SEDIMENTARY FRACTIONATIONS BETWEEN AL, TI, AND ZR AND THE GENESIS OF STRONGLY PERALUMINOUS GRANITES

Sedimentary sorting produces alternating shales and sandstones with co mplementary chemistries. The preferential partitioning of quartz and z ircon in the coarse-grained fraction of the sediments results in a str ong fractionation between SiO2, Zr, and other components (Al2O3, TiO2 ...). As a consequence, shales may be defined chemically as having hig her Al2O3/SiO2 and TiO2/Zr ratios than their source rocks; sandstones have the opposite characteristics and vary in composition according to the maturity of their source and the intensity of sorting processes. Most sedimentary suites display a good correlation between Al2O3/SiO2 and TiO2/Zr. This result is used to evaluate an average bulk compositi on for recent clastic materials and to model their composition on a te rnary Al-Ti-Zr diagram. Because the Al2O3/TiO2 ratio of igneous rocks is highly sensitive to magmatic differentiation, igneous and sedimenta ry trends crosscut in the Al-Ti-Zr diagram. In the case of strongly pe raluminous granites (SPG), genetic models based on crustal melting and restite separation are in conflict with the observed Al-Ti-Zr relatio ns because: (1) most SPG suites are displaced toward low TiO2/Zr ratio s when compared to their potential metasedimentary sources, and (2) ma ny SPG are too rich in Zr and Ti to be produced from calc-alkaline fel sic sources in the restite model. These features suggest that the bulk Zr and Ti budget of SPG is dominated by (relatively hot) melt contrib utions, not by recycled solid materials.