Paper No. 77-10
Presentation Time: 10:50 AM
CONTRASTING RECORDS OF MONAZITE FLUID ALTERATION IN NORTHERN SCOTLAND
Monazite is an extremely useful mineral for quantifying the timing and conditions of metamorphism using U-Pb geochronology and trace element compositions. In addition to participating in metamorphic reactions, monazite is also known to undergo dissolution and reprecipitation in the presence of fluid, thereby providing an opportunity to understand fluid infiltration events in metamorphic rocks. We present monazite data from the Sgurr Beag nappe in northern Scotland that preserve evidence for two different signatures of fluid recrystallization. Initial crystallization of monazite throughout the Sgurr Beag nappe occurred at c. 750 Ma. All dated samples experienced subsequent partial isotopic resetting toward younger ages (600 Ma and younger), likely as a result of fluid-mediated reprecipitation. A fluid-mediated resetting mechanism is indicated by textures observed in monazite, typically with homogenized rims that truncate and penetrate into interior compositional zoning, and occasional porosity and reaction rims. Additionally, many partially reset monazite are entirely included within garnets that grew at c. 750 Ma based on consistent parageneses with monazite and xenotime.
Although monazite textures, geochronologic data and metamorphic assemblages are broadly similar across the Sgurr Beag nappe, there are two clearly different compositional signatures in these monazite grains. In four samples, partially reset monazite is high in Y and heavy rare earth elements (HREE) and has low Th/U ratios of 3-4, due to higher relative U contents. In two samples, partially reset monazite is low in Y+HREE and has high Th/U ratios of 5-11. These two compositional trends in monazite appear to be associated with distinct metamorphic textures in thin section. Samples with high Y+HREE monazite consistently exhibit garnet breakdown to plagioclase, quartz and biotite. Samples with low Y+HREE monazite typically have garnet reacted to chlorite. We interpret the low Y+HREE, high Th/U monazite to have formed by fluid-mediated reprecipitation during chlorite-grade hydration, while high Y+HREE monazite resetting occurred at higher temperatures. This work highlights the value of thorough petrography and electron microprobe imaging.