Mantle Section Research Articles

Paragenesis of multiple platinum-group mineral populations in Shetland ophiolite chromitite: 3D X-ray tomography and in situ Os isotopes

Chromitite from the Harold’s Grave locality in the mantle section of the Shetland ophiolite complex is extremely enriched in Ru, Os and Ir, at µg/g concentrations. High-resolution X-ray computed tomography on micro-cores from these chromitites was used to determine the location, size, distribution and morphology of the platinum-group minerals (PGM). There are five generations of PGM in these chromitites. Small (average 5µm in equivalent sphere diameter, ESD) euhedral laurites, often with Os-Ir alloys, are totally enclosed in the chromite and are likely to have formed first by direct crystallisation from the magma as the chromite crystallised. Also within the chromitite there are clusters of larger (50µm ESD) aligned elongate crystals of Pt-, Rh-, Ir-, Os- and Ru-bearing PGM that have different orientations in different chromite crystals. These may have formed either by exsolution, or by preferential nucleation of PGMs in boundary layers around particular growing chromite grains. Thirdly there is a generation of large (100µm ESD) composite Os-Ir-Ru-rich PGM that are all interstitial to the chromite grains and sometimes form in clusters. It is proposed that Os, Ir and Ru in this generation were concentrated in base metal sulfide droplets that were then re-dissolved into a later sulfide-undersaturated magma, leaving PGM interstitial to the chromite grains. Fourthly there is a group of almost spherical large (80µm ESD) laurites, hosting minor Os-Ir-Ru-rich PGM that form on the edge or enclosed in chromite grains occurring in a sheet crosscutting a chromitite layer. These may be hosted in an annealed late syn- or post magmatic fracture. Finally a few of the PGM have been deformed in localised shear zones through the chromitites.The vast majority of the PGM – including small PGM enclosed within chromite, larger interstitial PGM and elongate aligned PGM – have Os isotope compositions that give Re-depletion model ages approximately equal to the age of the ophiolite at ∼492Ma. A number of other PGM – not confined to a single textural group – fall to more or less radiogenic values, with four PGM giving anomalously unradiogenic Os corresponding to an older age of ∼1050Ma. The 187Os/188Os isotopic ratios for PGM from Cliff and Quoys, from the same ophiolite section, are somewhat more radiogenic than those at Harold’s Grave. This may be due to a distinct mantle source history or possibly the assimilation of radiogenic crustal Os.

A new occurrence of titanian (hydro)andradite from the Nagaland ophiolite, India: Implications for element mobility in hydrothermal environments

Titanium-bearing (hydro)andradites from hydrothermally altered mantle section of the Nagaland ophiolite, India have been investigated for mineral chemical study. In serpentinized peridotite they occur in association with accessory chromites or dusty magnetites and the TiO2 content in them is relatively low (<0.35wt%). (Hydro)andradites in magnetite pods, hosted within serpentinized peridotite have a range of TiO2 content (up to 13.77wt%) and occur typically within veins and veinlets intricately traversing the pods. The TiO2-rich variants of the (hydro)andradites within the veins occur either as large, polyhedral, margin-parallel zoned, mostly isolated grains within an andradite-rich porous matrix, or in clusters of equant, complexly zoned spherulites, resembling a fissure-fill. Besides (hydro)andradites as the most abundant component, the remaining of the veins consists mostly of serpentine and chlorite with relict titanites and perovskites. Magnetite pods are almost monomineralic, composed of large chromian magnetite crystals with accessory ilmenites occurring at the grain boundaries. Textural and mineralogical evidences suggest the transformation of these pods prior to the formation of (hydro)andradites from chromitite protoliths that have undergone extensive Ca-metasomatism together with host peridotites. We envisage at least two hydrothermal episodes for the mineralogical modifications where the formation of (hydro)andradites is related to the last event. The textural and mineral chemical characteristics of the entire assemblage as a whole demonstrate a gradual, temporal evolution in the composition of the metasomatic fluid from the onset of serpentinization to the end. This new occurrence of titanian andradites from chromitite protolith, hitherto not reported has major implications on the mobility of elements in hydrothermal environments.

Metallogeny of the Chrome Ores of the Xerolivado-Skoumtsa Mine, Vourinos Ophiolite, Greece: Implications on the genesis of IPGE-bearing high-Cr chromitites within a heterogeneously depleted mantle section

