'T :MY! _'I E LLZED .. 1l"L ~n E

UlUL\&\'. - cologie;) I "ill II' point 10 II formatj U wl(ler Tn amorphieeon titioll' of an import."U1 f:ml I II c Pogallo Lllit>:I, euttill Ib c Mas iccio deiI,u hi:l a high gr.tde m t:\lllorphic mp1ex ncar Lak Maggiore (Nor b ru Italy).

ylouit from tbi faul b IV U lIIore cr I . de,,-clop yukill o(l.li· I' ry.lalli~. lioll ill the matrix bul 1101 i1\ III porphJTochlSls,

110 !logTe of r ery tallizal'ion II J..I u.l1s l;POll th mUllll'a.l ompo9ition of titrocks aff eted by the enln III's.

II the li bt of thes ob_ l""tt iOlls tbe terndllology of 11"stic mylonit i.seri . :illy I' vi w('d. Th (1U11101' propo~' a distinction!) tw 1 Mas/omylollit with

SYll.kiJlQlllllti.c I' ~ry tallizati II onl.l' ill III lJlatri~, 811d I' Crl/8/(/l/iud mylollitel,

II h I' r~r." t-allizMion nl 0 af/' porph)'l'oc1asl mId i. 11 jally YOUJl"" I'

f hall edaela j .

u

una d' tinzion 1m. blMtomiloniti eh raJlOmiloniti r'eri lollizzale

i porfiro 1a ti ed un t nom no

Int?oduction.

de:!'neorieri-

ri ristalliz,·

elle qualiI1zialmente

Til geological mapTlinO' f tllat part 0,' thw)lich li b tween th sso)a aile.Ital ) r('v aled tb pI' n' of 3 major"'all m ».

d i Laghi»(N rthern

: tJl «Po-

( ) Ist;tnl~ ill 1lincralogia P trogrnfi:l Geocllillli (lell "w"er 'Ia di Mi-l' I , LavoTO eseguito nc]1 nmhito dell rieerehe d I ellITo!li tudi sulla lran-

rnfia P Iro rafia it I Ipi utrali di ~ilnDo del .•.R

30 A. BOKIA:o>I

This fault divides the metamorphic complex ill this area into two

tectonic unit!>: these do not coincide WitJI the lithologic units commonlydistillg'uishrd (c. !"rie dei I.aghi:t " c Serie Diorito_kinzigitica:t) butconsist of a southeastern wnt', to which the name c Ceneri Zone) of

the Swiss geologists ClUI be extended, and of n. northwestern zone en­tirely formed by rocks of t11(' c Seric Diorito-kinzig,iticl\.:t.

The l>Onthcl1l sect.ioll of that PI1J·t of the Il Pogilllo I;ille» whiell hllsbeen mapp"d up t.o now W<lS l'ccl!.ntly described by HORLANI (1970a). 'rheI'CSU1tS of tlliJ:! illvcstigatioll indicate that this tectonic line is old, cer­tainly pre-Alpine and prob1tbly Hercynian, This age can be inferred

from the fact tlUlt its Fonnntion Follows the Herc;ynian folding, sinceit cuts across thr pilUlar and linear structures originated by this fold­ing while it t'SlWllljall:," precool'S the intrusion of the Grauiti"s of theLakes.

The connection between these late·Tlercynian granites and the faultis e\'ident; equally e\'idcnt l'lp)>cars tJle connection between the grtUlitefonnatioll Wid the static [iunl phase of the Hercynian metamorphismwhere a.na.Wctic phellomenSL of ~ren.t importance fire not lacking (Bo­RUNT & P}-:YRONEI~ PA<lLI'\NI, 1968),

'rhe goologiclIl evidence therefore points to a fOlTlmtion of the«Pogallo Line:t under metamorjlh ic conditions: in the strnctural evo­lution of tile whole compl"x it seems to represent a phase of uplift ofthe Hercynian orogenic lK-It, since the final static mrtnmorphism showslower press:UI'(' ehar8cl('l'S, drmonstrated by Ih(' inslability of the kyfl­nite-bearing asse·mblages.

