Geocronologia da capa carbonática e proveniência do intervalo Ediacarano Superior-Cambriano da bacia Araras-Alto Paraguai, sul do cráton Amazônico

Abstract

The Araras-Alto Paraguai basin, on the Southern Amazonian craton includes Cryogenian-Ediacaran-Cambrian deposits related to the Marinoan glacial (ca. 635 Ma) and post-glacial events (ca. 635-600 Ma) and initial stages of Western Gondwana. The glaciogene diamictites of the Puga Formation are directly overlaid by the Neoproterozoic cap carbonate of the lower Araras Group, evidencing the rapid icehouse-greenhouse transition in the context of the Snowball Earth theory. The Araras Group is unconformably superimposed by the Cambrian siliciclastics of the Raizama Formation, lower Alto Paraguai Group. The succession is bounded by U-Pb maximum depositional ages of < 652 ± 5 Ma (basement - Cuiaba Group) and < 528 ± 9 Ma (upper Alto Paraguai Group). Carbonate Pb-Pb ages of 627 ± 32 Ma and 622 ± 33 and δ13C negative excursions in the cap carbonate are coherent with these time estimates. Correlation of the isotopic data of the Araras-Alto Paraguai basin with δ13C and 87Sr/86Sr global curves indicates an age of ca. 614 Ma for the eroded upper limit of the Araras Group. The presence of skolithos at the base of the Alto Paraguai Group confirms the Cambrian age (<530 Ma), implying a stratigraphic gap of ca. 80 x 106 yr between the groups. The available data are still sparse, allowing only imprecise timing of the geologic events in the succession. New δ13C isotopic data with the typical cap carbonate negative signature, with values oscillating between -7 and -4 ‰ complemented the δ13C curves available for the succession, extending stratigraphic correlations over the inner platform of the Araras Group. The LA-ICP-MS U-Pb dating of calcite crystal fans, interpreted as aragonite pseudomorphs, provided the first diagenetic age of 624 ± 6 Ma of the cap carbonate. Textural relations and heavier isotopic signal in the fans (δ13C = -6.80 a -5.85, δ18O = -7.0 a -5.80) compared to the micrite (δ13C = -7.25 a -5.85, δ18O = -8.9 a -7.5), indicates dissolution-reprecipitation under the influence of oxidizing fluids in the diagenetic environment. Such a change in the composition of the diagenetic fluids may be related to the sea level fall during deposition of the highstand system tract of the cap carbonate sequence. The diagenetic age demonstrates that the depositional and diagenetic processes of the cap limestone, including micrite and pyrite precipitation, inversion of aragonite to calcite, burial neomorphism, chemical compaction and calcite dissolution-reprecipitation within the crystal fans lasted ca. 11 ± 6 x 106 yr. Although a similar spectrum of detrital zircon ages indicates the same cratonic source areas for the studied succession, there is a general higher proportion of ages older than 1.25 Ga for the upper Araras Group compared to the lower Alto Paraguai Group. This result corroborates the erosive hiatus between the groups, indicating a relative increase of sources from the SW-SE (<1.25 Ga, Sunsás belt) in detriment of sources to the N-NW (> 1.25 Ga, other terrains of the Amazonian craton) of the basin during the Ediacaran-Cambrian transition. On the other hand, in the cratonic domain, absence of ages younger than 1.38 Ga and paleocurrents persistently to SE for the lower Alto Paraguai Group suggest intensification of the siliciclastic influxes from the NNW. Integration of the sedimentological, chemostratigraphic, geochronological and tectonic data allowed to propose five evolutionary steps for the Ediacaran-Cambrian succession of the Araras-Alto Paraguai Basin: 1) Ice melting with deposition of the cap dolostone (635-632 Ma); 2) Thermal expansion of the ocean with deposition of the cap limestone and highstand system tract of the cap carbonate sequence (ca. 632-615 Ma); 3) Regression with transference of accommodation space to the East (ca. 615- 600 Ma); 4) Complete exposition of Southern Amazonian craton (600 – 530 Ma); and 5) Sea level rise with implantation of a siliciclastic platform (ca. 530-520 Ma). The obtained geochronological and chemostratigraphic data allowed a better stratigraphic characterization of the Araras-Alto Paraguai sedimentation exposed in the southern Amazonian craton, revealing an important part of the geological evolution of the primordial epicontinental seas related to western Gondwana assembly.