Coexistence of abyssal and ultra-depleted SSZ type mantle peridotites in a Neo-Tethyan Ophiolite in SW Turkey: Constraints from mineral composition,whole-rock geochemistry (major–trace–REE–PGE), and Re–Os isotope systematics

We present new, whole-rock major and trace element chemistry, including rare earth elements (REE), platinum-group elements (PGE), and Re–Os isotope data from the upper mantle peridotites of a Cretaceous Neo-Tethyan ophiolite in the Mu?la area in SW Turkey. We also report extensive mineral chemistry data for these peridotites in order to better constrain their petrogenesis and tectonic environment of formation. The Mu?la peridotites consist mainly of cpx-harzburgite, depleted harzburgite, and dunite. Cpx-harzburgites are characterized by their higher average CaO (2.27 wt.%), Al₂O₃ (2.07 wt.%), REE (53 ppb), and ¹⁸⁷Os/¹⁸⁸Os_(i) ratios varying between 0.12497 and 0.12858. They contain Al-rich pyroxene with lower Cr content of coexisting spinel (Cr# = 13–22). In contrast, the depleted harzburgites and dunites are characterized by their lower average CaO (0.58 wt.%), Al₂O₃ (0.42 wt.%), and REE (1.24 ppb) values. Their clinopyroxenes are Al-poor and coexist with high-Cr spinel (Cr# = 33–83). The ¹⁸⁷Os/¹⁸⁸Os_(i) ratios are in the range of 0.12078–0.12588 and are more unradiogenic compared to those of the cpx-harzburgites.Mineral chemistry and whole rock trace and PGE data indicate that formation of the Mu?la peridotites cannot be explained by a single stage melting event; at least two-stages of melting and refertilization processes are needed to explain their geochemical characteristics. Trace element compositions of the cpx-harzburgites can be modeled by up to ~ 10–16% closed-system dynamic melting of a primitive mantle source, whereas those of the depleted harzburgites and dunites can be reproduced by ~ 10–16% open-system melting of an already depleted (~ 16%) mantle. These models indicate that the cpx-harzburgites are the products of first-stage melting and low-degrees of melt–rock interaction that occurred in a mid-ocean ridge (MOR) environment. However, the depleted harzburgites and dunites are the product of second-stage melting and related refertilization which took place in a supra subduction zone (SSZ) environment. The Re–Os isotope systematics of the Mu?la peridotites gives model age clusters of ~ 250 Ma, ~ 400 Ma and ~ 750 Ma that may record major tectonic events associated with the geodynamic evolution of the Neo-Tethyan, Rheic, and Proto-Tethyan oceans, respectively. Furthermore, > 1000 Ma model ages can be interpreted as a result of an ancient melting event before the Proto-Tethys evolution.     

Geochemistry of platinum-group elements and mineral composition in chromitites and associated rocks from the Abdasht ultramafic complex,Kerman, Southeastern Iran

The Abdasht complex is a major ultramafic complex in south-east Iran (Esfandagheh area). It is composed mainly of dunite, harzburgite, podiform chromitites, and subordinate lherzolite and wehrlite. The podiform chromitites display massive, disseminated, banded and nodular textures. Chromian spinels in massive chromitites exhibit a uniform and restricted composition and are characterized by Cr# [= Cr / (Cr + Al)] ranging from 0.76 to 0.77, Mg# [= Mg/(Mg + Fe^2 +)] from 0.63 to 0.65 and TiO₂ < 0.2 wt.%. These values may reflect crystallization of the chromian spinels from boninitic magmas. Chromian spinels in peridotites exhibit a wide range of Cr# from 0.48 to 0.86, Mg# from 0.26 to 0.56 and very low TiO₂ contents (averaging 0.07 wt.%). The Fe^3 +# is very low, (< 0.08 wt.%) in the chromian spinel of chromitites and peridotites of the Abdasht complex which reflects crystallization under low oxygen fugacities.The distribution of platinum group elements (PGE) in Abdasht chromitites displays a high (Os + Ir + Ru)/(Rh + Pt + Pd) ratio with strongly fractionated chondrite-normalized PGE patterns typical of ophiolitic chromitites. Moreover, the Pd/Ir value, which is an indicator of PGE fractionation, is very low (< 0.1) in the chromitites.The harzburgite, dunite and lherzolite samples are highly depleted in PGE contents relative to chondrites. The Pd_N/Ir_N ratios in dunites are unfractionated, averaging 0.72, whereas the harzburgites and lherzolites show slightly positive slopes PGE spidergrams, together with a small positive Ru anomaly, and their Pd_N/Ir_N ratio averages 2.4 and 2.3 respectively. Moreover, the PGE chondrite and primitive mantle normalized patterns of harzburgite, dunite and lherzolite are relatively flat which are comparable to the highly depleted mantle peridotites.The mineral chemistry data and PGE geochemistry indicate that the Abdasht chromitites and peridotites were generated from a melt with boninitic affinity under low oxygen fugacity in a supra-subduction zone setting. The composition of calculated parental melts of the Abdasht chromitites is consistent with the differentiation of arc-related magmas.     

