Alkali Gabbro Research Articles

Petrology of alkali gabbro from the Messum Crater, Namibia (Paranà-Etendeka Province) – Melting of metasomatized carbonated lithospheric mantle sources

Petrology of alkali gabbro from the Messum Crater, Namibia (Paranà-Etendeka Province) – Melting of metasomatized carbonated lithospheric mantle sources

On the coexistence of rift-related silica-undersaturated and silica-saturated alkaline rocks – The cretaceous Messum alkaline complex (Namibia) revisited

The ~132 Ma-old Messum igneous complex, a circular structure of ~18 km diameter, belongs to the Damaraland intrusive complexes that intruded into the Pan-African Damara basement of Namibia. The plutonic rocks of the core of the complex comprise olivine-gabbro, biotite-gabbro, nepheline-syenite and quartz-syenite as well as minor alkali gabbro and coexist with the Etendeka volcanic sequences of the Goboboseb Mountains, comprising basalts (Tafelkop and Tafelberg types) that are overlain by deposits of several quartz latite eruptions. New major and trace element and Sr, Nd and Pb isotope data are reported for nepheline-syenite and quartz-syenite. Major and trace element data are consistent with the development of nepheline-syenites through fractional crystallization processes. The nepheline-syenite samples have initial 87Sr/86Sr of 0.7046–0.7049 and initial εNd ranging from +0.53 to +1.47, indicating a mantle source with time-integrated lithophile element depletion. Initial Pb isotopes are rather uniform (206Pb/204Pb:17.89–18.14, 207Pb/204Pb: 15.56–15.58, 208Pb/204Pb: 37.78–38.00). Uniform initial Sr and Pb isotope ratios for nepheline-syenites suggest that the syenite magmas evolved through fractional crystallization from an inferred mafic alkaline magma without crustal contamination. However, the range in Nd isotope composition is too large to be related to crystal fractionation alone and imply a heterogeneous source with respect to the Nd isotope composition. The quartz-syenites show a large range in initial εNd (+1.0 to −4.4) and initial 87Sr/86Sr (0.7047–0.7115). Initial Pb isotope ratios are also heterogeneous (206Pb/204Pb: 18.29–18.74, 207Pb/204Pb: 15.60–15.65, 208Pb/204Pb: 37.43–38.49). These data, together with major and trace element constraints and results from AFC modeling substantiate the production of the quartz-syenites by fractional crystallization of clinopyroxene, amphibole and K-feldspar accompanied by assimilation of up to about 30 % pre-Pan African lower crust. Isotope and chemical data do not support derivation from a single batch of magma undergoing contamination, but suggest that a large magma body at depth evolved largely by fractionation with batches of melt extracted from this chamber being variably contaminated at lower crustal levels.

Tithonian mafic intrusions in north-central Newfoundland: link to Atlantic rifting?

The small volume, Mesozoic alkali gabbro intrusions of the Budgell Harbour and Dildo Pond stocks and associated alkaline lamprophyre dykes in Notre Dame Bay in Newfoundland are an example of onshore magmatism that may be associated with North Atlantic Ocean opening. Chemical abrasion isotope dilution thermal ionization mass spectrometry U–Pb dating of zircon from Budgell Harbour Stock drill core samples yielded a weighted average 206Pb/238U age of 147.9 ± 0.5 Ma (95% confidence interval, mean square weighted deviation = 0.10). Five 40Ar/39Ar laser step-heating, single phlogopite grain analyses from the Budgell Harbour and Dildo Pond stocks, plus a lamprophyre dyke, yielded ages ranging from 146.3 ± 0.2 to 149.5 ± 0.5 Ma. The data demonstrate a ca. 148 Ma (Jurassic and Tithonian) alkaline magmatic event in Newfoundland, contemporaneous with rifting and offshore basin formation. These new age data and published determinations from magmatic rocks on conjugate margins and adjacent regions reveal long-lived episodic magmatism in the embryonic North Atlantic rift environment. We propose a new model whereby magmatism occurred in Notre Dame Bay at the convergence of older crustal-scale faults and localized Moho depth variations that may have triggered lithospheric mantle melting through isothermal, distal, and edge-driven upwelling related to regional extension.

