A new petro-structural map of the Monte Mucrone metagranitoids (Sesia-Lanzo Zone, Western Alps)

Figures & data

Figure 1. Tectonic sketch of the Sesia-Lanzo Zone redrawn after Compagnoni et al. (1977), Passchier et al. (1981), Spalla & Zulbati (2003) and location of the SLZ in the Alpine chain (inset).

Table 1. Mineral associations and dominant textures in metagranitoids and surrounding rocks of Monte Mucrone southern slope.

	Rock-type	Modal composition	Coronitic texture	Tectonitic texture	Mylonitic texture
Metaintrusives	Green-type metagranitoid: Jd or Omp, Grt, Wm and Gln fine to medium grained bearing metagranitoid	Qz 30–50%, Cpx (Jd/Omp) 20–30%, Wm 15–20%, Grt 15–20%, Zo 5% and Gln 5%		green colored rock medium grained with gneissic spaced foliation evidenced by Wm and Cpx SPO	fine grained mm spaced foliation evidenced by Wm and Cpx SPO
	Grey-type metagranitoid: Jd, Grt and Wm bearing medium to coarse grained metagranitoid with relics of igneous Bt, Ksp and Aln	Qz 50%, Jd 25%, Wm 15%, Grt 5% and Zo 5%. Estimated igneous composition: Qz 40%, Pl 40%, Ksp 20% and Bt 10%	grey colored rock with preserved hypidiomorphic igneous texture, with withish aggregates of Jd + Qz and Grt rimming Bt microsites		localized mm thick shear zones marked by Act, Wm and Ep SPO
	Metaquartzdiorite: Omp, Grt, Wm and Gln medium to coarse grained bearing metaquartzdiorite	Omp 30%, Grt 25%, Qz 20%, Wm 10% and Gln 10%. Igneous composition: Pl 30%, Qz 20%, Ksp 10% and mafic minerals 40%	medium to coarse grained rock with randomly oriented Omp, Ph and Gln	tectonitic to mylonitic textures occur at the boundaries of metaquartzdiorite lenses with foliation marked by Gln, Grt and Ph SPO
	Metaaplite-metapegmatoid: Jd-bearing fine grained metaaplite and Jd, Wm and Grt bearing coarse grained metapegmatoid	Qz 50%, Jd 25%, Wm 15%, Grt 5% and Zo 5%	same of grey-type metagranitoid s.s.	gneissic texture with foliation marked by Ph and elongated Qz-rich domains
	Leucocratic metagranitoid: Ab, Wm and Gln medium to fine grained leucocratic metagranitoid	Qz 40%, Ab 40% and Wm 20%		spaced to continuous tectonitic to mylonitic foliation, medium to fine grained, marked by Ph SPO and elongated Qz domains
	Eclogite: Gln, Wm and Zo bearing fine grained eclogite	Omp 35%, Grt 25%, Gln 20%, Wm 10% and Zo 10%	massive to very poorly foliated rock with S2 foliation marked by Gln		localized mm thick shear zones marked by Act, Wm and Ep SPO
Paraderivates	Micaschist: Wm, Grt, Gln, Cpx (Jd/Omp) and Zo bearing fine grained micaschist	Wm (15–40%), Qz (15–50%), Gln (5–25%), Grt 15%, Cpx 10% and Zo 5%		micaschist always have a tectonitic to mylonitic pervasive texture, with mm spaced continuous foliation marked by Qz, Cpx, Gln, Wm and Zo. Foliation can be reactivated as slip plane by successive deformation stages
	Paragneiss: alternances of Qz and Grt bearing layers and mafic layers	Qz and Grt bearing layers: Qz 65%, Grt 15%, Wm 15% and Gln 5%; mafic layers: Gln 50%, Omp 25%, Grt 15%, Qz 5% and Wm 5%	gneissic texture inhereted by pre-Alpine metasediments is marked by alternating layers	in less developed tectonitic texture the tectonitic fabrics are concentrated in Qz and Grt layers marked by Wm SPO. In more developed tectonite and in mylonitic texture a mm spaced foliation occurs in all layers marked by Qz, Wm, Cpx and Gln SPO
	Porphyric gneiss: Qz, Ab, Wm and Ksp fine to medium grained rock with coarse grained Ksp crystals	Qz 35%, Ab 25%, Wm 20% and Ksp 20%		gneissic texture with foliation marked by Wm SPO and by elongation of Qz and Ksp-rich domains
	Zoisitite: Zo, Qz, Wm and Gln bearing fine grained zoisitite with Grt	matrix (70% volume) composed of Zo 35%, Qz 15%, Wm 10%, Gln 5% and Omp 5%, with ten cm sized Grt blast 30%	granoblastic texture		mylonitic foliation marked by Zo, Wm and Gln enveloping Grt blasts
	Glaucophanite: Omp, Grt and Wm bearing porphiric fine grained glaucophanite	Gln 50%, Omp 20%, Grt 20% and Wm 10%. Locally Qz occurs		foliation marked by Gln and Wm SPO with randomly oriented Omp and Grt
	Quartz-dominated high-pressure vein: Wm, Grt, Zo and Gln bearing coarse grained quartzite	Qz 90–95% with Wm, Grt, Gln and Zo accessories		spaced foliation marked by Wm and Zo SPO

