Jadeite metagranite

The Sesia-Lanzo Zone: The Sesia Zone is a large Austroalpine continental terrane located in the Internal Western Alps (Fig. 1). The Sesia Zone (also known as the Sesia-Lanzo Zone) is the most easterly unit of the axial belt of the Western Alps. It is bounded by the Insubric Line (locally called External Canavese Line) to the east, the Piemonte Zone to the west, and the Lanzo Ultramafic Massif to the south. The Sesia Zone is a composite unit that consists predominantly of a polymetamorphic basement intruded by early Permian granites and gabbros. The Sesia Zone underwent Alpine under HP condition (ca. 1.4-1.6 GPa and 500-600 °C); during Late Cretaceaous to Early Palaeocene times, ca. 70-65 Ma.

Fig.1: (a) Tectonic outline of the Alps. (b) Simplified geological map of the Sesia-Lanzo zone. The table shows protoliths, physical conditions of Alpine metamorphism and suggested metamorphic ages for each unit. Modified after Zucali and Spalla (2011).

Based on petrographic and structural investigations, the Sesia-Lanzo Zone is subdivided into three main complexes:

Eclogitic Micaschist Complex: The Eclogitic Micaschists Complex (EMC) consists of a polycyclic basement, which consists of paragneisses, minor metamafic rocks, orthogneisses and impure marbles. This basement is intruded by abundant Carboniferous to Permian granitoids and minor gabbroic rocks. Granitoids are dominant in the northeastern part of the Sesia Zone; they are traditionally labelled "Sesia Gneiss". The EMC reached maximum pressures less than 2GPa at temperatures lower than 600°C.

Gneiss Minuti Complex: The Gneiss Minuti Complex (GMC) mainly consists of orthogneisses interbedded with more mafic layers rich in albite, white mica, chlorite, epidote, and actinolite. In several exposures, such as in the lower Aosta Valley, the orthogneisses appear to have derived from granitoids of probable Permian age which intruded into a locally preserved polycyclic basement. In contrast, in other locations, such as the Orco Valley and the southern Sesia Zone, the Gneiss Minuti Complex does not show any trace of pre-Alpine metamorphism and probably developed at the expense of a Mesozoic sedimentary sequence comprising predominant meta-arkose and raremarble, calcschist, and metachert.

Second Diorito-Kinzigitic Zone: The Second Diorito-Kinzigitic Zone (IIDK) consists of slivers of well-preserved pre-Alpine amphibolite-facies garnet-sillimanite-biotite micaschists with local migmatitic leucosomes, minor amphibolite, marble and a small harzburgite body. Alpine re-equilibration under blueschist-facies conditions is restricted to the margins of the slivers or to narrow shear zones. The 2DK crops out discontinuously along the contact between the EMC and the GMC.

Jadeite metagranite

High-pressure metamorphic assemblages in metagranitic massifs of the Western Alps were first described by Lefévre & Michard (1965), Dal Piaz et al. (1972) and Compagnoni & Maffeo (1973). Compagnoni & Maffeo (1973) were the first to describe the metagranites from Monte Mucrone, which show a partial transformation from a pre-Alpine igneous assemblage to an Eo-Alpine high-pressure assemblage.

Fig.2: Simplified map of Mt Mucrone modified from Cenki-Tok, et. al (2011).

Permian granites of Monte Mucrone (Fig.2), occur in the Polymetamorphic Basement Complex of the Sesia-Lanzo Zone and were metamorphosed during an Eo-Alpine event. The metagranites (Fig.3) now contain an assemblage of jadeite-zoisite-quartz±phengite±kyanite which replaced igneous plagioclase, along with garnet coronas around relict igneous or metamorphic titanian biotite.

Jadeite is formed from plagioc1ase breakdown during high-pressure and, generally, low-temperature metamorphism, through the reaction:

NaAlSi₃O₈ (Albite) = NaAlSi₂O₆ (Jadeite) + SiO₂ (Quartz).

Fig.3: Schematic representation of the metamorphic reaction from granite to jadeite metagranite. The igneous plagioclase is replaced by an assemblage of jadeite-zoisite-quartz and biotie is surrounded by garnet coronas. Qtz = Quartz; Plg = Plagioclase; Afs = K-Feldspar; Bt = Biotite; Jd = Jadeite; Zoi = Zoisite.

