Protracted (>60 Myrs) thermal evolution of a Neoproterozoic metasedimentary sequence from eastern Borborema Province (NE Brazil): Thermal and rheological implications for orogenic development

Abstract

The thermal evolution of orogenic belts is quite varied. While some show rapid cooling, others record high temperatures for tens of millions of years. Here, we report the latter situation based on U-Pb zircon and monazite dating of paragneiss and micaschist from the Surubim Complex (eastern Borborema Province, NE Brazil). The Borborema Province and its African counterparts (Cameroon belt and Nigerian Shield) formed during amalgamation of West Gondwana in the Late Neoproterozoic (Brasiliano/Pan-African Orogeny). The location of the Surubim Complex in the central core of this large orogenic system offers the opportunity to study tectonic processes at work in the inner regions of ancient orogens. Basement-involved thick-skinned tectonics, regional low-angle foliation related to NW-directed thrusting, transcurrent shear zones and intrusion of granitic plutons characterize the Brasiliano Orogeny in the study area. Zircon data indicate provenance of the Surubim Complex mainly from proximal sources, predominantly 2.2–2.0 Ga-old basement rocks (including an orthogneiss here dated at 2132 ± 11 Ma), and from Neoproterozoic extension-related granitoids. The ages of the youngest detrital zircon grains range from 690 to 650 Ma. The weighted mean age (647 ± 4 Ma) of low Th/U zircon overgrowths in one sample is tentatively interpreted as dating syn-extensional metamorphism given the small age difference to the youngest detrital zircon population. A time lag of c. 17 Myrs, between 647 Ma and 630 Ma, suggests a temperature drop due to contractional deformation. The ages of the oldest magmatic grains from a leucosome sample and of monazite grains from a paragneiss sample, together with data from the literature, show that temperatures high enough to promote local anatexis during development of the regional low-angle foliation were reached around 630 Ma. Afterwards, as indicated by zircon and monazite growth and/or recrystallization, temperatures remained relatively elevated until the transition to the transcurrent regime, at c. 590 Ma, and persisted so up to c. 563 Ma. The results outline a protracted thermal history spanning more than 60 Myrs. This evolution is comparable to the tectonothermal evolution of long-lived hot orogens, where persistence of high temperature conditions for a long time has been related to the formation of orogenic plateaus.