Deel 15-11-1978 LEIDSE GEOLOGISCHE MEDEDELINGEN, 51, Aflevering 2, pp. 313-329,

Petrological outline of an area near

( Valley, Western Italian Alps)

BY

L.D. Minnigh

Abstract

Several rocktypes and their metamorphic mineral growth are described from an area on the western border of the Sesia-Lanzo Zone

in the Orco Valley (NW ). It is argued that in some rocks (garnet-rich gneisses and micaceous gneisses) pre-Alpine metamorphic

minerals are present, in other rocks (carbonate-bearing schists, albite-chlorite gneisses) such minerals are rare or absent. For the latter

it is rocks therefore difficult to establish whether they are strongly retrograded Alpine basement rocks, or rocks belonging to the suite

of ophiolitic schistes lustrés. The two possibilities are discussed.

INTRODUCTION pically they can be recognized by the abundance of gar-

net, blue-grey quartz, the presence of metapegmatoid The is situated the external investigated area on (wes- veins and the reddish alteration colours (Minnigh, 1977).

tern) border of the Sesia-Lanzo Zone, and lies in the Near the contacts with the adjacent rocks, the gneisses Oreo Valley (NW Italy) (Fig. 1). The rocks of the area become more foliated, and mylonites, variable in thick-

comprise several types of gneiss, carbonate-bearing ness, occur.

schists, impure marbles and micaschists. The garnet-rich gneisses consist mainly of quartz, al-

Previous work dealing with the area has been publish- bite, white-mica and garnet. Glaucophane, chloritoid, ed Stella Novarese and Franchi several minerals and chlorite by (1894). (1894) epidote-group are present

(1911), and resulted in 1:25.000 geological maps (sheet

42, Foglio Ivrea of the Italian Geological Survey is ba-

sed on their work). Michel (1953) has described rocks

belonging to the Massif, the surrounding

metasedimentary rocks, and rocks belonging to the Se-

sia-Lanzo Zone. Elter (1971) has made a detailed study of the stratigraphy of the schistes lustres (calcescisti).

The aim of the present study is to unravel the meta-

Similar morphic history of the area. metamorphic pan-

geneses from different parts of the area are placed in the

same metamorphic episode, indicated by Mi, M2, etc. The numbers indicate the interpreted chronological or-

der.

Four main rock-units are distinguished:

I. garnet-rich gneisses

2. micaceous gneisses

3. albite-chlorite gneisses

4. carbonate-bearing micaschists and associated rocks.

GARNET-RICH GNEISSES

Introduction

Immediately north of Sparane and in a few other areas

(see Fig. 2) a reddish weathered, garnet-rich gneiss oc- curs. Amphibolitic rocks have been found in some pla- ces within the gneisses.

Description ofthe garnet-rich gneisses

The rocks have a poorly developed foliation. Macrosco-

Fig. 1. Tectonic sketch map of the northwestern Alps (from * Geologisch en Mineralogisch Instituut der Rijksuniversiteit, Carraro et al., 1970; Dal Piaz et. al., 1971; Compagnoni et al., Garenmarkt lb. Leiden, The Netherlands. 1977). 314

Fig. 2. Geological map of the area described with the main lithological rock types. In the northeast the II DK-Klippe of Vasario is

indicated. 315

Fig. 3. Garnet rim around white mica. This is probably an indication of the reaction biotite -> white mica + garnet.

in variable amounts; rutile is sometimes, sphene is often these fragments the outlines of a former porphyroblast

In sometimes inferred. I is present as accessory. thin section the poorly develo- can be Commonly garnet re-

ped foliation is defined by zoisite, fine-grained mica and crystallized into a slightly pink-coloured, idiomorphic

ribbons of quartz. garnet II. This later generation also occurs as corroded

than but have not been Garnet-porphyroblasts are present in different habits (less garnet I) porphyroblasts

and generations. The oldest garnets (I) are extensively broken.

broken into rounded, cloudy fragments. From clusters of Local regular alignments of rutile are present within

these garnets. From other areas Compagnoni has de-

II monstrated that garnet forms after biotite (Compag-

noni & Maffeo, 1973; Compagnoni, 1977). During this

transformation exsolved rutile needles remain from the

biotite and are now seen enclosed in the garnets. In one

rims been observed white case garnet have around mica

(Fig. 3); this is an indicationof biotite transforming into

white-mica and garnet, as is seen elsewhere (Compag-

noni & Maffeo, 1973). However, brown biotite has not

been found in these rocks in the area discussed. Garnet

I as well as garnet II alter into chlorite, apparently ac-

companied by some sphene (Fig. 4).

