The Tatric crystalline unit of the Western Carpathians in northern Slovakia displays an inverted metamorphic sequence where high-grade migmatite and orthogneiss units are overlying lower-grade mica schists. Enclosed within the migmatites are lenses of eclogite-bearing amphibolites. Conventional geothermobarometry coupled with isochemical modeling constrained P-T paths that exhibit contrasting metamorphic histories for rock units that are now heterogeneously interleaved. Relict eclogite facies assemblages with occasionally preserved omphacite record post-peak pressure conditions of 1.7-1.8 GPa followed by near isothermal decompression at ~750 °C leading to intensive re-equilibration of eclogites at high-pressure granulite facies conditions and development of diopside + plagioclase symplectitic textures. New ID-TIMS Sm-Nd dating of garnet separated from the omphacite-bearing eclogite yields a whole rock-garnet isochron age of 337 ± 10 Ma, with an epsilon Nd isotopic composition of +8.3. While major element profiles across the garnets display little variation, the trace element distribution shows a typical HREE enrichment profile and a slight core to rim disparity with LREE and MREE concentrations higher in the cores and higher HREE in the rims. Granulite-facies migmatites that host the eclogite boudins record lower pressure metamorphic conditions of 1.2 GPa at ~750 °C and a similar retrograde path. The lower-grade micaschists reached metamorphic conditions of 0.8 GPa at ~650 °C. Monazite U-Pb analysis from a migmatite surrounding the eclogite boudins yields one population of ca. 380 Ma age. Another migmatite away from the eclogite yields two populations monazite ages. A robust 340 ± 11 Ma monazite U-Pb age is indistinguishable from our garnet age and U-Pb SIMS age of zircons in the anatectic leucosome of the migmatite (347 ± 7 Ma). We interpret the ca. 340 Ma ages to represent the exhumation of the deep crustal root of the Variscan orogen into the middle crust coeval with anatexis. A younger monazite U-Pb age of 300 ± 16 Ma is consistent with 40Ar/39Ar thermochronology data of ca. 310 Ma that is likely indicative of the Late Carboniferous I-type magmatism and cooling in the Tatric block. Cooling rates calculated by garnet diffusion modeling yield estimates of ~30 °/Ma. This exhumation was likely tectonically forced by the action of a rigid indentor which prompted the weak lower crust to be heterogeneously extruded to mid-crustal levels at a time coeval with anatexis and subsequently extruded with mid-crustal material to the upper crust.

In order to understand the life cycle of a mountain range, it is crucial to identify and quantify the processes that influence the rate of denudation, sediment flux through the landscape, and the resulting changes in relief over long time scales in tectonically-inactive regions. Geologic history and the quartz-rich lithologies make the southern Appalachian Mountains an ideal location for terrestrial cosmogenic nuclide (TCN) measurements aimed at studying erosion and denudation processes in an evolving post-orogenic landscape. We used in situ-produced TCN measurements of Beryllium-10 (10Be) to determine the denudation rate in ten catchments along the southern Appalachians. The locations selected are all within the east-draining Blue Ridge escarpment in North Carolina and Georgia. In five of the ten catchments we sampled two grain sizes, gravel and sand. In the remaining five catchments we sampled one grain size, sand. Our analysis provided erosion rates of 15 to 26 mm Ky-1 for the 0.025 to 0.050 cm sand samples and 12 to 20 mm Ky-1 for 3 to 8 cm gravel samples. We analyzed these TCN measurements in the context of several basin metrics, including slope and relief, derived from a digital elevation model (DEM). Our results provide evidence that most surficial basin metrics are not good predictors of denudation rates at a global scale, but can aid in predictions at a regional level. This finding supports the dynamic equilibrium hypothesis of landscape evolution and casts doubt on the possibility to estimate basin-wide denudation rates and watershed sediment supply at a global scale from simple metrics of basin morphology.

Remains of a mammoth, other Pleistocene fauna, and a wooden bowl were recovered from the Vernor site located in Clute, Brazoria County on the Texas Gulf Coast. Stratigraphy, sedimentology, and geochronology were used to establish the depositional history of the site. The geologic evidence suggests that these sediments were deposited during a period of fluvial activity by an ancient meander belt of the Brazos River, known today as Oyster Creek, which characterized this region during the Late Pleistocene and Early Holocene. Organics associated with the wooden bowl were radiocarbon dated to 4205 + 30 yr B.P. (UCIAMS-12039), while sand grains associated with the remains of the mammoth were dated using the luminescence technique to 66,000 + 7000 yr B.P. (UIC1383). According to these dates and their positions in the stratigraphic record, it was established that the mammoth and other Pleistocene age fauna preceded human occupation, and are not contemporaneous with the wooden bowl.

