ABSTRACT LXXVI, April 2007 n.2:

Abstract - The Variscan metamorphic basement of the westernmost part of the Southern Alps consists of the Ivrea-Verbano Zone and Serie dei Laghi separated by the Cossato-Mergozzo-Brissago Line, a subvertical, late-Variscan ductile shear zone. The paper deals with the complex history of magmatism, deformation, and metamorphism during pre-Alpine and Alpine evolution of the Serie dei Laghi, a metasedimentary sequence hosting older and younger granites. Rb-Sr whole-rock (WR) isochron (466±5 Ma) and Pb-Pb single zircon evaporation ages (458±6 Ma and 463±4 Ma) on metagranites date the emplacement of the older intrusive series, whereas Rb-Sr muscovite ages (311-325 Ma) approach the Carboniferous metamorphism (331-340 Ma). Rb-Sr WR isochrons (277±8 Ma) and biotite ages (276-281 Ma) on granitic plutons date the emplacement of the younger intrusive series. However, both intrusive series have some rejuvenated biotite ages (up to 170 Ma). Such ages suggest opening of the Sr isotope system, though the timing of this event cannot be determined precisely via Rb-Sr systematics. The metasediments reached incomplete whole-rock Sr isotopic homogenisation at 288±99 Ma, very close to the age of the Permian event recorded by the granites. Ordovician U-Pb zircon ages have been ascribed to infiltration of residual granitic melts. Moreover, zircons of 1003±2 Ma reflect the presence of old detrital components. Finally, most of the intrusive, metaintrusive, and metasedimentary rocks plot on well-defined linear correlations in both the initial Pb-Pb and the ²⁰⁶Pb/²⁰⁴Pb(i) vs. ²³⁸U/²⁰⁴Pb diagrams. These correlations, as well as ²³⁸U/^204Pb ratios far higher than those reported for crustal rocks, are interpreted as due to recent opening of the Pb isotope system, which caused either an increase in the U/Pb ratios and/or a rough homogenisation of the Pb isotopes. However, this process affected the Sr isotope system only at the mineral scale (post-Variscan biotite ages), while whole-rock isochrons are still preserved in both metaintrusive and intrusive rocks. An estimated age of 26±10 Ma is obtained by applying an “inverse” approach to the Pb data. Such a widespread process, which involved most of the igneous, metaigneous, and metasedimentary rocks of the Variscan belt, implies a large-scale mobilisation of fluids that could be attained only through the reactivation of the many pre-Alpine faults dissecting the Serie dei Laghi.

Giuseppe Cassinis, Luciano Cortesogno, Laura Gaggero, Cesare Perotti and Ausonio Ronchi - Volcanic products from the Early Permian Collio Basin (southern Alps) and their geodynamic implications.

Abstract -The typical Early Permian Collio Basin, in the central Southern Alps between eastern Lombardy and western Trentino, developed in a widespread transtensional regime, likewise all the Permian continental basins of this region, and was associated with the emplacement of volcanic products. The basal plateau, up to about 130 m thick, is made up of calc-alkaline rhyolitic ignimbrites interpreted as early anatectic magmas from the lower crust, thickened after the Variscan collision. However, andesite lithic inclusions suggest that a volcanic phase with intermediate composition occurred prior to the conspicuous silica rich-volcanism. The source area of these subaerial igneous flows, also known as “Lower Quartz Porphyries” in the literature, was probably between the southwestern margin of the basin and the present lower Camonica Valley. A pile, less than 100 m thick of vitric and crystal tuffs, marked at the top by diffuse accretionary lapilli, follows above. Later, the overlying alluvial-to-lacustrine Collio Formation was interfingered with several subaqueous thin volcaniclastic key-beds, suitable for correlation. They normally consist of (1) a lower, amalgamated coarse-sandy to gravelly crystal-rich massive turbiditic subunit, 10–20 and more metres thick, which displays isolated outsize clasts of the underlying Collio Formation, as well as of volcanic and metamorphic rocks; and (2) an upper, well-bedded sandy-pelitic subunit, generally less than 3 m thick and rich in outsize porphyritic fragments, which suggest deposition from fine-grained turbidity currents. These so-called “Dasdana Beds” originated from one or more igneous centres at the eastern part of the Collio Basin beyond the Caffaro Valley, near the Brescia-Trento border, rapidly thinning westwards. In particular, the main areas feeding the igneous activity were located along the ENE Val Trompia lineament, north of the Bagolino-Riccomassimo “knee-fold”. Here, the subvolcanic bodies cut the crystalline basement and part of the overlying Lower Permian succession and flowed into the basin floor from the Dosso del Bue lava dome (up to about 200 m thick). Eastwards, from Riccomassimo to Darzo (upper Val Sabbia), close to the intersection between the Val Trompia and the Lower Giudicarie lines, the Early Permian igneous activity is more developed and shows textural heterogeneities probably related to the subvolcanic occurrence of the bodies. The last igneous event of the basin is represented by further Early Permian rhyolite/rhyodacite massive ignimbrites (“Auccia Volcanics”), which filled up the Collio Basin. The top of this upper volcaniclastic plateau, probably exposed for more than 20 Ma, displays clear erosional surfaces and palaeosols in many parts of the basin, before deposition of the fluvial Verrucano Lombardo-Val Gardena red beds, which mark the end of the Late Variscan igneous activity all over the Southern Alps.

