RENDICONTI DELJ...\ soclETA ITAUANA 01 MINERALOGlA E PETROLOGlA, 1988, VoL 43·2, pp. 237-262

Granites as indicators in paleogeodynamics

JEAN-PIERRE !'uPIN1.IIbollltOUe de Petrologie-Mineralogie, FlcuJ~ des Sciences, Pan; VI1rwc, 06034 Nice CcdeJ: (France)

ASSTllAcr. - The study of the: spatial distribution ofgnnitie rocks in different orogenic belts lew 10 defineffillgmltie polarities from zircon typolOSY dltl. A moreor less complete magmatie zoning with the successionof tonalites Ind 4a calc-alkaline gnnites, analeetiegranites 1·2·3, cak-1l1kaline and K-calc..aJka.linc: graniles4b-e, K subalkaline granites , can be: observed.Abundance: of an.atectie rocks. f .values of calc-a1kaline:,subalkaline rocks are depending upon the relativeimportance: and dlrl.tion of mantle-CJUSt interactionsduring subductioD<ollision procasc:s.

k, UJOnls: M.p.tic wning. on:JBCIlic: granites, lonalites.zircon typOlogy.

LES GRANITES, MARQUEURS ENPALEOGEDYNAMIQUEREsuME.. - L'elUdc: de: la rep.nition sp-tiaIc: des rochesgranitiques de diff~ntsdomaines orog6Uques conduit~ dHinir des polaritb magmatiques basees sur lesdonnees typologiqlll:S des popuIations de zircons. UnezonaJite magmatique plus OIl moins compl~te de tonaliteset granites caleo·a1ca1ins 4a, granites anatcctiques 1-2-3,granites calco·alcalins et calcoalcalins potassiques 4 b·c,granites subalealins potassiques , peut ainsi etreobserv~e. L'abondance des roches analcctiques, lesindices f des rodles calco-alcwnes et subalcalinesdependent lugemenl de I'importanee relative et de ladurtt des interacdons manteau'Q'Oute pendanl lesprocessus de subduction et de collision.

MOll clh: zonalile magmatique. granites orog~niques,

lonalites, typologie du zircon.

I. Introduction

Magmatic zoning is known for a long rime(KUNO, 1959; lAKES and WJ-DTE, 1972;

MCKENZIE and CHAPPELL, 1972; GILL andGORTON, 1973; LEFEVRE, 1979) from thecharac[eristics of volcanites in (relatively)recent orogenic domains. The spatialdis[ribution of the magmatism often yields aclear polarity with increasing contents of K,Rb, Ba, Ti, Th, U and light R.E.E. whengoing away from the trench zone inconvergent margin environments (continentalmargins and island arcs).

In ancient orogenic zones of subduction­collision, most of the volcani[es disappeared,but in contras[ granitoids are more: or less wellreptesented. Such d~p produced orogenicplutonites, especially granites, linked [0 d~p

structures of the orogens, are: able [0 givefundamental cri[eria to discuss the platetecmnics. Some results had been obtainedfrom the geochemical data on calc-alkalinegranites (BA'fEloJtAN and DoDGE, 1970; DoDGE,1972; BAIRD e[ al., 1974; KIKUCHI inMtYASHIRO, 1975), [he use of isotopes (SINHAand ZiETZ, 1982), iron oxydes (ISHIHARA,1977, 1980) or zircon typology (PtwIN, 1981,1985; WERNICK et al., 1987).

But this implies the availability of a goodgenetic classification fot granites. Recent andimportant progresses have been teginered inthis domain (BowoEN et al., 1984; CHAPPELLand WHITE, 1974; DmlER et al., 1982;ISHlHAJlA, 1977; UMEYRE, 1980; UMEYREand BoWDEN, 1982; ORSINI, 1976, 1980;PuRCE et al., 1987; PITcHER, 1979; PuPm,1980, 1981, 1985).

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population type. Main types and subtypes arerelated on a square board with two variables(Fig. 1), depending upon the relativedevelopment of the crystalline faces.

For each sample, in the 0.050-0.160 mmfraction, the typologic distribution has beendetermined on the basis of the examinationof 100 unbroken zircon crystals wheneverpossible, and the coordinates (A, 1) and theT.E.T. have been calcul"ed (Fig. 2) [lA, tJrepresent the mean point of the population:

n. The typology method

Zircon typology is a cheap and easy method(~PIN . and TuRCO, 1972a; PuPIN, 1976).ZtrCOn 15 genernlly well distributed in granitesand the zircon population of one sample isgenerally enough to characterize anhomogeneous granitic body (PupyN 1985).Zircon. gi~es information on the 'primarycrystallizauon of the host rock and is generallynot affected by deuteric phenomena(hydrothermal. alteration, weathering, tectonicand metamorphic events, ...) (Pupm. 1976).