Table 2. Mineral assemblages marking fabrics related to successive deformation stages (D1, D2 …) in metaintrusives and metasediments of Monte Mucrone southern slope.

	Rock-type	pre-D1	D1	D2	D3	D4	D5	D6
Metaintrusives	Green-type metagranitoid	Ap, Aln and Zrn	S1 gneissic massive foliation marked by Qz, Omp/Jd, Ph, Grt, Zo and Rt	coronitic growth of Cpx, Ph, Czo and Qz mainly associated to crenulation	10 cm-thick shear zones producing lenticular Qz and Wm. Brs formation on boudinaged Omp	ten m scale open parallel folds. Ab, Act, Ep and Wm replacement on Cpx, Gln, Zo and Ph	mm-thick shear zones with widespred growth of Chl, Wm, Ab, Act, Bt, Ep and Ttn	gentle folds with axial plane disjunctive foliation marked by Ab, dark Amp, Kfs(Adl) and Pg replacing Jd
	Grey-type metagranitoid	igneous texture preserved with Bt, Aln, Ap and Zrn relics	coronitic substitution of Pl by Qz + Jd + Zo aggregates, Bt and Ksp partially replaced by Ph. Grt rims igneous Bt	ten m scale isoclinal folds, coronitic growth of Omp on Cpx. Ph recrystallization of older Wm	cm-thick shear zones. S3 marked by Ph, Brs, Czo and Ttn	ten m scale open parallel folds. Ab, Act, Ep and Wm replacement on Cpx, Gln, Zo and Ph	mm-thick shear zones with widespred growth of Chl, Wm, Ab, Act, Bt, Ep and Ttn	ten m scale gentle folds
	Metaquartzdiorite	igneous texture preserved with Bt, Aln, Ap and Zrn relics	granoblastic texture of Omp and Grt with interstitial Qz, Gln and Ph	ten m scale isoclinal folds, coronitic reaction forming Qz, Omp, Gln, Ph, Grt, Czo and Rt	m-thick shear zones on the lithologic boundaries. S3 marked by Qz, Ph, Brs, Grt, Czo, Mg-Chl and Ttn	ten m scale open parallel folds. Ab, Act, Ep and Wm replacement on Cpx, Gln, Zo and Ph	mm-thick shear zones with widespred growth of Chl, Wm, Ab, Act, Bt, Ep and Ttn	ten m scale gentle folds
	Metaaplite and metapegmatoid	igneous texture preserved with Bt, Aln, Ap and Zrn relics	coronitic substitution of Pl by Qz + Jd + Zo aggregates, Bt and Ksp partially replaced by Ph. Grt rims igneous Bt	ten m scale isoclinal folds, coronitic growth of Omp on Cpx. Ph recrystallization of older Wm	cm-thick shear zones. S3 marked by Ph, Brs, Czo and Ttn	ten m scale open parallel folds. Ab, Act, Ep and Wm replacement on Cpx, Gln, Zo and Ph	mm-thick shear zones with widespred growth of Chl, Wm, Ab, Act, Bt, Ep and Ttn	ten m scale gentle folds
	Leucocratic metagranitoid	/	S1 cm spaced continuous foliation marked by Ph and elongated Qz domains	ten m scale isoclinal similar folds. Partial dynamic recrystallization of Ph and growth of Gln parallel to fold axes	/	open to tight folds, crenulation of hinge zones and dynamic ricrystallization of Qz, Ab, Act, Wm and Ep	mm-thick shear zones with widespred growth of Chl, Wm, Ab, Act, Bt, Ep and Ttn	ten m scale gentle folds
