Resumo

MARTINEZ, Juan C et al. Late-Neoproterozoic hydrothermal fluid activity in the Tandilia belt, Argentina. Rev. Asoc. Geol. Argent. [online]. 2013, vol.70, n.3, pp.410-426. ISSN 0004-4822.

In the Barker - Villa Cacique area, Tandilia belt, alteration mineral assemblages were studied by petrography, XRD and EMPA at three different stratigraphic levels: (1) a phyllic alteration at the unconformity palaeoproterozoic basement-neoproterozoic sedimentary succession (TLPU); (2) an advanced argillic alteration in the Las Águilas Formation (middle level); and (3) a phyllic alteration on pyroclastic rocks of the Olavarría Formation (upper level). Special emphasize was placed on the chemical characterization of K-white micas and chlorites. Secondary K-white micas of altered migmatites, from the Las Aguilas and Olavarría Formations have a low paragonite content (Na* <0.13) with a distinctly higher phengite content (Si/Al(tot) ~1.6) in the latter. Secondary Fe-chlorites in arenites at the TLPU and from pyroclastic rocks of the Olavarría Formation have a high Fe/(Fe+Mg) ratio between ~0.80 and 0.88. K/Ar mineral ages (K-white micas and Na-bearing alunite) constrained the hydrothermal fluid flow between ca. 590 and 620 Ma, post-dating diagenesis. Illite crystallinity and chlorite geothermometry of altered and dated rocks suggest upward decreasing temperatures from ~350 - 300 ºC to ~250 ºC, markedly higher than the expected diagenetic temperatures and correlative with unlikely deep depths (>9.5 km) for the entire sedimentary pile. Hot fluids would rise from deep-seated realms, metamorphic and/or hidden igneous sources. A correlation with a Brasiliano thermo-tectonic event is hypothesized.

Palavras-chave : K-white mica and chlorite chemistry; Kübler Index; Geothermometry; Hydrothermal-diagenetic and metamorphic realms.

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