Genesis and Geodynamic Significance of Chromitites from the Fuchuan Ophiolite, Southern China, as Evidenced by Trace Element Fingerprints of Chromite, Olivine and Pyroxene

The Fuchuan ophiolite is located in the northeasternmost segment of the Neoproterozoic Jiangnan orogen and consists mainly of harzburgites, with minor dunites, pyroxenite and gabbro veins and dykes. In order to investigate the genesis and tectonic setting of the Fuchuan ophiolite and chromitites, in situ analyses of unaltered chromites and silicates were carried out. Trace element analyses of unaltered chromites from the Fuchuan chromitites indicate the parental magma is of mid-ocean ridge basal...     

Mylonitized peridotites of Songshugou in the Qinling orogen,central China: A fragment of fossil oceanic lithosphere mantle

The Songshugou mylonitized peridotites within the Qinling Group metamorphic rocks in Central China are distributed in the northern part of the Shang-Dan Suture Zone (SDSZ) and contain abundant dunites and harzburgites. The dunites were intensely deformed and mylonitized converting the coarse-grained type to medium- and fine-grained types which contain prominent lenticular structure and relict olivine (Ol) porphyroclasts. Mineralogical and geochemical compositions suggest that the protoliths of the mylonitized peridotites were coarse-grained peridotites of lithospheric mantle origin. The harzburgites occur as enclaves within mylonitic peridotites in the form of lenses or veins. The orthopyroxenes in harzburgites were formed at the expense of Ol and have similar compositions to those of metasomatized harzburgites, characterized by low Al₂O₃, CaO and Cr₂O₃ contents. The harzburgites exhibit the gently U-type REE patterns with enriched incompatible elements (Rb, Ba, Sr, Zr and Hf), suggesting the metasomatic origin. The obvious ductile deformation of the large porphyroclastic orthopyroxene (Opx) suggests that the metasomatism occurred before the deformation. Ductile shearing deformation is indicated by the small fold structures and net-style ductile shearing zones within the Songshugou peridotite massif. The process is also result in the alignment of elongated Ol grains from initially coarse-granular via porphyroclastic to fine-granular texture. The relatively low Fo olivine, together with high Al₂O₃, and CaO contents and the abnormally low total PGE abundance in the fine-grained dunites suggest the ingress of melt/fluid during the mylonitization. The presences of significant amount of amphibole in the peridotites indicate the ingress of hydrous fluids. In general, the Songshugou peridotites have similar compositional characteristics with peridotites of Oman and Troodos ophiolites which are fragments of oceanic lithosphere mantle. One coarse-grained dunite has a T_RD age of 875 Ma. Additionally two stages Paleozoic T_RD ages are obtained from medium-grained and fine-grained dunites (491 Ma and 550 Ma; 446 Ma and 476 Ma). The broadly coeval nature of mylonitization with progressive metamorphism of surrounding amphibolites suggested that the Songshugou peridotites were generated before the early Paleozoic deformation. Our data, combined with the previous work on the surrounding HP/UHP metamorphic rocks, demonstrate that the Songshugou mylonitized peridotites represent fragments of the Neoproterozoic fossil oceanic lithospheric mantle that experienced extensive deformation during the Early Paleozoic subduction processes.     

Using platinum-group elements and Au geochemistry to constrain the genesis of podiform chromitites and associated peridotites from the Soghan mafic–ultramafic complex,Kerman, Southeastern Iran

The podiform chromite deposit of the Soghan mafic–ultramafic complex is one of the largest chromite deposits in south-east Iran (Esfandagheh area). The Soghan complex is composed mainly of dunite, harzburgite, lherzolite, pyroxenite, chromitite, wehrlite and gabbro. Olivine, orthopyroxene, and to a lesser extent clinopyroxene with highly refractory nature, are the primary silicates found in the harzburgites and dunites. The forsterite content of olivine is slightly higher in dunites (Fo₉₄) than those in harzburgites (Fo₉₂) and lherzolites (Fo₈₉). 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 very high Cr-numbers (80–83.6), Mg-numbers (62–69) and very low TiO₂ content (averaging 0.17 wt.%) for which may reflect the crystallization of chromite from a boninitic magma. The Fe^3 +-number is very low, down to < 0.04 wt.%, in the chromian spinel of chromitites and associated peridotites of the Soghan complex.PGE contents are variable and range from 80 to 153 pbb. Chromitites have strongly fractionated chondrite-normalized PGE patterns, which are characterized by enrichments in Os, Ir and Rh relative to Pt and Pd. Moreover, the Pd/Ir value which is an indicator of PGE fractionation ranges from < 0.08 to 0.24 in chromitite of the Soghan complex. These patterns and the low PGE abundances are typical of ophiolitic chromitites and indicating a high degree of partial melting (about 20–24%) of the mantle source. Moreover, the Pd_N/Ir_N ratios in dunites are unfractionated, averaging 1.2, whereas the harzburgites and lherzolites show slightly positive slopes PGE spidergrams, together with a small positive Ru and Pd anomaly, and their Pd_N/Ir_N ratio averages 1.98 and 2.15 respectively.The mineral chemistry data and PGE geochemistry, along with the calculated parental melts in equilibrium with chromian spinel of the Soghan chromitites indicate that the Soghan complex was generated from an arc-related magma with boninitic affinity above a supra-subduction zone setting.     

