Two New Methods for Geochemical Discrimination of Mafic-Intermediate Volcanic Rocks: A Proof of Concept Study Using Data from the Sierra Nevada Foothills and Klamath Mountains, California

Two new probabilistic, N-dimensional methods for geochemical discrimination developed to determine the original tectonic setting of volcanic rocks have recently been proposed (Snow, JGR; Shragge and Snow, AJS). Here, we present two case studies to demonstrate the effectiveness of these new methods; one from the North Fork terrane in the Klamath Mountains of California and one from the Penon Blanco arc in the Sierra Nevada Foothills of California. The North Fork terrane in the southern Klamath Mountains comprises two distinct volcanic suites. The first are bright green to gray-green metabasalts that are massive to locally pillowed, amygdaloidal, and vesicular with a primary igneous assemblage of clinopyroxene, hornblende, and plagioclase, that are interstratified with chert-argillite and volcanogenic metasediments (Scherer et al., in press). The second are greenstone blocks in a serpentinite matrix melange that have been tectonically removed from their original setting (Scherer et al., in press). Both of the new methods suggest an OIB origin for the first suite of volcanics and an arc origin for the second. This is consistent with the findings of Scherer et al. The Penon Blanco arc comprises two volcanic suites: a lower section of massive-pillowed plagioclase-phyric basalts (Jasper Point Formation), ~900m thick, that are overlain by 50m of radiolarian chert, and a 3500m thick upper section of massive-bedded clinopyroxene + plagioclase phyric flow-breccias and volcaniclastics (Penon Blanco Formation) that are unconformably overlain by the Mariposa formation. The entire package has been metamorphosed to prehnite-pumpellyite facies. The Penon Blanco arc was originally interpreted by Bogen (1985) as MORB (Jasper Point Formation) overlain by arc volcanics (Penon Blanco Formation). Our analysis of these volcanics suggests that both formations represent island arc volcanism. Application of the two new methods for discrimination demonstrate that both can be reasonably applied to metabasalts in addition to the fresh basalts they were designed with. Furthermore, since these methods are both probabilistic, they provide a measurement of certainty that was not available using previous discrimination methods.