HR: 0800h
AN: T11B-0580    [Abstracts]
TI: Geochemical Characteristics of Plutons Emplaced in a Contracting Arc and Evidence for the Development of Gravitationally Unstable Residual Arc Rocks, Coast Mountains, B.C.
AU: * Girardi, J D
EM: jgirardi@email.arizona.edu
AU: Patchett, P
EM: patchett@email.arizona.edu
AU: Ducea, M N
EM: ducea@email.arizona.edu
AU: Gehrels, G
EM: ggehrels@email.arizona.edu
AB: New geochemical, Nd, and Sr data from plutonic rocks of the Coast Mountains Batholith show that these rocks were derived from both primitive and somewhat compositionally evolved sources. Preliminary results indicate that arc plutons ranging in age from 9 to 188 Ma contain varying amounts of slightly evolved or older primitive terrane material with initial εNd ranging from +1 to +7 and initial 87Sr/86Sr ratios mainly in the range of 0.703 - 0.705. A minimum in initial εNd ranging from +1 to +4 at 85 - 95 Ma is interpreted to reflect crustal thickening and increased lower crustal assimilation into magmas as terranes were progressively thrust eastward. Similarly at 50 - 55 Ma very thick hot crust during a magmatic flare up event promoted the assimilation of lower crustal material. Maximum εNd of +5 to +7 from 68 - 82 Ma occurs due to decreased lower crustal assimilation into magmas during a period when magmatism occurred in a more restricted zone. Initial 87Sr/86Sr ratios corroborate Nd data and also indicate a cyclic pattern of assimilating slightly more compositionally evolved materials. The observed cycle of initial εNd and 87Sr/86Sr in these rocks reflects fundamental changes in the arc thermal/petrologic processes through time. It is known that to generate large quantities of intermediate composition crust requires the production of a complimentary ultramafic residue. Geochemical characteristics, specifically La/Yb ratios which steadily increase from 180 - 50 Ma, of these rocks indicate thickening crust and varying amounts of garnet as a residual phase in what would be a dense granulitic - eclogitic residual root susceptible to gravitational instabilities. From preliminary geochemical data it is apparent that the incorporation of slightly evolved or possibly older primitive terrane materials may be an important process during magmatic flare-ups, promoting the formation of garnet rich residues. The fate of these dense residues has important implications for our understanding of fundamental tectonic processes which occur in continental arcs.
DE: 1030 Geochemical cycles (0330)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
SC: Tectonophysics [T]
MN: 2007 Fall Meeting