VOLUME  2 - 1992

The metamorphic-hosted precious- and base-metal deposits of the Bottino - Valdicastello region (Apuane Alps, Tuscany): an overview

 

 M. Benvenuti 1, G. Cortecci 2, P. Costagliola 3, P. Lattanzi 1, G. Ruggieri 1, G. Tanelli 4

 

 1. Dipartimento di Scienze della Terra dell´Università di Firenze, via G. La Pira 4, 1-50121 Firenze, Italy

2. Dipartimento di Scienze della Terra dell´Università di Pisa, via S. Maria 53, 1-56126 Pisa, Italy

3. Museo di Mineralogia e Litologia, Università di Firenze, via G. La Pira 4, 1-50121 Firenze, Italy

4. Dipartimento di Scienze della Terra dell´Università Federico 11 in Napoli, via Mezzocannone 8, 1-80138 Napoli, Italy

 

Abstract

In the Apuane Alps district, precious - and base-metal mineralization is essentially restricted to metamorphic-hosted vein-type orebodies that have been exploited in the past for lead, silver and minor zinc and, eventually, eold at Pollone, Buca dell´Angina, Bottino-Gallena and S. Barbara mines. The geologic features of these polymetallic (±barite) orebodies suggest that they formed throughout the Tertiary Apenninic event at syn- (Bottino-Gallena, S. Barbara and Pollone), late -, and post-tectonic stages (Buca dell´Angina). Mineralogy commonly includes pyrite, galena, sphalerite, chalcopyrite, tetrahedrite and gangue quartz + barite, fluorite, chlorite, and carbonates. Native gold has been observed at Buca dell´Angina and, possibly. at Bottino-Gallena; silver is usually contained in galena and tetrahedrite. Fluid inclusions from gangue quartz, fluorite, barite and apatite from various deposits
may be grouped into two distinct populations:
1) CO2-bearine fluid inclusions with low to moderate salinities (<15 wt % NaCl eq.) and Th around 240 °C;
2) CO2 -free fluid inclusions with variable Th (110-230 °C) and much hieher salinities (up to >30 wt % NaCl eq.).
The first group is interpreted to represent fluids of synmetamorphic ori-ein trapped at PT conditions close to metamorphic peak (T=400°-450°C, P=3-4 Kbars), whereas the second population points to high-salinitv fluids probably introduced at a late - to post-orogenic stage. Stable isotope systematics is also in good agreement with a metamorphic oriein of mineralizing fluids. Isotopic temperatures calculated at Pollone are in the order of 400 °C. The above data, together with textural relationships concur to suggest that most precious - and base-metal minerals were deposited by fluids having comparable temperatures (around 400 °C) and salinities (5-12 % NaCl eq.), d8 in the range 7-17 %o, Pfluid Pload and possibly being in isotopic, but not thermal equilibrium with host rocks.   On the basis of the above "empirical" model. an outline of a "conceptual" model is presented, according to which base - and precious-metal mineralization in the southern Apuane Alps was emplaced largely in a synmetamorphic stage by fluids of metamorphic nature. The fluids isotopically equilibrated with basement rocks at depth, uprised along shear zones, interacted at shallower levels (around 10 kilometers) with host rocks and/or metal pre-concentrations, and finally deposited their metal content under favorable environmental conditions.

 

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Volcanologic concerns of the siliceous metasedimentary xenoliths included in historic lava-flows of Lanzarote (Canary Islands)

 V. Araña , M. A. Bustillo

 Dipartimento de Geologìa. Museo Nacional de Ciencias Naturales. C.S.I.C. Josè Gutierrez Abascal 2, 28006 Madrid, Spain.

 

Abstract

A large number of siliceous metasedimentary xenoliths appears in some historic lava flows of Lanzarote (Canary Islands). In most cases those xenoliths are constitute of a-cristobalite, a-tridymite, calc-silicates (wollastonite 2M and diopside) or paragenesis of calc-silicates and a-cristobalite. The presence of microfossils ghosts, detrital grains and sedimentary structures reveal that all these xenoliths are formed by thermal metamorphism of sedimentary rocks, many of them rich in radiolarians. The variable amount of radiolarians and the other components (calcareous microfossils, micrite, mudstones) determine the formation of silica minerals or
calc-silicates during the thermal transformations. The thermal transformation would have taken place in the contact of the sedimentary rock with an intrusive body or shallow magmatic chamber corresponding to an endogenous stage of the 1730-1736 eruption. The high temperatures reached in this process, together with the mineralogical composition of the siliceous sedimentary rock, rich in opal-CT (porcellanite), determine the formation of tridymite and cristobalite instead of quartz. The information obtained from the study of the transformed sedimentary xenoliths also allows us the quantification of depth and temperature of the magmatic chamber, which coincide with those obtained by other methods.

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 Cooling history of basalts and its effects on the kinetics of olivine crystallization

 P. Armienti 1, M. T. Pareschi 2

 1. Dipartimento di Scienze della Terra, via S. Maria 53, 56126 Pisa, Italy

2. Centro di Studio per la Geologia Strutturale e Dinamica dell´Appennino, CNR, via S. Maria 53, 56126 Pisa, Italy

 

Abstract

The population densities of olivine in primitive basic lavas of Lanzarote display different trends originated during definite steps of the cooling history of the magmas. The growth of olivines in these systems is consistent with a model of diffusion controlled growth at variable degrees of undercooling; nucleation process has been described as controlled by heterogeneous mechanisms. The approach adopted to model olivine nucleation and growth allows to evaluate the times involved in magma migration to the surface and to estimate minimum times of storage of melts in deep reservoirs before the rise.

 

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Hydrothermal equilibria in the active Mofete geothermal system, Phlegraean Fields, Naples, Italy

 S. Balducci 1, W. Chelini 2

 1. AQUATER S.p.a., R & D Department, 1-61047, San Lorenzo in Campo, Pesaro, Italy 

2. AGIP S.p.a., Geologia del Sottosuolo, 20097 San Donato Milanese, Italy 

 

Abstract

The study of authigenic minerals and of geothermal fluids from Mofete hydrothermal system allows the evaluation of fluid-mineral equilibria at depth. The application of accurate speciation-solubility codes (Wolery, 1979) to the reconstructed geothermal fluid and of activity-composition models to the authigenic condensed phases leads to the following conclusions:
1) the geothermal fluid is internally buffered by the authigenic minerals and reaches local equilibrium at all investigated depths;
2) the equilibration temperatures deduced by activity of dissolved species in the fluid are coincident with the values observed in situ;
3) the recharge fluid is most probably marine-water already modified in its composition by the reaction trend envisaged by Bischofi and Seyfried (1978);
4) the composition of the fluid at several depths within the geothermal system indicates progressive approach to local equilibrium, through continuous exchanges with wall rock phases. The process can be conveniently described by a set of concomitant irreversible exchange reactions at different rates.

