ABSTRACT LXXVI, December 2007 n.3:

Abstract - Ultramafic ejecta preserved as xenoliths in 1906 pyroclastic deposits of Mount Vesuvius are essentially clinopyroxene-bearing dunites and clinopyroxenites. Some have been tectonized and exhibit mosaic-equigranular and porphyroclastic textures typical of mantle peridotites. Dunites show a orthocumulate-heterogranular texture consisting of euhedral to subeuhedral olivine (Fo90-88), locally coexisting with greenish pyroxene of diopsidic composition, and interstitial yellowish glass essentially of phono-tephritic composition (Mg# 48-62). Few large crystals of forsteritic olivine may show deformation bands (“strain lamellae”) and twinning as well as decompression textures that reflect a deep environment of formation. Some olivine clots exhibit abundant microinclusions of Cr-spinel (Cr-hercynite and chromite) and deep brownish to clear glass. Primitive liquids included in forsteritic olivine have a trachybasaltic composition (Mg# up to 71-75). Locally, microinclusions of phlogopite are coexisting with glasses of phono-tephritic and basaltic trachy-andesitic composition (Mg# ranging 29-65). A late, acicular, seldom radiating brown-greenish clinopyroxene of salitic composition may form around large olivine crystals and coexists with minute secondary olivine (Fo86-85), plagioclase (An78-76) and rarely leucite.
Thermobarometric estimates obtained by means of a grid of selected reactions indicate that dunitic materials equilibrate with primitive, moderately hydrous trachybasaltic melts (1.5-2.5 wt% H2O) at ~12.4-13 kbar for temperatures ranging 1250-1300°C. Equilibration occurs in proximity and/or below the crust-mantle transition, located at a depth of 30-36 km below Mount Vesuvius. More differentiated tephritic melts will react with the ultramafic nodules and equilibrate at about 3 kbar and 1220-1170°C. These regimes constrain a sector of an upper reservoir (whose top is located at about 9-10 km depth) where deep cumulitic materials are momentarily stored before being erupted during paroxysmal eruptions.

P. COMIN-CHIARAMONTI, C. B. GOMES, A. CUNDARI, F. CASTORINA AND P. CENSI - A review of carbonatitic magmatismin the Paraná-Angola-Namibia (PAN) system.

