Mafic-ultramafic Bodies Research Articles

Deccan Volcanism and Related Seismic Unrest in the Koyna–Warna Region, India

Abstract Koyna–Warna is a region of low-tectonic deformation and normal surface heat flow (∼40 mW/m2) in the Deccan volcanic province, India, where low-to-moderate magnitude earthquakes have continued to occur in the last 60 yr. These earthquakes are uniquely restricted to an 11×16 km2 area and confined to the upper crust between 3 and 9 km depth. Located at the last phase of the interaction of India with the Reunion mantle plume ∼65 Ma ago responsible for extensive volcanism, the cause of sustained seismicity in Koyna region is debated. Using the shear-wave velocity model derived through the joint inversion of the receiver function and surface-wave data from the seismic zone, we propose that earthquakes in the Koyna region occur due to stress concentration arising because of the high-density magma intrusions in the shallow crust at 3–9 km. The high-density mafic-ultramafic body exerts gravitationally induced stress of about ∼12 to 15 MPa. The continuation of earthquakes in the deeper part is inhibited by the possible fluid-filled mush zone imaged as a low-velocity layer at a 9–17 km depth. The magma intrusion as dike can induce a cycle of normal faulting in the overlying rock mass, as observed in the Warna region. We present the first evidence of an extremely high velocity (>4.7 km/s) layer at 40–50 km below Moho, interpreted as the presence of eclogite–peridotite responsible for producing Deccan magma in large volume.

Petrogenesis and Ni–Cu–(PGE) prospectivity of the Mount Ayliff Complex in the Karoo Igneous Province: new insights from the Ingeli and Horseshoe lobes

The Mount Ayliff Complex comprises five cognate mafic–ultramafic bodies emplaced along the southern margin of the Kaapvaal Craton. This study examines the Ingeli and Horseshoe lobes and assesses their magmatic sulfide prospectivity with respect to the Insizwa lobe, which hosts massive sulfides at its Waterfall Gorge occurrence. The Ingeli lobe consists of 465 m of olivine–chromite cumulates overlain by 340 m of (olivine-)gabbronorite, while the Horseshoe lobe consists of 45 m of olivine gabbronorite overlain by 5 m of gabbro. The Ingeli lobe possesses sparsely disseminated sulfides at its mafic–ultramafic transition, whereas disseminated sulfides are present throughout the Horseshoe lobe. A petrogenetic model is proposed, where magma accumulated and fractionated nickeliferous olivine and chromite in an upper-crustal staging chamber hosted by Proterozoic basement rocks. Magma then ascended and deposited olivine–chromite cumulates at the level of the complex. Prolonged magma flux in the staging chamber facilitated the assimilation of basement rocks, triggering sulfide saturation and the crystallization of Ni-poor olivine. Contaminated magma then ascended and interacted with pre-existing cumulates, depositing sulfide melt that may have backflowed as magmatic activity waned. Basaltic magma then flowed over the ultramafic cumulates, depositing disseminated sulfides whilst undergoing closed-system fractionation. Basal depressions and underlying feeder structures are the most prospective locations for magmatic sulfide mineralization.

Determination of structural lineaments of the mafic-ultramafic belt of the Southern Argentine Precordillera

In central-western Argentina, an Early Paleozoic belt including mafic-ultramafic bodies and marine metasedimentary rocks occurs along the western margin of the Precordillera and in the eastern Frontal Cordillera. In this paper, we present the analysis of aeromagnetic data and its processing focused on mapping geological limits, structures and the relation at depth of the southern Precordillera. With this main objective were applied filtering algorithms, reduction to the pole, tilt derivative, analytic signal, and particularly, the use of Improved Logistic Function (IL) as a new method of edge detection, in combination with the inversion map obtained from the Magnetization Vector Inversion (MVI) method. The results allow us to describe at depth the main trend of the Southern Precordillera Mafic Ultramafic Belt as two separated bocks, one in Peñasco and another in Cerro (Co.) Cortaderas, passing through Cerro (Co.) Pozos with a possible NNE-SSW direction. The combination of this techniques also can be used as prospecting guide to Pb–Zn–Ag vein deposits and High sulfidation Au-Epitermal deposits.

Utilizing Random Forest algorithm for identifying mafic and ultramafic rocks in the Gameleira Suite, Archean-Paleoproterozoic basement of the Brasília Belt, Brazil

Geological mapping techniques have continuously advanced, particularly with the increasing use of remote data acquisition methods, such as aerogeophysical data analysis and the use of machine learning techniques, both has become essential for mapping and understanding geological environments. Mafic-ultramafic bodies, which may be linked to metallic ore deposits, can be mapped using these innovative techniques. In Brazil, the Tocantins Province is reknown for its primary Cu–Co–Ni–Cr ore deposits, which are found in mafic-ultramafic rocks. The Tocantins Province contains several mafic-ultramafic bodies that have received less attention than the well-known deposits in the Brasília Belt. The Gameleira Suite is a geological unit located in the basement of the North Brasília Belt. Our study aims to use machine learning to identify new potential mafic-ultramafic occurrences in the Gameleira Suite. To achieve this, aerogeophysical surveys data, including magnetometry and radiometry, were used in conjunction with remote sensing. The data collected from aerogeophysics surveys were compiled into a database. The Random Forest algorithm was used to analyze this database, with 1.96% (from a total of 84.535 samples) used for training and generating the predictive map. Three approaches were used to verify and evaluate the data: analyzing magnetic lineaments to assess the influence of structural tectonic factors on body positioning, collecting field samples, and utilizing Magnetic Vector Inversion (MVI) to analyze the magnetic properties of the projected bodies at depth. This was done because the Gameleira Suite bodies display magnetic remanence. Our findings suggest that the utilization of these methods, in conjunction with the verification techniques employed, aids in the mapping and identification of mafic-ultramafic rocks. Machine learning can improve cartography by identifying new occurrences or indicating areas that need more detailed mapping. The use of additional geological knowledge and information not included in the model is crucial because the predictive map does not inherently represent geological truth. It is necessary to interpret the results from a geological perspective.

