Forearc Peridotites Research Articles

Origin of the Zedang and Luobusa Ophiolites, Tibet

Abstract:The Zedang and Luobusa ophiolites are located in the eastern section of the Yalung Zangbo ophiolite belt, and they share similar geological tectonic setting and age. Thus, an understanding of their origins is very important for discussion of the evolution of the Eastern Tethys Ocean. There is no complete ophiolite assemblage in the Zedang ophiolite. The Zedang ophiolite is mainly composed of mantle peridotite and a suite of volcanic rocks as well as siliceous rocks, with some blocks of olivine‐pyroxenite. The mantle peridotite mainly consists of Cpx‐harzburgite, harzburgite, some lherzolite, and some dunite. A suite of volcanic rocks is mainly composed of calc‐alkaline pyroclastic rocks and secondly of tholeiitic pillow lavas, basaltic andesites, and some boninitic rocks with a lower TiO2 content (TiO2 < 0.6%). The pyroclastic rocks have a LREE‐enriched REE pattern and a LILE‐enriched (compared to HFSE) spider diagram, demonstrating an island‐arc origin. The tholeiitic volcanic rock has a LREE‐depleted REE pattern and a LILE‐depleted (compared to HFSE) spider diagram, indicative of an origin from MORB. The boninitic rock was generated from fore‐arc extension. The Luobusa ophiolite consists of mantle peridotite and mafic‐ultramafic cumulate units, without dike swarms and volcanic rocks. The mantle peridotite mainly consists of dunite, harzburgite with low‐Opx (Opx < 25%), and harzburgite (Opx > 25%), which can be divided into two facies belts. The upper is a dunite‐harzburgite (Opx < 25%) belt, containing many dunite lenses and a large‐scale chromite deposit with high Cr2O3; the lower is a harzburgite (Opx >25%) belt with small amounts of dunite and lherzolite. The Luobusa mantle peridotite exhibits a distinctive vertical zonation of partial melting with high melting in the upper unit and low melting in the lower. Many mantle peridotites are highly depleted, with a characteristic U‐shaped REE pattern peculiar to fore‐arc peridotite. The Luobusa cumulates are composed of wehrlite and olivine‐pyroxenite, of the P‐P‐G ophiolite series. This study indicates that the Luobusa ophiolite was formed in a fore‐arc basin environment on the basis of the occurrence of highly depleted mantle peridotite, a high‐Cr2O3 chromite deposit, and cumulates of the P‐P‐G ophiolite series. We conclude that the evolution of the Eastern Tethys Ocean involved three stages: the initial ocean stage (formation of MORB volcanic rock and dikes), the fore‐arc extension stage (formation of high‐Cr2O3 chromite deposits and P‐P‐G cumulates), and the island‐arc stage (formation of calc‐alkaline pyroclastic rocks).

11B-rich fluids in subduction zones: The role of antigorite dehydration in subducting slabs and boron isotope heterogeneity in the mantle

Serpentinites form by hydration of mantle peridotite and constitute the largest potential reservoir of fluid-mobile elements entering subduction zones. Isotope ratios of one such element, boron, distinguish fluid contributions from crustal versus serpentinite sources. Despite 85% of boron hosted within abyssal peridotite being lost at the onset of subduction at the lizardite-to-antigorite transition, a sufficient cargo of boron to account for the composition of island arc magma is retained (c. 7μgg−1, with a δ11B of +22‰) until the down-going slab reaches the antigorite-out isograd. At this point a 11B-rich fluid, capable of providing the distinctive δ11B signature of island arc basalts, is released. Beyond the uniquely preserved antigorite-out isograd in serpentinites from Cerro del Almirez, Betic Cordillera, Spain, the prograde lithologies (antigorite–chlorite–orthopyroxene–olivine serpentinite, granofels-texture chlorite-harzburgite and spinifex-texture chlorite-harzburgite) have very different boron isotope signatures (δ11B=−3 to +6‰), but with no significant difference in boron concentration compared to the antigorite-serpentinite on the low P–T side of the isograd. 11B-rich fluid, which at least partly equilibrated with pelagic sediments, is implicated in the composition of these prograde lithologies, which dehydrated under open-system conditions. Serpentinite-hosted boron lost during the early stages of dehydration is readily incorporated into forearc peridotite. This, in turn, may be dragged to sub-arc depths as a result of subduction erosion and incorporated in a mélange comprising forearc serpentinite, altered oceanic crust and pelagic sediment. At the antigorite-out isograd it dehydrates, thus potentially providing an additional source of 11B-rich fluids.