An ~angement of given prismatic andpyramidal crystal faces constitutes a

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GRANITES AS INDlCAlORS IN PALEQGEODYNANlCS 239

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peralkaline medium favouring thedevelopment of (101) (Pupm 1976, 1980;PUPIN and TURco, 1975).

Fig. 3. _ Zircon crYJlallizarion period withcorn:sponding tcrnpcrtturc:s n:latcd 10 me T value5 inthe K·$Im-De.line granitic rocks of PbJmanach, Brittany(granileJ of Tr.xli~ros (la), La Qute (lb), Kerlco.­Canton (2a), Saint·Samson (2e) and aplitc of Trcgastel(All.

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The T.E.T. (Typological EvolutionaryTrend) is drawn through the different Apoints calculated for each value of LT (Pupm,1976, p. 62)].

The T.E.T. represents the chronology ofcrystallization of the different types andsubtype:s during the magmatic stage. It canbe deduced from the observation of crystalzoning along sections parallel or perpendicularto the c axis: under transmitted light forancient crystals (PuPIN and TURco, 1970;PuPIN, 1976) or with fission tracks techniquesto reveal Uranium distribution and crystalgrowth for recent crystals (CARPENA et al.,1987; GAGNOL, 1987).

The typologic data give direct informationon the crystallization medium:

T index is directly and positivelycorrelated with the temperature of zirconcrystallization; a geothermometric scalewas thus proposed (PuPlN and TURco,1972b).A index is correlated with the chemistry,a peraluminous medium favouring thedevelopment of {2l1l, an alkaline or

240 J.p. PlIPlN

Another factor corresponds to the waterpressure which modifies the period of zirconcrystallization. An illustration (Fig. 3) can begiven by the K·subalkaline granite series(PuPtN, 1981) of Ploumanach (BARRJiRE,1977) with an early crystallization of zircon(now limpid crystals) in the coarse grainedpink granites (Traouieros, La C1arte)crystallized from dry magmas to the latecrystallization of zircon in the fine grainedgranites (Kerleo-Canton, Saim-Samson) andaplites where crystals att richtt in U and nowmetamict.

A genetic classification for granitic rocksbased on zircon typology had been proposed(PUPIN, 1980). At this time, other studiespermit to define more precisely (from about600 samples) the domains corresponding tothe mean points distribution of the differentgenetic types (FIg. 4). lbis classification showssome advantages related notably to thepossibility of a good separation betweengranites of crusta! or mainly crusta! originand mantle derived or hybrid mantle +crust granites, especially hololeucocraticdifferentiates from the eale-alkaline, K-

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GRANITES AS INDICATORS IN PhLEOGEODYNAMICS 241

ealealkaline. subalkaline or alkaline series.The eale-alkaline granites belong to the low

K (<< tonalitic») and normal K (<<granodioritic »)series (LAMEYRE and BOWOEN, 1982). Themonzonitic association or high K series(ORSINl, 1976; PAGEL and LETERRIER, 1980;DEBON and LEFORT, 1983) corresponds to theK-calcalkaline series (sometimes qualified as

I.A200

magnesiopotassic relatively to the compositionof biotite: Le. Balagne. Corsica and Ballons,Vosges, France; ORSINI, 1976, 1980; PAGELand LETERRIER, 1980) and the subalkalineseries (ferropotassic; i.e. Ploumanach;BARRIERE. 1977; BARRIERE and CHAURIS,1971; DuPUIs. 1972).

For each population, the position of the

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Fig. ,. - Distribution of T.E.T. for granitic rocks: granites of crwta! or mainly cruSta! origin (0, ca!c·alka!inegranites (2), K-ealc·alkaline granites (2a), subalkaline granites (3), subsolvus alkaline granites (4), hypersolvus alkalinegranites ell.

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Fig. 6. - 1.ol.:adon of the studied area in Mexico (al: zone: I. upper volcanic supergroup province in the SierraMadu Occidental, n. castern Chihuahua and weSlernmost Texas provinces, Ill. alkalic province of Teans-PeeosTexas, after McDowELL and CLABAUGH, 1979. Distribution of the analyzed samples in the Q-A-P diagram ­normative data - (h); 1. Crctaccous-early Tertiary volcanic and batholithic rocks, 2 and 3. mid Tertiary volcanicrocks. Cross section of the Sierra Madre Ck:cidental betwttn MilZadan and thl: Guadania Valley (c) after McDoWFl.l.and Ct.AIlAUGH, 1979, with location of the studied samples: 1 _ 102-45 m.y., 2. 28-23 m.y., 3 _ 32-28 m.y.