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.     

Petrology and geochemistry of high Cr# podiform chromitites of Bulqiza,Eastern Mirdita Ophiolite (EMO), Albania

The ultramafic massif of Bulqiza, which belongs to the eastern ophiolitic belt of Albania, is a major source of metallurgical chromitite ore. The massif consists of a thick (> 4 km) sequence, composed from the base upward of tectonized harzburgite with minor dunite, a transitional zone of dunite, and a magmatic sequence of wehrlite, pyroxenite, troctolite and gabbro. Only sparse, refractory chromitites occur within the basal clinopyroxene-bearing harzburgites, whereas the upper and middle parts of the peridotite sequence contain abundant metallurgical chromitites. The transition zone dunites contain a few thin layers of metallurgical chromitite and sparse bodies are also present in the cumulate section. The Bulqiza Ophiolite shows major changes in thickness, like the 41–50 wt.% MgO composition similar with forearc peridotite as a result of its complex evolution in a suprasubduction zone (SSZ) environment. The peridotites show abundant evidence of mantle melt extraction at various scales as the orthopyroxene composition change from core to rim, and mineral compositions suggest formation in a forearc, as Fo values of olivine are in 91.1–93.0 harzburgite and 91.5–91.9 in dunite and 94.6–95.9 in massive chromitite. The composition of the melts passing through the peridotites changed gradually from tholeiite to boninite due to melt–rock reaction, leading to more High Cr# chromitites in the upper part of the body. Most of the massive and disseminated chromitites have high Cr# numbers (70–80), although there are systematic changes in olivine and magnesiochromite compositions from harzburgites, to dunite envelopes to massive chromitites, reflecting melt–rock reaction. Compositional zoning of orthopyroxene porphyroblasts in the harzburgite, incongruent melting of orthopyroxene and the presence of small, interstitial grains of spinel, olivine and pyroxene likewise attest to modification by migrating melts. All of the available evidence suggests that the Bulqiza Ophiolite formed in a suprasubduction zone mantle wedge.     

Melt Events in A Nascent Mantle Wedge: Implication from the Luobusa Ophiolite,Tibet

The compositions of minerals and whole rocks of the Luobusa ophiolite in South Tibet, a fragment of Neo‐Tethyan forearc lithosphere, is used to investigate the magmatic evolution of nascent mantle wedges in newly‐initiated subduction zones. Clinopyroxenes in the Luobusa peridotites all have diopsidic compositions, and their Al₂O₃ contents vary from ～ 2% in the dunites and refractory harzburgites to 2‐4% in the cpx‐bearing harzburgites. The REE of clinopyroxenes in the harzburgites have left‐sloping patterns with contents comparable to those in abyssal peridotites that have experienced 5‐15% partial melting. Chromites in the Luobusa chromitites have the highest Cr#s (～ 80) and TiO2 contents (0.1‐0.2%), and those in the cpx‐bearing harzburgites have the lowest Cr#s (20‐60) and TiO2 contents (0‐0.1%), whereas those in refractory harzburgites and dunites have intermediate compositions. Cpx‐bearing and refractory harzburgites show spoon‐and U‐shaped REE patterns, respectively, and their HREE distribution patterns suggest at least 15%‐ 20% partial melting. The REE patterns of dunites and high‐Cr chromitites vary from spoon‐ to U‐shaped and require 15‐30% partial melting in their mantle sources to produce their parental melts. Our dataset reveals that the nascent Luobusa mantle wedge was first infiltrated by slab‐derived fluids and later refertilized by transitional lava‐like melts, resulting in cpx‐bearing harzburgites. Partial melting in the deeper cpx‐bearing mantle generated high‐Ca boninitic to arc picritic melts, which interacted with the peridotites in the uppermost mantle to generate high‐Cr chromitites, dunites and some refractory harzburgites. Lithological variation from cpx‐bearing to refractory harzburgites in forearc ophiolites is the result of multi‐stage melt events rather than increasing degrees of partial melting. Intermittent slab rollback during subduction initiation induces asthenospheric upwelling and high heat flux in nascent mantle wedges. Elevated geothermal gradients play a more important role than slab dehydration in triggering Mg‐rich magmatism in newly‐initiated subduction zones.     

