Lamprophyric Rocks Research Articles

Some notes on the IUGS classification of lamprophyric rocks

Some notes on the IUGS classification of lamprophyric rocks

Neoproterozoic potassic andesitic and lamprophyric rocks in the tupanci area, southernomost Brazil: Petrology and associated mineralization

Au and sulfide mineralizations occur in the Sul-Rio-Grandense Shield, and important occurrences of these metals are reported and attributed to the Lavras do Sul Shoshonitic Association (LSSA). Several authors point to a significant participation of andesitic and lamprophyric rocks in the formation of these deposits. The Tupanci area has the presence of andesites and lamprophyres, stratigraphically correlated to LSSA volcanic rocks (Hilário Formation), representing the northernmost exposures of this magmatism. The LSSA is one of the most complete Neoproterozoic Shoshonitic associations (ca. 592-590 Ma) in southern Brazil, containing plutonic, volcanic and hypabyssal rocks, related to the post-collisional stage of the Brasiliano/Pan-African orogeny. The andesitic rocks of the Tupanci area occur in the form of the lava flows, overlayed by volcanoclastic deposits, product of the reworking of the volcanic unit. Lamprophyres occur in the form of dykes, with N-NW direction, which are intruded in rocks of the metamorphic basement and in andesitic lava flows, being texturally separated and classified as spessartite and lamprophyric lapilli-tuff. The singular occurrence of the lamprophyric lapilli-tuff stands out, is analogous to Kimberley-type pyroclastic kimberlitic intrusions, depicting explosive manifestations linked to LSSA lamprophyres. The andesites and lamprophyres that ocour in Tupanci area show characteristics of high-K calc-alkaline or shoshonitic rocks, evidenced by LILE and LREE enrichment, negative Ti and Nb anomalies, plus positive Pb anomalies and the absence of Eu anomalies. These attributes reinforce the relationship of andesitic and lamprophyric rocks with related LSSA rocks. The presence of Au and Pt grains in the matrix of lamprophyric lapilli-tuff, highlights the economic importance of these rocks for the mineral occurrences in the region, where future studies can better define the metallogenetic potential for the rocks of the Tupanci area.

Adamas as diamond and its occurrence near Ancient Philippi in Macedonia, northern Greece

The adamas of Ancient peoples is proved to be the precious diamond. An ancient and uconventional diamond occurrence, the mines of Philippi in Macedonia (northern Greece), is geologically explained here. The most probable source is believed to be flat subduction-related (shoshonitic) lamprophyres that originated at a depth of >110 km within the mantle in the diamond stability field. The thick lithospheric mantle beneath Greece supports the occurrence of diamondiferous lamprophyric rocks in this area. However, no lamprophyre outcrops have been found, except for a few lamprophyre dikes 14 km SE of Philippi. Another explanation for the occurrence would implicate detrital diamonds that have been preserved in the local greenschist facies rocks. The XRD analysis of Philippi paragneiss showed the existence of the mantle mineral magnesiowüstite, which is a possible diamond indicator. The Philippi occurrence deserves detailed investigation involving alluvial sampling and ground magnetic surveys to locate the diamond source.

Lamprophyre Rocks in the Nassara Gold Deposit, Southwest Burkina Faso: Characteristics and Implication for Mining Exploration

Lamprophyre Rocks in the Nassara Gold Deposit, Southwest Burkina Faso: Characteristics and Implication for Mining Exploration

Mineralization-tectonic Relationship in the JbelTirremi Fluorite Mine (Taourirt Region - Eastern Morocco): Contribution of Remote Sensing for Geological Exploration

The Tirremi fluorite deposit, located in the Taourirt region (Eastern Morocco), morphologically constitutes a mainly carbonate dome of the Jurassic age, crossed by numerous dykes of lamprophyric rocks of Eocene age. The tectonic study coupled with the study of lineaments extracted from band 2 of the sentinel-2 image shows that the main directions, in terms of frequency and size, are NE-SW and E-W to ENE-WSW. The other directions, N-S and NNW-SSE, are satellite faults with respect to the potential faults; but they are economically important since they give rise to fluorite-rich veins. Similarly, analysis of fracturing (faults, tension cracks, dykes, etc.) allows us to identify the Palaeostresses of stress responsible for the structuring of the Tirremi region. These phases, classified in chronological order based on field observations, are perfectly compatible and correlatable with previous work done in the area.

