New stratigraphical data on the Middle-Late Jurassic biosiliceous sediments from the Sicanian basin, Western Sicily (Italy)

The reported data present the stratigraphy of several sections across a Middle-Late Jurassic Radiolaritic Unit, well exposed in different thrust sheets pertaining to the Maghrebian chain of Southwestern Sicily. The aim was to define the chronostratigraphical distribution of the Jurassic biosiliceous sedimentation in the Sicanian palaeogeographical zone, a deep water basin belonging to the Southern Tethys continental margin. The radiolarian biostratigraphy indicates that the switching from carbonate to siliceous sedimentation in the Sicanian Basin is referable to the Bajocian, as shown by the section of Campofiorito, near Corleone. The biostratigraphical dataset allows the correlation between the onset of biosiliceous sedimentation and the fall of biodiversity in the Sicanian basin with the carbonate productivity crisis, indicated by the highest eutrophication that affected Western Tethys during Middle Jurassic times. Editorial handling: J.-P. Billon-Bruyat & M. Chiari (Guest)     

Direct measurement of 3D elastic anisotropy on rocks from the Ivrea zone (Southern Alps, NW Italy)

Lower crustal and upper mantle rocks exposed at the earth's surface present direct possibility to measure their physical properties that must be, in other cases, interpreted using indirect methods. The results of these direct measurements can be then used for the corrections of models based on the indirect data. Elastic properties are among the most important parameters studied in geophysics and employed in many fields of earth sciences. In laboratory, dynamic elastic properties are commonly tested in three mutually perpendicular directions. The spatial distribution of P- and S-wave velocities are then computed using textural data, modal composition, density and elastic constants. During such computation, it is virtually impossible to involve all microfabric parameters like different types of microcracking, micropores, mineral alteration or quality of grain boundaries.

In this study, complete 3D ultrasonic transmission of spherical samples in 132 independent directions at several levels of confining pressure up to 400 MPa has been employed for study of selected mafic and ultrabasic rocks sampled in and nearby Balmuccia ultrabasic massif (Ivrea zone, Southern Alps, NW Italy). This method revealed large directional variance of maximum P-wave velocity and different symmetries (orthorhombic vs. transversal isotropic) of elastic waves 3D distribution that has not been recorded on these rocks before. Moreover, one dunite sample exhibits P-wave velocity approaching to that of olivine single crystal being interpreted as influence of CPO.     

Age, Geochemistry and Petrogenesis of the Ultramafic Pipes in the Ivrea Zone, NW Italy

Pipe-like ultramafic bodies, hosting Ni–Cu–PGE sulphidedeposits, intrude the Main Gabbro and the roof metasedimentsof the Ivrea Zone, NW Italy. These bodies were emplaced at 287± 3 Ma and represent the last mantle-derived melts associatedwith an underplating event that largely drove the crustal evolutionof this area during the late Carboniferous (     

Phase equilibrium modelling of the amphibolite to granulite facies transition in metabasic rocks (Ivrea Zone,NW Italy)

The development of thermodynamic models for tonalitic melt and the updated clinopyroxene and amphibole models now allow the use of phase equilibrium modelling to estimate P–T conditions and melt production for anatectic mafic and intermediate rock types at high‐T conditions. The Permian mid‐lower crustal section of the Ivrea Zone preserves a metamorphic field gradient from mid amphibolite facies to granulite facies, and thus records the onset of partial melting in metabasic rocks. Interlayered metabasic and metapelitic rocks allows the direct comparison of P–T estimates and partial melting between both rock types with the same metamorphic evolution. Pseudosections for metabasic compositions calculated in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O (NCKFMASHTO) system are presented and compared with those of metapelitic rocks calculated with consistent end‐member data and a–x models. The results presented in this study show that P–T conditions obtained by phase equilibria modelling of both metabasic and metapelitic rocks give consistent results within uncertainties, allowing integration of results obtained for both rock types. In combination, the calculations for both metabasic and metapelitic rocks allows an updated and more precisely constrained metamorphic field gradient for Val Strona di Omegna to be defined. The new field gradient has a slightly lower dP/dT which is in better agreement with the onset of crustal thinning of the Adriatic margin during the Permian inferred in recent studies.     

