Geochemical constraints on the petrogenesis of the Salinian arc, central California: Implications for the origin of intermediate magmas

Magmatic arcs are the primary locations where continental crust is distilled to an intermediate, calc-alkaline composition. The root zones of continental arcs are thought to be the primary sites of magmatic differentiation, yet few deeply exhumed arc sections are available for direct study. The Coast Ridge Belt of central coastal California provides an exceptional opportunity to directly observe the cumulative effects of melting, mixing, assimilation, and homogenization related to construction of the Latest Cretaceous California arc. We present new major and trace element chemistry, as well as radiogenic isotopic ratios determined on Coast Ridge Belt assemblages representative of 20 to 30km crustal levels. Late Cretaceous (ca. 93 to 81Ma) gabbro, diorite, tonalite, and granodiorite of the Coast Ridge Belt are calc-alkaline, some exhibit cumulate characteristics, and all show enriched isotopic compositions (Sr_i=0.7061 to 0.7092 and ε_Nd=+1.4 to -5.9). Rare earth element patterns in igneous and metaigneous rocks of the Coast Ridge Belt suggest that they are sourced deeper than the ~25km paleodepth of the exposed section, but probably not significantly below 40km. Underplating of basaltic melts derived from evolved lithospheric mantle provides the most satisfactory mechanism explaining geochemical and field evidence for partial melting and assimilation of metasedimentary framework rocks to yield gabbroic to dioritic magmas, followed soon thereafter by remelting to produce more silicic magmas. We suggest that basaltic underplating provided a source of heat to the base of the Salinian crust, leading to thermal weakening and downward flow of melt-fertile intra-arc supracrustal assemblages, thereby igniting the magmatic pulse that formed the Salinian arc.