Abstract
Abstract This paper analyzes mineralogical, geochemical, and geochronological aspects, along with the effect of hydrothermal/metasomatic overprints, to identify the presence of primary phosphate as well as depositional and paleoenvironmental conditions in marble and calcsilicate sequences recrystallized under transitional amphibolite-granulite metamorphic conditions in the Tanque Novo-Ipira Complex within the Salvador-Curaca Orogen, northeastern Sao Francisco Craton, state of Bahia, Brazil. Petrographic studies have identified up to 10 vol.% disseminated apatite and whole-rock P2O5 contents up to 3.2 wt.%. Post-depositional events affected the lithofacies to varying degrees. Late hydrothermalism did not modify the rare earth element and yttrium (REEY) patterns considerably. When normalized to Post-Archean Australian Shale (PAAS), these lithofacies are marked by flat pattern REEY, true negative Ce anomalies, and positive Y and Gd. The highly variable Eu anomalies were inherited from the source composition but may have been affected by interaction with fluids. U-Pb LA-ICP-MS (laser ablation multicollector inductively coupled plasma mass spectrometry) ages indicate a maximum depositional age of 2128 Ga, as well as Paleoproterozoic and Neoarchean sources. Samples with anomalous phosphorus show Y/Ho ratios >30 and Ce/Ce* anomalies between 0.53 and 1.0 with an average of 0.70, suggesting a sub-oxic environment for phosphate precipitation.
Highlights
Calcsilicate rocks and marbles are common in metamorphic terrains associated with orogenic belts and are often intercalated with other pelitic and siliciclastic metasediments
Based on considerations associated with detrital contamination, diagenesis, and hydrothermalism, we suggest using the rare earth elements and yttrium (REEY) composition of marbles and calcsilicate rocks to characterize deposition conditions
Calcsilicate rocks and marbles of the Tanque Novo-Ipirá Complex are derived from the regional metamorphism of carbonates and siliciclastic sediments
Summary
Calcsilicate rocks and marbles are common in metamorphic terrains associated with orogenic belts and are often intercalated with other pelitic and siliciclastic metasediments. The decarbonation reactions that occur during the medium- to high-grade metamorphism of these rocks generate a variety of Ca–Mg-rich silicates, such as zoisite, tremolite, grossular, amphibole, diopside, and olivine (Bucher and Grapes 2010). These rocks are of great importance due to their metallogenic potential, especially for phosphate and skarn-type deposits, besides calcite mining. This complex mineralogical association can preserve several original geochemical signatures that, combined with field data, can be used for reconstructions, determination of paleoenvironmental conditions, and stratigraphic correlations. The first Paleoproterozoic phosphogenic pulse produced phosphorites that may contain less than 18 wt.% P2O5 (Pufahl 2010, Papineau 2010, Papineau et al 2013, Hiatt 2015)
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