Petrogenesis of the Paleoproterozoic (Orosirian) A-type granites of Carajás Province, Amazon Craton, Brazil: Combined in situ Hf[sbnd]O isotopes of zircon

Abstract

We present new LuHf and Oxygen isotopic analyses and complementary whole-rock Nd isotopic data for three A-type Paleoproterozoic suites (1880 Ma to 1857 Ma; Serra dos Carajás, Velho Guilherme, and Jamon) and related granites of the Carajás Province, eastern Amazon Craton, Brazil. Zircon grains from all the granites have remarkably restricted initial 176Hf/177Hf and strongly negative εHf(t) values ranging from −9 to −18, and δ18O fairly homogeneous varying from 5.50‰ to 7.00‰. Small differences were observed internally in the plutons or between them. These differences can result for contrasts in the crustal domains of the Carajás Province that were the source of the granites or of local contamination processes. Hf model ages indicate Paleoarchean sources (3.3 Ga to 3.6 Ga) with a minor contribution from Mesoarchean (3.0 Ga to 3.2 Ga) melts for these granites. This model ages are older than the exposed Archean country rocks of the Orosirian granites of the Carajás Province and more investigation is needed to verify the real existence of that older Archean crust. On the other hand, Nd isotope data and geochemical-petrological studies indicate that the Carajás granites derived from Mesoarchean granitoid sources and this hypothesis should be retained. The studied samples have HfO isotopic compositions that overlap within error, and evidence of contamination (crustal assimilation or mixing) for a mantle-derived magma cannot be seen. These plutons crystallized from magmas generated by melting of pre-existing igneous rocks with possibly in the Velho Guilherme Suite a minor contribution from a supracrustal (metasedimentary) component is envisaged. The Nd, Hf, and O isotope compositions of the Paleoproterozoic granites of Carajás Province clearly attest to an igneous ancient crustal source in the origin of their magmas. This anorogenic granites represent a large volume of granitic magmatism generated in the late Palaeoproterozoic in the eastern of the Amazon Craton. This work shows the importance of the Archean source in the generation of the A-type Paleoproterozoic granites from Carajás province. These granites are an example of A-type granites derived essentially from crustal sources without significant influence of mantle magmas except as a source of heat.