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Título

THE QUEST FOR A ROBUST MAXIMUM DEPOSITIONAL AGE OF THE SERRA DA SERPENTINA GROUP, GUANHÃES BASEMENT INLIER: STATE OF THE ART AND NEW DETRITAL U-Pb AGES

Texto do resumo

The Serra da Serpentina Group (SSG) is exposed in its typical location in the western limit of the Guanhães Basement Inlier (GBI). It comprises the basal Rhyacian(?) ultramafic Serro-Alvorada de Minas Suite, the Rio Mata Cavalo metavolcanosedimentary sequence, the intermediate metapelitic Meloso Formation, and the upper banded iron formations and subordinate dolomites that encompass the Serra do Sapo Formation. The SSG is unconformably covered by the Statherian Serra de São José Group (robust MDA~1700 Ma; Lower Espinhaço Sequence). The unconformity is characterized by quartzites and rudaceous rocks containing reworked folded-iron formation and dolomite fragments derived from the erosion of the SSG, indicating tectonic deformation of the Serra da Serpentina Group previous to the deposition of the Serra de São José Group. This evidence, in combination with the recent geochemical, chronological, and field data suggests the deposition of the SSG is linked to sediment accumulation in a back-arc basin, formed during the Rhyacian-Orosirian Minas-Bahia Orogen. The 1999 ± 16 Ma MDA for the SSG was established based on a single Orosirian detrital zircon grain (SHRIMP U-Pb date) recovered from a fine-grained quartzite purportedly belonging to the Meloso Formation. Recent mapping and structural analysis, however, lead to a radical re-interpretion of the stratigraphic position of this horizon that counts the Orosirian grain as belonging to the age spectrum of the Lower Espinhaço Sequence, and the quartzite as a tectonic slice of the Espinhaço Supergroup. In the interior of the GBI the iron formations of the Serra da Serpentina Group are frequently associated with medium to coarse-grained quartzites, which yielded a faintly older MDA (SHRIMP U-Pb 2.18 Ga). The combination of the Rhyacian detrital age of the SSG with the presence of folded-iron formation clasts in the Lower Espinhaço Sequence supports the hypothesis that the Serra da Serpentina Group was deposited and deformed during the Minas-Bahia orogenic event. Our ongoing goal is to constrain the depositional age of the Serra da Serpentina Group by (a) determining the crystallization age of the basal mafic/ultramafic rocks using baddeleyite U-Pb dating, and (b) establishing a robust maximum depositional age through analysis of multiple representative samples. Here, we present a new detrital zircon age spectrum from a medium-grained micaceous quartzite tectonically interlayered with the Serra do Sapo iron formation (UTM: 693929/7920276). Seventeen out of 50 analyses yielded concordant ages (>95%) and produced Archean (12%), Siderian (6%), and Rhyacian (82%) age clusters. The sample yielded a youngest graphical peak date at 2074 Ma (n=4), and a youngest grain cluster at 2(2072.6 ± 7.5 Ma; n=3) pointing to a Rhyacian MDA, consistent with the Minas-Bahia orogeny. Our ongoing additional studies are converging towards determining a more robust maximum depositional age of the SSG.

Palavras Chave

Serra da Serpentina Group; Detrital age; Guanhães Basement Inlier