Gabbro Complex Research Articles

A new magmatic crystallization age constraint for the Petrota gabbroic complex from the Jurassic Evros ophiolite, eastern Circum-Rhodope Belt, NE Greece

The Circum-Rhodope Belt is a major tectonic unit that frames both the Rhodope and the Serbo-Macedonian zones of the Alpine orogen in the northern Aegean region. In the eastern part of the Circum-Rhodope Belt, the supra-subduction zone Evros ophiolite is an important crustal component that spans 176 Ma to 169 Ma magmatic activity, which cooled down to 550 °C around 164 Ma according to published radiometric dating. This incomplete ophiolite contains in its upper crustal section gabbroic complexes, to- gether with massive and pillow lavas and dykes. In this contribution, in one of these gabbroic complexes, namely the Petrota gabbroic complex, a cross-cutting basalt dyke was dated to refine the timing of the gabbro crystallization and the temporal spread of the magmatic evolution for the whole complex. The basalt dyke yielded magmatic crystallization age at 164.5±0.93 Ma as derived from U-Pb zircon geochronology. This new crystallization age confirms the previous age for the Petrota gabbro crystallization at 169±2 Ma, but extends the span of the magmatic evolution within 7–8 Myr-lasting time interval (including the analytical errors) for the complex. The latter time interval is comparable in span to the magmatic evolution of another Evros ophi- olite gabbro occurrence at the town of Didymotycho in northeastern Greece.

Fossil oceanic core complexes in the Alps. New field, geochemical and isotopic constraints from the Tethyan Aiguilles Rouges Ophiolite (Val d\u2019H\xe9rens, Western Alps, Switzerland)

Exhumation of basement rocks on the seafloor is a worldwide feature along passive continental margins and (ultra-) slow-spreading environments, documented by dredging, drilling or direct observations by diving expeditions. Complementary observations from exhumed ophiolites in the Alps allow for a better understanding of the underlying processes. The Aiguilles Rouges ophiolitic units (Val d’Hérens, Switzerland) are composed of kilometre-scale remnants of laterally segmented oceanic lithosphere only weakly affected by Alpine metamorphism (greenschist facies, Raman thermometry on graphite: 370–380 °C) and deformation. Geometries and basement-cover sequences comparable to the ones recognized in actual (ultra-) slow-spreading environments were observed, involving exhumed serpentinized and carbonatized peridotites, gabbros, pillow basalts and tectono-sedimentary cover rocks. One remarkable feature is the presence of a kilometric gabbroic complex displaying preserved magmatic minerals, textures and crosscutting relationships between the host gabbro and intruding diabase, hornblende-bearing dikelets or plagiogranite. The bulk major and trace element chemistry of mafic rocks is typical of N-MORB magmatism (CeN/YbN: 0.42–1.15). This is supported by in-situ isotopic signatures of magmatic zircons (εHf = + 13 ± 0.6) and apatites (εNd = + 8.5 ± 0.8), determined for gabbros and plagiogranites. In-situ U–Pb dating was performed on zircons by laser ablation-ICP-MS, providing ages of 154.9 ± 2.6 Ma and 155.5 ± 2.8 Ma, which are among the youngest for oceanic gabbros in the Alps. Our study suggests that the former Aiguilles Rouges domain was characterized by tectonism and magmatism resembling present-day (ultra-) slow-spreading seafloor. It also suggests that the Tethyan lithosphere is laterally segmented, with punctuated magmatism such as the Aiguilles Rouges gabbros and carbonated ultramafic seafloor covered by basalts and Jurassic tectono-sedimentary deposits.

Zircon U-Pb geochronology of the newly discovered Kulabiye Cu-Ni sulfide-mineralized gabbroic complex, East Junggar Basin in Xinjiang, China

Zircon U-Pb geochronology of the newly discovered Kulabiye Cu-Ni sulfide-mineralized gabbroic complex, East Junggar Basin in Xinjiang, China

How American Mineralogist and the Mineralogical Society of America influenced a career in mineralogy, petrology, and plate pushing, and thoughts on mineralogy's future role

