low-Ca Boninite 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.

Ultra-Refractory Peridotites of Phanerozoic Mantle Origin: the Papua New Guinea Ophiolite Mantle Tectonites

AbstractHarzburgites and dunites forming the base of the Late Cretaceous–Paleocene Papuan Ultramafic Belt (PUB) and Marum ophiolites of Papua New Guinea (PNG) are among the most refractory mantle peridotites on Earth. We present a new integrated dataset of major element, bulk plus mineral trace element and Re–Os isotopic analyses aimed at better understanding the genesis of these peridotites. The PUB harzburgites contain olivine (Fo92–93), low-Al enstatite (less than or equal to 0.5 wt. % Al2O3 and CaO), and Cr-rich spinel (Cr# = 0.90–0.95). The Marum harzburgites are less refractory with olivine (Fo91.9–92.7), enstatite (~0.5–1.0 wt. % Al2O3 and CaO), minor clinopyroxene (diopside), and spinel (Cr# = 0.71–0.77). These major element characteristics reflect equivalent or greater levels of melt depletion than that experienced by Archean cratonic peridotites.Whereas bulk-rock heavy rare earth element (HREE) abundances mirror the refractory character indicated by the mineral chemistry and major elements, large-ion lithophile elements indicate a more complex melting and metasomatic history. In situ olivine and orthopyroxene REE measurements show that harzburgites and dunites have experienced distinct melt-rock interaction processes, with dunite channels/lenses, specifically, showing higher abundances of HREE in olivine. Distinctive severe inter-element fraction of platinum group elements and Re result in complex patterns that we refer to as ‘M-shaped’. These fractionated highly siderophile element (HSE) patterns likely reflect the dissolution of HSE-rich phases in highly depleted peridotites by interaction with subduction-related melts/fluids, possibly high-temperature boninites.Osmium isotope compositions of the PNG peridotites are variable (187Os/188Os = 0.1204 to 0.1611), but fall within the range of peridotites derived from Phanerozoic oceanic mantle, providing no support for ancient melt depletion, despite their refractory character. This provides further evidence that highly depleted peridotites can be produced in the modern Earth, in subduction zone environments. The complex geochemistry indicates a multi-stage process for the formation of the PNG mantle peridotites in a modern geodynamic environment. The first stage involves partial melting at low-pressure (&amp;lt;2 GPa) and high-temperature (~1250°C–1350°C) to form low-K, low-Ti tholeiitic magmas that formed the overlying cumulate peridotite–gabbro and basalt (PUB only) sequences of the ophiolites. This is inferred to have occurred in a fore-arc setting at the initiation of subduction. Later stages involved fluxing of the residual harzburgites with hydrous fluids and melts to form replacive dunites and enstatite dykes and interaction of the residual peridotites in the overlying mantle wedge with high-temperature hydrous melts from the subducting slab to generate the extremely refractory harzburgites. This latter stage can be linked to the eruption of low-Ca boninites at Cape Vogel, and other arc-related volcanics, in a nascent oceanic island arc. Both ophiolites were emplaced shortly after when the embryonic oceanic island arc collided with the Australian continent.

Origin and tectonic implications of boninite dikes in the Shiquanhe ophiolite, western Bangong Suture, Tibet

Abstract The Shiquanhe ophiolite, hosting newly discovered boninite dikes, is located in the western part of the Shiquanhe-Jiali ophiolite sub-belt (SJO sub-belt) in the Meso-Tethyan Bangong-Nujiang suture zone (BNSZ), Tibet. The boninite dike samples are plagioclase-free, have a porphyritic texture with pyroxene phenocrysts, and a microcrystalline groundmass. Characteristically they have high MgO (7.24–8.59 wt%), SiO2 (54.02–61.49 wt%), low TiO2 (0.15–0.24 wt%), CaO/Al2O3 (0.42–0.58) and are enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs). They display slightly concave “spoon-shaped” REE pattern and positive anomalies in Zr and Hf as well as negative Nb, Ta, and Ti anomalies which are similar to the typical low-Ca boninite in Cape Vogel, Papua New Guinea and elsewhere. These characteristics indicate that the Shiquanhe boninite originated from a depleted mantle source replenished by subducted sediments after the extraction of tholeiitic melts. The mantle source of the Shiquanhe boninites is harzburgite, and its melting degree is estimated to be approximately 20–35%. About 87.5% of melts from the depleted harzburgite mixed with about 12.5% of molten subducted sediments, forming the Shiquanhe boninite dikes in a forearc environment. This study on newly discovered boninite dikes further provides crucial evidence of island arc affinity of SJO in the Bangong Meso-Tethys. It gives another example of the relatively rare examples of forearc ophiolites in the Precambrian to Phanerozoic orogenic belts around the Earth.

