Monazite Dating of Prograde and Retrograde P-T-d paths in the Barrovian terrane of the Thaya window, Bohemian Massif
Abstrakt
Monazite laser ablation-split-stream inductively coupled plasma-mass spectrometry (LASS) was used to date monazite in situ in micaschists of the Moravian zone in the Thaya window, Bohemian Massif. Petrography and garnet zoning combined with pseudosection modelling show that rocks from staurolite-chlorite, staurolite, kyanite and kyanite-sillimanite zones record burial under a moderate geothermal gradient from 4 kbar and similar to 530 degrees C to 5 kbar and 570 degrees C, 6 kbar and 620 degrees C, 8 kbar and 640 degrees C, and 8 kbar and 650 degrees C, respectively.
In the kyanite and kyanite-sillimanite zones, garnet rim chemistry and local replacement of garnet by sillimanite-biotite aggregates point to re-equilibration at 6 kbar and 640 degrees C. Monazite abundance increases with metamorphic grade from 5 mu m in the staurolite- zone to > 100 mu m in the kyanite zone.
This is interpreted as the onset of prograde monazite growth at staurolite grade, and continued prograde monazite growth after the kyanite-in reaction, compatible with conditions of about 5.5 kbar and 570 degrees C and 7.5 kbar and 630 degrees C. Monazite is zoned, showing embayments and sharp boundaries between zones.
The ages from three samples range from 344 to 330 Ma, irrespective of metamorphic grade. The dates from monazite inclusions are interpreted as the ages of the staurolite-and kyanite-in reactions along the prograde path at 340 and 337 Ma, respectively.
The monazite in the matrix is interpreted as dating the prograde crystallization at 340-337 Ma, and then being affected by recrystallization at 332 Ma. The two groups of data, for 340-337 and 332 Ma, are significantly different when only their in-run uncertainties (+/- 1-3 Myr) are compared and indicate a 9 +/- 3 Myr period of monazite (re) crystallization.