U-PB GEOCHRONOLOGIC EVIDENCE OF A LATE CRETACEOUS PROTOLITH AGE FOR THE SWAKANE GNEISS, NORTH CASCADES, WA

The crystalline core of the Cretaceous North Cascades arc contains large tracts of deep crust recording pressures of 9-11 kbar. These island arc and oceanic terranes are thought to have been juxtaposed by ~96 Ma. However, the origin of the deepest unit, the Swakane Gneiss, is enigmatic because unlike other terranes in the North Cascades, the Swakane Gneiss lacks arc-related plutons, contains Precambrian zircons, and has Proterozoic Nd depleted mantle model ages.

Metasedimentary rocks in the Swakane Gneiss have a variety of detrital zircons morphologies. Single zircon, U-Pb ID-TIMS analyses yield discordant (15-70 %) Pb-Pb dates that range from 1178 to 1610 Ma; however, the dominant population of zircons is concordant and Late Cretaceous. Cathodoluminescence images of representative zircons show oscillatory zoned (igneous) cores with chaotically zoned (metamorphic) overgrowths. Zircons plucked from a grain mount after imaging have a range of crystallization dates (73.5-1610 Ma). Even the youngest population of zircons has oscillatory zoned cores interpreted to indicate that they are magmatic and were subsequently deposited in the Swakane protolith. The geochronological data place a maximum age on the deposition of the Swakane protolith at 73 Ma, the age of the youngest detrital grains. The timing of metamorphism has been difficult to determine. Zircons from a deformed leucocratic dike yield a crystallization age of 68 Ma. The composition of similar dikes suggests they are locally derived and therefore metamorphism and partial melting of the Swakane Gneiss likely occurred ca. 68 Ma.

These results indicate that the Swakane Gneiss is allochthonous with respect to the arc and was likely deposited between the arc and the North American craton with Mesoproterozoic zircons derived from the craton and Late Cretaceous zircons possibly derived from the arc itself. The Swakane Gneiss was involved in only the latest stages of arc evolution and could not have contributed to the generation of the overlying arc sheets and plutons. The deposition of this far-traveled unit, followed by rapid burial to >30 km and subsequent exhumation by ~45 Ma indicates that exhumation and juxtaposition of the Swakane Gneiss in the final stages of arc development may have been accommodated by extensional collapse.