BIMODAL HYPABYSSAL INTRUSIONS FOLLOWING GRANITIC PLUTON EMPLACEMENT IN A COLLISIONAL TERRANE SUTURE ZONE, NORTHERN TALKEETNA MOUNTAINS, SOUTH-CENTRAL ALASKA

Magmatism was prevalent along the suture zone between the accreted Wrangellia terrane and southern Alaska. The suture zone includes Late Paleozoic-Cretaceous oceanic gabbro, basalt, and pelitic sedimentary rocks. These rocks are intruded by Paleocene granitic plutons that yield 9 new zircon U-Pb ages of 62.1 to 57.4 Ma, coincident with 62-60 Ma peak metamorphism and deformation in the suture zone and confirming these as collisional plutons. The granitic plutons are peraluminous, have intraplate geochemistry, and some are similar to A-type granites. These traits, together with km-wide migmatite zones that mingle with the plutons, indicate that the granitic magmas formed largely as melts of suture zone sedimentary rocks.

The granitic plutons are cross-cut by basaltic and acidic dikes which form a late-stage bimodal shallow intrusive suite. One rhyolite dike yields a zircon U-Pb age of 57.0 Ma. The basaltic dikes are tholeiitic with moderate Mg# of 0.4 to 0.55 and have geochemical affinity between E-MORB and OIB. Some contain mm-scale olivine-bearing xenoliths. The basaltic dikes formed as partial melts of the oceanic mantle of the accreted Wrangellia terrane. The acidic dikes are peraluminous and have relatively flat REE patterns and negative Eu anomalies, similar to the REE of S-type granites (La/Yb 0.5 to 14, with La 20-50x chondrite). A set of the acidic dikes are coarse-grained and contain abundant myrmekite, a few modal percent garnet, and trace tourmaline, cordierite, and muscovite. These acidic magmas formed by melting of suture zone pelitic rocks and also included residual phases of the earlier granitic magmas.

The basaltic dikes confirm mantle involvement during collisional magmatism. We envision that mantle upwelling occurred at the end of terrane accretion (62-60 Ma) possibly due to delamination of thickened suture zone lithosphere and/or slab detachment. This led to widespread heating and partial melting of sedimentary rocks to form the granitic magmas. Adiabatic processes and/or fluid release from pelitic rocks then promoted partial melting of Wrangellia mantle to form the basaltic intrusions. Shallow pooling of mafic magmas then resulted in further partial melts of upper crustal sedimentary rocks which were co-emplaced with residual granitic magmas to form the last phase of acidic intrusions.