The Ruby-Humboldt Range in Northeastern Nevada exposes the deepest crust in the western portion of the Sevier Hinterland. The product of unique brittle and ductile accommodations, this block of lower crustal rock is a window into the processes of continental thickening and extension. The structure of the northern tip of the Ruby-Humboldt Range core complex is dominated by a large recumbent fold nappe with a southward closeure cored by Paleoproterozoic-Archean gneissic complexes with complex interdigitated field relationships that record polyphase continental metamorphism. Amphibolite-grade metapelitic rocks within the core and Winchell Lake nappe record a wide range of zircon age dates of metamorphic events the oldest of which at ~2.5 Ga is recorded in adjacent orthogneiss as a crystallization age. At least two younger metamorphic events are recorded within this orthogneiss, most significantly at 1.7-1.8 Ga, an event previously unpublished for this region that links it to Wyoming province activity in addition to inherited component of detrital cores up to 3.7 Ga in age that is among the oldest ages reported in Nevada. The youngest overprint of cretaceous metamorphic overgrowth ranges fro 60-90 Ma in age based on zircon rims in the aforementioned units as well as three garnet amphibolites that intrude the core of the nappe and are interpreted to be metabasic bodies.

Indiana University-Purdue University Indianapolis (IUPUI) / Tills from major ice streams (Institute, Foundation, Academy, Recovery, and Slessor) of the Weddell Sea Embayment contain detrital zircons with distinct U-Pb age populations that can be used as a provenance tool to better understand ice stream dynamics. U-Pb ages of detrital zircons were measured in 21 samples of onshore till, erratics, and bedrock of potential source rocks, and 12 samples of offshore till. Grains were analyzed by LA-ICPMS at the University of Arizona (n=5447). Relative probability U-Pb age density plots of till in moraines along the Institute Ice Stream have dominant Grenville (1070 Ma) and secondary Ross/Pan-African peaks (560 Ma, 630 Ma). The Foundation and Academy show prominent Ross/Pan-African peaks (500-530 Ma and 615-650 Ma). The Recovery transports zircons with prominent 530 Ma and 635 Ma peaks along the southern margin, and 1610 and 1770 Ma along the northern margin. The Slessor carries zircons with prominent populations at 1710 Ma and secondary 2260-2420 Ma. U-Pb ages in zircons from offshore till samples show a general trend of fewer Mesozoic ages from west to east. The western most core, PS 1423, has dominant Jurassic populations while cores 1197 and 1278 have a high proportion of early Ross/Pan-African ages relative to Grenville ages. The similar zircon age distributions between PS 1278 and the Foundation Ice Stream tills suggest that the Foundation switched to an easterly flow path around Berkner Island (BI) at some point during the LGM. In the eastern Weddell Sea (PS 1400), there was a near absence of Proterozoic zircon age populations carried by the Slessor and northern side of the Recovery. Another unexpected find was a lack of Grenville ages in PS 1423 relative to the Institute tills. The U-Pb data in this study provides a basis for two possible LGM ice flow reconstructions. In the first, the Institute flowed west around the unnamed isolated bedrock highs, deposited tills between PS 1423 and PS 1197, providing a westerly flow path around BI for the Foundation. In the second, the Institute flows over the subglacial topography and deposited till closer to PS 1197, forcing the Foundation east around BI.

