Paleomagnetism and isotopes of Hf in Lower Cretaceous rocks of the Guerrero Terrain, Chamela Bay and Cocinas Island (Jalisco, Mexico): tectonic implications
The southern extension of the Western Cordillera of North America, in western Mexico, is integrated by accreted terranes of oceanic affinity, conforming the Guerrero superterrane. This tectonic elements are composed of volcanic and volcano-sedimentary rocks from the Upper Jurassic to the Cretaceous, resting on a meta-sedimentary basement of supposed Triassic age of schists with tuffs and basalts with MORB chemistry. In the Zihuatanejo subterrane, the Tecalitlán Formation of the Lower Cretaceous is known to crop out in Chamela Bay and Cocinas Island (Jalisco); the Puerto Vallarta batholith to the north is inferred to intrude rocks of this formation. This unit is composed mainly of andesitic volcanic flows, epiclastic deposits from tuffs to breccias, and some sandstone intervals. From this sequence, 35 sites were collected for paleomagnetism, two samples for U-Pb geochronology and one for Hf analysis with the objective of establishing its paleogeographic relationship with the rest of Mexico, its absolute age, and the nature of the crust of the Jalisco block of the Zihuatanejo subterrane. The results obtained show that the volcanic rocks of the Tecalitlán Formation are andesites and dacites with a U-Pb age in zircons of 120 ± 1 Ma. The Hf isotopes in zircon present positive εHf initial values between +7.2 and +11.5, and Hf model ages TDM2C between 442 and 714 Ma (average 557 Ma), indicating that there is no significant contamination by evolved continental crust, as observed in eastern Mexico (Oaxaquia). The natural remanent magnetization of these untis is multivectorial. Three paleomagnetic components were identified, where a dual polarity C component is interpreted as the characteristic one with mean of D = 341.0° and I = 39.8° (n = 25 sites, k = 15.1, α95= 7.7°) that indicates a northern paleolatitude of 21.8° ± 3.8°. Comparing the paleolatitude obtained in this study with the paleolatitudes expected for the Aptian, as calculated from reference poles of recent global compilations (28° to 26.2°), it is concluded that the western part of the Guerrero superterrane was close to its current latitudinal position with respect to the North American craton by 120 Ma. The Hf isotopes are consistent with models that interpret the Zihuatanejo subterrane as an intraoceanic arc, and not with models that suggest separation of a continental fragment from Oaxaquia by opening of a back arc basin.