HYBRIDIZATION BETWEEN FELSIC AND MAFIC MAGMAS IN CALC-ALKALINE GRANITOIDS - A CASE-STUDY IN NORTHERN SARDINIA, ITALY

Two examples of felsic-mafic association resulting from a magma mingling mechanism have been studied in Carboniferous calc-alkaline granitoids from Sardinia using mineralogical and geochemical techniques: (1) the centimetric to decimetric magmatic mafic enclaves from the reversely zoned tonalitic Santa Restituta pluton; this type of enclave, which is common in calc-alkaline granitoids, represents blobs of mafic magma incorporated into a silicic host magma; (2) the Osidda mafic complex, about one kilometre in diameter, enclosed in the Benettuti granodiorite. This study intends to compare these two types of felsic-mafic association in order: (1) to clarify the nature of the interaction processes which occurred between the two magmatic components; (2) to test the influence of the degree of fragmentation of the mafic component into the felsic matrix on the intensity of such processes. The Santa Restituta enclaves (SRE) and the Osidda mafic complex (OMC) exhibit the same general mineral assemblage: plagioclase + biotite +/- hornblende +/- quartz. This assemblage is similar to that of the corresponding host granitoids; however, the SRE and the OMC are characterized by a greater abundance of biotite and hornblende. The mineralogical study reveals the presence of two types of crystals. The first type corresponds to minerals typical of the mafic or the felsic component. In the mafic magma they are represented by remnants of An-rich plagioclase, by hornblende clots (presumably pyroxene relicts) and by orthopyroxene (observed only in one facies of the OMC). These crystals represent remnants of phenocrysts present in the mafic component prior to the mingling event. In the felsic component, the first type of crystals is represented by quartz "ocelli" which are sometimes incorporated in the mafic magma as xenocrysts surrounded by a reaction rim of biotite and hornblende. The second type comprises minerals which are found indiscriminately in both magmatic components and which formed during the mingling event ("mingling minerals"). They include: plagioclase (An52-35), biotite +/- hornblende +/- quartz. Mineralogical data indicate that thermal equilibrium occurred between the mafic magma and the felsic matrix. That equilibration was reached in the SRE, is shown by the compositional similarity of the mingling minerals of the enclaves and their host rocks. However, due to a lesser degree of fragmentation, thermal equilibration was not reached in the OMC. Geochemical data. however, do reveal close relationships between the two magmas which appear therefore, to have both been hybridized by mutual chemical interaction. A chemical comparison between the SRE and the OMC shows that, in both cases, the same qualitative interaction mechanisms operated between the host granitoids and, either a voluminous mafic body (Osidda) or a highly fragmented mafic component (Santa Restituta enclaves). However, the intensity of hybridization is minor in the OMC as compared with the SRE. It is suggested that hybridization of the host granitoids involves: (1) mass-to-mass mixing due to disaggregation and dispersal of the mafic magma in the host granitoid, resulting from mechanical stirring during the pluton's genesis (from the initial meeting of the two magmas to the pluton's final emplacement): (2) loss of certain elements (Si, K, Rb, Ba, Li, Zr, P, Nb, REE) which migrated towards the mafic magma through diffusion (due to compositional gradients) +/- fluid transfer. As a consequence, the composition of the host granitoids is linked to that of their magmatic enclaves: the enclaves with the lowest FeO(tot)/MgO ratios are enclosed in the least felsic granitoids and vice versa. Hybridization of the mafic component involves: (1) mechanical transfer of crystals (quartz) from the host (2) enrichment in numerous elements migrating from the felsic component; this is expressed mineralogically by the crystallization of numerous anhedral phases such as quartz (SiO2), biotite (K, Rb, H2O), hornblende (H2O), apatite (P), and zircon (Zr); (3) loss of Ca and Sr by the mafic component. The nature of the processes of chemical migration are discussed. The importance of the continuous renewal of the contact surface between the two components is emphasized. sized. The original compositions of the interacting mafic and granitic components become, therefore, highly modified. This phenomenon can probably be generalized to apply to calc-alkaline granitoids as a whole, which are in fact characterized by the common occurrence of magma mingling.