Igneous Rocks of Deep Sea Drilling Project Leg 61, Nauru Basin

Abstract

During Leg 61 of the Deep Sea Drilling Project the deepest (1068.5 m) hole was drilled in the Nauru Basin. It penetrated rocks of two magmatic cycles. The first ranged from Pleistocene to Oligocene and is represented by thin ash layers, composed of fragments of fresh scoriaceous alkalic and tholeiitic basalts, and crystal fragments of olivine, chrome-diopside, spinel, and feldspars. Specific features of the rocks are high calcium content (up to 14 wt.% CaO) and titanium content (up to 4 wt. % TiO 2), accompanied by high alkalinity. Rare varieties of titanium-rich (up to 30 wt.% TiO 2) magnetite and Fe-Ca-anorthoclase are present among the fragments. According to the rock and mineral assemblage, the fragments from the ash layers may be considered to be derivatives of a subaerial, Hawaii-type volcano. In the interval from Cenomanian to Hauterivian, a tholeiite sill complex was formed, the upper part of which (562-728 m) is represented by essentially holocrystalline dolerites alternating with the hyaloclastites, whereas in the lower part (728-1068 m), thin-layered, pillow-like basalt sills are dominant. Rocks of the upper part are characterized by high iron and titanium contents, and show considerable fractionation (differentiated tholeiites) as compared with the homogeneous, low-titanium and magnesia-rich basalts of the lower part (primitive tholeiites). Consolidation of dolerites was culminated by the formation of quartz-potassium feldspar granophyric aggregates. According to the compositions of liquidus spinel and olivine, crystallization of the magma began at T ~ 1210 to 1240°C and/ θ2) 10~8 to 1O~8• 5 atm. Within the low-temperature veins in dolerites, silica-bearing magnetite (up to 3.5% SiO 2) was discovered. On the basis of mineralogical-geochemical features, the Nauru sill complex is classified as an oceanic trap formation.

Chromian Spinel Composition and the Platinum-Group Minerals of the Pge-Rich Loma Peguera Chromitites, Loma Caribe Peridotite, Dominican Republic

“…Field of chromitite composition from Ural-Alaskan chromitites according to Garuti et al (2005). C) Comparison of TiO 2 (wt%) and Al 2 O 3 (wt%) contents in chromite from Loma Peguera with compositions of chromian spinel in volcanic and plutonic complexes from various geotectonic settings: Nauru basin and Manihiki oceanic plateaus after Tokuyama & Batiza (1981), Shcheka (1981) and Clague (1976). Data on Alaskan-type complexes are from the compilation of Batanova et al (2005).…”

Chemistry of chromian spinel in volcanic rocks as a potential guide to magma chemistry

“…(C) Oceanic plateau basalts. Data sources: Manihiki Plateau, Clague (1976); Nauru Basin, Shcheka (1981), Tokuyama and Batiza (1981); Ontong-Java Plateau, Stoeser (1975). (1981), Clague et al (1980), Evans and Wright (1972), Hawkins and Melchior (1983), Wilkinson and Hensel (1988); Other intraplate tholeiites, Gunn etal.…”

Secondary Minerals in Basalt from the Costa Rica Rift, Holes 501 and 504B, Deep Sea Drilling Project Legs 68, 69, and 70

“…Where thin flows and pillow lavas have erupted, secondary minerals do not form in the deuteric stage but begin to originate during a later stage of heating. Therefore, during the formation of off-ridge basement in Cretaceous Nauru Basin (Shcheka, 1981;Shcheka and Kurenzova, in press), the conditions of secondary mineral formation and those of cation flux into the ocean and sediments were probably different from those at the Costa Rica Rift.…”