Mineral chemistry and geothermo-barometry based on amphibole of alkali gabbro dykes of Zarigan, northeast of Bafq

Mineral chemistry and geothermo-barometry based on amphibole of alkali gabbro dykes of Zarigan, northeast of Bafq

Zirconium-REE mineralization characteristic in Sakju and Ryongpho intrusions of Sakju Complex distributed in the western area, DPR Korea

Alkali intrusions of Sakju Complex distributed in the western area, DPR Korea were formed in mid-Proterozoic era. The Zirconium-REE mineralization occurs in Sakju and Ryongpho intrusions of Sakju Complex. Zr and Hf are the typical lithophile elements and always exist together under the closed relation to alkali and acid igneous rock. In our country, Zr and Hf mineral resources are exploited in the coast placer deposits and some in the alluvial placers. In addition, deposits related to alkali syenite, carbonatite and pegmatite deposites are found. The contents of Zr, Hf and REEs in intrusions of the Sakju complex are much higher than the average contents of the earth's crust and those in alkali rocks. The REE values in the constituent rocks of Ryongpho intrusion, from earlier differentiation phase to later phase, gradually decrease in the order of alkali pyroxenite to alkali gabbro to alkali amphibole syenite to alkali feldspar syenite. The constituent rocks have the similar chondrite-normalized REE distribution patterns, i.e., rich in LREE and deficient in HREE, and no or weak negative anomaly of Eu. REEs are concentrated in the fluorite zone experienced Ca–F metasomatism in alkali pyroxenite and alkali gabbro to form a medium-high grade REE ore body. The carbonatite in Sakju intrusion and alkali metasomatic belts heavily experienced the albitization in Ryongpho intrusion are unique useful mineral resources capable of recovering simultaneously Hf, Sr, REEs as well as Zr.

High Arctic Large Igneous Province Alkaline Rocks in Canada: Evidence for Multiple Mantle Components

Abstract The Cretaceous High Arctic Large Igneous Province (HALIP) in Canada, although dominated by tholeiites (135–90 Ma), contains two main groups of alkaline igneous rocks. The older alkaline rocks (∼96 Ma) scatter around major fault and basement structures. They are represented by the newly defined Fulmar Suite alkaline basalt dykes and sills, and include Hassel Formation volcanic rocks. The younger alkaline group is represented by the Wootton Intrusive Complex (92·2–92·7 Ma), and the Audhild Bay Suite (83–73 Ma), both emplaced near the northern coast of Ellesmere Island. Fulmar Suite rocks resemble EM-type ocean island basalts (OIB) and most show limited crustal contamination. The Fulmar Suite shows increases of P2O5 at near-constant Ba–K–Zr–Ti that are nearly orthogonal to predicted fractionation- or melting-related variations, which we interpret as the result of melting composite mantle sources containing a regionally widespread apatite-bearing enriched component (P1). Low-P2O5 Fulmar Suite variants overlap compositionally with enriched HALIP tholeiites, and fall on common garnet lherzolite trace element melting trajectories, suggesting variable degrees of melting of a geochemically similar source. High-P2O5 Hassel Formation basalts are unusual among Fulmar rocks, because they are strongly contaminated with depleted lower crust; and because they involve a high-P2O5–Ba–Eu mantle component (P2), similar to that seen in alkali basalt dykes from Greenland. The P2 component may have contained Ba–Eu-rich hawthorneite and/or carbonate minerals as well as apatite, and may typify parts of the Greenlandic sub-continental lithospheric mantle (SCLM). Mafic alkaline Audhild Bay Suite (ABS) rocks are volcanic and hypabyssal basanites, alkaline basalts and trachy-andesites, and resemble HIMU ocean island basalts in having high Nb, low Zr/Nb and low 87Sr/86Sri. These mafic alkaline rocks are associated with felsic alkaline lavas and syenitic intrusions, but crustally derived rhyodacites and rhyolites also exist. The Wootton Intrusive Complex (WIC) contains geochemically similar plutonic rocks (alkali gabbros, diorites and anatectic granites), and may represent a more deeply eroded, slightly older equivalent of the ABS. Low-P2O5 ABS and WIC alkaline mafic rocks have flat heavy rare earth element (HREE) profiles suggesting shallow mantle melting; whereas High-P2O5 variants have steep HREE profiles indicating deeper separation from garnet-bearing residues. Some High-P2O5 mafic ABS rocks seem to contain the P1 and P2 components identified in Fulmar–Hassel rocks, whereas other samples trend towards possible High-P2O5 + Zr (PZr) and High-P2O5 + K2O (PK) components. We argue that the strongly alkaline northern Ellesmere Island magmas sampled mineralogically heterogeneous veins or metasomes in Greenlandic-type SCLM, which contained trace phases such as apatite, carbonates, hawthorneite, zircon, mica or richterite. The geographically more widespread apatite-bearing component (P1) could have formed part of a heterogeneous plume or upwelling mantle current that also generated HALIP tholeiites when melted more extensively, but may also have resided in the SCLM as relics of older events. Rare HALIP alkaline rocks with high K–Rb–U–Th fall on mixing paths implying strong local contamination from either Sverdrup Basin sedimentary rocks or granitic upper crust. However, the scarcity of potassic alkaline HALIP facies, together with the other trace element and isotopic signatures, provides little support for a ubiquitous fossil sedimentary subduction-zone component in the HALIP mantle source.