Published detrital zircon geochronological data was compiled to form the Global Detrital Zircon Database (GDZDb). This database provides a reference block for provenance analysis by future detrital zircon geochronological studies. This project entailed three subprojects: 1. crustal growth/crustal recycling patterns, 2. a provenance study of the Triassic Dry Fork Formation of the Danville-Dan River Rift basin of Virginia and North Carolina, and 3. sample size issues in detrital zircon studies. The global detrital zircon age frequency distribution exhibits six prominent, statistically significant peaks: 3.2-3.0, 2.7-2.5, 2.0-1.7, 1.2-1.0, 0.7-0.5, and 0.3-0.1 Ga. These peaks are also observed when the data is sorted for continent of origin, the tectonic setting of the host sediment and for modern river sediments. Hf isotope model ages were also incorporated into the database where grains were dated with both U-Pb and Hf isotopes. The Hf isotope model ages suggest that the majority of detrital zircons U-Pb ages reflect crustal recycling events that generated granitic magmatism, as most grains exhibited Hf isotope ages that are much older than the corresponding U-Pb age. The Triassic Dry Fork Formation was sampled from a site in southern Virginia in the Danville-Dan River Basin. The detrital zircon age frequency distribution for this formation was strongly unimodal with a peak at 400-450 Ma and a paucity of Grenville-age zircons. Comparison of the Dry Fork sample to published east coast data and to the North American record (from the GDZDb) illustrate the unusual nature of the Dry Fork Formation sample. It is probable that older Grenville zircons were blocked from the rift valley by the rift shoulder. Using the GDZDb a study of sample size was conducted in order to estimate the best sample size to use when trying to constrain the maximum age of sedimentation of the host sediment. Rift basins and active margins exhibited smaller offsets from the youngest zircon grain age to host sediment maximum age than observed in samples from passive margins. This study recommends that at least 50 grains need to be age dated on average in order to best constrain the age of the host sediment. / Ph. D.

The Tahorakuri Formation was introduced as a stratigraphic term to simplify the sometimes complex and inconsistent naming conventions in subsurface deposits within the geothermal fields of the central Taupo Volcanic Zone (TVZ). It consists of all volcaniclastic and sedimentary deposits between the ~350 ka Whakamaru-group ignimbrites and the greywacke basement that cannot be correlated with known ignimbrites. As such, it represents a long period in which relatively little is known about the volcano-tectonic history of the TVZ. The thesis focuses on the Tahorakuri Formation at Ngatamariki and Rotokawa geothermal fields and the implications for the volcano-tectonic evolution of the TVZ. Drill cuttings from wells NM5 and NM6 are re-examined, and new U-Pb zircon dates from the Tahorakuri Formation are presented and implications discussed. Potassium feldspars identified in the drill cuttings from NM5 were examined by Raman spectroscopy and electron microprobe (EMP) analysis. Although petrographically many of the feldspars appear similar to sanidine, a primary volcanic mineral phase, this showed them to be adularia which formed during hydrothermal alteration. Raman spectroscopy was found to be ideal for analysing a large number of grains quickly, with the spectral peak at ~140 cm⁻¹ being particularly useful for identifying adularia as it is absent in sanidine. EMP analysis was found to be somewhat slower, but definitively identified the feldspars as adularia, with typical potassium-rich compositions of Or₉₄-Or₉₉. U-Pb dating shows that the Tahorakuri Formation formed over a very long time, with pyroclastic deposits ranging from 1.89 - 0.70 Ma. This was followed by a period with little or no explosive volcanism until ~0.35 Ma during which sediments were deposited at Ngatamariki. The periods at ~1.9 Ma and ~0.9 Ma were particularly active phases of pyroclastic deposition, with the second phase likely correlating with the Akatarewa ignimbrite. The oldest deposits overlie a large andesitic composite cone volcano. Significant subsidence of the andesite must have preceded emplacement of the silicic deposits, indicating that rifting within the central TVZ may have started earlier than previously thought. While the origin of the deposits is uncertain, the distribution of the oldest deposits outcropping at the surface, as well as the likely early initiation of rifting, would suggest a source within the TVZ is likely.

No description available.