 

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Reconstruction and numerical simulation of the lahar of the 1877 eruption

of Cotopaxi volcano (Ecuador)

 F. Barberi 1, P. Caruso 1, G. Macedonio 2, M. T. Pareschi 2, M. Rosi 1 

1. Dipartimento di Scienze della Terra, Università di Pisa, via S. Maria 53, 56126 Pisa, Italy

2. Centro di Studio per la Geologia Strutturale e Dinamica dell´Appennino, C.N.R., via S. Maria 53, 56126 Pisa, Italy

 

Abstract

The lahar generated by snow and ice melting during the 1877 Cotopaxi explosive eruption has been simulated up to a distance of about 140 km south of the volcano along Rio Cutuchi valley. A simple numerical model assuming homogeneous flow with constant lahar volume (no erosion, no deposition) has been used. The maximum heights actually reached by the 1877 lahar in several points along Rio Cutuchi at increasing distance from the volcano were reproduced by the simulation. Comparison with the 1980-1982 Mt. St. Helens and the 1985 Nevado del Ruiz lahars allowed us to infer the total volume (150*106 m3) of the 1877 Cotopaxi lahar. The volume of a lahar that would be generated nowadays by an 1877-like eruption is estimated to be about 2/3 of that of 1877, due to the corresponding summit glacier surface reduction. Latacunga, a major town located 43 km downstream of the crater, would be struck, one hour and a half after the lahar generation, by a 10 m high lahar wave. In light of the frequency of past events, there is a probability of 0.62 for such an eruption to occur in 1992.

 

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Davyne from Zabargad (St. John´s) Island: peculiar chemical and structural features

 E. Bonaccorsi, S. Merlino and M. Pasero

 Dipartimento di Scienze della Terra, Università di Pisa, via S. Maria 53, 1-56126 Pisa, Italy

 

Abstract

Davyne is a feldspathoid which belongs to the cancrinite-group of minerals. A K-free and Cl-rich variety of davyne from Zabargad (St. John), Island, Red Sea, is described from a chemical and structural point of view. It has space group P63, a = 12.736(2), c =5.336(1) A, chemical formula Na5.5Ca2.5Si6Al6024Cl2.5(SO4). The cages present the characteristic Ca-Cl-Ca-Cl... chains, whereas
the large channels of the structure contain sodium and minor calclum cations, sulfate groups and addltional chlonne anions: the peculiar distribution of the ions in the channels is described and discussed. The large crystals of davyne have been probably formed through the metasomatic action of hypersaline fluids on the metabasalts of the island and were subsequently affected by an altera-
tion process which produced a lot of phases among which the most abundant is a phyllosilicate of "septichlorite"-type.

 

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Peridotites from the Chain Fracture Zone in the Equatorial Atlantic: a preliminary report

 E. Bonatti 1,2,3, D. Daniele 1, A. Peyve 4 

 1. Dipartimento di Scienze della Terra, via S. Maria 53, 56126 Pisa, Italy

2. Istituto di Geologia Marina, CNR, via Zamboni 67, 40127 Bologna, Italy

3. Lamont Doherty Geological Observatory, Palisades, NY, USA

4. Geology Institute, Russian Academy of Sciences, Moscow, CSI

 

Abstract

Serpentinized peridotites were recovered from 14 sites in the Chain transform zone (1°S, in the equatorial Atlantic) during the first PRIMAR (Russian-Italian Mid Atlantic Ridge Project) expedition, and during a 1967 expedition of the research vessel Pillsbury of the University of Miami. Preliminary data on the chemistry of mantle-equilibrated minerals (olivine, ortho-and clino-pyroxene and
spinel) were obtained in a few samples from 6 sites. Spinel 100 Cr/(Cr+Al) shows a wide range of values, from ~ 18 to >50. Al203 in orthopyroxene shows a rough inverse correlation with spinel 100 Cr/(Cr+Al). These variations may reflect in part variations in the degree of melting of the upper mantle and in part secondary metasomatic reactions. These preliminary data suggest in general than the Chain F.Z. upper mantle is more depleted than the mantle underlying the Romanche F.Z. immediately to the north, confirming the existence of broad regional heterogeneities in the composition of the upper mantle beneath the Atlantic.

 

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The Permian intrusive rocks of Serie dei Laghi Western Southern Alps)

 A. Boriani 1, V. Caironi 1, E. Giobbi Origoni 1, R. Vannucci 2

 1. Dipartimento di Scienze della Terra, via Botticelli 23, 20133 Milano, Italy

2. Dipartimento di Scienze della Terra and CNR - Centro di Studio per la Cristallochimica e la Cristallografia, via Bassi 4, 27100  Pavia, Italy

 

Abstract

Small bodies of Lower Permian mafic intrusives (Appinites) and large granite plutons (Graniti dei Laghi) occur at the boundary between the upper crustal Serie dei Laghi and the lower crustal Ivrea Verbano Zone, in the western Southern Alps (Northern Italy). Published and new petrographic and geochemical data on many intrusive bodies are illustrated and discussed. The proposed genetic
model envisages a genetic relationship for mafic and acidic intrusives in the Ivrea mafic magma, which evolved through: fractional crystallization, assimilation of lower crust and mixing with anatectic melts, tapping of the deep magma chamber at different stages. Assimilation of upper crustal rocks and crystal/liquid fractionation at shallow depth seem to have played a minor role. The space relationship between intrusions and regional tectonics suggests an important role of Lower Permian wrench faulting in their ongin, evolution and emplacement.

