Precambrian Age Determinations in the Western San Gabriel Mountains, California

Isotopic age determinations (U-Th-Pb) on zircons from pegmatite and granophyre dikes in the western San Gabriel Mountains have yielded concordant ages of about 1,200 million years. From structural and petrological relations, this is both a minimum and a probable age for the San Gabriel Mountains anorthosite complex. It is also a minimum age for the Mendenhall gneiss, a retrograded "charnockitic" granulite which has been intruded by the anorthosite complex. Several zircon fractions from the gneiss yield consistent but moderately discordant apparent ages as follows: Pb^(206)/U^(238) = 1.295m.y., Pb^(207)/U^(235) = 1.355m.y., Pb^(207)/Pb^(206) = 1.440m.y. The Pb^(207)/Pb^(206) age is considered minimal for the zircon and, considering genetic uncertainties, a possible minimum age for the parental rock of the gneiss. These age determinations provide the first compelling evidence of Precambrian rocks on the west coast of North America. Their intimate involvement in the great fault systems of southern California complicates our understanding of the history of this part of the continental crust. It is indicated that a significant point on the Pacific margin of the continent originated more than 1,200 million years ago and has persisted without destructive modification to the present