Crystal chemical variations in Ba-rich biotites from gabbroic rocks of lower crust (Ivrea Zone,NW Italy)

Biotites from mafic rocks occurring at different stratigraphic levels of the Ivrea-Verbano Mafic Complex are studied. The rocks are gabbros and diorites. All the biotites are intermediate between phlogopite and annite [0.282 (up to 7.14 and 9.32 wt%, respectively) with respect to those of the diorites (up to 1.26 and 6.26 wt%, respectively). Systematic compositional variations support the substitution model 2 ^IV Si+( ^IV R²⁺)2 ^IV Al+ ^VI Ti (R²⁺=Fe+Mg+Mn) in gabbros and ^IV Si+ ^VI Al ^IV Al+ ^VI Ti in diorites. A predominance of disordered stacking sequences, coexisting with 1M, 2M ₁ and 3T polytypes was observed in all biotites. It was possible to carry out structural refinements only on three biotites-2M ₁ from diorites (R-values between 2.68 and 3.77) and one biotite-1M from gabbros (R-value=3.09). It was shown that: (1) the reduced thickness of the tetrahedral sheet in Ba-rich biotites supports the coupled substitution ^IV Si+ ^XII K ^IV Al+ ^XII Ba; (2) the interlayer site geometry is affected by the whole layer chemistry and does not reflect only local chemical variations; (3) in two samples of the 2M ₁ polytype, the M(1) octahedral site is larger and more distorted than the M(2) sites because of the preferential ordering of Fe²⁺ in the M(1) site, whereas one sample shows complete cation disorder in the octahedral sites. Biotite-1M shows that Fe²⁺ can also be located in the M(2) site. Some of the differences between the biotites of gabbros and diorites (e.g. Ba concentration and exchange vectors) may be linked to the host rock composition and to its crystallization process. Biotite occurs in trace amounts in gabbros and its crystallization is related to the interstitial melt which contributed to the adcumulus growth of the main rock forming phases and became highly enriched in K, Ba and Ti. Diorites are the result of equilibrium crystallization of a residual melt rich in incompatible elements, where biotite is a major constituent.     

Western Alpine back-thrusting as subsidence mechanism in the Tertiary Piedmont Basin (Western Po Plain, NW Italy)

Basin formation dynamics of the Tertiary Piedmont Basin (TPB) are here investigated by means of cross-section numerical modelling. Previous works hypothesised that basin subsidence occurred due first to extension (Oligocene) and then to subsequent loading due to back-thrusting (Miocene). However, structural evidence shows that the TPB was mainly under contraction from Oligocene until post Pliocene time while extension played a minor role. Furthermore, thermal indicators strongly call for a cold (flexure-induced) mechanism but are strictly inconsistent with a hot (thermally induced) mechanism. Our new modelling shows that the TPB stratigraphic features can be reproduced by flexure of a visco-elastic plate loaded by back-thrusts active in the Western Alps in Oligo-Miocene times. Far-field compression contributed to the TPB subsidence and controlled the basin infill geometry by enhancing basin tilting, forebulge uplift and erosion of the southern margin of the basin. These results suggest that the TPB subsidence is the result of a combination of mechanisms including thrust loading and far-field compressional stresses.     

