MANTLE MELTING AND PLATE TECTONIC CONTROLS ON MAGMATISM IN THE CASCADE ARC: A PETROLOGIC PERSPECTIVE

Experiments on a primitive magnesian andesite, basaltic andesite and high-alumina olivine tholeiite from the Mt. Shasta- Medicine Lake region, N. California have been carried out over a range of pressure and temperature conditions and H₂O contents. Partial melting of mantle wedge peridotite fluxed by volatiles from the subducted oceanic lithosphere has played an important role in the origin of Mt. Shasta region primitive lavas. Extensive hydrous mantle melting produces H₂O-rich (>4.5 to 6 wt. % H₂O) melts that are in equilibrium with a refractory harzburgite (olivine + orthopyroxene, > 25 % melting) residue at shallow mantle depths (~ 1 GPa). Trace elements and H₂O are contributed from a slab-derived fluid and/or melt. In contrast, shallow mantle melting under anhydrous conditions has led to the production of primitive high-alumina olivine tholeiites throughout the arc and back arc in the S. Casacades. These melts are in equilibrium with a more fertile spinel lherzolite and represent smaller degress of shallow melting (1 GPa, ~ 5 - 7 % melting) under anhydrous conditions. These two types of magmas are erupted in close spatial and temporal association from the Mt. Shasta region west to Medicine Lake and south to the Lassen region.

The magmatic flux from these southernmost Cascade volcanoes in the past million years dwarfs rest of the arc volcanism in the north Cascades (the back-arc Newberry Center is an exception). The arc volcanic centers to the north are smaller by a factor of ~10. Both hydrous and anhydrous melts occur in these volcanic centers, and suggest similar melting processes occur (again, Newberry may be the exception), but the magmatic flux is significantly less.

The explanation of the difference in magma generation rates between north and south may lie in the subducted lithosphere that is supplied beneath the mantle wedge. In the southern Cascades the subduction of the Blanco Fracture zone and fracture zones in the Gorda Plate may provide a larger supply of H₂O bound in serpentinized mantle lithosphere that reaches depths of 200 km beneath the wedge before it releases its H₂O, fluxing magma generation, lowering mantle viscosity and promoting mantle flow. The more abundant supply of H₂O from the subducted lithosphere may be the key ingredient that leads to a higher rate of melt generation.