Mafic-ultramafic Complex Research Articles

An Early Paleozoic Ultramafic Complex in the North Wulan Metamorphic Complex, North Qaidam: Contraints on the Nature of the Alaskan‐type Continental Arc Root

AbstractOrogenic peridotite is an important component of orogenic belts and retains crucial information on mantle magmatic activity, slab subduction, and melt or fluid metasomatism. To determine the source of the mantle‐derived parental magma of the peridotite and to investigate the metasomatism that it experienced, we undertook an integrated study of the petrography, whole‐rock major‐ and trace‐element compositions, in situ zircon U–Pb geochronology, and mineral major‐ and trace‐element compositions of an early Paleozoic ultramafic complex in the North Wulan area of North Qaidam. The Halihatu ultramafic–mafic complex is composed of dunite, pyroxene peridotite, and gabbro, which are characteristic of Alaskan‐type complexes. The dunite yields a weighted mean 206Pb/238U age of 479 ± 5 Ma (MSWD = 0.7), which reflects the age of the metasomatism rather than the crystallization age of the ultramafic magma. The peridotites have high Mg# (89.8–91.8) and Cr contents (2419–5190 ppm), low Al2O3 (0.20–1.68 wt.%) and Ni (289–1012 ppm) contents, and high olivine Fo contents (87–91), suggesting a large degree (~15%–22%) of partial melting of lithospheric ultramafic rocks followed by variable degrees of fractional crystallization of olivine and pyroxene. This is consistent with estimates of 15%–22.3% partial melting calculated using the Cr# of spinel crystals and with the low Yb (0.04–0.33 ppm) and Y (0.72–1.29 ppm) contents of clinopyroxene crystals. Whole‐rock trace‐element patterns show enrichment in large ion lithophile elements and depletion in high field strength elements, along with high Al2O3 (2.10–6.47 wt.%) and low TiO2 (0.01–0.21 wt.%) contents of clinopyroxene crystals, suggesting an arc magma cumulate trend. These features, along with the high olivine Fo contents (87–91 ppm), imply that the Halihatu peridotite is an Alaskan‐type crustal cumulates derived from Mg‐rich hydrous basaltic melts. The high estimated fO2 (FMQ +1.97 to FMQ +3.81) further supports the idea that they formed in an arc setting. The Ni/Co and Ni/Mn ratios and cumulate textures of the olivine, quenched boundaries between mafic and felsic melts, and the occurrence of tremolite and phlogopite reflect interactions between the Halihatu peridotite and injected silicate and carbonatitic melts in the lower crust. Therefore, we propose a new cumulate‐infiltration model for the petrogenesis of Alaskan‐type ultramafic complexes, which improves our understanding of the nature of Alaskan‐type continental arc root.

Geochronology, geochemistry and Sr-Nd isotope compositions of the Niubiziliang mafic-ultramafic complex in the South Altyn orogen, NW China: Implications for Ni-Cu mineralization and tectonic setting

The Niubiziliang mafic-ultramafic complex located in the South Altyn orogen consists of lherzolite, plagioclase-bearing lherzolite, olivine websterite, and gabbro. The intrusion contains a newly discovered zone of magmatic Ni-Cu sulfide mineralization, which is categorized into three units (Nos. 1, 2, and 3) based on discernible morphological features, with drilling investigations revealing sulfide mineralization in complexes Nos. 2 and 3 in both mafic and ultramafic rocks. The rocks of the Nuibiziliang complex have olivines with Fo 76.9–82.4 and Ni contents between 79 and 1957 ppm. The clinopyroxenes have variable Al2O3 contents between 0.52 and 5.05 wt%, TiO2 contents between 0.02 and 1.30 wt%, and NaO contents from 0.08 to 1.27 wt%. The geochemical variations are consistent with the intrusion belonging to a subalkaline magma series and the major element variations are consistent with igneous differentiation from a single magma type. The rocks display enrichment in light rare-earth elements and large-ion lithophile elements (K, Ba, and Sr) and depletion in high-field-strength elements (Ti, Zr, Hf, Ta, and Nb) and heavy rare-earth elements, which are typical features of enriched mid-ocean ridge basalt (E-MORB). These rocks exhibit enriched Sr–Nd isotope compositions with low (87Sr/86Sr)i ratios and mostly low positive εNd(t) values, indicating that the Niubiziliang complex originated from the partial melting of the asthenospheric mantle with up to 10% contamination by continental crust. Fractional crystallization enriched sulfur in the magma during the initial stage of magmatic evolution, but crustal contamination was the main factor causing sulfide saturation in the magma. Laser ablation inductively coupled plasma-mass spectrometry U-Pb dating of baddeleyites in the ultramafic rocks of the Niubiziliang complex yielded an age of 452±12 Ma for the crystallization of the magma. This age is similar to the age of eclogites formed during exhumation for the South Altyn orogen and indicates that the mafic-ultramafic magmatism formed in a rift setting by a process of break-up of an oceanic crustal slab at depth in the mantle.

