The effect of contact metamorphism on lead in potassium feldspars near the Eldora Stock, Colorado

The isotopic composition and the concentration of lead have been determined for samples of potassium feldspar from the quartz monzonite of the Eldora stock and from Precambrian pegmatites in the surrounding Idaho Springs Formation near Boulder, Colorado. The isotopic data show that lead entered the potassium feldspar up to 1000 feet from the stock, which is about 10,000 feet across. The isotopic results also prove that not all the lead that entered the feldspar could have come from the quartz monzonite. The isotopic constitution of the lead in potassium feldspars was more easily affected by the contact metamorphism than the Rb-Sr system of the feldspars or the K-Ar system of hornblende but less easily than the K-Ar and Rb-Sr systems of biotite. The transition of microcline to orthoclase in pegmatites of the Idaho Springs Formation took place without major effect on the isotopic composition or the concentration of lead. Comparison of the Rb-Sr and lead isotope data indicates that lead can be added to the feldspars without solution and redeposition of the feldspar. Isotopic data on Precambrian feldspars from Colorado and New York and feldspars from the Laramide quartz monzonite are not compatible with generation of all their lead from an ‘infinite reservoir’ with fixed present-day values of U/Pb and Th/Pb. The existence of such a reservoir, however, is not precluded, for the departures could be caused by contamination at the time of intrusion. The variations found for the Precambrian rocks could also have been caused by metamorphism subsequent to intrusion.