The transition from the end of the Ordovician to the beginning of the Silurian Period is characterized by the glaciation of the Gondwana paleocontinent, eustatic sea level change, a perturbation to the global carbon cycle and one of the ve major mass extinctions of the Phanerozoic Eon. Due to signi cant sea level fall, the Ordovician-Silurian (O-S) boundary is often marked by hiatus and exposure in the shallow marine geologic record (e.g., Copper et al. [2013]). Two locations that host stratigraphic succession close to the boudary are Anticosti Basin of Quebec (Canada, e.g., Desrochers et al. [2010]), and the carbonate mounds of the Siljan ring district (Dalarna County, Sweden, e.g., Ebbestad et al. [2015]). The exact timing and dynamics of the glaciation and mass extinction are yet to be understood. Similarly, the interplay between those events and the carbon cycle perturbation are still unclear. As a result, there is a serious need for radiometric age constraints in this crucial part of the Paleozoic Era. The acquisition of more radiometric dates, achieved in this study, aims to address the present dearth of absolute dates close to the boundary. The dates produced in this study represent the first modern geochronologic constraints on the O-S boundary, leveraging the development of the EARTHTIME initiative and the latest U-Pb dating techniques that have improved accuracy and allowed for dating of single zircon crystals at <=0.1% precision level. Here I present two new U-Pb zircon ages obtained via bentonite dating. The first bentonite, 443.61+-0.52 Ma (2, including analytical, tracer calibration and decay constant uncertainties) was collected from the base of the Lousy Cove Member, Ellis Bay Formation (Anticosti Island, Quebec, Canada). The second one, 443.28+-0.50 Ma (including analytical, tracer calibration and decay constant uncertainties) comes from a karstic void within the Boda Core Facies of the Boda Formation (Dalarna County, Sweden). U-Pb geochronology (chemical abrasion, isotope dilution, thermal ionization mass spectrometry: CA-ID-TIMS) on single zircons was used to obtain these ages. These results are the closest radiometric ages to the current O-S boundary (compared to any time constraints in the 2012 Geologic Time Scale) and allow to signifcantly reduce the uncertainty of the current age boundary (443.8+-1.5, Cohen et al. [2018]). Furthermore these absolute ages have been used to make models that explore drivers of Earth system change, such as an end-Ordovician global carbon cycle perturbation. / Graduate / 2022-07-07

abstract: Establishing the timing of impact crater formation is essential to exploring the relationship between bolide impact and biological evolution, and constraining the tempo of planetary surface evolution. Unfortunately, precise and accurate impact geochronology can be challenging. Many of the rock products of impact (impactites) contain relict, pre-impact phases that may have had their isotopic systematics completely reset during the impact event, only partially reset, or not reset at all. Of the many isotopic chronometers that have been used to date impactites, the U/Pb zircon chronometer (ZrnPb) seems least susceptible to post-impact disturbances, and ZrnPb dates are typically much more precise than those obtained using other chronometers. However, the ZrnPb system is so resistant to resetting that relict zircons in impactites often yield dates that reflect the igneous or metamorphic ages of the target rocks rather than the age of the impact itself. The present study was designed to answer a simple question: is there a straightforward sample collection and analysis strategy for high-accuracy ZrnPb dating of an impact structure if the impactites collected from it may contain inherited zircons? To study this, ZrnPb dates were determined for impactites from a single crater with a well-constrained impact age: the West Clearwater Lake impact structure, located at Lake Wiyâshâkimî, Québec, Canada. The amount of ZrnPb resetting and the mechanisms responsible for resetting varied amongst the samples. Each sample characteristically contained either: newly crystallized zircons from the impact melt ("neocrystalline"), relict zircons ~50-100% reset, or, relict zircons ~0-50% reset. The variably reset relict zircons define a discordia line from ~2700 Ma to ~286 Ma – consistent with the ages of the target rock and the impact, respectively (Schmieder et al., 2015a; Simard, 2004). ZrnPb measurements from the neocrystalline zircons provided a new preferred impact age of 286.64 ± 0.35 Ma (2σ), a ~10x improvement in precision. The characteristics of the West Clearwater ZrnPb data vary between samples yet become easily interpretable as a whole, showing that efforts to measure robust, precise impact ages benefit from strategies that prioritize applying multiple analytical techniques to multiple types of impactite from the same crater. / Dissertation/Thesis / Masters Thesis Geological Sciences 2019

