Petrology and Tectonic Significance of Gabbros, Tonalites, Shoshonites, and Anorthosites in a Late Paleozoic Arc-Root Complex in the Wrangellia Terrane, Southern Alaska

Plutonic rocks intrusive into the late Paleozoic Tetelna Formation of southern Alaska are the underpinnings of the late Paleozoic Skolai arc of the Wrangellia Terrane. There are four groups of intrusive rocks within the Skolai arc: (1) Gabbro-diorite plutons that contain gabbroic to anorthositic cumulates along with a differentiated series of gabbros and diorites of basaltic to andesitic composition; (2) Silicic intrusions including tonalite, granodiorite, and granite; (3) Monzonitic to syenitic plutonic rocks of the Ahtell complex and related dikes and sills; (4) Fault-bounded bytownite anorthosite of uncertain age and association. These anorthosites may be related to post-Skolai, Nikolai Greenstone magmatism. The silicic rocks yield discordant U-Pb zircon ages of 290-320 Ma (early to late Pennsylvanian). Relative age relations suggest that the oldest intrusive rocks are the gabbro-diorite plutons, the youngest are the monzonitic rocks, and that the silicic rocks span this range. The gabbro-diorite pluto...