Investigations of mineral textures and zoning as evidence for open system processes during magmatic evolution have always been a centerpiece of petrological studies and have provided some of the best evidence for magma mixing and crustal contamination for many decades (e.g., Milch 1905; Kuno 1936; Eichelberger 1975; Sato 1975; Anderson 1976). In fact, evidence from mineral studies was instrumental in the acceptance of magma mixing as important petrological process ever since it was initially proposed by Bunsen (1851). In recent years, mineral studies are invigorated by the development of high-precision, high-resolution analytical instruments and...

The Columbia River Basalt Group (CRBG) is the world’s youngest continental flood basalt province, presumably sourced from the deep-seated plume that currently resides underneath Yellowstone National Park in the northwestern United States. The earliest-erupted basalts from this province aid in understanding and modeling plume impingement and the subsequent evolution of basaltic volcanism. We explore the Picture Gorge Basalt (PGB) formation of the CRBG, and discuss the location and geochemical significance in a temporal context of early CRBG magmatism. We report new ARGUS-VI multicollector ⁴⁰Ar/³⁹Ar incremental heating ages from known PGB localities and additional outcrops that we can geochemically classify as PGB. These ⁴⁰Ar/³⁹Ar ages range between 17.23 ± 0.04 Ma and 16.06 ± 0.14 Ma, indicating that PGB erupted earlier and for longer than other CRBG main-phase units. These ages illustrate that volcanism initiated over a broad area in the center of the province, and the geochemistry of these early lavas reflects a mantle source that is distinct both spatially and temporally. Combining ages with the strongest arc-like (but depleted) geochemical signal of PGB among CRBG units indicates that the shallowest metasomatized backarc-like mantle was tapped first and concurrently, with later units (Steens and Imnaha Basalts) showing increased influence of a plume-like source.

Occurrences of high-Mg andesite (HMA) in modern volcanic arcs raise the possibility that significant volumes of continental crust could be directly derived from Earth's mantle. Such rocks are commonly associated with subduction of young, warm oceanic lithosphere or occur in areas heated by mantle convection. A relatively rare occurrence near Mt. Shasta in the Cascades volcanic arc has been considered to represent one such primary mantle-derived magma type, from which more evolved andesitic and dacitic magmas are derived. Recognition that the Shasta area HMA is actually a hybrid mixed magma, calls into question this notion as well as the criteria upon which it is based. We report new chemical and mineralogical data for samples of the Shasta HMA that bear on the components and processes involved in its formation. Several generations of pyroxenes and olivines are present along with different generations of oxide minerals and melt inclusions. The most magnesian olivines (Fo₉₃) exhibit disequilibria textures, exotic melt inclusions, and reaction rims of Fo₈₇ composition; these crystals along with spongy, ~Mg# 87 orthopyroxene crystals are interpreted to be xenocrystic and do not signify a primitive mantle derivation. The groundmass is andesitic with moderate MgO content, and melt inclusions of similar compositions are hosted by equilibrium olivine (~Fo₈₇). The bulk magma (whole rock) is more magnesian, but primarily due to incorporation of mafic minerals and ultramafic xenolith debris. We propose that the exotic crystal and lithic debris in these rocks is derived from (1) dacitic magmas of possible crustal derivation, (2) prograded ultramafic rocks in the underlying crust, and (3) random lithic debris and crystals derived from conduit wall rocks and earlier intruded magmas within the feeder plexus beneath Shasta. The HMA is inferred to represent a mixture between evolved dacitic and primitive basaltic magmas as well as incorporation of xenolithic crystal cargo. There is no compelling evidence that HMA is present in large volumes, and it is not considered to be an important parental liquid to more evolved magmas at Shasta.