Timing of Neoproterozoic Glaciation, implication for the Snowball Earth hypothesis

On top of the Proterozoic Sao Francisco Craton of central eastern Brazil, numerous signs of neoproterozoic glaciations have been found which would had covered an area of at least 910,000 km² (Pflug and Schöll, 1975). These sequences contain typical traces of glaciation, such as diamictites, varvites with dropstones, glacial streaming etc., and discordantly overly Archean to Mesoproterozoic rocks. The glacial deposits themselves are overlain by Neoproterozoic calcerous and psamopelitic sediments of the BambuÌ Group. Because of the lack of data age determination and subsequent correlation of the potential sources remained up to now uncertain.

The aim of our study is focussed on the age determination of the glacial deposits of the Jequitai Formation of Minas Gerais and the Caboclo Formation in western Bahia. For this study we have collected phosphate-bearing columnar stromatolite and different dolomitic and/or massive black limestones together with the diamictites from the eastern Irecê Basin. Clay minerals (< 2.0 µm) were recovered from one diamictite sample and were divided in three different grain-size fractions and were dated by K-Ar and Rb-Sr method. The K-Ar age obtained for the coarse fraction (0.8-2.0 µm) was 515 ± 13 Ma; the middle-sized fraction (0.4-0.8 µm) showed an age of 502 ± 12 Ma; and the finest fraction (0.0-0.4 µm) yielded an age of 512 ± 12 Ma. The Rb-Sr ages obtained increases with the increasing size fraction. The age is 516 Ma for the finest fraction, 565 Ma for the middle-size fractions, and the coarse-grained fractions have ages of 649 Ma and 701 Ma. The youngest age was determined on the finest fraction which is composed by authigenic illite/smectite and illite, and it represents the time of crystallization of these minerals; this age is coincident with the time of the fluid percolation event (Babinski et al., 1999a, b). The older age (701 Ma), which was determined on the coarser fraction which is made up almost exclusively by authigenic chlorite, is interpreted as the crystallization age of this clay mineral. The other ages are probably mixing ages which reflect the varying proportions of the different clays minerals present in these fractions. Although Pb-Pb, K-Ar, and Rb-Sr ages of ca. 500 Ma were determined on the samples and they represent the time of post-depositional events that took place in the basin, the Rb-Sr age of 700 Ma determined on authigenic chlorites from the carbonate rock of the Salitre Formation provides its minimum depositional age. Based on this age we can suggest that the glacial event recorded by the diamicitites underlying the Salitre Formation is Sturtian (ca. 780 Ma). Additionally, Pb-Pb evaporation ages determined on zircons recovered from a diamictite from the Macaúbas Group (Minas Gerais State) show values of ca. 900 Ma (Buchwaldt et al, 1999) reinforcing the above hypothesis. This result gives us a general idea about the duration of the glacial even within this Neoproterozoic sequence.

In order to reconstruct and define geochemically the provenance and the tectonic settings of the potential sources of the glaciogenic deposits, seven samples in a vertical profile of the cap dolostone of the Salitre Formation in western Bahia were collected. The cap dolostone is suitable for this purpose because of its fine grained silicate mineral phases, which are a homogenized record of the glaciogenic deposits. This is geochemically well documented with the chondrite normalized REE patterns of all collected rocks. They all show similar Eu/Eu* (0.80 ± 0.08), Ce/Ce* (1.02 ± 0.02) and La_N/Yb_N (10.23 ± 0.41) ratios although the absolute concentration in REE is slightly variable.

The REE, HFSE and Th are known to be reliable provenance indicators (Taylor and McLennan, 1985) because they tend to be transferred unfractionated from provenance material into the sedimentary site. These elements are also relatively immobile during weathering, diagenesis and metamorphism (Wronkiewicz and Condie, 1987, Cullers et al., 1987, McLennan, 1989) and their realibility can be proved by the relation between Th/Sc and Zr/Sc ratios. In the analysed cap dolostone samples this relation shows the absent of enrichment of heavy mineral which would be an indication of sedimentary sorting during deposition.

These diagrams demonstrate that the source rocks may be granodioritic in composition and were derived of a continental island arc tectonic enviroment. In relation of La versus Th this observation is not fully confirmed because the island arc seems to have strong affinities to oceanic island composition. Nevertheless with both characterization diagrams we are able to exclude for shure the enviroments of passive margin and an active continental margin regime. To compare this event with other glacial events worldwide other diamictites namely from Australia and Scotland are momentarily under investigation.