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.

Low-Ca boninite formation by second-stage melting of spinel harzburgite residues at mature subduction zones: new evidence from veined mantle xenoliths from the West Bismarck Arc

We report the abundances of major and lithophile trace elements and volatiles (H2O, Cl and S) in orthopyroxenite veins cutting mantle-derived, spinel harzburgite xenoliths from the active Ritter volcano in the West Bismarck Arc (Papua New Guinea). The veins preserve sulfide-bearing glass coexisting with crystals. The glass formed by the quench of residual liquids left after crystallisation of abundant orthopyroxene, variable amounts of clinopyroxene, and minor olivine and spinel from silicate melts intruding spinel harzburgite. The glass compositions range from low-Ti (≤ 0.2 wt% TiO2), intermediate to magnesian (2–5 wt% MgO) and high-Mg# (0.45–0.6) andesite to dacite, with Mg# = Mg/(Mg + Fet) where Fet indicates all Fe treated as Fe2+. The glass is depleted in moderately incompatible lithophile trace elements and contains 1.2 to 2.5 wt% H2O, 400–1800 ppm Cl and 100–400 ppm S. Thermo-barometric calculations indicate that the original vein-forming melts intruded the shallow sub-arc mantle lithosphere between 0.5 and 1 GPa, where they partially crystallised (50–60%) to form residual liquids during cooling from ~ 1200 °C down to ~ 650 °C. Our data and calculations show that West Bimarck orthopyroxenite veins are of high-temperature magmatic origin and bear no relation to the carrier magmas of the xenoliths. Instead, petrological modelling demonstrates that the veins formed from the intrusion of primitive low-Ca boninite (LCB) melts in the sub-arc mantle lithosphere. These melts were originally produced by low degrees of melting (≤ 5%) of spinel harzburgite at ≥ 1360 °C and ≤ 1.5 GPa. Some key features of the parental LCB vein-forming melts from West Bismarck (e.g. lithophile trace element signatures and moderate volatile abundances and oxygen fugacity) suggest that, contrary to the widely held belief, their formation does not require persistent mantle wedge fluxing by slab-derived components. Instead, we propose that adiabatic decompression melting of spinel harzburgite residues may also form LCB in the sub-arc mantle. West Bismarck veins are very similar to those previously described in mantle xenoliths from the Kamchatka Arc; active boninite magmatism in these mature arcs shows that the generation of these melts is not as restricted as previously thought. In fact, the growing evidence for the prevalence of boninite in sub-arc mantle peridotites is consistent with the presence of boninite-like geochemical components in many island arcs.

Variation in sub-arc mantle oxygen fugacity during partial melting recorded in refractory peridotite xenoliths from the West Bismarck Arc

