Ultra‐high temperature, mid‐crustal level, contact metamorphism imprinted on granulite facies paragneisses by a norite intrusion (São Gabriel da Baunilha, Araçuaí orogen, southeast Brazil)

Abstract

AbstractLarge and hot intrusions may imprint relatively wide aureoles of ultra‐high temperature (UHT >900°C) contact metamorphism even on granulite facies rocks at mid‐crustal levels, especially on those still somewhat hydrated rocks like biotite‐rich paragneisses. After reaching UHT conditions, even followed by rapid cooling, such contact aureoles record reaction textures and newly formed mineral assemblages. Near intrusive contacts, high‐T to UHT contact aureoles present highly recrystallized hornfelsic rocks exhibiting conchoidal fracture that may also preserve inherited features. This paper describes a contact metamorphic aureole developed under high to ultra‐high temperatures at intermediate P conditions, imposed by a mafic intrusion on granulite facies paragneiss. In the Espírito Santo State (SE Brazil), many post‐collisional intrusions (the 525–480 Ma G5 supersuite) occur in the back‐arc region of the Araçuaí orogen. Among them, a greatly homogeneous norite forms a large intrusion hosted by Al‐rich migmatitic paragneisses previously metamorphosed to granulite facies during the collisional stage. Zircon U‐Pb (SHRIMP) data yield the age of 523 ± 7 Ma for the norite crystallization. This intrusion imprinted a contact metamorphism aureole on the migmatitic paragneiss, forming Al‐rich hornfelses free of biotite, near the intrusive contact, followed by recrystallized garnet‐rich rocks with relict gneissic fabrics progressively enriched in biotite towards the distal aureole. The Al‐rich hornfelses are very tenacious, massive, fine‐grained rocks with conchoidal fracture, resinous lustre, intergrowth of feldspars, simplectites of cordierite and hercynite, and show an increase calcium content in plagioclase towards the intrusion. The hornfelses comprise two mineral assemblages: (I) plagioclase–garnet–orthopyroxene–quartz and (II) plagioclase–garnet–quartz–cordierite–sillimanite–hercynite–K‐feldspar. Calculated temperatures for host rocks range from ~1,010°C, for a hornfels collected at 1 m from the intrusive contact, to ~732°C, for a paragneiss located at 262 m from the intrusion. Using Theriak‐Domino software, phase‐equilibrium modelling for the hornfels assemblage (II) provides temperatures between 970 and 1,010°C, at pressures from 5.6 to 6.0 kbar, characterizing UHT contact metamorphism at mid‐crustal level (21–22 km depth). Neocrystallized zircons with soccer ball morphology aged at 520 ± 19 Ma, and P–T conditions of 845°C at 6.35 kbar for a recrystallized paragneiss record the contact metamorphism at least up to 250 m distance from the intrusion. The presented data constrain a P–T–t peak ~5.8 kbar – 990°C – 520 Ma for the contact metamorphism while the P–T–t peak at 7.5 kbar – 785°C – 560 Ma was related to the collisional metamorphism of regional paragneisses in the literature. Therefore, the pressure–time relation between those metamorphic peaks gives a pressure difference of 1.7 kbar (~6.3 km thickness of continental crust) over 40 Ma, corresponding to low decompression (42.5 bar/Ma) and exhumation (157 m/Ma) rates from the collisional peak to the post‐collisional stage, in the studied region.