Platinum-group elements distribution and spinel composition in podiform chromitites and associated rocks from the upper mantle section of the Neoproterozoic Bou Azzer ophiolite, Anti-Atlas, Morocco

The distribution of platinum-group elements (PGEs), together with spinel composition, of podiform chromitites and serpentinized peridotites were examined to elucidate the nature of the upper mantle of the Neoproterozoic Bou Azzer ophiolite, Anti-Atlas, Morocco. The mantle section is dominated by harzburgite with less abundant dunite. Chromitite pods are also found as small lenses not exceeding a few meters in size. Almost all primary silicates have been altered, and chromian spinel is the only primary mineral that survived alteration. Chromian spinel of chromitites is less affected by hydrothermal alteration than that of mantle peridotites. All chromitite samples of the Bou Azzer ophiolite display a steep negative slope of PGE spidergrams, being enriched in Os, Ir and Ru, and extremely depleted in Pt and Pd. Harzburgites and dunites usually have intermediate to low PGE contents showing more or less unfractionated PGE patterns with conspicuous positive anomalies of Ru and Rh. Two types of magnetite veins in serpentinized peridotite, type I (fibrous) and type II (octahedral), have relatively low PGE contents, displaying a generally positive slope from Os to Pd in the former type, and positive slope from Os to Rh then negative from Rh to Pd in the latter type. These magnetite patterns demonstrate their early and late hydrothermal origin, respectively. Chromian spinel composition of chromitites, dunites and harzburgites reflects their highly depleted nature with little variations; the Cr# is, on average, 0.71, 0.68 and 0.71, respectively. The TiO₂ content is extremely low in chromian spinels, <0.10, of all rock types. The strong PGE fractionation of podiform chromitites and the high-Cr, low-Ti character of spinel of all rock types imply that the chromitites of the Bou Azzer ophiolite were formed either from a high-degree partial melting of primitive mantle, or from melting of already depleted mantle peridotites. This kind of melting is most easily accomplished in the supra-subduction zone environment, indicating a genetic link with supra-subduction zone magma, such as high-Mg andesite or arc tholeiite. This is a general feature in the Neoproterozoic upper mantle.     

High-Al and high-Cr podiform chromitites from the western Yarlung-Zangbo suture zone,Tibet: Implications from mineralogy and geochemistry of chromian spinel,and platinum-group elements

On the basis of their mineral chemistry, podiform chromitites are divided into high-Al (Cr# = 20–60) (Cr# = 100 1 Cr/(Cr + Al)) and high-Cr (Cr# = 60–80) varieties. Typically, only one type occurs in a given peridotite massif, although some ophiolites contain several massifs that can have different chromitite compositions. We report here the occurrence of both high-Cr and high-Al chromitite in a single massif in China, the Dongbo mafic-ultramafic body in the western Yarlung-Zangbo suture zone of Tibet. This massif consists mainly of mantle peridotites, with lesser pyroxenite and gabbro. The mantle peridotites are mainly composed of harzburgites and minor lherzolites; a few dike-like bodies of dunite are also present. Seven small, lenticular bodies of chromitite ores have been found in the harzburgites, with ore textures ranging from massive through disseminated to sparsely disseminated; no nodular ore has been observed. Individual chromitite pods are 1–3 m long, 0.2–2 m wide and strike NW, parallel to the main trend of the peridotites. Chromitite pods 3, 4, and 5 consist of high-Al chromitite (Cr# = 12–47), whereas pods 1 and 2 are high-Cr varieties (Cr# = 73 to 77). In addition to chromian spinel, all of the pods contain minor olivine, amphibole and serpentine. Mineral structures show that the peridotites experienced plastic deformation and partial melting. The mineralogy and geochemistry of the Dongbo peridotites suggest that they formed originally at a mid-ocean ridge (MOR), and were later modified by suprasubduction zone (SSZ) melts/fluids. We interpret the high-Al chromitites as the products of early mid-ocean ridge basalt (MORB) or arc tholeiite magmas, whereas the high-Cr varieties are thought to have been generated by later SSZ melts.     

High-Cr and High-Al Podiform Chromitites,Western China: Relationship to Partial Melting and Melt/Rock Reaction in the Upper Mantle

Podiform chromitites include both high-Cr and high-Al varieties with distinctly different geochemical characteristics. A comparison of high-Cr and high-Al deposits in western China has demonstrated that both varieties are magmatic in origin and that chromite compositions reflect the degree of partial melting in the mantle source area. The chromitites of the Sartohay ophiolite of Xinjiang Province have chromites with low Cr numbers (<70) and are hosted in highly depleted harzburgites. In both deposits melt/wall rock interaction has produced highly depleted dunite envelopes around the chromitites. In Sartohay, high-Al magmas reacted with lherzolites to produce high-Al dunites and harzburgites, whereas in Luobusa the reaction between more refractory melts and depleted harzburgites yielded only highly depleted dunite envelopes. This study suggests that high-Al deposits can occur in weakly depleted mantle sequences (lherzolite ophiolite type or transitional type) that are locally depleted by melt/rock reaction in the immediate vicinity of the chromitite pods.     

