Sedimentation, metamorphism and granite generation in a back-arc region: Records from the Ediacaran Nova Venécia Complex (Araçuaí Orogen, Southeastern Brazil)

Abstract

The Nova Venécia migmatite-granulite-granite Complex (NVC) in the core of the Araçuaí Orogen (AO, 630–480Ma), southeast Brazil, exposes a mid-crustal section with abundant evidence for high-grade metamorphism linked to production, extraction and emplacement of peraluminous melts. Although the AO represents the textbook example of a confined orogen, there is surprisingly a lack of detailed studies on its metamorphic evolution related to widespread granitogenesis occurring from the Neoproterozoic to early Paleozoic. In this study, we combine U–Pb geochronology and metamorphic petrology to constrain the evolution of the NVC migmatitic metasedimentary granulites, from deposition to high-grade metamorphism, and to correlate the metamorphic history of the terrain with the several episodes of granite magmatism (G1–G5) in the AO. The sedimentation of the NVC can be bracketed within a maximum 13My period, between its maximum depositional age at ca. 606Ma and the intrusion of early syn-collisional granitoids at 593Ma. Compilation of available U–Pb data shows that the bulk of the magmatic rocks in the AO (G1+G2 rocks) crystallized contemporaneously over a period of 15 My (ca. 595–570Ma) with a peak at ca. 575Ma. Although it is inferred a protracted period of crustal heating in the AO (from ca. 640–480), U–Pb ages of metamorphic and magmatic zircons and monazites suggest at least two major heat pulses at ca. 593–560 and 523–495Ma. The timing of peak regional metamorphism is constrained from 575 to 560Ma, which temporally overlaps with the crystallization of the youngest granitoids. Phase equilibrium modeling of metasedimentary granulites from three different localities within the NVC, indicates that all areas record similar peak P–T conditions of 750–850°C and 5300–7500bar. This is followed by high temperature retrograde evolution to 640–800°C and 4500–6000bar. A post-collisional thermal event linked to the intrusion of large norite bodies (520–480Ma) is recorded in our metagreywackes (monazite U–Pb) and in granites (monazite and zircon U–Pb) from 523 to 495Ma.