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

Piacentini, Thiago and Vasconcelos, Paulo M. and Farley, Kenneth A. (2013) ^(40)Ar/^(39)Ar constraints on the age and thermal history of the Urucum Neoproterozoic banded iron-formation, Brazil. Precambrian Research, 228 . pp. 48-62. ISSN 0301-9268.

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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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Farley, Kenneth A.0000-0002-7846-7546
Additional Information:© 2013 Elsevier B.V. Received 6 January 2012; Received in revised form 3 December 2012; Accepted 3 January 2013; Available online 16 January 2013. This research project is possible thanks to the grant issued to T.P. by CNPq (GDE200895/2009-5). We would like to express our gratitude to the mining companies of the Urucum district (Urucum Mineração, Mineração Corumbaense Reunida, and Vale) for allowing us access and geological information. We are grateful to Xiaodong Deng, from the GPMR-CUG for the SEM images. We are also grateful to staff of the CMM-UQ, particularly Ronald Rasch and Ying Yu for their assistance during the EMPA analysis, and, Benjamin Cohen and David Thiede (UQ-AGES), during the isotopic analysis. T.P. also thanks Bernardo Freitas for insightful discussions. The painstaking review of Umberto Cordani, an anonymous reviewer, and the editor Randal Parrish, greatly improved the manuscript.
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Subject Keywords:Ar-40/Ar-39; Neoproterozoic; Banded iron-formation; Urucum; Brasiliano
Record Number:CaltechAUTHORS:20130528-090838756
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Official Citation:Thiago Piacentini, Paulo M. Vasconcelos, Kenneth A. Farley, 40Ar/39Ar constraints on the age and thermal history of the Urucum Neoproterozoic banded iron-formation, Brazil, Precambrian Research, Volume 228, May 2013, Pages 48-62, ISSN 0301-9268, 10.1016/j.precamres.2013.01.002.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:38680
Deposited By: Tony Diaz
Deposited On:03 Jun 2013 18:30
Last Modified:03 Mar 2020 13:01

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