Clinopyroxene-bearing Harzburgite Research Articles

Evidence for multiple stages of serpentinization from the mantle through the crust in the Redwood City Serpentinite mélange along the San Andreas Fault in California

The active plate boundary of the California margin includes the active San Andreas Fault System and the active mountain building of the California Coast Ranges where ultramafic rocks are also commonly found. There are numerous hypotheses for the origin and the timing and emplacement mechanisms of such rocks that ultimately came from Earth's mantle, but typically the mineralogical, structural, and geochemical features of fresh serpentinite, and their reaction and deformation histories were unknown due to heavy weathering of most of the exposures. We investigated a block-and-matrix serpentinite mélange in Redwood City, California on the San Francisco Peninsula that is proximal to an active strand of the San Andreas Fault. The mélange is composed of 6 m- to cm-sized serpentinite blocks surrounded by fine-grained serpentine matrix. The blocks are polygonal in shape and show a remarkable internal mineralogical structure that has recorded at least three stages of serpentinization of a clinopyroxene-bearing harzburgite protolith. The largest, freshest polygonal serpentinite blocks contain a core comprised of surviving peridotite minerals plus lizardite (lz) ± antigorite (atg) + brucite (brc) + magnetite (mag) domains. The cores are surrounded by a ~10 cm thick peripheral rim of lizardite + magnetite. The reaction boundaries between rims and cores are parallel to the external polygonal surfaces of the blocks. These blocks are also sheathed by thin skins of sheared lizardite + chrysotile (ctl). The higher-temperature lz/atg + brc + mag serpentinization represented by the core mineralogy (Stage A) probably occurred on a large scale in the mantle and the hydration metamorphism producing the lz + mag mineralogy in the rims (Stage B) was likely triggered by the growth of a system of approximately planar fractures under crustal conditions that was enabled by the ingress of water at high pressure and that defined the polygonal block shapes as well as the Stage-A to Stage-B reaction boundaries. Stage-A serpentinization reactions are internally balanced except for addition of H2O, Cs and Rb as indicated by core mineral composition. The Stage-B serpentinization was open system with CaO, Cs and Rb migrating outside the blocks with addition of H2O, U, Pb ± LREE. The sheared lizardite skin probably formed during the ascent of the RCS through the crust. Several classes of mineral-filled veins were found in both cores and rims and indicate fluid pressures approaching lithostatic pressures that enabled these opening- mode fractures to occur. We posit that such high-pressure hydrothermal conditions were important in promoting the serpentinization reactions as well as enabling a weak rheology consistent with a cold-intrusion/diapiric emplacement mechanism. These factors give insights into where these mantle rocks came from, the sources of the water causing serpentinization, and the deformation mechanisms by which they come to Earth's surface. Similarities of the structures of serpentinite blocks reported in this paper with those in blocks from other serpentinite bodies in the San Francisco Bay Area (Lewis and Kirby, 2015 AGU Abstract T13H-05) suggests that the processes inferred from our investigation are regional in spatial extent.

Pyroxenite veins in spinel peridotites of the Unnavayam sheet, Kuyul ophiolite terrane (Koryak Upland): Origin and setting of formation

Pyroxenite veins in mantle peridotites of the Unnavayam sheet of the Kuyul ophiolite terrane (Koryak—Kamchatka folded area) are composed of clinopyroxenite and websterite crystallized from a boninite-like melt. The host clinopyroxene-bearing spinel harzburgites are moderately depleted residues, whose mineral compositions and conditions of formation correspond to those of peridotites from mid-oceanic and back-arc spreading centers. Mantle peridotites of the Unnavayam sheet may have formed a part of the mantle wedge above a subduction zone and have been intruded by boninitic melts at a certain stage of their evolution.

