High Degrees Of Partial Melting Research Articles

Serpentinised Dunites of the Neoarchean Shimoga Greenstone Belt, Western Dharwar Craton, India: Insights on Ni‐PGE Mineralisation and Genesis

ABSTRACTUltramafic rocks of the Archean greenstone belts worldwide are potential hosts for Cu‐Ni, precious metal deposits like platinum group elements (PGEs) and gold. This study highlights the geochemical evidence and genesis of Ni‐PGE mineralisation in the ultramafic rocks of Shimoga greenstone belt of the Western Dharwar Craton, India. Petrographically, the studied rocks are identified to be serpentinised dunites, while their geochemical signatures indicate komatiitic affinity. Presence of disseminated sulphides and pronounced serpentinisation in these rocks suggest a combination of Type II (disseminated sulphides) and Type IV (post‐magmatic alterations) komatiite‐related Ni–Cu‐PGE deposits. Chondritic Al2O3/TiO2 (9.65–16.38) ratios, superchondritic Gd/YbN (1.3–1.6), depleted high‐field‐strength elements (HFSEs) (Zr, Hf), enrichment of LREE over HREE and negative Nb–Ta anomalies reflect the generation of parental melts from plume‐sourced Al‐depleted komatiites with significant crustal contamination during their emplacement. The major, trace element and PGE relationships (FeO vs. MgO, Cu vs. Pd, Ni/Cu vs. Pd/Ir) infer the origin of sulphur undersaturated primary melts through moderate to higher degrees of partial melting followed by crustal assimilation that led to PGE enrichment. These observations suggest their formation from melts derived from greater depths of the upper mantle (> 400 km) at high pressure (> 10 GPa), wherein, the mantle residue retained majorite garnet. The high Ni (avg. Ni = 6511 ppm) substantiated by high Kambalda ratio ([Ni:Cr] × [Cu:Zn] = 5.6), Ni/Cr ratios (> 1) with high concentration of PGEs (avg. ∑PGE = 3078 ppb) confirm the fertile/mineralised nature of the komatiitic source and potential Ni‐PGE mineralization in the ultramafic rocks of the Shimoga greenstone belt.

Tectonic evolution of the Early Paleozoic proto-East Asian continental margin: Inference from Paleozoic metamorphic rocks in the South Kitakami belt, northeast Japan

To constrain the incipient Pacific-type orogeny and tectonic processes in the Early Paleozoic proto-East Asian continental margin, the Motai–Matsugadaira–Yamagami (MMY) metamorphic rocks in the South Kitakami belt, northeast Japan were investigated. They are divided into two different types: amphibolite-facies rocks associated with serpentinite and blueschist-facies rocks associated with pelitic and psammitic schists. Three geochemical groups are identified from the MMY metamorphic rocks. Groups 1 and 2 resemble geochemical characteristics of mid-ocean ridge basalt and continental arc rocks, respectively. Group 3 exhibits considerable depletion of highly incompatible elements, which is caused by the high degree of partial melting of a hot mantle plume. The zircon U–Pb ages of Group 1 indicate that the protoliths experienced amphibolite-facies metamorphism soon after their formation in the Early Ordovician. Group 2 exhibits a coeval zircon U–Pb age with Group 1. The age distribution of detrital zircons in the MMY psammitic schists shows a peak of 500–400 Ma, the presence of Archean to Neoproterozoic zircons, and the youngest Late Devonian zircon. The following model is proposed for the tectonic evolution of the proto-East Asian continental margin: (1) the formation of an arc in the eastern margin of the South China craton in the Cambrian to Ordovician; (2) the subduction of a spreading axis and an oceanic plateau at the same time as the continental arc formation; (3) the consumption and subduction of arc materials by tectonic erosion; and (4) the formation of the Carboniferous accretionary complex and high-pressure metamorphic rocks under steady oceanic plate subduction. The proposed tectonic evolution model may also be applicable to equivalent Early Paleozoic rocks in southwest Japan.

Synchronous felsic volcanism and carbonate sedimentation as a setting for VMS deposits localization at the Salair terrane, NE Central Asian Orogenic Belt

The Salair terrane located in the northern part of the Central Asian Orogenic Belt (CAOB) contains many epithermal deposits including volcanogenic massive sulphide (VMS) deposits. The host rocks mainly consist of felsic volcanic rocks such as lavas, volcanic breccias and tuffs that associated with thick strata of carbonates. In this study, we present zircon U–Pb ages, whole rock geochemistry, and Nd isotope data from the volcanic rocks, and results of geochemical and isotope (Sr, C, O) studies of carbonates to constrain their age and petrogenesis, and to characterize the tectonic setting. Zircon U–Pb dating reveals that the ore-bearing felsic lavas have age of 519.3 ± 1.9 Ma, while the felsic tuffs have age of 516.0 ± 0.9 Ma. These volcanic rocks are characterized by high SiO2 and Na2O contents, enrichment in light rare-earth elements, remarkable negative Eu anomalies, and pronounced depletion in Nb, Ta, P, and Ti. They have depleted ɛNd(t) values ranging from +4.9 to +6.3, and young two-stage Nd model ages (from 0.82 to 0.64 Ga). The felsic volcanic rocks from the Salair terrane are interpreted as highly evolved I-type magmatic rocks that might be produced by high degree partial melting of juvenile lower crust without a significant contribution of ancient crust and without crustal reworking.Felsic volcanism was accompanied by the formation of thick strata of carbonates. These carbonates are marine limestones with Mg/Ca ratios less than 0.007, δ18O(SMOW) values from 17.1 to 23.8 ‰ and δ13C(PDB) values between –0.9 and +0.9. Their Sr isotope composition varies in a narrow range within 0.70844–0.70859 that interpreted as representing proxy for coeval seawater. These values are consistent with depositional ages of 520–510 Ma and confirms the synchronicity of the formation of carbonates and felsic volcanism. Based on the regional geology and geochemistry, the ore-host rocks of the Salair terrane were formed in the back-arc setting where the marine transgression occurred as a result of graben subsidence. It is important for better understanding epithermal deposits in the northern CAOB and might provide new insights about prospecting the VMS deposits in similar tectonic settings.

