Deep crustal extensional faulting in the Ivrea Zone of Northern Italy

The Ivrea Zone in Northern Italy consists of amphibolite and granulite facies metasedimentary, metabasic, and ultrabasic rocks, and is thought to represent a section through the deep continental crust. Detailed mapping of the high grade part of this zone has revealed a series of high temperature shear zones, which, after the effects of later low temperature faulting and folding have been removed, appears to accommodate extension of the sequence. The shear zones formed at temperatures similar to those at which thermal equilibration occurred in the host rocks, and locally show evidence for development under prograde conditions. These observations fit well with thermal effects calculated from a simple heat flow model for extensional faulting. In this model the effects of shear heating and displacement of cool hanging-wall rocks against hotter footwall rocks are calculated.

These observations indicate that faulting as a mode of failure may be important during extension of the lower continental crust, especially when basic rocks are the dominant components of the rock pile. A restored section through the extended lower crust as observed in the Ivrea Zone is presented, and the extent to which the features observed are able to explain seismic reflectivity of the lower crust is discussed.     

Stress-induced natural transformation of ortho- to clinohypersthene in metagabbros of the Ivrea Zone,Northern Italy

Summary Orthopyroxenes of a high temperature protomylonite of the Ivrea Zone, Northern Italy show twin like polysynthetic lamellae parallel to {210} of the hypersthene host. The transformation is caused by plastic deformation under high metamorphic conditions which has resulted in dynamic recrystallization of pyroxene and plagioclase. The lamellae consist of clinohypersthene. The twin plane and the lamellar clino-ortho-inversion of hypersthene due to natural deformation have not been described hitherto.

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Stressinduzierte natürliche Transformation von ortho- zu Klinohypersthen in Metagabbros der Ivrea-Zone, Norditalien

Zusammenfassung Orthopyroxene aus hoch temperierten Protomyloniten der Ivrea-Zone, Nord Italien zeigen polysynthetische Lamellen parallel {210} des Wirtes. Ihr Erscheinungsbild entspricht Deformationszwillingen. Die Lamellen sind invertiert zu Klinohypersthen. Die Ortho-Klino-Transformation ist auf eine Hochtemperaturdeformation zurückzuführen, wie dies anhand der dynamischen Rekristallisation der Pyroxene und Plagioklase bewiesen werden kann. Weder die speziellen Verwachsungsflächen noch die Ortho-Klino-Inversion durch natürliche Deformation wurden bisher beschrieben.

     

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     

Fluid Flow During Contact Metamorphism of Ophicarbonate Rocks in the Bergell Aureole, Val Malenco, Italian Alps

Contact-mctamorphic assemblages in ophicarbonate from the Bergellaureole correspond either to model isobaric invariant T-XCO₂points [Atg-Cal-Di-Tr-Fo (6 samples) and Atg-Cal-Tr-Fo-Dol (2)]or to isobaric univariant T-XCO₂, curves [Tr-Cal-Di-Atg (18),Tr-Dol-Atg-Cal (1), Atg-Cal-Fo-Di (1), and Atg-Cal-Tr-Fo (1)].Calcite-dolomite thermometry and mineral-fluid equilibria inthe invariant assemblages record T=440–540C at P=3•5kbar. Equilibrium metamorphic fluids were very H₂O rich withX CO₂,=0•001–0•027. In the invariant assemblagesTr + Fo were produced by prograde decarbonation-dehydrationreactions. In contrast, measured modes and reaction texturesin samples with univariant assemblages indicate thai Tr wasproduced by carbonation reactions. The apparent paradox of simultaneousdecarbonation reactions in the model isobaric invariant assemblagesand carbonation reactions in univariant assemblages is resolvedby local mineral-fluid equilibrium and fluid flow through ophicarbohatesin the direction of decreasing temperature as the aureole heated.Time-integrated flux (q) was computed from measured reactionprogress in 28 samples for models of both horizontal and verticaldown-temperature flow. Results are similar, with q decreasingrapidly from (0•2–5•1) 10⁵ cm³ fluid/cm² rock1•3–1•7 km from the intrusion to 0–0•610⁵cm³/cm² at 1•8–4•0 km. The decrease in q ismore consistent with vertical than horizontal flow. Variationsin time-integrated flux of more than an order of magnitude arerecorded by samples from the same outcrop. The absence of carbonatein adjacent metaperidotite indicates that flow was confinedto the ophicarbonate. Channelized, spatially heterogeneous,vertical flow can be explained by the brecciation and strongvertical foliation of the ophicarbonate relative to surroundingmassive metaperidotite. Generation of metamorphicfluids by decarbonation-dehydrationreactions within the ophicarbonates explains larger averageflux 1–2 km from the intrusion compared with more distalpoints. KEY WORDS: Bergell; contact metamorphism; fluid flow; ophicarbonate ^*Telephone: (410) 516-8121. Fax: (410) 516-7933     