Petrological studies of neoproterozoic serpentinized ultramafics of the nubian shield: spinel compositions as evidence of the tectonic evolution of egyptian ophiolites

Abstract The mafic-ultramafic rocks of the Gabal El-Degheimi area, Central Eastern Desert of Egypt, are parts of an ophiolitic section. The ophiolitic rocks are dismembered and tectonically enclosed within, or thrust over, island arc assemblages. Serpentinites, altered slices of the upper mantle, represent a distinctive lithology of the dismembered ophiolites. Some portions of the serpentinized rocks contain fresh relicts of primary minerals such as chromian spinel and olivine. The abundance of bastite and mesh textures suggests harzburgite and dunite protoliths, respectively, for these serpentinites. Some fresh cores of chromian spinel are rimmed by ferritchromite and Cr-magnetite. The development of alteration rims around chromian spinel cores indicates their formation during prograde alteration and under oxidizing conditions during lower amphibolite facies metamorphism. Fresh chromian spinels are characterized by high contents of Cr2O3 (48.92-56.74 wt. %), Al2O3 (10.29-20.08wt. %), FeO (16.24-28.46 wt. %) and MgO (4.89-14.02 wt. %), and very low TiO2 contents (<0.16 wt. %). The analyzed fresh chromian spinels have high Cr# (0.62-0.79) characteristic of spinels in mantle peridotite that has undergone some degree of partial melting. The data presented here suggest that the mantle peridotites of the Gabal El-Degheimi area are similar to forearc peridotites of suprasubduction zone environments.

Metasomatic hydration of the Oeyama forearc peridotites: Tectonic implications

In contrast to the widely recognized aspects of serpentinization, initial stages of hydration and tectonic processes of unserpentinized peridotites are still unclear, but have important implications for understanding the lithospheric architecture of supra-subduction zones. This study provides petrological evidence from the Oeyama ophiolite, SW Japan, of the effects of high-temperature metasomatic hydration immediately before the cooling and ductile deformation of forearc peridotites. Key findings in this study are: 1) complex association of high-temperature metasomatic minerals: tremolitic amphibole, cummingtonite, phlogopite, chlorite, olivine and orthopyroxene in veins and in mylonites; 2) the systematic variation in Si and Na+K contents of the tremolitic amphibole, corresponding to its mode of occurrence and mineral association; and 3) the presence of thin (<0.7mm) veins of fine-grained olivine accompanied by a narrow diffusion zone of the host primary olivine. On the basis of petrography and mineral chemistry, the temporal sequence of hydration and deformation of the Oeyama ophiolite is considered as follows: 1) infiltration of slab-derived fluids, causing decomposition of primary pyroxene and chemical modification of primary olivine, 2) metasomatic formation of variable modal amounts of amphibole, phlogopite, chlorite, vein-forming olivine and secondary orthopyroxene at 650–750°C; 3) early-stage mylonitization of the hydrous peridotites in localized shear zones; and 4) syntectonic serpentinization at 400–600°C to form serpentinite mylonites. Paragenesis and amphibole compositions suggest comparable temperature conditions for metasomatism and early-stage mylonitization. Mylonitization occurred exclusively in hydrous peridotites, and the peridotite mylonites were preferentially overprinted by syntectonic serpentinization. Diffusion profiles of olivine cut by a vein suggest rapid cooling immediately after the metasomatic fluid infiltration. From these observations and calculations, it is concluded that the exhumation of the forearc peridotites was closely related to the infiltration of high-temperature metasomatic fluids and hydration occurred under a wide range of temperature conditions.

Two ultramafic rock types in the Hongseong area, South Korea: Tectonic significance for northeast Asia

Several isolated lensoidal mafic and serpentinized ultramafic bodies occur in the Hongseong area, South Korea, which is the Korean extension of the Dabie–Sulu collisional belt of China and the northern margin of the South China Block. The mafic and ultramafic bodies are important in any interpretation of the Neoproterozoic tectonics of northeast Asia related to the amalgamation and break-up of the Rodinia supercontinent. The Singok, Gwangcheon, and Hongseong ultramafic rocks are significant bodies of ultramafic rock in the Hongseong area. The orthopyroxenes in the Singok harzburgite plot mostly in the forearc peridotite fields of major element classification diagrams. The Singok olivines also classify as forearc peridotite on the basis of their Fo and NiO contents. The spinels of the Singok harzburgite plot within the mantle array on an Al2O3 versus Cr2O3 diagram, and they fall in the ophiolite and supra-subduction zone peridotite (SSZ) fields on several tectonic discrimination diagrams. These data indicate that the Singok harzburgite is a segment of residual mantle that formed in a SSZ tectonic setting before the amalgamation of the Rodinia supercontinent as the Bibong and Baekdong ultramafic rocks in the Hongseong area. The spinels from the Gwangcheon harzburgite and Hongseong dunite have similar geochemical characteristics with those from ultramafic rocks formed in the Lesvos incipient rift. They plot in the mantle array on the Al2O3 versus Cr2O3 diagram and have higher Mg/(Mg+Fe2+) than abyssal peridotites and lower Cr/(Cr+Al) than SSZ and forearc peridotites. These data together with field evidence indicate that the Gwangcheon and Hongseong ultramafic rocks formed in a continental rift tectonic setting during the break-up of the Rodinia supercontinent. These two types of ultramafic rock from the Hongseong area, which formed in different tectonic settings, are correlated well with the two periods of Neoproterozoic magmatic activity along the northern margin of the Yangtze Block which experienced a subduction and continental rifting stages before and after the amalgamation of the Rodinia supercontinent, respectively.

