Caracterização geológica, geoquímica e geocronológica da suíte plutônica enoproterozóica da região de Serrinha, porção centro-leste do maciço São José de Campestre, sudeste do RN

The of Serrinha plutonic suite, northeastern portion of the Borborema Province (NE Brazil), is characterized by a voluminous and diversified magmatism of Neoproterozoic age, intrusive in the Archean to Paleoproterozoic gneissic-migmatitic basement of the São José de Campestre massif. Field relations and petrographic and geochemical data allowed us to individualize different lithologic types among this plutonic suite, which is represented by intermediate to mafic enclaves, porphyritic diorites, porphyritic granitoids, porphyritic granodiorites, microporphyritic granites and dykes/sheets of microgranite. The intermediate-to-mafic enclaves occur associated with porphyritic granitoids, showing mixture textures. The porphyrytic diorites occur as isolated bodies, generally associated with intermediate-to-mafic enclaves and locally as enclaves within porphyritic granites. The granodiorites represent mixing between an intermediate to mafic magma with an acidic one. The micropophyritic granites occur as isolated small bodies, generally deformed, while the microgranite dykes/sheets crosscut all the previous granitoids. A U-Pb zircon age of 576 + 3 Ma was obtained for the Serrinha granite. This age is interpreted as age of the peak of the regional ductile deformational event (D3) and of the associated the E-W Rio Jacu shear zone, which control the emplacement of the Neoproterozoic syntectonic plutons. The porphyrytic granitoids show monzogranitic composition, transitional between peraluminous and metaluminous types, typically of the high potassium subalkaline-calc-alkaline series. The intermediate-to-mafic enclaves present vary from quartz diorite to tonalite/granodiorite, with metaluminous, shoshonitic affinity. The diorites are generally quartz-monzodiorite in composition, with metaluminous, subalkaline affinity. They display coarse-grained, inequigranular, porphyrytic texture, with predominance of plagioclase phenocrystals immersed in a matrix composed of biotite and pyroxenes. The microporphyrytic granites are essentially monzogranites of fine- to medium-grained texture, whereas microgranite dikes/sheets varying from monzogranites to syenogranites, with fine to media texture, equigranular. The diversified magmatism occurring at a relatively small surface associated with shear zones, suggests lithospheric dimensions for such structures, with magma extractions from different depths within the lower crust and upper mantle. The geological, geochemical and geochronological characteristics of the Serrinha plutonic suite suggest a pos-collisional geodynamic context for the Neoproterozoic magmatism. Thermobarometric data show emplacement conditions in the range 5-6 kbar (AlTamphibole) and 730-740°C (plagioclase-amphibole) for the porphyrytic granitoids (Serrinha body) and the intermediate-to-mafic enclaves

Geological evolution of the basement rocks in the east-central part of the Rondonia Tin Province, SW Amazonian craton, Brazil: U-Pb and Sm-Nd isotopic constraints

