Supra-subduction mantle pyroxenites in an infant subduction system: the New Caledonia ophiolite record
Articolo
Data di Pubblicazione:
2021
Citazione:
Supra-subduction mantle pyroxenites in an infant subduction system: the New Caledonia ophiolite record / E. Ferrari, A. Secchiari, A. Montanini, D. Cluzel. - (2021). ((Intervento presentato al convegno vEGU21: Gather Online : 19–30 April tenutosi a (online) nel 2021 [10.5194/egusphere-egu21-15299].
Abstract:
Pyroxenites constitute the major form of heterogeneity in the upper mantle. Their occurrence in
supra-subduction zone settings is mostly testified by veins and layers in refractory ophiolitic
peridotites, where they represent a crucial witness of melt migration in the forearc/subarc
environment [1,2]. The New Caledonia ophiolite hosts one of the largest forearc mantle section
worldwide, providing a unique perspective into upper mantle processes. The sequence is
dominated by ultra-depleted harzburgites [3], locally overlain by mafic-ultramafic cumulates
[4,5,6]. The harzburgites are highly refractory residues that register a multi-phase evolution,
including fluid-assisted melting in a forearc environment and contamination by fluid- and melt
inputs triggered by Eocene subduction [1]. Pyroxenitic rocks intruding the harzburgites are only
known in the Bogota peninsula shear zone, which records HT deformation along a paleotransform
fault [7]. In this contribution, we report a comprehensive petrological and geochemical
characterization on a new set of pyroxenites from this locality. The pyroxenites (~5-15 cm-thick)
generally cut the peridotite foliation at variable angles, but concordant, locally boudinaged, layers
also occur. Pyroxenite textures range from cumulitic to porphyroclastic or granoblastic-polygonal.
The studied samples mostly consist of amphibole-bearing (5-44 vol.%) websterites, with variable
amounts of orthopyroxene (27-67 vol.%) and almost constant clinopyroxene contents (~ 25-29
vol.%). Minor olivine-bearing orthopyroxenites are also present. Accessory phases include high-Ca
(An= 82-86 mol%) plagioclase, Cr-rich spinel (Cr# = 50-61), sulfides and, occasionally, apatite.
Pyroxenes displays high Mg# (Mg# Opx= 91-92; Mg# Cpx= 84-93), coupled with low Al2O3
contents (0.97-1.92 wt% and 1-2.42 wt% for orthopyroxene and clinopyroxene, respectively).
Amphibole is high Mg# edenite. Application of conventional pyroxene thermometry yield
equilibration temperatures ranging between 930-1040°C, comparable to the enclosing
harzburgites (~ 950°C), whereas amphibole-plagioclase geothermometer provides lower
temperatures (~ 800°C). Bulk rock composition of the websterites show variable Mg# (82-91) and
REE concentrations ranging between 1 to 10 times chondritic values. They are characterized by flat
to LREE-depleted (LaN/SmN 0.28-0.92) patterns, coupled to weak MREE-HREE fractionation
(GdN/YbN = 1.73-1.92) and Eu negative anomalies. By contrast, orthopyroxenites display notably
lower concentrations (0.1≤REE≤1 chondrite abundances). As a whole, clinopyroxene REE patterns
of the websterites mirror bulk rocks at higher absolute values. Putative melts in equilibrium with
clinopyroxene indicate strongly enriched compositions (up to 300 times chondritic values) coupled
to variable LREE-HREE fractionation (LaN/LuN = 3-19) and flat to fractionated HREE (GdN/LuN 1-2).
Such enriched liquids, which show some analogies with pre-obduction adakite-like dikes [8], have
never been recorded in the MTZ cumulitic sequence of the New Caledonia ophiolite and shed new light on the magmatic activity in the early stage of subduction.
Tipologia IRIS:
23 - Pubblicazione su portale
Elenco autori:
E. Ferrari, A. Secchiari, A. Montanini, D. Cluzel
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