Petrology and geochronology of the Calmallí pluton: Insights to the suture zone between island arc and continental crusts in the southern Peninsular Ranges batholith, Baja California, México

Abstract

The Peninsular Ranges batholith (PRB) of Baja California peninsula, México, is a good example of a compositionally zoned batholith. Its transversal zoning is associated with a transition between island arc to continental arc magmatism, which is a striking feature along the PRB. In this study, we concentrate on the Calmallí and Piedra Blanca plutons, which are the southernmost exposures of the PRB. The Calmallí pluton is a concentrically zoned body, its nucleus is formed by gabbro-diorite and its periphery by quartz diorite-tonalite. Harker diagrams of major and minor elements and other major elements indexes suggest that the differentiation process could be explained by fractional crystallization. The Calmallí plutonic rocks are magnesian, calcic and metaluminous, common features in batholiths emplaced in island arc settings. The REE flat pattern of gabbros (normalized to chondrites) indicates a mantellic source; diorites and tonalites have subparallel REE patterns with respect to the gabbros. One diorite and two tonalites have εHfi values ranging from +9.4 ± 1.2 to +3.7 ± 1.2 (2σ). The positive values support a depleted mantle source, but some isotopic variability suggests contamination. UPb zircon data of one diorite and two tonalites yield crystallization ages of 99.6 ± 1.7 Ma, 102 ± 1.3 Ma, and 104.8 ± 1.6 Ma (2σ), respectively, indicating an emplacement period of 5 Ma. The Calmallí pluton intrudes a volcanic and volcanoclastic sequence that was deformed and metamorphosed before the pluton emplacement. This sequence is correlated to a Jurassic island arc exposed in the “El Arco” region. Magmatic foliations in the periphery rocks are markedly parallel to the metamorphic foliations, suggesting that preexisting deformational structures favored the magmatic emplacement of the Calmallí pluton. Eastward of the Calmallí pluton crops out the Piedra Blanca pluton, which is formed by biotite-hornblende granodiorite and biotite granite. These rocks are classified as magnesian, calcic and mainly peraluminous. Strongly fractionated REE patterns suggest a different source (compared to the Calmallí pluton), similar to typical La Posta-type plutons from the northern PRB, which are emplaced in a thickened crust of continental affinity. The petrologic differences between both plutons may represent a transition in the magmatic activity during the Cretaceous (105-100 Ma), where Calmallí pluton has geochemical signatures of island arc activity and Piedra Blanca pluton represents a transition to continental arc activity.