It is debatable whether oxygen fugacity (fO2), the usual measure of the oxidation state of a system, can vary during partial melting in the Earth's mantle or not. Notably, samples of mantle peridotite recovered from lavas and pyroclastic deposits in island arcs are mostly more oxidized than those from other tectonic settings. However, the petrological history of sub-arc mantle peridotites, in particular the respective extents to which partial melting and post-melting metasomatism have controlled their fO2 record, are elusive. It has remained unclear whether the oxidized peridotites have preserved the oxidation state of a melt-depleted, residual mantle wedge or not.Here we report Mössbauer spectroscopy and EPMA measurements of Fe valence states in spinel (Fe3+/∑Fespinel where ∑Fe refers to Fe3++Fe2+) in a suite of markedly unaltered, sub-arc mantle-derived, harzburgite and dunite xenoliths from the active Ritter volcano (West Bismarck Arc, Papua New Guinea). These rocks, together with similar suites from the Kamchatka and Izu-Bonin arcs, have recently been interpreted to be residues of partial melting in the mantle wedge. The Fe3+/∑Fespinel decreases from 0.27 ± 0.04 to 0.11 ± 0.01 with increasing degrees of melt depletion in the West Bismarck sample suite, as monitored by decreasing Al2O3 (from 0.72 to 0.29 wt%) and modal percentage of orthopyroxene (from ~28 to ~7 wt%) in bulk rocks. Importantly, Fe3+/∑Fespinel in the most melt-depleted, orthopyroxene-poor residual samples are significantly lower (down to 0.11 ± 0.01) than those in melt-percolated harzburgite (0.29 ± 0.04) and dunite melt channel-cumulates (0.20 ± 0.01) found at the same sampling sites. The calculated fO2 in West Bismarck residual samples decreases from +1.7 ± 0.2 to −0.5 ± 0.2 log units relative to the synthetic fayalite-magnetite-quartz redox buffer (∆logfO2(FMQ)) with Al2O3 and orthopyroxene contents. The upper-end ∆logfO2 for the least melt-depleted, orthopyroxene-rich residual samples are consistent with those for sub-arc mantle harzburgite xenoliths from the Kamchatka and Izu-Bonin arcs recording similar melting degrees, but also those for more fertile lherzolite and harzburgite rocks from the northeastern Japan Arc. In turn, the most melt-depleted, orthopyroxene-poor residual samples have ∆logfO2 similar to the upper bound recorded in abyssal peridotites.Taking literature data into consideration, the fO2 spectrum recorded by the West Bismarck sub-arc mantle peridotite suite is modelled here by a two-stage partial melting process. The first-stage oxidation state is near-buffered from lherzolite to orthopyroxene-rich harzburgite by fluxed-melting with volatile-rich, slab-derived components at ca. FMQ + 0.5 to FMQ + 1.5 during the generation of high-partial melting degree, picrite-boninite-andesite oxidized liquids. The second-stage oxidation state is un-buffered during re-melting of residual harzburgite accompanying generation of low- to moderate-degree partial melts such as low-Ca boninite; these magmas preserve more variable fO2 extending to lower values (FMQ and below) owing to the progressive removal of Fe3+ from their sources with increasing melting degree. The second-stage melting event likely occurs during adiabatic decompression of residual spinel harzburgite to the uppermost mantle wedge.The data in this study support the general hypothesis that Fe3+/∑Fespinel and fO2 of residual peridotite (and of the melts formed at equilibrium) can vary during partial melting in the Earth's mantle. These results further provide direct source evidence for the controls of various subduction zone melting processes on the oxidation state of different types of primary arc melts. Melt depletion of mantle wedge sources can result in a progressive decrease in the fO2 of liquids subsequently extracted from these sources, but only in the absence of oxidized, Si- and volatile-rich components. These components are presumably derived from the subducted slab and effectively buffer fO2 during fluxed-melting. The observed fO2 variability in sub-arc mantle peridotites worldwide likely reflects the combination of fluxed- and adiabatic decompression melting in the mantle wedge.

Characterization of the accreted Ophiolite slices of Rutland island, Andaman sea: Evolution in a Suprasubduction zone setting

