high-Ca Boninites Research Articles

Heterogeneous mantle source compositions for boninite from Bonin and Troodos, evidence from iron isotope variations

Boninite has been widely accepted to be the melting product of highly refractory mantle with the addition of slab-derived fluids. Previous studies have found that boninite, like other subduction-related magmas (e.g., mature arc basalts), has generally consistent chondrite-like iron isotope composition (c. 0.03‰), which is on average lighter than those of oceanic basalts. However, it remains unclear whether the lighter iron isotope composition of subduction-related magmas is inherited from the depleted mantle source or caused by the addition of slab-derived oxidized fluids. Moreover, boninite has highly varied element patterns (e.g., Yb, Zr), reflecting varying degrees of mantle depletion and different contributions of slab-derived fluids. Given the limited data for iron isotope composition of boninite so far available, it is thus required to systematically study iron isotope composition of boninite with different compositions to further understand the petrogenesis of boninite and the iron isotope variation of subduction-related magmas. Here we report iron isotope compositions of boninite from two classic suites on Earth, i.e., those from Bonin islands and submarine forearc (low-Ca boninite; CaO/Al2O3 mainly <0.75) and Troodos ophiolite complex, Cyprus (high-Ca boninite: CaO/Al2O3 = c. 0.85). The δ56Fe values of the Bonin boninite vary from −0.046 ± 0.003‰ to 0.078 ± 0.031‰ (2SD, SD = standard deviation of 4 times repeated analysis; with an average of 0.03 ± 0.03‰, n = 14). The δ56Fe value of the Troodos boninite is relatively constant, i.e., from 0.046 ± 0.010‰ to 0.091 ± 0.016‰ with an average of 0.06 ± 0.01‰ (n = 7). Bonin forearc basalts (FABs) were also analyzed for comparison with δ56Fe of ~0.10‰, except for those FABs with higher Th/U ratio and Cu contents (δ56Fe = 0.01–0.05‰). Together with previously reported δ56Fe data for boninite from New Caledonia (low-Ca boninite with CaO/Al2O3 = ~ 0.4, δ56Fe = ~ +0.03‰), the lower δ56Fe values of the Bonin boninite than Bonin FAB and Troodos boninite reflect the lighter iron isotope composition of the more depleted mantle source. Based on relationships between different indicators for various fluids (e.g., Ba/La, Zr/Sm) and modeling, Bonin boninite is likely produced by partial melting of the refractory mantle with an addition of ~5–10% melts from sediments and oceanic crust, while the formation of Troodos boninite can be ascribed to the melting of less refractory mantle, which has been previously metasomatized by ~5% fluids derived from oceanic crust and sediments.

Ophiolitic Pyroxenites Record Boninite Percolation in Subduction Zone Mantle

The peridotite section of supra-subduction zone ophiolites is often crosscut by pyroxenite veins, reflecting the variety of melts that percolate through the mantle wedge, react, and eventually crystallize in the shallow lithospheric mantle. Understanding the nature of parental melts and the timing of formation of these pyroxenites provides unique constraints on melt infiltration processes that may occur in active subduction zones. This study deciphers the processes of orthopyroxenite and clinopyroxenite formation in the Josephine ophiolite (USA), using new trace and major element analyses of pyroxenite minerals, closure temperatures, elemental profiles, diffusion modeling, and equilibrium melt calculations. We show that multiple melt percolation events are required to explain the variable chemistry of peridotite-hosted pyroxenite veins, consistent with previous observations in the xenolith record. We argue that the Josephine ophiolite evolved in conditions intermediate between back-arc and sub-arc. Clinopyroxenites formed at an early stage of ophiolite formation from percolation of high-Ca boninites. Several million years later, and shortly before exhumation, orthopyroxenites formed through remelting of the Josephine harzburgites through percolation of ultra-depleted low-Ca boninites. Thus, we support the hypothesis that multiple types of boninites can be created at different stages of arc formation and that ophiolitic pyroxenites uniquely record the timing of boninite percolation in subduction zone mantle.

Dunites in the mantle section of the Oman ophiolite – The boninite connection

Dunites in the mantle section of the Oman ophiolite contain olivines which show both a wide range of compositions (Fo86.2 to Fo94) and very high magnesium numbers. These data are combined with experimental liquidus olivine-melt data to show that the range of olivine compositions requires that the dunites formed from a range of melt compositions and that some of these melts were very magnesian, with MgO concentrations up to MgO = 18–20 wt%. These observations are consistent with the finding of MgO-rich melt inclusions from chromitites in the mantle section of the Oman ophiolite (Rollinson et al., Lithos, 2018). It is proposed that the high Mg-olivines formed in equilibrium with high-Ca boninites, found in the upper section of the pillow lavas sequence of the ophiolite. A model is developed whereby high MgO boninites fractionated olivine through a process of melt-rock reaction/fractionation with the enclosing harzburgite to create a range of dunitic compositions and evolved boninitic lavas. Field evidence shows that the emplacement of boninites was late in the evolution of the Oman ophiolite indicating that the mantle dunites of boninitic origin formed late in the history of the ophiolite. High-Ca boninites form through the shallow, hydrous melting of the mantle wedge in a subduction setting and the presence of boninitic dunites and lavas further supports the view that the Oman ophiolite formed in a fore-arc setting through subduction-induced spreading.

