Matthew Rioux

Magmatic development of an intra-oceanic arc: High-precision U-Pb zircon and whole-rock isotopic analyses from the accreted Talkeetna arc, south-central Alaska

The accreted Talkeetna arc, south-central Alaska, is an archetypal example of an intra-oceanic arc crustal section. Arc-related units include all levels of a lithospheric column, from residual mantle harzburgites to sub-aerial volcanic rocks, and provide a rare opportunity to study intrusive arc processes directly. We present the first high-precision U-Pb zircon ages and an extensive new data set of 143Nd/144Nd and 87Sr/86Sr isotopic analyses from Talkeetna arc plutonic rocks. These data provide new insight into the timing and extent of Talkeetna arc magmatism, the tectonic development of the arc, and the role of preexisting crustal material in the generation of arc magmas. New analyses from the exposed arc crustal section in the Chugach Mountains indicate that the Talkeetna arc began to develop as a juvenile [ϵNd(t) = 6.0–7.8 and 87Sr/86Srint = 0.703379–0.703951] intra-oceanic arc between 202.1 and 181.4 Ma. This initial arc plutonism was followed ca. 180 Ma by a northward shift in the arc magmatic axis and generation of a large plutonic suite in the Talkeetna Mountains. Plutons from the eastern Talkeetna Mountains yield U-Pb zircon ages of 177.5–168.9 Ma and are isotopically similar to the Chugach Mountains intrusions [ϵNd(t) = 5.6–7.2 and 87Sr/86Srint = 0.703383–0.703624]. However, plutons from the western Talkeetna Mountains batholith have more evolved initial isotopic ratios [ϵNd(t) = 4.0–5.5 and 87Sr/86Srint = 0.703656–0.706252] and contain inherited xenocrystic Carboniferous–Triassic zircons. These data are interpreted to represent assimilation of adjacent Wrangellia crust into arc magmas and require amalgamation of the Talkeetna arc with the Wrangellia terrane by ca. 153 Ma. As a whole, the combined U-Pb zircon and isotopic data from the Chugach and Talkeetna Mountains indicate that the main volume of Talkeetna arc magmas formed with little or no involvement of preexisting crustal material. These observations justify the use of the Talkeetna arc as a type section for intrusive intra-oceanic arc crust.

Subduction erosion of the Jurassic Talkeetna-Bonanza arc and the Mesozoic accretionary tectonics of western North America

The Jurassic Talkeetna volcanic arc of south-central Alaska is an oceanic island arc that formed far from the North American margin. Geochronological, geochemical, and structural data indicate that the arc formed above a north-dipping subduction zone after ca. 201 Ma. Magmatism migrated northward into the region of the Talkeetna Mountains ca. 180 Ma. We interpret this magmatism as the product of removal of the original forearc while the arc was active, mainly by tectonic erosion. Rapid exhumation of the arc after ca. 160 Ma coincided with the sedimentation of the coarse clastic Naknek Formation. This exhumation event is interpreted to reflect collision of the Talkeetna arc with either the active margin of North America or the Wrangellia composite terrane to the north along a second north-dipping subduction zone. The juxtaposition of accreted trench sedimentary rocks (Chugach terrane) against the base of the Talkeetna arc sequence requires a change from a state of tectonic erosion to accretion, probably during the Late Jurassic (before 150 Ma), and definitely before the Early Cretaceous (ca. 125 Ma). The change from erosion to accretion probably reflects increasing sediment flux to the trench due to collision ca. 160 Ma.

Zircon dating of oceanic crustal accretion.

Most of Earths present-day crust formed at mid-ocean ridges. High-precision uranium-lead dating of zircons in gabbros from the Vema Fracture Zone on the Mid-Atlantic Ridge reveals that the crust there grew in a highly regular pattern characterized by shallow melt delivery. Combined with results from previous dating studies, this finding suggests that two distinct modes of crustal accretion occur along slow-spreading ridges. Individual samples record a zircon date range of 90,000 to 235,000 years, which is interpreted to reflect the time scale of zircon crystallization in oceanic plutonic rocks.