Abstract-Mesozoic to Cenozoic allkaline-carbonatitic complexes from southern Brazil, Angola and Nambia occur along main tectonic lineaments. In general, the alkaline-carbonatite complexes show intrusive/subintrusive, subcircular or oval shaped structures and are indicative of high upwelling energy. Processes of liquid immiscibility from trachytic-phonolitic liquids, starting from parental alkaline mafic magmas are believed to have generated carbonatitic liquids, as suggested by field relationships and geochemical characteristics.
Ca-, Mg- and Fe-carbonatites are widespread even in the same complex. The occurrences comprise three main chronogroups, i.e. 1) Early Cretaceous (Eastern Paraguay; Brazil, Ponta Grossa Arch and Anitápolis; Angola and Namibia); 2) Late Cretaceous (Brazil, Ponta Grossa Arch, Lages and Alto Paranaíba; Namibia); 3) Paleogene, Brazil and Namibia. Two principal types of associated alkaline rocks are represented, i.e. plagioleucitites l.s. (Eastern Paraguay; Brazil: Ponta Grossa Arch; Angola and Namibia) and kamafugites l.s. (Brazil: Alto Paranaíba and Lages; Namibia). Significant variations in O-C isotope compositions are found in primary carbonates, the variations being mainly due to isotope exchange between carbonates and H₂O-CO₂-rich hydrothermal fluids, whereas magmatic processes, i.e. fractional crystallization or liquid immiscibility, probably affect the d ¹⁸O and d¹³C values by not more than 2d‰. The isotope exchange model implies that the most significant isotopic variations took place in a hydrothermal environment, e.g. in the range 400-80°C, involving fluids with CO₂/H₂O ratio ranging from 0.8 to 1. Sr-Nd-Pb isotope systematics highlight heterogeneous mixtures between HIMU and EMI mantle components, similar to the associated alkaline rocks and the flood tholeiites of the Paraná-Angola-Etendeka (Namibia) system. This is also consistent with Re-Os systematics on selected mafic samples from the Alto Paranaíba alkaline-carbonatite province.The data relative to the noble gases suggest that the source(s) are similar to other mantle derived magmas (e.g. HIMU and MORB) and that the carbon of carbonatites is unlikely to be subduction-related carbon, and support a C-O fractionation model starting from mantle-derived sources. In spite of the strong variation shown by C-O isotopes, Sr-Nd-Pb-Os isotopic systematics could be related to an isotopically enriched source where the chemical heterogeneities reflect a depleted mantle “metasomatized” by small-volume melts and fluids rich in incompatible elements. These fluids are expected to have promoted crystallization in the mantle of K-rich phases that gave rise to a veined network variously enriched in LILE and LREE (cf. Foley, 1992b). The newly formed veins (enriched component) and peridotite matrix (depleted component) underwent a different isotopic evolution with time as reflected by the carbonatitic rocks. These conclusions may be extended to the whole Paraná-Angola-Etendeka system, where isotopically distinct parent magmas were generated following two main enrichment events of the subcontinental lithospheric mantle at 2.0-1.4 and 1.0-0.5 Ga, respectively, as also supported by Re-Os systematics. The mantle sources preserved the isotopic heterogeneities over a long time, suggesting a non-convective lithospheric mantle beneath different cratons or intercratonic regions. Overall the data indicate that the alkaline-carbonatitic magmatism originated from a significant, but small scale heterogeneous subcontinental mantle. In this scenario, the Tristan da Cunha, Walvis Ridge-Rio Grande Rise and Vitória-Trindade hotspot tracks might reflect the accomodation of stresses in the lithosphere during rifting, rather than continuous magmatic activity induced by mantle plumes beneath the moving lithosphere.

R. COMPAGNONI, F. ROLFO, F. MANAVELLA AND F. SALUSSO - Jadeitite in the Monviso meta-ophiolite, Piemonte Zone, Italian western Alps.

Abstract-Following the search of jadeitite in the alluvial deposits at the mouths of the main alpine valleys, a systematic field survey around Monviso, italian western Alps, led to the discovery of a primary jadeitite outcrop. The jadeitite occurs as a tectonic block about 1 m3 in volume near Punta Rasciassa, and is embedded in a serpentinised lherzolite of the basal serpentinite unit of the Monviso meta-ophiolite. On hand specimen, the jadeitite is a very pale grass green, fine-grained massive rock surrounded by a darker retrogression margin and crosscut by coarser grained phlogopite-rich pockets and irregular discontinuous veins. A portion of the boudin consists of a peculiar rock with pegmatoid grain-size. Microscopically, the fine grained jadeitite appears more heterogeneous with clinopyroxene composition ranging from jadeite to omphacite. Three main portions may be distinguished, which consist of clear clinopyroxene, clinopyroxene with accessory rutile, and dusty clinopyroxene, respectively. In all portions the matrix clinopyroxene occurs as aggregates of interlocked stumpy prismatic crystals with sharp compositional zoning, which locally contain larger cloudy clinopyroxene relics. Zircon is a typical ubiquitous accessory mineral. The coarser-grained veins and pockets consist of randomly oriented clinopyroxene idioblasts included in xenoblastic albite or aggregates of phlogopite and Mg-chlorite. In both the rock matrix and the veins a poikiloblastic allanitic epidote locally occurs. The pegmatoid portion consists of Cr-bearing omphacite and chlorite + phlogopite domains. Very rare garnet porphyroblasts locally occur, which contain inclusions of ilmenite and minor apatite.
The occurrence of rutile and the equilibration temperature deduced by the Y content of garnet suggest eclogite-facies conditions. The ubiquitous presence of zircon suggests that the jadeitite derived from a former felsic dyke (plagiogranite?), originally intrusive into upper mantle peridotite, which experienced a significant metasomatism during peridotite serpentinisation.