Chrome ore deposits comprise less than 1% of the volume of a pervasively serpentinized dunite body (∼3.5km3) that constitutes the Xerolivado-Skoumtsa Mine of the Vourinos Ophiolite in northwestern Greece. Ores have been examined to aid determination of their geological mode of occurrence, their mineralogy and mineral chemistry (Cr-spinel and olivine), and platinum-group element (PGE) geochemistry. The ore bodies are highly deformed and of the schlieren type, where Cr-spinel-rich and serpentine (after olivine)-rich layers (1–20cm thick) alternate. The Cr-spinel displays a limited range in Cr# [100×Cr/(Cr+Al)] atomic ratio (77–85) and TiO2 content (0.05–0.22wt%). The Mg# [100×Mg/(Mg+Fe2+)] of Cr-spinel shows a wider variation (52–74) indicating that subsolidus (MgSp-Fe2+Ol) re-equilibration occurred. Remnant fresh olivine in the serpentine-rich layers is typical forsterite [Fo#: 100×Mg/(Mg+Fe2+)=93–95]. The Cr-rich chromitite ores have total PGE and Au abundances that range between 74.3 and 205ppb: they show a general enrichment in Ir and Ru over PPGE (Rh, Pt and Pd) resulting mostly in a negatively-sloping C1 chondrite-normalized PGE profile with a quite strong Ru peak. Field observations indicate that the schlieren banding of the deformed ore bodies is a tectonometamorphic feature acquired within ductile deformation of the ophiolitic slab. Geochemical calculations demonstrate that the parental melts of the Xerolivado-Skoumtsa high-Cr chromitites were boninitic. Their high Ru content and variation in Cr#Sp values indicate a polygenetic origin from geochemically similar, but spatially distinct melt inputs. These melts originated within a hydrated mantle wedge beneath a forearc basin within an evolving lithospheric slab. We emphasize that the PGE budget of the Xerolivado-Skoumtsa chromitites was not inherited from interactions of the migrating melts with their parental harzburgites, but it is an intrinsic feature of the deep-seated and heterogeneously depleted mantle source of these magmas.

Chemical homogeneity of high-Cr chromitites as indicator for widespread invasion of boninitic melt in mantle peridotite of Bir Tuluha ophiolite, Northern Arabian Shield, Saudi Arabia

The Bir Tuluha ophiolite is one of the most famous chromitite-bearing occurrences in the Arabian Shield of Saudi Arabia, where chromitite bodies are widely distributed as lensoidal pods of variable sizes surrounded by dunite envelopes, and are both enclosed within the harzburgite host. The bulk-rock geochemistry of harzburgites and dunites is predominately characterized by extreme depletion in compatible trace elements that are not fluid mobile (e.g., Sr, Nb, Ta, Hf, Zr and heavy REE), but variable enrichment in the fluid-mobile elements (Rb and Ba). Harzburgites and dunites are also enriched in elements that have strong affinity for Mg and Cr such as Ni, Co and V. Chromian spinels in all the studied chromitite pods are of high-Cr variety; Cr-ratio (Cr/(Cr+Al) atomic ratio) show restricted range between 0.73 and 0.81. Chromian spinels of the dunite envelopes also show high Cr-ratio, but slightly lower than those in the chromitite pods (0.73–0.78). Chromian spinels in the harzburgite host show fairly lower Cr-ratio (0.49–0.57) than those in dunites and chromitites. Platinum-group elements (PGE) in chromitite pods generally exhibit steep negative slopes of typical ophiolitic chromitite PGE patterns; showing enrichment in IPGE (Os, Ir and Ru), over PPGE (Rh, Pt and Pd). The Bir Tuluha ophiolite is a unimodal type in terms of the presence of Ru-rich laurite, as the sole primary platinum-group minerals (PGM) in chromitite pods. These petrological features indicates that the Bir Tuluha ophiolite was initially generated from a mid-ocean ridge environment that produced the moderately refractory harzburgite, thereafter covered by a widespread homogeneous boninitic melt above supra-subduction zone setting, that produced the high-Cr chromitites and associated dunite envelopes. The Bir Tuluha ophiolite belt is mostly similar to the mantle section of the Proterozoic and Phanerozoic ophiolites, but it is a “unimodal” type in terms of high-Cr chromitites and PGE-PGM distribution.

The origin of Patagonia revealed by Re-Os systematics of mantle xenoliths

We present mineral chemistry and whole rock major, trace, and platinum group element (PGE) concentrations, and Re-Os isotope data for eighteen mantle xenoliths carried to the surface of southern Patagonia (45°–52°S) by Paleocene to Pleistocene alkaline basalts in seven localities scattered widely across southern South America. The new data along with those previously published show that peridotites derived from the lithospheric mantle of the Deseado Massif (DM), southern Patagonia, have compositions indicative of higher degrees of partial melt extraction compared to those from surrounding regions. Re-depletion model ages (TRD) of mantle xenoliths from the DM (n=20) range from 0.5 to 2.1Ga, with an average of 1.5Ga. In contrast, samples from the surrounding areas (n=39) have a wider range of Re-depletion ages from 0.0 to 2.5Ga, with an average of 1.0Ga. Similar geochemical characteristics are recognized between the lithospheric mantle section of the DM and that beneath East Griqualand (∼1.1Ga), south-eastern Africa, which is related to the Proterozoic Namaqua-Natal Province. The Re-Os systematics of the mantle xenoliths are indicative of Meso to Paleoproterozoic ages for partial melting and stabilization processes of the lithospheric mantle of southern Patagonia, which are considerably older than the crystallization ages obtained for the scarce basement rocks of Patagonia (<0.6Ga). In addition, published elastic thickness (Te) estimates of southern South America show maximum values on the submerged continental region located between the DM and the Malvinas/Falkland Islands, where Grenville-age metamorphic rocks are exposed. These geochemical and geophysical results suggest that southern Patagonia and the Malvinas/Falkland Islands and plateau constitute an integrated and relatively rigid continental block formed mainly during the Meso to Paleoproterozoic as part of the supercontinent Rodinia.To the north, the formation of the North Patagonian Massif (NPM) seems to be contemporaneous with that of the DM. Nevertheless, its ancient lithospheric mantle and lower crust appear to have been widely eroded and replaced by relatively young convecting mantle, possibly during the Carboniferous collision between the DM and the NPM. The differences between the DM and NPM lithosphere histories apparently controlled the subsequent formation and distribution of Jurassic epithermal Au-Ag deposits.