Sedimentology and provenance of the Upper Jurassic Naknek Formation, Talkeetna Mountains, Alaska: Bearings on the accretionary tectonic history of the Wrangellia composite terrane

Analysis of the Upper Jurassic Naknek Formation in the Talkeetna Mountains, Alaska, documents synorogenic sedimentation in a forearc basin along the outboard (southern) margin of the allochthonous Peninsular terrane during accretion to the western North American continental margin. New geochronologic, sedimentologic, and compositional data defi ne a two-part stratigraphy for the Naknek Formation. Microfossil, megafossil, and U-Pb clast ages document early Oxfordian to early Kimmeridgian deposition of the lower 690 m of the Naknek Formation and early Kimmeridgian to early Tithonian deposition of the upper 225 m of the Naknek Formation. Lithofacies and paleocurrent data from the lower Naknek Formation demonstrate initial deposition on a high-gradient, southward-dipping basin fl oor. Submarine mass fl ows deposited poorly sorted, cobble-boulder conglomerate in proximal fan-delta environments. Gravelly mass fl ows transformed downslope into sandy turbidity currents on a muddy prodelta slope. During early Kimmeridgian to early Tithonian time, fan-delta environments were replaced by lower gradient marine shelf environments characterized by deposition of cross-stratifi ed sandstone and bioturbated mudstone. Source-diagnostic clasts, feldspathic sandstone compositions, southwarddirected paleocurrent indicators, and U-Pb zircon ages of plutonic clasts (167.6 ± 0.3 Ma; 166.5 ± 0.2 Ma, 164‐159 Ma, 156.2 ± 0.4 Ma) indicate that the Naknek Formation was derived primarily from volcanic and plutonic source terranes exposed along the northern basin margin in the southern Talkeetna Mountains. Geologic mapping documents the Little Oshetna fault, a newly identifi ed northward-dipping reverse fault that bounds the northern margin of the Naknek Formation in the Talkeetna Mountains. The concentration of boulder-rich mass-fl ow deposits in the footwall of the fault in combination with geochronologic and compositional data suggest that sedimentation was coeval with Late Jurassic shortening along the fault and exhumation of plutonic source terranes exposed in the hanging wall of the fault. From a regional perspective, coarse-grained forearc sedimentation and pluton exhumation along the outboard (southern) segment of the Peninsular terrane were coeval with crustal-scale shortening and synorogenic sedimentation in retroarc basins along the inboard (northern) margin of the Wrangellia terrane (Kahiltna, Nutzotin, and Wrangell Mountains basins). We interpret the regional and synchronous nature of Late Jurassic crustal-scale deformation and synorogenic sedimentation in south-central Alaska as refl ecting either initial collision of the Wrangellia and Peninsular terranes with the former continental margin of western North America or amalgamation of the two terranes prior to collision.

Paleomagnetic and geochronological evidence for large-scale post–1.88 Ga displacement between the Zimbabwe and Kaapvaal cratons along the Limpopo belt

Proterozoic reconstructions of the Kaapvaal and Zimbabwe cratons have been limited by the scarcity of precisely dated paleomagnetic poles for the Zimbabwe craton. We present new U-Pb baddeleyite and apatite dates from two diabase sheets that have previously yielded paleomagnetic data from the Mashonaland igneous province in the Zimbabwe craton. Discordant baddeleyite analyses yield upper intercept dates of 1871.9 ± 2.2 and 1882.7 +1.6/–1.5 Ma. Apatite data from the same samples give less precise but statistically indistinguishable dates, providing direct constraints on the post-magmatic thermal history of the diabases. The new U-Pb dates and other recently published baddeleyite dates from the Mashonaland province are coeval with mafic magmatism in the adjacent Kaapvaal craton (1879–1872 Ma), but paleomagnetic poles from the two intraplate suites differ by 39°, suggesting that the two cratons underwent substantial relative motion after ca. 1.88 Ga. Paleomagnetic reconstructions are consistent with >2000 km of lateral displacement being accommodated in the Limpopo orogenic belt that separates the two cratons.

