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The phase relations of primitive magnesian andesites and basaltic andesites from the Mt. Shasta region, N California have been determined over a range of pressure and temperature conditions and H 2 O contents. The experimental results are used to explore the influence of H 2 O and pressure on fractional crystallization and mantle melting behavior in subduction zone environments. At 200-MPa H 2 O-saturated conditions the experimentally determined liquid line of descent reproduces the compositional variation found in the Mt. Shasta region lavas. This calc-alkaline differentiation trend begins at the lowest values of FeO*/MgO and the highest SiO 2 contents found in any arc magma system and exhibits only a modest increase in FeO*/MgO with increasing SiO 2 . We propose a two-stage process for the origin of these lavas. (1) Extensive hydrous mantle melting produces H 2 O-rich (>4.5-6 wt% H 2 O) melts that are in equilibrium with a refractory harzburgite (olivine + orthopyroxene) residue. Trace elements and H 2 O are contributed from a slab-derived fluid and/or melt. (2) This mantle melt ascends into the overlying crust and undergoes fractional crystallization. Crustallevel differentiation occurs under near-H 2 O saturated conditions producing the distinctive high SiO 2 and low FeO*/MgO characteristics of these calc-alkaline andesite and dacite lavas. In a subset of Mt. Shasta region lavas, magnesian pargasitic amphibole provides evidence of high pre-eruptive H 2 O contents (>10 wt% H 2 O) and lower crustal crystallization pressures (800 MPa). Igneous rocks that possess major and trace element characteristics similar to those of the Mt. Shasta region lavas are found at Adak, Aleutians, Setouchi Belt, Japan, the Mexican Volcanic Belt, Cook Island, Andes and in Archean trondhjemite-tonalite-granodiorite suites (TTG suites). We propose that these magmas also form by hydrous mantle melting.

DefinitionsD VSMOW is the difference between the measured D/H ratio of a sample and the D/H ratio of Vienna Standard Mean Ocean Water (VSMOW) expressed in per mil (‰):