OLIGOCENE BASALTS OF EASTERN OREGON AS PROBES INTO A PRE-CRBG MANTLE IN EASTERN OREGON AND POSSIBLY CONNECTION TO A NEW “RE-EMERGING PLUME MODEL”

Voluminous Columbia River Basalt Group (CRBG) volcanism starting at ~17 Ma may not be due to the impingement of a newly arriving deep plume but instead due to a secondary mantle upwelling of a mantle plume that before was responsible for generating the Siletzia LIP along the Pacific coast (Wells et al., 2014). Therefore, mafic volcanism in eastern Oregon predating CRBG may provide important clues about the state-of-the mantle as well as mantle sources prior to flood basalt volcanism.

While some research has suggested the correlation of central Oregon’s Oligocene calderas to the migrating Yellowstone plume, this work was completed on rhyolites and did not use basaltic lavas as the more primary magmatic signals of a melting anomaly. Our study has identified Oligocene basalts exposed south of the Aldrich and Strawberry Mountains, and to the east near Ironside, Oregon. Basalts are geochemically wide-ranging from alkali basalts, basalts characterized by a Nb-Ta peak on a mantle-normalized trace element diagram, and calc-alkaline basalts similar to those of the nearby Strawberry Volcanics. However, the older (23.5-25 Ma) calc-alkaline basalts have noticeably lower radiogenic Sr ratios than basalts of the Strawberry Volcanics. Overall geochemical similarities between mid-Miocene and Oligocene basalts suggest they may have a similar petrogenetic origin.

Basaltic magmas are the ultimate driver of more evolved magmatism when mafic underplating leads to crustal melting or more directly as material source during fractionation to andesites. Additional evidence for mafic magmatism comes from newly recognized, yet widespread, dacites and andesites of late Oligocene/early Miocene age (25 to 19 Ma), exposed in a broad area between Burns-John Day-Unity.

Oligocene mafic volcanism of eastern Oregon could provide critical insight into the state of mantle melting shortly before the main flood basalt volcanism, by providing insights into the spatial variability of mantle components, location of the mantle melting, and influence on mid-Miocene magmatism in light of a migrating plume model.

Wells, R. et al., 2014, Geosphere, 10(4),692-719.