High Concentrations Of Incompatible Elements Research Articles

The origin of the ultrahigh-pressure Tso Morari complex, NW Himalaya: implication for early Paleozoic rifting

AbstractThe origins and age distribution of the Himalayan high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks are critical for understanding the pre-Himalayan history. Although the protoliths to the UHP Tso Morari eclogites in Ladakh, NW Himalaya are believed to be the Permian Panjal volcanics, the geochronological evidence is absent. Here, we demonstrate that the protoliths of the UHP Tso Morari Complex formed in a continental rift setting at the Indian margin associated with the northern East Gondwana during the Early Paleozoic. Zircon U–Pb dates from eight gneisses and one garnet amphibolite indicate the Early Paleozoic bimodal magmatism of 493–476 Ma, which could be associated with the separation of South China from North India. Except for arc-related eclogites found in the Nidar ophiolite, the eclogites and amphibolites are rift-related, exhibiting enriched light rare earth elements and high concentrations of incompatible elements, along with evidence for crustal contamination. Our findings support the previously reported diversity in the sources and ages of the protoliths of the Himalayan HP–UHP metamorphic rocks along the orogen.

In-situ trace element and Li-isotope study of zinnwaldite from the Degana tungsten deposit, India: Implications for hydrothermal tungsten mineralization

Zinnwaldite, a Li, Rb-mica is a common magmatic or hydrothermal mineral in many tungsten deposits. In this study, we use the trace element and Li-isotope composition of zinnwaldite from Degana, the largest tungsten deposit of India, to constrain the source and evolution of the ore fluid, the precipitation mechanism of W and the roles of fractional crystallization, fluid exsolution and fluid-rock interaction in the mineralization process. Textural evidence suggests that the mineralization in the Degana rocks involved at least two stages of fluid infiltration. The earlier of the two was multi-pulsed and responsible for the primary tungsten and zinnwaldite mineralization. The second hydrothermal stage caused partial dissolution and reprecipitation of both wolframite as well as zinnwaldite, reflected in the patchy zones that replace both minerals. The ore fluid had high concentration of Li, Rb, Cs, Nb, and Ta as evident from zinnwaldite chemistry. The δ7Li of the ore fluid (+14 to +19 ‰), estimated from the Li-isotope composition of zinnwaldite in mineralized veins (δ7Li = +12 to +17‰), is significantly heavier compared to upper/middle continental crust, ruling out the possibility of a metamorphic fluid source. The high concentration of incompatible elements including Cs, extremely low K/Rb (10–15), is best explained by its growth from fluids exsolved from an enriched, late-fractionated granitic melt. Alteration of the host granite, formation of greisen, large-scale albitization and muscovitization are indicative of fluid-rock interaction, which might have increased the fluid pH, destabilizing fluoride complexes of W, resulting in the precipitation of wolframite. The shift in the chemical and Li-isotope composition from the early mica to late altered ones in the greisen indicates interaction of the hydrothermal fluid with the host rocks. A U-Pb Concordia age of 838 ± 9 Ma retrieved from magmatic domains of zircon from the Degana granite dates it emplacement and represents the maximum age of the W-mineralization.

Enrichment of Incompatible Elements in Alkaline Syenites in Large Igneous Provinces Due to Magma Replenishment and Reactive Porous Flow in a Mush Reservoir

AbstractUnderstanding the petrogenesis of alkaline syenites is important for constraining the mechanisms of rare earth element (REE) and rare-metal mineralization. Here we report a detailed petrological and geochemical study of early Permian syenitic rocks from the southwestern Tarim large igneous province (TLIP) in the Wajilitag area (China). We use these data to investigate the complex magmatic processes responsible for the enrichment of incompatible elements (e.g. REEs and rare metals) in these rocks. The Wajilitag syenitic rocks comprise early hornblende syenite (281 Ma) and later nepheline syenite (278 Ma), both of which are spatially and temporally associated with mafic intrusions (i.e. gabbro and diabase). These syenitic rocks show continuous variations in major elements with the mafic rocks. They also have similar Sr–Nd–Hf isotopic compositions with the nearby mafic rocks, denoting that the Wajilitag syenitic rocks should be derived from the melts represented by these mafic rocks. However, these syenitic rocks, particularly the nepheline syenites, are typically characterized by extreme enrichment of incompatible elements, which is hard to be accounted by simple fractional crystallization of mafic magmas. Some clinopyroxene phenocrysts in the Wajilitag nepheline syenites show oscillatory zoning with strikingly increasing and then gradually decreasing MgO contents, which recorded replenishment of mafic magma at the interval. Low-MgO clinopyroxene mantles and/or rims have much higher incompatible element contents (e.g. Nb, Ce, and Zr) and ratios (e.g. Ce/Y) than their cores, which could reflect melt injection controlled by reactive porous flow in a mush reservoir of a crustal magma chamber. Such processes may also cause the high Ce and/or Nb contents of the mantles and/or rims of zoned titanite and apatite phenocrysts. The injection of reactive porous flow melts is the key process that produces the extreme enrichment of incompatible elements in the alkaline syenites from the western TLIP, as well as other plume-related alkaline syenites that host world-class REE and rare-metal deposits.

