Archaean Sedimentary Rocks of northwestern Ontario
0
Citation
0
Reference
10
Related Paper
Sulfur Cycle
Cite
Citations (63)
Andesites
Ultramafic rock
Cite
Citations (15)
Abstract Volcanoes produce probably the most spectacular geological phenomena on Earth. Any of their eruptions can have a strong consequence on the surrounding environment, often captured in great detail in the sedimentary records of volcanically active regions. In addition, flank landslides and background erosive processes affecting volcanic sequences release volcanic particles that circulate within sedimentary environments up to billions of years after their generation. Therefore, exploring volcanically influenced sedimentary environments is an exciting and challenging scientific exercise requiring insights across multiple geological disciplines, drawing upon an increasing varied range of expertise and analytical approaches from across the geoscientific community. This book aims to provide an updated collection of works that illustrate the state-of-the-art in this topic, and to define the future directions of the geological sciences in utilizing and interpreting sedimentary records of volcanism.
Geologic record
Cite
Citations (2)
Greenstone belt
Batholith
Lithophile
Cite
Citations (1)
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
Cite
Citations (5)
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
Cite
Citations (67)