Origin of paired extension-compression during rotational rifting: An early Paleogene example from the northeast Atlantic region and its implications

Abstract The formation of paired extension-compression (PEC) postulated by rotational kinematics of rift propagation is demonstrated by analogue models but rarely observed in nature. In our study of the early Paleogene continental rift in the northeast Atlantic, a PEC is proposed based on the northeastward propagation and the coeval compression at the rift tip. The propagation is deduced from tectono-magmatic trends, including along-axis development of magmatism, and migration of tectonic faulting inward and toward the rift tip. Where this propagation terminated, we documented coeval extension and compression in the form of a horst-and-graben system (H&G) and V-shaped anticline (VA), respectively. Given their structural characteristics and spatiotemporal relationships with the rift, their origin is best illustrated by a three-stage model: (1) Rifting initiated at the site of mantle upwelling and propagated northeastward in the Paleocene. (2) The rift tip was stalled by an elevated mafic-ultramafic body at the Barents Shelf, which led to forward projection of the rift’s driving force to create the H&G and the VA (PEC), dissipating the along-axis force component. (3) Domination of axis-perpendicular components then promoted orthogonal extension and sheared margin development. Our study suggests that PEC has a crucial role in both termination of propagation and rift-mode conversion.

Geochemistry of extremely modified chromites from the chrysotile asbestos-bearing Zvishavane Ultramafic Complex, south central Zimbabwe

ABSTRACT The Zvishavane Ultramafic Complex (ZUC) is a metamorphosed ultramafic-mafic body exposed within the Zvishavane gneiss complex in south central Zimbabwe. The complex hosts Africa's largest mine and reserves of high-grade chrysotile asbestos. Rock types of the ZUC range from serpentinites, metadunites, and metagabbros. Accessory chromites, which are altered to ferrian chromites, are hosted in metadunites and serpentinites. Chromium number, Cr# (100*Cr/(Cr + Al)), is extremely high, and ranges from 90.6 to 99.5. Low magnesium number, Mg# 100*Mg/(Mg + Fe2+), ranges from 5.1 to 8.4. Most ZUC ferrian chromites occur as inclusions or marking the outlines of relict olivine crystals, and in veinlets traversing the serpentinites. On 100*Fe3+/(Al + Fe3+) versus 100*Fe2+/(Mg + Fe2+) diagram, the chromites all plot in the magnesioferrite field, and in the ferrian chromite field on a Fe3+-Cr-Al ternary diagram. On wt% Al2O3 versus wt% Cr2O3 and 100*Cr/(Cr + Al) versus 100*Mg/(Mg + Fe2+) plots, chromites all plot in the modified trend fields. The bulk of the chromites were metamorphosed at temperatures of ≤500°C, and no samples experienced temperatures exceeding 550°C. A plot of Fe3+/(Fe3++ Fe2+) versus wt% MnO is indicative of hydrous metamorphism of ZUC chromites. It is interpreted that extensive modification of the ZUC chromites occurred during hydrothermal alteration, under conditions more oxidising than quartz-fayalite-magnetite but less than magnetite-hematite, which accompanied the formation of high-grade chrysotile asbestos for which the ZUC is famous. Four granitic plutons exposed within 50km of ZUC were likely the source of hydrothermal fluids responsible for extensive hydrothermal alteration, which likely led to the formation of high-grade chrysotile asbestos, and almost completely modifying the chromites in the process. Compositions of accessory chromites in chrysotile asbestos meta-ultramafic bodies elsewhere are likely to be extremely modified.

Geological Characterization, Lithogeochemistry and the Metallogenic Potential for Chromium of the Riacho do Mocambo Mafic-Ultramafic Body, Northeast of the São Francisco Craton, BA, Brazil

In the geotectonic context of the Salvador-Curaçá Orogen, north portion of the São Francisco Craton, an association of mafic-ultramafic (M-UM) rocks was identified and described in this paper as the Riacho do Mocambo Mafic- Ultramafic Body (RMMUB). Despite being located approximately 60 km from the Vale do Jacurici Complex (VJC), the host of Brazil’s largest reserves of Cr, the RMMUB has never been associated with this Complex in regional geologic mapping projects. When it is mentioned in the bibliography, the M- UM rocks of the RMMUB are genetically related to the São José do Jacuípe Suite (SJJS). While the VJC is described as differentiated sills, associated with a synorogenic to a tardi-orogenic event, the SJJS is interpreted as fragments of an Archean-Paleoproterozoic oceanic crust or as a Gabbro- Anorthosite Stratiform Complex. Such contrasting genesis raised doubts about the RMMUB’s origin and field work along with geochemical analyses were carried out in order to better understand the possible source of the RMMUB. In the field, the RMMUB exhibits an elongated shape of small thickness (7 km of extension by less than 100 m of apparent thickness), displayed concordantly with the Tanque Novo-Ipirá Complex metasediments. In the mapped outcrops it is possible to observe the rhythmic and gradual alternation amid the lithotypes of the RMMUB, varying from serpentinite to metagabbro, suggesting that it is a layered igneous body. The geochemical results support the primitive aspect of the ultramafic rocks of this body (MgO up to 38 wt.%; Ni up to 2972 ppm; Cr up to 7799 ppm) and suggest that the RMMUB shows distinctive characteristics from the SJJS, but similar ones with magma of the VJC such as geochemical signatures, source, depth, and tectonic environment. The discovery of this new M-UM body in an area of great metallogenic fertility opens a potential for the identification of new Cr mineralization and magmatic sulfides of Ni, Cu, and EGP, in the Salvador- Curaçá Orogen, São Francisco Craton, the northeast region of the state of Bahia.

