Ultramafic Section Research Articles

Serpentinised Mantle Section of Neoproterozoic Ophiolite at Al‐Barramiya District, North Arabian‐Nubian Shield: Tectono‐Magmatic Evolution and Metamorphism

ABSTRACTThis research focuses on the field observations, petrography, mineral chemistry and geochemistry of the serpentinised peridotite of Al‐Barramiya ophiolitic sequence to place constraints on their magmatic history and their geodynamic evolution. Al‐Barramiya ophiolitic rocks are a dismembered ophiolite which was strongly deformed and metamorphosed under greenschist to lower amphibolite facies. They comprise a mantle section dominated by highly serpentinised peridotite with less metapyroxenite and chromitite, as well as a crustal portion represented by metagabbros. Along shear zones, the ophiolite sequence was affected by several types of alteration. Extensive carbonate alteration is common in the ultramafic section, resulted in talc carbonates, listvenites and magnesite, while rodingitisation is common in the metagabbro resulted in rodingite. Despite the extensive serpentinisation, some fresh relics of primary mantle minerals such as Cr‐spinel, olivine and pyroxenes are preserved sporadically in the serpentinised peridotite. Few Cr‐spinel crystals are sometimes surrounded by subhedral flakes of Cr‐chlorite (kämmererite) that was formed due to replacement of Cr‐spinel during later alteration or regional metamorphism. The serpentinite samples are depleted in the total REE (0.56–1.19 ppm) with slightly negative to slightly positive Eu anomalies (0.89–1.28). The fresh cores of Cr‐spinel have Cr# mostly > 60, and the relics of pyroxenes and olivine are Mg‐rich suggesting that the Al‐Barramiya serpentinites are residual to high degrees of melt extraction. The estimated degrees of partial melting range between 18.2% and 20.7%. All the mineralogical and geochemical characteristics of the ultramafic section of the Al‐Barramiya ophiolites are most consistent with its formation in a fore‐arc setting.

From onset of Cadomian subduction to forearc-arc-continent collision: Insights from the Neoproterozoic Calzadilla Metaophiolite (SW Iberia)

ABSTRACT The Cadomian basement of SW Iberian Massif appears in the so-called Ossa-Morena Complex, which contains several allochthonous terranes that are key for deciphering the evolution of the Gondwana margin during the Neoproterozoic. In this context, the Calzadilla Ophiolite represents the remnants of an oceanic Ediacaran lithosphere formed by a boninitic mafic crust that overlies a serpentinized ultramafic section, the latter being the biggest outcrop of Cadomian serpentinites in Europe. In this work, the petrological and geochemical nature of the mantle section is analysed, as well as the tectonothermal evolution of both, the mafic crust and the ultramafic section. The algebraic analysis of whole rock geochemistry of the serpentinites indicates that the protolith corresponds to harzburgite. The geochemical fingerprint of the serpentinites, as well as the primary spinel crystals composition, shows that peridotites underwent high degrees of partial melting (up to 16%) in a forearc setting. Thermodynamic modelling and average PT methods were applied to serpentinites and mafic dykes from the ultramafic section as well as to amphibolites from the mafic section, both of which experienced progradation up to 4–5 kbar and 525–575°C. According to the data, the protoliths of the Calzadilla Ophiolite were formed in a peri-Gondwanan arc system developed during the subduction of oceanic crust below Gondwana. The roll-back of the subducting slab would induce the opening of the forearc basin at c. 602 Ma, protolith age of amphibolites from crustal section, with an extensive partial melting of the mantle wedge and the formation of boninitic-affinity magmas that constitute the mafic section of the Calzadilla Ophiolite. Metasomatism and serpentinization of the mantle wedge would occur via melt- and fluid–rock interactions. A compression-dominated stage at c. 540 led to the emplacement of the Calzadilla Ophiolite onto the Cadomian magmatic arc, with the development of the amphibolite-facies metamorphic imprint.

Generation and exhumation of granitoid intrusions in the Penjween ophiolite complex, NW Zagros of the Kurdistan region of Iraq: Implications for the geodynamic evolution of the Arabian-Eurasian collision zone

