Boninitic Magmas Research Articles

Recycling of crustal Os in nascent subduction zones revealed by ophiolitic boninites

Olivine-rich lavas in the Margi area, Cyprus, are part of the boninitic Upper Pillow Lava series of the Troodos ophiolite. Re-Os isotope compositions of both whole rock and olivine phenocrysts of these lavas have been measured to determine the initial 187Os/188Os isotope compositions of the primary boninitic magma. The Margi lavas have high and variable whole rock Re-Os isotope compositions (187Re/188Os: 1.060–8.613; 187Os/188Os: 0.1307–0.1599), with (87Sr/88Sr)90Ma (0.705936 ± 0.001021), εNd(90 Ma) (5.6 ± 2.5), and δ18O (8.59 ± 0.38 ‰). In contrast, olivine phenocrysts, which host spinel and melt inclusions, have low and constant 187Os/188Os from 0.1301 to 0.1336 and low 187Re/188Os from 0.1350 to 1.557. Initial 187Os/188Os of these olivine phenocrysts (0.1300 ± 0.0041) is lower than that of whole rock (0.1373 ± 0.0135). These facts indicate that the olivine phenocrysts faithfully record initial 187Os/188Os of the primary boninitic magmas, which are comparable to the 187Os/188Os of primitive mantle, whereas whole rock Re-Os isotopes have been affected by secondary processes after olivine crystallization. Further geochemical modeling indicates that, although Os can be mobilized during slab dehydration, the contribution of crustal Os is insufficient to significantly raise the 187Os/188Os of the mantle wedge in a nascent subduction zone. Overall, our findings provide new insights about Os recycling in the early stages of subduction.

New geochemical and age constraints ( 40 Ar/ 39 Ar and U–Pb) on forearc intrusive rocks from the New Caledonia Ophiolite (SW Pacific): diversity of melts generated at hot subduction inception

The New Caledonia Ophiolite is cross-cut by coarse- to medium-grained pyroxenite and hornblende gabbro/diorite dykes intruded between 55.5 and 50 Ma (U–Pb zircon and 40 Ar/ 39 Ar hornblende), whereas the finer grained dolerites of tholeiitic affinity are younger (50–47 Ma). The production of hornblende gabbros/diorites was modelled by moderate degrees (20–40%) of partial melting of the high-temperature amphibolites of the metamorphic sole. The end-member compositions (hornblendites and anorthosites) resulted from solid-state phase segregation of crystal mushes within tectonically active magmatic conduits. Cascade reactions of the slab melts with mantle wedge peridotites successively formed clinoenstatite-bearing boninite magmas, which fed gabbronorite cumulate lenses at the mantle–crust transition; in turn, the clinoenstatite-bearing boninite melts reacted with peridotites to form websterites. The youngest magmas (of tholeiitic affinity) appeared c. 6 Myr after the inception of subduction when the cooler subducting slab plunged more steeply. Incipient slab retreat allowed corner flow, triggering low-pressure hydrous melting of the uplifted asthenosphere. The early stages of forearc magmatism were closely associated with transcurrent shear zones, which recorded oblique subduction inception. The early Eocene tectonic and magmatic features of the New Caledonia Ophiolite provide evidence for a north- or NE-dipping hot (forced) subduction zone in the SW Pacific, notably distinct from the slightly younger west-dipping Izu-Bonin–Marianna cold (spontaneous) subduction system. Supplementary material: Field pictures, additional diagrams, GPS location of dated samples, whole-rock geochemical and isotope data, Ar/Ar data, U–Pb zircon data, and distribution coefficients ( K d ) of trace elements used for modelling are available at https://doi.org/10.6084/m9.figshare.c.6949265 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

Cadomia origins: paired Ediacaran ophiolites from the Iberian Massif, the opening and closure record of peri-Gondwanan basins

Abstract The recent discovery of Ediacaran ophiolites in the SW Iberian Massif has made it possible to pinpoint the evolution of the Cadomian basement of Europe. The Calzadilla and Mérida ophiolites (gabbroic protoliths dated at c. 600 and 594 Ma, respectively) have geochemical characteristics typical of supra-subduction zone ophiolites. They are interpreted as originating during the initial opening of a forearc basin with boninitic magmatism (Calzadilla), followed by the formation of a back-arc basin with arc-tholeiites (Mérida). Widening of the back-arc led to the rifting and drifting of a section of the active continental margin (Cadomia). Closure of these oceanic domains initiated rapid contraction, culminating in the collision of Cadomia with Gondwana ( c. 590–540 Ma). The application of a PANALESIS model to this palaeogeographic setting confirms the plausibility of Cadomian rifting and the likely opening of broad oceanic domains. It also confirms the final collision of Cadomia with Gondwana, although the synthetic and regional data disagree in the precise chronology of the convergence and collision of Cadomia with the West Africa Craton. This work shows that the evolution of the Cadomian basement is much more complex than traditionally considered.