Indiana University-Purdue University Indianapolis (IUPUI) / A broad range of samples were collected from the Huron-Erie Lobe, Lake Michigan Lobe, Saginaw Lobe, and Tipton Till Plain of northern Indiana to determine the provenance of Laurentide Ice Sheet till in the Midwest U.S. during the Illinoian and Wisconsinan glaciations. U-Pb age distributions from approximately 300 detrital zircons (DZ) were used as provenance indicators for each till sample. Till from the Lake Michigan Lobe and was found to be largely homogenized. The distinct lobe DZ age distributions are the Lake Michigan Lobe till with a dominant ~1465 Ma peak, the northern Huron-Erie Lobe till with a dominant ~1060 Ma and a secondary peak at ~1450 Ma, the southern Huron-Erie Lobe till with nearly equal peaks at ~1435 Ma, ~1175 Ma, and ~1065 Ma, and the southern Saginaw Lobe till with a dominant peak at ~1095 Ma. Those four DZ age distributions were treated as endmembers in a nonlinear least-squares mixing model to calculate the contribution of each lobe to till in the Tipton Till Plain. Huron-Erie and Saginaw lobe tills were found to be the primary components of the Tipton Till Plain, and Lake Michigan Lobe till was only found in the western Tipton Till Plain. Zircons from the Saginaw Lobe till increased 39 % in the eastern Tipton Till Plain between the Illinoisan and Wisconsinan glaciations. The mixing model was also applied to relate the DZ age distributions of the lobes to bedrock within and near their flow paths. When comparing nearby bedrock to each lobe’s till, mixing model results, yield an approximate maximum transport distance between 500 and 630 kilometers for the matrix vii fraction of till in the Lake Michigan, Huron-Erie, and Saginaw lobes. Samples for the southern Huron-Erie Lobe indicate that the most of the zircon ages within the southern Huron-Erie Lobe till in Indiana were specifically entrained between Niagara County, New York and east-central Indiana. Within the model’s error, 93 – 100 % of the detrital zircons in each of the three lobes are relatable to nearby Paleozoic and Precambrian sedimentary and metamorphic bedrock formations.

