Geochemistry and petrology of late Eocene Cascade Head and Yachats Basalt and alkalic intrusions of the central Oregon Coast Range, U.S.A.

Abstract

The Oregon Coast Range underwent extension and basaltic magmatism accompanying tectonic rotation during the Tertiary. Basaltic magmatism was expressed initially in submarine eruptions, but mostly in younger subaerial flows. A tholeiitic basalt series, represented by Yachats units, evolved from a depleted, low K, Rb source through a process of fractional crystallization and magma mixing. An alkalic basalt series, represented by Cascade Head units, evolved primarily by fractional crystallization from parental magmas created by lower degrees of partial melting at greater depth than the Yachats series. Camptonite, found in sills and dikes, was formed by even smaller degrees of melting of sources similar to those of Cascade Head. Small nepheline syenite intrusions, emplaced as sills into middle Eocene Tyee Formation, appear to have formed through extensive fractional crystallization of parental basanite, which is sparsely represented in the Yachats and Cascade Head basaltic lava units. Individual syenite intrusions are not directly related to each other, as each intrusion represents a relatively small batch of magma separated at different evolutionary stages from underlying magma chambers. P–T segregation conditions were estimated for primitive Coast Range basalts. These include depths of ~ 120–160 km and temperatures of 1600–1700 °C for the alkalic series represented primarily by Cascade Head units and depths of ~ 60–120 km with temperatures of ~ 1500–1600 °C for tholeiitic units. A few analyses from other Coast Range igneous units (Tillamook and Goble volcanics) suggest T–P segregations at shallower depths (~ 55–33 km) and lower temperatures (~ 1400 °C). Paired Nd and Sr isotopic analyses of four Coast Range samples fall in the enriched MORB field. The depths and high temperatures indicated for the Central Oregon Coast ancestral basalts require magma generation in the garnet peridotite stability field, with the alkalic series probably originating below the subducting Juan de Fuca plate. This would suggest an origin for the magmas in a slab window or mantle plume environment. We further suggest that major extension documented for the Coast Range may have triggered upwelling of mantle asthenosphere giving birth to a mantle plume.