Mantle-wedge alteration facilitates intra-oceanic subduction initiation along a pre-existing fault zone

Intra-oceanic subduction initiation may occur where a transform fault or a fracture zone is located between the younger and older lithospheres. However, the mechanisms and conditions controlling the initiation of subduction remain debated. Here, we present two-dimensional visco–elasto–plastic numerical models of vertically-forced (or spontaneous) subduction initiation along such a pre-existing fault zone where slab-derived fluids enter the overlying mantle wedge. The models show that just after sinking of the older lithosphere, hydrothermal alteration to form serpentine (antigorite) or talc occurs within a juvenile, thin mantle wedge, resulting in the development of a low-viscosity shear zone (1019–1020 Pa s) at its base. The low-viscosity shear zone acts to decouple the mantle wedge from the slab, thereby facilitating slab subduction. Continued slab subduction generates shallow (<50 km depth) flux melting in the inner part of the mantle wedge at 3–5 Myr after the start of the model run, consistent with the timing of boninitic magmatism after the onset of subduction initiation in the Izu–Bonin–Mariana system. Our models that incorporate water release from the slab require average fracture zone (or transform fault) shear strengths of ≤7 MPa to attain a self-sustaining subduction zone. The fault strengths might be achieved in nature where seawater infiltrates deep into the fault zone.

Formation and implication of a Cr-bearing spinel seam by interaction between alkali basaltic melt and lherzolite

It has long been known that an indisputable fact that chromite/spinel deposits are genetically related exclusively with komatiitic, tholeiitic or boninitic magmas derived from high-degree mantle melting. Here we report formation of a seam of Cr-bearing spinel through interaction between an alkali basaltic magma and fertile lherzolite. The thin seam occurrence of spinel resembles concordant podiform chromitites in ophiolites and stratiform chromitites in layered intrusions. We show that the Cr-bearing spinel seam formed by melt-rock reaction that observed in ophiolitic peridotites. The most significant difference is that the Cr-bearing spinel produced from the alkali basaltic melt-lherzolite interaction is extremely rich in Al with Cr# values of 3–6. We consequently re-classify chromitite into four types based on their Cr# values namely: high-Cr (chromite Cr# > 70), intermediate (Cr# = 70–50), high-Al (Cr# = 50–40), and extremely high-Al (Cr# < 10) with a Cr# gap between 40 and 10. The chromite/spinel compositions correspond closely to diagnostic parental magmas and the lithologies of the host peridotites.

Subduction and Oceanic Magmatism Records in Plutonic Rocks of the Kamchatsky Mys Ophiolite, Eastern Kamchatka

The paper presents petrographic, mineralogical, and geochemical data on dunites, pyroxenites, peridotites, and gabbroids of the Kamchatsky Mys ophiolite. These data were acquired to distinguish cogenetic assemblages of igneous rocks, gain an insight into their geodynamic settings, and test various criteria of genetic links between the different magmatic rocks of ophiolites. The ultramafic and mafic rocks are shown to belong to two series, which differ in the compositions of the primary minerals, bulk rocks, and estimated trapped melts. The rocks of these series are found out to have been produced by geochemically different melts in different geodynamic settings, and during different episodes of mantle magmatism. The rocks of the high-Ti series (gabbro of the Olenegorsk massif, dunite and melanogabbro xenoliths in them, and vein gabbro in these xenoliths) crystallized from N-MORB melts in an oceanic spreading center. The rocks of the low-Ti series (dunite, pyroxenite, and gabbro veins in the residual spinel peridotites of the Mount Soldatskaya massif, as well as pyroxenite, peridotite, and gabbro alluvium and diluvium in the central and western parts of the peninsula) crystallized from water-rich boninite melts in relation to initial subduction magmatism. Taken into account the absence of boninite lavas from the Kamchatsky Mys ophiolite, the plutonic ultramafic rocks (including the rocks of the veins) might be the only evidence of subduction boninitic magmatism in the ophiolites. It was demonstrated that conclusions about the geodynamic settings of plutonic ultramafic and mafic rocks and recognition of cogenetic relations of these rocks with spatially associated basalts are more reliable when derived from the compositions of the trapped melts, which are estimated from their bulk geochemistry and primary mineral compositions, than when they are based on the mineral compositions only.

