Implications of Sm-Nd model ages and single grain U-Pb zircon geochronology for the age and heritage of the Swakane gneiss, Yellow Aster Complex, and Skagit gneiss, North Cascades, Wash
- Univ. of Texas, Austin, TX (United States). Dept. of Geological Sciences
Uncertainties regarding the protolith, age, and tectonic affinity of the Swakane gneiss (Swg), Skagit gneiss (Skg), and Yellow Aster complex (YAc) have long been obstacles to deciphering the tectonic history of the North Cascades. Single-grain zircon U-Pb data and Sm-Nd model ages now demonstrate these units are genetically unrelated metasediments derived from Early Proterozoic and younger sources. Zircon populations from Swg samples include rounded, subrounded, and euhedral grains. Three of four analyzed grains define a well-correlated discordia array that intercepts concordia at 1.4 Ga whereas the fourth grain has a 1.6 Ga 207Pb/206Pb age. the Sm-Nd depleted mantle model ages of two samples of Swg are 1.18 Ga and 1.27 Ga. The authors suggest that the Swg gneiss is a metaclastic rock derived chiefly from rocks of Middle Proterozoic age. A single analyzed zircon crystal extracted from garnet-biotite yields a nearly concordant age of 0.14 Ga. The YAc consists of fragments of metaplutonic rocks and quartz-pyroxene gneisses of uncertain protolith that are imbricated with Paleozoic and Mesozoic rocks of the Cretaceous Northwest Cascades thrust system. U-Pb data from 6 single zircons extracted from mylonitic quartz-pyroxene gneiss are interpreted to indicate the presence of Grenville orogen-derived grains of ca 0.9--1.3 Ga age whereas one analyzed zircon has an ca 0.6 Ga age. Analyses of abraded zircons from a post-mylonitic pegmatite dike indicate pegmatite crystallization at ca0.22 Ga and imply the presence of ca1.85 Ga xenocrystic zircons.
- OSTI ID:
- 6136111
- Report Number(s):
- CONF-921058--
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Journal Name: Geological Society of America, Abstracts with Programs; (United States) Vol. 24:7; ISSN GAAPBC; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
580000* -- Geosciences
ACTINIDES
AGE ESTIMATION
DEVELOPED COUNTRIES
ELEMENTS
GEOLOGIC FORMATIONS
GEOLOGIC HISTORY
GNEISSES
ISOTOPE DATING
ISOTOPE RATIO
LEAD
METALS
METAMORPHIC ROCKS
MINERALS
NEODYMIUM
NORTH AMERICA
RARE EARTHS
ROCKS
SAMARIUM
SILICATE MINERALS
TECTONICS
URANIUM
USA
WASHINGTON
ZIRCON