L. FIORA, M. CARANDO AND R. SANDRONE- Multimedia petrographic guide of the city of Torino, Italy.

Abstract - This paper describes a petrographic guide of Torino, organized as a multimedia CD-ROM. It considers the monuments and historical buildings that are especially interesting or significant as to the stones used and from the historical or artistic point of view. Specific lithologies are described in filig cards which illustrate the geological, petrographical, lithoapplicatory and durability features. The CD-rom also includes a short note on the geological setting of Piedmont stones, a comprehensive bibliography and five geological maps.

A. FORNELLI, F. MICHELETTI AND G. PICCARRETA- The Neoproterozoic-Early Cambrian felsic magmatism in Calabria (Italy):inferences as to the origin and geodynamic setting.

Abstract - Some of the Alpine Units of the nappe pile of Calabria (southern Italy) include blocks of Variscan continental crust. Among these, the Castagna, Sila and Aspromonte Units containing felsic augen gneisses are here considered. These augen gneisses are abundant in the Castagna and Aspromonte Units, whereas are scarce in the lower crust segment of the Sila Unit. Spot U-Pb zircon-dating revealed Latest Precambrian-Early Cambrian emplacement of the magmatic protoliths of the augen gneisses from the three units with Neoproteozoic, Palaeoproterozoic and Archean inheritances. Geochemistry and Sr, Nd isotope signatures indicate the protoliths formed in orogenic setting through mixing of crust-mantle melts. The high-K calc-alkaline to shoshonitic nature of protoliths points to a transition between compressive and extensional tectonic setting or even to a collapsed intracontinental orogen formed by amalgamated Cadomian terranes and Gondwana craton. The coeval emplacement of similar melts in the three different units suggests they were portions of the same Pan-African-Cadomian block in Precambrian times. Sr and Nd isotope signatures evidence that these magmas cannot be derived from the partial melting of the now exposed restitic metapelites of the lower crust segment of the Sila Unit. More likely the basement containing the augen gneisses was eradicated from the source of the crustal melts during the Variscan orogeny and records recycled terranes derived from West African Craton.

M.L. FREZZOTTI AND S. FERRANDO - Multiphase solid inclusions in ultrahigh-pressure metamorphic rocks: a petrographic approach.

Abstract -The study of fluid inclusions in ultra high-pressure (UHP) rocks does not represent an easy task, since most peak inclusions are destroyed during post-metamorphic uplift. Peak metamorphic conditions are in fact largely outside the isochore fields even for the densest fluid solutions, and the exhumation P-T paths of the host rocks strongly favour chemical interaction with the host mineral. Despite these difficulties, a number of documented examples show that inclusions in these metamorphic rocks might be preserved, providing valuable information on the composition of fluids at UHP, which are precursor to subduction fluids. Most peak inclusions in UHP rocks are multiphase solid (MS), containing different silicates ± salts, and no or very subordinate amount of water. A petrographic approach to MS inclusion studies in UHP rocks is proposed, consisting of a number of successive investigation steps (i.e. identification, chronology, selection, and interpretation). Since MS inclusions represent an integral part of UHP metamorphic phase assemblages, they should be systematically investigated during any petrographic study of such rocks.

C. GROPPO AND R. COMPAGNONI - Metamorphic veins from the serpentinites of the Piemonte Zone, western Alps, Italy: a review.

Abstract - Petrographic and micro-Raman analysis of metamorphic veins occurring in serpentinites from the Piemonte Zone (western Alps) has allowed us to recognise seven different vein generations, developed at different P-T-X conditions. The first vein generation (balangeroite + magnetite + FeNi-alloys) formed during the Alpine prograde evolution, whereas the second vein generation (diopside + Ti-clinohumite + olivine + antigorite + Mg-chlorite) is related to the high pressure (eclogite-facies) metamorphic peak. All the others vein generations developed during the retrograde evolution, at different P-T conditions, moderate for types 3 (carlosturanite + antigorite + diopside + garnet) and 4 (antigorite + diopside) and low or very low for types 5 (chrysotile), 6 (tremolite + calcite) and 7 (brucite + magnetite and talc + calcite). Detailed petrographic analysis of the metamorphic veins occurring in the serpentinites gives information complementary to the conventional thermobarometric estimates based on the associated basic rocks and consequently is a powerful method to reconstruct the P-T path of the ophiolitic units.