Tectonic discrimination of chromian spinels, olivines and pyroxenes in the Northeastern Jiangxi Province ophiolite, South China

The peridotites of the Northeastern Jiangxi Province Ophiolite (NJO), including dunite, harzburgite and clinopyroxene-bearing harzburgite, are strongly altered under serpentinization, except for minor aggregations of partially-altered olivines, chromian spinels and pyroxenes. The forsterite content of olivines in dunites (Fo 93.6) is slightly higher than in harzburgites (Fo 91.4). Chromian spinels in harzburgites and dunites are very refractory, with restricted chemical compositions of high-Cr varieties. The unaltered cores of chromian spinels have low Al2O3 and TiO2 content, and display a large range of Mg# (100× [Mg/(Mg + Fe)], 41–64) and Cr# (100× [Cr/(Cr + Al)], 53–83) values, suggesting that the peridotites originated from a highly-depleted mantle. The spinels plotted in “olivine-spinel mantle array” (OSMA) diagram and Cr# versus. Mg# diagram both indicate the peridotite of NJO experienced a >25 % partial melting. The positive correlation between the Cr# and the TiO2 content probably resulted from the reaction between boninitic melt and mantle peridotite, as a consequence of melt-mantle interaction within the arc setting. The oxygen fugacity (ƒO2), calculated using chromian spinel-olivine pairs, indicates that the peridotites in the NJO were formed under relatively low oxidizing conditions quite different from those commonly found in supra-subductions zones (SSZ). This might be explained by the reaction between fore-arc magmas and residual mantle in a back-arc oceanic basin during a rapid subduction process. The Neoproterozoic subduction between the Yangtze and Cathaysia blocks was therefore probably rapid, and the addition of water and other volatiles to the mantle wedge beneath the island arc would have enhanced melting, leading to the production of highly depleted boninitic melts.

Geochemical record of subduction initiation in the sub-arc mantle: Insights from the Loma Caribe peridotite (Dominican Republic)

The Loma Caribe peridotite body is mainly composed of serpentinized spinel harzburgites and lherzolites and minor orthopyroxene-bearing dunites and dunites. Modal proportions, mineral and whole-rock major and trace element compositions generally coincide with those of abyssal mantle rocks from mid-ocean ridges for the lherzolites, and refractory supra-subduction peridotites for the harzburgites and dunites. The clinopyroxene-bearing harzburgites have intermediate compositions that overlap with those of residual mantle from both these settings. Major elements in the peridotites were mostly undisturbed by serpentinization and/or seafloor weathering whereas light rare earth elements (LREE) and large ion lithophile elements (LILE) were enriched by syn- and/or post-melting interaction with fluids/melts. Major element variations indicate that protoliths of the Loma Caribe peridotites mostly melted at 1–2GPa and 1250°C–1500°C, as normal mid-ocean ridge and supra-subduction zone mantle. The MREE/HREE fractionations in both whole rocks and clinopyroxene can be explained by initial low (5%–6%) fractional melting of a garnet lherzolite source followed by variable (5%–20%) melting in the spinel stability field. The lherzolites and clinopyroxene-bearing harzburgites are residues of increasing melting triggered by increasing addition of slab fluids to a spinel peridotite source, while melting of the harzburgite protoliths was likely the result of focused flux of slab hydrous melts. The dunites and orthopyroxene-bearing dunites are products of pyroxene dissolution in residual peridotites caused by reaction with two different subduction-related melts, probably the parental magmas of Early Cretaceous low-Ti island arc tholeiites (IAT) and boninites from Central Hispaniola, respectively. We conclude that the geochemical heterogeneity of the Loma Caribe peridotites records shifting conditions of melting during the development of subduction beneath the incipient Greater Antilles paleo-island arc in the Early Cretaceous.

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 (>4km) 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 41–50wt.% 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.

Peridotites from the Kamchatsky Mys: evidence of oceanic mantle melting near a hotspot

A suite of mantle peridotites sampled in the Kamchatsky Mys includes spinel lherzolite, clinopyroxene-bearing harzburgite, and harzburgite. Mineral chemistry of olivine, chromian spinel, and clinopyroxene show strongly correlated element patterns typical of peridotite formed by 8% to more than 22% partial melting. Clinopyroxene in the Kamchatka peridotites is compositionally different from that of both abyssal and suprasubduction varieties: Clinopyroxene in lherzolite is depleted in LREE relative to abyssal peridotite and that in harzburgite has very low LREE and Sr unlike the subduction-related counterpart. These composition features indicate that the rocks ultra-depleted in basaltic components originated in the vicinity of a hotspot, possibly, proto-Hawaiian plume, which provided high temperature and melting degree of the MORB source mantle at mid-ocean ridge.