On the basis of geologic, petrologic, and U-Pb geochronologic data the basement rocks in the east-central part of the Rondonia Tin Province (RTP, southwestern Amazonian craton) are grouped into five lithologic associations: (1) tonalitic gneiss (1.75 Ga); (2) enderbitic granulite (1.73 Ga); (3) paragneiss; (4) granitic and charnockitic augen gneisses (1.57-1.53 Ga); and (5) fine-grained granitic gneiss and charnockitic granulite (1.43-1.42 Ga). The first three are related to development of the Paleoproterozoic Rio Negro-Juruena Province and represent the oldest crust in the region. The tonalitic gneisses and enderbitic granulites show calc-alkaline affinities and Nd isotopic compositions (initial epsilon(Nd) = +0-1 to -1.5; T-DM of 2.2-2.1 Ga) that suggest a continental arc margin setting for the original magmas. The paragneisses yield T-DM values of 2.2-2.1 Ga suggesting that source material was primarily derived from the Ventuari-Tapajos and Rio Negro-Juruena crusts, but detrital zircon ages and an intrusive granitoid bracket deposition between 1.67 and 1.57 Ga. The granitic and charnockitic augen gneisses show predominantly A-type and within-plate granite affinities, but also some volcanic arc granite characteristics. The initial epsilon(Nd) values (+0.6 to +2.0) indicate mixing of magmas derived from depleted mantle and older crustal sources. These rocks are correlated to the 1.60-1.53 Ga Serra da Providencia intrusive suite that reflects inboard magmatism coeval with the Cachoeirinha orogen located to the southeast. The fine-grained granitic gneiss and charnockitic granulites represent the first record of widespread magmatism at 1.43-1.42 Ga in northern Rondonia. Their geochemical signatures and the slightly positive initial epsilon(Nd) values (+0.7 to +1.2) are very similar to those of the most evolved granites of the calc-alkaline Santa Helena batholith farther southeast. U-Pb monazite and Sm-Nd whole-rock-garnet ages demonstrate that a high-grade tectonometa-morphic episode occurred in this region at 1.33-1.30 Ga. This episode attained upper-amphibolite conditions and is interpreted as the peak of the Rondonian-San Ignacio orogeny. The U-Pb and Sm-Nd data presented here and data published on rapakivi granites elsewhere indicate that the east-central part of the RTP is a poly-orogenic region characterized by successive episodes of magmatism, metamorphism, and deformation between 1.75 and 0.97 Ga. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Upper Pleistocene to Holocene peatland evolution in Southern Brazilian highlands as depicted by radar stratigraphy, sedimentology and palynology

Paleoenvironmental interpretation of proxy data derived from peatlands is largely based upon an evolutionary model for ombrotrophic bogs, in which peat accumulates in still environments. Reports on proxies obtained from minerotrophic fens, where hydrologic inputs are variable, are less common. In this study, a highland peatland in southern Brazil is presented through ground penetrating radar (GPR) and sedimentological, palynological and geochronologic data. The radar stratigraphic interpretation suggests a relatively complex history of erosion and deposition at the site since the beginning of Marine Isotope Stage 3 (MIS 3) interstadial period. In spite of this, radar stratigraphic and palynologic interpretations converge. Electromagnetic reflections tend to group in clusters that show lateral coherence and correlate with different sediment types, while pollen grains abound and are well preserved. As a result, the study of minerotrophic fens provides a source of proxies. suggesting that ombrotrophic bogs are not the only reliable source of data in wetlands for palynological analysis. (C) 2012 University of Washington. Published by Elsevier Inc. All rights reserved.

Provenance signature of a forearc basin modified by spreading ridge subduction: Detrital zircon geochronology and detrital modes from the Paleogene Arkose Ridge Formation, southern Alaska