Depuis au moins 540 Ma deux grands systèmes de subduction coexistent sur Terre : d'une part, les systèmes de subduction-collision (chaînes Hercynienne, Himalayenne ou Alpine) et d'autre part, les systèmes de subduction de type péri-pacifique. Pour ces derniers, l'avant-arc constitue une zone clef pour retracer l'évolution de la subduction au cours du temps. En effet ces zones au contact avec le slab peuvent enregistrer des événements tectoniques et/ou des conditions métamorphiques variées (e.g. formation de « paired metamorphic belts »), qui sont autant d'indicateurs du contexte géodynamique. Le massif métamorphique de El Oro en Equateur est un exemple exeptionnel où une section complète et basculée de l'avant-arc Triasique est préservée. L'ensemble est constitué d'une série métasédimentaire de bas à haut grade métamorphique intrudée par des granitoïdes de type S, juxtaposé avec un laccolithe gabbroïque et des schistes bleus. Ce travail de thèse s'est concentré sur l'étude du métamorphisme de haute-température basse-pression et ses relations les schistes bleu. Afin de contraindre l'événement tectono-métamorphique affectant l'avant-arc Equatorien au Trias et la formation d'une "paired metamorphic belt", nous avons utilisé des outils structuraux, métamorphiques, géochimiques, géochronologiques et de modélisation thermique. Nos résultats montrent que durant cette période l'avant-arc Equatorien connait un intense épisode de fusion partielle en régime extensif. La base de la croûte est migmatisée sur une épaisseur de 10km. Les estimations Pression-Température indiquent que les conditions de fusion partielle varient de 4.5 kbar et 650°C pour la partie supérieure métaxitique et jusqu'à 7.5 kbar et 720°C pour la partie inférieure diatexitique. La gradient géothermique inféré est divisé en deux segments : un segment supérieur caractérisé par un gradient de 40°C/km et un segment inférieur caractérisé par un gradient quasi-isothermique. L'absence de paragénèse de ultra-haute température est attribuée à la grande fertilité du protolithe métasédimentaire. Les résultats géochimiques montrent que les plutons granodioritiques sont issus d'un mélange entre : (1) les liquides de fusion partielle produit par la réaction de deshydration de la muscovite des métasédiments et (2) un magma basique. Les âges U-Pb sur zircons et monazites révèlent que l'événement anatectique fût bref entre 229 et 225 Ma. La source de chaleur à l'origine de l'événement thermique est attribuée à la mise en place d'un pluton gabbroïque à ~ 230 Ma en base de croûte. Successivement, se sous-plaque les schistes-bleu refroidissant rapidement l'avant-arc. L'événement anatectique observé dans le massif de El Oro au Trias s'insrit à plus grande échelle au sein d'une large anomalie thermique affectant l'ensemble du continent sud Américain entre 260 et 220 Ma. Durant cette période la marge est un soumise à un régime extensif accompagné d'un important magmatisme d'origine crustal, principalement en position d'arc et d'avant-arc. Nous attribuons cette anomalie thermique d'ampleur continental à une "avalanche mantellique". A la lumière du contexte géodynamique globale nous inteprétons la formation de la paired metamorphic belt de El Oro à la rupture du slab. / Since about 540 Ma, two subductions systems co-exist on Earth: the subduction-collision systems (Hercynian, Himalayan or Alpin belts) and the circum-pacific subduction system. For the last the forearc region constitutes a key zone to understand the dynamic of the subduction. Indeed the forearc region in contact with the slab may records various tectonics events and/or metamorphic conditions (e.g. formation of paired metamorphic belt). Theses geological records are direct evidences of the linked geodynamical context. The El Oro metamorphic complex in Ecuador is a unique example where a whole Triassic forearc section is tilted and well preserved. The complex is made of low to high grade metasedimentary rocks intruded by S-type granitoids, juxtaposed with gabbroic rocks and blueschists. This study is focused on the high-temperature metamorphism and its retionaships with the high-pressure metamorphism. In order to constrain the tectono-metamorphic affecting the forearc region and the formation of a paired metamorphic belt we used strutural, metamorphic, geochemical, geochronological and themal modeling studies. Our results show that during Triassic times the Ecuadorian forearc underwent a strong episode of partial melting in extentional context. The migmatized part of crust is 10 km thick. Pressure-Temperature estimates indicate that partial melting started at 4.5 kbar and 650°C for the upper metatexitic part until 7.5 kbar and 720 °C for the lower diatexitic part. The resulting geothermal gradient exhibits two parts: an upper part caracteristed by a thermal gradient of 40°C/km and a lower part caractérized by a near-adiabatic gradient. The absence of ultra-high tempetature paragenesis is attributed to the high fertility of the metasedimentary protolith. Geochemical results show that granodiorite made of a miwing between: (1) the melt extacted under muscovite dehydration melting and (2) a basic magma. U-Pb ages on zircon and monazite reveal that the anatectic event was short, between 229 and 225 Ma. The origin of the thermal anomaly is attributed to the emplacement of the gabbroic plutonic unit at ~230 at root level. Successively, the blueschites are underplating triggering a strong coolng of the forearc region. The anatectic recorded in the El Oro metamorphic complex at Triassic times is part of a larger thermal anomaly affecting the whole south american margin between 260 and 220 Ma. During this period the margin is under extentional conditions and exhibit a strong S-type magmatic activity. This magmatism is mainly located in arc and forearc position. We attribute this large-scale thermal anomaly to slab fall in the lower mantle. In the light of the geodynamical context, we suggest that the formation of the El Oro paired metamorphic belt is related to slab breakoff.

No description available.

