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.