Unraveling the Early Volcanic History of the Pacific Northwest: 

Geochemical Evidence for Widespread Melting of the Deep Crust 

Matthew Loewen and Jeffrey Tepper 

University of Puget Sound Geology Department 

Abstract: 

Fifty million years ago, during a time period known as the Eocene, the Pacific Northwest was home to a volcanic mountain range with chemical traits similar to the Cascades but extending across an area three times as wide. Formation of such a wide volcanic belt would have required a fundamentally different process than the subduction zone present today. Concurrent with this widespread magmatism in NE Washington, voluminous basaltic lava flows erupted in what are now the Olympic Mountains. Various hypotheses have been proposed to account for local aspects of this Eocene volcanism, but to date there has been no comprehensive explanation. 

Our study looks at the chemical and isotopic compositions of granitic rocks across NE Washington in an effort to determine how they formed.  In particular, we are using what we know about the behavior of elements during melting and crystallization of magmas to evaluate the idea that this magmatic episode was triggered by the subduction of a divergent plate boundary, forming a “slab window” that then passed beneath the Pacific Northwest.

Our data reveal that these Eocene rocks are not derived from the mantle but instead originated by the re-melting of older rocks in the deep crust. The “Cascade-like” chemistry of these rocks is actually a relict signature related to their original, much earlier, formation. A massive region-wide heating event 50 million years ago would have been required to drive this melting. Such heating is consistent with the high heat flow expected above a “slab window”.