EVIDENCE FOR AN ANATECTIC ORIGIN OF EARLY TERTIARY GRANITIC PEGMATITES IN THE BLACK MOUNTAINS, DEATH VALLEY, CA

Early Tertiary granitic pegmatites exposed in the footwalls of the Badwater and Copper Canyon turtlebacks, in the Black Mountains of Death Valley, have mineralogy, geochemical signatures, and field relations suggesting that they are muscovite class and were derived directly from metamorphic basement rocks that comprise the uplifted and exposed cores of the turtlebacks, rather than from fractional crystallization of a granitic pluton. Pegmatites in both turtleback footwalls have simple mineralogy and lack rare element minerals that reflect concentration of incompatible elements by fractionation processes. Trace element abundances of Ca, Sr, and Ba are high, whereas Rb and Cs are low. Chondrite-normalized REE abundances in the pegmatites have flat patterns, no negative Eu anomaly, and are just slightly greater than chondrite. The pegmatites are generally conformable to the foliation of their host rocks, which include high pressure Barrovian sequence amphibolite facies pelitic schists, marble, arkosic quartzite, amphibolite, and gneiss. There is no evidence of a suitable granitic parent for either group of pegmatites.

Similarities in age and structural relations suggest that pegmatites in both turtlebacks were produced during the same Laramide-age deformation event. However, differences in mineralogy, geochemistry, and zoning probably reflect, at least in part, the vastly different thermal histories of the two turtlebacks during the late Tertiary. They may also indicate different parental source contributions. Pegmatites exposed in the Mormon Point turtleback to the south also lack rare element mineralization and have similar field relations, but are undated and display distinctly different geochemical signatures.