Mineral assemblages stable during successive deformations and classical thermo-barometrical estimates have been used in order to define the physical conditions of the evolving metamorphism. Thermo-barometers were only applied to mineral pairs in mutual contact and with rational boundaries.
Pressure and temperature stability fields of metamorphic assemblages or reaction equilibria were calculated using Thermocalc (Holland and Powell, 1990) and Perplex (Connolly, 1990). Activities for Thermocalc calculations were obtained using Ax (Holland and Powell, 2000).
T - estimates have been obtained using the calibrations of the Grt-Cpx thermometer (Ai, 1994; Ellis and Green, 1979; Krogh, 1988; Krogh, 2000; Powell, 1985; Sengupta et al., 1989) widely applied also to eclogite facies rocks (see Carswell and Harley, 1990). Grt-Cpx thermometer, applied to the post-D1a , post-D1b and syn-D2 pairs of Grt-OmpI, in eclogites and Lws-bearing eclogites, yields a T interval of 550 ± 80°C for P from1.5 to 2.0 GPa; according to Phillippot and Kienast (Philippot and Kienast, 1989) the large interval of temperature, obtained using Grt-Cpx thermometer, may be due to the dependence of the estimated temperature with respect to Fe3+ content in OmpI. The thermometer based on the Ti content in amphiboles (Otten, 1984) has been applied to Amp-Chl schists, giving results spanning between 570 ± 10°C. Minimal P can be estimated applying the barometer calibrated on the basis of the XJd content of omphacites in eclogites (Carswell and Harley, 1990; Holland, 1980). This calibration applied to the Omp underlying the S2 foliation of eclogites gives a P of 1.5 ± 0.1 GPa.
T and P of the superposed metamorphic assemblages are also constrained by the comparison with existing petrogenetic grids and recalculated grids using internally consistent datasets (Connolly, 1990; Holland and Powell, 1990): the pressures attained during the D1 stage can be constrained by the omphacite-in reaction reported by Poli (Poli, 1993; Poli and Schmidt, 1995) in basaltic system (Fig. 7).
The univariant reactions AmpI + Ep + GrtI = OmpI + GrtII + Pg + Qtz, Omp + Ep + Grt = Lws + Grt and WmI (Pg) + Ep = Lws + OmpI constrain the post-D1a stage, as well the Lws break-down reactions Lws = Ky + Ep and Pg = Omp + Ky + V (Fig. 7).
The post-D1b metamorphic assemblage Omp + Amp + Wm + Ky + Ep + Rt ± Qtz is also constrained by the univariant curves Lws = Ky + Ep and Pg = Omp + Ky + V at P2.0 GPa and T600°C
The D2 stage is constrained by the univariant reactions Omp + Ky ± V = Pg ± Ep ± Amp, Lws = Ep + Ky and by the general omphacite-in reaction in eclogites: in figure 7 the reactions defining the stability field for D2 paragenesis, and their references, are reported.
The reactions Ank + Amp + H2O = Chl + Cpx + CO2 and Dol + Amp + H2O = Chl + Cpx + CO2 (calculated using Thermocalc) for ultramafites also agree with the P-T interval of the syn-D2 assemblages.
Syn-D3 retrograde evolution can be well constrained in eclogites by the breakdown of OmpI (aJd=0.4) following the reactions Jd + Qtz = Ab, Omp + Grt = Amp + Grt + Ep and by the Grt-in reaction after Liu et al. (1996).
Figure 7. P-T-d-t path
A) P-T-d-t path of the Ivozio complex rocks where coloured boxes represent P-T conditions estimated with respect to successive deformation stages (e.g. D1, post-D1a and post-D1b ). The reactions of amphibole decomposition (Amp-out), of plagioclase disappearance (Pl-out) and of garnet appearance (Grt-in) and the wet solidus are after the experiments of Liu et al. (1996) on the olivine-normative basalt-H2O system at the amphibolite to eclogite transition; Amp, Zo/Czo and Lws stability fields are after Poli and Schmidt (1995) and Schmidt and Poli (1998). Ky = Sil, And = Sill and And = Ky are after Spear (1993); Omp-in are after Poli (1993) and Schmidt (1993) respectively for experiments on basaltic and tonalitic systems; the reaction (a) Cpx + Ep + Grt = Lws + Grt after Poli and Schmidt (1997), the reaction Pg = Omp + Ky + V and Lws + Omp = Pg + Zo + Qtz are after Poli (1993). Equilibria Amp + GrtI + Ep = Omp + GrtII and Ab = Jd + Qtz for eclogites have been recalculated using Thermocalc (Holland and Powell, 1990); equilibria Chl + Ttn + Qtz + Wm = Cpx + Rt + Wm + V for schists, Dol + Cpx + Atg + Rt = Ilm + Di + V(COH) for serpentinites and Dol + H2O = Chl + Cpx +CO2 , Ank + Amp + H2O = Chl + Cpx +CO2 for ultramafites have been calculated using Peprlex (Connolly, 1990). Dashed lines correspond to the T and P intervals inferred with thermo-barometers (see text).
B) P-T-t path of the Ivozio complex compared with successive T/depth ratios and a standard stable geotherm (Vi ; Spear, 1993). Absolute age data referred to successive deformation-metamorphic re-equilibration stages are deduced from literature as discussed in the text.