Petrology of the Basaltic Rocks Drilled on Leg 33 of the Deep Sea Drilling Project

Abstract

Six flow units of basalt were penetrated at the base of Hole 315A, located near Fanning Island in the Line Islands chain; intercalated sedimentary rocks, if present, were not recovered. Ten flow units of basalt were penetrated in Hole 317A, located on the high Manihiki Plateau; four of these flow units are intercalated with thin beds of baked volcaniclastic siltstone. Basement was not reached in Hole 316, near Christmas Island in the Line Islands chain, nor in Hole 318, between Ahe and Arutua atolls in the Tuamotu Islands chain, but centimeter-sized pebbles of volcanic rocks were recovered from breccia beds at both these sites. The basaltic rocks in Hole 315A are highly altered, chiefly to montmorillonite group minerals and calcite, but not to the extent that their textures, original mineralogy, and major-and minor-element abundances could not be established. The flow units show textural differences, but their composition is similar to basalts of the oceanic island type, intermediate between Hawaiian tholeiite and alkalic basalt. The pebbles of Hole 316 may also be of this type. These basalts are much less alkalic than the amphibolebearing basalts drilled in the more northerly Line Islands on Leg 17, which appear to resemble Hawaiian post-erosional basanites and nephelinites. It would appear from existing analyses that the Line Islands are composed of basaltic rocks that are very similar to those of the Hawaiian chain.Basalts drilled in Hole 317A differ in character from those of Hole 315A. They are less altered, and their textures, mineralogy, and major and minor element compositions unquestionably show them to be basalts of the oceanic ridge tholeiite type. They resemble basalts drilled beneath the Ontong-Java Plateau during Leg 30. Basalts from both areas are similar to rocks forming at present-day spreading centers, and do not support the contention that these plateaus are subsided microcontinents. The basalts are, however, exceedingly vesicular, and must have been erupted at water depths of less than 400 meters, or perhaps even subaerially. This implies that the Manihiki Plateau has subsided between 3000 and 3500 meters during the last 110 to 120 m.y.Basaltic pebbles from the northwestern end of the Tuamotu chain are also of the oceanic island type. Age data down the chain are scanty, and it is not possible at present to say whether this part of the Tuamotu chain is of the progressively propagating type.

Site 803

“…The closest resemblance of the Site 803 lavas is to the few available basalts from other Pacific plateaus (see Fig. 45, for example); in particular, those from the Manihiki Plateau (Jackson et al, 1976) and the single flow of Ontong Java basalt recovered at Site 289 (Stoeser, 1975). They also are quite similar to lavas from the islands of Malaita and Santa Isabel (J.…”

“…Hence, above this depth the log density measurements average about 1.4 g/cm 3 , compared with core measurements of about 1.6 g/cm 3 (see , 1980), Hole 504B on the Costa Rica Rift (Kempton et al, 1985), and the Nauru Basin (Floyd, 1986;Saunders, 1986). Averages for Manihiki Plateau lavas ("M"; Jackson et al, 1976) and Malaita basement basalts ("Ma"; J. Mahoney, unpubl.…”

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

“…Clinopyroxene, some approaching titanaugite in composition, is the dominant phenocryst phase, followed by olivine and minor Plagioclase. These rocks are similar in incompati-ble-element composition to the mildly alkalic basalts drilled during Leg 20, Holes 170 and 171 (on Horizon Guyot; Bass et al, 1973) and Leg 33 in Hole 315A near Fanning Island in the central Pacific (Jackson et al, 1976).…”

Site 807

“…Similar basalts were found on the Manihiki Plateau (Jackson et al, 1976), in the Nauru Basin to the east of Site 807 (e.g., Floyd, 1986), and on the island of Malaita and Santa Isabel at the western edge of the Ontong Java Plateau (J. Mahoney, unpubl.…”