Subduction-related origin of the volcanic rocks of the Eocene Clarno Formation near Cherry Creek, Oregon
by Jeffrey B. Noblett, Department of Geology, Colorado College, Colorado Springs, Colorado 80903
Terrestrial calc-alkaline of the Eocene Clarno Formation in north-central Oregon appear to be a good example of subduction-zone magmatism. Mapping of an area along the John Day River near Cherry Creek showed that the many individual flows could be placed into three petrographic types. The earliest lavas are highly porphyritic two-pyroxene basaltic andesites. The key feature of the middle portion of the Clarno Formation is a group of nonporphyritic, quartz-bearing basaltic andesites, which have also been noted as single units in other stratigraphic columns throughout the Clarno. The nature of their origin suggests they may be correlated as a time horizon across the Clarno. An angular unconformity and thick saprolite separate the gently folded lower Clarno beds from the much less voluminous upper Clarno volcanic rocks. A dome of hornblende andesite is the most notable feature of these later flows.
Textural and chemical evidence suggests that the porphyritic lavas formed as hydrous melts that rose from a subducted slab and interacted with the overlying mantle, whereas the nonporphyritic lavas formed by partial melting of anhydrous quartz eclogite followed by rapid ascent. New age data (Robinson, 1979, personal communication) suggest that the Clarno is older than 40-42 m.y. Subduction during this time was fairly rapid and at about a 30° angle. Estimated temperatures on the surface of this plate match temperatures at which the Clarno andesites could have formed. A model of magmatism arising from the shallowly dipping hydrous portion of the subducted plate, with one brief increase in dip (about 5°) generating the quartz lavas, can explain the general characteristics of Clarno volcanism.