T.E.T. permits to precise the belonging to agenetic group (Fig. 5). Only one sample isnecessary for obtaining a characteristic trend.

m. The Magmatic zoning

The mean points distribution of zirconpopulations from granites sampled in the samearea visualizes very clearly the contrastpreviously mentioned by PITCHER (1979) foracidic magmas in «alpinotype», «andinotype»and «hercynotype» environments. Moreover,the comparison between the differenttypologic characteristics (mean points A, t;T.E.T.) lead to define magmatic zonings inancient orogeru; (PUPIN, 1981, 1982, 1983,1985; PUPIN and WERNICK, 1987; WERNICK

et al., 1987).Several examples of magmatic zoning are

proposed. They are characterized by theabundance or scarcity of granitic bodies and/orassociated rhyolites, the importance of crustal

anatexis, the temperature and water contentof the melts - especially concerning calc­alkaline granites:

Cainozoic period: Mexico, NewCaledonia, Western Alps, Elba-Tuscany;Hercynian cycle: French Massif Central,Brittany, Corsica-Provence, Morocco;Caledonian cycle: BelgiumBrasiliano cycle (late Precambrian):southern Brazil.

1 - Mexico (Sierra Madre Occidental)

The analysed rocks had been sampled in theSierra Madre Occidental of Mexico along across-section between Mazatlan and Durango(Fig. 6a). The rocks belong to the three mainunits described by McDOWELL and KEIZER

(1977), McDoWELL and CLABAUGH (1979)(Fig. 6c) and differ in ages:- The oldest ones Cretaceous to early

Tertiary (102-45 m.y.), correspond to the

GRANITES !IS INDICATORS IN PALEOGEODYNAMICS 243

western calc-alkaline vo1canites (andesites,dadtes, rhyolites) and batholithic rocks(granodiorites and diorites);the youngest ones are Tertiary (32-23m.y.) tufs and rhyolites (32-28 m.y.: UanoGrande to Guadania Valley in the west;28-23 m.y.: El Palmito to 11ano Grandein the central part). In the Q.A.P.diagram, these rocks show increasing silicaand alkali contents when going away fromthe trench (Fig. 6b).

Considering the mean points distributionin the typological diagram (Fig. 7 and 8), thesame zoning is observed. It can be noticedthat:

1) anatectic magmas are absent;2) the populations have high t indices

indicating high temperatures of crystallization

from relatively dry magmas;3) the mean points distribution_shows a

linear inferior limit with increasing T indicescorrelated with increasing depth of thesubduction zone.

2 - Brazil (Dam Feliciana belJ)

The Dom Feliciano fold belt (BrasUjanocycle, late Precambrian) runs parallel to theatlantic coast of southern Brazil (SantaCatarina, Rio Grande do SuI) and Uruguay.

A zircon typology study (nearly onehundred samples) together with lithochemicaland petrographic data (PuPIN and WERNICK,

1987; WERNICK et al., 1987) allowed therecognition of the following main groups ofgranitoids (Fig. 9) in the brazilian part of this

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Fig. 8 - Mean points clintibution of magmatic rocks from Muico (Sitrl'll Madre Occidenlal) and Bruil (DomFe/id.no belt).

belt:1) a) anatectic granites arxf b) diorires,

tonalites and low tempc:I1ltu.te cale-alkaline(Coastal batholith in Rio Grande do SuI);

2) mroium to high tempc:ratUtt cale-alkalinegranitoids of normal to K-rich composition(central part of the magmatic arc);

3) subalkaline K-rich granitoids and syenites,common in the western part of the belt;

4) alkaline granites.Thus, zircon typology reveals a clear

magmatic zoning, this bdng more completein Rio Grande do SuJ. The mean pointsdistribution shows mainly high f indices (Fig.Sb) and the same oblique linear inferior limitthan for the volcanic rocks from Mexico.

The different domains defined by thetypological evolutionary trends (mean T.E.T.and T.E.T. distribution; Fig. lOa, b)~ quiteequivalent to these for similar rocks fromHercynian and Permian plutonism of westernEurope. The similarity with the Mexican

orogenic area is reinforced by the scarcity ofcrustal anatectic rocks which is in strongcontrast to the great amounts of crustalgranites occurring in the French Hercynides(PupIN, 1985). As in Mexico, the increasingdepth of the supposed subduction zone ijoSTet al., 1984; Fig. 11) is correlated with thedevelopment of the [100] prism (Fig. 12), thatis to say the increasing of t indices. Thegeotectonic polarity indicated by the loweroblique line in the zircon typological diagrampoints to a former trench situated east to thepresent shoreline.

3 - Western A*,s

In the Western Alps, alpine granitic rocksare rare and associated with tonalites in theBergell and Adamello massifs. The calc­alkaline magmatism had been proposed (i.e.GIRAUD, 1983) to be related with thedisappearance of a limited oceanic crust in the

GRANITES AS INDICATORS IN PALEOGEODYNAMlCS 245

piedmontese domain (Fig. 13). In such amodel, these granites, granodiorites andtonalites were emplaced very near the suturezone.

