Browsing by Author "Wooden, Joseph L."

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    Leveraging detrital zircon geochemistry to study deep arc processes: REE-rich magmas mobilized by Jurassic rifting of the Sierra Nevada arc
    (Elsevier, 2021-07) Clemens-Knott, Diane; DeGraaff Surpless, Kathleen; Barth, Andrew P.; Wooden, Joseph L.; Earth Science, School of Science
    Anomalous trace element compositions of Middle to Late Jurassic detrital zircon separated from Sierra Nevada forearc and intra-arc strata reveal processes of differentiation occurring within the deep arc lithosphere. REE-Sc-Nb-Ti-Hf-U-Th covariations define three populations of atypically REE-rich grains that we interpret as crystallizing from (1) differentiates produced by olivine+clinopyroxene+plagioclase+garnet±ilmenite fractionation; (2) mixing between mafic arc magmas and partial melts of Proterozoic Mojave province crust; and (3) compositionally transient, low Gd/Yb magmas generated by hornblende resorption during decompression. We interpret a fourth population of Middle Jurassic to Early Cretaceous zircons having REE contents similar to “typical” arc zircon but with atypically high Gd/Yb ratios as having crystallized from partial melts of recycled arc crust and from deep-arc differentiates that evolved down-temperature through hornblende saturation. We hypothesize that latest Jurassic extension ripped open the arc, facilitating upward migration and eruption of geochemically anomalous zircon-bearing magmas. The anomalous compositions relative to “typical” arc zircon imply that these zircons and their host magmas rarely reach the upper arc crust, where eruption and/or erosion would release their zircon cargo to the clastic system. Focusing on the trace element compositions of zircons of syn-extensional age represents a productive new strategy for learning about deep magmatic reservoirs and early differentiation pathways within the thick lithosphere of continental margin arcs.
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    Protolith age of the Altar and Carnero complexes and latest Cretaceous–Miocene deformation in the Caborca–Altar region of northwestern Sonora, Mexico
    (2019-03) Jacobson, Carl E.; Jacques-Ayala, César; Barth, Andrew P.; García y Barragán, Juan Carlos; Pedrick, Jane N.; Wooden, Joseph L.; Earth Sciences, School of Science
    In the Caborca–Altar area of northwest Sonora, variably deformed and metamorphosed sedimentary and volcanic rocks crop out in a northwest-southeast–trending belt (El Batamote belt) at least 70 km long. We obtained detrital zircon U-Pb ages from two distinctive components of the belt near Altar, here termed the Altar complex and Carnero complex. Zircon ages for metasandstone and metaconglomerate matrix from the Altar complex indicate a Late Cretaceous maximum age of sedimentation, with at least part of the complex no older than 77.5 ± 2.5 (2σ). Pre-Cretaceous detrital zircons in the complex were derived largely from local sources, including Proterozoic basement, the Neoproterozoic–Cambrian miogeocline and the Jurassic arc. The detrital zircon ages and lithologic character of the Altar complex suggest correlation with the Escalante Formation, the uppermost unit of the Upper Cretaceous El Chanate Group. In contrast, one sample from the Carnero complex yielded a Middle Jurassic maximum depositional age and a detrital zircon age distribution like that of the Jurassic eolianites of the North American Cordillera. The Carnero complex may correlate with the Middle Jurassic Rancho San Martín Formation but could also be a metamorphosed equivalent of the Upper Jurassic Cucurpe Formation, Upper Jurassic to Lower Cretaceous Bisbee Group, or El Chanate Group derived by recycling of Jurassic erg sandstones. The Late Cretaceous age for the Altar complex protolith contradicts models that relate deposition of the entire El Batamote protolith to a basin formed by oblique slip along the Late Jurassic Mojave-Sonora megashear. Instead, the belt is best explained as an assemblage of Middle Jurassic to Upper Cretaceous formations deformed and locally metamorphosed beneath a northeast-directed Laramide thrust complex. Potassium-argon and 40Ar/39Ar ages confirm previous inferences that deformation of El Batamote belt occurred between the Late Cretaceous and late Eocene. A second phase of deformation, involving low-angle normal faults, occurred during and/or after intrusion of the ~22-21 Ma Rancho Herradura granodiorite.
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    Testing models of Laramide orogenic initiation by investigation of Late Cretaceous magmatic-tectonic evolution of the central Mojave sector of the California arc
    (Geological Society of America, 2021-11) Economos, Rita C.; Barth, Andrew P.; Wooden, Joseph L.; Paterson, Scott R.; Friesenhahn, Brody; Wiegand, Bettina A.; Anderson, J. Lawford; Roell, Jennifer L.; Palmer, Emerson F.; Ianno, Adam J.; Howard, Keith A.; Earth Sciences, School of Science
    The Mojave Desert region is in a critical position for assessing models of Laramide orogenesis, which is hypothesized to have initiated as one or more seamounts subducted beneath the Cretaceous continental margin. Geochronological and geochemical characteristics of Late Cretaceous magmatic products provide the opportunity to test the validity of Laramide orogenic models. Laramide-aged plutons are exposed along a transect across the Cordilleran Mesozoic magmatic system from Joshua Tree National Park in the Eastern Transverse Ranges eastward into the central Mojave Desert. A transect at latitude ∼33.5°N to 34.5°N includes: (1) the large upper-crustal Late Cretaceous Cadiz Valley batholith, (2) a thick section of Proterozoic to Jurassic host rocks, (3) Late Cretaceous stock to pluton-sized bodies at mesozonal depths, and (4) a Jurassic to Late Cretaceous midcrustal sheeted complex emplaced at ∼20 km depth that transitions into a migmatite complex truncated along the San Andreas fault. This magmatic section is structurally correlative with the Big Bear Lake intrusive suite in the San Bernardino Mountains and similar sheeted rocks recovered in the Cajon Pass Deep Scientific Drillhole. Zircon U-Pb geochronology of 12 samples via secondary ionization mass spectrometry (SIMS) (six from the Cadiz Valley batholith and six from the Cajon Pass Deep Scientific Drillhole) indicates that all Cretaceous igneous units investigated were intruded between 83 and 74 Ma, and Cajon Pass samples include a Jurassic age component. A compilation of new and published SIMS geochronological data demonstrates that voluminous magmatism in the Eastern Transverse Ranges and central Mojave Desert was continuous throughout the period suggested for the intersection and flat-slab subduction of the Shatsky Rise conjugate deep into the interior of western North America. Whole-rock major-element, trace-element, and isotope geochemistry data from samples from a suite of 106 igneous rocks represent the breadth of Late Cretaceous units in the transect. Geochemistry indicates an origin in a subduction environment and intrusion into a crust thick enough to generate residual garnet. The lack of significant deflections of compositional characteristics and isotopic ratios in igneous products through space and time argues against a delamination event prior to 74 Ma. We argue that Late Cretaceous plutonism from the Eastern Transverse Ranges to the central Mojave Desert represents subduction zone arc magmatism that persisted until ca. 74 Ma. This interpretation is inconsistent with the proposed timing of the docking of the Shatsky Rise conjugate with the margin of western North America, particularly models in which the leading edge of the Shatsky Rise was beneath Wyoming at 74 Ma. Alternatively, the timing of cessation of plutonism precedes the timing of the passage of the Hess Rise conjugate beneath western North America at ca. 70–65 Ma. The presence, geochemical composition, and age of arc products in the Eastern Transverse Ranges and central Mojave Desert region must be accounted for in any tectonic model of the transition from Sevier to Laramide orogenesis.