Primary composition, alteration, and origin of Cretaceous volcaniclastic rocks, East Mariana Basin (Site 585, Leg 89).

Abstract

The volcaniclastic rocks of Site 585 Unit VI are subdivided into a lower tholeiitic and an upper basaltic sequence. The rocks are dominantly former vitric basaltic tuffs and minor lapilli-stones, with lesser crystals and basaltic lithic clasts, mixed with shallow-water carbonate debris. The lower sequence is mostly plagioclase- and olivine-phyric with lesser amounts of Ti-poor clinopyroxene, whereas the upper sequence is mainly clinopyroxene- and olivine-phyric with minor plagioclase. All rocks have undergone low-T alteration. Microprobe analyses are given for primary phases (plagioclase, kaersutitic amphibole, biotite, magnesiochromites, titanomagnetite and clinopyroxenes), secondary phases (palagonite-altered glass shards, chlorites, chlorite/smectites, smectites, smectite/illite, celadonite, natrolite, Na-chabazite, analcite, albite, K-feldspar, apophyllite and gyrolite), glass shards and glass inclusions (5 in pyroxene, 1 in plagioclase).- C.M.L.