OSR Journal of Student Research

Article Title

Origin of the serpentinite bodies in the Domenigoni Mountains, Diamond Valley Lake


Zachary Andrews


Within the Domenigoni Mountains in Hemet, California are six meta-serpentinite bodies which were first described by Morton (2003), most of the mountain range is composed of metapelitic rocks than range from low-grade schist to high-grade gneiss. It is very uncommon for isolated serpentinites, that have an ultramafic (high Mg and low SiO2) composition, to be surrounded by meta-pelitic rocks that have a felsic (low Mg and high SiO2) composition. Common origins for serpentinites include ophiolite sequences, seafloor serpentinization of oceanic crust, obduction of seafloor crust on land, xenoliths in basalt, and metamorphism of mafic/ultramafic slab during subduction. None of the surrounding rock bodies are consistent with such origins. As of yet no one has published detailed petrographic and mineralogical analysis of the rock bodies. To help understand this odd field relation of the meta-serpentinite bodies I performed detailed petrographic analysis including x-ray diffraction (XRD), scanning electron microscope (SEM), and optical microscopy to characterize the rocks. The rock samples collected were three serpentinites, results determined the rocks contain serpentine, talc, and forsterite. Serpentine minerals are not antigorite which indicates low pressure that is consistent with simultaneous metamorphism of pelites and the serpentine protolith. The presence of talc could be from silica metasomatism from surrounding SiO2-rich fluids from pelites, or formed from thermal decomposition of serpentinite. Origin of serpentinite protolith is still unclear, it is possible forearc diapir because bedding throughout the mountain range is still intact except around serpentinite bodies which is consistent with diapir rising into mountain range only disrupting bedding locally.

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