The Redox State of Incipient Oceanic Subduction Zones: An Example From the Troodos Ophiolite (Cyprus)

AbstractTo understand the oxygen fugacity (fO2) during subduction initiation, we examined proto‐arc boninites and associated mantle peridotites from the Troodos ophiolite, Cyprus. The Troodos ophiolite represents an exhumed piece of oceanic lithosphere that formed during subduction initiation in Neo‐Tethys in Late Cretaceous. Dunites and surrounding harzburgites in the Kokkinorotsos podiform chromite deposit of the ophiolite were formed by reactions between peridotites and ascending boninitic magmas. The olivine‐chromite oxybarometers show that the fO2 values (∆QFM −0.41 ± 0.37) of the dunites are slightly higher than those (∆QFM −0.85 ± 0.52) of the harzburgites under mantle conditions, indicating that the mantle fO2 was raised limitedly by percolation of the boninitic magmas. Meanwhile, the boninitic upper pillow lavas in the Margi area also recorded relatively reduced fO2 values (∆QFM −0.52 ± 0.20) during olivine‐chromite crystallization, comparable to the fO2 of mid‐ocean ridge basalts. Shallow‐level processes (i.e., magma ascent, crystallization, crustal assimilation, and degassing) appear to have limited influence on the estimated fO2 values in Troodos. The similarly low fO2 values of the mantle peridotites and boninitic magmas suggest that the Troodos primary boninitic magmas were perhaps not as oxidized as the Izu‐Bonin‐Mariana (IBM) proto‐arc boninitic magmas, though both mantle sources have captured the slab fluids released during subduction inception. This difference might be attributed to the different nature of the slab components (i.e., a more reduced subduction input for Troodos). Our study thus suggests that subducted slabs might not always release oxidized fluids, and the oxidation processes might be different for each nascent subduction zones.

Sluggish Slab Rollback at the Early Stage of Flux Melting During Subduction Initiation: Li Isotopic Evidence From the Coto High‐Al Chromite Deposit, Zambales Ophiolite, Philippines

AbstractThe compositions of chromitites and dunites from Moho transition zone (MTZ) of the Coto block of the Zambales ophiolite, Philippines, are used to investigate the geodynamic transition from anhydrous to hydrous magmatism during subduction initiation (SI). Chromite grains in the chromitites have Cr# values [100 × Cr/(Cr + Al)] and TiO2 contents ∼35–50 and 0.05–0.30 wt.%, respectively, intermediate between those of chromite in typical MORB‐like lavas (Cr#, ∼20–60; TiO2, ∼0.6–1.7 wt.%) and boninites (Cr#, ∼70–85; TiO2, &lt;0.4 wt.%). Olivine grains in the dunites have δ7Li values varying from ∼−2‰ to +21‰ with most between +10‰ and +15‰, beyond that of normal mantle (+4 ± 2‰) but comparable to those of some arc lavas (up to +12‰). The data set indicates that parental magmas of the high‐Al chromitites originated from hydrated harzburgitic mantle sources and formed temporally between MORB‐like and boninitic magmatism during SI, resulting from the early stage of flux melting in the Zambales proto‐forearc mantle. Modeling of Li diffusion reveals that the MTZ cooled down at a minimum rate of 0.1°C/yr in order to preserve the large δ7Li variation of olivine in the dunites, comparable to the thermal conditions below ultra‐slow to slow spreading ridges. Such a stage of transitional magmatism, although displaying notable slab contributions, took place at a sluggish period of slab rollback and asthenospheric upwelling, leading to a trough level of heat flow and magma production during the entire course of SI.

Geochemistry of Waziristan Ophiolite Complex, Pakistan: Implications for Petrogenesis and Tectonic Setting

The Waziristan ophiolite complex (WOC), a segment of paleo-suture zone between the Indo-Pakistan plate to the south-east and the Afghan microplate to the north-west, is primarily composed of serpentinized dunites and serpentinites after harzburgites, crosscut by pyroxenites. The crustal portion dominantly comprises basalts and dolerites, with less abundant plagiogranites. Whole-rock XRF and trace element ICP-MS analyses were undertaken to determine the degree of melt extraction and subsequent geochemical depletion or enrichment of peridotite protoliths. The investigated peridotites are extremely refractory and show high Mg# values (0.88–0.92), low Al2O3 (0.19–0.96 wt%), total alkali values (0.02 &lt; Na2O + K2O ≤ 0.20 wt%), CaO (0.04–0.73 wt%), and TiO2 (0.001–0.017 wt%), but are rich in Cr (up to 3550 ppm) and Ni (up to 2340 ppm). Bulk REE modelling suggests that Waziristan peridotites underwent a high degree (15%–25% melting) of melt extraction in the closed system of spinel-bearing peridotite facies at shallow depths above a subduction zone and, in their chemistry (Sc, REE, Al2O3, and MgO), resemble Izu-Bonin-Mariana (IBM) forearc peridotites. Their U-shaped REE patterns and trace elements resemble peridotites derived from supra-subduction zone (SSZ) settings, especially a forearc setting, and also reflect metasomatism by slab-derived fluids. The ultramafic rocks preserve depletion in REE content (0.03 &lt; ΣREECN &lt; 0.60), indicating the highly depleted nature of refractory peridotites. The basalts (Mg#, 0.48-0.68) are similar in composition to typical boninite and show low to moderate MgO (6.2–13.0 wt%), low total alkali content (0.01–4.45 wt%) and TiO2 (0.13–0.17 wt%), but are higher in Al2O3 (≈11.9 wt%). They show a sub-alkaline affinity and possess geochemical signatures that are transitional from island arc basalt (IAB) magmas to boninitic magmas due to the changes from an extensional to a compressional regime during the initiation of subduction. These geochemical signatures suggest the formation of basalts from melts, resulting from the re-melting of the depleted mantle during intra-oceanic subduction initiation. The higher Th/Nb and V/Ti ratios of the studied basalts further confirm their generation in an SSZ setting characterized by subduction-derived fluids under higher oxidizing conditions. The mafic-ultramafic rocks of the WOC were, therefore, likely formed during intra-oceanic subduction initiation (forearc spreading) in the SSZ setting; they were exhumed along a thrust fault, and obducted onto the forearc region as result of the collision between the Indian plate and the Afghan microplate.