The Penjween ophiolite complex represents a remnant of the Neotethys oceanic crust that was preserved in the Zagros Suture Zone of NE Iraq. The complex is composed of dunite, harzburgite, and pyroxenite at the bottom, gabbro in the middle, and diorite at the top. Minor peraluminous granitiod intrusions of different types were identified in the complex. These granitoids are classified into trondhjemite and pegmatitic granite. Zircon grains from these intrusions show different ages and geochemistry, pointing at differences in their genesis and composition of the parent melts. The trondhjemite has an age of ~90 Ma giving the minimum age of the ultramafic section. This indicates that the mafic rocks were underlain by a source capable of yielding silica-saturated sodic granitoids. The pegmatitic granite with high potassium content, has a UPb zircon age of ~46 Ma, suggesting derivation from a non-mafic source melt at a younger age. The (UTh)/(HePb) double dating on zircons from the trondhjemite and pegmatitic granite intrusions indicate a comparable exhumation age of ~23 Ma. These results suggest that two episodes of magmatism occurred prior to the final closure of the Neotethys. In the Late Cretaceous event, melting of a mafic source formed trondhjemites, whereas in the middle Eocene event, possibly melting of the associated sedimentary rocks with the downgoing Neothethys oceanic slab formed pegmatitic granite. The similar exhumation time of the two genetically different granitoids suggests that the Penjween ophiolite complex was exhumed by the beginning of Miocene, potentially as a consequence of the Arabian-Eurasian plate collision.

A structural approach to the genesis of chrome ores within the Vourinos ophiolite (Greece): Significance of ductile and brittle deformation processes in the formation of economic ore bodies in oceanic upper mantle peridotites

The understanding of ophiolites as hosts to economic–scale chromitite ore deposits requires the integration of research relating their geochemical metallogenesis with studies of the mode and nature of their lithospheric-scale deformation. The Neotethyan Vourinos ophiolite in Northern Greece represents a well-preserved ophiolite complete with sheeted dikes, an in-situ Moho transition zone, and a nearly 8-km-thick upper mantle sequence. Chromitite bodies and sub-economic chrome ores occur throughout its ultramafic section, and above and below the petrological Moho; but, economic concentrations of chromitite are restricted to a region referred to as the “metalliferous zone” within the upper mantle domain. Ductile deformation, which developed synchronously with mineral re-equilibration and late magmatic activity, re-concentrated chromite grains in mesoscopic fold noses and created massive-textured ore bodies. Re-equilibration of chromite and chromite–olivine grains on microscopic–scale during this ductile deformation produced geochemical patterns that are compatible with strain distribution. Whether re-equilibration varied with strain rate in the mantle lithosphere due to regional heterogeneous conditions, or whether the change in deformation–induced modal distribution of the mineral aggregate more strongly affected the process of re-equilibration is yet to be determined. The richest ore bodies in Vourinos are represented by the schlieren ores of the Xerolivado Mine, and occur as extensive lenses of mylonitic chromitite, marking the highest degree of deformation within the complex. These textures display patterns of re-equilibration geochemical halos and transition from ductile into brittle deformation conditions.

Suprasubduction-zone origin of the podiform chromitites of the Bir Tuluhah ophiolite, Saudi Arabia, during Neoproterozoic assembly of the Arabian Shield

The ultramafic section of a dismembered ophiolite is exposed at Bir Tuluhah, in the north-central part of the Arabian Shield. It is penetratively serpentinized and locally carbonate-altered to talc‑carbonate and quartz‑carbonate rocks (listvenite) along shear zones and fault planes. Despite the high degree of mineral replacement, preserved mesh and bastite textures and fresh relics of primary Cr-spinel and olivine show that the protoliths were mainly harzburgite with minor dunite, with sparse massive chromitite bodies of various forms and sizes. Olivine inclusions in the chromitite lenses have higher forsterite content and NiO concentrations than fresh olivine relics in the host harzburgites and dunites, due to subsolidus re-equilibration. Cr-spinels in the chromitites have higher Cr# (0.74–0.82) than those hosted in dunite (0.72–0.76) or harzburgite (0.55–0.66). The scarce Cr-spinel crystals in harzburgite that have Cr# < 0.6 are interpreted to represent the population least affected by melt-rock interaction. The chromitite bodies are interpreted to have formed just below the contact between the oceanic crust and mantle sections (i.e., the petrologic Moho). The primary olivine (high Fo and Ni content) and Cr-spinel core compositions (high Cr# and low TiO2 content) of the Bir Tuluhah serpentinized peridotite are typical of modern supra-subduction zone (SSZ) fore-arc peridotites and consistent with crystallization from boninitic magma. The multistage petrogenesis leading to the chromitite bodies begins with moderate to high degrees of melt extraction from the protoliths of the serpentinized harzburgites, followed by reaction with melt compositions that evolved from tholeiite to boninite and left dunite residues. The massive Cr-rich chromitites in the Bir Tuluhah ophiolite are most probably the residues of such interaction between depleted harzburgite and ascending melts; mixtures of the reacted melts formed boninites, which became saturated with chrome-rich spinel and crystallized chromite pods before ascending past the Moho. We offer a novel thermodynamic model of this mixing and reaction process that quantifies the yield of Cr-spinel.

