U-Pb zircon geochronology of plutonism in the northern Peninsular Ranges batholith, southern California

Abstract

Utilizing both sensitive high-resolution ion microprobe (SHRIMP) and conventional isotope dilution-thermal ionization mass spectrometry (ID-TIMS) methods, crystallization and/or emplacement ages have been obtained for a suite of Cretaceous intermediate-composition plutonic samples collected along a roughly E-W-trending traverse through the northern Peninsular Ranges batholith. Previously noted petrologic, mineralogic, and textural differences delineated four major zonations from west to east and raised the need for detailed geochemical and isotopic work. U-Pb zircon geochronology establishes that these zonations are essentially temporally separate. Mean 206Pb/ 238U ages date the three older zones from west to east at 126-107 Ma, 107-98 Ma, and 98-91 Ma. Despite petrologic differences, a relatively smooth progression of magmatism is seen from west to east. A fourth zone is defined by magmatism at ca. 85 Ma, which represents emplacement of deeper-level plutons east of the Eastern Peninsular Ranges mylonite zone in an allochthonous thrust sheet in the northeastern Peninsular Ranges batholith. The age data presented here differ slightly from those presented in earlier work for similar rocks exposed across the middle and southern portions of the Peninsular Ranges batholith in that our data define a relatively smooth progression of magmatism from west to east, and that the transition from western-type to eastern-type plutonism is interpreted to have occurred at ca. 98 Ma and not at ca. 105 Ma. The progressive involvement of older crustal components in the enrichment of eastern Peninsular Ranges batholith-type magma sources is documented by the occurrence of Proterozoic zircon inheritance within samples of the eastern part of the batholith. Show more