Phlogopite Phenocrysts Research Articles

Tracking the Ti4+ substitution in phlogopite by spectroscopic imaging: A tool for unravelling the growth of micas at HP-HT conditions

Phlogopite solid-solutions have a wide pressure–temperature (P-T) stability field and are ubiquitous in a wide variety of geological settings, from deep lithosphere magmatic environments to upper crust metamorphic domains. Phlogopite composition represents therefore a valuable physical–chemical archive and may provide important information regarding its crystallization and the petrogenesis of the host-rock. In this paper we examine the phlogopite phenocrysts from the well-known Fort Regent mica-bearing lamprophyre minette from St. Helier (Island of Jersey, UK). Phlogopite phenocrystals from lamprophyres generally show normal-step and continuous compositional zoning, however those from the Fort Regent minette show a peculiar texture characterized by dark brown high-Ti (average TiO2 ≈ 8.5 wt.%) cores enveloped by euhedral low- to mid-amplitude zonation due to oscillatory contents in Ti, Fe and Mg. Thermo-barometry modelling based on biotite-only composition yields relatively high P-T estimates (T ≈ 970 ± 54 °C at P ≈ 0.73 ± 0.13 GPa) for cores whereas lower values (T ≈ 790 ± 54 °C at P ≈ 0.29 ± 0.13 GPa) are obtained for the outer rims. Comparable temperatures (T ≈ 1075 ± 54 °C) but extremely high and anomalous pressure values (P ≈ 1.82 ± 0.13 GPa) are obtained for the yellowish inner rims. The combination of electron micro probe (EMP) analysis and single-crystal infra-red (FTIR) imaging in the OH-stretching region shows that the exceptional and oscillatory Ti contents are due to the Ti-vacancy substitution, typical of crystallization and growth processes of HP/HT environments. Raman imaging provides additional insight for this process, confirming the dominant dioctahedral nature for the Ti-Fe-rich cores and outer rims. Interpretation of thermobaric estimates obtained from the phlogopite composition-only model, based on the fine-scale compositional evolution, shows that pressure–temperature values from low-Ti high-Mg domains should be carefully evaluated because the substitution mechanisms during the dark mica growth are not univocally related to pressure–temperature variation of the crystallizing environment. Our results demonstrate how a multidisciplinary approach based on the combination of chemical investigations and vibrational spectroscopies could represent a valuable tool to evaluate pressure–temperature estimates from biotite composition-only thermo-barometry models and therefore to correctly unravel HP/HT petrogenetic processes at a very fine scale.

Petrogenesis of Eocene Lamprophyre Dykes in Northern Qiangtang Terrane, Tibetan Plateau: Implications for the Tethyan Mantle Metasomatism and Tectonic Evolution

Post-collisional (ultra)potassic lamprophyre dykes are the key probes for understanding mantle metasomatism and reconstructing tectonic evolution. In this study, we present new petrological, geochronological, geochemical and zircon Lu-Hf isotopic data for lamprophyre dykes in the northern Qiangtang terrane (central Tibet), aiming to constrain their petrogenesis and geodynamic setting. The studied lamprophyres are minettes with phenocrysts of siderophyllite and phlogopite, which intrude into Triassic granite of 236.9 Ma. These lamprophyres yield zircon U-Pb ages of 39.7–40.9 Ma. They exhibit high contents of K2O (7.61–8.59 wt.%) and ultrapotassic features with high K2O/Na2O (11.43–14.38) ratios. They are characterized by increased values of Mg# (69.1 to 72.1) and high concentrations of compatible elements (e.g., Cr = 277–529 ppm, Ni = 232–322 ppm), which are diagnostic of mantle-derived primitive magma. The studied lamprophyres have a high abundance of rare earth elements (∑REE = 902–1061 ppm) with significantly fractionated REE patterns ((La/Yb)N = 66.3–100.6), and they are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), but depleted in high field strength elements (HFSE) (e.g., Nb, Ta and Ti) and heavy rare earth elements (HREE) with enriched zircon Hf isotopes (εHf(t) from −6.40 to 3.80). This indicates their derivation from an enriched mantle source which was metasomatized by subduction-related fluids and sediment-derived melts. A petrogenetic study suggests that the lamprophyres were generated by the partial melting of a phlogopite-bearing lherzolite within the garnet stability field. We propose that the Cenozoic ultrapotassic mafic rocks in the central Tibetan Plateau originated in the lithospheric mantle metasomatized by the subduction-related components, and are the magmatic response to the detachment of the subducted Tethyan slab.

