SDSU Contributions to the GSA Memoirs 211, 2014; Peninsular Ranges Batholith, Baja California and Southern California. Edited by Douglas M. Morton, U.S. Geological Survey and Department of Earth Sciences, University of California, Riverside.
GSA Memoirs 211, 2014Peninsular Ranges Batholith, Baja California and Southern California
Douglas M. Morton, U.S. Geological Survey and Department of Earth Sciences, University of California, Riverside, California 92521, USA
Fred K. Miller, U.S. Geological Survey, 904 West Riverside Ave., Spokane, Washington 99201, USA
Santiago Peak volcanics: Early Cretaceous arc volcanism of the western Peninsular Ranges batholith, southern California
Charles T. Herzig and David L. Kimbrough
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 εSr (−18 to +5) and εNd (+7.5 to +0.1) values for the lavas plot along the mantle array. Silicic lavas have higher εSr and εNd 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 40Ar/39Ar 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.
A west-to-east geologic transect across the Peninsular Ranges batholith, San Diego County, California: Zircon 176Hf/177Hf evidence for the mixing of crustal- and mantle-derived magmas, and comparisons with the Sierra Nevada batholith
Stirling E. Shaw, Victoria R. Todd, David L. Kimbrough, and Norman J. Pearson
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 (HfTAM) 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 δ18O 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 176Hf/177Hf 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 176Hf/177Hf (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.
The Rattlesnake Valley and Oriflamme Canyon plutons: Key temporal markers in the Jurassic and Cretaceous development of the transition zone of the Peninsular Ranges batholith
Cassady Bethel-Thompson, Jon Sainsbury, Jason W. Ricketts, and Gary H. Girty
Along ~33°N latitude, we mapped a segment of the transition zone of the Peninsular Ranges batholith. Based on structural fabric and plutonic ages, we subdivided the mapped area into western and eastern domains. The structural fabric of the western domain is characterized by brittle to brittle-ductile faults overprinting an earlier-formed ductile fabric dominated by a series of upright tight to isoclinal F1 folds, and S1, a spaced to continuous pervasively developed cleavage and gneissic foliation. Faults overprinting F1 and S1 within the western domain include the conjugate dextral Green Valley and sinistral Indian Exhibit faults. The ca. 130 Ma Rattlesnake Valley pluton crosscuts F1 and S1 and is not cut by these faults. The brittle faults within the western domain do not extend eastward beyond the Sunrise Highway fault, the frontal fault defining the western boundary of the eastern domain and the Sunrise Highway–Oriflamme Canyon shear zone. The Sunrise Highway fault is represented by an ~1–3-m-thick protomylonitic rind along the margin of the Rattlesnake Valley pluton. The Sunrise Highway–Oriflamme Canyon shear zone includes a tabular and platy segment of the Julian Schist, the ca. 116 Ma protomylonitic Oriflamme Canyon pluton, and a poorly exposed high-grade segment of the Harper Creek Gneiss. S2, within the Julian Schist segment, is a penetrative, NW-striking, steeply NE-dipping phyllonitic cleavage. It can be traced eastward into the syntectonic Oriflamme Canyon pluton, where it is represented by protomylonitic foliation. C-surfaces dipping ~30°–40° eastward consistently deflect S-surfaces in an east-over-west sense. Following development of the Sunrise Highway–Oriflamme Canyon shear zone, between ca. 94 Ma and ca. 80 Ma, a large segment of the Harper Creek Gneiss was thrust westward over the Oriflamme Canyon pluton along the brittle Chariot Canyon fault.
