Palaeoproterozoic assembly of the São Francisco craton, SE Brazil : new insights from U–Pb titanite and monazite dating.

Abstract

Isotopic U–Pb titanite and monazite data from the southern São Francisco craton better constrain the timing of the tectono-thermal event that led to the amalgamation of the craton and the crust that forms the basement of its fringing orogenic belts in the Palaeoproteroic. The data obtained from assemblages exposed in the Quadrilátero Ferrífero mining district and adjacent Palaeoproterozoic Mineiro belt magmatic arc terrane reveal two different age populations: a first metamorphic event between 2772 and 2613 Ma, followed by recrystallization or/and isotopic resetting in the interval of 2080–1940 Ma. The partial preservation of Neoarchaean ages in the craton interior suggests that the Palaeoproterozoic metamorphism did not exceed the minimum closure temperature for the titanite (between 650 C and 700 C), which is also reinforced by the absence of Palaeoproterozoic metamorphic zircons in the southern São Francisco craton. Combining new and existing Palaeoproterozoic data, we infer that the Archaean crust of the southern craton as well as the surrounding magmatic arcs were affected by a long-lived metamorphic event from ca. 2100 Ma to 1940 Ma. These age interval includes an episode of syn-collisional metamorphism between 2100 and 2070 Ma, which represents the amalgamation of the Archaean nuclei of both the São Francisco and Congo cratons, along with magmatic arcs and microcontinents. This collision led to closure of the large Palaeoproterozoic Minas basin, followed by orogenic collapse and development of a dome-and-keel architecture in time interval of 2070–2050 Ma. A period of slow cooling ( 1 C/Ma) succeeded these events and lasted until ca. 1940 Ma. Our results correlate with the Palaeoproterozoic metamorphic ages obtained in the various blocks forming the northeastern sector of the craton.