Potassium isotope fractionation during silicate-carbonatite melt immiscibility and phlogopite fractional crystallization

Abstract Potassium (K) isotopes have been used as a tracer of K recycling in the Earth, but K isotope fractionation during magma evolution is poorly constrained. Here, we present K isotope data for a magmatic suite of alkaline silicate-carbonatite affinity. The suite was formed from liquid-liquid immiscibility and subsequent phlogopite fractionation. The K isotopic signatures of different rock types are in the following order: alkaline silicate lavas (δ41K = –0.424 to 0.090‰) > carbonatitic silicate lavas (δ41K = –0.640 to –0.035‰) > carbonatites (δ41K = –0.858 and –0.258‰). Phlogopite phenocrysts in the silicate lavas are isotopically lighter (δ41K = –0.628 to –0.534‰) than the lavas in which they occur (Δ41KPhlogopite-whole rock = –0.502 to –0.109‰). Correlations between δ41K values and chemical proxies of melt immiscibility and phlogopite fractionation indicate that K isotopes are significantly fractionated by both processes at a ~0.6‰ magnitude. Such K isotope variation overlaps the range of δ41K in arc lavas. Compilations of literature data further confirm the critical roles of melt immiscibility and phlogopite fractionation in K isotope variations of high-K lavas (K2O >1 wt%) from post-collision orogenic and intra-continental settings. In comparison, basaltic arc lavas are depleted in K2O (mostly <1 wt%) and lack evidence of significant phlogopite fractionation. The K isotope variations of arc lavas are mainly controlled by their mantle sources, which were metasomatized by melt or fluid released from the subducting slab. Therefore, K recycling and K isotope variation are controlled by distinct mechanisms in different tectonic settings.

A Mantle Plume Connection for Alkaline Lamprophyres (Sannaites) from the Permian Tarim Large Igneous Province: Petrological, Geochemical and Isotopic Constraints

AbstractThe origin of lamprophyres associated with large igneous provinces (LIPs) remains controversial, particularly whether they are derived by direct melting of mantle plumes, or from previously metasomatized domains in thermally perturbed subcontinental lithosphere. Here, we report the petrological and geochemical characteristics of a recently identified suite of alkaline lamprophyres (sannaites) that represent the final pulse of magmatism in the Permian Tarim LIP in NW China. The sannaites display porphyritic texture with phenocrysts of olivine, clinopyroxene, hornblende, phlogopite, and titanomagnetite in a groundmass of plagioclase, clinopyroxene, nepheline, hornblende, biotite, and titanomagnetite with minor pyrite and apatite. Carbonate ocelli and almost pure albite in the groundmass are interpreted to have crystallized from immiscible carbonate and hydrous fluids, respectively, produced by late-stage magmatic segregation. The rocks show low to moderate SiO2 (37.7–49.3 wt.%) and MgO (2.74–9.91 wt.%), together with high Fe2O3T (up to 22.7 wt.%) and alkali contents (up to 9.02 wt.% Na2O + K2O). They are characterized by high incompatible element abundances, especially a marked enrichment in large-ion lithophile elements (Rb and Ba) and light rare-earth elements (e.g. La and Ce) relative to P and high-field-strength elements (e.g. Ti). They show a relatively restricted range of δ66Zn values between 0.22‰ and 0.46‰ with an average of 0.37 ± 0.04‰ (2SE, n = 10), which is marginally heavier than that of MORBs (0.27 ± 0.05‰). Their (87Sr/86Sr)t values range from 0.7035 to 0.7061, εNd(t) from −0.97 to +5.62, and δ26Mg from −0.36‰ to −0.17‰ (n = 8), the latter being consistent with those of global MORBs. Based on the correlation between Zn isotopes and TiO2–FeO concentrations, we infer that the heavy Zn isotopes in some of the sannaites resulted from fractional crystallization of Fe–Ti oxide minerals. The whole rock geochemical features of these rocks (negative K anomalies and enrichment in large-ion lithophile elements) and rhyolite–MELTS simulations suggest that the primary magmas of the sannaites were derived from an amphibole-bearing enriched lithospheric mantle source. Metasomatism and related formation of amphibole-bearing metasomatized mantle may be linked to sublithospheric melts/fluids derived from the Tarim plume in the earlier stages of plume activity, rather than slab-derived fluids or carbonate melts as suggested in previous studies for other alkaline mantle-derived magmas. Partial melting may have been triggered by the thermal input from the Tarim plume during a later stage. This study suggests that exotic, alkali-rich magmas can be produced during the multi-stage evolution of large mantle plumes, involving complex cycles of lithospheric mantle metasomatism and later melting of previously enriched domains.

Reconstruction of magma chamber processes preserved in olivine-phlogopite micro-ijolites from the Oldoinyo Lengai, Tanzania