A microcosm of modern crust formation: Evidence from zircon ages, Hf[sbnd]O and Nd[sbnd]Sr isotopes and bulk geochemistry of the Menglian Batholith, SE Tibet

Modern continental crust has evolved to a more potassic, granitic composition than early continental crust, which comprises largely sodic TTG-suite magmas. The present paradigm holds that the latter are largely “juvenile” (in the sense that the time from mantle extraction to felsic crust production is comparatively short, of the order 10–100 Ma) while the former represent recycled older crust of igneous or sedimentary composition. The data from high-Mg diorites, tonalites, granodiorites and potassic granites of the 125–115 Ma Menglian Batholith (SE Tibet) exemplify the modern situation and can therefore be used to constrain current crust formation processes. These rocks have higher concentrations of incompatible elements than magmatic rocks from typical continental arc settings, with a continuum of increasing K2O/Na2O ratios, SiO2, K2O, Rb, and Th concentrations juxtaposed with decreasing MgO, CaO, and Sr. They consistently record both higher zircon δ18O values than mantle values and decoupled NdHf isotope systems caused by the interaction of subducted sediments with the mantle wedge. Petrogenetic mechanisms that connect the suite include crystal fractionation within the diorites, melting of the lower crust induced by advection of heat and water by the diorites, and high-level fractionation of the tonalite-granodiorite suite to produce the high-silica granites. Therefore, this example of modern fertile continental crust had a five-stage evolution: (1) subduction-enrichment of the mantle source, (2) mantle melting to produce mafic magmas that pooled in or below the lower crust, (3) mafic magma differentiation to produce the tonalite-granodioritic magmas controlled by crystal-liquid equilibria, (4) crustal melting and admixture to the evolving felsic magmas and (5) final high-level fractionation and melt extraction to produce the silicic extreme, enriched in incompatible elements such as Rb, Th and K. This model could be a general mechanism for how modern mature continental crust evolves. Importantly, it indicates a significant role for mafic magmas and thence a more important role for juvenile additions than is generally accepted.

Significance of silicate liquid immiscibility for the origin of young highly evolved lithic clasts in Chang’E-5 regolith

Highly evolved lithology distributes across the Moon sparsely but serves as a critical record of the extensive differentiation processes of lunar magmas. In contrast to the Apollo-returned highly evolved rocks that essentially formed before the end of the Nectarian period, silicic lithologies detected by remote sensing within the nearside Procellarum KREEP Terrane (PKT) have cratering model age as young as ∼2.5 Ga (Chevrel et al., 2009). The formation mechanism of the young silicic magmatism remains enigmatic. Here we present a detailed study of lithic clasts with highly evolved compositions from the northwestern PKT returned by Chang’E-5 mission. Two different types of highly evolved lithic clasts were recognized: (a) Type A clasts predominately consist of granophyric intergrowths of K-feldspar and quartz. They are highly depleted in incompatible elements (except for K, Rb, Cs, and Ba) and have a V-shaped REE pattern, which can be explained by silicate liquid immiscibility (SLI) following the fractionation of merrillite from a KREEP-like melt. The microtextural features of quartz in Type A clasts indicate that they could have crystallized through relatively slow cooling at temperature below 870 °C, supporting a shallow intrusive origin. The silicic intrusion exposed in the interior, rim, and ejecta of Aristarchus crater has a cratering model age of ∼2.5–3.7 Ga, which could be the source for Type A clasts; (b) Type B clast has little MgO, high incompatible element concentrations, and an REE pattern inclined to the right. Thermodynamic calculations indicate that Type B clast likely formed through SLI of the ∼25% residual melt of Em3 basalts in the Chang’E-5 landing region. This is consistent with the crystallization age of 2.57 ± 0.26 Ga for the zirconolite in Type B clast. The highly evolved samples from Chang’E-5 regolith provide new evidence that SLI may have played an important role in the young highly evolved intrusive bodies’ formation on the Moon. Furthermore, our thermodynamic modeling results show that compared to KREEP basalt, partial melting of quartz monzodiorite/monzogabbro at ∼930–1000 °C can produce melts with composition close to lunar granites and felsites. Thus, if a series of silicic volcanisms distributed mostly within the PKT was generated through this mechanism, quartz monzodiorite/monzogabbro may also widely distribute within the lunar nearside upper crust.

Tectono-Thermal History of the Neoarchean Balehonnur Shear Zone, Western Dharwar Craton (Southern India)

Abstract A widely spaced Neoarchean shear zone network traverses the granite-greenstone terrains of the Western Dharwar craton (WDC). The NNW-SSE trending Balehonnur shear zone traverses the largest part of the preserved tilted Archean crustal ensemble in the Western Dharwar craton (WDC) from the amphibolite-granulite transition in the south to greenschist facies in the north and eventually concealed under Deccan lava flows. Published tectonic fabrics data and kinematic analysis, with our data reveal a sinistral sense of shearing that effectuate greenstone sequences, Tonalite-Trondhjemite-Granodiorite Gneisses (TTG), and Koppa granite as reflected in variable deformation and strain localization. A profound increase of strain towards the core of the shear zone in the ca. 2610 Ma Koppa granite is marked by a transition from weak foliation outside the shear zone through the development of C-S structures and C-prime fabrics, mylonite to ultramylonite. The mineral assemblages in the Koppa granite and adjoining greenstone indicate near peak P-T conditions of 1.2 Gpa, 775-800°C following a slow cooling path of 1.0 GPa and 650°C. Field-based tectonic fabrics data together with U-Pb zircon ages reveal that the Koppa granite emplaced along the contact zone of Shimoga-Bababudan basin ca. 2610 Ma, coinciding with the emplacement of ca. 2600 Ma Arsikere-Banavara, Pandavpura, and Chitradurga granites further east which mark the stabilization of WDC. Significant variation in major element oxide (SiO2 = 56-69 wt.%) together with high content of incompatible elements (REE, Nb, Zr, and Y) and high zircon crystallization temperatures (~1000°C) of Koppa granite suggests derivation by partial melting of composite sources involving enriched uppermost mantle and lower crust. The development of widely spaced shear zones is probably linked to the assembly of eastern and western blocks through westward convergence of hot oceanic lithosphere against already cratonized thick colder western block leading to the development of strain heterogeneities between greenstone and TTGs due to their different mineral assemblages leading to rheological contrast in the cratonic lithologies.