Petrology and geochemistry of a boninite dyke from the western Bastar craton of central India

The Dongargarh Supergroup along with the basal Amgaon Gneissic Complex constitutes the northwestern part of the central Indian Bastar craton. In the present study, we report a new finding of a boninite dyke intruded in the Amgaon gneisses of this area. The dyke composed of mainly pyroxenes, amphiboles and subordinate amount of plagioclase. The higher contents of \(\hbox {SiO}_{2}\) (51–54 wt.%), MgO (12–14 wt.%), Ni (375–473 ppm), Cr (1416–1580 ppm) and very low \(\hbox {TiO}_{2}\) (0.2–0.4 wt.%) are consistent with the boninite nature of the dyke as well as the unevolved primary nature of the source magma. The extraordinarily high CaO content (15.97–17.7 wt.%) with higher \(\hbox {CaO/Al}_{2}\hbox {O}_{3}\) (3.13–3.96) ratios classifies it as high-Ca boninite. The trace element ratios including Zr/Ti, Ti/V, Ti/Sc and Ti/Yb further show its geochemical similarity with the Archaean boninite. The dyke also shows negative high-field strength element (Nb, Ta and Ti) anomalies which are the characteristics of the boninite rocks reported elsewhere and along with the enriched light rare earth element pattern, it shows more affinity particularly with the northern Bastar boninite dyke. The mineralogical and geochemical similarities of the boninite dykes from the Bastar craton indicate a widespread boninitic event during the Palaeoproterozoic having a similar origin. These boninite dykes indicate the preservation of subduction-related signatures in the lithospheric mantle beneath the Bastar craton at the time of its evolution or may be during the convergence of the Bastar and Bundelkhand cratons.

The role and conditions of second-stage mantle melting in the generation of low-Ti tholeiites and boninites: the case of the Manihiki Plateau and the Troodos ophiolite

High-Mg, low-Ti volcanic rocks from the Manihiki Plateau in the Western Pacific share many geochemical characteristics with subduction-related boninites such as high-Ca boninites from the Troodos ophiolite on Cyprus, which are believed to originate by hydrous re-melting of previously depleted mantle. In this paper we compare the Manihiki rocks and Troodos boninites using a new dataset on the major and trace element composition of whole rocks and glasses from these locations, and new high-precision, electron microprobe analyses of olivine and Cr-spinel in these rocks. Our results show that both low-Ti Manihiki rocks and Troodos boninites could originate by re-melting of a previously depleted lherzolite mantle source (20–25% of total melting with 8–10% melting during the first stage), as indicated by strong depletion of magmas in more to less incompatible elements (Sm/Yb 0.5). In comparison with Troodos boninites, the low-Ti Manihiki magmas had distinctively lower H2O contents ( 2 wt% in boninites), ~ 100 °C higher liquidus temperatures at a given olivine Fo-number, lower fO2 (ΔQFM + 0.2) and originated from deeper and hotter mantle (1.4–1.7 GPa, ~ 1440 °C vs. 0.8–1.0 GPa, ~ 1300 °C for Troodos boninites). The data provide new evidence that re-melting of residual upper mantle is not only restricted to subduction zones, where it occurs under hydrous conditions, but can also take place due to interaction of previously depleted upper mantle with mantle plumes from the deep and hotter Earth interior.

Primary Silica-rich Picrite and High-Ca Boninite Melt Inclusions in Pyroxenite Veins from the Kamchatka Sub-arc Mantle

