OFF-AXIS SNAKE RIVER PLANE MAGMATISM ALONG AN ACTIVE EXTENSIONAL DETACHMENT: AN EXAMPLE FROM THE JIM SAGE VOLCANIC SUITE, SOUTHERN ID

The Miocene Jim Sage Volcanic (JSV) suite consists of intercalated flows of anhydrous rhyolitic lavas and rhyolitic ignimbrites, and is capped by a basaltic flow. The rhyolites erupted from a series of small eruptive centers along the Raft River Detachment – Albion Fault (RRD-AF) system which was responsible for exhuming the Albion and Raft River metamorphic core complexes between 13.3 and ~7 Ma. The volcanic rocks were constrained in the Raft River Basin, constituting the top 1000 m of the syn-extensional basin fill. The rhyolites are divided into two members that are separated by a middle tuff or a volcanoclastic breccia. The basal contacts of the two members are indicative of deposition in a shallow lake or on top of wet sediments.

The two rhyolitic members show little chemical zonation, small variation in crystallinity (20-30%), and subtle chemical differences between the lower and the upper rhyolite. Zircon U-Pb SHRIMP-RG geochronology indicates that the lower rhyolite has an age range of 9.5 to 9.3 Ma, while the upper rhyolite is 8.2 Ma. Their age, geochemistry and anhydrous phenocryst assemblage (Plag>CPX>oxides>quartz) are very similar to the rhyolites of the Central Snake River Plane (SRP), indicating that the JSV suite is connected to the eruptive centers of the SRP.

The location of the rhyolites on the upper plate of the RRD-AF, the N-S alignment of the volcanic edifice and the eruptive centers and domes, together with their proximity to the SRP province leads to the interpretation that the JSV suite represents the surface expression of a series of deep N-S trending dikes that intruded south from the main SRP province along the RRD-AF, while the detachment was actively exhuming the metamorphic core complex. Thus the JSV suite provides important evidence about the interaction of a large silicic igneous province with an active extensional detachment system.