Phase equilibrium constraints on a deep crustal metamorphic field gradient: metapelitic rocks from the Ivrea Zone (NW Italy)

The metamorphic rocks of the Ivrea Zone in NW Italy preserve a deep crustal metamorphic field gradient. Application of quantitative phase equilibria methods to metapelitic rocks provides new constraints on the P–T conditions recorded in Val Strona di Omegna, Val Sesia and Val Strona di Postua. In Val Strona di Omegna, the metapelitic rocks show a structural and mineralogical change from mica‐schists with the common assemblage bi–mu–sill–pl–q–ilm ± liq at the lowest grades, through metatexitic migmatites (g–sill–bi–ksp–pl–q–ilm–liq) at intermediate grades, to complex diatexitic migmatites (g–sill–ru–bi–ksp–pl–q–ilm–liq) at the highest grades. Partial melting in the metapelitic rocks is consistent with melting via the breakdown of first muscovite then biotite. The metamorphic field gradient in Val Strona di Omegna is constrained to range from conditions of ～3.5–6.5 kbar at ≈650 °C to ～10–12 kbar at >900 °C. The peak P–T estimates, particularly for granulite facies conditions, are significantly higher than those of most earlier studies. In Val Sesia and Val Strona di Postua, cordierite‐bearing rocks record the effects of contact metamorphism associated with the intrusion of a large mafic body (the Mafic Complex). The contact metamorphism occurred at lower pressures than the regional metamorphic peak and overprints the regional metamorphic assemblages. These relationships are consistent with the intrusion of the Mafic Complex having post dated the regional metamorphism and are inconsistent with a model of magmatic underplating as the cause of granulite facies metamorphism in the region.     

Anatexis and fluid regime of the deep continental crust: New clues from melt and fluid inclusions in metapelitic migmatites from Ivrea Zone (NW Italy)

We investigate the inclusions hosted in peritectic garnet from metapelitic migmatites of the Kinzigite Formation (Ivrea Zone, NW Italy) to evaluate the starting composition of the anatectic melt and fluid regime during anatexis throughout the upper amphibolite facies, transition, and granulite facies zones. Inclusions have negative crystal shapes, sizes from 2 to 10 μm and are regularly distributed in the core of the garnet. Microstructural and micro‐Raman investigations indicate the presence of two types of inclusions: crystallized silicate melt inclusions (i.e., nanogranitoids, NI), and fluid inclusions (FI). Microstructural evidence suggests that FI and NI coexist in the same cluster and are primary (i.e., were trapped simultaneously during garnet growth). FI have similar compositions in the three zones and comprise variable proportions of CO₂, CH₄, and N₂, commonly with siderite, pyrophyllite, and kaolinite, suggesting a COHN composition of the trapped fluid. The mineral assemblage in the NI contains K‐feldspar, plagioclase, quartz, biotite, muscovite, chlorite, graphite and, rarely, calcite. Polymorphs such as kumdykolite, cristobalite, tridymite, and less commonly kokchetavite, were also found. Rehomogenized NI from the different zones show that all the melts are leucogranitic but have slightly different compositions. In samples from the upper amphibolite facies, melts are less mafic (FeO + MgO = 2.0–3.4 wt%), contain 860–1700 ppm CO₂ and reach the highest H₂O contents (6.5–10 wt%). In the transition zone melts have intermediate H₂O (4.8–8.5 wt%), CO₂ (457–1534 ppm) and maficity (FeO + MgO = 2.3–3.9 wt%). In contrast, melts at granulite facies reach highest CaO, FeO + MgO (3.2–4.7 wt%), and CO₂ (up to 2,400 ppm), with H₂O contents comparable (5.4–8.3 wt%) to the other two zones. Our results represent the first clear evidence for carbonic fluid‐present melting in the Ivrea Zone. Anatexis of metapelites occurred through muscovite and biotite breakdown melting in the presence of a COH fluid, in a situation of fluid–melt immiscibility. The fluid is assumed to have been internally derived, produced initially by devolatilization of hydrous silicates in the graphitic protolith, then as a result of oxidation of carbon by consumption of Fe³⁺‐bearing biotite during melting. Variations in the compositions of the melts are interpreted to result from higher T of melting. The H₂O contents of the melts throughout the three zones are higher than usually assumed for initial H₂O contents of anatectic melts. The CO₂ contents are highest at granulite facies, and show that carbon‐contents of crustal magmas are not negligible at high T. The activity of H₂O of the fluid dissolved in granitic melts decreases with increasing metamorphic grade. Carbonic fluid‐present melting of the deep continental crust represents, together with hydrate‐breakdown melting reactions, an important process in the origin of crustal anatectic granitoids.     

