INGEOSUR   20376
INSTITUTO GEOLOGICO DEL SUR
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
GENESIS OF FLUORAPATITE AGGREGATES IN DOLOMITES, QUARTZITES, LIMESTONES AND PELITES OF THE OLAVARRÍA AREA, TANDILIA RANGES, ARGENTINA
Autor/es:
DRISTAS, JORGE A.; MARTINEZ, JUAN CRUZ; MASSONNE, HANS -JOACHIM; THEYE, THOMAS; GRAFF, AILÍN AYELÉN
Lugar:
Heidelberg
Reunión:
Congreso; The 23rd Latin American Colloquium on Earth Sciences; 2014
Institución organizadora:
Universität Heidelberg
Resumen:
At the acid hydrothermal alteration front (pyrophyllite-kaolinite paragenesis) of the most intensely altered migmatitic basement from the Barker, La Juanita and San Manuel areas, Tandilia ranges, neither primary apatite nor monazite were recognized, though they are normally present in the same but unaltered rocks. However, epigenetic apatite veinlets and fluorapatite-rich aggregates have been identified in different levels at the unconformably overlying sedimentary sequence of the Barker area. This Neoproterozoic-Early Paleozoic sedimentary succession is better exposed in several quarries of dolomite (upper Villa Mónica Fm.) and limestone (Loma Negra Fm.) at the Sierras Bayas-Olavarría area. At the basal contacts of both carbonatic lithologies, fluorapatite-rich aggregates and secondary quartz-rich aggregates are commonly present. Previous works on the Olavarría area (Martínez et al., 2009; Gómez Peral et al., 2011) mention that hydrothermal activity affected the basement and the overlying stratigraphic sequence. The orthoquartzites of the Villa Mónica Fm. at the upper contact with the dolomite bed show clear textural evidences of dissolution and replacement of syntaxial quartz overgrowths and clasts by fluorapatite-rich aggregates (+ chlorite + sericite + secondary quartz + goethite). Secondary quartz-rich aggregates (+ chlorite + sericite + goethite) also fill dissolution cavities of the orthoquartzite. Finally, calcite veinlets cross-cut both types of aggregates and replace the syntaxial quartz overgrowth and clasts of the orthoquartzite. Ondulose extinction in quartz clasts and their syntaxial quartz cement points to slightly deformed rocks. Trails of secondary bi-phasic fluid inclusions (L+V) cross-cutting quartz clasts and their cement, can also be recognized. Under the microscope, the micritic dolomite of the stromatolithic dolomite bed is replaced by large idiomorphic dolomite crystals (50-80 microns), filled interstitially by secondary quartz + goethite + fluorapatite. The original stromatolithic structure of the dolomite is also destroyed by the presence of centimetric micro-breccias, which include clasts of corroded orthoquartzite, fluorapatite-rich aggregates and secondary quartz-rich aggregates. Late calcite cemented and partially replaced clasts and aggregates of the micro-breccias. The calcite cement contains small irregular patches of sericite and also exhibits low-temperature fluid inclusions. We suggest that the acid front of hydrothermal fluids is loaded with phosphate from the destruction of original apatite and monazite in the igneous-metamorphic basement rocks, as well as with fluoride from fluorite. The fluids migrated upwards through breccias and contacts to different stratigraphic levels and precipitated fluorapatite-rich aggregates (+ chlorite + sericite + secondary quartz), when the acid solutions were neutralized by carbonatic lithologies.