Predicting olivine formation environments using machine learning and implications for magmatic sulfide prospecting

Abstract Global volcanic and plutonic olivines record the compositional characteristics and physicochemical conditions of the parental magmas. Thus, they have significant potential for use as petrogenetic discriminators of the olivine formation environment and prospecting indicators for potential host rocks of magmatic sulfide deposits. Several data visualization approaches have been proposed by researchers to determine olivine origins. However, they can only discriminate specific olivine populations and require the incorporation of trace elements for which data are lacking globally. In this study, a machine-learning method consisting of the random forest algorithm and the synthetic minority oversampling technique (SMOTE) is used to discriminate the crystallization environments of olivine and predict the sulfide potential of olivine-bearing mafic-ultramafic intrusions. We employ a global data set of 24 341 olivine samples from 12 environments to determine the contents of MgO, FeO, Ni, Ca, Mn, and Cr and the Fo number [100 × Mg/(Mg+Fe)]. The results indicate that the proposed method can classify olivine into genetically distinct populations and distinguish olivine derived from mineralized intrusions from that derived from sulfide-barren intrusions with high accuracies (higher than 99% on average). We develop a dimensionality reduction algorithm to visualize the olivine classifications using low-dimensional vectors and an olivine classifier (accessible at http://101.33.204.62:8080/olivine_web/main.html, China University of Geosciences, Beijing). The model is used successfully to identify the contributions of distinct sources to regional magmatism using olivines from the late Permian picrite and basalt along the western margin of the Yangtze block (SW China) and to predict the sulfide potential of the newly discovered Qixin mafic-ultramafic complex in the southern Central Asian Orogenic Belt (NW China). The findings suggest that the proposed approach enables the accurate identification of olivine origins in different formation environments and is a reliable indicator suitable for global Ni-Cu-platinum group element (PGE) exploration.

Pd,Hg-Rich Gold and Compounds of the Au-Pd-Hg System at the Itchayvayam Mafic-Ultramafic Complex (Kamchatka, Russia) and Other Localities

The unique minerals of the Au-Pd-Hg system in gold grains from heavy concentrates of the Itchayvayam placers and watercourses draining and ore samples of the Barany outcrop at the Itchayvayam mafic–ultramafic complex (Kamchatka, Russia) were investigated. Gold grains from watercourses draining and heavy concentrates of the Itchayvayam placers contain substitution structures formed by Pd,Hg-rich low-fineness gold (Au0.59–0.52Pd0.24–0.25Hg0.17–0.23, 580‰–660‰) and Pd,Hg-poor high-fineness gold (Au0.94–0.90Pd0.02–0.04Hg0.03, 910‰–960‰). Potarite (PdHg) without and with impurities (Au < 7.9, Cu < 3.5, Ag < 1.2 wt.%), Ag-poor high-fineness gold (Au0.91Ag0.09, 950‰), Ag,Pd,Hg-bearing middle-fineness gold (Au0.75Ag0.08Pd0.09Hg0.08—Au0.88Ag0.09Pd0.02Hg0.01, 820‰–930‰), and Pd,Hg-rich low-fineness gold with minor contents Ag and Cd (Au0.51–0.55Pd0.25–0.22Hg0.21–0.16Ag0.03–0.06Cd0.01, fineness 580‰–630‰) were observed as individual microinclusions in the ore samples of the Barany outcrop. XRD and EBSD study results show that the Pd,Hg-rich low-fineness gold is isotypic to gold and has the same structure type, but different cell dimensions. According to data obtained for the Itchayvayam and some deposits and ore occurrences with Pd,Hg-bearing gold, the stable ternary phases and solid solutions of the following compositions in the Au-Pd-Hg system have been identified: Pd,Hg-poor gold (Au0.94–0.90Pd0.02–0.04Hg0.03), Pd,Hg-rich gold (Au0.59–0.52Pd0.24–0.25Hg0.17–0.23), Au-potarite (PdHg0.62Au0.38—Pd1.04Hg0.96—Au0.80Pd0.68Hg0.52), and Au,Hg-bearing palladium (Pd0.7Au0.3Hg0.1). The genesis of Pd,Hg-rich gold is insufficiently studied. We supposed that the meteoric waters or low-temperature hydrotherms rich in Pd and Hg could lead to the replacement Pd,Hg-poor gold by Pd,Hg-rich gold. High concentrations of Pd in Pd,Hg-bearing gold indicate a genetic relationship with mafic–ultramafic rocks.

