Proterozoic crustal evolution at the southern margin of the amazonian craton in the state of mato grosso, Brazil: evidence from RbSr and K:Ar data

Abstract

Abstract Geologic mapping on a 1:100,000 scale in the Sao Jose dos Quatro Marcos (SJQM) area, southwest of the state of Mato Grosso, central-western Brazil, showed a remarkable variety of Precambrian lithologic units. Predominant are grey tonalitic gneisses with a RbSr isochron age of 1971±770 Ma, an initial 87 Sr 86 Sr ratio of 0.7071, and a KAr biotite cooling age of 1500 Ma. The gneisses gneisses show interlayered “first-generation” amphibolite lenses and bands (KAr amphibole age of 1500 Ma), and some rare relicts of grey gneissic granulites, together with associated pink augen gneisses. Rare outcrops of deformed banded calc-silicate rocks are recrystallized by contact metamorphism. These units are intruded by a granitoid suite constituted by early, usually somewhat deformed tonalites and granodiorites, and late massive pink granodiorites and monzo- and syegogranites (RbSr isochron age=1472 ± 19 Ma, i.r.=0.7037). They show enclaves of grey gneisses and “second-generation” amphibolites. A large amphibolite body to the west is also considered part of this second-generation basic sequence. Scattered rhyolite outcrops to the east represent the southernmost remnants of the volcanic Rio Branco Group. The grey gneisses, here formally identified as the ”Sao Jose dos Quatro Marcos Gneiss“ and probably derived from Lower Proterozoic tonalites, were complexly deformed and recrystallized, at ∼ 2.0 Ga, into predominantly amphibolite-facies rocks. The first- and second-generation amphibolites document two distinct commingling episodes, separated by at least 0.5 Ga, each directly associated with the corresponding host-rock granitoid magmatism. The porphyritic granites were converted into pink augen gneisses before 1500 Ma and are thus not contemporaneous to the 1380 Ma syn-kinematic San Ignacio granitoids from eastern Bolivia. Calc-silicate rocks are interpreted as remnants of supracrustal sequences, perhaps related to the Quatro Meninas Group. The 1500 Ma granitoids, formally named the “Jaboti Intrusive Suite”, invaded under a syn- to late-kinematic regime, determining a unique thermal-magmatic event in this region.