Two partial melting events as recorded by the U-Th-Pb chronometer in monazite: LA-ICPMS in situ dating in metapelites from the Bulgarian Central Rhodopes

In the Bulgarian Central Rhodopes, the lower part of the metamorphic pile is dominated by migmatitic orthogneisses having recorded fluid-assisted partial melting at 650-700ºC / 6-8 kbar. Several zircon and monazite U-Pb ages around 36-38 Ma have been reported, interpreted as dating the crystallization of melts. In the area of Chepelare, this pile is exposed as a ~5 km-thick north-dipping monocline. Structures document top-to-SW shearing developed during and subsequent to anatexis. In the middle part of the section, the ~1 km thick Chepelare Shear Zone (CSZ) reflects late Eocene syn-metamorphic thrusting and exposes a variegated rock assemblage of highly sheared migmatitic gneisses hosting discontinuous layers of marbles, garnet-kyanite gneisses, metabasites, and ultramafics.
   In order to constrain the P-T-time evolution of this variegated rock assemblage, we present new petrological and geochronological data obtained from garnet-kyanite gneisses. The samples represent melt-depleted residual granulite composed of zoned garnet and kyanite porphyroblasts of centimeter size in a low-portion matrix of K-feldspar, quartz and biotite. The latter forms retrograde rims around garnet, and together with kyanite, defines a rough foliation. In some samples fibrolite partially replaces synfolial biotite. The accessory mineral assemblage comprises monazite (up to 400 μm), apatite, zircon, rutile, ilmenite, staurolite, and graphite, found in the matrix, and as single or polyphase solid inclusions in garnet and kyanite porphyroblasts. Polyphase inclusions mark core-rim boundary in zoned kyanite and consist of K-feldspar, quartz, monazite, apatite, rutile, graphite, ± zircon, ± biotite. Planar faces of mineral grains suggest crystallisation of trapped melt. Graphite nucleation indicates participation of carbon-saturated fluids.
   U-Th-Pb analyses on monazite were performed by means of LA–ICPMS in thin sections. The results yield two age groups related to the textural position of the monazite grain (included in a garnet or kyanite porphyroblast vs. in the matrix). Mesozoic ages, between 137 and 142 Ma, are most common. They were obtained in all monazite included in garnet as well as in polyphase inclusions in kyanite. P-T estimates based on the metamorphic record preserved in garnet and kyanite suggest granulite facies anhydrous melting, or low a_H2O fluid participation, at > 800ºC / > 1.2 GPa, that produced peritectic garnet (and probably kyanite) together with a K-rich melt. In the same samples, Cenozoic ages between 38 and 42 Ma were obtained in the outer rim of monazite grains located in the matrix. These monazites also preserve Mesozoic ages in the grain core. The Cenozoic ages relate to the tectonometamorphic event that led to widespread fluid-assisted partial melting in adjacent orthogneisses. During this event, because they represent a melt-depleted residue with respect to a previous higher grade melting event, the garnet-kyanite gneisses remained unfertile and preserved good petrological and geochronological record of the older event. Nevertheless, they also recorded the Cenozoic event in at least two ways, namely the growth of fibrolite at the expense of biotite, and the partial recrystallization of monazite grains located outside large porphyroblasts.
   Finally, an interesting result of this study is the first documentation, in the Bulgarian Central Rhodopes, of a Late Jurassic-Early Cretaceous high-grade metamorphic event that is also known from the Greek part of the Rhodopes Mountains (e.g., in the hanging wall of the Nestos Shear Zone).