Occurrence of Contrasting skarn formations in dolomites of the Traversella deposit (Ivrea,Italy)

Metasomatic columns hosted in dolomitic marbles in the thermal aureole of the Traversella monzodiorite (Ivrea, Italy) differ by their mineralogy and/or mineral composition. Three groups have been distinguished. In group A, the first zone always contains forsterite-calcite and the second zone contains clinopyroxene. The last zone is made of wollastonite (A1), andradite-rich garnet (A 2) or grossular-rich garnet (A 3). In group B, tremolite instead of forsterite occurs in the first two zones. In group C columns, there are only two zones and clinohumite or chondrodite characterize the first zone.Field, petrographic and chemical data demonstrate that the occurrence of contrasted mineral zonations in the same protolith has not been induced by local heterogeneities in the dolomitic marbles. The presence of tremolite in group B instead of forsterite is due to the lower temperature prevailing in the external part of the contact aureole. Chemical data as well as - diagrams suggest that the columns in groups A and C were formed through the interaction of the dolomitic marbles with fluids with different _{Fe 2}O₃, _{Al 2 O 3} and _F. A fluid with low _{Al 2 O 3} and _{Fe 2}O₃ is assumed for the metasomatic column with wollastonite (A 1) and different Al₂O₃/Fe₂O₃ values in the fluid are responsible for the formation of columns A 2 and A 3. The stability of clinohumite or chondrodite in group C columns has been induced by a higher _F of the fluid than in the other groups. The presence during the prograde metasomatic stage of distinct fluids differing by their _{Al 2 O 3}/_{Fe 2}O₃ (columns A 2 and A 3) is likely to have been induced by variations in the source composition. Moreover, the low A1 and Fe content in column A 1 could be due to the reaction of the dolomitic marble with a fluid previously depleted in these components during percolation and reaction with the country rocks.As shown by isocon diagrams and Gresens's equation, skarn formation has locally induced both mass and volume change. The volume decreased in the wollastonite zone of A 1 (19%) and in the Fo-Cc zone of A2 (17%). Mass is always increased except in the wollastonite of A 1 where it is decreased. Mass balance of major elements has shown that Ca is likely to be a perfectly inert component and that Si is always strongly increased. Moreover, Fe is increased in A 2 and A 3 whereas Al is only increased in A 3. Mg is leached in the internal part of column Al.Quantification of chemical potential diagrams using different values of _{Al 2 O 3 Fe 2 O 3}, X _F and T suggests that the successive zones observed in all types of columns can be obtained along an increase of _{SiO 2} towards the inner zones. A simultaneous decrease in _MgO is inferred for group A columns.     

Intractions of mantle and crustal magmas in the southern part of the Ivrea Zone (Italy)

In the southern part of the Ivrea Zone (Italy), the majority of the Mafic Formation is composed of: 1. amphibole-bearing gabbro; 2. a series of rocks ranging from norites to charnockites; 3. leucocratic charnockites. In the proximity of metasedimentary septa within the Mafic Formation, the igneous lithologies are in many places intimately and chaotically intermingled, giving rise to a marble-cake structure. Whole-rock chemistry, and oxygen and strontium isotopic compositions indicate that the mafic and felsic rocks are dominated by mantle and crustal sources respectively. The norite-charnockite suite may be modelled as the mixing product of basic and acid melts. Abundant plastic deformation structures suggest that mafic and hybrid rocks experienced an important tectonic event during or soon after their crystallization. Melting of crustal country rocks continued after the deformation event and produced the undeformed leucocratic charnockites. The study area exemplifies some of the possible effects of the intrusion of a large volume of basic magma into hot crust.     

