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SHRIMP U-Pb zircon age constraints on the Late Archaean tectonostratigraphic architecture of the Eastern Goldfields Superterrane,Yilgarn Craton, Western Australia
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NatalieKositcin
、
StuartJ.A.Brown
、
MarkE.Barley
、
BryanKrapez
、
KevinF.Cassidy
、
DavidC.Champion
Reassessment of Late Archaean tectonostratigraphic architecture of the Eastern Goldfields Superterrane is based on 35 new SHRIMP U-Pb zircon ages of supracrustal sequences. The superterrane comprises at least five terranes with ∼2810 to ≤2657 Ma volcanic and sedimentary rocks amalgamated by∼2660 Ma. Each terrane has distinctive stratigraphy, volcanic facies, geochemistry and timing of volcanism. The KalgoorlieTerrane comprises a >2715 to ∼2692 Ma plume-influenced, but deep-marine, mafic-ultramafic succession (Kambalda Sequence) overlain unconformably by a 2686±5 to ≤2666±6Ma arc-related, deep-marine felsic volcaniclastic-shale succession of tonalite-trondjemite-diorite (TTD) affinity (Kalgoorlie Sequence). Felsic volcaniclastic rocks of the Agnew Domain and northern Yandal Belt are included in the Kalgoorlie Terrane, based on their similar age and geochemistry. The terrane also contains enclaves of >2740 Ma volcanic sequences that are remnants of autochthonous basement to the Kambalda Sequence.Arc-related bimodal and intermediate volcanic rocks of the Gindalbie Terrane, ranging in age from 2693±4 to 2676±5 Ma, are coeval with but geochemically distinct from the Kalgoorlie Sequence. They overlie sequences similar in age and composition to volcanic rocks of the Kurnalpi Terrane. The Kurnalpi Terrane comprises deep-marine, calc-alkalic mafic and intermediate volcanic rocks (Kurnalpi Sequence) ranging in age from 2715±5 to 2704±4 Ma, overlain conformably by mafic to ultramafic lavas (Minerie Sequence). Although of overlapping age, arc-related rocks in the Kambalda and Kurnalpi-Minerie Sequences are geochemically different. SHRIMP dating has identified older volcanic sequences in the Laverton (≤2808±8 Ma) and Duketon-Burtville (2805±5 Ma) Terranes, with komatiite-hosted Ni-sulphide deposits atWindarra, in the Laverton Sequence, linked to mafic-ultramafic magmatism ∼100 million years older than in the Kalgoorlie Terrane. Older sequences in the Laverton Terrane are autochthonous basement to the Kurnalpi Sequence.Synorogenic siliciclastic sequences occur as erosional remnants either in synclinal structures or adjacent to terrane-scale faults, and overlie a regional-scale unconformity. Detrital zircon ages indicate deposition after 2657±4 Ma, placing important age constraints on the principal shortening event (D2) and peak metamorphism. Detrital zircon ages also indicate older sources at ∼2730, ∼2800, ∼2950, ∼3000 Ma and, less commonly, >3400 Ma. Many zircon age-populations correspond to known supracrustal and crustal rocks in the Yilgarn Craton, but some have no known source in the Eastern Goldfields Superterrane. Superterrane-wide distribution and evidence for erosion of all crustal and supracrustal rocks ≥2660 Ma in age requires deposition of these sequences after terrane amalgamation, most likely synorogenic to collision between the Eastern Goldfields Superterrane and the Yilgarn Craton.
Provenance and age delimitation of Quadrila´tero Ferrı´fero sandstones based on zircon U–Pb isotopes
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Le´oA.Hartmann
、
IssamuEndo
、
MarcosTadeuF.Suita
、
Joa˜oOrestesS.Santos
、
Jose´CarlosFrantz
、
Maurı´cioA.Carneiro
、
NealJ.McNaughton
、
MarkE.Barley
The Quadrila´tero Ferrı´fero has some of the largest iron and gold deposits in the world and is a major geotectonic unit of the Sa˜o Francisco Craton in Brazil. U–Pb zircon SHRIMP geochronology of six detrital sedimentary and metasedimentary rocks (114 zircon crystals, 118 spot analyses) has improved the understanding of the sedimentary processes and provenance ages of both rocks and the associated iron formation. The age of deposition of the iron formation is constrained between 2.58 and 2.42 Ga. The presence of an old Paleoarchean crust is dated in detrital zircon crystals, including the oldest zircon in South America (3809G3 Ma). Only high-Th/U, magmatic zircon crystals are present in the dated sedimentary rocks, and these indicate that the crust of the region was formed mostly during the Jequie´ cycle (six age peaks between 3055 and 2635 Ma). This time span ofw420 m.y. is similar to the duration of a long-lived Wilson cycle. Most of the Rio das Velhas Basin was filled during approximately 30 m.y. between 2746 and 2717 Ma, though volcanism probably started earlier. The youngest detrital zircon age from the Minas Supergroup indicates that the intracratonic basin fill, including the iron formation, was deposited after 2580 Ma. Therefore, the crust was cratonized shortly after the intrusion of minor granitic bodies at around 2.62–2.58 Ga. A large gap in orogenic activity is indicated by the absence of zircon ages of 2580–2182 Ma.
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