Geodynamic evolution of the Southern Brasília orogen (SE Brazil): new petrochronological insights from UHT and HP metamorphic rocks

Abstract

The Southern Brasília orogen is one of the orogens formed around the São Francisco paleocontinent during the West Gondwana amalgamation. Its formation is interpreted to have been the result of the subduction of the passive margin of the São Francisco plate under the Paranapanema plate, which subsequently evolved into a collisional orogeny that formed nappe systems verging to the east. Certain tectonic domains play a key role towards understanding the subduction to collision evolution of the southernmost portion of the Southern Brasília orogen, including the ultra-high temperature metamorphosed magmatic arc rocks of the Guaxupé nappe, of which ages and isotopic signatures of magmatism and metamorphism are poorly constrained and for which the heat source for metamorphism remained a matter of debate, as well as the retro-eclogites (e.g., from Pouso Alegre), which as (ultra)-high pressure mafic rocks can provide insights into subduction, collision and exhumation processes. As a general characteristic of the rocks from the Guaxupé nappe, zircon data spread in a Concordia diagram. Textural analyses were thus coupled to U-Pb zircon chronology and Lu-Hf ratios to determine the crystallization age of their protoliths. It was thereby revealed that this nappe hosts, besides magmatic arc rocks, a so far undescribed basement rock and a ca. 790 Ma opdalite of uncertain origin. A banded mafic granulite displays a crystallization age of 2559 ± 66 Ma for a juvenile protolith (Hf (2550) =+2.7 to +10.0). Most of its zircon rims record the Brasiliano Event at ca. 600700 Ma and 15 % of all grains register an unusual event at ca. 2.4 Ga. The same approach applied to the opdalite revealed the first 786 ± 10 Ma-igneous rock in the Guaxupé nappe (Hf(786)= -12.6 to -13.4). Younger magmatic activity, which is ascribed to continental arc activity, is recorded by a banded mafic granulite at 691 ± 3 Ma (Hf(690) = -8.2 to -12.6) and by mafic granulite enclaves, synchronous to anatexis, at 664 ± 9 Ma (Hf(660)= -5.9 to -11.4). Comparison of the relative distribution of metamorphic zircon data from different samples permitted the identification of an 80 m.y. long-lived metamorphism with three main zircon crystallization stages: (i) 670650 Ma, (ii) 640630 Ma, and (iii) 615590 Ma. The first stage is interpreted as subduction-related, with basic magma emplacement that triggered ultra-high temperature metamorphism. The two following stages were linked to collision and exhumation. Thus, the ultra-high temperature (998 ± 23 ºC obtained by orthopyroxene-clinopyroxene thermometry) is interpreted to have taken place in an arc-related setting. Peak metamorphic conditions of 690 ± 35 °C and 13.5 ± 2.8 kbar for the retro-eclogite were obtained by phase equilibria modelling, and compositions of the garnet and omphacite (Jd20, reconstructed composition). Monazite and metamorphic zircon from the host rocks were dated at ca. 630 Ma and their growth was linked to peak metamorphic conditions similar to the ones recorded in the retro-eclogite. Local retrogression into symplectite and corona proceeded coevally at 595 ± 25 °C and 4.8 ± 1.5 kbar. A zircon age of 603 ± 7 Ma obtained for metamorphic zircons rims was linked to retrogression. The low maximum pressure of 14 kbar and the high geothermal gradient do not necessarily support subduction process-related metamorphism but rather favor a continental collision setting. The calculated exhumation rate of 1 mm.y1 for the clinopyroxene-garnet amphibolite, and high geothermal gradient for the symplectite and corona formation stage indicate that exhumation was tectonically driven. The combined investigation on rocks from the Guaxupé nappe and Pouso Alegre permitted constraining a subduction-related metamorphism from 670640 Ma, which was followed by peak-collisional metamorphism at ca. 630 Ma and decompression accompanied by widespread migmatization until ca. 600 Ma.