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Título

EXOTIC CRUSTAL FRAGMENTS AND SEDIMENT HERITAGE IN THE SOUTHERN GOIAS MAGMATIC ARC

Texto do resumo

INTRODUCTION
Orogenic accretion involving ancient fragments of Paleoproterozoic crust was already considered for the Southern Goiás Magmatic Arc (SGMA) and has been reinforced by recent data. Some basal metavulcanosedimentary units of the area also show Orosirian maximum depositional age, what suggests they are part of another exotic crustal material preserved.

OBJECTIVES
The goal of this work is to review some published geochronological data that register relic crust or sediment heritage in the Arenópolis and Bom Jardim Complex and display new geochronological and lithogeochemical data of exotic terranes represented by trachytic rocks.

METHODOLOGY
Geological, chemical and geochronological data were compiled from literature of the Southern Goiás Magmatic Arc and organized in a GIS.
U-Pb analyses were performed using LA-MC-ICP-MS Neptune (Thermo-Finnigan) and the lithogeochemistry samples were prepared by the GEOSOL Ltda, with major oxides digested by lithium tetraborate fusion and measured by X-Ray Fluorescence, while minor and trace elements were digested by multi-acid digestion or lithium metaborate fusion and analyzed in ICP-MS.

RESULTS AND DISCUSSION
To advance the discussion of relict crust present in the SGMA, the Morro Selado, Aldeinho and Morro do Urubu Formation, characterized from a stable coastal platform towards a slope depositional and volcanogenic environment, is suggested to no longer be associated to the neoproterozoic Bom Jardim Complex. A geochronological analysis of a quartzite sample from the Morro Selado Formation (CPRM, 2021) reveals a maximum depositional age of 1876 Ma and has significant contributions of Paleoproterozoic and Archean zircon grains.
Inherited zircon crystals in Arenópolis Complex are quite non-uniform, highlighting that different portions of the arc received different input depending on the proximity with the relict crust. The central part of this complex presents significant heritage contribution while as the south very little heritage contribution.
Meanwhile near Jaupaci Complex a yet unpublished U-Pb dating of 1777 ± 6 Ma on a microporphyritic trachyte reveals additional evidence of the influence of paleoproterozoic crustal fragments. From the geochemical standpoint, other five samples nearby this dating presents the same characteristics classified as miaskitic A-type rock, phonolitic (Le Bas et al., 1986), high incompatible element content, including large ion lithophile elements, flat on HREE and minimum estimates of crystallization temperature above 866.1 °C, therefore poor in inheritance grains. This alkaline rock has average silica arround 53.43%, high Na2O and K2O concentrations (8.23% and 6.42%, respectively) and low concentrations of MgO and CaO (0.38% and 1.52%, respectively). In terms of petrographic characteristics, it was well described in CPRM (2021).

CONCLUSION

A maximum depositional age of 1876 Ma correlated to continental sedimentation patterns and associated volcanic rocks followed by 1.77 Ga alkaline unit along the SGMA can be correlated to environmental settings of early to evolved rift stage, respectively.
Despite arguments on relating these units to a Statherian taphrogenesis process, they can be preserved like relicts of Paleoproterozoic extend margins. It is strongly recommended to constrain those proposals with new studies.

Palavras Chave

relict crust; orogenic accretion; Southern Goiás Magmatic Arc