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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. % TiO2), accompanied by high alkalinity. Rare varieties of titanium-rich (up to 30 wt.% TiO2) 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/ θ 2 ) 10~ 8 to 1O~• atm. Within the low-temperature veins in dolerites, silica-bearing magnetite (up to 3.5% SiO2) was discovered. On the basis of mineralogical-geochemical features, the Nauru sill complex is classified as an oceanic trap formation. INTRODUCTION During the DSDP Leg 61 and part of Leg 62, two closely spaced holes (462, 462A) were drilled in the Nauru Basin, western equatorial Pacific. They penetrated igneous rocks of two magmatic cycles. From 30 to 120 meters below sea floor, three thin ash layers composed of products of alkali-basalt eruptions were cored. Lower, ashes consisting of redeposited material of tholeiite basalts were recovered from 562 to 1068.5 meters. Inasmuch as vertical sections of both holes are identical, a joint characterization of them is given.

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