A detailed petrographic and mineralogical investigation of olivine-phlogopite micro-ijolite xenoliths from Oldoinyo Lengai, Tanzania indicates a complex evolutional history. These xenoliths consist of diverse textural subdomains characterized by minerals ranging from early-formed olivine, through diopside-hosted perovskite and phlogopite, to evolved aegirine-augite and titanite. Thermometry and mineral compositions in the subdomains suggest crystallization temperatures from 1070–970 °C to 850–700 °C at plutonic pressures and SiO2-activities controlled by perovskite-titanite equilibria. Double coronas are a characteristic textural feature of the olivine-phlogopite micro-ijolite, consisting of olivine cores surrounded by an inner clinopyroxene corona and an outer phlogopite corona. These double coronas might have formed during early magma chamber processes, including magma movement to a subsequent chamber resulting in dissolution of olivine with subsequent crystallization and accumulation of diopside and phlogopite. Diopside−aegirine-augite compositional zonation indicates several magma injections followed by cooling periods, during the formation of micro-ijolite groundmass. Mg# (80–83) and Ca (0.1–0.3 in wt%) contents of olivine together with the presence of primary melt inclusions in clinopyroxene, phlogopite, and nepheline indicate a magmatic origin from a possible parental olivine-nephelinite melt. There is evidence for subsolidus, or near-solidus, re-equilibration processes as indicated by the reaction of olivine with titanite forming symplectitic textures of ilmenite and diopside with minor zirconolite. Ti-exchange between phlogopite phenocrysts and other Ti-bearing minerals (perovskite, titanite, magnetite) resulted in ∼750 °C equilibrium temperatures for phlogopite, which are much lower than mafic magmatic (>900 °C) conditions. Calculated subsolidus temperatures suggest crystallization of olivine-phlogopite micro-ijolites over a 10–20 km depth interval.

Petrogenesis of Ultramafic Lamprophyres from the Terina Complex (Chadobets Upland, Russia): Mineralogy and Melt Inclusion Composition

The mineral composition and melt inclusions of ultramafic lamprophyres of the Terina complex were investigated. The rocks identified were aillikites, mela-aillikites, and damtjernites, and they were originally composed of olivine macrocrysts and phenocrysts, as well as phlogopite phenocrysts in carbonate groundmass, containing phlogopite, clinopyroxene and feldspars. Minor and accessory minerals were fluorapatite, ilmenite, rutile, titanite, and sulphides. Secondary minerals identified were quartz, calcite, dolomite, serpentine, chlorite, rutile, barite, synchysite-(Ce), and monazite-(Ce). Phlogopite, calcite, clinopyroxene, Ca-amphibole, fluorapatite, magnetite, and ilmenite occurred as daughter-phases in melt inclusions. The melt inclusions also contained Fe–Ni sulphides, synchysite-(Ce) and, probably, anhydrite. The olivine macrocrysts included orthopyroxene and ilmenite, and the olivine phenocrysts included Cr-spinel and Ti-magnetite inclusions. Crystal-fluid inclusions in fluorapatite from damtjernites contain calcite, clinopyroxene, dolomite, and barite. The data that were obtained confirm that the ultramafic lamprophyres of the Terina complex crystallized from peridotite mantle-derived carbonated melts and they have not undergone significant fractional crystallization. The investigated rocks are considered to be representative of melts that are derived from carbonate-rich mantle beneath the Siberian craton.

Oxide minerals of Kaavi kimberlites (Finland)

The mineralogical and petrogeochemical features of the Neoproterozoic kimberlite rocks of the Lahtojoki and Niilonsuo pipes of the Kaavi cluster (Kaavi-Kuopio, Finland) have been studied, differences in their petrogeochemical composition, quantitative and chemical composition of oxide minerals of deep (mantle) and kimberlite genesis have been revealed. The kimberlites of the pipes are moderately titanic, but the TiO2 content in the kimberlites of Niilonsuo is higher (2.11 wt.%) than in the kimberlites from the breccia of the Lahtojoki pipe (1.07 wt.%). The kimberlites of the Niilonsuo pipe also differ in higher concentrations of Fe2 O3 , Ca, P, K, Rb, V, Nb, Ba, Th, U, Ta and REE. In the Lahtojoki kimberlite breccias the main TiO2 concentrator mineral is magnesian ilmenite (13,3—15,2 wt.% MgO; 0,5—4,4 wt.% Cr2 O3 ), (macrocrysts up to 4 mm); the fine-grained matrix of rocks contains small grains of rutile, chromespinelides, Mn-ilmenite and sometimes titanomagnetite. Macrocrystals of magnesian ilmenite have been not found in the kimberlites of the Niilonsuo pipe, perovskite acts as the main mineral of titanium, and chromespinelids and titanomagnetite are less common. Long-term crystallization of relatively large (up to 200 μm) perovskite grains proceeded according to estimates using an Nb-Fe-perovskite oxybarometer under a wide range of oxygen fugacity (fо2 ) of the kimberlite melt (NNO from -3,8 to 5,1). Chromespinelids from the groundmass of kimberlite pipe rocks differ in composition, but have the same specific zonality — enrichment of Al and Mg in the edge zones of crystals, which is possibly due to the dissolution of phlogopite phenocrysts in the rising kimberlite melt. In addition to oxide minerals, djerfisherite is widely distributed in the groundmass of kimberlites of the Niilonsuo pipe, the composition of which for the rocks of the body has been described for the first time. The combination of features of oxide mineralization indicates unfavorable conditions for the preservation of diamonds during their transportation by kimberlite melt.

