A re-examination of the typology of peral uminous granite

types in intracontinental orogenic belts

Carlos VilIaseca, Luis Barbero and Vktor Herreros

A BSTRAcr: Convenlional rock classification diagrams do not distinguish the variery of peraJumi­

nollS rock �ries. Mortover. peraluminClIl.� granite Iypes have nOl been clearly discriminated in rooenl revisions- The srudy of several peraluminous series in differenl iDtraooDUnental orogen..ic bellS reveals th:it four distinct groups can be defined. Using an A-B dil!lgraro, these four groups are:

(I) rughly peraluminou! graniloids (hP) cbaracterised by high A values and typified by an increase in pcraJuminosity toward t he mOst malic vurieullS; (1) moderately peraluminous graniLOids (mP) whicb o.ccupy the intermediate field :and generaUy show increasing peraJuro.inosity t owa rds the most f!:lsic varieties; (3) low peraluminous gran.itoids (JP) which plot in the lowest p.an of tbe pcralwninous field dtfi.n.ing negalive slope trends.; (4) bigbly felsic pcraluminou:\ graniles UP) with po.orly defined

va ria tion trends.

In inlracontinentaJ orogenic belts, tbe genesis of pcraluminous granitic series is favoured by the

abundance of (enile cruslal prol oliths. mainly mctapditcs. mctaigneous rocks and mctagrcywlIckcs.

The difficulty of auaining temperatures in C"X� o( 950°C al lowcr cJUSTal Icvcl� during (he (octonothermal evolution or Ihjckened CIlJ.SI. inhibits 1 he panial melting of 010r-c basic sources. A Ithough the physica.l Pllramctc� of the mehing prote:S! in fluence I heir chemical and mineralogical characteristics.. SQurtt rock composition ultimately detemlinc.s t he degree of pcraJuminosily of the granitic series.

J<. EY WOR DS: collisional orogenic magmatism. crustal protolil hs. granite elllSSificalion.

Conventional rock classilie:ation diagrams (e.g. TAS and KlO-SiOl plots) do not distinguish the vari<!ty of peralumi­

nou;; cock series and r6COUr$e i� frequently made to the peraJuminosity indcJ( o( Shand (1927 J. Also, recent alphabetic

classifications of graniloids (M [SA classilk..ation) which Jrc

more genetically biaSed do nol overcome the simple dichotomy between peralurninous and nletaluminow £erie� Rocks in which the maleculM proportion of AI,Ol is higher than (!l0 + NalO + J( 10 arc termed peraJurninous (Shand 1927). UGually. but not n�arily. they ha,·c nonn3tive corundum

and such charactetlftie modal phases as biotile, mU£OOvite,

garnet.. cordierilc "nd aJumii\ium silicates. Most (elsic igneous rocks are pcraluminous (Ocbon & � Fort 198]). bUI generally the A 1,0) excess is due 10 biotit e alone (Millet 1985). Igneous rocb containing i1luminium pha.<c! other tban biDtitc. though common and widesp�ild. £Ire subordinat.e. The difference in

degree of peraluminosity in (clslc igneous rocks has been discus.>c<l by several aUlhorI (Debon & Le Fon 1983; Miller

198 S) in order to est tlblish d iffercn ( penl u rn i /"IOu S series and, more import.Bntly. �ince lhe publication of tbe 1- and S-type

classitkation scheme of Chnppell & Whjte ( '974). (0 discuss

pctrogcnct ie problems remlcd 10 the origin and signific.ancx: of I hose: igneou� rocks with st rongjy peraJuminous compositions.

Deboo & Le Fon (19&]) used a bInary diagram with

a per2.lumin01ity parameter A = Al-(K + Na + 2Ca) and u

diR"erenliation indel B = Fe + Mg + Ti and suggesled criteria (ar acbieving El rigClrous chemical-minera.logical definilion of

81uminous associations.. but wit hOllt Slating how many types of peralu01inous series can be found.

M i U\:r (198.5) proV<lsOO a daM; flea t ion of peralu minau! gronites using Ibe A FM diagram in which he esLabli�hcd (WO main helds: strongly pcralllminous (Ps) and w.:::akly peralumi­

nous (?w) serics. t.ach with different mineralogical charllcler·

Isties. This k.ind of chu.sification diagram bas the problem that several igneous serie;s cross both the Ps and Pw fields as !.he differentiation index increases.