The Bazman and Taftan volcanoes of southern Iran: Implications for along-arc geochemical variation and magma storage conditions above the Makran low-angle subduction zone

Bazman and Taftan are two Late Miocene-Quaternary volcanoes of the Makran continental arc formed by the subduction of Arabian oceanic lithosphere beneath southern Eurasia. This study compares their bulk rock compositions as well as major and trace element contents of major mineral phases. Bazman is composed of a geochemically heterogeneous suite of low to medium-K basic to acidic compositions, while Taftan exposes more homogenous mainly andesitic rocks of medium to high-K affinity. Both volcanoes are characterized by the enrichment of LILE compared to HFSE and depletion of Nb, Ta, and Ti, reminiscent of subduction zone magmatism. At a given SiO2 content, the Taftan volcanics have higher concentrations of incompatible elements and, La/Yb, Th/Yb, and Sr/Y ratios (adakitic signature), here interpreted as being related to the more enriched mantle source, higher contribution of subducted sediments in magma genesis and thicker continental crust beneath Taftan. Thermobarometric calculations based on mineral chemistry (amphibole, orthopyroxene, clinopyroxene, and plagioclase) indicate pre-eruptive T = ∼1050–800 °C, P = 250–100 MPa and H2Omelt > 3.5 wt%. Bazman lavas record slightly lower P and T, and higher H2Omelt, suggesting a shallower magma reservoir and thus a more developed magma plumbing system. Trace element abundances in minerals show consistent distinctions between the two volcanoes, e.g., amphibole is REE-enriched in Taftan over Bazman samples. The features of trace element compositions and zoning patterns of minerals (amphibole, plagioclase and, pyroxene) along with available major element data are suggestive of both recharged magmatic systems and convective self-mixing magma chambers.

Peritectic and phenocrystic garnet accumulation and the origin of strongly peraluminous granitic rocks: The Flagstaff Lake Igneous Complex, Maine

The Flagstaff Lake Igneous Complex of Maine contains a metaluminous phase that ranges from diorite to granite, and three phases of strongly peraluminous, garnet-bearing rocks with silica contents that range between 43 and 59 wt%. These peraluminous rocks have increasing amounts of normative corundum with increasing maficity, indicating that they do not represent liquid compositions, rather garnet accumulation accounts for their bulk-rock A/CNK compositions. Garnet in these rocks displays a variety of textures, inclusion patterns, and trace element compositions. The Loon Lake phase is characterized by garnet with abundant, randomly distributed apatite inclusions. Some grains have spiral inclusion patterns that are peritictic grains formed during partial melting of source rocks while they were undergoing differential stress. In contrast, the Quarry Phases A and B contain inclusion-free garnet with euhedral grain boundaries. These grains have trace element profiles indicative of crystal fractionation processes and are interpreted to be phenocrysts. Some of these phenocrysts have inclusion poor, resorbed cores that are mantled by inclusion-free overgrowths. The cores have lower concentrations of compatible trace elements and higher concentrations of incompatible elements than the mantles. These cores are resorbed phenocrysts from an evolved magma that were mingled in a more primitive, garnet crystallizing magma. Also present in Quarry Phase B rocks are a minor population of inclusion-rich garnets with random inclusion distributions. These are also interpreted as peritectic garnet, entrained from the source, or from dehydration melting of country rock biotite. They are mantled by inclusion-free garnet with the same composition as the phenocrysts and are phenocrystic overgrowths. Thus, the original Flagstaff Lake anatectic melts were mildly peraluminous with the current bulk-rock compositions of the garnet tonalites dominated by the accumulation of variable portions of phenocrystic garnet or the entrainment of peritictic garnet, both of which generated the increasing peraluminosity with increasing maficity of the complex. Magmatic, mafic enclaves present in the peraluminous rocks are similar to the metaluminous, main phase rocks of the complex, suggesting that the coeval basaltic magmas provided the heat for anataxis to generate the peraluminous magmas.

Petrography and Geochemistry of the Harzburgite Xenoliths of Lake Guinnadji and Ngao Djalsoka Volcano from the Dibi Area (Adamawa Plateau - Cameroon)

Petrographic and geochemical studies have been carried out on the peridotite xenoliths included in the pyroclastite deposits from the Lake Guinnadji and Ngao Djalsoka volcano close to the Dibi locality in the Adamawa plateau, central Cameroon. The peridotite xenoliths exhibit protogranular and porphyroclastic textures, and are composed of olivine, orthopyroxene and minor amount of clinopyroxene, amphibole (?) and spinel, typical of harzburgite type. ICP-MS and ICP-OES analyses of representative samples show that they are in the range of those of the primitive mantle origin (42.4-45.0 wt.% MgO, 65.7-79.0 wt.% normative olivine). Decreasing SiO2, TiO2, Al2O3 and CaO contents vs increasing MgO trends evidence depletion from more fertile mantle by extraction of basaltic melts, leaving a refractory residue of harzburgite composition. High contents of REE and incompatible elements point out secondary enrichment processes which have affected the peridotite xenoliths. For this, metasomatism caused by silicate fluids is invoked. In that way, peridotites of Dibi area witness of the nature and the evolution of lithospheric mantle under the Adamawa plateau.