The c Poga.Ilo l~i.ne:t is markE'rl in the field by a. layer of mylo­nites some-limes retlching tile thickness of hundred m('ters; since thefault plaJle cuts ac~ the S<"histosity of the country rocks, the mylo­nites are deriy('(l from a. wide rlll1~e of lithologic t.ypes.

'rwo importnnt conditiOlls IIrc therf>fore present in these mylo­nil,('8:

G.) fOITIlllt ion lUH]CI' l11f'ullllorphic conditions,

b) wide compositional l'all~C of the rocks i.nvolved.

To these must be l'lddcd thc almost complete Inek of retrogressivereactiolls or noomineralizatiOlls in the mylonites,

.A detailed sun'ey of tilesP roeks seemed to me worthy of interest;fl"Olll the Yery Jx,:rililling this stully revealed thnt the tenninology of

m,ASTOMYLOl'ITES AND lt8CIWSTA(,IAU.:D 11YLQKln:s :37

«mylonit€s I. s.» is far from being adequate to a non.ambigu(HlS defi­nition of these rocks. The problem is not only rc.strictcd to the tenni­llology but also involves the intel'Jwetation of the relationships betweendefonnation alld rccl'.rstllllization.

Terminology of the mylonites I. $.

It does Hot seem necessary to re-examine carefully the termino­logy of these rocks, since the detailed studies of .MALARODA (1946),l-(r(oPF (1931) lUld othcl"S are still completely valid today, especialJy asregiu'ds the rocks where t11C eataclnsis pl"evails over reerystaJlizationby far. From not always clear and wlequ.ivocal sources these autJlOr8drew a ctassific.a.tion stilJ adequate to the petrographic demands. .Aconfirmation can be obtflined fl'om the reading of the recent book ofSPRY (1969), on metamorphic tcxtw·t'S (p. 229). Spry introduced theamendments of Hsu (1955) and CHRIS1'1.£ (1963) but essentially ac­cepted Knopf's classification of the mylonites 8.S.

:Ua1aroda accepted t.he two terms« Mylon..itgne.iss» (Ql...""E."<SEL, 1917)and c Blastomylonite» (SANDER, 1912) fOE' rocks where reerystaJliza­tion plays a more important rote. l\:lylonitgneis8 was considered syuo­nimoul'l to c augen-schist» (I...\PWOWrD, 1885) and «flaser rock» (TYR­RELL, 1926). POl' Malaroda the differenee between the two types de­pellds Oil the degree of rccrysw.llization which, ill thc blastomylon..itcs,also a.£.fects tile porphyroe1asts.

fn my opinion the t.erm q: £laser ('ock» is to be preferred to tile ob­solete q: mylonitgneiss» bnt does not apply to true mylonitic rocks,also in the light of tlle studi<.-'S of KA'rz (1968) 011 tlle formation of theflas.er fabric in the griulUlites of Uont Tremblant Park (Quebec); theprocess is recognized as essentia.lly crystallob1astie without a true ca­ulelasis. '1'he prefix mylonit- therefore seems UlUleees.<;ary.

1\Ialaroda '$ definition of blastomylonite is: (a rock in which)q: ollly minor traces of cataclasis are prcscnt while not only the wholemlLtrix, but also t.he porphyroclnJ;ts have been more or Jess recrystal­lized.

It is impol'blllt to observe tllat. while Sander very clearly definedother terms (see for example: 4: Phyllonit. », SANDER, 1911), he did notdefine the tenn blastomrlonite e<luaUy well. MallY autllol'S attributeddifferent definitions to Sander or used the tenn according to tlleir

