Early to Middle Jurassic San Andres-Cedros plutonic suite, western coast of Baja California, Mexico: Geochemical and isotopic evidence for an island arc extending to the central peninsula

摘要

In the Vizcafno peninsula and Cedros Island region, west-central part of the Baja California peninsula, Mexico, Upper Triassic ophiolitic sequences are intruded by arc-related Mid-Jurassic plutons. We analyze the similar to 150 Ma old San Roque pluton (SRp) in the Vizcaino peninsula and plutonic rocks (similar to 165 Ma) of the Punta Norte zone (PN) in the Cedros Island. The modal composition of the almost 10 km(2) extending San Roque pluton varies from biotite-hornblende quartz diorite to biotite-hornblende tonalite intruded by granodioritic dykes. Intrusive rocks are subalkaline, calcic/calc-alkaline and metaluminous, akin to island arc granite setting. The primitive initial eNd values of +8.9 and + 9 of SRp quartz diorite and tonalite are indicative of a depleted mantle source without crustal assimilation. The sequence intrudes amphibolites that are interpreted as metavolcanic remnants of the oceanic-type crust of the Vizcafno peninsula ophiolite. One amphibolite sample is characterized as magmatically poorly evolved, with flat REE patterns, and MORB-type initial eNd values of +10.1. The PN plutonic rocks of Cedros Island crop out at its northern edge. Their modal compositions are hornblende quartz diorite, hornblende leucotonalite, and microtonalitic dykes. These rocks are classified as subalkaline, magnesian, and calcic, typical island arc granites. The REE patterns tend to be subparallel, with progressive enrichments in REE abundances, suggesting fractional crystallization resulting from the evolution of a single magmatic system. Microtonalite dykes and leucotonalite yield initial eNd values of + 7.8 to +8, indicating that the PN rocks were produced from magmas associated with a depleted mantle source. These values are in agreement with the correlation of epsilon Sr-(165 ma) vs. epsilon Nd-(165 Ma) where all samples plot in the depleted mantle field. The PN plutonic rocks intruded pillow basalts with basaltic dykes. Their initial epsilon Nd and epsilon Sr plot in the depleted mantle field indicative of island arc tholeiites.The SRp and PN plutonic rocks are correlated with the coeval El Arco system (similar to 160 Ma) in the central part of the Baja California peninsula. They show the same magmatic evolution in a primitive oceanic island arc environment derived from a depleted mantle with no crustal assimilation. We suggest that all the Jurassic sequences of the southern Peninsular Ranges batholith belong to the same arc.