Palaeoproterozoic tectonic evolution of the Alto Tererê Group, southernmost Amazonian Craton, based on field mapping, zircon dating and rock geochemistry

Abstract New geochemical and geochronological U–Pb and Sm–Nd data from amphibolites of the Alto Terere Group, which are of Palaeoproterozoic age, are presented. The amphibolites are exposed in the central-eastern portion of the Rio Apa Block, southern Amazonian Craton, Mato Grosso do Sul, Brazil, and are composed of hornblende, plagioclase, quartz, biotite, cummingtonite and epidote. The amphibolites are subdivided into three lithofacies: (i) thinly banded amphibolites (metabasalts), (ii) coarse- and medium-grained amphibolites with relic subophitic texture (metagabbros), and (iii) amphibolites with relic cumulate texture (metapyroxenites). Chemical data also suggest the subdivision of the amphibolites into three different types. These rocks yield a U–Pb zircon age of 1768 ± 6 Ma and are therefore older than rocks of part of the Rio Apa Complex. Their Sm–Nd model ages range between 2.89 and 1.88 Ga, and their eNd (T) values range between −3.40 and + 3.74. Chemical analyses of these rocks indicate SiO 2 concentrations between 45.23 and 50.65 wt.%, MgO concentrations between 4.34 and 8.01 wt.%, TiO 2 concentrations between 0.91 and 1.74 wt.%, weakly fractionated rare-earth element (REE) patterns with mild depletion in heavy REEs, enrichment in large-ion lithophile elements (LILEs) and high-field-strength element (HFSEs), negative Nb, Ta and Co anomalies, positive Ba and Pb anomalies, low Ce concentrations, high Rb/Y ratios and low Th/La and Hf/Sm ratios. These features reflect metasomatism of the mantle wedge produced by sediments from the subducted plate. Various degrees of melting mark the evolution of the parent basic magmas, although subordinate crustal contamination may also have occurred. The geochemical signature of the amphibolites corresponds to that of tholeiitic basalts generated in an extensional back-arc-basin environment. The deposition in the basin apparently ceased during the first episode of compression and deformation at approximately 1.68 Ga, and the main deformation was most likely related to the San Ignacio Orogeny, which dates to approximately 1.3 Ga.