Reworking of aged mafic crust in a Palaeoarchaean layered complex inferred from coupled Sm-Nd and Lu-Hf isotope systematics, Stolzburg Complex, Barberton Greenstone Belt, South Africa

The Stolzburg Complex is a prominent and well-preserved Palaeo- to Mesoarchaean layered ultramafic-mafic complex in the vicinity of the Barberton Greenstone Belt. Whole rock Sm-Nd, Lu-Hf and 142Nd isotopic data obtained from a variety of lithologies, and augmented by major and trace element geochemistry, are presented to examine petrogenesis, magma source composition, and mantle differentiation as well as the geodynamic setting of emplacement. Geochemically, all samples are characterized by unfractionated normalized trace element patterns (La/Luchond. = 0.7–2.3, La/Gdchond. = 0.6–1.7, Sm/Luchond. = 0.9–1.4) associated with weak HFSE anomalies (Nb/[0.5Th + 0.5La]PRIMA = 0.5–2.3). Trace element concentrations vary as a function of mineralogy, but differences in trace element compositions are unsystematic across lithologies. The coherence of trace element characteristics is consistent with a common mantle source that underwent moderately large degrees of melting at mantle pressures within the spinel stability field. Whole rock Sm-Nd and Lu-Hf data yield well-defined apparent isochrons corresponding to ages of 3367 ± 62 (n = 12, MSWD = 1.5) and 3396 ± 36 (n = 11, MSWD = 2.1), respectively. Initial Hf and Nd isotopic compositions vary from +3.3 to +5.7 and + 0.9 to +1.7, respectively, indicative of derivation from slightly to moderately depleted mantle source(s). Isochron ages and a regressed initial Hf isotopic composition of +4.0 ± 0.9 (at 3.4 Ga) contrast with an established U-Pb emplacement age (zircon, titanite) of 3.25 Ga and εHf values for zircon of −2.5 to +3.0. Hydrothermal alteration or weathering, incorporation of evolved pre-existing (continental) crust and fractionation by high-pressure phases prior to melt extraction can all be ruled out to have significantly affected isotope systematics. Protracted magma chamber activity and non-synchronous emplacement of unrelated gabbroic magmas also fail to fully account for the discrepancy between whole rock and mineral age and isotopic data. Instead, the coherence in apparent isochron ages and variability of initial Hf and Nd isotopic composition is adequately explained by reworking (partial or near-complete remelting) of aged mafic-ultramafic (likely oceanic) crust in staging chambers with compositions indistinguishable from newly formed magmas. Thus, apparent isochrons represent mixing lines and do not record timing of crystallization. A subset of samples was also analyzed for 142Nd isotopic compositions to trace preserved mantle heterogeneities caused by early Archaean crust-mantle differentiation. The results (μ142Nd = −2.3 ± 2.2 to +1.7 ± 2.1) are not resolvable from the modern mantle value, indicating that 142Nd-enriched or depleted mantle reservoirs had not remained isolated from convective homogenization in the asthenosphere, or the volume of mantle underlying the layered complex was too small to sample mantle heterogeneities. The Stolzburg Complex was possibly emplaced in ancient oceanic lithosphere, followed by pervasive but inhomogeneous Ca-metasomatism as documented by exposed rodingites.

Integrated subsurface investigation for magmatic sulfide mineralization in Betul Fold Belt, central India

Magnetotelluric (MT), Electrical Resistivity Tomography (ERT), Time-Domain Induced Polarization (TDIP), Geochemical and Geological studies followed by drilling and down-hole logging were undertaken with in the Betul Fold Belt (BFB) in Central India, to demarcate zones of magmatic Ni-Cu-PGE sulfide mineralization. The BFB is predominantly composed of circular to elongate gabbro bodies of the Padhar Mafic-Ultramafic Complex, intruded into a sequence of bimodal volcanic rocks and quartzites. Near-surface samples of ultramafic rocks were subjected to precise geochemical analysis and scanned by an electron microscopy with an energy dispersive spectrometer (SEM-EDS). This work indicated the presence of pyrite, pyrrhotite, chalcopyrite, pentlandite, and minor amounts of W-Cd bearing boweiite and palladenite assemblage. These minerals are regarded as favorable to the occurrence of Ni-Cu-PGE sulfide mineralization. MT data derived from two profiles were analyzed and modeled using 2D and 3D inversion algorithms. The robust conductivity anomalies obtained from the MT model have been interpreted coupled with electrical tomography, geology, and geochemistry data. The near-surface shallow depth conductors observed in the ERT sections are interpreted as the sulfide mineralized zones. They corroborate the MT results. These conductive zones reflect the occurrence of the magmatic Ni-Cu-PGE bearing sulfide mineralization associated with rocks of the mantle-derived Padhar Mafic Ultramafic Complex. This geophysical data, in conjunction with petrological and geochemical analysis of drill core samples have allowed the identification of the origin and paragenesis of sulphide mineralization in the study area. Geochemical studies suggest that the parental magma was generated from a subduction modified, metasomatized and an enriched mantle source that was subsequently emplaced in a magmatic continental arc setting. The interpreted conductors, observed at shallow depths (∼200-300 m), have been generated by secondary hydrothermal fluid circulation leading to vein formation in the host Padhar Mafic-Ultramafic Complex. The MT and electrical tomography models delineate the geological boundaries of the sulfide-bearing mineralized deposits in the BFB.