Detailed mapping of a rocks110 km 2 area of the Rutland Island has demarcated four dismembered thrust slices of Cretaceous ophiolite which are interleaved with Eocene sediments. The ophiolite sequence is represented by tectonite, chromitite pods, transitional peridotite, layered ultramafic-mafic, intrusives of plagiogranite-diorite suite and extrusives of basalt and boninite, capped by pelagic sediments. The layered ultramafics and the massive chromitites are characterized by high Mg-olivine (Fo 92-93 ), high Mg-orthopyroxene (En 88–90 ), low CaO (X Ca M2 = 0.01 to 0.03), low alumina (Al 2 O 3 = 0.66 to 1.20 wt %) and high Cr-chromites, which is typical of boninitic melts in suprasubduction zone (SSZ) environments. The estimated Al 2 O 3 contents of the parental melt (10.80 wt%) correspond to a boninitic parental melt. The Type-2 low-Ca boninite extrusives with SiO 2 < 58 wt%, MgO = 8.4 to 9.5 wt% is the extrusive equivalent of boninitic parental lava. As boninitic rocks show higher depletion of Zr and Y than the basaltic rocks, the source rock for boninite was more depleted than that of basalts. Tectonites showing embayed clinopyroxene grains and irregular shaped Cr-spinel as residual phases preserve records of partial melting. The degree of partial melting of the mantle tectonite has been estimated as ~ 20%, based on Cr content of the spinel. Petrographic features of the transitional peridotites show dissolution of orthopyroxene and clinopyroxene grains and development of small fresh olivine grains along the embayed margins and fractures, displaying melt-rock interaction effects in the mantle rocks. The melt-rock interaction features as well as the bivariant relation of TiO 2 wt% vs. Al 2 O 3 wt% of the chromites reflect the evolution of the Rutland ophiolite in a SSZ setting.

High-Mg Adakite and Low-Ca Boninite from a Bonin Fore-arc Seamount: Implications for the Reaction between Slab Melts and Depleted Mantle

DEPARTMENT OF RESOURCES AND ENVIRONMENTAL ENGINEERING, WASEDA UNIVERSITY, TOKYO 169-8555, JAPAN FUKADA GEOLOGICAL INSTITUTE, TOKYO 113-0021, JAPAN CENTER FOR NORTHEAST ASIAN STUDIES, TOHOKU UNIVERSITY, SENDAI 980-8576, JAPAN DEPARTMENT OF EARTH AND PLANETARY SCIENCE, TOKYO INSTITUTE OF TECHNOLOGY, TOKYO 152-8551, JAPAN GUANGZHOU INSTITUE OF GEOCHEMISTRY, CHINESE ACADEMY OF SCIENCES, GUANGZHOU 510640, CHINA KOCHI INSTITUTE FOR CORE SAMPLE RESEACH, JAPAN AGENCY FOR MARINE^EARTH SCIENCE AND

A new petrogenetic model for low‐Ca boninites: Evidence from veined sub‐arc xenoliths on melt‐mantle interaction and melt fractionation

Low‐Ca boninites (LCB) are arc‐related magmatic rocks enriched in large ion lithophile elements (LILE), light rare earth elements (LREE), Zr and Hf relative to medium to heavy REE (MREE‐HREE). These signatures are commonly attributed to a unique slab‐derived agent that metasomatized a depleted mantle source but their origin in such an agent remains enigmatic. We report andesite‐hosted refractory spinel harzburgite xenoliths from the Kamchatka arc, which contain a series of orthopyroxene‐rich veins; these veins range in thickness, the contents of clinopyroxene and amphibole and degrees of reaction with the host. Vein minerals in reaction zones with host harzburgites show progressive depletion in MREE‐HREE at constant Zr‐Hf and develop patterns (U‐shaped REE with Zr‐Hf spikes) that mimic those of LCB. Major and trace element modeling suggests that these veins (1) formed from a high‐temperature (≥1400°C), MgO‐rich (∼30 wt.%) and silicic (∼54 wt.% SiO2) initial melt, strongly depleted in Al2O3, TiO2and alkalis and (2) record fractionation of the initial melt to form hydrous liquids; the initial melt was equilibrated with metasomatized Kamchatka harzburgites and was similar in trace element abundances to island arc tholeiite (LREE‐depleted to flat LREE‐MREE patterns, low Nb and Ta but no significant Zr‐Hf anomalies, high LILE). We argue that primary magmas of LCB formed by fluid‐fluxed (LILE‐rich and [Nb, Ta]‐depleted fluid) melting of a cpx‐free, highly refractory (≥30% melt extraction) harzburgitic source similar to arc peridotites from Kamchatka and elsewhere. This peculiar source may explain the distinctive major element features of LCB primary magmas. We propose that the primary magmas of LCB develop their characteristic trace element patterns through fractionation and reaction with refractory peridotites in the mantle wedge. Slab‐related components alone may explain the high LILE in LCB but not their distinctive REE patterns and positive Zr‐Hf anomalies.

