Rb—Sr and Sm—Nd geochronology of lower Proterozoic granite—greenstone terrains in French Guiana,South America |
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| Authors: | Gerard Gruau Herve Martin Bernard Leveque Raymond Capdevila Alain Marot |
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| Institution: | 1. CAESS, Université de Rennes, Institut de Géologie, 35042 Rennes Cedex France;2. BRGM, BP 552, 97302 Cayenne Cedex, Guyane France |
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| Abstract: | Nine samples of metavolcanic rock from the lower parts of greenstone belts in central French Guiana (the Paramaca series) and 14 granitic samples from the intrusive gneisses (the Degrad Roche and Arawa gneisses) were selected for Sm—Nd and Rb—Sr analysis.The Sm—Nd results from the metavolcanic series (including two tholeiites, five peridotitic komatiites and two andesites) yield an isochron age of 2.11±0.09 (2 σ) Ga with an initial ratio (INd) of 0.51002±9 (2 σ), corresponding to ?Nd(T) = + 2.1 ± 1.8. This isochron is interpreted as representing the age of initial volcanism of the Paramaca series. Acid intrusives were dated by the Rb—Sr method. A whole rock Rb—Sr isochron, including data points from both the Degrad Roche and Arawa gneisses, yields an age of 2.00±0.07 (2 σ) Ga with initial ratio (ISr value) of 0.7019±4 (2 σ). This result is considered to be the time of emplacement of the orthogneiss protoliths.The positive εNd value (+ 2.1 ± 1.8) obtained from the metavolcanic rocks of French Guiana suggests that their mantle sources have evolved in reservoirs slightly depleted in Light Rare Earth Elements (LREE). This result confirms the possible existence of ancient LREE-depleted reservoirs within the lower Proterozoic mantle. Moreover, the high εNd(T) value for these rocks excludes any significant crustal contamination during magma genesis.The French Guianese orthogneisses yield a low ISr value (0.7019±4 (2 σ)) which, together with geochemical considerations, suggests that their granitic protoliths could have originated by partial melting of short-lived crustal precursors of basaltic to granodioritic composition.The present geochronological and isotopic study suggests that the Guiana Shield may represent a major continental accretion event during the lower Proterozoic. |
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