6.1. Formation of the plutonic sequence by fractional
crystallization
The mafic and ultramafic rocks of the Rio Boba plutonic sequence exhibit
textures varying from adcumulate to orthocumulate. The cumulate textures
are the product of solid-liquid separation processes, evidenced by modal
and grain-size layering from decimeter to millimeter scale. Cumulate
textures imply fractional crystallization in a magmatic system as the
main differentiation process. In this situation, it is not surprising
that the variation in the whole-rock major and trace-element composition
of the rock is controlled by the cumulate phases.
Plagioclase is the dominant phase in the cumulate gabbronorites. The
whole-rock Al2O3 and CaO contents are
the result of plagioclase fractionation. The variable, but always
present, positive Eu anomaly clearly reflects the cumulate nature of the
gabbronorites and troctolites. The absence of a clear positive Eu
anomaly in the pyroxenites suggests that plagioclase was not present in
the primary melt in equilibrium with the residual mantle. Also, the
absence of a Eu anomaly in the related Puerca Gorda volcanic rocks
indicates that plagioclase accumulation processes did not affect them,
which is consistent with the absence of plagioclase phenocrysts. The
effects of the plagioclase fractionation can be visualized with the help
of diagrams of whole-rock trace elements ratios. In Fig. 9, the trend in
Sr/Y appears generally to be the result of plagioclase fractionation in
the cumulate gabbronorites and in the more evolved oxide gabbronorites,
analogously to the Sr/Y trend described in the Talkeetna arc (Green et
al., 2006). The diagram also shows that for a similar value of Mg#, the
Sr/Y ratio is generally higher due to the plagioclase accumulation in
gabbroic rocks than in Puerca Gorda mafic volcanic rocks, whose magmatic
evolution was not primary controlled by the fractionation of this
mineral.
Fe-Ti oxides (magnetite-ilmenite) are also major phases in the gabbroic
rocks and their crystallization largely controlled the whole-rock
FeOT and TiO2 of the oxide gabbronorites
and related mafic volcanic rocks. This is particularly evident in the
trace-element patterns of Fig. 10, where the oxide gabbronorite samples
have pronounced positive Ti anomalies, and the mafic volcanic rocks of
Puerca Gorda exhibit complementary negative Ti anomalies. Although the
parent magma was probably depleted in Ti relative to HREE in the source,
the crystallization of Fe-Ti oxides within the gabbronorites and
particularly in the oxide gabbronorites gave rise to magmas depleted in
TiO2 that formed the volcanic sequence. In the Fig. 9,
the crystallization of V-rich, Fe-Ti oxides in the gabbronorites is
reflected by a trend of increasing Ti/Zr and decreasing V/Ti from the
more primitive gabbronorites to the more evolved oxide gabbronorites. As
Zr appears to be controlled almost exclusively by fractionation,
increasing of the Ti/Zr ratio monitors the Fe-Ti oxide accumulation in
the oxide gabbronorites, which does not take place in volcanic rocks.
The trends of variation in Ti/Zr and V/Ti in the gabbroic rocks of Rio
Boba are also recorded in the plutonic and volcanic rocks of Talkeetna
arc section (Fig. 9), which have been interpreted by Green et al. (2006)
as a strong signature of Fe-Ti oxide fractionation.