Metamorphic path constrained by metapelitic rocks from the inner Aiuruoca-Andrela ˆndia nappe, south of the Sa ˜o Francisco craton, SE Brazil Luciana Pascarelli Santos * , Mario da Costa Campos Neto, Carlos Henrique Grohmann Instituto de Geocie ˆncias, Universidade de Sa ˜o Paulo, Rua do Lago, 562, Butanta ˜, Cidade Universita ´ria, CEP 05508-900 Sa ˜o Paulo, SP, Brazil Received 1 December 2003; accepted 31 December 2003 Abstract A Nappe system south to southwest of the Sa ˜o Francisco Craton represents the southern extension of the Brası ´lia belt and describes an inverted metamorphic pile of greenschist facies toward amphibolite facies. The Aiuruoca-Andrela ˆndia nappe is one of the nappes of this system. The hind portion of the Aiuruoca-Andrela ˆndia nappe, south of Caxambu and Aiuruoca (MG), consists of a structural-metamorphic domain transported toward the E-NE. There is a metamorphic transition, from the kyanite zone to kyanite and sillimanite coexistence, until the sillimanite zone. Metapelitic rocks preserve high-pressure parageneses (Rt – Ky – Grt – Ms – Bt – Pl – Qtz) and contain retrograde eclogitic rocks. Sil – Pl – Qtz coronitic intergrowths around garnets are common decompressive textures. Kyanite schists register the P max of 11 kbar at 660 8C and define a decompressive path until 6–7.5 kbar at 650 8C. These P–T conditions represent the equilibrium in S 2 schistosity (amphibolite facies) and the beginning of the cooling path in the Ky – Sil transition. The decompressive path suggests an extrusional process, immediately after burying at about 60 km. Exhumation controlled by convergent events, related to the Sa ˜o Francisco Plate subduction and tectonic erosion, took these units, isothermally, to higher levels (20 – 33 km). Later, the metamorphic path shifted toward near-isobaric cooling. q 2004 Elsevier Ltd. All rights reserved. Keywords: Neoproterozoic; High-pressure nappe; Metamorphic path; Nappe exhumation 1. Introduction Late thermal activity during metamorphism is thought to be responsible for the main chemical equilibrium estab- lished between rock-forming minerals. In many collision- type orogens, such as the high-Himalayas, the crust undergoing subduction experiences mid-T and high-P conditions, whereas during the collision stage parts of this crust that have been exhumed to mid-crustal levels experience high-T mid-P conditions that overprint the assemblages formed during the subduction stage (Hodges et al., 1993; Vannay and Hodges, 1996; Vannay and Grasemann, 1998). The chemical diffusion between micas and garnets at high temperature during the collision stage tends to obscure the previously high-temperature conditions registered by the favorable mineral assemblage (Rt – Ky – Grt–Na-rich Pl–Ms–Qtz) in metapelites acquired during the early continental subduction (mineral abbreviations after Kretz (1983) and Spear (1995)). The preservation of an inverted thermal gradient, unrelated to the post-meta- morphic thrust stacking (Stephenson et al., 2000), requires fast exhumation processes controlled by instantaneous thrusting (Karabinos and Ketcham, 1988), coeval hinterland directed normal shearing, and erosion (Hodges et al., 1996; Chemenda et al., 1995, 1996; Coleman and Hodges, 1998). On the other hand, this inverted metamorphic gradient can also develop on the footwall of an extruding ductile and hot unit (Beaumont et al., 2001). These processes are characterized by retrograd equilibria related to unloading and are registered by an isothermal decompression metamorphic path, mainly preserved in surviving basic- intermediate eclogitic rocks (Pognante et al., 1993; Caby, 1994; Guillot et al., 1997; Mo ¨ller, 1998; Sigoyer et al., 2000). The partial melting of the deeper or inner portion of the orogen seems to occur during the late collision stage, when the exhumation allowed the isotherms to rise. Sil – Cdr can be formed at this stage, and dehydration melting of thickened crust can produce migmatites and leucogranites 0895-9811/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsames.2003.12.006 Journal of South American Earth Sciences 16 (2004) 725–741 www.elsevier.com/locate/jsames * Corresponding author. Fax: þ 55-3091-4258. E-mail addresses: pascarel@usp.br (L.P. Santos), camposnt@usp.br (M. da Costa Campos Neto).