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 arc magmatism in a subduction setting. The sequence consists of refractory harzburgites, overlain by ultramafic (dunites and wehrlites) and mafic lithologies (gabbronorites). The gabbronorites occur in the upper part of the sequence as decimetre to metre-size sills. They are mainly formed (≃ 55 to 70 vol%) of Ca-rich plagioclase (An 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 (WR) compositions exhibit 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, FME enrichments are observed for WR and clinopyroxene. 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 ≤ εNdi ≤ + 13.1), together with radiogenic Pb isotopic ratios, support an origin from a DMM source variably modified by slab fluids. We propose that the geochemical signature of the New Caledonia gabbronorites reflect emplacement of primitive, non-aggregated, magma batches in the lower fore-arc crust, during the first phases of arc formation.

The contrasting geochemical message from the New Caledonia gabbronorites: insights on depletion and contamination processes of the sub-arc mantle in a nascent arc setting / A. Secchiari, A. Montanini, D. Bosch, P. Macera, D. Cluzel. - In: CONTRIBUTIONS TO MINERALOGY AND PETROLOGY. - ISSN 0010-7999. - 173:8(2018 Jul 30), pp. 66.1-66.26. [10.1007/s00410-018-1496-8]

The contrasting geochemical message from the New Caledonia gabbronorites: insights on depletion and contamination processes of the sub-arc mantle in a nascent arc setting

A. Secchiari
Primo
;
2018

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 arc magmatism in a subduction setting. The sequence consists of refractory harzburgites, overlain by ultramafic (dunites and wehrlites) and mafic lithologies (gabbronorites). The gabbronorites occur in the upper part of the sequence as decimetre to metre-size sills. They are mainly formed (≃ 55 to 70 vol%) of Ca-rich plagioclase (An 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 (WR) compositions exhibit 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, FME enrichments are observed for WR and clinopyroxene. 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 ≤ εNdi ≤ + 13.1), together with radiogenic Pb isotopic ratios, support an origin from a DMM source variably modified by slab fluids. We propose that the geochemical signature of the New Caledonia gabbronorites reflect emplacement of primitive, non-aggregated, magma batches in the lower fore-arc crust, during the first phases of arc formation.
English
Intra-oceanic arcs; New Caledonia; Ophiolite; Sr–Nd–Pb isotopes; Ultra-depleted gabbros;
Settore GEO/07 - Petrologia e Petrografia
Articolo
Esperti anonimi
Pubblicazione scientifica
Goal 14: Life below water
   Melt-rock reaction and melt migration in the MORB mantle through combined natural and experimental studies
   MINISTERO DELL'ISTRUZIONE E DEL MERITO
   2015C5LN35_002
30-lug-2018
Springer
173
8
66
1
26
26
Pubblicato
Periodico con rilevanza internazionale
miur
MIUR
Aderisco
info:eu-repo/semantics/article
The contrasting geochemical message from the New Caledonia gabbronorites: insights on depletion and contamination processes of the sub-arc mantle in a nascent arc setting / A. Secchiari, A. Montanini, D. Bosch, P. Macera, D. Cluzel. - In: CONTRIBUTIONS TO MINERALOGY AND PETROLOGY. - ISSN 0010-7999. - 173:8(2018 Jul 30), pp. 66.1-66.26. [10.1007/s00410-018-1496-8]
reserved
Prodotti della ricerca::01 - Articolo su periodico
5
262
Article (author)
si
A. Secchiari, A. Montanini, D. Bosch, P. Macera, D. Cluzel
File in questo prodotto:
File Dimensione Formato  
Secchiari_et_al-2018-Contributions_to_Mineralogy_and_Petrology 1.pdf

accesso riservato

Tipologia: Publisher's version/PDF
Dimensione 6.88 MB
Formato Adobe PDF
6.88 MB Adobe PDF   Visualizza/Apri   Richiedi una copia
Pubblicazioni consigliate

I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.

Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2434/946356
Citazioni
  • ???jsp.display-item.citation.pmc??? ND
  • Scopus 28
  • ???jsp.display-item.citation.isi??? 26
social impact