Origin of Alteration Patterns in Accessory Chromites from the Kudada Metaperidotites, East Singhbhum District (Jharkhand, India)

Abstract The metamorphosed ultramafic-mafic bodies of the Kudada area are located close to the Singhbhum Shear Zone (SSZ) in eastern India, where the major rock types are talc-magnesite schist and serpentinite with accessory chromite and magnetite veins. The ultramafic bodies and associated metavolcanic rocks are part of the northern extension of the Early Archean Gorumahisani greenstone belt and belong to the Iron Ore Group (IOG) supracrustal sequence. This study reveals intense compositional variability in accessory chromites of serpentinite with core composition of chromites are characterized by the variable Cr# [Cr/(Cr+Al)] = 0.53 - 0.82 and Mg# [Mg/(Mg+Fe2+)] = 0.01 - 0.17. Compositional variability on the scale of a single chromite grain occurs in the form of multi-stage zoning. To identify the patterns of compositional zoning, chromites of serpentinite are subdivided into four types depending on their grain size, reflectivity of different rims, intensity of fracture, and porosity, and supported by in-situ mineral chemistry. The type-I chromites are less fractured and non-porous variety showing the outermost chrome magnetite rim envelops the inner ferritchromit rim. Porosity is mainly developed in the type-II grains where the inner ferritchromit is formed surrounding the pore spaces. The type-III chromites are small clustered grains having ferritchromit core and chrome magnetite rim while the type-IV grains are completely altered to chrome magnetite. Textural relations and mineral chemistry indicate that metamorphism and activities of H2O and CO2-rich hydrothermal fluids during tectonic evolution of the Singhbhum Shear Zone (SSZ) might have caused these zoning patterns and compositional variabilities in accessory chromites of the Kudada area. Cation exchange between chromite and silicate minerals along with intra-grain cation diffusion within different Cr-spinel zones further intensified these processes.

Petrogenesis of PGE mineralised intrusions in the floor of the northern Bushveld Complex

Abstract The floor rocks of the northern lobe of the Bushveld Complex host several sill-like mafic-ultramafic bodies. In the present paper we evaluate whole rock data generated by exploration companies for sills on the farms Townlands, Amatava, Uitloop, Turfspruit and Rietfontein, located to the north of Mokopane, in order to constrain the origin of the sills and their mineralisation. Key observations include: (i) The sills have geochemical affinities to the Lower Zone (LZ) or Lower Critical Zone (LCZ). (ii) Most sills are enriched in sulphides and platinum-group elements (PGE) relative to most other LZ and LCZ cumulates. (iii) Most PGE mineralised intrusives have been emplaced into the carbonaceous-pelitic Duitschland Formation. (iv) The sills are spatially associated with the Mokopane gravity anomaly, possibly representing a major feeder zone to the Bushveld Complex. (v) The sills show evidence for assimilation of the sedimentary host rocks in the form of locally elevated δ34S, incompatible trace element contents and the presence of carbonaceous and pelitic country rock xenoliths. (vi) There is no correlation between PGE abundance and indicators of crustal contamination. Based on these data we propose that in the vicinity of the putative Mokopane feeder zone relatively fertile, unevolved magmas ascended through the crust initially as dykes. When intersecting the relatively fissile Duitschland Formation the mode of magma emplacement changed to one of sills. This facilitated contamination with sulphide- and graphite-rich carbonate and shale, triggering sulphide melt saturation. The sulphides were locally entrained and upgraded within the sills before precipitating, likely within flow dynamic traps.

Isotopic investigations of the Nova-Bollinger Ni–Cu–Co deposit in the Fraser Zone, Albany-Fraser Orogen, Western Australia

