Geochemistry and Sm-Nd isotopic characteristics of bimodal volcanic rocks of Juscelandia, Goias, Brazil: Mesoproterozoic transition from continental rift to ocean basin

The ca. 1.28 Ga Juscelandia Sequence, in Goias, Brazil, comprises a unit of bimodal metavolcanic rocks, amphibolite and felsic gneiss, intercalated with metapelite and metachert. The bimodal nature of the magmatism, is evident as the metavolcanic rocks have basalt and rhyolite compositions. Bimodality is also shown by the trace elements and Nd isotope data. Amphibolites yield positive epsilon(Nd)(T) values that can be separated into two groups. In the first group, amphibolite samples from lower and middle portion of the sequence are characterized by epsilon(Nd) between +2.8 and +5.4. These fairly positive values and enrichment in LREE and LILE suggest they were derived from a depleted mantle source with subsequent assimilation of continental crustal material. The second group is formed by amphibolite of the upper portion of the sequence that have EM around +6.0 and their low LILE and LREE contents resemble normal mid-ocean ridge basalts (N-MORB). Initial Nd-143/Nd-144 ratios represent the composition of the depleted mantle at 1.28 Ga. Felsic rocks yield negative epsilon(Nd)(T) values, between ca. -2.8 and -4.5, and Paleoproterozoic model ages (T-DM values in the interval between 1.8 and 2.1 Ga). The original felsic magmas represent the product of melting of older (Paleoproterozoic) continental crust with trace element chemistry indicating that the felsic magmas were the product of continental crustal melts with small mantle contribution. Previous models proposing an oceanic setting for the origin of magmatism, of the Juscelandia rocks, within a mid-ocean ridge or back-arc basin are not supported by the data presented here. Transition from a continental rift to an ocean basin represents the most likely tectonic setting for the origin of the Juscelandia rocks. (C) 2003 Published by Elsevier Science B.V.