ENIGMATIC JURASSIC FOREARC MAGMATISM IN THE PENINSULAR RANGES BATHOLITH, BAJA CALIFORNIA AND ITS RELATIONSHIP TO THE NAZAS ARC

Late Jurassic orthogneisses occur in Paleozoic, miogeoclinal units and deep-water, early Mesozoic greenschist- to lower amphibolite-grade metasedimentary units in the eastern and central parts, respectively, of the Peninsular Ranges batholith (PRB) of southern and Baja California. Fossil and detrital zircon data suggest formation of these units along the North American margin. Paleozoic units were likely deposited along the Paleozoic passive margin, while Mesozoic turbidite sequences, which were variably deformed within an accretionary prism, were probably deposited in a forearc or trench setting, or across the trench on the subducting plate.

U-Pb ages from orthogneisses near San Diego and further south in the Sierra San Pedro Mártir suggest almost coeval magmatism at ~164 Ma between latitude 33°N and 30.5°N. In addition several presently undated orthogneisses farther south in the Sierra Calamajue at 29.5° N and near Bahia Los Angeles at 29°N are thought to be the same age. These sheeted bodies are largely biotite-muscovite orthogneisses with relict igneous textures. Existing geochemical data from intrusions near San Diego show concentrated LREE and depleted HREE (La/Lu >7), elevated d¹⁸O (>10 ‰) and ⁸⁷Sr/⁸⁶Sr ratios >0.706.

Restoring Neogene extension places the PRB along the NW coast of present-day mainland Mexico. Even so, the belt of Jurassic magmatism, which is coeval with continental margin arc magmatism in California, southern Arizona, and the Nazas arc of Mexico, lies well to the SW or west of the subduction-related Triassic-Jurassic continental margin arc. Although Jurassic intra-arc rifting is evident in parts of the southern Cordillera, widespread Jurassic extensional structures are absent in the PRB. The punctuated and widespread character of the intrusive event and the geochemical compositions of the intrusions are best explained by subduction of a trench-parallel spreading ridge. Thus, we speculate that ridge subduction along the Cordilleran margin caused magmatism in the forearc accretionary prism. This implies the existence of another, not yet described plate that was subducted along the North American margin. The size of this plate and the effects of ridge subduction on plate convergence and intra-arc rifting in the Jurassic continental margin arc remain uncertain.