During the late Cenozoic mafic lavas were emplaced contemporaneously with Basin and Range extension near Battle Mountain, Nevada. They display wide ranges in major and trace elements contents and Sr, Nd, and Pb ratios. The temporal and compositional characteristics of the basalts indicate that the effects of crustal contamination on the compositions of the younger lavas has decreased relative to the older lavas. Assimilation and fractional crystallization (AFC) modeling suggests that their compositional heterogeneity cannot be attributed solely to crustal contamination, and instead most likely reflects their derivation from a heterogeneous mantle source region.
Regional variations in the trace element ratios (Rb/La and Ba/Zr) and Sr, Nd and Pb isotopic compositions of mafic lavas from the western U.S. indicate they were derived from a heterogeneous lithospheric mantle. Crustal contamination and subduction related processes appear to be unable to reproduce the range in $\sp{87}$Sr/$\sp{86}$Sr and low Rb/La of the lavas. Therefore, the variation in the basalts' isotopic ratios most likely reflects their derivation from a heterogeneous mantle source region. Plots of $\sp{87}$Sr/$\sp{86}$Sr versus $\sp{207}$Pb/$\sp{204}$Pb or $\sp{207}$Pb/$\sp{207}$Pb clearly illustrates the compositional characteristics of their source regions.
The Snake River Plain and southern Sierran Province basalts display elevated $\sp{87}$Sr/$\sp{86}$Sr and varied Pb isotopic ratios indicating they were derived from a heterogeneous ancient lithospheric source with high time integrated Rb/Sr and U/Pb ratios. The low $\sp{87}$Sr/$\sp{86}$Sr and Pb isotopic ratios of the suggest the Colorado Plateau-Eastern Transition Zone basalts were derived from a heterogeneous ancient lithospheric source with low time integrated Rb/Sr and U/Pb ratios. Basalts from the Basin and Range, Oregon Plateau, and northern Sierran Province display a positive correlation between their $\sp{87}$Sr/$\sp{86}$Sr versus $\sp{208}$Pb/$\sp{204}$Pb or $\sp{207}$Pb/$\sp{204}$Pb ratios, which appears to reflect two component mixing between accreted depleted oceanic-type mantle and an enriched mantle component.
| Identifer | oai:union.ndltd.org:RICE/oai:scholarship.rice.edu:1911/16558 |
| Date | January 1992 |
| Creators | Lum, Clinton Chew Lun |
| Contributors | Leeman, William P. |
| Source Sets | Rice University |
| Language | English |
| Detected Language | English |
| Type | Thesis, Text |
| Format | 232 p., application/pdf |
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