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Melting Relations of Brown-Hornblende Mylonite from St. Paul's Rocks under Water-Saturated and Water-Undersaturated Conditions to 30 Kilobars

Crystalline brown-hornblende mylonite, with a composition similar to some basanites and nephelin-ites, was crushed and reacted with excess water and with no water added (about 2% water present) in sealed platinum capsules in a piston-cylinder apparatus from 10-30 kb. The solidus with excess water, and liquidus curves for excess water and for 2% water, are presented, together with stability limits (within 50°C brackets) in the melting intervals for amphibole, plagioclase, clinopyroxene, garnet, olivine, rutile, nepheline, and zoisite. With increasing water content and consequent decreasing liquidus temperature on the water-undersaturated liquidus surface, the number of liquidus and near-liquidus minerals increases, the field of olivine extends to higher pressures, and garnet becomes stabilized at pressures above 20 kb. Orthopyroxene was not found. Amphibole is stable at the liquidus between 10 and 21 kb with excess water at 1,075°C, and between 10 and 16 kb with 2% water at 1,175°C. These results combined with previous interpretations are consistent with the following tentative petrogenetic history for St. Paul's Rocks. A water-undersaturated olivine-basanite magma rose from a depth of about 100 km at a temperature of 1,200-1,300°C, and differentiated until it reached a depth somewhere between 45-70 km, where the magma had reached the composition of brown-hornblende mylonite. At a temperature between 1,050°C and 1,000°C amphibole was the dominant mineral being precipitated. Exsolution of water from the magma became fixed in the mantle peridotite as amphibole, and locally may have caused incipient melting of the peridotite. This process may have initiated upward movement of the heterogeneous assemblage of peridotite and largely crystallized basanite magma, which rose as a near-solid diapiric intrusion with temperature possibly remaining as high as 1,000°C up to 7 km deep, where intense mylonitization began during the final ascent and shallow emplacement.