Partial melting and genesis of HP graphite-bearing granulites in the Intermediate Unit of the Central Rhodope Metamorphic Province, Greece

Three main tectono-lithostratigraphic Units are pilled up in the Greek part of the Rhodope Metamorphic Province. The Lower Unit (Pangaion complex), with continental passive margin affinities, is composed of orthogneisses of Permo-Carboniferous magmatic age overlain by amphibolite facies marbles and minor schists. The Intermediate Unit is an assemblage of strongly deformed -in parts ultramylonitic- and variably migmatized lithologies of oceanic and trench affinity (amphibolites, eclogites and metatrondjemites bearing MORB and arc signatures, metacherts, phengitic quartzites, biotite gneisses, pelites, psammites and calcsilicates), into which large pods of ultrabasites and ~300 Ma orthogneisses are tectonically intercalated. Biotite (± amphibole) gneisses, dominating the upper part of the Intermediate Unit, are interpreted as trench filling metagreywakes of mainly volcanic origin, dragged down and accreted to the overriding plate of a subduction zone active during the Late Jurassic. Above them, the Upper Unit is composed of orthogneisses of Late Jurassic magmatic age, probably the edifice of a volcanic arc built above this subduction zone.
   Although HP (kyanite field) amphibolite facies parageneses characterise the ITU, some evidence of UHP metamorphism has been reported from ex-eclogitic pods and some pelites in it. These may represent samples of deeply subducted material returned by some mechanism from mantle depths and tectonically emplaced at shallower levels. Evidence however also exists that the dominantly HP amphibolite facies parageneses in the ITU overprint earlier HP (kyanite, cpx) granulite facies ones, which were imposed coevally with partial melting in this unit. For the now exposed subducted oceanic and trench lithologies of the ITU, this implies a prolonged residence and thermal relaxation near the base of an overthickened crust, apparently after continental collision. This could only be achieved by an abnormally slow collapse and levelling of the orogen above, probably as a result of preserving a high crustal relief due to Cretaceous shortening (?thrusting) in the Rhodope.
   Focused in an area near Sminthi village, partial melting phenomena in the ITU related with the HP granulite event are examined in some detail. The dominant rock type in this area is a migmatized dark coloured Bt-gneiss. Leucosomes of variable sizes and distribution forms have been separated from the mesosomatic gneisses and are interbedded with or cut as veins the surrounding gneisses and marbles. (?)Syn- to post-melting intense deformation affects both the mesosomatic gneisses and the leucosomes forming boudins and tight isoclinal folds. From the migmatitized gneisses two representative rock types bearing evidence of an early HP granulite facies event have been studied in more detail. The first is a medium grained, mesosome dominated metatexitic migmatite with Bt + Pl + Qtz + Kfs + Cpx(Amp) + Gt + accessories (All, Ttn, Ap, Gr, Py(Ght), rounded zircons). The second is a coarse grained metatexitic paragneiss with Phl + Gt(Bt) + Ky(minorSill) + Pl + Qtz + Kfs + Rt + Gr. Melting should have been anhydrous; likely dehydration melting reactions are:
   a) Bt + Pl + Qtz = Cpx + Gt ±Kfs + melt.
   b) Phe + Pl + Qtz = Phl + Als + Gt + melt
   c) Bt + Als + Qtz + Pl = Gt ± Kfs + melt.
   The leucosomes are anhydrous pegmatites or aplites, bearing mostly Di-Hd cpx (when hosted in marbles) or garnet (when hosted in biotite gneisses). Graphite and sulphides are ubiquitous in both mesosomes and leucosomes, suggesting highly reduced conditions during melting.