Island arc picrites and boninites are magnesian magmatic rocks believed to be generated by high degrees of melting of depleted mantle sources fluxed by subduction-derived, volatile-rich components. These magmas can be probes of both the mantle wedge protoliths and subduction components, but are rare among other, usually more evolved, types of arc lavas. Furthermore, many arc picrites and boninites show evidence for late-stage differentiation prior to or during eruption, masking their primary, mantle-derived geochemical signatures. We report textural and chemical data on spinel-hosted melt inclusions of mantle origin in amphibole-bearing websterite veins cross-cutting spinel harzburgite xenoliths from the active andesitic Avacha volcano (south Kamchatka, Russia). The data are used to constrain the composition and origin of melts that formed the websterite veins in the sub-arc lithospheric mantle. The melt inclusions typically contain euhedral orthopyroxene and clinopyroxene and occasionally minor amphibole in silicate glass. The melt inclusions were homogenized using heating stages and gas-mixing furnaces. The homogenized glasses range from subalkaline primitive silica-rich picrite and high-Ca boninite (>15 wt % MgO, 48–54 wt % SiO2) to rhyolite. High-Ca boninite glasses have moderate volatile and low heavy rare earth element contents and elevated Cs, Rb, Ba, U, Sr, and Li abundances, with extremely high U/Th. In turn, the glasses display no negative spikes in the high field strength elements Nb, Ta, Zr, Hf, and Ti. We show that the silica-rich picrite and high-Ca boninite liquids in this study formed by high degrees of melting (>25%), at volatile under-saturation, of hybrid melt-depleted but silica-rich mantle sources at ≥1·5 GPa. The hybrid sources formed in two stages: first, by extraction of ∼15% melt from the convecting mantle to form a refractory protolith, which was subsequently enriched in silica via interaction with subduction-derived components prior to or during remelting in the mantle wedge. The subduction-derived components were enriched in fluid-mobile elements and probably oxidized. Overall, our results suggest that silica-rich picrites and high-Ca boninites can be primary melts in mature subduction zones and differentiate within the mantle wedge and the deep arc crust to form more evolved andesite magmas.

Platinum Group Elements (PGE) geochemistry of komatiites and boninites from Dharwar Craton, India: Implications for mantle melting processes

High MgO volcanic rocks having elevated concentrations of Ni and Cr are potential hosts for platinum group elements (PGE) owing to their primitive mantle origin and eruption at high temperatures. Though their higher PGE abundance is economically significant in mineral exploration studies, their lower concentrations are also valuable geochemical tools to evaluate petrogenetic processes. In this paper an attempt has been made to evaluate the PGE geochemistry of high MgO volcanic rocks from two greenstone belts of western and eastern Dharwar Craton and to discuss different mantle processes operative at diverse geodynamic settings during the Neoarchean time. The Bababudan greenstone belt of western and Gadwal greenstone belt of eastern Dharwar Cratons are dominantly composed of high MgO volcanic rocks which, based on distinct geochemical characteristics, have been identified as komatiites and boninites respectively. The Bababudan komatiites are essentially composed of olivine and clinopyroxene with rare plagioclase tending towards komatiitic basalts. The Gadwal boninites contain clinopyroxene, recrystallized hornblende with minor orthopyroxene, plagioclase and sulphide minerals. The Bababudan komatiites are Al-undepleted type (Al2O3/TiO2=23–59) with distinctly high MgO (27.4–35.8wt.%), Ni (509–1066ppm) and Cr (136–3036ppm) contents. These rocks have low ΣPGE (9–42ppb) contents with 0.2–2.4ppb Iridium (Ir), 0.2–1.4ppb Osmium (Os) and 0.4–4.4ppb Ruthenium (Ru) among Iridium group PGE (IPGE); and 1.4–16.2ppb Platinum (Pt), 2.8–19ppb Palladium (Pd) and 0.2–9.8ppb Rhodium (Rh) among Platinum group PGE (PPGE). The Gadwal boninites are high-Ca boninites with CaO/Al2O3 ratios varying between 0.8 and 1.0, with 12–24wt.% MgO, 821–1168ppm Ni and 2307–2765ppm Cr. They show higher concentration of total PGE (82–207ppb) with Pt concentration ranging from 13 to 19ppb, Pd between 65 and 180ppb and Rh in the range of 1.4–3ppb compared to the Bababudan komatiites. Ir, Os and Ru concentrations range from 0.6 to 2.2ppb, 0.2 to 0.6ppb and 1.4 to 2.6ppb respectively in IPGE. The PGE abundances in Bababudan komatiites were controlled by olivine fractionation whereas that in Gadwal boninites were influenced by fractionation of chromite and sulphides. The Al-undepleted Bababudan komatiites are characterized by low CaO/Al2O3, (Gd/Yb)N, (La/Yb)N, with positive Zr, Hf, Ti anomalies and high Cu/Pd, Pd/Ir ratios at low Pd concentrations suggesting the derivation of parent magma by high degrees (>30%) partial melting of mantle under anhydrous conditions at shallow depth with garnet as a residual phase in the mantle restite. The komatiites are geochemically analogous to Al-undepleted Munro type komatiites and their PGE compositions are consistent with Alexo and Gorgona komatiites. The S-undersaturated character of Bababudan komatiites is attributed to decompression and assimilation of lower crustal materials during magma ascent and emplacement. In contrast, the higher Al2O3/TiO2, lower (Gd/Yb)N, for Gadwal boninites in combination with negative Nb, Zr, Hf, Ti anomalies and lower Cu/Pd at relatively higher Pd/Ir and Pd concentrations reflect high degree melting of refractory mantle wedge under hydrous conditions in an intraoceanic subduction zone setting. Higher Pd/Ir ratios and S-undersaturation of these boninites conform to influx of fluids derived by dehydration of subducted slab resulting into high fluid pressure and metasomatism of mantle wedge.