Spatial variations in cooling rate in the mantle section of the Samail ophiolite in Oman: Implications for formation of lithosphere at mid-ocean ridges

To understand how the mantle cools beneath mid-ocean ridge spreading centers, we applied a REE-in-two-pyroxene thermometer and major element thermometers to peridotites from the Wadi Tayin massif in the southern part of the Samail ophiolite in the Sultanate of Oman, which represent more than 10 km of structural depth beneath the paleo-Moho. Closure temperatures for REEs in pyroxenes deduced from the REE-in-two-pyroxene thermometer (TREE) decrease smoothly and systematically with depth in the section, from >1300 °C near the crust to <1100 °C near the metamorphic sole, consistent with previously observed, similar variations in mineral thermometers with lower cooling temperatures. Estimated cooling rates decrease from ∼0.3 °C/y just below the crust–mantle transition zone (MTZ) to ∼10−3 °C/y at a depth of six km below the MTZ. Cooling rates derived from Ca-in-olivine thermometry also decrease moving deeper into the section. These variations in cooling rate are most consistent with conductive cooling of the mantle beneath a cold overlying crust. In turn, this suggests that hydrothermal circulation extended to the MTZ near the axis of the fast-spreading ridge where the igneous crust of the Samail ophiolite formed. These observations are consistent with the Sheeted Sills model for accretion of lower oceanic crust, and with previous work demonstrating very rapid cooling rates in the crust of the Wadi Tayin massif. Our observations, combined with previous results, suggest that efficient hydrothermal circulation beneath fast spreading centers cools the uppermost mantle from magmatic temperatures to <1000 °C as quickly as tectonic exhumation at amagmatic spreading centers. In contrast, thermometers sensitive to cooling over lower temperature intervals indicate that the Wadi Tayin peridotites cooled more slowly than tectonically exhumed peridotites sampled near the seafloor along mid-ocean ridges. Hydrothermal cooling of the crust may have waned, so that the crust–mantle package cooled more slowly, whereas rapid cooling of abyssal peridotites during tectonic exhumation continued to seafloor temperatures.

Mantle hornblendites of Naein ophiolite (Central Iran): Evidence of deep high temperature hydrothermal metasomatism in an upper mantle section

The Naein ophiolite is the most complete ophiolitic exposure in Cental Iran and considered as a remnant of the Mesozoic Central East Iranian microcontinent (CEIM) confining oceanic crust. In the northeastern part of this ophiolite (Darreh Deh area) within the mantle peridotites, a few hundred meters below the top of the Moho transition zone (MTZ), the hornblendites are present as dykes (former cracks and joints) from a few millimeters to nearly 50 cm wide. They have sharp boundaries with the surrounding mantle harzburgites and dunites. These hornblendites are pale green and coarse-grained in hand specimen and composed of magnesio-hornblende (Mg# = 0.93), chlorite (penninite and clinochlore, Mg# = 0.95), Cr-spinel (chromite, Cr# = 0.67 and Mg# = 0.55), tremolite, calcite and dolomite. Tremolites were formed by retrograde metamorphism of hornblendes. Calcite and dolomite occur as late-stage veins. Very high amount of primary hydrous phases (~94 vol % hornblende and chlorite), as well as peculiar mineralogical and chemical characteristics of the Naein ophiolite mantle hornblendites, do not match a magmatic origin. They are possibly products of the reaction between mantle peridotites and seawater-originated supercritical fluids, rich in silicate components. The presence of primary hydrous phases (hornblende and chlorite) may reveal high activity of H2O in the involved solution. The chemical composition of chromite in the hornblendites is near to the average chromite composition from the surrounding harzburgite and dunite. This suggests that the main source of Cr should be chromites of nearby peridotites, which were totally or partly dissolved by hydrothermal fluids. The positive anomaly of Eu in the chondrite-normalized REE patterns of hornblendes, high modal abundance of Ca-rich hornblende, as well as presence of calcite and dolomite, point to seawater ingression through the gabbros in to the uppermost mantle peridotites. The higher value of MgO than CaO, presence of high-Cr chromite and Cr-enrichment of hornblendes and chlorites indicate a higher contribution of peridotites rather than gabbros to the chemical characteristics of the involved fluids. This study shows that circulation of possibly seawater-derived high temperature hydrous fluids in the upper mantle can leach and provide necessary elements to form hornblendite in joints and cracks of the uppermost mantle.

