U-PB ZIRCON AGE OF DYKES IN THE ROOF OF THE BLACK PEAK INTRUSIVE COMPLEX, NORTH CASCADES

The contact zones of large batholiths can preserve extensive records of the structural, magmatic, and other processes associated with pluton emplacement. The roof of the Cretaceous Black Peak Intrusive Complex (BPIC), part of the crystalline core of the Cascades, is well exposed and reveals a structurally complex region with abundant hydrothermal mineralization, variable contact geometry and an extensive suite of crosscutting dyke rocks. The BPIC was intruded into surrounding metasediments at mid to shallow crustal depths as an elliptical-shaped body between ~87-92 Ma.

Field relations show intrusion of these dykes postdate the main tonalitic phase of the pluton emplaced here, the Crescent Mountain unit, at ~91.4 Ma and are the latest identified phase of magmatism. However regionally the pluton is also cross cut by dyke swarms of Eocene age (ca. 45 Ma). In many cases these dykes are modally and texturally similar to the dykes exposed in the roof zone. Thus prior to devising petrogenetic models that relate dyke rocks to the emplacement of the BPIC it is also important to verify that dyke rocks are Cretaceous in age.

Two samples of dyke rocks obtained from the roof zone east of Crescent Mountain, WA were dated using zircon U-Pb and laser ablation ICP-MS. Both samples represent small (< 1m) thick dykes that demonstrably cross cut tonalite. One dyke is porphyritic in texture the other is pegmatoidal.

We obtained 206Pb-238U weighted mean ages of 89.4 ± 0.9 and 89.9 ± 1.4 Ma (95%), with minor inheritance up to 140 Ma. These ages show that dykes are Cretaceous in age and relate to assembly of the Black Peak pluton. However the dykes are also younger than the local tonalite, suggesting that they may relate to a later phase of intrusion of the pluton – possibly the voluminous Stiletto Mountain unit intruded between ~90-91 Ma. In this case the roof zone of the pluton may have acted as a structural trap for intrusion of a variety of dykes.