 

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Kinetical modelling of exsolution textures in igneous pyroxenes

 E. Brizi 1, M. Mellini 2

 
1. Dipartimento di Scienze della Terra, piazza Università, 06100 Perugia, Italy2. Dipartimento di Scienze della Terra, via delle Cerchia 3, 53100 Siena, Italy

 

Abstract

Exsolution microstructures have been studied, for a few selected igneous pyroxenes, by transmission electron microscopy (TEM) Textures range from ill-defined fluctuating decomposition products up to coarse lamellar intermixing. Minimal crystallization temperatures as well as maximal exsolution temperatures were estimated from analytical electron microscopy data. The growth process was kinetically modelled starting from the available experimental constants, modified to take account of the variable pyroxene composition by calculating the free energy contents for the different phases, expressed within a reduced two-components system. After calculation of the transformation, TTT diagrams were plotted and time was defined based upon the known exsolution temperature. Isothermal cooling data were transformed to continuous cooling rates, that appear to increase from achondritic and terrestrial subvolcanic pyroxenes (40-50°C per year) to volcanic pyroxenes (1400°C per year). When data from the same rock are compared, progressively decreasing cooling rates are observed moving from exsolution to intracrystalline site-partitioning, therefore, the geospeedometric combination of events with different closure temperatures may possibly provide a sharper definition of
past thermal regimes.

 

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Helicopter borne COSPEC SO2 flux measurement

 T. Caltabiano 1, G. Guiduzzi 2, S. Leuzzi 2, R. Romano 1 

 1. International Institute of Volcanology, C.N.R., piazza Roma 2, 95123 Catania, Italy

2. Italian Navy, Maristaeli, Catania, ltaly

 

Abstract

A COSPEC type correlation spectrometer has been used to measure the sulfur dioxide flux (SO2) emitted from active volcanoes by performing a planar scan (traverse) of the area covered by the flux. This study describes two helicopter borne measuring methods:
1) using the COSPEC in such a way that its field of view intersects the rotor blade plane,
2) with the field of view in oblique direction during a particular flight trajectory.
Physical and computation problems encountered during flux determination are underlined, especially those regarding the second method. Therefore, the type of volcano, its orography and estimated emission rate should indicate the most suitable measuring method.

 

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Gas geobarometry in boiling hydrothermal systems: a possible tool to evaluate the hazard of hydrothermal explosions

 G. Chiodini 1, R. Cioni 2, M. Guidi 3, L. Marini 3, B. Raco 2, G. Taddeucci 2 

 1. Dipartimento di Scienze della Terra, Università di Perugia, piazza dell´Università , 06100 Perugia, Italy

2. Istituto di Geocronologia e Geochimica Isotopica, CNR, via Cardinale Maffi 36, 56127 Pisa, Italy

3. Geotermica Italiana srl, lungarno Mediceo 16, 56127 Pisa, Italy

 

Abstract

The gas geobarometric methodology based on CO, H2, CH4, CO2 and H2O contents seems to be an effective tool to evaluate the hazard of hydrothermal explosions, as indicated by its application to the fumaroles of Guagua Pichincha crater (Ecuador), Vulcano beach (Aeolian Islands, Italy) and Solfatara crater (Phlegraean Fields, Italy), all fed by boiling hydrothermal aquifers. At Guagua Pichincha crater an increase in fluid pressure sufficient to break the overburden was observed three months before the onset of the hydrothermal eruption of April-June 1990. After the explosive event which started this hydrothermal eruption, fluid pressure decreased at lower values, likely close to hydrostatic pressure. At Vulcano beach remarkable increment in fluid pressure was recorded in 1985. However fluid pressure remained below the local pressure threshold for hydrothermal explosion. This pressure change was probably caused by a larger input of deep hot fluids in the hydrothermal aquifer. A concurrent change in composition, suggesting a larger contribution of deep fluids, was observed in crater fumaroles. During the 1982-1984 bradyseismic crisis at Phlegraean Fields, fluid pressure decreased shortly after the beginning of anomalous seismicity. Opening of shallow fractures induced by seismicity and/or ground uplift is likely the cause of such depressurization. At that time the risk of hydrothermal explosions was therefore low.

 

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Dynamics of the A.D. 79 eruption: stratigraphic, sedimentological and geochemical data on the successions from the Somma-Vesuvius southern and eastern sectors

 R. Cioni, P. Marianelli, A. Sbrana

 Dipartimento di Scienze della Terra, via S. Maria 53, 56126 Pisa, Italy

 

Abstract

Several stratigraphic sections from the eastern and southern sectors of Somma-Vesuvius were studied in order to reconstruct the eruptive dynamics of the Plinian phase and of the subsequent phreatomagmatic phase. Eight eruption units have been defined, each  related to deposits of a single eruptive pulse or of an eruptive phase whose depositional mechanism can be unequivocally defined. Some pyroclastic flow deposits have been singled out, and their role in the eruptive dynamics has been defined. The pumice fallout of the Plinian phase shows grain-size vertical variations which correlate well between the proximal and medial sections and could be related to non-steady processes during emission and/or deposition. The white-grey pumice transition corresponds to the emplacement of a thin pyroclastic flow unit dispersed only in the more proximal sites, with an intermediate composition between white and grey pumice end member. According to Sigurdsson et al. (1985) at least four episodes of partial column collapse occur during the grey pumice Plinian fallout phase. The compositional variations in the fallout deposit parallel the grain-size vertical changes and can be explained in terms of the withdrawal dynamics from a stratified, compositionally zoned magma chamber. The starting of the phreatomagmatic phase is everywhere marked by the deposits of a highly turbulent, highly diluted pyroclastic flow, showing a very peculiar bedset similar to that described for the 1902 nuèes ardentes of Mt. Pelèe. This pyroclastic flow could be related to the total collapse of a phreatomagmatic cloud following a vanishing Plinian phase. Evidences of a caldera collapse phase are shown by some lithic-enriched debris flow deposits at the outlets of the main paleovalleys. A second turbulent, phreatomagmatic pyroclastic flow with more mafic, phono-tephritic scoriae follows the caldera collapse and precedes the final, "wet" phase, where residuals, more evolved magma bodies are tapped from the collapsed magma chamber.

 

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Nicolaus Steno: Geologiae fundator

 C. Cipriani

 Museo di Storia Naturale dell´Università di Firenze, sez. di Mineralogia, via La Pira 4, 50121 Firenze, Italy

 

Abstract

The main events of Niels Stensen´s life, his scientific work and his apostolic ministry are outlined. A particular emphasis is given to his activity in earth sciences, which was so important that Stensen may be called the founder of Geology.

