Monazite geochronology in melt-percolated UHP meta-granitoids: An example from the Erzgebirge continental subduction wedge, Bohemian Massif
Abstrakt
Coupled age and trace element mapping in monazite grains using the Laser-Ablation Split-Stream ICP-MS technique was carried out for a sequence of samples from (U)HP crustal section in the Eger Crystalline Complex interpreted as a Variscan continental subduction wedge. The sampled section comprises banded orthogneiss with monazite protolith ages of 486 +- 3[10] Ma.
The orthogneiss was heterogeneously transformed to migmatite, HP felsic granofels and U(HP) fine-grained Ky-Kfs granulite during coupled deformation and percolation of silicate melts in a subduction channel. Age of the (U)HP metamorphism and related melt percolation in the granulite is dated by Th-rich and HREE-poor monazite cores at 353.6 +- 2.7[8] Ma, that are in equilibrium with HREE-rich garnets.
Th-rich cores of monazites in the migmatite, granofels and granulite that lack the (U)HP signatures show younger ages of 347.8 +- 3.4[8] Ma, 345.8 +- 2.4[8] Ma and 339.3 +- 1.3[7] Ma, respectively. The monazite rims with lower Th and higher HREE contents than the monazite cores are likely a result of (re)crystallization in equilibrium with melt that partly dissolved the HREE-rich prograde garnet during decompression.
This (re)crystallization is constrained by ages of 344.2 +- 1.6[8] Ma in (U)HP granulites and 338-340 +- [8] Ma in migmatites, granofelses and granulites without the (U)HP signature. The groups of ages from 353 Ma to 338 Ma, are significantly different when only their in-run uncertainties (7-10 Ma) are compared and indicate a 15 +- 3 Ma period of monazite (re)crystallization from burial/peak to exhumation.