Early Carboniferous P-T path from the Upper Gneiss Unit of Haut-Allier (French Massif Central) - Reconstructed by geothermobarometry and EMP-Th-U-Pb monazite dating

Abstract

In the Haut-Allier region (French Massif Central), a Variscan inverted metamorphic sequence is made up by crustal nappes. A high-grade Upper Gneiss Unit (UGU) was thrusted over a Lower Gneiss Unit (LGU) and an amphibolite-facies para-autochthonous Micaschist Unit (MU). Growth-zoned garnets with distinct Mn-Mg-Fe-Ca and trace-element zonation trends occur in kyanite garnet gneisses at Agnat (UGU). The porphyroblasts have been characterised by automated energy-dispersive X-ray spectral mapping with SEM, by electron microprobe and LA-ICPMS analyses. Microstructurally-controlled geothermobarometry based on cation exchange and net transfer reactions was used to reconstruct a syn-deformational and clockwise P-T path for garnet crystallization in the UGU. The P-T path passed maximal pressures at 700 °C/13 kbar and then maximal temperatures at ∼800 °C/11 kbar in the stability field of kyanite + K-feldspar. The final stage of the P-T path is a marked decompression from 10 to 5 kbar at 700-750 °C. The timing of this P-T evolution in the UGU has been constrained by electron-microprobe Th-U-Pb monazite dating (CHIME). A detailed interpretation of ages and Y-contents of monazites enclosed in a syncrystalline-rotated Mg-rich garnet allowed to relate a marked pressure decrease along a late stage of the P-T path to ∼330 Ma. Simple 1D forward numerical modeling with variations of vertical velocity and geothermal gradient confirmed that the onset of monazite crystallization at ∼360 Ma should give a maximal age of the early P-T evolution. A distinct group of Y-rich monazites could be the relic of this Late Devonian event. Most monazite ages from the UGU, the LGU and the MU in the Haut-Allier region range between 360 and 320 Ma, with isochrons for single samples giving ages from 332 to 338 Ma. The prograde-retrograde P-T-t evolution in the UGU appears as an independent Early Carboniferous metamorphic cycle, which was related to a continental collision. It post-dated a Silurian HP-(UHP) event and a subsequent Early Devonian migmatization in the UGU.