Post-magmatic Alteration Research Articles

Molybdenum isotope evidence for recycled oceanic crust in the mantle sources of continental intraplate mafic lavas, Emeishan Large Igneous Province

The chemical and lithological heterogeneities of Earth's mantle in oceanic settings have long been attributed to the presence of recycled materials such as subducted crust, oceanic or subcontinental lithosphere mantle and sediment, as evidenced by Mid-Ocean Ridge Basalts (MORB) and Ocean Island Basalts (OIB). However, the extent to which recycled materials contribute to mantle heterogeneity in continental settings remains uncertain. Here we present systematic Sr-Nd-Pb-Mo isotope data for the Pingchuan picritic porphyries and associated mafic rocks (basalts and gabbros) in the central Emeishan Large Igneous Province, southwestern China, to resolve this issue. In contrast to the gabbro samples that show continental crust contamination in variations of δ98/95Mo (relative to NIST SRM 3134, −0.35 ± 0.03 to 0.03 ± 0.02 ‰), the Pingchuan picritic porphyries and basalts show a large range of δ98/95Mo (−0.32 ± 0.01 to −0.15 ± 0.06 ‰) and broadly negative trend between δ98/95Mo and Ce/Mo ratios, which cannot be explained by post-magmatic alteration, crustal contamination, fractional crystallization, olivine accumulation, metasomatism or fluid-activity in the mantle source. They were most likely derived from incongruent melting of a heterogeneous mantle source, i.e., a deep mantle with chondritic-like δ98/95Mo (−0.15 ± 0.01 ‰) containing recycled oceanic crust (+ sediment) with light δ98/95Mo (< −0.32 ‰). The basalts display low δ98/95Mo (−0.32 ± 0.01 to −0.24 ± 0.04 ‰), high 87Sr/86Sri (0.7046 to 0.7059), TiO2, Sm/Yb, Dy/Yb and Ce/Mo, pointing to characteristics of melts produced by MORB-type eclogite (+ sediment) at relatively low degrees of partial melting. With increasing degrees of partial melting, melts produced by peridotite (87Sr/86Sri = 0.7035 to 0.7043, chondritic-like δ98/95Mo = −0.18 ± 0.03 to −0.15 ± 0.06 ‰) come to dominate, resulting in low TiO2, Sm/Yb, Dy/Yb and Ce/Mo of the Pingchuan picritic porphyries. From a global perspective, continental basalts also plot on the negative trend defined by the OIB datasets (δ98/95Mo vs. Ce/Mo). As such, Mo isotope may provide a robust tracer of recycled oceanic crust in shaping mantle heterogeneities that occur in both oceanic and continental settings.

In situ geochemistry of apatite: Petrogenetic and tectonic interpretations of Jurassic felsic magmatism in the Yanbian area, NE China

Geochemical analyses of individual minerals provide more detailed insights into the petrogenesis of igneous rocks than whole-rock analyses. This study conducted in situ geochemical and Nd isotope analyses of apatites from 10 Jurassic granitic plutons in the Yanbian area, NE China, to establish the petrogenesis and regional tectonic evolution. The results indicate that the apatite geochemistry of Jurassic granitoids in the Yanbian area was controlled primarily by the composition of parental melt. Post-magmatic alteration may lead to geochemical decoupling between apatite and parental melt, while Nd isotopes exhibit some resilience to such alterations. Apatites from Early Jurassic granitoids display characteristics that are consistent with an I-type origin, whereas those from Middle and Late Jurassic granitoids exhibit an adakitic affinity. Variations in apatite compositions indicate the fractional crystallization of other rare earth element (REE)-bearing minerals during magma evolution. The early crystallization of plagioclase and allanite led to decreases in Sr and Th contents in apatite, respectively, resulting in a negative Eu anomaly and light REE depletion. The fractional crystallization of titanite and hornblende resulted in the depletion of middle REE in apatite. Hornblende is regarded as the main residual phase in the magma source of Middle and Late Jurassic adakitic granitoids in the Yanbian area. Apatite Nd isotopic compositions suggest that the Jurassic granitoids in the Yanbian area originated from two crustal sources: the Central Asian Orogenic Belt and the North China Craton. Additionally, increasing trends in apatite Sr/Y and (La/Yb)N ratios from the Early to Late Jurassic suggest a gradual thickening of the regional crust, which is likely driven by the continentward migration of the subduction zone associated with the Paleo-Pacific Plate.