Origin of high-Cr podiform chromitites from Kabaena Island, Southeast Sulawesi, Indonesia: constraints from mineralogy and geochemistry

ABSTRACT The East Sulawesi Ophiolite, one of the three largest ophiolites in the world, contains important podiform chromitites. However, the origin of these chromite deposits is not well constrained. Here, we present the detailed mineralogy and in situ chemistry of chromite and solid inclusions within chromite for podiform chromitites from the Kabaena Island, Southeast Sulawesi. Chromite grains have low TiO2 (0.17–0.27 wt%) and Al2O3 (13.4–15.1 wt%) contents and high Cr# [Cr/(Cr+Al), molar] of 0.70–0.74 and Mg# [Mg/(Mg+Fe2+), molar] of 0.69–0.75, which resemble those of other high-Cr podiform chromitites worldwide. The MORB-normalized patterns for some selected major-trace elements of chromite show slightly positive slopes from Al2O3 to Mn. Chromite-hosted silicate inclusions include olivine, clinopyroxene, orthopyroxene and amphibole and are characterized by higher Mg/Fe ratios than those of host peridotites. The geothermobarometry of chromite-hosted clinopyroxene-orthopyroxene pairs indicates that the estimated T-P conditions of the parental magma of the Kabaena chromitites are 950–1010°C and 7.0–8.4 Kbar. The extreme-Mg-rich olivine inclusions (Fo >96) with extremely high Ni (5300–8200 ppm) and Cr (1500–6700 ppm) contents are interpreted to have crystallized from high-Mg and Cr boninitic melts, and the Fo contents were then elevated by subsolidus re-equilibration (Fe–Mg and Fe–Ni exchange) with chromite. Sulfide inclusions contain base metal minerals containing millerite with rare pentlandite and chalcopyrite, and platinum-group minerals that include the laurite-erlichmanite series, Ir–Ni monosulfide, irarsite and cuproiridsite. This sulfide assemblage reveals that their parental magmas experienced the evolution of high T and low f(S2) to low T and high f(S2) from the early to late stage. Finally, combined with the palaeogeographic reconstruction in Sulawesi, we propose that the high-Cr chromitites in this region were formed by the reaction of depleted mantle and boninitic magma beneath a juvenile island arc, implying the initiation of a new subduction in the Miocene.

Geochemical constraints on the multistage evolution of the Yilashan ophiolite, central Tibet

The Yilashan ophiolite in the southern part of the central Bangong-Nujiang suture zone comprises variably deformed harzburgite, dunite, and less abundant gabbro and pyroxenite pods and crosscutting veins. Cr-spinels in podiform chromitite bodies are characterized by high Cr# (64.3–74.3), high Mg# (58.2–67.5), and consistently low contents of TiO2 (≤0.12 wt%). Primary melt calculations designate parental boninitic magmas with Al2O3(liquid) contents of 11.67–13.49 wt%, and (FeO/MgO)liquid values of 0.92–1.12.Whole-rock geochemical data of fresh pyroxenite samples are characterized by negative K, Ta, Nb and Zr anomalies, no Eu anomaly, and chondrite-normalized REE patterns comparable to the N-MORB melts. The gabbro samples, in contrast, are high-K calc-alkaline, with elevated contents of LILE (Rb, K, Th) and depleted HFSE (i.e., Nb, Ta, P, Ti), consistent with subduction-related magmas. The fractionated chondrite-normalized REE patterns, high LREE/HREE values, and negative Eu anomalies of these gabbroic rocks are similar to the island arc basalts. Pyroxenite shows high PPGE/IPGE values, variable Pd/Ir (55.7–316.7), and left-leaning primitive mantle-normalized PGE pattern, whereas massive chromitite is enriched in IPGE relative to PPGE. The PGE patterns of pyroxenite and chromitite, though distinct, depict significant fractionation and partial melting in their formation. The elevated contents of PPGEs in pyroxenite suggest derivation from sulfur-saturated magmas as chromitite crystallization scavenged the compatible IPGEs from the parental melt. This assumption is supported by clinopyroxene in pyroxenite with variable Al2O3 contents and variable Cr2O3 contents (0.32 to 0.73 wt%). For the fractionated gabbro unit, zircons yielding a concordia U-Pb age of 162.8 ± 3.0 Ma and widely variable εHf(t) values (−9.2 to +12.0) suggest formation from heterogenous or reworked source materials during a Middle Jurassic subduction increment in the Neo-Tethyan Ocean.