Relics of 2.6–2.5 Ga oceanic crust from the ultramafic-mafic complex of Goa, western India: Magmatic response to a progressive subduction system

Subduction initiation in the Early Earth and the onset of plate tectonics are topics of wide interest in understanding the formation and evolution of continents and supercontinents on our planet. Here we report results from integrated petrological, geochemical and geochronological (Zircon U Pb ages) studies on an ultramafic-mafic complex from the Bondla-Usgao-Durigini (BUD) sectors of Goa to address its petro-tectonic evolution and correlation with Neoarchean crustal growth processes and cratonization events. The ultramafic-mafic cumulate sequence from BUD sectors of Goa comprising dunite, peridotite, troctolite, olivine gabbro and gabbro represents a dismembered crustal section of oceanic lithosphere formed either in a MOR or SSZ fore-arc extensional regime. The geochemical attributes marked by boninitic-IAT trend, depleted to enriched mantle compositions, selective enrichment in fluid-mobile LILE and LREE over HFSE enunciate a multistage petrogenetic evolution associated with a SSZ fore-arc tectonic setting. The ocean-continent transition featured by the geochemical signatures can be translated in terms of oceanic crust formation in a juvenile extensional fore-arc in a SSZ setting, its maturation and accretion at active continental margin during Neoarchean ocean basic closure and arc-continent collision. The studied ultramafic-mafic suite fingerprints the transition of depleted mantle wedge to a LILE-LREE enriched metasomatized one through influx of subduction components and hydrous arc magmatism typical of SSZ oceanic lithosphere formed above a nascent subduction zone. The magmatic precursors of the studied lithologies were derived from a transitional depleted to enriched mantle regime marked by hydration and metasomatism of a depleted MOR-type mantle by the influx of subduction-derived fluids. Zircon grains in the peridotite sample yield positive εHf(t) of +3.2 to +7.6, together with T DM C of 2610–2877 Ma suggesting depleted mantle source. The gabbro sample shows positive εHf(t) values of +3.7 to +6.3 and T DM C age of 2731–2868 Ma also attesting to depleted mantle source. Zircon U Pb ages of 2602.42 ± 0.82 Ma for peridotite and 2479–2684 Ma for gabbro represent the magma emplacement time and Neoarchean-early Paleoproterozoic melt activity. The highly variable ages from zircon grains in the studied samples might suggest multiple stages of metasomatism and melt-rock interaction in a long-lived sub-continental lithospheric mantle. The tectonic evolution can be equated with Neoarchean global subduction-accretion-collision and arc-continent amalgamation events. This study deciphers a 2.6–2.5 Ga Neoarchean oceanic crust formation and magmatic response to subduction initiation, arc maturation and accretion processes correlatable with global crustal growth processes and amalgamation of western Dharwar Craton (WDC) and Antongil-Masora blocks into the Greater Dharwar Craton (GDC). • Ultramafic-mafic cumulate sequence from Goa represents dismembered SSZ oceanic crust. • Depleted to enriched melt composition reflects mantle heterogeneity. • Paleo-ocean closure and arc accretion culminated to GDC formation. • 2.6–2.5 Ga age coincides with Neoarchean global subduction-accretion event.

Petrogenesis and geochemical characteristics of pyroxenite dykes in and around Salem Mafic–Ultramafic complex, southern India: an arc-related origin of Alaskan-type

Petrogenesis and geochemical characteristics of pyroxenite dykes in and around Salem Mafic–Ultramafic complex, southern India: an arc-related origin of Alaskan-type

Origin of highly variable and unusually low δ7Li in mineral separates from ultramafic intrusive rocks in a convergent tectonic setting in the Tibetan plateau