A main foliation bends vague (Sm) around both gene-

rations of garnet.

Brown hornblende is observed in several thin sec-

tions of amphibolitic rocks. The mineral is mostly alter-

ed, so that only brown stains and concentrations of

sphene along the cleavage planes of the hornblende are

visible (Fig. 5). Brown hornblende is older than

glaucophane (see below) and is enclosed II Fig. 4. Garnet transformation into chlorite in a garnet-rich by garnet

has been gneiss. White-micas (thin lines) bend around the garnet. (Fig. 6). This same phenomenon observed by

Chl=chlorite; ep= epidote; q=quartz; sph =sphene. Dal Piaz et al. (1971) and Compagnoni (1977). 316

Fig. 5. Rutile concentrations along the cleavage planes of a brown hornblende, due to alteration.

Fig. 6. Brown hornblende (br. hbl) and clinopyroxene (clpx) enclosed in garnet(gt), indicating a high-grade metamorphism. 317

The have been obser- A similar following alteration reactions glaucophanes (II). succession has been repor-

ved: ted from the Sanbagawa metamorphic belt (Iwasaki,

Brown hornblende—￿ chlorite + white-mica + zoisite I960). Gosso (1977) has observed two generations of

Brown hornblende—» colourless hornblende + sphene ± glaucophane, whereby the older has recrystallized into

epidote smaller glaucophanes.

Brown hornblende-*- colourless glaucophane + sphene + In some samples the formation of glaucophane I after

epidote brown hornblende (third alteration reaction of brown

The reaction the hornblende mentioned be inferred. is second probably represents same above) can Sphene

phenomenon mentioned as decolouring of the brown oriented parallel to the (110) cleavage of the former

brown hornblendeby Dal Piaz et al. (1971). hornblende (Fig. 7), which is a very distinctive

Glaucophane is a common metamorphic mineral in feature and different from the biotite transformation

It I the garnet-rich gneisses. is a very pale glaucophane described previously. Inclusions of garnet and rutile-

with 2Vx = 45-50° and a weak the bi- biotite dispersion (r

refringence is somewhat higher (A -0.02) than for the Tourmaline has been observed only once as an inclu- coloured observed in other of sion. more glaucophanes rocks Glaucophane I and II alter via a fibrous barroisitic

this area; these properties indicate a Mg-rich glaucopha- hornblende into actinolite. Also fine aggregates of mica

ne (Miyashiro, 1957). and twinned albite have been observed as alteration

Several different habits of glaucophane as well as zo- products.

ning within one porphyroblast have been observed. Chloritoid is commonly present as a main metamor-

1 have colour- mineral. Like Large porphyroblasts ( mm) always a nearly phic glaucophane, chloritoid is also devel-

less core, sometimes the rim is pale-blue. Often, also oped in two generations, and the older (chloritoid I) is small, pale-blue glaucophanes are present. The smaller normally larger than the younger one (chloritoid II) (Fig.

glaucophanes always bend around the larger porphy- 8). Chloritoid can be an important mineral in defining

roblasts. It is therefore concluded that the colourless the main foliation.

glaucophanes are older than the slightly coloured ones, The coarse and generally extensively twinned chlori-

and that the coloured rim around the large glaucophanes toid 1 is pale-grey with weak pleochroism (x=grey-blue,

is formed with smaller = (1) contemporaneous the y=green-blue, z yellow-colourless). The optic axial

7. Concentrations of Fig. sphene indicate a former brown hornblende, now transformed into glaucophane I. 318

Fig. 8. Micro-boudinage of chloritoid I. Smaller chloritoid II is parallel to the foliation

Fig. 9. Isoclinal intrafolial folds of zoisite in a garnet-rich gneiss. 319

This angle is variable (2Vz=20-45°), dispersion r>v, and the B. the Val Vogna-Valle di Gressoney Klippe.

birefringence A =0.006. Klippe has been described in detail by Dal Piaz et al.