Spinel-free dunites as a proxy to komatiitic melt activity in the mantle

High-Mg dunites, which are spinel-free or spinel-poor, occur as veins that cut across a pod of chromitite within the mantle section deep below the Moho transition zone of the northern Oman ophiolite (Wadi Rajmi). Only thick (>50cm) veins were considered in order to avoid the effects of subsolidus Mg–Fe2+ exchanges with the chromitite wall rock on the compositions of olivine. The spinel-free dunites contain especially high-Mg (Fo>93) olivines, whereas olivines in the spinel-poor dunites have lower Fo contents (Fo91–93). In addition, chromian spinels in the spinel-poor dunites display high values of Cr# up to 0.8 [Cr#=Cr/(Cr+Al) atomic ratio]. We found that these komatiitic dunites formed via an olivine fractional crystallization process, and not by replacement, and that the chromitites served as isolating capsules where melts were preserved from melt–rock reactions with peridotite minerals. The olivines show a continuous trend in their chemistry from the spinel-free dunites, through the spinel-bearing dunites, to the high-Ca boninites described from the northern Oman ophiolite (Fizh Block). This strongly suggests a genetic linkage between the high-Mg spinel-free or spinel-poor dunites and the high-Ca boninites, which respectively represent cumulates and the resultant fractionated melts of a primary komatiitic melt. In this paper, we challenge the prevalent models that rule out the genesis of komatiites in supra-subduction zones and shed light on the conditions of the mantle at an early stage of arc evolution.

Consequences of Channelized and Diffuse Melt Transport in Supra-subduction Zone Mantle: Evidence from the Voykar Ophiolite (Polar Urals)

The well-preserved, 6 km thick mantle section of the Voykar ophiolite in the Polar Urals contains numerous dunite bodies as well as dunite and pyroxenite veins within the host harzburgites. These rocks provide evidence of a composite asthenosphere-lithosphere history of partial melting, plastic deformation, multi-stage melt migration and melt-rock interaction.We investigated the petrology and geochemistry of multiple samples of the different mantle lithologies to define the sequence of mantle melting and melt migration events, as well as the composition of the percolating melts. Spinel harzburgites sampled far from dunite bodies and pyroxenite veins have fairly homogeneous bulk-rock, olivine and Cr-spinel compositions and are interpreted as residues after 14-16% of partial melting, most probably at a mid-ocean ridge. Near the contacts with the dunite bodies and pyroxenite veins, spinel peridotites demonstrate distinct compositional changes marking different stages of melt migration in a supra-subduction environment. At the earliest stage, which probably took place in the lithosphere-asthenosphere boundary of the forearc mantle at temperature between 1050 and 1200°C and a pressure of 1-1.7 GPa, the dunite bodies formed as a result of stress-driven focused melt flow. The latest stage melts moved in cracks under a conductive cooling regime within the lithospheric mantle section when it was horizontally displaced towards the trench. The trace element composition of the melts that migrated through the mantle section during dunite formation have geochemical characteristics like those of high-Ca boninites. The role of the slab-derived component progressively increased through time and late-stage, pyroxenite-forming melts were conspicuously rich in SiO 2 and H 2 O. These low-viscosity melts impregnated the surrounding harzburgites, modifying or obliterating their primary composition.

Boninites as windows into trace element mobility in subduction zones

Boninites are subduction-related rocks originating from re-melting of highly depleted mantle sources left after extraction of tholeiitic melts. Due to their depleted nature, the incompatible trace element inventory of boninites is virtually entirely inherited from slab components without a significant contribution from the refractory mantle wedge. Thus, boninites constitute an excellent window into processes controlling trace element mobilization at the slab–mantle wedge interface. In order to constrain the behaviour of trace elements in subduction zones with a special emphasis on high field strength elements, we analyzed low-Ca boninites and associated tholeiitic basalts from Cape Vogel, Papua New Guinea (PNG) and compare them with compositions of high-Ca boninites and associated tholeiitic basalts from Cyprus. High-precision HFSE (Nb, Ta, Zr, Hf, W) concentration data of the boninites and associated tholeiitic basalts were obtained by isotope dilution. Major, trace element, and Sr–Nd–Hf–Pb isotope compositions clearly document a significant contribution of slab-derived melts involved in the petrogenesis of the PNG boninites, whereas only fluid-like subduction components were involved in the petrogenesis of the PNG basalts and the Cyprus suite. Low-Ca boninites from PNG are derived from a more refractory mantle source (∼21% depletion) than the high-Ca boninites from Cyprus (∼11% depletion) and their respective tholeiitic precursors (<10% depletion). In agreement with the more depleted nature of their mantle source, boninites exhibit a significantly stronger overprint by slab components. High-precision HFSE data indicate that, in comparison to LILE, a somewhat lower but measurable mobilization of all investigated HFSE in both slab-derived fluids and melts is evident. Modelling calculations demonstrate that the subduction components dominate the LILE budget and also largely control LREE and HFSE abundances in the boninite sources. Notably, the increasing influence of slab-derived fluids results in a decrease of the negative Nb–Ta anomaly, most likely reflecting a similar mobility of Nb–Ta and LREE at higher pressures near the critical point of fluid-melt miscibility. Ratios of Zr/Hf and Nb/Ta in the melt-like slab components dominating in the sources of the PNG boninites were probably fractionated in equilibrium with garnet–amphibolitic mafic oceanic crust. HFSE ratios in the Cyprus boninites are best explained by dehydration of subducted pelagic sediments in the absence of Ti-rich phases such as rutile. Our results also confirm previous assertions that the mobility of HFSE decreases in the order Sb > W–Mo > Nb–Ta > Zr–Hf. Furthermore, Mo–W systematics may provide a potential novel tracer for the amount, composition and redox state of subducted pelagic sediments that contribute to the geochemical budget of intra-oceanic arc systems.