C. GROPPO, D. CASTELLI AND F. ROLFO- HT, pre-Alpine relics in a spinel-bearing dolomite marble from the UHP Brossasco-Isasca Unit (Dora-Maira Massif, western Alps, Italy).

Abstract - A dolomite marble from the polymetamorphic complex of the UHP eclogite-facies Brossasco-Isasca Unit (southern Dora-Maira Massif), which preserves relics of a HT pre-Alpine, Spl-Dol-Ilm-bearing assemblage partially converted into a Crn + Chl + (Arg) + Rt UHP assemblage during the Alpine evolution, has been petrologically investigated. Its pre-Alpine P-T-X(CO₂) evolution is discussed using P-T phase diagram projections for H₂O-CO₂ mixed volatile systems. The P-T-X(CO₂) modelling suggests that the Dol + Crn = Cal + Spl + F decarbonation reaction is the most likely spinel-producing reaction along the pre-Alpine prograde path. In the mixed-volatile model system, a X(CO₂) > 0.60 has been inferred as the equilibrium fluid composition of this univariant assemblage. Comparison of the P-T-X(CO₂) pre-Alpine and Alpine evolutions shows that CO₂-rich fluids involved in the pre-Alpine reactions are substantially different from the acqueous fluids produced during the UHP Alpine stage.

L. MELLUSO, V. MORRA, P. BROTZU, L. FRANCIOSI, C. GRIFA, M. LUSTRINO, P. MORBIDELLI, H. RIZIKY AND M. VINCENT- The Cenozoic alkaline magmatism in central-northern Madagascar:a brief overview.

Abstract - The Cenozoic volcanic rocks of Madagascar were emplaced as lavas, pyroclastic rocks, dykes, and plugs, and range in composition from olivine melilitites, basanites and alkali basalts, to phonolites, trachytes and rhyolites. The ultrabasic-basic lithologies are dominant, in particular basanites and tephrites, with less abundant, mildly evolved, compositions. These rocks form at least three different magma lineages of broadly sodic affinity. Fractional crystallization of the observed phenocryst phases is the most reasonable petrogenetic model for the genesis of the more evolved compositions. The mantle-derived magmas were likely generated by variable degrees of partial melting of incompatible element-enriched mantle sources, possibly located in the deep lithosphere or in the asthenosphere.

G.B. PICCARDO, G. RANALLI, M. MARASCO AND M. PADOVANO- Ultramafic pseudotachylytes in the Mt. Moncuni peridotite (Lanzo Massif, western Alps): tectonic evolution and upper mantle seismicity.