Upper Paleocene–Eocene boulder conglomerate, cross-stratified sandstone, and laminated carbonaceous mudstone of the Arkose Ridge Formation exposed in the southern Talkeetna Mountains record fluvial-lacustrine deposition proximal to the volcanic arc in a forearc basin modified by Paleogene spreading ridge subduction beneath southern Alaska. U-Pb ages of detrital zircon grains and modal analyses were obtained from stratigraphic sections spanning the 2,000 m thick Arkose Ridge Formation in order to constrain the lithology, age, and location of sediment sources that provided detritus. Detrital modes from 24 conglomerate beds and 54 sandstone thin sections aredominated by plutonic and volcanic clasts and plagioclase feldspar with minor quartz, schist, hornblende, argillite, and metabasalt. Westernmost sandstone and conglomerate strata contain <5% volcanic clasts whereas easternmost sandstone and conglomerate strata contain 40 to >80% volcanic clasts. Temporally, eastern sandstones andconglomerates exhibit an upsection increase in volcanic detritus from <40 to >80% volcanic clasts. U-Pb ages from >1400 detrital zircons in 15 sandstone samples reveal three main populations: late Paleocene–Eocene (60-48 Ma; 16% of all grains), Late Cretaceous–early Paleocene (85–60 Ma; 62%) and Jurassic–Early Cretaceous (200–100 Ma; 12%). A plot of U/Th vs U-Pb ages shows that >97% of zircons are <200 Ma and>99% of zircons have <10 U/Th ratios, consistent with mainly igneous source terranes. Strata show increased enrichment in late Paleocene–Eocene detrital zircons from <2% in the west to >25% in the east. In eastern sections, this younger age population increases temporally from 0% in the lower 50 m of the section to >40% in samples collected >740 m above the base. Integration of the compositional and detrital geochronologic data suggests: (1) Detritus was eroded mainly from igneous sources exposed directly north of the Arkose Ridge Formation strata, mainly Jurassic–Paleocene plutons and Paleocene–Eocenevolcanic centers. Subordinate metamorphic detritus was eroded from western Mesozoic low-grade metamorphic sources. Subordinate sedimentary detritus was eroded from eastern Mesozoic sedimentary sources. (2) Eastern deposystems received higher proportions of juvenile volcanic detritus through time, consistent with construction of adjacent slab-window volcanic centers during Arkose Ridge Formation deposition. (3)Western deposystems transported detritus from Jurassic–Paleocene arc plutons that flank the northwestern basin margin. (4) Metasedimentary strata of the Chugach accretionaryprism, exposed 20-50 km south of the Arkose Ridge Formation, did not contribute abundant detritus. Conventional provenance models predict reduced input of volcanic detritus to forearc basins during exhumation of the volcanic edifice and increasing exposure ofsubvolcanic plutons (Dickinson, 1995; Ingersoll and Eastmond, 2007). In the forearc strata of these conventional models, sandstone modal analyses record progressive increases upsection in quartz and feldspar concomitant with decreases in lithic grains, mainly volcanic lithics. Additionally, as the arc massif denudes through time, theyoungest detrital U-Pb zircon age populations become significantly older than the age of forearc deposition as the arc migrates inboard or ceases magmatism. Westernmost strata of the Arkose Ridge Formation are consistent with this conventional model. However, easternmost strata of the Arkose Ridge Formation contain sandstone modes that record an upsection increase in lithic grains accompanied by a decrease in quartz and feldspar, and detrital zircon age populations that closely match the age of deposition. This deviation from the conventional model is due to the proximity of the easternmost strata to adjacent juvenile volcanic rocks emplaced by slab-window volcanic processes. Provenance data from the Arkose Ridge Formation show that forearc basins modified by spreading ridge subduction may record upsection increases in non-arc, syndepositional volcanic detritusdue to contemporaneous accumulation of thick volcanic sequences at slab-window volcanic centers. This change may occur locally at the same time that other regions of the forearc continue to receive increasing amounts of plutonic detritus as the remnant arc denudes, resulting in complex lateral variations in forearc basin petrofacies and chronofacies.

Sedimentology, Depositional Age, and Provenance of Sedimentary and Volcanic Rocks Exposed Along Willow Creek, Eastern Susitna Basin, South-Central Alaska: Implications for Modification of a Forearc Basin by Spreading Ridge Subduction

This is the first detailed study of the westernmost portion of the outcrop belt, which extends along the western flank of the Talkeetna Mountains and includes thick, well-exposed outcrops along Willow Creek in the eastern Susitna basin. New sedimentologic, compositional, and geochronologic data were obtained from stratigraphic sections within Arkose Ridge Formation strata at Willow Creek. This data combined with new geologic mapping and geochronologic data from Willow Bench and Kashwitna River Bluff (north of Willow Creek), and from the Government Peak area (east of Willow Creek), help constrain depositional processes and source terranes that provided detritus to the westernmost Arkose Ridge Formation strata.

