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.