北京周口店官地杂岩变质-深熔作用的锆石 LA-ICP-MS U-Pb定年

应用LA-ICP-MS技术测得北京周口店官地杂岩深熔成因浅色体和其源岩--斜长角闪岩中的锆石U-Pb年龄分别为(2 548+23/-24) Ma和(2 538+34/-35) Ma.这表明,官地杂岩在新太古代末期发生了高级区域变质作用和深熔作用.定年结果为华北陆块的东部地块新太古代末期的克拉通化事件提供了进一步的变质年代学证据.     

Metamorphism, Fluid Flow and Partial Melting in Pelitic Rocks from the Onawa Contact Aureole, Central Maine, USA

Field, petrologic and geochemical data were used to characterizefluid infiltration and partial melting during metamorphism ofpelitic rocks in the contact aureole of the Onawa pluton, centralMaine, USA. Mineral assemblages delineate five metamorphic zoneswithin the contact aureole: chlorite zone, andalusite–cordierite(a–c) zone, alkali feldspar zone, sillimanite zone andleucocratic-vein (l–v) zone. The sequence of observedmineral assemblages and mineral–fluid reactions calculatedby mass balance is similar to those observed in other contactaureoles. Pressure of contact metamorphism is 3 kbar, on thebasis of optimum geothermobarometry calculations. Metamorphictemperatures vary from 500C in the andalusite–cordieritezone to 65OC in the leucocratic-vein zone. Data from fieldobservations, mineral textures, observed reaction stoichiometry,geothermometry and major-element geochemistry suggest that theleucocratic veins of the l-v zone represent crystallized, partialmelts. Two overall calculated mineral reactions are responsiblefor vein formation: which can be modeled as combinations of two NKFMTASH meltingreactions: Progress of (M1) and (M2) was measured in eight samples, andreaction (M1) is the dominant melt-forming reaction in all samples.Partial melting (and vein formation) was therefore driven byinfiltration of the l-v zone by H²O-rich fluids. Calculatedtime-integrated fluid fluxes for l-v zone samples range from09 10⁴ to 31 10⁴ mol/cm², and flow was in the directionof increasing temperature. KEY WORDS: pelites; contact metamorphism; fluid infiltration; partial melting; Onawa Pluton; Maine; USA ^*Corresponding author. Telephone:(516) 632–8192. Fax (516)632–8240 e-mail: gsymmes{at}ccmail.sunysb.edu     

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 (     

P-T-t Path of Mafic Granulite Metamorphism in Northern Tibet and Its Geodynamical Implications

Mafic granulites have been found as structural lenses within the huge thrust system outcropping about 10 km west of Nam Co of the northern Lhasa Terrane, Tibetan Plateau. Petrological evidence from these rocks indicates four distinct metamorphic assemblages. The early metamorphic assemblage (M1) is preserved only in the granulites and represented by plagioclase+hornblende inclusions within the cores of garnet porphyroblasts. The peak assemblage (M2) consists of garnet+clinopyroxene+hornblende+plagioclase in the mafic granulites. The peak metamorphism was followed by near-isothermal decompression (M3), which resulted in the development of hornblende+plagioclase symplectites surrounding embayed garnet porphyroblasts, and decompression-cooling (M4) is represented by minerals of hornblende+plagioclase recrystallized during mylonization. The peak (M2) P-T conditions of garnet+ clinopyroxene+plagioclase+hornblende were estimated at 769-905℃ and 0.86-1.02 GPa based on the geothermometers and geobarometers. The     

Local disequilibrium of plagioclase in high-temperature shear zones of the Ivrea Zone, Italy

