U-PB DETRITAL ZIRCON AND SM-ND GARNET GEOCHRONOLOGY OF METASEDIMENTARY ROCKS FROM THE SOUTHERN PENINSULAR RANGES BATHOLITH, BAJA CALIFORNIA, MEXICO: EARLY JURASSIC DEPOSITION FROM CONTINENTAL SOURCES AND MIDDLE JURASSIC METAMORPHISM

Cretaceous (128-92 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, closest to the paleotrench, consists of Jurassic meta-igneous rocks with island arc affinity. Toward the continent, the eastern belt is formed by amphibolite, para- and orthogneiss, schist, calcsilicate, marble, and other undifferentiated metamorphic rocks. The sedimentary protoliths are probably associated with Triassic-Jurassic back-arc assemblages or Paleozoic continental slope and basin sediments. However, the depositional age, provenance, and timing of metamorphism are still unknown. Here, we present U-Pb LA-ICPMS detrital zircon ages of five paragneiss samples and two schists. In addition, we present Sm-Nd ID-TIMS isochrones of leached garnet and whole rock (WR) to establish the timing of metamorphism. Detrital zircon geochronology of five samples from the western belt and two from the eastern belt indicates Early Jurassic maximum depositional ages (195-180 Ma) with zircon populations that were shed from similar sources. The compiled zircon data show four peaks, suggesting provenance from the Permo-Triassic Cordilleran arc and the Appalachian, Pan African, and Grenville orogens. Thus, the sedimentary protoliths formed from continentally derived material with Laurentian and peri-Gondwanan affinity. Only one biotite-schist shows local provenances with a maximum depositional age of ca. 155 Ma, which probably formed in an intra-arc basin isolated from continental influence. One biotite-garnet paragneiss yielded a Sm-Nd garnet-WR age at 157 ± 5.7 Ma, which is coeval with metamorphic zircon growth detected in two samples, indicating a Middle Jurassic metamorphic event. Our results provide Early Jurassic maximum depositional ages of the metasedimentary host rocks of the southern PRB and detrital zircon evidence that ties their sedimentary protoliths to the ancient continental margin of Mexico since the Early Jurassic. The Sm-Nd garnet geochronology provides the first confident dates constraining the time of metamorphism in this region.