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

Abstract

This paper presents new geochronologic, isotopic, and geochemical data bearing on the evolution of Proterozoic crust in SW Mato Grosso state, Brazil, which is at the southern end of the ≈1.6–1.8 Ga Rio Negro–Juruena orogenic belt of the Amazonian Craton (Amazonia). Our data define three major crustal events: (i) the Alto Jauru terrane occurs in the eastern part of the region and is comprised of island arc-related rocks with U/Pb ages from 1.79 to 1.74 Ga. These rocks have ε Nd( t) values range from +2.8 to +2.0 with crustal residence ages ( T DM) from 1.93 to 1.78 Ga, indicating a mainly juvenile signature at 1.8 Ga; (ii) the Cachoeirinha suite occurs in the central part of the region and consists of calc-alkaline plutons emplaced into Alto Jauru terrane host rocks. These plutons have U/Pb ages from 1.56 to 1.54 Ga and ε 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 comprised of the Alto Jauru terrane; (iii) the Santa Helena batholith occurs in the western part of the region and is a large, elongate body of calc-alkaline rocks ranging from granodiorite to highly evolved granite. These units yield U/Pb ages from 1.45 Ma to 1.42 Ga, with ε Nd( t) values ranging from +4.1 to +2.6 and crustal residence ages ( T DM) from 1.70 to 1.50 Ga. The 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 geologic relationships suggest that these rocks are part of the crust into which the Santa Helena batholith was emplaced. We interpret this batholith as the magmatic core of a juvenile arc accreted to the edge of the Alto Jauru terrane, with incorporation of some older crust (Alto Jauru terrane) in the east and derivation from mainly juvenile crust (Rio Alegre domain) in the west. The Rio Branco suite occurs to the east of exposed Alto Jauru terrane rocks as large hills protruding through younger Aquapeı́ Group sedimentary rocks. 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 hinterland derivation from Alto Jauru terrane basement during development of the Santa Helena batholith. Several undeformed 1.5–1.4 Ga granitic plutons occur within the Alto Jauru terrane; these are also regarded as inboard manifestations of subduction related magmatism associated with accretion of the Rio Alegre domain and formation of the Santa Helena batholith. The siliciclastic Aguapeı́ Group was deposited sometime between 1.0 and 1.4 Ga; it overlies 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 west it is deformed and metamorphosed in the NNW trending Aguapeı́ thrust belt. Farther west the Aguapeı́ Group is horizontal, undeformed, and overlies the Paleoproterozoic Paragua block in Bolivia, where it has been correlated with the Sunsás Group. The Aguapeı́ thrust belt has K/Ar cooling ages of about 930 Ma and is apparently a foreland fold and thrust belt formed by reactivation of an older rift basin during the 1.0 Ga Sunsás orogeny, which occurs west of the Paragua block in Bolivia.