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 Baja California. Most are of two common types: coarse-grained garnet-bearing biotite granite and xenolith-rich hornblende tonalite. 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 indicate 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 American craton since Aptian-Albian time and prior to the opening of the Gulf of California at 5 Ma.

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