Fluid-induced disturbance of the monazite Th–Pb chronometer: In situ dating and element mapping in pegmatites from the Rhodope (Greece, Bulgaria)

The monazite Th–Pb chronometer is widely used in a variety of geological contexts but may be affected by interaction with fluids. In the central part of the Rhodope Massif (Greece, Bulgaria), synfolial pegmatite veins were emplaced within two major synmetamorphic shear zones. In the Chepelare shear zone (CZS), which displays no evidence of post-metamorphic fluid flow, consistent monazite Th–Pb ages of ∼ 36 Ma are interpreted as dating vein emplacement. Higher in the metamorphic pile, zircon and monazite give U–Th–Pb ages of ∼ 42 Ma for vein emplacement. In the Nestos Shear Zone (NSZ) pegmatites show evidence of post-emplacement interaction with fluids during greenschist facies ductile deformation. Evidence for this includes the precipitation of calcite in microscale tension gashes within feldspar porphyroclasts, and the oxygen isotope disequilibrium between quartz and feldspar. Aqueous carbonic fluids deriving from carbonate lithologies in the footwall of the NSZ are identifiable in the pegmatites by the high δ18O value of quartz (up to 21.8‰) and the presence of calcium in calcite veinlets. In these samples, some of the monazite grains show large intragrain scattering of Th–Pb age spanning up to ∼ 12 Ma, with intergrain scattering reaching ∼ 16 Ma (from ∼ 39 to ∼ 55 Ma). Within individual grains, age domains correlate with chemical heterogeneities, and some show a characteristic Ca-excess. These chemical and isotopic alterations are interpreted to be caused by interaction with fluids derived from carbonate lithologies. Complementary U–Pb data on zircons from the NSZ pegmatites yield mostly Mesozoic ages related to an older metamorphic cycle, and an age of ∼ 48 Ma for one grain with typical magmatic zoning. The Th–Pb ages of 49.3 ± 1.6 to 54.9 ± 1.7 Ma probably relate to the emplacement of the pegmatites. The spread of younger Th–Pb ages (from 38.6 ± 1.1 to 46.2 ± 1.6 Ma) probably reflects the period of fluid circulation during progressive cooling to greenschist facies conditions, or variable perturbations of the monazite isotope system during fluid-assisted greenschist facies deformation. This is consistent with 39Ar–40Ar mica ages of ∼ 32 to 34 Ma, interpreted as a tighter constraint for the timing of the greenschist facies metamorphism. This study illustrates the capacity of monazite to record distinct events in a single rock and highlights the need for identifying the potential involvement of fluids in order to interpret monazite ages.