Jurassic ophiolite formation and emplacement as backstop to a subduction-accretion complex in northeast Turkey, the Refahiye ophiolite, and relation to the Balkan ophiolites

The eastern Mediterranean region within the Tethyan realm shows a high concentration of ophiolites with contrasting times of formation and emplacement along the belt: In the Balkans, the ophiolites formed during the early to medial Jurassic, and were obducted during the late Jurassic, whereas in Turkey and farther east, structurally intact Jurassic ophiolites are rare and Jurassic ophiolite obduction is unknown. Here we report a structurally intact, large ophiolite body of early Jurassic age from NE Turkey, the Refahiye ophiolite, located close to the suture zone between the Eastern Pontides and the Menderes-Taurus block. The Refahiye ophiolite forms an outcrop belt, 175 km long and 20 km wide, and is tectonically bound by the late Cretaceous ophiolitic melange to the south, and by the North Anatolian Transform Fault against the Triassic low-grade metamorphic rocks to the north. Early to medial Jurassic very low- to low-grade metamorphic rocks, interpreted as intraoceanic subduction-accretion complexes, occur either beneath the ophiolite or as thrust slices within it. The ophiolite body within the studied section is made up of mantle peridotite (clinopyroxene-bearing harzburgite and minor dunite) crosscut by up to 20 cm thick veins of clinopyroxenite and later dikes/pods/stocks of gabbro ranging in size from 2 m to several hundreds of meters. The gabbro is represented by two distinct types: (i) cumulate gabbro, and (ii) non-cumulate gabbro with locally well-developed igneous foliation. Within the non-cumulate gabbro or enclosing peridotite, there are up to 5 m and 50 cm-thick veins of trondhjemite and pegmatitic gabbro, respectively. LA-ICP-MS dating on zircons from two trondhjemite samples yielded weighted mean ages of ∼184 ± 4 Ma and 178 ± 4 Ma (2σ), respectively, suggesting formation during early Jurassic time. Formation in a suprasubduction-zone forearc setting is inferred from (i) wide-ranging pyroxene and spinel compositions in the peridotites as documented in most suprasubduction-zone ophiolites, (ii) arc tholeiitic signature of the non-cumulate gabbros, and (iii) association of the ophiolite with the coeval subduction-accretion complexes. Emplacement of a trapped forearc ophiolite above its own subduction-accretion complex as a backstop is proposed based on a series of field relationships such as (i) intimate association of the unsubducted suprasubduction-zone ophiolite with coeval accretionary complexes, (ii) absence of unambiguous relationship to the southern Atlantic-type continental margin, and (iii) absence of any stratigraphic indications for the ophiolite obduction in the southern Atlantic-type continental margin during Jurassic time. This is a clear difference from the Jurassic ophiolites in the Balkans that were obducted over the Atlantic-type continental margin. This difference in mode of emplacement is most probably related to the greater distance of the intra-oceanic subduction zone to the Atlantic-type continental margin than it was in the Balkans, which is commensurate with the greater width of the Tethys in the east during Jurassic time.

Zircon U–Pb age and geochemical constraints on the origin of the Birjand ophiolite, Sistan suture zone, eastern Iran

The Birjand ophiolite is one of the ophiolitic complexes in the northern Sistan suture zone, eastern Iran, which marks the closure of an enigmatic branch of the Neotethys Ocean (called the Sistan Ocean) associated with the collision between the Lut and Afghan continental blocks. Ophiolitic rocks in the suture zone occur as collisional block-to-block geological terranes, or as blocks within tectonic mélange. We present zircon U–Pb age, and whole-rock geochemical and Sr–Nd isotopic data of the Birjand ophiolite to understand its formation age and magma genesis in this poorly constrained part of the Alpine–Himalayan orogenic belt. Varieties of the ophiolitic rocks include clinopyroxene-bearing harzburgites, harzburgites, dunites, massive and pillow basalts, dolerites, gabbros and leucogabbros. Using laser ablation inductively coupled plasma-mass spectrometry, zircon separates from two leucogabbros yield U–Pb ages of 113±1 and 107±1Ma, which are interpreted to represent the formation ages of the ophiolite. The clinopyroxene-bearing harzburgites can be explained as melting residues after low to moderate extraction of MORB-type melts, whereas the harzburgites require more than one melting event to explain their trace element depletion. Most mafic rocks of the ophiolite display MORB-like trace element patterns and high εNd(t) (+3.4 to +8.4), features consistent with derivation from MORB-source mantle. Magma genesis can be explained by low to moderate degrees of melting of moderately depleted spinel peridotite and variable interaction with enriched mantle melts. The Birjand ophiolite represents a MORB-type ophiolite and part of the Middle Cretaceous oceanic lithosphere of the Sistan Ocean that closed during the destruction of a narrow arm of the Neotethys Ocean.

