Ijolite versus carbonatite as sources of fenitization

Ijolite-carbonatite complexes are ubiquitously surrounded of an aureole of metasomatically altered rocks. The process of alteration is termed fenitization and is generally caused by peralkaline fluids emanating from cooling alkaline, i.e. ijolite and carbonatite magmas. Ijolites and carbonatites normally occur together and attempts to determine the source of the fenitizing fluids may therefore lead to controversial, if not erroneous, conclusions.
Mineralogical and chemical data of fenites from Oldoinyo Lengai (Tanzania), Fen (Norway), and Alnö (Sweden) are reviewed in the present paper in order to reveal the main factors controlling the fenitization around ijolite and carbonatite. Despite the overall alkaline nature of the process, variables such as XCO₂ of the fluid, activity gradients of SiO₂, Al₂O₃ and CaO, FeO/MgO ratio, f O₂ and temperature gradients may differ, producing distinctive patterns of fenitization around the two magmatic sources. The ijolitic-type fluid has low XCO₂, high activities of alkalies, SiO₂ and Al₂O₃, and low activity of CaO. The f O₂ evolves along the hm-mt buffer conditions and the temperature falls gradually with distance from the magmatic source. The carbonatitic-type fluid has high XCO₂, high activities of alkalies and CaO, and low activities of SiO₂ and Al₂O₃. Temperatures and f O₂ are initially high, but decrease sharply with distance from the source. Moreover, the CO₂-rich fluid may complex and transport the REE.     

Rosia Poieni copper deposit,Apuseni Mountains,Romania: advanced argillic overprint of a porphyry system

The Rosia Poieni deposit is the largest porphyry copper deposit in the Apuseni Mountains, Romania. Hydrothermal alteration and mineralization are related to the Middle Miocene emplacement of a subvolcanic body, the Fundoaia microdiorite. Zonation of the alteration associated with the porphyry copper deposit is recognized from the deep and central part of the porphyritic intrusion towards shallower and outer portions. Four alteration types have been distinguished: potassic, phyllic, advanced argillic, and propylitic. Potassic alteration affects mainly the Fundoaia subvolcanic body. The andesitic host rocks are altered only in the immediate contact zone with the Fundoaia intrusion. Mg-biotite and K-feldspar are the main alteration minerals of the potassic assemblage, accompanied by ubiquitous quartz; chlorite, and anhydrite are also present. Magnetite, pyrite, chalcopyrite and minor bornite, are associated with this alteration. Phyllic alteration has overprinted the margin of the potassic zone, and formed peripheral to it. It is characterized by the replacement of almost all early minerals by abundant quartz, phengite, illite, variable amounts of illite-smectite mixed-layer minerals, minor smectite, and kaolinite. Pyrite is abundant and represents the main sulfide in this alteration zone. Advanced argillic alteration affects the upper part of the volcanic structure. The mineral assemblage comprises alunite, kaolinite, dickite, pyrophyllite, diaspore, aluminium-phosphate-sulphate minerals (woodhouseite-svanbergite series), zunyite, minamyite, pyrite, and enargite (luzonite). Alunite forms well-developed crystals. Veins with enargite (luzonite) and pyrite in a gangue of quartz, pyrophyllite and diaspore, are present within and around the subvolcanic intrusion. This alteration type is partially controlled by fractures. A zonal distribution of alteration minerals is observed from the centre of fractures outwards with: (1) vuggy quartz; (2) quartz + alunite; (3) quartz + kaolinite ± alunite and, in the deeper part of the argillic zone, quartz + pyrophyllite + diaspore; (4) illite + illite-smectite mixed-layer minerals ± kaolinite ± alunite, and e) chlorite + albite + epidote. Propylitic alteration is present distal to all other alteration types and consists of chlorite, epidote, albite, and carbonates. Mineral parageneses, mineral stability fields, and alteration mineral geothermometers indicate that the different alteration assemblages are the result of changes in both fluid composition and temperature of the system. The alteration minerals reflect cooling of the hydrothermal system from >400 °C (biotite), to 300–200 °C (chlorite and illite in veinlets) and to lower temperatures of kaolinite, illite-smectite mixed layers, and smectite crystallization. Hydrothermal alteration started with an extensive potassic zone in the central part of the system that passed laterally to the propylitic zone. It was followed by phyllic overprint of the early-altered rocks. Nearly barren advanced argillic alteration subsequently superimposed the upper levels of the porphyry copper alteration zones. The close spatial association between porphyry mineralization and advanced argillic alteration suggests that they are genetically part of the same magmatic-hydrothermal system that includes a porphyry intrusion at depth and an epithermal environment of the advanced argillic type near the surface.Editorial handling: B. Lehmann     

