Primary Composition, Alteration, and Origin of Cretaceous Volcaniclastic Rocks, East Mariana Basin (Site 585, Leg 89)

Abstract

An upper Aptian to middle Albian series of volcaniclastic rocks more than 300 m thick was drilled at Site 585 in the East Mariana Basin. On the basis of textural and compositional (bulk-rock chemistry, primary and secondary mineral phases) evidence, the volcaniclastic unit is subdivided into a lower (below 830 m sub-bottom) and an upper (about 670-760 m) sequence; the boundary in the interval between is uncertain owing to lack of samples.The rocks are dominantly former vitric basaltic tuffs and minor lapillistones with lesser amounts of crystals and basaltic lithic clasts. They are mixed with shallow-water carbonate debris (ooids, skeletal debris), and were transported by mass flows to their site of deposition.The lower sequence is mostly Plagioclase-and olivine-phyric with lesser amounts of Ti-poor clinopyroxene. Mineralogical and bulk-rock chemical data indicate a tholeiitic composition slightly more enriched than N-MORB (normal mid-ocean ridge basalt). Transport was by debris flows from shallow-water sites, as indicated by admixed ooids. Volcanogenic particles are chiefly moderately vesicular to nonvesicular blocky shards (former sideromelane) and less angular tachylite with quench Plagioclase and pyroxene, indicating generation of volcanic clasts predominantly by spalling and breakage of submarine pillow and/or sheet-flow lavas.The upper sequence is mainly clinopyroxene-and olivine-phyric with minor Plagioclase. The more Ti-rich clinopyroxene and the bulk-rock analyses show that the moderately alkali basaltic composition throughout is more mafic than the basal tholeiitic sequence. Transport was by turbidity currents. Rounded epiclasts of crystalline basalts are more common than in the lower sequence, and, together with the occurrence of oxidized olivine pseudomorphs and vesicular tachylite, are taken as evidence of derivation from eroded subaerially exposed volcanics. Former sideromelane shards are more vesicular than in the lower sequence; vesicularity exceeds 60 vol.% in some clasts. The dominant clastic process is interpreted to be by shallow-water explosive eruptions.All rocks have undergone low-temperature alteration; the dominant secondary phases are "palagonite," chlorite/ smectite mixed minerals, analcite, and chabazite. Smectite, chlorite, and natrolite occur in minor amounts. Phillipsite is recognized as an early alteration product, now replaced by other zeolites. During alteration, the rocks have lost up to 50% of their Ca, compared with a fresh shard and fresh glass inclusions in primary minerals, but have gained much less K, Rb, and Ba than expected, indicating rapid deposition prior to significant seafloor weathering.

Petrography and Geochemistry of Graded Volcaniclastic Sediments and Their Clasts, Leg 129

“…The shard classification used by Schmincke and coworkers has been confirmed and used in this study Fisher and Schmincke, 1984;Viereck et al, 1986). The same range of shard types was encountered in both Miocene and Cretaceous deposits.…”

“…The highly intricate and delicate cuspate shapes of large shards controlled by vesicle margins are commonly preserved, while large multichambered vesicular scoriae are common in both the Miocene and the Cretaceous volcaniclastites of Sites 800 and 801. Vesicles are invariably infilled with radiating fibrous smectites, as described previously from other sites (for example, Viereck et al, 1982Viereck et al, , 1986Floyd, 1986a), although zeolites and sometimes calcite (especially at Site 801) has been noted.…”

“…The occurrence of volcaniclastic rocks in sediments of the deep oceans has frequently been recorded in cores obtained by deep-sea drilling (for example, Bowles et al, 1973;Schmincke et al, 1978;Scheidegger et al, 1978;Houghton, 1979;Kelts and Arthur, 1981;Schmincke, 1981Schmincke, ,1983aSchmincke, , 1983bRea and Vallier, 1983;Floyd, 1986aFloyd, , 1986bViereck et al, 1986). In the western Pacific Ocean, especially Sites 800, 801, and 802 (Lancelot, Larson, et al, 1990).…”

“…One example from the Cretaceous (Sample 129-800A-33R-4, 146-150 cm) was selected for detailed analysis because of the range of different sites of alteration shown by the smectites in plagioclase, olivine pseudomorphs, zoned vesicles, and matrix, with various colored clay minerals (Table 2 on range of mineral replacement is similar to that described by Viereck et al (1986), but in the present study the sites of alteration were analyzed separately. The data indicate a variety of compositions which appear to be site specific.…”

“…Studies of the alteration of basaltic glass in pillow lavas under low temperature conditions abound, but less so for hyaloclastites (for example, Floyd and Rowbotham, 1986;Schmincke, 1983a;Viereck et al, 1982Viereck et al, ,1986.…”