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.

ANDRADITE-UVAROVITE SOLID SOLUTION FROM HYDROTHERMALLY ALTERED PODIFORM CHROMITITE, RUTLAND OPHIOLITE, ANDAMAN, INDIA

Compositions of chromium-bearing garnet with little grossular content belong to the uvarovite–andradite solid-solution series. They are extremely rare in nature. Here, we report the occurrence of chromian andradite with up to 50 mol.% uvarovite component from hydrothermally altered podiform chromitite of the Rutland ophiolite, Andaman Island, India. The mantle tectonites of this ophiolite are characterized by a broad spectrum of peridotitic rocks ranging from depleted harzburgite to clinopyroxene-bearing harzburgite that hosts impersistently developed podiform chromite. Chromian garnet is concentrated at the margins of serpentine-rich veins that dissect the chromitite pods. The mineral assemblages are suggestive of low-temperature hydrothermal alteration of the host peridotite. The formation of chlorite and tremolite is simultaneous, and linked to the process of magnesiochromite oxidation. Garnet formed by reaction between tremolite and chromian spinel, which, during its growth, engulfed the tremolite needles.

Mineral chemical composition and geodynamic significance of peridotites from Nain ophiolite, central Iran

The peridotites from north of the town of Nain in central Iran consist of clinopyroxene-bearing harzburgite and lherzolite with small lenses of dunite and chromitite pods. The lherzolite contains aluminous spinel with a Cr number (Cr# = Cr/[Cr + Al]) of 0.17. The Cr number of spinels in harzburgite and chromitite is 0.38–0.42 and 0.62, respectively. This shows that the lherzolite and harzburgite resulted from 16 ± 2.3 kbar for lherzolites and estimated fo2 is 10−1±0.5 for peridotites. Discriminant geochemical diagrams based on mineral chemistry of harzburgites indicate a supra-subduction zone (SSZ) to mid-oceanic ridge (MOR) setting for these rocks. On the basis of their Cr # , the harzburgite and lherzolite spinels are analogous to those from abyssal peridotites and oceanic ophiolites, whereas the chromites in the chromitite (on the basis of Cr# and boninitic nature of parental melts) resemble those from SSZ ophiolitic sequences. Therefore, the Nain ophiolite complex most likely originated in an oceanic crust related to supra-subduction zone, i.e. back arc basin. Field observations and mineral chemistry of the Nain peridotites, indicating the suture between the central Iran micro-continent (CIM) block and the Sanandaj–Sirjan zone, show that these peridotites mark the site of the Nain–Baft seaway, which opened with a slow rate of ocean-floor spreading behind the Mesozoic arc of the Sanandaj–Sirjan zone as a result of change of Neo Tethyan subduction regime during middle Cretaceous.

Peridotites from the Khoy Ophiolitic Complex, NW Iran: Evidence of mantle dynamics in a supra-subduction-zone context

