The Hercynian batholith of central and northern Portugal and adjacent parts of Spain is dominantly calc-alkalic granodiorite and granite. Minor massifs of gabbroic to tonalitic rocks occur locally associated with these granitic rocks.

In this paper, the origins of some of these mafic bodies and of the granite itself are discussed.

Analytical data for both major and trace elements are presented for a series of rocks, from the Aregos region, northern Portugal, ranging from hornblende-biotite tonalite to muscovite-biotite granite.

This series shows a distribution of major and trace elements paralleling those observed in series of differentiation of calc-alkalic rocks. However, the more mafic rocks, in particular, show variations in their element contents incompatible with an origin by differentiation.

The tonalites and most granodiorites of the series are hybrid rocks formed by reaction of a granitic magma with mafic material. The petrographic and chemical evidence show that the granites crystallized from a melt, and the origin by anatexis of this granitic magma is discussed.

The distribution of trace elements in the rocks of this hybrid series is largely conditioned by fractionation by the mineral phases and, therefore, the trends of variation are similar to those observed in other series, whereas the contents of some elements (Na, Li, Rb, Cs) for comparable rock types, are different, giving the Aregos series some distinctive geochemical features.

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