The Fraser Zone of the Albany-Fraser Orogen of Western Australia contains an extensive suite of mafic and mafic–ultramafic intrusions. This includes the Nova-Bollinger intrusions (ca 1300 Ma), which host economic Ni-sulfide mineralisation, a prime example of magmatic sulfide liquids emplaced into the deep crust under ambient granulite facies conditions. Sulfur, Sr and Sm–Nd isotopes, and trace elements have been used to evaluate the role of crustal contamination of the parental magmas to Nova-Bollinger and other sulfide-bearing mafic–ultramafic bodies within the Fraser gabbro suite. All ore types have δ34S values between +1.6 and +8.0‰ VCDT with a positively skewed distribution and a median value of +2.3‰ VCDT, anomalous relative to uncontaminated mantle values near 0‰ VCDT. No significant Δ33S anomalism was observed. The initial 87Sr/86Sr and 147Sm/143Nd ratios of most of the Nova-Bollinger igneous rocks have an age of formation of 1300 Ma, which is similar to those of CHUR1300Ma and record limited contamination by lower continental crustal material. The country rocks of the Snowys Dam Formation, locally enriched in iron and sulfur, are likely candidates as crustal contaminants. These metasedimentary sulfides show similar ranges in values for both δ34S and Δ33S to the ores, but their Sr isotopic range is greater than observed in the host intrusions at Nova-Bollinger and in most of the other sampled intrusions. Bollinger globular sulfides with ɛSr values of 63.1 and 144.5 and ɛNd values of −5.7 and −1.8 are suggestive of localised crustal contamination from adjacent wall rock marbles, but otherwise high ɛSr country rocks do not appear to have been dominant contaminants. Bulk assimilation of the country rocks, as modelled in both silicate and sulfide portions of the system, of up to 30% contamination of a primitive mantle melt by siliceous country gneisses, would have been sufficient to produce the degree of crustal contamination recorded in the Nova-Bollinger ores, and more regionally within the Fraser gabbros. Beyond indications that S was crustally derived, it is not possible to identify a specific source, but Snowys Dam sediments are the most likely. There is no compelling evidence for Archean S in the ores, unsurprisingly given the mid-Proterozoic age. It is likely that much of the contamination of the magma and generation of sulfide liquid took place during lateral flow through linked sill complexes within the deeply buried Snowys Dam metasediments, prior to the emplacement of the magmas to form the orebodies. KEY POINTS Nova-Bollinger Ni–Cu–Co sulfide ores have δ34S values between +1.6 and +8.0‰ VCDT, with a median value of +2.3‰ VCDT, anomalous relative to uncontaminated mantle values but similar to the range in the country rock metasediments. Values are consistent with equilibration of sulfide xenomelts at R factors of 200–700. There is no detectable mass-independent fractionation of δ33S in the Nova-Bollinger ores. The initial 87Sr/86Sr and 147Sm/143Nd ratios of most of the Nova-Bollinger igneous rocks at the age of formation of 1300 Ma are similar to those of CHUR1300 Ma and record contamination of a primitive mantle melt by up to 30% lower continental crustal material. Unusual globular ores at Bollinger have anomalous ɛSr indicating localised contamination from adjacent wall rock marble; however, this contamination is not pervasive through the orebody. It is likely that much of the contamination of the magma and generation of sulfide liquid took place during lateral flow through linked sill complexes within the deeply buried Snowys Dam metasediments, prior to the emplacement of the magmas to form the orebodies.

Origin of ultramafic–mafic bodies on the Isles of Lewis and Harris (Scotland, UK): Constraints on the Archean–Paleoproterozoic evolution of the Lewisian Gneiss Complex, North Atlantic Craton

The Lewisian Gneiss Complex (LGC) is a tonalite-trondhjemite-granodiorite (TTG)-dominated fragment of the North Atlantic Craton (NAC) in northwest Scotland. End-member models describe the LGC as representing either a continuous piece of Archean crust or up to 12 geologically distinct Archean terranes, with interpretations sitting on a spectrum between these end-members. There is particular uncertainty over the correlations between the Archean–Paleoproterozoic magmatic and metamorphic events recorded by mainland part of the LGC and the part exposed on the Outer Hebridean islands of Lewis and Harris. In this paper, we present the results of field mapping, petrography, and major, trace and platinum-group element (PGE) bulk-rock geochemistry for four ultramafic–mafic bodies in Lewis and Harris, namely: Maaruig, Loch Mhorsgail, Coltraiseal Mor and Beinn a’ Chuailean. We consider the effects of metamorphism and element mobility, their petrogenesis, and potential correlations with ultramafic–mafic rocks elsewhere in the LGC. Our data indicate that the studied ultramafic–mafic rocks can be subdivided into two petrologically distinct groups. Metaperidotites and metapyroxenites from Maaruig and Loch Mhorsgail are interpreted as Archean (>2.8 Ga) cumulates distinct from anything currently identified in the mainland LGC, with this interpretation based on distinctive modal layering, a discordance with surrounding TTG gneiss, fractionated PGE patterns ([Pd/Ir]N = 1.3–6.6) and negative HFSE anomalies ([Nb/La]N = 0.2–0.8). Metagabbronorites from Coltraiseal Mor and Beinn a’ Chuailean, which also exhibit negative high field strength-element (HFSE) anomalies ([Nb/La]N = 0.2–0.7) and show mildly fractionated ([Pd/Ir]N = 1.2–2.8) PGE patterns, most likely represent deformed Paleoproterozoic dykes. These occurrences could be correlatives of a suite of ca. 2.4 Ga mafic dykes exposed throughout the mainland LGC (the Scourie Dykes), with the Outer Hebridean occurrences having experienced more intense Paleoproterozoic (Laxfordian) deformation/reworking. These interpretations suggest that the LGC lithologies of Lewis and Harris were proximal to the mainland LGC’s Central Region by the early Paleoproterozoic but raises the possibility that they were distinct crustal blocks in the Mesoarchean.

