Aeromagnetic survey of the San Gabriel Anorthosite Complex, San Gabriel Mountains, southern California

The Precambrian San Gabriel Anorthosite Complex, San Gabriel Mountains, southern California, is composed of genetically related anorthosite, gabbro, and norite. Aeromagnetic studies and field observations indicate that the gabbro and norite underlie the anorthosite.

Field relations of the three units and textures of primary igneous features indicate that final stages of crystallization occurred, in part, by crystal settling. These observations and a geophysical interpretation of one magnetic anomaly as a feeder dike for the complex suggest that the anorthosite complex may have formed in two stages: (1) intrusion of crystal mush through the feeder dike, possibly by a mechanism of flow differentiation, and (2) final crystallization, after intrusion, by crystal settling. Subsequently, the complex was tectonically raised to its present position.