Rutile-alignments from former biotites, broken garnet (1971).

I and sometimes glaucophane I have been observed as C. the Klippe of Vasario (Orco Valley).

inclusions in chloritoid I. A commonly observed altera- Recently also other small occurrences of II DK rocks

tion reaction of chloritoid is: have been found (Compagnoni et al., 1977; Boriani et

chloritoid—» white-mica + chlorite + quartz ± opaques ± al., 1976). The garnet-rich gneisses are interpreted as

epidote (?) rocks belonging to II DK (Minnigh, 1977), however the

discussed is in- In the mylonitic rocks near the contacts of the garnet- Alpine metamorphic imprint in the area

rich gneisses with the adjacent rocks, porphyroblasts of tense, and therefore the pre-Alpine mineral assemblage DK-bodies. glaucophane I and chloritoid I are commonly larger than is not as striking as in other II

elsewhere.

Epidote-group minerals are common. Clinozoisite is

MICACEOUS GNEISSES an important component that defines the main foliation.

Probably zoisite is older because isoclinal intrafolial

folds are present (Fig. 9); epidote sometimes is present Introduction from the as an accessory. The mica-rich gneisses differ in several aspects

alteration colours a well- Large xenoblastic rutile is present in a few places; II DK-rocks: are grey-brown,

I. foliation is and therefore probably this mineral has the same age as garnet developed always present, and Possible pseudomorphs after sillimanite are present as folds are common, blue-grey quartz meta-pegmatoid

very fine elongate mica in quartz-rich domains (Com- veins are absent and garnet is scarce. The rocks occur sometimes small lenses pagnoni, pers. comm.). Fresh sillimanite has not been in several places in the area, as

found. (<0.5 m) intercalated with other rocktypes.

In one thin section of metabasite Ca-clinopy roxene

has been found (Fig. 6); both, pyroxene and brown Description

and hornblende are armoured by garnet and thus preserved. Macroscopically, mica, feldspar, quartz, sometimes In thin Two generations of white- mie a are present. The ol- chloritoid and glaucophane can be distinguished.

dest mica I is very fine-grained and gives rise in certain section the micaceous gneisses are coarser grained than main constituents places to a vague foliation (Si). White-mica II is coarser the garnet-rich gneisses. The are mica and albite; and is one of the minerals that defines the main foliation quartz, a slightly green (phengite) some-

m m (S ); sometimes it is seen that S is a crenulation cleav- times epidote, chloritoid, glaucophane or garnet are pre-

rutile in amounts than age. sent. Spheneand are present greater

Fine-grained quartz is commonly concentrated in in the II DK-gneisses.

shows the features as in the II DK- lenses or ribbons. The numerous sutured quartz sub- Garnet I same grains have undulose extinction. In the mylonites mortar gneisses. In a few places a small idiomorphic garnet II is

textures are common. Albite is common and overgrows present, sometimes as a recrystallized product of garnet

Sm. I. Both garnets are older than Sm.

Of the above-mentioned minerals, garnet I, biotite. Glaucophane and chloritoid as in the garnet-rich

and sillimanite, brown hornblendeand clinopyroxene suggest gneisses, are present in two generations, the older

high-grade metamorphic conditions, and are different porphyroblasts are the larger ones. Both minerals have

is from the later high-pressure and low-temperature regi- also more intense colours, and dispersion more pro-

me. Metamorphic minerals found in other areas in the nounced. Glaucophane alters into a blue-green amphi-

Sesia-Lanzo Zone (e.g. Carrara et al., 1970; Dal Piaz et bole but, like chloritoid, is also replaced by chlorite.

