Rhyacian-Orosirian transitional arc-collisional magmatism in the Archean Piedade Microcontinent, southern São Francisco Paleocontinent, Brazil
Mariana CarvalhoHenrique BrunoSamuel Moreira BersanCraig StoreyCatherine MottramGlenn ChapmanMike FowlerCláudio de Morisson ValerianoCristiano LanaCarla NetoMônica Heilbron
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This volume contains a series of papers that cover a wide range of aspects, including geophysics, structure and tectonics, atmosphere, origin of life, biosphere, deep mantle geochemistry, early oceans, microbial ecology, on the development of the Earth in the first 2000 Ma of its history.The aim of this publication is to facilitate future discussions and understanding of this area of research. This book is divided into three parts: • Geophysical and petrological constraints on Archaean lithosphere • Models of cratonic evolution and modification • Constraints on the Archaean environment Subjects covered include the chemical and biological controls on the atmosphere and oceans, early controls on the carbon cycle and photosynthesis, petrologic, isotopic, tectonic and seismic evidence for the composition and structure of Archaean lithosphere. This volume should be of interest to geologists and geophysicists who work on the Archaean, and students at all levels.
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Greenstone belt
Batholith
Lithophile
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This thesis summarises the results of four original papers concerning U-Pb geochronology and geochemical evolution of Archaean rocks from the Kuhmo terrain and the Nurmes belt, eastern Finland. The study area belongs to a typical Archaean granite-greenstone terrain, composed of metavolcanic and metasedimentary rocks in generally NS trending greenstone belts as well as a granitoid-gneiss complex with intervening gneissic and migmatised supracrustal and plutonic rocks. U-Pb data on migmatite mesosomes indicate that the crust surrounding the Tipasjarvi-Kuhmo-Suomussalmi greenstone belt is of varying age. The oldest protolith detected for a migmatite mesosome from the granitoid-gneiss complex is 2.94 Ga, whereas the other dated migmatite protoliths have ages of 2.84–2.79 Ga. The latter protoliths are syngenetic with the majority of volcanic rocks in the adjacent Tipasjarvi-KuhmoSuomussalmi greenstone belt. This suggests that the genesis of some of the volcanic rocks within the greenstone belt and surrounding migmatite protoliths could be linked. Metamorphic zircon overgrowths with ages of 2.84–2.81 Ga were also obtained. The non-migmatised plutonic rocks in the Kuhmo terrain and in the Nurmes belt record secular geochemical evolution, typical of Archaean cratons. The studied tonalitic rocks have ages of 2.83–2.75 Ga and they have geochemical characteristics similar to low-Al and high-Al TTD (tonalite-trondhjemite-dacite). The granodiorites, diorites, and gabbros with high Mg/Fe and LILE-enriched characteristics were mostly emplaced between 2.74–2.70 Ga and they exhibit geochemical characteristics typical of Archaean sanukitoid suites. The latest identified plutonic episode took place at 2.70–2.68 Ga, when compositionally heterogeneous leucocratic granitoid rocks, with a variable crustal component, were emplaced. U-Pb data on migmatite leucosomes suggest that leucosome generation may have been coeval with this latest plutonic event. On the basis of available U-Pb and Sm-Nd isotopic data it appears that the plutonic rocks of the Kuhmo terrain and the Nurmes belt do not contain any significant input from Palaeoarchaean sources. A characteristic feature of the Nurmes belt is the presence of migmatised paragneisses, locally preserving primary sedimentary structures, with sporadic amphibolite intercalations. U-Pb studies on zircons indicate that the precursors of the Nurmes paragneisses were graywackes that were deposited between 2.71 Ga and 2.69 Ga and that they had a prominent 2.75–2.70 Ga source. Nd isotopic and whole-rock geochemical data for the intercalated amphibolites imply MORB sources. U-Pb data on zircons from the plutonic rocks and paragneisses reveal that metamorphic zircon growth took place at 2.72–2.63 Ga. This was the last tectonothermal event related to cratonisation of the Archaean crust of eastern Finland.
Protolith
Migmatite
Greenstone belt
Hadean
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ABSTRACT U-Pb zircon geochronology of Mesoproterozoic (Subjotnian) rapakivi complexes in central Sweden yields: 1526 ± 3 Ma (Mullnäset), 1524 ± 3 Ma (Mårdsjö), 1520 ± 3 Ma (Nordsjö) and 1497 ± 6 Ma (Rödön). Together with complexes further S in Sweden, they constitute the westernmost, youngest (1·53−1·47 Ga) belt of rapakivi magmatism in the Fennoscandian shield. The low initial ε Nd values (−8·9 to −4·8) of all studied Subjotnian basic, intermediate and silicic rocks, require an input from an old (Archaean) low-radiogenic source component, as evidence for Palaeoproterozoic protoliths in the age range 2·5−2·1 Ga is lacking in this region. Crustal, early Svecofennian + Archaean (roughly 30−40%) sources are suggested for the Subjotnian A-type granites and syenites, where the granites derive from undepleted, granodioritic, and the syenites from monzodioritic (±depleted crustal) protoliths. The basic rocks originate from a depleted mantle acquiring the enriched Nd isotopic signatures during interaction with an Archaean lower crust (20−40%), largely depleted after rapakivi melt extraction. Pb isotope data from feldspars ( 207 Pb/ 204 Pb to 15·018−15·542) support the presence of Archaean components in the magmas. The results indicate that an Archaean basement is underlying relatively wide areas of Svecofennian formations in central Sweden. This old basement section was most likely rifted off the Archaean craton in the NE in Palaeoproterozoic times.
Protolith
Geochronology
Basement
Baltic Shield
Radiogenic nuclide
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Cenozoic magmatism in the North American Cordillera has attracted the attention of volcanologists, structural geologists, and petrologists for more than a century. From horseback reconnaissance in the 1800s, when a hand lens was the analytical instrument of choice [e.g., von Richthofen , 1868; King , 1878] to today's studies, where mass spectrometers, electron microprobes, and computers are essential tools, attention has focused on the broad regional extent, large volumes, and complexly shifting space‐time‐composition patterns of Tertiary magmatism in the Cordillera. Cordilleran igneous rocks provide key information on regional geologic history, primary geologic constraints on many types of ore deposits, and the basis for development of broad concepts applicable to global magmatism and plate tectonics. Studies of Cordilleran igneous rocks have been especially intense in the past few decades, fueled by improved analytical techniques and by societal concerns such as the search for geothermal and mineral resources and evaluation of volcanic hazards.
Volcanology
Petrogenesis
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