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Title: Rb-Sr and Sm-Nd whole rock analyses of basalts of the Grao Para Group, Serra dos Carajas, Brazil

Abstract

The Grao Para Group at Serra dos Carajas in the Guapore Shield of Brazil is a 6 km thick sequence of bimodal metavolcanic rock with interbedded iron formations. Rhyolites are a minor part of the low grade meta-volcanic section, with the bulk consisting of subalkaline basalt, basaltic andesite, and shoshonite. Rb-Sr whole rock analyses of 7 basalts yield an age of 2687 +/- 54 Ma. This agrees well with an age of 2758 +/- 39 Ma from zircon U-Pb analyses of the interbedded rhyolites. Except for the allocthonous Imataca Complex of Venezuela, these dates for the Grao Para Group are the first well-constrained Archean ages from the Amazonian Craton. An even older age for the basement to the Grao Para Group is also implied. Sm-Nd whole-rock analyses of four of these basalts did not yield an isochron because of the limited range of /sup 147/Sm//sup 144/Nd values, but element of/sub Nd/ values relative to CHUR were calculated using the Rb-Sr age. The typical LREE-enrichment, lack of Ce depletion or spilitic alteration of alkalies, and the high Sr initial ratios, may indicate that these isotopic patterns were derived by incorporation of some older continental crust in the mafic melts. These datamore » together with an element of/sub Sr/ of +63.6 might indicate significant seawater exchange with volcanic rocks derived from mantle with chondritic REE patterns. This demonstrates the presence of significant continental crust in the Archean and seawater-volcanic rock interaction in a rifting environment similar to modern analogs.« less

Authors:
; ;
Publication Date:
Research Org.:
Univ. of New Hampshire, Durham (USA)
OSTI Identifier:
6459197
Alternate Identifier(s):
OSTI ID: 6459197
Report Number(s):
CONF-8510489-
Journal ID: CODEN: GAAPB
Resource Type:
Conference
Journal Name:
Geol. Soc. Am., Abstr. Programs; (United States)
Additional Journal Information:
Journal Volume: 17; Conference: 98. annual meeting of the Geological Society of America, Orlando, FL, USA, 28 Oct 1985
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; BASALT; CHEMICAL COMPOSITION; PETROGENESIS; BRAZIL; GEOCHEMISTRY; GEOLOGIC FORMATIONS; ISOTOPE RATIO; NEODYMIUM ISOTOPES; PRECAMBRIAN ERA; QUANTITATIVE CHEMICAL ANALYSIS; RUBIDIUM ISOTOPES; SAMARIUM ISOTOPES; SEAWATER; STRONTIUM ISOTOPES; VOLCANISM; ALKALI METAL ISOTOPES; ALKALINE EARTH ISOTOPES; CHEMICAL ANALYSIS; CHEMISTRY; DEVELOPING COUNTRIES; GEOLOGIC AGES; HYDROGEN COMPOUNDS; IGNEOUS ROCKS; ISOTOPES; LATIN AMERICA; OXYGEN COMPOUNDS; RARE EARTH ISOTOPES; ROCKS; SOUTH AMERICA; VOLCANIC ROCKS; WATER 580400* -- Geochemistry-- (-1989); 580300 -- Mineralogy, Petrology, & Rock Mechanics-- (-1989)

Citation Formats

Olszewski, W.J. Jr., Gibbs, A.K., and Wirth, K.R. Rb-Sr and Sm-Nd whole rock analyses of basalts of the Grao Para Group, Serra dos Carajas, Brazil. United States: N. p., 1985. Web.
Olszewski, W.J. Jr., Gibbs, A.K., & Wirth, K.R. Rb-Sr and Sm-Nd whole rock analyses of basalts of the Grao Para Group, Serra dos Carajas, Brazil. United States.
Olszewski, W.J. Jr., Gibbs, A.K., and Wirth, K.R. Tue . "Rb-Sr and Sm-Nd whole rock analyses of basalts of the Grao Para Group, Serra dos Carajas, Brazil". United States.
@article{osti_6459197,
title = {Rb-Sr and Sm-Nd whole rock analyses of basalts of the Grao Para Group, Serra dos Carajas, Brazil},
author = {Olszewski, W.J. Jr. and Gibbs, A.K. and Wirth, K.R.},
abstractNote = {The Grao Para Group at Serra dos Carajas in the Guapore Shield of Brazil is a 6 km thick sequence of bimodal metavolcanic rock with interbedded iron formations. Rhyolites are a minor part of the low grade meta-volcanic section, with the bulk consisting of subalkaline basalt, basaltic andesite, and shoshonite. Rb-Sr whole rock analyses of 7 basalts yield an age of 2687 +/- 54 Ma. This agrees well with an age of 2758 +/- 39 Ma from zircon U-Pb analyses of the interbedded rhyolites. Except for the allocthonous Imataca Complex of Venezuela, these dates for the Grao Para Group are the first well-constrained Archean ages from the Amazonian Craton. An even older age for the basement to the Grao Para Group is also implied. Sm-Nd whole-rock analyses of four of these basalts did not yield an isochron because of the limited range of /sup 147/Sm//sup 144/Nd values, but element of/sub Nd/ values relative to CHUR were calculated using the Rb-Sr age. The typical LREE-enrichment, lack of Ce depletion or spilitic alteration of alkalies, and the high Sr initial ratios, may indicate that these isotopic patterns were derived by incorporation of some older continental crust in the mafic melts. These data together with an element of/sub Sr/ of +63.6 might indicate significant seawater exchange with volcanic rocks derived from mantle with chondritic REE patterns. This demonstrates the presence of significant continental crust in the Archean and seawater-volcanic rock interaction in a rifting environment similar to modern analogs.},
doi = {},
journal = {Geol. Soc. Am., Abstr. Programs; (United States)},
number = ,
volume = 17,
place = {United States},
year = {1985},
month = {1}
}

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