Bibliography

• Ibarguchi, J. I. G. (1995). Petrology of jadeite metagranite and associated orthogneiss from the Malpica-Tuy allochthon (Northwest Spain). European Journal of Mineralogy, 403-416.
• Tropper, P., Essene, E. J., Sharp, Z. D., & Hunziker, J. C. (1999). Application of K-feldspar-jadeite-quartz barometry to eclogite facies metagranites and metapelites in the Sesia Lanzo Zone (Western Alps, Italy). Journal of Metamorphic Geology, 17, 195-210.
• Cenki-Tok, B., Oliot, E., Rubatto, D., Berger, A., Engi, M., Janots, E., & Robyr, M. (2011). Preservation of Permian allanite within an Alpine eclogite facies shear zone at Mt Mucrone, Italy: Mechanical and chemical behavior of allanite during mylonitization. Lithos, 125(1), 40-50.
• Zucali, M., & Spalla, M. I. (2011). Prograde lawsonite during the flow of continental crust in the Alpine subduction: Strain vs. metamorphism partitioning, a field-analysis approach to infer tectonometamorphic evolutions (Sesia-Lanzo Zone, Western Italian Alps). Journal of Structural Geology, 33(3), 381-398.

Photo
Fibrous, gray jadeite (after plagioclase) and biotite rimmed by small garnet. Monte Mucrone, Sesia-Lanzo zone, Italy. PPL image, 2x (Field of view = 7mm)	Fibrous, gray jadeite (after plagioclase) and biotite rimmed by small garnet (isotropic). Monte Mucrone, Sesia-Lanzo zone, Italy. XPL image, 2x (Field of view = 7mm)	Fibrous, gray jadeite (after plagioclase) and biotite rimmed by small garnet. Monte Mucrone, Sesia-Lanzo zone, Italy. PPL image, 2x (Field of view = 7mm)
Quartz with polygonal texture and biotite. Monte Mucrone, Sesia-Lanzo zone, Italy. XPL image, 2x (Field of view = 7mm)	Fibrous, gray jadeite (after plagioclase) and biotite rimmed by small garnet. Monte Mucrone, Sesia-Lanzo zone, Italy. PPL image, 2x (Field of view = 7mm)	Fibrous, gray jadeite (after plagioclase), quartz and biotite rimmed by small garnet (isotropic). Monte Mucrone, Sesia-Lanzo zone, Italy. XPL image, 2x (Field of view = 7mm)
Fibrous, gray jadeite (after plagioclase) and biotite rimmed by small garnet. Monte Mucrone, Sesia-Lanzo zone, Italy. PPL image, 2x (Field of view = 7mm)	Fibrous, gray jadeite (after plagioclase) and biotite rimmed by small garnet. Monte Mucrone, Sesia-Lanzo zone, Italy. PPL image, 2x (Field of view = 7mm)	Fibrous, gray jadeite (after plagioclase) and biotite rimmed by small garnet. PPL image, 10x (Field of view = 2mm)
Biotite rimmed by small garnet. Monte Mucrone, Sesia-Lanzo zone, Italy. PPL image, 10x (Field of view = 2mm)	Biotite rimmed by small garnet. Monte Mucrone, Sesia-Lanzo zone, Italy. PPL image, 10x (Field of view = 2mm)	Fibrous, gray jadeite (after plagioclase) and biotite rimmed by small garnet. Monte Mucrone, Sesia-Lanzo zone, Italy. PPL image, 10x (Field of view = 2mm)
Fibrous, gray jadeite (after plagioclase) and biotite rimmed by small garnet. Monte Mucrone, Sesia-Lanzo zone, Italy. PPL image, 10x (Field of view = 2mm)	Fibrous, gray jadeite (after plagioclase) and biotite rimmed by small garnet (isotropic). Monte Mucrone, Sesia-Lanzo zone, Italy. XPL image, 10x (Field of view = 2mm)	Fibrous, gray jadeite (after plagioclase) and biotite rimmed by small garnet. Monte Mucrone, Sesia-Lanzo zone, Italy. PPL image, 10x (Field of view = 2mm)
Fibrous, gray jadeite (after plagioclase) and biotite rimmed by small garnet. Monte Mucrone, Sesia-Lanzo zone, Italy. PPL image, 10x (Field of view = 2mm)	Fibrous, gray jadeite (after plagioclase). Monte Mucrone, Sesia-Lanzo zone, Italy. PPL image, 10x (Field of view = 2mm)	Fibrous, gray jadeite (after plagioclase) and biotite rimmed by small garnet. Monte Mucrone, Sesia-Lanzo zone, Italy. PPL image, 10x (Field of view = 2mm)
Fibrous, gray jadeite (after plagioclase) and biotite rimmed by small garnet. Monte Mucrone, Sesia-Lanzo zone, Italy. PPL image, 10x (Field of view = 2mm)