Application of Landsat 8 OLI for Mapping the Deccan Traps of Kachchh, Gujarat

In the present study, Landsat 8 Operational Land Imager multispectral data covering Gujarat's Kachchh district have been analyzed to map the Deccan Traps. The study area is covered by different lithological units from Middle Jurassic to Holocene rock formation. Deccan Volcanics of Upper Cretaceous to Eocene age consist of Alkali Gabbro, Andesitic Trachyte, Basalt Flows, Intertrappean sediments, Dolerite dykes and Olivine Gabbro. The study aims to distinguish the basalts of Deccan Volcanics using remote sensing data. The basalts were segregated using image processing techniques such as false color composite based on the optimal index factor, band rationing, minimum noise fraction and principal component analysis. Conclusively, entire techniques used differentiated the basalts from other lithological units. The data were validated using laboratory reflectance spectra of rock samples. The map of basalt in Kachchh district was created from the processed Landsat 8 OLI image manifesting different flows, and it covers an area of total 1913.95 sq km.

Characteristics of tectono-magmatic alkali gabbros in northern Fathabad, Zarand (NW Kerman): based on the pyroxene mineral chemistry

Characteristics of tectono-magmatic alkali gabbros in northern Fathabad, Zarand (NW Kerman): based on the pyroxene mineral chemistry

Major and trace elements behaviour in two weathering profiles developed on gabbroic rocks in Meka'a and Abiete (south Cameroon, central Africa)

The present work deals with the behaviour of chemical elements in two weathering profiles developed on tholeiitic gabbro of Meka'a and alkali gabbro of Abiete, in southern Cameroon. The weathering profiles were characterized based on the colour, structure, texture, mineralogical and geochemical compositions. Elements mobility and redistribution during weathering processes are evaluated. The region is under an equatorial climate with four seasons strongly influenced by oceanic monsoon in the coastal zone. Geologically, the study area is located within the Ntem complex (Congo craton). Fresh rocks display a granular texture made up of plagioclase crystals, pyroxene, amphibole, biotite, apatite, chlorite, ilmenite and magnetite. The profile at Meka'a is made up of a coarse saprolite, a fine saprolite and a loose clayey horizon while that of Abiete comprises four horizons: coarse saprolite, fine saprolite, nodular, and loose clayey horizons. Common minerals are goethite, kaolinite, quartz and anatase. Halloysite is specific to Meka'a while gibbsite and hematite occur only at Abiete.In the weathered profile of Meka'a, organic matter contents decrease downwards while pH values increase. Fe2O3, Al2O3, TiO2, P2O5, Ba and Cu are partially removed from the top of the profile and accumulated in the saprolite layers whereas Cs, Th, HFSE and LREE (La, Ce, Pr) contents increase upwards. Transition elements and HREE are all leached out of the profile.At Abiete, OM content oscillates around 3.5 from the surface to the nodular horizon parallel with the accumulation of Al2O3 and Fe2O3. The nodular horizon shows a peculiar feature where abundant OM and an excellent drainage have induced the decrease of the pH value from 4.5 to 3 with concomitant leaching of Al2O3, Fe2O3, and many trace elements such as REE and HFSE. In the saprolite horizons, the OM content oscillates with concomitant accumulation of Al2O3 and Fe2O3. Positive Ce anomaly observed in the saprolitic set of Meka'a profile and in the coarse saprolite and the superficial clayey horizon at Abiete is assigned to oxidizing conditions in these levels.

