FLOW CHEMISTRY AND VENT ALIGNMENTS FROM THE JORDAN VALLEY VOLCANIC FIELD, OREGON: INSIGHTS INTO THE EVOLUTION OF MONOGENETIC VOLCANO FIELDS

Monogenetic Volcano Fields (MVF) consist of discrete volcanoes (such as cinder cones and small shields) that erupt only once in their lifetime. They are found in a wide variety of tectonic settings, but are particularly common in extensional environments. Such fields represent long-lived, low-volume magmatism. The JVVF is a monogenetic volcano field located in southeastern Oregon. Tectonically, it lies in the transition zone between the northern part of the Owyhee Plateau and the southern edge of the Miocene Oregon-Idaho graben. It consists of more than 10 major vents; mostly small, scutulum-type shields, but also some cinder cone remnants. Some of the main vents have associated smaller vents / spatter cones. Only a few of these have been radiometrically dated, however, geomorphic evidence suggests ages ranging from > 5 Ma to less than 5 Ka.

The erupted lavas include high alumina olivine tholeiites (HAOT) and transitional to mildly alkaline basalts. The compositions of lavas from each vent are distinct and cannot be related to each other through any simple differentiation process. One of the vents, the Coffeepot Crater shows compositional variations within its eruptive products. Two chemically distinct lava types were erupted, based on major and trace element as well as Sr isotopic composition. Petrographic variations within different lava flows from some other vents (e.g., the Rocky Butte) have also been documented. Similar observations have very recently been reported from other MVF. This suggests that compositional heterogeneity within monogenetic volcanoes may be more common than has been previously recognized.

Distinct alignments are recognized for vents from the JVVF. Particularly interesting is the NNW-SSE alignment of four vents (ranging in age from > 1 Ma to < 5 Ka). Similarly, three vents are aligned along an E-W trend. Once again, such alignments are observed in several MVF. The magmas probably follow pre-existing zones of weakness such as faults and fractures, particularly if these are favorably aligned with respect to the local and regional tectonic stresses. The relatively lower number of vents along alignments and their discrete geochemistry in the JVVF provides an excellent laboratory to answer some fundamental questions regarding source heterogeneity and plumbing systems of MVF in general.