THE DINNER CREEK TUFF AND OTHER MID-MIOCENE RHYOLITES AT THE MAGMATIC FOCAL ZONE OF THE COLUMBIA RIVER BASALT GROUP

A not often appreciated part of Columbia River Basalt Group (CRBG) volcanism is the closely associated silicic volcanism. Large rhyolite vent complexes form a belt that stretches over 300 km from near Baker City, Oregon to northern Nevada. Voluminous silicic eruptions from these centers span the time frame between 16.5 to 15 Ma, coincident with the most voluminous CRBG flood basalts. Here, we focus on silicic volcanism in the northern 100 km segment, which overlaps the proposed location of CRBG crustal magma reservoirs (Wolff et al., Nature Geoscience, 2008). Our current data indicate silicic volcanism here began at 16.4 Ma. Initial calc-alkaline compositions become more Fe-rich and A-type like with time. One of the major northern rhyolite centers generated the voluminous Dinner Creek Ash-flow Tuff. Although the vent complex is yet to be mapped in detail, the source of the Dinner Creek Tuff lies between Castle Rock and Iron Mtn. An exposure of more than 70 m thick rheomorphic Dinner Creek Tuff at Iron Mtn. defines the northern near-vent facies. Several hundred thousand years of silicic volcanism produced multiple fall and ash-flow deposits at this complex. Duration of activity is currently defined by new Ar-Ar dates ranging from 15.9±0.13 to 15.4±0.16 (2σ) Ma on feldspar separates from Dinner Creek Tuff ignimbrites. Some younger tuffs with subtle chemical differences have nearly identical ages ranging between 15.45±0.16 and 15.38±0.17 (2σ) Ma. Welded tuff lithics with Dinner Creek Tuff compositions indicate recycling of older tuffs and provide further evidence for multiple ignimbrite eruptions. Ash-flow tuffs that erupted from the Dinner Creek Tuff center extend over an area of more than 20,000 km² and, through our new correlations, include the Mascall Ignimbrite of the greater Paulina Basin and the Pleasant Valley Tuff of the Baker valley. Other correlative ash-flow tuffs extend from the Silvies River north of Burns, to John Day, Prairie City, and Unity, and as far north as La Grande suggesting an eruption volume of several 100’s of km³ which is also suggested by Dinner Creek Tuff fallout extending from northern Nevada (Nash et al., EPSL, 2008) to Umatilla county, Oregon.