NEW DATA ON ROCKS OF THE MAGELLAN SEAMOUNTS, WESTERN PACIFIC

Abstract

The aim of work done on the 9th cruise of the R/V Akademik Mstislav Keldysh over the Magellan Seamounts in the central part of the East Mariana Basin was to determine the origin of guyots (tablemounts). For this purpose, the Ita May Tai and Ioan2 guyots were studied. The rock samples raised were large, separate, iron- and manganese-encrusted blocks - the fragments not only of massive and, less commonly, pillow and brecciform lava flows, but also of breccias with a phosphate-carbonate cement. The volcanic rocks of the Ita May Tai and the Ioan Guyots can be subdivided into extrusive basalt and volcaniclastic varieties. The high TiO2 content, plus the high sum of the alkalies and the low Na2O/K2O ratio of these rocks, as well as their petrography allow them to be assigned to the alkalic basalt series. They can be divided into four groups. These four alkalic basalt groups represent a rock series, in which, from the first group to the fourth, the sum of alkalies and the fraction of potassium increase while the Na2O/K2O ratio and, to a lesser extent, the iron content significantly decrease. Additional aspects of the subject are discussed.