Origin of S-type granites coeval with I-type granites in the Hellenic subduction system, Miocene of Naxos, Greece

Abstract

The island of Naxos in the Cyclades preserves a core complex developed by Miocene detachment faulting associated with extension of continental crust inboard of the Hellenic subduction zone. The extension has unroofed Miocene hornblende-biotite granite and leucogranite, previously defined as I-type and S-type granite respectively. Crystalline basement exposed by extension includes partially migmatised ortho- and paragneisses principally with Hercynian protolith ages and intercalated high-grade metasedimentary and metavolcanic rocks with Mesozoic protoliths. Most leucogranites, in addition to quartz, feldspar and biotite contain muscovite, tourmaline and garnet (almandine-spessartine). Mineral assemblages suggest crystallisation pressures < 2.5 kbar, with stability of muscovite and tourmaline resulting from high B, Mn and possibly F in the magma. Two types of leucogranite can be distinguished on the basis of rock chemistry, mineral chemistry and Nd/Sm isotopes. Earlier leucogranites (type II) have high LREE:HREE ratio, high LILE, low Y and Nb and ϵNd ∼-10 and appear to be a partial melting product of Hercynian paragneiss. Leucogranite dykes within the I-type granite appear similar. Later leucogranites (types Ia, Ib) have low LREE:HREE ratio and ϵNd ∼-7 and appear to be derived from melting of both Hercynian paragneisses and younger metasediments. The major element composition of rare leucogranite type Ic suggests derivation from partial melting of orthogneiss. These varied sources result in significant variation in mineral assemblages in the leucogranites.