Exhumation of an anatectic complex by channel flow and extrusion tectonics: structural and metamorphic evidence from the Porto–Viseu Metamorphic Belt, Central-Iberian Zone

T Bento Dos Santos,C Meireles,N Ferreira,P Ferreira,B Cotrim,J A Ferreira,P Castro,A Ribeiro,I Pereira,F Guimarães,J F Rodrigues

doi:10.1007/s00531-021-02067-z

Abstract

Structural and metamorphic analyses of the Porto–Viseu Metamorphic Belt (PVMB), an anatectic complex located in the Central-Iberian Zone (CIZ), Iberian Variscan Massif, constrain the kinematic features of the polyphase Variscan deformation, as well as the boundary conditions associated with the exhumation and emplacement of the belt onto the CIZ’s lower-grade metasediments. A sharp metamorphic contrast is evident between the lowest grade rocks of the anatectic complex—garnet-staurolite mylonitic schists with a minimum metamorphic peak at P = 5.7 kbar and T = 635 °C—and the host chlorite-biotite metasediments of the CIZ. Additional P–T estimates for metatexites in the PVMB provided P = 7.6–7.9 kbar and T = 770–810 °C, confirming high-grade metamorphic peak conditions. The temperature jump between the anatectic complex and the low-grade host rocks exceeds 400 °C if the anatectic core of the belt (metatexites, diatexites, and S-type granites) is considered, implying a vertical difference of 10–17 km. The PVMB is limited in the NE by a D2 shear zone displaying normal kinematics, which has been explained so far by extensional tectonics. However, the SW boundary is limited by a D2 shear zone showing reverse kinematics. This pair of coeval and antithetical ductile shear zones bordering the PVMB, associated with the increase of the metamorphic grade towards the axial domain of the belt, is consistent with anatectic channel flow, followed by extrusion tectonics of the PVMB within an overall D2 compressive setting.

Highlights

Anatectic complexes are the result of partial melting of the middle and lower crust under conditions of highgrade metamorphism and are usually associated in space to syn-orogenic granitic bodies
Cambrian low-grade metasediments intruded by syn- to post-tectonic Variscan granitoids
The Porto – Viseu Metamorphic Belt (PVMB) is essentially composed of migmatites in its anatectic core formed at T > 800 oC, and displays staurolite-bearing schists in its borders formed at T = 635 oC and P = 5.7 kbar

Summary

Introduction

Anatectic complexes are the result of partial melting of the middle and lower crust under conditions of highgrade metamorphism and are usually associated in space to syn-orogenic granitic bodies Brihni 2007; Bento dos Santos et al 2011a; Pereira et al 2017b) For this reason, many anatectic complexes present the highest temperature record of an orogenic belt, which makes their study crucial to understanding. In the western sector of the CIZ, some anatectic complexes have been described: the Porto-Viseu Metamorphic Belt (PVMB; Reavy et al 1991; Valle Aguado et al 2005; Silva 2014); the Figueira de Castelo Rodrigo -. 60 Lumbrales Anatectic Complex (FCR-LAC; Iglesias & Ribeiro 1981; Escuder Viruete et al 1994; Fernández & Pereira 2016; Pereira et al 2017a; Ferreira et al 2019; 2020); the Miranda do Douro

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Conclusion