STRUCTURE AND LITHOLOGIES OF DEEP LEVELS WITHIN THE NORTH CASCADES CONTINENTAL MAGMATIC ARC, NORTH CASCADES, WA

The crystalline core of the North Cascades of Washington exposes deep levels of a Mesozoic continental magmatic arc, and is a valuable location for studying metasedimentary rocks emplaced into the deep levels of arcs. The 9-12 kb Swakane Biotite Gneiss is the deepest unit recognized, and represents Late Cretaceous clastic rocks with a maximum depositional age (MDA) of 93-81 Ma; it was emplaced into the arc by underplating or fore-arc thrusting. The poorly studied, informally named Spider Glacier unit appears to dip beneath the Swakane Gneiss. Previous workers interpreted the unit as migmatitic gneiss of uncertain affinity, part of the oceanic Napeequa complex, or part of the Cascade River-Holden unit. The latter unit contains Triassic arc and Cretaceous clastic rocks with a maximum depositional age (MDA) of 97 Ma. Our new U-Pb dates on Spider Glacier detrital zircons yield an Early Cretaceous (107 Ma) MDA precluding correlation with the Paleozoic to Triassic Napeequa complex, and suggesting that the unit is at least in part older than the Swakane Gneiss.

The Spider Glacier unit was mapped at 1:12,000 to elucidate its lithology, structure and relationships to adjacent units. The unit is composed of biotite ± garnet schist, biotite-hornblende ± garnet schist, quartz-bearing amphibolite, stromatic metatexite, and rare diatexite and micaceous quartzite. One- to 50-cm-wide lenses of metasomatized peridotite and hornblende gabbro occur at some locations. Foliation defined by compositional layering, aligned mafic minerals and elongate quartz aggregates strikes NW to NE and dips 37-74° W. Abundant felsic bodies occur as stromatic leucosomes and felsic sheets. The Spider Glacier unit contains at least two generations of structures. Early deformation is marked by foliation, and recumbent, tight to isoclinal folds of felsic bodies with axial planes parallel to foliation. Late outcrop-scale, upright folds fold foliation, and have variable trends and plunges.

The Spider Glacier unit is more heterogeneous than the Swakane Gneiss, but there are some common lithologies. Foliations in the unit are generally concordant and parallel to the Swakane contact, but are more widely folded. To reconcile the similarities and differences, we speculate that the units may have formed and been emplaced beneath the arc by similar processes.