5.3. Trace elements
The pyroxenites and gabbroic rocks are highly depleted in terms of REE
and other trace elements, having concentrations lower than those of
N-MORB and in some cases below 0.1 times N-MORB (Appendix E). Overall,
REE values (ΣREE) systematically increase as follows: clinopyroxenites
and websterites (3.2-9.0), to olivine gabbronorites (2.8-5.8),
troctolites (2.0-8.8), gabbronorites (4.2-16.3) and oxide gabbronorites
(4.3-16.7). In the N-MORB (N) normalized diagrams of the Fig. 10, all
samples show remarkably parallel trace element patterns. They are
characterized by an enrichment in LILE (Rb, Ba, K, Pb and Sr, but
generally no Th) relative to the HREE, Ti and Y, and have high fluid
mobile/immobile element ratios (i.e., Ba/La, Sr/Nd and Pb/Ce
<<1). Such features are commonly attributed to an
aqueous fluid component in the source (Pearce & Peate, 1995). The
patterns also show pronounced negative anomalies in HFSE (i.e. Ta, Nb,
Zr and Hf) which are typical of subduction-related magmas (Pearce &
Peate, 1995). It should be noted that the lack of correlation between
LILE contents and the loss on ignition (not shown) suggest that
metamorphism and low-T alteration did not influenced the trace element
abundances of these rocks.
In the N-MORB normalized diagramas (Fig. 10a), the clinopyroxenites and
websterites shown generally a LREE depletion
(0.10<LaN/NdN<1.12)
and flat HREE segments
(0.60<SmN/YbN<0.97).
Pyroxenites do not present a clear Eu anomaly [Eu*=0.88-1.42, where
Eu*=
(EuC/(SmC+GdC)0.5)].
The troctolites also display a LREE depletion
(0.12<LaN/NdN<1.02)
and a sub-horizontal to moderate HREE depletion
(0.75<SmN/YbN<2.92).
These rocks exhibit a moderate to pronounced positive Eu anomaly
(Eu/Eu*=1.48-2.15), reflecting their plagioclase-cumulate nature.
Compositionally, they are comparable to the intermediate troctolites of
the Puerto Plata complex (Fig. 10b). The gabbronorites have a pronounced
LREE depletion
(0.18<LaN/NdN<0.54)
and a flat to slight HREE depletion
(0.72<SmN/YbN<1.4).
The pronounced LREE depletion
(0.19<LaN/NdN<0.72)
and the flat HREE segment
(0.87<SmN/YbN<1.18)
is also characteristic of the trace element patterns of the oxide
gabbronorites. Gabbronorites and oxide gabbronorites present a moderate
Eu anomaly (Eu/Eu*=1.17-1.66 and 1.18-2.03, respectively) indicative of
plagioclase accumulation. Compositionally, the gabbronorites of the Rio
Boba sequence are comparable to the lower and upper gabbronorites of the
Puerto Plata complex, as well as the more primitive plutonic rocks of
the Talkeetna Arc (Fig, 10c, d). The Ti anomaly relative to HREE is
slightly negative in the pyroxenites, troctolites and gabbronorites.
However, the evolved oxide gabbronorites show a marked positive Ti
anomaly, related to the late crystallization of Fe-Ti oxides (Fig. 10e).
Given the high Mg#, the significant LREE depletion, very low
TiO2 concentrations and low HREE values, indicate a
strongly depleted mantle source for both pyroxenites and gabbroic rocks
of the Rio Boba sequence and/or high-degrees of partial melting.
The trace element compositions of the Puerca Gorda mafic metavolcanic
rocks have been reported by Escuder-Viruete et al. (2011c). Their
patterns are characterized by a moderate to strong LREE enrichment
(1.5<LaN/NdN<2.2)
and HREE depletion
(1.2<SmN/YbN<2.4)
(Fig. 10f). These metavolcanic rocks show a prominent negative Ti
anomaly, but they lack an Eu anomaly (Eu/Eu*=0.90-1.12). The LREE
depletion, low-TiO2 and lower Ti/V values, as well as
lower HREE levels, suggest that the source for these rocks was strongly
depleted mantle and/or the protoliths were affected by high degrees of
partial melting. Their trace element patterns are comparable to low-Ti
IAT and boninites of the Lower Cretaceous Puerto Plata (Los Caños Fm)
and El Cacheal complexes (Fig. 10h; Escuder-Viruete et al., 2014), as
well as boninites from the Marianas, New Caledonia and Izu-Bonin
fore-arc (Pearce & Peate, 1995; Pearce & Reagan, 2019).