Platinum-Group Elements and Nb-Ta geochemistry of the Deccan basalts from Kumbharli and other Ghat sections and the Koyna Seismic Zone, Maharashtra (India): Crustal contamination and implication for sulfide saturation of the magma

The Deccan basalts from Kumbharli, Nadlapur, Bijaynagar, Hazarmacchi, and Surli Ghat sections (surface samples) and the Koyna borehole-7 (KBH-7, sub-surface samples) from Maharashtra (western India) demonstrate a two-stage crystallization process. Olivine and chromite were crystallized earlier in a shallow crustal magma chamber while clinopyroxene and plagioclase phenocrysts were crystallized in the sub-volcanic conduits forming the crystal mush, and carried to the surface by the evolved Fe-Ti-rich ascending basaltic magma. The high modal abundance of plagioclase at the top and clinopyroxene at the bottom part of the Kumbharli Ghat section is responsible for an increase in Sr and a decrease in Sc content towards the upper flows. The surface and sub-surface basalt samples contain Pd = 5–31 ppb, Pt = 4–15 ppb, Ir = 1–4 ppb, Ru = 1–3 ppb, and Rh = 3–5 ppb. Negative relations of Pd/Ir, Pd/Ru, and Pd/Rh with MgO from both sets of samples indicate partitioning of Ir, Ru, and Rh to early fractionated cumulus chromite (or olivine). Magnetite is a late crystallizing phase from the evolved Fe-Ti-rich basaltic magma. With an increasing modal abundance of magnetite, there is an increase of FeO total , TiO 2 , and V with Pd indicating fractionation of Pd by magnetite. Geochemical characters show that the surface basalt samples are less contaminated compared to the sub-surface KBH-7 basalts. High Nb (6.8–13.6 ppm) and low Ta (0.2–0.37 ppm) content and elevated Nb/Ta ratios of the surface samples from the ghat sections indicate the possible interaction of the basaltic magma with the carbonate metasomatized subcontinental lithospheric mantle (SCLM). On the other hand, a high degree of partial melting with limited crustal contamination can also elevate the Nb/Ta ratios of the basalts from the ghat sections. Deccan basalts from the study areas do not bear the signature of sulfide supersaturation and subsequent immiscible sulfide segregation. This is evidenced by their high chalcophile element concentrations (Ni = 79–103 ppm, Cu = 121–259 ppm) and ratios like Cu/Zr (≥ 1), Ni/MgO (13.7–21.8) with low Cu/Pd (2140–29938) compared to other Ni-Cu (PGE) sulfide mineralized continental flood basalts. The lack of crustal sulfur in the various contaminants is perhaps the main reason for this sulfide-undersaturated character of the Deccan basalts of the current study.

Early terrestrial and lunar anorthosites: Comparative geochemistry and evolutionary processes

In a paper in 1970, Brian Windley first recognised that early terrestrial and lunar anorthosites both have calcic plagioclase, and low TiO2and high CaO and Al2O3contents. Despite these similarities, the geochemistry of early terrestrial and lunar anorthosites has not been rigorously compared and contrasted. To this end, we compiled 425 analyses from 212 early terrestrial anorthosite occurrences and 306 analyses from 16 lunar anorthosite occurrences. This was supplemented by a compilation of plagioclase anorthite (An) contents and pyroxene Mg# from early terrestrial and lunar anorthosites. Early terrestrial anorthosites have lower whole-rock An contents but similar Mg# to lunar anorthosites. The CaO contents of lunar anorthosites are higher than those of early terrestrial anorthosites for a given MgO and Al2O3content, early terrestrial anorthosites have higher SiO2contents than lunar anorthosites at a given MgO content, and lunar anorthosites have higher Eu/Eu* anomaly ratios yet broadly similar La/Yb and Nd/Sm ratios than early terrestrial anorthosites. Some early terrestrial anorthosites have less fractionated chondrite-normalised rare earth element (REE) patterns and less prominent positive Eu anomalies than lunar anorthosites. Lunar anorthosites have higher plagioclase An contents, yet a similar range of pyroxene Mg# compared to their early terrestrial counterparts. Some early terrestrial anorthositesare more fractionated than some lunar anorthosites. Our interpretations imply that most early terrestrial anorthosites crystallised from basaltic parental magmas that were generated by high-degree partial melting of sub-arc asthenosphere mantle wedge sources that were hydrated by slab-derived fluids, with the remainder being associated with mid-ocean ridge and mantle plume settings. Some of the arc-related early terrestrial anorthosites were influenced by crustal contamination. In addition, early terrestrial anorthosites originated from partial melting of the mantle at various depths with variable garnet residua, whereas lunar anorthosites formed without any significant garnet residua. Lower plagioclase CaO contents and pyroxene Mg# in early terrestrial anorthosites can be explained by higher proportions of clinopyroxene and olivine fractionation in terrestrial magma chambers than in the lunar magma ocean where orthopyroxene and olivine fractionation occurred. Low TiO2contents in both terrestrial and lunar anorthosites reflect rutile and/or ilmenite fractionation.