Age determination of ODP Leg 120 samples

Conventional K-Ar and 40Ar/39Ar analyses on whole-rock samples are reported for basaltic samples retrieved on the Central and Southern Kerguelen plateaus during Ocean Drilling Program Leg 120. Sites 747, 749, and 750 recovered basalts from the plateau basement, whereas Site 748 drilled a lava flow interbedded with sediments of probable Albian age. The freshest core basalts from the basement yielded dates falling in the 110-100 m.y. interval. Sample 120-749C-15R-3 (26-31 cm) gave conventional K-Ar, total fusion, and plateau 40Ar/39Ar ages that are closely concordant: 111.5 ± 3.2 m.y., 109.9 ± 1.2 m.y., and 109.6 ± 0.7 m.y., respectively. Sample 120-750B-15R-5 (54-60 cm), when taking into account the analytical uncertainties, yields conventional K-Ar and 40Ar/39Ar plateau ages that can be considered similar: 101.2 ± 7.5 and 118.2 ± 5 m.y., respectively. Inspection of the 39Ar/40Ar vs. 36Ar/40Ar diagram does not reveal the occurrence of an initial argon component of radiogenic composition in the two samples. Accordingly, our results suggest that the formation of the basement of the Central Kerguelen Plateau was closed at 110 m.y.. Furthermore, these results are in agreement with a K-Ar age of 114 ± 1 m.y. mentioned in the literature for a basalt dredged in the 77°E Graben. The still scant amount of data indicates that the outpourings of the Central Kerguelen Plateau correspond rather well with widespread continental magmatism in Gondwanaland that is believed to mark the incipient opening of the eastern Indian Ocean. This implies a huge head for the mantle plume at the source of these liquids. Nevertheless, on land and at sea the exact duration of magmatism remains unknown. Therefore, a catastrophic pattern similar to that currently invoked for the Deccan Traps at the end of the Cretaceous, though possible, is not yet required by present geochronologic data.

(Table) Argon isotope ratios and argon ages of rocks obtained from the Chernorud zone, western Baikal area

Structural-petrologic and isotopic-geochronologic data on magmatic, metamorphic, and metasomatic rocks from the Chernorud zone were used to reproduce the multistage history of their exhumation to upper crustal levels. The process is subdivided into four discrete stages, which corresponded to metamorphism to the granulite facies (500-490 Ma), metamorphism to the amphibolite facies (470-460 Ma), metamorphism to at least the epidote-amphibolite facies (440-430 Ma), and postmetamorphic events (410-400 Ma). The earliest two stages likely corresponded to the tectonic stacking of the backarc basin in response to the collision of the Siberian continent with the Eravninskaya island arc or the Barguzin microcontinent, a process that ended with the extensive generation of synmetamorphic granites. During the third and fourth stages, the granulites of the Chernorud nappe were successively exposed during intense tectonic motions along large deformation zones (Primorskii fault, collision lineament, and Orso Complex). The comparison of the histories of active thermal events for Early Caledonian folded structures in the Central Asian Foldbelt indicates that active thermal events of equal duration are reconstructed for the following five widely spiced accretion-collision structures: the Chernorud granulite zone in the Ol'khon territory, the Slyudyanka crystalline complex in the southwestern Baikal area, the western Sangilen territory in southeastern Tuva, Derbinskii terrane in the Eastern Sayan, and the Bayankhongor ophiolite zone in central Mongolia. The dates obtained by various isotopic techniques are generally consistent with the four discrete stages identified in the Chernorud nappe, whereas the dates corresponding to the island-arc evolutionary stage were obtained only for the western Sangilen and Bayankhongor ophiolite zone.

Tectonomagmatic evolution of Western Amazonia: Geochemical characterization and zircon U-Pb geochronologic constraints from the Peruvian Eastern Cordilleran granitoids

