The obducted sections of intraoceanic arc lithosphere represent a favourable setting to investigate the crust generation in nascent arcs. The New Caledonia ophiolite hosts a mafic-ultramafic igneous sequence overlying harzburgitic rocks that underwent very high degrees of partial melting in a forearc setting (Marchesi et al., 2009; Secchiari, 2016). This sequence has been interpreted as a crust-mantle section in a nascent arc (Pirard et al., 2013). The igneous rocks include dunites, wehrlites and mafic rocks (mainly gabbronorites). Dunites and wehrlites derive from melt/peridotite interactions involving primitive arc tholeiites and boninitic liquids, whereas the mafic rocks are the result of cumulus processes (Marchesi et al., 2009; Pirard et al., 2013). The samples of this study are olivine gabbronorites occurring in the upper part of the sequence as metre-sized sills. Subhedral Ca-rich plagioclase (An = 90-96 mol. %) is the dominant phase (50-80 vol %). Mg-rich olivine (5-15 vol %, Fo = 87-89 mol %) occurs as irregularly shaped crystals. Cpx (15-20 vol%) is generally rimmed by interstitial to poikilitic Opx (5-15 vol %). Cpx shows high Mg# (88-92), low Al 2 O 3 (1.5-2.4 wt%) and negligible TiO 2 and Na 2 O contents. Bulk rock compositions are characterized by high Mg# (86-93) and very low incompatible trace element contents (e.g. LREE concentrations mostly below 0.1 times chondrite). They show marked LREE depletion, nearly flat and low HREE (Yb N = 0.2-0.9) and positive Eu anomalies. The highly depleted nature of these rocks is mirrored by Cpx trace element composition. In contrast, fluid-mobile trace elements are relatively enriched and distinct Pb and Sr positive anomalies are observed in both whole rocks and Cpx. The calculated compositions of putative liquids in equilibrium with the most primitive gabbronorites are consistent with melting of a refractory peridotite source in the spinel stability field. The inferred melts have high Mg# of 75-76 and very low REE, Nb, Zr, Hf contents. On the other hand, Ba, Pb and Sr spikes in the calculated liquids argue for the involvment of a subduction-related component. Overall, the liquids show geochemical signatures akin to olivine boninite melts with lower absolute concentrations of LREE and other incompatible trace elements (Ba, Pb, Sr, Zr, Hf). We propose that the New Caledonia gabbronorites reflect injection of primitive, ultradepleted (“boninitelike”) magma batches in the lower forearc crust, possibly before aggregation, during the first phases of arc formation. The investigated gabbronorites provide homogeneous initial Nd isotope compositions (e Nd ranging between +8.2 and +9.4), coupled with high 208Pb/207Pb and 207 Pb /Pb ratios, falling in a domain intermediate between DMM and Pacific sediments. The heterogeneous Pb isotopic signature of the gabbronorites most likely reflect a subarc asthenospheric mantle source variably modified by subduction-related fluids.
Subduction-related ultradepleted melts in a nascent arc: geochemical and isotopic evidence from the intrusive sequence of the new caledonia ophiolite
MACERA, PATRIZIA;
2016-01-01
Abstract
The obducted sections of intraoceanic arc lithosphere represent a favourable setting to investigate the crust generation in nascent arcs. The New Caledonia ophiolite hosts a mafic-ultramafic igneous sequence overlying harzburgitic rocks that underwent very high degrees of partial melting in a forearc setting (Marchesi et al., 2009; Secchiari, 2016). This sequence has been interpreted as a crust-mantle section in a nascent arc (Pirard et al., 2013). The igneous rocks include dunites, wehrlites and mafic rocks (mainly gabbronorites). Dunites and wehrlites derive from melt/peridotite interactions involving primitive arc tholeiites and boninitic liquids, whereas the mafic rocks are the result of cumulus processes (Marchesi et al., 2009; Pirard et al., 2013). The samples of this study are olivine gabbronorites occurring in the upper part of the sequence as metre-sized sills. Subhedral Ca-rich plagioclase (An = 90-96 mol. %) is the dominant phase (50-80 vol %). Mg-rich olivine (5-15 vol %, Fo = 87-89 mol %) occurs as irregularly shaped crystals. Cpx (15-20 vol%) is generally rimmed by interstitial to poikilitic Opx (5-15 vol %). Cpx shows high Mg# (88-92), low Al 2 O 3 (1.5-2.4 wt%) and negligible TiO 2 and Na 2 O contents. Bulk rock compositions are characterized by high Mg# (86-93) and very low incompatible trace element contents (e.g. LREE concentrations mostly below 0.1 times chondrite). They show marked LREE depletion, nearly flat and low HREE (Yb N = 0.2-0.9) and positive Eu anomalies. The highly depleted nature of these rocks is mirrored by Cpx trace element composition. In contrast, fluid-mobile trace elements are relatively enriched and distinct Pb and Sr positive anomalies are observed in both whole rocks and Cpx. The calculated compositions of putative liquids in equilibrium with the most primitive gabbronorites are consistent with melting of a refractory peridotite source in the spinel stability field. The inferred melts have high Mg# of 75-76 and very low REE, Nb, Zr, Hf contents. On the other hand, Ba, Pb and Sr spikes in the calculated liquids argue for the involvment of a subduction-related component. Overall, the liquids show geochemical signatures akin to olivine boninite melts with lower absolute concentrations of LREE and other incompatible trace elements (Ba, Pb, Sr, Zr, Hf). We propose that the New Caledonia gabbronorites reflect injection of primitive, ultradepleted (“boninitelike”) magma batches in the lower forearc crust, possibly before aggregation, during the first phases of arc formation. The investigated gabbronorites provide homogeneous initial Nd isotope compositions (e Nd ranging between +8.2 and +9.4), coupled with high 208Pb/207Pb and 207 Pb /Pb ratios, falling in a domain intermediate between DMM and Pacific sediments. The heterogeneous Pb isotopic signature of the gabbronorites most likely reflect a subarc asthenospheric mantle source variably modified by subduction-related fluids.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
