ISOTOPIC AND CHEMICAL EVIDENCE FOR THREE ACCRETIONARY MAGMATIC ARCS (1.79-1.42 GA) IN THE SW AMAZON CRATON, MATO GROSSO STATE, BRAZIL

Abstract

Twenty-one U/Pb ages of granitoids in the SW Amazon craton define three crustal accretionary events during the Paleo- and Mesoproterozoic that represent significant portions of the Rio Negro-Juruena Province and the Rondonian/San Ignacio Province. Two events refer to the Rio Negro-Juruena Province: The Alto Jauru greenstone belt comprises acid volcanics and tonalite to granite gneisses with U/Pb ages from 1790 to 1750 Ma. Sm/Nd isotopic data (e Nd(t) from +2.6 to +2.2 and T DM from 2.00 to 1.80 Ga) indicate a volcanic arc with juvenile signatures for these units. The second event (Cachoeirinha arc) comprises granites to tonalites with U/Pb ages from 1580 to 1530 Ma. Sm/Nd results (e Nd(t) . from +3,7 to -1,3 and T DM from 2.05 to 1.74 Ga) suggest a continental margin magmatic arc setting, with significant involvement of the Alto Jauru crust in the genesis of the respective magmas. The Santa Helena suite, which refers to Rondonian/San Ignacio Province, comprises granite to tonalite rocks, with calc-alkaline trends with U/Pb ages varying from 1460 to 1420 Ma and T DM from 1.70 Ma to 1.50 Ma (e from +4.1 to +2.6). They were generated in a magmatic arc adjacent to the older (1790 to 1530 Ma) continental margin that partially participated in the Santa Helena magma genesis.