Platinum‐group mineral and silicate inclusions in the low‐Al chromitites of the Manipur ophiolite, northeast India: Implications on Cr‐PGE mineralization in nascent subduction zone settings

Low‐Al chromitites from the Manipur ophiolite contain a wide range of mineral inclusions, including hydrous and anhydrous silicates, sulphides, and platinum group minerals (PGM). The common inclusions are primary silicates, such as clinopyroxene, orthopyroxene and secondary silicate inclusions, such as chlorite, serpentine, and garnet. These inclusions are small (&lt;30 μm in PGM, &lt;100 μm in silicates) and randomly distributed throughout their host chromite grains. The shape and texture indicate some inclusions (clinopyroxene, orthopyroxene, PGMs) trapped during the magmatic stage and most inclusions (chlorite, serpentine, and uvarovite) trapped during recrystallization of chromite. Clinopyroxene was discrete inside the chromite, with Cr2O3 content ranging from 0.62 to 1.77 wt.%, and Al2O3 varying between 0.96 to 1.63 wt.%. Orthopyroxene is enstatite (En93Wo2Fs4), Cr2O3 varies from 0.58 to 1.8 wt.% and the minor amount of CaO (0.41–0.81 wt.%), and Al2O3 (0.58–0.83 wt.%). Uvarovite is present in two forms: discrete crystal inclusions (anhydrous) in chromites and fractures (hydrous) as fillings. The chlorite occurs as lath shaped, and patchy phase is Cr‐rich variety ‐ kammererite (clinochlore) and penninite with variable Cr2O3 (0.22–7.25 wt.%) and FeO (2.5–3.9 wt.%). The PGM in the studied chromite are laurite (Ru, Os, Ir)S2. The parental melts of chromitites had low TiO2 (0.16–0.48 wt.%) and Al2O3 (10.18–14.58 wt.%) contents, and primary mineral inclusions, suggesting an arc‐like geochemical affinity, and the estimated melt composition is comparable with that of boninitic magma.

Ion-probe (SIMS) U-Pb geochronology and geochemistry of the Upper Cretaceous Kızıldağ (Hatay) ophiolite: Implications for supra-subduction zone spreading in the Southern Neotethys

The Kızıldağ (Hatay) ophiolite in southern Turkey represents a complete, although fault-dissected, remnant of oceanic lithosphere that formed within the South Tethyan ocean during the late Cretaceous. The Kızıldağ forms part of the Upper Cretaceous belt which includes the Troodos (Cyprus), Baer-Bassit (Syria), Amanos (S Tukey), S Iran and Semail (Oman) ophiolites. Ion-probe (SIMS) dating of seven samples of crustal rocks (cumulate gabbro, isotropic gabbro and isolated dykes in mantle tectonite), and a plagiogranite intrusion provides important clues concerning the temporal development of the emplaced oceanic crust. Single grain 206Pb/238U dates that overlap within analytical uncertainty for four samples, including the plagiogranite (93.83 ± 0.46 Ma), the isotropic gabbro (92.9 ± 0.52 Ma) and the isolated dykes (92.54 ± 0.44 Ma to 93.6 ± 0.75 Ma), are interpreted as magmatic crystallisation ages, and suggest that the Kızıldağ ophiolite formed within 1-2 Ma. Three other samples with single grain 206Pb/238U dates that are outside the range of analytical uncertainty yielded nearly identical lower intercept ages of 94.2 ± 2.5 Ma to 94.4 ± 0.97 Ma for two cumulate gabbros and 90.0 ± 6.4 Ma for an isotropic gabbro. Comparison of the new and published radiometric ages of the Kızıldağ suggest that this ophiolite is ∼1.5 Ma older than previously believed, and is similar to the crystallisation ages of plagiogranites from the Troodos (Cyprus) and the Semail (Oman) ophiolites. The new age data emphasise the value of dating a range of ophiolitic rocks. Geochemically, the crustal rocks of the Kızıldağ ophiolite formed from boninitic magmas (cumulate gabbros and isolated dykes) and from island arc tholeiitic magmas (isotropic gabbro). The new whole-rock chemical data support a subduction-initiation (fore-arc) setting for the Kızıldağ ophiolite, in common with the Troodos, Semail, Baër-Bassit and other Upper Cretaceous ophiolites of the South-Tethyan region.