Geochemistry and tectonic evolution of the Late Cretaceous Gogher–Baft ophiolite, central Iran

The Late Cretaceous Gogher–Baft ophiolite is one of the best preserved remnants of Neo-Tethyan oceanic lithospheric within the inner Zagros ophiolite belt. The ophiolite comprises from bottom to top, harzburgites, pegmatite and isotropic gabbroic lenses within the mantle sequence, pillowed to massive basalts to dacites and pyroclastic rocks associated with blocks of pelagic limestone and radiolarite. Basaltic to dacitic sills crosscut the pyroclastic rocks. The ophiolite sequence is overlain by Turonian–Maastrichtian pelagic limestones (93.5–65.5Ma). Mineral compositions of harzburgites are similar to those of fore-arc peridotites and overlap with abyssal peridotites. Most Gogher–Baft ophiolite magmatic rocks show supra-subduction zone affinities, except for some E-MORB type lavas. The geochemical characteristics suggest that Gogher–Baft ophiolite magmatic rocks were generated during subduction initiation. These show progressive source depletion leading to the formation of MORB to boninitic magmas. Early E-MORB-type pillow lavas may have originated by melting mantle that was not affected by subduction components as the Tethyan oceanic plate began to sink beneath Eurasia as subduction began in the Late Cretaceous. Initial εNd (t) values range from +2.6 to +9 for Gogher–Baft magmatic rocks. Samples with radiogenic Nd overlap with least radiogenic MORBs and with Oman and other Late Cretaceous Tethyan ophiolitic rocks. The initial 87Sr/86Sr ratios range from 0.7048 to 0.7057, indicating modification due to seafloor alteration. Radiogenic 207Pb/204Pb isotopic compositions (systematically above the NHRL) and less radiogenic Nd isotopic compositions suggest the involvement of sediments in the mantle source in some magmatic rocks. Our results for Gogher–Baft ophiolite and the similarity of these to other Iranian Zagros ophiolites suggest a subduction initiation setting for the generation of these magmatic rocks.

Depth, degrees and tectonic settings of mantle melting during craton formation: inferences from major and trace element compositions of spinel harzburgite xenoliths from the Udachnaya kimberlite, central Siberia

We examine major and trace element compositions of whole-rocks and minerals of 18 spinel harzburgite xenoliths from the Udachnaya kimberlite in the central Siberian craton. The samples are fresh, modally homogeneous and large enough to provide representative whole-rock samples (>100g); all residual spinel peridotites in our collections that satisfied these criteria were selected for this study. Their Mg# ranges from 0.92 to 0.93. Thirteen xenoliths have 6–22% orthopyroxene (opx) and 0–3% clinopyroxene (cpx); five are opx-rich (31–43%) and contain 3–5% cpx. The low-opx Udachnaya harzburgites define regular co-variation trends on major and minor oxide plots and appear to be pristine melt extraction residues. Their Al–Fe and Al–Mg# trends, in combination with experimental results on melting of fertile peridotites, indicate an origin by ∼40% of polybaric decompression fractional melting between 7−4GPa and ≤1–2GPa. In line with Cr and HREE abundances, garnet was stable in the residues at early melting stages. Thus, the spinel facies cratonic peridotites situated at >40 to 130km (1–4GPa), are residues of melting that took place over a broader depth range; the residues probably underwent density-driven segregation from the denser ambient mantle to form the base of cratonic nuclei. The opx-rich Udachnaya harzbrugites cannot be produced by dry or hydrous melting at 1–7GPa and might be related to fractionation or metamorphic segregation of initial high-P melting residues and their interaction with interstitial liquids retained in the residues during gravity-driven upwelling. Our results, in comparison with published data on arc and fore-arc peridotites, show no evidence that significant enrichments in silica (opx) could be produced during melting or metasomatism in subduction settings, or that initial melting residues were dunites, which were later transformed to harzburgites by melt-rock reactions.