Geochemistry and PGE of the lower mineralized Zone of the Waterberg Project, South Africa

The Waterberg deposit is located north of the Northern Lobe of the Bushveld Complex in South Africa and represents a large, high-grade, new Platinum-Group-Element (PGE) discovery. The first comprehensive study of the lower ultramafic section of the area has been completed, and whole-rock chemical analyses and PGE data are presented.The section studied comprises mineralized harzburgite and marginal orthopyroxenite, overlain by troctolite grading into gabbroic rocks. Whole-rock analyses show geochemical variations typical of differentiated assemblages of cumulus olivine, plagioclase, and pyroxene. Normalized trace element data display HREE depletion, strong positive Eu and Sr anomalies and LREE enrichment. The negative anomalies for Th, Rb, Nb, and Ta are typical for rocks of the Bushveld Complex. Normalized PGE distributions are fractionated (Pd/Ir 177), Pd-enriched, and Au-poor.Emplacement of the magmas is believed to have commenced with west-east trending, finger-like intrusions, followed by lateral dilation and emplacement of sulfide droplet-bearing, ultramafic magmas. This was followed by a second phase of intrusions, characterized by sheet-like bodies of troctolite. Fractionation of these magmas led to the development of gabbroic rocks that make up the top of the succession. The Waterberg Project is located in the Southern Marginal Zone of the Limpopo Belt. This position in a structurally active area may have facilitated the creation of space for initial magmas.It is argued, that the mafic to ultramafic succession of the Waterberg Project does not represent a simple marginal extension of the Northern Lobe, nor does it directly correlate with the Platreef. It shares geological features but represents a separate magmatic basin.

Petrological and geochemical characteristics of the Samar Ophiolite ultramafic section: implications on the origins of the ophiolites in Samar and Leyte islands, Philippines

ABSTRACTCretaceous ophiolites and ophiolitic fragments occur in the Samar and Leyte islands in eastern central Philippines. The Samar Ophiolite is a complete crust–mantle sequence exposed in southern Samar, whereas the Tacloban and Malitbog ophiolite complexes are, respectively, located in the northeastern and southwestern portions of the nearby Leyte island. Despite the close proximity of these islands, the genetic relationship of these ophiolites and ophiolitic complexes, if any, remains to be elucidated. We present here new petrographic and geochemical data on the harzburgites and dunites of the ultramafic section of the Samar Ophiolite. These mantle peridotites are highly depleted residues which have low modal pyroxene content, high spinel Cr# (=0.62–0.79), and slightly enriched light rare earth element abundance with depletion in Zr and Ti. Such characteristics are typical of supra-subduction zone peridotites and strongly contrast with the abyssal signatures of the Tacloban and Malitbog ophiolite complexes. The absence of a structure between these adjacent ophiolite fragments initially hints that they form a single oceanic crust. However, with our new results, we suggest other possible mechanisms that could explain the relationship of these ophiolites.

Localized slip controlled by dehydration embrittlement of partly serpentinized dunites, Leka Ophiolite Complex, Norway

Dehydration of partly or completely serpentinized ultramafic rocks can increase the pore fluid pressure and induce brittle failure, a process referred to as dehydration embrittlement. However the extents of strain localization and unstable frictional sliding during deserpentinization are still under debate. In the layered ultramafic sections of the Leka Ophiolite Complex in the Central Norwegian Caledonides, prograde metamorphism of serpentinite veins led to local fluid production and to the growth of Mg-rich and coarse-grained olivine with abundant magnetite inclusions and δ18O values 1.0–1.5‰ below the host rock. Embrittlement associated with the dehydration caused faulting along highly localized (<10 μm-wide) slip planes near the centers of the original serpentinite veins and pulverization of wall rock olivine. These features along with an earthquake-like size distribution of fault offsets suggest unstable frictional sliding rather than slower creep. Structural heterogeneities in the form of serpentinite veins clearly have first-order controls on strain localization and frictional sliding during dehydration. As most of the oceanic lithosphere is incompletely serpentinized, heterogeneities represented by a non-uniform distribution of serpentinite are common and may increase the likelihood that dehydration embrittlement triggers earthquakes.

Petrography and geochemistry of Cenozoic sedimentary sequences of the southern Samar Island, Philippines: Clues to the unroofing history of an ancient subduction zone