;18 A. BORIA~[

personal opinion. STAUB (1915) used the tenn to define rocks in which

l'ecrystallizatioll only affected the matrix <Uld 110t the porphyroclasts.WENK (1934) attributes to Sander the following definition « ... gamodeI' Will T('il regen('ri(,l'te )1ylollit~, involving a postkinclllatic recry­

staUi:m.tion of the J'oek.SPRY'S (1969) definition is rather simuul': «Blastomylonite is a

mylonite in whieh late recrystalliza.tion is flO pronounced that the ori­ginal cat.a.elastic nature can only be recognized with difficulty, if at

all. It L'> cOIl\'eniCllt to distillguish between hartschiefcl' ill which onlymatrix has beell reclJ'stallized ami bwsfolllywnitc in which both the

matrix and lenticles 01' porph,'iJ'odllst,> have been recrystnllizcd~. It isto be noted tllaf while Spry mentions Knopf in the definition (togetherwith a wrong reference to Sa,llder), KNOPF (1931) says that « the eyeo;RI'e stilJ calaelastie although t.he paste hils erysw.llized. A blastomylo­

nite is produced by a deformation which is partly ruptuml and partlycrystallobtnstie, 1lOt by a rehcali11fj crystallization of a previoltsly my­lOl1itized rock ~.

Moreover it St.><:.lllS unjustified to insert the term hadschiefer in thetable of p. 229 (SPRY, 1969) as intermediate type between mylonitesand blastomylonites. The origjJl1J definition of «hartsehiefer» has

mainly a textural meaning and I'efers onty in part to a more or les.<;advanecd recrystallization.

«"Mylonitgnciss» and «Hartsehiefer ~ are therefore rat.her flimsyand disused terms. rt remains blastolJ~)flO1lite but its definition is sowide lind vague aR to be referred to rocks with vet'y different fabricand histor~',

Prom the given definitions by the eliHel'ent authors for the termmylonite, and first of ,111 by its crcatOJ', LAI'WORTD (1885) such a termmust be u.sed to indicate rocks connected with nOll-penetrative discon­

1inuities origillaled by ol'uslling and milling alollg horizons of restrictedthickness.

lIfylonitcs, thongll schistose by definition, in contl'wt with cat<t­elnsites, arc dark-colored and massiw rocks sometimes with a flintyappea.rance scattered with more or less abundllllt porph,yroclasts andthat hal'dly pennit 11. l'Ccognit.ioll of th£' nature o,r the ol'igiltal rock, Thesuffix or the prefix .m~'lonit- must therefore indicate mylonite-like rockseonnected with more or less old faults.

BI,,\STO?<IYLO:-IITES AN!) I~ECRYST,\LL1Z£n MYLONITES 3!'

ThClr fabric does not match the concept of a. pcnetrative defor­mation (in a large doma.in such as a. pre-, para-, or postcryst.e'llllncfolding), It is in fact lmown that many rocks originAted by rcgionalmctmnorphism contain relict, minerals belollgiJlg to older erystallobla.­

stie phases and showing post-crystalline deformations (for example:a.ugen gneisses, flaser gneis.<>es, granulites etc.). 1 thcrefore considerit unjustifiod to ca.ll all these rocks blastomylonites.

It is my opinion that a blastomrlOllite Illust be intended as c theproduce of crushing of prccxistinl! rocks along faults which were aetiveunder metamorphic conditions, for example during a i;tlltiC episodc

following a foldulg" phase with perll'tratve regional clmracter ».'rhe crystallobhlstic charact,f'l' of such mylonites is thel'e.fOre es­

sentially synkincmatic, even if at the end of defonllatioll the survivalof the met.'lJIlorphie condit,ions mig-ht C<'LUS(, morc or less evident, equi­

librium fabrics Ul the mylonitic matrix. 1t is also Illy OpiJlion thatunccrt.aiu.ities i.n blastomylonite definjtion were originated by a seantyknowledge of the distinctive criteria between synkinc-Il11Ltie and staticfabl'ics; t1ICse criteria aJ'e now wcll known from the rceellt contribution

to the inwl'pretation of micl'ostructurcs1>upplied by other sciences, suchllS ccrllmics alld metallurgy,

Given the geologic l'elevunce of thc blastQmylonilt"c clwracter ofthe mylonites from the different fn,ults, espeeiall.y those of regional

importance, it seems worth while to consider in detail the various fea­tures that characterize ~Uld the factor1> t,hat determine the observablefnbrics.

