The crustal structure of the Peninsular Ranges batholith can be divided geophysically into two parts: (1) a western mafic part that is dense, magnetic, and characterized by relatively high seismic velocities (>6.25 km/s), low heat flow (<60 mW/m 2 ), and relatively sparse seismicity, and (2) an eastern, more felsic part that is less dense, weakly magnetic, and characterized by lower seismic velocities (<6.25 km/s), high heat flow (>60 mW/m 2 ), and abundant microseismicity. Potential-field modeling indicates that the dense, mafic part of the batholith extends to depths of at least 20 km and likely to the Moho. The magnetic anomalies of the western part of the batholith extend south beyond the spatially extensive exposures of the batholith to the tip of the Baja California peninsula, which suggests that the mafic part of the batholith projects beneath Cenozoic volcanic cover another 400 km. The linearity and undisrupted nature of the magnetic belt of anomalies suggest that the western part of the batholith has behaved as a rigid block since emplacement of the batholith. The batholith may have influenced not only the development of the Gulf of California oblique rift, but also strike-slip faulting along its northern margin, and transtensional faulting along its western margin, likely because it is thermally and mechanically more resistant to deformation than the surrounding crust.

Nine U-Pb zircon ages were determined on plutonic rocks sampled from surface outcrops and rock chips of drill core from boreholes within the greater Los Angeles Basin region. In addition, lead-strontium-neodymium (Pb-Sr-Nd) whole-rock isotopic data were obtained for eight of these samples. These results help to characterize the crystalline basement rocks hidden in the subsurface and provide information that bears on the tectonic history of the myriad of fault systems that have dissected the Los Angeles region over the past 15 m.y. Seven of the nine samples have U-Pb ages ranging from 115 to 103 Ma and whole-rock Pb-Sr-Nd isotopic characteristics that indicate the crystalline basement underneath the greater Los Angeles Basin region is mostly part of the Peninsular Ranges batholith. Furthermore, these data are interpreted as evidence for (1) the juxtaposition of mid-Cretaceous, northern Peninsular Ranges batholith plutonic rocks against Late Cretaceous plutonic rocks of the Transverse Ranges in the San Fernando Valley, probably along the Verdugo fault; (2) the juxtaposition of older northwestern Peninsular Ranges batholith rocks against younger northeastern Peninsular Ranges batholith rocks in the northern Puente Hills, implying transposition of northeastern Peninsular Ranges batholith rocks to the west along unrecognized faults beneath the Chino Basin; and (3) juxtaposition of northern Peninsular Ranges batholith plutonic rocks against Late Cretaceous plutonic rocks of the Transverse Ranges along the San Jose fault in the northern San Jose Hills at Ganesha Park. These mainly left-lateral strike-slip faults of the eastern part of the greater Los Angeles Basin region could be the result of block rotation within the adjacent orthogonal, right-lateral, Elsinore-Whittier fault zone to the west and the subparallel San Jacinto fault zone to the east. The San Andreas fault system is the larger, subparallel, driving force further to the east.

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-SiO 2 granites; plutons in this zone contain magnetite as an accessory mineral. Most plutonic rocks have initial 87 Sr/ 86 Sr (Sr i ) values <0.7045, initial 206 Pb/ 204 Pb (Pb i ) <19, δ 18 O <9‰, and positive initial epsilon Nd (ε Ndi ). 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 Sr i values of 0.7045–0.7050, δ 18 O <9‰, and positive ε Ndi . 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 Sr i values of 0.7051–0.7057, δ 18 O >9‰, and negative ε Ndi . 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 Sr i >0.7060, mostly in the range of 0.7061–0.7076, Pb i >19, δ 18 O >9‰, and a large negative ε Ndi . 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 ε Ndi , and Sr i 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 Sr i 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 Sr i 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.

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 206 Pb/ 238 U 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.

Conventional potassium-argon (K-Ar) ages were obtained on biotite from samples of granitic rocks collected at as regular spacing as outcrop and sample suitability permitted across the entire northern exposed part of the Peninsular Ranges batholith. Uranium-lead (U-Pb) ages on zircons range from 6 to 24 m.y. older than roughly corresponding conventional K-Ar biotite ages. The U-Pb zircon ages are considered to be emplacement or near-emplacement ages and provide a basis for using the conventional biotite ages to approximate variations in cooling history. Contouring of the biotite cooling ages shows the same west-to-east younging trend that earlier regional dating studies have shown. Contours generated by these earlier regional studies produced a relatively smooth, even age gradient across the batholith. Biotite cooling age contours generated by the much more closely spaced data set used here suggest a more complicated cooling history and show strong digressions from the smooth, even regional-scale gradient. Along much of their respective traces, the right-lateral strike-slip Elsinore and San Jacinto fault zones cut granitic rocks of the batholith; we have used the age contours as datums to estimate maximum offset across the Elsinore fault and to support a proposed offset based on geologic mapping evidence on the San Jacinto fault. These estimates, based on offsets of the age contours, may differ from true offsets because of uncertainties related to (1) inability to establish the dip of the age contours, (2) possible vertical components of offsets on faults, and (3) paucity of samples in some areas. Cooling age contours are offset 12 km across the Elsinore fault zone, a figure in keeping with offsets of 10–15 km based on detailed geologic mapping. Likewise, alignment of contour features at the north end of the Perris block with those at the north end of the San Jacinto block requires restoration of 29 km of right-lateral displacement, i.e., essentially the same as documented offsets based on geologic mapping.

The thermochronology for several suites of Mesozoic metamorphic and plutonic rocks collected throughout the northern Peninsular Ranges batholith (PRB) was studied as part of a collaborative isotopic study to further our understanding of the magmatic and tectonic history of southern California. These sample suites include: a traverse through the plutonic rocks across the northern PRB ( N = 29), a traverse across a central structural and metamorphic transition zone of mainly metasedimentary rocks at Searl ridge ( N = 20), plutonic samples from several drill cores ( N = 7) and surface samples ( N = 2) from the Los Angeles Basin, a traverse across the Eastern Peninsular Ranges mylonite zone ( N = 6), and a suite of plutonic samples collected across the northern PRB ( N = 13) from which only biotite 40 Ar/ 39 Ar ages were obtained. These geochronologic data help to characterize five major petrologic, geochemical, and isotopic zonations of the PRB (western zone, WZ; western transition zone, WTZ; eastern transition zone, ETZ; eastern zone, EZ; and upper-plate zone, UPZ). Apparent cooling rates were calculated using U-Pb zircon (zr) and titanite (sphene) ages; 40 Ar/ 39 Ar ages from hornblende (hbl), biotite (bi), and K-feldspar (Kf); and apatite fission-track (AFT) ages from the same samples. The apparent cooling rates across the northern PRB vary from relatively rapid in the west (zr-hbl ~210 °C/m.y.; zr-bio ~160 °C/m.y.; zr-Kf ~80 °C/m.y.) to less rapid in the central (zr-hb ~280 °C/m.y.; zr-bio ~90 °C/m.y.; zr-Kf ~60 °C/m.y.) and eastern (zr-hbl ~185 °C/m.y.; zr-bio ~180 °C/m.y.; zr-Kf ~60 °C/m.y.) zones. An exception in the eastern zone, the massive San Jacinto pluton, appears to have cooled very rapidly (zr-bio ~385 °C/m.y.). Apparent cooling rates for the UPZ samples are consistently slower in comparison (~25–45 °C/m.y.), regardless of which geochronometers are used. Notable characteristics of the various ages from different dating methods include: (1) Zircon ages indicate a progressive younging of magmatic activity from west to east between ca. 125 and 90 Ma. (2) Various geochronometers were apparently affected by emplacement of the voluminous (ETZ and EZ) La Posta–type plutons emplaced between 99 and 91 Ma. Those minerals affected include K-feldspar in the western zone rocks, biotite and K-feldspar in the WTZ rocks, and white mica and K-feldspar in rocks from Searl ridge. (3) The AFT ages record the time the rocks cooled through the AFT closure temperature (~100 °C in these rocks), likely due to exhumation. Throughout most of the northern traverse, the apatite data indicate the rocks cooled relatively quickly through the apatite partial annealing zone (PAZ; from ~110 °C to 60 °C) and remained at temperatures less than 60 °C as continued exhumation cooled them to present-day surface temperatures. The ages indicate that the western “arc” terrane of the WZ was being uplifted and cooled at ca. 91 Ma, during or shortly after intrusion of the 99–91 Ma La Posta–type plutons to the east. Uplift and cooling occurred later, between ca. 70 Ma and ca. 55 Ma, in the central WTZ, ETZ, and EZ rocks, possibly as upwarping in response to events in the UPZ. The UPZ experienced differential exhumation at ca. 50–35 Ma: Cooling on the western edge was taking place at about the same time or shortly after cooling in the younger samples in the ETZ and EZ, whereas on the east side of the UPZ, the rocks cooled later (ca. 35 Ma) and spent a prolonged time in the apatite PAZ compared to most northern traverse samples. Apparent cooling rates from Los Angeles Basin drill core samples of plutonic rocks show that four are similar to the WTZ thermal histories, and two are similar to the WTZ histories, indicating that the eastern part of the Los Angeles Basin area is underlain by mainly western zone PRB rocks. Thermal histories revealed by samples from Searl ridge indicate that the WTZ magmatism intruded the metasedimentary rocks prior to their deformation and metamorphism at ca. 97 Ma. Both low-grade schists and metasandstones of the western side of the ridge and high-grade gneisses of the eastern side of the ridge have thermal histories consistent with eastern zone rocks—suggesting a temporal/thermal relationship between the western transition zone and the eastern zones. Limited ages from six samples across the Eastern Peninsular Ranges mylonite zone (EPRMZ) indicate that this zone underwent cooling after emplacement of the youngest UPZ rocks at 85 Ma, suggesting that thrusting along the EPRMZ was either coeval with emplacement of the UPZ plutonic rocks or occurred shortly afterwards (~10–15 m.y.). Alternatively, the EPRMZ thrusting may have occurred at temperatures under ~180 °C at yet a later date. The geochronology presented here differs slightly from previous studies for similar rocks exposed across the middle and southern portions of the PRB, in that our data define a relatively smooth progression of magmatism from west to east, and the transition from western, oceanic-arc plutonism to eastern, continental arc plutonism is interpreted to have occurred at ca. 99–97 Ma and not at ca. 105 Ma.

Within the duration of the U.S. Geological Survey (USGS)–based Southern California Areal Mapping Project (SCAMP), many samples from the northern Peninsular Ranges batholith were studied for their whole-rock radioisotopic systematics (rubidium-strontium [Rb-Sr], uranium-thorium-lead [U-Th-Pb], and samarium-neodymium [Sm-Nd]), as well as oxygen (O), a stable isotope. The results of three main studies are presented separately, but here we combine them (>400 analyses) to produce a very complete Pb-Sr-Nd-O isotopic profile of an arc-continent collisional zone—perhaps the most complete in the world. In addition, because many of these samples have U-Pb zircon as well as argon mineral age determinations, we have good control of the timing for Pb-Sr-Nd-O isotopic variations. The ages and isotopic variations help to delineate at least four zones across the batholith from west to east—an older western zone (126–108 Ma), a transitional zone (111–93 Ma), an eastern zone (94–91 Ma), and a much younger allochthonous thrust sheet (ca. 84 Ma), which is the upper plate of the Eastern Peninsular Ranges mylonite zone. Average initial 87 Sr/ 86 Sr (Sr i ), initial 206 Pb/ 204 Pb ( 206 Pb i ), initial 208 Pb/ 204 Pb (average 208 Pb i ), initial epsilon Nd (average ε Ndi ), and δ 18 O signatures range from 0.704, 18.787, 38.445, +3.1, and 4.0‰–9.0‰, respectively, in the westernmost zone, to 0.7071, 19.199, 38.777, −5, and 9‰–12‰, respectively, in the easternmost zone. The older western zone is therefore the more chemically and isotopically juvenile, characterized mostly by values that are slightly displaced from a mantle array at ca. 115 Ma, and similar to some modern island-arc signatures. In contrast, the isotopic signatures in the eastern zones indicate significant amounts of crustal involvement in the magmatic plumbing of those plutons. These isotopic signatures confirm previously published results that interpreted the Peninsular Ranges batholith as a progressively contaminated magmatic arc. The Peninsular Ranges batholith magmatic arc was initially an oceanic arc built on Panthalassan lithosphere that eventually evolved into a continental margin magmatic arc collision zone, eventually overriding North American cratonic lithosphere. Our Pb-Sr-Nd data further suggest that the western arc rocks represent a nearshore or inboard oceanic arc, as they exhibit isotopic signatures that are more enriched than typical mid-ocean-ridge basalt (MORB). Isotopic signatures from the central zone are transitional and indicate that enriched crustal magma sources were becoming involved in the northern Peninsular Ranges batholith magmatic plumbing. As the oceanic arc–continental margin collision progressed, a mixture of oceanic mantle and continental magmatic sources transpired. Magmatic production in the northern Peninsular Ranges batholith moved eastward and continued to tap enriched crustal magmatic sources. Similar modeling has been previously proposed for two other western margin magmatic arcs, the Sierra Nevada batholith of central California and the Idaho batholith. Calculated initial Nd signatures at ca. 100 Ma for Permian–Jurassic and Proterozoic basement rocks from the nearby San Gabriel Mountains and possible source areas along the southwestern Laurentian margin of southern California, southwestern Arizona, and northern Sonora strongly suggest their involvement with deep crustal magma mixing beneath the eastern zones of the Peninsular Ranges batholith, as well as farther east in continental lithospheric zones. Last, several samples from the allochthonous, easternmost upper-plate zone, which are considerably younger (ca. 84 Ma) than any of the rocks from the northern Peninsular Ranges batholith proper, have even more enriched average Sr i , 206 Pb i , 208 Pb i , and ε Ndi signatures of 0.7079, 19.344, 38.881, and −6.6, respectively, indicative of the most-evolved magma sources in the northern Peninsular Ranges batholith and similar to radioisotopic values for rocks from the nearby Transverse Ranges, suggesting a genetic connection between the two.

We studied the formation of low-initial-Sr felsic plutons by using data from the Early Cretaceous western Peninsular Ranges batholith near Escondido, California. The systematically sampled Escondido plutons have a uniformly low initial 87 Sr/ 86 Sr isotope ratio of Sr i < 0.704, but a wide range of SiO 2 compositions, from 46 to 78 wt%, which fall in three distinct groups: 20% gabbros, 35% tonalites, and 45% granodiorites. These low-Sr i plutons are unique in having undergone one cycle of mantle melting to give basalt composition rocks, and a second cycle of arc basalt melting to give a range of SiO 2 plutons, but no third cycle of melting and contamination by old continental crust to yield high-Sr i rocks. After doing two-cycle partial melting and fractional crystallization calculations, it was recognized that mixing of gabbro and granodiorite magmas was necessary to yield the tonalites. The linear data pattern on Harker diagrams is interpreted as resulting from mixing of mafic magma from partial melting of the mantle and felsic magma from partial melting of the lower crust to form intermediate magma. These plutons provide a simplified two-cycle Phanerozoic example of the petrogenetic process for forming continental crust.

The Santiago Peak volcanics, in the northern Santa Ana Mountains, are the northernmost exposures of the Santiago Peak–Alisitos magmatic arc present along the western edge of the Peninsular Ranges batholith. Remnants of deeply eroded volcanic sequences in the Santa Ana Mountains consist of subaerial basaltic-andesite to rhyolite lavas, rare basalt, welded tuff, and pyroclastic rocks that were emplaced across deformed Middle Jurassic turbidites. Subalkaline lavas have mixed calc-alkaline and tholeiitic affinities. Relatively primitive ε Sr (−18 to +5) and ε Nd (+7.5 to +0.1) values for the lavas plot along the mantle array. Silicic lavas have higher ε Sr and ε Nd values in comparison to mafic lavas. Parental magmas were derived from hydrous melts of relatively depleted mantle wedge, followed by fractionation and the assimilation of up to 10% crustal materials. The whole-rock compositions, isotopic data, and U/Pb and 40 Ar/ 39 Ar ages (128–110 Ma) of the Early Cretaceous Santiago Peak volcanics and related Estelle Mountain volcanics overlap with emplacement ages of plutons of the western zone of the Peninsular Ranges batholith. The volcanic rocks are interpreted as the volcanic component of the arc plumbing system of the batholith. Arc rocks are in turn unconformably overlain by a forearc sequence of Upper Cretaceous through Tertiary strata that indicate deep erosion of the Santiago Peak volcanics by 95 Ma. Volcanic clasts of Turonian age within the forearc sequence yield U/Pb ages of 108–106 Ma. Age data and whole-rock geochemistry of the volcanic clasts indicate that they were eroded from supracrustal volcanic rocks located farther east within the Elsinore block.

The Cretaceous Green Acres layered igneous complex, northeast of Winchester, California, is composed of a suite of olivine- and hornblende-bearing gabbros in the Peninsular Ranges batholith within the Perris tectonic block. A consistent mineral assemblage is observed throughout the complex, but there is considerable textural and modal heterogeneity. Both preclude a consistent set of principles based on appearance and mineralogy on which to delineate map units. Distinct changes in the chemistry of olivine, pyroxene, and hornblende, however, serve to define discrete mappable units, and the complex has been divided into five geochemical map units on this basis. Limited whole-rock data show the Green Acres complex is chemically comparable to other Peninsular Ranges batholith gabbroic rocks, and rare earth element (REE) concentrations and patterns are typical of magmas generated in convergent margin settings. For the complex as a whole, olivine is Fo 80–35 , plagioclase is An 100–64 , clinopyroxene is Wo 49–41 En 48–38 Fs 18–6 and Wo 36–26 En 65–42 Fs 30–8 , and orthopyroxene is Wo 5–0 En 78–42 Fs 50–21 , where Fo is forsterite, An is anorthite, Wo is wollastonite, En is enstatite, and Fs is ferrosilite. The Mg/(Mg + ΣFe) atomic ratio in hornblende ranges from 0.84 to 0.50. Magmatic lineations and modal and textural layering are prevalent throughout the complex. Mineral chemistry does not change in any systematic way within and between layers in any map unit. Although the strike of layering varies, in any map unit at any given location it is the same in all units irrespective of intrusive order. Thin dikes, typically late-stage hornblende gabbro, commonly intrude parallel to layering. The strikes of magmatic lineations and modal layers are consistent with the populations of strikes of fabrics in the metamorphic basement as well as tectonic features in surrounding, postgabbro granitic rocks. These relations imply that the regional state of stress at the time of gabbro emplacement played a role in layer formation in conjunction with thermal and hydraulic pressure perturbations.

The Lakeview Mountains pluton is a concordant teardrop-shaped pluton located at a marked deflection of the structural grain of the prepluton rocks within the northern Peninsular Ranges batholith. This dynamically emplaced 100 Ma pluton lies within the western transition zone and consists of biotite-hornblende tonalite that lacks K-feldspar. The pluton is characterized by ubiquitous schlieren that range from black hornblende-biotite rock to near-white quartz-plagioclase rock, imparting an extreme outcrop-scale mineral and chemical heterogeneity to the pluton. Geometrically, the schlieren define three structural sets; one is concordant, and the other two constitute a northeast- and northwest-oriented conjugate set. The orientation of the concordant schlieren resulted from the outward expansion of the pluton, and the orientation of the conjugate set is in response to regional stresses at the time of emplacement. Based on chemical analysis of systematically collected samples, the pluton consists of two chemically distinct parts. Initially emplaced magma formed an ellipsoidal body concordant with the regional northwest structural grain. This early-emplaced magma formed a zoned body having a relatively potassic core and a mafic outer part. Later-emplaced magma expanded the pluton to the north-northeast, deflecting the regional structural grain of the batholith, and forming the teardrop-shaped outline of the composite pluton. The later-emplaced magma was more mafic than the initial magma, producing a more mafic core and a relatively higher-potassium outer part. Variations in major and trace elements, specific gravity, magnetic susceptibility, and magnetite content, in addition to aeromagnetic and pseudogravity anomalies, all show similar patterns within the pluton. Bodies of hypersthene gabbro, large masses of melanocratic and leucocratic tonalite, and numerous potassic granitic pegmatite dikes are concentrated in the more mafic part of the pluton, and interpreted as the last to crystallize. The leucocratic and melanocratic tonalite bodies are interpreted to be late-emplaced giant schlieren. Initial 87 Sr/ 86 Sr ratios (Sr i ) have only subtle, limited systematic variation, reflecting a relatively uniform magma source. Rb/Sr ratios also are relatively constant from the early- to late-emplaced magma, indicating the absence of, or only slight, fractional crystallization from the early- to late-emplaced magma. Sr i values of the hypersthene gabbro, mafic enclaves, and granitic pegmatites are the same as the tonalite of the pluton. The deflection of the regional structural grain by outward expansion of the pluton is interpreted to be a result of dynamic emplacement of the magma. Highly attenuated mafic enclaves in a prepluton mixed granitic rock unit that partly surrounds the pluton are also interpreted to have developed in response to outward expansion during the dynamic emplacement. A comb-layered gabbro, located along the contact of the earlier-formed part of the pluton, is interpreted as an early water-rich magma emplaced in a border area of the pluton protected from primary flow and dynamic strain. The high-potassium pegmatite bodies are interpreted to have formed from residual, immiscible, water-charged fluids derived from the low-potassium tonalite magma, which are concentrated in the mafic last part of the pluton to crystallize.

Tonalite of the Lakeview Mountains pluton is extremely heterogeneous in modal composition at both locality and outcrop scales. We applied an X-ray diffraction (XRD) method of modal analysis to 335 whole-rock powders previously used to study variability in major-element chemistry. We developed sampling levels appropriate for XRD modal study, mapped compositional gradients, recognized contrasts between chemical and modal variability, and tested the XRD method's suitability for petrologic investigations. Modal variability was determined at differing scales ranging from drill-core samples and hand specimens to 610 m 2 subareas of the pluton. Compositing of three hand samples reduced variance within subareas of this size sufficiently for mapping modal composition; the equivalent number found by previous studies for major-element chemical composition was nine. For this pluton, the X-ray method permits a single machine analysis to substitute for point counting at least 18 standard thin sections in each subarea in order to map compositional gradients. Because potassium occurs significantly only in biotite, close similarity of trend surfaces for potassium and biotite provides confirmation of the efficacy of XRD modal analysis in petrologic studies.

At 100 Ma, about midway through the history of the emplacement of the northern part of Peninsular Ranges batholith, a regional-scale deformation zone 1 developed that juxtaposed two major prebatholithic units. Deformation occurred as subduction transitioned from beneath oceanic crust to beneath continental crust. The deformation zone is well exposed at Searl Ridge, where the intensity of deformation attendant to the juxtaposition of the two prebatholithic units progressively increased from west to east over a distance of 5.5 km. In this 5.5-km-wide zone, a series of three progressive structural transpositions is recorded in the metasedimentary rocks. Many outcrops have two intersecting planar fabrics and a linear fabric produced by the intersecting planar fabrics. West to east within the deformation zone, the rock fabric changes from phyllitic to schistose, and the grade of metamorphism increases from greenschist to lower granulite. Plutonic rocks predating the deformation zone at the west edge of Searl Ridge were likewise deformed locally into rocks having a pronounced gneissic fabric. The transposition displacement produced a 300 °C gradient over a 4-km-wide zone of low-pressure, high-temperature, Buchan-type metamorphic mineral assemblages. The 300 °C gradient is based on biotite forming at a temperature of ~425 °C, and K-feldspar at ~750 °C. A 3–4 kbar pressure gradient across the deformation zone is inferred from a crystallization pressure of 2 kbar for the 120 Ma Domenigoni Valley pluton, located at the western edge of the deformation zone, and a 5–6.3 kbar pressure for the 100 Ma Lakeview Mountains pluton, emplaced in the eastern part, the latter being coeval with the formation of the deformation zone. If temperature increase is attributable entirely to difference in depth, and assuming an idealized 0.3 kbar increase in pressure per 1 km increase in crustal depth, the vertical displacement component of deformation within the deformation zone was 10–13 km.

Twenty-four samples were collected from prebatholithic metasedimentary rocks along Searl Ridge, the north rim of the Diamond Valley Reservoir, Domenigoni Valley, centrally located in the northern Peninsular Ranges of southern California. These rocks exhibit progressive metamorphism from west to east across fundamental structural discontinuities now referred to as a “transition zone.” Documented structural and mineralogical changes occur across this metamorphic gradient. Sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) U-Pb ages were obtained from detrital zircons from metasedimentary rocks through the transition zone. To the west, metapelitic and minor metasandstone units yielded numerous concordant 206 Pb/ 238 U ages between 210 and 240 Ma, and concordant 207 Pb/ 206 Pb ages at 1075–1125 Ma, 1375–1430 Ma, and 1615–1735 Ma, although distinct differences in provenance were noted between units. A few older 207 Pb/ 206 Pb ages obtained were ca. 2250 Ma and ca. 2800 Ma. Rocks of the eastern part of the transition zone include high-grade paragneisses that yielded numerous concordant 206 Pb/ 238 U ages between 103 and 123 Ma and between 200 and 255 Ma, and concordant 207 Pb/ 206 Pb ages at 1060–1150 Ma, 1375–1435 Ma, and 1595–1710 Ma. Some zircon results from these high-grade gneisses are marked by distinct Pb-loss discordia with lower-intercept ages of ca. 215 Ma and Paleoproterozoic upper-intercept ages. Younger ages between 100 and 105 Ma are mainly obtained from rims of some zircon grains that are characterized by low Th/U values (<0.1) and high U contents (>1000 ppm), indicating the likelihood of metamorphic zircon growth at that time. The similarity of zircon age populations between western and eastern units through the transition zone indicates that this fundamental structure probably dissects sediments of the same basin. This supposition is further supported by initial whole-rock Pb-Sr-Nd isotopic data that show similar average initial 206 Pb/ 204 Pb (18.65 to 18.9), 87 Sr/ 86 Sr (0.713 to 0.718), and ε Nd (−7 to −12) values for both the western and eastern units—values that also indicate the presence of significantly older crustal material in their provenance. Magmatic zircons from a diorite dike that crosscuts the foliation, but is itself subsequently metamorphosed, yielded a SHRIMP-RG concordia age of 103.3 ± 0.73 Ma, which is within agreement of an isotope dilution–thermal ionization mass spectrometry (ID-TIMS) U-Pb age of 103.37 ± 0.25 Ma. A postmetamorphic, cross-cutting pegmatite yielded discordant U-Pb zircon age data, but euhedral, glassy monazite from the pegmatite yielded a slightly discordant 207 Pb/ 235 U age of 101.85 ± 0.35 Ma and a Th-Pb age of 97.53 ± 0.18 Ma, suggesting that this pegmatite was injected during or just after deformation ceased. The age and initial Pb-Sr-Nd signature for the dioritic dike indicate it was produced during the transition zone plutonism elsewhere in the northern Peninsular Ranges batholith, whereas the pegmatitic dike was derived from crustal anatexis. Collectively, these results indicate that this sequence of metasedimentary rocks was derived from mainly a Late Permian to Early Triassic igneous provenance that probably intruded Proterozoic crust. The sequence was subsequently metamorphosed during deformation of the Cretaceous continental margin at ca. 105 to 97 Ma.

