Geochemical and isotopic constraints on petrogenesis: The Serra da Freita pluton, a typical granite body from the Portuguese Hercynian collision belt

摘要

The Hercynian Serra da Freita pluton in north-central Portugal is emplaced within a sillimanite-bearing flychoid sequence of graywackes and pelitic schists, here tested as a possible protolith for the granite. There is a marked overlap in most major-element values between the schists and the granite; however, high K values preclude anatexis of pelite, and the 3-fold Na enrichment shown by the granites could be achieved by 25%-30% partial melting of immature graywacke. Depletions of Ba, Nb, Ce, Zr, Ti, and Y in the granite reflect retention of these elements in the source. Restite separation may have occurred to some limited extent; however, enrichment in Rb may be due to late-stage fractional crystallization. Overlapping 18O values for both schists and granites at +10 to +14, combined with an initial 87Sr/86Sr of 0.7136 for the pluton, strongly support the local graywacke-rich Beira Schists as the most likely protolith for this strongly peraluminous granite magma.