The mylonites of the K Pogallo line n,

The mylonites of thc «Pogallo TJine» originate from different rocktypes, thc most interesting of which are:

0-) a.ugen gneisses,

b) biohte-sillinHulite sehists,

c) amphibolires.

A,ltycn Yllt>i,~scs. These rocks belong to the «Serie dei Laghi» wherethe,r crop out in elongat.ed bodies that are part of folds with very steepaxes; till now it was not possible to state if they are in the cores or

-I

in the limb of th e fol alth uah j ill ill r likely tha h y

f rm the r .In thin' ti n th myI nit . f auapn !!ll i

b)' tbp 1'ela ,j" ar ity of mi a alll abundan fj formed by quartz and ern. h t Id par with 0 mira.

m~'loni . _ lin') t pol~'g nal aod trougl)' i-orient 1 qUMbht) h ] f 11k,: I' (fine ,un t1nrk part) :wd mi·:\

The porph~'r 'm ts are of lni.r ·lin aod plagioebl .

uartz ha b J tmtirely r CI'Y t<tlli;t, d and . h w a rat11 ' p lyg l1al

iabric; quart:' lay r ar bent ar und porph rocla t . The ins rti noi the quartz I lat· rev all; th 5 'onO' j o-o,ri ntation i this min n .H cr' tullizatic)I\ i· 11 r for , lIkin ma ic v Il if po t-kinematic graub undary miPOT<l ion ri mat d a qua i-p Iya unl xtur.

Tb r h d f 1d1 par (Fi!{. :.) II YI ize mall r thanquartz and d n app a1' ha,' b~ recr:- talliz . ce he grainhow a rand m ptical ori n a j n. rushed f par nonnally m·

BLAS'l'OMYLONlT AND HE RY TALLIZED MYLONITE. 41

round.. porphyr clasts. There is 110 trace of r crystallization in the por­phyroclasts which always shQW strong post-crystalline deformations.

B1:otite-sillimanite schists. This rock typ b long to the «SerieDiorito-kinzig-itica» and, in th e tion of th «Pogallo Line» undercon ideration, crops out on its western sidt'o In this case porphyroclasts

Fig. 2. - Augen gueiss mylonite. A plagioclase porphyroclnst surrounded b)'fine grained feldspar ma l'ix. Qua.. z has beeu cIea.rly recrystaJlized. Orossed nieolso X.

consist of mica, plagioclase "'al'llet and sometime taul'olite. Quartz,crushed plagioclas and miea lake form the matrix (Fig. 3).

The presence of mi a amon'" -ehe porphyroclasts is due to its greatabundance in the original rock. '£h ratio porpbyroclastsjmatrix is

l,i"'hly variable and d pends mainly upon tl,e relative abundance ofplagioclas in the dif erent layers.

C 'ushed plagioclase is rather scare in the matrix while quartz is

abundant and shows a notable degree of recry tallization and iso-

n.,,,tiCOllti S.I.M.P. - 6

42 A. BORIANI

orientation; in soml" lllstanCes the quartz grains coalesce in broad mo­nocry talline 1 nses or ribbons (Fig. 4). Mino,' recrystallization can beobserved also in tlle mica flakes of the matrix.

ArnphiboWes. Amphibolite. ar pI' sent in both the units ut bythe fault altllOugh t.hey are by far more abundant in the scllists of the

Fig. 3. - Bioti eo ilIi.lllanite sc.hist mylOllite. Note the tability of biotite silli·manito ne dl (centel'), stal11"olite amI feldspar, which hows little, if any, trace ofaltemtioll. Plu,ne po.J. light 35 ><.

« serie diorito-kinzigitiea ». They form intercalatons of limited thick­n '; mylonites from these rocks an be recognized by their darkgreen col I' in contrast to the daJ:k gray or brown of the other mylo­nites.