Crystal-Liquid Segregation in Silicocarbonatite Magma Leads to the Formation of Calcite Carbonatite

Abstract A suite of silicocarbonatite and lamprophyre rocks from SW Ireland, with mantle affinity and primitive composition, are used as a proxy for parental carbonated silicate magmas to model early magmatic evolution. Reconstruction of volatile ratios is validated using global occurrences. At 1200°C, the point at which melts transition from ionic liquids with exceptionally low viscosity (0.06 PaS) to covalently polymerised liquid (viscosity up to 1.3 PaS) is 33 mol% SiO2. Incremental and significant increase in magma density accompanies magma ponding, due to dehydration of magmas from model molar CO2/(CO2 + H2O) of 0.60 in plutonic settings to 0.75 for initial subvolcanic magmas. Magma-crystal density differences dictate that repeated influxes of magmas into an inflating magma chamber sustain a mechanical boundary layer between dense (silicate and oxide) mineral layers and a calcite ± phlogopite flotation assemblage. The range of critical CO2 concentration at which calcite floats (10–13 wt% CO2) may be extended by the presence of additional volatiles and fluid bubbles. The model accommodates a range of phenomena observed or inferred for alkaline/carbonatite complexes, including the following: 1, a growing calcite-dominated flotation assemblage with an apparently early magmatic mineralisation; 2, a residual liquid with high concentrations of incompatible metals; 3, variable carbonatite–pyroxenite–phoscorite rock relations; and 4, multiple phases of overprinting metasomatism.

Geochemistry and Sr–Nd isotopic studies of Paleoproterozoic ( c. 2.3 Ga) meta-lamprophyre from the Rapuru area, Nellore Schist Belt, southern India: implications for back-arc basin magmatism and its relevance to the Columbia supercontinent assembly

Abstract This study reports on two lamprophyre dykes from the Rapuru area along the margin of the Eastern Dharwar Craton (EDC) and the Nellore Schist Belt (NSB). The Rapuru lamprophyre (RL) dykes are situated along the southern extension of the Prakassam Alkaline Province (PAP). The RL dykes are deformed, yet still preserve a porphyritic–panidiomorphic texture, with mica phenocrysts, and amphibole and feldspars in the groundmass. Geochemically, the RL dykes have a low Mg# (0.28–0.37), and Ni (30–60 ppm) and Cr (119–228 ppm) concentrations that indicate their evolved nature, such as for other reported lamprophyres from the PAP and EDC. This is further supported by Sr–Nd isotopic ratios that show an affinity towards a mid-ocean ridge basalt (MORB)–ocean island basalt (OIB)-like signature and a juvenile magmatic nature. The RL seems to have been affected by two major influences, namely, the primary source region, which is geochemically juvenile similar to the compositional field of enriched-MORB, and the continental lithosphere. Such magmas are known to have formed in a back-arc-basin environment. The initial 87 Sr/ 86 Sr ratio ( c. 0.7012–0.7045) and initial ε Nd ratios (3.13–7.93) are in line with back-arc basin basalts recorded in other parts of the world. The field observations and bulk-rock Sr–Nd radiogenic isotope values in the present study support the Paleoproterozoic nature of the RL. This concurrence of juvenile radiogenic isotopes and fluid-related trace element compositions apparently suggest dehydration of a subducted-slab-triggered metasomatism of the overlying mantle wedge in a subduction-related geodynamic setting. Such intrusive lamprophyre rocks of older ages are limited in India as well as other parts of the world. The 2.1 and 1.8 Ga rocks are widely considered to represent the initial accretion and final break-up of an erstwhile Columbia supercontinent assembly. We argue that the RL were formed in the Paleoproterozoic during the waxing stages of the Columbia supercontinent assembly in a back-arc basin environment, most probably due to the low degree of partial melting of the asthenosphere–lithospheric interaction caused by the introduction of an influx of subduction components into the arc–back-arc basin system.

