GARNET-BEARING GRANITIC PLUTON IN A COLLISIONAL TERRANE SUTURE ZONE, NORTHERN TALKEETNA MOUNTAINS, SOUTH-CENTRAL ALASKA

A granitic pluton (10’s km²) emplaced in the suture zone of the Wrangellia composite terrane (WCT) in southern Alaska is cross-cut by small-scale (<1 m) garnet-bearing granitic dikes. This is the first known report of garnet-bearing granitic rocks in this region. The garnet-bearing dikes contain an average of 2.5% garnet that ranges from <1 mm to 2 mm in diameter. Garnet grains are subhedral to euhedral, show embayed and sieve textures, and contain quartz intergrowths, all indicating primary igneous garnet. The remaining mineralogy of these dikes includes 33% quartz, 12% alkali feldspar, 26% plagioclase, 7% biotite, and trace percentages of muscovite, hornblende, zircon, apatite, and titanite. The mineralogy of the host granitic pluton is 25% quartz, 13% alkali feldspar, 46% plagioclase, 15% biotite and trace percentages of zircon, hornblende, apatite, and titanite. Myrmekite and graphic texture are abundant within the garnet-bearing dikes and are less prevalent in the host granitic rocks.

The garnet-bearing dikes and host granite are peraluminous, high K calc-alkaline, and transitional between S- and A-type granites. They also have low to moderate chondrite-normalized light rare earth element enrichment and primitive-mantle normalized 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 granitic host is geochemically similar to other collisional igneous rocks in the WCT suture zone and is consistent with a model in which granitic magmas formed by assimilation-fractional crystallization from an enriched mafic source that partially melted and assimilated aluminum-rich crustal rocks of the suture zone (Kahiltna assemblage flysch). The garnet-bearing dikes represent small volumes of late-stage magma that formed as partial melts of Kahiltna flysch, yielding modal garnet and muscovite, and releasing fluids which contributed to the formation of the myrmekite and graphic textures either as exsolution or metasomatic reactions. The granitic pluton and garnet-bearing granitic dikes confirm an episode of collisional magmatism in the WCT suture zone that involved partial melting of upper- to mid-level sedimentary crustal rocks.