COLLISION, DEEP BURIAL, AND MAGMATISM ON WESTERN NORTH AMERICA: IMPLICATIONS FOR THE TIMING OF MID-CRETACEOUS MARGIN GROWTH

The timing and dynamics of terrane accretion along the western margin of North America in the Early to mid-Cretaceous remains a subject of intense debate. In particular, several models exist for motion of the southern Coast Mountains batholith/Insular (Alexander + Wrangellia) superterrane during this time. These models include: terrane accretion, major sinistral motion of the terrane, and terrane accretion followed by rifting and re-accretion. To evaluate these models, we investigate the continent margin for evidence (or lack thereof) a major compressional event at this time.

New U-Pb zircon geochronology, field observations, and whole-rock geochemistry from north-central Washington suggest a period of plutonism and partial melting inboard of the Coast Mountains batholith/Insular superterrane in the mid-Cretaceous. This event was defined by large-scale intrusion of the Okanogan Range batholith into the Okanogan-Spences Bridge arc and migmatization in the nearby Chelan Migmatite Complex (CMC), an assemblage of partially melted meta-igneous rocks. The extent of magmatism in both the CMC and the Okanogan Range batholith imply this was a regional event, potentially stretching over 200 km.

Geochronology on 7 samples from the CMC, along with published data, show evidence for multiple periods of zircon growth and resorption. These data suggest that igneous protoliths for the migmatite intruded during the Jurassic, but rare zircon cores are as old as 261 Ma. Most samples contain abundant zircon rims with ages ranging between approximately 118 Ma and 107 Ma, suggesting that migmatization occurred during this time. These dates agree with published (114-111 Ma) and new crystallization ages on the Okanogan Range batholith and suggest that partial melts produced in the CMC may have contributed to magmatism in the Okanogan Range batholith.

Existing P-T estimates and fieldwork on the CMC and adjacent supracrustal units indicate burial to approximately 25-30 km prior to 90 Ma. Together, these data suggest that deep burial and migmatization of the CMC, along with magmatism in the Okanogan Range batholith, may have been the result of a major mid-Cretaceous compressional tectonic event in the region.