Lamprophyre as the Source of Zircon in the Veneto Region, Italy

Discrete zircons, up to 9 mm in length, occur in alluvial deposits from the Veneto area. They are likely derived from the disaggregation of lamprophyric rocks belonging to a regional, pervasive dyke-swarm. Zircon and REE phases occur in both alkaline lamprophyres and connate calcite-bearing felsic lithics and their debris in lamprophyre breccia. We present 36 new complete U–Pb and trace element analyses of zircons and associated inclusions. We used a statistical approach on a larger dataset using new and literature data to evaluate the confidence figure to give an estimation of age of zircons. Inclusions suggest a genetic link with an S–CO2–ZrO–BaO–SrO–CaO-rich fluid/melt possibly associated with carbonate-rich alkaline parental magma and a metasomatised mantle source. This paper confirms the importance of calcite–syenite and lamprophyre genetic link and zircon magmatic origin, in contrast with hydrothermal and metamorphic zircons. U–Pb dating by LA-ICP-MS provides time constrains (40.5–48.4 Ma, Lutetian), consistent with the age of the alkaline magmatic event. Trace element data indicate a link to anorogenic magmatism associated with mantle upwelling. Complex zoning is highlighted by cathodoluminescence images. The Veneto zircons are helpful for regional geological information and may have commercial potential as a critical resource for green technologies.

Lithium isotopes in kimberlites, lamproites and lamprophyres as tracers of source components and processes related to supercontinent cycles

Abstract Our pilot study reveals potential Li isotope fingerprints recorded in the Mesoproterozoic ( c. 1.4–1.1 Ga) kimberlites, lamproites and lamprophyres from the Eastern Dharwar Craton and Paleocene (62 Ma) orangeite from the Bastar Craton in India. The new data are interpreted in the context of available Li isotope composition of lamproitic to lamprophyric rocks occurring in Variscan (Bohemian Massif) and Alpine–Himalayan (SW Tibet) orogenic belts formed in response to Gondwana–Pangea amalgamation and break-up. As a result of the development of supercontinents, kimberlites from the Eastern Dharwar Craton and ‘orangeite’ from the Bastar Craton show clear presence of a component with a heavy Li isotope signature (δ 7 Li up to 9.7‰) similar to ancient altered oceanic crust, whereas the Eastern Dharwar Craton lamproites (2.3–6.3‰) and lamprophyres (3.3–6.7‰) show Li isotope signatures indicative of a dominant contribution from heterogeneous lithospheric mantle. Variscan lamprophyric to lamproitic rocks and post-collisional mantle-derived (ultra)potassic volcanic rocks from SW Tibet, i.e. rocks from the orogenic belts outside the cratonic areas, are characterized by a clear Li isotope shift towards an isotopically lighter component (δ 7 Li as low as –9.5‰) comparable with the involvement of evolved continental crust and high-pressure metamorphic rocks in their orogenic mantle source. Such components with isotopically light Li are strikingly missing in the source of cratonic kimberlites, lamproites and lamprophyres.

Simple steps for the detection and classification of different lamprophyric rocks: a case study from Greece

Abstract This study aims to help non-expert geologists in locating, classifying and analyzing lamprophyres, the exotic and complex rocks. The process includes three basic steps, a macroscopic study of the formation, petrographic microscopy and lastly whole-rock and trace element analyses of the samples. Fourteen Greek lamprophyric rocks with distinct characteristics were used. Conventional lamprophyres were considered along with rarer alkali minettes, meta-lamprophyres, lamproschists, appinites and para-lamproites.