The Aegean Sea is considered to be a classic back-arc basin. Back-arc basins may develop by active processes including retreat of the overriding plate or upwelling from the subducting slab. Alternatively, back-arc basins may develop as passive responses to regional tensional stresses. The Biga Peninsula of western Turkey provides an opportunity to explore and test these models. The Biga region is characterized by granitoid plutons of Cretaceous to Miocene age that may provide insight into the nature of extension. This study focuses on understanding the evolution of three of these plutons, the Kozak, Eybek, and Kestanbolu. Geochemical and geochronological data and cathodoluminescence (CL) images of the rocks and zircons were acquired. The first in situ (in thin section) ion microprobe U-Pb ages of zircon, and the first zircon ages ever reported from the Kozak and Eybek plutons are presented. Zircon ages range from 36.5±6.6 Ma to 17.1±0.7 Ma (238U/206Pb, ±1) with two ages from a single grain of 280±18 Ma and 259±14 Ma. Samples from the Kozak and Eybek plutons are magnesian, calc-alkalic, and metaluminous, whereas the Kestanbolu rocks are magnesian, alkali-calcic, and metaluminous with one ferroan sample. The Rb vs. (Y+Nb) diagram suggests the Kozak and Kestanbolu plutons have a volcanic arc source, whereas the Eybek pluton records a within plate setting. CL imagery documents magma mixing, brittle deformation, and fluid- rock interactions based upon cracked plagioclase cores, cross-cutting microcracks, and fluid reaction textures of myrmekite and red rims on alkali feldspar. The plutons were generated following the collision of the Sakarya continent with the Anatolide-Tauride block. Geochemical data suggest the Kozak and Kestanbolu granitoids were generated by fluid flux melting from dehydration of the subducting slab of the Anatolide-Tauride block. The Kestanbolu granitoid intruded into the Vardar Suture north of this collision, whereas the Eybek pluton was created within the lithosphere during exhumation of the Kazdağ Massif. The Eocene - Oligocene zircon ages indicate emplacement and initial crystallization of the plutons. Early Miocene ages indicate ongoing extension in the region at this time and are consistent with earlier interpretations that subduction slab roll-back along the Hellenic arc formed the extensional environment in the region at this time. / text

In mineral deposits the application of the Re-Os system has evolved on two fronts; as a geochronometer in molybdenite, and as a tracer of the source of metals by direct determination of the source of Os contained in the ore minerals. Results obtained from a wide variety and types of mineral deposits indicate that ore minerals in most deposits contain a high initial osmium composition, compared to the mantle value at the time of ore formation. The Re-Os data presented here for the Platreef, South Africa, adds to the growing notion that the crust plays a fundamental role in the formation of mineral deposits and as a source of ore minerals. Additional data from the Zambian Copperbelt illustrate the utility of the Re-Os system as a geochronometer of sulfide mineralization. Two isochron ages of ca. 825 Ma and 575 Ma are consistent with a long-lived period of multistage mineralization linked to basin evolution and support a model where brines play a fundamental role in the formation of sediment-hosted stratiform deposits.Numerous new Re-Os molybdenite ages have recently been reported; however, the behavior of Re and Os in molybdenites is still poorly understood and controversy remains regarding the possible disturbance of the Re-Os isotopic system. Previous studies indicate that the Re-Os system in molybdenites, and in other sulfides, can experience disturbance by Re and Os loss or Re gain (both examples of open system behavior), and that the analysis of these altered samples yields equivocal ages. Through replicate analyses of samples and/or comparison with other robust dating techniques, such as the U-Pb geochronometer, it is possible to differentiate between Re-Os molybdenite ages reflecting a mineralization age or a post depositional event. Once the reliability of the Re-Os molybdenite analyses is proven, it is possible to constrain the timing of mineralization and the identification of multiple molybdenite mineralization events, information that is relevant in assessing the longevity of porphyry systems.The examples presented in this work support the use of the Re-Os isotopic system as an important geochemical tool in the understanding of mineral deposits.

This dissertation contains studies that use various geochronometric and thermochronometric techniques to better understand the post-magmatic evolution of Sierra Nevada, California. (U-Th)/He ages in apatite and zircon from Sierran batholithic rocks are used to constrain the Cenozoic exhumation of the northern part of the range. Zircon and apatite ages determined from the same samples revealed relatively rapid cooling and exhumation rates (0.2 - 0.8 km/My) from ~ 90 to 60 Ma, followed by tectonic quiescence and slow exhumation (0.02 - 0.04 km/My) from the late Paleocene to present. In addition to the thermochronology of basement lithologies, the detrital zircon geochronology of grains from preserved Eocene fluvial sediments in the central and northern Sierra Nevada was performed. U-Pb ages of detrital zircons from the deposits were found to have distributions closely matching age-area estimates of Mesozoic plutons in the Sierra Nevada, suggesting that Eocene river systems were draining local Sierran catchments and likely had steeper axial gradients than has been proposed. Provenance analysis of the Eocene sediments is used to provide constraints on the paleotopography of the Sierra Nevada and inferred range-wide Cenozoic uplift.In addition to the Sierra Nevada work, this dissertation also contains studies that focus on the development of the K-Ca system as a geochronometric technique suitable for dating the deposition of sedimentary sequences. We present a new method for measuring Ca isotopic ratios using a multi-collector ICP-MS equipped with a hexapole collision cell. Isobaric argon interferences are minimized via gas phase reactions in the collision cell. The reproducibility of Ca ratio measurements is found to be ~ 0.02 % (RSD), which is comparable to high precision TIMS techniques and an order of magnitude improvement over single collector ICP-MS techniques using a similar reaction cell method. K-Ca ages of glauconite and K-rich evaporites are determined in order to evaluate the usefulness of the K-Ca system as a sedimentary geochronometer. K-Ca ages in both glauconite and K-salts are found to be variable and significantly younger than documented depositional ages. Reported ages, however, are thought to be recording important basinal thermal histories and recrystallization events.