Laura Buzzi and Laura Gaggero - Petrogenesis of post – orogenic Late Paleozoic andesite magmatism: a contribution from the Ligurian Alps (Italy).

Abstract - TThe post-orogenic subalkaline andesite to K-andesite volcanism, developed within intramontane basins after the collapse of the Variscan orogen, originated in a geodynamic environment significantly different from that of continental or island arc settings. However, the major, trace and rare earth element compositions are not particularly effective in discriminating any clear affinity, and suggest a complex genetic hypothesis. In southwestern Europe, andesites are preserved in the Alpine units of the Ligurian Alps, and belong to successions characterised by the constant occurrence of a basal rhyolitic ignimbrite, followed by andesites, and conspicuous dacites-rhyodacites (or diorites), all intercalated within lacustrine sediments. The trace element ratios and initial Sr and Nd isotopic compositions of the intermediate and outer Briançonnais andesites were compared with potential magma sources, and allowed to exclude Andean-type and OIBs magmas. The initial (290 Ma) ¹⁴⁴Nd/¹⁴³Nd and ⁸⁷Sr/⁸⁶Sr for the Ligurian Briançonnais andesite fitted the composition issuing from the assimilation-fractional crystallisation model, starting from a subcontinental, lithospheric mantle-type parental magma through the addition of ~80 wt% of a crustal metasedimentary component to the picrobasaltic composition. The fractionating assemblage for the SLM parental picrobasalt consisted of 40 wt% olivine, 10 wt% orthopyroxene, 40 wt% clinopyroxene and 10 wt% plagioclase. We assumed that such melt, generated at some 3 GPa and ˜1400 °C in equilibrium with a garnet lherzolite, was the parental liquid of the Briançonnais calc-alkaline magmatism. A crust/mantle separation model age of 1.26 Ga was calculated from the Nd isotopic composition, possibly matching the Elzevirian Orogeny event (~1250-1190 Ma), at the southeastern margin of Laurentia.

Giovanni B. Piccardo - Evolution of the ultra-slow spreading Jurassic Ligurian Tethys: view from the mantle.

Abstract - The Jurassic Ligurian Tethys was floored by tectonically exhumed mantle peridotites, discontinuously covered by MORB lava flows. It was characterised by along-axis alternance of: 1) a-volcanic segments, showing direct exposure of mantle peridotites below the oceanic sediments, and 2) volcanic segments, showing a volcanic cover on top of mantle peridotites. Ophiolitic peridotites from the Ligurian Tethys show strong petrological and geochemical heterogeneity, varying from rather fertile lherzolites, to strongly depleted harzburgites and dunites, to refertilised plagioclase peridotites. Peridotites from Ocean Continent Transition (OCT) settings are mostly fertile lherzolites, which derive from the sub-continental lithospheric mantle of the Europe-Adria system. They were isolated from the convective mantle and accreted to the sub-continental thermal lithosphere starting from Proterozoic times, prior and independently to rifting and drifting in the Ligurian Tethys realm. Peridotites from More Internal Oceanic (MIO) settings are mostly represented by pristine sub-continental lherzolites, which have been transformed into reactive spinel peridotites, impregnated plagioclase peridotites and replacive spinel harzburgites and dunites by melt-peridotite interaction processes. Stratigraphic-structural features (i.e. mantle at the sea-floor and alternance of a-volcanic and volcanic segments) and petrologic features of both magmas (i.e. presence of mildly enriched and alkaline melts) and peridotites (i.e. extreme compositional heterogeneity induced by melt percolation and melt-peridotite reaction processes) indicate the close similarity of the Jurassic Ligurian Tethys with modern ultra-slow spreading ridges (Gakkel and South-West Indian Ridges). Petrological and geochronological data on the Ligurian Tethys ophiolitic peridotites and gabbros allow the following scenario to be documented: 1) Lithosphere extension started during Triassic and caused exhumation of sub-continental lithospheric mantle (as old as Proterozoic), by means of km-scale shear zones; 2) Lithosphere extension and thinning induced almost adiabatic upwelling and partial melting on decompression of the underlying asthenosphere, which started most probably during Early Jurassic times; 3) MORB melts from the asthenosphere percolated and reacted with the sub-continental lithospheric mantle, forming melt-modified, depleted/enriched transitional peridotites; 4) Prior to sea-floor exposure, lithospheric and transitional peridotites were intruded by aggregated MORB magmas, from 180 Ma in OCT peridotites to 164-160 Ma in MIO peridotites; 5) Oceanic refractory peridotites, coeval and cogenetic with Jurassic MORB melts, were accreted to the thermal lithosphere. Across-axis variation (from OCT to MIO settings) of mantle peridotites describe the different evolution stages of the basin, i.e.: 1) Exhumed Sub-continental Mantle characterises the OCT zones and represents the rifting stages dominated by subsolidus extension and thinning of the Europe-Adria lithosphere; 2) Percolated Subcontinental Mantle mostly characterised the more external MIO settings and represents the drifting stage after inception of asthenosphere partial melting on decompression along the axial zone of the future oceanic basin; 3) Percolated Oceanic Mantle, which is cogenetic with the Jurassic MORB melts, is sporadically present at More Internal MIO settings and represents the oceanic stage, after complete failure of the continental crust, when Jurassic refractory residua are emplaced at the sea-floor of the basin.