In thin section (Fig. 5) they show very peculiar textural cllarac­tel'S in which porphyro lasts and matrix axe formed by the same mi­nerals, i.e. hornblend and plaaioclwe'. Biotite :is veTy scarce and ispresent either enclosed in th amphibole porphyroclasts or as tin.y flakesin the matrix. PlaO'ioclase i the principal n tituent of the matrix

BLASTOMYLON 'r 43

and is completely recrystallized, as is shown also by its rather uniformoptical orientation. Recrystallization is clearly synkinematic and boun­daries have interlobat hape; also amphibole and biotite in the matrixshow traces 0 syn- <IDd late-kinemabc recl'ystaUization (see for exam­ple the polygonal arche of hornblende and biotite around the por­phyro la ts, Fig. 6).

:E ig. 4. - Biotite·sillltnll,lIite SCllist nlJ'loni . The recrystallized qua.rlz grai.us oftil matrix sometimes coalesce to £OTl11 a 1I10UO 'r)' !alline lens (center). Crosseduieol 60 x.

The observation of thes hre mylonitizcd different rock typesenables seveiral conclusions '0 be l' a hed on the behaviour of mineralsduring mylonitization under metamorphic conditions.

Spry divides th minerals of cry tallin· 1'0 kg into three groups:

1) brittle minerals uch as feld par,

2} somewhat du('til mineral with orne O'hde prop rties suchas mica, amphibole pyro,xene and olivine,

3) min ral which recrystallize easily, su h as quartz and calcite.

44 A. BQRIANI

It may be shown that in the presence of minerals of group 3) thosebelonging to groups ]) and 2) be'have xactly a anticipat d by thiscIa ilication; feld..;;pars form porphyroclast or are cru. hed in a matrixwithout recrystallization; micas are twi.t d and fra.y d and the I' ult-

Fig. 5. - mphibolit.e mylonit.e. Pu"plIyt'oela t. l.lJld matri..x a.re form :t by thsame two millO.raJ , which arc pTe ent in two gellprations a 1 re·kinematic an 1 aynkinenHl.tic one. Crossed nillols 35 :-:.

ing tiny flalres are ·til'red up in I ntie-ular 01' ribbon-like aggregate to­gether 'with f ld. par O'raiu.. nly part of this mica recrystallizes, butwh.n it' particularly abundant it al 0 forms porphyl'oclasts.

When quartz is ab nt, uch a in the amphibolite·, plaO'ioclamust bebav' und r metamorphic condition lik a min>ral of group 3),i.e. it recry tallize' in the matrix originating a kind of cry, talloblastimortar fabric with porphyr'oclast: showing po t-crystalline deformationsurround d by mall grains of 'ynkinemati recrystallization.

FTOm the characters observed it can be conclud d tha.t. under SUI'-

I:lLASTO)'ln.oXI1'ES AXl> lU'X:RYSTALL1Y.E!) ),IYI.oXITES 45

vlVlng metamorphic condition.!';, itS .!';ustained by thc geologie evidenceand by the absence of retrogressiYe metamorphism, mylonites are Ior­med whtll a s.rllkinematic retryl>tnllization in tlie ?J1atrix and that thenature or recrystaJliz.ed min('rllis depends upon tho relative abun-

.'ig. 6. - Amphibolite mylonile. Tho llJupbibole hail reeryataJlbed (eenter) as ...ellu plagiodlLlK' in the groundmaa. 0t0Med nieo1.8 80 X.

dlUlce of the l>proies showing differcllt behaviour ulHler a strong de­ronllRtion.

Porphyrocillsts are 1ltwcr recrystallized and always maintain thestmin ('llcrg;r accumulated during deformation. Tllis chnracter canbe defined btastoJJlyuYIlilic lind its evnluation must be based on the ob­8et'\'lttiOll of mylollit.es deri"ing from different lithologic types.

The geologie rele\'flllee of the blastomylonitic chllTllctcr of the my­10llit('l; present in a fault is obvious as it enables tile wlraveIling ofthe cOllnection of Ilon.penctrative tectonic discontinuities with the me­tamorphic events.

46

Blastomylonites and recrystallized mylonites.