A new find of boninite dyke from the Palaeoproterozoic Dongargarh Super group: Inference for a fossil subduction zone in the Archaean of the Bastar craton, Central India

The Dongargarh Supergroup (DSG), a bimodal Large igneous province (LIP), is one of the Palaeoproterozoic greenschist facies-metamorphosed volcano-sedimentary belts in the Bastar craton of the Central Indian shield. Two contrasting models are in vogue for the generation of the mafic volcanics from the DSG - a continental rifting model and an arc related model. In this paper, we report the occurrence of a boninite dyke from the Bijli rhyolite Formation, which is the lower volcanic horizon in the Nandgaon Group of the DSG. The boninite dyke is characterised by high magnesium (MgO : 18.32-18.80 wt.%), primitive Mgnumber (Mg# > 80), abundance of silica (SiO 2 : 51.63-51.95 wt.%), high Ni (~369 ppm), Cr (~2703 ppm), extremely low titania (TiO 2 : 0.04 wt.%), enrichment of LREE over MREE and HFSE and pronounced negative anomalies in Nb, Ti and Zr on primitive mantle normalized multi-element plots. The Dongargarh boninite dyke is inferred to have been derived from a primary magma and shares geochemical characteristics of modern- as well as Archaean-boninites. It comes under the high-Ca boninite category and displays distinct geochemical traits compared to the so far reported boninites from the Bastar craton. Its petrogenesis necessitates a two stage-model involving a refractory mantle as well as fluids derived from subducted sediments. Crustal assimilation (contamination) or a direct plume-derived melt cannot account for its observed geochemical characters. Even though we cannot constrain the generation of the mafic volcanics of DSG vis-a-vis rifting vs convergence with the available data, the occurrence and geochemistry of the boninite dyke indeed demonstrates that this domain represents a fossil subduction zone.

Thick sections of layered ultramafic cumulates in the Oman ophiolite revealed by an airborne hyperspectral survey: Petrogenesis and relationship to mantle diapirism

Using the HyMap instrument, we have acquired visible and near infrared hyperspectral data over the Maqsad area of the Oman ophiolite (~ 15 × 60 km). This survey allowed us to identify and map the distribution of clinopyroxene-rich cumulates (inter-layered clinopyroxenites and wehrlites) whose occurrence was previously undocumented in this area. The cumulates reach several hundred meters in thickness and crop out at distances exceeding 15 km on both sides of the Maqsad former spreading centre. They occur either in mantle harzburgites, as km-sized layered intrusions surrounded by fields of pegmatitic dykes consisting of orthopyroxene-rich pyroxenite and gabbronorites, or at the base of the crustal section where they are conformably overlain by cumulate gabbros. These ultramafic cumulates crystallized from silica- and Mg-rich melts derived from a refractory mantle source (e.g. high Cr#, low [Al 2O 3], low [TiO 2]). These melts are close to high-Ca boninites, although, strictly speaking, not perfect equivalents of present-day, supra-subduction zone, boninites. Chemical stratigraphy reveals cycles of replenishment, mixing and fractional crystallization from primitive (high Mg#) melts, typical of open magma chambers and migration of inter-cumulus melts. The TiO 2 content of clinopyroxene is always low (≤ 0.2 wt.%) but quite variable compared to the associated pegmatites that are all derived from a source ultra-depleted in high field strength elements (HFSE). This variability is not caused by fractional crystallization alone, and is best explained by hybridization between the ultra-depleted melts (parent melts of the pegmatites) and the less depleted mid-ocean ridge basalts (MORB) parent of the dunitic–troctolitic–gabbroic cumulates making up the crustal section above the Maqsad diapir. We propose that, following a period of magma-starved spreading, the Maqsad mantle diapir, impregnated with tholeiitic melts of MORB affinity, reached shallow depths beneath the ocean ridge. This diapir induced melting of the formerly accreted and hydrothermally altered lithosphere. At this stage, these boninitic-like lithospheric melts crystallized as pegmatitic dykes. As the diapir continued to rise, the amount of MORB reaching shallow depths increased, together with the surrounding temperature, leading to the formation of magma chambers where the crystallization of layered cumulates became possible. These cumulates remained rich in pyroxene and devoid of plagioclase as long as the contribution of MORB-derived melts was moderate relative to the lithospheric-derived melts. As the contribution of MORB to the refilling of the magma chamber increased, gabbroic cumulates started to crystallize.