Compositional variability of spinel-group minerals from the shergol serpentinized peridotites along indus suture zone,ladakh himalaya (India): constraints on tectonomagmatic history

The Shergol ophiolitic peridotites along ISZ, Ladakh Himalaya are serpentinized to various degrees and are harzburgite in composition. Electron microprobe analyses of spinels from Shergol Serpentinized Peridotites (SSPs) were carried out in order to evaluate their compositional variation with alteration. Chemical discontinuity was observed from core to rim in analyzed spinel grains with Cr-rich cores rimmed by Cr-poor compositions. From unaltered cores to rims it was observed that Cr³⁺# and Fe³⁺# increases while Mg²⁺# decreases due to Mg²⁺ − Fe²⁺ and Al³⁺ (Cr³⁺) − Fe³⁺ exchange with surrounding silicates during alteration. These peridotites contain Al-rich spinels forming subhedral to anhedral grains with lobate and corroded grain boundaries; altered to ferritchromite or magnetite along cracks and boundaries by later metamorphism episode. The unaltered Cr-spinel cores are identified as Al-rich and are characterized by lower values of Cr³⁺# (0.34–0.40), high Al³⁺# (0.58–0.68) and Mg²⁺# (0.52–0.70). Mineral chemistry of these Al-rich Cr-spinels suggest that host peridotites have an affinity to abyssal and alpine-type peridotites. High TiO₂ concentration of magmatic Cr-spinel cores are in agreement with MORB melt-residual peridotite interaction. Presence of unaltered magmatic Cr-spinel cores suggest that they do not have re-equilibrated completely with metamorphic spinel rims and surrounding silicates. Cr-spinel core compositions of SSPs suggest an ophiolitic origin derivation by low degrees of melting of a less-moderate depleted peridotite in a mid-ocean ridge tectonic setting. Based on textural and chemical observations the alteration conditions of studied spinel-group minerals match those of transitional greenschist-amphibolite facies metamorphism consistent with estimated metamorphic equilibration temperature of ∼ 500–600 ^°C.     

Chemical variations of abyssal peridotites in the central Oman ophiolite: Evidence of oceanic mantle heterogeneity

Basal peridotites above the metamorphic sole outcropped around Wadi Sarami in the central Oman ophiolite give us an excellent opportunity to understand the spatial extent of the mantle heterogeneity and to examine peridotites−slab interactions. We recognized two types of basal lherzolites (Types I and II) that change upward to harzburgites. Their pyroxene and spinel compositions display severely variations at small scales over < 0.5 km, and encompass the entire abyssal peridotite trend; clinopyroxenes (Cpxs) show wide ranges of Al₂O₃, Na₂O, Cr₂O₃ and TiO₂ contents. Primary spinels show a large variation of Cr# [= Cr/(Cr + Al)] from 0.04 to 0.53, indicating various degrees of partial melting. Trace-element compositions of peridotites and their pyroxenes also show a large chemical heterogeneity in the base of the Oman mantle section. This heterogeneity mainly resulted from variations of partial-melting degrees due to the change of a mantle thermal regime and a distance from the spreading ridge or the mantle diapir. It was overlapped with subsolidus modification during cooling and fluid metasomatism prior and/or during emplacement. The studied peridotites are enriched in Rb, Cs, Ba, Sr and LREE due to fluid influx during detachment and emplacement stages. Chondrite (CI)-normalized REE patterns for pyroxenes are convex upward with strong LREE depletion due to their residual origin, similar to abyssal peridotites from a normal ridge segment. The Cpxs are enriched in fluid mobile elements (e.g., B, Li, Cs, Pb, Rb) and depleted in HFSE (Ta, Nb, Th, Zr) + LREE, suggesting no effect of melt refertilization. Their HREE contents, combined with spinel compositions, suggest two melting series with 1–5% melting for type II lherzolites, 3– < 10% melting for type I lherzolites and ~ 15% for harzburgites. Hornblendes are enriched in fluid-mobile elements relative to HFSE + U inherited from their precursor Cpx. The clinopyroxenite lens crosscuts the basal lherzolites, forming small-scale (< 5 cm) mineralogical and chemical heterogeneities. It was possibly formed from fractional crystallization of interstitial incremental melt that formed during decompression melting of a normal MORB mantle source. The studied peridotites possibly represent a chemical heterogeneity common to the mantle at an oceanic spreading center.     

