4.1. Major elements
The major element composition of minerals was obtained by EMPA. Representative EMPA data of minerals, instrumental details and analytical conditions are given in the Appendix C of the Data Repository (Escuder-Viruete et al., 2021). During the analysis, magmatic minerals were carefully distinguished from those recrystallized by metamorphic processes.
Olivine grains are compositionally unzoned and have the same composition in a given rock sample. In the pyroxenites, the Mg# values for olivine range from 77.8 to 85.4, with an average of 81.1 (Fig. 6). In the gabbroic rocks, olivine has Mg# values of 77.1-83.8 (average 78.6) in the olivine gabbronorites, 69.7-79.9 (average 76.2) in the troctolites, and 69.8-72.1 (average 70.5) in the oxide gabbronorites. The Mg# versus NiO diagram (Fig. 6) shows that olivine in some pyroxenites and gabbronorites has relatively high Mg# (~85) and NiO (0.15 wt.%), comparable to the olivine found in the SSZ mantle pyroxenites of Solomon Islands (Berly et al., 2006). These olivine compositions are close to the most evolved values on a mantle differentiation trend, along which Mg# and NiO both decrease, as defined in the Fig. 6 by the olivine compositions of the Puerto Plata and La Cuaba harzburgites of the Caribbean island arc (Escuder-Viruete et al., 2014; Escuder-Viruete & Castillo-Carrión, 2016). The mantle differentiation trend follows the compositional fields of olivine in the mantle peridotites of Oman (Bodinier & Godard, 2007) and the Cabo Ortegal (Santos et al., 2002). In contrast, the olivine in most of the pyroxenites and gabbroic rocks has significantly lower Mg# (~70-80) and NiO concentrations (<0.1 wt.%), comparable to olivine in the lower crustal gabbronorites of Talkeetna arc (Green et al., 2006). These compositional relations indicate olivine crystallization from an already differentiated melt, following a crustal differentiation trend. The decrease of Mg# in olivine broadly reflect the crystallization of gabbronorites, troctolites and oxide gabbronorites as melts progressively evolve.
Spinel is rare in the pyroxenites. It is Cr and Al-rich [Cr#>0.5; Cr#=100xCr/(Cr+Al)] in the clinopyroxenites and more Al-rich (Cr#<0.5) in part of the websterites. Spinels in the rest of websterites and gabbronorites are Mg-Al-rich hercynite, very poor in Cr (Cr#<0.05). The plastically deformed and recrystallized gabbronorites typically contain ilmenite grains, as well as exsolved ilmenite-magnetite pairs. TiO2 contents in magnetite from these gabbronorites range between 5.2 wt.% and 3.0 wt.%.
Clinopyroxene has a relatively limited compositional variation, both in the pyroxenites and the gabbronorites. It ranges in composition from Al-Cr diopside to Al-Fe diopside and does not show systematic zoning in individual grains (Appendix C). Clinopyroxene has Mg# values of 83.7-89.1 (average 86.0) in the clinopyroxenites and 85.9-89.6 (average 87.6) in the websterites. In the gabbroic rocks, clinopyroxene has Mg# values of 82.4-88.6 (average 85.0), 86.7-88.0 and 73.6-86.8 (average 79.2) in the gabbronorites, troctolites and oxide gabbronorites, respectively. The Fig. 7b shows that these Mg# values are lower than those of the clinopyroxenes in the Puerto Plata and La Cuaba harburgites, SSZ (fore-arc) mantle peridotites and pyroxenites. However, clinopyroxene compositions in the Rio Boba sequence overlap those of the Solomon Islands mantle pyroxenites. In Fig. 7b, the clinopyroxenes define in each group of rocks a sub-parallel crustal fractionation trend, from high Mg# and low Al2O3 (1.6 wt.%) to lower Mg# and higher Al2O3(3.6 wt.%), overlapping the compositional fields of arc-related crustal pyroxenites and mafic cumulates. The Cr2O3 contents range between 0.04 and 0.6 wt.% and are generally correlated with Mg#, with the exception of the oxide gabbronorites which have very low Cr2O3 (<0.1 wt.%). On the other hand, the clinopyroxenes have very low-Ti in all analyzed samples, particularly in the websterites and troctolites, similar to those of island arc cumulates and unlike the more TiO2-rich clinopyroxene compositions of the ocean-ridge cumulates (Fig. 7d). TiO2 increasing up to 0.45 wt.%, with decreasing Mg#, also delineating a fractionation trend. This trend is followed at a lower Mg# by the composition of clinopyroxene in the mafic and intermediate lavas of the Puerca Gorda and Los Caños Formation.
In the TiO2-Na2O-SiO2/100,ternary diagram of the Fig. 7a (Beccaluva et al., 1989), clinopyroxene compositions of the Rio Boba sequence are compared with the reference fields for diverse basaltic lavas in ophiolites as reported by Saccani and Photiades (2004). The clinopyroxenes of the pyroxenites, troctolites and gabbronorites plot in the fields of boninites, fore-arc basalts/basaltic andesites and island arc tholeiites (IAT), while the clinopyroxenes of the oxide gabbronorites fall exclusively in the IAT field due to the relative larger content in Na2O. In this diagram, clinopyroxene compositions from Puerca Gorda metavolcanic rocks and from Puerto Plata gabbroic rocks also display chemical compositions comparable to clinopyroxenes from boninitic basalts and intra-oceanic, fore-arc basalts/basaltic andesites.
Orthopyroxene compositions correlate with coexisting clinopyroxene compositions in a given plutonic rock type, but have slightly lower Al2O3 and slightly lower Mg# values (Fig. 7c). In the clinopyroxenites, orthopyroxene has a narrow compositional range, with high Mg# of 85.2-87.0 (average 85.8) and low Al2O3 (1.36-1.54 wt.%), which is different of those from the abyssal and SSZ mantle peridotites. In the websterites, orthopyroxene have Mg# values of 81.2-82.1 and low Al2O3 of 2.1-2.79 wt.%. Orthopyroxene compositions from the pyroxenites plot in the fields of arc crustal pyroxenites and SSZ mantle pyroxenites of Solomon Islands (Fig. 7b). Orthopyroxene has low Al2O3 (1.45-3.1 wt.%) and Mg# values of 73.6-82.1 (average 76.1) in the troctolites, 80.0-80.6 in the gabbronorites and 74.5-74.9 in the oxide gabbronorites. As in the case of clinopyroxene, the overall orthopyroxene compositions define a crustal fractionation trend in which the Al2O3 slightly increases with decreasing Mg#, along the fields of arc crustal pyroxenites and arc-related mafic cumulates (Fig. 7c). On the other hand, TiO2 in the orthopyroxene are very low, ranging from 0.08-0.16 wt.% in the pyroxenites to 0.02-0.18 wt.% in the gabbroic rocks. Cr2O3 is very low and range between 0.32 and 0.01 wt.%.
Plagioclase is an interstitial phase in the pyroxenites and the most abundant phase in the troctolites and gabbronorites. However, there is minimal intra-grain zoning or variation in anortite content (XAn ). Measured XAn ranges between 0.90 to 0.98 in the pyroxenites, 0.94 to 0.99 in the troctolites, and 0.90 to 0.94 in the gabbronorites (Appendix C). Some of the plagioclases analyzed in the gabbronorites show rims slightly more calcic than the cores.