Abstract - The small ultramafic body at Mt. Moncuni consists of impregnated plagioclase peridotites, metre-scale masses of spinel dunites and harzburgites, and widespread gabbroic and porphyritic dykes. Metre- to decametre-scale shear zones cut the plagioclase peridotites and deform the pre-existing gabbroic dykes. Within the shear zones and in the host peridotite, millimetre- to decimetre-wide, decimetre- to metre-long, veins of pseudotachylytes are present, both concordant and discordant to the tectonite-mylonite foliation of shear zones. Late coarse-grained porphyritic dykes cut across both deformed plagioclase peridotites and shear zones.
The formation of decametre-scale shear zones and the sequence of metamorphic assemblages developed in the shear zones, under increasing fluid activity, from amphibole-bearing plagioclase-peridotite-facies to amphibolite-facies, show that these rocks underwent progressive exhumation to shallower lithospheric levels, under progressively decreasing pressure and temperature conditions.
Millimetre-wide pseudotachylyte veins are concordant with the fault planes of the shear zones (fault-vein type A) showing ultra-fine grained cryptocrystalline/microcrystalline and microlitic/spherulitic textures and rather large (up to 10-15% by volume) amounts of clastic olivine grains or aggregates and lithic mylonitic clasts. Decimetre-wide, metre-long, pseudotachylyte veins (injection-vein type B) cut discordantly the foliation of the host deformed peridotite and exhibit spinifex textures and limited presence (<1% by volume) or absence of exotic lithic aggregates. Pseudotachylytes type B show peridotitic major and trace element bulk rock compositions.
The occurrence of ultramafic pseudotachylytes is consistent with the occurrence of upper mantle earthquakes. The formation of millimetre-wide ultra-fine grained type A pseudotachylyte veins suggests that the fault planes acted as loci for the seismogenic release of the accumulated shear stress. The co-seismic shear heating and the associated temperature increase produced the strongly localized, almost complete melting of the host deformed peridotite, forming melts with peridotitic composition. Strong localized heating of the host rock (up to 1450°C) was associated with intrusion of parental melts of type B pseudotachylytes, where rapid crystallization produced spinifex textures, typical of komatiitic (peridotitic) magmas. The mutual relations between the early (i.e., pre-shear zones) MORB-type gabbroic dyke intrusion, the formation of shear zones and pseudotachylytes, and the crosscutting of the late (i.e., post-shear zones) MORB-type porphyritic dyke intrusion give clear time constraints for the upper mantle seismicity. The investigated pseudotachylytes are records of Jurassic earthquakes related to extensional faults in the upper mantle, during its exhumation from sub-continental lithospheric levels towards the sea-floor of the Jurassic Ligurian Tethys.

G.B. PICCARDO, A. ZANETTI, A. PRUZZO AND M. PADOVANO- The North Lanzo peridotite body (NWItaly): lithospheric mantle percolated by MORB and alkaline melts.

Abstract - The North Lanzo peridotite body is mostly composed of fertile spinel lherzolites showing diffuse pyroxenite bands. These lithologies typically preserve older (lithospheric) spinel-facies assemblages, but most of them experienced a widespread recrystallization at plagioclase-facies conditions. In particular, spinel pyroxenites are deeply recrystallized to plagioclase+olivine-rich assemblages. The high pyroxenes modal contents of the lherzolites and the diffuse occurrence of pyroxenite banding indicate that this mantle section escaped important modifications by reactive interaction (i.e. pyroxene dissolution and olivine precipitation) of undersaturated MORB-type melts at spinel-facies conditions, prior to the subsolidus partial equilibration, as widespread in the South Lanzo peridotite body. According to the petrographic evidence, the early plagioclase-bearing assemblages originated during subsolidus partial recrystallization, likely during the emplacement of the mantle sequence at relatively shallow levels. Field, structural and compositional features suggest that hectometre-scale areas of the North Lanzo peridotites were successively enriched in plagioclase + olivine + pyroxenes as a result of large diffuse percolation and impregnation mostly operated by aggregate MOR-type melts.
This indicates that asthenospheric melts migrated via porous flow through the North Lanzo peridotites when they had been already exhumed to plagioclase-facies conditions, i.e. to relatively shallow levels in the mantle lithosphere. Replacive, strongly pyroxene-depleted, spinel harzburgites cut in the form of channels through both spinel-plagioclase peridotites and impregnated plagioclase peridotites. They have small interstitial magmatic pyroxenes showing clear alkaline affinity. This indicates that, after diffuse percolation and impregnation by MOR-type melts, the North Lanzo peridotites underwent focused percolation along strongly pyroxene-depleted channels of alkaline melts, deriving from garnet-bearing fertile mantle sources.
Available data indicate that the North Lanzo spinel-facies lherzolite protolith represent the ancient sub-continental lithospheric mantle, which was isolated from the convective mantle, as previously argued on the basis of Sr-Nd isotopic data. These peridotites record a composite tectonic-metamorphic and melt-related evolution that indicates that this piece of sub-continental lithospheric mantle has been exhumed to shallow levels, it has been subsequently pervasively percolated and impregnated by asthenospheric MOR-type melts, and finally it has been percolated along focused channels by alkaline melts. Field and structural evidence indicates, thus, the close relationships between lithosphere extension, asthenosphere decompressional partial melting and lithosphere melt percolation during the rifting stages of the Ligurian Tethys basin.