Petrogenesis of a Hybrid Cluster of Evolved Kimberlites and Ultramafic Lamprophyres in the Kuusamo Area, Finland

Abstract Kimberlites are often closely associated, both in time and space, with a wide variety of alkaline ultramafic rock types, yet the question of a genetic relationship between these rock types remains uncertain. One locality where these relationships can be studied within the same cluster is the Karelian craton in Finland. In this study we present the first petrographic, mineral and whole-rock geochemical results for the most recently discovered kimberlite cluster on this craton, which represents an example of the close spatial overlap of kimberlites with ultramafic lamprophyres. The Kuusamo cluster incorporates seven bodies [Kasma 45, Kasma 45 south, Kasma 47, Kalettomanpuro (KP), Kattaisenvaara (KV), Dike 15 and Lampi] distributed along a 60 km NE–SW corridor. Hypabyssal samples from KV, KP, Kasma 45 and Kasma 47 consist of altered olivine macrocrysts and microcrysts and phlogopite phenocrysts in a groundmass of perovskite, apatite, spinel, ilmenite, serpentine, and calcite. These petrographic features combined with mineral (e.g. Mg-rich ilmenite, Al–Ba-rich, Ti–Fe-poor mica) and whole-rock incompatible trace element compositions (La/Nb = 0·8 ± 0·1; Th/Nb = 0·07 ± 0·01; Nb/U = 66 ± 9) are consistent with these rocks being classified as archetypal kimberlites. These Kuusamo kimberlites are enriched in CaO and poor in MgO, which, combined with the absence of chromite and paucity of olivine macrocrysts and mantle-derived xenocrysts (including diamonds), suggests derivation from differentiated magmas after crystal fractionation. Samples from Lampi share similar petrographic features, but contain mica with compositions ranging from kimberlitic (Ba–Al-rich cores) to those more typical of orangeites–lamproites (increasing Si–Fe, decreasing Al–Ti–Ba), and have higher bulk-rock SiO2 contents than the Kuusamo kimberlites. These features, combined with the occurrence of quartz and titanite in the groundmass, indicate derivation from a kimberlite magma that underwent considerable crustal contamination. This study shows that crustal contamination can modify kimberlites by introducing features typical of alkaline ultramafic rock types. Dike 15 represents a distinct carbonate-rich lithology dominated by phlogopite over olivine, with lesser amounts of titaniferous clinopyroxene and manganoan ilmenite. Phlogopite (Fe–Ti-rich) and spinel [high Fe2+/(Fe2+ + Mg)] compositions are also distinct from the other Kuusamo intrusions. The petrographic and geochemical features of Dike 15 are typical of ultramafic lamprophyres, specifically, aillikites. Rb–Sr dating of phlogopite in Dike 15 yields an age of 1178·8 ± 4·1 Ma (2σ), which is considerably older than the ∼750 Ma emplacement age of the Kuusamo kimberlites. This new age indicates significant temporal overlap with the Lentiira–Kuhmo–Kostomuksha olivine lamproites emplaced ∼100 km to the SE. It is suggested that asthenospheric aillikite magmas similar to Dike 15 evolved to compositions akin to the Karelian orangeites and olivine lamproites through interaction with and assimilation of MARID-like, enriched subcontinental lithospheric mantle. We conclude that the spatial coincidence of the Kuusamo kimberlites and Dike 15 is probably the result of exploitation of similar trans-lithospheric corridors.

Alteration and mineralization styles of the orogenic disseminated Zhenyuan gold deposit, southeastern Tibet: Contrast with carlin gold deposit

Orogenic disseminated and Carlin gold deposits share much similarity in alteration and mineralization. The disseminated orogenic Zhenyuan Au deposit along the Ailaoshan shear zone, southeastern Tibet, was selected to clarify their difference. The alteration and mineralization from the different lithologies, including meta-quartz sandstone, carbonaceous slate, meta-(ultra)mafic rock, quartz porphyry and lamprophyre were researched. According to the mineral assemblage and replacement relationship in all types of host rocks, two reactions show general control on gold deposition: (1) replacement of earlier magnetite by pyrite and carbonaceous material; (2) alteration of biotite and phlogopite phenocrysts in quartz porphyry and lamprophyre into dolomite/ankerite and sericite. Despite the lamprophyre is volumetrically minor and much less fractured than other host rocks, it contains a large portion of Au reserve, indicating that the chemically active lithology has played a more important role in gold precipitation compared to structure. LA-ICP-MS analysis shows that Au mainly occurs as invisible gold in fine-grained pyrite disseminated in the host rocks, with Au content reaching to 258.95 ppm. The diagenetic core of pyrite in meta-quartz sandstone enriched in Co, Ni, Mo, Ag and Hg is wrapped by hydrothermal pyrite enriched in Cu, As, Sb, Au, Tl, Pb and Bi.Different host rock lithology has much impact on the alteration and mineralization features. Carbonate and sericite in altered lamprophyre show they have higher Mg than those developed in other of host rocks denoting that the carbonate and sericite incorporated Mg from phlogopite phenocrysts in the primary lamprophyre during alteration. The ore fluid activated the diagenetic pyrite in meta-quartz sandstone leading the hydrothermal pyrite enriched in Cu, Mo, Ag, Sb, Te, Hg, Tl, Pb and Bi, but the hydrothermal pyrite in meta-(ultra)mafic rock is enriched in Co and Ni as the meta-(ultra)mafic rock host rock contain high content of Co and Ni. However, Au and As shear similar range in both types of host rocks indicating that these two elements most likely come from the deep source fluid rather than the host rocks. It was shown in the disseminated orogenic gold deposit that similar hydrothermal alteration with mineral assemblage of carbonate (mainly dolomite and ankerite), sericite, pyrite and arsenopyrite develops in all types of host rocks. This is different from the Nevada Carlin type, in which alteration is mainly dissolution and silicification of carbonate host rock. On the other hand, Au mainly occur as invisible gold in both disseminated orogenic and Carlin gold deposits.