Lithium isotopes are increasingly used as a tracer for the sources of mafic-ultramafic intrusive rocks, but the validity of such approach has never been evaluated directly. We have addressed this issue by analyzing mineral separates from an arc-type mafic-ultramafic complex (namely Xiarihamu) in the Tibetan plateau for Li isotopes. The Xiarihamu complex consists of a gently-dipping, ultramafic chonolith in the center and older gabbro in the margins. The chonolith was interpreted to be a feeder for olivine-charged magma by previous studies. Our new data show minor to strong isotopic disequilibrium between coexisting minerals in the chonolith, with lower δ7Li in clinopyroxene than in coexisting orthopyroxene by up to 7‰. The total range of δ7Li in the pyroxene and olivine separates together is from 5.0‰ to −16.2‰, which is significantly different from that of known arc lavas in the world (3.6 ± 1.2 ‰). Samples from a subvertical drill core of ∼190 m in length penetrating the chonolith show a concentric decrease of δ7Li in orthopyroxene from −16.2‰ in the middle to +1.5‰ and + 3.8‰ in both ends. The δ7Li values of mineral separates show no correlation with whole-rock SrNd isotopes and incompatible trace element ratios, as well as olivine forsterite contents and orthopyroxene Mg/(Mg + Fe) molar ratios, indicating that the observed great variability of δ7Li in the minerals was not caused by variable subduction inputs or magma differentiation alone. A positive correlation exists between orthopyroxene δ7Li and whole-rock LOI, illustrating that serpentinization-talc alteration caused δ7Li in the residual mineral to increase, contributing to the great variability of δ7Li on the mineral scales. Subducting sediment input during magma generation and crustal contamination together cannot adequately account for the observed extremely low δ7Li (−2 to −16.2‰) in the minerals from many of the rock samples as well as the unusually low average δ7Li (−3.5‰) of all of the samples, because the δ7Li of the crust and subducting sediments are much higher. Kinetic fractionation of Li isotopes in response to a temperature gradient across the chonolith as well as to subsequent diffusion of Li from interstitial melt to the mineral phases is the best explanation. Since the kinetic fractionation is independent of tectonic settings, such effect is inevitably present in all mafic-ultramafic intrusions on Earth, illustrating that the effect of kinetic Li isotope fractionation must be considered before using Li isotopic data to constrain mantle sources and magmatic evolution, especially if mineral rather than whole-rock data are employed.

Thenmudiyanur Mafic–Ultramafic Complex from the north eastern margin of Southern Granulite Terrane, India: A possible Neoproterozoic Alaskan-type Complex

Thenmudiyanur Mafic–Ultramafic Complex from the north eastern margin of Southern Granulite Terrane, India: A possible Neoproterozoic Alaskan-type Complex

Mapping of mafic-ultramafic rocks in SMUC-SGT, India using ASTER & Sentinel-2A satellite images

The Salem Mafic–Ultramafic Complex (SMUC) is a major complex in southern Granulite Terrane (SGT), in the northern part of the Cauvery Suture Zone (CSZ), India, also known as the Chalk Hills of Salem. The present study aims to review the efficiency of satellite image processing and related techniques to map the Mafic-Ultramafic rocks in Salem Mafic-Ultramafic Complex (SMUC). This mapping is done using atmospherically corrected Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and Sentinel-2A satellite image data. The processes employed were image enhancement techniques like Band Combinations (BC), Principal Component Analysis (PCA), and Minimum Noise Fraction (MNF) to map the various rock types. Band combinations constructed from the output of PCA and MNF in RGB format showed promising results in discriminating various rock units. The Spectral Angle Mapper (SAM) and Support Vector Machine (SVM) classifications techniques were used to classify ASTER and Sentinel-2A satellite images for the study area. The classified lithologies showed strong match with the lithology map published by the Geological Survey of India (GSI), other researches, and field investigation. Particularly, SVM classification of Sentinel-2A MNF + DEM showed 90% overall accuracy when compared with field investigation. The study proved that the processing of VNIR and SWIR bands of ASTER and Sentinel-2A Satellite images coupled with DEM and ground investigation has excellent potential in mapping Mafic-Ultramafic rocks in SMUC SGT.

Formation of Late Paleoproterozoic Gaositai Hornblendite in Northern North China Craton: Evidence from Zircon U-Pb Isotopes and Amphibole Trace Elements

Paleoproterozoic tectonic evolution of the northern North China Craton has been a hot research topic. We firstly identified a 1.85 Ga hornblendite from the Gaositai mafic–ultramafic complex, in northern Hebei. Systematic studies of petrology, zircon U-Pb geochronology, and in situ mineral major and trace elements of hornblendite are the key to revealing the petrogenesis of the Paleoproterozoic ultramafic rock and the tectonic evolution of northern North China Craton. LA-ICP-MS zircon U-Pb dating suggests the Gaositai hornblendite formed at 1851 ± 44 Ma. The late Paleoproterozoic ultramafic rocks, together with coeval post-collisional granites, formed a bimodal igneous assemblage. Both hornblende and its equilibrium melt compositions show strongly fractionated HREE patterns, relative enrichments in LREEs and LILEs, and depletions in HREEs and HFSEs. The phlogopite-bearing hornblendite magma could have originated from a hydrous garnet-facies mantle source metasomatized by slab-derived silicate melt. Furthermore, the variations of major and trace elements in hornblende from core to rim also reveal the mineral fractional crystallization and magma recharge. Zircon trace elements, melt composition equilibrium with hornblendes, and the bimodal igneous assemblage suggest that the generation of the Gaositai Paleoproterozoic hornblendite was likely the product of post-collisional extension related to the collision between eastern and western North China blocks.