Peridotite-chromitite complexes in the Eastern Desert of Egypt: Insight into Neoproterozoic sub-arc mantle processes

The Neoproterozoic peridotite-chromitite complexes in the Central Eastern Desert of Egypt, being a part of the Arabian-Nubian Shield, are outcropped along the E–W trend from Wadi Sayfayn, Wadi Bardah, and Jabal Al-Faliq to Wadi Al-Barramiyah, from east to west. Their peridotites are completely serpentinized, and the abundance of bastite after orthopyroxene suggests harzburgite protoliths with subordinate dunites, confirmed by low contents of Al₂O₃, CaO and clinopyroxene (< 3 vol%) in bulk peridotites. The primary olivine is Fo89.3–Fo92.6, and the residual clinopyroxene (Cpx) in serpentinites contains, on average, 1.1 wt% Al₂O₃, 0.7 wt% Cr₂O₃, and 0.2 wt% Na₂O, similar in chemistry to that in Izu-Bonin-Marian forearc peridotites. The wide range of spinel Cr-number [Cr/(Cr + Al)], 0.41–0.80, with low TiO₂ (0.03 wt%), MnO (0. 3 wt%) and Y_Fe [(Fe^3 +/(Cr + Al + Fe^3 +) = 0.03 on average)] for the investigated harzburgites-dunites is similar to spinel compositions for arc-related peridotites. The partial melting degrees of Bardah and Sayfayn harzburgites range mainly from 20 to 25% and 25 to 30% melting, respectively; this is confirmed by whole-rock chemistry and Cpx HREE modelling (~ 20% melting). The Barramiyah peridotite protoliths are refractory residues after a wide range of partial melting, 25–40%, where more hydrous fluids are available from the subducting slab. The Neoproterozoic mantle heterogeneity is possibly ascribed mainly to the wide variations of partial melting degrees in small-scale areas, slab-derived inputs and primordial mantle compositions. The Sayfayn chromitites were possibly crystallized from island-arc basaltic melts, followed by crystallization of Barramiyah chromitites from boninitic melt in the late stage of subduction. The residual Cpx with a spoon-shape REE pattern is rich in both LREE and fluid-mobile elements (e.g., Pb, B, Li, Ba, Sr), but poor in HFSE (e.g., Ta, Nb, Zr, Th), similar to Cpx in supra-subduction zone (SSZ) settings, where slab-fluid metasomatism is a prevalent agent. The studied chromitites and their host peridotites represent a fragment of sub-arc mantle, and originated in an arc-related setting. The systematic increase in the volume of chromitite pods with the increasing of their host-peridotite thickness from Northern to Southern Eastern Desert suggests that the thickness of wall rocks is one factor controlling the chromitite size. The factors controlling the size of Neoproterozoic chromitite pods are the thickness, beside the composition, of the host refractory peridotites, compositions and volumes of the supplied magmas, the amount of slab-derived fluids, and possibly the partial melting degree of the host peridotites.     

Regularities and mechanism of formation of the mantle lithosphere structure beneath the Siberian Craton in comparison with other cratons

Regularities of the mantle structure beneath the Siberian Craton were determined using the monomineral thermobarometry and common Opx-Gar methods. Samples were taken from 80 pipes from the Siberian Craton and in comparison 70 pipes from worldwide kimberlites. The largest pipes contain several dunite layers in the lower part of lithospheric mantle which are responsible for the diamond grade. The lithospheric mantle consists of two major parts divided at a depth of 4.0 GPa by a pyroxenite layer. Major intervals determined for the mantle beneath Udachnaya and Mir are: 1) 8.0–6.5 GPa harzburgites, eclogites and dunitic veins; 2) 6.5–5.5 GPa sheared peridotites, low-Cr pyroxenites, dunites; 3)in 5.5–4.0 GPa interval there are 4–6 layers of harzburgitic paleoslabs; 4) 4.0–3.5 GPa the pyroxenites lens; 5) upper layered Sp-Gar peridotite sequence including a trap of basaltic and other silicate melt cumulates at 3.0–2.0 GPa. The lithospheric mantle beneath seven different tectonic terrains in Siberia is characterized by TRE geochemistry and major elements of peridotitic clinopyroxenes. The mantle in Magan terrain contains more fertile peridotites in the South (Mir pipe) than in North (Alakit) which were metasomatized by subduction-related melts producing Phl and Cpx about 500–800 Ma ago. Daldyn terrain is essentially harzburgitic in the west part (abyssal peridotite) but in the east in Upper Muna (East Daldyn terrain) the mantle is more differentiated and in general more oxidized. The Markha terrain (Nakyn) contains depleted but partly refertilized harzburgites, subducted pelitic material and abundant eclogites. Circum-Anabar mantle is ultradepleted in the lower part but in the upper regions it has been fertilized by fluid-rich melts very enriched in incompatible elements. The P-Fe# diagrams (and other components) reveal different structure of mantle columns in each terrain. They are subvertical for the mantle sampled by Devonian pipes. Beneath Mesozoic pipes the mantle has been affected by melt percolation caused by the Siberian Superplume which created continuous Fe-enrichment in the upper part. The models of continent growth and evolution are briefly discussed. In general the geothermal regime and mantle heating is negatively correlated with the thickness of lithosphere. The sheared peridotites under Udachnaya and other kimberlite pipe are likely to have formed after the intrusion of protokimberlite volatile rich (hydrous) melts and hydraulic fracturing. This mechanism is responsible for the origin of asthenospheric lenses.Progressive melting especially in the pervasive zones may be responsible for the creation of 3-4 upper asthenospheric lens near mostly before 4.0 GPa which may be accompanied by mantle diapirism. Such a lens is the trap for the kimberlites in Siberia in Mesozoic time and in rifted intracontinental areas and margins.     

