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.