Constraints on the source of Siberian Trap magmas from Mo isotope evidence

The relative contributions of asthenospheric mantle, lithospheric mantle, and continental crust in the genesis of the Siberian Traps Large Igneous Province (ST-LIP) remain poorly constrained. Most models invoke partial melting of asthenospheric mantle within a mantle plume with an inventory of recycled crustal material, with or without melting of subcontinental lithospheric mantle, and crustal contamination during ascend through the continental lithosphere. A greater understanding of this topic is of fundamental importance because the ST-LIP basalts are associated with the large Permian-Triassic extinction and the world’s largest magmatic Ni-Cu-platinum group element sulfide resources (Ni-Cu-PGE), the Norilsk-Talnakh mining camp. The ∼250 ± 2 Ma Siberian Traps at Norilsk contain a classic sequence of basaltic rocks that provide a spatial and stratigraphic context for changing chemistry of the eruptive products. We present a detailed geochemical and Sr-Nd-Hf-Mo isotopic investigation of ST-LIP volcanic rocks and associated sedimentary rocks, including coal and anhydrite, from the Norilsk area. The 98Mo/95Mo isotope ratios (reported as δ98Mo ratio relative to NIST SRM 3134) vary significantly from −0.62 to 0.07 ‰ for the older basalt formations and −0.41 to 0.03 ‰ for the younger basalt formations. The range of δ98Mo and its correlation with the other geochemical tracers cannot be explained by post-magmatic alteration, magmatic differentiation, or sulfide fractionation as Mo behaves as a lithophile element under these magmatic conditions. We suggest that the δ98Mo range can be explained by interaction of the plume with subcontinental lithospheric mantle modified by subduction processes. This is particularly prominent in the earlier Ivakinsky to Gudchikhinsky formations where light δ98Mo values coupled with low Mo/Ce can be explained by contributions from a dehydrated eclogitic component, whereas more rare heavy δ98Mo values and high Mo/Ce likely require contributions from a fluid metasomatized mantle source. Later magmas of the Nadezhdinsky formation show clear evidence of crustal contamination in their combined Mo-Sr-Nd-Hf isotope and trace element systematics, while the later more voluminous Morongovsky type magmas are shallower, large degree melts with limited crustal interaction. Our data shows the usefulness of Mo isotopes in deciphering magma sources of large igneous provinces eruptions.

Kaapvaal lamproite melts (aka orangeites): A new type of alkali-carbonate liquid? insights from olivine-hosted multiphase inclusions (Silvery Home, South Africa)

Kaapvaal lamproites (aka orangeites) are a group of volatile-rich (H2O, CO2), micaceous, ultrapotassic igneous rocks that are unique to the Kaapvaal craton in southern Africa. However, the composition of the melts that give rise to these rocks remains poorly understood due to overprinting effects of contamination by mantle and crustal material, volatile exsolution, fractional crystallisation and post-magmatic alteration. Consequently, this lack of reliable data on the initial composition of the Kaapvaal lamproite melts hampers our understanding of their source, petrogenesis and ascent mechanisms.Olivine is a common mineral comprising the Kaapvaal lamproites that has both xenocrystic (i.e., mantle) and magmatic origins. Multiphase inclusions (melt/fluid) entrapped within olivine have been consistently demonstrated as an effective tool for gaining fundamental insights into the composition and evolution of melts that produce both kimberlites and lamproites, prior to processes, such as eruption, devolatization and syn-/post-magmatic alteration.In this study, multiphase inclusions of both secondary and pseudosecondary origin hosted in olivine from a Kaapvaal lamproite (Silvery Home, South Africa) provide novel insights into the composition of the melt(s) that initially transported olivine to the surface and then crystallised after emplacement to form the lamproite groundmass. The inclusions in our study contain daughter mineral assemblages consisting of diverse Ca-Mg carbonates, including K-, Na-, Ba-, and Sr-bearing varieties, moderate K-rich silicates (phlogopite, tetraferriphlogopite), and subordinate oxides, phosphates, sulphides, sulphates, and halides. Based on these daughter mineral assemblages, we suggest that the composition of the melt entrapped by olivine was SiO2-poor, Ca-Mg carbonate-rich and contained elevated concentrations of K, Na, Ba, Sr, P and Cl. The mineral and reconstructed melt compositions are in stark contrast to the mineral association of the groundmass and the bulk-rock composition of the Silvery Home lamproite, respectively. We suggest that alkali-/alkali-earth carbonates, phosphates, sulphides, sulphates, and halides represented a potentially significant, or even dominant, component of the melt that crystallised the Silvery Home lamproite but were likely removed by degassing and/or interaction with syn-/post-magmatic fluids. We show that olivine-hosted multiphase inclusions from the Silvery Home lamproite share many compositional similarities to melt inclusions hosted in olivine from kimberlites but are distinct from ‘classic’ cratonic olivine lamproites worldwide.

