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).