Buds of Santonian magmatism associated with Marion hotspot in southern India

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

REE mineralisation within the Ditrău Alkaline Complex, Romania: Interplay of magmatic and hydrothermal processes

The Ditrău Igneous Complex (north-east Romania) is a tilted Mesozoic alkaline intrusion (~19 km diameter), with enrichments in rare earth elements (REE), niobium, and molybdenum. It has the potential to contribute to a secure and sustainable European REE mining industry, ensuring supply security for these critical metals. The complex comprises a sequence of ultramafic rocks, alkali gabbros, diorites, syenites, nepheline syenites and alkali granites. These units have been significantly modified by sub-solidus interaction with late-stage magmatic fluids and are cut by secondary mafic dykes. The complex was subsequently cut by REE-mineralised carbonate-rich veins. Geochemical and petrological data, including apatite mineral chemistry, from the alkaline igneous rocks, dykes and veins within the Ditrău Complex, have been used to assess the interplay of magmatic processes with late-stage magmatic and hydrothermal fluids, and the effects of these processes on element remobilisation and concentration of critical metals. Only limited critical metal enrichment was achieved by magmatic processes; the REE were preferentially incorporated into titanite and apatite in ultramafic cumulates during primary crystallisation, and were not enriched in evolved magmas. A hydrothermal system developed within the Ditrău Complex magma chamber during the later stages of magmatic crystallisation, causing localised alteration of nepheline syenites by a sodium-rich fluid. Mafic dykes subsequently acted as conduits for late stage, buoyant potassic fluids, which leached REE and HFSE from the surrounding syenitic rocks. These fluids percolated up and accumulated in the roof zone, causing the breakdown of nepheline to K-rich pseudomorphs and the precipitation of hydrothermal minerals such as zircon and pyrochlore within veins. REE mineralisation within the Ditrău Complex is hosted in the latest hydrothermal phase, mineralised carbonate-rich veins, which cross-cut the complex. Monazite is the main REE-bearing phase, it crystallised from a late REE- and carbonate-rich fluid with pH controlled REE deposition.

Petrogenetic evolution of Cretaceous Samchampi-Samteran Alkaline Complex, Mikir Hills, Northeastern India: Implications on multiple melting events of heterogeneous plume and metasomatized sub-continental lithospheric mantle