LITHIUM IN THE SUBSOIL OF UKRAINE Part 7. Genesis of lithium-bearing objects: rare-metal granites of the Ukrainian Shield

In the genetic sense, the following lithium-bearing objects have been characterized: vein Li-F granites and areal Liznykivsky granites of the Korosten pluton, rare-metal granites of the Kamianomohylsky complex, and ongonites of the Azov region. Most researchers consider Li-F granites as the final phase of magmatism of the Korostensky complex. Our conclusion differs from the mainstream view: vein Li-F granites of the pluton are products of crystallization of deep fluid flows that penetrated into the fissure cavities of the pluton rocks after the formation of magmatic massifs. In our opinion, the Liznykivsky granites are metasomatic formations that have developed on different-age rapakivi-like biotite-amphibole granites under the influence of deep fluid flows. The rare-metal granites of the Kamianomohylsky complex form three massifs in the Azov region: Kamiana Mohyla, Katerynivsky and Starodubivsky. Initially, all granite massifs were formed in the process of crystallization of magmatic melt, which later changed to metasomatic alteration of granites. Post-magmatic changes in granites that contributed to the development of rare metal mineralization include: 1) biotitization; 2) microclinization; 3) early albitization; 4) greisenization; 5) late albitization. According to the data of thermobarogeochemical studies, the stage-by-stage development of the post-magmatic alteration of the Kamiana Mohyla granitoids was substantiated and physicochemically characterized: microclinization (500-600 °С), albitization (430 °С), topazization (430 °С), silicification (430-440 °С), sulfidization (320 °С), late albitization (290-420 °С), crystallization of xenotime (200-210 °С), vein fluoritization and the growth of fluorite crystals (125-155 °С). The crystallization of ongonites took place at high temperature, which was 1140 °C at the final stage of quartz phenocrysts growth.

LITHIUM IN THE SUBSOIL OF UKRAINE Part 7. Genesis of lithium-bearing objects: rare-metal granites of the Ukrainian Shield

In the genetic sense, the following lithium-bearing objects have been characterized: vein Li-F granites and areal Liznykivsky granites of the Korosten pluton, rare-metal granites of the Kamianomohylsky complex, and ongonites of the Azov region. Most researchers consider Li-F granites as the final phase of magmatism of the Korostensky complex. Our conclusion differs from the mainstream view: vein Li-F granites of the pluton are products of crystallization of deep fluid flows that penetrated into the fissure cavities of the pluton rocks after the formation of magmatic massifs. In our opinion, the Liznykivsky granites are metasomatic formations that have developed on different-age rapakivi-like biotite-amphibole granites under the influence of deep fluid flows. The rare-metal granites of the Kamianomohylsky complex form three massifs in the Azov region: Kamiana Mohyla, Katerynivsky and Starodubivsky. Initially, all granite massifs were formed in the process of crystallization of magmatic melt, which later changed to metasomatic alteration of granites. Post-magmatic changes in granites that contributed to the development of rare metal mineralization include: 1) biotitization; 2) microclinization; 3) early albitization; 4) greisenization; 5) late albitization. According to the data of thermobarogeochemical studies, the stage-by-stage development of the post-magmatic alteration of the Kamiana Mohyla granitoids was substantiated and physicochemically characterized: microclinization (500-600 °С), albitization (430 °С), topazization (430 °С), silicification (430-440 °С), sulfidization (320 °С), late albitization (290-420 °С), crystallization of xenotime (200-210 °С), vein fluoritization and the growth of fluorite crystals (125-155 °С). The crystallization of ongonites took place at high temperature, which was 1140 °C at the final stage of quartz phenocrysts growth.

Mantle-like to low oxygen isotopes in zircon from the mid-Cretaceous high-silica granites reveal unweathered basement recycling along the present coastal area of SE China

Zircon oxygen isotope compositions can provide potential constraints on the origin of granitic magmas. In this work, we report new data of zircon UPb dating, trace elements and HfO isotopes for two types of mid-Cretaceous high-silica granites from Zhoushan archipelago in Zhejiang Province, SE China. The in-situ zircon data are carefully screened for radiation damage and post-magmatic alteration. The analyzed magmatic zircon grains indicate that the Zhoushan calc-alkaline granites (ZCAG; 101–92 Ma) have mantle-like δ18O values of 5.29–5.88 ‰, in contrast to the Zhoushan peralkaline granites (ZPAG; 91–89 Ma) with relatively low δ18O of 4.42–4.6 ‰. The slight decreases in δ18O and εHf(t) from zircon center to periphery of both groups suggest that they may have consistent oxygen isotopic compositions in their parental magma sources and have gone through assimilations to hydrothermally altered wall rocks in the shallow crust. The mantle-like δ18O values in zircon are widespread in the Cretaceous felsic rocks in the present coastal area of SE China, which, in combination Hf isotopes, indicates the possible existence of a buried unweathered Proterozoic basement in the eastern part of the Cathaysia Block. This work highlights the role of deep crustal magma sources, assimilation and post-magmatic radioactive damage on the formation of mantle-like to low δ18O signals in zircons, and has fundamental implications for understanding the formation of widespread mid-Cretaceous felsic rocks along the present coastal area of SE China.