Syn-convergent low-angle normal-sense detachments (LANDs) are found in many orogens around the world. However, those tectonic processes which result in their formation are little known. The South Tibetan detachment system (STDS) is the best-studied example worldwide of a syn-convergent LAND, and formed in the Miocene due to the continental collision of India and Asia. In Bhutan, eastern Himalaya, the STDS is duplicated. Here I investigate the tectonic history of the inner STDS and particularly the outer STDS in Bhutan, to determine whether the duplicated STDS can be explained by or used to constrain models of Himalayan orogenesis. A range of geochronometric, thermochronologic, petrologic, structural, thermobarometric, thermometric, and isotopic tools are used to constrain: the onset and cessation of motion on the outer STDS; the cessation of motion on the inner STDS; the peak metamorphic conditions in the hanging wall and footwall of the outer STDS; the pressure-temperature-time paths of tectonites in the hanging wall and footwall of the outer STDS; the structural history of the hanging wall rocks of the outer STDS, and; the paleogeographic affinity of the hanging wall rocks of the outer STDS. The results of these studies are compared to thermo-mechanical models of Himalayan- type continental collision. Similarities in model predictions of the type and timing of structures, peak metamorphic conditions of hanging wall and footwall tectonites, pressure-temperature-time paths, and other regional tectonic observations lead to two main conclusions. 1. The STDS is a system of three main types of LANDs: those that formed during channel flow of low-viscosity mid-crustal rocks, those that formed by extrusion of cooled channel rocks to the surface, and those that formed by destabilization of the upper crust above a dome of mid-crustal channel rocks. 2. The STDS was duplicated by underthrusting of a crustal ramp into the Himalayan orogen since early Miocene. The underthrusting led to extrusion of a dome of weak mid-crustal above a previously-extruded channel. The crustal ramp may be local to the eastern Himalaya due to higher convergence and/or erosion rates, or due to local underthrusting of relatively strong crust behind weaker crust.

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名古屋大学年代測定総合研究センターシンポジウム報告

The Tatric crystalline unit of the Western Carpathians in northern Slovakia displays an inverted metamorphic sequence where high-grade migmatite and orthogneiss units are overlying lower-grade mica schists. Enclosed within the migmatites are lenses of eclogite-bearing amphibolites. Conventional geothermobarometry coupled with isochemical modeling constrained P-T paths that exhibit contrasting metamorphic histories for rock units that are now heterogeneously interleaved. Relict eclogite facies assemblages with occasionally preserved omphacite record post-peak pressure conditions of 1.7-1.8 GPa followed by near isothermal decompression at ~750 °C leading to intensive re-equilibration of eclogites at high-pressure granulite facies conditions and development of diopside + plagioclase symplectitic textures. New ID-TIMS Sm-Nd dating of garnet separated from the omphacite-bearing eclogite yields a whole rock-garnet isochron age of 337 ± 10 Ma, with an epsilon Nd isotopic composition of +8.3. While major element profiles across the garnets display little variation, the trace element distribution shows a typical HREE enrichment profile and a slight core to rim disparity with LREE and MREE concentrations higher in the cores and higher HREE in the rims. Granulite-facies migmatites that host the eclogite boudins record lower pressure metamorphic conditions of 1.2 GPa at ~750 °C and a similar retrograde path. The lower-grade micaschists reached metamorphic conditions of 0.8 GPa at ~650 °C. Monazite U-Pb analysis from a migmatite surrounding the eclogite boudins yields one population of ca. 380 Ma age. Another migmatite away from the eclogite yields two populations monazite ages. A robust 340 ± 11 Ma monazite U-Pb age is indistinguishable from our garnet age and U-Pb SIMS age of zircons in the anatectic leucosome of the migmatite (347 ± 7 Ma). We interpret the ca. 340 Ma ages to represent the exhumation of the deep crustal root of the Variscan orogen into the middle crust coeval with anatexis. A younger monazite U-Pb age of 300 ± 16 Ma is consistent with 40Ar/39Ar thermochronology data of ca. 310 Ma that is likely indicative of the Late Carboniferous I-type magmatism and cooling in the Tatric block. Cooling rates calculated by garnet diffusion modeling yield estimates of ~30 °/Ma. This exhumation was likely tectonically forced by the action of a rigid indentor which prompted the weak lower crust to be heterogeneously extruded to mid-crustal levels at a time coeval with anatexis and subsequently extruded with mid-crustal material to the upper crust.