The Samchampi (26° 13′N: 93° 18′E)-Samteran (26° 11′N: 93° 25′E) alkaline complex (SSAC) occurs as an intrusion within Precambrian basement gneisses in the Karbi-Anglong district of Assam, Northeastern India. This intrusive complex comprises a wide spectrum of lithologies including syenite, ijolite-melteigite, alkali pyroxenite, alkali gabbro, nepheline syenite and carbonatite (nepheline syenites and carbonatites are later intrusives). In this paper, we present new major, trace, REE and Sr-Nd isotope data for different lithologies of SSAC and discuss integrated petrological and whole rock geochemical observations with Sr-Nd isotope systematics to understand the petrogenetic evolution of the complex. Pronounced LILE and LREE enrichment of the alkaline-carbonatite rocks together with steep LREE/HREE profile and flat HREE-chondrite normalized patterns provide evidence for parent magma generation from low degree partial melting of a metasomatized garnet peridotite mantle source. LILE, HFSE and LREE enrichments of the alkaline-silicate rocks and carbonatites are in agreement with the involvement of a mantle plume in their genesis. Nb-Th-La systematics with incompatible trace element abundance patterns marked by positive Nb-Ta anomalies and negative K, Th and Sr anomalies suggest contribution from plume-derived OIB-type mantle with recycled subduction component and a rift-controlled, intraplate tectonic setting for alkaline-carbonatite magmatism giving rise to the SSAC. This observation is corroborated by enriched 87Sr/86Srinitial (0.705562 to 0.709416) and 143Nd/144Ndinitial (0.512187 to 0.512449) ratios for the alkaline-carbonatite rocks that attest to a plume-related enriched mantle (~EM II) source in relation to the origin of Samchampi-Samteran alkaline complex. Trace element chemistry and variations in isotopic data invoke periodic melting of an isotopically heterogeneous, metasomatized mantle and generation of isotopically distinct melt batches that were parental to the different rocks of SSAC. Various extents of plume-lithosphere interaction also accounts for the trace element and isotopic variations of SSAC. The Srinitial and Ndinitial (105Ma) isotopic compositions (corresponding to εNd values of −6.37 to −1.27) of SSAC are consistent with those of Sung Valley, Jasra, Rajmahal tholeiites (Group II), Sylhet Traps and Kerguelen plateau basalts.

Fluid inclusion studies on the Koraput Alkaline Complex, Eastern Ghats Province, India: Implications for mid-Neoproterozoic granulite facies metamorphism and exhumation

Following ultrahigh temperature granulite metamorphism at ∼1Ga, the Eastern Ghats Province of India was intruded by the Koraput Alkaline Complex, and was subsequently re-metamorphosed in the granulite facies in the mid-Neoproterozoic time. Fluid inclusion studies were conducted on silica undersaturated alkali gabbro and syenites in the complex, and a pre-metamorphic pegmatitic granite dyke that intrudes it. High density (1.02–1.05g/cc), pseudo-secondary pure CO2 inclusions are restricted to metamorphic garnets within the gabbro and quartz within the granite, whereas moderate (∼0.92–0.95g/cc) and low density (∼0.75g/cc) secondary inclusions occur in garnet, magmatic clinopyroxene, plagioclase, hornblende and quartz. The isochores calculated for high density pseudo-secondary inclusions pass very close to the peak metamorphic window (∼8kbar, 750°C), and are interpreted to represent the fluid present during peak metamorphism that was entrapped by the growing garnet. Microscopic round inclusions of undigested, relict calcite in garnet suggest that the CO2 present during metamorphism of the complex was internally derived through carbonate breakdown. Pure to low salinity (0.00–10.1wt% NaCl equivalent) aqueous intra-/intergranular inclusions showing unimodal normal distribution of final ice-melting temperature (Tm) and temperature of homogenization (Th) are present only in quartz within the granite. These represent re-equilibrated inclusions within the quartz host that were entrapped at the metamorphic peak. Rare, chemically precipitated graphite along the walls of carbonic inclusions is interpreted as a post-entrapment reaction product formed during decompression. The fluid inclusion evidence is consistent with rapid exhumation of a thickened lower crust following the mid-Neoproterozoic granulite facies metamorphic event. The study suggests that mantle CO2, transported by alkaline magma into the crust, was locked up within carbonates and released during granulite metamorphism.

Evaluation of phase chemistry and petrochemical aspects of Samchampi-Samteran differentiated alkaline complex of Mikir Hills, northeastern India