From onset of Cadomian subduction to forearc-arc-continent collision: Insights from the Neoproterozoic Calzadilla Metaophiolite (SW Iberia)

ABSTRACT The Cadomian basement of SW Iberian Massif appears in the so-called Ossa-Morena Complex, which contains several allochthonous terranes that are key for deciphering the evolution of the Gondwana margin during the Neoproterozoic. In this context, the Calzadilla Ophiolite represents the remnants of an oceanic Ediacaran lithosphere formed by a boninitic mafic crust that overlies a serpentinized ultramafic section, the latter being the biggest outcrop of Cadomian serpentinites in Europe. In this work, the petrological and geochemical nature of the mantle section is analysed, as well as the tectonothermal evolution of both, the mafic crust and the ultramafic section. The algebraic analysis of whole rock geochemistry of the serpentinites indicates that the protolith corresponds to harzburgite. The geochemical fingerprint of the serpentinites, as well as the primary spinel crystals composition, shows that peridotites underwent high degrees of partial melting (up to 16%) in a forearc setting. Thermodynamic modelling and average PT methods were applied to serpentinites and mafic dykes from the ultramafic section as well as to amphibolites from the mafic section, both of which experienced progradation up to 4–5 kbar and 525–575°C. According to the data, the protoliths of the Calzadilla Ophiolite were formed in a peri-Gondwanan arc system developed during the subduction of oceanic crust below Gondwana. The roll-back of the subducting slab would induce the opening of the forearc basin at c. 602 Ma, protolith age of amphibolites from crustal section, with an extensive partial melting of the mantle wedge and the formation of boninitic-affinity magmas that constitute the mafic section of the Calzadilla Ophiolite. Metasomatism and serpentinization of the mantle wedge would occur via melt- and fluid–rock interactions. A compression-dominated stage at c. 540 led to the emplacement of the Calzadilla Ophiolite onto the Cadomian magmatic arc, with the development of the amphibolite-facies metamorphic imprint.

Magmatic (Cr-Ni-PGE) and secondary/hydrothermal (emerald-peridot-rodingite-nephrite jade) mineralization associated with mafic-ultramafic rock complexes of Pakistan

Mafic magmatism has repeatedly taken place in Pakistan throughout its geologic history from the earliest Proterozoic to Recent. Much of it, however, does not have associated metallic mineralization. This paper concerns mineralization associated with mafic-ultramafic plutonic rock occurrences in the western- and northern suture zones of Pakistan, which demarcate the collision boundary between the northwestern Indian plate and Eurasian blocks. We discuss the petrology of the ophiolites and basaltic magma-related chromite deposits, showings of PGE and Ni minerals, and secondary mineral deposits (emerald and peridot gems, rodingite, and nephrite jade) associated with mafic-ultramafic complexes of the two suture zones. Chromite with high Cr#s (68 to 85) occurs in fair quantity and is mostly considered to be genetically related to basaltic magma of subduction-related environments. PGE and nickeliferous minerals are insignificant in quantity; PGE's are formed by a higher degree of partial melting and melt-rock reaction, whereas Ni-sulphides are the product of serpentinization of the ultramafic rocks. Emerald occurs in several localities in the ISZ, whereas the peridot has so far been reported from only one locality. Emerald is associated with Mg-carbonate ± quartz ± fuchsite ± talc ± serpentine, and peridot (Fo 89-92 ) is associated with chrysotile ± magnesite ± talc ± magnetite ± ludwigite. Both are formed via hydrothermal alteration of ultramafic rocks through the introduction of hydrous fluids incorporating CO 2 , Be, B, K, Al, and possibly Na. In view of the undeformed nature of the emerald and peridot and the 23 Ma age of emerald formation, the mineralization appears to post-date deformation and may be of Late Oligocene-Early Miocene age. The metasomatising fluids were either derived from the subducting Indian plate crust underneath the Kohistan arc or were related to 23-26 Ma silicic magmatism in this area of the NW Indian plate. The nephrite jade in the ophiolite mélange is hosted in serpentinites and has disseminated grains of chrome spinel with elevated Fe/(Mg + Fe) ratios (0.10–0.18), implying low-grade metamorphism of ultramafic rocks (protolith of serpentinite) with loss of Al and introduction of SiO 2 during accompanying metasomatism. The rodingite in Dargai ophiolite could have formed by the action of seawater channeled through fractures and cracks in rocks. However, their mineral assemblages indicate that they formed as a result of the influx of Ca, Al, and Na-bearing hydrothermal fluids related to the intrusion of gabbros/sheeted dykes in the serpentinized ultramafic rocks, resulting in Ca-metasomatism.