The Trinity ophiolite (California): the strange association of fertile mantle peridotite with ultra-depleted crustal cumulates

The Trinity ophiolite (California): the strange association of fertile mantle peridotite with ultra-depleted crustal cumulates

Boninites and Adakites from the Northern Termination of the Tonga Trench: Implications for Adakite Petrogenesis

Adakitic rocks were recovered by dredging from the northern termination of the Tonga Trench during the 1984 voyage of the R.V. Natsushima and the 1996 voyage of the R.V. Melville. These contain magmatic zircons that have been dated at 2� 5M a by U^Pb methods, indicating that they are contemporaneous with boninite magmatism previously described from this area. This is the first time adakites and high-Ca boninites have been reported from the same active tectonic setting. The Tonga adakites are classified as high-SiO2 adakites, and are compositionally consistent with an origin as partial melts of subducted Pacific oceanic crust and sediment. Geochemical modelling suggests that the adakites are not involved in the petrogenesis of the Tongan high-Ca boninites. However, the recovery of adakites and boninites from the termination of the northern Tonga Trench suggests that both magma types are related to the unique tectonic setting of this region, where a transition from steep subduction to a transform fault plate boundary has created a slab window with an associated slab edge. Boninites are generated as a result of hot Samoan plume mantle moving through the slab window and subsequently being fluxed by H2O-rich fluids from the subducting Pacific oceanic crust. The Tonga adakites, in contrast, result from the direct melting of the slab edge as a result of the juxtaposition of the subducting slab against hot mantle derived from the Samoan plume.

Juxtaposition of adakite, boninite, high-TiO 2 and low-TiO 2 basalts in the Devonian southern Altay, Xinjiang, NW China

We present petrographic and geochemical data on representative samples of the Devonian adakite, boninite, low-TiO 2 and high-TiO 2 basalts and associated rocks in the southern Altay areas, Xinjiang, NW China. These volcanic rocks mostly occur as tectonic blocks within suture zones between the Siberian and Junggar plates. Adakite occurs in the Suoerkuduke area ca. 40 km south of Fuyun, and actually represents a poorly-sorted massive volcaniclastic deposit, mostly consisting of a suite of hornblende andesite to pyroxene andesite. The geochemical features of the adakite suggest its generation by melting of subducted oceanic crust. Boninite occurs in the Saerbulake area ca. 20 km southwest of Fuyun, as pillowed lava or pillowed breccia. It is associated with high-TiO 2 basalt/gabbro and low-TiO 2 basalt. The boninites are metamorphosed, but contain relict clinopyroxene with Mg# (=100*Mg/(Mg+Fe)) of 90–92, and Cr 2O 3 contents of 0.5–0.7 wt% and chromian spinel with Cr/(Cr+Al) ratio of 0.84. The bulk rock compositions of the boninites are characterized by low and U-shaped REE with variable La/Yb ratios. They are classified as high-Ca boninite. The Cr-rich cpx phenocryst and Chromian spinel suggests that the boninites were formed by melting of mildly refractory mantle peridotite fluxed by a slab-derived fluid component under normal mantle potential temperature conditions. Basaltic rocks occur as massive flows, pillowed lavas, tuff breccia, lapilli tuff and blocks in tectonic mélanges. Together with gabbros, the basaltic rocks are classified into high-TiO 2 (>1.7 wt%) and low-TiO 2 (<1.5 wt%) types. They show variable trace element compositions, from MORB-type through transitional back-arc basin basalt to arc tholeiite, or within plate alkalic basalt. A notable feature of the Devonian formations in the southern Altay is the juxtaposition of volcanic rocks of various origins even within a limited area; i.e. the adakite and the boninites are associated with high-TiO 2 and low-TiO 2 basalts and/or gabbros, respectively. This is most likely produced by complex accretion and tectonic processes during the convergence in the Devonian–Carboniferous paleo-Asian Ocean between the Siberian and Junggar plates.