 

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Mantle domains beneath the Red Sea region: inferences from isotopic and geochemical characteristics of Oligo-Miocene magmatism in Yemen 

 L. Civetta 1, S. Conticelli 2, M. D´Antonio 1, L. La Volpe 3, P. Manetti 2, G. Poli 4

 1. Dipartimento di Geofisica e Vulcanologia, largo San Marcellino 10, 1-80138 Napoli, Italy

2. Dipartimento di Scienze della Terra, via La Pira 4, 1-50121 Firenze, Italy

3. Dipartimento Geomineralogico, via E. Orabona 4, 1-70124 Bari, Italy

4. Dipartimento di Scienze della Terra, piazza Università, 1-06100 Perugia, Italy

 

Abstract

Two main groups of basaltic rocks with slightly different chemical and petrological characteristics have been recognized among the Oligo-Miocene volcanics (30-18 Ma) of northern Yemen. The first group includes basalts from different parts of northern Yemen with low incompatible element abundances and moderate TiO2 (1-3%) contents. To this group belong all the studied transitional and alkali basalts from the Sa´dah area, north of San´a (Northern Yemen Trap Series), the mafic dykes outcropping in the Yemen coastal plain (Tihama), together with some basalts from the region south of Sumara (Southern Yemen Trap Series). The second group is made by transitional basalts, characterized by high TiO2 (>4%) and incompatible element abundances from the central and southern portions of the Yemen Trap Series.  The geochemical and isotopic data evidenced that the volcanics from the central and southern parts of the Yemen plateau were generated mostly by extensive AFC process, involving a Pan-African type crust; the degree of contamination increases southwards.
Although, the less differentiated volcanics (SiO2<52%) still show evidence of crustal contamination, the basic ones, with (87Sr/86Sr)i values lower than 0.7038 allow the recognition of two different geochemical reservoirs in the evolution of their mantle source. The source of high-Ti transitional basalts was characterized by the presence of a within plate type geochemical component. Conversely, the source of Tihama basaltic dykes has a strong MORB signature, tending to the Red Sea Type MORB end member.

 

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Steady state magma supply in the last 3400 years of Vesuvius activity

 L. Civetta 1, R. Santacroce 2 

 1. Dipartimento di Geofisica e Vulcanologia, Università di Napoli, largo San Marcellino 10, 80138 Napoli, ltaly

2. Dipartimento di Scienze della Terra, Università di Pisa, via Santa Maria 53, 56126 Pisa, Italy

 

Abstract

Geochemical and Sr-isotopic data are provided on volcanic products erupted in the last 3400 years of the Vesuvius activity. During this interval the volcano was characterized by periods of open conduit conditions [persistent strombolian activity, frequent lava effusions and sporadic more voluminous phreatomagmatic "Final Eruptions"; tephritic to phonolitic tephritic erupted magmas] alternating with long rest periods (obstructed conduit) interrupted by powerful Plinian and Subplinian eruptions (phonolitic to tephritic phonolitic magmas). Isotopically different magmas were involved in each eruption whatever its size but "extreme" 87Sr/86Sr values (0.7072 and 0.7081) characterize few tephritic samples only and have been never measured in mildiy and highly evolved Plinian and Subplinian magmas. Most of Vesuvius magmas erupted in either open conduit or Plinian-Subplinian conditions are the result of repeated magma mixing and fractionation processes occurred in shallow magma reservoirs. The depth of the reservoir controls its size and recharge-time as well as the evolution degree of the stored magma (all increasing with depth). Trace of volatile-poor, crystal-rich residua of magma chamber processes left in the reservoir have been constantly recognized in the younger products of the following eruption emptying reservoirs of similar depth: the first erupted pyroclastic deposits of a Plinian eruption inherit in fact the geochemical and isotopic inprint of the last erupted products of the preceding Plinian event; and the same occurs during the "Final Eruptions" in open conduit conditions. No differences are apparent between the magma supply modalities of Vesuvius in open and obstructed conduit conditions, a part the depth and size of the magma reservoir. All data are coherent in indicating that Vesuvius was fed in the last 3400 years through the repeated arrival of relatively small (10-15x109 kg) tephritic magma batches ("feeding units"). whose examples were recognized among products of 1906 and 472 eruptions.

 

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Coexistence de "verres" alcalins et tholèiitiques saturès en CO2 dans les olivines des hyaloclastites d´Aci Castello (Etna, Sicile, Italie). Arguments en faveur d´un manteau anormal et d´un rèservoir profond

 R. Clocchiatti 1, J. Weisz 1, M. Mosbah 1, J. C. Tanguy 2

1. Groupe des Sciences de la Terre, Laboratoire Pierre Sue CEA-CNRS, CE Saclay, 91191 Gif sur Yvette Cdex, France

2. Universitè Paris Vl et C.N.R.S., 4, Avenue de Neptune, 94100 Saint Maur des Fossès, France

 

Abstract

Aci Castello hyaloclastites, with theoleiitic affinity, are the oldest and primitive magmas of the Pre-Etnean edifice. They took place during a submarine eruption of Surtseyan type. Olivine crystals (Fo87-80) contain dense CO2 fluid inclusions (dmax = 0.88) and melt inclusions of both alkali basalt and tholeiite compositions. Tholeiitic glass inclusions are the most abundant and are characterized by high contents of dissolved volatiles (K2O = 1.7% ; S = 1400-900 ppm; Cl = 1100-700 ppm). Volatile contents in the alkali basaltic inclusions are the same that in the recent alkali hawaite basalts (H2O = 2.7% ; S = 3800-2200 ppm; Cl = 4000-1800 ppm). CO2 contained both in dense fluid inclusions and in the shrinkage bubble of melt inclusions evidences the over saturated character of the liquids. With the measured densities, the PCO2min is estimated to be close of 700 MPa. Consequently, 24 km is estimated as the minimal depth of CO2 entrapment; this depth is consistent with the supposed magma chamber. Variations of composition agree with the behaviour of the major elements during a partial melting process. We can adopt two interpretations to explain the two liquids
coexistence:
1) melt inclusions have recorded a mixing process between an alkaline and tholeiitic liquid in the deep magma chamber,
2) melt inclusions have recorded "in situ" peridotite fusion process.
The exceptionally high volatile contents in these two types of inclusions compared with inclusions contained in olivines from basalts and oceanic tholeiites, more the CO2 oversaturation seem characters inherited from the mantle source. These data are perfecly consistent with the previous works. The unusual character of the Etnean lava source is underlined.

