Finero Harzburgite

The Ivrea-Verbano Zone (Fig.1) belongs to the Southern Alps and is separated at the NW from the Alpine terrains by a major fault, the Insubric line. Other tectonic lines (the Pogallo and Cossato-Mergozzo-Brissago lines juxtapose to the SE the Ivrea-Verbano with the amphibolite-facies terrains and igneous plutons of another unit of southern Alps, the Serie dei Laghi Zone. Geophysical data indicate that the Ivrea-Verbano Zone extends under the Po plane, where it is subhorizontal. Near the surface, the Ivrea-Verbano terrains steeply dip to the SE and turn to subhorizontal at a depth of 20-30 km. This sequence was vertically uplifted during the Mesozoic crustal thinning and tilted to the actual subvertical position during the Alpine collision. The Ivrea-Verbano Zone is constituted by three main units: the Kinzigite Formation, the Mafic Complex, the Mantle Peridotites.

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Fig.1: Ivrea-Verbano Geological map. IL = Insubric Line; CMBL = cossato-mergozzo-bissago Line; PL = Pogallo Line. From G.Rivalenti (1999)



Kinzigite Formation
It is an amphibolite- to granulite-facies volcano-sedimentary sequence where the dominant sedimentary protolith was pelitic. Other sedimentary components were arenaceous sediments and limestones (actually marbles), these latter expecially useful as marker horizons revealing the regional structure. The dominant igneous component of the volcano-sedimentary sequence was the basalt, actually transformed into amphibolite and pyribolite in the amphibolite- and granulite-facies domains, respectively. The granulite-facies and amphibolitefacies metamorphism record temperatures higher than 800oC and lower than 700oC and pressures between 0.6 - 1.1 GPa and 0.4 - 0.7 GPa, respectively.

Mafic Complex
It is intrusive into the Kinzigite Formation and primary contacts are preserved in several occurrences (Fig.2). It has its maximum thickness in the southern part of the Ivrea-Verbano Zone, expecially in Val Sesia, where it may reach 11 km. In central Ivrea-Verbano it occurs as sills of variable thickness, whereas the northernmost part of Ivrea-Verbano has a composite nature, dominated by sill structures in the deeper part, overlain by a huge mass of mafic and ultramafic rocks representing another, possibly genetically unrelated, intrusion.

Since 1975 it was shown that the Val Sesia section consists, from base to top, of a layered series, a huge and virtually homogeneous gabbro (Main Gabbro unit), and an upper dioritic zone (Diorite unit) in contact with the metasedimentary sequence. The layered series is on its turn distinguished into a basal (BZ), an intermediate (IZ) and upper (UZ) zone. The basal and intermediate zones consists of alternating ultramafic and mafic (gabbro) layers, which crystallized under a pressure gradient documented by the coexistence of clinopyroxene and spinel in BZ and olivine and plagioclase in IZ. The upper zone has only rare ultramafic layers and the lithology is dominantly gabbro-anorthosite. The MG unit is constituted by a hornblende-bearing norite. Although diorite predominate in the uppermost Diorite unit, monzonitic, or even granitic (leucocharnokites) compositions occur in tectonically controlled domains.Lenses and major horizons (septa) of the Kinzigite Formation are very common in the layered series and a major metasedimentary horizon occurs at the IZ top.

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Fig.2: Ivrea-Verbano mafic complex. From S.Sinigoi (1994)



Finero Harzburgite

Some Authors suppose that an eclogite-facies subducted (of probably Variscan age) plate produces H2O ± undersatured, silica rich, containing some CO2. These melts infiltrate the overlying harzburgitic mantle and, because of the higher temperature of rocks in contact with the subducted slab, become heated during decompression (at 2GPa the temperature of the mantle wedge may be well in excess of that of amphibole stability). The viscosity of the melt decreases at decompression and this favours infiltration of large mantle volumes. If the rock-melt ratio is high, the melts will reach a temperature close to that of the peridotite. Two situations are envisaged depending on whether the temperature is lower or higher than that required for inducing initial melting in the resulting hydrous peridotite system:

The Finero Peridotite massif is a Harzburgite that suffered a dramatic metasomatic enrichment resulting in the pervasive presence of Amphibole and Phlogopite and in the sporadic occurence of apatite and carbonate (dolomite) bearing domains.

The Finero harzburgite, 11 x 1 km wide, is the most north easterly lens of the peridotite bodies occurring close to the western margin of the Ivrea Verbano mafic formation. The Finero body markedly differs in its geochemical and isotopic characteristics from the other Ivrea peridotites, the major being those of Balmuccia and Baldissero. These latter are depleted Lherzolites showing absent (Baldissero) or weak (Balmuccia) metasomatic effects limited to the regions at pyroxenite contacts (Rivalenti et al. 1995).

1. If the temperature is lower, melt would react with the peridotite during percolation, increasing the amount of orthopyroxene and forming new reaction phases. The trace element geochemistry would be controlled by percolation-reaction processes, resulting in marked concentration gradients, and eventual direct evidence of the slab-derived melt may be provided by glass inclusions in the peridotite minerals. This situation, which cannot apply to Finero, is normally well documented in mantle xenoliths.

2. If the resulting temperature is above that of the incipient melting of the hydrous peridotite system, the slab derived melt equilibrates with the harzburgite and a crystal mush consisting of harzburgite and a silica saturated, hydrous melt is formed. During cooling, the crystal mush crystallizes producing the observed sequence of mineral phases and their observed chemical characteristics. In this context pyroxenites are regions of higher concentration of the melt in equilibrium with the harzburgite and not passage-ways through which exotic melts percolated.

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Foliation in the Finero Harzburgite.



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Finero Harzburgite, Olivine (pale green), Cr-diopside (Emerald green) and Phlogopite.