Abstract My geologic research began at Carleton College. I studied heavy minerals in some midcontinent orthoquartzites, publishing my very first paper in American Mineralogist in 1954. As a master's candidate at the University of Minnesota, I investigated igneous differentiation in a diabase-granophyre sill of the Duluth Gabbro Complex. Later, in a Ph.D. program at Johns Hopkins University, I became Joe Boyd's apprentice at the Geophysical Laboratory (GL), and for a time was phase-equilibrium god of the Na-amphiboles. Doctoral research earned me an offer of a UCLA assistant professorship as a mineralogist in 1960. There, I continued pursuing amphibole P-T stability relations in lab and field. My glaucophane phase equilibrium research would later be found to have instead crystallized Na-magnesiorichterite. However, amphibole research led me to map field occurrences of HP-LT (high P-low T) blueschists of the Franciscan Complex. Thus, when plate tectonics emerged in the late 1960s, I was deep in the subduction zone. My recent studies focused on the petrology and geochemistry of oceanic crustal rocks, Californian calc-alkaline arcs, and coesite ± microdiamond-bearing crustal margin rocks in various parts of Eurasia. Other works treated global mineral resources and population, mineralogy and human health, and early Earth petrotectonic evolution. I tried to work on important problems, but mainly studied topics that fired my interest. For the future, I see the existential challenge facing humanity and the biosphere as the imperative to stop our overdrafting of mineral resources. This will require reaching a dynamic equilibrium between the use and replenishment of near-surface resources (i.e., nutrients) essential for life. Earth scientists are planetary stewards, so we must lead the way forward in life-supporting mineral usage, recycling, substitution, and dematerialization. In any event, sustainable development will soon return to the Earth's Critical Zone of life because Mother Nature—the ruling terrestrial economist—abhors long-term overdrafting of resources1.

Reply to comment by Michard et al. on “The Mesozoic Margin of the Maghrebian Tethys in the Rif Belt (Morocco): Evidence for Polyphase Rifting and Related Magmatic Activity”

Key Points Opening of the Maghrebian Tethys during a major Middle Jurassic rifting phase Late Triassic to Early Jurassic CAMP‐related magmatism Non‐oceanic nature of the Mesorif Gabbroic Complex and absence of an overlaying oceanic sequence

The Ashizuri granite-alkaline gabbro complex in the forearc of a Paleogene accretionary complex, Shikoku, Japan: Constraints on evolution by zircon U-Pb age and trace element composition

The Ashizuri granite-alkaline gabbro complex in the forearc of a Paleogene accretionary complex, Shikoku, Japan: Constraints on evolution by zircon U-Pb age and trace element composition

The Mesozoic Margin of the Maghrebian Tethys in the Rif Belt (Morocco): Evidence for Polyphase Rifting and Related Magmatic Activity

AbstractThe Rif belt (northern Morocco) is a mountain chain located at the junction between the Mediterranean and Central Atlantic Domains. Although the Rif belt underwent important Cenozoic (i.e., Alpine) shortening, remnants of the Mesozoic North African rifted margin are preserved in its external zones. This contribution aims to characterize the Mesozoic architecture and polyphase rifting history of this rifted margin. We present detailed field evidence and geochronological data from two palaeogeographic zones (Mesorif and Intrarif) preserving remnants of the former North African distal margin. The Mesorif conserves lithostratigraphic associations characterized by mafic intrusive rocks overlain by dismembered and discontinuous blocks of Lower Jurassic carbonates covered by Middle to Upper Jurassic sediments. U‐Pb zircon dating of four samples from this gabbroic complex shows ages close to the Triassic‐Jurassic boundary (195–200 Ma). The gabbros were emplaced within the continental crust at the end of the first Triassic rift event and exhumed shortly after during a second Middle Jurassic rift event, which presents exceptional rift‐related structures. The most distal part of the margin is exposed in the Intrarif. In this unit, the Beni‐Malek serpentinized peridotites exhibit ophicalcites with uppermost Jurassic limestones resting conformably on top, suggesting that exhumation of the mantle occurred at the distal part of the North African margin at this time. When integrated, these new evidences enable us to discuss the evolution of the western part of the North African rifted margin and its relations with the Moroccan Atlantic margin and Tethys system.