The Alkaline Lamprophyres of the Dolomitic Area (Southern Alps, Italy): Markers of the Late Triassic Change from Orogenic-like to Anorogenic Magmatism

AbstractWe present the first complete petrological, geochemical and geochronological characterization of the oldest lamprophyric rocks in Italy, which crop out around Predazzo (Dolomitic Area), with the aim of deciphering their relationship with Triassic magmatic events across the whole of the Southern Alps. Their Mg# of between 37 and 70, together with their trace element contents, suggests that fractional crystallization was the main process responsible for their differentiation, together with small-scale mixing, as evidenced by some complex amphibole textures. Moreover, the occurrence of primary carbonate ocelli suggests an intimate association between the alkaline lamprophyric magmas and a carbonatitic melt. 40Ar/39Ar data show that the lamprophyres were emplaced at 219·22 ± 0·73 Ma (2σ; full systematic uncertainties), around 20 Myr after the high-K calc-alkaline to shoshonitic, short-lived, Ladinian (237–238 Ma) magmatic event of the Dolomitic Area. Their trace element and Sr–Nd isotopic signatures (87Sr/86Sri = 0·7033–0·7040; 143Nd/144Ndi = 0·51260–0·51265) are probably related to a garnet–amphibole-bearing lithosphere interacting with an asthenospheric component, significantly more depleted than the mantle source of the high-K calc-alkaline to shoshonitic magmas. These features suggest that the Predazzo lamprophyres belong to the same alkaline–carbonatitic magmatic event that intruded the mantle beneath the Southern Alps (e.g. Finero peridotite) between 190 and 225 Ma. In this scenario, the Predazzo lamprophyres cannot be considered as a late-stage pulse of the orogenic-like Ladinian magmatism of the Dolomitic Area, but most probably represent a petrological bridge to the opening of the Alpine Tethys.

Chemistry and Sr-Nd isotope signature of amphiboles of the magnesio-hastingsite–pargasite–kaersutite series in Cenozoic volcanic rocks: Insight into lithospheric mantle beneath the Bohemian Massif

Amphibole phenocrysts, xenocrysts and cumulate xenoliths from Cenozoic volcanic rocks of the Bohemian Massif (BM) belong to the magnesio-hastingsite–pargasite–kaersutite series. Their host rocks are mostly basaltic lavas, stocks, dykes and breccia pipe fills, less commonly also felsic rocks. Felsic rocks with amphibole cumulate xenoliths represent differentiated magmas which have undergone polybaric fractionation of the mafic minerals. The calculated p–T conditions suggest that almost all amphiboles crystallized in a relatively narrow temperature range (1020–1100 °C) at depths of ~20–45 km (0.7–1.2 GPa) during the magma ascent. These p–T estimates are compatible with the published experimental data on the stability of kaersutite. We therefore suggest the presence of a deep magma chamber situated close to the crust–mantle boundary where amphibole xenoliths to megacrysts could have formed. Nevertheless, crystallization of rare amphibole rims during the magma ascent was observed in a hornblendite cumulate in sodalite syenite from “Giegelberg”. The lowest concentration of incompatible elements in the amphiboles was found in xenocrysts in alkaline basaltic rocks and mantle xenoliths and megacrysts, followed by phenocrysts/xenocrysts in lamprophyric rocks, xenocrysts of cumulates in felsic rocks, and phenocrysts in subvolcanic rocks. Amphibole compositional and Sr-Nd isotope characteristics resemble those of amphiboles from metasomatic clinopyroxene/amphibole veins in mantle peridotites. The initial 143Nd/144Nd and 87Sr/86Sr ratios of amphiboles (0.51266–0.51281 and 0.70328–0.70407, respectively) are similar to those of their whole rocks (0.51266–0.51288 and 0.70341–0.70462, respectively). Amphiboles of the magnesio-hastingsite–pargasite–kaersutite series of the BM are mostly chemically homogeneous, with no pronounced Mg-Fe fractionation and zoning. The amphiboles are characterized by relatively homogeneous εNd(i) = +1.4 to +3.8 values: only a single sample from the České Středohoří Volcanic Complex (CSVC) yielded a negative εNd(i) (−0.6). This testifies to locally elevated proportions of recycled Variscan crustal material during melting of mantle peridotites rich in clinopyroxene–amphibole veins. These veins were formed by metasomatic fluids enriched in High Field Strength Elements (HFSE) and are isotopically similar to Enriched Mantle1 (EM1)-type mantle. Amphibole host rocks occur in areas with a significant concentration of basaltic magmas in rift zones along lithospheric block boundaries of the BM. Lithospheric mantle beneath such zones was probably strongly influenced by metasomatic fluids during the formation of clinopyroxene–amphibole veins in mantle peridotite that facilitated the generation of basaltic magma with amphibole.