Francesco Menna, Gianfranco Principi, Benedetta Treves, Silvia Podetti, Francesca Garfagnoli and Simone Corti - The pre-orogenic tectonic history of the Bracco gabbroic massif: review and news.

Abstract - We present some preliminary data on the oceanic tectono-magmatic history and deformation structures of the Bracco gabbroic Massif, recently collected through detailed structural fieldwork in the Bracco Pass - Mt. San Nicolao area (Eastern Liguria). The Bracco Massif (Vara Supergroup, Internal Ligurian Units) represents a fragment of Jurassic oceanic lithosphere. It is largely made of gabbros intruded in serpentinitic lherzolites and affected by very low-grade orogenic metamorphism (prehnite-pumpellyite facies); the gabbroic mass often shows a cumulitic texture and it is locally characterised by the presence of ductile shear zones, displaying a peculiar HT-LP oceanic metamorphic imprint. It is crosscut by basalt dykes and hornblende-oligoclase veins. We collected new data on the attitudes of the magmatic layering, shear zones, basalt dykes and hornblende-oligoclase veins, focusing on those areas where the Alpine to post-Alpine deformation is scarce, so that the pre-orogenic structural features were preserved and easier to interpret. The data show that the younger oceanic structures (basalt dykes and hornblende-oligoclase veins) have a homogeneous trend in the whole studied area, whereas the magmatic layering and the ductile deformations (mylonites, flaser) have very different attitudes in the eastern and western sectors of the Bracco area. These sectors are separated by a NW-SE normal fault that runs immediately westwards from Mt. San Nicolao. We also present a discussion on the tectonic significance of the ductile structures and an interpretation of the structural data in the geodynamic context of an ocean ridge near a transform fault intersection.

Lorenzo Fedele , Michele Lustrino, Leone Melluso, Vincenzo Morra and Fosco d’Amelio - The Pliocene Montiferro volcanic complex (central-western Sardinia, Italy): geochemical observations and petrological implications

Abstract -The Montiferro volcanic complex is one of the several districts of Middle Miocene-Quaternary magmatism of Sardinia. It was active between 3.9 and 1.6 Ma, with a climax at 3.6 Ma. Erupted rocks have a wide spectrum of compositions, ranging from basic and ultrabasic to highly evolved lithotypes, belonging to three different magmatic suites: 1) strongly alkaline sodic series; 2) mildly alkaline sodic series; 3) tholeiitic series. The products of the strongly alkaline series are mainly basanites, erupted both at the first and at the last stages of Montiferro activity. Strongly alkaline magmas are likely to have risen rapidly to the surface, as testified by the common occurrence of mantle xenoliths and by the absence of differentiated products. The mildly alkaline series ranges from hawaiites to trachytes and phonolites through very abundant mugearites and few benmoreites. Primitive magmas of this series are likely to have ponded at crustal levels, experiencing both differentiation and crustal contamination processes. Finally, the tholeiitic series is mainly represented by rare basaltic andesites and a few basaltic trachyandesites straddling the alkaline/subalkaline boundary. Incompatible trace elements, ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotopic ratios are consistent with the derivation of these products from a common mantle source, represented by a spinel-lherzolite. Primitive magmas of three magmatic suites may have generated by different degrees of partial melting of this source, progressively increasing from strongly alkaline (~1-3% partial melting) to mildly alkaline (~5-7%) and tholeiitic products (~10-12%). Geochemical modellings suggest that this source is represented by a DMM-like lithospheric mantle, metasomatised by small inputs of various crustal components.