Geochemistry and petrogenesis of Neoarchaean high-Mg low-Ti mafic igneous rocks in an intracratonic setting, Central India craton: Evidence for boninite magmatism

The southern Central India craton (also known as Bastar craton) has experienced several episodes of mafic magmatism during the early Precambrian time. These mafic igneous rocks are emplaced as dyke and volcanic rocks. A set of mafic dyke swarm and few exposures of mafic volcanics, both emplaced in the Neoarchaean time, contain high-SiO2 (>52 wt%), high-MgO (>8 wt%), and low-TiO2 (mostly <0.5 wt%) and classified as boninites on IUGS classification scheme. Al2O3/CaO ratio classifies these high-Mg mafic igneous rocks into high-Ca and low-Ca (Type 3) boninites. High-Ca boninites show typical boninitic geochemical characteristics, whereas low-Ca variety shows geochemical characteristics similar to the high-Mg norites. Siliceous high-magnesium basalts (SHMB) show similar geochemical properties as observed for the high-Mg norites. Early Precambrian high-Mg norites are reported from the rift setting environment but, on the other hand, boninitic magmatism represents an enigmatic style of magmatism and commonly formed at convergent margin settings. Most boninites are Phanerozoic in age and rarely reported from the Archaean terrains. The established geological and tectonic setting of the Archaean Bastar craton clearly suggests existence of a stable continental rift environment in the region since the Archaean time. Thus, report of Neoarchaean boninite-like rocks in an intracratonic setting is an important feature noticed in the Archaean terrain of Indian peninsula. A highly refractory mantle source region is required to form boninitic magma. Refractory lithosphere may be result of extensive extraction of mafic magma during Mesoarchaean. The preservation of refractory reservoir beneath the Bastar craton for long period of time corroborates a high degree of crustal stability during the Meso-Neoarchaean time. Trace element modelling suggests that high-Mg mafic igneous rocks under study are product of different pulses of boninitic magma produced by ∼20-25% melting of a refractory mantle source.

EXPERIMENTAL HYDROUS PARTIAL MELTING OF HARZBURGITE AT SHALLOW PRESSURES: IMPLICATIONS ON HYDROUS LATE-STAGE MAGMATIC PROCESSES WITHIN THE MANTLE/CRUST TRANSITION OF THE OCEANIC LITHOSPHERE

EXPERIMENTAL HYDROUS PARTIAL MELTING OF HARZBURGITE AT SHALLOW PRESSURES: IMPLICATIONS ON HYDROUS LATE-STAGE MAGMATIC PROCESSES WITHIN THE MANTLE/CRUST TRANSITION OF THE OCEANIC LITHOSPHERE

The case for Archaean boninites

Rare Archaean light rare earth element (LREE)-enriched mafic rocks derived from a strongly refractory mantle source show a range of features in common with modern boninites. These Archaean second-stage melts are divided into at least two distinct groups—Whundo-type and Whitney-type. Whundo-type rocks are most like modern boninites in terms of their composition and association with tholeiitic to calc-alkaline mafic to intermediate volcanics. Small compositional differences compared to modern boninites, including higher Al2O3 and heavy REE (HREE), probably reflect secular changes in mantle temperatures and a more garnet-rich residual source. Whundo-type rocks are known from 3.12 and 2.8 Ga assemblages and are true Archaean analogues of modern boninites. Whitney-type rocks occur throughout the Archaean, as far back as ca. 3.8 Ga, and are closely associated with ultramafic magmatism including komatiites, in an affiliation unlike that of modern subduction zones. They are characterised by very high Al2O3 and HREE concentrations, and their extremely depleted compositions require a source which at some stage was more garnet-rich than the source for either modern boninites or Whundo-type second-stage melts. Low La/Yb and La/Gd ratios compared to Whundo-type rocks and modern boninites either reflect very weak subduction-related metasomatism of the mantle source or very limited crustal assimilation by a refractory-mantle derived melt. Regardless, the petrogenesis of the Whitney-type rocks appears either directly or indirectly related to plume magmatism. If Whitney-type rocks have a boninitic petrogenesis then a plume related model similar to that proposed for the modern Tongan high-Ca boninites might apply, but with uniquely Archaean source compositions and source enrichment processes. Second-stage melts from Barberton (S. Africa –3.5 Ga) and ca. 3.0 Ga rocks from the central Pilbara (Australia) have features in common with both Whundo- and Whitney-types, but appear more closely related to the Whitney-type. Subduction zone processes essentially the same as those that produce modern boninites have operated since at least ~3.12 Ga, while a uniquely Archaean boninite-forming process, involving more buoyant oceanic plates and very inefficient mantle-source enrichment, may have occurred before then.

