Cretaceous tholeiitic volcanic rocks from the Western

character, is constructed almost entirely of Cretaceous sedimentary and volcanic rocks. The Diabase Group consists of tholeiitic basalts, dolerites and gabbros, with thin intercalations of chert and shale. Two major sedimentary sequences, one predominantly arenaceous, the other argillaceous, are conformable with the volcanic rocks. Diabase Group rocks possess a limited compositional range and show only minor magmatic evolution. The main geochemical characteristics include low concentrations of LIL elements, light-depleted to slightly light-element-enriched chondrite-normalized REE patterns, and high Nb and Ta contents relative to N-type MORB. Detailed studies of minor and trace elements indicate that the rocks have a transitional nature, similar to T-type MORB. The Western Cordilleran sequence probably formed in an extensive oceanic flood basalt province, similar to the Caribbean or Nauru Basin, adjacent to the margin of South America. The western margin of Central and South America is characterized by Upper Mesozoic to Lower Tertiary basaltic volcanic rocks and intercalated metasediment