The isotopic composition of lead in microclines from the Llano uplift, Texas

Abstract

The isotopic composition of lead and the concentrations of lead and uranium in microclines were determined for some igneous and metamorphic rocks from the Llano uplift, Texas. At the time of their crystallization, all the microclines from the igneous rocks had rather similar lead isotopic compositions (Pb206/Pb204 = 16.6 to 17.0, Pb207/Pb204 = 15.4 to 15.5, Pb208/Pb204 = 36.4 to 36.7), which yield model ages very close to their Rb-Sr age of 1000 to 1100 million years. Conversely, the microlines from the Valley Spring gneiss exhibit a wide range in isotopic composition (Pb206/Pb204 = 17.1 to 19.1, Pb207/Pb204 = 15.5 to 15.7, Pb208/Pb204 = 36.8 to 38.2), which is suggestive of a more complex history. Most of the lead in the plutonic rocks was probably obtained from a rather uniform source, which is possibly related to some deep crustal or upper mantle layer and which also provides the reservoir of so-called ‘conformable’ ore lead. Some special mechanism of selective mobilization and extensive mixing would have been required if the presently exposed metamorphic host rocks contributed a significant part of the lead to the igneous rocks. Several total rocks were analyzed and their Pb206/Pb204 ratios were calculated as a function of time by means of the presently observed U/Pb ratio. In the analyses of an igneous and a metaigneous rock, it was found that the Pb206/Pb204 ratio in the rock became equal to that in the microcline approximately 1100 million years ago, which suggests initial lead homogeneity and subsequent closed system conditions. The microcline and total rock from four metamorphic rocks, however, were calculated to have equal lead isotopic compositions at times other than that reasonably attributable to some tectonic event. Uranium and lead migration and initial lead inhomogeneity in these rocks are considered as possible causes of their discrepant behavior.