It to al., 1971; Compagnoni et al., 1977), and interpreted as Clinozoisite is sometimes abundant. is parallel

pre-Alpine, are comparable with the high-grade as- Sm, together with phengite. Epidote crystals are more

II semblage from the garnet-rich gneisses. common than in the DK-gneisses; epidote overgrows

Certain characteristics of the rocks, such as mylonitic Sm.

veins, reddish alteration col- contacts, meta-pegmatoid Rutile and sphene are locally important compo- ours, pre-Alpine minerals, show great similarities with nents. 'Strings' of sphene-grains sometimes define Sm, rocks that Carraro et al. (1970) and Dal Piaz et al. (1971) which is characteristic of these rocks.

have interpreted as belonging to the 'II Zona Diorito- Fine-grained mica is often present as a mineral older

kinzigitica' (II DK). Carraro et al. (1970) have interpre- than Sm. A coarse greenish mica (phengite) defines the

ted the II DK occurrences in the Sesia-Lanzo Zone as main foliation.

rooted the II remnants of a nappe in Ivrea-Verbano Zone Near the contacts with the DK-gneisses, in some

('Zona Diorito-kinzigitica') (Southern Alps). They have localities, pseudomorphs after lawsonite have been described several Klippen of these rocks (Fig. I): found.

A. the northern part of the Sesia-Lanzo Zone between The normally equant, medium-grained quartz distin-

Rima San Giuseppe and the Ossola Valley. Here, partly guishes this rock from the II DK-gneisses. Sometimes autochthonous and allochthonous II DK occurs, accord- exaggerated grain growth is developed. Often zones

ing to Isler&Zingg(1974). oblique to Sm show undulose extinction. The grain 320

boundaries lobate sutured in the sometimes are (not as garnet-rich tite; actinolite, graphite and sphene are pre-

sent too. The foliation is defined mica gneisses). by and, if pre- minerals actinolite. Zircon, apatite and opaque are common ac- sent,

cessories. Small found in a few garnets are places and are not

corroded or broken, but are retrograded to chlorite.

ALBITE-CHLORITE GNEISSES Pale actinolite, always lying parallel to the main fo-

liation, is sometimes present in fairly large amounts. Introduction The twinned albite rarely seems to be present in two

In Italian literature the albite-chlorite gneisses are generations: sometimes the foliation bends around albite

minuti' sometimes albite includes known as 'gneiss (= fine-grained gneisses) and (I), (II) minerals that are lying

are interpreted as a special type of 'Sesia-gneiss' (e.g. parallel to the foliation.

Dal Piaz has Dal Piaz et al., 1971; et al., 1972; Compagnoni Epidote commonly a yellow rim of pistacite

et term and al., 1977). Here a descriptive without an inter- is sometimes pre Sm, or parallel to Sm, but can also

of and is include Sm. pretation possible origin age preferred. Single pistacite crystals always overgrow the albite-chlorite is in the foliation. few instances Coarse-grained gneiss common In a allanite is present as an present area. The colour of the rock varies with the accessory mineral.

of albite: mica define amount grey-green (little albite, much chlorite) Small, xenomorphic (I)-flakes an old fo- to light-grey with a greenish lustre (abundant albite). liation. Crenulation of this plane gives rise to the main foliation. The rock occurs mainly together with carbonate schists, Coarse phengite (II) lies parallel to Sm. An ir- micaceous crenulation of Sm in certain gneisses (Fig. 10) or very light-coloured regular places gives rise to a

gneisses. Layers of these rocks have a variable thick- new, poorly developed S-plane. The coarse phengite II

from is often rimmed ness several centimetres to tens of metres. Some- by green biotite. times there is a quick repetition of this lithology due to Chlorite is darker than in the micaceous gneisses or intensive folding. the garnet-rich gneisses and has anomalous purple-blue

interference colours.

Description Green biotite forms mostly rims around phengite, but is also within The gneisses are mainly composed of albite, chlorite, present phengite in distinct zones,

mica and and bio- to the Sometimes flakes of greenish some epidote, quartz green aligned parallel cleavage.

Fig. 10. Contact of greenish albite-chlorite (ab-chl.) gneisses and brown micaceous gneisses. 321

Albite with Fig. 11. porphyroblast graphite-inclusions showing that the main foliation is a crenulation cleavage.

