Magmatically induced metamorphism and deformation in the Kigluaik gneiss dome, Seward Peninsula, Alaska

• Published in: Tectonics (Washington, D.C.) Vol. 13; no. 3; pp. 515 - 527
• Main Authors: Amato, Jeffrey M., Wright, James E., Gans, Phillip B., Miller, Elizabeth L.
• Format: Journal Article
• Language: English
• Published: American Geophysical Union 01-06-1994; Blackwell Publishing Ltd
• Subjects: INE, USA, Alaska, Seward Peninsula; INE, USA, Alaska
• ISSN: 0278-7407; 1944-9194
• DOI: 10.1029/93TC03320

Field observations and U‐Pb isotopic data from plutonic and high‐grade metamorphic rocks within the Kigluaik gneiss dome on the Seward Peninsula, Alaska, document a Late Cretaceous age of peak metamorphism and shed light on the relationship between fundamentally mantle‐derived magmatism and the development of the gneiss dome. The dome consists of upper‐amphibolite‐facies to granulite‐facies metasedimentary rocks mantling the granitoid Kigluaik pluton. The main deformational fabric in the dome is a well‐defined, moderately dipping foliation and compositional layering that contains a pervasive, east‐west trending mineral‐elongation lineation defined by sillimanite and hornblende. Leucosomal segregations in migmatite are boudinaged and isoclinally folded, and late‐stage tension gashes are filled with melt, demonstrating that most deformation occurred at peak metamorphic temperatures. The large pluton in the core of the dome consists of a granitic cap overlying a mafic to intermediate root. Mafic pillows with crenulate margins and spectacular magma mingling textures indicate that the two magmas were coeval. The Kigluaik pluton is largely undeformed and discordant, although some dikes possess a weak deformational fabric. The lack of quench textures at the margins of the pluton and very limited alteration in adjacent wall rock suggest that the pluton was emplaced while country rocks were still at high temperatures. U‐Pb analyses of three fractions of zircon from a highly strained and concordant garnet‐bearing granite orthogneiss yield an intrusive age of 105 Ma; therefore significant deformation in the dome must have occurred after 105 Ma. Several U‐Pb analyses of monazite from metapelite and pegmatite (derived from partial melting of metapelite) yield an age of 91 ± 1 Ma for high‐temperature metamorphism and deformation. U‐Pb analysis of eight fractions of zircon from the Kigluaik pluton shows that it crystallized at 92 ± 2 Ma. Thus there is both field and geochronologic evidence for coeval magmatism, metamorphism, and deformation at about 92 Ma, which may represent the latest stages of a more protracted tectonic event. We present a model for gneiss dome development wherein a large, silicic to intermediate magmatic diapir with its mantling gneisses ascended from ∼35 km to ∼15 km during an event associated with crustal extension in northern Alaska. We emphasize the close link between plutonism and gneiss dome development and the fundamentally mafic character of this plutonism.