ABSTRACT Ferroan granite is a characteristic rock type of the Laurentian margin. It is commonly associated with anorthosite and related rocks. Ferroan granites are strongly enriched in iron, are alkalic to alkali-calcic, and are generally metaluminous. These geochemical characteristics reflect their tholeiitic parental magma source and relatively reducing and anhydrous conditions of crystallization. Their compositions distinguish them from arc magmas, which are magnesian and calcic to calc-alkalic. Ferroan granite magmas are hot, which promotes partial melting of their crustal wall rocks. Assimilation of these silica-rich and peraluminous melts drives the resulting magmas to higher silica and aluminum saturation values. Where Proterozoic ferroan granites intrude Archean crust, their mantle component is readily identified isotopically, but this is more difficult where they intrude relatively juvenile crust. Ferroan granite forms in tectonic environments that allow partial melts of tholeiitic mantle to pond and differentiate at or near the base of the crust. Phanerozoic examples occur in plume settings, such as the Snake River Plain and Yellowstone, or under certain conditions involving slab rollback, such as those that formed the Cenozoic topaz rhyolites of the western United States or ferroan rhyolites of the Sierra Madre Occidental. It is possible that the long-lived supercontinent Nuna-Rodinia, of which Laurentia was a part, formed an insulating lid that raised underlying mantle temperatures and created a unique environment that enabled emplacement of large volumes of mafic melt at the base of the crust. Ascent of felsic differentiates accompanied by variable crustal assimilation may have created large volumes of Proterozoic ferroan granite and related rocks.

ABSTRACT Phanerozoic continental subduction zones have produced thick continental crust composed almost entirely of granitoid plutons. While ideas about how plutons form have evolved from models that envisioned large, highly molten magma bodies, the exact processes involved remain debated. Geochronology and seismology have led to the view that plutons form by incremental emplacement; stacked sills represent one type of incremental model whereby granitoids grow top-down by sills underplating their predecessor. Still, many questions remain unanswered, including why sill-like contacts are not often seen in more mature plutons, why the mafic residuum is not observed with many granitoid plutons, why some plutons are compositionally zoned (and others are not), and why geochemical characteristics of intrusions systematically change during magmatic cycles. Here, we propose a hypothesis for the construction of batholiths by amalgamation of plutons formed in a two-stage process. During stage 1, intermediate-composition sills underplate previous sills, forming a moving reaction zone mafic complex that produces a thickening granitoid as the process moves downward. The top of this mafic complex also releases a water-rich, low-temperature silicate liquid (LTSL), which begins ascent by reactive porous flow. During stage 2, the upward flux of LTSL further differentiates the overlying granitoids, increasing silica by 5%–10% and resulting in linear “mixing-like” behavior on Harker diagrams. Multiple plutons building downward in a magmatic cycle thicken the crust, leading to garnet gabbro mafic complexes forming at ~20 km depth. Their high density leads to delamination and net silicification of continental crust. The continuous flux of LTSL up through the arc crust can explain many geochemical spatial-temporal trends found in magmatic cycles, including Pb isotope evolution and increasing Fe 3+ , and provides a mechanism for addition of water to the upper crust, leading to sustained volcanism through time.