Geochronology, petrogenesis and tectonic implications of the newly discovered Cu–Ni sulfide-mineralized Yueyawan gabbroic complex, Kalatag district, northwestern Eastern Tianshan, NW China

The early Permian Cu–Ni sulfide-bearing mafic-ultramafic intrusions from North Xinjiang in NW China help to provide constraints on the late Paleozoic crustal growth in the Central Asian Orogenic Belt (CAOB) and improve the understanding of the genesis of magmatic sulfide mineralization in orogenic settings and the identification of future prospecting targets. The Cu–Ni sulfide-mineralized Yueyawan gabbroic complex in Kalatag represents a new breakthrough for Cu–Ni prospecting in covered areas in the northwestern part of the Eastern Tianshan orogenic belt, North Xinjiang. Rocks from this intrusion have similar LA-ICP-MS zircon U–Pb ages (282.4 ± 1.1–281.3 ± 1.2 Ma) to other mafic-ultramafic intrusions containing magmatic sulfide mineralization in North Xinjiang. These early Permian gabbroic intrusions are thus important targets for Cu–Ni mineralization exploration in this region. The rocks of the Yueyawan gabbroic complex are a ca. 1 km2 elongated NW-trending crescent-shaped at the junction of two E-W- and NW-SE-trending faults. The intrusion has the morphology of a conduit tube. The rocks include troctolite, olivine gabbro, hornblende gabbro, and gabbro, and the Cu–Ni sulfides occur as steeply dipping (65–70°) lenses with disseminated textures in the contact zone between the olivine gabbro and hornblende gabbro, indicating that sulfide precipitation was coeval with the crystallization of the magmas giving rise to these rocks. The rocks investigated in this study are gabbroic in composition; they are characterized by moderate concentrations of SiO2 (44.8–48.8 wt%) and low to moderate concentrations of MgO (1.69–13.4 wt%, Mg# values of 39.6–73.3), Ni, and Cr. Furthermore, the rocks have high positive εHf(t) values (+13.1 to +15.9) and flat rare earth element patterns ((La/Yb)N = 2.01–2.89), with marked positive Eu anomalies, high Ti/Zr ratios (70–164), and low Ce/Pb (3.65–14.83) and Nb/Ta values (9–16), indicating very restricted amounts (5–10%) of crustal assimilation. This complex was likely derived from 10 to 20% partial melting (as evidenced by the low Sm/Yb values of 1.48–2.02 and La/Sm values of 1.61–2.23) of weakly slab melt-metasomatized (as evidenced by the low Ba/Nb and Ba/Zr ratios) subcontinental lithospheric mantle (with 2% garnet and 2% spinel lherzolite) at depths greater than 85 km in a post-collisional extensional source that contained sufficient Ni and Cu for the saturation and segregation of immiscible sulfides to concentrate the metals. The magma underwent sulfide saturation and segregation together with olivine fractionation (as evidenced by Ni and TFe2O3 increase and SiO2, CaO, and Zr decrease with increasing MgO content) in the conduit-type intrusion. Therefore, regional fault structures, ore-bearing rock types, and geochemical ratios that reflect high degrees of partial melting are features of the Cu–Ni-mineralized gabbroic rocks in adjacent areas that help support exploration.

Variations of Major and Minor Elements in Pt–Fe Alloy Minerals: A Review and New Observations

