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.