Constraints on the bulk composition and root foundering rates of continental arcs: A California arc perspective

Garnet pyroxenites are the most common deep lithospheric xenolith assemblages found in Miocene volcanic rocks that erupted through the central part of the Sierra Nevada batholith. Elemental concentrations and isotope ratios are used to argue that the Sierra Nevada granitoids and the pyroxenite xenoliths are the melts and the residues/cumulates, respectively, resulting from partial melting/fractional crystallization at depths exceeding 35-40 km. The estimated major element chemistry of the protolith resembles a basaltic andesite. Effectively, at more than about 40 km depth, batholith residua are eclogite facies rocks. Radiogenic and oxygen isotope ratios measured on pyroxenites document unambiguously the involvement of Precambrian lithosphere and at least 20-30% (mass) of crustal components. The mass of the residual assemblage was significant, one to two times the mass of the granitic batholith. Dense garnet pyroxenites are prone to foundering in the underlying mantle. An average removal rate of 25-40 km³/km Myr is estimated for this Cordilleran-type arc, although root loss could have taken place at least in part after the cessation of arc magmatism. This rate is matched by the average subcrustal magmatic addition of the arc (∼23-30 km³/km Myr), suggesting that the net crustal growth in this continental arc was close to zero. It is also suggested that in order to develop a convectively removable root, an arc must have a granitoid melt thickness of at least 20-25 km. Residues of thinner arcs should be mostly in the granulite facies; they are not gravitationally unstable with respect to the underlying mantle.