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Lucky we're here boys! Sounds like our friend is on top of the subduction zone living on the churned up hills of the old plate sutured to the North American Plate. Serpentenite (pic related fig. a) comes from the very bottom of the crust. It is pretty rare for a base of a tectonic plate to be turned up in such a way that you can go out and walk on it, but on the western mountain belt the ongoing collision of plates gives us some cool views as to what comes up from the depths. Ultramafic minerals are really rich in Mg and Fe, these ions are preferentially precipitated out of the melts first in the form of olivines and oxides, then the chemistry rollercoaster takes you down Bowen's Reaction Series (pic related fig. b) where you can start getting your pyroxenes and plagioclase!!!
Ok so olivine is usually what the serpentine is made from, it's crystal structure is most stable at the pressure and temperatures of the base of the crust so as soon as they start ascending they recrystallize. As soon as ultramafic rock gets wet is serpentenizes into the serpentine family of minerals (serpentine, antigorite, lizardite), you even get some asbestos too.
I fucking love ophiolites they are based, it is where platinum, rhodium, chromite, nickel, and many more interesting metals come from.