P. ROSSETTI, A. AGANGI, D. CASTELLI, M. PADOAN AND R. RUFFINI- The Oligocene Biella pluton (western Alps, Italy): new insights on the magmatic vs. hydrothermal activity in the Valsessera roof zone.

Abstract - The composite Biella pluton is part of an Oligocenevolcano-plutonic complex whose origin is connected to the Alpine subduction-collision processes and that emplaced at shallow crustal levels within the eclogite-facies rocks of the Austroalpine Sesia-Lanzo Zone. In the roof zone of the pluton, small satellite igneous bodies are set within the Sesia-Lanzo country rocks, close to the main Biella pluton, and range in composition from quartz alkali feldspar syenite, quartz monzodiorite and monzogabbro, to quartz diorite and gabbronorite. Their geochemical features, including the REE patterns, are coherent with the calc-alkaline to shoshonitic affinity recognized in the whole volcano-plutonic complex. Field and petrographic data suggest that these bodies represent earlier crystallization/differentiation products of the Biella primary magma(s), which underwent contact metamorphic recrystallization during the multistage emplacement of the main pluton. Tourmaline-bearing hydrothermal breccias and different types of hydrothermal veins (including quartz-plagioclase-, quartz-tourmaline- and ankerite-quartz-sulphides-bearing veins) occur within both the intrusive rocks (satellite bodies + the main pluton) and their Sesia-Lanzo Zone country rocks. Both field relationships and vein assemblages suggest a close connection between the late-magmatic evolution of the Biella pluton and the multistage, boron-rich hydrothermal activity.

R. SACCHI, M. ALENE, M. BARBIERI AND A. CONTI- On the Palaeozoic Tillite of the Adigrat Group (Tigrai, Ethiopia).

Abstract - A flat-lying sequence containing diamictite convincingly interpreted as tillites in the literature, occurs at the base of the (Mesozoic) Adigrat Sandstone in the Tigrai Region (Northern Ethiopia). The age of the tillite is controversial, two options (Ordovician and Permian) being put forward by previous authors. As an outcome of petrographic study, the pebbles displayed widespread, previously unreported occurrence of volcanics, thus strengthening the Permian options on the ground of regional geology. Isotopic analyses of C and O allowed comparison with tillites of the underlying, Neo-Proterozoic basement and proved the lack of the extreme excursion of d¹³C that in the Neopoterozoic tillite were seen as supporting the “Snowball Earth” hypothesis for the “Sturtian” age glaciation. Rb-Sr radiometric dating was also performed and yielded results compatible with a Permian age, though facing difficulties connected with intimate intermingling of clasts and matrix, and deuteric alteration.

M. SCAMBELLURI, N. MALASPINA AND J. HERMANN- Subduction fluids and their interaction with the mantle wedge: a perspective from the study of high-pressure ultramafic rocks.

Abstract - We review three case studies emphasizing the role of ultramafic rocks in the recycling of volatiles and trace elements at convergent plate margins. Serpentinites are major water carriers in subduction zones and their breakdown liberates large quantities of water at sub-arc depths. The incompatible elements incorporated during oceanic serpentinization are released into the fluid phase produced once antigorite dehydrates to olivine + orthopyroxene. Importantly, the antigorite breakdown can trigger either wet melting or production of supercritical fluids in altered basalts and sediments. The produced fluid phases incorporate substantial amounts of incompatible element, initially residing in the crustal reservoirs. The fluid phase which exits the slab is highly reactive with respect to the overlying, silica undersaturated, mantle rocks. This leads to formation of reactive (ortho)pyroxenite layers, which filter the uprising hydrous melt/supercritical fluid to produce aqueous, solute-rich solutions. This fluid has equilibrated with peridotites and is mobile in the mantle. A consequence of these subduction fluid/mantle reactions is that the mantle wedge domains overlying the slabs can be heterogeneous in composition and layered, due to the presence of reactive pyroxenite bodies. Another aspect regards the debate whether supercritical fluids or hydrous melts are effective media for trace element transport. Since both agents are saturated in silica, they will react with the silica-undersaturated mantle wedge peridotites to produce aqueous, incompatible trace element-rich residual fluids. Hence, while hydrous melt and/or supercritical fluids are important for scavenging incompatible elements from the slab, they may not be the agents that transfer the metasomatic subduction signature to the inner parts of the mantle wedges.

