Late Cretaceous volcaniclastic rocks from the Walvis Ridge, southeast Atlantic, Leg 74 ( DSDP).

Abstract

Volcaniclastic rocks of Late Cretaceous age occur in 4 out of 5 sites (525, 527, 528, 529) drilled on the crest and the NW flank of the Walvis Ridge during Leg 74. They are mostly interlayered with and overlie basement in the lowermost 10-100 m of the sedimentary section. Rocks from Holes 525A and 528 were studied megascopically and microscopically, by XRD, and XRF chemical analyses of whole- rock major and trace elements were undertaken. The sedimentary structures of volcaniclastic layers and their occurrence within deep sea calcareous oozes indicate a mass flow origin. Although diagenetic alteration of the volcaniclastic rocks is pronounced, there were no drastic changes in the chemical composition of the rocks, except for pronounced increases in K and Rb and decreases in Ca and Fe. The basaltic volcaniclastic rocks very much resemble basement basalts in that they are moderately evolved tholeiites derived from an LIL-enriched mantle source with Zr/Nb ratios (Hole 528) of 5-6. This, in conjunction with the interbedding of volcaniclastic rocks and basement lavas, indicates contemporaneous seamount or island and basement volcanic activity involving magmas derived from similar sources.-from Authors