Early Jurassic S-type granitoids in the Nyainqêntanglha Range, South Tibet: A record of slab roll-back of subducted Neo-Tethyan Ocean

The Gangdese orogenic belt in southern Tibet records a succession of arc-trench-basin assemblages formed by the Neo-Tethyan subduction and the subsequent India-Asian collision. However, the geochronology and geodynamics of S-type granites in the northern part of the Gangdese belt are disputed. This paper describes ~190 Ma S-type granitoids outcropped in the Nyainqêntanglha Range, South Tibet, focusing on their petrogenesis and their relationship with contemporaneous magmatism. There are three distinct groups of granitoids, based on field relationships, petrology, and geochemistry. The coarse-grained Group I and II granitoids are large, sheet-like bodies and exhibit high contents of incompatible elements and negative Eu anomalies. The fine-grained Group III granites form small laccoliths and have low REE contents and positive Eu anomalies. The negative εNd(t) and εHf(t) values, abundant inherited zircons, moderately light REE and LILE enrichment, and HFSE depletion indicate that the Damxung granitoids were derived mainly by the partial melting of a metapelitic source. Further studies proposed that group I and II samples with high contents of incompatible elements and negative Eu anomalies represent fractionated melts, whereas group III samples with low REE contents and positive Eu anomalies represent cumulates. The age distribution pattern of inherited zircons supports the interpretation that the Lhasa terrane rifted away from the northern margin of the Indian plate. Considering previously published data in the Lhasa terrane, we suggest that the Early Jurassic magmatism could be related to a period of subducted oceanic slab roll-back, which provides new insights into the evolution process of the Neo-Tethyan ocean realm.

Appinitic and high Ba[sbnd]Sr magmatism in central Brazil: Insights into the late accretion stage of West Gondwana

Large amounts of high BaSr magmatism can be produced as the result of late accretion or decompression in the final stages of continental arc evolution in orogenic systems and leads to continental crustal growth. This type of magmatism often occurs associated with mega-shear zones and displays geochemical signatures that indicate magmas deriving from the melting of enriched lithospheric mantle. Such enrichment may be an indication of slab-derived fluid and/or melt interaction, released from subducted material. The high BaSr granitoids of the Brasiliano Paraguay Belt, Tocantins Province, central Brazil, comprise two suites that crystallized approximately 40 My apart from each other. Both suites are composed of metaluminous to slightly peraluminous rocks (ASI = 0.64–1.09) with magnesian character, and present high Ba (~1430 ppm), Sr (~800 ppm), K2O (~4 wt%), LREE and LILE contents. The negative εNd(T) values (−6.32 and − 6.65) associated with high contents of incompatible elements indicate an evolution from partial melting of mantle-derived magma previously metasomatized by subducted slab fluids. Crystallization ages of 557 ± 5.7 Ma and 554 ± 2.9 Ma have been obtained for the older, high MgK calc-alkaline suite. It evolved from high Cr and Ni mantle-derived appinitic magma to coeval granodiorites by crystal fractionation accompanied by minor crustal contamination. The younger, 515 ± 3.6 Ma-old suite displays an elevated concentration in some HFSEs, show alkali-calcic affinity and is represented by high-K monzogranites. A model proposed for the two hybrid suites is based on a slab break-off and a subsequent lithospheric delamination after 40 My, where the heat flux generated by mantle upwelling led to a partial melting of the enriched subcontinental lithospheric mantle beneath the continental crust. The older calc-alkaline suite represents the end process of magmatic arc accretion in the central Tocantins Province. The younger one is compatible with a transitional shoshonitic to A-type affinity, associated to post-collisional collapse stage of the orogen at Cambrian times. The petrogenesis of the investigated rocks requires an ocean closure phase at ca. 590 Ma, which could be linked to the consumption of the Clymene Ocean toward Amazonian Craton, recording a new collisional event in the central West Gondwana.

Carbonatite-Like Rock in a Dike of the Aikhal Kimberlite Pipe: Comparison with Carbonatites of the Nomokhtookh Site (Anabar Area)

Abstract ––A dike of rock similar in composition to carbonatites has been found in the Aikhal diamondiferous pipe of the Alakit–Markha field of the Yakutian kimberlite province (YaKP). The fine-grained rock of essentially carbonate composition (dolomite and calcite) rich in thin-platy phlogopite contains minerals typical of carbonatites: monazite, baddeleyite, and pyrochlore. In the high contents and distribution of incompatible elements the rock differs significantly from kimberlites and is transitional from kimberlites to carbonatites. The content of incompatible elements in this rock is 3–5 times lower than that in carbonatite breccias of the pipes in the Staraya Rechka kimberlite field of the YaKP (Nomokhtookh site). The compositions of accessory trace element minerals from the Aikhal dike rock and the Nomokhtookh carbonatite breccias are compared. An assumption is made that the high contents of incompatible elements in the carbonatite-like rock, which caused the crystallization of accessory minerals, are due to the differentiation of kimberlite melt/fluid. The high Sr isotope ratios indicate that the rock altered during hydrothermal and metasomatic processes. The obtained data on the composition of the carbonatite-like rock cannot serve as an argument for the genetic relationship between the Aikhal kimberlites and typical carbonatites. The genetic relationship between kimberlites and carbonatites in the northern fields of the YaKP remains an open issue.

