Abstract

New geochronologic data from the El Arco porphyry copper deposit, Baja California, Mexico, yield a Middle Jurassic crystallization U-Pb age of 164.7 ± 6.5 Ma for zircons from a granodiorite porphyry that hosts the Cu-Au-(Mo) mineralization, and four Re-Os molybdenite ages yield a weighted average age of 164.1 ± 0.4 Ma. These ages disagree with previous K-Ar age determinations that suggest a correlation of the intrusive rocks of El Arco-Calmalli area with the eastern Peninsular Ranges batholith of Cretaceous age. Instead, the volcanic-plutonic rocks of the El Arco-Calmalli area are more properly associated with the San Andres-Cedros volcanic-plutonic complex (∼166 Ma). El Arco-Calmalli block likely was part of the intraoceanic arc system described in the Vizcaino-Cedros region and accreted to the continent during the Cretaceous (98-110 Ma). The presence of a continental Jurassic magmatic arc in the mainland and the occurrence of El Arco-Calmalli (San Andres-Cedros) volcanic-plutonic complex indicate two Jurassic magmatic arcs at this latitude (continental and oceanic). © 2006 Elsevier Ltd. All rights reserved.