Subduction: The recycling engine room for global metallogeny

The formation of ore deposits, within mineral systems that represent highly anomalous concentrations of economic elements in localized parts of the Earth’s crust, first requires an enriched and available source of those elements. In early Earth history, initial enrichment in the upper mantle was arguably through mantle plumes that even recycled metals from the core-mantle boundary. From the Mesoarchean onwards, the development of plate tectonics allowed subduction processes to be increasingly important in both recycling of elements from the upper mantle into the lower crust and from there into the upper crust where mineral deposits could form and be exploited. The most obvious expression of this recycling process is the direct association of porphyry-high-sulfidation epithermal-skarn Cu-Au-Ag +/- Mo systems and inboard fractionated granite-associated Sn-W systems with magmatic-hydrothermal fluids derived from subduction-related magmas in convergent margins, where epithermal Au-Ag systems also developed due to high thermal gradients. Syn-subduction VMS Cu-Zn-Pb(Au) systems were accreted at convergent margins. Most orogenic gold systems relate to fluids released through devolatilization of down-going subduction slabs and overlying metal-rich sediment wedges. More indirect associations relate to the ability of subduction zones to cause structural damage and metasomatize and fertilize sub-continental mantle lithosphere along the margins of bordering craton margins. Orogen collapse in the final stages of the subduction-related orogenic event led to asthenosphere upwelling and formation of proximal intrusion-related and more distal Carlin-type gold systems in continental basins on the craton margins. During subsequent orogenic events, previously metasomatized and fertilized mantle lithosphere was reactivated to release hydrothermal fluids to form many of the orogenic gold systems in eastern China. Reactivation of the metasomatized lithosphere also resulted in development of post-subduction hybrid or alkaline magmas that generated giant magmatic Kiruna-type Fe-P, carbonatite-related REE-Nb or Cu-P, and lamproite-related diamond systems, together with magmatic-hydrothermal IOCG mineral systems near craton margins. Subduction-damaged craton margins and their rift-related marginal basins became the sites of formation of Zambian Copper Belt-type deposits and the craton margins were reactivated during subsequent rifting allowing intrusion of giant Ni-Cu mafic–ultramafic bodies along lithosphere-tapping fault zones at those margins. Thus, subduction acts as the recycling engine room or factory responsible for many of the global giant mineral systems that are currently exploited.

Geochemical evolution of the Lebowa Granite Pluton in western Bushveld Igneous Complex, South Africa: More insight into the evolution of bimodal A‐type granitoid

The ~2.05 Ga Bushveld Igneous Complex (BIC) in the Kaapvaal Craton, South Africa, with an areal extent of ~65,000 km2, represents one of the largest bimodal A‐type granitoid suites in the world. The igneous activity of this complex begun with the Rooiberg Group of basalt–rhyolite bimodal volcanism, followed by the emplacement of the ultramafic–mafic Rustenburg Layered Suite (RLS), and finally the granitoid rocks of the Rashoop Granophyre (RG) and Lebowa Granite Suite (LGS). In this study, we present our new borehole description and whole‐rock chemical data on the LGS from the relatively unknown western BIC to constrain the origin of the LGS and try to evaluate their geochemical relationship to the Rooiberg Group felsic volcanics. In the western BIC, the LGS is locally represented by the hydrothermally altered Paalkraal Granite and the Kenkelbos Granite that together form a central dome‐like feature. The Veekraal Granite forms the peripheral part of the dome. These granitoids are, in general, coarse‐grained, porphyritic, or equigranular, showing granophyric intergrowth, and consist of quartz, perthitic microcline, and albitic plagioclase with minor opaque and interstitial hornblende/biotite. Our whole‐rock chemical data on western LGS, along with those from the Groblersdal area, eastern BIC (taken as reference), form a cluster in the ‘granite’ field in the normative Ab–An–Or diagram with mean Or/(Ab+Or) ~0.5 (0.11), and are peralkaline to weakly peraluminous in composition. These A‐type granitoids are strictly ‘ferroan’ and calc‐alkalic to alkali‐calcic in nature, and evolved through partial melting as evident mainly in process identification plots. In bulk chemical composition, the LGS and high‐SiO2 Rooiberg Group rhyolites overlap with the experimental melts generated during dehydration melting of tonalite–granodiorite, indicating the Archaean Kaapvaal TTG as their possible source. Constraints from experimental petrology further suggest the pressure and temperature of this relatively anhydrous (≤1 wt% H2O) melting were 2–1 kbar (i.e., ≤7.5 km crustal depth) and ~920°C, respectively. As suggested before, the parent magma of the voluminous mafic–ultramafic body (RLS) of the BIC, forming a voluminous magma chamber in shallow crustal depth, provided the heat for this partial melting event. The following igneous event as evident from field description in the literature, our new borehole description and whole‐rock chemical data, were a limited chemical mixing between the parent basaltic magma of the Rooiberg Group and the parent LGS magma under anorogenic condition to generate the metaluminous felsic magma represented by the Rooiberg Group dacite at present. This anorogenic mixing, perhaps guided by chemical diffusion and lasted for ≤950 ka, modified the major oxide compositions (except Na), and/or transitional elements (Sc, V, Co, and Ni), LILEs (Rb, Sr, and Ba), REEs, HFSE (Y, Zr, Nb, and Ta), actinide (Th and U), and possibly Sr‐isotopic compositions of both the parent LGS and Rooiberg Group basaltic magmas. The petrogenetic model observed for the BIC is perhaps unique in explaining geochemical and isotopic variations observed in bimodal A‐type granitoid/volcanic suites like the Karoo Igneous Province.

