THE MG ISOTOPIC SIGNATURE OF GRANITES AND THE ROLE OF WEATHERING IN CONTINENT FORMATION

Continental crust is too Si-rich and Mg-poor to have derived directly from mantle melting, which generates basaltic rather than felsic magmas. Converting basalt to more felsic compositions requires additional processes involving Mg loss, which are believed to be dominated by internal igneous differentiation. However, internal igneous differentiation alone may not be able to generate granites, the most silicic endmember making up the upper continental crust. Here we show that Mg contents of granites from the Peninsular Ranges Batholith in southern California are isotopically heavy compared to basaltic magmas and correlate positively with Si content and O, Sr and Pb isotopes and negatively with Mg content. These geochemical systematics indicate that the granites form in part from melting of ancient pre-existing crust that must have been substantially modified earlier in its history by low temperature processes, such as chemical weathering, resulting in leaching of Mg and fractionation of Mg isotopes. These observations suggest that in addition to internal igneous differentiations, chemical weathering in the Earth’s surface likely plays an important role in the evolution of the continental crust’s major element composition.