The composite zircon-xenotime-monazite-allanite assemblage in the leucogranite from the Low Tatra Pluton, Western Carpathians: Interplay of melts and fluids

The highly peraluminous Zamostska Hola leucogranite (ZHG) forms a small, dyke-like body within the granodiorites of the Prasiva type in the composite, 353-351 myr Low Tatra Pluton (LTP), Central Western Carpathians, Slovakia. The ZHG fabric recorded syn-magmatic brecciation, multiple influxes of fluid-rich melt, and fluid-rock interaction. The accessory mineral assemblage includes zircon, xenotime-(Y), monazite-(Ce) and allanite-(Ce). The zircon in the ZHG shows very low Zr/Hf and Th/U; apparently, primary zircon is euhedral, oscillatory zoned and P, Al, Ca, Fe, U, Y, HREE-rich; however, most of the zircon grains are to variable extents altered by dissolutionreprecipitation process, and show depletion or redistribution of trace elements within the crystal. The xenotime-(Y) is spatially associated with zircon and varies from the euhedral, igneous-like to strongly irregular secondary grains formed from the zircon dissolution in the presence of P-bearing fluid. Some zircon and xenotime-(Y) grains are high in As. The monazite-(Ce) shows morphology typical for early-magmatic origin, and alteration is restricted to huttonite-enriched, U, Ca, Y-depleted rims and patches; however, the entire crystals are unusually high in F up to 0.8 wt %. The monazite-(Ce), followed by the trace element-rich zircon and euhedral xenotime-(Y), crystallized from peraluminous, F, U, Y + HREE-enriched melt and resemble ones from highly fractionated granites or pegmatites. The alteration of primary accessory minerals and formation of the secondary xenotime-(Y) is related to an influx of alkali-enriched, fluid-dominated magma. The reaction of REE in fluid with brecciated and altered plagioclase and micas led to the local formation of secondary allanite-(Ce). The magmatic age of the ZHG monazite is 345 +/- 2.5 Ma and emphasizes its formation in the final stage of the prolonged evolution of the LTP; but whether the leucogranite represents residual differentiates of the magma of hosting Prasiva type, or independent magmatic pulses, remains an unresolved issue. The exceptional accessory phases assemblage records mineral-melt-fluid interactions in the late magmatic-hydrothermal transition, which is the critical ore-forming phase, and overall characteristics of the ZHG minerals shares attributes of those in the fertile intrusions and provides arguments for the close relationship between the composite magmatism and the ore petrogenesis in the LTP.