The granodiaites and granites are rypicallyealc-alkaline of trend 4a (Fig. 14a), with lowf indices indicating low temperatures ofcrystallization and a relatively high waterptessure. The two-mica Novate graniteappears to be of anatectic origin and so, notrelated with the previous trend.

If we compare the plutonites of Adamellowith the trends obtained for andesitic suites(andesites, dacites, rhyolites) (GIRAUD et al.,1980) of central volcanoes in the world, the

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accordance is good and the diorites-tonalitesappear to be plutonic equivalents of theandesites-dacites (Fig. 14b). Gtanodioritesand granites follow this trend at lowet findices, confirmjng the fractionalcrystallization suites.lt is noticeable that thesegranites are clearly distinct from the calc·alkaline granites with highet f indices, oftenlinked with huge volumes of calc-alkalineignimbrites which crop out generally at greaterdistances from the suture zone.

4 - New Caledonia

A similar example with a relatively limiled

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Fi,. 9. - DUtribution of main genetic grotlp$ of plutonic rocl<s in the Dom Feliciano bdt, Brazil: anatectic granites(1), aIe·alkaline granites of the 4a traxl (Pupm, 1980) (2) and associated diorites ab), Car.;apava cak-alkalinegranite aa}, wc-alkaline and K-ealc·alkaline granites of the 4b-< trends m, 5UbaIblinc granitcs (4) and associated5)'cnites (4al, alkaline granites (j).

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246 J.P. PUPIN

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subduction and a very little distance Ixtw~nthe trench zonr: and the intrusions may begiven by New Caledonia during Alpineorogeny. Tonal.ite5 and granodiorites wereintruded at 25 m.y. just after the emplacementof the peridotites nappe (GUILLOU in

A.u.E<;RE. 1980; PARIS, 1981) (Fi8. 15). Alonalite of Saint-Louis and thrtt granites ofSaint-Louis and Koum were studied. Theserocks are typical of the caIc-alkaline trend 4aand quite similar to those of the Adamellomassif (Fig. 15).

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GRANITES AS INDICATORS IN PALEOGEODYNAMlCS 247

Ft,. 12. - Typologic mocIific-tions in:Iicatinc geocIynamic: polaritie$: 1-4: Dan Fdic:ia:oo bdl, Brazil (1: 5 I, anatectic81anite, Coastal Balholith; 2: 512, calc-alkalinc granile, 4. type, Coastal Bcholilh; J: 518, K-a.lc·alkalinc granile,Valsungana; 4: 524, subalkaline 81l11litc, Rio Chacao), '-8: Elb.-TllSCf.ny, haly (': GI, calc..Ik.alinc sranite, 4atype, Monte Capannc, Elba; 6: 52, anatectic sranite, Giglia; 7: 519, calr:::-alkaline granite, Gavorrano; 8: 524,subalkalinc la alkaline: volcan.itts, Monte Amiata and Vitorehiano); 9-12: Mas.sif Central, France (9: 5', caJc·alkalincgr1lnodiarite, 4a type, 5idobre; 10: Ll, anatcctic Icucogranite, Saint.5ylvestrc; 11: SI, anatecli!; manwgranile,Margeride; 12: 519, K<a1c.alkalinegranite, Aigoual, Cevennes). The r1I111e of crynalsizes is 0.14·0.2' mm.

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GRANITES AS INDICATORS IN PALEOGEOOYNAM1CS 249

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Fig. 14. - (a) Mean points distribution of granodiorites (Gd) and granites from Adamello (4a eale·alkaline trend;data after HANSMANN et aL, 1983, HANSN\ANN, 1986, PuPlN , unpubl.). Similar results had been obtained by BARTHet aI., 1987 on the Rensc:n pluton, S. Tirol. N: Novate two mica granite. (b) Comparison hctween the plutonicseries of AdamelJo: quartz diorites, tonalites (d), granodiorites (gd), granites (g) and the andesitic series from thecentral volcanoes (differentiation T.E.T. indicated by arrows _ GIRAUD et al., 1980 _: andesites (A), dacites(D), cordierite.bearil18 dacitc:s (cD), myodacites·rhyolites (R). Granitic eaJc.-alkaline field (stock 4) and related calc·alkaline ignimbrites (CAI) after PuPl:N, 1980. For references about AdameUo, see BIANCHI et al., 1970, C.U.I..EGARland DAL PtAZ, 197.3, Dvpvy et aI., 1982.

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Humbolt

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Fig. 15. ~ (a) Geodynamic setting in New Caledonia (afler GUILLOU in AlLE(;RE. 1980); pre (al) and POSt (a2)eo.Oligocene. (b) Samples location and (c) mean points distribution of tonalite and granodioritc:s from Saint-Louisand Koum (4a ealc·alkaline trend).