The Cenozoic sedimentary sequences of southern Samar Island in eastern Philippines were examined to understand the unroofing history of an ancient arc terrane. Petrographic and geochemical data revealed varying degrees of inputs from the ophiolite basement and differences in modal compositions. The sedimentary units are mostly made up of lithic fragments. The Late Oligocene to Early Miocene Daram Formation contains more chert and volcanic fragments whereas the late Middle Miocene to Early Pliocene Catbalogan Formation is dominantly composed of ultramafic components. These variances are correspondingly reflected in the geochemical signatures of these two sedimentary formations. The Catbalogan Formation clastic rocks have higher volatile-free MgO and Fe2O3 values (average: 8.4% for both oxides) compared to the Daram Formation samples (average: 5.1 and 6.3%, respectively). Geochemical variations are also reflected in the Co, Cr and Ni values: the Catbalogan Formation samples reflect higher concentrations (Co: 15–57ppm; Cr: 231–1094ppm; Ni: 84–484ppm) compared to the Daram Formation samples (Co: 24–32ppm; Cr: 234–418ppm; Ni: 212–323ppm). These observations suggest that the Daram Formation eroded and transported more of the crustal portions of the ophiolite, while the younger Catbalogan Formation represents a later exhumation and subsequent erosion of the ultramafic section. An oceanic island arc (OIA) setting is proposed for the two formations based on several tectonic discrimination diagrams (e.g., Th-La-Sc, La vs. Th). The OIA signature is further supported by their smooth chondrite-normalized rare earth element (REE) patterns with no obvious Eu anomaly as well as LREE enrichment which are typical of sediments deposited in OIA setting. Based on the dominantly ophiolitic provenance of the Daram and Catbalogan formations, the post-emplacement history of the nearby Samar Ophiolite is constrained during the Late Oligocene to Early Pliocene period.

A fore-arc setting of the Gerf ophiolite, Eastern Desert, Egypt: Evidence from mineral chemistry and geochemistry of ultramafites

The Gerf ophiolite is the largest mantle-derived complex in the Arabian–Nubian Shield (ANS). This ophiolitic complex extends for tens of kilometers in the south Eastern Desert (SED) of Egypt as part of the Allaqi–Heiani and Oneib-Sol Hamed suture zones. The ultramafic section of the Gerf ophiolite comprises serpentinites, serpentinized peridotites and minor pyroxenites. All rocks contain relics of original magmatic phases. The elevated Cr# (>0.84) of Cr-spinels indicates that these rocks represent highly-depleted mantle residues after high degrees of melt extraction.Mineral and bulk-rock chemistry show that the Gerf ophiolite suite represents fragments of oceanic lithosphere that developed in fore-arc setting in a supra-subduction zone (SSZ) environment. The pyroxenites have a LREE-enriched pattern relative to the serpentinites while the serpentinized peridotites display depleted patterns [average (La/Yb)n=0.56)]. Modeling of LREE suggests that the LREE-enriched pyroxenites and serpentinites could have been produced via contamination of their mantle source by crustal material and/or subduction-related slab fluids during the mantle evolution in a SSZ setting or soon after ophiolite assemblage obduction onto the continental crust. In contrast, the LREE-depleted serpentinized peridotites could have been generated through MORB melt/mantle rock reaction.

Talc–carbonate alteration of ultramafic rocks within the Leka Ophiolite Complex, Central Norway

Petrographic and geochemical analyses of partly and completely serpentinized and carbonated peridotites within the ultramafic section of the Leka Ophiolite Complex have been used to elucidate the evolution of alterations and identify possible fluid sources. The alterations show no evidence for any major deformation and are located along low-angle structures that were formed in a late stage of the structural evolution of the ophiolite complex. Modeling of mineral equilibria in the SiO2–MgO–FeO–Fe2O3–CaO–H2O±CO2 system has been utilized to constrain the conditions during serpentinization and carbonation.The partly altered peridotites consist of the mineral assemblage olivine–clinopyroxene–serpentine–magnetite–brucite and formed at temperatures <410°C by infiltration of aqueous fluids. Completely serpentinized rocks with the mineral assemblage serpentine–magnesite–magnetite–dolomite formed at temperatures <500°C and low XCO2 (≤0.05) by the breakdown of the minerals in the partly altered peridotites. Talc–carbonate rocks formed by the breakdown of the serpentine in the previously formed serpentinite rock at temperatures <550°C and elevated XCO2, which resulted in the stabilization of the assemblage talc–magnesite–magnetite–dolomite. Carbon isotope values determined for dolomite from crosscutting carbonate lenses within the talc–carbonate rock yield δ13C values of ~−5 indicative of a mantle source for the carbon required for the carbonation. Oxygen isotope values δSMOW18O of ~10.8–11.3‰ together with initial 87Sr/86Sri=400Ma values of 0.7029 and 0.7063, suggest dehydration of rocks with mantle affinity as a source for the fluids. Based on analytical results and field observations we propose that the formation of the talc- and carbonate-bearing alteration zones is caused by the focused infiltration of fluids that originated at the bottom of already partly serpentinized ophiolite complex during extension-driven burial at the late stage of the Caledonian orogeny.