The results of a coupled, in situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb study on zircon and geochemical characterization of the Eastern Cordilleran intrusives of Peru reveal 1.15 Ga of intermittent magmatism along central Western Amazonia, the Earth's oldest active open continental margin. The eastern Peruvian batholiths are volumetrically dominated by plutonism related to the assembly and breakup of Pangea during the Paleozoic-Mesozoic transition. A Carboniferous-Permian (340-285 Ma) continental arc is identified along the regional orogenic strike from the Ecuadorian border (6 degrees S) to the inferred inboard extension of the Arequipa-Antofalla terrane in southern Peru (14 degrees S). Widespread crustal extension and thinning, which affected western Gondwana throughout the Permian and Triassic resulted in the intrusion of the late- to post-tectonic La Merced-San Ramon-type anatectites dated between 275 and 220 Ma, while the emplacement of the southern Cordillera de Carabaya peraluminous granitoids in the Late Triassic to Early Jurassic (220-190 Ma) represents, temporally and regionally, a separate tectonomagmatic event likely related to resuturing of the Arequipa-Antofalla block. Volcano-plutonic complexes and stocks associated with the onset of the present Andean cycle define a compositionally bimodal alkaline suite and cluster between 180 and 170 Ma. A volumetrically minor intrusive pulse of Oligocene age (ca. 30 Ma) is detected near the southwestern Cordilleran border with the Altiplano. Both post-Gondwanide (30-170 Ma), and Precambrian plutonism (691-1123 Ma) are restricted to isolated occurrences spatially comprising less than 15% of the Eastern Cordillera intrusives. Only one remnant of a Late Ordovician intrusive belt is recognized in the Cuzco batholith (446.5 +/- 9.7 Ma) indicating that the Famatinian arc system previously identified in Peru along the north-central Eastern Cordillera and the coastal Arequipa-Antofalla terrane also existed inboard of this parautochthonous crustal fragment. Hitherto unknown occurrences of late Mesoproterozoic and middle Neoproterozoic granitoids from the south-central cordilleran segment define magmatic events at 691 +/- 13 Ma, 751 +/- 8 Ma, 985 +/- 14 Ma, and 1071-1123 +/- 23 Ma that are broadly coeval with the Braziliano and Grenville-Sunsas orogenies, respectively. Our data suggest the existence of a continuous orogenic belt in excess of 3500 km along Western Amazonia during the formation of Rodinia, its ``early'' fragmentation prior to 690 Ma, and support a model of reaccretion of the Paracas-Arequipa-Antofalla terrane to western Gondwana in the Early Ordovician with subsequent detachment of the Paracas segment in form of the Mexican Oaxaquia microcontinent in Middle Ordovician. A tectonomagmatic model involving slab detachment, followed by underplating of cratonic margin by asthenospheric mantle is proposed for the genesis of the volumetrically dominant Late Paleozoic to early Mesozoic Peruvian Cordilleran batholiths.

U Pb and Sm Nd geochronologic studies of the eastern Borborema Province, Northeastern Brazil: initial conclusions