Thermal history and parental magma composition of early olivine gabbros of the Troodos ophiolite, Cyprus

Samples of an early olivine gabbro in the lower plutonic sequence of the Troodos ophiolite were studied to constrain their thermal history and the compositions of their parental magma. These rocks exhibit typical orthocumulate textures and petrographic evidence of reaction between a migrating interstitial magma and a crystal mush, including embayed margins of olivine crystals and irregular ragged grain boundaries of clinopyroxenes, indicating dissolution of olivine and crystallization of clinopyroxene, followed by crystallization of plagioclase within the framework of olivine and clinopyroxene. All the minerals exhibit relatively homogeneous compositions, without any clear zoning in either their major or trace elements, indicating that the minerals and migrating magma reached equilibrium. The rare earth element (REE) distribution between clinopyroxene and plagioclase records an equilibrium temperature of about 1300 °C, but the Mg-exchange geothermometer yielded a lower temperature of about 1000 °C. These different closure temperatures indicate a rapid cooling history with a cooling rate of −2 °C/year log units. The low TiO 2 contents and additional trace element compositions of clinopyroxene show that the parental magma of the gabbro was related to boninitic magmas of the ophiolite. Consequently, we show that some boninitic lavas in the upper oceanic crust have been affected by melt-rock reaction process during their ascent through the lower crust, and therefore their composition cannot be directly used to constrain the composition of the primary boninites. • Gabbros of the Troodos ophiolite display petrographic evidence of reaction between a migrating magma and a crystal mush. • Trace elements of clinopyroxene indicate that the migrating magma is related to boninites in the UPL. • Some boninitic lavas affected by melt-rock reaction process cannot represent composition of the primary boninite.

Mineralogy and PGE geochemistry of chromitites and peridotites of the sapat complex in the indus suture zone, northern Pakistan: implications for magmatic processes in the supra-subduction zone

ABSTRACT The Sapat Complex in Northern Pakistan contains remnants of the northern Neo-Tethys Ocean, presently exposed along the Indus Suture Zone. The mantle peridotites of the Sapat Complex include harzburgites, dunites, and subordinate lherzolites. Harzburgites are the dominant peridotite variety over dunites. The dunites are hosted by harzburgites and occur exclusively as ‘envelopes’ surrounding chromitite pods. The podiform chromitites show disseminated, banded, and massive textures. Chromitites exhibit variable Cr# [Cr/(Cr + Al)] and Mg# [Mg/(Mg + Fe2+)], which range from 0.76 to 0.77, and from 0.64 to 0.66, respectively, while TiO2 contents are <0.2 wt.%. These features perhaps reflect crystallization of the chromian spinel from a boninitic magma. Similarly, chromian spinel in peridotites manifest a wide range of Cr# and Mg#, from 0.49 to 0.83 and 0.41 to 0.57, respectively, and are characterized by very low TiO2 values, averaging at 0.1 wt.%. Chromian spinel of chromitites and peridotites of the Sapat Complex have also very low Fe3+# (<0.01), which indicate their crystallization under low oxygen fugacities. The platinum group elements (PGE) distributions show high (Os + Ir + Ru)/(Rh + Pt + Pd), very low Pd/Ir values, and are defined by a prominently fractionated chondritic normalized PGE pattern, hence, this deposit is a typical example of an ophiolitic chromitite. The studied peridotites are highly depleted in PGE compared to chondritic values. The PdN/IrN values, averaging to 1.5 in dunites are unfractionated, while PGE spidergrams of harzburgites and lherzolites depict minor positive slopes, a minor positive Ru anomaly, and have average PdN/IrN values of 2.3 and 2.4, respectively. Furthermore, the harzburgites, dunites, and lherzolites display generally flat chondritic and primitive mantle normalized PGE patterns, and therefore, are nearly identical to highly depleted mantle peridotites. The mineralogical and PGE geochemical imprints of Sapat Complex chromitites and peridotites establish a strong affinity to supra-subduction zone ophiolites. Moreover, calculated parental melts of the chromitites and various geochemical discrimination diagrams elucidate that the chromitites were derived from boninitic magma produced by melting of depleted mantle peridotites in an oceanic arc, characterized by low oxygen fugacity, similar to a supra-subduction zone tectonic setting. This research highlights the use of mineralogy and geochemical compositions of chromitites and peridotites to reveal deep magmatic processes in a supra-subduction zone environment.

