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Título

EDIACARAN CRUSTAL REWORKING IN THE BORBOREMA PROVINCE, NE BRAZIL: U-PB DATA FROM ZIRCON AND GARNET

Texto do resumo

The Borborema Province (BP) in NE Brazil formed a large continental landmass after the amalgamation of the Congo and São Francisco cratons. It is composed of vast Paleoproterozoic gneissic basement, including Archean remnants, and voluminous Neoproterozoic granitoids and metasupracrustals, separated or delimited by crustal scale high temperature shear zones, the most prominent ones being the Transbrasiliano (TL), Patos (PL), Picuí - João Câmara (PJCL), and Pernambuco (PL). In this contribution, we report U-Pb zircon and garnet ages of Ediacaran granite intrusions, and neosomes to the West and East of the PJCL and along the PL. U-Pb zircon data were acquired at the Universidade de Campinas on an ICP-MS Element XR (Thermo Scientific) coupled to an Excite193 laser ablation system (Photon Machines) with HelEx ablation cell. U-Pb garnet ages were obtained using a Neptune Plus Thermo Finnigan coupled with a Photon-Machines 193 nm laser system of the Universidade Federal de Ouro Preto. A mylonitic porphyritic granodiorite (ANP17, 6.6131oS / 36.7385oW, Jardim do Seridó region, north of the PL and west of the PJCL) gave a 206Pb/238U zircon age of 585±6 Ma (n=11, MSWD 3.9). In the same region, a garnet-bearing granite (C10, 6.6139oS / 36.7752oW) crosscutting the flat lying regional S2 structure of the Seridó micaschist gave 206Pb/238U zircon age of 590±3 (n=8, MSWD 0.9). Both samples have inherited zircons with mostly Paleoproterozoic ages, with some Archean zircons in C10. The Catingueira granite (AP66A, 7.1204oS / 37.6071oW emplaced along the PL presents one zircon population close to the Concordia with 206Pb/238U age of 573±14 Ma (n=8, MSWD 6.3); most zircons, however, are inherited with ages from Neoproterozoic to Archean (670-2643 Ma). A garnet-bearing leucosome from an Archean orthogneiss of the Archean São José do Campestre Domain (ASJCD) east of the PL yielded 207Pb/206Pb peak ages of 3402±3, 3057±6, and 1994±15 Ma (S82, 6.1714oS / 35.8386oW; n=71). Another nearby garnet-bearing leucosome from an Archean paragneiss (CLZ17B, 6.2219oS / 35.7779oW) gave 207Pb/206Pb ages 2890±5 (n=42) and 605±2 Ma (n=12). U-Pb data of garnet from granitic leucosome S82 referred to above furnished a lower intercept at 565±22 Ma (n=12, MSWD 3.3). Garnet from calcsilicate diopside-rich rocks 3A (6.6771oS / 36.7726oW) and ES923 (6.2181oS / 35.7204oW) from the ASJCD yielded lower intercepts at 572±12 Ma (n=30, MSWD 1.0) and 577±6 Ma (n=31, MSWD 4.0), respectively. The ages obtained clearly show a time interval of 600-570 Ma for the main high-temperature Ediacaran tectono-metamorphic event and plutonism in both Paleoproterozoic and Archean domains. Metamorphic associations (cordierite, sillimanite, andalusite) in metasupracrustal units and crustal (peraluminous and subalkaline) intrusions are coeval with mantle derived gabbro to diorite, and apparently are controlled by deep ductile shear zone suggesting a relevant regional heating during the Ediacaran. Adiabatic melting synchronous to post-orogenic uplift and mantle plume are the mechanisms proposed for the context described. The collision between continental blocks north of the PL resulting in the transpression of the Seridó Belt culminated in directional movements along deep ductile NE-SW directed shear zones (e.g., PJCL) in the part of the BP.

Palavras Chave

Archean to Paleoproterozoic; crustal reworking; tectonic context