 The occurrence of such basic magmas in a volcanic setting dominated by dacites appears to be a consequence of local tectonics, which involve the extension of the crust compared to the compressional regime farther west.Originating in the mantle, the magmas quickly ascended in faults and were contaminated by crustal material.The mantle itself had been modified before by crustal material added by delamination of the lower crust and subduction erosion.