Partial Melting in the Lower Crust: New Constraints on Crustal Contamination Processes in the Central Andes

Abstract

The granitoids of the Idaho Batholith were generated in the late Cretaceous when the crust in that area was 60–70 km thick, and they therefore constrain models for crustal melting and crustal contamination in areas of unusually thickened crust, such as the Central Andes. The volumetrically dominant granodiorites and granites exhibit unfractionated Rb/Ba and Sr/Nd, relatively low P, Ti, Y and HREE abundances, and Sr, Nd and Pb isotopes consistent with partial melting of crustal source rocks with no significant contribution from mantle derived magmas. It is argued that they were generated by partial melting of tonalitic source rocks, in the presence of residual garnet, at depth in the continental crust, and that they may therefore be used to evaluate lower crustal contamination processes in the Central Andes.For the Central Andes there are now a significant number of Nd isotope analyses which offer tight constraints on the Nd isotope composition of any crustal endmember. Mass balance considerations indicate that if crustal contamination was responsible for the Nd and Sr isotope ratios of selected andesites in the Central Andes, the amounts of contamination are ~ 35–70%. Moreover, the uncontaminated magmas must themselves have had relatively high Nd (and Sr) abundances, whether that reflects fractional crystallization and/or source or partial melting processes. It would appear that lower crustal contamination models for the Central Andes andesites either imply that the isotope ratios were changed with little affect on the observed trace element patterns, or that the added contaminant is relatively low in SiO₂ and has none of the characteristic features of the lower crustal melts from the Idaho Batholith.