New Sm/Nd data from different lithologic units of the Ribeira Belt (RB), southeastern Brazil, provide important information about the crustal architecture and geologic evolution of the region. Nd data of basement gneisses of the RB indicate that they represent Paleoproterozoic crustal growth, whereas those of the supracrustal sequences suggest provenance from both Paleoproterozoic and younger sources. On the basis of their Nd signatures, felsic brasiliano plutons derived largely from Paleoproterozoic basement, but systematic variations in their T DM model ages as a function of their chemistry may reflect differences in fractionation, vertical lithospheric heterogeneity, or possibly some contributions from the asthenospheric mantle.
The production of juvenile continental crust was larger in the Proterozoic than Archean. During Proterozoic the production was about 64%, while during Archean was around 34% of the total volume. The great accretion from mantle to crust happened between 2,2 to 1,9Ga corresponding 35% of the total volume of the actual continental crust. During Archean an intense recycling between mantle and continental crust took place, while in Paleoproterozoic the rate of mantle accretion to continental crust was larger than the assimilation. During MesoNeoproterozoic little accretion of juvenile material occurred, predominating crustal reworking. Consequently the chemical depletion rate (fractionation) of the upper mantle was smaller in Archean than Proterozoic. The isotopic data show a certain increase of the depletion degree of Sm-Nd and Rb-Sr in the upper mantle that began at 2.2 Ga as a reflex of the strong differentiation of the upper mantle to continental crust.
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