Genesis and tectonic setting of ophiolitic chromitites from the Dehsheikh ultramafic complex (Kerman,southeastern Iran): Inferences from platinum-group elements and chromite compositions

Chromitite bodies of various sizes associated with dunite envelopes have been found in the Dehsheikh ultramafic massif, in the southeastern part of the outer Zagros ophiolite belt. The chromitites occur as layered and lenticular bodies, and show both magmatic and deformational textures, including massive, disseminated, banded and nodular types. The Dehsheikh chromitites display a variation in Cr# [100 × Cr / (Cr + Al)] from 69 to 78, which is typical of high-Cr chromitites. The Al₂O₃ and TiO₂ contents of chromites range from 10.3 wt.% to 16.9 wt.% and 0.12 wt.% to 0.35 wt.%, respectively. The Al₂O₃, TiO₂, and FeO/MgO values calculated for parental melts of Dehsheikh chromitites are within the range of boninitic melts. Chondrite-normalized distribution patterns of platinum-group elements show relative enrichments in Ru, Ir, and Os, and depletions in Rh, Pd, and Pt that are typical of chromitites associated with ophiolites formed by high degrees of mantle partial melting. The presence of Na-rich amphibole inclusions in chromite grains, together with the mineralogical and chemical composition of the chromitites and estimates of their parental melt compositions are used to help establish the tectono-magmatic setting. It is shown that the Dehsheikh massif is an ophiolite formed in a suprasubduction zone setting. We suggest that the composition of the rocks in this section was influenced by hydrous partial melts which might be formed in the subduction zone. Variable melt/rock interaction produced melt channel networks in the dunite which allowed the parental melt of the chromitite to percolate through them. Similar characteristics have been observed in other ophiolite complexes from the outer Zagros Iranian ophiolitic belt; these are believed to be the product of magmatism in a fore-arc environment.     

西昆仑库地蛇绿岩富Al型豆荚状铬铁矿床成因

西昆仑库地蛇绿岩发育小规模的铬铁矿床,矿体呈豆荚状和层状、似层状,均与纯橄岩紧密伴生。这些纯橄岩主要由橄榄石和副矿物尖晶石组成,与方辉橄榄岩相比,橄榄岩中的橄榄石粒径粗(平均2.5mm),Mg#(88~90)低,这与它们全岩低Mg#(90)值,富Al_2O_3、TiO_2、Cr_2O_3、Fe_2O_3相吻合,与熔融残余成因的纯橄岩明显不同,反映了其很可能是由熔体与方辉橄榄岩反应而成。矿体主要由块状、浸染状及脉状铬铁矿石组成;铬铁矿石中的尖晶石具有低而相对稳定的Cr#(43~56),低于富铬型铬铁矿矿床中的铬铁矿(Cr#60)。块状矿石与纯橄岩呈突变接触,矿石中的尖晶石呈浑圆状,包裹有较多橄榄石、辉石等硅酸盐矿物及角闪石等含水硅酸盐矿物;浸染状铬铁矿石中的尖晶石与橄榄石颗粒构成交织结构,或呈云朵状,沿橄榄石颗粒边界相互连接,矿石的结构构造显示了熔/岩反应成因特征。通过计算分析,我们认为该区富铝型铬铁矿石是由拉斑玄武质熔体与地幔橄榄岩反应而成,由于熔体中含有较高的H_2O,参与反应的熔体可能源于弧后扩张脊环境。     

Compositional signatures of SSZ-type peridotites from the northern Vourinos ultra-depleted upper mantle suite,NW Greece