Compositional variations of major and minor elements were examined in Pt–Fe alloys from various geological settings and types of deposits, both lode and placer occurrences. They included representatives of layered intrusions, Alaskan-Uralian-(Aldan)-type and alkaline gabbroic complexes, ophiolitic chromitites, and numerous placers from Canada, USA, Russia, and other localities worldwide. Pt–Fe alloy grains in detrital occurrences are notably larger in size, and these are considered to be the result of a special conditions during crystallization such as temperature, pressure, geochemistry or time. In addition, the number of available statistical observations is much greater for the placer occurrences, since they represent the end-product of, in some cases, the weathering of many millions of tonnes of sparsely mineralized bedrock. Typically, platinum-group elements (PGE) present in admixtures (Ir, Rh, and Pd) and minor Cu, Ni are incorporated into a compositional series (Pt, PGE)2–3(Fe, Cu, Ni) in the lode occurrences. Relative Cu enrichment in alloys poor in Pt implies crystallization from relatively fractionated melts at a lower temperature. In contrast to the lode deposits, the distribution of Ir, Rh, and Pd is fairly chaotic in placer Pt–Fe grains. There is no relationship between levels of Ir, Rh, and Pd with the ratio Σ(Pt + PGE):(Fe + Cu + Ni). The compositional series (Pt, PGE)2–3(Fe, Cu, Ni) is not as common in the placer occurrences; nevertheless, minor Cu and Ni show their maximums in members of this series in the placer grains. Global-scale datasets yield a bimodal pattern of distribution in the Pt–Fe diagram, which is likely a reflection of the miscibility gap between the ordered Pt3Fe structure (isoferroplatinum) and the disordered structure of native or ferroan platinum. In the plot Pt versus Fe, there is a linear boundary due to ideal Pt ↔ Fe substitution. Two solid solution series are based on the Ir-for-Pt and Pd-for-Pt substitutions. The incorporation of Ir is not restricted to Pt3Fe–Ir3Fe substitution (isoferroplatinum and chengdeite, plus their disordered modifications). Besides, Ir0 appears to replace Pt0 in the disordered variants of (Pt–Ir)–Fe alloys. There is a good potential for the discovery of a new species with a Pd-dominant composition, (Pd, Pt)3Fe, most likely in association with the alkaline mafic-ultramafic or gabbroic complexes, or the mafic units of layered intrusions. The “field of complicated substitutions” is recognized as a likely reflection of the crystallochemical differences of Pd and Ir, extending along the Ir-Pd axis of the Ir–Pd–Rh diagram. The inferred solid solution extends approximately along the line Ir–(Pd:Rh = 2:3). Minor Pd presumably enters the solid solution via a coupled substitution in combination with the Rh. An Ir-enrichment trend in Pt–Fe alloys typically occurs in the Alaskan-type complexes. The large size of the Pt–Fe nuggets associated with some of these complexes is considered to be related to an ultramafic-mafic pegmatite facies, whereas significant Pd-enrichment is characteristic of gabbroic source-rocks (e.g., Coldwell Complex), resulting in a markedly different trend for the Pt versus Fe (wt.%). However, based on our examination of a large dataset of Pt–Fe alloys from numerous origins, we conclude that they exhibit compositional overlaps that are too large to be useful as reliable index-minerals.

Buds of Santonian magmatism associated with Marion hotspot in southern India

Imprints of Marion hotspot magmatism representing the final stages of rifting between India and Madagascar from the Gondwana assembly occur scattered in several localities of southern India. In this study, we report evidence for rift‐related Santonian magmatism from alkali gabbro in the Madurai Block and present mineralogical, petrological, zircon U–Pb geochronological, and whole‐rock geochemical data on the Mundanmudi gabbroic complex. The rock shows mesocumulate textures with plagioclase laths forming the triple junctions and porphyritic texture with hornblende and clinopyroxene as phenocrysts. Geochemically, the gabbros are characterized by moderate to high SiO2 (50–58 wt.%), alkalies (4.14–7.00 wt.%), and high TiO2 (1.71–4.22 wt.%), with calc‐alkaline to high‐K alkaline affinity. Chondrite‐normalized REE patterns are highly fractionated with LREE enrichment relative to HREE and absence of Eu anomalies. The primitive mantle normalized trace element patterns have LILE enrichment relative to HFSE, exhibiting negative anomalies at Nb, Zr, and Ti. The HFSE relationships indicate a within‐plate tectonic setting for the genesis of these gabbros. The major element tectonic discrimination plots suggest MORB‐OFB affinity which is also supported by the Nb–Th relationship endorsing the divergent tectonic processes responsible for the generation of these gabbros. The zircon U–Pb mean age of ca. 85 Ma represents the magmatic emplacement age within a rift‐related tectonic setting during the Santonian (late Cretaceous). Based on the LREE and HREE relationship, it is inferred that the parental magma might have been derived at garnet lherzolite depths above the Marion plume, possibly correlatable with the large igneous province generated at this time in Madagascar. Our study provides new evidence for the final phase of the magmatism at ca. 85 Ma related to India–Madagascar rifting.

Petrogenesis and geodynamic setting of the Neoarchaean Haines Gabbroic Complex and Shebandowan greenstone belt, southwestern Superior Province, Ontario, Canada