Foreword to the thematic set on ‘Melilitic, alkaline and calc-alkaline lamprophyric rocks. Centenary celebration of the first description of polzenites by K. H. Scheumann’

Foreword to the thematic set on ‘Melilitic, alkaline and calc-alkaline lamprophyric rocks. Centenary celebration of the first description of polzenites by K. H. Scheumann’

پترولوژی، ژئوشیمی و کانی سازی سنگهای آتشفشانی ترشیری و توده های نفوذی و نیمه نفوذی همراه آنها در منطقه ارغش- قاسمآباد (شمالشرق ایران) با گرایشی ویژه به سن و منشأ گرانیت ها

منطقه ارغش- قاسم آباد در شمال‌شرق ایران و در حاشیه زون ساختاری سبزوار قرار دارد. رخنمونهای ترشیری شامل گرانیت و کوارتز مونزودیوریت پورفیری همراه با سنگهای آتشفشانی از نوع داسیت و آندزیت هستند. در منطقه مورد مطالعه، دایک‌هایی از نوع کوارتز گابرو و کوارتز مونزودیوریت، سنگهای قدیمی تر را مورد نفوذ قرار داده اند. گرانیت‌ها و سنگهای آتشفشانی، میزبان رگه های کوارتز-‌کلسیت طلا و آنتیموان دار هستند. رگه های کانی سازی عمدتا در اطراف دایک‌ها و در نیمه جنوبی منطقه دیده می شوند. مطالعات ژئوشیمیایی حاصل از این پژوهش نشان می دهد که گرانیت‌ها ماهیت کالک‌آلکالن پتاسیم بالا داشته، و از نوع گرانیتوئیدهای انتقالی بین دو نوع A و I هستند. سنگهای آتشفشانی نیز با ترکیب آداکیتی، سرشار از استرانسیم و باریم می‌باشند. دایک‌های منطقه مورد مطالعه، ویژگیهای سنگهای لامپروفیری را نشان می دهند. سن سنجی اورانیم- سرب بر روی زیرکن در گرانیت، سن 2/2 ± 4/55 میلیون سال (ائوسن زیرین) را نشان می دهد. نسبتهای 87Sr/86Sr و eNd اولیه در گرانیت‌ها به‌ترتیب 704142/0 و 84/5+ می-باشند. داده های نسبتهای ایزوتوپی 87Sr/86Sr اولیه، وجود ماگمایی با منشأ گوشته تهی‌شده را تأیید می کند. سن به‌دست آمده نشان می دهد کانی سازی طلا- آنتیموان از ائوسن زیرین جوانتر است. براساس شواهد صحرایی، ارتباط کانی سازی با دایک‌های لامپروفیری، بیش از ارتباط آن با سنگهای آداکیتی است.