The Wrangellia composite terrane is one of the largest fragments of juvenile crust added to the North American continent since Mesozoic time, and refining its accretionary history has important implications for understanding how continents grow. New U-Pb geochronology and Hf isotopes of detrital zircons from Late Jurassic-Late Cretaceous strata from the forearc of the Wrangellia composite terrane allows more insight on the tectonic and paleogeographic history of the terrane. Our stratigraphically oldest samples from the Late Jurassic Naknek Formation have a detrital zircon U-Pb signature dominated by Early and Late Jurassic grains (195-190 Ma; 153-147 Ma). Hf isotopic compositions of these grains are juvenile to intermediate (εHf(t)=4.5-14.7). Disconformably above the Naknek Formation are two poorly understood units Ks and Kc. The Ks unit is dominated by Early to Late Jurassic grains (159-154 Ma) with a few Paleozoic grains (347-340 Ma). Hf isotopic compositions of Carboniferous-Jurassic grains are juvenile to intermediate (εHf(t)=6.0-18.8). The overlying Kc unit has Late to Early Jurassic zircons (198-161 Ma), and an increase in Paleozoic ages (374-323 Ma). Hf isotopic compositions of these grains are juvenile to intermediate (εHf(t)=4.5-14.7). Samples from the Matanuska Formation have major Late Cretaceous grains (90-71 Ma), and minor Early Cretaceous (137-106 Ma), Late to Early Jurassic (200-153 Ma), Paleozoic (367-277 Ma), and Precambrian grains (2597-1037 Ma). Hf compositions have a wider range from both the Late Cretaceous grains (εHf(t)=-1.5-14.9) and Paleozoic-Precambrian grains (εHf(t)=-23.7-16.3). Our results suggest an evolving provenance from Late Jurassic to Late Cretaceous time for the Wrangellia composite terrane forearc basin. The Late Jurassic Naknek Formation samples were dominantly derived from a juvenile to intermediate Jurassic igneous sediment source. During Early Cretaceous time, there is a slight increase in the number of Paleozoic grains in the Ks and Kc unit samples. The Early Cretaceous sediments have a mostly positive Hf isotopic compositions suggesting exhumation of Jurassic and Paleozoic juvenile igneous sediment sources. By Late Cretaceous time, our data illustrates another increase in Paleozoic grain abundances, in addition to the introduction of Precambrian grains, all with widely variable Hf isotopic compositions. We interpret this to reflect a larger sediment flux from the interior of Alaska where more evolved igneous rocks of that age are found.

This dissertation research addresses the tectonism of continental crust during ocean basin closure, suturing between continental landmasses, and collisional orogenesis. The new data and insights presented here were gathered through localized geologic investigations of the Tibetan Plateau of central Asia. This area of central Asia is an ideal location to study these fundamental tectonic processes because it has been the locus of numerous Tethyan ocean basins and terminal collisions between continents during Phanerozoic accretion of Gondwana-derived landmasses onto the southern margin of Eurasia. In this work, I propose, in many orogens, that high-pressure (HP) metamorphism of continental rocks may mark the early stages of the suturing process between continental landmasses rather than the culmination of suturing. This insight has been acquired from a geologic-, geochronologic-, and thermochronologic-based investigation of the HP-near ultrahigh-pressure bearing Triassic metasedimentary metamorphic belt in central Tibet. This work shows near synchronous continent-continent collisions between landmass adjacent to the Paleo-Tethys ocean prior to its final closure in Late Triassic time. In addition, this work shows that Mediterranean-style tectonics may be more widespread during accretionary tectonics than previously thought. A comparison between the distribution of the HP bearing metamorphic belt, autochthonous crystalline basement, and geophysical images of Tibet suggests that a Mesozoic tectonic feature may be controlling the structure and distribution of melt within the middle crust of the Tibetan Plateau. This concept underscores the importance of inherited tectonic frameworks on the evolution of orogenic plateaus. Work in southwest Tibet, along the India-Asia suture zone, highlights the complex behavior of continental crust during collisional orogenesis. This work identifies previously undocumented magmatism, crustal antexis, and high-grade metamorphism along the India-Asia suture. In this work I attribute these observations to the initial interactions between Indian, Asian, and subducting Neo-Tethys oceanic lithosphere.

