Chapter 17: Partial melting of metabasites in the contact aureoles of gabbroic plutons in the Smartville Complex, Sierra Nevada, California

Metamorphosed volcanic and hypabyssal rocks (metabasites) have been partially melted in the contact aureoles of gabbroic plutons in the Smartville Complex of northern California. The melting occurred in the upper crust of an active arc at pressures of less than 3 kbar and temperatures that may have exceeded 900 °C. The resultant rocks resemble intrusive breccias and consist of pyroxene hornfels of metamorphic origin and clearly igneous diorite. Coexisting hornfels and diorite appear to be in mineralogical equilibrium. The bulk compositions of the hornfels and of the hornfels-diorite mixed rocks as a whole are depleted in alkalis and silica and enriched in Fe and Mg (restitic) with respect to their inferred greenstone protoliths. The diorite veins and layers in the mixed rocks are unlike any other intrusive rocks in the Smartville Complex in that they have refractory bulk compositions, but evolved mineral compositions. A partial melt origin for the mixed rocks is consistent with their high temperatures of formation, the inferred equilibrium between hornfels and diorite, and the estimated major-element chemistry of the hornfels. The refractory chemistry of the diorite veins and layers and the restitic character of the mixed rocks as a whole suggest that a melt has been extracted from the mixed rocks. Although the mixed rocks physically resemble intrusive breccias, they are by most definitions migmatites. Some Smartville Complex tonalites may be related to melts extracted firom these rocks.