The Borborema Province of NE Brasil comprises the central part of a wide Pan-African-Brasiliano orogenetic belt that formed as a consequence of late Neoproterozoic convergence and collision of the São Luis-West Africa craton and the São Francisco-Congo-Kasai cratons. New Sm Nd and U Pb results from the eastern part of this province help to define the basic internal architecture and pre-collisional history of this province, with particular emphasis on delineating older cratonic terranes, their fragmentation during the Mesoproterozoic, and their assembly into West Gondwana during the Pan African-Brasiliano orogeny at ca. 600 Ma. The region can be divided into three major geotectonic domains: a) Rio Piranhas-Caldas Brandão massif, with overlying Paleoproterozoic to Neoproterozoic supracrustal rocks, north of the Patos Lineament; b) the Archean to Paleoproterozoic São Francisco craton (SFC) to the south; and c) a complex domain of Paleoproterozoic to Archean basement blocks with several intervening Mesoproterozoic to Neoproterozoic fold belts in the center (south of Patos Lineament and north of SFC). The northern and central domains comprise the Borborema Province. Archean basement gneiss and Transamazonian granulite of northern SFC are exposed in the southern part of the central domain, underlying southern parts of the Sergipano fold belt. Basement in the Rio Piranhas massif appears to consist mostly of Transamazonian (2.1 to 2.2 Ga) gneissic rocks; Nd model ages (TDM) of ca. 2.6 Ga for 2.15 Ga gneisses indicate a substantial Archean component in the protoliths to these gneisses. The Caldas Brandão massif to the east yields both Transamazonian and Archean U Pb zircon and Nd (TDM) ages, indicating a complex architecture. Metasedimentary rocks of the Jucurutu Formation yield detrital zircons with original crystallization ages as young as 1.8 Ga, indicating that these rocks may be late Paleoproterozoic and correlate with other ca. 1.8 Ga cratonic supracrustal rocks in Brazil such as the Roraima Group and Espinhaço Group. Most metavolcanic and pre-Brasiliano granitic units of the Sergipano (SDS), Pajeú-Paraíba (SPP), Riacho Pontal (SRP), and Piancó-Alto Brígida (SPAB) fold belts in the central domain formed ~ 1.0 ± 0.1 Ga, based on U Pb ages of zircons. Nd model ages (TDM) for these same rocks, as well as Brasiliano granites intruded into them and large parts of the Pernambuco-Alagoas massif, are commonly 1.3-1.7 Ga, indicating that rocks of the fold belts were not wholly derived from either older (> 2.1 Ga) or juvenile (ca. 1.0 Ga) crust, but include mixtures of both components. A simple interpretation of Brasiliano granite genesis and the Nd data implies that there is no Transamazonian or Archean basement underlying large parts of these fold belts or of the Pernambuco-Alagoas massif. An exception is a belt of syenitic Brasiliano plutons (Syenitoid Line) and host gneisses between SPAB and SPP that clearly has a Transamazonian (or older) source. In addition, there are several smaller blocks of Archean to Transamazonian gneiss that can be defined within and among these fold belts. These blocks do not appear to constitute a continuous basement complex, but appear to be isolated older crustal fragments. Our data support a model in which ca. 1.0 Ga rifting was an important tectonic and crust-forming event along the northern edge of the São Francisco craton. Our data also show that significant parts of the Borborema Province are not remobilized Transamazonian to Archean crust, but that Mesoproterozoic crust is a major feature of the Province. There are several small remnants of older crust within the area dominated by Mesoproterozoic crust, suggesting that the rifting event created several small continental fragments that were later incorporated into the Brasiliano collisional orogen. We cannot at present determine if the Rio Piranhas-Caldas Brandão massifs and the older crustal blocks of the central domain were originally part of the São Francisco craton or whether some (or all) of them came from more exotic parts of the Proterozoic Earth. Finally, our data have not yet revealed any juvenile terranes of either Transamazonian or Brasiliano age. © 1995.

Isotope geochemical and geochronologic studies of basement units in the Central Western Carpathians (Slovakia)