Arc building and maturation of the Lombok Island, East Sunda Arc

Intra-oceanic subduction zones are major sites of crust–mantle material exchange and crustal growth; however, the processes responsible for the magmatic evolution and transformation of nascent arcs into mature oceanic arcs with thicker crust are debated. We present mineral chemistry, zircon UPb age and Hf isotopic data, and whole-rock chemical and Sr–Nd–Hf–Pb isotopic data for Cenozoic volcanic rocks on Lombok in the East Sunda Arc, Indonesia, to investigate their petrogenesis and arc maturation. SIMS zircon UPb analyses of lavas from southern Lombok yield an early Oligocene age of ∼30 Ma. The lavas can be divided into three groups based on their ages and geochemical characteristics: group I (late Eocene) and II (∼30 Ma) lavas from southern Lombok, and group III lavas (∼3–0 Ma) from northern Lombok. The lavas are characterized by a progressive enrichment in K2O and incompatible element contents, and NdHf isotopic compositions with time. Group I lavas are tholeiitic basalts with low K2O contents (0.15–0.17 wt%) and Indian-MORB like rare earth element (REE) patterns and NdHf isotopic compositions (εNd = +7.4; εHf = +14.5), enrichment in large-ion lithophile elements (e.g., Rb, U, and K), and typical forearc basalt (FAB) geochemical characteristics. Group II lavas are low-K basaltic andesites to dacites that yield flat chondrite-normalized REE patterns [(La/Sm)N = 0.9–1.4], uniform initial 87Sr/86Sr ratios (0.7038–0.7042), and positive εNd(t) (+5.9 to +6.8) and εHf(t) (+13.9 to +14.4) values. Group III lavas are typically medium- to high-K calc-alkaline basalt to andesite and are enriched in light REEs [(La/Sm)N = 1.6–3.3] with uniform initial 87Sr/86Sr ratios (0.7038–0.7042) and slightly low εNd(t) (+3.9 to +5.4) and εHf(t) (+10.5 to +12.5) values. Progressive enrichment in incompatible element contents and NdHf isotopic compositions over time, from FAB to high-K calc-alkaline lavas, reflect the maturation of the East Sunda Arc. Sr–Nd–Pb–Hf isotopic mixing models, along with trace element ratios (e.g., Ba/Th, La/Yb, Th/Yb), suggest that increasing amounts of sediments were incorporated into the mantle sources of the magmas from group I (∼0%) to group II (<0.5%) and group III (0.5%–1.0%) lavas. Partial melting models show that the formation of group I and II lavas can be reproduced by high-degree (∼10%) partial melting of spinel peridotite, whereas the group III lavas were formed by low-degree (∼5%) partial melting of spinel peridotite. We propose that the evolution of Lombok arc magmas was controlled mainly by the fluxes of subducted slab material, along with decreasing degrees of partial melting as a result of crustal thickening over time.

Petrogenesis of flood basalts and shield basanite from Mertolemariam- Abamineos area, northwestern Ethiopian plateau: Assessments for mantle source variations

Petrographic and geochemical data (major and trace elements) are presented for Oligocene flood basalts and Miocene shield lavas from the Mertolemariam-Abamineos area, northwestern Ethiopian Plateau to examine the petrogenesis of the erupted magmas and the nature of mantle source compositions. The Mertolemariam flood basalts have mainly aphyric and plagioclase phyric textures. The Abamineos shield lavas have sparsely clinopyroxene phyric to highly plagioclase-clinopyroxene phyric textures. Geochemical classification shows that the Mertolemariam Oligocene flood basalts are sub-alkaline in composition, whereas the Abamineos Miocene shield lavas are highly alkaline in composition. The Abamineos shield has a unique composition (i.e., basanites) compared with all other shield basalts (transitional to alkaline) in the northwestern Ethiopian Plateau. Major and trace element compositions display two distinct trends between Mertolemariam flood basalts and Abamineos shield basanites. Fractional crystallization, partial melting, and crustal contamination of a homogeneous mantle source cannot explain the compositional variations between the Mertolemariam flood basalts and the Abamineos shield basanites. The trace element composition suggests that the Mertolemariam Oligocene flood basalts were more likely generated from the mixing of OIB (mantle plume) and E-MORB (enriched asthenosphere) mantle components. The Abamineos Miocene shield basanites were derived from the mantle plume (OIB) component. LREE/MREE and LREE/HREE indicate that both groups possibly originated from a mantle source within the stability field of spinel and garnet. In comparison, the Mertolemariam flood basalts were formed by a higher degree of partial melting from relatively shallow depths than the Abamineos shield basanites. We propose a scenario that explains the magmatic genesis in the northwestern Ethiopian Plateau: volcanism initiated by the initial arrival of the mantle plume (OIB-like) beneath the lithosphere, which comes across a geochemically fertile and enriched MORB (E-MORB) mantle component in the upper asthenosphere. The hot mantle plume triggered melting of the fertile and enriched MORB, and then melting occurred in the plume (OIB-like) at depth in the stability field of spinel and garnet. Melts from the mantle plume (OIB-like) and E-MORB components mixed to produce sub-alkaline flood basalts during the Oligocene in the northwestern Ethiopian Plateau Subsequently, melts from the advanced upwelling mantle plume (OIB-like) produced Miocene shield lavas in the northwestern Ethiopian Plateau.

Geochemistry and zircon U-Pb geochronology of diabase, gabbros, anorthosites and ultramafic rocks from the South Andaman Island ophiolite suite on the South-Eastern margin of the Indian plate

The South Andaman Island is located on the southeastern fringe of the Indian plate. Tectonically, the islands are associated with the Burma-Andaman-Java subduction zone of the Indonesian arc system. Several Mesozoic ophiolitic exposures are preserved in the Burma-Andaman-Java subduction zone. This study reports new petrological, geochemical, and zircon U-Pb geochronological data for the diabases, gabbros, anorthosites, and harzburgites of the ophiolite suite exposed in the South Andaman Island. The diabase is composed of plagioclase and clinopyroxene exhibiting ophitic textures. Anorthosite mainly contains plagioclase (> 90 vol%), pyroxenes and Fe-oxides. Gabbros have plagioclases and clinopyroxenes occurring as the major minerals. Harzburgites consist of olivine, orthopyroxene, clinopyroxene and spinel. Geochemical investigation reveals that these rocks were formed through crystal fractionation and accumulation processes from partial melt generated from a spinel lherzolite source. Gabbro, diabase, and anorthosites are formed by ∼1–5% melting of the, whereas depleted harzburgites were generated by slightly higher degrees of partial melting (∼10–20%) of the spinel lherzolite source. Trace element modelling reveals the influence of mid-oceanic ridge basalt (MORB)-melt interaction and subduction-related mantle alterations.The anorthosite and gabbro samples yield zircon 206Pb / 238U weighted mean ages of 93.2 ± 1.0 Ma and 442.5 ± 4.0 Ma, 383.9 ± 3.3 Ma respectively. However, the harzburgite sample records multiple age population including Paleo-Mesoproterozoic grains ranging in age from 1216 to 1931 Ma as well as Neoproterozoic and Phanerozoic grains of 607 Ma, 594 Ma, 237 Ma, 105 Ma, and 85 Ma suggesting a long-lived supra subduction zone in the Burma-Andaman-Java subduction zone where the South Andaman ophiolite suite was formed, evolved, and was emplaced.

