6.2. Experimental constraints on parental melt, phase
crystallization sequence, pressure conditions and water content
Experimental studies indicate that fractional crystallization of
anhydrous, mantle derived, tholeiitic liquids in the temperature range
of 1060-1330 ºC at 0.7 GPa (lower crust conditions) and 1.0 GPa (base of
the arc crust conditions) produces phase relations in proportions and
compositions that explain the characteristics of ultramafic to mafic
lower crustal cumulate rocks (Müntener et al., 2001; Villiger et al.,
2004, 2007; Müntener & Ulmer, 2018). Although the temperature of first
appearance of each phase varies for each phase assemblage, the
crystallization sequence is similar at 0.7 and 1.0 Gpa. With falling
temperature in the experimental run (Fig. 9), the crystallization
sequence begins with olivine and spinel as liquidus phases at 1300 ºC
and continues with the appearance of olivine, spinel, clino and
orthopyroxene, until the disappearance of olivine at 1240 ºC. The first
appearance of plagioclase is at 1210 ºC at both 0.7 and 1.0 GPa,
coprecipitating with spinel, clinopyroxene and ortopyroxene. Between
1210 ºC and 1180 ºC, plagioclase and spinel crystallize
(orthopyroxene-out). At 1060 ºC the stable assemblage is clinopyroxene,
plagioclase and ilmenite (± quartz). This crystallization sequence is
controlled by the peritectic reaction olivine + liquid = orthopyroxene
and the early plagioclase saturation (e.g. Müntener et al., 2001).
Therefore, the experimentally obtained crystallization sequence for
anhydrous tholeiitic melts explains the association of mafic and
ultramafic rocks in the Rio Boba plutonic sequence, where the pyroxene
crystallization precedes plagioclase crystallization. In this sense, the
modal compositions, mineral chemistry and whole-rock compositions of the
Rio Boba pyroxenites and gabbroic rocks represent a cumulate sequence
formed by fractionation of tholeiitic magmas with very low initial
H2O in the lower crust of the arc. Melts evolved along
the simplified crystallization sequence of olivine → pyroxenes →
plagioclase → Fe-Ti oxides (± quartz).
Several arguments support the formation of the Rio Boba plutonic rocks
following this crystallization sequence. (1) Mg# and NiO in olivine
decrease progressively from the pyroxenites and troctolites to the
olivine gabbronorites and oxide gabbronorites. (2) The decrease in Mg#
and the increase in Al2O3 and
TiO2 in the orthopyroxene and clinopyroxene are
negatively correlated from the pyroxenites to gabronorites and oxide
gabronorites. (3) The Mg# decrease in the spinel, which varies in
composition from Cr-rich spinel to hercynite, culminating in Fe-Ti
oxides in the most evolved rocks. (4) Anorthite-rich, anhedral
plagioclase occurs between cumulus olivine and pyroxenes in the
pyroxenites, which is attributed to the entrapment of melt among cumulus
phases. (5) The crystallization (and accumulation) of the successive
mineral phases of the sequence exerts a control on the variation of the
whole-rock major-element compositions
(Al2O3, CaO, FeOT and
TiO2). (6) The incompatible trace elements
concentrations (e.g. Th, HFSE and REE) increase with the decrease in
Mg#, both in clinopyroxene and in whole-rock, from the clinopyroxenites
and websterites to troctolites and gabbronorites (as well as the related
Puerca Gorda volcanic rocks). (7) The magmatic amphibole is very scarce
or absent, appearing only as a late magmatic phase.
For these reasons, we propose that the Rio Boba plutonic sequence is of
cumulus origin and was controlled by fractional crystallization (and
post-cumulus melt entrapment), as follows. The initial precipitation of
olivine and Cr-rich spinel was followed by the appearance of
clinopyroxene and orthopyroxene, giving rise to olivine clinopyroxenite
and websterite cumulates. Residual melts evolved through a fractional
crystallization-, initially controlled by olivine separation, which led
to the formation of olivine-free websterites. Subsequent melts were
controlled by the appearance of An-rich plagioclase, and clinopyroxene,
resulting in the development of the gabbronorites. The appearance of
Fe-Ti oxide also plays a major role in the late-stage fractional
crystallization process and gave rise to the oxide gabbronorite.
Accordingly, the absence of magmatic amphibole and garnet in the
crystallization sequence implies a very low initial H2O
content in the magma (e.g. Alonso-Perez et al., 2009), and constraints
the formation of the cumulate sequence to intermediate pressures typical
of the lower arc crust (<1.0 GPa; Jagoutz et al., 2011).
However, the Rio Boba troctolites recorded a crystallization sequence in
which the crystallization of olivine and plagioclase precedes that of
pyroxene. Therefore, although volumetrically less important, troctolitic
gabbros represent a distinctive cumulate sequence formed by
fractionation of anhydrous tholeiitic magmas at lower pressures
(<0.45 GPa; Villiger et al., 2007).