Cretaceous crustal melting records of tectonic transition from subduction to slab rollback of the Paleo-Pacific Plate in SE China

The Mesozoic geology of SE China has been widely considered to relate to subduction of the Paleo-Pacific Plate beneath the Eurasia, accompanied with extensive felsic magmatism. However, the relationship between the origin of the voluminous crust-derived magmatism and the Paleo-Pacific slab subduction remains unclear. Here we present petrology, zircon U-Pb-Hf isotope data, and whole-rock geochemical and isotopic compositions of two episodes of Cretaceous silicic volcanic rocks (~114–113 Ma for Group 1 and ~ 100–93 Ma for Group 2) from southeastern Zhejiang Province in SE China, to provide new constraints on the geodynamic transition of the Paleo-Pacific slab subduction. Both volcanic groups show similarly unradiogenic whole-rock εNd(t) and zircon εHf(t) values (εNd(t) = −8.6 to −6.4 and εHf(t) = −8.5 to −0.7 for Group 1, and εNd(t) = −7.1 to −6.1 and εHf(t) = −7.8 to −2.2 for Group 2). The limited emplacement of coeval mafic magmas and absence of mafic microgranular enclave and intermediate lavas suggest an insignificant role of mantle-derived melt either as a parental magma for differentiation or a mafic component during magma mixing. Instead, the low MgO and high concentrations of SiO2 and incompatible elements and the evolved Nd and Hf isotopic compositions indicate derivation of these two groups of silicic magmas from similar hybrid crustal sources containing the metasedimentary rocks of Cathaysia basement and the newly accreted arc crust. Group 1 rocks contain a higher proportion of hydrous minerals (e.g., biotite) and Sr/Y and La/YbCN ratios but lower zircon saturation temperature than Group 2. Such variations in mineral and geochemical compositions could be mainly attributed to the change of melting conditions from a relatively deeper, colder and water-fluxed crust to a shallower, hotter and less hydrous state. By combining our new results and previous studies of the late Mesozoic volcanic rocks in SE China, we propose that a geodynamic transition from subduction to slab rollback of the Paleo-Pacific Plate was responsible for such P-T-H2O variations of the crustal source.

Idiomorphic oikocrysts of clinopyroxene produced by a peritectic reaction within a solidification front of the Bushveld Complex

Orthopyroxenite cumulates throughout the Critical Zone of the Bushveld Complex commonly contain prominent euhedral crystals of bottle-green augite, typically around 1 cm in size and surrounded by “haloes” of nearly pure plagioclase. On close examination, the augite grains can be seen to be oikocrysts, containing extensively resorbed chadacrysts of orthopyroxene, indicating an effective peritectic reaction relationship between the two pyroxenes. A detailed textural study of one such layer from the UG3 Unit in the Eastern Bushveld reveals some distinctive grain-scale features, including the presence of extensively 3D-interconnected chains of thousands of cumulus chromite grains that surround the orthopyroxene grains and extend through the augite oikocrysts. The high degree of interconnectivity is remarkable in view of the relatively low (5 vol.%) modal proportion of chromite in the sample. Other noteworthy features include zoning of the oikocrysts towards higher incompatible element contents in the rims and the presence of the plagioclase haloes around the augite oikocrysts. These haloes are essentially narrow zones devoid of orthopyroxene inclusions developed within large plagioclase oikocrysts that overgrow all the other phases. We propose a mechanism whereby the plagioclase oikocrysts grow early in communication with the main magma body within an initial crystal mush of orthopyroxene and chain-textured chromite. The clinopyroxene oikocrysts grew in the remaining pore space in such a way that dissolution of orthopyroxene occurred within a narrow few-mm wide chemical boundary layer ahead of the advancing oikocryst margin. The relative rates of crystallisation and dissolution were controlled by limited chemical diffusion through the boundary layer such that orthopyroxene grains more than a few grain diameters away showed no reaction at all. The plagioclase haloes developed as a result of continuing growth of the plagioclase oikocrysts to overtake the growing pyroxene oikocrysts, locking in the orthopyroxene-depleted boundary layer, and preserving euhedral cumulus morphologies away from the pyroxene oikocrysts. This texture represents a circumstance whereby oikocrysts in the same rock develop at different overlapping stages: plagioclase oikocrysts forming early in diffusive connection with the magma column, and clinopyroxene forming peritectic poikilitic textures within residual liquid pockets chemically isolated from the overlying magma body.

