Framework and petrogenesis of the northern Peninsular Ranges batholith, southern California
Published:January 01, 2014
D.M. Morton, F.K. Miller, R.W. Kistler, W.R. Premo, C-T.A. Lee, V.E. Langenheim, J.L. Wooden, L.W. Snee, B.L. Clausen, P. Cossette, 2014. "Framework and petrogenesis of the northern Peninsular Ranges batholith, southern California", Peninsular Ranges Batholith, Baja California and Southern California, Douglas M. Morton, Fred K. Miller
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The Peninsular Ranges batholith north of latitude 33°N consists of five distinctive longitudinal batholith zones. Four zones are autochthonous—a western zone, western transition zone, eastern transition zone, and an eastern zone. The fifth zone, the upper-plate zone, is allochthonous. The western zone, western transition zone, eastern transition zone, and eastern zone are contiguous products of Cretaceous subduction transitioning from a Mesozoic oceanic-arc setting to continental margin arc setting. Within the autochthonous zones, the nature and geochemistry of plutons record changes reflecting subduction proceeding from west to east over a 35 m.y. period. The allochthonous upper-plate zone is structurally located above the regional Eastern Peninsular Ranges mylonite zone. Host rocks for the western zone, western transition zone, and eastern transition zone are mostly Mesozoic, and host rocks of the eastern zone are Paleozoic. The composition of the plutons reflects changes in magma originating in shallow oceanic crust in the western zone to a deeper continental marginal setting in the eastern zone and upper-plate zone. Several aspects of the upper-plate zone rocks set them apart from the autochthonous batholithic rocks.
Western zone magmatism occurred during an extensional subduction phase that involved Mesozoic oceanic crust. Plutons were emplaced passively from 126 Ma to 108 Ma, forming 47.9% of the area of the autochthonous batholith at a rate of 2.7% per million years. Geochemical variation is greater in the western zone than it is in the other zones. Rock compositions range from gabbro to high-SiO2 granites; plutons in this zone contain magnetite as an accessory mineral. Most plutonic rocks have initial 87Sr/86 Sr (Sri) values <0.7045, initial 206Pb/204 Pb (Pbi) <19, δ18O <9‰, and positive initial epsilon Nd (εNdi).
By 111 Ma, conditions for pluton emplacement began to change radically from extensional to compressional as subduction encountered older continental crust. The boundary between the western zone and western transition zone is marked clearly by a change in the magnetic properties, which are highly magnetic in the western zone to weakly magnetic in the transition zones. Western transition zone plutons, which have affinities with the western zone plutons, constitute 13.5% by area of the autochthonous batholith and formed over 13 m.y. at a decreased rate of batholith formation, 1% per million years. Plutons of the western transition zone are characterized by Sri values of 0.7045–0.7050, δ18O <9‰, and positive εNdi. Deformation of the prebatholithic rocks was intense at 100 Ma, as the plutonism of the western transition zone ended and emplacement in the eastern transition zone began. From 99 to 93 Ma, the rate of magma emplacement accelerated, forming 2.4% per million years by area of the northern part of the autochthonous batholith. The eastern transition zone plutons, having affinities with the eastern zone plutons, have Sri values of 0.7051–0.7057, δ18O >9‰, and negative εNdi. Most eastern transition zone plutons were emplaced in a less dynamic setting than the western transition zone plutons.
By 98 Ma, subduction had transitioned eastward as plutons were emplaced in continental crust. The rate of magma emplacement increased to form the eastern zone over 7 m.y., or a rate of batholith growth of 3.4% per million years by area. There is considerable temporal overlap in the magma emplacement of the eastern transition zone and the eastern zone. Combined eastern transition zone and eastern zone magmatism produced 39% (by area) of the autochthonous batholith in 8 m.y. at a rate of ~5% per million years. The 102 Ma gabbro body is not considered in this analysis. Eastern zone plutons are characterized by Sri >0.7060, mostly in the range of 0.7061–0.7076, Pbi >19, δ18O >9‰, and a large negative εNdi.
The allochthonous granitic sheets that constitute the upper-plate zone include batholithic rocks ranging in age from 92 to 75 Ma; most are in the range of 86–75 Ma. These granitic rocks have a more restricted range of geochemistry than those in the other zones; they are magnetite-bearing rocks, unlike the ilmenite-bearing granitic rocks of the transition zones and eastern zone, and they have large negative εNdi, and Sri in the range of 0.7076–0.7084. During the Late Cretaceous, the Eastern Peninsular Ranges mylonite zone developed in the eastern part of the Peninsular Ranges Province, deforming granitic rocks of the eastern part of the eastern zone. Following mylonitization, westward displacement on a series of low-angle thrust faults placed sheets of metamorphic and plutonic rock above the Eastern Peninsular Ranges mylonite zone, forming the upper-plate zone.
Compatible elements decrease west to east across the batholith, and incompatible elements increase. Geochemical variation shows that magma forming the western part of the batholith had a shallow and primitive source compared with the eastern part, which had a deeper and more-evolved continental component. The frequency distribution of Sri in the batholith is bimodal, having a peak of 0.7038 in the western zone, reflecting the oceanic crustal source, and a peak of 0.7072 in the eastern zone, reflecting increased incorporated continental crust sources. Only a small part of the batholith has Sri values between 0.7055 and 0.7065, indicating a relatively sharp boundary between oceanic and continental crust. Linear arrays on Harker diagram indicate that geochemical variation within the batholith is from magma mixing and not magmatic differentiation. Our data are most simply explained by the Cretaceous arc transitioning from a Mesozoic oceanic-arc setting to a continental margin setting.
Figures & Tables
Peninsular Ranges Batholith, Baja California and Southern California
- alkaline earth metals
- host rocks
- igneous rocks
- isotope ratios
- magnetic properties
- metamorphic rocks
- North America
- Peninsular Ranges Batholith
- plate tectonics
- plutonic rocks
- rare earths
- Southern California
- stable isotopes
- subduction zones
- United States