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wsw CORSiCA ELBA TUSCANY

~=e"m,. :5__._- 05.5'N'

70km cFig. 16. - Mean points distribution (a) of ~tudied samples from Elba·Tuscany (b): A _ Monte Amiata, E. Elba,G _ Giglio, Ga. Gl!vorrano, R__ ROCCllSlrada, sv. San Vincem:o, V • Vitorchiano; 1 • alkali potassic volcanismof the Roman Province {1,7 m.y. to ~esentl, 2: Corsica massif, 3: intral;tuStal acidic magmalism of the TuscanProvince (8,7.0,4 m.y.), 4; Northern Tyrrhenian geosuture, 5: main tensional faults (or grabens) (b after BARIlERI

and INNOCENTI, 1980, simplified). Cross section with velocity depth (I;) after LETZ et al., (1977).

5 - Elba-Tuscany

A more complex example concern themagmarism of Elba-Tuscany in Italy since 8million years. Composed of plutonites (Elbagranite and microgranites, Gavorrano andBotra ai Marmi granites) and volcanites (SanVincenzo, Roccastrada, Monte Amiata)(RI'ITMAN, 1962; Dupuy, 1970; MORELu,1975; Dl GlROLAMO, 1978) (Fig. 16), thismagmarism is interpreted as a lithosphereconsumption zone during the presumedrotation of the Corsica-Sardinia block (as afragme.ot of the European plate) up to itscollision with the Adriatic microplate(MORELU, 1975, BocCALETTI and GUAZZONE,1972). As hybrid granitic rocks exist (ealc­alkaline granite of Monte Capanne, Elba), itcould be assumed that a very limitedsubduction-like process worked at the

beginning of the underthrusting of theCorsican plate. The development ofmagmatism followed in space and time the endof the underthrusting during several m.y. (8m.y. - Elba - to 0.2 m.y.. Monte Amiata .).

From west to eaSt, zircon typology gives agood zoning with successively: calc·alkalinegranite (Monte Capanne) _and micrograniticdykes of Elba with low T indices near thepresumed suture zone (BARBERI andINNOCENTI, 1980), anatectic granites andrhyolitic ignimbrites such as Giglio, SanVincenzo and Roccastrada, calc-alkalinegranites such as Gavorrano and finally K·subalkaline or alkaline volcanites such asMonte Amiata or Viterbo. These last rockscould belong to the alkaline magmatism ofCentral Italy (LocARDI and Ml'rrEM:PERGHER,1969; BARBEIU and INNOCENTI, 1980; TURl,1987).

GRANITES AS INDICATORS IN PALEOGEODYNAMlCS 251

So, when going away from the westernsuture zone, a very clear inc~ of f indicesappear (Fig. 17a), directly linked with thedepth of the Moho from 40 to 65 km givenby geophysical data after LETZ et al., (1977)(Fig. 16). The modifications registered on

zircon typology are presented on Fig. 12.

6 - French MtJssij CentrtJ/ tJnd Brittany

In this Hercynian domain of westernEurope, a very clear zoning can be deduced

suturezone

o

I.T.

100I

300kmI

Elba-Tuscany

300

800

F.M.C.200

«

- ..* \J 0 0 «! 00 «

0

~ « 00

0~00

• • •• •• •« • • •• • • •• • 1 .. ..• • • ••• • •••0 ,

7000

Fig. 17. - Relationship between f indices and dinlIlOeS from the supposed suture zones in Elba-Tuscany andthe French Massif Central (F.M.C.): calc-alkaline granile:s, 4a trend (squares), anateedc granites of groups 1 (stars),2 (open circles) and } (filled circles), calc·alkaline and K-calc-alkaline granites (triangles), alkaline and continentaltholeiitic rocks {encircled stars}.

l.P: PUPIN

Fig. 18. - Distribution of graniles with t values ofzirCQn populations < 4'0 (shaded [I]) and > 4'0 (black[2]) in the French Massif Central; V • the late Hercynianempla~ Velay anatecric diapiric dome (AUTRAN andPETERLONGO, 1979) (after PuPlN, 198').

from zircon typology data ever whenconsidering the populations with i indiceslower or greater than 450 (Fig. 18) (P'uPIN,1985).

An increasing of t indices clearly appearsfrom west to east in the same way than inElba-Tuscany (Fig. 17b). In detail, the FrenchMassif Central shows from west to east thesuccession of lonalites and calc-alkalinegranites with low i indices, anatectic granitesof the groups 1 to J (PUPIN, 1980), calc­alkaline and K calc·alkaline granites withhigher T indices crystallized from hotter anddrier magmas (PupIN, 1981, 1985).

Moreover, a very clear bimodality of theorogenic magmatism, as well as in Brittany,is underlinecl by zircon typology data (Fig. 19).

