Sr,Nd,Pb and trace element systematics of the New Caledonia harzburgites:Tracking source depletion and contamination processes in a SSZ setting

The New Caledonia ophiolite(Peridotite Nappe)consists primarily of harzburgites,locally overlain by mafic-ultramafic cumulates,and minor spinel and plagioclase lherzolites.In this study,a comprehensive geochemical data set(major and trace element,Sr-Nd-Pb isotopes)has been obtained on a new set of fresh harzburgites in order to track the processes recorded by this mantle section and its evolution.The studied harzburgites are low-strain tectonites showing porphyroclastic textures,locally grading into protomylonitic textures.They exhibit a refractory nature,as attested by the notable absence of primary clinopyroxene,very high Fo content of olivine(91-93 mol.%),high Mg#of orthopyroxene(0.91-0.93)and high Cr#of spinel(0.44-0.71).The harzburgites are characterised by remarkably low REE concentrations(<0.1 chondritic values)and display"U-shaped"profiles,with steeply sloping HREE(DyN/YbN=0.07-0.16)and fractionated LREE-MREE segments(LaN/SmN=2.1-8.3),in the range of modern fore-arc peridotites.Geochemical modelling shows that the HREE composition of the harzburgites can be reproduced by multi-stage melting including a first phase of melt depletion in dry conditions(15%fractional melting),followed by hydrous melting in a subduction zone setting(up to 15%-18%).However,melting models fail to explain the enrichments observed for some FME(i.e.Ba,Sr,Pb),LREE-MREE and Zr-Hf.These enrichments,coupled with the frequent occurrence of thin,undeformed films of Al2 O3,and CaO-poor orthopyroxene(Al2O3=0.88-1.53 wt.%,CaO=0.31-0.56 wt.%)and clinopyroxene with low Na2 O(0.03-0.16 wt.%),Al2 O3(0.66-1.35 wt.%)and TiO2(0.04-0.10 wt.%)contents,point to FME addition during fluid-assisted melting followed by late stage metasomatism most likely operated by subductionrelated melts with a depleted trace element signature.Nd isotopic ratios range from unradiogenic to radiogenic(-0.80<εNdi≤+13.32)and negatively correlate with Sr isotopes(0.70257≤87Sr/86Sr≤0.70770).Pb isotopes cover a wide range,trending from DMM toward enriched,sediment-like,compositions.We interpret the geochemical signature displayed by the New Caledonia harzburgites as reflecting the evolution of a highly depleted fore-arc mantle wedge variably modified by different fluid and melt inputs during Eocene subduction.

Multi-stage rodingitization of ophiolitic bodies from Northern Apennines(Italy): Constraints from petrography, geochemistry and thermodynamic modelling

The investigated mantle bodies from the External Ligurians(Groppo di Gorro and Mt.Rocchetta)show evidences of a complex evolution determined by an early high temperature metasomatism,due to percolating melts of asthenospheric origin,and a later metasomatism at relatively high temperature by hydrothermal fluids,with formation of rodingites.At Groppo di Gorro,the serpentinization and chloritization processes obliterated totally the pyroxenite protolith,whereas at Mt.Rocchetta relics of peridotite and pyroxenite protoliths were preserved from serpentinization.The rodingite parageneses consist of diopside+vesuvianite+garnet+calcite+chlorite at Groppo di Gorro and garnet+diopside+serpentine±vesuvianite±prehnite±chlorite±pumpellyite at Mt.Rocchetta.Fluid inclusion measurements show that rodingitization occurred at relatively high temperatures(264-334℃ at 500 bar and 300-380℃ at 1 kbar).Garnet,the first phase of rodingite to form,consists of abundant hydrogarnet component at Groppo di Gorro,whereas it is mainly composed of grossular and andradite at Mt.Rocchetta.The last stage of rodingitization is characterized by the vesuvianite formation.Hydrogarnet nucleation requires high Ca and low silica fluids,whereas the formation of vesuvianite does not need CO2-poor fluids.The formation of calcite at Groppo di Gorro points to mildly oxidizing conditions compatible with hydrothermal fluids;the presence of andradite associated with serpentine and magnetite at Mt.Rocchetta suggests Fe^3+-bearing fluids with fO2 slightly higher than iron-magnetite buffer.We propose that the formation of the studied rodingite could be related to different pulses of hydrothermal fluids mainly occurring in an oceancontinent transitional setting and,locally,in an accretionary prism associated with intra-oceanic subduction.

New Caledonia Ophiolite, Marginal Rifting to Fore-arc Evolution

The New Caledonia Ophiolite(Peridotite Nappe), represents about one third of the island’s surface(i.e. 5 500 km2). The ophiolite is composed of harzburgites, dunites, lherzolites, minor mafic-ultramafic cumulates, and various dykes and sills. The mantle section underwent a polyphase evolution, which involved prominent depletion and re-fertilization. The oldest events are probably recorded by abyssal-type lherzolites of the northern massifs, which bear traces of moderate partial melting. Plagioclase lherzolites were formed by shallow entrapment of highly depleted MORB melt in residual spinel lherzolites. Nd isotope compositions are consistent with derivation from an asthenospheric mantle source that experienced a recent MORB-producing depletion. This evolution was most likely accomplished during the late Cretaceous breakup of the eastern Australian margin. The harzburgite-dunite association, which forms the bulk of Peridotite Nappe was probably formed through a multistage magma-producing process. Harzburgites composition may have be obtained by a first phase of ~15% dry fractional melting, followed by 15%–18% hydrous melting in a supra-subduction zone setting. Variable εNd negatively correlate with 87Sr/86Sr, while Pb isotopes cover a wide range, trending from depleted mantle towards enriched, sediment-like, compositions. Such signatures likely reflect the evolution of a highly depleted forearc mantle wedge variably modified by different fluid and melt inputs during Eocene subduction. The harzburgite-dunite set is overlain by a dunite transition zone ~300 m thick, in turn discontinuously covered by cumulate lenses consisting of layered pyroxenites, dunites, and wherlites at the base and gabbronorites/websterites on top. The mafic cumulates crystallized from primitive, ultra-depleted melts in the nascent lower fore-arc crust. In particular, FME enrichments and Nd-Pb isotopes support an origin from a refractory mantle source modified by slab fluids for the gabbronorite-forming melts. The Peridotite Nappe has been extensively serpentinized(40% to 100%) with extremely scarce occurrences of unserpentinized rocks. Lizardite, brucite, magnetite and minor chrysotile developed from joints and intra-granular cooling cracks in a near-static environment. Serpentine-coated joints and peridotite foliation have been thereafter reopened and injected by various felsic, mafic and ultramafic supra-subduction melts emplaced within a narrow time interval(55–50 Ma), immediately after subduction inception at 56 Ma, i.e. the age of granulite-facies metamorphic sole. The youngest magmatic event is represented by island-arc tholeiite dykes dated at 50 Ma. A widespread set of antigorite and tremolite-bearing veins crosscut all previous structures in a progressively cooling forearc environment. The former are synkinematic crack seals, which display highly radiogenic, sediment-like 87Sr/86Sr ratios suggesting direct derivation of fluids from the subduction zone, while the latter bear mantle-like isotopic signatures and probably originated from the interaction of wall rocks with Ca-rich fluids released by Eocene dykes or fluids that leached them. Finally, continental subduction and obduction occurred during the 44–34 Ma interval and were accompanied by the development of the HP-LT metamorphic belt of northern New Caledonia, which constrains the polarity of subduction.