城子河组的新材料及其地层意义

黑龙江省密山县金沙农场北部鹿山地区被钻孔揭示的城子河组剖面产有海相一半成水相双壳类、沟鞭藻，孢子花粉及植物化石，这些新材料进一步证明城子河组的时代应属早白垩世早中期，从而为侏罗～白垩系界线划分及鸡西群与龙爪沟群对比提供了新的生物地层依据。     

New chronological and geochemical constraints on the genesis and geological evolution of Ponza and Palmarola Volcanic Islands (Tyrrhenian Sea, Italy)

A new geochronological and geochemical study of the volcanic rocks of the Ponza and Palmarola Islands, Pontine Archipelago, has been carried out. This archipelago is located along the boundary between the Italian continental shelf and the opening Tyrrhenian basin. It is a key area to study volcanism related to the opening of the Tyrrhenian Sea. Ponza is the oldest felsic magmatic manifestation in the central Tyrrhenian area. Previous studies suggested that Ponza volcanic activity began before 5 Ma. Twenty-five new K–Ar ages constrain the volcanic activity (rhyolitic hyaloclastites and dykes) to the last 4.2 Ma, with two episodes of quiescence between 3.7 and 3.2 Ma and between 2.9 and 1.0 Ma. A new volcanic episode dated at 3.2–2.9 Ma has been identified on the central and southern Ponza, with emplacement of pyroclastic units. At 1.0 Ma, a trachytic episode ended the volcanic activity. The near island of Palmarola exhibits rhyolitic hyaloclastites and domes dated between 1.6 and 1.5 Ma, indicating that the island was entirely built during the Early Pleistocene in a short span of time of ca. 120 ka. Although only 6–8 km apart, the two islands display significantly different geochemical signatures. Ponza rhyolites show major and trace element compositions representative of orogenic magmas of subduction/collision zones: high-K calc-alkaline and metaluminous rhyolites (Agpaitic Index [AI] and Alumina Saturation Index [ASI] <1), high LILE/HFSE (Th/Ta=16–21) and LREE/HFSE ratios (La/Nb>3), and Nb–Ta negative anomalies. In Palmarola, the orogenic character is also present, but much less marked than in Ponza: rhyolites have a peralkaline character (AI>1), lower LILE/HFSE (Th/Ta=11–15), low LREE/HFSE ratios (La/Nb=1–2) close to those of anorogenic lavas, and the Nb–Ta negative anomalies are almost absent. Y/Nb ratios indicate different magmatic sources, one similar to island-arc or active continental margin basalts for Ponza rhyolites, and the others probably involving an OIB type component for Palmarola rhyolites and Ponza trachytes. Palmarola volcanics represent a transitional magmatism: although a preserved collisional geochemical imprint, they show geochemical features approaching those of anorogenic lavas erupted in a within-plate context. The change of magmatism evidenced in this study can be related to the tectonic evolution of the area. Indeed, Hf, Ta and Rb contents suggest that the oldest Pliocene rhyolites of Ponza would emplace in a syn- to late-collisional setting, while the younger Pleistocene rhyolites of Palmarola would be emplaced in a post-collisional setting in which the orogenic character (Th/Ta) decreases and mantle influence (Nb/Ta) increases. Geochemical modeling strongly suggests that the Palmarola rhyolites represent the waning stages of a subduction-related magmatism. The K–Ar datings allow us to estimate precisely the transition of magmatism to last less than 1.3 Ma. The transitional magmas may be the result of the upwelling of asthenospheric mantle inducing melting of a metasomatized lithospheric mantle and the mixing between these two sources. This upwelling could occur during the extension of the Tyrrhenian basin, caused by the slab retreat and steepening, or during a process of slab break-off starting in the Pliocene.     

Multiple slump scars in the Tortonian type area (Piedmont Basin, northwestern Italy)

Multiple intraformational discordances, recognized in the Tortonian (Upper Miocene) type area, occur at the top of a shelf sequence, directly overlain by slope sediments. The discordances display smooth, concave upward lower surfaces and are covered by sediments identical to the surrounding strata. Inferred lateral extent of single discordances is 500–700 m. Thickness of sediments overlying the discordances ranges from 5 to 25 m. The intraformational discordances are interpreted as scars formed by downslope sliding of whole packets of strata. The slumps may have been triggered by tectonic oversteepening of shelf sediments due to tilting of the sea floor. The slump scars appear comparable in position and size to some small scale modern examples recorded on margins near the shelf-break.     

