GRANITIC AND VOLCANIC EPISODES IN A COLLISIONAL SETTING, CLARK CREEK IGNEOUS FIELD, NORTHERN TALKEETNA MOUNTAINS, ALASKA

The Clark Creek igneous field is situated within a regional belt of plutons and volcanic rocks across the suture zone of the Wrangellia composite terrane (WCT). Zircon U-Pb ages by LA-ICPMS show early Paleocene emplacement of granodiorite to tonalite (63.1 +/- 1.1), late Paleocene emplacement of granite (59.0 +/- 1.0), and younger shallow intrusions and volcanic rocks (57.0 +/- 0.6 for andesite dike). The e. Paleocene plutons mingle with migmatite and are intruded by basaltic plugs (km –scale) that contain granite and gneiss xenoliths. Both sets of plutons are cross-cut by basalt, andesite, and acidic dikes (m-scale). The volcanic unit is 100’s m thick and contains andesite and rhyolite lavas, vitric-lithic tuff, and dacite-rhyolite pyroclastic flow breccias with clasts of granite, gneiss, and argillite.

Basaltic samples have TiO2/Yb and Nb/Yb ratios like E-MORB, La/Yb ratios of 7.7 to 9.8, elevation of fluid-mobile elements (FME) (e.g., Cs, Ba, U, Pb) and paired Nb-Ta depletion. The late Paleocene acidic volcanic and granite samples have La/Yb ratios of 11 to 13, negative Eu anomalies, FME enrichment, and strong depletion of P and Ti compared to the mafic samples. The early Paleocene granodiorite samples are distinctly different; they are depleted in heavy rare earth elements (La/Yb ratio up to 44), have high Sr/Y ratio (80 to 139), and do not show P or Ti depletion. All of the acidic samples are peraluminous and ferroan indicating their forming under low oxygen fugacity consistent with intraplate settings as opposed to active arc environments.

Our new ages are consistent with the timing of magmatism across the entire WCT suture zone with early Paleocene granitic magmatism (ca. 61-63 Ma) coinciding with peak collision of the WCT. Magmas at this time were generated in part as lower crustal melts with adakitic and intraplate characterisitcs. Late Paleocene magmas (ca. 57-59 Ma) were generated post-collisionally from a combination of WCT mantle and crustal sources and were emplaced through the earlier plutons. Arc-like chemical affinity was most likely inherited from metasomatized mantle that was remnant from Late Cretaceous subduction. The Clark Creek igneous field is a case example of rapid and short-lived episodes of overlapping plutonic and volcanic activity in a collisional tectonic setting.