Fig. 10. N-MORB-normalized trace-element plots for mafic and ultramafic
rocks of the Rio Boba plutonic sequence, as well as for other regionally
related volcanic rocks. (a) Pyroxenites, (b)troctolites, (c) olivine
gabbronorites, (d) gabbronorites, (e) oxide gabbronorites, (f) Puerca
Gorda Schists, (g) main geochemical groups of Lower Cretaceous volcanic
rocks in Hispaniola, and (h) mafic volcanic rocks from El Cacheal
complex and Los CaƱos Fm of Puerto Plata ophiolitic complex (data from
Escuder-Viruete et al., 2006, 2011c, 2014, and this work).
MORB-normalizing values are from Sun and McDonough (1989). Boninite
compositions are from the ODP Leg 125 (Pearce et al., 1992; Pearce and
Peate, 1995; Taylor and Nesbitt, 1995; Pearce and Reagan, 2019). See
text for explanation.
Overall, the major element composition of the gabbronorites is similar
to the Talkeetna Arc rocks, though some of the more evolved Talkeetna
samples have lower CaO, TiO2 and FeOTfor similar Mg#. The pyroxenites display a restricted compositional
range and can be compared with the pyroxenites of the Solomon Islands,
although they have a lower Mg#.
The experimental models for anhydrous fractional crystallization of
primitive tholeiitic basalt at the base of the crust (1.0 GPa) and at
shallover crustal conditions (0.7 GPa) show a continuous differentiation
trend from high Mg# cumulates (dunite, lherzolite and websterite) to
evolved, low Mg# liquids. Gabbroic samples follow a trend subparallel
to the experimental liquid line of descent in the Mg# 80-40 interval.
This trend is continuous with no gaps in Mg#. The Rio Boba gabbroic
rocks plot away from the experimental crystallization lines at 0.7 and
1.0 Gpa in the CaO, TiO2 and FeOT vs.
Mg# diagrams, reflecting a variability that may be related to
fractional crystallization. These differences may be due to a different
starting basalt composition and/or the elimination of all solid phases
in each single fractionation step and/or the constant pressure
conditions followed in the modeling. With some exception, the
dunite-wehrlite-pyroxenite cumulates obtained in the modeling of
anhydrous fractional crystallization have higher Mg# values than the
Rio Boba pyroxenites, suggesting that these pyroxenites are products of
the crystallization of already evolved mantle-derived magmas. This is
consistent with the lower Mg# values with respect to the SSZ mantle
pyroxenites of the Solomon Islands.
The concentrations of Cr, Ni, V and Sc are higher in pyroxenites and
progressively decrease from the gabbronorites to the oxide gabbronorites
(Appendix E). The concentrations of these elements in the Puerca Gorda
mafic metavolcanic rocks are similar to those in the gabbronorites. With
some exception of the more evolved oxides gabbronorites, the mafic
plutonic rocks of the Rio Boba sequence and the metavolcanic rocks of
Puerca Gorda have Ti/V values equal to, and lower than, chondrite
(<10), which values are similar to those of the boninites,
suggesting high depletion in the mantle source. Zr concentration shows
an incompatible behavior increasing from very low levels in the
pyroxenites and gabbronorites to higher concentrations in the mafic
metavolcanic rocks.