Proterozoic geologic evolution of the SW part of the Amazonian Craton in Mato Grosso state, Brazil

This paper presents new geochronologic, isotopic, and geochemical data bear ing on the evolution of Proterozoic crust in SW Mato Grosso state, Brazil. which is at the southern end of the approximate to 1.6-1.8 Ga Rio Negro-Jur uena orogenic belt of the Amazonian Craton (Amazonia). Our data define thre e major crustal events: (i) the Alto Jauru terrane occurs in the eastern pa rt of the region and is comprised of island are-related rocks with U/Pb age s from 1.79 to 1.74 Ga. These rocks have epsilon (Nd)(t) values range from +2.8 to +2.0 with crustal residence ages (T-DM) from 1.93 to 1.78 Ga, indic ating a mainly juvenile signature at 1.8 Ga; (ii) the Cachoeirinha suite oc curs in the central part of the region and consists of calc-alkaline pluton s emplaced into Alto Jauru terrane host rocks. These plutons have U/Pb ages from 1.56 to 1.54 Ga and epsilon (Nd)(t) values ranging from +1.0 to -0.8, with crustal residence ages (T-DM) from 1.88 to 1.75 Ga. We interpret this orogen as the roots of a continental margin arc built upon basement compri sed of the Alto Jauru terrane, (iii) the Santa Helena batholith occurs in t he western part of the region and is a large. elongate body of calc-alkalin e rocks ranging from granodiorite to highly evolved granite. These units yi eld U/Pb ages from 1.45 Ma to 1.42 Ga. with epsilon (ND)(t) values ranging from +4.1 to +2.6 and crustal residence ages (T-DM) from 1.70 to 1.50 Ga. T he Rio Alegre domain occurs west of the Santa Helena batholith and includes juvenile 1.52 to 1.47 Ga volcanic and mafic plutonic rocks. Regional geolo gic relationships suggest that these rocks are part of the crust into which the Santa Helena batholith was emplaced. We interpret this batholith as th e magmatic core of a juvenile are accreted to the edge of the Alto Jauru te rrane. with incorporation of some older crust (Alto Jauru terrane) in the c ast and derivation from mainly juvenile crust (Rio Alegre domain) in the we st. The Rio Branco suite occurs to the east of exposed Alto Jauru terrane r ocks as large hills protruding through younger Aquapei Group sedimentary ro cks. It consists of gabbro and granophyric rocks with U/Pb ages of 1.47 Ga (gabbro) and 1.43 Ga (granophyre) and inherited older Nd, suggesting hinter land derivation from Alto Jauru terrane basement during development of the Santa Helena batholith. Several undeformed 1.5-1.4 Ga granitic plutons occu r within the Alto Jauru terrane. these are also regarded as inboard manifes tations of subduction related magmatism associated with accretion of the Ri o Alegre domain and formation of the Santa Helena batholith. The siliciclas tic Aguapei Group was deposited sometime between 1.0 and 1.4 Ga: it overlie s Jauru terrane basement. rocks of the Santa. Helena batholith, and the Rio Alegre domain. In the east, it is flat-lying and undeformed. but in the we st it is deformed and metamorphosed in the NNW trending Aguapei thrust belt . Farther west the Aguapei Group is horizontal, undeformed, and overlies th e Paleoproterozoic Paragua block in Bolivia, where it has been correlated w ith the Sunsas Group. The Aguapei thrust belt has K/Ar cooling ages of abou t 930 Ma and is apparently a foreland fold and thrust belt formed by reacti vation of an older rift basin during the 1.0 Ga Sunsas orogeny, which occur s west of the Paragua block in Bolivia. (C) 2001 Elsevier Science B.V. All rights reserved.