As reported in the literature, during the Late Jurassic to Early Cretaceous, the Farallon and North American plates converged in an oblique sinistral fashion, and then sometime between broadly ca. 140 Ma and ca. 125 Ma, convergence between the two plates became nearly orthogonal. This period of normal convergence continued until ca. 115 Ma. We speculate that (1) the tight to isoclinal upright folds and S1 cleavage characteristic of the western domain are the intra-arc record of the transition from oblique sinistral convergence to orthogonal convergence; (2) during and following emplacement of the Rattlesnake Valley pluton, the intra-arc strain field produced by the interaction of the Farallon and North American plates became weaker; and (3) the formation of the Sunrise Highway–Oriflamme Canyon shear zone was the result of heat and fluids softening the areas immediately adjacent to the intruding Oriflamme Canyon pluton, and thus weakening them to the point that they yielded to the horizontal contraction imposed by normal convergence. As suggested by other investigators, brittle movement along the Chariot Canyon fault some 22–36 m.y. following the development of the Sunrise Highway–Oriflamme Canyon shear zone was likely generated by removal of lithospheric mantle during Laramide shallow subduction. This process destabilized the overlying crust and triggered erosion, localizing brittle shortening along the Chariot Canyon fault.
Upper Jurassic Peñasquitos Formation—Forearc basin western wall rock of the Peninsular Ranges batholith
David L. Kimbrough, Patrick L. Abbott, Duane C. Balch, Sarah Hosken Bartling, Marty Grove, J. Brian Mahoney, and Robert F. Donohu
Improved depositional age constraints and stratigraphic description of rocks in San Diego require designation of a new Upper Jurassic formation, herein named the Peñasquitos Formation after its exposures in Los Peñasquitos Canyon Preserve of the city of San Diego. The strata are dark-gray mudstone with interbedded first-cycle volcanogenic sandstone and conglomerate-breccia and contain the Tithonian marine pelecypod Buchia piochii. Laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon 206*Pb/238U ages of 147.9 ± 3.2 Ma, 145.6 ± 5.3 Ma, and 144.5 ± 3.0 Ma measured on volcaniclastic samples from Los Peñasquitos and Rancho Valencia Canyons are interpreted as magmatic crystallization ages and are consistent with the Tithonian depositional age indicated by fossils. Whole-rock geochemistry is consistent with an island-arc volcanic source for most of the rocks.
The strata of the Peñasquitos Formation have been assigned to the Santiago Peak volcanics by many workers, but there are major differences. The Peñasquitos Formation is marine; older (150–141 Ma); deformed everywhere and overturned in places; and locally is altered to pyrophyllite. In contrast, the Santiago Peak volcanics are nonmarine and contain paleosols in places; younger (128–110 Ma); undeformed and nearly flat lying in many places; and not altered to pyrophyllite. The Peñasquitos Formation rocks have also been assigned to the Bedford Canyon Formation by previous workers, but the Bedford Canyon is distinctly less volcanogenic and contains chert, pebbly mudstones, and limestone olistoliths(?) with Bajocian- to Callovian-age fossils.
Here, we interpret the Peñasquitos Formation as deep-water marine forearc basin sedimentary and volcanic strata deposited outboard of the Peninsular Ranges magmatic arc. The Upper Jurassic Mariposa Formation of the western Sierra Nevada Foothills is a good analog. Results of detrital zircon U/Pb dating from an exposure of continentally derived sandstone at Lusardi Creek are consistent with a mixed volcanic-continental provenance for the Peñasquitos Formation. A weighted mean U/Pb age of 144.9 ± 2.8 Ma from the youngest cluster of detrital grain ages is interpreted as the likely depositional age. Pre-Cordilleran arc zircon age distributions (>285 Ma) are similar to Jurassic deposits from the Colorado Plateau, with dominant Appalachian-derived Paleozoic (300–480 Ma), Pan African (531–641 Ma), and Grenville (950–1335 Ma) grains, consistent with derivation either directly, or through sediment recycling, from the Colorado Plateau Mesozoic basins and related fluvial transport systems. Appalachian- and Ouachita-like detrital zircon age distributions are characteristic of Jurassic Cordilleran forearc basins from northeast Oregon to west-central Baja California, indicating deposition within the same continent-fringing west-facing arc system.
The Sierra San Pedro Mártir zoned pluton, Baja California, Mexico
R Gordon Gastil, David L. Kimbrough, Joan M. Kimbrough, Marty Grove, and Masaaki Shimizu
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 87Sr/86Sr initial isotopic compositions of 0.7038–0.7050 and δ18O 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.