The Khoy Ophiolitic Complex as a part of the Tethyan ophiolites is exposed in the northwestern part of the Iranian-Azerbaijan province, extending to the Anatolian ophiolites in southeastern Turkey. Petrography, geochemistry and microstructural studies of the residual mantle sequence in the Khoy Ophiolitic Complex provide important information about the degree of partial melting and deformation in the oceanic mantle lithosphere. Ultramafic tectonites dominantly composed of lherzolite and clinopyroxene-bearing harzburgite (TiO 2 = 0.012–0.024 wt.%; Al 2O 3 = 1.36–1.81 wt.%). Chondrite-normalized rare-earth-element patterns are characteristically U-shaped. These peridotites can be divided into two types: (1) type 1 peridotites with Al-rich spinels (Cr number of 0.16–0.26, and Mg number of 0.64–0.76), resembling the fertile abyssal peridotites, supposed to have originated as the residue from <15% partial melting and mid-ocean ridge (MORB) magma extraction; (2) type 2 peridotites, representing characteristics of the depleted abyssal or supra-subduction-zone peridotites, with Cr-rich spinels (Cr number of 0.31–0.60 and Mg number of 0.51–0.72). This type of peridotite has undergone >20% partial melting, followed by segregation of basaltic magmas. Microstructural fabrics of olivine grains in peridotites highlight a sequence of dislocation creep on the (0 1 0) [1 0 0] slip system, plus subsidiary slip along the (0 0 1) [1 0 0] slip system. These systems, as well as coarse and fine-grained porphyroclastic textures, indicate deformation at high temperatures of ∼1000–1250 °C. The observed subsidiary (0 0 1) [1 0 0] slip system is considered to have been triggered by elevated H 2O activity, and that deformation phases took place in a wet subduction-related environment. The geochemical and microstructural data suggest that the mantle sequence of the Khoy Ophiolitic Complex is consistent with a supra-subduction-zone environment in relation to a slow-spreading back-arc basin.

Composition and origin of peculiar pyropes from lherzolites: evidence for the evolution of the lithospheric mantle of the Siberian Platform

More than 1000 pyropes from the Muza (J3) and Ivushka (D–C) (northeastern Siberian Platform, and Khorkich (Mz) (southwestern part of the platform) kimberlite pipes, alluvial deposits of the Muna-Markha area, and granular peridotites of the Udachnaya pipe have been analyzed for major and some minor elements. As a result, a group of pyropes was distinguished whose composition is not typical of the lherzolite paragenesis (LAC pyropes). They are predominant in the Muza pipe and are widespread over the world. This group is described as a separate paragenetic type. In all known cases, LAC pyropes belong to granular clinopyroxene-bearing harzburgites, and in situ conditions for this suite are typically below 50 kbar and 1000 °C. Our own and literature data suggest that LAC pyropes may appear when the magmas corresponding to the high-degree melting of the primary magma affect the depleted peridotites of the lithosphere mantle. The character of paleogeotherm and distribution of LAC pyropes in kimberlites and secondary collectors of varying age that occur on the Siberian Platform indicate that the lithosphere mantle was considerably thinner in the northeast and the rocks characterized by LAC pyropes played an increasingly important role in this region in the period from Paleozoic to Mesozoic and that these rocks were abundant in the lithosphere mantle of the platform’s interior. These facts as well as a considerable change in the rock composition in the lithospheric mantle and in the southwestern part of the platform in the same range of time suggest that the effect of the Permian-Triassic Siberian plume on the lithospheric mantle of the platform considerably changed its composition and structure in its separate parts.

COMPOSITIONAL VARIATIONS AS A RESULT OF PARTIAL MELTING AND MELT-PERIDOTITE INTERACTION IN AN UPPER MANTLE SECTION FROM THE ORTACA AREA, SOUTHWESTERN TURKEY

The bulk composition (major and trace elements) and the composition of minerals in peridotites of the Ortaca area, south-western Turkey, have been investigated to understand the processes controlling lithological and compositional variations in the dismembered mantle section, containing clinopyroxene-bearing harzburgite, harzburgite and dunite. The Ortaca peridotites vary from almost fertile clinopyroxene-bearing harzburgite (1.92-2.42 wt\% Al2O3, 2.00-2.64\% CaO) with a lower Cr# of the spinel phase (<24) to depleted harzburgite and dunite (0.17-1.60\% Al2O3, 0.32-1.71\% CaO) with a higher Cr# of the spinel phase (31-76). Systematic variations of major and trace elements of whole rocks and mineral phases are related to varying degrees of partial melting (up to 30\%). The depleted harzburgite and dunite display LREE-enriched primitive-mantle-normalized patterns, suggesting mantle metasomatism, probably by fluid-bearing melt derived from the subducted slab, due to the melt-rock interaction process, whereas clinopyroxene-bearing harzburgite show HREE-enriched patterns. The Ortaca peridotites have low temperatures of equilibration relative to abyssal peridotites, because of H2O-assisted diffusional equilibration in the suprasubduction zone. They are characterized by oxygen fugacities Δlog FMQ between -1.61 and +0.74. Clearly, the clinopyroxene-bearing harzburgites define an increasing oxygen fugacity with almost constant Cr#, whereas the refractory harzburgite and dunite show a strong positive correlation between oxygen fugacity and Cr#. In view of the different measurements on minerals and whole rocks, and as a result of calculated temperatures and oxygen fugacities, we contend that the peridotites were subsequently affected by percolating hydrous boninitic melt, from which the high-Cr, low-Ti chromitites were formed, surrounded by a dunite envelope. It is likely that such an envelope of dunite resulted from interaction between melt and harzburgite, which caused the local removal of orthopyroxene within the mantle wedge above the suprasubduction zone.