The structural evolution of the Asinara Island (NW Sardinia,Italy)

The metamorphic basement of the Asinara island represents a key area of the Sardinia Variscan segment, because it displays an almost complete cross-section through the inner part of the Sardinia Variscan belt, where different tectono-metamorphic complexes have been juxtaposed along narrow belts of high-strain concentration. Detailed field mapping coupled with preliminary studies on the structural and metamorphic features of this small island, allow to draw a better picture of the structural frame issued from the Variscan collision in the inner zone of the belt. Three deformation phases related to crustal thickening in a compressive and transpressive, partitioned tectonic regime, followed by a later phase of extensional deformation have been recognised. In spite of a general HT/LP metamorphic overprint, linked to the post-collisional deformation phases, a relic Barrovian zoneography is still detectable. The Barrovian assemblages are preto syn-kinematic with respect to the D2 deformation phase, and pre-date the third, contractional tectonic event.

The HT/LP assemblage indicates a static growth of weakly deformed by the last deformation events. The complex geometry of the fabric associated to the D2 and D3 deformation events suggests an heterogenous deformation history with a monoclinic geometry characterized by switching of the stretching lineation orientation and a contrasting sense of displacement, probably controlled by a northward partitioned pure shear.     

Glacier fluctuations in the western Alps during the Neoglacial,as indicated by the Miage morainic amphitheatre (Mont Blanc massif,Italy)

Holocene glacier variations pre‐dating the Little Ice Age are poorly known in the western Alps. Studied for two centuries, the Miage morainic amphitheatre (MMA) is composed of three subconcentric sets of c. 25 moraines. Because of its location and of a dominant mode of morainic accretion, the MMA is a well‐preserved marker of the glacier dynamics during the Neoglacial. Radiocarbon dates were obtained by digging and coring in inter‐ morainic depressions of the MMA and through a deep core drilling in a dammed‐lake infill (Combal); complementary data for the inner MMA were obtained by lichenometry and dendrochronology. Radiocarbon chronology shows that (i) the MMA not only pre‐dates the Little Ice Age (LIA), but was built at least since 5029–4648 cal. yr BP (beginning of the Neoglacial); (ii) outer sets of moraines pre‐date 2748–2362 cal. yr BP; (iii) the MMA dammed the Lake Combal from 4.8 to 1.5 cal. kyr BP, while lakes/ponds formed inside the moraines (e.g. from 2147–1928 to 1506–1295 cal. yr BP). The ‘Neoglacial model’ proposed here considers that the MMA formed during the whole Neoglacial by a succession of glacier advances at 4.8–4.6 cal. ky BP (early Neoglacial), around 2.5 cal. ky BP (end of Göschener I), at AD 600–900 (end of Göschener II) and during the LIA, separated by raising phases of the right‐lateral moraine by active dumping because of the Miage debris cover.     

Permian high-temperature metamorphism in the Western Alps (NW Italy)

International Journal of Earth Sciences - During the late Palaeozoic, lithospheric thinning in part of the Alpine realm caused high-temperature low-to-medium pressure metamorphism and partial...     

