REE of Neoproterozoic phosphate concretions and their diagenetic implications: Tandilia System, Argentina

Abstract

Two phosphogenetic events are recognized in the Neoproterozoic Tandi(ia System based on the preservation of concretionary phosphate P205 abundances ranging between 25 to 35%. The o(der of the two is atop the quartz-arkosic facies association of the Villa Mónica Formation (Gómez Peral et al., 2005). The age of this unit is until controversial since it was considered Tonian-Cryogenian for stromatolite assemblages (Poiré, 1993) and Ediacarian regarding acritarch assemblages (Gaucher et al., 2005). Further investigation on micropaleontology of the phosphate concretions is considered in order to assess the age of these rocks. The younger phosphate level is at the base of the Cerro Negro Formation (Ediacarian) in association with a widespread karstic surface. In both cases these phosphate concretionary levels are related to relative sea level fall probably as a result of g(aciations. Tota( REE contents in phosphate concretions range from 311 to 1010 ppm in the Villa Mónica Formation and from 290 to 1471 ppm in the Cerro Negro Formation. These values are comparab(e with averages reported for similarly-aged phosphogenetic events wor(dwide. Cerium is unusually enriched in the Villa Monica samples (Ce/La vary from 2.3 to 3.2), which may be attributed to either diagenetic alteration or an increase in the detrital component. All samples show a progressive depletion in REE abundance patterns, and positive Eu anomalies (ranging from 1. to 1.4). Eu anomalies may be considered as real since there is no positive correlation between Ba and Eu. The overall REE results suggest reducing conditions in deep marine environments, which may be re-lated to stratification of seawater in the Villa Monica basin. In the Cerro Negro samples Ce/La varies from 1.3 to 1.9, imp(ying little enrichment in Ce, and negative Ce anomalies (Ce/Ce*) range from -0.15 to -0.18; no Eu an-oma(ies are observed. These results are consistent with oxic sea water conditions in mixed platform (marls, silty sandstone - mud limestones) facies. Ce/Ce* are typically accepted as primary signatures if there is no correlation between LaN/SmN and when these ratios are above 0.35. In our sample set five samples show LaN/SmN slightly lower than 0.35. Furthermore, diagenetic processes may result in the correlation between Ce/Ce* and DyN/SmN, and positive correlation between Ce/Ce* and REE contents. These correlations do not appear and REE abund-ances likely reflect paleoseawater redox conditions. This study suggests that marine depositional conditions where phosphates accumulated were very different in these two discrete stratigraphic levels. The REE patterns from selected unaltered samples of the Villa Mónica Formation indicate a marine depositional environment with anoxic characteristics which could be linked with a stratified ocean probably related to the Cryogenian. On the other hand, phosphate concretions of the Cerro Negro Formation are associated with oxygenated seawater more characteristic of Later Ediacarian-Lower Cambrian oceans.