The southern part of Brirtany ischaracterized by Hercynian anatectic granitesof groups 1-2, the northern by anatecticgranites of group 3, followed by calc-alkaline,K calc·alkaline and subalkaline granites with

high t values (Puprn, 1985).As previously, modifications on zircon

typology are presented Fig. 12.

IV. The plutonic zoning model

Results obtained on sevetal orogenicdomains, clearly distinct in space and time,show chat we can propose for plutonic rocks(granitoids) che model of a more or lesscomplete magmatic zoning. This zoningreveals the ancient geodynamic polarity andis composed, from the trench or the suturezone to the external part of the overthrustingplate, of:

tonalites and calc-alkaline granites (4atrend, PuPtN, 1980) with low i indicescorresponding to the andesitic suites;anatectic granites of groups 1-2 (aluminousleucogranites and (sub) autochtonousmonzogranites);mainly anatectic porphyritic granites ofgroup 3 with idiomorphic cordierite;cale-alkaline and K cale-alkaline graniteswith high T indices of group 4 (4b-4c);subalkaline granites of group 5.

Often a tensional magmatism follows intime the orogenic one and sometimes thesesuccessive magmatic aclivities show someoverlapping with contemporaneous orsubconremporaneous caIc-alkaline or anatecticand alkaline products. In such a case, thealkaline acidic magmas are frequentlyperturbated by the very recent crustalorogenic activity and the feldspar mineralogyis generally subsolvus (Le. Corsican subsolvusgranites, BONIN, 1980; early Permian alkalinedykes and vo!canites, Provence-Esterel,PuPIN, 1976 and unpublished data; Kagenfe1sgranites, REVE, 198); Baveno granites,northern Italy, CAlRONI, 1985; Combeynotgranite, Pe1voux massif, Western Alps,COSTARELLA, 1987). The alkaline intrusionscan constitute logically a last zone after theK calc·alkaline and/or subalkaline ones, as insouthern Brazil. (WERNlCK et al., 1987);nevertheless, they often overlap the orogenicmagmatic arc, mainly the zones of caIc-alkalineand K calc·alkaline granites with high jindices zircon populations (Le. Provence­Corsica, PuPIN, 1985).

N

I

Montpetlieroo 50km

~

GRANITES AS INDlCAroRS IN PALEOOWDYNAMICS 25}

I.A200 700 200 700

100

• •~ .. • •

IT :: ~ .., ." .. .... ••"''''~

••

• • •• e e e•.. '••....-,

0

800

MASSI F CENTRAL N.125 BRITTANY N.65

Fig. 19. - Mean points distribution of zircon populalioll5 from Hercynian.odic; magm8tic rocks from Ihe FrenchMas.sif Ccntnl. (I) 8Ild BritWly (b). Black circles .. gnnitcs, cnrirded $18I'S _ Bn:vcnnc gnnitC$, black $WS. rhyolitC$,N .. number of elI8mincd umplC$.

The plutonic zoning mood can be appliedto other orogenic sites, with the aim ofdetermining geodynamic polarities. Threeexamples can thus be presented; they concernthe Hercynian gr-anitoids of Provence-Corsicaand Morocco and the Caledonian magmatismof Belgium.

1 - Corsica-ProvenceThe two successive main magmatic

activities of Corsica: the orogenic calc-alkalineHercynian one and the anorogenic Perrnianone are very well separated by the typologicdiagram (Fig. 20). ID comparison with calc­alkaline granites and rhyolites of Mexico­Brazil and French Massif Central-Brittany,Corsica clearly appears to be nearer the secondcase and thus \V'Ould correspond to a fragmentof a collisional chain. This collision couldfollow only a relatively limited subductionprocess.

A zoning exists in Corsica, with the higherT indices registered in the north-east, thelower in the south-west. It fits weil with themajor petrographic and geochemical domainsdefined by ORSINI (1976, 1980). This zoningis better defined when considering togetherCorsica and Provence, with Corsica in itsprerotational position (Fig. 21). From west toeast, the complete zoning corresponds totonalites and associated eale-alkaline granitesof Figarets-Hcmitan, anatectic intrusiveporphyritic gnmites of Rouet - Pl.an de la Tour- Gigaro and cale-alkaline granites of Saint­Antonin and Camarat (PuPtN, 1976; TOUREet al., 1983; AMENZOU and PuPIN, 1986;AMENZOU, 1988). A west·east polarity canthus be deduced in conformity with thepolarities found for Brittany and the FrenchMassif Central (PuP'IN, 1985), the suture zonebeing probably in the west as proposed byLAMEYRE and AUTRAN (1980).

