THE GRANDE RONDE BASALT, COLUMBIA RIVER BASALT GROUP: FLOW VOLUMES AND COMPOSITIONS THROUGH TIME

The peak of Columbia River Basalt Group (CRBG) volcanism occurred during Grande Ronde Basalt (GRB) time, approximately 15.6 Ma, and lasted about 0.42 + 0.18 Ma (Barry et al, 2010) over four polarity intervals (R1, N1, R2, N2). Approximately 151,000 km³ basalt erupted and covered more than 169,600 km² of the province.

The Grande Ronde Basalt consists of at least 120 major basalt flows that can be grouped into 24 members defined by composition and physical characteristics. Eruptions began during the R1 interval; R1 flows have a total volume of about 30,400 km³. The N1 flows have a total volume of approximately 27,900 km³. The R2 flows are the most extensive of all the GRB flows and have a total volume of 57,900 km³. The N2 flows are the youngest and extend from near the eastern margin of the province to the Pacific Ocean. The N2 flows have a total volume of approximately 35,000 km³. Individual flows in the members range in volume from several hundred km³ to well over 10,000 km³.

The greatest volume of GRB erupted during the R2 polarity interval and represents about 40% of the GRB. The largest volume of eruptions peaked with the R2 Wapshilla Ridge Member which has about 38,000 km³ of basalt. From the initial eruption of GRB with the Buckhorn Springs Member to the Wapshilla Ridge Member, the volume of basalt for each member is relatively constant. After that volumes are highly variable.

GRB averages 56 wt% SiO_2, and 4.0 wt% MgO. MgO and SiO₂ show an inverse cyclical pattern over time. In addition, there is an inverse relationship between how primitive the composition of a member is and the volume of basalt erupted. The highest the SiO₂ and incompatible elements occur in the largest eruptions peaking in the Wapshilla Ridge Member. The most primative flow has a volume of 300 km³ and a MgO content of nearly 6 wt% and imediately follows the R2 flows.