Several peridotites in coastal southwestern Oregon contain the high-pressure mineral assemblage, forsterite, aluminous enstatite and diopside, and spinel. Textural relations and mineral chemistry indicate that the peridotites were recrystallized at high temperatures (1100° to 1200° C) over a range of pressure decreasing from 19 to 5 kbars. It is proposed that the peridotites represent upper mantle material brought from depth beneath a Mesozoic ocean ridge system, transported eastward by a spreading oceanic plate, and then emplaced by thrust faulting in the western margin of the Cordillera during late Mesozoic time.

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