Thermal aureole provoked by the intrusion of the Totoró pluton over micaschists of the Seridó Group, Ediacaran of the Borborema Province, NE Brazil

Abstract

The Borborema Province, NE Brazil, is marked by significant Ediacaran plutonism. In this geological context, the Totoró pluton, Rio Grande do Norte, produces an extensive metamorphic aureole along the contact with micaschist of the Seridó Group. This contribution reports data from field work, petrography, mineral chemistry and rock petrophysics aiming to characterize this aureole. The Totoró pluton is an NNE-SSW elongate mafic to felsic body including diorite, gabbro-norite, granodiorite/tonalite (the main facies), biotite-bearing equigranular to porphyritic granite, and fine- to medium-grained granite. Geothermometric calculations based on mineral chemistry permitted estimating pressures of 1.6 – 2.7 kbar and temperature of 800 – 900ºC for the emplacement of the pluton. The thermal aureole can reach up to 2 km away from the contact and consists of: (i) migmatized schist immediately adjacent to the contact, often with leucosomes subconcordant with the S2 tectonic fabric of the host rocks; (ii) an intermediate zone with sillimanite (± cordierite + garnet + biotite); (iii) a zone with cordierite (± staurolite ± andalusite + garnet + biotite); (iv) an external zone of fine-grained micaschist. Petrophysical modelings reveal that the temperature next to the pluton reached 688 – 756ºC. Minerals of the metamorphic aureole define a low angle S2 schistosity (D2/M2 event in amphibolite to pyroxene hornfels facies), which are overprinted by open to tight folds, that evolve via strong transposition to generate mylonites in NE-SW to N-S directed shear zones, which represent the last ductile event (D3/M3). The results indicate that some regional batoliths (e.g., Acari, Totoró, Umarizal) were emplaced in a relatively shallow crust that provided locally very high geothermal gradient.