Fractional crystallization and mantle-melting controls on calc-alkaline differentiation trends

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.