Arc–Backarc Exchange Along the Tonga–Lau System: Constraints From Volatile Elements

Abstract Analyses of olivine-, pyroxene- and plagioclase-hosted melt (now preserved as glass) inclusions (MI) in submarine glasses and subaerial scoria from seven volcanoes along the volcanic front of the Tonga arc provide the first comprehensive dataset including volatile contents (H2O, CO2, S, Cl and F) for this arc. Maximum water contents at each volcano are typical (3–5 wt %) of other arc volcanoes worldwide, and within each volcano, water generally correlates with sulfur, consistent with degassing but not diffusive re-equilibration. The Tonga arc is notable for the eruption of magmas sourced from strongly depleted upper mantle, including boninites in the northern half of the active arc. A key question has been whether such boninites are derived from high degrees of melting actively occurring under the arc, driven by high mantle temperatures and water contents, or from high integrated degrees of melting including melting of depleted mantle in the backarc. This study supports the latter view, based on sub-arc mantle melt fractions of 17–23%, calculated from primary melt water contents and mantle-melt thermobarometry. Such fractions are not high enough to generate boninites from a typical, fertile mantle source in a single melting stage, and melt fractions do not increase north of 22°S, where the boninitic magmas appear. Instead, the northern Tonga arc boninites reflect high cumulative degrees of melting (&amp;gt;30%, with respect to fertile mantle) and occur along the volcanic front where adjacent backarc spreading is fast, proximal and mature. Slab tracers such as the Ba/La ratio peak at around 20–21°S, in the central part of the Tonga arc. A Ba/La peak also appears at about the same latitude in the Lau backarc basin. This peak is consistent with the coolest slab temperatures beneath the arc and backarc at this latitude, as the slab warms to the north due to an increase in mantle potential temperature, and to the south due to less heat extraction from melting. A new concept developed here is cooling of the mantle wedge by melting, due to the supply of the heat of fusion and melt removal. Such a process can cool the mantle by more than 100 degrees, and thus affect slab heating as well. We find a critical distance of slab influence in the Lau spreading centers; back-arc magmas erupted &amp;gt;70 km from the arc are &amp;gt;210 km above the slab, beyond the point of slab dehydration, and show no enrichment in Ba/La. The coupled dynamics of the Tonga arc-Lau basin are rich in their connections, and demonstrate how melting processes beneath the back-arc and arc can affect slab processes, which in turn can affect the composition of fluids that feed the arc and backarc.

Chromite compositional variability and associated PGE enrichments in chromitites from the Gomati and Nea Roda ophiolite, Chalkidiki, Northern Greece

The Gomati and Nea Roda ophiolites are located into the Serbo-Macedonian massif of the Chalkidiki peninsula (Northern Greece). The present work focuses on the variability of platinum-group elements (PGEs), geochemistry, spinel mineral chemistry, and platinum-group minerals (PGMs) — base metal minerals (BMMs) assemblage in chromitites of three Gomati localities (St. George, Tripes, and Limonadika) and Nea Roda. The studied chromitites show variable textures and are heavily altered. Primary silicates are almost completely replaced by chlorite, and chromite rims are altered into ferrian chromite. The variability of spinel mineral chemistry in terms of Cr# [Cr/(Cr + Al)] and Mg# [Mg/(Mg + Fe2+)], and the PGE contents, argues for a genesis in a supra-subduction setting (SSZ), at different stratigraphic positions in the ophiolite section. Chromitites from Tripes have the lowest Cr# (0.5–0.6) and the highest PGE contents (3516 ppb), similar to some chromitites formed in small magma chambers in the cumulate sections above the Moho. The high PGE contents of Tripes chromitites are due to an IPGEs-enriched melt derived from critical melting of mantle peridotites. Limonadika and St. George show the highest Cr# (0.77–0.96 and 0.74–0.87, respectively) and variable PGE contents (175 ppb and 383 ppb on average respectively), compatible with a genesis from boninitic magmas in the mantle section. Nea Roda chromitites have intermediate to high Cr# (0.66–0.75) and low PGE contents (135 ppb on average) and show similarities to other intermediate chromitites formed from evolving magma sources at subduction initiation. BMMs detected in both ophiolites are primary (pentlandite) and secondary (mainly millerite and heazlewoodite) sulfides. All the detected PGMs are primary, crystallized from the melt, and entrapped into chromite, and they are mainly laurites. In the studied chromitites, the absence of alloys indicates that the circulating fluids during chloritization were at high fS2 and fO2, and did not remobilize the PGEs. The same fluids are probably responsible for the low-T crystallization of an uncommon suite of arsenides and antimonides at St. George.

Refertilization of depleted mantle peridotite in the Nagaland–Manipur ophiolite, north‐east India: Constraints from PGE, mineral, and whole‐rock geochemistry

