The role of fluids in the monazite record during successive partial melting events: a textural, chemical and in situ dating study in Grt-Ky gneisses of the Central Rhodope (Bulgaria, Greece)

International audienceMonazite is considered to be resistant to diffusive Pb loss at high temperatures and thus, it is particularly adapted to record various stages during a sequence of high-temperature geological events. This study focuses on Grt-Ky gneisses from the lower part of the metamorphic pile in the central part of the Rhodope Metamorphic Complex, in the areas of Chepelare (Bulgaria) and Sidironero (Greece). The outcrops of both regions have experienced a polycyclic evolution during Alpine times, with at least two stages of high temperature metamorphism. According to P-T estimates, the first event involved granulite facies dehydration melting that produced peritectic garnet and kyanite together with a K-rich melt. The second event relates to widespread fluid-assisted partial melting. The latter is well known in adjacent rocks, where it is dated at ~36-50 Ma, but is poorly expressed in our samples, which well preserve the early granulite facies assemblages. Monazite is present in all samples, included in early porphyroblasts such as garnet and kyanite or in the matrix. Matrix monazites are associated with white mica, rutile and biotite (Greek part) and sillimanite and biotite (Bulgarian part). Matrix monazite grains show fluid-assisted dissolution-recrystallisation features with pronounced Y-zoning correlated to age domains. Y-poor domains, dated by LA-ICPMS method (208Pb/232Th ages) between 130 and 155 Ma, represent the largest part of each grain, while Y-rich domains, dated between 40 and 50 Ma, occur either as thin discontinuous rims (< 15 μm) or as small single grains surrounding the Mesozoic grains or filling white mica cleavages. The low P and REE-content of the surrounding minerals suggests that the Cenozoic domains essentially crystallized at the expense of the Mesozoic domains. In addition, garnet being the Y-richest mineral in the samples, its fluid-assisted resorption is the most likely mean to provide Y involved in the Cenozoic domains. The origin of the fluid is not clearly defined: external fluid infiltration or fluid produced by the recrystallisation of H2O-bearing minerals. Regardless, this fluid interaction was responsible for the partial dissolution of Mesozoic monazite grains, as well as for garnet resorption, and the precipitation of newly-formed Y-rich monazite during mid-Cenozoic times