Implications of initial Pb isotopic ratios for source characteristics of Proterozoic rocks in the Llano Uplift and Trans-Pecos Texas
- Univ. of Texas, Austin, TX (United States). Bureau of Economic Geology
- Univ. of Texas, Austin, TX (United States). Dept. of Geological Sciences
Newly determined Pb isotopic ratios of HF acid-leached feldspars from Middle Proterozoic igneous and metaigneous rocks from the Llano Uplift and Trans-Pecos Texas suggest that these rocks are derived primarily from mantle of petrologically primitive crust. U-Pb geochronologic studies indicate pre-Grenville magmatism ranges in age from 1.35 to 1.23 Ga, whereas post-tectonic plutonism is 1.16 to 1.07 Ga. Initial Pb data from post-tectonic plutons plot along a 1.1 Ga 206-Pb/Pb-204--Pb-207/Pb-204 isochron, indicating that sources of these rocks have a range of U/Pb ratios or that radiogenic Pb was not entirely removed by leaching. Analyses from metaigneous rocks have a greater range. Several sequentially leached feldspars yield 1.1 Ga isochrons, suggesting that they incorporated radiogenic Pb during metamorphism. Initial ratios for the Texas rocks fall below the regional Pb-206/Pb-204--Pb-207/Pb-204 isochron for 1.7--1.4 Ga rocks of Arizona and New Mexico. At 1.35 to 1.1 Ga the isotopic ratios of these rocks would have been higher than the Texas initial ratios. This suggests that similar crust played, at most, a minor role in magmagenesis. In contrast, the Texas initial Pb ratios are within the range of ratios reported for the 1.5--1.3 Ga Granite-Rhyolite Province of the central US. Initial Pb isotopic ratios fall slightly below mantle model curves, and 1.3 to 1.1 Ga isochrons fitted to the data intersect these curves at 1.3 to 1.1 Ga model ages. The Pb-208/Pb-206 data for all samples suggest a source with Th/U [le] 4. The 1.35 to 1.23 Ga metaigneous samples appear to be derived from geochemically primitive sources, possibly in ensialic arc settings. Post-tectonic plutons may have similar juvenile sources but also could have assimilated or been derived from local 1.35 to 1.23 Ga crust.
- OSTI ID:
- 5808242
- Report Number(s):
- CONF-921058-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 24:7; Conference: 1992 annual meeting of the Geological Society of America (GSA), Cincinnati, OH (United States), 26-29 Oct 1992; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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