CONTAMINATION OF SILICIC MAGMAS AND FRACTAL FRAGMENTATION OF XENOLITHS IN PALEOCENE PLUTONS ON KODIAK ISLAND, ALASKA

Abstract

Paleocene plutons on Kodiak Island, in Alaska, contain host-rock xenoliths that have undergone varying degrees of fragmentation, metamorphism and assimilation in the surrounding granitoid rocks, and therefore provide an excellent natural laboratory to study the processes of xenolith fragmentation and magma contamination. Kodiak Island has two belts of Paleocene plutons: the Kodiak batholith that runs along the axis of the Island, and the trenchward belt, which is comprised of plutons and dikes intruded into the Ghost Rocks Formation south of the Contact fault. Within the Kodiak batholith, there are two different populations of xenoliths, one near its exterior margin and another near its center. The exterior xenoliths are relatively rare but, where present, are angular and have retained sedimentary structures. Xenoliths from the center of the Kodiak batholith are abundant, rounded, gneissic, garnet- and sillimanite-bearing, and had an extended residence-time in the batholith. In the trenchward belt, the Shaft Peak pluton contains a high abundance of Ghost Rocks Formation xenoliths that underwent extensive fragmentation at length scales ranging from >10 m to a submillimetric scale. The frequency-size distributions of xenolith in both the Kodiak batholith and the Shaft Peak pluton can be modeled by power-law functions and can be described by renormalization-group fractal methods of fragmentation. Our qualitative descriptions and the fractal analysis indicate that the Shaft Peak xenoliths underwent catastrophic fragmentation and were in a state of disequilibrium when its system froze, whereas the Kodiak batholith xenoliths, once they fragmented down to a diameter of <10 cm, approached thermal and chemical equilibrium with the surrounding magma.