INFERRED EARLY JURASSIC DEPOSITION (U-Pb IN DETRITAL ZIRCON) FOLLOWED BY MIDDLE JURASSIC AND CRETACEOUS METAMORPHISM (Sm-Nd IN GARNET) OF METASEDIMENTARY ROCKS FROM THE SOUTHERN PENINSULAR RANGES BATHOLITH, BAJA CALIFORNIA, MEXICO

Cretaceous (131-92 Ma) plutonic rocks intruded two different NW-SE oriented belts of host rocks in the southernmost tip of the Peninsular Ranges Batholith. The western belt, likely close to the paleotrench, consists of Jurassic meta-igneous rocks with oceanic-arc affinity. Toward the continent, the eastern belt is comprised of amphibolite, para- and orthogneiss, schist, calcsilicate, marble, and undifferentiated metamorphic rocks, in which the sedimentary protoliths are interpreted as Triassic-Jurassic back-arc assemblages or Paleozoic continental slope and basin sediments. Here, we present U-Pb LA-ICP-MS detrital zircon data of five paragneisses and two schists for maximum depositional ages and provenance analysis estimations. In addition, we obtained Sm-Nd ID-TIMS isochron ages of leached garnet and whole-rock (WR) to establish the timing of metamorphism. Detrital zircon geochronology of three samples from the western belt and two from the eastern belt indicates Early Jurassic maximum depositional ages (MDA; 193-177 Ma), whereas one sample from the western belt has a Late Triassic MDA (208 Ma), with zircon populations that were shed from similar sources. The compiled zircon data show four age modes, suggesting provenance from the Permo-Triassic Cordilleran arc and the Appalachian, Pan African, and Grenville orogens. Based on these observations, we consider that the two proposed belts of host rocks from the southern PRB share a similar origin where the sedimentary protoliths were deposited in ocean basins fed by continentally derived material with peri-Gondwanan affinity. One biotite-garnet paragneiss yielded a Sm-Nd garnet-WR age at 160 ± 2 Ma, coeval with metamorphic zircon growth (ca. 166 Ma) in the same sample, indicating a Middle-Late Jurassic metamorphic event. The other samples yielded Sm-Nd garnet-WR ages of 105 ± 3, 114 ± 2, and 214 ± 13 Ma. The Cretaceous ages are coeval with plutonism, whereas the Triassic age is the oldest metamorphic age documented in the Baja California Peninsula. The detrital zircon provenance ties the sedimentary protoliths to the ancient continental margin of Mexico since the Late Triassic. The Sm-Nd garnet-WR geochronology provides the first well-constrained dates for the timing of metamorphism in this region, which reveals coeval metamorphism and magmatism.