Geochemistry and petrogenesis of the Late Cretaceous Haji‐Abad ophiolite (Outer Zagros Ophiolite Belt, Iran): implications for geodynamics of the Bitlis–Zagros suture zone

The Haji‐Abad ophiolite in SW Iran (Outer Zagros Ophiolite Belt) is a remnant of the Late Cretaceous supra‐subduction zone ophiolites along the Bitlis–Zagros suture zone of southern Tethys. These ophiolites are coeval in age with the Late Cretaceous peri‐Arabian ophiolite belt including the Troodos (Cyprus), Kizildag (Turkey), Baer‐Bassit (Syria) and Semail (Oman) in the eastern Mediterranean region, as well as other Late Cretaceous Zagros ophiolites. Mantle tectonites constitute the main lithology of the Haji‐Abad ophiolite and are mostly lherzolites, depleted harzburgite with widespread residual and foliated/discordant dunite lenses. Podiform chromitites are common and are typically enveloped by thin dunitic haloes. Harzburgitic spinels are geochemically characterized by low and/or high Cr number, showing tendency to plot both in depleted abyssal and fore‐arc peridotites fields. Lherzolites are less refractory with slightly higher bulk REE contents and characterized by 7–12% partial melting of a spinel lherzolitic source whereas depleted harzburgites have very low abundances of REE and represented by more than 17% partial melting. The Haji‐Abad ophiolite crustal sequences are characterized by ultramafic cumulates and volcanic rocks. The volcanic rocks comprise pillow lavas and massive lava flows with basaltic to more‐evolved dacitic composition. The geochemistry and petrology of the Haji‐Abad volcanic rocks show a magmatic progression from early‐erupted E‐MORB‐type pillow lavas to late‐stages boninitic lavas. The E‐MORB‐type lavas have LREE‐enriched patterns without (or with slight) depletion in Nb–Ta. Boninitic lavas are highly depleted in bulk REEs and are represented by strong LREE‐depleted patterns and Nb–Ta negative anomalies. Tonalitic and plagiogranitic intrusions of small size, with calc‐alkaline signature, are common in the ophiolite complex. The Late Cretaceous Tethyan ophiolites like those at the Troodos, eastern Mediterranean, Oman and Zagros show similar ages and geochemical signatures, suggesting widespread supra‐subduction zone magmatism in all Neotethyan ophiolites during the Late Cretaceous. The geochemical patterns of the Haji‐Abad ophiolites as well as those of other Late Cretaceous Tethyan ophiolites, reflect a fore‐arc tectonic setting for the generation of the magmatic rocks in the southern branch of Neotethys during the Late Cretaceous. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Investigation on mantle peridotites from Neyriz ophiolite, south of Iran: geodynamic signals

Neyriz ophiolite in Abadeh Tashk area appears as four major separated massifs in an area with 125 km2, south of Iran. Peridotites including harzburgite, dunite, and lesser low-Cpx lherzolite are the major constituents of the ophiolite with very minor mafic rocks. Usual gabbros of ophiolite complexes are virtually absent from the study area. Mineral modality associated with bulk rock and mineral chemistry of the peridotites show a progression from fertile to ultra-refractory character, reflected by a progressive decrease in modal pyroxenes and in Al2O3, CaO, SiO2, Sc, Ta, V, and Ga values of the studied rocks by approaching chromite deposits. The Neyriz peridotites vary from low-Cpx lherzolite (MgO, 41.97–43.1 wt.%; Al2O3, 0.8–1.3 wt.%) with low content of Cr# spinel (36.7–37.6) and Fo olivine (90.79–91.5) to harzburgite (MgO, 44.31–45.25 wt.%;Al2O3, 0.29–0.45 wt.%; Cr# spinel, 58.2–73.45; Fo olivine, 91.23–91.56), and then to dunite (MgO, 45.9–49.2 wt.%; Al2O3, 0.18–0.48 wt.%) with higher content of Cr# spinel (74.34–79.36) and Fo olivine (91.75–94.68). Compared to modern oceanic settings, mineral and rock composition of low-Cpx lherzolite plot within the field of mid-ocean-ridge environment, whereas those of harzburgite and dunite fall in the field of fore-arc peridotites. As a result of the studies on minerals and whole rock chemistry along with rock interrelationships, we contend that the peridotites were subsequently affected by percolating hydrous boninitic melt from which the high-Cr–Mg, low-Ti chromitites were formed within mantle wedge above the supra-subduction zone in a fore-arc setting.

Significance of chromian spinels from the mantle sequence of the Andaman Ophiolite, India: Paleogeodynamic implications

