Peritectic phase entrainment and magma mixing in the late Miocene Elba Island laccolith-pluton-dyke complex (Italy)

The comparison between the major element chemical variability exhibited by the granitic rocks of the Elba Island laccolith-pluton-dyke complex (Italy) and the composition of relevant fluid-absent experimental melts, indicate that Elba rocks have Fe, Mg, Ti and Ca contents that are too high to represent crustal melts derived from sources considered typical for granitic magmas and likely to be abundant in the Earth's crust. Therefore, the origin of the Elba Island laccolith-pluton-dyke complex demands the addition of a ferromagnesian, Ti- and Ca-rich component to the melt. Various authors, on the basis of textural and chemical data, have interpreted the chemical variability exhibited by the Elba Island granitic rocks as reflecting progressive hybridization of an original crustal melt with mantle-derived magma(s). However, a simple mantle-crustal magma mixing hypothesis is challenged by the observation that some elements (e.g. Ti and Ca) are highly correlated with Fe+Mg, while others (e.g. Sr, K 2O, Na 2O) are not, as well as by the scattered major and trace element composition exhibited by both mafic microgranular enclaves and dykes cutting all the other units of the complex. This contribution focuses on reconsidering the role of mantle-derived magmas in the petrogenesis of the Elba Island intrusive system from the perspective of the constraints imposed by crustal melt compositions. On the basis of the major- and trace element geochemical data we propose that at least part of the compositional variations displayed by the Elba Island intrusive complex is primary, i.e. it reflects the magma composition that ascended directly from the source. Following this hypothesis, the final composition of magmas may be controlled by two main factors: (i) the stoichiometry of the melting reaction(s) and the composition of reactant phases in the source, that control the composition of the anatectic melt; (ii) the degree of entrainment of the peritectic assemblage, the character of which will be dependent on (i), as well as the P, T conditions of melting. Moreover, major element data indicates that the oldest igneous rocks of the complex were formed through biotite fluid-absent melting of metasedimentary rocks that had a significant fraction of Al-rich clays (e.g. metapelites) while the youngest ones are likely to be generated by coupled biotite and amphibole fluid-absent melting of immature volcanogenic sediments of andesitic-dacitic composition.