Geochemical Characteristics of Mafic Dykes from Wairagarh Area, Western Bastar Craton, Central India

The NW-SE trending mafic dykes intrude the Amgaon gneisses and Dongargarh granite in Wairagarh area of the Western Bastar Craton (WBC). These dykes consist of mainly the clinopyroxene, plagioclase feldspar and minor amphiboles and titano-magnetite. These are metamorphosed to green schist facies. The mafic dykes show tholeiitic trend on multivariate geochemical plots and moderate range of variation in SiO2 (50.15 to 52.84 wt.%), MgO (5.13 to 7.64 wt.%), Fe O (12.97 to 15.08 wt.%), Al O (12.74 to 14.48 wt.%) and the Mg# value ranges from 37.74 to 51.21. Strong positive correlation of Sr, Th, Hf, U, Ga, Ta, Pb, Cs, Rb and HREEs negate the effect of post magmatic alteration. Negative Nb, Ta and Ti with positive Zr and Hf anomalies suggest the effect of assimilation and crustal contamination, which is also supported by Ce/Nb (0.23-0.33) equivalent to the crustal value. Negative ΔNb value suggests depleted mantle source for the mafic dykes of the WBC. Restricted range of (Gd/Yb)N (1.05-1.67), (Dy/Yb)N (0.94-1.25) and (Sm/Yb)N (1.22-2.26) advocate that these mafic dykes were derived from spinel lherzolite source. About 5-15% degrees of partial melting of depleted mantle source is envisaged for the generation of the mafic dykes. The field and geochemical evidences suggest that the mafic dykes of Wairagarh area, WBC were emplaced within continental rift environment. Keywords: Mafic dykes, Geochemistry, Wairagarh, Bastar Craton, Central India

Příspěvek k petrografii a mineralogii lokality Vyskočilka (Praha-Malá Chuchle, silur Barrandienu)

Classical locality Vyskočilka in the Barrandian Basin displays a well-exposed section of black Silurian shales penetrated by a ca. 10-m thick diabase sill. We examined the petrographic character and mineral composition of the intrusive body and enclosing shales, as well as the hydrothermal veins cutting the rocks. The diabase is a rock with basic affinity (paleobasaltoid), which was strongly influenced by post-magmatic alterations including albitization and zeolitization of feldspars as well as chloritization of clinopyroxenes and Fe-Ti spinelides. Host sediments show variously intense thermal overprint at the contact with the sill. The predominating Silurian shales are rarely accompanied by laminites, formed by siliciclastic material, calcite, apatite and probably also a pyroclastic component. A minor component of all sediments are Ti-rich chromspinelides, which are classified as chromite, magnetite, and spinel. Chromspinelides form isolated euhedral single crystals reaching up to 50 µm in size, as well as their intergrowths and fragments, which all do not display marks of abrasion due to transport. We therefore presuppose that they represent relics of ultrabasic pyroclastic material (volcanic ash), which was deposited during sedimentation of Silurian sediments. There are known few sills of Silurian meimechites in the Prague Basin containing chromspinelides with comparable chemical composition, which can thus represent a likely source of chromspinelides. The vein hydrothermal mineralizations, present at the study site, probably originated during different geological processes. The calcite veins with minor quartz and hydrocarbons likely formed during Variscan tectono-thermal overprint of the area. By contrast, the vein mineralization containing calcite, quartz, titanite, anatase, and Zn-Fe-Ni-Cu sulphides was likely associated with contact-metamorphic processes related to intrusion of diabase.

Trace element and Nd isotope analyses of apatite in granitoids and metamorphosed granitoids from the eastern Central Asian Orogenic Belt: Implications for petrogenesis and post-magmatic alteration

We present in situ trace element and Nd isotopic data of apatites from metamorphosed and metasomatized (i.e., altered) and unaltered granitoids in the Songnen and Jiamusi massifs in the eastern Central Asian Orogenic Belt, with the aim of fingerprinting granitoid petrogenesis, including both the magmatic and post-magmatic evolution processes. Apatites from altered granitoids (AG) and unaltered granitoids (UG) are characterized by distinct textures and geochemical compositions. Apatites from AG have irregular rim overgrowths and complex internal textures, along with low contents of rare earth elements (REEs), suggesting the re-precipitation of apatite during epidote crystallization and/or leaching of REEs from apatite by metasomatic fluids. εNd(t) values of the these apatites are decoupled from zircon εHf(t) values for most samples, which can be attributed to the higher mobility of Nd as compared to Sm in certain fluids. Apatites from UG are of igneous origin based on their homogeneous or concentric zoned textures and coupled Nd-Hf isotopic compositions. Trace element variations in igneous apatite are controlled primarily by the geochemical composition of the parental melt, fractional crystallization of other REE-bearing minerals, and changes in partition coefficients. Sr contents and Eu/Eu* values of apatites from UG correlate with whole-rock Sr and SiO2 contents, highlighting the effects of plagioclase fractionation during magma evolution. Apatites from UG can be subdivided into four groups based on REE contents. Group 1 apatites have REE patterns similar to the host granitoids, but are slightly enriched in middle REEs, reflecting the influence of the parental melt composition and REE partitioning. Group 2 apatites exhibit strong light REE depletions, whereas Group 3 apatites are depleted in middle and heavy REEs, indicative of the crystallization of epidote-group minerals and hornblende before and/or during apatite crystallization, respectively. Group 4 apatites are depleted in heavy REEs, but enriched in Sr, which are features of adakites. Some unusual geochemical features of the apatites, including the REE patterns, Sr contents, Eu anomalies, and Nd isotopic compositions, indicate that inherited apatites are likely to retain the geochemical features of their parental magmas, and thus provide a record of small-scale crustal assimilation during magma evolution that is not evident from the whole-rock geochemistry.

