Apparent undisturbed paleoposition of mid-Cretaceous plutons emplaced across a Jurassic island arc segment in the southernmost part of the Peninsular Ranges batholith, Baja California, Mexico

Abstract

The Calmallí (CA), Piedra Blanca (PB), and El Barril (EB) plutons were emplaced during the Late Cretaceous at the southern limit of the Peninsular Ranges batholith (PRB) at the present latitude of ∼28°N. These plutonic suites are juxtaposed, showing contrasting compositions, probably associated with different basement rocks. The region is separated by the high magnetic values in the western zone compared with the lowest magnetic values in the eastern zone. The western CA pluton intrudes a Jurassic island arc assemblage, whereas host rocks outcrops of the PB and EB are absent, but it is inferred that they intruded into the eastern margin of the Jurassic arc. The CA plutonic suite has a ca. 100 Ma old mafic core surrounded by ca. 102–105 Ma old tonalities. Such zonation is a typical feature of many plutons of the western PRB. The PB and EB plutons show granodioritic to granitic compositions of ca. 92 Ma. We compare the paleoposition of the CA-EB section with respect to the San José, and San Pedro Mártir plutons (∼31°N), and the Compostela pluton (∼29°N) which yield clockwise rotations of 31°–45°, and 16.4°, respectively. These northern plutons record rotations by tectonic deformation at suture zones between the Alisitos island arc and the older North America craton. Based on the paleomagnetic observations in the transition zone between western and eastern type plutons, we analyzed 15 sites from the CA, nine from the PB, and 23 from EB to evaluate their paleoposition. We obtained the characteristic remanent magnetizations (ChRM) following the detailed stepwise alternating magnetic field (AF) and thermal demagnetization methods. Each pluton was compared to the reference pole of North America at 100 Ma and 90 Ma. The paleopoles do not differ much from the cratonic references since the data indicate an apparent clockwise rotation of 8.1° ± 8.8° and a flattening of −13.9° ± 10.6° for CA pluton and an apparent clockwise rotation of 1.43° ± 8.3° and −14.2° ± 9.8° of flattening for combined paleopole of PB and EB plutons. Statistically, the paleopoles displacement are not significative, similar to five cretaceous plutons located NNW of the study area, suggesting that the mid-Cretaceous plutonic suites of the southern PRB remained stable since their emplacement in a period between 128 Ma and 90 Ma.