The role of boron and fluids in high temperature, shallow level metamorphism of the Chugach Metamorphic Complex, Alaska

Abstract

The possible role of boron (B) involvement in granite equilibria and generation of melts during crustal metamorphism has been a focus of speculation in recent literature. Most of the evidence for such involvement derives from experimental data which implies that the addition of B will lower the temperature of the granite solidus. Also the presence of tourmaline has a minor effect on the temperature of the solidus. Further indirect evidence that B may be involved in partial melting processes is the observation that granulites are commonly depleted in B, whereas the B content of low grade metapelites can be high (up to 2000 ppm). Researchers' measurements of the whole-rock B contents of granulites from the Madras region, India are low, ranging from 0.4 to 2.6 ppm. Ahmad and Wilson suggest that B was mobilized in the fluid phase during granulite facies metamorphism of the Broken Hill Complex, Australia. Thus, it appears that during the amphibolite to granulite transition, B is systematically lost from metasediments. The B that is released will probably partition into the vapor phase and/or melt phase. Preliminary measurements imply that the boron content of rocks in the Chugach Metamorphic Complex is not sufficient to influence the processes of partial melting at low pressures