Petrological evidence of an anhydrous carbonatitic peridotite source for the Xiaogulihe ultrapotassic volcanic rocks, northeastern China

This study presents new olivine and whole-rock geochemical data for the Xiaogulihe ultrapotassic volcanic rocks of northeastern China. These data provide insights into the petrogenesis of the rocks of the Erkeshan–Wudalianchi–Keluo–Xiaogulihe (EWKX) potassic–ultrapotassic volcanic belt. The petrography and whole-rock geochemical compositions of the Xiaogulihe volcanic rocks are indicative of formation from magmas that primarily underwent olivine + clinopyroxene fractionation with insignificant crustal contamination and post-magmatic alteration. The high-Fo (87–90) olivine within the Xiaogulihe volcanic rocks has high Ni (2050–3800 ppm) and Mn (1190–1750 ppm) contents, high 10000Zn/Fe values (12–15), low Ca (1310–1800 ppm) contents, and low Fe/Mn (56–72) and Mn/Zn (10–14) values. Their high Mn contents and low Fe/Mn values together with their low FC3MS (<0.65; FC3MS = FeO/CaO – 3MgO/SiO2), low FCKANTMS (<0.37; FCKANTMS = ln(FeO/CaO) − 0.08ln(K2O/Al2O3) − 0.052ln(TiO2/Na2O) − 0.036ln(Na2O/K2O) ln(Na2O/TiO2) − 0.062(ln(MgO/SiO2))3–0.641(ln(MgO/SiO2))2–1.871ln(MgO/SiO2) − 1.473) and low CaO–Al2O3–Fe2O3Total values are indicative of derivation from a mantle source containing refractory peridotite. The Xiaogulihe olivines have slightly higher Zn/Fe and lower Mn/Zn values than expected for olivines formed from typical peridotite-derived melts, suggesting that the source for these lavas was previously metasomatized by interaction with Zn-rich sedimentary carbonatitic melts. The low magmatic water contents of the Xiaogulihe lavas (<1 wt% determined by use of the Ca-in-olivine geohygrometer) is also indicative of derivation from an anhydrous mantle source. These data suggest that the Xiaogulihe ultrapotassic lavas represent partial melts of anhydrous refractory subcontinental lithospheric mantle peridotite. The K-rich nature of these volcanic rocks is indicative of derivation from the K-rich reservoir of the mantle, which is closely associated with carbonatitic metasomatism, indicating the role of this process in the generation of the K-rich EWKX volcanics.

Crystallization order effects on inclusion assemblages in magmatic accessory minerals and implications for the detrital record