Petrology and tectonic evolution of late Paleozoic mafic-ultramafic sequences and the Leones Pluton of the Eastern Andean Metamorphic Complex (46-47°S), southern Chile

The metamorphosed mafic-ultramafic sequences of the Eastern Andean Metamorphic Complex outcropping in the Patagonian Andes are critical to disclose the late Paleozoic tectonic evolution of the southwestern margin of Gondwana. In the study area, mafic-ultramafic bodies are thrusted onto polydeformed metasedimentary rocks and intruded by the mid-Carboniferous composite Leones Pluton. The metabasalts (mostly tremolite-chlorite schists and amphibolites) show N-MORB and BABB chemical affinities pointing that formed part of an oceanic crustal section with components of the marginal basin, emplaced after the main pulse of Devonian arc magmatism, possibly in a retreating convergent margin. Interleaved serpentinites consist of serpentine polymorphs (antigorite, lizardite, and late chrysotile) and magnetite, with variably distributed minor amounts of chlorite, tremolite, and traces of ilmenite. Serpentinites have high Cr, Ni, Ti, and Yb contents, and show slightly enriched LREE and flat HREE patterns with a noticeable Eu positive anomaly. Mineralogical and geochemical features indicate that olivine-rich clinopyroxene-spinel-bearing peridotites were metamorphosed in a newly formed east-dipping subduction zone. The closure of the marginal basin continued with the tectonic underthrusting and tectonic juxtaposition of mafic-ultramafic rocks within an accretionary wedge. The tectonic cycle of the oceanic basin finished with the intrusion of mid-Carboniferous subduction-related plutons and pluton-driven thermal metamorphism.

Suprasubduction zone ophiolite fragments in the central Appalachian orogen: Evidence for mantle and Moho in the Baltimore Mafic Complex (Maryland, USA)

AbstractSuprasubduction zone (SSZ) ophiolites of the northern Appalachians (eastern North America) have provided key constraints on the fundamental tectonic processes responsible for the evolution of the Appalachian orogen. The central and southern Appalachians, which extend from southern New York to Alabama (USA), also contain numerous ultramafic-mafic bodies that have been interpreted as ophiolite fragments; however, this interpretation is a matter of debate, with the origin(s) of such occurrences also attributed to layered intrusions. These disparate proposed origins, alongside the range of possible magmatic affinities, have varied potential implications for the magmatic and tectonic evolution of the central and southern Appalachian orogen and its relationship with the northern Appalachian orogen. We present the results of field observations, petrography, bulk-rock geochemistry, and spinel mineral chemistry for ultramafic portions of the Baltimore Mafic Complex, which refers to a series of ultramafic-mafic bodies that are discontinuously exposed in Maryland and southern Pennsylvania (USA). Our data indicate that the Baltimore Mafic Complex comprises SSZ ophiolite fragments. The Soldiers Delight Ultramafite displays geochemical characteristics—including highly depleted bulk-rock trace element patterns and high Cr# of spinel—characteristic of subduction-related mantle peridotites and serpentinites. The Hollofield Ultramafite likely represents the “layered ultramafics” that form the Moho. Interpretation of the Baltimore Mafic Complex as an Iapetus Ocean–derived SSZ ophiolite in the central Appalachian orogen raises the possibility that a broadly coeval suite of ophiolites is preserved along thousands of kilometers of orogenic strike.

Carajás Mineral Province - Example of metallogeny of a rift above a cratonic lithospheric keel

The Carajás Mineral Province (CMP) is located on the southeastern margin of the Amazonian Craton, in northern Brazil. It is the largest producer of high-grade iron ore and a major world supplier of copper, nickel, manganese and gold. This article presents the CMP as a product of oblique rift evolution (2.76–2.06 Ga) controlled by the Canaã and Cinzento strike slip system. The Carajás rift is hosted in the tonalite-trondhjemite-granodiorite (TTG)-greenstone Rio Maria terrains (3.0–2.85 Ga). It overlies a subcratonic lithospheric mantle (SCLM) keel as indicated by seismic tomography and is filled by the Carajás plutonic-volcanic sedimentary sequences (CPVSS) that host the CMP. The rifting stage (Grão Pará Group) began with intense tholeiitic fissure volcanism with basalt and rhyolite flows, coeval intrusions of alkali granites and mafic-ultramafic bodies, BIFs, shales and restricted tuff beds. The Azul and Águas Claras Formations consist mainly of shales and sandstones deposited during the sag stage. The calc-alkaline signature of the Grão Pará metavolcanic rocks was demonstrated to reflect the composition of the melted TTG basement. The high magnesium and platinum-group elements (PGE) contents of Puma and Onça mafic-ultramafic complexes indicate extensive partial fusion, conditions that are indicative of a plume origin. We are proposing that CMP's IOCG deposits were formed from saline carbonate fluids exsolved from deep-seated metasomatic mantellic magmas (MMM), channelled and concentrated along jogs and step-overs in strike-slip fault systems in the Carajás rift.The CMP comprises three main ore-forming stages: (1) a plume stage, ca. 2.76–2.73 Ga, creating supergiant exhalative sedimentary biogenic Fe deposits (Serra Norte, Serra sul and Serra Leste), syngenetic exhalative Cu–Au–Zn (Pojuca deposit), exhalative Mn (Antonio Vicente), Ni-PGE-reef deposits in a magma mixing zone of layered intrusions (e.g. Luanga deposit), and Siqueirinho IOCG deposit; (2) a metasome stage including crustal extension and mantle flash melting (60–100 km depth), ca. 2.68–2.50 Ga, forming the most important IOCG deposits like Salobo, Alemão and Igarapé Bahia. and (3) a sag stage, ca. 2.68–2.06 Ga, with sedimentary Mn deposited in a redox-controlled stratified oceanic environment. The deposits related to granitic intrusion, ca. 1.88 Ga, with Cu-(Au–Mo–W–Sn) greisen (Breves deposit) and Cu-(Au–Mo–F) veins (Gameleira and Alvo 118 deposits) are associated with the A-type magmatism that affected the entire central Amazonian Craton. These deposits are, in contrast, related to an intracratonic silicic large igneous province (SLIP) and have no relation to the Archean Carajás cratonic lithosphere keel rift evolution. The CMP is directly related to the thinning of the lithosphere above a mantle plume, beneath a sub-continental lithospheric mantle (SCLM) keel.

