Formation and evolution of metapelitic-derived melts within anatectic complexes: Geochemical constraints and inferences on the protoliths and geodynamics of the Porto-Viseu Metamorphic Belt, Central Iberian Zone (central-north Portugal)

Abstract

Migmatite formation is dependent of several and inter-related factors, such as P-T conditions, protolith composition and the abundance of volatiles, which usually hinders the full understanding of anatectic progression and how it relates to orogenic evolution. We have constrained the petrological evolution of the anatectic complex (mostly metatexites and diatexites) of the Porto-Viseu Metamorphic Belt in the Central Iberian Zone (CIZ) using petrography, geochemical and isotopic data.

Major and trace element geochemistry allowed us to conclude that the studied metatexites were formed through water-present melting of muscovite from metapelitic schists, whereas diatexites were mostly formed by dehydration-melting of muscovite and, occasionally, by the dehydration-melting of biotite. Additionally, melting evolution from incipient (metatexites) to pervasive (diatexites and anatectic granitoids) included SiO₂ enrichment and elemental depletion (except for Ca, P, Na and K). The geochemical evolution of these elements during melting progression was essentially controlled by fractionation/melting of plagioclase, K-feldspar, mica and apatite, whereas REE abundance was constrained by accessory minerals, such as monazite, apatite and zircon. In both cases, the geochemical control was variable and dependent on the fate of these mineral phases during the progression of melting, as well as the variable degree of melt loss or gain, with diatexites and granites being the result of substantial melt gain.

The ⁸⁷Sr/⁸⁶Sr₃₂₀ values have a large dispersion (0.708–0.766), which is common for rocks that experienced these metamorphic conditions, whereas the εNd₃₂₀ values are less variable, roughly from −3 to −9 (diatexites: −8.24 to −2.96; metatexites: −8.79 to −5.15; staurolite-bearing schists: −7.57 to −5.19; granite: −8.21), being both isotopes independent of lithological type or degree of melting. Therefore, we interpret their Sr isotopic variation as mostly related to secondary alteration processes, whereas Nd isotopes, which are usually more immobile, should correlate to the initial protolith. In fact, their isotopic results are broadly consistent to the values of other CIZ metasediments that did not experience such high metamorphic conditions and melting (Douro-Beiras Supergroup), being most likely derived from these rocks, namely the Douro Group.