The Planalto da Serra lithotypes are represented by 600 Ma plugs and necks of glimmerites and carbonatites. Phlogopite and/ or tetraphlogopite are the most abundant minerals of the glimmerites. Carbonatites comprise granular hypidiomorphic alvikites and brecciod beforsites. Diopside and melanitic garnet are restricted to the glimmerites, whereas pyroclore occur only in carbonatites. Perovskite, tremolite, magnetite, pyrite, apatite, titanite, chlorite and serpentine are common accessories of both lithologies. Under high CO2 and H2O activity glimmerites and carbonatites gave rise to hydrotermalites, in which chlorite and serpentine are the predominant minerals (up to 70%).They derived from tremolite, diopside and sometimes from olivine. Pyrite is a common accessory. Glimmerites, alvikitic dykes and hydrotermalites have kamafugitic affinity. Sr-Nd isotopes and incompatible trace-element data, characterized high LREE/HREE fractionation suggest that the K-ultramafic alkaline and carbonatite rocks originated from a varied metasomatized source mantle, characterized by Sr radiogenic enrichment. Crustal contamination is negligible or absent. Geochronological data of the Planalto da Serra rocks and its relationship with the low-grade metamorphic rocks of the Cuiabá Group make improbable the presence of the Clymene Ocean in this area. The age determination also rules out the geochronological relationship between the Planalto da Serra intrusions and the Mesozoic alkaline bodies from the Azimuth 125° lineament, and makes possible to relate its emplacement with to one of the most important extensional periods of Neoproterozoic times, which is characterized by the separation of Laurentia and the Amazonian Craton. The TDM model ages suggest that the Planalto da Serra melts derived from the remobilization of subcontinental lithospheric mantle that had been enriched by small-volume K-rich melt fractions at the Early to Late Neoproterozoic, which makes highly improbable the hypothesis of mantle plume tail, given that a subcontinental mantle enrichment, controlled by an Early Neoproterozoic event, produced similar ultramafic alkaline products at 600 and 80-90 Ma.
Petrology of potassic alkaline ultramafic and carbonatitic rocks from Planalto da Serra (Mato Grosso State), Brazil
DE MIN, ANGELO;SLEJKO, FRANCESCA FEDERICA;
2014-01-01
Abstract
The Planalto da Serra lithotypes are represented by 600 Ma plugs and necks of glimmerites and carbonatites. Phlogopite and/ or tetraphlogopite are the most abundant minerals of the glimmerites. Carbonatites comprise granular hypidiomorphic alvikites and brecciod beforsites. Diopside and melanitic garnet are restricted to the glimmerites, whereas pyroclore occur only in carbonatites. Perovskite, tremolite, magnetite, pyrite, apatite, titanite, chlorite and serpentine are common accessories of both lithologies. Under high CO2 and H2O activity glimmerites and carbonatites gave rise to hydrotermalites, in which chlorite and serpentine are the predominant minerals (up to 70%).They derived from tremolite, diopside and sometimes from olivine. Pyrite is a common accessory. Glimmerites, alvikitic dykes and hydrotermalites have kamafugitic affinity. Sr-Nd isotopes and incompatible trace-element data, characterized high LREE/HREE fractionation suggest that the K-ultramafic alkaline and carbonatite rocks originated from a varied metasomatized source mantle, characterized by Sr radiogenic enrichment. Crustal contamination is negligible or absent. Geochronological data of the Planalto da Serra rocks and its relationship with the low-grade metamorphic rocks of the Cuiabá Group make improbable the presence of the Clymene Ocean in this area. The age determination also rules out the geochronological relationship between the Planalto da Serra intrusions and the Mesozoic alkaline bodies from the Azimuth 125° lineament, and makes possible to relate its emplacement with to one of the most important extensional periods of Neoproterozoic times, which is characterized by the separation of Laurentia and the Amazonian Craton. The TDM model ages suggest that the Planalto da Serra melts derived from the remobilization of subcontinental lithospheric mantle that had been enriched by small-volume K-rich melt fractions at the Early to Late Neoproterozoic, which makes highly improbable the hypothesis of mantle plume tail, given that a subcontinental mantle enrichment, controlled by an Early Neoproterozoic event, produced similar ultramafic alkaline products at 600 and 80-90 Ma.Pubblicazioni consigliate
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