Conditions and timing of high-pressure Variscan metamorphism in the South Carpathians, Romania

High-pressure (HP) metamorphic rocks, including garnet peridotite, eclogite, HP granulite, and HP amphibolite, are important constituents of several tectonostratigraphic units in the pre-Alpine nappe stack of the Getic–Supragetic (GS) basement in the South Carpathians. A Variscan age for HP metamorphism is firmly established by Sm–Nd mineral–whole-rock isochrons for garnet amphibolite, 358±10 Ma, two samples of eclogite, 341±8 and 344±7 Ma, and garnet peridotite, 316±4 Ma.

A prograde history for many HP metamorphic rocks is documented by the presence of lower pressure mineral inclusions and compositional zoning in garnet. Application of commonly accepted thermobarometers to eclogite (grt+cpx±ky±phn±pg±zo) yields a range in “peak” pressures and temperatures of 10.8–22.3 kbar and 545–745 °C, depending on tectonostratigraphic unit and locality. Zoisite equilibria indicate that activity of H₂O in some samples was substantially reduced, ca. 0.1–0.4. HP granulite (grt+cpx+hb+pl) and HP amphibolite (grt+hbl+pl) may have formed by retrogression of eclogites during high-temperature decompression. Two types of garnet peridotite have been recognized, one forming from spinel peridotite at ca. 1150–1300 °C, 25.8–29.0 kbar, and another from plagioclase peridotite at 560 °C, 16.1 kbar.

The Variscan evolution of the pre-Mesozoic basement in the South Carpathians is similar to that in other segments of the European Variscides, including widespread HP metamorphism, in which P–T–t characteristics are specific to individual tectonostratigraphic units, the presence of diverse types of garnet peridotite, diachronous subduction and accretion, nappe assembly in pre-Westphalian time due to collision of Laurussia, Gondwana, and amalgamated terranes, and finally, rapid exhumation, cooling, and deposition of eroded debris in Westphalian to Permian sedimentary basins.     

Alpine polyphase tectono-metamorphic evolution of the South Carpathians: A new overview

The main terrains involved in the Cretaceous–Tertiary tectonism in the South Carpathians segment of the European Alpine orogen are the Getic–Supragetic and Danubian continental crust fragments separated by the Severin oceanic crust-floored basin. During the Early–Middle Cretaceous times the Danubian microplate acted initially as a foreland unit strongly involved in the South Carpathians nappe stacking. Multistage folding/thrusting events, uplift/erosion and extensional stages and the development of associated sedimentary basins characterize the South Carpathians during Cretaceous to Tertiary convergence and collision events. The main Cretaceous tectogenetic events responsible for contraction and crustal thickening processes in the South Carpathians are Mid-Cretaceous (“Austrian phase”) and Latest Cretaceous (“Laramide” or “Getic phase”) in age. The architecture of the South Carpathians suggests polyphase tectonic evolution and mountain building and includes from top to bottom: the Getic–Supragetic basement/cover nappes, the Severin and Arjana cover nappes, and Danubian basement/cover nappes, all tectonically overriding the Moesian Platform. The Severin nappe complex (including Obarsia and Severin nappes) with Late Jurassic–Early Cretaceous ophiolites and turbidites is squeezed between the Danubian and Getic–Supragetic basement nappes as a result of successive thrusting of dismembered units during the inferred Mid- to Late Cretaceous subduction/collision followed by tectonic inversion processes.