Contents of Volume

There are four Neoproterozoic ophiolitic belts in southern Ethiopia—Megado, Kenticha, Moyale-El Kur and Bulbul. The mafic rocks, which form the bulk of these belts, are dominantly subalkaline, low-K, low-Ti tholeiitic basalts (LOTI). This study, for the first time, shows occurrences of boninites in the Moyale-El Kur Belt and in the Geleba area, in addition to the already known occurrence in the Megado Belt. The mafic rocks of the Bulbul Belt consistently exhibit high-Ti, high-K calc-alkaline basaltic geochemistry, similar to that reported from the Kenticha Belt. Several samples from the Geleba area and Moyale-El Kur Belt also show high-K, high-Ti calc-alkaline geochemistry.The REE patterns of the Megado low-Ti tholeiites are similar to arc-tholeiites of the Southern Sandwich Islands, whereas the Moyale tholeiites show patterns similar to back-arc tholeiites of the Scotia Sea Rise. The most distinctive features in spider diagrams of the LOTI include the selective enrichment of Sr and Ba, and the relative lack of enrichment for K, P, Zr, Ti, Ce, Sm±Y, similar to those of oceanic basalts from supra-subduction zone (SSZ) settings, where boninitic and tholeiitic magma mixing could occur.The boninites are dominantly high-Ca boninites and are more akin to tholeiites and boninites from known marginal basins, such as the Mariana fore-arc basin. The presence of boninites in association with low-Ti tholeiites in the Moyale-El Kur Belt suggests that this belt also represents an ophiolite sequence formed in a SSZ setting similar to the Megado ophiolite. Concentrations of most immobile elements (Ti, Nb, P, Ce, Zr, Th, V) in the Megado rocks are lower than in the Moyale rocks. This may imply that the magnitude of subduction was less during the formation of the Moyale ophiolite than in the case of the Megado ophiolite.On tectonic discrimination diagrams, the Megado and Geleba mafic rocks and the boninitic rocks of the Moyale-El Kur belt consistently show fore-arc basin affinity, whereas the Moyale-El Kur tholeiites plot into fore-arc basin and back-arc basin fields. The Bulbul mafic rocks consistently have calc-alkaline basaltic affinity, suggestive of a continental-arc setting. However, high-K calc-alkaline basalts are also known from a SSZ setting, such as the New Hebrides.The geochemical interpretation presented in this study, together with discussion of lithological association and geochronological and structural data, is used to decipher the tectonic evolution of the Neoproterozoic of southern Ethiopia.

Geochemistry of the mafic rocks of the ophiolitic fold and thrust belts of southern Ethiopia: constraints on the tectonic regime during the Neoproterozoic (900–700 Ma)

There are four Neoproterozoic ophiolitic belts in southern Ethiopia—Megado, Kenticha, Moyale-El Kur and Bulbul. The mafic rocks, which form the bulk of these belts, are dominantly subalkaline, low-K, low-Ti tholeiitic basalts (LOTI). This study, for the first time, shows occurrences of boninites in the Moyale-El Kur Belt and in the Geleba area, in addition to the already known occurrence in the Megado Belt. The mafic rocks of the Bulbul Belt consistently exhibit high-Ti, high-K calc-alkaline basaltic geochemistry, similar to that reported from the Kenticha Belt. Several samples from the Geleba area and Moyale-El Kur Belt also show high-K, high-Ti calc-alkaline geochemistry. The REE patterns of the Megado low-Ti tholeiites are similar to arc-tholeiites of the Southern Sandwich Islands, whereas the Moyale tholeiites show patterns similar to back-arc tholeiites of the Scotia Sea Rise. The most distinctive features in spider diagrams of the LOTI include the selective enrichment of Sr and Ba, and the relative lack of enrichment for K, P, Zr, Ti, Ce, Sm±Y, similar to those of oceanic basalts from supra-subduction zone (SSZ) settings, where boninitic and tholeiitic magma mixing could occur. The boninites are dominantly high-Ca boninites and are more akin to tholeiites and boninites from known marginal basins, such as the Mariana fore-arc basin. The presence of boninites in association with low-Ti tholeiites in the Moyale-El Kur Belt suggests that this belt also represents an ophiolite sequence formed in a SSZ setting similar to the Megado ophiolite. Concentrations of most immobile elements (Ti, Nb, P, Ce, Zr, Th, V) in the Megado rocks are lower than in the Moyale rocks. This may imply that the magnitude of subduction was less during the formation of the Moyale ophiolite than in the case of the Megado ophiolite. On tectonic discrimination diagrams, the Megado and Geleba mafic rocks and the boninitic rocks of the Moyale-El Kur belt consistently show fore-arc basin affinity, whereas the Moyale-El Kur tholeiites plot into fore-arc basin and back-arc basin fields. The Bulbul mafic rocks consistently have calc-alkaline basaltic affinity, suggestive of a continental-arc setting. However, high-K calc-alkaline basalts are also known from a SSZ setting, such as the New Hebrides. The geochemical interpretation presented in this study, together with discussion of lithological association and geochronological and structural data, is used to decipher the tectonic evolution of the Neoproterozoic of southern Ethiopia.