Petrology of basaltic rocks from the São Paulo ridge, southwestern Atlantic Ocean

Abstract

Seven basaltic rocks dredged from the São Paulo ridge, southwestern Atlantic Ocean, and one pebble recovered from the ridge during DSDP Leg 39, display alteration ranging from seawater weathering to greenschist facies metamorphism. Bulk compositions of the most highly metamorphosed samples show high Na 2O contents (about 4–5 wt.% at 49–52 wt.% SiO 2), and analyses of minerals in them revealed albite to be the feldspar component; accordingly, five rocks are classified as spilites. Trace-element abundances, namely Zr (∼180 to 288 ppm), and analyses of relict clinopyroxene (Fs 11–19 Wo 36–40) are compatible with seven basaltic samples being transitional between the tholeiitic and alkalic suites, and one belonging to the alkalic suite. The alkalic affinities of these rocks are critical to defining an east-west fracture zone expressed by the São Paulo ridge and forming the southern boundary of the São Paulo plateau. The spilitization likely occurred from relatively high temperatures (hydrothermal and contact metamorphism) in the fracture zone, and is possibly associated with a shift in the location of an early mid-ocean ridge.