THE VOLUME AND RARE-EARTH CONCENTRATIONS OF MAGMAS GENERATED DURING FINITE STRETCHING OF THE LITHOSPHERE

We present a model of lithospheric stretching and associated melting t o predict the volume and rare earth element composition of basaltic ma gmas generated during rifting The model differs from previous approach es in two ways: (1) we assume a two-layer lithospheric stretching mode l in which the amount of stretching in the crust and lower lithosphere may differ; (2) we allow for multiple instantaneous episodes of exten sion to occur over a specified time interval. Melt volumes are compute d from the syn- and post-rift subsidences that account for the thickne ss of the sedimentary sequence. The concentrations of rare earth eleme nts are calculated using an incremental melding model incorporating va riations of the melt fraction with depth. The predicted melt thickness and rare earth concentrations are most sensitive to variations in the basal mantle temperature, magnitude of subcrustal stretching, and tim e dependence of deformation. Given moderately high mantle temperatures (>1450 degrees C) and large amounts of stretching in the upper mantle, the model is capable of generating alkaline melts, and predicts the t ransition to tholeiitic magmatism observed in many continental rifts. We compare some of the model predictions with observations an the igne ous and tectonic history of the North Sea Rift and Labrador margin (ea stern Canadian margin).