Troodos ophiolite mantle section exposed along Atalante Geo-Trail, Troodos Geopark, Cyprus

Abstract The following paper presents the field trip through the unique mantle sequence composed of peridotites and exposed along Atalante Geo-Trail in Troodos Geopark, Cyprus. This manuscript briefly summarises data from many papers and presents authors’ own field observations, in order to provide information on evolution of enigmatic Troodos mantle peridotites.

Na-rich character of metasomatic/metamorphic fluids inferred from preiswerkite in chromitite pods of the Khoy ophiolite in Iran: Role of chromitites as capsules of trapped fluids

We report a new occurrence of preiswerkite, a rare Na- and Al-rich trioctahedral mica (Na-analog of eastonite), from a high-Al (Cr# of chromite ~0.4) chromitite pod of the Khoy meta-ophiolite in northwestern Iran. The preiswerkite is euhedral and associated with Ca–Cr–Al-garnet, clinochlore, titanite, and calcite. Preiswerkite and associated minerals fill cavities in the chromitite pod and are absent in the surrounding peridotites. The preiswerkite-rich aggregate is possibly representative of aqueous fluid supplied to the mantle section of the Khoy ophiolite. Peraluminous conditions, which are indispensable for preiswerkite formation, were established only in the pool of fluid trapped by the high-Al chromitite. The fluid and preiswerkite precipitates were isolated from the peridotite by the chromitite capsule. The Na-rich fluid was a precursor of the serpentinization fluid. The fluid is an analog of the fluid released from the subducting slab, which can precipitate jadeite under high-pressure conditions.

Refertilization Processes in the Subcontinental Lithospheric Mantle: the Record of the Beni Bousera Orogenic Peridotite (Rif Belt, Northern Morocco)

Correlations between major and minor transition elements in tectonically emplaced orogenic peridotites have been ascribed to variable degrees of melt extraction and melt–rock reaction processes, leading to depletion or refertilization. To elucidate how such processes are recorded in the subcontinental lithospheric mantle, we processed a large geochemical dataset for peridotites from the four tectono-metamorphic domains of the Beni Bousera orogenic massif (Rif Belt, northern Morocco). Our study reveals that variations in bulk-rock major and minor elements, Mg-number and modal mineralogy of lherzolites, as well as their clinopyroxene trace element compositions, are inconsistent with simple partial melting and mainly resulted from different reactions between melts and depleted peridotites. Up to 30% melting at < 3 GPa and cryptic metasomatism can account for the geochemical variations of most harzburgites. In Grt–Sp mylonites, melting and melt–rock reactions are masked by tectonic mixing with garnet pyroxenites and subsolidus re-equilibration. In the rest of the massif, lherzolites were mostly produced by refertilization of a refractory protolith (Mg-number ∼ 91, Ol ∼ 70%, Cpx/Opx = 0·4) via two distinct near-solidus, melt–rock reactions: (1) clinopyroxene and orthopyroxene precipitation and olivine consumption at melt/rock ratios <0·75 and variable mass ratio between crystallized minerals and infiltrated melt (R), which are recorded fairly homogeneously throughout the massif; (2) dissolution of orthopyroxene and precipitation of clinopyroxene and olivine at melt/rock ratios ≤1 and R = 0·2–0·3, which affected mainly the Ariegite–Seiland and Seiland domains. The distribution of secondary lherzolites in the massif suggests that the first refertilization reaction occurred prior to the differentiation of the Beni Bousera mantle section into petro-structural zones, whereas the second reaction was associated with the development of the tectono-metamorphic domains. Our data support a secondary, refertilization-related origin for most lherzolites in orogenic peridotite massifs.

Mineralogy and composition of chromitites and host peridotites from the Colkahan ultramafic complex (Nazdasht mine), Kerman, southeastern Iran