 

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Soil degassing in tectonically active areas of Mt. Etna

 W. D´Alessandro 1, R. De Domenico 1, F. Parello 2, M. Valenza 2

1. Istituto di Geochimica dei Fluidi, CNR, via Torino 27/D, 90I33 Palermo, Italy

2. Istituto di Mineralogia Petrografia e Geochimica, via Archirafi 36, 90123 Palermo, Italy 

 

Abstract

An evaluation of the soil gas distribution around the main faults of Mt. Etna was performed. The primary objective of this study was to identify, by geochemical techniques, active faults through soil degassing. Soil gas samples were collected in several profiles across the main faults systems (Pernicana fault, Fondo Macchia area, S. Venerina area, Linera and Acireale areas, S. Maria di Licodia area, Bronte area). Gas measurements were performed for CO2, H2, CO, CH4 and Rn. The results obtained seems to confirm that the eastern area of the volcano, characterized by high seismic activity, represents also the principal degassing area. Chemical concentration values and isotopic data (d3C) of CO2 point out, at least for some areas, a deep origin for this gas.

 

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Volcanic activity and drainage network evolution of the Alban Hills area (Rome, Italy)

 D. De Rita 1, R. Funiciello 1, C. Rosa 2

 1. Dipartimento di Scienze della Terra, Università degli Studi di Roma "La Sapienza", p.le Aldo Moro 5, 00185 Roma, Italy

2. Via Goffredo Ciaralli 102, 00156 Roma, Italy

 

Abstract

In this paper we investigate how the emplacement of large pyroclastic flows modified the drainage network of the Alban Hills area (25 km South of Rome, Central Italy) during the Middle Pleistocene. We focus our attention on the explosive events related to the first phase of activity of the Alban Hills volcanic complex (Tuscolano-Artemisio (T.A.) phase, 700-338 ka), during which about 98% of the total volume of pyroclastic and lava products is believed to have been emplaced. The major component of this study is the analysis of the logs of wells drilled mainly for water supply purposes and distributed over the whole Alban Hills area. Such data allowed us to draw structural maps such as the structural map of the base of the volcanic products and the map of the underlying sediments lithofacies, the isopach map of the volcanic products and the isopach and structural maps of the base of the main pyroclastic flow units. By the comparison of these mapc we could conclude that, when the volcanic activity started, the Alban Hills drainage network was already strongly delineated by extension related to the sinking of the Tyrrhenian Sea margin. This primitive drainage network included a northern drainage basin (Sabatini and Sabina areas) and a southern one (Valle Latina). Three main river valleys may tentatively be identified as ancestral to the present drainage network: the PaleoTiber, the PaleoAniene and the PaleoSacco rivers. Activity in the Alban Hills volcanic complex caused strong alterations to this primitive drainage network through emplacement of a large volume of lava and pyroclastic flows. The alterations are mainly morphological and include reversal of river flow directions and shifting of rivers and their valleys. The strongest changes resulted from emplacement of the II T.A. pyroclastic flow unit (34 km3 of erupted products, De Rita et al., 1988b), but sin and post volcanic tectonism has had considerable local influence as well. Several features suggest that tectonic activity continued during and after volcanism and influenced the geomorphic evolution of the area.

 

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A petrogenetic model for the coexistence of alkali basalts and tholeiites in the Plio-Pleistocene volcanism of Iblean plateau (South-East Sicily, Italy)

 R. De Rosa, R. Mazzuoli, T. Trua

 1. Dipartimento di Scienze della Terra, Università della Calabria, Arcavacata di Rende, Cosenza, Italy

 

Abstract

During the Plio-Pleistocene within the Iblean area the volcanism occurs close to the suture zone of African and Eurasian plates along fissure systems oriented NNE-SSW due to a wrenching phase which affected all the area during the Middle-Pliocene. During this period the volcanism assumes a dichotomic character and together with basanites and alkali basalts (LKG basalts), which characterized the Iblean volcanism since Cretaceous time, tholeiites and alkali basalts with relatively higher K content (HKG basalts) became dominant. We suggest that a complex interplay of processes has given rise to the broad spectrum of compositions shown by the Plio-Pleistocene basalts of the Iblean plateau. A possible model involves a progressive upwards melting through the upper mantle. The partial melting process is initiated in an enriched upper mantle (as OIB mantle source), under high pressure conditions, low H2O/CO2 where the kaersutitic amphibole remained in the solid phase, in these conditions, low degree of partial melting produced the LKG basalts. In response to variations of the tectonic regime, the partial melting process moves upwards and occurs under lower pressure conditions and higher H2O/CO2 , where the amphibole partecipated to the liquid formation. Depending on the degree of partial melting both tholeiites and HKG basalts are generated.

 

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Valorisation de deux sables cotiers du Sud-Vietnam

F. Dimanche

Universitè de Liège, Laboratoires de Gèologie Appliquèe, Avenue des Tilleuls 45, B 4000 Liège, Belgique

 

Abstract

The paper is concerned with two sandy ores from Vietnam. The first one is convenient for glass purpose, the second could furnish Zr Ti and RE oxides. The methods for qualitative examination are chemical, optical and granulometric ones. Some genetic comments are proposed.

 

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Geological evolution and metamorphic petrology of the basal sequences

of eastern Elba (complex II)

 S. Duranti 1, R. Palmeri 2, P.C. Pertusati 1, C.A. Ricci 2 

 1. Dipartimento Scienze della Terra, via S. Maria 53, 56126, Pisa, Italy

2. Dipartimento Scienze della Terra, via delle Cerchia 3, 53100, Siena, Italy

 

Abstract

Elba island is a fragment of the northern Apenninic chain and is characterized by the superposition of several tectonic units belonging to different paleogeographic domains with different styles and metamorphic histories. The study of the structural and metamorphic evolution of the rocks belonging to Complex II (according to Trevisan, 1950) provided the following results. a) Five lithostratigraphic units were distinguished: Capo Arco Schist, Ortano Porphyroid, Ortano Marble, Punta dell´ Acquadolce Carbonate-Schist, Rio Marina Serpentinite. b) A carbonatic layer containing radiolaria, calpionella and foraminifera within the Punta dell´Acquadolce Carbonate-Schist, enabled this unit to be dated to the upper part of the lower Cretaceous and to be attributed by lithological analogy to the "Palombini shales" formation of the Ligurian realm. c) Three tectonic units of different  paleogeographic domains characterize the study area. The first and lowest is a portion of Hercynian basement, the second a belt of marble of Tuscan affinity, and the third a sequence of Ligurian affinity. d) There is evidence of three deformation phases D1, D2, D3. The first (D1) is the most important regional deformation phase, while D2 and D3 are only of local significance and probably linked to the emplacement of Mt. Capanne granodiorite (6.2 Ma) and Porto Azzurro monzogranite (5.1 Ma), respectively. e) A low-grade metamorphism is associated with D1 and D2. The main foliation S2 is characterized by the synkinematic growth of index minerals like Ms + Chl; similar metamorphic conditions can be reconstructed for D1 on the basis of the mineral assemblage of the microlithons within S2, representing the previous regional foliation S1. f) A postkinematic contact metamorphism developed because of the emplacement of the Porto Azzurro monzogranite. This metamorphism was studied in pelitic, carbonatic and ultramafic rocks. It ranges from greenschist facies to amphibolite facies and increases from NNE to SSW. Comparisons of the critical assemblages in the different
rocks indicate conditions ranging from 300°C to 600-650°C and pressures around 2kbar.