The Trinity ophiolite (California): the strange association of fertile mantle peridotite with ultra-depleted crustal cumulates

The Trinity ophiolite (California): the strange association of fertile mantle peridotite with ultra-depleted crustal cumulates

Atypical stratiform sulfide-poor platinum-group element mineralisation in the Agnew – Wiluna Belt komatiites, Wiluna, Western Australia

A new style of komatiite-associated sulfide-poor platinum-group element (PGE: Os, Ir, Ru, Rh, Pt, Pd) mineralisation has been identified at Wiluna in the strongly nickel sulfide (NiS) mineralised Agnew – Wiluna Greenstone Belt, Western Australia. The komatiite sequence at Wiluna is ∼200 m thick and comprises a basal pyroxenite layer, a thick ortho-to-mesocumulate-textured peridotite core, which is overlain by rhythmically layered wehrlite, oikocrystic pyroxenite and thick upper gabbroic margins. Pegmatoid and dendritic (harrisitic) domains are common features, whereas spinifex-textured horizons and flow-top breccias are absent. The presence of anomalous PGE-enriched horizons (ΣPt – Pd = 200 – 500 ppb) in the oikocrystic pyroxenite and in the layered melagabbro and gabbronorite horizons directly overlying the wehrlite unit is due to the presence of fine-grained (1 – 10 μm) platinum-group minerals (PGMs). More than 70 PGM grains were identified, and a considerable mineralogical variability was constrained. However, only Pd – Pt-bearing phases were identified, whereas no Ir – Ru-bearing PGMs were found in any of the sections examined. Interestingly, all PGMs are not in paragenetic association with sulfides, and only sulfide-poor/free intervals contain significant PGM concentrations. The whole-rock PGE sequence largely reflects the PGM distribution. It is hypothesised that the Pd – Pt enrichment in the oikocrystic pyroxenite and melagabbros and the overall Ir – Ru depletion in the upper mafic section of the sequence are the result of extensive olivine and chromite crystallisation in the basal ultramafic section. PGE saturation was driven by extensive crystallisation of silicate and oxide phases in a sulfide-undersaturated environment. The crystallisation of clinopyroxene in the oikocrystic pyroxenite horizon may have triggered the formation of Pt – Pd-bearing alloys and arsenides, which were the first PGMs to form. Stratiform sulfide-poor PGE mineralisation at Wiluna is more similar in stratigraphic setting, style and composition to PGE-rich sulfide-poor mineralisation zones in thick differentiated intrusions, rather than to other PGE-enriched zones in komatiite-hosted systems, where PGE enrichment is directly associated with accumulations of magmatic sulfides.

Multistage evolution of the Jijal ultramafic–mafic complex (Kohistan, N Pakistan): Implications for building the roots of island arcs

The ultramafic–mafic complexes located at the base of obducted paleo-island arcs are commonly interpreted as evidence for intra-crustal fractionation of primitive (high-Mg#) mantle melts. The present study, realized on the Jijal mafic–ultramafic basal section of the Cretaceous Kohistan oceanic arc (N Pakistan), discards a crystal fractionation model from a single parental magma to take into account the geochemical and isotope variations observed between the plutonic crust and the underlying ultramafic section. The basal ultramafic rocks, i.e. dunites, wehrlites and clinopyroxenites, show high Mg#, extremely depleted REE patterns, no marked HFSE anomalies and high LILE enrichment. Cpx and leached whole rocks from this unit yield restricted ε Nd values (+ 5.5 to + 6.8), a large ε Sr range (− 5.9 to + 8.8) and mainly radiogenic 207Pb/ 204Pb ratios. The overlying mafic section, i.e. gabbroic rocks, displays low Mg# (46 < Mg# < 54), enriched REE patterns and marked HFSE anomalies. These plutonic rocks have also very homogeneous Sr and Pb isotopic compositions defining a restricted domain, which does not overlap that of the ultramafic samples. Moreover, Cpx from the basal ultramafics yield an Sm–Nd isochron at 117 ± 7 Ma interpreted as dating crystallization of the ultramafic section. Together, these results indicate that the mafic rocks on one hand and the ultramafic rocks on the other hand, originate from separate sources with specific emplacement underneath the subduction zone. A lherzolite lens, collected from the top of the mafic section, plots on the early Cretaceous Sm–Nd isochron and therefore yields the same 143Nd/ 144Nd initial ratio. This suggests that the ultramafic part of the lower Kohistan arc complex sampled the initiation stages of the subduction with production of magmas with boninite-like features. At this stage, the mantle wedge has been already modified by percolation of the first slab-derived fluids. On such a scenario the lherzolite lens would correspond to the lithospheric mantle underneath the arc before its initiation. A multi-stage history is proposed for the evolution for the Kohistan arc through ∼ 30 Ma. Stage #1 reflects the spontaneous initiation of subduction associated with extensive boninitic affinity magmatism, stages #2 and #3 are related to arc building by tholeiitic melts and, a fourth step (stage #4) corresponds to the intra-crustal differentiation.

