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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 plutons are similar to gabbroic plutonic rocks in modern and other ancient island arc complexes. They record the differentiation of calc-alkaline basalt to andesite by the fractionation of plagioclase, pyroxene, olivine, and Fe-Ti oxides. The silicic rocks do not appear to be related to either the gabbros or the monzonites. They may represent partial melts of Skolai arc crust. The monzonitic rocks of the Ahtell complex have shoshonitic chemistry. Similar shoshonitic rocks are widespread in both the Wrangellia terrane and the neighboring Alexander terrane and intrude the contact between the two. In modern oceanic arcs, shoshonitic rocks are typically associated with tectonic instability occurring during the initial stages of subduction or just prior to or during termination or flip of an established subduction zone. The nature of any tectonic instability which may have led to the cessation of subduction in the Skolai arc is unclear. Possibilities include collision of the arc with a ridge, an oceanic plateau, another arc, or a continental fragment. One possibility is that the shoshonitic magmatism marks the late Paleozoic amalgamation of Wrangellia and the Alexander terrane. The scarcity of arc rocks predating the shoshonites in the Alexander terrane supports this possibility, but structural corroboration is lacking.