Fig. 12. Albite porphyroblast with complex inclusion pattern of graphite. Same thin section as Fig. 11. 322

green biotite are present, probably representing com- to mica-schists into albite-chlorite gneisses have been

pletely recrystallized phengites. observed in several localities. Other rock types, in close

In certain places (north of the Valley) association with the carbonate-bearing mica schists, are

graphite-rich albite-chlorite gneisses occur. The graph- mica schists, sometimes rich in quartz, albite-chlorite

ite is fine coloured present as alignments parallel to an S-plane. gneisses, very light gneisses, impure marbles

Spectacular inclusion patterns in albite show that Sm is a and chert layers. Due to the variable lithology and well-

crenulation cleavage (Figs. 11 and 12). developed schistosity, several generations of folds can

Tourmaline and apatite are sometimes common be observed.

accessories.

In the albite-chlorite gneisses of the area discussed, Description

relics of pre-Alpine metamorphism, as found elsewhere The main constituents of the carbonate schists and mica schists (e.g. K-feldspar or chessboard albite), are absent. Alla- are albite, white mica, chlorite, epidote (pista-

nite has been found in only a few places where the al- che), quartz and carbonate minerals. Variable quantities

bite-chlorite gneisses are associated with micaceous of actinolite, sphene and graphite are present. The main

gneisses, probably indicating an igneous origin of the foliation is a crenulation cleavage and is defined by

rocks. white-mica, graphite, chlorite aggregates, carbonate

concentrations and, if present, actinolite needles. Sm is

CARBONATE-BEARING MICA SCHISTS often irregularly crenulated, and a new surface, defined

by axial surfaces of the crenulations, is present.

Introduction Small is sometimes idiomorphic garnet present, ex-

Carbonate-bearing schists known in Italian literature as tensively replaced by chlorite. 'calcescisti' schistes (= lustres) occur in many places in In several places close to the garnet-rich gneisses on

the present area. The carbonate-rich parts commonly the eastern side of the Ribordone Valley, pseudomorphs

are extensively weathered and therefore the discontin- after metamorphic minerals are common. The regular

is often visible. In the small uous layering schists, shape of original lawsonite is sometimes well preser-

white sometimes folded several defor- formed quartz-veins, by ved. Normally the pseudomorphs are by flakes mations characteristic. are of irregular white-mica and small amounts of epidote.

Intercalations with other rocks are commonly obser- The foliation clearly bends around the pseudomorph

ved. Gradual transitions from carbonate-bearing schists (Fig. 13).

13. Fine Fig. aggregate of mainly white-mica forms a pseudomorph after lawsonite. 323

Chlorite or albite-actinolite aggregates after glauco- Green stilpnomelane (2Vx=ca. 10°; pleochroism

h p a n e are common. Sometimes glaucophaneis still preser- x=yellow-green, y=green, z=blue-green) has occasion-

ved and two possible generations may be recognized. The ally been found in the carbonate schists.

older(glaucophane I) has a blue colour (darker than in the II Medium-grained quartz is present in variable

DK-gneisses or micaceous gneisses) and occurs together amounts. There are often mutual foam-textures, which

with clinozoisite. Glaucophane II is present as a light-col- occur also between quartz and carbonate (Fig. 14). rim Zoned tourmaline is oured around glaucophane I or as small light-coloured yellow-brown always present

porphyroblasts; sometimesitis arecrystallized glaucophane as a sometimes important accessory.