 

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Morphological, chemical and structural study of robinsonite  (Pb4Sb6S13)

from Alpi Apuane, Italy

 M. Franzini, P. Orlandi, M. Pasero

 Dipartimento di Scienze della Terra, Università di Pisa, via S. Maria 53, 56126 Pisa, Italy

 

Abstract

The first Italian occurrence of the rare lead-antimony sulfosalt robinsonite, Pb4Sb6S13, is reported from the Pitone marble quarry Apuan Alps, Tuscany. Robinsonite is monoclinic, space group I2/m, a = 23.637(7) A, b = 3.969(1), c = 24.406(7), b = 93 73(3)°. A morphological study, based on 45 indexed faces, led to the following parametric ratios: a:b:c = 5.821:1:6.018, with a b value of 93.70°. A microprobe analysis indicated for robinsonite a chemical composition very close to the ideal stoichiometry, with only minor substitution of Sb by As, resulting in the formula Pb4Sb5.69As0.31S13. The X-ray single crystal structural study of robinsonite (R = 0.065, Rw = 0.061 for 1753 reflections) confirmed the isostructurality of robinsonite with its synthetic analogous. Pb and Sb are fundamentally coordinated by 5 S in a square pyramidal configuration; pyramids share faces to give rise to cross-linked ribbons with different width (either 4 or 12 pyramids) which develop along b.

 

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Geothermal exploration in the west of the Nestos delta (Northern Greece) 

 M. Fytikas 1, N. Kolios 2

 1. Aristotele University, Department of Geology, Thessaloniki, Greece

2. Institute of Geology and Mineral Exploration, Messoghion 70, Athens, Greece

 

Abstract

A very interesting and extensive geothermal area was defined to the west of the Nestos River, having high thermal gradients (3 - 4 times than normal). In this area there is (a) a good secondary hot (70°C) reservoir 600m deep m neogenic sediments and (b) a probable medium enthalpy reservoir at greater depths. The water of the first reservoir contauns NaCl due to the formahon characteristics (brackish sediments). It is possible that the main reservoir has better water quality than the first.

 

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Genesis and classification of the rocks of the Tuscan Magmatic Province: thirty years after Marinelli´s model 

 F. Innocenti 1,2, G. Serri 1,2, G. Ferrara 1,5, P. Manetti 3,4, S. Tonarini 5

1. Dipartimento di Scienze della Terra, Università di Pisa, via S. Maria 53, 56126 Pisa, Italy

2. Centro di studio per la Geologia Strutturale e Dinamica dell´Appennino, CNR, via S. Maria 53, 56126 Pisa, Italy

3. Dipartimento di Scienze della Terra, Università di Firenze, via La Pira 4, 50121 Firenze, Italy

4. Centro di studio per la Mineralogia e Geochimica dei Sedimenti, via La Pira 4, 50121 Firenze, Italy

5. Istituto di Geocronologia e Geochimica Isotopica, CNR, via Cardinale Maffi 36, 56127 Pisa, Italy

 

Abstract

In addition to the Marinelli´s proposal, the ´Provincia Magmatica Toscana (PMT), as defined here, includes also the Sisco sill, the volcanic rocks of the Capraia Island, the Vercelli seamount and the buried plutons of the northern Tyrrhenian sea. This definition does not imply any genetical link, nevertheless the use of the term PMT is still recommended only in the meaning of a temporal and spatial association, because of a wide variability of petrogenetic affinities, including crustal - and mantle-derived magmas has been recognized in this province. The igneous activity of the northern Apennine comprises the PMT and ´Provincia Magmatica Romana´ (PMR) and took place in four phases separated in space and time, progressively younger from west to east: Phase I, ~ 14 Ma (Sisco-Corsica); Phase II, 7.3-6.0 Ma (Monte Capanne, Montecristo, Vercelli, Capraia I); Phase III, 5.1-2.2 Ma (Porto Azzurro Giglio, Campiglia, Gavorrano, Castel di Pietra, Monteverdi, San Vincenzo, Orciatico, Montecatini Val di Cecina, Capraia II, Roccastrada and Tolfa district); Phase IV, 1.3-0.1 Ma (Radicofani, Cimini, Torre Alfina, Amiata, as well as the NW PMR). Most of the acidic, dominantly crust-derived plutonic and volcanic rocks of the Tuscan region show the evidence of mixing with K-rich subcrustal magmas. Geochemical data on primitive rocks (Mg#>65) show the existence of two groups of mantle-derived magmas. The rocks of the group I (ol-hy and Q-normative, mostly lamproites, ultrapotassic high-Mg latites and shoshonites: saturated trend) are considered to be derived by partial melting at low pressure (<50 km) of strongly (lamproites) to moderately depleted phlogopite harzburgite sources produced by reaction of residual peridotites with a potassic-silica-rich melt with high 87Sr/86Sr (~0.717) and Ce/Sr (>0.3), and low 143Nd/144Nd (~0.5121-0.5120) and Ba/La (<20) values formed by partial melting of subducted carbonate-free material of the upper crustal reservoir (e.g., non-restitic felsic granulites). We propose that the rocks of the saturated trend are formed by partial melting of the mechanical boundary of the Tuscan continental lithosphere (MBL). The rocks of the group II are critically undersaturated, mostly leucitites, tephritic leucitites, leucite basanites, melilitites (undersaturated trend); trace element modelling indicates that three components were involved in the genesis of their mantle source: A) an OIB-like mantle, B) a component with very high Sr, Ca and Sr/Ce values, probably carried by a melt related to subducted carbonates; C) a recently added K-rich crustal component, relatively well constrained to high 87Sr/86SrSr (>0.712), low 143Nd/144Nd (<0.5120) and Ba/La (<20) and Ce/Sr (~0.10 . 0.25).   We propose a geodynamic model grounded on these petrological and geochronological constraints. The northern Apennine is considered a trench/arc/back-arc system enterely developed in an intra-continental collisional setting; in the model, the driving mechanism of this orogenesis is the delamination and subduction of the Adriatic continental lithosphere. As this process is still ongoing, the igneous activity of the northern Apennine should not be considered extinct.