The mantle section of the Andaman Ophiolite between Rutland Island in the south and north Andaman in the north has been studied. The restitic peridotite of Rutland Island is comprised of depleted harzburgite to clinopyroxene-bearing harzburgite whereas that in middle and north Andaman is mostly less-depleted, lherzolite-dominated mantle. Chromian spinels from the mantle section of the Andaman Ophiolite belong to four major groups: Group-1 — spinels from Rutland chromitite pods, characterized by high Cr# [=Cr/(Cr+A1) atomic ratio] (~0.71); Group-2 — spinels from north Andaman chromitite pods with two subgroups, 2A — characterized by high Cr# (0.70–0.79) and 2B — characterized by medium Cr# (0.48–0.51) and high Mg# [=Mg/(Mg+Fe2+) atomic ratio] (0.74–0.77); Group-3 — residual accessory grains from Rutland peridotite, characterized by medium Cr# (0.37–0.55) and low Mg# (0.41–0.54); and Group-4 — residual accessory grains from middle and north Andaman peridotites, characterized by low Cr# (0.09–0.23) and high Mg# (0.71–0.81).Group-1 chromian spinels of Rutland Island and Group-2A of north Andaman are likely to have crystallized from boninitic melts, formed by relatively high degrees of partial melting, whereas Group-2B varieties from north Andaman are from tholeiitic melts, formed by lower degrees of melting. The chemistry of the residual accessory chromian spinels (Groups 3 and 4) suggests that the mantle peridotites of Rutland Island towards the south are similar to fore-arc peridotites of suprasubduction zone environments whereas those of north Andaman are less depleted. This spatial and/or temporal directional change in spinel compositions may reflect variations linked to the melting history where the same sliver of oceanic mantle underwent different styles of melting in different tectonic settings at different points in time. The coexistence of both chromitite types, with high- and low-Cr spinels, in the same area from north Andaman possibly reflects temporal variations of separate melt intrusions produced through specific melting stages. It is inferred that a similar geodynamic setting to the present-day also existed during the Cretaceous period with a fore-arc at Rutland Island and a back-arc towards the north at north Andaman.

Origin of Neoproterozoic ophiolitic peridotites in south Eastern Desert, Egypt, constrained from primary mantle mineral chemistry

The ophiolitic peridotites in the Wadi Arais area, south Eastern Desert of Egypt, represent a part of Neoproterozoic ophiolites of the Arabian-Nubian Shield (ANS). We found relics of fresh dunites enveloped by serpentinites that show abundances of bastite after orthopyroxene, reflecting harzburgite protoliths. The bulk-rock chemistry confirmed the harzburgites as the main protoliths. The primary mantle minerals such as orthopyroxene, olivine and chromian spinel in Arais serpentinites are still preserved. The orthopyroxene has high Mg# [=Mg/(Mg + Fe2+)], ~0.923 on average. It shows intra-grain chemical homogeneity and contains, on average, 2.28 wt.% A12O3, 0.88 wt.% Cr2O3 and 0.53 wt.% CaO, similar to primary orthopyroxenes in modern forearc peridotites. The olivine in harzburgites has lower Fo (93−94.5) than that in dunites (Fo94.3−Fo95.9). The Arais olivine is similar in NiO (0.47 wt.% on average) and MnO (0.08 wt.% on average) contents to the mantle olivine in primary peridotites. This olivine is high in Fo content, similar to Mg-rich olivines in ANS ophiolitic harzburgites, because of its residual origin. The chromian spinel, found in harzburgites, shows wide ranges of Cr#s [=Cr/(Cr + Al)], 0.46−0.81 and Mg#s, 0.34−0.67. The chromian spinel in dunites shows an intra-grain chemical homogeneity with high Cr#s (0.82−0.86). The chromian spinels in Arais peridotites are low in TiO2, 0.05 wt.% and YFe [= Fe3+/(Cr + Al + Fe3+)], ~0.06 on average. They are similar in chemistry to spinels in forearc peridotites. Their compositions associated with olivine’s Fo suggest that the harzburgites are refractory residues after high-degree partial melting (mainly ~25−30 % partial melting) and dunites are more depleted, similar to highly refractory peridotites recovered from forearcs. This is in accordance with the partial melting (>20 % melt) obtained by the whole-rock Al2O3 composition. The Arais peridotites have been possibly formed in a sub-arc setting (mantle wedge), where high degrees of partial melting were available during subduction and closing of the Mozambique Ocean, and emplaced in a forearc basin. Their equilibrium temperature based on olivine−spinel thermometry ranges from 650 to 780 °C, and their oxygen fugacity is high (Δlog ƒO2 = 2.3 to 2.8), which is characteristic of mantle-wedge peridotites. The Arais peridotites are affected by secondary processes forming microinclusions inside the dunitic olivine, abundances of carbonates and talc flakes in serpentinites. These microinclusions have been formed by reaction between trapped fluids and host olivine in a closed system. Lizardite and chrysotile, based on Raman analyses, are the main serpentine minerals with lesser antigorite, indicating that serpentines were possibly formed under retrograde metamorphism during exhumation and near the surface at low T (<400 °C).

Neoproterozoic ophiolitic peridotites along the Allaqi-Heiani suture, South Eastern Desert, Egypt

The Wadi Allaqi ophiolite along the Egyptian-Sudanese border defines the southernmost ophiolitic assemblage and suture zone in the Eastern Desert. Ophiolite assemblages comprise nappes composed mainly of mafic and ultramafic rocks that were tectonically emplaced and replaced by serpentine and carbonates along shear zones probably due to CO2-metasomatism. Serpentinites, altered slices of the upper mantle, represent a distinctive lithology of dismembered ophiolites of the western YOSHGAH suture. Microscopically, they are composed of more than 90 % serpentine minerals with minor opaque minerals, carbonate, brucite and talc. The mineral chemistry and whole-rock chemical data reported here indicate that the serpentinized peridotites formed as highly-depleted mantle residues. They show compositions consistent with formation in a suprasubduction zone environment. They are depleted in Al2O3 and CaO similar to those in fore-arc peridotites. Also, high Cr# (Cr/ (Cr+Al)) in the relict chrome spinels (average ~0.72) indicates that these are residual after extensive partial melting, similar to spinels in modern fore-arc peridotites. Therefore, the studied serpentinites represent fragments of an oceanic lithosphere that formed in a fore-arc environment, which belongs to an ophiolitic mantle sequence formed in a suprasubduction zone.

