Geochemistry of Tuscan archipelago granitoids, Central Italy: the role of hybridization and accessory phase crystallization in their genesis

This paper reports major, trace, and rare-earth-element data for selected samples from the Elba, Giglio, and Montecristo plutons, which lie in the Tuscan Archipelago (Italy). On the basis of field and petrologic constraints, all the rocks can be divided into five facies and range in composition from granodioritic to alkali-granitic, with a strong predominance of monzogranites. Tuscan Archipelago granitoids are clearly collision-related rocks, and they straddle the I and S types of Chappell and White. However, mineralogical and chemical characteristics indicate that the more basic rocks are broadly I type, while the high-silica rocks are S type. Genetic models that relate these magmas through the segregation of their major and accessory mineral phases are inconsistent with major, trace, and REE element abundant data. Geological, petrographical, and geochemical constraints show that the microgranular mafic enclaves found within the rocks are blobs of mafic, high-temperature magma chilled within a cooler, more silicic host, but a simple two-end-member mixing process fails to explain the geochemical characteristics of the plutons, either as a group or singly. A two-stage petrogenetic model explains all the geological, petrographical, and geochemical features shown by these intrusive rocks: first, a Mixing plus Crystal Fractionation (MFC) process involving a crustal peraluminous magma and a basic magma of probable subcrustal origin, and second, a simple mixing process between the same crustal peraluminous magma and different evolved magmas derived from the basic magma during the first stage. The second stage can be complicated by possible superimposed processes. The acid end-member could be the Leucocratic Facies found in the Giglio pluton, while the basic end-member should be similar to the Capraia rocks on the basis of age and geochemical characteristics. The model proposed is also able to explain the dichotomy between S and I granite characteristics. Trace element data support aplites genesis by a crystal fractionation process starting from the most evolved monzogranites and crystallizing an assemblage consisting of plagioclase, K-feldspar, quartz, monazite, and zircon.