In the Pilgangoora Belt of the Pilbara Craton, Australia, the 3517 Ma Coonterunah Group and 3484-3468 Ma Carlindi granitoids underlie the 3458 Ma Warrawoona Group beneath an erosional unconformity, thus providing evidence for ancient emergent continental crust. The basalts either side of the unconformity are remarkably similar, with N-MORB-normalised enrichment factors for LILE, Th, U and LREE greater than those for Ta, Nb, P, Zr, Ti, Y and M-HREE, and initial e(Nd, Hf) compositions which systematically vary with Sm/Nd, Nb/U and Nb/La ratios. Geological and geochemical evidence shows that the Warrawoona Group was erupted onto continental basement, and that these basalts assimilated small amounts of Carlindi granitoid. As the Coonterunah basalts have similar compositions, they probably formed likewise, although they were deposited >60 myr before. Indeed, such a model may be applicable to most other early Pilbara greenstone successions, and so an older continental basement was probably critical for early Pilbara evolution. The geochemical, geological and geophysical characteristics of the Pilbara greenstone successions can be best explained as flood basalt successions deposited onto thin, submerged continental basement. This magmatism was induced by thermal upwelling in the mantle, although the basalts themselves do not have compositions which reflect derivation from an anomalously hot mantle. The Carlindi granitoids probably formed by fusion of young garnet-hornblende-rich sialic crust induced by basaltic volcanism. Early Archaean rocks have Nd-Hf isotope compositions which indicate that the young mantle had differentiated into distinct isotopic domains before 4.0 Ga. Such ancient depletion was associated with an increase of mantle Nb/U ratios to modern values, and hence this event probably reflects the extraction of an amount of continental crust equivalent to its modern mass from the primitive mantle before 3.5 Ga. Thus, a steady-state model of crustal growth is favoured whereby post ~4.0 Ga continental additions have been balanced by recycling back into the mantle, with no net global flux of continental crust at modern subduction zones. It is also proposed that the decoupling of initial e(Nd) and e(Hf) from its typical covariant behaviour was related to the formation of continental crust, perhaps by widespread formation of TTG magmas.

Thesis (M.S.)--Indiana University, 2009. / Title from screen (viewed on February 2, 2010). Department of Earth Sciences, Indiana University-Purdue University Indianapolis (IUPUI). Advisor(s): Andrew P. Barth, Gabriel M. Filippelli, Kathy Licht. Includes vitae. Includes bibliographical references (leaves 102-110).

In the Pilgangoora Belt of the Pilbara Craton, Australia, the 3517 Ma Coonterunah Group and 3484-3468 Ma Carlindi granitoids underlie the 3458 Ma Warrawoona Group beneath an erosional unconformity, thus providing evidence for ancient emergent continental crust. The basalts either side of the unconformity are remarkably similar, with N-MORB-normalised enrichment factors for LILE, Th, U and LREE greater than those for Ta, Nb, P, Zr, Ti, Y and M-HREE, and initial e(Nd, Hf) compositions which systematically vary with Sm/Nd, Nb/U and Nb/La ratios. Geological and geochemical evidence shows that the Warrawoona Group was erupted onto continental basement, and that these basalts assimilated small amounts of Carlindi granitoid. As the Coonterunah basalts have similar compositions, they probably formed likewise, although they were deposited >60 myr before. Indeed, such a model may be applicable to most other early Pilbara greenstone successions, and so an older continental basement was probably critical for early Pilbara evolution. The geochemical, geological and geophysical characteristics of the Pilbara greenstone successions can be best explained as flood basalt successions deposited onto thin, submerged continental basement. This magmatism was induced by thermal upwelling in the mantle, although the basalts themselves do not have compositions which reflect derivation from an anomalously hot mantle. The Carlindi granitoids probably formed by fusion of young garnet-hornblende-rich sialic crust induced by basaltic volcanism. Early Archaean rocks have Nd-Hf isotope compositions which indicate that the young mantle had differentiated into distinct isotopic domains before 4.0 Ga. Such ancient depletion was associated with an increase of mantle Nb/U ratios to modern values, and hence this event probably reflects the extraction of an amount of continental crust equivalent to its modern mass from the primitive mantle before 3.5 Ga. Thus, a steady-state model of crustal growth is favoured whereby post ~4.0 Ga continental additions have been balanced by recycling back into the mantle, with no net global flux of continental crust at modern subduction zones. It is also proposed that the decoupling of initial e(Nd) and e(Hf) from its typical covariant behaviour was related to the formation of continental crust, perhaps by widespread formation of TTG magmas.