Mineral composition in cognate inclusions in Late Miocene–Early Pliocene potassic lamprophyres with affinities to lamproites from the Denizli region, Western Anatolia, Turkey: Implications for uppermost mantle processes in a back-arc setting

Mineral assemblages of both cognate inclusions and host-lamprophyres with lamproitic affinity in Kocapınar district (Denizli), Western Anatolia, consist of phlogopitic mica, diopsidic clinopyroxene, K-feldspar, apatite, opaque and carbonates (calcite–dolomite). Two distinct types of cognate inclusions have been defined in host lamprophyres: clinopyroxene-rich (CCI) and clinopyroxene–phlogopite-rich (CPCI). Whole-rock compositions of inclusions show a near-primitive nature with high MgO (7.56–15.1wt.%), Cr (195–2270ppm), Ni (213–335ppm) contents and a potassic character [K2O (2.0–2.8wt.%)>Na2O (0.4–2.2wt.%)]. Data imply that the inclusions crystallized from magmas formed by melting of phlogopite-bearing pyroxenites in a peridotitic mantle source, however, the presence of reverse-zoning in clinopyroxenes with salitic Fe-rich green cores in host-lamprophyres and CPCI inclusions suggest that the Denizli lavas represent mixtures of distinct (probably ultrapotassic and alkali basaltic) magmas. Estimated geobarometric constraints inferred from clinopyroxene compositions in CCI and CPCI inclusions indicate moderate pressures of pyroxene crystallization (ranging between 1.7 and 2.2GPa and corresponding to 53–70km depths) under low pressure magma fractionation. Results reveal that i) the origin of Denizli lamprophyres with transitional (between arc-type and intra-plate-type) geochemical signatures is consistent with a shallow level mantle petrogenesis, rather than a deep-seated origin related to mantle convection, ii) the source was a highly refractory and metasomatized peridotitic mantle present at the base of the lower crust, iii) metasomatic agents that affected the mantle lithosphere beneath Denizli region are distinct from those beneath other western Anatolia orogenic centers, and iv) the transitional character of Kocapınar (Denizli) lamprophyric rocks were probably formed as a result of either underplating or contamination of asthenospheric magma at the base of the mantle lithosphere, or assimilation of delaminated continental edge-lithospheric mantle via ascending asthenosphere. Denizli lamprophyres are interpreted to have formed during the formation of a basin/graben structure in the Latest Miocene–Early Pliocene, just after Late Miocene exhumation of Menderes massif.

Relationship between chemical composition and magnetic susceptibility in the alkaline volcanics from the Isparta area, SW Turkey

Potassium-rich volcanic rocks in the Isparta area (SW Turkey) consist mainly of older (Pliocene) volcanic rock suites (e.g., lamprophyre, basaltic trachyandesite, trachyandesite, trachyte) and younger (Quaternary) caldera forming lava dome/flows (e.g., tephriphonolite, trachyte) and pyroclastics (ash/pumice fall deposits and ignimbritic flows). The magnetic susceptibility (K) was performed for both groups. The magnetic susceptibility value of the less evolved rocks characterized by SiO2 < 57 wt% (e.g., basaltic trachyandesite, tephriphonolite, lamprophyric rocks) and having mostly mafic phenocrysts such as pyroxene, amphibole, and biotite-phlogopite is over 10 (10−3 [SI]). Fine to medium-grained and subhedral to anhedral opaque minerals are scattered especially in the matrix phase of the less evolved volcanic rocks. However, the K value of the more evolved rocks (e.g., trachyte and trachyandesites) with SiO2 over 57 wt% vary between 0.1 and 28, but most of them below 10. SI values are negatively correlated with SiO2, Na2O, but positively correlated with Fe2O3, CaO, MnO, P2O5 and MgO contents, suggesting inverse variation of SI with fractionation of potassic magma. That is to say that less evolved volcanic rocks have relatively higher magnetic susceptibility values in the volcanic suite. Fine to medium-grained and subhedral to anhedral Fe-Ti oxides are scattered mainly in the matrix phase of the less evolved volcanics, presumably cause the pronounced relatively higher magnetic susceptibility.