This paper discusses whole‐rock geochemistry, mineral chemistry, and platinum group element (PGE) systematics of depleted mantle rocks (harzburgite and dunite) from the northern part of Nagaland–Manipur Ophiolite (NMO), north‐east India, to comprehend their source features, fractionation behaviour of PGE during magmatic evolution, and its tectonic origin. The studied ultramafic rocks are characterized by a low concentration of CaO (0.57–0.71 wt%), Al2O3 (0.18–0.92 wt%) with ∑REE of 1.135–2.702 ppm and high concentrations of MgO (38.70–44.21 wt%), Cr (1,843–4,572 ppm), and Ni (894–4,138 ppm). They show U‐shaped REE patterns [LREE and HREE enrichment (La/Sm)N = 1.85–4.11, (Dy/Yb)N = 0.51–0.85]. Olivine ranges Fo 88.18 to Fo92.23, whereas Cpx and Opx range En44.84 to En47.89 and En86.37 to En93.37 respectively. The chrome spinel Cr# [Cr/(Cr + Al)] and Mg# [Mg/(Mg + Fe2+)] are 0.47–0.83 and 0.31–0.60, respectively, which indicates recrystallization from a boninitic magma in a Supra‐Subduction Zone setting. Conventional thermometry indicates the equilibration temperatures of the dunite sample yielded high temperatures of ~850°C, suggesting their formation due to later interaction with high‐temperature percolating melts. The PGE contents in harzburgite are low (125.6–142.8 ppb) as compared to the dunite (248–360 ppb). They have high PPGE/IPGE and negative Pt* (Pt/Pt* = 0.73) anomaly, which is characteristic of re‐entry of PPGE into the system via reaction with percolating basaltic melt in the mantle wedge. Significantly higher concentration of PPGEs than IPGEs in the samples, indicating recrystallization of PPGEs with early sulphide fractionation. The presence of significant Rh and Pd enhancements relative to Pt in all samples suggests that Pt was removed during PGE fractionation. This could be one of the reasons for both harzburgite and dunite's sulphide undersaturation. PGE distribution in NMO ultramafic rocks was therefore validated as being governed by sulphide saturation in parental magma and altered not only by partial melting but also by fractionation during their production in the Supra‐Subduction Zone environment.

PALEOGEODYNAMICS OF MAGMATIC AND METAMORPHIC PROCESSES OF FORMATION OF ULTRAMAFIC ROCKS OF CHAGAN-UZUN MASSIF (OPHIOLITES OF GORNY ALTAI)

The Geochemical and mineralogical studies showed, that harzburgites of the Chagan-Uzun massif are restites with a degree of partial melting 15–20 %, which formed at temperatures 1520–1420 ° С under the conditions of the mid-oceanic ridge and transformed during the evolution of paleooceanic structures under the influence of magmatic processes at the initial stages of subduction and manifestation of boninite magmatism. The combined use of data on the geochemistry of rare and rare earth elements, as well as on the compositions of pyroxenes, Cr-spinels, melt inclusions in Cr-spinels and computational modeling, indicates the formation of clinopyroxenites of Chagan-Uzun ophiolites at the mid-oceanic ridge during crystallization of picrite and picrobasalt melts at temperatures 1315–1245 °C and pressures 4–2 kbar. The study of amphiboles showed high metamorphic parameters of transformation of harzburgites (5.1–1.9 kbar, 820–700 ° С ) and clinopyroxenites (2.6–1.4 kbar and 740–680 ° С ) of the Chagan-Uzun massif, typical for ultrabasites from the modern mid-oceanic ridges. In general, results of comprehensive studies made it possible to determine the sequence of paleogeodynamic processes of formation of ultramafic rocks of the Chagan-Uzun massif. Initial formation of harzburgites in the course of partial melting of mantle and crystallization of clinopyroxenites in the magma chamber occurred under the conditions of the mid-oceanic ridge. At the next stage, ultramafic rocks fell within the subduction-zone initiation area, where they were exposed to the influence of boninite melts.

First evidence for the subduction initiation and boninitic magmatism from the Armutlu Peninsula (NW Turkey): geodynamic significance for the Cadomian magmatic arc system of the Gondwanan margin

ABSTRACT The Cadomian Orogeny was active along the northern margin of Gondwanaland during the Late Neoproterozoic–Early Cambrian period. Remains of this orogenic activity are traced from Western Europe to the Himalayas, including some metamorphic basement rocks of Turkey. In this study, we provide new whole-rock and mineral chemistry, and zircon U–Pb age data for the metagabbro and metagranites in the Armutlu Peninsula, north of the Intra Pontide suture zone (IPSZ), which are the equivalent of the Cadomian basement of the İstanbul Zone (İZ). Geochemically, the metagabbros show an extreme depletion in Nb, Zr, Th, and Hf; low TiO2 contents; and high Mg# values exhibiting boninitic affinity, which are commonly associated with fore-arc settings. The metagranites are calc-alkaline and high-K calc-alkaline in character. They are slightly enriched in LREE and have similar features to shear-type plagiogranites. The yielded zircon U-Pb crystallization ages are 564 ± 4.4 Ma for metagabbros and 561 ± 9.5 Ma (Late Ediacaran) for metagranites. In this study, we provide additional information for the Cadomian basement of the İZ and the geodynamic evolution of Cadomian magmatism by discussing the tectonic setting for two geochemically contrast but almost coeval magmatic products.