Geochronologic Studies in the Eastern Mojave Desert, California

Abstract

Age determinations using the $$Ar^{40}-K^{40}$$ and $$Sr^{87}-Rb^{87}$$ methods were made on samples of muscovite, biotite, and potassium feldspar from four areas of igneous and metamorphic rocks of Precambrian or presumed Precambrian age in the eastern Mojave Desert. In the Marble Mountains the results indicate that the granite was emplaced approximately 1,400 million years ago. No positive evidence of a significant subsequent thermal metamorphism was obtained. The Rb-Sr age of potassium feldspar from a granite inclusion in a mafic dike suggests that short-term thermal metamorphism may not be an important mechanism in the consideration of strontium loss from potassium feldspar. The age of recrystallized biotite from the inclusion suggests that the dike was emplaced during the Jurassic period. In the Mountain Pass district the data suggest two earlier Precambrian events. The country rock was formed by regional metamorphism and pegmatite injection approximately 1,650 million years ago, whereas the younger potassium-rich igneous rocks and intrusive carbonate rocks apparently were emplaced approximately 1,400 million years ago. In Joshua Tree National Monument and the Kilbeck Hills no positive evidence for the Precambrian age of the metamorphic rocks was obtained.