Compositional variations, thermometry, and probable parental magmas of Archean chromite from the Sargur greenstone belt, Western Dharwar Craton (India)

The chromite and magnesite deposits from Talur, Sindhuvalli, and Doddakanya in the layered ultramafic-mafic plutonic sequence are a part of the Archean Sargur greenstone belt within the Western Dharwar Craton of the Indian shield. The ultramafic bodies in these three areas are highly sheared and comprise of serpentinized dunite with chromitite bodies along with harzburgite and pyroxenite. Detailed electron microprobe studies reveal intense compositional variability of the chromite grains in silicate-rich chromitite (~ 50 vol% of chromite) and serpentinite (~ 2 vol% of chromite) throughout the entire ultramafic sequence. In Talur, the chromites in massive chromitites (~ 60–70 vol% of chromite) are characterized by relatively lower Mg# (0.28–0.39) compared to chromites from massive chromitites in Sindhuvalli (Mg# = 0.63–0.69). The accessory chromite in dunite from Talur shows higher Cr#, Fe2O3, TiO2, and MnO content than other localities. The range of Fo content of cumulus olivine in dunites from Talur (Fo91-92) is relatively lower than the dunite from Doddakanya (Fo92-93) and net-textured chromitite from Sindhuvalli (Fo95-96). The range of Mg# in the intercumulus clinopyroxene in Talur net-textured chromitite varies between 94 and 99, whereas, in the Sindhuvalli net-textured chromitite, the intercumulus clinopyroxene shows a lower range of Mg# (= 96–97). Chromites in massive and silicate-rich chromitites, and from dunite contain several rounded to subhedral inclusions of silicate minerals of various sizes (~ 4–20 μm). Spherical inclusions of olivine (Fo94) are present in accessory chromites of dunite (Fo87). The Mg-rich clinopyroxene inclusions have Cr2O3 content from 1.5 to 2.3 wt%. The clinopyroxene inclusions in the Talur schlieren-banded chromitite are more magnesian (Mg# = 95–98) compared to the inclusions in chromites of the Sindhuvalli net-textured chromitite (Mg# = 96–97). The euhedral phlogopite inclusions in chromites from the Sindhuvalli net-textured chromitite are Mg-rich (~ 31.09 wt% MgO) with a significant amount of K2O (~ 9.28 wt%). The chlorite inclusion in chromites from the Talur schlieren-banded chromitite is also Mg-rich (~ 39.32 wt% MgO) with Al2O3 ~ 21.01 wt% and a significant amount of Cr2O3 (~ 3.51 wt%). Clinopyroxene (along with phlogopite) inclusions always occur as independent grain testifying their primary nature and might be genetically linked with the chromite crystallization from a primary high-Mg silicate magma. Chlorite and dolomite inclusions may be related to secondary hydrothermal alteration events. The average re-equilibration temperature of the olivine-chromite pairs in the Sindhuvalli net-textured chromitite and Doddaknaya dunite of around 545 °C represents the first stage of metamorphism under reducing conditions. Subsequently, the second hydrothermal episode involved fluids evolving to an average temperature of about 327 °C calculated from the accessory chromite and olivine pairs from Talur dunite, consistent with the abundance of magnesite veins throughout the Talur mines. The computed values of Al2O3, TiO2, and FeO/MgO ratio of the melt show that the parental melt associated with the chromitites in the Sargur greenstone belt has a close resemblance with the parental melt of the chromitites in the Nuggihalli schist belt, which is low-Al komatiitic. In the tectonic discrimination diagrams, most of the primary chromite compositions from Sargur plot within the field of chromites from komatiites from the Archean greenstone belts. Parental komatiitic magma fractionated in the subvolcanic conduits within the thin lithospheric crust and formed the sill-like layered ultramafic-mafic bodies with chromite deposits. These conduits fed the overlying extrusive system that developed the ultramafic-mafic volcanic sequence of the Sargur greenstone belt in the Western Dharwar Craton.

Seismic Evidence for Proterozoic Collisional Episodes along Two Geosutures within the Southern Granulite Province of India

Abstract The Southern Granulite Province of India had witnessed episodes of multiple tectonic activities, leading to sparsely preserved surface geological features. The present study is focused on unraveling the geodynamic evolution of this terrain through measurement of Moho depth and Vp/Vs ratio using data from a large number of broadband seismic stations. These results unambiguously establish three domains distinct in Moho depth and crustal composition. An intermediate to felsic crust with a 7–10 km step-in-Moho is delineated across the Moyar–Bhavani region. Anomalously high felsic crust with abrupt jump in Moho (~8–10 km) together with a dipping feature at deeper level characterizes the transition from eastern to southern segments of the Jhavadi–Kambam–Trichur region. By contrast, the central zone hosting the Palghat–Cauvery shear zone records uniform felsic crust and flat Moho. Drawing analogy from similar results in different parts of the globe, juxtaposition of petrologically dissimilar crustal blocks characterized by varied depths to the Moho is argued to point towards unambiguous presence of two distinct geosutures in the study area: one along the Moyar–Bhavani region and the other across the Jhavadi–Kambam–Trichur. This inference is corroborated by the presence of layered meta-anorthosite, related rock suites, and mafic-ultramafic bodies, supporting the view of a suprasubduction setting in the Moyar–Bhavani region. The Jhavadi–Kambam–Trichur area is marked by operation of the Wilson cycle by way of sparsely preserved geological features such as the presence of ophirags (ophiolite fragments), alkali syenites, and carbonatites. Geochronological results suggest that the suturing along Moyar–Bhavani took place during the Paleoproterozoic and that along Jhavadi–Kambam–Trichur was during the late Neoproterozoic.