Several chromitite pods of variable sizes are hosted in dunite-harzburgite of the about 14 km2 mining district of Nazdasht, in the Colkahan ultramafic complex, located in the Haji Abad-Esfandagheh district in the Kerman province, southeastern Iran. Olivine, orthopyroxene, and to a lesser extent clinopyroxene of a highly refractory nature, are the primary silicates found in the dunites and harzburgites. The matrix minerals between chromian spinel grains are serpentinized olivine and pyroxene, Cr-rich chlorite (kaemmererite), uvarovite, and veins of magnesite. Chromian spinel mainly occurs as massive chromitite pods and as thin massive chromitite bands together with minor disseminations in dunites and harzburgites. Chromian spinels in massive chromitites show relatively high Cr-numbers (0.60–0.82), Mg-numbers (0.54–0.74) and very low TiO2 contents (averaging 0.16 wt.%) which may reflect the crystallization of chromite from a boninitic magma. The Fe3+-number is very low, down to <0.17 wt.%, in the chromian spinel of chromitites and associated peridotites of the Colkahan ultramafic complex. The harzburgite and dunite contain accessory chromian spinel with Cr-numbers of about 0.66 and 0.69, respectively. The Colkahan dunites and harzburgites are high depleted with low CaO (0.01–0.47 wt.%) and Al2O3 (0.29–0.64 wt.%) compared to primitive mantle. The chemical and mineralogical characteristics of these chromitites have a strong affinity to podiform chromitites in the mantle section of supra-subduction zone ophiolites. Calculated parental melts of the chromitites and geochemical discrimination diagrams based on mineral chemistry of peridotites are consistent with the differentiation of arc related magmas with boninitic affinity and support the supra-subduction paleogeodynamic setting of Haji Abad-Esfandagheh district previously proposed by some authors.

Tectonic significance and redox state of Paleoproterozoic eclogite and pyroxenite components in the Slave cratonic mantle lithosphere, Voyageur kimberlite, Arctic Canada

Mantle-derived eclogite and pyroxenite xenoliths from the Jurassic Voyageur kimberlite on the northern Slave craton in Arctic Canada were studied for garnet and clinopyroxene major and trace element compositions, clinopyroxene Pb and garnet O isotopic compositions, and garnet Fe3+/ΣFe contents. The Voyageur xenoliths record a wide range of pressures, but are cooler compared to mantle xenoliths derived from the nearby, coeval Jericho kimberlite. The CaO, TiO2 and Zr contents of Voyageur eclogites increase with depth, which is also observed in northern Slave peridotite xenoliths, demonstrating ‘bottom-up’ metasomatic processes within cratonic mantle lithosphere.The Voyageur eclogites have positive Eu anomalies, flat HREEN patterns, and major element compositions that are consistent with ultimate origins from basaltic and gabbroic protoliths within oceanic lithosphere. Clinopyroxene Pb isotope ratios intercept the Stacey-Kramers two-stage terrestrial Pb evolution curve at ca. 2.1Ga, and form an array towards the host kimberlite, indicating isotopic mixing. The 2.1Ga eclogite formation age broadly overlaps with known Paleoproterozoic subduction and collision events that occurred along the western margin of the Slave craton. Unlike the eclogites, the Voyageur pyroxenites contain garnet with distinctive fractionated HREEN, sinusoidal REE patterns of calculated bulk rocks, and clinopyroxene with 206Pb/204Pb ratios that intercept the Stacey-Kramers curve at 1.8Ga. This suggests a distinct origin as Paleoproterozoic high-pressure mantle cumulates. However, the pyroxenite Pb isotope ratios fall within the eclogite array and could also be explained by protoliths formation at ca. 2.1Ga followed by minor isotopic mixing during mantle metasomatism. Thus, an alternative scenario involves pyroxenite formation within the mantle section of Paleoproterozoic oceanic lithosphere followed by variable metasomatism after incorporation into cratonic mantle lithosphere. This model allows for a linked petrogenesis of the Voyageur eclogites (crust) and pyroxenites (mantle) as part of the same subducting oceanic slab.Oxygen fugacity determinations for one pyroxenite and ten eclogite xenoliths show a range of 3 log units, from −4.6 to −1.6 ΔFMQ, similar to the range observed for nearby Jericho and Muskox eclogites (ΔFMQ −4.2 to −1.5). Importantly, the northern Slave eclogite and pyroxenite mantle components are highly heterogeneous in terms of redox state provided that they range from reduced to oxidized relative to Slave peridotite xenoliths.Moreover, the Voyageur eclogites do not exhibit any trend between oxidation state and equilibration depth, which contrasts with the downward decrease in fO2 shown by Slave and worldwide cratonic peridotite xenoliths. Our investigation of mantle eclogite and pyroxenite fO2 reinforces the important influence of recycled mafic components in upper mantle processes, because their high and variable redox buffering capacity strongly controls volatile speciation and melting relations under upper mantle conditions.