Petrology and geochemistry of the Andaman ophiolite: melt–rock interaction in a suprasubduction-zone setting

Abstract: The Andaman ophiolite occurs as thrust slices in the outer arc of the Andaman–Java subduction zone. This ophiolite preserves the mantle sequence, layered ultramafic–mafic rocks, intrusive and extrusive rocks. The mantle sequence is represented by serpentinized lherzolite and harzburgite, hosting dunite and chromitite pods. The low Cr-number (0.2–0.4), Cr-number–TiO 2 relation of the chromites, oxygen fugacity ( f O 2 ) values ( Δ log f O 2 (FMQ) = −1.90) and trace elements of mantle peridotites indicate a mid-ocean ridge basalt–suprasubduction-zone (MORB–SSZ) setting. The MORB mantle underwent a low degree of melting ( c . 10–15%) and interacted with the subduction-zone melts. Melt–rock interaction of the peridotites in a suprasubduction zone is demonstrated by the replacement of pyroxene grains by olivine grains (Fo 90 ), composition of chromites and oxygen fugacity ( Δ log f O 2 (FMQ) = –1.90 to +2.16, where FMQ is the fayalite–magnetite–quartz buffer). The chromite composition of chromitite pods (Cr-number 0.72–0.75), f O 2 levels and trace elements for layered peridotites, and occurrence of the extrusive rocks as low-Ca boninite and island arc tholeiitic (IAT) basalt indicate interplay of both boninite and IAT melts for the Andaman ophiolite. The MORB mantle of the subducting Indian plate accreted into the mantle wedge and then melting of the accreted mantle produced boninite melt at the first stage and tholeiitic melts at the second stage. Supplementary material: Sample co-ordinates from the Andaman ophiolite, mineral analysis data for all the samples, modal compositions of the units of the Andaman ophiolite and details of the laboratory analyses including details of the standard are available at http://www.geolsoc.org.uk/SUP18459 .

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.

Global Intracratonic Boninite-Norite Magmatism during the Neoarchean—Paleoproterozoic: Evidence from the Central Indian Bastar Craton

Although boninites and boninite-like rocks formed throughout Earth history and have been emplaced in different tectonic settings, they are most common in Phanerozoic settings where they are associated with subduction-related magmatism. However, Neoarchean—Paleoproterozoic noritic intrusions, which have bulk-rock geochemistry closely similar to boninites, occur in many Archean terrains, chiefly emplaced in intracratonic settings. Thus, noritic intrusions are probably derived from melts similar to those of boninites. The best examples of such boninite-norite associations occur in the Bushveld, southern West Greenland, East Antarctica, Wyoming, northwest Scotland, and the eastern Fennoscandian shield. A similar boninite-norite suite also is present in the central Indian Bastar craton. These high-Mg mafic igneous rocks were emplaced as dikes and volcanic rocks during Neoarchean—Paleoproterozoic time in an intracratonic rift. They contain high Si (>52 wt%), high Mg (>8 wt%), and low Ti (mostly <0.5 wt%) and are true boninites. Based on Al2O3/CaO ratios, these rocks may be classified as high-Ca and low-Ca boninites. The high-Ca rocks show typical boninitic geochemical characteristics, whereas the low-Ca variety shows geochemical characteristics similar to high-Mg norites. Bulk-rock compositions of these two varities of high-Mg igneous rocks suggest a comagmatic relationship through a high-magnesium mafic melt derived from a metasomatized, highly refractory mantle. Very close geochemical similarities exist between the Bastar and boninite-norite suites worldwide. Spatial and temporal correlation suggests that such magmatism occurred globally during the Neoarchean—Paleoproterozoic. Many Archean terrains were united as a supercontinent as expanded Ur and Arctica at this time, and its rifting gave rise to numerous mafic dike swarms, including boninite-norite.

