Read Full Article (file size: 882433 bytes)    Cited by

TECTONICS, VOL. 23, TC6009, doi:10.1029/2003TC001577, 2004

Protolith age of the Swakane Gneiss, North Cascades, Washington: Evidence of rapid underthrusting of sediments beneath an arc

Jennifer E. P. Matzel

Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA

Samuel A. Bowring

Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA

Robert B. Miller

Department of Geology, San Jose State University, San Jose, California, USA

Abstract

The metamorphic core of the North Cascades largely comprises island arc and oceanic terranes juxtaposed prior to circa 96 Ma magmatism. However, the tectonic affinity of the structurally deepest terrane, the 9–12 kbar Swakane Gneiss, is distinctly different from other terranes in the core; it is not intruded by arc-related plutons and contains abundant Precambrian zircons. New U-Pb analyses of detrital zircons from the Swakane Gneiss yield dates from 73 Ma to 1610 Ma with a dominant Late Cretaceous population. These data indicate that the Swakane protolith was deposited as late as 72.5 ± 0.6 Ma, the ²⁰⁶Pb/²³⁸U date of the youngest detrital grain. Following deposition, the gneiss was intruded by peraluminous leucogranite sheets that may represent partial melt derived from the gneiss at near- to postpeak P-T conditions. One sheet yielded a U-Pb crystallization age of 68.36 ± 0.07 Ma, which indicates that the gneiss was deeply buried within 5 Myr of deposition. Two possible mechanisms considered for this rapid burial (∼7 mm/yr) are overthrusting of a forearc or back arc basin by older crystalline rock or underthrusting of trench sediments during low-angle subduction. The model involving overthrusting of a forearc or back arc basin is most consistent with thermobarometric and isotopic data and the regional geologic setting of the Cascades core. Rapid burial of the Swakane protolith is coincident with burial of sediments that formed the Pelona, Orocopia and Rand schists of southern California; however, differences in peak metamorphic temperatures indicate that conditions of burial must have varied along the plate margin.

Received 25 August 2003; accepted 30 September 2004; published 30 December 2004.

Keywords: Cascades; Swakane Gneiss; U/Pb geochronology; detrital zircon; burial mechanism; rates.

Index Terms: 1035 Geochemistry: Geochronology; 8110 Tectonophysics: Continental tectonics—general (0905); 8150 Tectonophysics: Plate boundary—general (3040).

Read Full Article (file size: 882433 bytes)    Cited by

Citation: Matzel, J. E. P., S. A. Bowring, and R. B. Miller (2004), Protolith age of the Swakane Gneiss, North Cascades, Washington: Evidence of rapid underthrusting of sediments beneath an arc, Tectonics, 23, TC6009, doi:10.1029/2003TC001577.

Copyright 2004 by the American Geophysical Union.