Fig. 9. Mg# versus Al2O3 (a), TiO2 (b), CaO (c), FeOT (d), Sr/Y (e),
Ti/Zr (f) and V/Ti (g) variation diagrams for mafic and ultramafic rocks
of the Rio Boba plutonic sequence and mafic metavolcanic rocks of the
Puerca Gorda Schists (see also Escuder-Viruete et al., 2011c). All data
on anhydrous basis in wt.%. Gray and light gray fields correspond to
low-, intermediate- and high-Ca boninites from the ODP Leg 125 (Crawford
et al., 1989: Pearce et al., 1992; Pearce and Peate, 1995; Taylor et
al., 1994) and SSZ mantle pyroxenites of Solomon Islands (Berly et al.,
2006). Plutonic rocks from the Early to Middle Jurassic Talkeetna Arc
section (Greene et al., 2006) are plotted for comparisons with a
well-documented arc crustal sequence analog. Points and lines join melts
obtained experimentally for the fractional crystallization of anhydrous,
mantle derived, tholeiitic melts at 0.7 and 1.0 GPa (liquid lines of
descent from Villiger et al., 2004, 2007). See text for explanation.
As the Mg# decreases, Al2O3 and CaO
first define a rapid increase in the pyroxenites
(Al2O3 from 4.7 to 14.0 wt.%), followed
by a regular decrease in the gabbroic rocks
(Al2O3 from 22 to 16 wt.%), with a
minimal compositional overlap between gabbronorites and oxide
gabbronorites. This change in trend of
Al2O3 and CaO coincides with the
initiation of plagioclase crystallization. These rocks, however, have an
obvious plagioclase cumulate component and their compositions are
therefore strongly controlled by the cumulate phases. This compositional
effect due to plagioclase crystallization and cumulate formation matches
the lower Al2O3 and CaO contents of the
mafic volcanic rocks of Puerca Gorda Schists, which for similar Mg#
values do not commonly contain abundant phenocrysts. This suggests that
these volcanic rocks are the extrusive equivalents of the liquids in
equilibrium with the cumulates.
The degree of magmatic fractionation is also expressed with the
progressive increase in TiO2 and FeOTwith decreasing Mg#. However, all the studied samples have very low
TiO2, in particular pyroxenites (0.10-0.24 wt.%),
troctolites (<0.1 wt.%) and olivine gabbronorites (0.03-1.21
wt.%), which are similar to those of the mafic metavolcanic rocks of
the Puerca Gorda and the primitive boninites of the ODP Leg 125. The
relatively higher TiO2 of oxide gabbronorites may be due
to the accumulation of Fe-Ti oxides in these more evolved magmas.