Pyroxene–Cr-spinel exsolution in mantle lherzolites of the Syum-Keu ophiolite massif (Arctic Urals)

We consider the composition and microstructures of pyroxene–Cr-spinel exsolution products from lherzolites of the deep mantle section of the Syum-Keu ophiolite massif in the Arctic Urals. Enstatite and diopside from the lherzolites contain lamellae and diverse micron segregations of Cr-spinel. We have first determined the composition of Cr-spinel lamellae and the composition trend of pyroxenes undergoing exsolution. The zonal inhomogeneity of their composition is expressed as a decrease in the contents of Cr2O3, Al2O3, and Na2O from the core of coarse grains free of exsolution structures to the sites with Cr-spinel segregations. The Cr/(Cr + Al) value in the Cr-spinel lamellae varies from 0.23 to 0.33, and the Mg/(Mg + Fe) value, from 0.61 to 0.67. The degree of iron oxidation in the formed Cr-spinel is low (0.10–0.19) and virtually does not depend on the Cr content in this mineral. We calculated the temperature and pressure of the beginning and completion of pyroxene–Cr-spinel exsolution for the equilibrium enstatite–diopside (of different generations) pairs and established the participation of H2O fluid in the exsolution. During the exsolution, the temperature decreases from 970 to 650–700 °C and the pressure grows from 0.9 to 1.3 GPa. The exsolution of pyroxene and migration of elements proceeded under high-temperature solid-plastic flow of mantle material, probably in the local zones where peridotites were subjected to strong shearing strains caused by the above high-velocity flow. The zone with these processes was located in the lithospheric mantle (mantle wedge) above the subduction zone. Migration of ore components from silicates and formation of new Cr-spinel grains contributed to the formation of chromite segregations.

Zircon U-Pb geochronological constraints on rapid exhumation of the mantle peridotite of the Xigaze ophiolite, southern Tibet

The Xigaze ophiolite outcrops in the central segment of the Yarlung Zangbo suture zone, southern Tibet. It is characterized by large amounts of ultramafic units with minor mafic rocks. The mafic rocks consist of gabbros, diabases and basalts. The gabbroic rocks of the Xigaze ophiolite occur as layered bodies or isotropic dikes intruding into mantle sections. Hectometric-sized gabbroic bodies are well-preserved in Dazhuqu, Baigang and Jiding. However, their formation time and generation mechanism are not systematically proposed or well understood. In this study, nine samples of mafic rocks from the Xigaze ophiolite, including seven gabbros and two rodingites (altered from diabases), were selected for in situ zircon U-Pb and Hf isotopic analyses. The geochemical feature suggested that these rocks formed by the intrusion of melts of normal mid-ocean ridge (N-MORB) type. The U-Pb data yielded identical ages of 124–129Ma within uncertainties. Positive zircon εHf(t) values indicated that these samples had an origin of depleted mantle source. Combined with previous studies on mafic dikes, amphibolite blocks and radiolarian cherts, it can be concluded that the Yarlung Zangbo ophiolites formed over a short period of time from 119 to 132Ma. Hence, a rapid exhumation of the mantle peridotites and gabbroic rocks of the Xigaze ophiolite may have occurred to get intrusion of the diabase dikes and sills. It excludes the existence of a long-term ancient magma chamber or lens. It is more likely that the gabbroic rocks are a series of plutonic intrusions beneath a fossil slow-spreading ridge, rather than products of magma chambers. Therefore, the “Chapman detachment model” may be applied to the generation of the Yarlung Zangbo ophiolites.

Alakit and Daldyn kimberlite fields, Siberia, Russia: Two types of mantle sub-terranes beneath central Yakutia?

Mineral data from Yakutian kimberlites allow reconstruction of the history of lithospheric mantle. Differences occur in compositions of mantle pyropes and clinopyroxenes from large kimberlite pipes in the Alakit and Daldyn fields. In the Alakit field, Cr-diopsides are alkaline, and Stykanskaya and some other pipes contain more sub-calcic pyropes and dunitic-type diamond inclusions, while in the Daldyn field harzburgitic pyropes are frequent. The eclogitic diamond inclusions in the Alakit field are sharply divided in types and conditions, while in the Daldyn field they show varying compositions and often continuous Pressure–Temperature (P–T) ranges with increasing Fe# with decreasing pressures. In Alakit, Cr-pargasites to richterites were found in all pipes, while in Daldyn, pargasites are rare Dalnyaya and Zarnitsa pipes. Cr-diopsides from the Alakit region show higher levels of light Rare Earth Elements (LREE) and stronger REE-slopes, and enrichment in light Rare Earth Elements (LREE), sometimes Th-U, and small troughs in Nb-Ta-Zr. In the Daldyn field, the High Field Strength Elements HFSE troughs are more common in clinopyroxenes with low REE abundances, while those from sheared and refertilized peridotites have smooth patterns. Garnets from Alakit show HREE minima, but those from Daldyn often have a trough at Y and high U and Pb. PTXfO2 diagrams from both regions show similarities, suggesting similar layering and structures. The degree of metasomatism is often higher for pipes which show dispersion in P–Fe# trends for garnets. In the mantle beneath Udachnaya and Aykhal, pipes show 6–7 linear arrays of P–Fe# in the lower part of the mantle section at 7.5–3.0 GPa, probably reflecting primary subduction horizons. Beneath the Sytykanskaya pipe, there are several horizons with opposite inclinations which reflect metasomatic processes. The high dispersion of the P–Fe# trend indicating widespread metasomatism is associated with decreased diamond grades. Possible explanation of the differences in mineralogy and geochemistry of the mantle sections may relate to their tectonic positions during growth of the lithospheric keel. Enrichment in volatiles and alkalis possibly corresponds to interaction with subduction-related fluids and melts in the craton margins. Incorporation of island arc peridotites from an eroded arc is a possible scenario.