Geochemistry and Origin of Listwanites in the Sartohay and Luobusa Ophiolites, China

Gold-bearing listwanites of the Sartohay ophiolite, northwestern China, consist mainly of talc, magnesite, chlorite, quartz, and dolomite, locally accompanied by chromian mica (fuchsite). They formed along NE-trending faults by hydrothermal alteration of serpentinized peridotites. The original protoliths were clinopyroxene-bearing harzburgites and lherzolites containing relatively aluminous magnesiochromite. Compared to their protoliths, listwanites generally have lower MgO and significantly higher CaO, TiO2, and total rare-earth elements. Gold mineralization occurs chiefly in large quartz veins with grades up to several tens of g/t. Listwanites in the Luobusa ophiolite, Tibet, form a narrow band along the southern boundary fault of the ophiolite. Some of these listwanites have relict textures, suggesting derivation from serpentinized peridotite; however, adjacent harzburgites and clinopyroxene-bearing harzburgites are very fresh, with less than 1% serpentinization. The Luobusa listwanites consist essentially of talc and magnesite with lesser amounts of quartz. Most are indistinguishable geochemically from the unserpentinized protoliths except for having notably higher loss on ignition. None of these rocks are mineralized. The Sartohay listwanites are spatially associated with granitic intrusions dated at 301 Ma. They were altered by relatively high temperature fluids, as indicated by homogenization temperatures of fluid inclusions in quartz ranging from 340 to 360°C. Those in the Luobusa ophiolite are not associated with any known igneous activity and are believed to have formed at somewhat lower temperatures. Although both varieties are completely altered and have generally similar mineral assemblages, only those of Sartohay show significant geochemical modification and gold mineralization. Both types have been moderately silicified and are easily recognized by their light brown color and resistant outcrops.

Comments on the paper “Ochreous laterite: a nickel ore from Punta Gorda, Cuba” by Oliveira et al.