M.I. SPALLA AND A.M. MAROTTA- P-T evolutions vs. numerical modelling: a key to unravel the Paleozoic to early-Mesozoic tectonic evolution of the Alpine area.

Abstract - The pre-Alpine continental crust of the Alps preserves Permian-Triassic magmatic and high-temperature (HT) metamorphic evolutions, which overprinted records of Variscan subduction and collision-related metamorphism. The occurrence of numerous Variscan eclogites in the pre-Alpine continental crust, presently belonging to different structural domains, indicates that part of the Variscan suture zone occurs in the Alpine belt. The late Variscan evolution took place from 340 to 300 Ma, and therefore the igneous and metamorphic signatures up to Upper Carboniferous may represent the record of the late orogenic evolution. On the contrary, different authors interpreted the HT metamorphism associated with gabbro to granite intrusions younger than 290 Ma as the effect of Permian-Triassic late-orogenic collapse or continental rifting. The goal of this study is to reduce the ambiguity about the geodynamic significance of the Permian-Triassic HT metamorphism and igneous activity in the Alpine continental crust, with the support of numerical modelling of: ocean subduction, continental collision, lithospheric detachment and subsequent gravitational thermal relaxation. Comparison of the model predictions with structural and petrologic data has driven the successive model refinements to improve the fit. The best fit model predictions show a rather good agreement with natural data (coincidence of age, P-T values and rock compositional affinity) up to late-Variscan times. The poor agreement during the Permian-Triassic evolution suggests that, with respect to the thermal state established during the post-collisional gravitational evolution, an additional positive heat anomaly is necessary to induce the thermal state indicated by natural P-T estimates.

P. ULMER- Differentiation of mantle-derived calc-alkaline magmas at mid to lower crustal levels: experimental and petrologic constraints.

Abstract - This contribution compares experimentally derived liquid-lines-of-descent on primary, hydrous calc-alkaline magmas ranging from picrobasaltic to basaltic andesite at pressures of 1bar to 15 kbar with geochemical evolutionary trends of two contrasting, subduction related, continental arc systems, namely the Adamello batholith and the Cascadian Arc system. Field, petrologic and geochemical data of both systems are indicative for polybaric crystallization over basically the entire crustal column. This is clearly supported by experimental data that reveal that the majority of basic to intermediate rock compositions are not only close to the liquid-line-of-descent indicating that crystal retention in plutonic systems is not very efficient in basaltic to intermediate magmas, but also that the mean crystallization pressures obtained from CaO-MgO diagrams are in the range 5-10 kbar consistent with deep-level (crust-mantle boundary) and intermediate crustal level (15-25 km) magma storage areas where the principal crystallization-driven differentiation took place. In the case of the southern Adamello, subtle differences in the liquid-lines-of-descent are fully consistent with experimental data and trace element geochemistry indicating that different rock suites (plutons) have acquired their major and trace element characteristics at different crustal levels. The principal differences in crystallization-differentiation with increasing depth is the decreasing plagioclase stability leading to delay of the onset of plagioclase crystallization and the enhancement of both earlier clinopyroxene and amphibole crystallization with increasing depth shifting derivative liquids closer to the metaluminous / peraluminous limit or even within the peraluminous field with increasing pressure, fully consistent with the geochemical evolution of the southern Adamello granitoids that partly evolve to peraluminous compositions that cannot be linked to extensive crustal assimilation.