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Finero Harzburgite, Olivine (pale green), Cr-diopside (Emerald green) and Phlogopite.



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Cr-diopside vein in the Finero Harzburgite.



Bibliography



• Chemical evolution of a large mafinc intrusion in the lower crust, Ivrea-verbano zone, Northen Italy. S.Sinigoi, James. E. Quick, Diane clemens-knott, Adriano Mayer, Gabriella Demarchi, Maurizio mazzucchelli, Luisa negrini and Giorgio Rivalenti. Journal of gophysical research, vol. 99 (1994)
• The Finero Phlogopite-peridotite massif: an exemple of subduction-relataed metasomatism. Alberto Zanetti, Maurizio Mazzucchelli, Giorgio Rivalenti, Riccardo Vannucci. Contrib.minerl.petrol. (1999)

Photo
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Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 1x (Field of view = 9mm)
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Olivine and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 1x (Field of view = 9mm)
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Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 1x (Field of view = 9mm)
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Olivine and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 1x (Field of view = 9mm)
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Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 1x (Field of view = 9mm)
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Olivine and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 1x (Field of view = 9mm)
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Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 1x (Field of view = 9mm)
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Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 1x (Field of view = 9mm)
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Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 1x (Field of view = 9mm)
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Olivine (colorles), pyroxene (with cleavage) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 1x (Field of view = 9mm)
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Olivine, pyroxene (with cleavage) and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 1x (Field of view = 9mm)
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Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 1x (Field of view = 9mm)
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Olivine (colorles), pyroxene (with cleavage) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)
finero2017(2).jpg

Olivine, pyroxene (with cleavage) and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)
finero2017(3).jpg

Olivine (colorles), pyroxene (with cleavage) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)
finero2017(4).jpg

Olivine, pyroxene (with cleavage) and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)
finero2017(5).jpg

Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)
finero2017(6).jpg

Olivine and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)
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Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)
finero2017(8).jpg

Olivine and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)
finero2017(9).jpg

Olivine, pyroxene (with cleavage) and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)
finero2017(10).jpg

Olivine, pyroxene (with cleavage) and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)
finero2017(13).jpg

Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)
finero2017(14).jpg

Olivine and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)
finero2017(17).jpg

Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)
finero2017(15).jpg

Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)
finero2017(16).jpg

Olivine and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)
finero2017(19).jpg

Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)
finero2017(20).jpg

Olivine and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)
finero2017(21).jpg

Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)
finero2017(22).jpg

Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)
finero2017(23).jpg

Olivine and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)
finero2017(24).jpg

Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)
finero2017(25).jpg

Olivine and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)
finero2017(26).jpg

Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)
finero2017(27).jpg

Olivine and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)
finero2017(28).jpg

Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)
finero2017(29).jpg

Olivine and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)
finero2017(30).jpg

Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)
finero2017(31).jpg

Olivine and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)
finero2017(32).jpg

Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)
finero2017(33).jpg

Olivine and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)
finero2017(34).jpg

Olivine (colorles), pyroxene (with cleavage) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)
finero2017(35).jpg

Olivine, pyroxene (with cleavage) and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)
finero2017(36).jpg

Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)
finero2017(62).jpg

Pyroxene (with cleavage) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 10x (Field of view = 2mm)
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Pyroxene (with cleavage) and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 10x (Field of view = 2mm)
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Olivine (colorles), pyroxene (with cleavage) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 10x (Field of view = 2mm)
finero2017(65).jpg

Olivine (colorles), pyroxene (with cleavage) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 10x (Field of view = 2mm)
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Olivine, pyroxene (with cleavage) and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 10x (Field of view = 2mm)
finero2017(69).jpg

Olivine (colorles), pyroxene (pale green) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 10x (Field of view = 2mm)
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Olivine (colorles), pyroxene (with cleavage) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 10x (Field of view = 2mm)
finero2017(76).jpg

Olivine, pyroxene (with cleavage) and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 10x (Field of view = 2mm)
finero2017(77).jpg

Olivine (colorles), pyroxene (with cleavage) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 10x (Field of view = 2mm)
finero2017(81).jpg

Olivine (colorles), pyroxene (with cleavage) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 10x (Field of view = 2mm)
finero2017(82).jpg

Olivine, pyroxene (with cleavage) and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 10x (Field of view = 2mm)
finero2017(85).jpg

Olivine (colorles), pyroxene (with cleavage) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 10x (Field of view = 2mm)
finero2017(87).jpg

Olivine (colorles), pyroxene (with cleavage) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 10x (Field of view = 2mm)
finero2017(88).jpg

Olivine, pyroxene (with cleavage) and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 10x (Field of view = 2mm)
finero2017(89).jpg

Olivine (colorles), pyroxene (with cleavage) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 10x (Field of view = 2mm)
finero2017(92).jpg

Olivine (colorles), pyroxene (with cleavage) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 10x (Field of view = 2mm)
finero2017(93).jpg

Olivine (colorles), pyroxene (with cleavage) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 10x (Field of view = 2mm)
finero2017(67).jpg

Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 10x (Field of view = 2mm)
finero2017(68).jpg

Olivine and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 10x (Field of view = 2mm)
finero2017(73).jpg

Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 10x (Field of view = 2mm)
finero2017(74).jpg

Olivine and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 10x (Field of view = 2mm)
finero2017(79).jpg

Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 10x (Field of view = 2mm)
finero2017(80).jpg

Olivine and phlogopite crystals. Finero Harzburgite, Ivrea Verbano, Italy. XPL image, 10x (Field of view = 2mm)
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Olivine (colorles) and phlogopite (brown) crystals. Finero Harzburgite, Ivrea Verbano, Italy. PPL image, 10x (Field of view = 2mm)