The 2722 ± 2 Ma Archaean megacrystic anorthosite-bearing Haines Gabbroic Complex in the Wawa subprovince of the western Superior Province formed contemporaneously with the spatially associated Greenwater Assemblage of the Shebandowan greenstone belt, Ontario, Canada. Both field and petrographic observations indicate that the Haines Gabbroic Complex and the spatially and temporally associated ultramafic to intermediate volcanic and intrusive rocks of the Greenwater and Burchell assemblages experienced extensive metamorphic recrystallization and alteration as a consequence of greenschist-facies metamorphism. These metamorphic processes resulted in pervasive replacement of igneous minerals by metamorphic counterparts. Metamorphic alteration also mobilised LILE and transition metals and slightly reset the U-Th-Pb and Sm-Nd isotope systems. Consequently, the Haines Gabbroic Complex and Greenwater Assemblage have respective Sm-Nd errorchron ages of 2765 +110/‐280 Ma and 3108 +120/‐380 Ma.Major and trace element compositions suggest that the least altered anorthosites, leucogabbros and gabbros of the Haines Gabbroic Complex are not cogenetic with the Greenwater and Burchell assemblages and have distinct parental magmas and depleted mantle sources. The calc-alkaline andesites of the Burchell Assemblage, the tholeiitic basalts and gabbros of the Greenwater and Burchell assemblages, the transitional gabbros of the Greenwater and Burchell assemblages and the picrites, serpentinites and peridotites of the Greenwater Assemblage are all petrogenetically distinct and have different parental magmas and depleted mantle sources. The pyroxenites are genetically distinct from the other Greenwater Assemblage lithologies, the Burchell Assemblage and the Haines Gabbroic Complex. The Haines Gabbroic Complex crystallised from hydrous, Ca- and Al-rich tholeiitic parental magmas that fractionated from more primitive, hydrous boninitic parental magmas.The initial εNd values of the Haines Gabbroic Complex (εNd = +3.0 to +4.1) and the Greenwater (εNd = +1.8 to +4.1) and Burchell (εNd = +2.0 to +3.7) assemblages are indicative of variably depleted mantle sources. The major and trace element characteristics of the Haines Gabbroic Complex and the Greenwater and Burchell assemblages and geological relationships indicate that they formed in a Japan-style mature intra-oceanic continental arc with a large volume of TTGs. Intra-oceanic arc rifting was most likely triggered by slab rollback, followed by mantle plume emplacement. On the basis of rock association and geochemical data, we suggest that the Haines Gabbroic Complex and the Greenwater and Burchell assemblages represent a dismembered subduction-related Archaean ophiolite that marks a Neoarchaean suture zone between the Wawa and Wabigoon subprovinces.

Geochemistry and SHRIMP U‐Pb Zircon Dating of Mafic Rocks North of Zunhua City, Eastern Hebei, North China Craton: Paleoproterozoic Gabbro rather than Neoarchean Ophiolite

Abundant mafic‐ultramafic blocks and dikes occur in the area north of Zunhua City, eastern Hebei Province, and were previously suggested to be part of a late Archean ophiolitic assemblage. We employed SHRIMP zircon dating and a geochemical study on these mafic and surrounding rocks to test the ophiolite hypothesis. The SHRIMP data suggest that three metagabbro samples were metamorphosed at ∼1.8 Ga. Numerous ∼2.5 Ga zircons display strong oscillatory zoning, characteristic of zircons from granitoid rocks but not from gabbro, so we suggest that these are xenocrystic grains. The age of these xenocrystic zircons and their metamorphic rims suggests that these mafic blocks formed in Paleoproterozoic. The surrounding gneiss of intermediate composition also contains 2.5 Ga zircons with oscillatory zoning and 1.8 Ga metamorphic rims. Fractionated REE patterns and Nb, Ta, Zr, Hf negative anomalies to variable extent were observed in the mafic blocks and surrounding rocks, also supporting a significant difference in the chemistry of ophiolitic rocks. Our data suggest that many mafic blocks in northern Zunhua are not part of a late Archean ophiolite complex but part of a tectonically dismembered Paleoproterozoic intrusive gabbro complex. This study shows that late Paleoproterozoic metamorphism occurred in the western part of eastern Hebei Province.

Ankaramite: A New Type of High-Magnesium and High-Calcium Primitive Melt in the Magnitogorsk Island-Arc Zone (Southern Urals)

This work describes the geological position, mineral and chemical composition of high-Mg effusive ankaramites occurring as dykes and lava flows. They were found in the melange zone of the western margin of the Magnitogorsk island arc zone in the Southern Urals. Data on the liquidus association of phenocrysts and on the composition of the matrix of effusives are given. According to the data obtained, the conclusion was drawn that the ankaramites studied can be attributed to the primary island arc melts, which were not subject to essential differentiation. This type of effusives has not been distinguished previously among island arc volcanogenic formations of the Urals. It is shown that ankaramites can be considered to be primary melts parental for dunite–clinopyroxenites–gabbro complexes of Ural–Alaskan type. The occurrence of ankaramites in the Paleozoic island arc formations of the Urals indicates the wehrlite composition of the mantle as the reason for the extremely wide development of wehrlites and clinopyroxenites in different mafic–ultramafic complexes of the Urals.

