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Description
The Santiago Peak Volcanics are weakly metamorphosed volcanic and volcaniclastic rocks exposed discontinuously along the western margin of the Peninsular Ranges batholith. These rocks have been assigned a Late Jurassic age based on Tithonian Buchia Piochii fossils found in isolated marine volcaniclastic sections that are not representative of the generally andesitic flows and breccias that make up the bulk of the formation. Zircon U/Pb isotopic dates are presented for ten samples from widely separated exposures of the Santiago Peak Volcanics, as well as for five samples of plutonic rocks from the western part of the Peninsular Ranges batholith. Data from the metavolcanic rocks yield variably discordant ages with the 206*Pb/238U values ranging from 137 Ma to 119 Ma. Four of the ten Santiago Peak Volcanics samples yielded only slightly discordant results that suggest an Early Cretaceous crystallization ages for the zircon. The youngest well-determined age is 122 Ma. The new U/Pb age data combined with field relations and composition suggest that the Santiago Peak Volcanics represent a Late Jurassic to Early Cretaceous volcanic arc. Zircon U/Pb ages also indicate a closer relationship between the mostly subaerial Santiago Peak Volcanics and the mainly marine Alisitos Group of Aptian-Albian age, which crops out along the western edge of the Peninsular Ranges batholith in Baja California. These two formations are interpreted as different segments of the same volcanic arc, with the northern part having been more emergent than the southern part. All of the plutonic rocks yielded Cretaceous zircon U/Pb ages ranging from 137 Ma to 108 Ma. The 137 Ma age is based upon a single fraction from a hornblende-biotite granite from the Mission Gorge plutonic complex, and represents one of the oldest dated granitic plutons in the western part of the batholith. The new U/Pb zircon ages from metavolcanic and plutonic rocks of the batholith indicate that the extrusive and intrusive events overlapped in time. This provides support for previously proposed models that interpret the Santiago Peak Volcanics as the cover rocks of shallowly emplaced plutons along the western margin of the Peninsular Ranges batholith.