Trace element composition of amphibole and petrogenesis of hornblendites and plutonic suites of Cretaceous magmatic arcs developed in the Fuegian Andes, southernmost South America

The evolution of continental crust in convergent margins can be explored in southernmost South America (54-56°S). Plutonic rocks of the Fuegian Batholith and the rear-arc satellite Ushuaia Pluton were emplaced within the magmatic arc and the Fuegian fold-and-thrust belt, respectively. They record subduction zone processes in two distinct tectonic settings during the evolution of the Rocas Verdes Basin. We report new U-Pb zircon geochronology, bulk rock chemistry, Sr-Nd isotope data, and EPMA and in-situ LA-ICP-MS analyses of amphibole from ‘hornblendites’ and gabbroic-granitoid suites in order to evaluate the origin and evolution of the magmatic plumbing systems in the upper plate of the subduction zone. Textural relationships and amphibole compositions in hornblendite indicate crystallization at lower crustal depths with pressures of 7-8 kbar in the Fuegian Batholith and of 5-6 kbar in the Ushuaia Pluton. Lower Cretaceous suites of hornblendite and calc-alkaline hornblende-gabbro, diorite and tonalite in the Fuegian Batholith have εNdt values ranging between +2 and +4. They were emplaced within an island arc coeval with mid-oceanic type spreading in the Rocas Verdes back-arc basin. Isotope ratios and amphibole compositions in hornblendite indicate crystallization from primitive and hydrous sub-alkaline basaltic melts with relatively low LREE/HREE and low alkali contents. The Late Cretaceous plutons in the fold-and-thrust belt were emplaced after the tectonic juxtaposition of Rocas Verdes ophiolitic complexes. The Ushuaia Pluton, consisting of clinopyroxene-hornblende cumulates, hornblende-gabbro, diorite and monzodiorite, was emplaced during the waning stage of Late Cretaceous magmatism. In this case hornblendite amphiboles show high contents of alkalis, LREE and incompatible elements with a strong crustal affinity (Th, Ba, Rb). The enriched incompatible trace element patterns indicate their derivation from K-rich transitional magmas formed in supra-subduction settings. Chemical variations in amphibole from hornblendites and spatially related plutonic rocks are evaluated in terms of fluid flux from the subducted slab and partial melting of the sub-arc mantle, ultimately controlled by the thermal state of the subducted slab and convergence rates.

Геохимическая и Sr-Nd-Pb изотопная характеристики щелочных пород и карбонатитов Белозиминского массива (Восточный Саян)

Belaya Zima alkaline carbonatite complex (East Sayan, Russia) is a multiphase intrusion of the central type. The carbonatite stock is exposed in the central part of the complex, which is composed of different mineralized carbonatite. Alkaline silicate rocks form ring-shaped bodies surrounding the central carbonatite and are represented by melteigites and ijolites, dyke-shaped bodies of alkaline syenites. The article presents the results of Pb/Pb dating, petrological and geochemical studies, including radiogenic (Nd, Pb and Sr) isotopic data. The age value of 631 ± 11 Ma, obtained from the Pb/Pb isochron line for all rocks of intrusion, is consistent with the previous age determinations [643 ± 3 Ma, Yarmolyuk et al. 2005 and 645 ± 6 Ma, Doroshkevich et al., 2016; 645 ± 6 Ma, Salnikova et al., 2019]. The age is in the range of 700-600 Ma, the interval of the formation of alkaline carbonatite complexes located along the edge of the Siberian craton (Aldan-Stanovoy shield, Yenisei Ridge, East Sayan). Emplacement of the complexes is associated with the final stage of the break-up of the Rodinia supercontinent [Yarmolyuk et al., 2005]. Melteigites are more magnesium (Mg # = 28–35) in comparison with ijolites (Mg # = 24–30) and alkaline syenites (Mg # = 15–29), which proves the lowest degree of differentiation of the former. The low total Mg # of the Belaya Zima alkaline silicate rocks, their low nickel and chromium concentrations confirm the fact that olivine crystallization has completed. Early crystallization of perovskite in melteigites led to depletion of the residual melt in REEs and Nb and formation of REE-depleted ijolites and nepheline syenites. There are the evolution trend of HFSE and REEs from early calcite carbonatite to calcite-dolomite and ankerite carbonatites. The calcite and calcite-dolomite carbonatites have the highest contents of Nb-Ta and Zr-Hf and reflect the early crystallization of pyrochlore and zirconolite. Late ankerite carbonatites are enriched in REEs in comparison with calcite and calcite-dolomite carbonatites, are REEs are concentrated in rare earth carbonates and monazite (Ce). Isotopic data (87Sr/86Sr = 0.702672–0.703125 and eNd (Т) = 3.14–4.97) for rocks of the Belaya Zima complex indicate that primary melts were formed from a heterogeneous, moderately depleted mantle source. The high concentration of incompatible elements in the rocks (Sr and Nb, La/Sm ratio), the presence of carbonatites in the complex indicate that the possible mantle metasomatic agent had a carbonate initial composition. This conclusion is confirmed by the results of melt inclusions study in minerals from the Belaya Zima ijolites [Andreeva et al., 2007]. A high Gd/Yb ratio (up to 7) in the rocks indicates that they were derivatives of primary melts formed by low degree of partial melting of a garnet-bearing source.

