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

Conference · Tue Jan 01 04:00:00 EST 1985 · Geol. Soc. Am., Abstr. Programs; (United States)

OSTI ID:6459197

Olszewski, Jr, W J; Gibbs, A K; Wirth, K R

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.

• Research Organization: Univ. of New Hampshire, Durham (USA)
• OSTI ID: 6459197
• Report Number(s): CONF-8510489-
• Conference Information: Journal Name: Geol. Soc. Am., Abstr. Programs; (United States) Journal Volume: 17
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580300 -- Mineralogy
Petrology
& Rock Mechanics-- (-1989)
580400* -- Geochemistry-- (-1989)
ALKALI METAL ISOTOPES
ALKALINE EARTH ISOTOPES
BASALT
BRAZIL
CHEMICAL ANALYSIS
CHEMICAL COMPOSITION
CHEMISTRY
DEVELOPING COUNTRIES
GEOCHEMISTRY
GEOLOGIC AGES
GEOLOGIC FORMATIONS
HYDROGEN COMPOUNDS
IGNEOUS ROCKS
ISOTOPE RATIO
ISOTOPES
LATIN AMERICA
NEODYMIUM ISOTOPES
OXYGEN COMPOUNDS
PETROGENESIS
PRECAMBRIAN ERA
QUANTITATIVE CHEMICAL ANALYSIS
RARE EARTH ISOTOPES
ROCKS
RUBIDIUM ISOTOPES
SAMARIUM ISOTOPES
SEAWATER
SOUTH AMERICA
STRONTIUM ISOTOPES
VOLCANIC ROCKS
VOLCANISM
WATER