Petrography and Mineral Chemistry of Basalts from Holes 519A, 520, 522B, and 524—Leg 73 (South Atlantic) of the Deep Sea Drilling Project

Abstract

Basalt was encountered under sediments in four holes (519A, 520, 522B, and 524) on Leg 73. The rocks in three holes (519A, 520, and 522B) are typical somewhat evolved mid-ocean-ridge basalt (MORB) tholeiites that occur as pillows, flows, and sills. In Hole 524 there occur (1) an upper alkalic sequence of pillows and a sill enclosed in Cretaceous volcaniclastic sedimentary rocks and (2) a lower, moderately evolved, slightly alkalic tholeiite in a multiple sill, also injected into Cretaceous volcaniclastic rocks.The tholeiites of Holes 519A, 520, and 522B display chemical, mineralogical, and petrographic features that are characteristic of MORB found elsewhere. The chilled margins of flows and pillows display a sequence of quench textures, and the interiors of the thicker flows and sills have the intersertal, intergranular, subophitic, and ophitic textures normally found in such rocks. The rocks are finely microphyric, with pre-emptive Plagioclase and sparse olivine microphenocrysts. Only pillows were encountered in Hole 520, and the rocks of that hole are distinctive in that they show a variolitic structure not seen in any of the other holes of Leg 73.The tholeiites of Hole 524 are petrographically similar to those of Holes 519A to 522B, but they are more strongly microphyric, with all major silicate phases (olivine, Plagioclase, and clinopyroxene) present as abundant to common microphenocrysts. The alkalic rocks of Hole 524 are distinctive in that (1) Fe-Ti oxides are present as pre-eruptive microphenocrysts in addition to the silicate phases, (2) the Plagioclase grains are epitaxially mantled by alkalic feldspar, and (3) biotite and magnesioriebeckite occur as late-stage accessories.The mineral phases have compositions well within the ranges of typical MORB from many locations: Plagioclase microphenocrysts average An 76 _ 80 , olivine Fo 81 _ 87 , clinopyroxene microphenocrysts are close to En 46 Fs 12 Wo 4 4 and zoned toward modest ferrosilite enrichment, titanomagnetites have average 64.4 mol.% ulvospinel, and ilmenites average 8.2 mol.% hematite. Trace elements are likewise well within the ranges reported from other oceanic sites.Clinopyroxenes from the tholeiitic multiple sill display an unusual compositional zoning in which Mj cations (Fe, Mg, Mn, Ti, and Cr) show sympathetic variations from core to rim. There is a distinct drop in iron, manganese, and titanium and a concomitant increase in magnesium and chromium from inner core to outer core and then a strong reversal of these trends in the rims. The temporary reversal of the normal differentiation pattern from inner core to outer core in clinopyroxene occurs in both the early and late phases of the multiple sill and is not explained, although it may be related to variations in oxygen fugacity or the crystallization pattern of the Fe-Ti oxide minerals.

Gas filter-pressing origin for segregation vesicles in dykes

“…Similar features have been reported widely from abyssal tholeiites (e.g. Carman, Dietrich & Sommerauer, 1984;Shibata, DeLong & Walker, 1979), and a common origin is suggested for segregation vesicles in dolerite dykes and segregation vesicles in ocean-floor basalts. In contrast, segregation vesicles reported from subaerial lavas (Anderson el al.…”

“…They were termed segregation vesicles by Smith (1967) in view of their obvious genetic similarity to segregation veins. Segregation vesicles have been reported from abyssal tholeiites of the Atlantic Ocean (Baragar et al 1977;Bideau, Hekinian & Francheteau, 1977;Shibata, DeLong & Walker, 1979;Sato, 1979;Donnelly et al 1979, p. 65;Carman, Dietrich & Sommerauer, 1984), and from subaerial basaltic flows (Upton & Wadsworth, 1971;Anderson et al 1984).…”

Increase of chromium with fractionation in early formed pyroxene: Examples from boninite and Kasayama andesite.

“…Heterogeneous Cr distributions are com mon features of pyroxene phenocrysts in many primitive basalts (e.g., Ohashi, 1980 ;Carmen et al, 1984;Stakes et al, 1984). …”