Petrogenesis of the Sanshilipu gabbro complex in the Shangdan Suture: New constraints on evolution of the Qinling Orogen

The Sanshilipu gabbro complex is located along the Shangdan Suture, which is important to understanding the evolution of the Qinling Orogen, Central China. The complex is mainly composed of norite, gabbro, and gabbro–diorite. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon dating yields a weighted mean 206Pb/238U age of 414 ± 2.8 Ma, interpreted to represent the crystallization age of the gabbro complex. The complex has SiO2 contents ranging from 48.6 to 51.3 wt% with enrichment of Na relative to K, showing calc‐alkaline feature. The studied samples are characterized by an enrichment of large‐ion lithophile elements (Ba, Sr, and Pb) and depletion of HFSE (Nb, Ta, Zr, Hf, and Ti) and a large range of Nb/Ta ratios (3.7 to 11.8). They also have early flattened‐LREE distribution patterns. These geochemical characteristics indicate that the Sanshilipu gabbro complex was formed in an island‐arc environment. They have slightly enriched Sr–Nd isotope compositions with (87Sr/86Sr)i = 0.7047–0.7063 and ɛNd(t) from −0.44 to −4.63, indicating that the formation of the complex was involved in some subducted sediments and oceanic crust and fluid released from the subducted slab. The identification of island‐arc‐originated mafic rocks suggests that the onset time of the collision between the Yangtze Craton and the North China Craton should be younger than 415 Ma.

A double Fe-Ti oxide and Fe-sulphide liquid immiscibility in the Itsindro Gabbro Complex, Madagascar

The petrology and mineralogy of the Itsindro complex in south-central Madagascar has been investigated through samples obtained from the 320.7 m-deep Lanjanina borehole. The section consists of a 254 m-thick pyroxenite unit with interbedded gabbro layers that overlies a gabbro unit and is itself overlain by a 19 m-thick granite unit. Most of the structures are sub-horizontal. A weak magmatic layering is locally observed but at the scale of the core, the intrusion does not appear to be a layered complex.Pyroxenite and gabbro show a systematic disseminated mineralization consisting of Fe-Ti-P oxides and Fe-(Cu-Ni) sulphides that takes the form of ilmenite-titanomagnetite ± apatite and pyrrhotite ± chalcopyrite ± pentlandite. In the upper zone, from 90 to 72 m, sub-massive centimetre-to decimetre-sized layers of oxides and sulphides comprise a total of 16 m of sub-massive sulphide (the main mineralized zone). In this mineralized zone the oxide/sulphide ratio is close to 1/1. The sulphide is strongly dominated by pyrrhotite, which may locally contain inclusions of molybdenite crystals with the Re sulphide rheniite (ReS2). Oxides are generally euhedral, included in or attached to the Fe-sulphide, and also locally form sub-massive centimetre-sized bands.Apatite as a cumulus phase is ubiquitous. Locally it may account for 30% of the ore-rich samples and some samples consist of apatite-Fe-Ti oxides-Fe-Cu-Ni sulphides with virtually no silicate. Apatite is the main REE carrier but the total REE content remains low (<90 ppm).Mineral compositions and whole rock geochemistry indicate that the rocks are highly differentiated, and in spite of a relatively limited thickness, the differentiation process is observed. Two zones can be distinguished: from the bottom to 162.8 m we see a decrease in the Mg number of olivine and pyroxene, and a drop in TiO2 and Al2O3 for the latter. A reverse trend is then observed within the pyroxenite unit from the 162.8 m level upwards. The main mineralized zone is located in the upper part of this unit, about 70 m above this discontinuity.The highly differentiated Fe-Ti-P facies of the Lanjanina series in the Itsindro Gabbro Complex have been interpreted as corresponding to the Fe-Ti-P rich, Si-poor member remaining after an immiscible segregation of an evolved mafic magma. The granite dykes and the overlying granite unit represent the second, Si-rich member of the immiscibility process.The presence of large amounts of sulphide is attributed to sulphur contamination of the Fe-Ti-rich liquid. Fe-Ti oxides will tend to crystallize on the sulphide droplets and the accumulation of dense Fe-sulphides (liquid) and associated Fe-Ti oxides (solid) will result in this complex and unusual association taking the form of a net texture.