For chlorite, rela- I, accompanied by the transformationof clinozoisite into green biotite, and epidote the same

epidote. tionships with other minerals have been found as in the

albite-chlorite Chloritoid-porphyroblasts are also sometimes pre- gneisses.

sent as two generations. Chloritoid too is more intensely Gosso (1977) has pointed out the transformation of

coloured than in other rock units. The following retro- white-mica into al bite. In the present area the same

grade reactions have run to nearly completion (see also phenomenon has been observed in all rock units. This

Bearth, 1963): transformation is seen in Fig. 15, where an elongated al-

bite is chloritoid —￿ chlorite ± magnetite embedded in quartz, while in the continuation of

the is still chloritoid -» white-mica + epidote ± magnetite albite some mica preserved. The timing of

Actinolite is present as a rare transformation pro- this process deduced by Gosso is post B3; for the area

actinolite discussed this of the main duct of glaucophane, but needles of are also means post-crenulation (B2-)

present where no relationship with glaucophane is seen. foliation, a phenomenon that is seen in Fig. 16 (see also

Two generations of white-mica are commonly present, Fig. 11).

around as in the albite-chlorite gneisses. Sometimes a third ge- The oligoclase-rim albite, commonly observed

neration (white-mica III) is developed. In the axial sur- in northern parts of the Sesia-Lanzo Zone (e.g. Com-

faces of the crenulations of the main foliation, sporadic pagnoni, 1977; Gosso, 1977), has not been found in the

newly formed crystals have been found. It seems that present area.

white-mica III is of the same generation as green biotite.

Fig. 14. Slightly curved grain boundaries with triple junctions of carbonate (Ca) and quartz (Q). 324

Fig. 15. Formerly elongate white-mica embedded in quartz is nearly completely replaced by albite.

Fig. 16. Crenulated white mica is replaced by albite. Rutile- and graphite-fragmentsare enclosed in albite. 325

Fig. 17. Single-twinned albite phenocryst in a fine-grained albite-quartz matrix resembling a light-coloured gneiss.

Rocks in association with carbonate-bearing mica

schists

West of the Ribordone Valley carbonate schists are

quite common. They are intercalated with different

rocks: light-coloured gneiss, quartz-mica schist, thin

quartzites, and albite-chlorite gneisses. The thickness of

these layers varies considerably.

Light-coloured gneisses occur in several places

(Fig. 2). The rock is in contact with quartz-rich mica

schists or carbonate-bearing schists, and always with al-

bite-chlorite gneisses. The extreme light colour of the

gneiss, and the presence of feldspar-augen (up to 3 cm),

are striking features to distinguish the rock from other

gneisses.

Microscopically the feldspar-augen are large micro-

clines, chessboard albites or single-twinned albites (Fig.

17). The matrix consists mainly of albite and quartz; very chemical little white-mica and chlorite are present. A

investigation and the possible origin of this rock will be published elsewhere (Minnigh in prep.).

Thin layers of quartzite have been reported by R.

Prato, 1970, unpubl. map, Petrographical Inst., Turin) and lie in the vicinity of the light-coloured gneisses (Fig.

18). A rough description will be given. The variable

from less than 1 1 thickness, cm up to m, is due to fol-

is well-laminated, sometimes reddish, ding. It a some- Fig. 18. Outcrop pattern with the characteristic lithologies of

times yellow or grey-black coloured rock. The quartzite the region near Apperè. 326

Fig. 19. Spessartine-rich idiomorphic garnets of a meta-chert. The cores are rich in inclusions and give the garnets a ‘frog-spawn’

appearance.

M M M M Pre-Alpine(M-,) 2 3 4 5

1 II garnet

biotite

sillimanite

(?) K-fsp

clinopyroxene

brown hornblende

1 II crossite glaucophane

actinolite

1 II chloritoid allanite (zoisite) clinozoisite epidote pistacite epidote-group

white-mica Mg Fe-rich chlorite

rutile

sphene

andesine albite plagioclase

stilpnomelane

lawsonite

Table I. Time-table showing relative order of growth of the described metamorphic minerals 327

structural because of of minerals in the appears a good marker; however, scarcity pre-Alpine metamorphic gar-

in to lack of continuous outcrop and the sometimes very net-rich gneisses and micaceous gneisses respect

it other Carrara 1970 for small dimensions, it is difficult to trace over large areas (see e.g. et al., pre-Alpine

distances. The fine layering within the quartzite gives rise minerals in several II DK-bodies).