A presente dissertação apresenta uma investigação sobre a evolução tectono-metamórfica da Formação São Tomé, uma das unidades metassedimentares pertencente ao Grupo Rio Doce, no Orógeno Araçuaí. Essa unidade, de origem pelítica, ocorre em uma faixa com direção geral aproximadamente N-S, com vergência para NE, contrária ao cráton São Francisco neste setor do orógeno. Possui como litotipo principal quartzo-mica xisto, com proporções variadas de granada, estaurolita, sillimanita e plagioclásio. Duas fases deformacionais estão registradas através de uma foliação continua S1 do tipo clivagem de fratura e uma foliação S2 de clivagem de crenulação. As relações texturais indicam que parte das fases minerais se cristalizaram tanto durante o evento deformacional como em condições ausentes de tensão deviatórica, e registro de condições de pico metamórfico de 6 kbar e 650 °C. Os dados geocronológicos foram obtidos em monazita por meio de datações Th-UPb em microssonda eletrônica. Foram identificadas três populações distintas: (i) A população mais antiga de 641±32 Ma, 614±39 Ma e 607±47 Ma, a qual pode representar grãos detríticos de corpos ígneos, não mais aflorantes, relacionados a um possível estagio pré-colisional ou, tratar-se de monazitas metamórficas formadas em um primeiro evento térmico; (ii) População com idades intermediárias entre 560±32 Ma e 559±29 Ma, compatíveis com as idades do ápice metamórfico regional; (iii) População mais jovem de idades entre 501±28 Ma e 491±34 Ma, condizentes com a formação concomitante à geração dos corpos ígneos tardios. Ainda que não esteja esclarecido se essas idades estão relacionadas a eventos metamórficos distintos ou são reflexo da baixa taxa de resfriamento do orógeno, os três grupos de monazita com idades distintas estão bem estabelecidos. / The present dissertation presents an investigation on the tectono-metamorphic evolution of the São Tomé Formation, one of the metasedimentary units belonging to the Rio Doce Group, in the Araçuaí Orogen. This unit, of pelitic origin, occurs in a band with general direction approximately N-S, with vergence for NE, contrary to the São Francisco craton in this sector of the orogen. It has quartz-mica schist as main lithotype, with varying proportions of grarnet, staurolite, sillimanite and plagioclase. Two deformational phases are recorded through a continuous foliation S1 of the fracture cleavage type and a foliation S2 of crenulation cleavage. The textural relations indicate that part of the mineral phases crystallized both during the deformational event and in conditions absent of deviatore voltage, and record of metamorphic peak conditions of 6 kbar and 650 ° C. The geochronological data were obtained in monazite by means of Th-U-Pb dating in electron microprobe. Three distinct populations were identified: (i) The oldest population of 641 ± 32 Ma, 614 ± 39 Ma and 607 ± 47 Ma, which may represent detrital grains of igneous bodies, no longer outcrops, related to a possible pre- collisional or, being metamorphic monazites formed in a first thermal event; (ii) Populations with intermediate ages between 560 ± 32 Ma and 559 ± 29 Ma, compatible with regional metamorphic apex ages; (iii) Younger population between 501 ± 28 Ma and 491 ± 34 Ma, consistent with the formation concomitant with the generation of late igneous bodies. Although it is unclear whether these ages are related to distinct metamorphic events or are reflective of the low orogenic cooling rate, the three monazite groups of different ages are well established.