Abstract Microstructural and chemical analysis of plagioclase in 20 superficially similar amphibolite facies ductile shear zones in metagabbors and amphibolites of the Ivrea Zone in Italy reveals significant differences in An and Ba contents. Plagioclase, which was deformed at P-T conditions lower than those of the wall rocks, occurs in the following four different microstructural situations with different chemical compositions: (i) relatively undeformed porphyroclasts, (ii) dynamically recrystallized grains and subgrains rimming the porphyroclasts, (iii) infill of microcracks cross-cutting the porphyroclasts and (iv) fine-grained recrystallized grains in the matrix of the shear zones. The differences in the An and Ba contents are caused by partial chemical equilibration of plagioclase in the shear zones during and partly after deformation. Changes in An and Ba contents were caused by fluid-assisted grain-boundary migration recrystallization, as well as by solid-state diffusion, while fluid activity was high. The relation between the composition and microstructures of the plagioclase in the shear zones indicates that in the different shear zones, fluids ceased to be active during different stages in the late shear zone deformation history.
The interpretation of the variations in composition and microstructures reveals that only grains that developed by grain-boundary migration recrystallization and that are not adjacent to porphyroclasts reflect P-T conditions during the dominant shear-zone deformation.     

The Role of Water Retention in the Anatexis of Metapelites in the Bushveld Complex Aureole, South Africa: an Experimental Study

Highly restitic metapelites occur at the contact of the RustenburgLayered Suite (Bushveld Complex). On the basis of previous experimentalstudies, the high (     

Mineralogical Evidence for Fluid-Rock Interaction Accompanying Prograde Contact Metamorphism of Siliceous Dolomites: Alta Stock Aureole, Utah, USA

Contact metamorphism of siliceous dolomite in the southern partof the metamorphic aureole of the Alta stock (Utah, USA) producedthe prograde isograd sequence: talc (Tc), tremolite (Tr), forsterite(Fo), and periclase (Per). Calcite (Cc)–dolomite (Do)geothermometry and phase equilibria define a general progradeT–X(CO₂) path of decreasing X(CO₂) with rising temperaturefor the dolomite. High-variance assemblages typify the aureole.Per + Cc and Fo + Cc + Do characterize the inner aureole (Perand Fo zones), and Tr + Do + Cc and Tc + Do + Cc are widespreadin the outer aureole (Tr and Tc zones). Low-variance assemblagesare rare and the thickness of reaction zones (coexisting reactantand product minerals) at the isogradic reaction fronts are narrow(tens of metres or less). The mineral assemblages, calculatedprogress of isograd reactions, and the prograde T–X(CO₂)path all indicate that massive dolomite was infiltrated by significantfluxes of water-rich fluids during prograde metamorphism, andthat the fluid flow was down-temperature and laterally awayfrom the igneous contact. Fluid infiltration continued throughat least the initial retrograde cooling of the periclase zone.Down-T fluid flow is also consistent with the results of Cc–Dogeothermometry and patterns of ¹⁸O depletion in this area. Theclose spatial association of reacted and unreacted chert nodulesin both the tremolite and talc zones plus the formation of tremoliteby two reactions indicate that the outer aureole varied in X(CO₂),and imply that fluid flow in the outer aureole was heterogeneous.The occurrence of dolomite-rich and periclase (brucite)-absent,high-     

The effect of metasomatism on the Cr-PGE mineralization in the Finero Complex, Ivrea Zone, Southern Alps

The magmatic metasomatism that was responsible for producing chromitite–dunite bodies in the unusual phlogopite peridotite of the Finero Complex in Permian to Triassic times also influenced the Cr-platinum group elements (PGE) mineralization. At least the end stages of this metasomatism are recorded in compositional zoning of chromite grains in the podiform chromitite. Metasomatic melt, with or without vapor, reacted with chromite to produce core-to-rim Cr enrichment of extant chromite grains and was concurrent with pyroxene crystallization. Under conditions of lower melt/rock ratio, metasomatism resulted in core-to-rim Al enrichment in chromite and crystallization of amphibole between chromite and clinopyroxene. This early, high-temperature metasomatism is unrelated to the later and pervasive K-metasomatism that crystallized phlogopite and was associated with the intrusion of clinopyroxenite dikes that cut the peridotite. Much later, serpentinization of olivine locally depleted chromite in Al and enriched it in Fe and formed minor amounts of magnetite.The PGE, which are present mainly as laurite inclusions in chromite, were remobilized by the early metasomatism. This resulted in substantial variation in the PGE contents of chromitites and imposed a characteristic PGE pattern in which chondrite-normalized Os, Ir, Ru and Rh contents are high but Pt and Pd contents are low. The slopes of PGE chondrite-normalized concentration patterns are systematically related to absolute PGE abundance and to rock mode. Chromitites with low modal orthopyroxene, clinopyroxene, and amphibole exhibit negative PGE slopes and contain relatively high PGE concentrations, whereas chromitites rich in these silicate minerals have positive slopes and low PGE contents.     