SLAB-RELA TED MANTLE-METASOMATISM: A SIMS INVESTIGATION OF LLE COMPOSITION OF FINERO PHLOGOPITE-PERIDOTITE (W ALPS)

The Finero mafic-ultramafic complex (Ivrea-Verbano Zone, Western Italian Alps) offers a unique opportunity to study km-scale hydrous melt migration on a outcropping mantle body. The ultramafic mantle section is mainly formed by spinel harzburgites, which contain hydrous mineral assemblages (amphibole + phlogopite ± apatite) and show LILE (Cs, Rb, Ba, K, Sr, LREE) enrichments. Moreover, these peridotites are characterised by large radiogenic Sr and Pb and low radiogenic Nd (Voshage et al., 1987; Cummings et al., 1987; Hartmann and Wedepohl, 1993). Such petrochemical features have been supposed to be related to pervasive migration of slab-derived melts through a supra-subduction mantle wedge (e.g. Zanetti et al. 1999; Prouteau et al., 2001; Morishita et al., 2003). Alternative hypotheses involve the upward migration of different kinds of alkaline melts (from K-rich differentiates to carbonatites), in the frame of variable geodynamic environments (Grieco et al., 2001, 2004; Garuti et al., 2001; Zaccarini et al, 2004). A new SIMS investigation has been accomplished on clinopyroxenes (Cpx) from the mantle sequence of the Finero peridotite, in order to place further constraints on the nature of the ascending melt(s) and on the genetic and temporal relationships occurring among the different ultramafic units so far recognised. Micro-analytical characterisation of selected LILE, HFSE and REE was combined with the insitu determination of Li and B concentration (Light Lithophile Elements: LLE), because the fractionation of these elements represents a valuable geochemical marker about the presence of slab-derived components in uprising magmas (Ottolini et al., 2004). Five spinel-facies samples, namely one phlogopite pyroxenite, two apatite-free phlogopite harzburgites, one apatite- bearing phlogopite wehrlite and one chromitite, representative of the main Finero ultramafics were selected for this study. Pyroxenite is a pegmatitic websterite, in which clinopyroxene (the dominant phase), amphibole and orthopyroxene are up to 16, 13 and 6 mm long, respectively. Phlogopite is also present with crystals up to 3 cm long. Minerals do not show deformation. Amphibole and phlogopite normally occur as interstitial phases. Unlike pyroxenite, the peridotites show amphibole modal concentration exceeding that of clinopyroxene. The apatitefree phlogopite harzburgites (the dominant mantle lithology) have porphyroclastic textures. Olivines, and sometimes orthopyroxenes, show evidence of ductile deformation, which is absent in most of clinopyroxenes and in the hydrous minerals. In the apatite-bearing wehrlites, orthopyroxene is lacking or occurs only in trace amounts. They have a mosaic equigranular texture, consisting of crystals of equant dimension often with 120° triple junctions. Modal abundance of amphibole and phlogopite increases up to 25 and 4% by volume, respectively. Apatite occurs as interstitial grains, sometimes associated with dolomite, and within fractures cross-cutting large olivine. Dolomite is also present as inclusion in spinel. Dunite-chromitite bands have secondary coarse granular texture, modally dominated by very large olivine and spinel grains. Clinopyroxene and amphibole are randomly present as strongly anhedral interstitial phases. Locally, phlogopiterich layers occur. The B content is 3.9-9.7 ppm and 1.3-1.6 ppm in Cpx from phlogopite harzburgites and pegmatitic websterites, respectively. Such concentrations are rather large for mantle minerals and lie in the range of the lower continental crust (Leeman and Sisson, 1996). Conversely, Cpx from apatite-bearing wehrlite and dunite-chromitite bands have low B concentrations (~0.8 and ~0.6 ppm, respectively), which are consistent with those of the upper mantle (<1 ppm) (Chaussidon and Jambon, 1994). Li contents show a narrow range. However, Cpx from phlogopite harzburgites have lower Li concentrations (0.5- 1.1 ppm) than pegmatitic websterite (1.5-1.9 ppm) and apatite- bearing peridotite (1.6-1.7 ppm). The highest Li contents occur in Cpx from dunite-chromitite bands (2.0-2.4 ppm). Clinopyroxenes from websterite and phlogopite harzburgites are similar, being enriched in LILE and depleted in moderately incompatible trace elements (e.g. HREE; LaN/YbN = 15-25, YbN = 0.97-1.82 xC1). Differently, Cpx from apatite-bearing wehrlite are characterised by: i) relatively low LREE/HREE fractionation (LaN/YbN = 2, with YbN = 25 xC1); ii) marked negative HFSE anomalies. The combination of the LLE data on Cpx with the other trace elements relevant for geochemical modelling indicates the presence in the Finero mantle sequence of contrasting metasomatic overprints. In particular, the large B/Li ratios and B concentration shown by Cpx from phlogopite harzburgites and websterite confirm that they formed as the result of the migration of a melt having a crust-derived component. By contrast, the occurrence of crustal contributions is not apparent on the basis of the LLE compositions of the Cpx from apatite-bearing wehrlite and chromitite, which are more consistent with a derivation from basaltic melts of upper mantle origin. Further geochemical investigations are in progress to assure whether such different trace element compositions are actually due to the migration of several melts or can be the product of complex differentiation processes affecting a unique melt with crustal component.

