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Maria Emilia S. Della Giustina, Universidade de Brasília (Brazil)
Claudinei G. Oliveira, Universidade de Brasília (Brazil)
Márcio M. Pimentel, Universidade de Brasília (Brazil)
Luciana V. Melo, Universidade de Brasília (Brazil)
Reinhardt A. Fuck, Universidade de Brasília (Brazil)
Elton L. Dantas, Universidade de Brasília (Brazil)
Bernhard Buhn, Universidade de Brasília (Brazil)
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The Paleoproterozoic represents the main episode of crustal growth recorded in present-day continents. In the South-American Platform, two distinct accretionary phases are observed, one between ca. 2.3 and 2.1 and another at 2.1-1.8 Ga.
The Campinorte volcano-sedimentary sequence and associated plutonic unit represents this Paleoproterozoic scenario in the northern Brasilia Belt, Central Brazil. Quartz micaschist, with variable amounts of carbonaceous matter is the main rock type of this unit. Psammitic rocks include micaceous, thinly laminated quartzite, as well as orthoquartzite massive beds. Chemical deposits such as gondites and metacherts have been identified, and constitute metric lenses within the metasedimentary rocks. Felsic volcaniclastic rocks occur as small, elongate bodies interlayered within the metasedimentary sequence. Pyroclastic deposits are the most representative lithologies, and are composed of rhyolitic crystal metatuffs to metalapilli-tuffs. Rhyolites are rare and present idiomorphic K-feldspar and anhedral blue quartz phenocrysts, immersed in a fine-grained quartz-feldspar groundmass. This supracrustal unit has been submitted to at least two deformational events and has been metamorphosed under greenschist facies conditions.
The Campinorte Intrusive Unit includes metatonalites, metagranodiorites and metagranites. In general, the intrusions occur as NNE-trending elongate plutons showing varied degrees of deformation.
A LAM-ICPMS provenance study of zircon grains from a quartzite sample identifies a single juvenile source, and sets the maximum depositional age at ca. 2190 Ma. The metasedimentary rocks from the Campinorte sequence were generated, therefore, from the erosion of the surrounding Paleoproterozoic arc rocks.
U-Pb ID-TIMS analyses of a rhyolitic metatuff yield an upper intercept age of 2179 ± 4 Ma, which is interpreted as the age of crystallization of the original magma. This is compatible with the maximum depositional age of ca. 2.2 Ga for the supracrustal unit.
Zircon from the investigated granitic rocks present Rhyacian ages ranging from 2.18 to 2.16 Ga. No inheritance was observed, suggesting that no Archean crust has been involved in the genesis of the granites. The Sm-Nd data for plutonic rocks, with TDM model ages range between ca. 2.25 Ga and 2.36 Ga and positive εNd values, indicate the juvenile nature of the original magmas.
The lithological association combined with the isotopic/geochronologic data presented here demonstrate the similarity between the Campinorte volcano-sedimentary sequence and associated plutonic rocks and those observed in other Paleoproterozoic provinces described in the world, especially the Birimian Belt of the West African Craton and the Transamazonic Belt in the Guiana Shield. The data suggest therefore that the Campinorte Sequence and related intrusive unit are relict fragments of the Paleoproterozoic episode of crustal growth, exposed in the Goiás Massif.
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