Mineral inclusion assemblages in zircon are almost certainly a complex function of crystallization sequence, magma chemistry, and physical proximity to other crystallizing phases. While the latter may be largely random, the former two factors should have more systematic controls. To better constrain the effects of crystallization sequence, we investigated a suite of granitoids from the Cretaceous arc of southern California and from the Miocene Colorado River Extensional Corridor, ranging from tonalites to leucogranites. In particular we examined inclusions in a variety of accessory minerals to elucidate crystallization order effects. Apatite is almost universally overrepresented as an inclusion in the studied minerals relative to its volumetric abundance in the rock as a whole. Where crystallization order can be inferred, earlier phases usually either have a higher proportion of apatite inclusions (11 out of 20 cases) or show no significant differences in apatite content from later phases (7 cases). An increase in the proportion of apatite with progressive crystallization is observed in four out of the 20 samples with clear crystallization orders (two rocks out of the 20 contain both an increasing and a decreasing case among different mineral pairs and are included in both groups). We interpret these observations as showing the widespread early crystallization of apatite inclusions – likely by pileup of the slow-diffusing phosphorus complex along growing mineral grains. The lower apatite contents in late-crystallizing modal phases are also reflected in the very low apatite contents within late-crystallizing zircon from some studied mafic-intermediate units, contrary to earlier suggestions that apatite decreases as a proportion of the inclusion assemblage with increasing whole rock silica. Plagioclase inclusions in zircon are on average more albitic than matrix plagioclase. In some samples, sequences of plagioclase crystallization can be identified by albite content among 1) magmatic cores of matrix grains, 2) inclusions in mafic accessory minerals, 3) inclusions in zircon, and 4) post-magmatic alteration veins and exsolution lamellae. However, the relative proportions of quartz, K-feldspar, and plagioclase inclusions do not systematically vary in accessory minerals with crystallization order, suggesting other origins of the often-recognized modal shifts toward more quartz- and K-feldspar-rich inclusion assemblage compared to the matrix. Unlike Himalayan leucogranites, when muscovite is present in the matrix in our sample suite, it is a rare inclusion phase in later-crystallizing accessories and may, in some cases, even be absent in zircon even when present in other phases. It is possible to mistake non-magmatic inclusions in detrital grains for primary magmatic inclusions, and examination of internal host grain texture and geochronology (when applicable) is especially important in a detrital setting. These results should allow an improved framework for interpreting inclusion assemblages in detrital magmatic minerals and in interpreting the phase petrology of the host igneous rock. Given the remaining ambiguities in linking a number of observations (i.e., apatite contents, position on the quartz-alkali feldspar-plagioclase ternary, and the ratio of felsic to mafic silicate phases) to specific magmatic provenance, the identification of rare but petrologically significant inclusion phases such as muscovite, halides, Mn and Nb minerals, sulfides, or baddeleyite, may be more helpful in pointing to either highly evolved or more mafic provenances.

Magnesium and zinc isotopic evidence for the involvement of recycled carbonates in the petrogenesis of Gaussberg lamproites, Antarctica

Lamproites are rare mantle-derived peralkaline ultrapotassic rocks, and they are commonly geographically associated with the ultramafic lamprophyres and kimberlites. Their unique geochemistry and mineralogy make determining their mantle source and origin important because of the significance for inferring specific geodynamic processes. In this study, we further examine lamproite petrogenesis using new Mg and Zn isotopic data for the typical Gaussberg lamproites, Antarctica, the source of which were thought to be contributed by recycled crustal materials. Results show that these lamproites have lower δ26Mg (−0.44‰ to −0.39‰) and higher δ66Zn (0.36‰ to 0.39‰) values than terrestrial mantle (δ26Mg = −0.25 ± 0.04‰, δ66Zn = 0.18 ± 0.05‰). The post-magmatic alteration and crustal contamination as well as fractional crystallization and partial melting cannot account for these anomalous Mg and Zn isotopic values. By contrast, the involvement of sedimentary carbonates which are characterized by light δ26Mg (average approximately −2.0‰) and heavy δ66Zn (average ~ +0.91‰) values in their mantle source can explain these Mg and Zn isotopic anomalies. Quantitative modelling suggests that addition of 10–15% subducted dolomite into the source of Gaussberg lamproites can well reproduce their Mg and Zn isotopic values. The source component with light Mg and heavy Zn isotopic compositions can either be sub-continental lithospheric mantle metasomatized by carbonate melts or residue of subducted carbonate-bearing sediments after deep melting in the mantle transition zone. A lithospheric mantle contribution is indeed required to explain their strongly enriched radiogenic isotopic compositions. However, in terms of carbonate component, their positive ZrHf anomalies (Hf/Hf* = 1.28–2.19), and extremely high K/U (~40, 000) and Ba/Th (~400) ratios lead us to favor the latter deep recycling model in which the recycled carbonate-bearing sediments subducted as K-hollandite and majorite underwent partial melting within the mantle transition zone.

Phenakite and bertrandite: products of post-magmatic alteration of beryl in granitic pegmatites (Tatric Superunit, Western Carpathians, Slovakia)

AbstractThe beryllium silicate minerals phenakite and bertrandite have been identified in granitic pegmatite dykes of the beryl-columbite subtype of Variscan age (~340−355 Ma), associated with S- to I-type granitic rocks of the Tatric Superunit, Western Carpathians (Slovakia). The two beryllium silicates and associated minerals were characterised by electron microprobe analysis, back-scattered electron petrography and cathodoluminescence imagery, X-ray diffraction and micro-Raman techniques. Phenakite and bertrandite form euhedral-to-anhedral crystals and aggregates in irregular domains and veinlets replacing primary magmatic beryl. A detailed textural study revealed a close genetic association of phenakite and bertrandite with secondary fine-grained quartz, K-feldspar and muscovite. Locally, clay phyllosilicate minerals, (with compositions similar to those of Fe-dominant hydrobiotite, beidellite, nontronite and saponite) occur as the youngest minerals. During the post-magmatic (hydrothermal) stage of the pegmatites, infiltration of aqueous K-bearing fluids at T ≈ 200–400°C resulted in the breakdown of magmatic beryl to secondary assemblages containing phenakite and bertrandite.