Petrogenesis of carbonatitic lamproitic dykes from Sidhi gneissic complex, Central India

Petrographic, mineral chemical and whole-rock geochemical characteristics of two newly discovered lamproitic dykes (Dyke 1 and Dyke 2) from the Sidhi Gneissic Complex (SGC), Central India are presented here. Both these dykes have almost similar sequence of mineral-textural patterns indicative of: (1) an early cumulate forming event in a deeper magma chamber where megacrystic/large size phenocrysts of phlogopites have crystallized along with subordinate amount of olivine and clinopyroxene; (2) crystallization at shallow crustal levels promoted fine-grained phlogopite, K-feldspar, calcite and Fe-Ti oxides in the groundmass; (3) dyke emplacement related quench texture (plumose K-feldspar, acicular phlogopites) and finally (4) post emplacement autometasomatism by hydrothermal fluids which percolated as micro-veins and altered the mafic phases. Phlogopite phenocrysts often display resorption textures together with growth zoning indicating that during their crystallization equilibrium at the crystal-melt interface fluctuated multiple times probably due to incremental addition or chaotic dynamic self mixing of the lamproitic magma. Carbonate aggregates as late stage melt segregation are common in both these dykes, however their micro-xenolithic forms suggest that assimilation with a plutonic carbonatite body also played a key role in enhancing the carbonatitic nature of these dykes. Geochemically both dykes are ultrapotassic (K2O/Na2O: 3.0–9.4) with low CaO, Al2O3 and Na2O content and high SiO2 (53.3–55.6 wt.%) and K2O/Al2O3 ratio (0.51–0.89) characterizing them as high-silica lamproites. Inspite of these similarities, many other features indicate that both these dykes have evolved independently from two distinct magmas. In dyke 1, phlogopite composition has evolved towards the minette trend (Al-enrichment) from a differentiated parental magma having low MgO, Ni and Cr content; whereas in dyke 2, phlogopite composition shows an evolutionary affinity towards the lamproite trend (Al-depletion) and crystallized from a more primitive magma having high MgO, Ni and Cr content. Whole-rock trace-elements signatures like enriched LREE, LILE, negative Nb-Ta and positive Pb anomalies; high Rb/Sr, Th/La, Ba/Nb, and low Ba/Rb, Sm/La, Nb/U ratios in both dykes indicate that their parental magmas were sourced from a subduction modified garnet facies mantle containing phlogopite. From various evidences it is proposed that the petrogenesis of studied lamproitic dykes stand out to be an example for the lamproite magma which attained a carbonatitic character and undergone diverse chemical evolution in response to parental melt composition, storage at deep crustal level and autometasomatism.

First evidence of lamprophyric magmatism within the Subbetic Zone (Southern Spain)

Two drillings carried out at Cerro Prieto (Province of Malaga), together with additional geophysical data, revealed the existence of an igneous body formed of rock-types previously unknown in the Subbetic zone. The recovered rocks, emplaced under hypoabyssal conditions, are predominantly porphyric, containing olivine, diopside and TiO2-rich phlogopite phenocrysts (up to 1-2 mm in size) within a micro-to-hypocrystalline groundmass composed of alkali-feldspar, diopside, phlogopite and abundant magnetite, and could be classified as alkali minettes lam- prophyres. They contain numerous xenocrysts corroded by the magma and centimetric ultrafemic xenoliths deriv- ing from the mantle. Clinopyroxenes yield crystallisation temperatures from about 1150 to 1320o C and pressures ranging from about 4 to 17 kbar, suggesting 50 km as the minimum depth of the magma sources. The chemical compositions of these lamprophyres are similar to intra-plate alkali-basalts, derived from oceanic-island-basaltic-type highly metasoma- tized mantle sources. 40 Ar/ 39 Ar dating of a phlogopite mineral separate gave an age of 217±2.5 Ma. However, these 1 4

Origin of lamprophyres from the northern margin of the North China Craton: implications for mantle metasomatism

