A unique sequential melting mechanism for the generation of anatectic granitic rocks from the Penafiel area, northern Portugal

Abstract

Granitic rocks derived by sequential partial melting are very uncommon occurrences worldwide with only three known cases in Portugal (Tourém, Guarda–Sabugal and Penafiel) and one in Argentina (Achiras complex). The suite documented in this paper comprises two of the three main phases of a batholith in the Penafiel area. These Variscan peraluminous two-mica granite and granodiorites formed at 310 and 305Ma during and near the end of the Variscan D3 deformation and have the characteristics of S-type granites. None of the metamorphic country rocks have an adequate Sr isotopic composition to be a protolith. It is argued that progressive isotopic re-equilibration of crustal material, probably a metagreywacke for G1 and G3 with also some metapelitic contribution for G2, during granulization of the lower Variscan crust explains the difference in isotopic composition between outcropping granitic rocks and metamorphic country rocks. Biotite>muscovite granite G1 and biotite>muscovite granodiorite G3 have similar (87Sr/86Sr)i and εNdt values and were derived by sequential partial melting of the same source material. This is indicated by coherent trends of the whole rock major and trace element compositions, the compositions of feldspar, biotite, muscovite and tourmaline, the presence and type of xenocrystic mineral phases and xenoliths, and whole rock oxygen isotope compositions. This mechanism was the result of post-thickening Variscan extension and mantle upwelling. Another member of the same batholith is a biotite≈muscovite granodiorite (G2), with an intermediate age of 307Ma, but with geochemical characteristics suggesting that it corresponds to a chemically distinct pulse of magma, although likely stemming from a comparable, though more metapelitic, lower crustal source.