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

摘要

The Peninsular Ranges batholith (PRB) of Baja California peninsula, Mexico, 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 Calmalli and Piedra Blanca plutons, which are the southernmost exposures of the PRB. The Calmalli 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 Calmalli 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 eHfi values ranging from +9.4 +/- 1.2 to +3.7 +/- 1.2 (2 sigma). The positive values support a depleted mantle source, but some isotopic variability suggests contamination. U-Pb 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 (2 sigma), respectively, indicating an emplacement period of 5 Ma. The Calmalli 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 Calmalli pluton. Eastward of the Calmalli pluton crops out the Piedra Blanca pluton, which is formed by biotite-homblende 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 Calmallf 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 Calmalli pluton has geochemical signatures of island arc activity and Piedra Blanca pluton represents a transition to continental arc activity.