In have been to fold-interferencepatterns. Fold structures and deforma- the Sesia-Lanzo Zone the rocks subdi-

in vided several tion history, will be published in another paper (Minnigh, on lithologic and metamorphic grounds by

prep.). authors (e.g. Carrara et al., 1970; Dal Piaz et al., 1971;

The main component is fine-grained quartz. Micro- Dal Piaz et al., 1972; Compagnoni et al., 1977). The ge-

scopically the layering is defined principally by fine- neral picture of this subdivision, based on their obser-

is East grained idiomorphic, spessartine-rich garnets. If the gar- vations, a metamorphic zonation from to West.

net is inclusion-free the layer is macroscopically yellow. Internally (east), high-pressure and low-temperature

Often a cluster of reddish needles (possibly hematite) is metamorphism (eoalpine) and relics of pre-Alpine high

inclusion in the which the metamorphism are preserved in several present as garnet, gives garnet temperature

inclusion-rich In the external of the Se- a 'frog-spawn' appearance (Fig. 19). The rock types. (western) part coloured. sia-Lanzo Zone, late garnet layers are pink Alpine greenschist-facies meta-

Some of the minerals commonly found in the quartz- morphism is important. Relics of high-pressure-low-

ites are epidote, crossite, piemontite, stilpnomelane. The temperature metamorphic minerals are present, but no

Mn-rich minerals have been found. quartzite is probably a meta-chert, since traces of pre-Alpine metamorphism

the internal and external there is are an important component. Between part a zone

where, from east to west, eoalpine metamorphic influen-

metamor- ce decreases and late Alpine greenschist facies

DISCUSSION phism increases. In the same zone pre-Alpine relics be-

come rare or are lacking. This zone roughly corresponds

mineral with in From the above data on metamorphic growth a change lithology: externally, gneisses mainly of and relative time-relationships a time table can be com- composed albite, quartz, phengite, epidote ± green

piled (Table I). This time table conforms with observa- biotite, actinolite and stilpnomelane, known as 'gneiss micaschists with tions reported elsewhere (e.g. Compagnoni, 1977). The minuti', occur, while internally eclogite

oldest metamorphic minerals garnet I, biotite, brown facies paragenesis (eclogitic micaschists) dominate (Dal

hornblende, sillimanite, and clinopyroxene, clearly re- Piaz et al., 1972). rocks in localities present a metamorphism of high temperature and mode- Metagranitoid occur lying externally

and well in the Sesia-Lanzo Zone rate pressures, have been interpreted as the meta- as as internally (Callegari of morphic regime of pre-Alpine age (generally inferred to et al., 1976). These rocks comprise an important part

the basement. the meta- be Hercynian; see e.g. Boriani et al., 1976). pre-Alpine During subsequent

Microscopically it is difficult to establish whether the morphic and tectonic episodes, some of the rocks are

transformed into with metamorphic conditions after the pre-Alpine episode re- augen-gneisses, gneisses potas-

sium chessboard present events separate from one another, or a more or feldspar-relics (sometimes albite) or where and less continuous process of decreasing pressure and ris- gneisses original texture mineralogy are com-

ing temperature. The tendency from high-pressure-low- pletely transformed ('gneiss minuti') (Compagnoni et al.,

temperature conditions (glaucophane, chloritoid, lawso- 1977).

to this and the nite) to moderate pressures and higher temperatures (al- According interpretation terminology

biotite, is observed micaceous and albite-chlorite of the bite, green chlorite) a commonly gneisses gneisses feature in the Alps (Frey et al., 1974). From radiometric area discussed are 'gneiss minuti' or so-called 'Sesia-

Dal Piaz et Dal Piaz et al., et al. age determinations (Bocquet et al., 1974; al., gneisses' (e.g. 1972). Compagnoni

1972; Hunziker, 1974) two distinct metamorphic episo- (1977) have interpreted the light-coloured gneiss as a

des have been ascertained: eoalpine (the above cited Ivh leucocratic dyke derived from a granitoid stock.

started K-Ar in and Mi) with glaucophanes possessing ages The lithological units found the area discussed can

of 80-100 related to the be divided into with my, and late Alpine, normally two groups: one group pre-Alpine