The paper, “Ochreous laterite: a nickel ore from PuntaGorda, Cuba,” by Oliveira et al. (2001) presents acharacterization of oxidized nickel ore from the PuntaGorda deposit in the Moa district (eastern Cuba). Theauthors assume that the nickel ore derives from an ophioliticassociation (serpentinized ultramafic rocks, olivine gabbros,and plagiogranites) strongly affected by lateritization. Theynote that ‘no extensive study has so far been published aboutthe Ni mineralization in regard to the mineralogical andchemical nature of the ore’ and reach the conclusion thatnickel is associated with goethite and cobalt with manga-nese minerals. The authors find two ore types, Ni-rich(1.56–2.06%) and Ni-poor (0.24–0.96%), and infer that thetwo types are related to differences in the parent rocks.Oliveira et al. contend that the Ni-rich ore derives fromperidotites containing Ni-bearing orthopyroxenes and thatthe Ni-poor ore forms from peridotites containing Al-clinopyroxene depleted in Ni. Despite their interestingattempt, we believe these points require some criticalcomments that point to possible alternative interpretations.First, the Moa-Baracoa ophiolitic massif consists of alower unit of mantle tectonites made up of serpentinizedharzburgite with minor dunite, ‘impregnated peridotite’(plagioclase- and clinopyroxene-bearing peridotite), wehr-lite, and pyroxenite and a crustal sequence composed oflayered gabbro (olivine gabbro and gabbronorite) anddiscordant pillowed basalts and sediments of the Quivija´nFormation (Guild, 1947; Wadge et al., 1984; Ri´os andCobiella, 1984; Fonseca et al., 1985; Iturralde-Vinent, 1989,1996, 1998; Keer et al., 1999; Proenza et al., 1999a,b).Towards the top of the mantle tectonites, the harzburgitecontains increasing amounts of dunite, gabbro sills, andchromitite, which form elongated pseudotabular bodiesparallel to the foliation of the host harzburgite, as well asdiscordant dikes of gabbro and pegmatitic gabbro (Fig. 1).This upper zone has been considered as forming part of theMoho Transition Zone (MTZ) (Proenza et al., 1999a,b).Second, Oliveira et al. (2001) ignore many publishedstudies on the geological, mineralogical, and chemicalcharacteristics of the Cuban lateritic nickel deposits(Kudelasek et al., 1967; Vera, 1979; Formell and Oro,1980; Vershinin et al., 1984; Quintana-Puchol, 1985;Ostroumov et al., 1985, 1987; Cordeiro et al., 1987;Rojas-Puro´n et al., 1993; Rojas-Puro´n and Carballo, 1993;Capote et al., 1993; Almaguer and Zamarzry, 1993;Rojas-Puro ´n and Beyris, 1994; Rojas-Puron and Orozco,1995; Almaguer, 1995, 1996; Lavaut, 1998). These studiescontain extensive results about the Ni mineralization in theMoa district and conclude that Ni in ochreous laterite isassociated with goethite and Co with manganese minerals(asbolanes).Third, the differences in the mineralogical and chemicalcomposition of the Ni-rich and Ni-poor ores cannot beexplained by the presence of Ni-bearing orthopyroxene orNi-depleted, Al-rich clinopyroxene in the parent rock, asOliveira et al. (2001) conclude, because the Moa peridotitesare made up of only harzburgites and dunites (Proenza et al.,1999a,b), neither of which contains significant clinopyrox-ene. The average forsterite and NiO content of olivine in theharzburgites is 91.0 and 0.39 wt%, respectively, whereas indunites, these values are 91.5 and 0.38 wt% (Table 1). TheNi content of orthopyroxene is very low (NiO , 0.1 wt%),usually below the detection limit of the electron microprobe(Table 1). However, clinopyroxene-bearing harzburgites arevery scarce in the whole Moa-Baracoa massif. Therefore,the Ni-rich ore can only derive from harzburgite and dunite

Geochemical and geochronological constraints on the origin and emplacement of the Yarlung Zangbo ophiolites, Southern Tibet

AbstractThe Indus-Yarlung Zangbo suture zone in southern Tibet marks the Eocene collision of the Indian continent and the Lhasa Block of Eurasia. It is characterized, particularly in its central portion, by an east-west belt of ophiolitic and related oceanic volcanic and sedimentary rocks that form a number of structurally juxtaposed geological terranes. Although tectonically disrupted in many places, almost complete ophiolite sequences exist at Luobusa and Zedong in the east and near Xigaze in the west. In Luobusa, the ophiolite sequence is thrust over the Tertiary molasse deposits of the Luobusa Formation or onto plutonic rocks of the Gangdese batholith. A mantle sequence dominates the ophiolite massif and consists chiefly of harzburgite and clinopyroxene-bearing harzburgite with abundant podiform chromitites enveloped by dunite. The Luobusa ophiolite formed the basement to an intra-oceanic volcanic arc, the Zedong terrane, which developed between the Mid-Jurassic and Mid-Cretaceous. Farther to the west, complete ophiolite sequences exist at Dazhuqu and near Xigaze. These ophiolites have suprasubduction zone geochemical signatures but there is no apparent development of a volcanic arc. Sensitive high-resolution ion microprobe U-Pb zircon analyses yield an age of 126 Ma for the crystallization of a quartz diorite from the Dazhuqu massif. Amphibolites that occur as large blocks in mélanges at the base of the ophiolites are considered to be remnants of dynamothermal metamorphic soles produced early in the ophiolite obduction process. Ar/Ar geochronology on amphibole and biotite separates from these rocks yields ages of 80 and 90 Ma, respectively, for this event, which is considered to have occurred as the Indian continental margin entered the intra-oceanic subduction zone. Continued northward subduction of the remaining portion of the Neo-Tethyan ocean floor beneath the southern margin of Eurasia produced the Gangdese continental arc on the southern margin of the Lhasa Block and led to the final closure of the ocean with the collision of India and Eurasia in the Eocene.