Claudio Geloni and Giovanni Gianelli - A preliminary geochemical model of the fluid-rock interaction processes forming the eastern Liguria ophicalcite (northern Apennines, Italy)

Abstract - The Northern Apennine ophicalcites are likely the Jurassic equivalent of the present-day Lost City and Rainbow submarine hydrothermal systems, where complex tectonic, sedimentary and hydrogeochemical processes occur. The aim of this research work is to simulate the interaction of a peridotite of the Northern Apennines ophiolites with a reconstructed Jurassic seawater, and to verify whether the ophicalcite formation is enhanced or hampered. The Jurassic seawater has been reconstructed on the basis of literature data. The seawater during the Lower Jurassic was characterised by higher Ca/Mg ratios, lower SO₄ and higher HCO₃ contents, when compared with the present-day composition. The simulations have been conducted using the program REACT of the Geochemist Work Bench package, in a temperature range of 25-200 °C, and TOUGHREACT, at 25° and 100 °C for reactive flow modelling. The thermodynamic modelling results indicate that the alteration products are antigorite, brucite, chlorite, carbonates and andradite, whereas anhydrite does not reach conditions of oversaturation, and, therefore, precipitation. The indication is that the Jurassic seawater enhances the formation of calcite and does not allow a significant precipitation of anhydrite, which is formed when the present seawater composition is used. The occurrence of andradite, even at low temperature, is in agreement with the petrographic data of the Ligurian ophiolite. This opens the possibility of an ocean-floor metamorphism origin also for the garnet phase. Reactive flow modelling indicates that significant amount of calcite (up to approximately 15 vol%) can precipitate in a fractured serpentinized peridotite at low temperatures, compatible with the conditions of the uppermost part of the Levanto Breccia, close to a deep sea sedimentary environment and therefore permeated by cold waters.

Rosangela Bocchio- Barium-rich phengite in eclogites from the Voltri Group (northwestern Italy)

Abstract - A white mica with high barium content has been found for the first time in eclogites from a single body cropping out near the village of Vara Inferiore (Voltri Group, Italy). Petrographic observations reveal that the modal abundance of mica is low and that its crystallisation took place after the climax of the eclogite-facies metamorphism. This study reports data on bulk rock chemistry, obtained by ICP-MS, as well as the results of EMP and LA-ICP-MS analyses on major and selected trace elements carried out in situ on handpicked separate grains of mica. The most significant features of the bulk rock chemistry are the high content of Na₂O and the enrichment in LIL elements (Ba and Cs over K and Rb) compared with other eclogites of the area. The white mica is a “phengite” that contains 3.17 - 3.30 Si a.p.f.u, and 0.19 - 0.31 Mg a.p.f.u. All the analysed grains show marked heterogeneities in both major and trace element distribution. In particular, the content of BaO varies from 1.65 to 4.55 wt% and from 2.46 to 4.59 wt% in two examined samples, thus leading to a range of ˜4-12 % Ba atoms in the cation interlayer (I) of the mica. The high barium content in white mica is coupled with a high abundance in other LILE (Cs, Rb) and light elements such as Li and B. On the contrary, mica has low concentration of REE, Sr, P and HFSE (e.g. Nb, Zr, Ti, Y). Phengite chemistry indicates that its trace element signature is controlled by a fluid activity that also prevented the achievement of complete homogenisation.

Chiara Groppo and Roberto Compagnoni- Ubiquitous fibrous antigorite veins from the Lanzo Ultramafic Massif, Internal Western Alps (Italy): characterisation and genetic conditions

Abstract - Discontinuous veins, 1 to 20 cm thick and from few centimetres to several decimetres long, of fibrous antigorite are widespread in the serpentinites of the Lanzo Ultramafic Massif, Western Alps. The cross fibre antigorite veins consist of rigid and brittle bundles of fibres and typically show a banded structure parallel to the vein selvages. The fibrous antigorite has been characterised by optical microscopy, electron microscopy (SEM-EDS, EMPA and TEM), and vibrational spectroscopy (FTIR and µ-Raman). The determined value of m = 17 (m = number of tetrahedra along an entire wavelength) and thermobarometric data on the associated mafic rocks and rodingites, suggest that fibrous antigorite veins formed during exhumation at T = 350-400 °C and P = 0.3-0.6 GPa, i.e. under greenschist facies conditions. The fibrous habit of antigorite may be explained by two different mechanisms, i.e. the crack-seal process and the dissolution-precipitation creep mechanism. Both these processes are compatible with the banded structure of the studied veins and the estimated P-T conditions of formation.