Early Cretaceous thick-skinned tectonics was replaced by thin-skinned tectonics in Late Cretaceous. Thus, the former Middle Cretaceous “Austrian” nappe stack and its Albian–Lower Senonian cover got incorporated in the intra-Senonian “Laramide/Getic” stacking of the Getic–Supragetic/Severin/Arjana nappes onto the Danubian nappe duplex. The two contraction events are separated by an extensional tectonic phase in the upper plate recorded by the intrusion of the “Banatitic” magmas (84–73 Ma). The overthrusting of the entire South Carpathian Cretaceous nappe stack onto the fold/thrust foredeep units and to the Moesian Platform took place in the Late Miocene (intra-Sarmatian) times and was followed by extensional events and sedimentary basin formation.     

The petrology of the Ditrau alkaline complex, Eastern Carpathians

Summary ?The Ditrau complex in eastern Transsylvania, Romania is a large (ca. 18 km diameter) Mesozoic alkaline igneous complex generated in an extensional environment associated with a rifted continental margin. It comprises an eccentric arcuate suite of intrusions in which there was a generalised migration of focus from west to east. Whereas most of the complex consists of salic rocks (syenites, nepheline syenites and alkali granites) a spectrum of intermediate rock types (monzonites, monzodiorites and alkali diorites) grades to alkali gabbros. Isolated masses of ultramafic rocks may represent autoliths derived from early cumulates. The earliest components appear to be the ultramafic, gabbroic and dioritic rocks of the north-west whereas the large area of nepheline syenites in the east of the complex represents the youngest large-scale intrusive event. An interval of dyke intrusion and widespread hydrothermal alteration marked the end of activity. Rocks of contrasted composition commonly show intricate and complex geometric relationships. Those between mafic (especially alkali gabbroic and dioritic) facies and salic (syenite and quartz syenite) facies display pillowy forms suggesting synchronous emplacement of mafic and salic magmas with the former intruded into, and chilled against, the latter. Mixing, mingling and hybridisation in these pillowed associations has been widespread. Olivine is confined to some of the ultramafic rocks. The basic rocks contain diopsidic pyroxene and amphibole ranging from kaersutite through ferroan pargasite to hastingsite although edenitic and actinolitic varieties occur. Titanite is ubiquitous and is a major component in some facies of the basic rocks. The syenites consist of sodic plagioclase, alkali feldspar and hastingsite whereas the nepheline syenites comprise alkali feldspar, nepheline and aegirine-augite with accessory cancrinite, scapolite and sodalite. The complex is deduced to have been generated from primitive basanitic magmas, formed as small-fraction asthenospheric melts, with progressive evolution through to phonolitic residues. Fractional crystallisation is inferred to have involved olivine and spinel in the early stages, followed by the incoming of clinopyroxene and amphibole (with loss of olivine in increasingly hydrous residual melts). A generalised increase in Nb/Ta from basic to nepheline syenite compositions is ascribed to titanite fractionation. The divergence towards silica oversaturated products is attributed to crustal assimilation and concomitant fractional crystallisation of the basic magmas at a relatively early stage in the development of the complex. An overall rise in δ¹⁸O with increasing SiO₂ supports this conclusion. Evidence from the broad metamorphic aureole around the complex, the importance of amphiboles and extensive late-stage alteration of many of the rocks (with formation of e.g. scapolite, sodalite and cancrinite), suggests that the Ditrau magmas were notably volatile-rich. Factors responsible for the upwardly concave (chondrite-normalised) REE patterns exhibited by the salic rocks may include fractionation of minerals (kaersuite, titanite, apatite) preferentially removing MREE, accumulation of HREE-rich phases (zircon) and interaction with late-stage fluids enriched in HREE. The intrusive sequence and the inter-relationships of the basic and salic rocks suggest that stratified magma bodies may have been generated, with salic melts overlying denser basaltic melts. Mixing is inferred to have taken place during subsequent emplacement.

