HR: 14:55h
AN: V32F-06    [PDF]
TI: Geochronologic and Isotopic Constraints on Arc Dynamics, North Cascades, WA
AU: * Matzel, J
EM: jpmatzel@mit.edu
AF: Massachusetts Institute of Technology, Dept of Earth, Atmospheric and Planetary Sciences 77 Massachusetts Ave, Cambridge, MA 02139 United States
AU: Bowring, S A
EM: sbowring@mit.edu
AF: Massachusetts Institute of Technology, Dept of Earth, Atmospheric and Planetary Sciences 77 Massachusetts Ave, Cambridge, MA 02139 United States
AU: Miller, R B
EM: rmiller@geosun.sjsu.edu
AF: San Jose State Universtity, Geology Department One Washington Square, San Jose, CA 95192 United States
AB: The metamorphic core of the North Cascades arc records the Cretaceous to Paleogene history of crustal growth along a segment of the North American margin. The application of high precision U-Pb geochronology and isotope geochemistry to the study of this arc has revealed rapid changes in arc dynamics. Terrane accretion/assembly in the North Cascades predates the main phase of arc magmatism at 96-88 Ma, and is followed by magmatism in the eastern half of the core from 78-46 Ma. The 9-12 kbar, Swakane Gneiss underlies these island arc and oceanic terranes, but lacks arc-related plutons. U-Pb analyses of detrital zircons from the Swakane Gneiss reveal that the sedimentary protolith of the gneiss was deposited later than 72.6$\pm$0.6 Ma, the age of the youngest detrital grain, and post-dates voluminous arc magmatism. The gneiss was deeply buried $\sim$5 m.y. later as indicated by a 68.5 Ma dike that cross-cuts fabric in the gneiss, and this deep burial is coincident with a decrease in the volume of arc magmatism. Between 68 and 48 Ma, plutonism appears to be restricted to the eastern margin of the core near the Ross Lake fault zone. By ca. 45 Ma, the Swakane Gneiss was unroofed to near the surface, and several ca. 46 Ma arc plutons were emplaced throughout the eastern half of the core. Nd isotope data from granodioritic, tonalitic and gabbroic plutons that are spatially and temporally distributed within the arc indicate that all of the plutons were derived from or interacted with an older crustal component. Initial $\epsilon$Nd values of the ca. 46 Ma plutons (+1.5 to +2.9) are lower than $\epsilon$Nd values of the 96-65 Ma plutons (+2.9 to +6.0 when calculated at 46 Ma) and are consistent with the younger plutons either being derived from a more evolved crustal source or being composed of a greater proportion of crustally-derived melt. Changes in the Nd isotope signature may reflect changes in crustal architecture after burial of the gneiss. The mechanism of this rapid burial is not well-constrained, but two potential mechanisms include overthickening during closure of an arc basin or underthrusting of fore-arc sediments during subduction. Rapid burial of the Swakane sediments is coincident with burial of sediments that formed the Pelona and Orocopia schists in southern California, suggesting that similar processes may have occurred along the length of the continental margin from ~73-68 Ma.
DE: 1010 Chemical evolution
DE: 3640 Igneous petrology
DE: 8102 Continental contractional orogenic belts
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting