DIABASE VS DETRITAL ZIRCONS: THE AGE OF THE CRYSTAL SPRING FORMATION, SOUTHERN DEATH VALLEY, CA

The Crystal Spring Formation is the basal member of the Mesoproterozoic Pahrump Group in Death Valley, CA. It unconformably overlies 1.4 Ga A-type granite and is divided into lower, middle, and upper members. The upper member unconformably overlies limestone, dolomite, and chert in the middle member and consists of six siliciclastic units separated by five dolomite marker beds. All three members of the Crystal Spring Formation are intruded by ca 1.1 Ga diabase in the Kingston Range; the upper member is also intruded by diabase in the Owlshead Mountains. Six detrital zircons of 1,981 measured from sandstone and quartzite in the upper member of the Crystal Spring Formation yield U-Pb ages of 760-802 Ma. These anomalously young ages have been used to constrain the maximum depositional age of the Crystal Spring Formation, as well as the initiation of rifting of Rodinia and Neoproterozoic glaciation on a global scale. Our geologic and geochemical data suggest, however, that the anomalous U-Pb ages from the upper Crystal Spring Formation probably result from lead loss during the long magmatic and tectonic history of Death Valley. Our data emphasize the need to verify maximum depositional ages based on U-Pb ages of detrital zircons by independent methods such as regional geologic relations, Lu-Hf isotope systematics of the detrital zircons, and further geochemical modeling, especially where the minimum detrital ages represent such insignificant proportions of the zircon population.