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
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
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.
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
1979) to the reconstructed geothermal fluid and of activity-composition
models to the authigenic condensed phases leads to the following conclusions:
1. Dipartimento di Scienze della Terra, Università di Pisa, via S. Maria 53, 56126 Pisa, Italy
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.
Davyne is a feldspathoid which belongs to the cancrinite-group of
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
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
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
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
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:
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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
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.
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.
Prof. Giorgio Marinelli, who is now retiring, was and is
for us a cultural and human reference-point located
beyond any academic pettiness.
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 .