Geochemistry and tectonic significance of peridotites from the South Sandwich arc-basin system, South Atlantic

Petrographic and geochemical studies of peridotites from the South Sandwich forearc region provide new evidence for the evolution of the South Sandwich arc–basin system and for the nature of interactions between arc magma and oceanic lithosphere. Peridotites from the inner trench wall in the north-east corner of the forearc vary from clinopyroxene-bearing harzburgites, through samples transitional between harzburgites and dunites or wehrlites, to dunites. The harzburgites are LREE depleted with low incompatible element abundances and have chromites with intermediate Cr# (ca. 0.40). Modelling shows that they represent the residues from 15–20% melting at oxygen fugacities close to the QFM buffer. The dunites have U-shaped REE patterns, low incompatible element abundances and high Cr# (0.66–0.77). Petrography and geochemistry indicate that the latter are the product of intense interaction between peridotite and melt saturated with olivine under conditions of high oxygen fugacity (QFM + 2). The transitional samples are the product of lesser interaction between peridotite and melt saturated with olivine ± clinopyroxene. The data demonstrate that the harzburgites originated as the residue from melting at a ridge (probably the early East Scotia Sea spreading centre), and were subsequently modified to transitional peridotites and dunites by interaction with South Sandwich arc magmas. The second dredge locality, near the South Sandwich Trench–Fracture Zone intersection, yielded rocks ranging from lherzolite to harzburgite that could similarly have resulted from a two-stage melting and enrichment process, but involving a more fertile mantle residue and a reacting melt that is transitional between MORB and island arc tholeiite. The South Sandwich peridotites have a similar petrogenetic history to those from Conical Seamount in the Mariana forearc in the sense that both involved interaction between arc magma and pre-existing mantle lithosphere of different provenance. However, the precise compositions of the magma and mantle components vary from location to location according to the precise tectonic setting and tectonic history. Overall, therefore, data from the South Sandwich and Izu–Bonin–Mariana systems emphasise the potential significance of peridotite geochemistry in unravelling the complex tectonic histories of forearcs past and present.

Compositional heterogeneity in subduction-related mantle peridotites, Troodos massif, Cyprus

Geochemical mapping has revealed the presence of two distinct units within the mantle section of the Troodos massif, Cyprus. The first unit (U1) is composed of spinel lherzolite (Cr# [Cr/(Cr+Al)] of chrome spinels = 0.22‐0.28), which contains numerous dunite bodies accompanied by zones of clinopyroxene-bearing harzburgite (U1-harzburgite). The second unit (U2) mainly consists of refractory clinopyroxene-poor harzburgite (U2-harzburgite; Cr# of chrome spinels = 0.51‐0.70). The U1 spinel lherzolite is interpreted as residue after mid-ocean ridge basalt extraction, which was tectonically juxtaposed with U2. It was not involved in production of suprasubduction-type Troodos magmas, but has subsequently been modified by their percolation. The dunite bodies and related zones of U1-harzburgite might represent pathways of these melts in the mantle. The U2-harzburgite could have resulted from reaction between host rock and melts.

Omphacite-diopside vein in an omphacitite block from the Osayama serpentinite melange, Sangun-Renge metamorphic belt, southwestern Japan

AbstractOmphacite (Jd46.1–52.0Ae0–8.4Aug48.0–51.2) and diopside (Jd4.3–6.3Ae0–0.4Aug93.6–95.6) coexist in a vein cutting an omphacitite block in a serpentinite melange of the Oeyama ophiolite, central Chugoku Mountains. The compositional gap between omphacite and diopside is significantly wider than for other omphacitediopside pairs reported in the literature. The intergrowth texture of the omphacite-diopside vein suggests that the clinopyroxene pair was contemporaneously crystallized in the fracture from a Ca-bearing, alkali-rich fluid in a sub-solvus condition. Such a fluid may have been introduced from the surrounding serpentinized clinopyroxene-bearing harzburgite. The stability of omphacite and Al-rich pumpellyite in the matrix and the omphacite-diopside vein indicate that the earlier matrix recrystallization and later fracture filling may have both taken place under high-P-T condition during the melange-forming stage.