A geochemical traverse across the North Chilean Andes: evidence for crust generation from the mantle wedge

Abstract

Major and trace element and Sr- and Nd-isotope analyses are presented on 186-0 Ma magmatic rocks along an east-west traverse across North Chile at 22°S. ε Sr ranges from −25 to +100 and ε Nd from +6 to −9, but the low ε Nd and high ε Sr values are in rocks generated in the last 15 Ma. It is argued that previous discussions of petrogenesis in North Chile have been hampered because the changes in magma chemistry in this area of unusually thick crust reflect not one, but two processes. One results in a progressive shift of ε Sr from −25 to +20 and ε Nd from +6 to −6 in Jurassic to Recent rocks, which is accompanied by increasing Ta/Sm and Sr and decreasing Th/Ta. The second is largely confined to the younger rocks and it is characterised by ε Sr increasing up to +100 with increasing SiO 2 and decreasing Sr, and it results in relatively shallow trends on an ε Nd-ε Sr diagram. The preferred interpretation is that trend 1 is due to the mobilisation of old, late Proterozoic mantle lithosphere as magmatism migrated eastwards, and that trend 2 is due to crustal melting and contamination with differentiation in this area of thickened continental crust. It follows that the mantle wedge is the principal site of crust generation, and it is argued that < 20% of the Sr in the recent northern Chile rocks is derived from the subducted ocean crust.