ПЕТРОЛОГИЯ КЫЗЫР-БУРЛЮКСКОГО МАФИТ-УЛЬТРАМАФИТОВОГО МАССИВА (СЕВЕРО-ВОСТОК ЗАПАДНОГО САЯНА)

Актуальность исследования. В Западном Саяне установлены многочисленные интрузивные мафит-ультрамафитовые тела. Их формационная принадлежность и металлогеническая специализация до настоящего времени остаются до конца не выясненными и постоянно привлекают внимание как производственных, так и научных организаций, в связи с их металлогенической специализацией на хром, никель, медь, благородные металлы. Для выяснения их формационной принадлежности нами получены оригинальные результаты петрологического исследования мафит-ультрамафитового Кызыр-Бурлюкского массива, которые позволяют выявить условия его формирования, последующего преобразования в процессе длительной эволюции на уровнях верхней мантии–земная кора. Объекты: мафит-ультрамафитовый Кызыр-Бурлюкский массив. Он расположен в северо-восточной части Западного Саяна и, совместно с Калнинским и Эргакским массивами, является составной частью потенциально хромитоносных массивов Амыльского рудного района. Методы. Петрографическое изучение ультрамафитов проводилось на поляризационном микроскопе AxioScope-40 Carl Zeiss. Диагностика химического состава силикатных и рудных минералов выполнена методом рентгеноспектрального микроанализа с применением электронного сканирующего микроскопа Tescan Vega II LMU, оборудованного энергодисперсионным Oxford INCA Energy 350 и волнодисперсионным Oxford INCA Wave 700 спектрометрами. Результаты. Согласно проведенному петрологическому исследованию Кызыр-Бурлюкский массив представлен главным образом предельно деплетированными дунитами, сформировавшимися в верхней мантии, на уровнях которой они претерпели интенсивные высокотемпературные пластические деформации внутрикристаллическим трансляционным скольжением при подчиненной роли синтектонической рекристаллизации. В коровых условиях, под воздействием более поздних габброидных интрузивов, дуниты подвергались высокотемпературному метасоматозому с образованием пород верлитклинопироксенитовой ассоциации.

Petrogenesis, platinum-group element geochemistry and geodynamic evolution of the Cretaceous Chilas gabbros, Kohistan island arc, NE Pakistan

Kohistan, an intra-oceanic arc, formed due to the India-Asia collision-related tectonics. The arc is comprised of mafic-ultramafic and crustal lithologies. Chilas Complex, one of the major mafic-ultramafic body that lies at the base of the Kohistan arc, is dominantly composed of massive gabbronorites with local ultramafic-mafic intrusive bodies. The ultramafic rocks were considered as the products of fractional crystallization whereas gabbronorites were reported to have formed at the root zone of the magma chamber either from melts or due to fractional crystallization, based on major and trace element data. To understand the petrogenesis and origin of the Chilas gabbros, we conducted platinum group elements (PGEs) geochemistry combined with whole-rock major and trace element compositions. Geochemical data of the Chilas gabbros show tholeiite series, likely derived from a depleted lithospheric mantle source via peridotite melts. Presence of hornblende and relatively higher Al/Ti ratios of clinopyroxene in the Chilas gabbros suggest hydrous magma, ascribed to mantle wedge metasomatism by slab-derived fluids. Poor correlations between MgO vs. Ni, Cr, and V, Ni vs. Cr, Dy/Yb vs. SiO2, Sr/Y vs. Eu/Eu* and relatively low TiO2 indicate a minimal role of fractional crystallization. Low contents of LOI (0–3.24 wt%), Ce/Ce* (0.92–1.01), and positive relation of La, Ta, and Hf with Zr strongly support the insignificant impact of hydrothermal alteration. Enrichment of PGEs, such as Pt (0.63–13.75 ppb), Pd (0.43–13.87 ppb), and lower Pd/Ir ratios (0.76–40.04) suggest low degree partial melting of the parent magma. Higher Pd, Pd/Ir ratios compared to Pt and Pt/Pd, combined with enrichment of PPGE over IPGE, suggest the participation of slab-dehydrated fluids and mantle metasomatism however Cu/Ir vs. Ni/Pd, V-Pd/Ir and La-Pd/Ir ratios point towards olivine fractionation in the magma. Lower Cu/Pd ratios, relationships of Ru and Pt against Cr, and Os diminution corroborate S-undersaturation in parental magma. The PGEs contents and Pd/Pt ratios attest depleted mantle source for their production. In view of Cu/Zr and Cu/Pd ratios, it is substantiated that the sulphide segregation cannot be a promising reason for PGEs depletion. Therefore, we infer PGEs depletions in parental magma are due to PGEs-depleted mantle source, though sulfide retention in mantle source cannot be eliminated. The geochemical fingerprints in tectonic diagrams and negative anomalies of Nb and Zr in the studied samples provide credible facts that favour arc magmatism. Based on major and trace element data and PGEs values, we consider that the Chilas gabbros were formed from a lithospheric mantle that was metasomatized by fluids generated at the mantle wedge and derived from mafic melts in an arc setting. Geochemical data and their plotting on various tectonic discrimination diagrams indicate Chilas Complex as a remnant of the Neo-Tethyan oceanic arc system.