Cretaceous plutons of the Peninsular Ranges batholith, San Diego and westernmost Imperial Counties, California : intrusion across a Late Jurassic continental margin

The Peninsular Ranges batholith of southern California and Baja California was emplaced across the litho spheric boundary between North America and Pacific plates in the Jurassic and Cretaceous, In San Diego County, the locus of Cretaceous plutonism migrated eastward from oceanic lithosphere across the continental margin into early Mesozoic continental lithosphere Uplift and westward tilting of the Peninsular Ranges block associated with Late Cenozoic rifting and transform faulting resulted in au erosional depth profile from volcanic levels in the west to mid-crustal depths in the east. Our study of Cretaceous plutons in a west-to-east transect across southern San Diego County shows that granitic plutons have distinctive geophysical, geobarometric, mineralogical, geochemical, and isotopic characteristics that vary systematically with geographic position within the batholith, On the basis of these characteristics, granitic plutons are grouped into 10 plutonic suites, each comprising numerous plutons In the west-central part of the study area, five granitic suites and a suite of gabbroic rocks comprise a series of large, concentrically zoned plutonic complexes, possibly the roots of ring complexes. Mingling contacts between granitic and gabbroic plutons within these complexes indicate that mafic and silicic magmas were produced simultaneously during Cretaceous intrusion. Published and unpublished isotopic age studies indicate that plutons in the western zone are primarily of Early and middle Cretaceous age, whereas those in the eastern zone are middle to Late Cretaceous in age. Our delineation of western and eastern plutonic zones on the basis of the geographic distribution and compositions of plutonic suites corresponds closely with previously noted west-to-east batholithic asymmetries. Primary among these is a steep aeromagnetic- and gravity-anomaly gradient that coincides closely with the westernmost limit of Jurassic granites and their early Mesozoic wallrocks. This gradient is interpreted as the manifestation of an east-dipping fault of crustal dimensions that formed in the latest Jurassic-earliest Cretaceous (Shaw et al., this volume, Chapter 7). The distribution of Cretaceous plutonic suites, together with compositional changes in suites that intrude both western and eastern zones of the batholith, are considered to reflect the location of the fault-bounded Late Jurassic continental margin. Geochemical studies of the Cretaceous granitic suites characterize them as low-K₂0, low-Rb, calcic granites with aluminium saturation indices (ASI, i.e., molar Al����O����/[CaO+Na���O+K₂O] values) between 0.67 and 1.20, reflecting a significant mantle contribution in the source of the magmas. Systematic variations in the geographic distribution and composition of suites indicate that partial melting of chemically inhomogeneous, pH₂O -variable metaigneous rocks in the lower crust played a significant role in their origin. Constant rare earth element patterns and low Sr₁ values of the suites across the gravity-magnetic boundary suggest that source rocks of basaltic composition were present in the lower crust and/or as underplates from earlier subduction episodes beneath both western and eastern zones of the batholith. Compositional differences among five tonalite suites reflect (1) increasing crustal thickness flam the Early to Late Cretaceous, causing a phase transition of the zone of melting toward eclogitic assemblages, and/or (2) increasing depths of melting as the magmatic arc migrated eastward. Four leucogranite suites apparently represent fractionates of mafic melts, and/or partial melts of lithologically distinctive lower crustal rocks. The interaction of magmas with western and eastern lithosphere during ascent and emplacement was responsible for slight eastward increases in K₂O, Rb, initial ⁸⁷Sr/⁸⁶Sr ratios, δ¹⁸0 values, and radiogenic lead isotopes in suites that are present in both western and eastern zones, as well as for east-west differences in opaque oxide mineralogy and magnetic properties. The data of our study imply that a single Cretaceous magmatic arc migrated eastward across a pre-existing Late Jurassic-earliest Cretaceous lithospheric boundary, the distribution of known Cretaceous pluton ages does not require an age break, but rather suggests continuous eastward migration of a single magmatic arc. Published and unpublished Early Cretaceous pluton ages for the oldest Cretaceous granitic suite that stitched across the lithospheric boundary in this region constrain the minimum age of its formation as early EarIy Cretaceous.51 page(s