3-STAGE DECOMPRESSION-RELATED HALOKINESIS IN THE SUBALPINE RANGE (SE FRANCE) - FLUID-INCLUSION EVIDENCE IN ROCK-SALT

Rock salt outcrops from seven Triassic diapirs in the SE basin of Fran ce show two mineral parageneses : early generation-0 minerals are eith er diagenetic or neoformed during diapirism; later generation-1 minera ls are neoformed. Three populations of secondary halite-saturated CO2- bearing fluid inclusions (= type-S FI) are identified in quartz Q0, do lomite D1 or magnesite M1, and sedimentary dolostone D0, based on cont rasted ranges of dissolution temperature of halite (= Ts) : 400 degree s-250 degrees (population-0), 250 degrees-180 degrees (population-1) a nd 180 degrees-150 degrees (population-2). All type-S FI contain compl ex Ca-Mg-Na-K-bearing solutions, either Ca- or Mg-dominant or with int ermediate compositions. They also have constant degrees of halite and gas filling, their CO2-content is either high and variable (population s 0 and 1) or low (< 1 mole% CO2, population-2). At the diapir scale, the temperatures of vapour disappearance (= Tb) are variable and the T s are near constant. Reconstructed bulk V-X properties of type-S FI ar e as follows (in mole % and g.cm(-3), respectively) : for populations 0 and 1 FI, near constant compositions of 74 H2O-6.5 CO2-19.5 NaCl and 79 H2O-5 CO2-16 NaCl, and variable densities in the range 1.29-1.26, 1.26-1.24, respectively; for population-2 FI : 88 H2O-12 NaCl, CO2 < 1 , d = 1.14. In one diapir (Laragne), population-1 type-S FI are associ ated with FI showing a variable degree of gas and halite filling (type -VS FI), which yield extreme compositions such as 31.6 H2O-29.8 CO2-38 .6 NaCl, d = 1.295, -3 and 20.4 H2O-62.3 CO2-17.3 NaCl, d = 1 (in mole % and g.cm, respectively). Each FI population is related to a diapiri c stage. The reconstructed P-T trapping conditions of population-0, 1 and 2 FI in that order, are : > 300 degrees C, 270 degrees C, and 150 degrees C, respectively, and minimum fluid pressures always similar to 2 kbar. In detail at the diapir scale, the similar to 50 degrees C Tb -scatter of populations 0 and 1 FI indicates the trapping of fluids wi th constant composition and variable densities following the decompres sion related to halokinesis. The fact that the decompressing diapiric fluids are trapped in the two generations of neoformed minerals as pri mary (Perthuisot and Guilhaumou, 1983) or secondary FI (this study) su ggests that mineral neoformation was rapid and synchronous with diapir ism. At Laragne, associated population-1 type-S and type-VS FI suggest mixing between a H2O-NaCl (Ca-Mg-K) liquid and a CO2-rich end-member, with transient halite saturation. At Propiac, we provide evidence tha t these two fluid end-members (population-1 type-S and type-Lc FI) wer e injected separately without mixing. Finally, at Laragne, some popula tion-1 type-S FI are annular and probably represent type-S FI deformed in the anisotropic stress field consecutive to enhanced decompression . The first two halokinetic stages corresponding to populations 0 and 1 FI are related to the Oxfordian extensional episode and to the Late Cretaceous-Eocene tectonic event, respectively. At these two periods, fluid pressures exceeding 1.5 kbar were only reached in the deepest pa rts of evaporites. In addition, population-0 FI imply a thermal anomal y in the deep basin at the Oxfordian. The discrete population-2 FI at Laragne can be related to the Late Miocene Mediterranean activity.