The Samchampi-Samteran alkaline complex occurs as a plug-like pluton within the Precambrian granite gneisses of Mikir Hills, Assam, northeastern India and it is genetically related to Sylhet Traps. The intrusive complex is marked by dominant development of syenite within which ijolitemelteigite suite of rocks is emplaced with an arcuate outcrop pattern. Inliers of alkali pyroxenite and alkali gabbro occur within this ijolite-melteigite suite of rocks. The pluton is also traversed by younger intrusives of nepheline syenite and carbonatite. Development of sporadic, lumpy magnetite ore bodies is also recorded within the pluton. Petrographic details of the constituent lithomembers of the pluton have been presented following standard nomenclatorial rules. Overall pyroxene compositions range from diopside to aegirine augite while alkali feldspars are typically orthoclase and plagioclase in syenite corresponds to oligoclase species. Phase chemistry of nepheline is suggestive of Na-rich alkaline character of the complex. Biotite compositions are typically restricted to a uniform compositional range and they belong to ‘biotite’ field in the relevant classification scheme. Garnets (developed in syenite and melteigite) typically tend to be Ti-rich andradite, which on a closer scan can be further designated as melanites. Opaque minerals mostly correspond to magnetite. Use of Lindsley’s pyroxene thermometric method suggests an equilibration temperature from ∼450°–600°C for melteigite/alkali gabbro and ∼400°C for syenite. Critical assessment of other thermometric methods reveals a temperature of equilibration of ∼700°–1350°C for ijolite-melteigite suite of rocks in contrast to a relatively lower equilibration temperature of ∼600°C for syenite. Geobarometric data based on pyroxene chemistry yield an equilibration pressure of 5.32–7.72 kb for ijolite, melteigite, alkali pyroxenite, alkali gabbro and nepheline syenite. The dominant syenite member of the intrusive plug records a much higher (∼11 kb) equilibration pressure indicating a deeper level of intrusion. Major oxide variations of constituent lithomembers with respect to differentiation index (D.I.) corroborate a normal magmatic differentiation. A prominent role of liquid immiscibility is envisaged from field geological, petrographic and petrochemical evidences. Tectonic discrimination diagrams involving clinopyroxene chemistry strongly suggest within plate alkaline affinity for the parental magma which is in conformity with the regional plume tectonics.

New massif of Archean alkaline syenites in the Murmansk domain of the Kola peninsula

Alkaline magmatism typical of a mature continental crust is among the most important geodynamic markers in the Early Precambrian. The Late Archean alkaline rocks represented by alkali granites, alkali gabbros, syenites, nepheline syenites, and carbonatites are known at the ancient shields [1]. In the Kola Peninsula, such magmatism (with an age of 2.75‐2.61 Ga) in the form of alkali granite and alkali gabbro‐nepheline syenite complexes occurs in the Archean Keivy and Ingozero structures [2, 3]. No Precambrian alkaline complexes have been found yet in the Archean Murmansk domain of the Kola peninsula. The Late Archean Iokan’gskii intrusive complex of porphyritic lepidomelane‐ferrohstingsite granites was previously ascribed by Batieva [2] to the alkali granite complex. However, later studies [4] showed that it belongs to the complex of ancient rapakivilike granites of normal alkalinity. During recent studies of the granitoids from the Barents Sea shore of the Kola Peninsula in the area of Gremikha settlement and Mt. Ostrovnaya, the small Late Archean alkaline massif called Panejavr Massif after an nearby lake was discovered and studied. The Panejavr Massif is located within the Iokan’gskii block of the Murmansk domain, and its position is controlled by the junction of two faults (Fig. 1). The massif is contained among Late Archean migmatite‐granites and metabasite dikes that fill the NE-trending faults. The Panejavr Massif is a complex stock having an autonomous internal structure discordant to the host rocks (Fig. 1) and consisting of two chemically and mineralogically contrasting intrusive phases.

The origin of trachyte and pantellerite from Pantelleria, Italy: Insights from major element, trace element, and thermodynamic modelling