ロシア極東，Ust'-Belayaオライトのマントルかんらん岩から見えてきた前弧域かんらん岩の多様性

ロシア極東，Ust'-Belayaオライトのマントルかんらん岩から見えてきた前弧域かんらん岩の多様性

Petrology and geochemistry of peridotites in the Zhongba ophiolite, Yarlung Zangbo Suture Zone: Implications for the Early Cretaceous intra-oceanic subduction zone within the Neo-Tethys

The Zhongba ophiolite is located in the western part of the Yarlung Zangbo Suture Zone (YZSZ) separating Eurasia to the north from the Indian plate to the south. This ophiolite comprises a well-preserved mantle sequence dominated by harzburgites with minor dunites. Highly depleted modal, mineral and bulk rock compositions of the harzburgites indicate that they are residues after moderate to high degrees of partial melting (13–24%) mainly in the spinel-stability field. These rocks display typical U-shaped chondrite-normalized Rare Earth Element (REE) patterns and fractionated chondrite-normalized Platinum Group Element (PGE) patterns. These characteristics, in combination with their hybrid mineral and whole-rock compositions intermediate between those of abyssal and forearc peridotites, indicate melt-rock interaction resulting in the selective enrichment of LREE and Pd. We propose a two-stage model to explain the generation of the Zhongba harzburgites: 1) original generation from a MORB-source upper mantle, and 2) subsequent trapping as part of a mantle wedge above a subduction zone. Comparable observations from the ophiolitic massifs along the whole YZSZ allow us to propose that a ca. 2500-km long complex subduction system was active between India and the Lhasa terrane, Burma, and the Karakoram microcontinent within the Neo-Tethys during the Early Cretaceous, similar to the modern active intra-oceanic subduction systems in the Western Pacific.

Petrology and geochemistry of the Xiugugabu ophiolitic massif, western Yarlung Zangbo suture zone, Tibet

The Yarlung Zangbo Suture Zone (YZSZ), southern Tibet, is a discontinuous belt that is more than 2000km long, composed of the remnants of Neo-Tethyan Mesozoic ocean. One of these relicts is the Xiugugabu ophiolitic massif which is a mantle thrust sheet of more than 260km2 overlying the Cretaceous tectonic mélange south of the YZSZ in SW Tibet. The massif is composed of harzburgites and clinopyroxene–harzburgites with porphyroclastic and porphyromylonitic textures. In the southern part of the massif, peridotites were intruded by amphibole-bearing microgabbro and microgabbronorite sills. A diabase unit which is overlaid by a sedimentary sequence crops out on the NE flank of the massif.Mineral chemistry in harzburgites and clinopyroxene–harzburgites indicates compositions similar to abyssal and forearc peridotites. Peridotites are slightly LREE depleted to enriched with [La/Yb]CN 0.06–2.8 and [La/Sm]CN 0.34–2.64. These ultramafic rocks are inferred to be the residues of 5–25% of partial melting of a depleted mantle that has been enriched by percolating metasomatic melts in a suprasubduction environment.Amphibole–microgabbro and amphibole–microgabbronorite sills are mostly composed of brown to green amphibole, calcic plagioclase, clinopyroxene, ilmenite and orthopyroxene in gabbronorite. Textures and compositions of the brown amphiboles indicate a near-solidus high temperature hydrothermal origin (>800°C). These intrusive rocks are tholeiitic and show N-MORB type REE patterns ([La/Yb]NC 0.35–0.90), a LILE (mainly Th) enrichment and noticeable Nb, Ta and Ti negative anomalies. They have a suprasubduction affinity and were formed in a back-arc basin setting. The diabase unit outcropping to the NE of the massif is not directly related to the ultramafic and mafic ophiolitic rocks. The diabase shows LREE enriched patterns ([La/Yb]NC 8–8.9) and slight Nb, Ta and Ti negative anomalies. The diabase has an intraplate affinity and could have been derived from a mantle source enriched by subduction-related fluids. The absence of continental crustal assimilation indicates that these rocks were probably emplaced in the Jurassic, in an oceanic environment after the Triassic disaggregation of the Indian plate.The data are consistent with the recent geodynamic model proposed for the central part of the suture for the closure of the Neo-Tethys and suggest that the geodynamic evolution of the western part of the basin was comparable to the central part.