Mafic Magma Intraplating: Anatexis and Hybridization in Arc Crust, Bindal Batholith, Norway

The dioritic Velfjord plutons (     

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.     

Petrogenesis of the Anorthosite Dyke Swarm of Tromso, North Norway: Experimental Evidence for Hydrous Anatexis of an Alkaline Mafic Complex

The 456 ± 4 Ma Skattøra migmatite complex in thenorth Norwegian Caledonides consists of migmatitic nepheline-normativemetagabbros and amphibolites that are net-veined by numerousnepheline-normative anorthositic and leucodioritic dykes. Plagioclase(An_20–50) is the dominant mineral (85–100%) in thedykes and the leucosome, but amphibole is generally presentin amounts up to 15%. The following observations strongly suggestformation of the anorthositic magma by anatexis of the surroundinggabbro in the presence of an H₂O-bearing fluid phase: (1) themigmatites have plagioclase-rich (anorthositic) leucosomes andamphibole-rich restites; (2) crystallization of amphibole inthe anorthositic and leucodioritic dykes suggests high H₂O activity;(3) the presence of coarse-grained to pegmatitic dykes and miaroliticcavities indicates a fluid-rich magma; (4) hydration zones thatsurround many anorthosite dykes suggest that the magma probablyexpelled H₂O-rich fluids during crystallization. Water-saturatedmelting experiments at 0·5–1·5 GPa and temperaturesfrom 800 to 1000°C have been performed on a nepheline-normativegabbro to test the proposed petrogenesis of the Skattøraanorthosites. The glasses produced close to the solidus aretonalitic in composition, but they become richer in plagioclaseat higher temperatures. At and below 1·0 GPa, the residuesare composed of amphibole. Experiments above 1·0 GPaproduced residual garnet and/or zoisite in addition to amphibole,suggesting that the anorthositic dykes in the Skattøramigmatite complex formed below 1·25 GPa. The experimentsshow that the high Na₂O content of the anorthosite dykes canonly be produced if Na is added to the charges. The glass thatbest fits the composition of the Skattøra dykes was producedat 1·0 GPa and 900°C with 2 wt % Na(OH) added. KEY WORDS: anorthosite; dyke swarm; anatexis; experimental petrology     

Contact metamorphism of impure dolomitic limestone in the Boulder Aureole,Montana

Progressive metamorphism of impure dolomitic limestone in the 1.5 to 2.5 km wide contact aureole surrounding the northernmost portion of the boulder batholith has resulted in a consistent sequence of uniformly distributed zones of low-variance mineral parageneses separated by abrupt and distinctive isograds. In silica-undersaturated, aluminous marbles, the following mineral assemblages occur, in order of increasing grade: calcite-dolomite-calcic amphibole-chlorite, calcite-dolomite-calcic amphibole-chlorite-spinel, calcite-dolomite-calcic amphibole-chlorite-olivine-spinel, calcite-dolomite-chlorite-olivine-spinel, calcite-dolomite-olivine-spinel. The spatial distribution of parageneses and the occurrence of low-variance parageneses indicate buffering of the pore fluid composition by the local mineral assemblages. The observed sequence of mineral reactions and the spacing of isograds is in good agreement with experimental and calculated equilibria in terms of P-T-X _{CO 2}and temperatures of equilibration inferred from calcite-dolomite geothermometry, which range from 435 to 607 °C across the aureole.Microprobe analyses of coexisting minerals indicate attainment of exchange equilibrium. Calcic amphibole and chlorite coexisting with calcite and dolomite become progressively more aluminous with increasing grade; calcic amphibole changes rapidly from Al-poor tremolite to pargasite, while Al^IV in Cte increases from 2.0 to 2.3 atoms per 8 tetrahedral sites. Observed low-variance assemblages fix the activities of calcic amphibole and chlorite end-member components as a function of P and T, and hence the systematic compositional variation in these phases is not an independent variable, but is controlled by the local mineral assemblage.     

