin e basaltic magma, 2) partial melting of the continental crust, 3) frac tional crystallization of a basaltic to andesitic ''arc'' magma. Two d ifficulties limit the use of the Zr vs (Nb/Zr)N diagram for the geotec tonic identification of paleomagmatic rocks: the differentiated rocks emplaced in continental tholeiitic provinces plot in the field of lava s and plutons from continent/continent collision zones and the magmati sm associated with arc/continent collision zones spreads over the enti re diagram.