Ophiolite and volcanic arc assemblages on the Vizcaino Peninsula and Cedros Island, Baja California Sur, Mexico; Mesozoic forearc lithosphere of the Cordilleran magmatic arc.

Abstract

Mesozoic ophiolites in the Vizcaino Peninsula and Cedros Island region of Baja California Sur are suprasubduction zone Cordilleran-type ophiolites structurally juxtaposed with underlying high pressure-temperature subduction complex assemblages. The region is divided into three separate tectonostratigraphic terranes, but here we recognize stratigraphic, intrusive, and petrologic links between these terranes and interpret the evolution of the entire region within the same Late Triassic to Early Cretaceous tectonic framework. Several phases of extension are recognized, including two major phases that resulted in development of distinct ophiolite assemblages. The Late Triassic Vizcaino Peninsula Ophiolite (221+ or -2 Ma) represents the earliest stage of this history and comprises a complete spreading center sequence with depleted upper mantle and mafic crustal rocks, including sheeted dike complex. Jurassic arc magmatic rocks with low-Ti arc tholeiite and boninite geochemical affinities were intruded through and constructed on the Triassic ophiolite basement. Ultra-depleted arc-ankaramites on Cedros Island may represent an initial phase of arc rifting that was followed by major Middle Jurassic extension and production of the Cedros Island Ophiolite (173+ or -2 Ma). The Late Jurassic-Early Cretaceous Coloradito and Eugenia Formations contain mudflows and olistostrome blocks intercalated with arc volcanogenic sediment and rift-related pillow lavas; these units record extension and/or transtension and provide the earliest definite evidence of arc-continent interaction in the region. Middle Jurassic to Early Cretaceous arc plutonic rocks (ca. 165-135 Ma) were shallowly intruded into low greenschist-facies ophiolite and arc volcanic basement. Plutonic rocks range in composition from gabbro to granodiorite, but tonalite dominates. These intrusions are typical I-type Cordilleran batholithic rocks with relatively primitive arc geochemical affinities (initial (super 87) Sr/ (super 86) Sr range from approximately 0.704 to 0.706),but they are distinctly calcic in nature, a feature common to the adjacent Cretaceous Peninsular Ranges batholith. The Vizcaino-Cedros region correlates to ophiolitic terranes of the western Sierra-Klamath belt and Coast Ranges of California and Oregon that were constructed in part across the North American margin. Age, stratigraphic, and petrochemical data from the Vizcaino-Cedros region support previously proposed forearc rifting models developed for the U.S. sector of the Cordilleran orogen that interpret the ophiolite assemblages as autochthonous or parautochthonous forearc lithosphere constructed outboard of the Mesozoic continental margin arc.

31 samples from this reference