U-Pb ages of granitoid clasts in upper Mesozoic arc-derived strata of the Vizcaino Peninsula, Baja California, Mexico David L. Kimbrough Department of Geological Sciences, University of California, Santa Barbara, California 93106 James J. Hickey ARCO Oil and Gas Company, 2300 W. Piano Parkway, Piano, Texas 75075 Richard M. Tosdal U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 ABSTRACT Minor but widespread granitoid clasts occur in Late Jurassic to Early Cretaceous age volcanogenic strata of the Eugenia and Perforada Formations on the Vizcaino Peninsula of Bqja California. Most are of two common types: coarse-grained garnet-bearing biotite granite and xenolith-rich hornblende tonaHte. Concordia modeling of U-Pb zircon data from a biotite granite clast indicates crystallization at 150 ±3 Ma and a strong inherited component of Precambrian radiogenic Pb of 1.34 ±0.08 Ga. Data from a hornblende tonalite clast inclicate a slightly younger crystallization age and a small component of inherited radiogenic Pb. These clasts occur in association with quartzose sediments that contain abundant biotite probably derived from the same source as the biotite granite. This nonvolcanic component occurs in significant amounts throughout most of the Tithonian-Valanginian Eugenia Formation and in the overlying Aptian-Albian Perforada Formation. The apparent absence of a nearby suitable plutonic source for the granitic detritus, either from the Baja California Peninsula or from Sonora, Mexico, may reflect translation of the Vizcaino Peninsula relative to the North .Ameri- can craton since Aptian-Albian time and prior to the opening of the Gulf of California ai: 5 Ma. INTRODUCTION Mesozoic ophiolite, volcanic-arc, and subduc- tion-complex terranes occur on the Vizcaino Peninsula and nearby Cedros and San Benito islands along the Pacific continental margin of Baja California (Rangin, 1978; Barnes, 1982; Kimbrough, 1985; Fig. 1). Similar terranes form a dominant component of the Western Cordil- lera from Alaska to Mexico and comprise a broad accretionary belt, much of which is con- sidered allochthonous with respect to the North American craton (e.g., Coney et al., 1980). Crit- ical to understanding the paleogeographic signif- icance of accreted Mesozoic terranes is an understanding of their movement history with PE 125-6 Pta. Eugenia -PQ22 Pta. Quebrada ¡H Chapunes a Valle Fms. (Alb.-Sant.) H||| Asunción Fm. (Apt.-Alb.) Perforada Fm. (Apt.-Alb.) Eugenia Fm. (Tith.-Val. r=lj Sierra Placeres shear zone melange | San Andres arc complex (154-127 m.y. intrusives) Ophiolite 8 assoc. sedimentary rocks (Late Triassic) Serpentinite-matrix melange i 20 km Figure 1. Simplified geo- logic map of Mesozoic units on Vizcaino Penin- sula showing sampling localities. 26 GEOLOGY, v. 15, p. 26-29, January 1987