GEOCHEMICAL EVIDENCE FOR THE DEVELOPMENT OF A GARNET-BEARING CRUSTAL ROOT IN THE CRETACEOUS NORTH CASCADES MAGMATIC ARC, WA

Many arc magmas are depleted in HREEs, a feature that is often associated with the presence of garnet in the lower crust. This deep crustal garnet may be a residue left behind following partial melting or form during fractional crystallization of mantle-derived melts. Both of these processes require thickened arc crust in order for the lower crust to reach sufficient temperature and pressure conditions for garnet formation, meaning that it is likely that lower crustal composition and associated arc magmas evolve as crustal thickness changes. Understanding the changes that occur within an arc as the crust thickens requires detailed geochemical and isotopic work on well-dated intrusions.

The Black Peak intrusive complex (BPIC; 91.7-87 Ma) in the North Cascades of Washington offers a unique opportunity to link high-precision geochronology, whole-rock geochemistry, and oxygen isotopic data from the same samples in order to better understand how the arc changed over ~5 My.

Our data indicate that intrusions in the BPIC become more felsic and more HREE-depleted as they get younger: La/Yb_N changes from ~5 in the oldest dated rocks to >20 in the youngest. The lack of significant change in zircon δ¹⁸O over time suggests that thickening in the North Cascades arc was not related to any process involving supracrustal rocks (such as relamination or faulting). Instead, the data suggest that the depleted HREE signature in late (<90 Ma) felsic intrusions in the BPIC was caused by melt interaction with a garnet-bearing lower crust. This garnet-bearing lower crust was likely generated by the accumulation of the products of fractional crystallization (e.g., hornblende), ultimately thickening to the point where garnet was stable during magma generation. The gradual increase in garnet signature in the BPIC through time is interpreted as evidence for a progressively thickening crust in the North Cascades arc from 90 Ma to 87 Ma. This period also represents a major period of plutonism in this region. Intriguingly, the BPIC also reaches a “maximum” HREE depletion (and interpreted crustal thickness) immediately prior to the cessation of magmatism at approximately 87 Ma.