Reconstructing P-T paths from multi-stage garnets (Gran Paradiso nappe, Western Alps)

National audienceMost collision zones involve slices of continental crust that have been reworked during several, possibly unrelated, episodes. During the later metamorphic episodes, relics of the early history are progressively erased. However, traces of earlier metamorphic stages can be preserved either in low-strain domains, or within relic prekinematic phases in high-strain domains (e.g. garnets). Garnet-chloritoid-bearing micaschists from the Gran Paradiso massif (Western Alps) contain evidence of a polymetamorphic evolution. Detailed textural observations and detailed X-ray element maps reveal that two stages of garnet growth are present in the micaschists, interpreted as (i) relics of an early metamorphism of pre-Alpine age and (ii) newly-grown Alpine garnets, respectively. X-ray element maps for garnet 1 have revealed the occurrence of former fractures that cut across garnet 1, and were sealed during the growth of garnet 2. Furthermore, both generations of garnet preserve growth zoning. From THERMOCALC-based numerical modelling of mineral assemblages in P-T pseudosections, we infer that garnet 1 grew at increasing temperature and slightly increasing pressure, whereas garnet 2 grew at decreasing pressure and slightly increasing temperature. Estimated P-T conditions are about 620°C, 6 kbar for the peak of the pre-Alpine event, and of 490°C, 18-20 kbar for the pressure peak of the Alpine event. Modelling of the modal proportion and chemical composition of garnet (i) shows that the subsequent decompression (to 14-15 kbar at 550°C) must have been accompanied by moderate heating and (ii) does not support a stage of final temperature increase following decompressional cooling. This argues against a late thermal pulse associated with mantle delamination. Preservation of growth zoning in both generations of garnet and the limited amount of diffusive reequilibration at the boundary between the two garnets suggests that the rocks were subjected to fast burial and exhumation rates, consistent with data obtained from other internal Alpine units