Thermal and metasomatic rejuvenation and dunitization in lithospheric mantle beneath Central Europe – The Grodziec (SW Poland) case study

The 32Ma Grodziec nephelinite (Lower Silesia, SW Poland) contains xenolith of peridotite (mostly lherzolite) and clinopyroxenite/olivine clinopyroxenite composition. The forsterite content in olivine classifies these rocks into three groups: groups A and B consist of peridotites, while group C xenoliths are pyroxenitic cumulates. Group A xenoliths contain olivine Fo 87.90–91.8% and pyroxenes with high Mg# (~0.91–0.92); clinopyroxene is strongly LREE-enriched (LaN/LuN=2.19–17.74) and strongly impoverished in Zr, Hf and Ti relative to primitive mantle. The group B xenoliths (dunites and wehrlite) are orthopyroxene-free, olivine and clinopyroxene are less magnesian than those in the A group (Fo=85.2–87.2%, Mg#=0.86–0.88), clinopyroxene is less LREE-enriched (LaN/LuN=4.07–4.15) and only slightly impoverished in Zr, Hf and Ti. Group C xenoliths contain olivine with forsterite content from 78.6 to 86.6% and clinopyroxene of Mg# from 0.84 to 0.85, with LREE/trace element characteristics similar to those of B group (LaN/LuN=1.96–3.10).Group A xenoliths from Grodziec record migration of mixed carbonatite-alkaline silicate melts through the subcontinental lithospheric mantle beneath Lower Silesia, which preceded the migration of melts similar to the Grodziec nephelinite. The peridotitic protoliths were dunitized at the direct contacts with the migrating nephelinite melt and are now represented by group B. Group C pyroxenites originated in mantle conditions by crystal settling in places of transient nephelinite melt stagnation. The mantle section beneath Grodziec was reheated to ca 1000–1100°C. The Grodziec scenario is similar to that of Księginki (northern extension of Eger Rift, SW Poland), which shares a similar age of xenolith entrainment. Both sites show that the processes of mantle metasomatism and thermal rejuvenation of subcontinental lithospheric mantle were more intense during the Lower Oligocene volcanic climax compared to those recorded in younger (ca 20Ma) xenolith suites from Lower Silesia.

Sedimentary serpentinite and chaotic units of the lower Great Valley Group forearc basin deposits, California: updates on distribution and characteristics

ABSTRACTSedimentary serpentinite and related siliciclastic-matrix mélanges in the latest Jurassic to Lower Cretaceous lower Great Valley Group (GVG) forearc basin strata of the California Coast Ranges reach thicknesses of over 1 km and include high-pressure (HP) metamorphic blocks. These units crop out over an area at least 300 km long by 50 km wide. The serpentinite also contains locally abundant blocks of antigorite mylonite. Antigorite mylonite and HP metamorphic blocks were exhumed from depth prior to deposition in the unmetamorphosed GVG, but the antigorite mylonite may be mistaken for metamorphosed serpentinite matrix in localities with limited exposure. These olistostrome horizons can be distinguished from intact slabs of serpentinized peridotite associated with the Coast Range Ophiolite (CRO) or serpentinite mélanges of the Franciscan subduction complex (FC) on the basis of internal sedimentary textures (absent in CRO), mixing/interbedding with unmetamorphosed siliciclastic matrix and blocks (differs from CRO and FC), and preserved basal sedimentary contacts over volcanic rocks of the CRO or shale, sandstone, and conglomerate of the GVG (differs from CRO and FC). Even in the relatively well-characterized Palaeo trench–forearc region of the California Coast Ranges the GVG deposits are difficult to distinguish from similar units in the FC and CRO. In typical orogenic belts that exhibit greater post-subduction disruption, distinguishing forearc basin olistostrome deposits, subduction complex, and opholite mantle sections is much more difficult. Forearc basin olistostromal deposits have probably been misidentified as one of the other trench–forearc lithologic associations. Such errors may lead to erroneous interpretations of the nature of large-scale material and fluid pathways in trench–forearc systems, as well as misinterpretations of tectonic processes associated with HP metamorphism and exhumation of the resultant rocks.