Thermochronology of the Talkeetna intraoceanic arc of Alaska: Ar/Ar, U-Th/He, Sm-Nd, and Lu-Hf dating

As one of two well-exposed intraoceanic arcs, the Talkeetna arc of Alaska affords an opportunity to understand processes deep within arcs. This study reports new Lu-Hf and Sm-Nd garnet ages, Ar-40/Ar-39 hornblende, mica and whole-rock ages, and U-Th/He zircon and apatite ages from the Chugach Mountains, Talkeetna Mountains, and Alaska Peninsula, which, in conjunction with existing geochronology, constrain the thermal history of the arc. Zircon U-Pb ages establish the main period of arc magmatism as 202-181 Ma in the Chugach Mountains and 183-153 Ma in the eastern Talkeetna Mountains and Alaska Peninsula. Approximately 184 Ma Lu-Hf and similar to 182 Ma Sm-Nd garnet ages indicate that 25-35 km deep sections of the arc remained above similar to 700 degrees C for as much as 15 Myr. The Ar-40/Ar-39 hornblende ages are chiefly 194-170 Ma in the Chugach Mountains and 175-150 Ma in the Talkeetna Mountains and Alaska Peninsula but differ from zircon U-Pb ages in the same samples by as little as 0 Myr and as much as 33 Myr, documenting a spatially variable thermal history. Mica ages have a broader distribution, from similar to 180 Ma to 130 Ma, suggesting local cooling and/or reheating. The oldest U-Th/He zircon ages are similar to 137 to 129 Ma, indicating no Cenozoic regional heating above similar to 180 degrees C. Although the signal is likely complicated by Cretaceous and Oligocene postarc magmatism, the aggregate thermochronology record indicates that the thermal history of the Talkeetna arc was spatially variable. One-dimensional finite difference thermal models show that this kind of spatial variability is inherent to intraoceanic arcs with simple construction histories. Citation: Hacker, B. R., P. B. Kelemen, M. Rioux, M. O. McWilliams, P. B. Gans, P. W. Reiners, P. W. Layer, U. Soderlund, and J. D. Vervoort (2011), Thermochronology of the Talkeetna intraoceanic arc of Alaska: Ar/Ar, U-Th/He, Sm-Nd, and Lu-Hf dating, Tectonics, 30, TC1011, doi:10.1029/2010TC002798. (Less)

U‐Pb dating of interspersed gabbroic magmatism and hydrothermal metamorphism during lower crustal accretion, Vema lithospheric section, Mid‐Atlantic Ridge

New U/Pb analyses of zircon and xenotime constrain the timing of magmatism, magmatic assimilation, and hydrothermal metamorphism during formation of the lower crust at the Mid-Atlantic Ridge. The studied sample is an altered gabbro from the Vema lithospheric section (11°N). Primary gabbroic minerals have been almost completely replaced by multiple hydrothermal overprints: cummingtonitic amphibole and albite formed during high-temperature hydration reactions and are overgrown first by kerolite and then prehnite and chlorite. In a previous study, clear inclusion-free zircons from the sample yielded Th-corrected 206Pb/238U dates of 13.528 ± 0.101 to 13.353 ± 0.057 Ma. Ti concentrations, reported here, zoning patterns and calculated Th/U of the dated grains are consistent with these zircons having grown during igneous crystallization. To determine the timing of hydrothermal metamorphism, we dated a second population of zircons, with ubiquitous <1–20 µm chlorite inclusions, and xenotimes that postdate formation of metamorphic albite. The textures and inclusions of the inclusion-rich zircons suggest that they formed by coupled dissolution-reprecipitation of metastable igneous zircon during or following hydrothermal metamorphism. Th-corrected 206Pb/238U dates for the inclusion-rich zircons range from 13.598 ± 0.012 to 13.503 ± 0.018 Ma and predate crystallization of all but one of the inclusion-free zircons, suggesting that the inclusion-rich zircons were assimilated from older hydrothermally altered wall rocks. The xenotime dates are sensitive to the Th correction applied, but even using a maximum correction, 206Pb/238U dates range from 13.341 ± 0.162 to 12.993 ± 0.055 Ma and postdate crystallization of both the inclusion-rich zircons and inclusion-free igneous zircons, reflecting a second hydrothermal event. The data provide evidence for alternating magmatism and hydrothermal metamorphism at or near the ridge axis during accretion of the lower crust at a ridge-transform intersection and suggest that hydrothermally altered crust was assimilated into younger gabbroic magmas. The results of this study show that high-precision U-Pb dating is a powerful method for studying the timing of magmatic and hydrothermal processes at mid-ocean ridges.

