The Mesozoic Peninsular Ranges batholith, part of a long-lived Cordilleran subduction orogen, is located at a critical juncture at the southwest corner of cratonal North America. The batholith is divided into northern and southern segments that differ in their evolution. In this paper, we focus on the more poorly understood southern Peninsular Ranges batholith, south of the Agua Blanca fault at ~31.5°N latitude, and we compare its evolution with the better-known northern Peninsular Ranges batholith. Adding our new insights to previous work, our present understanding of the geologic history of the Peninsular Ranges consists of the following: (1) stronger connections between the Paleozoic passive-margin rocks in the eastern Peninsular Ranges batholith and similar assemblages in Sonora, Mexico, to the east and the Sierra Nevada batholith to the north that were originally proposed by earlier workers; (2) continuity of the Triassic–Jurassic accretionary prism and forearc basin assemblage from the northern Peninsular Ranges batholith through the southern Peninsular Ranges batholith; (3) possible synchronous subduction of an ocean ridge or ridge transform along the Peninsular Ranges batholith in late Middle Jurassic time; (4) continuity of the Early Cretaceous Santiago Peak continental arc from the northern Peninsular Ranges batholith along the entire margin, including the southern Peninsular Ranges batholith; (5) development of the Alisitos oceanic arc in Jurassic and possibly Triassic time, much earlier than originally thought; and (6) removal of part of the Santiago Peak assemblage in the southern Peninsular Ranges batholith during collision of the Alisitos terrane in latest Early Cretaceous time.

We report structural, paleomagnetic, and magnetic fabric data for mid-Cretaceous plutons of the Peninsular Ranges batholith along a transect at ~30°N latitude. Four plutons in the western sector are characterized by characteristic magnetizations residing in magnetite. In this sector, El Milagro, Aguaje del Burro, La Zarza, and San Telmo plutons yield a combined paleopole at 82.1°N, 169.7°E (K = 137.6, A 95 = 7.9°; n = 4–38 sites), which, rotated for closure of the Gulf of California, falls at 79.3°N, 179.5°E, and it is concordant with the North America reference pole. Plutons in the transition zone, between the eastern and western sectors of the Peninsular Ranges, have magnetizations residing in hematite. El Potrero and San José plutons yield highly discordant paleopoles, indicating apparent clockwise rotation (R) and flattening (F) of 33.0° ± 5.1° and −27.6° ± 6.1°, respectively (San José), and 46.1° ± 5.9° and −31.0° ± 7.0° (El Potrero). The discordance is best explained by west-down tilt of the crustal block between the Main Mártir thrust and the Rosarito fault, which are major compressional structures parallel to the trend of the Peninsular Ranges. The San Pedro Mártir pluton, a large La Posta–type pluton on the eastern sector of the transect, has magnetizations that reside primarily in hematite. The mean paleomagnetic pole (71.3°N, 335.5°E; K = 40.7 and A 95 = 7.2°) is slightly discordant, indicating westward tilt of ~15°. The different paleopoles obtained for individual plutons convincingly show that the Peninsular Ranges batholith has suffered internal deformation, which is more intense along the transition zone. The magnetic fabric for plutons representative of the western, eastern, and transitional sectors of the range show marked contrasts in the deformation recorded by anisotropy of magnetic susceptibility (AMS). Anisotropy is weakly developed in the western sector (El Milagro), very strongly developed in the transition zone (San José), and moderately developed in the eastern sector (Sierra San Pedro Mártir). Within the plutons, El Milagro fabrics record emplacement-related stress. In contrast, San José and San Pedro Mártir appear to record regional stress linked to evolution of the Main Mártir thrust. Overall, our data are consistent with rotation of the crustal block where Potrero and San José plutons are located; rotation was accommodated by major crustal faults in a compressional stress field, as the crustal block moved to occupy the space abandoned by the ascending (and westward expanding) San Pedro Mártir diapir batholith. The rotation could be related to interaction between the large Sierra San Pedro Mártir pluton and the Main Mártir thrust, or to mechanical controls such as wedging against a rigid salient.

The most striking feature of the eastern Peninsular Ranges batholith is the large volume of relatively homogeneous tonalite and low-K granodiorite distributed in a series of large zoned Late Cretaceous intrusive centers referred to as La Posta–type plutons. The Sierra San Pedro Mártir pluton in northern Baja California is an outstanding example, and this study was undertaken to test models for the origin of these large composite arc plutons as well as to investigate along-strike variability within this Late Cretaceous belt. The Sierra San Pedro Mártir pluton consists of a nested series of granitoids divided into hornblende, biotite, and muscovite zones that become progressively more felsic and younger inward to a slightly more mafic and lower-K muscovite core zone. Zircon and monazite U/Pb ages from each of the zones indicate composite assembly of the pluton over an ~7 m.y. time span (97–90 Ma), consistent with field evidence and internal compositional variability of the pluton. The Sierra San Pedro Mártir pluton consists of high-Na, high-Al calcic granitoids that contrast with high-K calc-alkaline granitoid intrusive suites typical of Sierra Nevada Late Cretaceous intrusive centers. Whole-rock major-element, trace-element, and rare earth element (REE) data from an ~20-km-long traverse from the margin to the core of the Sierra San Pedro Mártir pluton document compositions that closely match chemical characteristics of Archean high-Al tonalite-trondhjemite-granodiorite (TTG). REEs are in general strongly fractionated, with high (La/Yb) N ratios typical of high-Al TTG. However, large variations in heavy (H) REE abundances and light (L) REE/HREE abundance ratios within the Sierra San Pedro Mártir pluton are comparable to the total range of REE variability within the Peninsular Ranges attributed by previous workers to regional west to east variations across the batholith. High Sr contents and lack of strong Eu anomalies indicate a general lack of plagioclase in the source residue. Compositions are consistent with deep crustal or slab melting from a basaltic source region with residual garnet and amphibole. Hornblende-plagioclase thermobarometry indicates emplacement depths around 17 km and crystallization temperatures ranging from 650 °C to 700 °C. Unlike La Posta–type bodies to the north, which are exclusively ilmenite-series granitoids, the Sierra San Pedro Mártir pluton is partly magnetite-series rocks in the outer hornblende zone. Ilmenite-series rocks preferentially sequester Fe in biotite via Tschermak exchange. Relatively low 87 Sr/ 86 Sr initial isotopic compositions of 0.7038–0.7050 and δ 18 O whole-rock values of 8.5‰ in the Sierra San Pedro Mártir pluton are more typical of the western zone of the Peninsular Ranges batholith. The along-strike variation of La Posta–type centers may be correlated to progressive distancing from Proterozoic North American cratonal basement and/or diminishing contributions of subducted sediment and associated basement into the Cretaceous melt source region of the magmas.