254

r-----:-,-,---- I.A200

100

J.P. PllPlN

700 200 700

•

CORSICA C-A N.57 ALK N.48

Fig. 20. - Conia.: mean poinu disaibution of rircon popuJ..tions from (a) Hercynilll caJc·aIkallne dioritic (encircledSlats). granitic (black circles) and rhyol.itic (bI.-ck stan) rocks, migmatites (white Slars), (b) alkaline: Pennian granitic(black circles).oo rhyolitic (black sws) rocb (some rhyoltes from Esrcrd have been.x&ed); N .. rumbtt of examinedwnples.

o 20km~

@Sa;nt_Anlon;n

"""l- ---.J

- ,:\;:;:~~~.: - .

",'CE

SAftQ'N''''

CA,LYI

3 •

40506~ a

"""

bFig. 21. - Magmatic wning (a) of Ptovence-Conica with (1) metamorphic rocks, (2) Penman, (3) Hcrcynian lonalitesand associlued granites, (4) anatectic granites of group}; Rouet·Plan de I. Twr-Gigaro, (}) ealc·alkaline graniteswith f < :HlO, (6) caJc·alk.linc and K ale-alkaline granites with t > 500. Canica-Sardinia block in its pre-deriveposition (dala after WFSfPHAL et Ill., 1976, TOUIl2et al., 1981, Pt1PIN, 1985. AMEHzou et al., 1986l. Correspondinsmean points distribution of zircon popubtions (b). 4. cak-alkallne granites domain.

GRANITES AS INDICATORS IN oo.EOGEODYNAMlCS

0 60km, ,

OULMES

ZAER~.....

"0

"-...'"...~..."

I.A100

100

"­,

",\i.

800

T __ .... - - - - ..- - - - ...- - - - .. - --

••

•

•

(--1~ _ ;-...--.:e .-..._ ..- .,I /2.~/ • ! I ,

.. It"/-+ Z T// -+11

/ "I oj

\\ .,;/ /;\-+ 1-""--+";::'- -+.:.r • +,

'3-- If \\ ~/.. 4 0 i "

......./ • • Bol~ \ +•• i 0

ME- /5' I,/ I 0 1

+. i.. )t.. ··f ' •

:7 f· ..: / j

: -+. .;:-+/~ 1..... -::...:- ,

; ,800 .. + -+ .. ::"it:::::.J'--- ....Jb

I.T

Fig. 22. - MIglDItic zoning cl Hercynian grmiles in rhc: Middle Arlu of Morocco (a): map after VOCEL andWAJ.JtD, 197~: 1. younger rocb. 2. Pakozoic. J _ Hen:ynian gnniles, 4 '"' Precambrian. Mean points distribution(b) or zircon populatioN or ZIl& (Z). Oulrnb (om, Menl (ME) and Bowni. (80) graniles (for rdttenCU aboutMorocco, see VOGI!1 el al.• 1976, PIQut. 1979).

256 j.P. PUPIN

::::.:.:.:..:.:.:.:.:.:.:.:.:.:::::.::::::::::::::::.:.::::::::::::::::::::::::::::::::::::::::::::::.:.:.:.:.:.:.:.:.:.'

a

100

IT

800

100

+

+

+

+

+

+

I.A

+

+

+ +

•+•

•,+

,+,

•

·•. . . I, ,

••• , - i+ : __8_ _.1._....

800

b

rag. 2}. - Analyzd~ in BeIgi..-n (I): magmatic nxb (circled md Gcdinian saradstones (triqIes); B. Beabant,C - Condroz. R. Rocroi, SE. Sttpont, ST. SIavdoI:. Mean poinu dinriluion (b) al2ircon popub.tions: rugmaticrocks from Brabant-Coodroz (BCm). Rocroi (Km) and S1avdot (Sm), Gcdinian sandstones from the Condroz (Cl).Rocroi-5erpont (IU) and 5t.vdot (Ss) rqions. Fm rcfcre~about Belgium, I« CoRIN, 196', FlEltl!MANs, 1982).

GRANITES AS INDlCAlORS IN PALEOGEODYNAMlCS 257

c

AlpsNew Caledonia ""1Elba-Tuscan~:b===~

F. M.C.BrittanyProvence

CorsicaBelgiumMorocco

,

65

1-'-

4 4K>450

_:::::::t-=::t= __

=t===t---::..:.....~>- ~

.....!L.- Mexico

- - - - - Brazil /~+-~--j~~+-~--I-~~~

------

trench,suture

anat.Ton. ~,~'=-,4. 1·2 3

T<45O

Fis· 24. - SyntheU:: scheme of lI18glDacic zonins deduced from zircon typology data (1 to 6. corresponding graniticsroups, hatched lines _ poorly represented, stars _ rhyolites and rhyolitic isnimbrites. On the right: hatchedarea • main distribution fields of mean points, arrows. gcodynamic polarity when soing away from the trenchor suture ZOIle.

2 . Morocco

Four massifs werc= studiro among theHercynian granites of the Middle Atlas ofMorocco. The typologic data define a zoningwith a west-east polarity (Fig. 22): the low l'calc-alkaline granite of Zaer is followedeastwards by the anatectic granite of Oulmesand the high l' calc-alkaline granites of Men[and Boumia.