 

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Radiometric ages and chemical characterization

of the Galatean andesite massif, Pontus, Turkey

 J. Keller 1, D. Jung 2, F.-J. Eckhardt 3, H. Kreuzer 3

1. Mineralogisch-Petrographisches Institut der Universitat, Freiburg, Germany

2. Mineralogisch-Petrographisches Institut der Universitat, Hamburg, Germany

3. Bundesanstalt fur Geowissenschaften und Rohstoffe, Hannover, Germany

 

Abstract

The Galatean Volcanic Massif (GVM) north of Ankara is a dominantly Neogene andesite-dacite complex which developed with the closure of the Tethys ocean in the collision zone between the African plate in the south and Eurasia in the north. The calc-alkaline andesite-dacite association is accompanied by rhyolites and by undersaturated alkaline olivine basalts. In general lines, the rhyolites preceed the andesite-dacites, whereas the basalts occur late in the evolution as capping series. Lithospheric thickening and consumption resulted first in crustal anatexis, later in extensive mobilisation of andesitic melts in the zone of lithosphere subduction, and produced alkaline basalts from deeper mantle levels in the final stages. K-Ar dating, combined with fission-track age determinations on the same samples, places the rhyolites in the uppermost Oligocene. All analyzed andesites and dacites fall in the age range of 20-18 Ma. Alkaline olivine-basalts are the youngest members of the complex (11.6-9.5 Ma). Geochemically very similar basalts already occur earlier in the evolution at around 20 Ma, thus contemporaneous with the calc-alkaline suite. Separated from the dominantly Miocene- Pliocene activity of the GVM, is the earlier Galatean arc evolution for which one date of 73.2±1.2 Ma has been obtained. The presented radiometric data underline the geological distinction of two maior volcanic phases in this area of continental collision.

 

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Indicators of very-low grade metamorphism in metapelites from Bracco/Val Graveglia Unit (Ligurian Apennines, Northern Italy) and their relationships with deformation

 L. Leoni, M. Marroni, F. Sartori, M. Tamponi

 Dipartimento di Scienze della Terra, via S. Maria 53, 56126 Pisa, Italy

 

Abstract

Correlation between indicators of very-low grade metamorphism and deformation in a mega-structure of the Ligurian Apennines (Northern Italy) has been examined. Illite "crystallinity", polytypism and bo as well as chlorite "crystallinity" have been taken into account. Illite "crystallinity" values (Kubler index) ranging from 0.30° to 0.58° (D2Q) indicate diagenesis - anchizone boundary conditions. The value of this parameter averaged over samples from normal limb of the fold results significantly higher than the corresponding value for samples from inverted, more deformed limb of the structure. By contrast, no systematic variation of illite "crystallinity" within the limbs from the root to the hinge zone has been recorded. Also, no correlation between illite "crystallinity" and sample position in meso-folds has been observed. This would point to a close correlation between illite "crystallinity" and deformation on a large scale, at least in the diagenesis/anchizone boundary zone. The amount of 2M1 polytype (average value about 70%) and the mica bo parameter do not seem to be related to structural elements of the mega-structure. Chlorite "crystallinity" values range from 0.15° to 0.32° (D2Q). The average values of this parameter are substantially the same in the two limbs of the mega-fold.

 

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A unitary model for the long-term dynamics of Campi Flegrei

G. Luongo 1,2, E. Cubellis 2,3, S. M. Petrazzuoli 2

 1. Dipartimento Geofisica e Vulcanologia, largo S. Marcellino 10, 80138 Univ. Federico 11, Napoli, Italy

2. Osservatorio Vesuviano, via Manzoni 249, 80123, Napoli, Italy

3. C.N.R., Gruppo Nazionale Vulcanologia, via Nizza 128, 00198 Roma, Italy

 

Abstract

Our model of the formation of the Campi Flegrei caldera and its associated dynamics has been obtained by making use of both an analytical method and a numerical one. The formation of the caldera is fitted into a more extensive geodynamic process that affected the whole of the Campanian Plain. The data used for a reconstruction of the whole process (mantle upwelling, formation of the Campanian Plain´s graben, Phlegraean volcanism and formation of a caldera/calderas, recent dynamics of the Campi Flegrei and surrounding areas) concern crustal structures (viz. Moho depth, tectonics, etc.), volcanic history, evolution of bradyseism. The sequence of events recorded in the Campi Flegrei over the last 40.000 years shows a decrease in the size of eruptive phenomena and crustal deformations with time. The data from geophysical surveys show a crustal shortening in the Neapolitan volcanic area, its maximum values being found in the Phlegraean area. The data from geological surveys show a distension process affecting the Tyrrhenian margin of the Campanian Plain as well as a slow subsidence resulting in high thicknesses of sediments characterized by the same depositional facies. Based on such data, a unitary model that relates volcanism to regional dynamics has been worked out.
A numerical analysis carried out by the finite-element method made it possible to construct a model of the crust fracturation mechanism and work out the deformation and stress field associated with it. The formation of the caldera is obtained for a thrust value of about 4 kbar associated with a thrust source located at a depth of 7.5 km with a radius of about 4-S km. The expected uplift amounts to some hundred meters and a regional area is involved in the deformation process in agreement with the analytical model previously obtained. Summing up, the two models provide information about the size and depth of the source, the size and shape of the caldera associated with the process hypothesized, the size of the surrounding area involved and the acting stress fields together with associated deformations.

