Mineral compositions of a diverse suite of gabbroic rocks (anorthosites, leuco-troctolites, troctolites, leuco-olivine gabbros, olvine grabbos, leuco-grabbos, and gabbros) from the Mid-Cayman Rise spreading center indicate that these cumulate rocks have been produced by the crystal fractionation of basaltic liquids. Olivine compositions range from Fo87.8 to Fo73.0; plagioclase compositions range from An74 to An33; clinopyroxene Mg numbers range from 89.1 to 63.0. The covariations of these petrogenetic indicators are very similar to those produced during the experimental crystallization of mid-ocean ridge basaltic (MORB) magmas. Certain features of these gabbroic rocks, however, are inconsistent with the occurrence of this fractionation at low pressures (1 atm to 2 kbar). Many of the rocks have pyroxenes with Mg numbers higher than those formed during the low-pressure crystallization of MORBs.

The low-Ca pyroxene is orthopyroxene rather than pigeonite (the low-Ca pyroxene that crystallizes from MORBs at low pressures). Ilmenite occurs as a minor phase even in some gabbroic rocks with very forsteritic olivines (up to Fo87.3), which contrasts with results of phenocryst and experimental studies of MORBs that demonstrate the low-pressure crystallization of ilmenite only in highly fractionated magmas. Additionally, the olivine-out reaction that typically produces a low-Ca pyroxene at low pressures generally is not recorded in these rocks; olivine disappears by a reaction relationship, but a low-Ca pyroxene is not generally found in more fractionated gabbroic rocks. Although the experimental data are not available to conclusively demonstrate that these effects were produced at moderate pressures (5--10 kbar), the effects are similar to those predicted thermodynamically and to those observed in limited experimental studies within this pressure range. It is suggested, for this reason, that the most likely scenario for the production of these gabbroic rocks is the moderate-pressure crystallization of basaltic magmas within deep-seated magma chambers underneath this slow-spreading center. ¿ American Geophysical Union 1986