Petrogenesis and metallogenesis of the Kawuliuke Fe-P-Ti oxide-rich intrusive complex in the Kuluketage Block, northeastern Tarim Craton

The Kawuliuke Fe-P-Ti oxide-rich intrusive complex is one of the largest layered mafic–ultramafic complexes associated with Fe-P-(Ti) oxide deposits in the Kuluketage Block, northeastern Tarim Craton, NW China. Here, based on new field observations, whole-rock trace element, radiometric dating analyses as well as Sr-Nd-Hf isotopes we propose a non-cogenetic origin of the parental magmas of the mafic–ultramafic rocks and syenite in the Kawuliuke intrusive complex (KIC), both of which formed synchronously at circa 808 Ma.Geochemical characteristics and Sr-Nd-Hf-S isotopes ((87Sr/86Sr)i = 0.70437–0.70560; εNd(t) = -2.60 to −7.15; εHf(t) = -1.3 to −10.0; δ34S = -2.98 ‰ to + 2.36 ‰), in combination with minerals composition and previous research, strongly indicate that the mafic–ultramafic rocks of the KIC formed via extensive crystal fractionation/cumulation of a tholeiitic magma, possibly derived from partial melting of an enriched subcontinental lithospheric mantle previously modified by slab-released fluids in a continental rift setting. The Kawuliuke syenite, meanwhile, is interpreted as generated by the emplacement of syenitic melts from the differentiation of newly coeval underplating basaltic magmas at depth based on lines of evidence from petrography, geochemical signatures and Sr-Nd-Hf isotopes. Our newly presented enriched Sr-Nd-Hf isotopic signatures, together with previous research, suggest that the enriched subcontinental lithospheric mantle could be metasomatized by asthenosphere mantle melts in different degrees. We further show the Kawuliuke clinopyroxenite-related apatite-rich oxide ores were the products of normal fractional crystallization/cumulation of H2O-Fe-P-Ti enriched residual magma in high oxygen fugacity condition, which experienced high differentiation. The tempo-spatial relationships of the KIC and other regional coeval mafic rocks jointly suggest that the KIC was most likely formed in response to the proposed mid-Neoproterozoic mantle plume in Tarim Craton, which likely induced partial malting of, and likely mixed with, the metasomatized subcontinental lithospheric mantle, to form the KIC with clinopyroxenite-related Fe-P-Ti oxide mineralization.

Late Archean–Paleoproterozoic plate tectonics along the northern margin of the North China craton

AbstractThe North China craton of central Asia is a natural laboratory for investigating early Earth tectonic processes including subduction and large-scale horizontal crustal motions. However, it remains unclear how and when the North China craton formed from the amalgamation of several blocks and orogens including the Archean Western and Eastern blocks, the late Archean Central Orogenic Belt, and the Paleoproterozoic Northern Margin orogen. In this study, we integrated new and existing geological field observations, zircon and baddeleyite U-Pb geochronology, whole-rock geochemistry, and Sr-Nd-C-O isotope analyses along the northern margin of the North China craton to improve our understanding of its late Archean–Paleoproterozoic tectonic evolution. Observations show the Wuchuan ultramafic-mafic complex of the Northern Margin orogen contains a mixture of mylonite gneiss, serpentinized ultramafic blocks, and amphibolite pyroxenite xenoliths. Geochronology and geochemistry results suggest that the ca. 2.55–2.5 Ga development of the Wuchuan ultramafic-mafic complex was associated with subduction, which was followed by ca. 2.39 emplacement of lamprophyre dikes during continental rifting. Subsequent Paleoproterozoic subduction magmatism, continental collision, and post-orogenic rifting between the North China craton and southern margin of the Siberian craton are evidenced by ca. 2.0 Ga arc granitoids, ca. 1.87–1.85 Ga A-type, garnet-bearing granitoids and mafic dikes, and ca. 1.86 carbonatite. Ca. 2.0 Ga collision and ca. 1.87–1.85 Ga rifting were coincident with orogen-normal left-slip shear. Our findings show that the ca. 1.9 Ga North China-Siberia collision was a key event during the assembly of the Columbia supercontinent that occurred via modern plate-tectonic processes.

Spectrochemical and stable isotopic characteristics of magnesite deposits in Salem, Southern India: CO2 repository through supergene processes

Magnesite (MgCO3), a carbonate mineral, is considered as a carbon dioxide repository in geological formations and has important industrial applications. Magnesite can form through the alteration of magnesium-rich rocks in marine and non-marine geological settings. We investigated the magnesite deposits of the Salem mafic–ultramafic Complex (SMUC) in southern India through chemical and spectroscopic techniques. We characterize the magnesite mineralization based on visible-near infrared reflectance spectroscopic analyses, XRD, laser Raman analysis, and stable C-O isotope methods. Hyperspectral data show paired absorption at 2.3 and 2.5 µm, attributed to C-O bond vibration overtones and combination tones of C-O stretching and bending. The FTIR spectrum brings out frequencies of vibrations between the atomic bonds. The Laser Raman data show peaks at or near 212, 332, 735, 1096, and 1460 cm−1. The stable isotope data indicate that the magnesite mineralization resulted from the alteration of the ultramafic rocks by low-temperature (49 ± 3.3 °C) fluids. The source of carbon is correlated to the decarboxylation or dissolution of marine limestone. We propose a genetic model that invokes the percolation of meteoric water through weak planes and fractures into the ultramafic rocks.Our results provide a typical example of magnesite as a natural CO2 repository formed through supergene processes.