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.

Depleted arc volcanism in the Alboran Sea and shoshonitic volcanism in Morocco: geochemical and isotopic constraints on Neogene tectonic processes

Samples of volcanic rocks from Alborán Island, the Alboran Sea floor and from the Gourougou volcanic centre in northern Morocco have been analyzed for major and trace elements and Sr–Nd isotopes to test current theories on the tectonic geodynamic evolution of the Alboran Sea. The Alborán Island samples are low-K tholeiitic basaltic andesites whose depleted contents of HFS elements (∼0.5×N-MORB), especially Nb (∼0.2×N-MORB), show marked geochemical parallels with volcanics from immature intra-oceanic arcs and back-arc basins. Several of the submarine samples have similar compositions, one showing low-Ca boninite affinity. 143Nd/ 144Nd ratios fall in the same range as many island-arc and back-arc basin samples, whereas 87Sr/ 86Sr ratios (on leached samples) are somewhat more radiogenic. Our data point to active subduction taking place beneath the Alboran region in Miocene times, and imply the presence of an associated back-arc spreading centre. Our sea floor suite includes a few more evolved dacite and rhyolite samples with ( 87Sr/ 86Sr) 0 up to 0.717 that probably represent varying degrees of crustal melting. The shoshonite and high-K basaltic andesite lavas from Gourougou have comparable normalized incompatible-element enrichment diagrams and Ce/Y ratios to shoshonitic volcanics from oceanic island arcs, though they have less pronounced Nb deficits. They are much less LIL- and LREE-enriched than continental arc analogues and post-collisional shoshonites from Tibet. The magmas probably originated by melting in subcontinental lithospheric mantle that had experienced negligible subduction input. Sr–Nd isotope compositions point to significant crustal contamination which appears to account for the small Nb anomalies. The unmistakable supra-subduction zone (SSZ) signature shown by our Alboran basalts and basaltic andesite samples refutes geodynamic models that attribute all Neogene volcanism in the Alboran domain to decompression melting of upwelling asthenosphere arising from convective thinning of over-thickened lithosphere. Our data support recent models in which subsidence is caused by westward rollback of an eastward-dipping subduction zone beneath the westernmost Mediterranean. Moreover, severance of the lithosphere at the edges of the rolling-back slab provides opportunities for locally melting lithospheric mantle, providing a possible explanation for the shoshonitic volcanism seen in northern Morocco and more sporadically in SE Spain.

小笠原群島父島に産する低Ｃａボニナイト系列岩の２つの組成変化トレンド

Chichijima, located in Ogasawara (Bonin) arc, Japan has been known as the type locality of boninite. Meanwhile, extremely evolved volcanics (SiO2>70 wt%) also occur together with the high-magnesian andesites. The evolved rocks have been considered as the products of differentiation from the boninitic parental magma by fractional crystallization processes. The differentiation processes are considered to produce low-Ca boninite through bronzite-andesite, dacite, quartz-dacite and quartz-rhyolite. Some of the quartz-dacites, however, would not be necessarily regarded as differentiates from the dacite magma. Quartz-dacites are enriched in SiO2 for the fractional crystallization products from dacite. Some quartz-dacites show mineralogical evidences of magma mixing, while such features can not be observed in the dacites. Quartz phenocrysts in the silica rich quartz-rhyolite exhibit euhedral form, but quartz-dacites have corroded quartz phenocrysts. The compositions of mafic pyroxene phenocrysts in quartz-dacites are similar to those in bronzite-andesites. Groundmass pyroxenes in some quartz-dacites include orthopyroxene and augite. In contrast, pigeonite and augite are ubiquitous groundmass pyroxenes in the quartz-free dacites. These chemical and petrographical features imply that the quartz-dacites have calc-alkalic affinities, whereas the dacites have tholeiitic affinities. The tholeiitic trend of the dacites might be formed by fractional crystallization under low fO2 conditions, judging from the crystallization of Fe-rich pigeonite phenocrysts, and the quartz-rhyolite would be produced from the dacite magma by fractional crystallization in the tholeiite suite. Mixing calculations between bronzite-andesite and quartz-rhyolite reproduce the observed chemical variations of quartz-dacites. Therefore, calc-alkalic trend of the quartz-dacites might be formed by internal magma mixing between the bronzite-andesite magma and the quartz-rhyolite magma.