Trachyte and peralkaline rhyolite (pantellerite and comendite) frequently comprise the felsic end-member in bimodal suites in continental rift and oceanic island settings. In these settings, the relationship between the mafic (mildly alkaline, or transitional, basalt) and felsic lavas is ambiguous; major- and trace-element models and isotopic data are often consistent with an origin for felsic lavas from either fractional crystallization of transitional basalt or partial melting of alkali gabbro followed by fractional crystallization. In this paper, we present representative mineral analyses and whole-rock analyses from forty samples of a basalt–trachyte–pantellerite suite collected at Pantelleria, Italy, in the Strait of Sicily Rift Zone, and compare the results of major- and trace-element modelling with the results of thermodynamic (MELTS) modelling. From these results we conclude that metaluminous trachyte formed as a result of 70 to 75% low-pressure (0.1 GPa) fractional crystallization of an assemblage of plagioclase, clinopyroxene, olivine, magnetite, and apatite from a hydrous (1.0–1.5 wt.% H 2O) transitional basalt magma at relative oxygen fugacities approximately one log unit below the fayalite–magnetite–quartz buffer (FMQ−1). The “Daly gap”–a lack of intermediate (~ 49–62 wt.% SiO 2) volcanic rocks–at Pantelleria is concluded to be primarily the result of rapid differentiation through that interval. Relatively rapid crystallization at low pressure may have effected the partial degassing of water-saturated (~ 4 wt.% H 2O) metaluminous trachyte magma. Some metaluminous trachyte lavas have positive Eu anomalies, high K/Rb ratios, high concentrations of Ba, and low concentrations of incompatible trace elements; these are interpreted to be the result of up to 40% accumulation of alkali feldspar. Comenditic trachyte, pantelleritic trachyte, and pantellerite formed after an additional 20 to 80% fractional crystallization of an assemblage dominated by alkali feldspar from metaluminous trachyte magma. The most evolved pantellerite lavas and tuffs are the result of a total of ~ 95% fractional crystallization of transitional basalt, with phenocrysts that equilibrated at low temperatures (< 700 °C), low oxygen fugacities, and high (> 4 wt.%) water contents.

PARAGENETIC TYPES OF CARBONATITE AS INDICATED BY THE DIVERSITY AND RELATIVE ABUNDANCES OF ASSOCIATED SILICATE ROCKS: EVIDENCE FROM A GLOBAL DATABASE

Data on the diversity and relative abundance of igneous rock types associated with carbonatite have been compiled for 477 occurrences, which represent 90% of the 527 occurrences of carbonatite known to the authors. The carbonatites have been subdivided into magmatic carbonatites (84%) and carbohydrothermal carbonatites (16%) on the basis of their mineralogy and their intrusive or extrusive characteristics. The carbohydrothermal carbonatites are associated with a variety of silicate rock-types, but nepheline syenite and syenite are the characteristic ones. For the magmatic carbonatites, 24% of these localities have no associated igneous silicate rocks, whereas a diverse range of silicate rock-types are recognized to be associated with the other 76%. The silicate rocks associated with these carbonatites are listed, quantified and described. We distinguish seven main associations with silicate rocks. These are (in decreasing order of abundance): 1) nephelinite–ijolite, 2) phonolite – feldspathoidal syenite, 3) trachyte–syenite, 4) melilitite–melilitolite, 5) lamprophyre, 6) kimberlite, 7) basanite – alkali gabbro. A significant proportion of the nephelinite–ijolite, melilitite–melilitolite and carbonatite only associations also contain ultramafic rocks, which are interpreted as cumulates. Extrusive carbonatite is found at forty-six localities; these are characterized by the high proportion that contain mantle debris (xenoliths and xenocrysts) and the occurrence of melilite-bearing silicate rocks in more than one third of them. The presence of mantle debris in some carbonatite occurrences and the absence of associated silicate rocks from others are taken as evidence of derivation of these carbonatites directly from the mantle. Most other carbonatites are considered to have been generated by differentiation from magmas represented by the associated silicate rocks that are the result of partial melting in a metasomatized lithosphere. Carbonatites can be generated in a number of ways, but the close spatial and temporal association with a broad spectrum of the silicate melts implies a close relationship with them. However, certain associated silicate rocks are not strictly consanguineous with the carbonatite, but are interpreted as products of independent melting events at shallower levels in the lithosphere.

O maciço alcalino de Tunas, PR: novos dados geocronológicos

The NW-SE elongated Tunas intrusion, occupying an area of 22 km², crops out about 80 km from the city of Curitiba, and it is emplaced into Precambrian metavolcanic-sedimentary rocks of the Acungui Group. Five sub-circular volcanic structures are recognized. Petrographically, the massif mainly consists of syenites and alkali syenites with subordinate alkali gabbros, essexites, syenogabbros and syenodiorites; small late syenitic dykes and magmatic breccias are also found. Previous K-Ar data give a Late Cretaceous mean age of 82.2 Ma for Tunas rocks. In this article we present new U-Pb age determinations on zircon crystals from syenitic rocks (ID-TIMS, 82.7 ± 0.7 Ma; SHRIMP, 84.7 ± 1.2 Ma) which confirm that age, placing the massif into the Late Cretaceous group of alkaline intrusions tectonically associated with the Sao Jeronimo-Curiuva Lineament, a NW-SE-trending branch of the Ponta Grossa Arch in southern Brazil.