Multi-mode chemical exchange in seafloor alteration revealed by lithium and potassium isotopes

Elemental exchange during seafloor alteration exerts substantial control on the chemical compositions of seawater and oceanic crust, as well as the global elemental cycling. However, detailed processes and mechanisms for element transport and isotopic exchange remain unclear. Here, we report Li and K isotopic compositions across an altered MORB-like pillow basalt from Mariana Trench. We find large and systematic isotopic variations on a centimeter scale, with δ7Li decreasing from +9.07‰ to +6.21‰ and δ41K increasing from −0.12‰ to +0.33‰ from the rim to the core. The heavier-than-mantle Li and K isotopic compositions in the core likely inherit from an enriched mantle source rather than post-magmatic alteration processes. The low K and high Li isotopic compositions at the outmost rims result from adsorption during seawater-rock interactions. The basin-shaped elemental and isotopic variations were subsequently formed by chemical diffusion and spontaneous seawater penetration from the weathered rind to the interior of pillow basalt sample. This study uses a small-scale sampling approach and provides the first evidence for multi-mode chemical exchange in oceanic basalts and improves our understanding of the seafloor alteration processes.

Scavenging and release of REE and HFSE by Na-metasomatism in magmatic-hydrothermal systems

Exploitable or potentially exploitable deposits of critical metals, such as rare-earth (REE) and high-field-strength elements (HFSE), are commonly associated with alkaline or peralkaline igneous rocks. However, the origin, transport and concentration of these metals in peralkaline systems remains poorly understood. This study presents the results of a mineralogical and geochemical investigation of the Na-metasomatism of alkali amphiboles and clinopyroxenes from a barren peralkaline granite pluton in NE China, to assess the remobilization and redistribution of REE and HFSE during magmatic-hydrothermal evolution. Alkali amphiboles and aegirine-augites from the peralkaline granites show evolutionary trends from sodic-calcic to sodic compositions, with increasing REE and HFSE concentrations as a function of increasing Na-index [Na#, defined as molar Na/(Na+Ca) ratios]. The Na-amphiboles (i.e., arfvedsonite) and aegirine-augites can be subsequently altered, or breakdown, to form hydrothermal aegirine during late- or post-magmatic alteration. Representative compositions analyzed by in-situ LA-ICPMS show that the primary aegirine-augites have high and variable REE (2194–3627 ppm) and HFSE (4194–16,862 ppm) contents, suggesting that these critical metals can be scavenged by alkali amphiboles and aegirine-augites. Compared to the primary aegirine-augites, the presentative early replacement aegirine (Aeg-I, Na# = 0.91–0.94) has notably lower REE (1484–1972) and HFSE (4351–5621) contents. In contrast, the late hydrothermal aegirine (Aeg-II, Na# = 0.92–0.96) has significantly lower REE (317–456 ppm) and HFSE (6.44–72.2 ppm) contents. Given that the increasing Na# from aegirine-augites to hydrothermal aegirines likely resulted from Na-metasomatism, a scavenging-release model can explain the remobilization of REE and HFSE in peralkaline granitic systems. The scavenging and release of REE and HFSE by Na-metasomatism provides key insights into the genesis of globally significant REE and HFSE deposits. The high Na-index of the hydrothermal aegirine might be useful as a geochemical indicator in the exploration for these critical-metals.

PLAGIOCLASE ZONATION OF CLINOPYROXENITE–GABBRONORITE–DIORITE KAALAMO MASSIF (NORTHERN LADOGA AREA, RUSSIA)

The gabbroids of Kaalamo massif of the Northern Ladoga area contain plagioclases with a mixed type of zoning, which makes it possible to establish some features of crystallization and postmagmatic evolution of intrusive rocks. The study of minerals by the method of X-ray spectral microanalysis and numeric modeling of mineral formation during crystallization of rocks show ranges of compositions changes in plagioclases. The studied grains of plagioclase from the real samples were compared with theoretical (modelled) ones. The conclusions about the magmatic and postmagmatic evolution of gabbroids based on the data obtained were made. As the temperature dropped below the liquid’s of plagioclase, a solid solution precipitated from magma of ultrabasic – basic composition in the form of basic plagioclase with An 80 . Further, the Na-Si enrichment of plagioclase occurs due to fractional crystallization, in which the composition of the melt shifts to the side rich in silicic acid and sodium. Crystallization of plagioclase, reaching An 50 , is associated with mixing of the compositions of two magmas (fractionated early and new portions of the melt). In plagioclases An 50–40 , oscillatory zoning was formed due to diffusion processes at the crystal – melt interface during slow cooling of the intrusion and increased magma viscosity due to the shift of its composition to the silicic acid part. The latest plagioclases (to An 30 and lower) were formed during the recrystallization of magmatic plagioclases and reflect the stages of late-, postmagmatic alteration and superimposed regional metamorphism.

