INSIGHTS INTO THE ASSEMBLY OF THE PENINSULAR RANGES BATHOLITH AND THE NATURE OF PRIMITIVE BASEMENT THAT UNDERLIES ITS WESTERN AND SOUTHERN EXTENT

The Cretaceous Peninsular Ranges batholith (PRB) extends 1400 km along the southern and Baja California margin (34-23°N). The northeastern PRB is intruded into the Proterozoic continental margin (miogeocline) of SW Laurentia while its western and southern counterparts appear to be constructed upon comparatively primitive Triassic-Jurassic supra-subduction ophiolitic crust. The late Carboniferous-Permian collision zone between southern Laurentia and Gondwana extends roughly E-W through Sonora and intersects the PRB at 29°N. New detrital zircon age distributions integrated with existing data from the latest Neoproterozoic-Early Cretaceous prebatholith framework contains detritus fundamentally derived from: (1) SW North American craton; (2) the miogeocline; or (3) mainland Mexico. Some models for the PRB invoke arc-continent collision at ca. 115-100 Ma. Our sampling includes a number of locations that stratigraphically underlie and/or are intercalated with volcanic arc cover that crops out along the western margin of the PRB between 34-28°N. The provenance signature of these pre-to-syn volcanic strata all contain Laurentian or Gondwanan age distributions. These continental provenance signatures are also recognized in Late Jurassic, Early Cretaceous, and Late Cretaceous olistolith blocks and cobbles preserved in the Coloradito Fm, Eugenia Fm., and Valle Group within the Vizcaino region of west-central Baja. Collectively, these results require that the westernmost basement of the PRB were accessible by fluvial depositional systems or submarine slopes that dispersed craton-derived detritus. While the margin was likely highly extended setting during the Late Jurassic-Early Cretaceous, our results indicate that the PRB formed essentially in place and preclude the possibility that the western PRB represents a far-traveled, accreted oceanic island arc. In southernmost Baja, the prebatholithic framework appears contiguous with Gondwana-derived basement (e.g. Arteaga Complex) underlying the Guerrero Terrane of mainland Mexico. We conclude that the primitive basement that underlies southwestern Mexico correlates with equivalent suprasubduction ophiolitic rocks in the Magdalena, Vizcaino, and southern California borderland regions.