Supracrustal rocks of the Ami Group, together with coeval A-type granites r
epresent a ca. 1.77-1.58 Ga old continental rift in Brazil. Two granite fam
ilies are identified: the older (1.77 Ga) group forms small undeformed plut
ons, and the younger granites (ca. 1.58 Ga) constitute larger, deformed plu
Sr:Nd isotopic data for these rocks indicate that the magmatism is mostly p
roduct of re-melting of Paleoproterozoic sialic crust. Initial Sr ratios fo
r both granite families are ca 0.726 and 0.720. Most TDM model ages are bet
ween 2.58 and 1.80 Ga. epsilon (Nd)(T) values are between +3.6 and -11.9. A
rai volcanics are bimodal, with basalts and dacites/rhyolites interlayered
with continental sediments. The felsic volcanics show Nd isotopic character
istics which are very similar to the granites, and are also interpreted as
reworking of Paleoproterozoic crust.
Detrital sediments of the Arai Group revealed T-DM model ages between 2.4 a
nd 2.16 Ga, indicating that they are the product of erosion of Paleoprotero
The data indicate that the Arai rift system was established on crust that h
ad just become stable after the Paleoproterozoic orogeny.