The geochemical message from the New Caledonia gabbronorites: insights on depletion and contamination processes of the sub-arc mantle in a nascent arc setting
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Data di Pubblicazione:
2019
Citazione:
The geochemical message from the New Caledonia gabbronorites: insights on depletion and contamination processes of the sub-arc mantle in a nascent arc setting / A. Montanini, A. Secchiari, D. Bosch, P. Macera, D. Cluzel. ((Intervento presentato al convegno New Caledonia Peridotite Amphibious Drilling Workshop tenutosi a Montpellier nel 2019.
Abstract:
The New Caledonia ophiolite hosts one of the rare examples of crust-mantle sections built in a nascent arc
environment, providing the unique opportunity to investigate the first stages of magmatism in an intraoceanic
arc setting. The ophiolitic sequence dominantly consists of refractory harzburgites and dunites,
overlain by km-size lenses of ultramafic and mafic cumulates (dunites/pyroxenites, wehrlites and
websterites/gabbronorites in ascending order), which occur in the upper part of the sequence as decimetre
to metre-thick layers.
In this work, a comprehensive petrological, geochemical and Sr–Nd–Pb isotope investigation has been
carried out on the gabbronorites (Secchiari et al., 2018). They are mainly formed (≃ 55 to 70 vol%) of Carich
plagioclase (anorthite up to 96 mol%) and high Mg# (88–92), Al2O3-poor (1.5–2.4 wt%) clinopyroxene
(8–20 vol%), often rimmed by interstitial or poikilitic orthopyroxene (6–27 vol%). Mg-rich olivine (3–15 vol%,
Fo = 87–89 mol%) occurs as anhedral, resorbed crystals. Whole rock compositions display high Mg# (86–
92) and strikingly low trace element contents. They own LREE-depleted patterns, with nearly flat (0.82 ≤
DyN/YbN ≤ 1.00) and low HREE (YbN = 0.2–0.9) and positive Eu anomalies. Clinopyroxene trace element
chemistry mirrors the extreme depletion of the WR. By contrast, enrichments in FME (Fluid-Mobile Elements)
are observed for whole-rock and clinopyroxene.
FME enrichments and the high FME/immobile elements ratios shed light on the involvement of a subductionrelated
component during magma genesis. Geochemical models show that the gabbronorites crystallized
from primitive, ultra-depleted melts bearing evidence of fluid contamination processes, but with significantly
different geochemical signatures compared to boninitic rocks worldwide, i.e. lower LREE-MREE, and
absence of Nb depletion and Zr–Hf enrichments. Nd isotopes (+ 8.2 ≤ εNd(i) ≤ + 13.1), together with
radiogenic Pb isotopic ratios extending from DMM toward EM2/GLOSS-like compositions, support an origin
from a DMM source variably modified by slab fluids. We propose that the gabbronorites resulted from
melting a refractory mantle source, followed by crystallization of near-fractional single melt increments that were not homogenized before their emplacement in the fore-arc crust
Tipologia IRIS:
14 - Intervento a convegno non pubblicato
Elenco autori:
A. Montanini, A. Secchiari, D. Bosch, P. Macera, D. Cluzel
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