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 H2O contents.
The experimental results are used to explore the influence of H2O and
pressure on fractional crystallization and mantle melting behavior in
subduction zone environments. At 200-MPa H2O-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 SiO2 contents found in any arc magma system
and exhibits only a modest increase in FeO*/MgO with increasing SiO2.
We propose a two-stage process for the origin of these lavas. (1)
Extensive hydrous mantle melting produces H2O-rich (> 4.5-6 wt% H2O)
melts that are in equilibrium with a refractory harzburgite (olivine +
orthopyroxene) residue. Trace elements and H2O are contributed from a
slab-derived fluid and/or melt. (2) This mantle melt ascends into the
overlying crust and undergoes fractional crystallization. Crustal-level
differentiation occurs under near-H2O saturated conditions producing
the distinctive high SiO2 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 H2O contents (> 10 wt% H2O) 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.