Master Thesis presented by Lurdes Veloso , 1994, 181 p.

Abstract
The mafic microgranular enclaves (MME) associated with late hercynian granitoids from Braga - Vieira do Minho region (Centro Iberian Zone, Northern Portugal) have been studied using mineralogical and geochemical techniques. The Braga and Agrela granites have been emplaced about 307-310 Ma ago, largely controlled by the major Vigo - Régua shear zone. They are hybrid granitoids having the same mineralogical and geochemical composition and evolution.

The Braga and Agrela granites are frequently associated with MME which present a large variation in size, shape and petrographic characteristics. The same MME typology has been found in the two granitic plutons: (1) mesocratic enclaves with quartz "ocelli"; (2) fine-grained mesocratic enclaves; (3) fine- to medium-grained leucocratic enclaves.
The MME and the corresponding host granites display a similar mineral assemblage: plagioclase+biotite+quartz±K-feldspar. However, whole rock geochemical data reveal compositional differences between the MME and the associated granites. An evolution trend, without any significant gap, is observed in the sense fine-grained mesocratic enclaves -> fine- to medium-grained leucocratic enclaves -> felsic matrix. The samples from enclaves with quartz ÒocelliÓ do not always observe this general trend.
The integration of available data suggest an igneous origin for the MME. They represent blobs of a more mafic magma incorporated into a silicic host magma. Hybridization processes between the two magmatic components involve:

1. Modification of the physical parameters of crystallization
Mineralogical data indicate that thermal equilibrium occurred between the more mafic magma and the felsic matrix. However this equilibration was not totally reached in the enclaves with quartz "ocelli".

2. Mechanical transfer of crystals from the host granitoids
This hybridization mechanism is represented by: quartz xenocrysts, surrounded by a reaction rim of biotite; irregular cores of plagioclase phenocrysts, surrounded by dendritic and more calcic rims; K-feldspar xenocrysts.

3. Chemical exchanges
The geochemical characteristics of the studied enclaves are the result of: a migration of some elements from the felsic to the mafic component magma (Si, K, Rb); as a result of this process, a depletion through a dilution effect for other elements (Ca, Mg, Fe, Al) in the mafic component; a loss of Ca and Sr from this component. The main hybridization mechanism involved is believed to be chemical diffusion resulting from compositional gradients, possibly combined with the migration of fluids.