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Zusammenfassung ?Petrologie des Alkali-Komplexes von Ditrau in den Ost-Karpaten Der Ditrau-Komplex im ?stlichen Transsylvanien, Rum?nien, ist ein gro?er (ca. 18 km Durchmesser) mesozoischer Alkali-Komplex, der in einem extensionalen Umfeld im Zusammenhang mit dem Aufbrechen eines Kontinentalrandes entstanden ist. Es liegt eine bogenf?rmige, exzentrische Gruppe von Intrusionen vor, innerhalb derer der Fokus von West nach Ost gewandert ist. W?hrend der Gro?teil des Komplexes aus salischen Gesteinen (Syeniten, Nephelin-Syeniten und Alkali Graniten) besteht, liegen auch intermedi?re Gesteine (Monzonite, Monzodiorite und Alkali Diorite) vor, die in Alkaligabbros übergehen. Isolierte Massen von ultramafischen Gesteinen k?nnten Autolithe, die aus frühen Kumulaten abstammen, darstellen. Die ?ltesten Komponenten scheinen die ultramafischen, gabbroischen und dioritischen Gesteine des Nordwestens zu sein, w?hrend das gro?e Gebiet von Nephelin-Syeniten im Osten des Komplexes das jüngste Intrusionsstadium darstellt. Ein Intervall mit Gang-Intrusion und verbreiteter hydrothermaler Umwandlung markiert das Ende dieser Aktivit?t. Gesteine von gegens?tzlicher Zusammensetzung zeigen h?ufig komplizierte geometrische Beziehungen. Diejenigen zwischen mafischen (besondern alkaligabbroischen und dioritischen) Typen und salischen (Syeniten und Quarz-Syeniten) zeigen polsterartige Formen, die auf m?glicherweise gleichzeitige Platznahme von mafischen und salischen Magmen hinweisen; dabei dürften die ersteren die letzteren intrudiert haben. Mischung, Mingling und Hybridisation ist in diesen polsterf?rmigen Assoziationen weit verbreitet. Olivin ist auf einige der ultramafischen Gesteine beschr?nkt. Die basischen Gesteine enthalten diopsidischen Pyroxen und Amphibole, die von Kaersutit über “ferroan” Pargasit zu Hastingsit übergehen, obwohl auch edenitische und aktinolitische Variet?ten vorkommen. Titanit ist weit verbreitet und eine Hauptkomponente in einigen Typen der basischen Gesteine. Die Syenite bestehen aus sodischem Plagioklas, Alkali-Feldspat und Hastingsit, w?hrend Nephelin-Syenite, Alkali-Feldspat, Nephelin und Aeginin-Augit mit akzessorischem Cancrinit, Skapolith und Sodalit umfassen. Der Ditrau-Komplex dürfte aus primitiven basanitischen Magmen entstanden sein, die sich als “small-fraction” asthenosph?rischer Schmelzen bildeten, mit progressiver Evolution bis hin zu phonolitischen Residuen. Fraktionierte Kristallisation dürfte Olivin und Spinell in den Frühstadien betroffen haben, gefolgt vom Auftreten des Klinopyroxen und Amphibol (wobei Olivin in den zunehmend wasserreichen Restschmelzen verlorengeht). Eine allgemeine Zunahme in Nb/Ta von basischen zu nephelinsyenitischen Zusammensetzungen wird auf Titanit-Fraktionierung zurückgeführt. Die Entwicklung in Richtung Silika-übers?ttigter Produkte geht auf krustale Assimilation und fraktionelle Kristallisation des basischen Magmas in einem relativ frühen Stadium der Entwicklung des Komplexes zurück. Ein allgemeiner Anstieg in δ¹⁸O mit zunehmendem SiO₂ unterstützt diese Schlu?folgerung. Daten aus der breiten metamorphen Aureole des Komplexes, die Bedeutung der Amphibole und extensive Alteration im Sp?tstadium der Entwicklung vieler Gesteine (mit Bildung von Skapolith, Soldalit und Cancrinit) weist darauf hin, dass die Ditrau-Magmen sehr reich an volatilen Phasen waren. Die nach oben zu konkaven (Chondrit-normalisierten) SEE-Verteilungsmuster in den salischen Gesteinen dürften auf Mineralfraktionierung (Kaersuit, Titanit, Apatit) die vorzugsweise MSEE entfernt hat, Ansammlung von HSEE-reichen Phasen (Zirkon) und Wechselwirkungen mit sp?ten Fluiden, die an HSEE angereichert waren, zurückgehen. Die intrusive Abfolge und die Wechselbeziehungen zwischen den basischen und salischen Gesteinen legt nahe, dass geschichtete Magmenk?rper entstanden sind, wobei salische Schmelzen die dichteren basaltischen Schmelzen überlagert haben. W?hrend der darauf folgenden Platznahme dürfte Magmamixing stattgefunden haben.