Petrogenesis of post-orogenic Pan-African rare-element granitic pegmatites in the western Arabian-Nubian Shield, Aswan area, southern Egypt

• Aswan pegmatites evolved via fractional crystallization of the parental granitic magma . • The rare-element pegmatites were formed in a post-orogenic setting. • Post-magmatic hydrothermal fluids play an important role in their formation. Pegmatites with rare-element mineralization from the Aswan area in southern Egypt intruded the post-orogenic Pan-African granitic rocks and the associated gneisses. The granitic pegmatites show a regular pattern of concentric zoning and display a weak affinity to the Nb-Y-F (NYF) pegmatite family. These pegmatites consist mainly of K-feldspar, albite, quartz, and subordinate muscovite, accessory minerals include Nb-Ta oxide minerals, monazite-(Ce), xenotime-(Y), zircon, rutile, and magnetite. Field observations combined with the petrography, mineralogy and geochemistry of pegmatites and associated rocks in the Aswan area indicate that the pegmatites evolved via fractional crystallization of the parental granitic magma with an effective role for the post-magmatic alterations. The wide compositional variations of Nb-Ta minerals in term of their major and trace elements reflect the pegmatite formation stages. Primary magmatic generation of columbite-(Fe) occurs as homogeneous grains or showing oscillatory zoning with high Nb/Ta and Fe/Mn ratios. Late ixiolite shows patchy and irregular zoning with high Sc and W replacing the earlier regular texture, indicating metasomatic replacement. Electron microprobe U–Th–Pb dating of monazite from the studied pegmatites yielded post-orogenic Pan-African age of 578 ± 5 Ma, comparable with age of the fine-grained granite in this area, but it is younger than the previously estimated U–Pb zircon age of surrounding the Monumental granites and gneisses (∼600 to 620 Ma). The similarity in age and geochemical characteristics of the pegmatites and the fine-grained granite proved their genetic relationship.

First Evidence of the Post-Variscan Magmatic Pulse on the Western Edge of East European Craton: U-Pb Geochronology and Geochemistry of the Dolerite in the Lublin Podlasie Basin, Eastern Poland

The U–Pb measurements of youngest, coherent group of zircons from the Mielnik IG1 dolerite at the Teisseyre-Tornquist margin (TTZ) of East European Craton (EEC) in Poland yielded age of 300 ± 4 Ma. Zircon dated an evolved portion of magma at the late stage crystallization. It is shown that this isolated dyke from the northern margin of the Lublin Podlasie basin (Podlasie Depression) and regional dyke swarms of close ages from the Swedish Scania, Bornholm and Rügen islands, Oslo rift, Norway, and the Great Whine Sill in northeastern England, were coeval. They have been controlled by the same prominent tectonic event. The Mielnik IG1 dolerite is mafic rock with Mg-number between 52 and 50 composed of the clinopyroxene, olivine-pseudomorph, plagioclase, titanite, magnetite mineral assemblage, indicating relatively evolved melt. This hypabyssal rock has been affected by postmagmatic alteration. The subalkaline basalt composition, enrichment in incompatible trace elements, progressive crustal contamination, including abundance of zircon xenocrysts determines individual characteristics of the Mielnik IG1 dolerite. The revised age of dolerite, emplaced in vicinity of TTZ provides more evidences documenting the reach of the Permo-Carboniferous extension and rifting accompanied by magmatic pulses, that were widespread across Europe including the margin of the EEC incorporated that time into the broad foreland of the Variscan orogen.

Sr-bearing phosphates of the apatite supergroup from the alkaline rocks of the Konder pluton, Khabarovsk Krai

The paper presents a mineralogical description of Sr-bearing phosphates of the apatite supergroup found in eudialyte-aegirine-albite rocks of the Konder pluton, Khabarovsk krai. The Sr-dominant phases are associated with minerals of the lamprophyllite group and titanite, whereas the Ca-dominant phases are associated with calciocatapleite and kainosite-(Y), which are the products of the decomposition of eudialyte. The phosphates probably formed during post-magmatic alteration of host rock in a following sequence: stronadelphite &gt; fuorostrophite &gt; Sr-rich fuorapatite (fuorocaphite) &gt; Na-REE-fuorapatite. Their variable chemical composition indicates decreasing Sr, Ba and F contents and increasing Ca, Na, REE and (OH) contents during crystallization. The crystallization sequence could refect a decrease in alkalinity of the mineral-forming conditions. Keywords: stronadelphite, fuorostrophite, fuorocaphite, Sr-rich fuorapatite, fuorapatite, alkaline rocks, hydrothermal alterations, Konder pluton, alkaline-ultramafc complexes.