Lepontine phase (ca. 35 my) probably present on a re- metamorphic relics, and one without a trace of pre-Alpi- minerals. first consists of the gional scale (M4). In the present area omphacite, jadeite ne The group garnet-rich

or kyanite have not been found, therefore the eoalpine gneisses and the micaceous gneisses. Albite-chlorite

metamorphic grade is probably lower than in the more gneisses, carbonate-bearing schists and quartzites belong

northern and internal parts of the Sesia-Lanzo Zone, to the second group.

where micaschists in eclogite facies are abundant; it is The garnet-rich gneisses are interpreted as a Klippe of also possible that these minerals have been completely II DK. Elsewhere (Dal Piaz et al., 1971; Dal Piaz et al.,

retrograded (see Compagnoni, 1977). The late Alpine 1972; Compagnoni et al., 1977) the timing of the tectonic

event is expressed by the subsequent alterationof pre-Al- emplacement of II DK-rocks is argued to have been af-

pine and eoalpine metamorphic minerals, local formation ter the eoalpine high-pressure-low-temperature meta- of green biotite and stilpnomelane, and growth of albite morphism, because of lack of minerals representing that

(variable in intensity in the different rock units). This metamorphic event. This interpretation cannot be tested

greenschist facies metamorphism probably caused the in the area discussed, since no eoalpine metamorphic 328

It is 'jump' between the garnet-rich gneisses and the adjacent is suggested (Debenedetti, 1965). therefore propo-

rocks has been found. sed that the present cherts with (some of) the albite-

In the micaceous gneisses, garnet I is a remnant of chlorite gneisses belong to the 'vrais schistes lustres' albite-chlorite 'ozeanische Serie' of Bearth pre-Alpine metamorphism. In the gneiss- of Elter (1971) or the

has es, allanite as a possible pre-Alpine mineral, been (1976).

in few In structural of found very places only. the carbonate-bearing The complex metamorphic and history

schists and also in the quartzites no indications of pre- the rocks makes it difficult to determine whether con-

Alpine elements has been observed. tacts between different rocktypes are primary or not.

ba- The close association of carbonate-bearing micaschists Therefore the albite-chlorite gneisses may be partly

with meta-cherts and albite-chlorite gneisses is of great sement rocks, partly ovarditic gneisses belonging to the

in Detailed this interest. Frequently observed ovarditic gneisses the schistes lustres. investigations on rocktype

schistes lustres are interpreted as albitized lavaflows or are necessary to unravel the possible origin, may be the

tuffs (Nicolas, 1967; Elter, 1971; Compagnoni et al., presence of certain accessories such as allanite (believed

of tourmaline 1977), or as former spilitic rocks (e.g. Amstutz, 1968; to be igneous origin) and (an important

Vallance, 1960; see also Hughes, 1973), The commonly accessory of the carbonate schists) as is proposed by

strong resemblance between the albite-chlorite gneisses Compagnoni (pers. comm.) can be of help. and ovarditic gneisses is striking in several places. A definite interpretation of the rock assemblage de-

in the two The quartzites rich in manganese minerals area scribed above is not yet possible, but the possibilities basement rocks rocks discussed are probably meta-radiolarian cherts. The of origin (strongly retrogressed or possible environment of these cherts is that of the oph- belonging to the ophiolitic schistes lustres) deserve fur-

iolites of the schistes lustres. Investigations of Elter ther attention.

(1971) and Bearth (1976) on the schistes lustres of the

Gran Paradiso and the Zermatt region, respectively, ACKNOWLEDGEMENTS

in show the same rock-assemblages found the present

area. Manganese-rich cherts in schistes lustres have also The author is greatly indebted to P. W. C. van Calste-

P. been reported by Debenedetti (1965), Grunau (1965) and ren, R. Compagnoni, G. Gosso, R. Kuijper, B. Lom- which E. den P. F. Williams and H. J. Zwart for Dal Piaz (1969). In the Lanzo Valley near Ceres, bardo, Tex,

the lies in the continuation of the cherts of the Oreo Valley, the discussions and their suggestions during prepa-

Zambonini (1922) and Gennaro (1925) have reported ration of the manuscript.

manganese-rich rocks. For all these cherts a Malm-age

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