Structure and metamorphism beneath the obducting Oman ophiolite: Evidence from the Bani Hamid granulites, northern Oman mountains

The Cretaceous Semail ophiolite (northern Oman and the United Arab Emirates) includes an intact thrust slice of Tethyan oceanic crust and upper mantle formed above a northeast-dipping subduction zone that was the site of initiation of obduction. The normal metamorphic sole of the Semail ophiolite comprises a highly condensed sequence of hornblende + plagioclase ± garnet amphibolites with small enclaves of garnet + clinopyroxene granulites immediately beneath the mantle sequence peridotites, tectonically underlain by a series of epidote amphibolite and greenschist facies lithologies in a highly deformed ductile shear zone. Peak metamorphic conditions of 770–900 °C and 11–15 kbar indicate metamorphism at depths far greater than can be accounted for by the preserved thickness of the ophiolite (∼15 km). In the mountains of northern Oman, the 1.2-km-thick Bani Hamid thrust sheet is composed of intensely folded granulite and amphibolite facies rocks within mantle sequence peridotites, exhumed by late-stage out-of-sequence thrusting along the Bani Hamid thrust. The Bani Hamid thrust slice includes two-pyroxene quartzites (± hornblende, cordierite, sapphirine), diopside + andradite garnet + wollastonite + scapolite marbles and calc-silicates and amphibolites (hornblende + plagioclase ± clinopyroxene ± biotite) with localized partial melting, intruded by hornblende pegmatites. The Bani Hamid granulites represent metamorphosed cherts and calcareous turbidites probably derived from the distal Haybi Complex and Oman Exotic limestones, which have an alkali basaltic substrate. Metamorphic modeling using the program THERMOCALC in the system NCKFMASHTO (Na 2 O-CaO-K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O-TiO 2 -O) gives peak pressure-temperature conditions of 850 ± 60 °C and 6.3 ± 0.5 kbar, a pressure that is much lower than that of the metamorphic sole, suggesting a different origin. The 206 Pb/ 238 U zircon dates indicate that the gabbroic crust of the ophiolite formed by ridge magmatism from before 96.1 to 95.5 Ma. The 206 Pb/ 238 U zircon dates from the metamorphic sole range from 95.7 to 94.5 Ma, and suggest that metamorphism and melting was either synchronous with or slightly postdated ridge magmatism. The Bani Hamid granulites are younger; zircon and titanite U-Pb dates span ca. 94.5–89.8 Ma. Peraluminous granitic dikes intruding the mantle sequence peridotites are as young as 91.4 Ma and likely reflect localized partial melting of crustal material during the late stage of the obduction process. A minimum of 130 km shortening is recorded by restoration of the major folds within the Bani Hamid thrust sheet, and more than 30 km offset has occurred along the west-directed breaching out-of-sequence Bani Hamid thrust. These rocks may be representative of deep-level duplexes imaged on recent seismic sections across the mountains of northern Oman–United Arab Emirates.

EARTHTIME: Teaching Geochronology to High School Students in the USA

As part of the EARTHTIME outreach initiative, we have developed an educational module that teaches students about the basics of geochronology and how geologic time is measured. The exercises focus on the uranium-lead (U-Pb) dating method using the mineral zircon with applications to solving geological problems. During several Lab Day tutorials, students from local high schools attended a day of workshops, participating in hands-on exercises and a discussion of geochronology in earth science research. Student performance and learning impact were assessed using pre-tests (1 week before the event) and two post-tests (1 week after the event and 4 months after the event). These revealed that students greatly appreciated the hands-on exercises and that the exercises resulted in a significant increase in knowledge. We also developed and tested a “Lab Day on the road,” where scientists traveled to a local high school to introduce hands-on exercises and lead discussions related to geochronology. To further develop the module, a teacher workshop was conducted to identify educators’ needs and perspectives. The teaching methods, developed iteratively during 2 years of Lab Days, were incorporated in a Geochronology Lesson Plan and Material Kit. The finalized lesson plan is a 90- to 120-min educational module, downloadable at http://www.earth-time.org/Lesson_Plan.pdf, along with supporting spreadsheets and a video demonstration of the material. An EARTHTIME geochronology kit, linked to the lesson plan activities, is available by request to K-12 teachers in the USA.