Probable low-pressure intrusion of gabbro into serpentinized peridotite, northern California

Abstract

The Castle Lake gabbroic body intrudes the Trinity ultramafic sheet in northern California. The extent of concentric reaction zones of talc, tremolite, and chloritic blackwall on included blocks of peridotite correlates with increasing proportions of igneous hornblende in the gabbroic host; consequently, the reaction zones probably formed during initial cooling of the gabbro. The principal source of water in the gabbroic magma and in the reaction zones was probably a hydrothermal system which existed during and after crystallization of the magma. Blocks of pyroxenite in a succession of intrusive sheets of gabbroic rock indicate repeated intrusion of magma into its previously solidified margins. An increasing proportion of hornblende in the younger sheets correlates with a marked increase in the grain size. Variable distribution of dissolved and exsolved water and transitory minor decompression events can explain the grain-size layering in the younger sheets. Vugs as much as 5 mm in diameter in the pegmatitic hornblende gabbro are interpreted to be igneous in origin and, together with the patchiness of the metamorphism, suggest that the gabbro crystallized at a pressure of ∼2 kbar or less. Igneous hornblende (near solidus) and rare garnet are consistent with such a low pressure. The lack of sodium-rich plagioclase in the patchily metamorphosed gabbros suggests that the hydrothermal (metamorphic) fluids were poor in sodium (nonmarine or distilled marine). Because of the correlation between these igneous and metamorphic features, it is likely that the Castle Lake gabbro intruded its own hydrothermal envelope of serpentinized peridotite.