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Received October 20, 1998;/revised version accepted July 16, 1999     

Connexion porphyre cuprifère–épithermaux de type low-sulfidation : analyse structurale et représentation 3D du secteur de Bolcana,monts Apuseni,Roumanie

The AuPbZn low-sulfidation epithermal ore deposits of Troita, Trestia, and Magura (Apuseni Mountains, Romania) are spatially related to the Bolcana Cu-porphyry. In an attempt to demonstrate the connection between these mineralizations, a geometric study was made based on structural measurements and GOCAD^© geomodeller 3D representation of deposits. This study indicates that a specific spatial distribution of the different Au and PbZn veins of the epithermal deposits occurs around the Cu-porphyry, which cannot result from telescoped systems. To cite this article: O. Cardon et al., C. R. Geoscience 337 (2005).     

Volatiles associated with the alkaline – carbonatite magmatism at Alnö, Sweden: a study of fluid and solid inclusions in minerals from the Laångarsholmen ring complex

The Alnö alkaline-carbonatite complex consists in its northernmost part at Laångarsholmen of a ring-type intrusion composed of pyroxenite, sövite and ijolite, emplaced in that order. The intrusion is surrounded by a breccia zone. The petrography, mineral chemistry and fluid/solid inclusion studies suggest that the ring complex and the main intrusion at Alnö have had a somewhat different magmatic evolution, implying different evolution of fluid phases also. At Laångarsholmen, a mafic silicate magma started to crystallize Al-diopside of 0.11 CaTs (Tschermak’s) content during a mid-crustal stage of evolution (ca. 5–6?kbar and 1175°?C). At that stage, the mafic magma was coexisting with a Mg-bearing calcitic melt, recorded in the abundant inclusions, trapped by the crystallizing Al-diopside. The two immiscible melts appear to have separated at ca. 5?kbar and 1150°?C, in good agreement with recent experimental studies. The silicate magma crystallized di+ap+magnetite during its ascent, and was in contact with a saline hydro-carbonic fluid trapped as inclusions in diopside (di) and apatite (ap) (type B2 inclusions reluctant to dissolution up to 550°?C). As P_H2O started to increase, Fe-pargasite began to replace the pyroxene. It appears that the fluid present at that stage was aqueous and contained ca. 40%?NaCl. With decreasing PT, the fluid separated into two immiscible phases of high- and low-salinity (type B1 of 65%?NaCl and Cl of 7%?NaCl), respectively. At the shallow depths of the final emplacement, the composition of the fluid phase was most probably controlled by supply of meteoric water as indicated by the dilution trend of some B1 type inclusions. After separation, the carbonatite magma fractionated calcite+ap+dol (as shown by dolomite inclusions in early crystallizing apatite). Around 4?kbar, a CO₂-bearing aqueous fluid of low salinity (d=0.85) was coexisting with the melt, and became trapped in the apatite formed during the mid-crustal stage (type A1 fluid inclusions). The residual melt was emplaced into the shallow crust and gave rise to phlogopite-bearing sövite. Fluid inclusions (type A2) trapped in calcite and in recrystallized apatite indicate that the fluid phase evolved towards a late (Na+K) hydro-carbonic fluid during cooling at the shallow depths of the final emplacement. The ijolite does not show signs of liquid immiscibility with the sövite at Laångarsholmen, and exhibits mostly post-magmatic activity of fluid phases.     