The hidden link between dry ankaramitic melts and giant alkalic-type epithermal Au deposits: The Lihir Island example

The magmatic controls on the formation of giant epithermal Au deposits genetically associated with alkaline magmatic systems are not well understood. The general model, which attributes the generation of alkaline magmas to low-degree partial melting of metasomatized lithospheric mantle in a post-subduction tectonic setting cannot easily explain why some of these magmas are highly fertile for Au mineralization while others are not. This knowledge gap hampers the effective exploration of this economically important deposit type. In this study, we focused on the alkaline magmatism on Lihir Island and the Conical Seamount, Papua New Guinea, that gave birth to the Ladolam deposit and investigated the composition of silicate melt inclusions (SMIs) hosted in primitive volcanic rocks. Our findings offer important and novel insights into the early and deep magmatic processes that controlled the ore fertility of the melts that produced the largest known alkalic-type epithermal Au deposit on Earth. Olivine- and clinopyroxene-hosted primitive SMIs reveal nepheline-normative ankaramitic melt compositions derived from high-degree partial melting of metasomatized clinopyroxene-dominated lithologies, presumably amphibole-bearing clinopyroxenitic cumulates and metasomatically overprinted lithospheric mantle, both produced during earlier active subduction. These ultra-calcic parental melts are relatively low in H2O, S, and Cl, but enriched in Cu and highly oxidized. Due to their low water contents, significant clinopyroxene crystallization over a narrow temperature interval drove melt evolution from ankaramitic to alkaline compositions by strongly increasing incompatible element concentrations. Despite initially low H2O concentrations, evidence for high-temperature degassing is present in the form of low-density single-phase fluid inclusions co-existing with SMIs in clinopyroxene. This degassing event led to a strong decrease of S concentrations in the melt, whereas Cl and chloride-complexing elements remained largely unaffected. We propose that high-temperature volatile saturation as consequence of significant clinopyroxene crystallization is a key process for subsequent epithermal Au mineralization and can explain the efficient separation of Au from Cu. The complex tectonic setting of Lihir Island, marked by a reversal of subduction polarity, a tear in the slab beneath the island, and trans-lithospheric extensional structures, facilitated the generation and ascent of highly oxidized and relatively dry ankaramitic melts that ultimately exsolved a vapor-like, S- and presumably Au-rich fluid phase and might thus represent a prerequisite for alkalic-type epithermal Au mineralization.

Calcium isotopic composition of the bulk silicate Earth: A komatiite perspective

Calcium (Ca) isotopic composition of the mantle has been previously estimated as δ44/40Ca. However, δ44/40Ca of the mantle records both its stable isotopic composition and excess/deficit in radiogenic 40Ca that can be produced by the decay of 40K. Here, we present the high-precision Ca isotopic compositions of 16 Archean komatiites from five representative localities (Barberton, Munro, Abitibi, Yilgarn, and Taishan). Due to the high degrees of partial melting that led to their formation, these komatiites are used here to simultaneously estimate ε40/44Ca, δ44/40Ca, and δ44/42Ca of the bulk silicate Earth. The komatiites exhibit a resolvable negative ε40/44Ca relative to SRM915a, with an average of −0.6 ± 0.1 (2SE, 95 c.i.% if not specified). Given the lack of significant differences among the samples and the non-zero value of ε40/44Ca, we suggest using δ44/42Ca to report the stable Ca isotopic composition in this study. Excluding one sample (SD6/400), which may have been affected by alteration, the remaining komatiites display relatively homogeneous δ44/42Ca, ranging from 0.39 ± 0.02‰ (2SE, N = 16) to 0.49 ± 0.02‰ (2SE, N = 10), defining a mean of 0.44 ± 0.02‰ (2SE, N = 15). The absence of correlations between δ44/42Ca and LOI, Sr/Nd, (Dy/Yb)N, and (La/Sm)N among the komatiites indicates that post-eruption alteration, the influence of residual garnet, as well as prior melt extraction from the mantle source have had a negligible influence on their stable Ca isotopic compositions. Based on available experimental petrological data and isotope fractionation factors, it is demonstrated that komatiites were insignificantly fractionated from their mantle sources (i.e., <0.02% in δ44/42Ca). Their δ44/42Ca represents the ambient mantle at the time of segregation. Accordingly, we suggest that the average δ44/42Ca and ε40/44Ca of the komatiites measured here are representative of the bulk silicate Earth. Our new estimate can serve as a reference point for Ca isotopes as a two-dimensional tracer for sources and geological processes.

