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Late orogenic mafic magmatism in the North Cascades, Washington: Petrology and tectonic setting of the Skymo layered intrusion

Donna L. Whitney^1,, Jeffrey H. Tepper², Marc M. Hirschmann³ and Hugh A. Hurlow⁴

¹ Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA
² Department of Geology, University of Puget Sound, Tacoma, Washington 98416, USA
³ Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA
⁴ Utah Geological Survey, Salt Lake City, Utah 84114, USA

Correspondence: E-mail: dwhitney{at}umn.edu

The Skymo Complex in the North Cascades, Washington, is a layered mafic intrusion within the Ross Lake fault zone, a major orogen-parallel structure at the eastern margin of the Cascades crystalline core. The complex is composed dominantly of troctolite and gabbro, both with inclusions of primitive olivine gabbro. Low-pressure minerals in the metasedimentary contact aureole and early crystallization of olivine + plagioclase in the mafic rocks indicate the intrusion was emplaced at shallow depths (<12 km).

The Skymo rocks have trace-element characteristics of arc magmas, but the association of Mg-rich olivine (Fo_88–80) with relatively sodic plagioclase (An_75–60) and the Al/Ti ratios of clinopyroxene are atypical of arc gabbros and more characteristic of rift-related gabbros. A Sm-Nd isochron indicates crystallization in the early Tertiary (ca. 50 Ma), coeval with the nearby Golden Horn alkaline granite. Mantle melting to produce Skymo magma likely occurred in a mantle wedge with a long history of arc magmatism. The Skymo mafic complex and the Golden Horn granite were emplaced during regional extension and collapse of the North Cascades orogen and represent the end of large-scale magmatism in the North Cascades continental arc.

Key Words: gabbro • layered mafic intrusion • North Cascades • orogenic collapse • Skymo Complex