Tectonic setting and mineralisation potential of the Cowley Ophiolite Complex, north Queensland

Northeast Queensland contains multiple slices of mafic–ultramafic units, strung out along regional faults that mark major tectonic boundaries. One such complex is the Cowley Ophiolite Complex, which is situated along the Russell-Mulgrave Fault. The Cowley Ophiolite Complex is a differentiated mafic–ultramafic complex composed of gabbro, chlorite schist, anthophyllite schist and serpentinite. We interpret that the alteration assemblages observed throughout the Cowley Ophiolite Complex reflect an amphibolite facies metasomatic overprint within a supra-subduction zone setting. This interpretation is consistent with geochemical discrimination of the gabbro, and chromite grains from the anthophyllite schist. The Cowley Ophiolite Complex records a higher metamorphic grade than the surrounding Hodgkinson Formation, and we interpret this to reflect the allochthonous structural setting of the complex, positioned along an ancient subduction margin. This subduction margin is represented today by the Russell-Mulgrave Fault. Metasomatism and emplacement of the complex probably pre-dated deposition of the Mossman Orogen’s active margin successions. Our interpretation of an active subduction complex, which pre-dated the formation of the Mossman Orogen, suggests that the Russell-Mulgrave Fault is a Paleozoic, continental suture zone. The Cowley Ophiolite Complex presents little indication of economic mineralisation; however, much of the complex remains unexplored. KEY POINTS The Cowley Ophiolite Complex is a differentiated, mafic–ultramafic complex formed within a supra-subduction zone setting. Emplacement of the Cowley Ophiolite Complex, along the Russell-Mulgrave Fault, pre-dated the formation of the Hodgkinson Province. The Russell-Mulgrave Fault represents an ancient subduction margin, and is interpreted as a Paleozoic, continental suture zone.

The petrogenesis and emplacement mechanism of layered ultramafic-mafic complexes of the Barberton Greenstone Belt: Insights from the Stolzburg Complex, South Africa

Abstract Ultramafic-mafic layered complexes are important but not-well studied components of Archaean granitoid-greenstone terranes. In the vicinity of the Barberton Greenstone Belt (BGB), at least 27 such complexes are intimately associated with the supracrustal succession. The petrogenesis of one of these layered bodies, the Stolzburg Complex (SC), is explored, together with its relationship to the surrounding Barberton volcanic succession. Previous models for the origin of Barberton layered complexes proposed a variety of mechanisms, such as single chamber subvolcanic sills, ponded lavas, and alpine-type tectonites. In contrast, the present work suggests that emplacement mostly occurred as sheeted sills of crystal slurries into the country rocks. Unlike the subvolcanic sills model, whereby each complex grew through repetitive magma injection and differentiation in a single chamber, the preferred model regards the layered bodies as ‘stacks’ of discrete intrusions, where each magmatic unit represents a distinct sill. Through comparison of trace element geochemistry (i.e., trace element ratios and patterns), the Lower and Upper divisions of the SC are inferred to be petrogenetically related, but compositionally distinct from the enveloping Nelshoogte volcanic rocks. The trace element geochemistry of the Lower and Upper divisions of the complex is indistinguishable. While the SC ultramafic rocks display an Al-undepleted character, Nelshoogte metavolcanics can be classified as Al-depleted komatiites and komatiitic basalts.

3D modeling of the Esker intrusive complex, Ring of Fire intrusive suite, McFaulds Lake greenstone belt, Superior Province: Implications for mineral exploration