Abundant lamprophyre dykes occur near the northern margin of the North China Craton and offer a unique opportunity to study the nature of the mantle source. The dykes are minettes composed of phlogopite, sanidine and calcite. 40 Ar/ 39 Ar dating yields ages of 234 ± 2 and 222 ± 6 Ma. The lamprophyres are near-primary, mantle-derived ultrapotassic melts, having low SiO 2 (31.0 – 41.5 wt%) and high K 2 O (4.40 – 7.12 wt%) contents, high Mg# (62 – 84) and high contents of compatible elements. They are characterized by fractionated rare earth element patterns, radiogenic Sr and unradiogenic Nd isotope compositions ( 87 Sr/ 86 Sr i = 0.7070 – 0.7075; ε Nd ( t ) = −12.8 to −9.2). A small amount of mafic crustal rocks (<4.4%) may have been assimilated during magma ascent, as revealed by 187 Os/ 188 Os i ratios of 0.4548 – 0.8068. These data suggest that the lamprophyres originated from a low degree of partial melting of an enriched subcontinental lithospheric mantle source with abundant phlogopite, clinopyroxene and carbonate. The source has been metasomatized by carbonate- and potassium-rich fluids derived from carbonated sediments recycled via subduction of Palaeo-Mongolian oceanic slab beneath the North China Craton. Supplementary material: Microprobe analyses of biotite, feldspar and carbonate, 40 Ar– 39 Ar analytical data, and 36 Ar/ 40 Ar v. 39 Ar/ 40 Ar inverse isochron diagrams for phlogopite phenocrysts and groundmass from the Datong lamprophyres are available at https://doi.org/10.6084/m9.figshare.c.3574265 .

The final stages of kimberlite petrogenesis: Petrography, mineral chemistry, melt inclusions and Sr-C-O isotope geochemistry of the Bultfontein kimberlite (Kimberley, South Africa)

The petrogenesis of kimberlites is commonly obscured by interaction with hydrothermal fluids, including deuteric (late-magmatic) and/or groundwater components. To provide new constraints on the modification of kimberlite rocks during fluid interaction and the fractionation of kimberlite magmas during crystallisation, we have undertaken a detailed petrographic and geochemical study of a hypabyssal sample (BK) from the Bultfontein kimberlite (Kimberley, South Africa).Sample BK consists of abundant macrocrysts (>1mm) and (micro-) phenocrysts of olivine and lesser phlogopite, smaller grains of apatite, serpentinised monticellite, spinel, perovskite, phlogopite and ilmenite in a matrix of calcite, serpentine and dolomite. As in kimberlites worldwide, BK olivine grains consist of cores with variable Mg/Fe ratios, overgrown by rims that host inclusions of groundmass phases (spinel, perovskite, phlogopite) and have constant Mg/Fe, but variable Ni, Mn and Ca concentrations. Primary multiphase inclusions in the outer rims of olivine and in Fe-Ti-rich (‘MUM’) spinel are dominated by dolomite, calcite and alkali carbonates with lesser silicate and oxide minerals. Secondary inclusions in olivine host an assemblage of Na-K carbonates and chlorides. The primary inclusions are interpreted as crystallised alkali-Si-bearing Ca-Mg-rich carbonate melts, whereas secondary inclusions host Na-K-rich C-O-H-Cl fluids.In situ Sr-isotope analyses of groundmass calcite and perovskite reveal similar 87Sr/86Sr ratios to perovskite in the Bultfontein and the other Kimberley kimberlites, i.e. magmatic values. The δ18O composition of the BK bulk carbonate fraction is above the mantle range, whereas the δ13C values are similar to those of mantle-derived magmas. The occurrence of different generations of serpentine and occasional groundmass calcite with high 87Sr/86Sr, and elevated bulk carbonate δ18O values indicate that the kimberlite was overprinted by hydrothermal fluids, which probably included a significant groundwater component. Before this alteration the groundmass included calcite, monticellite, apatite and minor dolomite, phlogopite, spinel, perovskite and ilmenite. Inclusions of groundmass minerals in olivine rims and phlogopite phenocrysts show that olivine and phlogopite also belong to the magmatic assemblage. We therefore suggest that the crystallised kimberlite was produced by an alkali-bearing, phosphorus-rich, silica-dolomitic melt. The alkali-Si-bearing Ca-Mg-rich carbonate compositions of primary melt inclusions in the outer rims of olivine and in spinel grains with evolved compositions (MUM spinel) support formation of these melts after fractionation of abundant olivine, and probably other phases (e.g., ilmenite and chromite). Finally, the similarity between secondary inclusions in kimberlite olivine of this and other worldwide kimberlites and secondary inclusions in minerals of carbonatitic, mafic and felsic magmatic rocks, suggests trapping of residual Na-K-rich C-O-H-Cl fluids after groundmass crystallisation. These residual fluids may have persisted in pore spaces within the largely crystalline BK groundmass and subsequently mixed with larger volumes of external fluids, which triggered serpentine formation and localised carbonate recrystallisation.

Alkali basalts and enclosed ultramafic xenoliths near Ushuaia, Tierra Del Fuego, Argentina.

