Title
Late Orogenic Mafic Magmatism In The North Cascades, Washington: Petrology And Tectonic Setting Of The Skymo Layered Intrusion
Document Type
Article
Publication Date
5-1-2008
Publication Title
Geological Society Of America Bulletin
Department
Geology
Abstract
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.
Volume
120
Issue
56
pp.
531-542
ISSN
0016-7606
Citation
Whitney, Donna L., Jeffrey H. Tepper, Marc M. Hirschmann, and Hugh A. Hurlow. 2008. "Late orogenic mafic magmatism in the North Cascades, Washington: Petrology and tectonic setting of the Skymo layered intrusion." Geological Society Of America Bulletin 120(5-6): 531-542.