J - Belgium

A last example concerns the Caledonianmagmatism of Belgium with the study ofmagmatic rocks in the Brabam, Rocroi andStavdot massifs, and of Gedinian sedimentaryrocks around them (Fig. 23) (FlEREMANS,1982). These last detritals often showmonogenic or (sub) monogenic zirconpopulations exhibiting magmaticcharacteristics with euhedral crystalsindicating a limited transportation from thesource rocks.

The global results rc=veal a clear magmaticzoning with a polarity directed from thesouth/south-east to the north: tonalites andlow t calc-a1kaline granites or rhyoJites,anatectic granites and finally high l' calc­alkaline granites or rhyolites in the Brabantand Condroz areas. Such a zoning is quitesimilar to those previously observed forexample in the French Massif Central andBrittany.

Thus, the zircon typology method appliedto the Caledonian magmatism of Belgiumwould indicate the existence of a paleocollisionchain with a possible limited previoussubduction, the supposed paleosuture beingemplaced in the south or south-east.

V. Discussion and conclusion

The differc=m examples presented here indifferent orogenic contexts. with a more orless abundant magmatism, lead to theclassification in three main types (Fig. 24):

J.P. PUPIN

1) A long lasted subduction followed (ornot) by a very limited collision process. Thiscase is charactcoized by the absence or quasi­absence of aluminous granites derived fromthe anatexis of the sialic crust. Calc-alkaline,K calc-alkaline and subalkaline granites andrhyolites are well represented. The zirconpopulations have mainly very high t indices,with a linear inferior limit showing increasingof t indices from the trench rone towards theexternal domains of the overthrusting plateand giving the geodynamic polarity (Fig. 24al.

2) A very short lasted subduction orunderthrusting of an oceanic lithosphericfragment in a compressional context followed(or nO[) by a collisional process. Typicalplutonites are tonalites and cale-alkalinegranites of the 4a trend with low t indices,emplaced very near the trench or the suturezone. Western Alps and New Caledoniacorrespond to this case. The scarcity ofmagmatic rocks avoids the possibility to definea polarity (Fig. 24 b).

}) A short lasted subduction followed bymain collision processes. Anatectic graniticrocks are very abundant and associated withcalc-a1kaline, K calc-alkaline and more rarelysubalkaline granites. On a gene-aI mean, thet indices of the rocks are lowco than in case(I).

The zonings are more or less complete (Fig.24c) but in any case the geodynamic polarityis given by the increase of t indices from theancient trench or suture zone (French Massif­Central, Brittany, Provence-Corsica,Morocco, Belgium, Elba-Tuscany).

After the collision and the end of thesubduction, a lot of intrusions are representedby various calc-alkaline or K calc-alkalinegranites (late or post-tectonic). The peculiarrepetitive organization of such granites seemsto be related systematically with the memoryof the ancient subduction: such a possibilitycan be realized only if the subductionstructure persists a long time after thesubduction has stopped, or if the magmasmobilized at the end of the subduction onlyemplaced a long time later, in a tensionalregime. In any case, the lower t of thepopulations - observed when collisionprocesses are underlined by an important

anatectic magmatism - indicate a more orless important interaction (hybridization withanatectic products, role of crustal fluids)b=tween the mobilized crust and the differenttypes of granites from which the zoning isdeduced.

Thus, zircon appears to be a good tool todiscuss the origin of granites and granitesappear to b= good criteria to determine thepolarity of ancient orogens.

The magmatic zonings of granites andtonalites have been determined over distancesof about 150 to 400 km, quite similar todistances observed for volcanites in orogeniczones.

The frequencies and the typologicalcharacteristics of these granites vary largelywith the duration of the subduction, theimportance of the collision, the possibility oftensional events after the collision ­favouring the emplacement of various magmas(i.e. Elba-Tuscany) -, the compositions ofthe mother rocks, but this fact appears to b=only of second order considering the repetitivezonings observed.

The three main types observed rememb=rvery well, for acidic magmas, the trilogyproposed by PITcHER (1979) i!l Alpinotype,Andinotype and Hercynotype environments.

So granites, but also acid paleovolcanitesor orthogneisses (in which magmatic zirconpopulations are often preserved, Pt.wrn, 1976)are major possibilities for the understandingof ancient orogens. Such criteria, directlylinked with deep fundamental structures, areof interest notably for the study of verycomplex orogenic domains as i.e. theHercynian belt in western Europe, thepatchwork distribution of the magmatismprobably resulting from the involvment ofseveral microplates. Thus, acid magmatism islikely to provide excellent criteria of polaritycapable of solving the problems ofpaleogeodynamics and must b= taken intoaccount more widely in the furore.

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