Petrogenesis of the Early Cenozoic potassic alkaline complex of Morro de São João, southeastern Brazil

The Early Cenozoic alkaline complex of Morro de São João intrudes the Precambrian basement at the eastern end of the Rio de Janeiro coastline. It is mostly made up of melanocratic nepheline syenites and nepheline syenites, with subordinate clinopyroxenites, shonkinites (melasyenites), and alkali gabbros. Melanocratic nepheline syenites and nepheline syenites are cross-cut by thin dykes, ranging in composition from phonotephrite to phonolite. The mafic–ultramafic rocks are meso- to orthocumulates, with mineral compositions and whole-rock chemistry suggesting crystallization from a moderately evolved batch of magmas (potassic tephrite/phonotephrite). Melanocratic nepheline syenites and nepheline syenites likely represent magma–crystal mixtures, whereas a few fine-grained rocks roughly approximate liquid compositions; alkali syenites are typical cumulitic rocks. Major and trace element compositions reflect a genesis of clinopyroxenitic to shonkinitic rocks from tephritic parental magmas, which evolved toward phonotephritic and phonolitic compositions. Initial 87Sr/ 86Sr and 143Nd/ 144Nd ratios (0.7048–0.7061 and 0.51231–0.51239, respectively) overlap the values in the analog intrusions of the Serra do Mar province, supporting a genesis from similar parental magmas and source mantle. The dominant lithospheric components of the latter may be responsible for the potassic affinity and strongly incompatible element enrichment of the derived melts.

The felsic complex of the Vallehermoso Caldera: interior of an ancient volcanic system (La Gomera, Canary Islands)

A mid-to-upper Miocene felsic cone sheet dike complex, caldera collapse and dome intrusion, located around the Vallehermoso and Tamargada areas (North of La Gomera Island) is described. It constitutes one of the less-known volcanic episodes in the Canary Archipelago and especially in La Gomera Island. Based on detailed field and structural evidence, it is suggested that the cone sheet shows, at the surface, an almost circular geometry truncated to the NW sector. After the cone sheet dike complex formed, decompression of the interior of an assumed shallow magma chamber occurred due to the subsequent migration of felsic magma throughout the dikes of the cone sheet. This last event caused the roof of the magma chamber to collapse, leading to the formation of a 3- to 4-km-wide caldera. A breccia deposit formed during the collapse and filled a depressed central area which finally was affected by an episode of magmatic resurgence, leading to a later almost circular dome intrusion. The association of cone sheet, breccia deposits and domes constitutes the so-called “Trachytic–Phonolitic Complex” (TPC) and should be viewed as the root of an ancient intensely eroded volcanic edifice made of trachytes, nepheline–phonolites and minor Haüyne phonolites as the dominant materials. A description of the structural, petrological and geochemical features of the Trachytic–Phonolitic Complex and a discussion about their origin are considered in this paper. We also consider the correlation between another felsic intrusion of alkali gabbros and syenites (called Tamargada syenites) that also crops out in the Vallehermoso–Tamargada area with respect to the Trachytic–Phonolitic Complex. Our data suggest that this alkaline intrusion of alkali gabbros and syenites belongs to an earlier intrusive episode. On the basis of petrological, geochemical and detailed field studies, both trachytes and nepheline–phonolites included in the Trachytic–Phonolitic Complex most likely evolved from alkali mafic magmas from the Upper Old Basalt formation through a dominant fractional crystallization process. Additionally, haüyne phonolites could develop from local and later magmatic processes such as gas transfer that altered and changed a small part of the previously mentioned nepheline phonolites to haüyne phonolites.