EARLY CRETACEOUS MAFIC PLUTONIC ROCKS IN THE RUBY TERRANE AND THEIR CONNECTION TO THE PETROGENESIS OF THE RUBY BATHOLITH, CENTRAL ALASKA

The Ruby terrane of central Alaska consists of Paleozoic continental shelf rocks structurally overlain by thrust slices of Devonian to Jurassic oceanic crust, and is generally thought to record a continent-island arc collision. Over the 500-km length of the terrane, Early Cretaceous granitoid plutons emplaced during the waning stages of the collision constitute 50% of the outcrop. The petrogenesis of these plutons, which are collectively known as the Ruby batholith, has not yet been fully accounted for. Previous isotope geochemical work suggests a large contribution from crustal anatexis, and some workers have suggested that decompression melting during exhumation of the continental rocks could have produced the batholith. The Kokrines Hills in the south-central Ruby terrane expose an important, previously unreported suite of Early Cretaceous mafic to silicic plutonic rocks. Lithologies include gabbro, monzonite-syenite, tonalite, and the K-feldspar-megacrystic granite and granodiorite that are typical of the Ruby batholith. Biotite is ubiquitous in all rock types. Clinopyroxene, variably replaced by hornblende, is very common in the quartz-poor lithologies, constituting up to 40% of some gabbro samples. Field relations indicate that the mafic rocks are coeval with the silicic rocks and that the two magmas were significantly mixed. The country rock in the Kokrines is granulite facies and includes migmatites. Contacts between country rocks and mafic igneous rocks are compositionally gradational and never sharp, suggesting that heat from the mafic magma contributed to anatexis. Because the metamorphic petrology and geochronology of the country rock indicate that the Kokrines Hills expose a deeper structural level than the rest of the Ruby terrane, the mafic magmatic contribution described here may be representative of processes that operated in the genesis of the Ruby batholith at large. Preliminary major- and trace-element and isotope geochemical data will be presented.