Confronting Episodic vs. Continuous Heat Sources in Long-Lived Hot Orogens: Insights From Petrochronological Studies in the Nova Venecia Complex, Araçuaí Orogen (SE Brazil)

Abstract

Orogenic belts that sustain elevated temperatures at intermediate crustal depths for tens of millions of years are known as hot orogens. The evolution of these hot orogens is largely influenced by thermal maturation, primarily driven by the distribution of heat-producing elements (HPEs), such as K, Th and U in the overthickened crust. This process involves widespread anatexis, granulite facies metamorphism, extensive transfer of fluids and magma and large-scale crustal flow. Although most of the thermal evolution of hot orogens is controlled by HPEs, episodic heat transfer may also contribute to their geodynamic development. The Araçuaí orogen, located in southeastern Brazil, represents a Neoproterozoic hot orogen that exposes in its internal domain deeper levels of an overthickened crust, resembling the Tibetan plateau. Its evolution involves more than 120 million years of magmatism and metamorphism, including ultra-high-temperature metamorphism. Different geodynamic models have been suggested to explain the thermal evolution of this orogen, ranging from episodic heat sources within subduction-collisional orogeny to continuous long-lived heat sources in collisional or intracontinental settings. In this paper, we integrate detailed thermodynamic modelling, Lu–Hf and Sm–Nd garnet dating, and U–Pb and REE zircon data from an outcrop that includes granulite facies metasedimentary rocks (Nova Venecia complex) and an intrusive gabbroic stock (São Gabriel pluton). This key outcrop allows us to investigate the history of high-grade regional metamorphism within the internal domain of the Araçuaí orogen, as well as the thermal impact of the late-stage high-temperature magmatism on the later evolution of this hot orogen. Our data indicate that the studied rocks reached near-peak conditions (6.5–10 kbar and ~800°C) at 535–530 Ma and followed a near-isothermal decompression path to 5–6 kbar at ~530–520 Ma. Only local effects of contact metamorphism (~5 kbar and 900°C–1000°C) were observed along the contact between the gabbroic stock and its host rock. Based on this newly integrated dataset and the compilation of existing information, we argue that the Araçuaí orogen evolved from a single, continuous, long-lived heat source controlled by radiogenic decay from HPEs, rather than episodic advective heating from subduction/collision-related processes.