6.5. Conditions of formation of the mafic-ultramafic sequence
The coexistence of magmatic clinopyroxene and orthopyroxene provides an estimation of the pressure-temperature conditions of equilibration of the pyroxenites and gabbronorites, using the two-pyroxene thermometer and the enstatite-in-cpx barometer of Putirka (2008; updated in 2018). Calculated equilibrium temperatures for the pyroxenites and gabbronorites range from 854°C to 962°C (Fig. 12). Average temperatures calculated for clinopyroxenites (932±32 °C), websterites (889±13 °C), troctolites (861±5 °C), gabbronorites (921±20 °C), and oxide gabbronorites (882±25 °C), are within error, probably not distinguishable, and provide evidence for subsolidus recrystallisation at 840-930°C. These subsolidus temperatures are consistent with the occurrence of lobate grain boundaries, which are indicative of dynamic recrystallization at relatively high-temperatures (Passchier & Trouw, 1996).
However, the presence of exsolution textures in the pyroxenes of the pyroxenites and gabbroic rocks evidence a previous higher-temperature crystallization/cooling history. To estimate the temperature of crystallization, the composition of the original pyroxene was calculated from the complementary lamellae exsolutions. The area corresponding to the exsolutions relative to the host pyroxene was determined by analyzing images of grains displaying exsolution lamellae. Then the relative areas were combined with EMPA spot analyses of the individual phases to recalculate the pyroxene composition prior to exsolution. For amounts between 4 and 10% of orthopyroxene exsolution lamellae in clinopyroxene, the calculated temperature of crystallisation is significantly higher, as high as 950 °C-1078 °C in the clinopyroxenites and websterites, and 928 °C-1024 °C in the troctolites and gabbronorites. These crystallization temperatures for the original clinopyroxene are consistent with the experimental results of the fractional crystallization of anhydrous tholeiitic liquids in the temperature range between 1060 and 1330 ºC (at 0.7 GPa; Villiger et al., 2007).
The coronitic shells of orthopyroxene around olivine and symplectites of clinopyroxene + green spinel and/or amphibole + spinel between olivine and plagioclase, record the subsolidus cooling of the Rio Boba plutonic sequence from the granulite- to amphibolite-facies metamorphic conditions. Microstructural relationships, multiequilibrium thermobarometry and pseudosection analysis (in the NCKFMASH model system) suggest a P-T evolution of near isobaric cooling initially at ~0.7 GPa, accompanied by an increase in H2O activity (Fig. 12; Escuder-Viruete et al., in prep.). Microtextural relationships indicate that all these simplectites develop in both pyroxenites and gabbroic rocks after high-T ductile deformation.
The barometric calculations establish equilibrium pressures between 0.36 and 0.83 GPa (Fig. 12). These results are consistent with the absence of magmatic garnet in the Rio Boba plutonic sequence, and indicate that crystallization took place entirely at pressures below the stability limit of this mineral, which are of 0.6-0.8 GPa for temperatures of 800-1000 °C (at PH2O ~ Ptot ; Fig. 12). Average pressures calculated for clinopyroxenites (0.61±0.1 GPa), websterites (0.63±0.1 GPa), gabbronorites (0.76±0.13 GPa), and oxide gabbronorites (0.64±0.05 GPa) are similar within error, but clearly higher than those obtained for the troctolites (0.4±0.03 GPa). This suggest that the late intrusion of the troctolites took place at lower pressures (P ~ 4 GPa), after the intrusion and ductile deformation at high-T of the pyroxenites and gabronorites. This in turn has implications in the establishment of the magmatic evolution.