	Eclogite	/	granoblastic texture of Omp, Gln, Grt and Ph	coronitic growth of Omp, Gln, Grt and Ph	cm-thick shear zones. S3 marked by Ph, Brs, Czo and Ttn	ten m scale open parallel folds. Ab, Act, Ep and Wm replacement on Cpx, Gln, Zo and Ph	mm-thick shear zones with widespred growth of Chl, Wm, Ab, Act, Bt, Ep and Ttn	ten m scale gentle folds
Paraderivates	Micaschist	pre-Alpine Grt cores, Ap, Aln and Zrn	isoclinal similar folds. S1 marked by Qz, Cpx (Jd/Omp), Ph, Gln, Zo and Rt	m scale isoclinal similar folds, with associated S2 obliterating S1 marked by Qz, Omp, Ph, Gln, Grt, Czo and Rt	deformation distributed in all the volume with Qz, Ph and Brs grain size reduction	open to close folds with intense crenulation. Widespread growth of Ab, Wm, Qz, Act, Chl, Ep and Ttn	mm-thick shear zones with widespred growth of Chl, Wm, Ab, Act, Bt, Ep and Ttn	ten m scale gentle folds with minor kink folds
	Paragneiss	pre-Alpine layer alternation of metasediments and Grt cores, Ap, Aln and Zrn	coronitic growth of Omp, Grt, Gln, Ph, Zo and Rt. S1 marked by Qz, Cpx (Jd/Omp), Ph, Gln, Zo and Rt	m scale isoclinal folds. Coronitic growth of Qz, Omp, Ph, Gln, Czo and Rt	cm-thick shear zones. S3 marked by Ph, Brs, Czo and Ttn	ten m scale open parallel folds. Coronitic growth of Qz, Ab, Act, Chl, Ep and Wm	mm-thick shear zones with widespred growth of Chl, Wm, Ab, Act, Bt, Ep and Ttn	ten m scale gentle folds
	Porphyric gneiss	Ksp relics	isoclinal similar folds. S1 marked by Qz, Ksp, Ph, Gln, Zo and Rt	m scale isoclinal folds. Coronitic growth of Qz, Omp, Ph, Gln, Czo and Rt	cm-thick shear zones. S3 marked by Ph, Brs, Czo and Ttn	ten m scale open parallel folds. Coronitic growth of Qz, Ab, Act, Chl, Ep and Wm	mm-thick shear zones with widespred growth of Chl, Wm, Ab, Act, Bt, Ep and Ttn	ten m scale gentle folds
	Zoisitite	Aln	S1 cm spaced foliation marked by Ph, Zo, Gln and Qz	m scale isoclinal similar folds, with associated S2 marked by rotation of Ph and Gln	cm-thick shear zones. S3 marked by Ph, Brs, Czo and Ttn	open to close folds with crenulation between hinge zones. Abundant formation of Ep, Act, Wm, Qz, Chl and Ttn	mm-thick shear zones with widespred growth of Chl, Wm, Ab, Act, Bt, Ep and Ttn	ten m scale gentle folds
	Glaucophanite	/	S1 mm spaced foliation marked by Gln with granoblastic texture of Omp and Grt	/	cm-thick shear zones. S3 marked by Ph, Brs, Czo and Ttn	/	mm-thick shear zones with widespred growth of Chl, Wm, Ab, Act, Bt, Ep and Ttn	ten m scale gentle folds
	HP quartz vein	/	S1 cm spaced foliation marked by Ph, Zo, Gln and Grt trails	m scale isoclinal similar folds, S2 marked by Ph, Gln and Czo SPO	large deformed Qz crystals	/	mm-thick shear zones with widespred growth of Chl, Wm, Ab, Act, Bt, Ep and Ttn	ten m scale gentle folds

Compagnoni , R. , Dal Piaz , G. V. , Hunziker , J. C. , Gosso , G. , Lombardo , B. and Williams , P. F. 1977 . The Sesia-Lanzo Zone, a slice of continental crust with Alpine HP-LT assemblages in the Western Alps . Rend. Soc. It. Mineral. Petrol. , 33 : 281 – 334 .

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