Trace element composition of peridotites from the southern Mariana forearc: Insights into the geochemical effects of serpentinization and/or seafloor weathering

Peridotites from the southern Mariana forearc were sampled on the landward trench slope of the Izu-Bonin-Mariana (IBM) subduction zone by dredging. These mantle wedge peridotites underwent hydration by fluid derived from a dehydrated descending slab, and later interacted with seawater after emplacement at or near the seafloor. This study investigates how these two different rock-fluid interaction processes influenced trace element distribution in the southern Mariana forearc peridotites. We measured trace element concentrations of peridotites from the southern Mariana forearc. The southern Mariana forearc peridotites are characterized by a distinct seawater-like REE pattern with an obvious negative Ce anomaly, and La shows good correlation with other REEs (except Ce). In addition, there is a great enrichment of U, Pb, Sr and Li elements, which show a distinct positive anomaly relative to adjacent elements in the multi-element diagram. For the seawater-like REE pattern, we infer that REEs are mainly influenced by seawater during peridotite-seawater interactions after their emplacement at or near the seafloor, by serpentinization or by marine weathering. Furthermore, the anomalous behavior of Ce, compared with other rare earth elements in these samples, may indicate that they have undergone reactions involving Ce (IV) when the peridotites interacted with seawater. Positive U, Pb, Sr and Li anomalies are inferred to be related to seawater and/or fluids released during dehydration of the subducting slab.

Serpentinization of peridotites from the southern Mariana forearc

A detailed petrologic and mineralogic study was carried out on serpentinized peridotites dredged from the southern landward slopes of the Mariana Trench, in order to reveal the serpentinization process of these unusual rocks and to identify the sole presence of the mineral lizardite. The constituent minerals of these southern Mariana forearc peridotites are olivine, amphibole and spinel, as well as serpentine, chlorite and talc. Compared with serpentinite seamounts, the serpentinized peridotites from the southern Mariana forearc are characterized by the absence of magnetite and brucite, and the common presence of talc; besides, the serpentine mineral variety is simplex, only lizardite. Combining mineral chemistry and mineral phase relationships, we conclude that (1) the absence of magnetite in the serpentinized peridotites is due to incomplete serpentinization, other than magnetite, the iron end-member in olivine forms Fe-rich brucite and Fe-rich serpentine; (2) brucite is not stable with high silica activity, reacting with later SiO 2-rich fluid and then forming lizardite, leading to a lack of brucite in these serpentinized peridotites; (3) the occurrence of talc is the result of later SiO 2-rich fluid reactions with lizardite; and (4) the reason for the sole occurrence of lizardite is that the temperature condition of our study area was not high enough for the formation of antigorite (which is stable at >500 °C). Despite the broad overlap of lizardite and chrysotile in growth temperature, differences in the modes of occurrence of lizardite and chrysotile, such as the scarcity of H 2O, low porosity and permeability, as well as the actual situation of initial serpentinization in the study area, result in the absolute prevalence of lizardite over chrysotile in the area.

An early Palaeozoic supra-subduction lithosphere in the Variscides: new evidence from the Maures massif

Petrographic and geochemical studies of peridotites and melagabbros from the Maures massif (SE France) provide new constraints on the Early Palaeozoic evolution of the continental lithosphere in Western Europe. Peridotites occur as lenses along a unit rooted in the main Variscan suture zone. They are dominantly spinel peridotites and minor garnet–spinel peridotites. Spinel peridotites represent both residual mantle and ultramafic cumulates. Mantle-related dunites and harzburgites display high temperature textures, with olivine (Mg#0.90), orthopyroxene (Mg#0.90) and spinel (TiO2 < 0.2%; Cr#0.64–0.83) compositions typical of fore-arc upper mantle. Ultramafic cumulates are dunite adcumulates, harzburgite heteradcumulates and mesocumulates, melagabbro heteradcumulates and amphibole peridotites, with olivine (Mg#0.85–0.89), orthopyroxene (Mg#0.86–0.89) and Cr-spinel (TiO2 = 0.5–3.3%; Cr#0.7–0.98) compositions typical of ultramafic cumulates. Cr-spinel compositions of both spinel peridotite types suggest their genesis in a supra-subduction zone lithosphere. Core to rim zoning in spinel is related to the incomplete influence of regional metamorphism and serpentinisation. The covariation of major and minor elements with Al2O3 for cumulates is consistent with igneous processes involving crystal accumulation. Both mantle and cumulate dunites and harzburgites have U-shaped REE patterns and extremely low trace element contents, similar to peridotites from modern fore-arc peridotites (South Atlantic) and from ophiolites related to supra-subduction zones (Semail, Cyclops, Pindos, Troodos). Melagabbros also have U-shaped REE patterns similar to xenoliths from the Philippine island arc, but also similar to intrusive ultramafic cumulates from the Semail nappe of Oman related to a proto-subduction setting. A wehrlite has a REE pattern similar to that of amphibole peridotites reflecting metasomatism of clinopyroxene-bearing peridotites due to subduction-related fluids. The Maures spinel peridotites and melagabbros are therefore interpreted as the lowermost parts of a crustal sequence and minor residual mantle of lithosphere generated in a supra-subduction zone during Early Palaeozoic time. Garnet–spinel peridotites are chemically close to melagabbros, but have recorded high pressure metamorphism before their retrogression similar to spinel peridotites into amphibolites to greenschists facies metamorphism. They indicate burial to mantle depths of the margin of the supra-subduction lithosphere during the Early Palaeozoic continental subduction. Both peridotite types were exhumed during the Upper Palaeozoic continental collision. Comparable observations from other Variscan-related peridotites, in particular of the Speik complex of the Autroalpine basement, and a common age for the subduction stage allow extension of these regional conclusions to a broad area sharing the Cambrian suture zone, extending from the Ossa-Morena to the Bohemian massif.

