Petrographic and geochemical characterization and the significance of microgranular mafic inclusions in the Cima d'Astra pluton (southeast Alps)

The plutonic complex of Cima d'Asta is an example of hybrid magmatism, due to the interaction of mafic magmas of mantle origin and felsic crustal magmas, in a post-orogen extensional regime (lower Permian). Micro-granular mafic enclaves (IFM) of magmatic origin occur within the main pluton (monzogranite), and are concentrated into a narrow area near the top of Cima d'Asta pluton. The upper portion of the pluton is characterised by wide-spread alteration, detectable both in the monzogranite and in the IFM. The composition of IFM ranges from tonalite to granodiorite, rarely quartzdiorite. Their textures vary from pseudodoleritic to porphyritic and/or ocellar, because of xenocrysts from the granitic magma. The geochemistry of major and trace elements is typical of IFM-bearing granitoid rocks originated through complex interaction of coeval mafic and felsic magmas. The isotopic composition of Sr and the REE distribution are compatible with mingling and mixing mechanisms. The latter is meaningful also because rock with quite different evolution (SiO2 = 55-60% for IFM vs. 70% for monzogranites) show similar REE patterns, suggesting that in places an equilibrium between mafic blobs (enclaves) and monzogranite (matrix) was reached. Based on the whole data set, the IFM are interpreted as fragments of basic dikes intruded into the solidifying monzogranitic magma. The partially molten condition of the IFM during their incorporation into the granitic body is testified by their lobate shapes with small-scale crenulation and by the occurrence of partially resorbed quartz xenocrysts with ocellar texture. Mingling processes involving granitic and mafic magmas are documented also by the heterogeneity of IFM size, broken during the intrusion, and by quenching textures (acicular apatite and fine grain size in the IFM), caused by rapid cooling of the mafic magmas. Besides mechanical interactions, also chemical exchanges, up to a true hybridization, are suggested by the occurrence in places of a melanogranodiorite facies, with compositional characters intermediate between monzogranite and IFM. Moreover, the variability of the textural and compositional features of the IFM (still at the outcrop scale) and their occurrence as enclaves also within the melanogranodiorite facies, suggest that interaction and the hybridization developed through multiple and repeated intrusions of mafic magmas at different degree of evolution. Further evidence of chemical exchanges (migration of Si and K) are suggested by the occurrence in the IFM of poikilitic quartz and K-feldspar and by the abundance of biotite. The widespread alteration detected in the Vetta zone is not pervasive and have low intensity, suggesting that an opening of the system is not reliable. As a matter of fact the main mineralogical feature is the transformation of biotite into chlorite (and rare pyroxene into actinolitic horneblende), but the biotite is still present also in the most altered rocks. Moreover, no geochemical and isotopic (Sr) variations are detectable between rocks with different degrees of alteration. Such characteristics can easily be related to a late-post magmatic deuteric mechanism, that can be particularly active near the Vetta area, that represents the upper portion of the pluton.