Stable Isotopic Evidence for Fluid-Rock Interactions in the Ivrea Zone, Italy

Stable isotope compositions of Ivrea Zone marbles and associatedlithologies are in general heterogeneous. The oxygen isotopecomposition of quartz in pelites ranges from ¹⁸O +9 to + 17(SMOW) and does not vary systematically with metamorphic grade.Peridotites retain oxygen isotope signatures close to mantlevalues. Marble calcites vary in isotopic composition from ¹³C + 2(PDB),¹⁸0 +24(SMOW)to ¹³C –6(PDB), ¹⁸O + 13 (SMOW).Depletions in ¹⁸O and ¹³C may be explained dominantly by interactionwith fluids derived from within the observed metasedimentarysequence during prograde metamorphism. ¹⁸O and ¹³C show gradients of greater than 5/m across marblemargins and within marbles. The preservation of such isotopicgradients is not consistent with the long-term presence of grain-boundary-scaleinterconnected fluid films in and around marbles. There is ageneral lowering of ¹⁸O within individual marble bodies althoughlarge carbon and oxygen isotopic gradients are present. Calcitein marbles may attain oxygen isotope equilibrium, but rarelycarbon isotope equilibrium, with surrounding metapelites. Infiltrationof marbles must involve a component of channelized fluid flow. The general lack of isotopic equilibration within the sequencerequires channelized fluid flow and limited fluid-rock ratios.Large pervasive mantle to crust fluid fluxes are not consistentwith the observations. ^*Present address: Natural Environment Research Council, Polaris House, North Star Avenue, Swindon SN2 1EU, England     

Complex bedding geometries and facies associations of the turbiditic fill of a confined basin in a transpressive setting (Castagnola Fm., Tertiary Piedmont Basin, NW Italy)

ABSTRACT This paper details the influence of syndepositional tectonics in controlling the architecture of a well‐exposed confined turbiditic sandbody, which crops out in the eastern part of the Tertiary Piedmont Basin (Castagnola Basin, northern Italy). The Castagnola Basin was tectonically active during sedimentation of the sandbody, and the lateral distribution of turbidity‐current deposits has been used to constrain both how the basin subsided and the impact of basin topography on flow behaviour and deposition. The sandbody occurs in the lower member of an Upper Oligocene–Lower Miocene turbidite system (the Castagnola Formation). The sandbody is ≈30 m thick and can be followed laterally for ≈1·8 km; it shows onlap terminations onto both northern and southern basin margins. The outcrop is sufficiently large to allow a detailed analysis of the facies and geometrical heterogeneity, as viewed approximately parallel to the average palaeocurrent trend (SW–NE). Correlation between 41 sedimentological logs reveals the diachronous development of a succession of sandstone packages (subunits). Nine vertically stacked and laterally juxtaposed packages have been recognized (subunits B to I from oldest to youngest), which reflect changes in basin floor accommodation as a result of synsedimentary tectonism. Each package shows the development of different vertical stacking patterns with thinning‐ and ‐fining‐upward small‐scale sequences and variable lateral facies arrangements, as a consequence of the position relative to the basin margins. The geometry, stratigraphic relationships, facies distribution and palaeocurrent directions indicate that turbidite deposition during accumulation of most of the sandbody was controlled by (1) synsedimentary tilting of the basin slopes; (2) the distribution of structural and depositional relief within the basin; (3) the thickness and volume of the turbidite flows; and (4) the angle of impingement of turbidity currents against the basin slopes.     

Metamorphism and Anatexis in the Mafic Complex Contact Aureole, Ivrea Zone, Northern Italy

Emplacement of mantle-derived magma (magmatic accretion) isoften presumed or inferred to be an important cause of regionalgranulite facies metamorphism and crustal anatexis. The juxtapositionof mafic cumulates and regionally distributed granulite faciesrocks has led some to consider the Ivrea zone (northern Italy,Southern Alps) as an important exposure that demonstrates thiscausal relationship. However, regional PTt paths indicated bymetamorphic reaction textures and PT conditions inferred fromgeothermobarometry indicate that the emplacement of mafic plutonicrocks (Mafic Complex) at the Ivrea zone occurred during decompressionfrom ambient pressures at the regional thermal maximum. Fieldand petrographic observations, supported by PT estimates, indicatethat regional retrograde decompression and emplacement of theupper parts of the Mafic Complex probably accompanied extensionduring the Late Carboniferous–Early Permian. A spatiallyrestricted decompression-melting event accompanied final emplacement,depleting supracrustal rocks enclosed by an