A 3D geological model of the Archean Esker ultramafic–mafic intrusive complex of the Ring of Fire intrusive suite of northwestern Ontario (Canada) is presented, providing insight into the nature of its magmatic plumbing system and allowing for speculation on the formation of its valuable Ni-Cu-(PGE) and chromitite deposits. The 3D model was constrained by exploration drillholes and high-resolution aeromagnetic data, which allowed the interpretation of the broad internal structure of the intrusive complex and two shear zones, which define for a major part of their strike extent the contacts between ultramafic and overlying mafic intrusive rocks. Restoring the post-emplacement component of strike-slip displacement and subsequently rotating the intrusive complex back from its tilted to sub-horizontal orientation of emplacement, provides a three-dimensional rendition of a 14 km-long dominantly tabular-shaped intrusive complex. The complex is defined by intact basal intrusive contacts with localized keel-shaped ultramafic promontories parallel to a nearby blade-shaped magmatic conduit. The keels extend in a subparallel orientation to depths of up to 1 km into the underlying tonalitic rocks and coincide with significant offsets of the basal intrusive contact, leading to speculation that their intrusion may have been guided by pre-existing normal faults. Two of these magmatic conduits host Ni-Cu-(PGE) deposits/prospects that formed from gravitational segregation of sulfides. The chromitite deposits, hosted in ultramafic rocks at higher stratigraphic levels, were modeled based on drill hole assay data. Zones of dominantly massive to semi-massive chromitite were defined by the ≥ 35% Cr2O3 threshold, while intercalated sequences of chromitite in dunite/peridotite were defined by the ≥ 15% to ≤ 35% Cr2O3 percentage range. The 3D modeled surfaces fitted to these constraints exhibit ore zones with lateral extents from several hundreds of meters for massive bodies up to a maximum of 2 km for intercalated chromitite ore that are dominantly conformable to the layering of the ultramafic host rocks. The lack of lateral continuity of chromitite ore zones beyond this scale, suggests that the chromitite layers are not continuous along the entire strike extent of the Esker intrusive complex, which is consistent with bifurcations of the 3D modeled ore shells signifying pinch-outs of both the massive chromitite layers and the dunitic interlayers. This discontinuous distribution of chromitite ore resembles that in the stratiform deposits of the Stillwater Complex and may support recent models of mechanical segregation of chromitite from cotectic chromite-olivine slurries. Based on our 3D model, we propose that the Esker intrusive complex evolved as a series of individual intrusions that were emplaced during multiple magmatic pulses that eventually coalesced to form a composite ultramafic–mafic complex prior to being dissected and partly dismembered by post-ore shear zone deformation.

Early Paleozoic subduction fingerprints of the Paleo-Asian ocean in easternmost Central Asian Orogenic Belt (CAOB): Identification of the oldest Alaskan-type complex in the CAOB

Abstract The early-stage subduction records of the Paleo-Asian ocean are poorly preserved in the eastern segment of the Central Asian Orogenic Belt (CAOB), which hinders constraints on the evolution of the whole CAOB. This study presents new age data and zircon Hf-O isotopes as well as bulk-rock geochemistry of the Wuxing mafic-ultramafic complex in the Xingkai massif in northeastern China, which has been identified as Alaskan-type complex with aspects of field occurrence, petrological and mineral assemblages, and mineral chemistry in our recently published work. The results indicate that the complex formed mainly between 517 Ma and 510 Ma with a lithological sequence of Sanying clinopyroxenite (517 Ma), then Sanying gabbro (514 Ma), and Erying hornblendite (513 Ma), and finally Erying hornblende clinopyroxenite (510 Ma). The lithological formation sequence is consistent with the intrusive relations between lithological phases and their irrelevant major element compositions and variable trace element patterns of the bulk rocks. The εHf(t) values of zircon in two samples in this study vary from from +4.45 to +7.61 and from −11.8 to +4.42, respectively and tend to be more depleted with age. These features suggest that the Wuxing complex was a product of long-term arc magmatism and experienced significant ancient crustal assimilation in early-stage magmas and negligible contamination in later ones. The presence of 1222 Ma and 706 Ma inherited zircon grains implies existence of Proterozoic basement in the Xingkai massif and its continental arc setting in Cambrian. The Wuxing complex is the oldest Alaskan-type complex found so far in the entire CAOB and is a good witness of the Paleo-Asian oceanic subduction in the easternmost CAOB. The older age of the Wuxing complex compared to the regional Hongqiling intrusion is also compatible with its Alaskan-type nature and platinum-group element mineralization, which are distinct to the Permian-Triassic Ni-Cu sulfide deposit-hosting maficultramafic intrusions in the southern CAOB.

A bulk annealing and dissolution-based zircon concentration method for mafic rocks

Zircon geochronology is applied to a variety of geological problems to precisely and accurately date rocks via UPb decay. Zircon is most abundant and easily recovered in intermediate to felsic rocks, including the silicic eruptives of bimodal large igneous provinces, or fractionated granophyres in mafic-ultramafic complexes. However, the concentration of zircon crystals by conventional density and magnetic separation methods is inefficient and/or ineffective for medium- to fine-grained mafic rocks, due to their rarity, small grain size, common association with ferromagnetic minerals, and/or occlusion by paramagnetic modal minerals. To address these shortcomings, we have developed and tested a zircon concentration method that is based on a combination of physical separation and chemical dissolution. The sample is initially ground to sand-size particles and heavy minerals concentrated via density on a water table. The heavy mineral-rich fraction is annealed by heating at 900 °C for 60 h, and then put through a series of acid digestions: aqua-regia, hydrofluoric acid, aqua-regia and finally hydrochloric acid. This new method allows the concentration of an almost pure zircon aliquot, since zircon is one of just a few minerals that can survive this bulk rock acid attack. The result is an efficient extraction of analytically viable amounts of zircon for UPb geochronology using tens of grams of rock sample, representing an increase up to a hundred times the recovery rates of conventional separation techniques for zircon concentration.