Advecting heating by hot fluids of an Alpine fissure in Lauzière Granite (Belledonne massif, Western Alps)

Abstract. A multi-method approach investigation in the Lauziere granite, located in the external Belledonne massif of the French Alps, reveals unusually hot hydrothermal conditions in vertical open fractures (Alpine-type clefts), caused by advective heating. The host-rock granite shows sub-vertical mylonitic microstructures and partial retrogression at temperatures of 400 °C during Alpine tectonometamorphism. Novel zircon fission-track (ZFT) data in the granite give ages at 16.3 ± 1.9 and 14.3 ± 1.6 Ma, confirming that Alpine metamorphism was high enough to reset the pre-alpine cooling ages and that the Lauziere granite had already cooled below 240–280 °C and was exhumed to 10 km at that time. Novel microthermometric data and chemical compositions of fluid inclusions obtained on millimetric monazite and on quartz crystals from the same cleft indicate early precipitation of monazite from a hot fluid at T > 410 °C, followed by a main stage of quartz growth at 300–320 °C and 1.5–2.2 kbar. Previous Th-Pb dating of cleft monazite at 12.4 ± 0.1 Ma clearly indicates that this hot fluid infiltration took place significantly later than the peak of the Alpine metamorphism. Advective heating due to the hot fluid caused rejuvenation of the ZFT age at 10.3 ± 1.0 Ma in the cleft hanging wall. The results attest of highly dynamic fluid pathways, allowing the circulation of deep mid-crustal fluids, 150–250 °C hotter than the host-rock, affecting the thermal regime at the wall-rock of the Alpine-type cleft for a duration of 1–3 My. Such advecting heating may represent a pitfall source for exhumation reconstructions.