Re-melting of the Neoproterozoic juvenile mafic lower crust: The origin of adakitic and non-adakitic felsic suites in the southwest margin of Yangtze Craton

Eocene-Oligocene adakitic and non-adakitic shoshonitic felsic intrusions from the southwest margin of the Yangtze Craton provide a crucial geological record for studying crust-mantle interactions at the craton boundary. Despite their significance, the petrogenesis and source properties of these intrusions remain debated. This study suggests that the adakitic and non-adakitic features of these intrusions result from the re-melting of Neoproterozoic juvenile lower crust, as evidenced by zircon U-Pb dating, geochemical analyses, and petrogenetic investigations. The Ninglang-Yongsheng shoshonitic felsic intrusions, characterized by high Mg#, MgO, Ni, and Cr contents, exhibit isotopic characteristics akin to mantle-derived magma and juvenile mafic lower crust. Their U-Pb ages span 32–36 Ma, and they show similar patterns in major and trace elements, rare earth elements (REEs), and isotopes, with enrichment in large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs) like Nb, Ta, and Ti. These intrusions also demonstrate significant fractionation between light and heavy REEs, with a modest negative Eu anomaly. Their (87Sr/86Sr)i and εNd(t) values range from 0.7060 to 0.7069 and − 4.4 to −1.5, respectively, while Pb isotopic compositions vary slightly.This paper, along with prior research, has developed a genetic model for the Ninglang-Yongsheng shoshonitic felsic intrusions. The adakitic quartz monzonite porphyries likely originated from high-degree partial melting of the juvenile lower crust, whereas the non-adakitic shoshonitic syenite porphyries resulted from lower-degree melting. The composition of the juvenile lower crust may be garnet amphibolite-pyroxenite. This suggests distinct compositional variations within the Ninglang-Yongsheng juvenile lower crust. Moreover, the region's shoshonitic felsic intrusions have assimilated mantle-derived magma. In conclusion, the juvenile lower crust on the Yangtze Craton's southwest margin displays zonal characteristics, with the Liuhe-Beiya area's lower crust showing higher potassium content and more enriched isotopic signatures compared to the Ninglang-Yongsheng region.

Petrological and geochemical characteristics of the Low- and High-Hf Nam Meng dioritoid, northwest Vietnam: Implication for the mantle partial melting, mixing, and magmatic differentiation

This paper investigated the petrological, elemental, and isotope geochemical characteristics of the Nam Meng dioritoid to clarify the magma source and process. The Nam Meng massif comprises large amounts of dioritoids (gabbro-diorite and quartz diorite) and lesser amounts of granitoids (granodiorite and granite). The Nam Meng gabbro-diorite is a porphyritic texture with fine-grained plagioclase, amphibole, K-feldspar, and phenocryst biotite. The Nam Meng quartz diorite comprises coarse-grained plagioclase, amphibole, biotite, quartz, and K-feldspar. The Nam Meng gabbro-diorite contains higher plagioclase and lower biotite and quartz than the Nam Meng quartz diorite. The variation in petrography and mineralogy with the negative correlation between SiO2 contents with Al2O3, MgO, Al2O3 + MgO, T-Fe2O3, and CaO contents suggest a magmatic differentiation process. All the Nam Meng dioritoids have low ACNK (mostly less than 1.0), total alkaline (Na2O + K2O ≤ 6 wt.%) with Na2O/K2O ≥ 1, and negative anomalies of Ta, Nb, and Ti. Combined with the U-Pb zircon age of 290 Ma (Hieu et al. (2017), the Nam Meng dioritoid is thought to be an I-type granitic rock formed in the subduction stage of the Indosinian amalgamation event. The low La/Yb ratios (1.14-3.21) suggest that the mantle wedge that was melted to form the Nam Meng magma had a spinel peridotite composition. The εNd (290 Ma) of the Nam Meng dioritoid is close to bulk earth silicate at 290 Ma (-4.43 to 2.34). The 87Sr/86Sr (290 Ma) of the Nam Meng dioritoid varies in a wide range from low to intermediate (0.6987 to 0.7088), and the calculated Nd model age of the Nam Meng dioritoid is 1,258 ± 47 Ma. The Sr and Nd isotope data suggest that the Nam Meng spinel peridotite was a result of mixing between an ancient ocean crust, EM1, and EM2 that occurred in a paleo-subduction zone at Meso-Neoproterozoic Rodinia supercycle. The Hf contents of the low-Hf and high-Hf Nam Meng dioritoid series are 0.38-1.02 and 2.60-8.08, respectively. The LILE/Hf and HSFE/Hf ratios in the low-Hf Nam Meng dioritoids are high and have a strongly negative correlation with Hf. On the other hand, those ratios in the high-Hf Nam Meng dioritoids are low and have a weakly negative correlation with Hf. The Hf contents positively correlate with the degree of partial melting of the mantle wedge in the subduction zone. Therefore, the low Hf, high LILE/Hf, and HSFE/Hf ratios of the Nam Meng dioritoids could be derived from a low degree of partial melting of the mantle wedge. In contrast, the high Hf, low LILE/Hf, and HSFE/Hf ratios of the Nam Meng dioritoids could be produced by a high degree of partial melting of the mantle wedge.

Voluminous magma formation for the 30-ka Aira caldera-forming eruption in SW Japan: contributions of crust-derived felsic and mafic magmas

Understanding the origin, assembly, and evolution of voluminous magma that erupts in catastrophic caldera-forming eruptions (CCFEs) is a community imperative. A CCFE of the Aira caldera at 30 ka discharged over 350 km3 of magma, which can be grouped into petrographically and geochemically distinct types: voluminous rhyolite, small amounts of rhyodacite, and andesite magmas. To further understand the magma plumbing system of the Aira CCFE, we examined the geochemical characteristics of whole rock and plagioclase from its eruptive deposits. The trace element and 87Sr/86Sr signatures recorded in the plagioclase phenocrysts of these magmas indicate that the three magmas were originally produced by partially melting an identical source rock, which was estimated to be a mafic amphibolite with an 87Sr/86Sr signature of ∼0.7055 that comprised the lower crust. Melting of mafic amphibolite produced both felsic and mafic magmas by low and high degrees of partial melting, respectively. The mafic magma assimilated uppermost crustal materials and crystallized to produce an andesite magma type. The andesitic magma consists of phenocrysts (∼39 vol%) and melt with a dacitic (∼70 wt% SiO2) composition. The felsic magma mixed with ∼10% of the andesite magma and crystallized, forming the rhyolite magma. The mixing between the andesite and rhyolite magmas before the Aira CCFE produced the rhyodacite magma. The 30-ka Aira CCFE magmas were generated only by melting two kinds of crustal materials with different geochemical characteristics and had geochemical variations due to different conditions of partial melting and mixing between various crustal melts. The lack of definitive evidence of the mantle component mixing with the Aira CCFE magmas suggests that the mantle-derived magmas worked only as a heat source for crustal melting.

