Abstract

In much of the Andes there is a migration of the plutonic locus with time from the coast to the high Andres. As continental crust also thickens in that direction a simple model involving an increasing continental crust component in the magmas inboard has been put forward. This is commonly associated with increases in K, Rb, Th and Ce as well as Sr initial away from the continental margin. This model is examined at 9 °S in mid-north Peru in granites intruded between 100 and 3 Ma. In this sector of the Andes there is no change/increase in these elements inboard, but there is a marked increase in Na, Ta, Sr, LaN/YbN and possibly A/CNK. The latter is associated with high level deformation along the western margin and is not related to source. The increase in Na, Ta, Sr and LaN/YbN is due to a difference in depth of partial melting and a component of subcontinental lithosphere which becomes more important inboard. However both batholiths were produced from new basaltic crust formed in rather different settings, the Coastal Batholith from basinal rocks within the Albian marginal basin and the Cordillera Bianca from an underplate formed during the Miocene. The difference in composition between the two batholiths reflects the changing continental lithosphere inboard as well as the residual mineralogy and tectonic setting at the site of partial melting.