In cou

« Pogallofault 0 th

Fig. -. - 1f 1:lJl r:1ti· granulit my I uit ofporphyro I of p~'T03:ene ba been replacedphibole wi d lertur. r uieo

lin » (Finero).b. n w form nJll·

und r I w r m tamor­a la r ry talliza . n.

thbyty-

r

81,ASTOl!\'LOSIT!>S ANU H.£.CR\'STALLIZED l\!\'UJSITES 47

Blastomylonites or m,ylonites with Dlore or less pronounced bla­!fomylo7litte charade,. show lUI l'f$SCutially synkinematic recrystalliza.tion of the matrix, while recrystallized fIIywnitcs suffered & post..mylo­nitic recrystallization which also afCected the st.rained porph,rroclasts.

It might. be argued against this di,-ision that the two terms areequiYaient sinee both mean mylonite with reer:rstallil.1ttion. It is myopinion that the tenn blastom~'lollite eyokes a contemporaneousness ofdeformation and recrystalli.?.ation while tile second term is more suitableto indicate a sueCCS$ioll of the two phenomena.. Examples or static re­crystallization of strained 1)I"e-kinematic minerllis are shown by manytectonite6 such as, for example, the c Cenerigneiss)o (BORIANI, 1970b).

Almowlcdgemcllts.

The n.uthor wish<.\li to C~llrei:llJ hi" gmtitulle to Prot. n. "'llll\rodl~ lLlld tol>rof. II. J. Zwart for UHlir eritielll rCilding ot tIm Il1nlluserillt.

LITERATURE

BORIA.'H A. (19iOa) _ The c PogoI/o lille:t a_(1 it, co_lIul",,, IeWI Ille metalll.Ol"pllic

alld allaJulic phases 0/ c Maukcio dri Laghh bet.ccclI tile OuokJ Palley

olld LtJJ:c Maggio'rt (.\'orllln-ll Haly). BoiL Sot.. Goo!. It., 89, 4..15-433.BOIUASI A. (19IOb)· Thfl ltIicroftrlletHre 0/ CCClleriglleiU:t. Relld. Sot.. It. Min. e

Petro, 26, 487-501.

C!lIllSTIII J. Y. (1963) - Moi_e TA,.." zOlte ill tA~ Aui.'1l reg"'., .Vorlll Wed Scot·"'lid. Ullh-. Calif. Publ geol. Sei., 40, 3-&5....39.

Hsu K. J. (l!'~) - Grolllllilea Glild ",...I_lles 0/ IAe rtgioll a/)olll Cf(C(I_ga. (lIId

Sail ,dlltOIlW CaJlyou, Sa_ Ga.brid No"lItai..." Cali/onaia. Oni". Calif. Pub!.goo!. Sci., 30, 2:13·351.

KSOPr E. B. (1931) . Retrogrr:Wl:t: mdGmorplli8m Gild phyllollilizaliOll, Port J.Am. Jounl. of Set, 21, 1·2i.

T....II'WOIl.TIf C. (1885) - The high/alld cOlllrOIln-,y ilL British geology; ,I, COIUIC',cour,e Qlld COIIst!lllleIlU,. i\"nture, 32, 558-5;)9.

;\(.Il."'IODA R. (19-«;) . Nelli6iolle C «Uf./lorll(UllflllLo del/a 6i,tc/il«(lca dellfl leU/mitt adC/Of"lllaZWHe post-cr~t(llliIlO ~,\I ilouiti I. s. Aue!.). Tlelld. Soc. ],[i". It.,3, 150·171.

SASUr.R H. (1912) • Dber ei.liue GCstci,.I,oU.'llpl'e/1, ae, Ta14crllwtslclI(/cs. Jb. (lor k.k.

Ocol. R....,ielll:!., 62, 219·288.

SI'RY A. (11169) - Mfltomor/J/uG tt.tluru. Pergamon Prel,lS, Oxford elc., pp. 350.STAUB R. (191:;) . P~rogr(ll)ltiscllc UllluncllulIgcn iN. 1Ct,lIklru 1Jer"illage/)irge.

Viert. ~'IlL G6l!. Ziiriell, 55·336.

Wr..~K E. (1934) - Bei.triigc ,,"r Polrographk/ uJld Geologic du SihTCltoJ.·,."lallillS.s.],r.p.:M., ]4, 195-278.