Monomineral universal clinopyroxene and garnet barometers for peridotitic, eclogitic and basaltic systems

New versions of the universal Jd-Di exchange clinopyroxene barometer for peridotites, pyroxenites and eclogites, and also garnet barometer for eclogites and peridotites were developed. They were checked using large experimental data sets for eclogitic (∼530) and peridotitic systems (>650). The precision of the universal Cpx barometer for peridotites based on Jd-Di exchange is close to Cr-Tschermakite method produced by Nimis and Taylor (2000). Cpx barometer was transformed by the substitution of major multiplier for KD by the equations dependent from Al-Na-Fe. Obtained equation in combination with the thermometer of Nimis and Taylor (2000) allow to reconstruct position of the magma feeder systems of the alkali basaltic magma within the mantle diapirs in modern platforms like in Vitim plateau and other Southern Siberia localities and several localities worldwide showing good agreement of pressure ranges for black and green suites. These equations allow construct PTX diagrams for the kimberlite localities in Siberia and worldwide calculating simultaneously the PT parameters for different groups of mantle rocks. They give very good results for the concentrates from kimberlite lamproites and placers with mantle minerals. They are useful for PT estimates for diamond inclusions. The positions of eclogite groups in mantle sections are similar to those determined with new Gar–Cpx barometer produced by C. Beyer et al. (2015). The Fe rich eclogites commonly trace the boundary between the lower upper parts of subcontinental lithospheric mantle (SCLM) at 3–4 GPa marking pyroxenite eclogites layer. Ca-rich eclogites and especially grospydites in SCLM beneath Precambrian kimberlites occurs near pyroxenite layer but in younger mantle sections they became common in the lower parts. The diamondiferous Mg Cr-less group eclogites referring to the ancient island arc complexes are also common in the middle part of mantle sections and near 5–6 GPa. Commonly eclogites in lower apart of mantle sections are remelted and trace the high temperature convective branch. The Mg- and Fe-rich pyroxenites also show the extending in pressure trends which suggest the anatexic melting under the influence of volatiles or under the interaction with plums.

Mineral chemistry and petrology of mantle peridotites from the Guleman ophiolite (SE Anatolia, Turkey): Evidence of a forearc setting

The mantle section of Guleman ophiolite, southeast (SE) Turkey consists mainly of harzburgites and dunite lenses and large chromitite pods. The average Cr ratio = [100 × Cr/(Cr + Al) atomic ratio] of Cr-spinels in harzburgites and dunites is remarkably high (>63). The forsterite (Fo) content of olivine is between 90.9 and 92.3 in harzburgites and dunites. These features indicate that the harzburgites and dunites resulted from >35% of partial melting of a depleted mantle source. Discriminant geochemical diagrams based on the mineral chemistry of harzburgites indicate a supra-subduction zone (SSZ) origin. Orthopyroxene and clinopyroxene from the Guleman harzburgites have low CaO, Al2O3 and TiO2 contents, resembling those of depleted harzburgites from modern forearcs and contrasting with moderately depleted abyssal peridotites. Consequently, we propose that the Guleman peridotites formed in a forearc setting during the subduction initiation that developed as a result of northward subduction of the southern branch of the Neo-Tethys in response to the convergence between the Arabian and Anatolian plates.

Geochronological and mineral chemical constraints on the age and formation conditions of the leucogranite in the Mawat ophiolite, Northeastern of Iraq: insight to sync-subduction zone granite

Daraban leucogranite occurs as cactus-like dykes hosted in serpentinized harzburgite in the mantle section of the Mawat ophiolite, Kurdistan region, northeastern Iraq. It has a primary mineral assemblage of quartz + orthoclase + albite + tourmaline + muscovite, with Mg-rich biotite, phlogopite, zircon, ilmenite–titanohematite exsolution, xenotime, and monazite as the most abundant accessory minerals. New laser ablation inductively coupled plasma mass spectrometry U–(Th)–Pb dating of zircon, monazite, and xenotime reveal a single episode of leucogranite magmatism in the Mawat ophiolite at 92.6 ± 1.2 Ma. These data indicate that the intrusion ages of leucogranite rocks postdate the 105 ± 5 Ma formation age of Mawat ophiolite obtained by K–Ar hornblende method. The leucogranite magma originated by anatexis of pelagic sediments during the late Cretaceous subduction in the Neo-Tethys Ocean, leading to the formation of the Mawat ophiolite as part of the main Zagros ophiolite belt in Iraq and Iran. Tourmaline and biotite from leucogranite dykes were examined regarding their microchemistry and formation environment. Electron microprobe studies show that the tourmalines form mainly dravite–schorl solid solutions with a tendency to schorl compositions. Biotites in the leucogranite have bimodal composition represents by phlogopite and Mg-rich biotite. The tourmaline and biotite compositions, as well as field observations, appear to exclude a late-stage magmatic differentiation origin for the leucogranite. A probable source is S-type granitic magma rich in boron that resulted from the anatexis of silica-rich Ca-poor subduction wedge sediments like those of the Qulqula group. This intruded the ophiolites during the subduction stage. Calculated biotite and Fe–Ti oxide equilibria indicate that the parent magma formed along the subduction zone and solidified in the mantle wedge at a pressure 3.8–4.2 kbar, equivalent to 12.5–13.8 km depth.