At the southernmost part of Tierra del Fuego a few outcrops and erratic boulders of alkali basaltic rocks with ultramafic enclaves have been studied. Alkali basalt plugs or pipes hitherto identified are scarce, and host rocks are constituted by slates that belong to Mesozoic deposition. The petrography, texture and composition of the basalt and xenoliths were investigated by petrographic microscope and electron microprobe analysis. Xenocrysts of amphibole and alkali feldspar, phenocrysts of nepheline, olivine, spinel, phlogopite and Fe–Ti minerals (10 %) and a diversity of xenoliths, mainly lherzolitic, pyroxenite and wehrlitic nodules (15 %), but also from metamorphic rocks provenance, are contained in the basalt groundmass (75 %). This finer-grained material is made up of laths or needles of plagioclase, pyroxene, opaque minerals, apatite and glass, with intersertal, hyalopilitic and pilotaxitic. Locally, rock has an even granoblastic texture. Former amygdules are filled by analcite, zeolites, sodalite and calcite. The normative classification, based on nepheline content, conclude that this rock is an alkali basalt. The chemical classification, considering immobile elements as Zr/TiO2 versus Nb/Y indicate an alkali basalt too and plots over the TAS diagram fall in the foidite (Na-rich or nephelinite) and basanite fields. The REE patterns are fractionated (La/Yb primitive mantle normalized is approximately 30). The K–Ar isotopic technique on individual macrocrysts gave ages of 146 ± 5 Ma (amphibole) and 127 ± 4 Ma (alkali feldspar); and K–Ar whole rock datum reported 8.3 ± 0.3 Ma. Nevertheless, fertile samples show geochemical features typical of deep derived material thus, based on the position in the actual tectonic setting, indicate that the basalt is older than its isotopic age.

Petrology of parasitic and eccentric cones on the flanks and base of Somma-Vesuvius

Parasitic and eccentric cones on the flanks and at the base of Somma-Vesuvius potentially provide information on the plumbing system of the complex. We present geochemical data for minerals, glasses and rocks from the Pollena and Cercola cones. The rocks are phonotephrites and a basaltic trachyandesite. A volumetrically dominant type contains phenocrysts of olivine, clinopyroxene, leucite, plagioclase ± apatite ± Fe-Ti-oxides. A second type is phlogopite-phyric. Low magnesium-numbers (62–49) and Ni abundances (≤32 ppm) indicate that the primary magmas underwent crustal-level fractionation but the cores of olivine and clinopyroxene phenocrysts carry records of the parental magmas. Geochemical data indicate that the rocks form more than one magmatic lineage. Matrix glasses point to low-pressure fractionation trends towards decreasing melt silica-undersaturation. The phlogopite-phyric rocks were derived from more hydrous magmas than those lacking phlogopite phenocrysts, perhaps at higher pressures. Phenocryst assemblages are difficult to reconcile with published experimental work and it is likely that they are far from equilibrium assemblages. The cone magmas were probably derived from high levels within the main plumbing system via lateral transport.

Phlogopite/matrix, clinopyroxene/matrix and clinopyroxene/phlogopite trace-element partitioning in a calc-alkaline lamprophyre: new constrains from the Křižanovice minette dyke (Bohemian Massif)

Mineral/matrix and mineral/mineral partition coefficients were determined for clinopyroxene and phlogopite phenocrysts and their very fine-grained (chilled) groundmass from a Variscan calc-alkaline (agpaitic index = 0.6) lamprophyre dyke of minette composition from the Bohemian Massif (Krizanovice, Tepla-Barrandian Unit). This dyke is characterized by high MgO (8.9 wt. %; Mg-number = 69) at intermediate SiO2 content (53 wt. %). The partition coefficients (D) between clinopyroxene and matrix were determined for 23 elements by laser ablation-inductively coupled plasma-mass spectrometry. Calculated D-clinopyroxene/matrix values are very low, except for heavy rare earth elements (HREE) that range between 0.9 and 1.1. This suggests that HREE can be concentrated in clinopyroxene during crystallization from lamprophyre melt. In phlogopite, only 15 elements had contents above their respective detection limits. Phlogopite/matrix partition coefficients are on average higher than 1 (D-Ba = 1.1, D-Rb = 1.7 D-Ti = 1.5) and extremely low (<= 0.02) for light rare earth elements (LREE). On the other hand, D-phlogopite/matrix values for the majority of HREE could not be determined. Taken together, during simultaneous crystallization of clinopyroxene and phlogopite phenocrysts, Th, Zr, Hf, Y and LREE are preferentially partitioned into clinopyroxene and Ba with Rb and Ti into phlogopite.

The origin of shoshonites: new insights from the Tertiary high-potassium intrusions of eastern Tibet

