Petrography and mineral compositions of gabbros at DSDP Leg 60 Holes

This reaction, the extremely calcic Plagioclase, the occurrence of biotite and amphibole, and the association with greenschist facies metamorphic rocks suggest that crystallization of the gabbros occurred at elevated P(H2O). Comparisons with other calc-alkalic gabbro suites suggest pressures in excess of 4 kbar (about 12 km depth). The gabbros were exposed by the early stages of opening of the Mariana Trough and imply that considerable uplift may have attended rifting.

They were also subjected to hydrothermal alteration after breccia formation, resulting in formation of chlorite, epidote, actinolite, and prehnite. Temperatures of at least 200°C - and probably 350°C - were reached, and most likely could not have been attained without extrusion or intrusion of magmas nearby, even though no such rocks were cored.