GEOCHEMISTRY AND PETROLOGY OF LATE EOCENE CASCADE HEAD AND YACHATS BASALTS AND ALKALIC INTRUSIONS OF THE CENTRAL OREGON COAST RANGE, U.S.A

The Oregon Coast Range underwent extension and experienced anomalous forearc magmatism accompanying tectonic rotation during the Tertiary. This extension permitted the upwelling of underlying mantle, which underwent decompression melting to produce basaltic magmas, which were erupted, initially as submarine, but mostly as subaerial flows. A tholeiitic basalt series (basalt-basaltic andesite-trachyandesite-trachye) represented by Yachats units evolved from a depleted, low K,Rb source (possibly lower crust) through a process of fractional crystallization and mixing with mantle-derived melts. An alkalic basaltic series (ankaramitic basalt-picrobasalt-alkali basalt-hawaiite-mugearite-trachyandesite-trachyte) represented by Cascade Head units evolved primarily by fractional crystallization from mantle-derived parental magmas created by lower degrees of partial melting at greater depth than the Yachats series. Approximately 18 parts trachyte could be produced from 100 parts ankaramitic basalt. 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 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.