Tracing metallic pre-concentrations in the Limousin ophiolite-derived rocks and Variscan granites (French Massif Central)

The French Massif Central (FMC) is part of the West European Variscan belt. The economic and sub-economic ore deposits from the Limousin area are the consequence of intensive fluid-rock interactions related to a lithospheric delamination process that occurred during the Neo-Variscan Period (320–290 Ma). This study focuses on the fingerprints of fluid-rock interactions in the Limousin ophiolite-derived rocks and granitic massifs, in order to trace the source of trace elements transported by fluids involved in the formation of the Variscan ore deposits (including Sn, W, Au and U deposits). A series of compatible and incompatible metallic trace elements (Ag, As, Au, Bi, Li, Co, Cr, Cu, Ga, Ge, Ni, Pb, Sb, Sc, Sn, Ti, V, W) were analysed in a series of basic (metagabbros, basic dykes), ultrabasic (serpentinites) and felsic (granites and gneisses) rocks, as well as in the mineral assemblages, in order to determine their relationships with the Limousin ore deposits.Serpentinites and their mineral assemblages (serpentine, olivine, amphibole, chlorite and spinel) show high concentrations in compatible transition metals (Sc, V, Cr, Co, Ni, Zn, Cu) and low abundances of incompatible metals (Ge, Ga, Mo, Sn, Sb, W). Associated amphibolites (metagabbros and metamorphosed dykes) show high contents of compatible metals, though lower than ultrabasic rocks. Amphibole and, to a lesser extent, titanite, are the main hosts for metallic trace elements. Abundances in incompatible metals are generally low. Amphibole has low Sn and Sb contents, while higher Sn contents were measured in titanite. In a UHP zoisite-eclogite cropping out close to one of the ophiolite-derived massifs, zoisite, rutile, omphacite and retrograde amphibole show high concentrations of incompatible elements (Sn, Sb, W, As, Ga, Ge, Mo and Bi). However, it has been shown that high-temperature and low-temperature hydrothermal alteration that have affected the basic and ultrabasic protoliths is c. 100 Ma older than those that were responsible for the formation of ore deposits. For this reason, and as well because of their low W and Sn abundances, the ultrabasic and basic rocks cannot be considered as source for the Limousin WSn ore deposits.In granite and gneisses, the metallic trace elements are mainly hosted by biotite and muscovite. Quartz and feldspars only show minor amounts of Sc, Ga, Ge, Sn and Pb. The micas contain low amounts of Ge, As, Sb, Bi and variable amounts of Sc, Cu and Ga. Besides, micas display significant Sn and W contents. Variations in Sn and W in micas allow tracing the pre-mineralisation concentrations in the granitic massifs: there is a clear geographical correlation between the Sn and W composition of micas and the importance of the WSn ore deposits, arguing for a local source of pre-concentrations. Granites from the Vaulry WSn ore deposit display the highest W and Sn concentrations, while granites hosting the Au deposits in the southern part of the Limousin (St-Yrieix, Cheni, Lauriéras) show the smallest concentrations.

Slab-derived origin of tremolite–antigorite veins in a supra-subduction ophiolite: the Peridotite Nappe (New Caledonia) as a case study

Hydration of mantle peridotites provides information on the exhumation history and on the fluid regime accompanying exhumation of these rocks (reservoirs involved, fluid/rock ratios, temperature of interaction). Highly depleted harzburgites and dunites of the Peridotite Nappe of New Caledonia are crosscut by fractures, which have been pervasively serpentinized, producing lizardite, brucite, magnetite and minor chrysotile in a near-static environment, probably by sea water circulating in cooling joints. This event is generally referred to as “primary serpentinization”. In a next step, already serpentinized joints were re-opened to produce tension and shear cracks sealed by higher-temperature synkinematic fibrous minerals. Locally, tremolite-bearing veins and pockets, which do not display evidence for void infill, were generated by metasomatic replacement of the wall rock peridotite. Most veins only contain fibrous antigorite but some display tremolite-antigorite intergrowths or even pure tremolite. The latter rocks yield high contents of Ca and incompatible elements, which contrast with the overall depletion of the peridotite host rock and suggest contribution of an external source. Whole-rock geochemical and isotopic features (87Sr/86Sr, 18O and H) suggest that antigorite veins, which bear highly radiogenic Sr isotope signatures, were strongly influenced by fluids emitted by the subducted slab and associated sediments. In contrast, the geochemical and isotopic signatures of tremolite-bearing rocks suggest a genetic link with Early Eocene supra-subduction dykes and fluids that leached them. Calcium, strontium and REE-bearing oxidized fluids reacted with already serpentinized fracture wall rock as shown by fibre nucleation, chromite alteration and high Cr contents in tremolite. The bulk of syn-tectonic fluid-rock interaction associated with shear and tensile fracture development probably occurred at the onset of Eocene intra-oceanic subduction, when the buoyant lower plate (the South Loyalty Basin) obliquely forced its way beneath the nascent Loyalty fore-arc. Mobile elements extracted from supra-subduction dykes temporarily enriched the circulating fluids and generated tremolite, antigorite, Mg-chlorite and magnetite instead of antigorite and magnetite solely. Tremolite crystallization probably ceased due to the exhaustion of Ca-rich fluids in a globally cooling fore-arc environment.