Chondritic osmium isotopic composition of late Archean convecting upper mantle: Evidence from Zunhua podiform chromitites, Hebei, North China

Podiform chromite deposits are a characteristic feature of the mantle sequences of harzburgitic ophiolites. The chromites usually have very low Re and high Os contents, which makes it the most resistant phase remaining from the primary magmatic history of the ultramafic sections of ophiolites. The podiform chromite is one of the robust indicators of initial Os isotopic compositions of the ophiolites where podiform chromites were derived from, which provides strong evidence for the origin and evolution of oceanic lithosphere. The Re and Os contents and the Os isotopic compositions of seven podiform chromitites from Zunhua ophiolitic melange belt, North China are reported in this study. The Re contents range from 0.019 to 0.128 ng/g, Os from 8.828 to 354.0 ng/g, and the187Os/l88Os ratio from 0.11003 to 0.11145. Three massive chromitites among the sample set have very high Os contents (> 300 ng/g), and their187Os/188Os ratios range from 0.11021 to 0.11030, averaging 0.11026±0.00005 (σ), equivalent to a γOs=∼-0.12 ±0.06 at 2.6 Ga, which means that the Os isotopic composition of convecting upper mantle is chondritic in late Archean. It is the Os isotopic composition of podiform chromitites that are derived from the oldest ophiolite in the world till now.

Emplacement of the Resurrection Peninsula ophiolite in the southern Alaska forearc during a ridge‐trench encounter

The 57±1 Ma Resurrection Peninsula ophiolite of southern Alaska's Chugach terrane formed in a near‐trench environment as the Kula‐Farallon ridge was being subducted beneath North America. The magmatic sequence includes pillow lavas, sheeted dikes, gabbros, trondhjemites, and a poorly‐exposed ultramafic section. The lavas show mid‐ocean ridge basalt and arc‐like geochemical signatures, interpreted to reflect compositionally diverse melts derived from near‐fractional melting of a variably depleted mantle source, mixed with variable amounts of assimilated continentally‐derived flysch. A sedimentary sequence overlying the ophiolite, here named the Humpy Cove Formation of the Orca Group, preserves a continuous 3.6 plusmn;1.4 Ma record of turbidite sediments that were deposited on the ophiolite as it was transported to North America and emplaced in the Chugach accretionary prism. A transition from thinly bedded turbidites at the base of the section to thickly bedded turbidites is interpreted as a distal to proximal facies change recording the migration of the ophiolite toward the continent. Chemical trends vertically through the sedimentary sequence show an increasing biogenic component diluting terrigenous matter 950 m above ophiolitic basalt, signaling a change to more favorable biologic conditions or increased preservation of biogenic matter with proximity to the continent. Hydrothermal metals are not resolvable in sediments interbedded with pillow lavas or in sediments above the ophiolite, suggesting that they were diluted by turbidite sedimentation. The top of the section is truncated by the Fox Island shear zone, a 1 km thick, greenschist‐facies, west‐over‐east thrust related to the emplacement of the ophiolite into the accretionary wedge. The Fox Island shear zone is intruded by a 53.4±0.9 Ma granite, showing that the ophiolite formed, was transported to the North American continent, overthrust by a major accretionary prism‐related thrust, and intruded by granite all within 3.6±1.4 Ma. Using these estimates for the timing of formation and emplacement of the ophiolite, with palinspastic reconstructions and plate motion vector triangles, we suggest that the Resurrection Peninsula ophiolite was transported up to 500 km (461±79 km) relative to North America from the place of its formation at the Kula‐Farallon ridge prior to its emplacement into the North American margin. Since the best estimates of the amount of translation of the Chugach terrane come from paleomagnetism of the Resurrection Peninsula ophiolite, current estimates of margin‐parallel strike slip need to be reduced considerably.