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Isotopic and chemical composition of water, steam and gas samples of the natural manifestations of the island of Vulcano (Aeolian Arc, Italy)

 C. Panichi, P. Noto

 CNR, Istituto Internazionale per le Ricerche Geotermiche, piazza Solferino 2, 56100 Pisa, Italy

 

Abstract

The tritium, D, 180, Cl and SO 4 contents of the water samples delivered from shallow wells describe an underground water system that is subjected to an inflow of volcanic fluid in limited parts of the thermal area of Porto Levante. The main aquifer of the area appears to be a meteoric water body with a residence time of about 50 years. The isotopic composition of the steam collected at fumaroles F1 and F5 of the crater was recorded from 1978 to 1990, with a sampling frequency of about twice a year. Additional data, relative to the fumaroles F5HT, FO, F11, FA, FF and FB, were collected during 1989-1990. The overall isotopic variation ranges from +3.18 to +7.60 () and from - 14.0 to + 14.6 () in the d180 and dD values respectively. Changes seem to be correlated with variations in the thermal and hydraulic conditions of the volcanic system, as a consequence of a local increase of permeability caused by earthquakes in the area after 1978. The deuterium and oxygen-18 variations observed in the steam of F1 and F5 fumaroles before 1988 are ascribed to mixing processes involving a local "andesitic" water. After 1988 the temperature of the volcanic system increase up to 650°C, and the isotopic composition of the steam assumed dD values coincident with that of local seawater, while the d180 where enriched by 3.4 to 7.6 () with respect to seawater. The very high temperature steam is tritium-free (FF fumarole, 1989 = 0.2±0.4 T.U.), while the vapor delivered at lower temperatures contains appreciable amounts of tritium (F1-1986 contained 1.3±1.0 and F5-1987 contained 0.6±1.2 T.U.). Of the gas mixtures collected at the crater and the beach at Porto di Levante, only H2 shows significative variations on the isotopic composition. At the crater d values are -200±30 (), while at the beach it is almost constant at-575±15 (O). The CH4 values are instead homogeneous in both areas (d13C = -6.3±1 (O) and dD = -110+15 (O)). Because of the variation in the d13C value of carbon dioxide (from -0.7 to +4.1 (0)), and the observed dD values for the steam and thermal waters, the CH4-H2 and H2O-H2 pairs give isotopic temperatures slightly higher than 100°C for the hydrothermal system of Porto Levante and from 600 to 800°C for the volcanic crater. On the contrary, the 13C /12C ratios of CH4 remains frozen at the value corresponding to equilibration with associated CO2 at the temperature of a magmatic environment.

 

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Ligurian peridotites and ophiolites: from rift to ocean formation in the Jurassic Ligure-Piemontese basin

 G.B. Piccardo 1, E. Rampone 1, R. Vannucci 2

1. Dipartimento di Scienze della Terra, Università di Genova, corso Europa 26, 1-16132 Genova, Italy

2. Dipartimento di Scienze della Terra, Università di Pavia e CNR-Centro di Studio per la Cristallochimica e la Cristallografia, via Bassi 4, 1-27100 Pavia, Italy

 

Abstract

The results of structural petrologic and geochemical investigations on mantle ultramafics and associated basalts pertaining to the different paleogeographic domains of the Jurassic Ligure-Piemontese Basin (LPB) are discussed in the light of the current ideas on the geotectonic models for continental rift and oceanic inception. Mantle ultramafics from the Ligurian sector (Nothern Apennine and Voltri Massif) of the Alpine - Apennine system can be distinguished in two main groups: 1) fertile cpx-rich Iherzolites from the External Ligurides (EL-Northern Apennine) and the Erro-Tobbio (ET-Voltri Massif) units; 2) refractory cpx-poor peridotites from the Internal Ligurides (IL-Northern Apennine) units. The EL-ET Iherzolites record structural and compositional characters indicating for a composite history consisting of: i) an early super-solidus evolution and depletion by partial melting, presumably occurred during Proterozoic times when the Iherzolites acquired their compositional and isotopic MORB signatures, ii) a cooling stage followed by complete static equilibrium recrystallization to sp-facies assemblages, iii) a diffuse metasomatism by hydrous fluids producing disseminated Ti-rich pargasite, iiii) an almost sub- solidus, non-adiabatic uplift from deep lithospheric levels to the sea-floor, presumably starting from Triassic times.  The IL depleted peridotites solely record compositional features well consistent with residual mantle after MORB-generating fractional melting processes developed on an asthenospheric MORB source which upwelled adiabatically during the rifting stages of the LPB.  The MORB type basalts of the Ligurian sector of the LPB can be derived by variable degrees of fractional melting on an asthenospheric MORB mantle source, quite similar to the parental mantle to the IL peridotites. Early melts (i.e. some basaltic dykes crosscutting EL Iherzolites) possibly derived from lower melting degrees than the late IL basalts of the LPB oceanic lithosphere. The EL (and, most probably, the ET) Iherzolites can be considered as deep mantle material accreted from the asthenosphere to the Proterozoic lithosphere. They actually represent Triassic lithospheric sub-continental mantle evolved towards shallow levels during the rifting stages in the LPB. During continental rifting, the EL-ET Iherzolites followed a sub solidus, non-adiabatic P-T path: it is quite consistent with the thermal history expected for the foot-wall rocks of a lithosphere-scale dipping extensional detachment zone. All the available informations suggest that, similarly to some modern oceanic rifts and slow-spreading oceans, the LPB was characterized by an incipient rifting stage dominated by the passive and asymmetric extension of the lithosphere. This mechanism led to i) the tectonic denudation and sea-floor emplacement of the sub-continental lithospheric mantle, ii) the passive, almost adiabatic upwelling of the underlying asthenosphere. MORB type partial melts were produced by the ascending asthenosphere: they were firstly intruded in the overlying extended lithosphere and, later on, were emplaced at the sea-floor to form new oceanictype crust.

 

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Petrology of lamproitic rocks: evidence, hypotheses and problems 

G. Venturelli, S. Contini

Istituto di Petrografia, Università di Parma, viale delle Science 78, 43100 Parma, Italy

 

Prof. Giorgio Marinelli, who is now retiring, was and is

for us a cultural and human reference-point located

beyond any academic pettiness.

 

Abstract

During the last decade, many petrological and geochemical data have been collected on lamproites and rocks with lamproitic affinity, which, in places, may contain diamonds. This paper is a short review of the petrological problems concerning the genesis and evolution of these rocks. For lamproitic magmas, phase equilibria and melt inclusion thermometry suggest approximate temperatures of crystallization in the range 1250-650°C and largely variable redox conditions (from about IW + 2 log unit to NNO + 1 log unit). It is generally agreed that lamproitic magmas are generated from phlogopite-bearing harzburgite or phlogopite-bearing depleted Iherzolite at variable pressure (high and low pressure for olivine- and leucite-lamproites respectively). Prior to melting, the depleted source underwent enrichment in incompatible elements. The origin of the enriching agents, however, is poorly constrained .

 

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