Cretaceous to Miocene magmatism, sedimentation, and exhumation within the Alaska Range suture zone: A polyphase reactivated terrane boundary

AbstractThe Alaska Range suture zone exposes Cretaceous to Quaternary marine and nonmarine sedimentary and volcanic rocks sandwiched between oceanic rocks of the accreted Wrangellia composite terrane to the south and older continental terranes to the north. New U-Pb zircon ages, 40Ar/39Ar, ZHe, and AFT cooling ages, geochemical compositions, and geological field observations from these rocks provide improved constraints on the timing of Cretaceous to Miocene magmatism, sedimentation, and deformation within the collisional suture zone. Our results bear on the unclear displacement history of the seismically active Denali fault, which bisects the suture zone. Newly identified tuffs north of the Denali fault in sedimentary strata of the Cantwell Formation yield ca. 72 to ca. 68 Ma U-Pb zircon ages. Lavas sampled south of the Denali fault yield ca. 69 Ma 40Ar/39Ar ages and geochemical compositions typical of arc assemblages, ranging from basalt-andesite-trachyte, relatively high-K, and high concentrations of incompatible elements attributed to slab contribution (e.g., high Cs, Ba, and Th). The Late Cretaceous lavas and bentonites, together with regionally extensive coeval calc-alkaline plutons, record arc magmatism during contractional deformation and metamorphism within the suture zone. Latest Cretaceous volcanic and sedimentary strata are locally overlain by Eocene Teklanika Formation volcanic rocks with geochemical compositions transitional between arc and intraplate affinity. New detrital-zircon data from the modern Teklanika River indicate peak Teklanika volcanism at ca. 57 Ma, which is also reflected in zircon Pb loss in Cantwell Formation bentonites. Teklanika Formation volcanism may reflect hypothesized slab break-off and a Paleocene–Eocene period of a transform margin configuration. Mafic dike swarms were emplaced along the Denali fault from ca. 38 to ca. 25 Ma based on new 40Ar/39Ar ages. Diking along the Denali fault may have been localized by strike-slip extension following a change in direction of the subducting oceanic plate beneath southern Alaska from N-NE to NW at ca. 46–40 Ma. Diking represents the last recorded episode of significant magmatism in the central and eastern Alaska Range, including along the Denali fault. Two tectonic models may explain emplacement of more primitive and less extensive Eocene–Oligocene magmas: delamination of the Late Cretaceous–Paleocene arc root and/or thickened suture zone lithosphere, or a slab window created during possible Paleocene slab break-off. Fluvial strata exposed just south of the Denali fault in the central Alaska Range record synorogenic sedimentation coeval with diking and inferred strike-slip displacement. Deposition occurred ca. 29 Ma based on palynomorphs and the youngest detrital zircons. U-Pb detrital-zircon geochronology and clast compositional data indicate the fluvial strata were derived from sedimentary and igneous bedrock presently exposed within the Alaska Range, including Cretaceous sources presently exposed on the opposite (north) side of the fault. The provenance data may indicate ∼150 km or more of dextral offset of the ca. 29 Ma strata from inferred sediment sources, but different amounts of slip are feasible.Together, the dike swarms and fluvial strata are interpreted to record Oligocene strike-slip movement along the Denali fault system, coeval with strike-slip basin development along other segments of the fault. Diking and sedimentation occurred just prior to the onset of rapid and persistent exhumation ca. 25 Ma across the Alaska Range. This phase of reactivation of the suture zone is interpreted to reflect the translation along and convergence of southern Alaska across the Denali fault driven by highly coupled flat-slab subduction of the Yakutat microplate, which continues to accrete to the southern margin of Alaska. Furthermore, a change in Pacific plate direction and velocity at ca. 25 Ma created a more convergent regime along the apex of the Denali fault curve, likely contributing to the shutting off of near-fault extension-facilitated arc magmatism along this section of the fault system and increased exhumation rates.

Athapuscow aulacogen revisited: Geochronology and geochemistry of the 2046 Ma Union Island Group mafic magmatism, East Arm of Great Slave Lake, Northwest Territories, Canada

The Slave craton underwent widespread extension during the Paleoproterozoic. The East Arm basin of Great Slave Lake formed during this period and preserves a protracted record of sedimentation and volcanism in the southeastern Slave craton. Within the East Arm basin package, the Union Island Group represents voluminous mafic volcanism with subordinate sedimentary strata. We report the first high precision U–Pb zircon and baddeleyite dates for the Union Island Group; 2045.8 ± 1.0 Ma for a volcaniclastic unit from the lower basalt formation and 2042.7 ± 3.0 Ma for a diabase intrusion within volcaniclastic horizons in the lower basalt formation. We fundamentally revise the lower basin stratigraphy and demonstrate that the Union Island Group is the oldest identified supracrustal package in the East Arm basin, ∼120 Myr older than previously thought. This revised stratigraphy is supported by detrital zircon provenance age distribution in two Union Island Group sedimentary samples, which is dominated by 2.76–2.56 Ga ages, reflecting prominent input from the local Archean Slave basement.The Union Island Group contains two geochemically distinct mafic volcanic packages. The lower basalt package is predominantly alkaline, characterized by an OIB-like chemical signature with high and variable incompatible element contents (108–438 ppm Zr, 13–62 ppm Nb). In contrast, the upper basalt package is tholeiitic, contains much lower and uniform incompatible element contents (83–101 ppm Zr, 2–4 ppm Nb), and has an E-MORB-like chemical signature. Both packages display overlapping, depleted time-integrated εNd(i) values (+1.1 to +3.2). Petrological modeling suggests the lower package originated as OIB-like melts from the asthenosphere that interacted with a depleted reservoir; the upper package was produced by larger degree of partial melting of a similar mantle source, in addition to possible lithospheric input. The petrogenesis established in this study is consistent with a passive continental rift origin for the Union Island Group magmatism and is further supported by geochemical similarities with Proterozoic and Phanerozoic rift successions. We therefore propose that the 2046 Ma Union Island Group represents an incipient rift basin sequence formed during Paleoproterozoic extension that marked the initial breakup of the southern margin of the Slave craton from a pre-Laurentia supercontinent.