The first dating results for gabbro of the dunite–clinopyroxenite–gabbro complex of the Chistop massif (North Urals)

The first data on the Late Riphean age by U–Pb and Sm–Nd analysis (≥922 ± 14 and 686 ± 19 Ma, respectively) were obtained for rocks of the dunite–clinopyroxenite–gabbro complex of the Chistop massif in the Patinum-bearing Belt of the Urals. These data allow one to assume that the formation of the Ural paleoocean probably started immediately after the break-up of Rodinia.

Geology, U–Pb zircon geochronology, and geochemistry of PGE-bearing Neoarchean and Paleoproterozoic gabbroic rocks of the Peter Lake domain, southern Hearne craton, Canada

The Peter Lake domain, a component of the Hearne craton in Saskatchewan, includes several intrusive complexes ranging in age from 2.70 to 1.83 Ga. Two gabbroic complexes (Swan River and Porter Bay) together represent one of the largest accumulations of Precambrian mafic intrusion, both metamorphosed at amphibolite facies. Sulfide-poor PGE occurrences are found in both complexes and share many textural and lithological characteristics with magmatic contact-type PGE–(Ni–Cu) deposits such as the chaotic Lac des Iles pluton and with layered stratiform deposits of large layered intrusions such as Bushveld or Stillwater. Lithologically, both complexes are dominated by leucocratic gabbronorite and gabbro, locally characterized by cumulate layering, cross-bedding, brecciated, and pegmatitic textures. U–Pb zircon crystallization ages between 2562 and 2560 Ma were obtained for the Swan River complex. sulfide formation is interpreted to be of magmatic origin rather than remobilized, as had been speculated by exploration geologists, and is therefore of the same age as the host gabbro. Geochemical data support the interpretation of a mantle plume origin in a subduction environment for the Swan River complex. The Porter Bay complex is much more restricted in areal extent and a leucocratic gabbronorite yielded a U–Pb zircon age of 1913 ± 1 Ma, which is interpreted as the crystallization age and the time of sulfide formation. Major and trace element geochemical data of Porter Bay complex rocks show a considerably more evolved character than the Swan River complex, and is interpreted as indicating emplacement in a continental arc environment.

U–Pb geochronology of the Sondalo gabbroic complex (Central Alps) and its position within the Permian post-Variscan extension

The end of an orogenic cycle is commonly associated with a general extensional regime, widespread magmatism and complex metamorphic overprints. The Austroalpine domain in SE Switzerland and N Italy preserves a polyphase tectonic history spanning the Carboniferous Variscan orogeny, the Late Carboniferous–Early Permian post-Variscan extension and the Jurassic rifting. In this study, the late- and post-Variscan evolution of the Austroalpine domain is explored by constraining the timing of intrusion of the Sondalo gabbroic complex in the Campo unit. U–Pb zircon dating on magmatic and metamorphic rocks and trace element geochemistry in zircon are performed using laser ablation coupled with mass spectrometry. The ages of both magmatic and metamorphic rocks range between 289 ± 4 and 285 ± 6 Ma. Trace elements in zircon from magmatic rocks indicate that zircon grew in two different chemical systems: an initial one devoid of garnet and with already crystallized plagioclase, and a second one with an increasing modal amount of garnet and lacking (initial) plagioclase. The second system probably reflects mixing of the initial magma with melt derived from the surrounding partially molten metapelites. New results, existing age data and P-T estimates allow to describe the late- to post-Variscan evolution of the Austroalpine domain. These data are compared and integrated in the framework of the Permian geodynamic setting in Western Europe. We present new arguments to consider the “Permian event” in Europe as a process which was temporally distinct from the collapse of the Variscan orogen. Late-Variscan re-equilibration of the crustal thickness occurred between 310 and 290 Ma (i.e. during the collapse phase). Permian mafic magmas emplaced from 290 to 270 Ma formed either during or even after the latest stages of the Variscan collapse.

Continental gabbros in the Dent Blanche Tectonic System (Western Alps): from the pre-Alpine crustal structure of the Adriatic palaeo-margin to the geometry of an alleged subduction interface

Several Permian gabbro bodies are known in the continental-derived Dent Blanche Tectonic System (Western Alps). A new gabbroic complex, the Berrio Gabbro, is described in this paper, and compared with the other gabbro bodies of the Adriatic palaeomargin, as well as of the Tsaté Unit, a remnant of the Piemonte–Liguria ocean, immediately underlying the Dent Blanche Tectonic System. The Berrio Gabbro comprises ultramafic cumulates (clinopyroxenite, wehrlite and hornblendite) and mafic rocks (leucocratic and mesocratic gabbros). Ultramafic (serpentinite) and mafic rocks (gabbro) are also present in the oceanic Tsaté Unit of the Combin Zone. The ultramafic–mafic rocks of the Dent Blanche and the Tsaté are clearly distinguished by their mineralogy and their geochemistry. In addition, their behaviour during the Alpine deformation is distinct. Detailed mapping associated with geochemical characterization of the protoliths allows us to infer the structure of the Adriatic crust during the Permian extension and refute a model of mixing between oceanic and continental material along the hypothesized subduction interface. Supplementary material: Additional supporting information (deformation–mineral growth relationships and Alpine mineral analyses of the Berrio Gabbro basic and ultrabasic rocks) is available at https://doi.org/10.6084/m9.figshare.c.3673039 .

Influence of geology on groundwater–sediment interactions in arsenic enriched tectono-morphic aquifers of the Himalayan Brahmaputra river basin

Summary The present study interprets the groundwater solute chemistry, hydrogeochemical evolution, arsenic (As) enrichment and aquifer characterization in Brahmaputra River Basin (BRB) involving three geologically and tectono-morphically distinct regions located in northeastern India. These study regions consist of the northwestern (NW) and the northern (N) region, both located along the western and eastern parts of Eastern Himalayas and the southern (S) region (near Indo–Burmese Range and Naga hills) of the Brahmaputra basin which show distinct tectonic settings and sediment provenances in the Himalayan orogenic belt. Stable isotopic composition (δ2H and δ18O) in groundwater suggests that some evaporation may have taken place through recharging of ground water in the study areas. The major-ion composition shows that groundwater composition of the NW and N parts are between Ca HCO3 and Ca Na HCO3 while the S-region is dominated by Na Ca HCO3 hydrochemical facies. The major mineralogical composition of aquifer sediments indicates the dominant presence of iron(Fe)-oxide and oxyhydroxides, mica (muscovite and biotite), feldspar, pyroxene, amphibole, abundance of quartz and clay minerals whereas clay is predominantly present in sediments of S-aquifers. These mafic minerals, aluminosilicates and clay minerals might offer available reactive surface for As-adsorption and co-precipitatation with amorphous Fe. These associated adsorbed and co-precipitated As might be released due to reductive dissolution of Fe-oxide and oxyhydroxides in groundwater. These minerals are assumed to be possible sources of As in groundwater. The stability diagrams of groundwater data suggest that solute might have been introduced into groundwater from weathering of K-feldspar, plagioclase, pyroxene of Himalayan rocks, the Siwalik Group and Eastern Syntaxes in NW and N-regions. However, basic cations might be derived from weathering of K-feldspar, plagioclase, pyroxene and olivine those being major constituents in a gabbroic complex (ophiolite) and basalt terrain in S-region. The aquifers of S-region are severely contaminated with dissolved As compared to NW and N regions. Almost more than 92% of groundwater samples in the southern part (maximum 5.53 μM or 415 μg/L) are enriched with As, which draws a distinct difference from the NW and N parts of BRB aquifers. The redox-sensitive solutes (i.e., Fe, Mn, HCO3− and TOC) are positively correlated with As in NW and N-parts; whereas EH shows negative to very weak positive correlation which suggests that a redox-dependent mobilization plays important role in As liberation in NW and N parts of the basin. However, As in southern aquifers is not showing any correlation or weak negative correlation with redox-sensitive solutes; suggesting that multiple reactions and hydrogeochemical processes and their interaction control As mobilization and fate in the S-region of BRB. The occurrence of high concentrations of arsenic in groundwater of Brahmaputra basin is described through a crustal recycling model and tectonic movement between the Indian–Eurasian plates and Burmese micro-continents. As-enriched groundwater in Himalayan foreland basin in the BRB is probably a result of crustal evolution through which As is subsequently mobilized from aquifer matrix to solution in groundwater by water–sediment reaction under favorable biogeochemical conditions. The results of the study indicate geological control (i.e. change in lithofacies, tectonic set-up) on groundwater chemistry and distribution of redox-sensitive solutes such as As.