HR: 0800h
AN: V51C-0721    [Abstracts]
TI: Pervasive Crustal Melting on a Regional Scale: Sr-Nd Isotopic Evidence from Eocene Intrusions in NE Washington
AU: * Loewen, M W
EM: mwloewen@ups.edu
AF: Geology Department University of Puget Sound, 1500 N Warner, Tacoma, WA 98416-1048, United States
AU: Tepper, J H
EM: jtepper@ups.edu
AF: Geology Department University of Puget Sound, 1500 N Warner, Tacoma, WA 98416-1048, United States
AU: Asmerom, Y
EM: asmerom@unm.edu
AF: Dept. of Earth & Planetary Sciences University of New Mexico, 200 Yale Blvd., NE, Albuquerque, NM 87131, United States
AB: During the Eocene the Pacific Northwest was the site of a short-lived but voluminous and geochemically diverse magmatic episode, commonly termed the Challis event. To investigate the origins of this event we have measured whole rock Sr and Nd isotopic compositions of 12 plutonic and hypabyssal samples, ranging from basalt to two-mica granite, collected along a 250 km transect across NE Washington. This transect crosses the 0.706 line (Armstrong 1977), the boundary between dominantly Mesozoic crust to the west and older crust to the east. The results reveal a wide spread in isotopic compositions (⁸⁷Sr/⁸⁶Sr_m = 0.7041 - 0.7262; εNd_m = +3.8 to -18.5) with no systematic relationship between isotopic composition and bulk composition (e.g., MgO). This decoupling of isotopic composition and bulk chemistry suggests mixing between mantle and crustal melts was of minimal importance, and that these rocks are dominantly of crustal origin. The range in εNd_m also indicates melting of crustal sources that varied considerably in age. Samples with εNd_m > +2 range from basalt (13 wt.% MgO) to two-mica granite (0.3 wt.% MgO). Such juvenile εNd_m in a two-mica granite precludes significant involvement of ancient metasedimentary material and implies rapid intracrustal differentiation of a mantle-derived source, which may have been deep arc crust of Mesozoic age. At the other end of the spectrum, samples with εNd_m < -15 come from Springdale (⁸⁷Sr/⁸⁶Sr_m = 0.7071; εNd_m = -18.5) and Medical Lake (⁸⁷Sr/⁸⁶Sr_m = 0.7143; εNd_m = -15.2) at the eastern side of the study area. Data from these sites, both east of the 0.706 line, are similar to values reported for the nearby Silver Point Quartz Monzonite and attributed to melting of late Archean to Early Proterozoic crust (Whitehouse et al. 1992). Other samples analyzed in this study are broadly similar in isotopic composition (εNd_m = +1 to -8; ⁸⁷Sr/⁸⁶Sr_m = 0.706-0.709) to rocks of the Colville Igneous Province and probably formed by melting of Proterozoic arc crust (Morris 2000). Geographic variability in Sr-Nd data indicates that isotopically distinct crustal domains are juxtaposed laterally and/or vertically, in some cases on a small scale. The sample with the highest ⁸⁷Sr/⁸⁶Sr_m (0.7262; εNd_m = -13.3) is a dacite porphyry well west of the 0.706 line, while at Porcupine Bay adjacent plutons differ by almost 10 εNd units (-7.4 and 2.2). Ongoing work is designed to further characterize the crustal sources and better understand the nature of the thermotectonic event that drove such widespread crustal melting.
DE: 1020 Composition of the continental crust
DE: 1037 Magma genesis and partial melting (3619)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting