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The Peninsular Ranges batholith has been subdivided into two zones based on geochemical, geophysical, and lithologic parameters. Plutons in the eastern zone (La Posta–type) are typically larger and inwardly zoned from hornblende-bearing tonalite margins to muscovite-bearing monzogranite cores. U-Pb ages on zircon are generally in the 100 to 90 Ma range. They tend to be more discordant in the cores of the plutons and have upper concordia intercepts near 1300 Ma. Rb-Sr systematics on mineral separates yield an Sr i range from 0.7030 to 0.7044, although one pluton is reported to have a rim-to-core variation from 0.7043 to 0.7074. Whole-rock δ 18 O is lowest in the hornblende-bearing facies (8.3 to 10.9 per mil) and highest in the muscovite-bearing facies (10.2 to 11.8 per mil); the level of variation is pluton dependent. δ 18 O for quartz separates indicate an eastward-directed asymmetry toward heavier oxygen rather than the facies control observed in the whole-rock data. REE patterns from two plutons have nearly identical LREE enrichment and lack any Eu anomaly. Associated with the La Posta–type plutons are a series of small, compositionally restricted, garnet-bearing monzogranites. They are 1 to 5 m.y. younger than the surrounding La Posta–type plutons and contain zircons inherited from a 1200- to 1300-Ma source. Whole-rock δ 18 O values between 12.5 and 13.2 per mil and Sr i = 0.706 reflect a continental contribution to these magmas. La Posta–type melts were generated by subduction-related anatexis of amphibolite-or eclogite-grade oceanic crust. The relatively short emplacement interval and large size of the plutons suggest rapid separation of large volumes of melt from the source region under elevated P H 2 O. Rise toward the present erosional level occurred along the juncture between oceanic lithosphere and the older (ca. 1300 Ma) continental margin. Interaction with the continental crust produced the present-day eastward bias toward higher δ 18 O and zircon discordance.

The most striking feature of the eastern Peninsular Ranges batholith is the large volume of relatively homogeneous tonalite and low-K granodiorite distributed in a series of large zoned Late Cretaceous intrusive centers referred to as La Posta–type plutons. The Sierra San Pedro Mártir pluton in northern Baja California is an outstanding example, and this study was undertaken to test models for the origin of these large composite arc plutons as well as to investigate along-strike variability within this Late Cretaceous belt. The Sierra San Pedro Mártir pluton consists of a nested series of granitoids divided into hornblende, biotite, and muscovite zones that become progressively more felsic and younger inward to a slightly more mafic and lower-K muscovite core zone. Zircon and monazite U/Pb ages from each of the zones indicate composite assembly of the pluton over an ~7 m.y. time span (97–90 Ma), consistent with field evidence and internal compositional variability of the pluton. The Sierra San Pedro Mártir pluton consists of high-Na, high-Al calcic granitoids that contrast with high-K calc-alkaline granitoid intrusive suites typical of Sierra Nevada Late Cretaceous intrusive centers. Whole-rock major-element, trace-element, and rare earth element (REE) data from an ~20-km-long traverse from the margin to the core of the Sierra San Pedro Mártir pluton document compositions that closely match chemical characteristics of Archean high-Al tonalite-trondhjemite-granodiorite (TTG). REEs are in general strongly fractionated, with high (La/Yb) N ratios typical of high-Al TTG. However, large variations in heavy (H) REE abundances and light (L) REE/HREE abundance ratios within the Sierra San Pedro Mártir pluton are comparable to the total range of REE variability within the Peninsular Ranges attributed by previous workers to regional west to east variations across the batholith. High Sr contents and lack of strong Eu anomalies indicate a general lack of plagioclase in the source residue. Compositions are consistent with deep crustal or slab melting from a basaltic source region with residual garnet and amphibole. Hornblende-plagioclase thermobarometry indicates emplacement depths around 17 km and crystallization temperatures ranging from 650 °C to 700 °C. Unlike La Posta–type bodies to the north, which are exclusively ilmenite-series granitoids, the Sierra San Pedro Mártir pluton is partly magnetite-series rocks in the outer hornblende zone. Ilmenite-series rocks preferentially sequester Fe in biotite via Tschermak exchange. Relatively low 87 Sr/ 86 Sr initial isotopic compositions of 0.7038–0.7050 and δ 18 O whole-rock values of 8.5‰ in the Sierra San Pedro Mártir pluton are more typical of the western zone of the Peninsular Ranges batholith. The along-strike variation of La Posta–type centers may be correlated to progressive distancing from Proterozoic North American cratonal basement and/or diminishing contributions of subducted sediment and associated basement into the Cretaceous melt source region of the magmas.

The Peninsular Ranges of southern and Baja California are divided into a western, predominantly magnetite-bearing plutonic subprovince and an eastern, predominantly magnetite-free plutonic subprovince. The boundary that separates the two subprovinces corresponds roughly to the southwestern margin of the La Posta superunit, but in some places extends into the La Posta granitic province. Neither the pre–La Posta foliated granitic rocks nor the garnet- or muscovite-bearing rocks of the eastern Peninsular Ranges contain magnetite. The magnetite/ilmenite distinction occurs on three scales: regional variations that appear to be independent of host rock or individual plutons, variations paralleling modal facies within zoned plutons, and contact loss of magnetite in the outer margin of a pluton (from meters to more than a kilometer in width). Observations to date indicate that the regional distribution of magnetite- and ilmenite-series granitic rocks may result from generation of parental magma within the dehydration zone of a subduction plane. The gradation within zoned plutons probably results from a lowering of oxygen fugacity in the magma during progressive crystallization. The contact effect appears to be a consequence of reactions between the cooling pluton, the host rocks, and water-rich fluids from a variety of sources.

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.

The Catalina Schist underlies the inner southern California borderland of southwestern North America. On Santa Catalina Island, amphibolite facies rocks that recrystallized and partially melted at ca. 115 Ma and at 40 km depth occur atop an inverted metamorphic stack that juxtaposes progressively lower grade, high-pressure/temperature (PT) rocks across low-angle faults. This inverted metamorphic sequence has been regarded as having formed within a newly initiated subduction zone. However, subduction initiation at ca. 115 Ma has been difficult to reconcile with regional geologic relationships, because the Catalina Schist formed well after emplacement of the adjacent Peninsular Ranges batholith had begun in earnest. New detrital zircon U-Pb age results indicate that the Catalina Schist accreted over a ∼20 m.y. interval. The amphibolite unit metasediments formed from latest Neocomian to early Aptian (122–115 Ma) craton-enriched detritus derived mainly from the pre-Cretaceous wall rocks and Early Cretaceous volcanic cover of the Peninsular Ranges batholith. In contrast, lawsonite-blueschist and lower grade rocks derived from Cenomanian sediments dominated by this batholith's plutonic and volcanic detritus were accreted between 97 and 95 Ma. Seismic data and geologic relationships indicate that the Catalina Schist structurally underlies the western margin of the northern Peninsular Ranges batholith. We propose that construction of the Catalina Schist complex involved underthrusting of the Early Cretaceous forearc rocks to a subcrustal position beneath the western Peninsular Ranges batholith. The heat for amphibolite facies metamorphism and anatexis observed within the Catalina Schist was supplied by the western part of the batholith while subduction was continuous along the margin. Progressive subduction erosion ultimately juxtaposed the high-grade Catalina Schist with lower grade blueschists accreted above the subduction zone by 95 Ma. This coincided with an eastern relocation of arc magmatism and emplacement of the ca. 95 Ma La Posta tonalite-trondjhemite-granodiorite suite of the eastern Peninsular Ranges batholith. Final assembly of the Catalina Schist marked the initial stage of the Late Cretaceous–early Tertiary craton-ward shift of arc magmatism and deformation of southwestern North America that culminated in the Laramide orogeny.

ABSTRACT In the standard model, Cordilleran-type batholiths form beneath volcanic arcs in thickened crust, but our survey of modern and ancient continental arcs revealed most to be regions of normal to thinned crust, not zones of crustal thickening. This suggested to us that the standard batholithic paradigm is flawed. In order to better understand the batholiths, we explored (1) the 100–84 Ma La Posta and Sierran Crest magmatic suites of the Peninsular Ranges and Sierran batholiths, which formed after the 100 Ma Oregonian event due to closure of the Bisbee-Arperos seaway; (2) plutons and batholiths emplaced into the metamorphic hinterland of the 124–115 Ma Sevier event, which occurred in the Great Basin sector of the United States but, due to younger meridional transport, are now exposed in the Omineca belt and Selwyn Basin of Canada; and (3) Late Cretaceous–early Cenozoic intrusive rocks, such as the Coast, Idaho, and Boulder batholiths, which intruded a metamorphic hinterland during and after the Laramide event. The dominance of syn-to postdeformational emplacement and the distinctive slab failure–type geochemistry indicate that most, but not all, Cretaceous plutons within Cordilleran batholiths formed during and after arc-continent collision as the result of slab failure. We interpret whole-rock geochemistry, as well as radiogenic and stable isotopes, to indicate that slab failure magmas involve only minor amounts of crust and are derived mainly from plagioclase-absent melting of garnet-bearing rocks in the mantle. Some suites, such as the <100 Ma Oregonian Sierran and Peninsular Ranges batholiths, have evolved Nd and Sr isotopes compatible with old enriched subcontinental lithospheric mantle. The well-known 0.706 87/86 Sr i isopleth appears to separate rocks of Oregonian slab failure from rocks of older arc magmatism and is probably unrelated to any obvious crustal break; instead, it reflects involvement of old subcontinental lithospheric mantle in the slab failure magmas. To expand our findings we examined the geochemistry of Cenozoic slab window and Precambrian tonalite-trondhjemite-granodiorite suites and found them to share many similarities with the Cretaceous slab failure rocks. Because most Cretaceous plutons in the North American Cordillera appear to represent juvenile additions to the crust, we argue that substantial volumes of continental crust are formed by slab failure magmatism. Slab failure rocks, especially those emplaced within the epizone, are richly metalliferous and make excellent exploration targets.