The northern Vourinos massif, located in the Dinarides-Hellenides mountain belt in the Balkan Peninsula, forms a section of the so-called Neotethyan ophiolitic belt in the Alpine-Himalayan orogenic system. It is comprised mainly of a well-preserved mantle sequence, dominated by voluminous massive harzburgite with variable clinopyroxene and olivine modal abundances, accompanied by subordinate coarse- and fine-grained dunite. The harzburgite rock varieties are characterized by high Cr# [Cr/(Cr + Al)] values in Cr-spinel (0.47–0.74), elevated Mg# [Mg/(Mg + Fe²⁺)] in olivine (0.90–0.93), low Al₂O₃ content in clinopyroxene (≤1.82 wt.%) and low average bulk-rock concentrations of CaO (0.52 wt.%) and Al₂O₃ (0.40 wt.%), which are indicative of their refractory nature. In addition, dunite-type rocks display even more depleted compositions, containing Cr-spinel and olivine with higher Cr# (0.76–0.84) and Mg# (0.91–0.94), respectively. They also display extremely low average abundances of CaO (0.13 wt.%) and Al₂O₃ (0.15 wt.%). The vast majority of the studied peridotites are also strongly depleted in REE. Simple batch and fractional melting models are not sufficient to explain their ultra-depleted composition. Whole-rock trace element abundances of the northern Vourinos mantle rocks can be modeled by up to 22–31% closed-system non-modal dynamic melting of an assumed primitive mantle (PM) source having spinel lherzolite composition. The highly depleted compositional signatures of the investigated peridotites indicate that they have experienced hydrous melting in the fore-arc mantle region above a SSZ. This intense melting event was responsible for the release of arc-related melts from the mantle. These melts reacted with the studied peridotites causing incongruent melting of pyroxenes followed by considerable olivine and Cr-spinel addition in terms of cryptic metasomatism. This later metasomatic episode has obscured any geochemical fingerprints indicative of an early mantle melting event in a MOR setting. The lack of any MOR-type peridotites in the northern Vourinos depleted mantle suite is quite uncommon for SSZ-type Neotethyan ophiolites.     

Refractory- and metallurgical-type chromite ores,Zambales ophiolite,Luzon, Philippines

The Zambales ophiolite is the major source of chromite ore in the Philippines. The chromitites are concordant cumulates and are associated with distinct chromitite-bearing sequences within the mantle peridotites. Refractory and metallurgical chromite deposits are spatially separated and related to different lithologic associations, which crystallized from different parental magmas. — Refractory chromite ores (30–44 wt% Cr₂O₃; 20–30 wt% Al₂O₃) are linked with the peridotite-troctolite-olivine gabbro lineage. Two main types were found: (1) Al-rich refractory ores associated with harzburgites and feldspathic periodotites and (2) more Cr-rich varieties associated with lherzolites. — Metallurgical chromite ores (45–53 wt% Cr₂O₃; 12–18 wt% Al₂O₃) are linked with the peridotite-pyroxenite-norite lineage. Two main types were also found: (1) Cr-rich metallurgical ores associated with orthopyroxenites and (2) more Al-rich varieties related to clinopyroxenites. — The chemical composition of chromite within the deposits varies depending on the chromite/silicate ratios of the ore types and grades continuously into accessory chrome spinels in the wall-rock peridotites. — The geochemistry of accessory chrome spinels in various peridotites and mafic cumulates depends on the mineralogical composition and the stratigraphic position of their host rocks.New address: BEB Erdgas und Erdöl GmbH, Riethorst 12, D-3000 Hannover 51The terms chrome-spinels and chromite are used as follows: 1. Chrome-spinel is only used for those occuring as accessory minerals in various ultramafic and mafic rocks (= accessory chrome-spinels). Their chemical composition has been determined only by microprobe analysis. — 2. Chromite is used for ore and ore deposits (=chromitites); the chemical composition has been determined by wet chemistry (AAS) or by microprobe analysis     

Textures and geochemistry of the Saramta peridotites (Siberian craton): Melting and refertilization during early evolution of the continental lithospheric mantle

The Saramta peridotite massif is located within the Sharyzhalgai complex, SW margin of the Siberian craton. The Saramta massif was formed in the Archean and then juxtaposed with granulites of crystalline basement of the Siberian craton. The Saramta harzburgites are highly refractory in terms of lack of residual clinopyroxene, olivine Mg-number (up to 0.937), and spinel Cr-number (∼0.5), suggesting high degree of partial melting. Detailed study of their microstructures shows that they have extensively reacted with a SiO₂-rich melt, leading to the crystallization of orthopyroxene, clinopyroxene, amphibole and spinel at the expense of olivine. The major element compositions of the least reacted harzburgites are similar to the residues of refractory peridotites produced by the fractional melting (initial melting pressures >3 GPa and melt fractions ∼40%). Moreover, non-residual clinopyroxenes are highly depleted in Yb, Zr and Ti, but highly enriched in LREE. A two-stage history is proposed for the Saramta peridotite: (1) primitive mantle underwent depletion in the garnet stability field followed by melting in the spinel stability field; (2) refractory harzburgites underwent refertilization by SiO₂-rich melt in supra-subduction zone. Rare Saramta lherzolites probably formed from more refractory harzburgites as a result of such a melt–rock reaction. The Saramta peridotites are similar to low-T coarse-grained peridotites of subcratonic mantle. Processes of their formation, as reflected by textures and composition of minerals of the Saramta peridotites, are characteristic of the early stages of subcratonic mantle formation.