Melting processes, cooling rates, and tectonic settings of the Neo-Tethyan mantle: the in-situ mineral chemical record

Peridotites in the Yarlung Zangbo ophiolite (YZO) have the potential to elaborate the mantle dynamics that operated below the Neo-Tethyan oceanic crust. Here, we measured major and trace element concentrations of minerals (e.g., clinopyroxene, orthopyroxene, olivine, and spinel) in YZO harzburgites and lherzolites to trace the origin and evolution of the YZO. The rare earth element (REE) patterns of clinopyroxenes in lherzolites and harzburgites plot within the field of abyssal and forearc peridotites, respectively, indicating that the latter experienced a higher degree of partial melting. Based on the relative extents of light (LREE) and heavy (HREE) REE depletions in clinopyroxene, we classified the lherzolites into two groups (I and II). The group I clinopyroxenes are depleted in HREEs and enriched in LREEs and MREEs compared to those of group II. Meanwhile, the REE patterns of harzburgite clinopyroxenes are identical to that of group II, although they display lower REE concentrations. Models based on REE abundances in clinopyroxene suggest that group I lherzolites experienced ~8–11% partial melting in the spinel domain, whereas group II lherzolites formed after 5% melting in the garnet domain followed by 6–9% and 9–16% partial melting in the spinel domain, respectively. These high degrees of melting may suggest that the harzburgites formed in a supra-subduction zone (SSZ) setting. Surprisingly, the REE-in-two-pyroxenes thermometer (TREE) shows that the YZO peridotites record high-temperature cooling at 991–1218 °C which is lower than the conditions below mid-oceanic ridges (MORs), but compatible with recent reports of high closure temperatures in forearc environments. Furthermore, our TREE results constrain the cooling rate of the YZO peridotites to ~0.01–0.5 °C/yr, overlapping with rates reported for SSZ, MOR, and abyssal peridotites. By integrating the petrological, geochemical, and thermal features, we tentatively interpret the group I lherzolites accreted in an Early Cretaceous forearc during subduction initiation, whereas the group II lherzolites and the harzburgites formed by further melting of the earlier lherzolites in a SSZ setting.

Crustal control on the petrogenesis of adakite-like rocks

Adakite-like rocks have attracted extensive interest as they record magmatic processes and mantle-crustal material recycling, and because they are associated with major porphyry Cu deposits. However, their petrogenesis is still under debate. In this study, we compiled ca. 7000 sets of whole-rock geochemical and Nd isotopic data of Phanerozoic adakite-like rocks worldwide from published literature, with the aim to characterize geochemically adakite-like rocks from subduction, collision, and post-collision settings. Statistical analysis highlights the different lanthanides + Y patterns from various types of juvenile and mature crust. Modelling of the statistically-processed data suggest that pure fractionation of amphibole and/or garnet cannot account for the lanthanides + Y patterns of adakite-like rocks, whereas partial melting of mafic protoliths is a possible mechanism. This study supports a adakite-like magma generation model by partial melting of the eclogitic lower crust. The melt was then mixed with depleted mantle-derived basaltic melt, and subsequently fractionated. Compared with the adakite-like magma formation from the juvenile crust, that from the mature crust is featured by higher degree of partial melting, greater depths and higher water content. Additionally, secular peaks of La/Yb value (which reflects the depth of magma source) of adakite-like rocks worldwide suggest that the period of magmatic activity in deep-level magma reservoirs could be used as the indicator of either the final stages of the Phanerozoic orogenesis, or craton activation by subduction.

Development of a cambrian back-arc basin in the North Qilian orogenic belt: New constraints from gabbros in Yushigou ophiolite

Introduction: The North Qilian orogenic belt, as the Northern branch of the original Tethys tectonic domain, is important for reconstructing the tectonic evolution of the ancient Tethys. However, the tectonic history of the North Qilian orogenic belt remains controversial. This study addresses this issue from a geochemical perspective.Methods: In this study, a comprehensive analysis of the geochronology, whole-rock geochemistry, clinopyroxene mineral geochemistry, zircon Ti crystallization temperature, and gabbromagma temperature and pressure in the Yushigou ophiolite of the North Qilian orogenic belt was conducted to provide constraints on its tectonic evolution.Results and Discussion: Laser ablation inductively coupled plasma mass spectrometry zircon U-Pb dating results reveal that the gabbros have ages of 519 ± 3 Ma and 495 ± 4 Ma, belonging to the Cambrian period. Most of the studied gabbros exhibited geochemical characteristics of tholeiitic basaltic rocks with normal mid-ocean ridge basalt and island arc tholeiite dual geochemical affinities. The gabbros are interpreted to have formed by a high degree of partial melting of the depleted mantle spinel lherzolite. These results suggest that the back-arc basin of the North Qilian tectonic belt may have evolved to a relatively mature stage from 519 to 495 Ma. Overall, this study contributes to our understanding of the tectonic evolution of the North Qilian orogenic belt through geochemical analyses.

