Origin of Mesozoic and Tertiary granite in the Western United States and implications for Pre-Mesozoic crustal structure 1. Nd and Sr isotopic studies in the geocline of the Northern Great Basin
Mesozoic and Tertiary granite rocks in and adjacent to the northern Great Basin (NGB) in Nevada and Utah display a wide range of initial /sup 143/Nd//sup 144/Nd (epsilon/sub Nd/) and /sup 87/Sr//sup 86/Sr (epsilon/sub Sr/) values which vary regularly with geographic position. From the Klamath Mountains inland 500 km to central Nevada, granite epsilon/sub Nd/ values decrease regularly from +8 to -6 and correlate with epsilon/sub Sr/ values that increase from -20 to +60. In east-central Nevada, near the trace of the Roberts Mountains Thrust (RMT), the epsilon/sub Nd/ values decrease from -6 to an average of -18, while epsilon/sub Sr/ becomes highly variable with values generally greater than +100. These isotopic discontinuities correspond to the west-to-east facies transition from pelagic clastic sedimentary rocks to shelf carbonates and to the shift in the dominant granite bulk composition from metaluminous to peraluminous. In the eastermost NGB a second discontinuity in epsilon/sub Sr/ occurs with values dropping to approx.+60; average epsilon/sub Nd/ remains at -18. Combined with known aspects of NGB geology the isotopic data suggest that west of the RMT, granites formed via interaction of magma derived from a LREE-depleted pelagic sedimentary rock. Variations in /sup 87/Rb//sup 86/Sr with Sr, and /sup 147/Sm//sup 144/Nd with epsilon/sub Nd/, indicate that crystal fractionation accompanied assimilation, but that plagioclase was not an important fractioning phase. East of the RMT, granites appear to be primarily derived fom precambrian continental basement with little mantle input. The isotopic discontinuities near the RMT mark the western edge of precambrian basement and occur 100-200 km east of the 87Sr/86Sr ( = 0.7060) line of Kistleer and Peterman (1973). The epsilon/sub Nd/ discontinuity in the eastern NGB marks a boundary between Rb- depleted (granulite.) lower continental crust to the east, and basement that has no 'depleted' lower crust to the west.
- Research Organization:
- Department of Earth and Space Sciences, University of California, Los Angeles, California 90024
- OSTI ID:
- 6083182
- Journal Information:
- J. Geophys. Res.; (United States), Vol. 88:B4
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
GREAT BASIN
GEOLOGIC HISTORY
ISOTOPE RATIO
CONTINENTAL CRUST
FEDERAL REGION IX
GRANITES
MAGMATISM
MESOZOIC ERA
NEODYMIUM 143
NEODYMIUM 144
STRONTIUM 86
STRONTIUM 87
TERTIARY PERIOD
ALKALINE EARTH ISOTOPES
ALPHA DECAY RADIOISOTOPES
BETA DECAY RADIOISOTOPES
CENOZOIC ERA
EARTH CRUST
ELECTRON CAPTURE RADIOISOTOPES
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
GEOLOGIC AGES
HOURS LIVING RADIOISOTOPES
IGNEOUS ROCKS
INTERMEDIATE MASS NUCLEI
ISOMERIC TRANSITION ISOTOPES
ISOTOPES
NEODYMIUM ISOTOPES
NORTH AMERICA
NUCLEI
PLUTONIC ROCKS
RADIOISOTOPES
RARE EARTH NUCLEI
ROCKS
STABLE ISOTOPES
STRONTIUM ISOTOPES
USA
YEARS LIVING RADIOISOTOPES
580200* - Geophysics- (-1989)