Metamorphism and Fluid Infiltration of the Calc-silicate Aureole of the Beinn an Dubhaich Granite, Skye

The Tertiary Beinn an Dubhaich granite intruded at 75OC and05 kb into siliceous dolostones and limestones of the UpperDurness Group in Strath, Skye, with the consequent developmentof talc, tremolite, diopside, olivine, and periclase in thebulk of the aureole, and abundant fluoro-borosilicate skarnsimmediately adjacent to the pluton. With increasing grade thelimestones develop the mineral sequence talc, tremolite, diopside,and olivine, whereas the dolostones develop the sequence talc,tremolite, olivine, and periclase. The abundant chert nodulesin the dolostones take either of the two reaction paths, dependenton their size. Those below 2–7 mm in dimension followthe dolostone reaction path, whereas larger nodules follow thelimestone reaction path. The presence of monticellite in thehighest-grade rocks points to the flushing of the contact byvolumes of water-rich fluid, derived presumably from the granite.Consideration of low-grade olivine-bearing veins and fracturesin the dolostones also points to the presence of extremely water-richfluid in the more distal parts of the aureole. Using simplebox models with constant porosity, it is shown that the observedreaction progress in oli vine-grade and talc-grade rocks canonly be accounted for if the rocks were infiltrated by substantialvolumes of water-bearing fluid. Minimum estimates for volumesof infiltrated fluid show that rocks nearest the pluton wereprobably infiltrated by greater amounts of fluid than thosefurther away. Low-grade rocks which suffered greatest amountsof infiltration are the brecciated dolostones nearest the Thrust.     

Processes and Conditions During Contact Anatexis, Melt Escape and Restite Formation: the Huntly Gabbro Complex, NE Scotland

The Huntly Gabbro is one of a suite of large, Ordovician, syn-orogenic,mid-crustal, layered, mafic intrusions, emplaced into Proterozoicmetaclastic rocks of NE Scotland soon after the thermal peakof static, high-T, low-P regional metamorphism. This gabbroand its associated contact metamorphic rocks illustrate a varietyof processes operating during contact anatexis and subsequentmelt segregation and extraction. These processes may closelymirror those occurring at much larger scales in the deep crustduring high-grade regional metamorphism and the generation ofgranitic magmas. The emplacement of the Huntly mafic magma resultedin high-grade contact metamorphism and, locally, anatexis ofmetapelites, leading to the formation of migmatites. The migmatitesand country-rock schists were studied to establish the physicalconditions of metamorphism and anatexis, the nature of the meltingreactions, the compositions of the melts produced, and the extentto which melting was a closed- or open-system process. The country-rockschists immediately to the south of the Huntly Complex containmineral assemblages characteristic of the regional andalusitezone. Thermobarometry of an andalusite schist yields regionalmetamorphic conditions of 537 ± 42°C and 0·27± 0·12 GPa, consistent with previously publishedP–T estimates. The contact metamorphic rocks include sillimanitehornfelses, metatexites and diatexites. The metatexites consistof cordierite–K-feldspar hornfels melanosomes and K-feldspar-richgarnetiferous leucosomes. The diatexites consist of schollenof fine-grained granoblastic hornfels and metatexite suspendedin igneous-textured matrix rocks composed of abundant sub/euhedralgarnet, cordierite, plagioclase and, locally, orthopyroxene,with minor interstitial biotite, K-feldspar and quartz. Thehornfels melanosomes and schollen retained their structuralintegrity during partial melting, but the matrix rocks did not.In the highest-grade diatexites, the assemblage Grt + Opx +Crd + Hc + Pl characterizes both the hornfels schollen and thesub/euhedral minerals of the matrix rocks. Application of phaseequilibria to Opx-bearing rocks yields estimated peak-metamorphicconditions of 900 ± 50°C, 0·45 ± 0·1GPa and a_H2O < 0·3. The pressure estimate impliesan emplacement depth of     

Bulk composition and mineral parageneses of sapphirine-bearing rocks along a gabbro-lherzolite contact at Finero, Ivrea Zone, N Italy

Sapphirine occurs in a 3-5 m wide zone between amphibole-lherzolite and garnetiferous metagabbro at Finero in the Ivrea Zone, NW Italian Alps. Layers consisting of plag + hb + sa + cpx + opx + sp + gt are interbanded with spinel pyroxenites, which may contain sapphirine replacing spinel. All minerals are very magnesian, with X_Mg between 0.78 and 0.92. Bulk rock analyses suggest that precursors to the sapphirine-bearing rocks were igneous cumulates of plagioclase + olivine + hornblende + spinel. Up to 16wt% CaO does not inhibit sapphirine formation and it is the unusually Mg-rich nature of the host rocks which allows sapphirine development. The early igneous assemblage was replaced by one of cpx + sa + hb +± plag at a pressure of 9 ± 1 kbar and temperatures of 900 ± 50°C. Subsequent rapid uplift caused the instability of gt, gt + hb, hb and sa + cpx to form opx + plag ± sp ± sa symplectites.     

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.