Geochemical constraints on the origin of the Hegenshan Ophiolite, Inner Mongolia, China

The Hegenshan ophiolite in Inner Mongolia is a remnant of oceanic lithosphere of probable Devonian age. The ophiolite consists of several blocks composed chiefly of serpentinized ultramafic rocks with lesser amounts of troctolite and gabbro, and sparse lavas and dikes. The ultramafic rocks consist chiefly of depleted harzburgite and minor dunite and are interpreted as mantle tectonites. In the Hegenshan block dunite is relatively abundant and is typically associated with podiform chromitite. Both the chromite ore and the residual chromites in this body are relatively aluminous with average Cr numbers of 44–54. A few small chromite bodies and some of the residual chromites have much higher Cr numbers (72–76). Several blocks have well-layered cumulate sequences of gabbro and troctolite. Sheeted dikes are absent but small mafic dikes are common in some of the ultramafic sections. Most of the mafic dikes have flat chondrite-normalized REE patterns and are strongly depleted in incompatible elements, similar to depleted tholeiites from immature island arcs. The basaltic lavas of the Hegenshan ophiolite have two distinctly different chemical signatures—one similar to the mafic dikes and one similar to ocean island basalts. The entire complex was probably formed within an island arc–marginal basin system that was later accreted to the southern margin of the Siberian Altaids.

Os-isotope study of platinum-group minerals in chromitites in Alpine-type ultramafic intrusions and the associated placers in Borneo

Abstract187Os/186Os ratios were determined for in-situ laurite grains in Alpine-type chromitites and platinumgroup minerals (PGM) in the associated alluvial placers in Borneo, Indonesia/Malaysia. The Osisotope ratios of laurite grains in chromite defne an 187Os/186Os ratio for the 100 Ma mantle source of c. 1.04. Thelow 187Os/186Os ratios in all grains confirm the essential derivation of these platinum-group elements (PGE) from the mantle. A minor variation in 187Os/186Os ratios was detected among PGM from placers, but no variation was found within individual grains, including a grain with chemical inhomogeneity. The values are similar to those for PGM in the associated chromitites. The data are consistent with a detrital origin of PGM in placers: the placer PGM originated in the ultramafic section of ophiolities and the release of these grains from igneous rocks and their deposition in placers was almost entirely by mechanical processes.

Platinum-group element abundance and distribution in chromite deposits of the Acoje Block, Zambales Ophiolite Complex, Philippines

Platinum-group elements (PGE) occur in ore-grade concentration in some of the chromite deposits related to the ultramafic section of the Acoje Block of the Zambales Ophiolite Complex. The deposits are of three types: Type 1 - associated with cumulate peridotites at the base of the crust; Type 2 - in dunite pods from the top 1 km of mantle harzburgite; and Type 3 - like Type 2, but in deeper levels of the harzburgite. Most of the deposites have chromite compositions that are high in Cr with Cr/(Cr + Al) (expressed as chromium index, Cr#) > 0.6; high-Al (Cr# < 0.6) chromite deposits are relatively rare, occurring as interspersed orebodies in regions of Types 2 and 3. Sulfide-bearing Type 1 high-Cr deposits are PGE-enriched; the high-Al deposits have lowest Ir and Pd. Among the PGEs, Ir ranges from <20 to 550 ppb, Rh from <100 to 1100 ppb, Rh from 0.7 to 760 ppb, Pt from <1.0 to 5960 ppb and Pd from <0.5 to 8350 ppb. Chondrite-normalized (CN) PGE patterns are diverse, and include those with both negative slope (Ir > Pd, thought to be characteristic of PGE-barren deposits) and positive slope (Ir < Pd, characteristic of PGE-rich deposits). Iridium, Ru and Os commonly occur as micron-size laurite (sulfide) inclusions in unfractured chromite. Laurite and native Os are also found as inclusions in interstitial sulfides. Platinum and Pd occur as alloy inclusions (and possibly as solid solution) in interstitial Ni-Cu sulfides and as tellurobismuthides in serpentine and altered sulfides. Variability of PGE distribution may be explained by alteration, crystal fractionation or partial melting processes. Alteration and metamorphism were ruled out, because PGE contents do not correlate with degree of serpentinization or the abundance and type (hydroxyl versus non-hydroxyl) of silicate inclusions in chromite. Preliminary Os isotopic data do not support crustal contamination as a source of the PGEs in the Acoje deposits. The anomalous PGE concentrations in Type 1 high-Cr chromite deposits are attributed to two stages of enrichment: an early enrichment of their mantle source from previous melting events and a later stage of sulfide segregation accompanying chromite crystallization. High-Al chromite deposits which crystallized from basalts derived from relatively low degrees of melting owe their low PGE content to partitioning of PGEs in sulfides and alloys that remain in the mantle. High-Cr deposits crystallized from melts that were previously enriched with PGEs during early melting events of their mantle source; Pt and Pd ore concentrations (ppm levels) are attained by segregation of magmatic sulfides. The Acoje deposits indicate that ophiolites are a potential economic source of the PGEs.