Olivine-enriched melt inclusions in chromites from low-Ca boninites, Cape Vogel, Papua New Guinea: evidence for ultramafic primary magma, refractory mantle source and enriched components

The composition of primary magmas and their mantle sources can be successfully inferred from the study of melt inclusions trapped in spinel phenocrysts. This is particularly true in the case of severely altered rocks, in which spinel and spinel-hosted melt inclusions usually retain primary magmatic information. We report the results of the study of melt inclusions in high-Cr (Cr# 90–95), primitive (Mg# 65–78) spinel in the Palaeocene low-Ca boninites from Cape Vogel, Papua New Guinea. Melt inclusions are represented by the aggregate of skeletal olivine crystals in the residual glass. Raster beam electron microprobe analyses of melt inclusions demonstrated that they are broadly similar in composition to primitive orthopyroxene and to the most primitive boninites in this area, having (in wt.%): very high MgO (18–30), SiO 2 (53–61) and very low TiO 2 (0.04–0.19), Al 2O 3 (3–9), CaO (2–4), Na 2O (<0.9), K 2O (0.05–0.15) and CaO/Al 2O 3 (0.4–0.6). H 2O abundances in melt inclusions, analysed by an ion probe, are very high (1–2 wt.%), and they could have been even higher (∼3.5 wt.%) if the melts lost H 2O before crystallisation. Trace elements in melt inclusions, analysed by laser ablation ICPMS, have exceptional depletion in HREE (<1 PM) and significant enrichment in LREE over HREE (La/Yb 5–12), and Pb (Ce/Pb 2–12) and Zr (Zr N /Sm N 2–3.4) over REE. The compositions of melt inclusions correlate well with the compositions of host spinel showing the fractionation path of initial ultramafic melt. Cape Vogel primary melts could have originated from melting of extremely refractory hot (>1500 °C) harzburgitic mantle fluxed by subduction-related, H 2O-bearing enriched components. Trace element composition of these enriched components is estimated from melt inclusion compositions by mass balance calculations.

小笠原群島父島に産する高Ｃａおよび低Ｃａボニナイト

Chichijima, Bonin archipelago, is a Tertiary volcanic island approximately 1000 km south-south east of Tokyo, and has been known as the type locality of boninite. Two types of boninite are found in the Bonin archipelago: one is high-Ca boninite from the Mikazukiyama Formation (Mk-HCB) and the Marubewan Formation (Mr-HCB) in Chichijima, and another is low-Ca boninite from the Marubewan Formation (Mr-LCB), Anijima (Ani-LCB), and Mukojima (Muko-LCB). High-Ca boninite is distinguished by CaO content from dominant low-Ca boninite, and contains augite phenocrysts. The bulk MgO, Ni and Cr contents and the magnesian compositions of olivines and pyroxenes in Mk-HCB and Mr-LCB indicate their primitive nature. Bronzite phenocrysts in Mk-HCB and Mr-HCB are highly enriched in Wo molecule, implying existence of high-Ca primitive boninite magmas. Mr-HCB is a mixing product between a high-Ca boninite magma and a differentiate from Mr-LCB. Sr isotopic ratios and incompatible element ratios of Mr-HCB, Mr-LCB, Ani-LCB and Muko-LCB are nearly the same and therefore they could have been produced from the common source. In contrast, Mk-HCB has different Sr isotopic and incompatible element ratios from the others and then it may have been produced from different parental materials. Both REE model calculations and Cr-spinel compositions imply that the parental mantle of the Mr-LCB is more depleted than that of the Mk-HCB. The large enrichment of LILE and LREE in Mr-LCB compared to Mk-HCB suggests that Mr-LCB has been generated under more hydrous condition than Mk-HCB.