Magma mixing was an important process in the genesis of plutonic suites of the Peninsular Ranges batholith, San Diego County transect. Contrary to expectations, minimum Hf arc mantle model ages (Hf TAM ) calculated from Lu-Hf spot analyses of zircon from 15 granite samples and one gabbro sample indicate a Neoproterozoic component in granites from the western zone of the batholith and even older crustal components, including a Paleoproterozoic component, in those from the eastern zone. The delineation between western and eastern zones in the San Diego County transect of the batholith corresponds closely with a rapidly formed suture zone marked by the western limit of Jurassic S- and transitional I-S-type granites, magnetic and gravity anomalies, and the δ 1 8 O gradient. Zircon U-Pb ages, many reported herein for the first time, indicate that Early Cretaceous I-type plutons were emplaced into the western zone of the batholith and stitched across both the suture zone and the central belt of deformed Jurassic S-type and I-S-type granites. I-type plutons that intruded east of the suture zone are mainly Late Cretaceous in age. Zircon U-Pb ages, measured as much as possible from the same grains used for 176 Hf/ 177 Hf analyses, not only provide a record of crystallization ages but also of the degree of zircon inheritance—of which there is little for Cretaceous western-zone I-type granites. The variation in 176 Hf/ 177 Hf (εHf (t) ) values for the population of zircon grains from each plutonic sample is therefore interpreted to reflect the degree of magma mixing between crustal- and mantle-derived components between the time of melt generation and final pluton construction, a process that can only be reconciled with open-system chemical behavior. We consider the process of formation of the short-lived suture zone and the S-type granites of the Peninsular Ranges to be examples analogous to the short lived Bundarra Supersuite of the New England batholith (Jeon et al., 2012). The new Hf data of this study are compared to published Nd-Sm model age data for the Peninsular Ranges batholith and to new zircon Hf data for the Tuolumne intrusive suite of the Sierra Nevada batholith.

In the Peninsular Ranges batholith of southern California, a central belt of Jurassic metagranites was intruded by a Cretaceous magmatic arc that migrated from west to east across the belt. The Cretaceous batholith has been divided into western and eastern zones, zones that correspond to age, lithologic, geochemical, and geophysical zonations. In this study, density and magnetic susceptibility measurements performed on ~960 hand samples show that, in the eastern zone of the Peninsular Ranges batholith, values of magnetic susceptibility are uniformly low (<0.5 × 10 −3 cgs [centimeter-gram-second] units), while density values are in general lower and have less scatter than those in the western zone. A relatively sharp break between western and eastern zones indicates the existence of two crustal types separated by a tectonic suture: on the west, oceanic crust (mainly Mesozoic and older mantle and mantle-derived rocks) and on the east, continental crust (Neoproterozoic, Paleozoic, and early Mesozoic rocks). Previous studies in the San Diego County segment of the Peninsular Ranges batholith revealed petrologic distinctions between two Jurassic metagranite suites (S-type and transitional I-S type) and nine Cretaceous granite suites (exclusively I-type). The results of electron microprobe (EM) analyses of mafic minerals from Jurassic and Cretaceous plutonic rocks in general confirm plutonic suite subdivision. On biotite and hornblende variation diagrams, Early Cretaceous plutons tend to plot in distinct fields/trends that are characteristic of their various plutonic suites. Hornblende from three Early Cretaceous tonalite suites is Mg enriched, as expected from melts of mafic-intermediate composition that were H 2 O rich and contained hornblende as an early-crystallizing phase. Hornblende from gabbro plutons (Cuyamaca Gabbro) shows the greatest Mg enrichment for a given whole-rock SiO 2 value, reflecting cumulate processes in the evolution of gabbroic magmas. Biotite and hornblende from highly evolved leucomonzogranite-leucogranodiorite plutons assigned to three leucogranite suites have the most Fe- and Mn-rich compositions. Hornblende compositions of two Late Cretaceous tonalite suites overlap those of the Early Cretaceous tonalite suites, but, in general, Late Cretaceous hornblende does not show the extreme fractionation shown by hornblende of Early Cretaceous suites with similar SiO 2 contents. Biotite of two Jurassic plutonic suites has the most aluminous compositions of all Peninsular Ranges batholith suites, with biotite of the S-type Harper Creek suite markedly more Al rich than that of the transitional I-S–type Cuyamaca Reservoir suite. Complete overlap of Harper Creek biotite compositions with those of metasedimentary rocks of the Triassic–Jurassic Julian Schist indicates that partial melting of the latter was an appropriate source for Harper Creek magma. The existence of two Cuyamaca Reservoir biotite trends suggests that its parental magma originated by fractionation and contamination of an I-type magma by aluminous metasedimentary material, thus producing transitional I-S characteristics. All but one sample of hornblende from the Cuyamaca Reservoir suite falls in the subaluminous compositional range.