Mantle plume – volcanic arc interaction: consequences for magmatism, metallogeny, and cratonization in the Abitibi and Wawa subprovinces, CanadaThis article is one of a series of papers published in this Special Issue on the theme Lithoprobe — parameters, processes, and the evolution of a continent.

The Abitibi and Wawa subprovinces of the southern Superior Province differ in terms of the extent of pre-existing 2750 Ma sialic crust and relationships between mantle plume type (tholeiitic basalt – komatiite) and arc type (tholeiite to calc-alkaline basalt – andesite – dacite – rhyolite) volcanic successions but evolved in close proximity to each other. Isotopic data, evidence from the Kapuskasing uplift, continuation of major structures associated with large gold deposits from the Abitibi into the Wawa subprovince and the related occurrence of diamonds in lamprophyric rocks in both subprovinces point to a common evolution prior to and during orogeny. Differences preserved in supracrustal sequences of the two subprovinces suggest that the main petrogenetic controls on orogenic gold deposits and lamprophyre-hosted diamond deposits lay in the lower crust and upper mantle. Similar processes must also have been active where gold and diamonds are associated on other Archean cratons, such as the Slave and possibly the Kaapvaal craton. Based on evidence preserved in the Abitibi–Wawa orogen, rapid crustal growth at ∼2.7 Ga was linked to the interplay between plate tectonics and mantle plumes. Key indicators in the model developed for the Abitibi–Wawa arc are the co-existence of plume-related rock types, modern-style adakites, major gold deposits, and lamprophyre-hosted diamond occurrences, at least in cases where shoshonitic host magmas can ascend rapidly through the crust. All of these indicators are now identified on the Kaapvaal craton by 3.1 Ga and many recur together in Paleoproterzoic and younger terranes, suggesting a common mechanism for rapid crustal growth through much of Earth’s history. The variety of granitoid types found within the Abitibi–Wawa orogen demonstrates that local tectonic factors, rather than a hotter average upper mantle, were important in controlling the type of magmas formed. Based on the geodynamic history of the subprovince, mantle plume interaction with an existing volcanic arc and the subduction of oceanic plateau crust played an important role in the formation of magmas similar to Cenozoic adakites. Flat subduction beneath a mantle wedge was probably responsible for the generation of the adakites and also promoted diamond stability at shallow depths while enhancing the reservoirs for subsequent orogenic gold deposits. The history of magmatism and mineralization in the Abitibi and Wawa subprovinces precludes an early or gradual development of a cratonic keel, which instead must have coupled with crust during the latest stages of orogeny.

Occurrences of lamprophyric rocks in Greece

Occurrences of lamprophyric rocks in Greece

Kimberlite and related rocks from Garnet Lake, West Greenland, including their mantle constituents, diamond occurrence, age and provenance

