EVOLUTION OF A LATE CRETACEOUS-EARLY TERTIARY GRANITIC PLUTON AND ASSOCIATED GARNET-BEARING DIKES AND MIGMATITE IN A COLLISIONAL TERRANE SUTURE ZONE, TALKEETNA MOUNTAINS, SOUTH-CENTRAL ALASKA

Late Cretaceous-early Tertiary granitic plutons were emplaced into the Kahiltna flysch in the suture zone between the Wrangellia composite terrane (WCT) and the former margin of southern Alaska. The plutons have intraplate geochemistry, deviating from typical calc-alkaline arc rocks. This is the first detailed study of one of these plutons which includes a host granite, surrounding migmatite, garnet-bearing granitic dikes, and dacite and basalt shallow intrusions. The host granitic pluton contains, in decreasing abundance, plagioclase, quartz, alkali feldspar, and biotite with trace zircon, hornblende, apatite, and titanite. The migmatite is ~1 km wide and changes away from the pluton from a strongly foliated quartz-biotite gneiss to a blocky fabric of biotite-quartz schist intermingled with flow textured granitic gneiss and granitic veins. The garnet-bearing dikes cross-cut the host granite and migmatite and contain an average of 2.5% garnet with quartz, plagioclase, alkali feldspar, cordierite, and biotite and trace muscovite, tourmaline, zircon, apatite, and titanite with abundant myrmekite and graphic textures. The shallow intrusions cross-cut all of the above, representing a post-granitic phase of magmatism. Geochemically, the garnet-bearing dikes and host granite are peraluminous, high K calc-alkaline, and transitional with A-type granites. They have low to moderate chondrite-normalized light rare earth element enrichment, enrichment in some large ion lithophile elements, and depletion of Ba, Sr, Eu, and Ti (consistent with fractional crystallization of plagioclase and Fe-Ti oxides).

The migmatite, flow textures, and lack of discordant contacts indicate ~15 km depth of emplacement. The A-type affinity with the migmatite indicates high heat flow and partial melting of the pelitic Kahiltna assemblage during granitic magma petrogenesis in the WCT suture zone. Aluminous accessory minerals, myrmekite, and graphic textures in the garnet-bearing dikes indicate a greater degree of Kahiltna partial melting and metasomatic reactions during the final phase of granitic magma emplacement. Crustal melting in the WCT suture zone may have been induced by lithosphere thickening and delamination in the collisional suture zone and/or high heat flow related to a coeval slab window in southern Alaska.