The impact of heat waves on surface urban heat island and local economy in Cluj-Napoca city,Romania

The association between heat waves and the urban heat island effect can increase the impact on environment and society inducing biophysical hazards. Heat stress and their associated public health problems are among the most frequent. This paper explores the heat waves impact on surface urban heat island and on the local economy loss during three heat periods in Cluj-Napoca city in the summer of 2015. The heat wave events were identified based on daily maximum temperature, and they were divided into three classes considering the intensity threshold: moderate heat waves (daily maximum temperature exceeding the 90th percentile), severe heat waves (daily maximum temperature over the 95th percentile), and extremely severe heat waves (daily maximum temperature exceeding the 98th percentile). The minimum length of an event was of minimum three consecutive days. The surface urban heat island was detected based on land surface temperature derived from Landsat 8 thermal infrared data, while the economic impact was estimated based on data on work force structure and work productivity in Cluj-Napoca derived from the data released by Eurostat, National Bank of Romania, and National Institute of Statistics. The results indicate that the intensity and spatial extension of surface urban heat island could be governed by the magnitude of the heat wave event, but due to the low number of satellite images available, we should consider this information only as preliminary results. Thermal infrared remote sensing has proven to be a very efficient method to study surface urban heat island, due to the fact that the synoptic conditions associated with heat wave events usually favor cloud free image. The resolution of the OLI_TIRS sensor provided good results for a mid-extension city, but the low revisiting time is still a drawback. The potential economic loss was calculated for the working days during heat waves and the estimated loss reached more than 2.5 mil. EUR for each heat wave day at city scale, cumulating more than 38 mil. EUR for the three cases considered.     

Mass transfer and REE mobility during fenitization at Alnö, Sweden

The Precambrian migmatitic gneisses at Alnö have been altered to fenite by fluids emanating from alkaline and carbonatitic magmas intruded during early to middle Cambrian times. Fenitization, related to carbonatitic sources, was promoted by peralkaline, carbonate-rich fluids, in which the main chemical components and REE were mobile. Composition-volume relationships of progressively fenitized protolith suggest mainly isovolumetric equilibration, but a modest decrease of volume (6%) did occur in the highest grade of the process. The fenitizing fluids introduced essentially CaO, CO₂, Na₂O, and K₂O while removing SiO₂ and Al₂O₃. Different trends of fenitization, defined as sodic, potassic and intermediate, show differing REE distribution and abundance patterns. The sodic carbonate-rich fluid introduced all the REE, but the La/Lu ratio was high. The extreme REE enrichments of high-grade fenites are associated with the widespread formation of calcite, apatite and possibly titanite. The potassic carbonaterich fluid introduced essentially light REE, but produced also the redistribution of heavy REE in the high-grade fenites. REE distribution patterns of intermediate fenites suggest the re-equilibration of fenite with a highly oxidizing alkaline fluorine-rich fluid, possibly in a later post-magmatic episode.     

Fenitization at the Alnö carbonatite complex,Sweden; distribution,mineralogy and genesis

Re-mapping of the Alnö complex has radically reduced the area identified as fenite, in comparison with the classic work of Eckermann (1948). A marginal fenite zone, generally 500–600 m wide, is present around the complex, and the petrography and mineralogy of six selected key areas have been investigated in detail. Fenitization of the country rock migmatitic gneiss led to replacement of quartz, feldspars, biotite and chlorite by alkali pyroxene and amphibole, new generations of feldspars, calcite, titanite, fluorite and apatite. In some areas, however, a distinctive narrow band of fenitization, referred to as contact fenite, adjacent to large sövite dykes, contains mineral assemblages that include phlogopite, nepheline, melanite and wollastonite. Amphiboles in the fenites are richterite, katophorite, arfvedsonite and eckermannite. There is a very wide variation in the composition of pyroxenes which vary between diopside, aergirine-augite and aergirine. Although trends from aergirine to aegirine-augite and aegirine-augite to diopside have been defined, which are similar to those of other fenite localities, distinctive trends for the eastern part of the aureole have been identified that converge on aegirine, and approximate trends in some series of alkaline igneous rocks. Analyses of mica, garnet, wollastonite and feldspar are also presented and discussed. The mineralogical data are used to estimate the conditions of temperature, oxidation state, and activity of CO₂, H₂O and silica pertaining during the fenitization process. The fluids with which the fenites equilibrated were apparently different in composition in different parts of the aureole, and varied with time, implying more than one magmatic source. The various evolutionary trends identified in the pyroxenes and amphiboles, in particular, are explained in terms of two main fluid types, which emanated from ijolitic and carbonatitic magma sources.