^(40)Ar/^(39)Ar constraints on the age and thermal history of the Urucum Neoproterozoic banded iron-formation, Brazil

Abstract

Urucum is one of the youngest banded iron-formations (BIFs) yet its exact age remains uncertain. ^(40)Ar/^(39)Ar geochronology on late-diagenetic to early metamorphic cryptomelane from the Urucum sequence reveals a minimum depositional age of 587 ± 7 Ma. Metamorphic braunite age spectra yield flat segments defining apparent ages of 547 ± 3 Ma to 513 ± 4 Ma, interpreted as recrystallization or cooling ages. Metamorphic muscovite grains from a meta-arkose interbedded with the BIF yield reproducible plateau ages of 513 ± 3 Ma. Cryptomelane ages are interpreted to record the minimum timing of the extensional tectonic events that generated the graben systems at the southeastern border of the Amazon craton, including the Chiquitos-Tucavaca aulacogen and the Jacadigo Basin, which hosts the Urucum banded iron-formation. Structurally controlled hydrothermal alteration resulted in crystallization of braunite and muscovite during the interval between 547 and 513 Ma coeval with post-collisional decompression that resulted in granite emplacement (e.g., São Vicente Granite) in the Paraguay Belt. Subsequent uplift and erosion of the Precambrian sequence possibly succeeded metamorphism. A chemically distinct cryptomelane generation identified in surface samples yield ca. 60 Ma results. These ages provide evidence for supergene recrystallization after exhumation in the Mesozoic/Cenozoic. ^(40)Ar/^(39)Ar systematics implies that the oldest cryptomelane samples experienced Ar-loss probably via thermally induced diffusion. This suggests that the Urucum BIF was buried and heated to significant temperatures in the late Proterozoic, consistent with present mineralogy and isotopic composition that do not reflect the original characteristics of the depositional environment.

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