Neodymium, strontium, and oxygen isotopic variations in the crust of the western United States: Origin of Proterozoic continental crust and tectonic implications

Miscellaneous · Sun Jan 01 00:00:00 EST 1989

OSTI ID:7011271

Bennett, V C

Initial Nd isotopic ratios of crystalline rocks from an area of about 1.5 {times} 10{sup 6} km{sup 2} of the western United States have been determined in order to map Precambrian age province boundaries and thus document the growth and modification of the North American continent in the Proterozoic. Three age provinces have been delineated. It is demonstrated that large regions of Early Proterozoic continental crust were formed with anomalous isotopic compositions ({sup 143}Nd/{sup 144}Nd ratios lower than Early Proterozoic depleted-mantle). The variations in the initial {epsilon}{sub Nd} and {delta}{sup 18}O values correlate with each other, and correspond to the previously determined Nd isotopic provinces. The Pelona, Rand, Chocolate Mountain and Orocopia Schists are represented by 15 lithologically and structurally similar schist bodies exposed along the San Andreas and Garlock faults in southern California. The grayschists have measured {epsilon}{sub Nd} values from -1.7 to -11.7 with depleted-mantle model ages of 0.9 to 1.7 Ga. The Nd isotopic compositions can be modeled as variable mixtures of Early Proterozoic continental crust with a Mesozoic are component. The measured {sup 87}Sr/{sup 86}Sr ratios are from 0.7087 to 0.7129 and reflect the presence of an old continental source. Independent of age, the high initial {epsilon}{sub Nd} values ({sup +}9 {plus minus} 1.5) are consistent with derivation at an oceanic spreading center, either at a MORB or in a back-arc basin environment. The presence of both Early Proterozoic continental detritus and a younger sedimentary component in the grayschist protolith, and the MORB affinity of the metabasalts are compatible with formation of the protoliths of the Pelona and related schists in a Mesozoic basin adjacent to the southwestern United States continental margin.

• Research Organization: California Univ., Los Angeles, CA (USA)
• OSTI ID: 7011271
• Resource Relation: Other Information: Thesis (Ph. D.)
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
CALIFORNIA
CONTINENTAL CRUST
ORIGIN
TECTONICS
CONTINENTAL MARGIN
GEOCHEMISTRY
GEOLOGIC FAULTS
GEOLOGIC HISTORY
ISOTOPE RATIO
LITHOLOGY
NEODYMIUM 143
NEODYMIUM 144
OXYGEN 18
OXYGEN ISOTOPES
STRONTIUM 86
STRONTIUM 87
VARIATIONS
ALKALINE EARTH ISOTOPES
ALPHA DECAY RADIOISOTOPES
BETA DECAY RADIOISOTOPES
CHEMISTRY
EARTH CRUST
ELECTRON CAPTURE RADIOISOTOPES
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
FEDERAL REGION IX
GEOLOGIC FRACTURES
GEOLOGIC STRUCTURES
HOURS LIVING RADIOISOTOPES
INTERMEDIATE MASS NUCLEI
ISOMERIC TRANSITION ISOTOPES
ISOTOPES
LIGHT NUCLEI
NEODYMIUM ISOTOPES
NORTH AMERICA
NUCLEI
RADIOISOTOPES
RARE EARTH NUCLEI
STABLE ISOTOPES
STRONTIUM ISOTOPES
USA
YEARS LIVING RADIOISOTOPES
580000* - Geosciences