In dieser Arbeit werden geochronologische und isotopen-geochemische Daten zur Entwicklung der Zentralen Westlichen Karpathen präsentiert. Die Karpathen bilden die östliche Fortsetzung der Alpen und können in drei Alpine Grundgebirgsdecken unterteilt werden, von denen zwei, die Veporische und die Gemerische, bearbeitet wurden. In der Veporischen Einheit wurden polymetamorphe Grundgebirgseinheiten untersucht, um deren genaue Altersstellung zu definieren und sie isotopengeochemisch zu klassifizieren. Dagegen wurde in der der Gemerischen Einheit, welche die Veporische Einheit überlagert, ein spezialisierter S-Typ Granit im Detail untersucht, um die petrogenetischen Prozesse, die zur magmatischen Entwicklung dieses Granits geführt haben, zu identifizieren. U-Pb Datierungen an Zirkonen der Veporischen Grundgebirgseinheiten zeigen für die gesamte Veporische Einheit ordovizische Entsehungsalter an (440-470 Ma). Diese Datierungen revidieren publizierte kambrische Entstehungsalter dieses Grundgebirges. Die Isotopensignatur (epsilon Nd und 87Sr/86Sr) der ordovizischen Grundgebirgseinheiten, bestehend aus stark überprägten Amphiboliten und Gneissen, ist von der Signatur der sich im Norden anschliessenden Tatrischen Einheit gut unterscheidbar. Die Bleiisotopenzusammensetzung dieser Gesteine ist stark krustal geprägt und überschneidet sich mit der der Tatrischen Einheit. Zusammen mit den T-DM Altern sind diese Einheiten vergleichbar mit prävariskischen Einheiten der Alpen. Somit kann das ordovizische Grundgebirge zu den peri-Gondwana Terranen gezählt werden, die an einem aktiven Kontinentalrand im Norden von Gondwana gebildet wurden. In den Gesteinen der Veporischen Einheit wurde im Weiteren eine starke metamorphe überprägung und intensiver felsischer Magmatismus karbonischen Alters erkannt (320-350 Ma). Dieses Ereignis ist zeitgleich mit dem Magmatismus, welcher hauptsächlich in der sich im Norden anschliessenden Tatrischen Einheit beobachtet wird. Dieser gehört der variskischen Orogenese an. Intensive alpine Deformation und Metamorphose konnte in der südlichen Veporischen Einheit anhand der Einzelzirkondatierungen und der Isotopendaten der ordovizischen Einheiten nachgewiesen werden. Am Dlha Dolina Granit in der Gemerischen Einheit können starke Fraktionierungs- und Auto-Metasomatose-Effekte beobachtet werden. Durch die magmatische Fraktionierung wird eine Anreicherung der SEE erzeugt, wogegen die Metasomatose die SEE stark verarmt. Es kommt sogar zur Ausbildung eines Tetraden Effektes im SEE Muster, welche den starken Einfluss von Fluiden während der spät-magmatischen Phase belegt. Gesamtgesteins Pb-Pb Daten beschränken das minimale Intrusionsalter dieses Granites auf 240 Ma. Dieses Alter ist in guter übereinstimmung mit den Sr-Isotopendaten der magmatisch dominierten Gesteine, wohingegen die stark metasomatisch geprägten Gesteine ein zu radiogenes 87Sr/86Sri aufweisen. Während dieser Arbeit wurde intensiv mit der Blei-Isotopenzusammensetzung von Gesamtgesteinsproben gearbeitet. Um die Auswertung dieser Daten optimieren zu können wurde ein Computerscript für das GPL Programm Octave erstellt. Die Hauptaufgabe dieses Scripts besteht darin, Regressionen für geochronologische Anwendungen gemäss York (1969) zu berechnen. Ausserdem können mu und kappa-Werte für diese Regressionen berechnet und eine Hauptkomponentenanalyse, welche hilfreich für den Vergleich von zwei Datensätzen ist, durchgeführt werden. Am Ende der vorliegenden Arbeit wird die analytische Methode für einen Mikrowellen beschleunigten Säureaufschluss von granitoidem Material zur Bestimmung der Sr- und Nd-Isotopenzusammensetzung und der Elementkonzentrationen vorgestellt. Diese kombinierte Methode nutzt ein TIMS für die Sr und Nd Isotopenmessungen und eine Einzelkollektor-ICPMS zur Bestimmung der SEE, Rb und Sr Konzentrationen, welche mithilfe von relativen Sensitivitätsfaktoren gegenüber einem internen Standard quantifiziert werden. Diese Methode wird durch Messungen von internationalen Referenzmaterialien bewertet. Die Ergebnisse zeigen eine Reproduzierbarkeit von <10% für die Elementkonzentrationen und von <5% für Elementverhältnisse.

U-Pb (zircon) geochronologic analyses for rocks of the Transantarctic Mountains (Table 4.3-1)

The Byrd Glacier discontinuity us a major boundary crossing the Ross Orogen, with crystalline rocks to the north and primarily sedimentary rocks to the south. Most models for the tectonic development of the Ross Orogen in the central Transantarctic Mountains consits of two-dimensional transects across the belt, but do not adress the major longitudinal contrast at Byrd Glacier. This paper presents a tectonic model centering on the Byrd Glacier discontinuity. Rifting in the Neoproterozoic producede a crustal promontory in the craton margin to the north of Byrd Glacier. Oblique convergence of the terrane (Beardmore microcontinent) during the latest Neroproterozoic and Early Cambrian was accompanied by subduction along the craton margin of East Antarctica. New data presented herein in the support of this hypothesis are U-Pb dates of 545.7 ± 6.8 Ma and 531.0 ± 7.5 Ma on plutonic rocks from the Britannia Range, subduction stepped out, and Byrd Glacier. After docking of the terrane, subduction stepped out, and Byrd Group was deposited during the Atdabanian-Botomian across the inner margin of the terrane. Beginning in the upper Botomian, reactivation of the sutured boundaries of the terrane resulted in an outpouring of clastic sediment and folding and faulting of the Byrd Group.