The shoshonitic intrusions of eastern Tibet, which range in age from 33 to 41 Ma and in composition from ultramafic (SiO2 = 42 %) to felsic (SiO2 = 74 %), were produced during the collision of India with Eurasia. The mafic and ultramafic members of the suite are characterized by phenocrysts of phlogopite, olivine and clinopyroxene, low SiO2, high MgO and Mg/Fe ratios, and olivine forsterite contents of Fo87 to Fo93, indicative of equilibrium with mantle olivine and orthopyroxene. Direct melting of the mantle, on the other hand, could not have produced the felsic members. They have a phenocryst assemblage of plagioclase, amphibole and quartz, high SiO2 and low MgO, with Mg/Fe ratios well below the values expected for a melt in equilibrium with the mantle. Furthermore, the lack of decrease in Cr with increasing SiO2 and decreasing MgO from ultramafic to felsic rocks precludes the possibility that the felsic members were derived by fractional crystallization from the mafic members. Similarly, magma mixing, crustal contamination and crystal accumulation can be excluded as important processes. Yet all members of the suite share similar incompatible element and radiogenic isotope ratios, which suggests a common origin and source. We propose that melting for all members of the shoshonite suite was initiated in continental crust that was thrust into the upper mantle at various points along the transpressional Red River-Ailao Shan-Batang-Lijiang fault system. The melt formed by high-degree, fluid-absent melting reactions at high-T and high-P and at the expense of biotite and phengite. The melts acquired their high concentrations of incompatible elements as a consequence of the complete dissolution of pre-existing accessory minerals. The melts produced were quartz-saturated and reacted with the overlying mantle to produce garnet and pyroxene during their ascent. The felsic magmas reacted little with the adjacent mantle and preserved the essential features of their original chemistry, including their high SiO2, low Ni, Cr and MgO contents, and low Mg/Fe ratio, whereas the mafic and ultramafic magmas are the result of extensive reaction with the mantle. Although the mafic magmas preserved the incompatible element and radiogenic isotope ratios of their crustal source, buffering by olivine and orthopyroxene extensively modified their MgO, Ni, Cr, SiO2 contents and Mg/Fe ratio to values dictated by equilibrium with the mantle.

First Evidence of Lamprophyric Magmatism from the Konya Region, Turkey: a Genetic Link to High‐K Volcanism

AbstractIn the vicinity of Konya (Turkey), mafic, micro‐porphyritic sub‐volcanic rocks intrude into the Mesozoic units, which represents the only example of such a rock type in the region. 40Ar/39Ar dating of two whole rock samples from the sub‐volcanics gave ages of 13.72±0.13 and 12.40±0.11 Ma, suggesting temporal association to the Late Miocene‐Pliocene high‐K calc‐alkaline volcanism in the region. The mineral chemistry and geochemical data permit us to classify the rocks as “minette” lamprophyres. They include diopside and phlogopite phenocrysts in a microcrystalline groundmass composed of sanidine, phlogopite, diopside and titano‐magnetite. Segregation and ocelli‐like globular structures occur commonly in the samples. In terms of major elements, the lamprophyres are calcalkaline, and potassic to ultrapotassic rocks. All the lamprophyres display strong enrichments in LILE (Rb, Ba, K, Sr), radiogenic elements (Th, U) and LREE (La, Ce) and prominent negative Nb, Ta, and Ti anomalies on primordial mantle‐normalized trace element diagrams. Geochemical data suggest that the lamprophyres and high‐K calc‐alkaline rocks in the region derived from a subduction‐modified lithospheric mantle source affected by different metasomatic events. Lamprophyric magmatism sourced phlogopite‐bearing veins generated by sediment‐related metasomatism via subduction, but high‐K calc‐alkaline magmas are possibly derived from a mantle source affected by fluid‐rich metasomatism.

Petrological characterization of the mantle source of Mediterranean lamproites: Indications from major and trace elements of phlogopite

Lamproites are among the most enigmatic mantle generated melts, and are typically abundant in phlogopite. We investigated the major and trace element chemistry of phlogopite crystals from Mediterranean lamproite occurrences in Spain, Serbia and Turkey. The most primitive lamproite samples were selected in order to evaluate the generation of the different phlogopite populations in their distinct mantle setting(s).Phlogopite hosted in Mediterranean lamproites can be grouped into different compositional arrays on the basis of their major element chemistry using Al2O3, FeOT and TiO2 concentrations. Contents of Cr, F and presumably Ti allow discrimination of the macrocrysts into phlogopite phenocrysts, which are liquidus and near-liquidus phases, and mantle xenocrysts. Further distinction among different populations of mantle-derived phlogopite can be achieved, considering trace element and isotopic ratios. The latter also distinguish between crustal and asthenospheric imprints on the source of the lamproitic parental melt.

Comment on “New Ar–Ar ages of southern Indian kimberlites and a lamproite and their geochemical evolution” by Osborne et al. [Precambrian Res. 189 (2011) 91–103

Osborne et al. (2011; Precambrian Res. 189, 91–103) report new 40Ar/39Ar age results for two kimberlites (Muligiripalle pipe 5 and Tummatapalle pipe 13) and one lamproite (Pochampalle), from the Archaean Dharwar craton in southern India. Previous studies indicate that kimberlites and related rocks are highly susceptible to extraneous argon contamination, leading to anomalously old ages; although reliable 40Ar/39Ar ages can be obtained from step-heating analyses of groundmass phlogopite grains. Osborne et al. (2011) carried out single-step 40Ar/39Ar fusion analyses on phlogopite phenocrysts and xenocrysts. This approach is not favoured, because it does not allow identification of argon loss/gain effects, thus complicating data interpretation and reducing confidence in the reliability of the reported ages. In particular, it is suggested that the Pochampalle lamproite was emplaced <1400Ma ago, rather than ca.1500Ma, as suggested by Osborne et al. (2011). It is recommended that 40Ar/39Ar geochronology of kimberlite and lamproites include step-heating analyses of matrix phlogopite grains, with anomalous or contentious results verified using other dating methods.