In this article, we document a detailed analytical characterisation of zircon M127, a homogeneous 12.7 carat gemstone from Ratnapura, Sri Lanka. Zircon M127 has TIMS-determined mean U-Pb radiogenic isotopic ratios of 0.084743 +/- 0.000027 for Pb-206/U-238 and 0.67676 +/- 0.00023 for Pb-207/U-235 (weighted means, 2s uncertainties). Its Pb-206/U-238 age of 524.36 +/- 0.16 Ma (95% confidence uncertainty) is concordant within the uncertainties of decay constants. The delta O-18 value (determined by laser fluorination) is 8.26 +/- 0.06 parts per thousand VSMOW (2s), and the mean Hf-176/Hf-177 ratio (determined by solution ICP-MS) is 0.282396 +/- 0.000004 (2s). The SIMS-determined delta Li-7 value is -0.6 +/- 0.9 parts per thousand (2s), with a mean mass fraction of 1.0 +/- 0.1 mu g g(-1) Li (2s). Zircon M127 contains similar to 923 mu g g(-1) U. The moderate degree of radiation damage corresponds well with the time-integrated self-irradiation dose of 1.82 x 10(18) alpha events per gram. This observation, and the (U-Th)/He age of 426 +/- 7 Ma (2s), which is typical of unheated Sri Lankan zircon, enable us to exclude any thermal treatment. Zircon M127 is proposed as a reference material for the determination of zircon U-Pb ages by means of SIMS in combination with hafnium and stable isotope (oxygen and potentially also lithium) determination.

The St. Cyr klippe hosts well preserved to variably retrogressed eclogites found as sub-meter to hundreds of meter scale lenses within quartzofeldspathic schists in the Yukon-Tanana terrane, Canadian Cordillera. The St. Cyr area consists of structurally imbricated, polydeformed, and polymetamorphosed units of continental arc and oceanic crust. The eclogite-bearing quartzofeldspathic schists form a 30 by 6 kilometer thick, northwest-striking, coherent package. The schists consist of metasediments and felsic intrusives that are intercalated on the tens of meter scale. The presence of phengite and Permian age zircon crystallized under eclogite facies metamorphic conditions indicates that the eclogite was metamorphosed in situ with its quartzofeldspathic host. I investigated the metamorphic evolution of the eclogite-facies rocks in the St. Cyr klippe using isochemical phase equilibrium thermodynamic (pseudosection) modeling. I constructed P-T pseudosections in the system Na2O-K2O-CaO-FeO-O2-MnO-MgO-Al2O3-SiO2-TiO2-H2O for the bulk-rock composition of an eclogite and a host metatonalite. In combination with petrology and mineral compositions, St. Cyr eclogites followed a five-stage clockwise P-T path. Peak pressure conditions for the eclogites and metatonalites reached up to 3.2 GPa, well within the coesite stability field, indicating the eclogites reached ultrahigh-pressure conditions. Decompression during exhumation occurred with a corresponding temperature increase. SHRIMP-RG zircon dating shows that the protolith of the eclogites formed within the Yukon-Tanana terrane during early, continental arc activity, between 364 and 380 Ma, while the metatonalite protolith formed at approximately 334 Ma, during the Little Salmon Cycle of the Klinkit phase of Yukon-Tanana arc activity. Both the eclogites and the metatonalites were then subducted to mantle depths and metamorphosed to ultrahigh-pressure conditions during the late Permian, between 266 and 271 Ma. The results of our study suggest portions of the Yukon-Tanana terrane were subducted to high-pressure and ultrahigh-pressure conditions. This is the first report of ultrahigh-pressure metamorphism in the accreted terranes of the North American Cordillera. Petrological, geochemical, geochronological, and structural relationships link the eclogites at St. Cyr to other eclogite localities in Yukon, indicating the high-pressure assemblages form a larger lithotectonic unit within the Yukon-Tanana terrane.

No description available.