High degree partial melting of the metasomatized mantle: A possible source for the Eocene-Oligocene porphyry Cu-Au-Mo deposits in Lut block, Eastern Iran

The small (<50 Mt) Eocene porphyry Cu-Au deposits of Maher abad and Khupic and the Oligocene Cu-Mo deposits of ChahShaljmi and DehSalam are attributed to the subduction of the Sistan ocean crust beneath the Lut block of eastern Iran. The calculated Ce4+/Ce3+ ratios of Dehsalm and ChahShaljami zircons are clustered around 10–50 and 75–155, and the values for Maher abad and Khupic are clustered around 120–300 and 50–150, respectively. The calculated logfO2 values indicate FMQ buffer conditions for Eocene and Oligocene magmas (ΔFMQ < 2; ΔFMQ −1.8 to + 0.6 ± ∼1.5 for ChahShaljami; ΔFMQ −1.3 to −0.4 ± ∼1.5 for Dehsalm, and ΔFMQ −3.2 to −2.1 ± ∼1.5 for Maher abad; ΔFMQ −2.6 to −1.3 ± ∼1.5 for Khupic), which never reached the ideal values of the worldwide giant to large porphyries (ΔFMQ < 4; HM buffer). The 30–60% partial melting of a slab (∼10–25% garnet) can explain ChahShaljami and Dehsalm, while it is > 60% partial melting of a slab (garnet > 50%) + ∼10% of metasomatized garnet-lherzolite (∼10% garnet) for Maher abad and Khupic. High partial melting of a slab has produced magma poor in copper in the magmatic subarc of the Lut block. Our zircon data and whole-rock geochemical characteristics of the Eocene and Oligocene ore-bearing intrusions along with the kinematics studies of the subduction zones are well consistent with a west-directed subduction system in eastern Iran. In such a system, a higher subduction rate > convergence rate causes deep steep subduction and in the continuation roll-back of the slab occurs in response to the opening and closing of the back-arc basin. This event caused the thinning of the Lut continental crust, mantle delamination and hot asthenospheric upwelling. The ascent of magmas resulting from the partial melting of a hydrous slab through a hot mantle and thinned continent caused widespread volcanism, and large amounts of water, sulfur and, volatiles were released to the atmosphere in an island arc regime during the middle-upper Eocene. The poor in metal low-volume magmas resulting from the high partial melting of a low water warm/hot deepest slab slightly metasomatized the mantle that subsequently amphibole-FC process with low contamination in the MASH zones and their emplacement during the transition from the island- to normal continental arc in the upper Eocene-Oligocene formed upper Eocene high-K calc-alkaline I/A-type (ferroan) arc magmatism and lower Oligocene adakite-like high-K calc-alkaline-Shoshonitic I-type (magnesian) arc magmatism, led to the formation of small Cu-Au and Cu-Mo porphyry deposits in Lut block, respectively. These events occur in a particular tectonic condition where subduction and collision were accompanied by rotation of the block affected by both Indo-Asian and Arabian-Eurasian collisions. Our study implies that W-directed subduction systems do not have favorable conditions for the formation of large porphyry deposits such as the ideal down-dip thermal gradient of the oceanic slab, water content, depth of dehydration, degree of partial melting of oceanic slab, and thickened continental crust.

A greenstone belt in southeast Tibet: An accreted middle–late Permian oceanic plateau

Studies of accreted oceanic plateau sections provide crucial information on their structures, compositions, and origins. We investigate the petrogenesis of ultramafic–mafic rocks in the Tangjia–Sumdo greenstone belt of southeast Tibet using petrography, whole-rock geochemistry, and U-Pb zircon geochronology. These rocks are divided into four groups based on geochemical characteristics that include depleted and tholeiitic mafic rocks, transitional mafic rocks, enriched and alkaline mafic rocks, and picritic ultramafic rocks. Depleted and tholeiitic mafic rocks have the oldest crystallization ages (∼272 Ma), followed by picritic ultramafic rocks (∼270 Ma), transitional mafic rocks (267–254 Ma), and enriched and alkaline mafic rocks (252–250 Ma). Hafnium and neodymium isotope ratios of depleted and tholeiitic mafic rocks (εHf(t) = +13.1–+16.9; εNd(t) = +6.9–+7.1), transitional mafic rocks (εHf(t) = +1.8–+16.9; εNd(t) = +0.8–+5.5), enriched and alkaline mafic rocks (εHf(t) = +0.5–+5.4; εNd(t) = −1.5 to +1.9) and picritic ultramafic rocks (εHf(t) = +14.9–+17.2; εNd(t) = +7.8–+9.0) are similar to those of N-MORB, E-MORB, OIB and depleted-type picritic mafic rocks in other oceanic plateaus, respectively. The geochemical characteristics of the depleted and tholeiitic mafic rocks suggest that they formed by partial melting of depleted spinel lherzolite in a mid-ocean ridge setting, whereas the picritic ultramafic rocks suggest a high degree of partial melting of depleted lherzolite in a hot mantle plume head. The transitional mafic rocks formed by partial melting of moderately enriched garnet lherzolite. The youngest rocks (enriched and alkaline mafic rocks) formed by partial melting of a more enriched garnet lherzolite (compared to transitional mafic rocks) at relatively low temperatures. We propose that the depleted and tholeiitic mafic rocks represent normal oceanic crust of the Sumdo Paleo-Tethys Ocean and the transitional mafic rocks, enriched and alkaline mafic rocks and picritic ultramafic rocks are the fragments of the oceanic plateau, which were related to middle–late Permian mantle plume activity in the Sumdo Paleo-Tethys Ocean. We further suggest that the majority of the Tangjia–Sumdo greenstone belt represents a middle–late Permian oceanic plateau that reflects a previously unrecognized middle–late Permian mantle plume.