Unidades litológicas, em particular arenitos, muitas vezes, carecem de um posicionamento cronoestratigráfico preciso. Como os arenitos são importantes rochas-reservatório de hidrocarbonetos e aquíferos, a falta de exatidão nestas informações dificulta a exploração destes bens minerais. A datação relativa de rochas sedimentares pode ser obtida por análise do conteúdo fossilífero ou por correlação estratigráfica. Entretanto, em algumas rochas sedimentares, esta análise não é possível ou tem um caráter duvidoso. Este é o caso da Formação Marizal (Bacia do Recôncavo) que apresenta um histórico controverso sobre a real idade deposicional. A Formação Marizal é um arenito flúvio-eólico cuja idade é discutível e, por isso, sua posição na coluna estratigráfica (aproximadamente Albiniano/Aptiniano), ainda é questionável. Em algumas amostras são encontrados overgrowths de K-feldspatos e nos quais é possível aplicar a técnica de datação 40Ar-39Ar visando obter idades que possam ser relacionadas com processos ocorrentes nestes arenitos (em geral, deposição e/ou diagênese). Entre os minerais pesados existentes nas amostras da Formação Marizal, foram encontrados grãos de zircões. A datação U-Pb de zircões detríticos pode fornecer informações sobre a proveniência desta unidade. Assim, zircões da Formação Marizal foram analisados visando complementar as informações sobre esta unidade, permitindo uma melhor interpretação. Os overgrowths de K-feldspatos indicaram valor de 159.89 ± 23.96 Ma e, para o núcleo detrítico, 432.57 ± 11.89 Ma. O valor médio obtido em torno de 160 Ma, considerando-se que todos os cuidados analíticos e de seleção de amostra foram considerados, é mais antigo do que o esperado. Assim, este valor foi interpretado como indicativo de que o overgrowth teria sido desenvolvido numa rocha fonte sedimentar sendo posteriormente transportado. Esta idade pode ser relacionada a fase pré rifte da Bacia do Recôncavo. O valor confirma ideias existentes de remobilização do substrato da bacia durante a fase rifte. Como tem sido discutido, overgrowths de K-feldspato são estáveis e possíveis de serem transportados por pequenas distâncias, o que corrobora a interpretação acima. Já o valor obtido para o núcleo mostra a contribuição do Paleoprotrozóico adjacente à bacia, retrabalhado no Brasiliano. Em relação ao zircão, a idade do núcleo detrítico de 432,53± 6,54 Ma pode ser associada com a cobertura sedimentar do Paleoproterozóico retrabalhada no ciclo Brasiliano, também observada nos valores U-Pb definidos para os zircões. Em relação ao zircão, os dados indicam ausência aparente de fontes arqueanas. Os resultados mostram duas fontes principais para a sedimentação: uma Rhyaciana (Paleoproterozóico onde ± 53 % dos grãos são “Transamazônicos”) e outra Neoproterozóica-Cambriana (30% dos zircões são “Brasilianos”). / Sandstones represent the most important reservoir rocks and aquifers in many sedimentary basins. It is necessary to have a precise chronostratigraphic position in order to provide a better explotation of water or hydrocarbons. Traditionally, the relative dating of sedimentary units is obtained with fossil content or stratigraphic correlation. But in many sedimentary rocks these analyses are not possible and sometimes have a dubious interpretation. This is the case of the Marizal Formation (Recôncavo Basin) where many questions arise when the age of the unit is questioned. The Marizal Formation is a fluvio-eolic sandstone which has been associated with an Albian/Aptian age in the stratigraphic column, although very discussible. Samples of sandstones of Marizal Formation present an important diagenetic overgrowths around K-feldspar detrital cores and they are suitable to 40Ar-39Ar dating concerning the identification of processes in the sandstones (as diagenesis or depositional ages). Among the heavy mineral suite in the Marizal Formation, zircon grains are identified. The U-Pb dating of detrital zircons can provide information about the provenance of the unit allowing better interpretation to the Marizal Formation. The overgrowths of K-feldspar indicated a value of 159.89 ± 23.96 Ma and to the detrital core, 432.57 ± 11.89 Ma. The mean value obtained around 160 Ma, considering that all care and analytical sample selection were considered, is older than expected. So, this was interpreted as indicating that the overgrowth, have been developed in a sedimentary source rock being transported latter to the depositional site. This age may be related to pre-rift stage of the Recôncavo Basin. The value confirms previous ideas of remobilization of the substrate during the rift basin stage. As has been extensively discussed, overgrowths of K-feldspars are stable and can be transported by small distances, which corroborates the above interpretation. The value obtained to the detrital core can be associated with a Paleoproterozoic sedimentary cover reworked in the Brazilian cycle. For zircon U-Pb dating, the data indicate no apparent Archean sources. The results show two main sources for sedimentation: a Rhyacian (Paleoproterozoic where ± 53% of the grains are "Transamazonian") and another Neoproterozoic-Cambrian (30% of zircon are "Brazilian").