Observations of thickness, orientation and morphology and mineral chemistry of the principal diamondiferous intrusive sheet and associated bodies in the vicinity of Garnet Lake, Sarfartoq, West Greenland are reported. The principal body dips to the east on a NE/SW (true) trend and reaches a maximum thickness of 4.25 m. Multiple intrusive events are identified within the main sheet including sub-parallel bands occasionally exhibiting grain size sorting, cross-cutting layers and late-stage carbonate-rich emplacement, particularly at the contacts with country rock. Phenocrystic mineral assemblages and compositional measurements reveal two principal petrological types. The dominant type is an aillikite and the second rock type is a kimberlite. The kimberlite exhibits thin Ba-rich rims (towards kinoshitalite) on Al-rich phlogopite crysts, and an abundance of perovskite. Compositional zonation in groundmass spinels suggest a later transition towards an aillikite component. The aillikite is characterised by abundant phlogopite, heavily zoned with tetraferriphlogopite rims, transitional Type 1-Type 2 spinel compositions, rare Al,Ti-rich groundmass clinopyroxene and occasional exotic Sr-carbonate phases such as olekminskite. The Garnet Lake main sheet is characterised by mantle phases occurring as individual grains, most strikingly as garnet xenocrysts up to 5 mm and disaggregated mantle olivine crysts. Xenoliths occur rarely and are typically garnet dunites and garnet lherzolites. Heavy mineral separation reveals an abundance of G10D garnets and, whilst peridotitic garnets dominate, eclogitic G3D and G4D garnets also occur. Trace element compositions of garnet crysts reveal sinusoidal REE patterns in harzburgitic garnets however a component of flat and REE-enriched G11 garnets is apparent, reflecting significant mantle refertilisation. Thermorbarometric calculations on assemblages in Garnet Lake main sheet garnet lherzolites reveal equilibrium conditions clustering closely around 1258 °C and 6.20 GPa, equivalent to a depth of 195 km along a 41 mWm − 2 mantle geotherm. Garnet Lake xenoliths thus lie at the deepest extent of mantle samples recovered from western Greenland lamprophyric rocks and from well within the diamond stability field. Diamonds are typically colourless and occur as fragments or with a dominant octahedral morphology. Crysts exhibit very high quantities of nitrogen (up to 2133 ppm N) and yet are not strongly aggregated to IaB centres (typically about 33% aggregation). This suggests either a short residence time, low residence temperature or likely a combination of both. Employing the restricted range of equilibrium conditions exhibited by mantle xenoliths, calculated diamond formation ages peaking around 583–593 Ma are favoured. This is a short time before the 568 Ma Garnet Lake main sheet emplacement age which coincides with the age of the nearby Sarfartoq carbonatite complex. A model of mantle refertilisation and diamond formation culminating in conditions favourable for creating carbonate-rich melts acting as carriers of mantle constituents to the surface is presented. Data suggest close temporal, chemical and geographic associations between kimberlite, aillikite and carbonatite.

Enhancement of lamproite magnetic signatures

Lamproites are peralkaline, typically ultrapotassic (6 to 8% K2O) and magnesia-rich lamprophyric rocks of volcanic or hypabyssal origin and are now considered to crystallize from a distinct type of magma. Most lamproites occur in irregular, asymmetric craters that are generally rather shallow, usually less than 300 metres in depth, often in the shape of a champagne glass with crater diameters ranging from a few hundred metres to 1500 metres. An unusual feature is that volcaniclastic rocks in many lamproite craters are intruded by a magmatic phase that forms lava lakes or domes. Lamproite forms from a very explosive volatile-rich magma that forms deep down within the Earth (greater than 150 km) and rises rapidly to the surface because of the high fluid pressure. As it rises, it fractures the surrounding rock explosively and most lamproites contain numerous xenoliths from all levels of the mantle and crust that the lamproite has passed through on its way to the surface. The initial explosive phreatomagmatic stage of the eruption, powered by gases or boiling ground water, corrodes the hosting rock to form the champagne-glass shape. Particles of ash, lapilli, and pumice partially fill the crater and form a tuff ring and finally the crater fills with a lava pond from the degassed lamproite magma. These are typical maar-type diatremes, formed by an explosive reaction induced when hot, rising magma came into contact with subterranean water. Lamproites are small-volume magmas and there are relatively few lamproites known world wide, with less than 20 geological provinces, of which only seven are diamondiferous. Only olivine lamproites are diamondiferous, other varieties such as leucite lamproites presumably did not originate deep enough in the mantle to contain significant diamond content. Olivine lamproite pyroclastic rocks and dikes are the usual source of diamonds whereas diamonds are rarely found in the magmatic equivalents. Diamonds do not crystallize from the lamproite magma but are brought to the surface as the magma ascends rapidly to the surface, collecting fragments of the mantle and crust en route. The Kimberley region of Western Australia contains the only two lamproite-hosted diamond mines in the world (the Argyle Diamond Mine in the East Kimberley and Kimberley Diamond Company?s Ellendale 9, in the West Kimberley).