Ultra-refractory Domains in the Oceanic Mantle Lithosphere Sampled as Mantle Xenoliths at Ocean Islands

Many peridotite xenoliths sampled at ocean islands appear to have strongly refractory major element and modal compositions. To better constrain the chemistry, abundance and origin of these ultra-refractory rocks we compiled a large number of data for xenoliths from nine groups of ocean islands. The xenoliths were filtered petrographically for signs of melt infiltration and modal metasomatism, and the samples affected by these processes were excluded. The xenolith suites from most ocean islands are dominated by ultra-refractory harzburgites. Exceptions are the Hawaii and Tahiti peridotites, which are more fertile and contain primary clinopyroxene, and the Cape Verde suite, which contains both ultra-refractory and more fertile xenoliths. Ultra-refractory harzburgites are characterized by the absence of primary clinopyroxene, low whole-rock Al2O3, CaO, FeO/MgO and heavy rare earth element (HREE) concentrations, low Al2O3 in orthopyroxene (generally < 3 wt %), high Cr-number in spinel (0·3–0·8) and high forsterite contents in olivine (averages > 91·5). They are therefore on average significantly more refractory than peridotites dredged and drilled from mid-ocean ridges and fracture zones. Moreover, their compositions resemble those of oceanic forearc peridotites. The formation of ultra-refractory ocean island harzburgites requires potential temperatures above those normally observed at modern mid-ocean ridges, and/or fluid fluxed conditions. Some ultra-refractory ocean island harzburgites give high Os model ages (up to 3300 Ma), showing that their formation significantly pre-dates the oceanic crust in the area. A genetic relationship with the host plume is considered unlikely based on textural observations, equilibration temperatures and pressures, inferred physical properties, and the long-term depleted Os and Sr isotope compositions of some of the harzburgites. Although we do not exclude the possibility that some ultra-refractory ocean island harzburgites have formed at mid-ocean ridges, we favor a model in which they formed in a process spatially and temporally unrelated to the formation of the oceanic plate and the host plume. As a result of their whole-rock compositions, ultra-refractory harzburgites have a very high solidus temperature at a given pressure, low densities and very high viscosities, and will tend to accumulate at the top of the convecting mantle. They may be preserved as fragments in the convecting mantle over long periods of time and be preferentially incorporated into newly formed lithosphere.

Geochemical constraints on the provenance and depositional setting of sedimentary rocks from the islands of Chios, Inousses and Psara, Aegean Sea, Greece: implications for the evolution of Palaeotethys

The provenance and depositional setting of Late Palaeozoic and Early Mesozoic clastic sediments from the eastern Aegean archipelago are examined here for the first time using whole-rock geochemistry and composition of detrital chrome spinel. Major- and trace-element data for Late Palaeozoic and Permo-Triassic clastic sediments from the Lower and Upper Units of Chios are compatible with an acidic to intermediate source, minor input of (ultra)mafic detritus and recycling of older sedimentary components. Chondrite-normalized REE profiles are uniform with light REE enrichments (La N /Yb N c . 7.7), negative Eu anomalies (Eu/Eu* c . 0.67) and flat heavy REE patterns (Gd N /Yb N c . 1.5), indicating an upper continental crustal source and/or young differentiated arc material. Detrital chrome spinel from the clastic sediments of Chios has Cr-number (Cr/(Cr + Al)) values between 0.29 and 0.89 and Mg-number (Mg/(Mg + Fe 2+ )) values between 0.24 and 0.70, suggesting a probably mixed (ultra)mafic source involving ridge peridotites (mid-ocean ridge type), fore-arc peridotites and island-arc basalts. The metasediments from the islands of Inousses and Psara have similar whole-rock chemical signatures to those of Chios, although no evidence was found for an (ultra)mafic source. We conclude that both the Late Palaeozoic sediments from the Lower and Upper Units of Chios and the metasediments from Inousses and Psara were deposited in a continental island-arc setting until at least Late Permian times, probably at a single Palaeotethyan margin. They are interpreted to be allochthonous, tectonically transported to their present position by Late Mesozoic to Cenozoic orogenic processes.