Recently Barbaein (1996) made a revision of the genesis of peraluminous granite! in which the �t a rting point was the

consideration tlllll the most significant \'olllm� an: made up of only two typ.s prod uced by extensive anatexi, of crusla l rocks. However. we �how below ths! rrom detailed study of several orogenic segments it is possible to recDgnise at least four types of peraJuminous granitoid� We also al1c:mpt to show that the complex divcnity of felsic magmatism may be best c1a..ssi6ed by the use of ILIl A-a diagram.

t. Variety of peralllminous series

In order to establish the diversity of crust:tlly dcrivro

pcralurninous scrit.\, granitic s.amples from scveral areas have bun selccted and plotted on an A-8 (Deoon & Lc Fort 1l}83) diagram (Fig. I). First.. peraluminous graniloids trom the Spanish Central Region have been plolt.ed, as they have been

thoroughly studied from a mineralogical. isotopic and geo­chemical point of view ( ViIlasec.a & Barbero 1994; Villaseca el ill. 1998) and also because it is possible to distinguish the

(our rypc:s o( peraluminous granites that can be found in :an orogenic segment (Fig. I aJ. On Figu(e I b. several scril!S (rom the Lacblan Fold Belt in Australia an:: assembled. and on Figure le other granitic serid (rom differ-cnt orogenic segments are plolted for comparis(ln.

It is noticeable tha t, despite the {act that the four types of granitic series defined in the Spanish CenITal Region can be (ound in other orogcnic.: st:(;tor.l, granite n::scarche� have focused thcir attention on tbe simple pen.lurninous versus metaJurnioous dichotomy (associat� with the 1- and S--typc petmgeneuc discu�ion). or have distinguished only two types

200 tJ) �) Ofhw granll.S:

160 @

Cil' U 100 � +

� 50

� � q: 11

a

q:

-50

-1M

B=Fe+Mg+Ti

Figure I A-S di:.sram (madi�oo (rom DeMo &. Le ForI 19&) for di(f�rcJlI �r3nillc series: (a) Cenlr.o.l Spain Hereyni"n Bel!: Ly (uYO$ gran,(e. BJHbcr(l &. Vill;)sc;;a 1992). MV. NM un<.l AR (Mor3-Venl:l$. La.� NJV3S del �hirques and La Alalaya Real pIUIMS-. Villa=:.a el al. J998). (b) Ulchlan Fold Bdl suies: Co. S, Jnd Mo

(Coom.,'l. StralhMsj.: �fld Moroly� seri� Cbappell t!1 al. 1991). all and Ji (.BuUenb;&lIon& nnd Jindab)'llC. Hinc tI al. t91!l1 lnd Da (D:llgtly su,I.:. Wbilt t!1 01. 191): Ch�ppcll .. , al. 1991). Icl Other granil(s: To (TOIl(CUl. HollZ & Barbey (991), Ki (KiMman mile, C:1�r" &< LiOM (986). Ro (Roycre. Slnn; & Cuney 1993). Ou (Gutr.:l. Debor) &. Le FOH (98). Au (Mold:lnubi�1\ 18 l·typ.: iUIlC. Liew;,1 vi. 19891. Li (Umousin, Leger e, nl. 19901. Hi IHim.ala�.an Icucogranll�'i, D�boll &. Le Fon 19�J). Ts (Trois Scisn.:ul'S, Wic:kh:lfll 19871. and Vel (Vd��. Wi!liam�n �I al. t9971. Fidd� :tre: l,P (highly peraluminous). mP (mooenllely peralutTlinou�l. IP (low pcr.aluminousl. {P (fdsi.: pcrnluminous). DOlled line OD IiJlure I a is Ihe liS boundary hl1�.

of peraluminous grani(es·. peraluminous &l'1lnodiorites and analcclic Icucogra nites (Lameyre 1988). Ba rbarin (1996) has named the1C: two Iypes CPGs ( pe mluminous cordierite-bcaring tonalite'S to monwgranites) and M PGs (pcraluminous IwO mica mon20gra nites and leucogranitcs), respeclively.

1.1. Highly peralumioous graoiloids (hP) In this field. granitic series wllh a high degree ofpcraluminosity pial as dennin& 1\ typiul positive trend. i.e. increasing peraluminosity towards the mOst roalic varietie�. This corre.i­pond s to Ihe typical S·tyre lrend (Whlle & Chappell (988).

Nevertheless, only a rew S-typc suites or sUpersUitC5 from the Lachlan Fold Belt clearly plot i.n this field. panicularly I he Coom:>. a.nd Bu lien balon S su i tcs {ram I he I<..osei usk ('> ba I h Oli I h (H.inc et al. 1978: Cbappcll el al. 1991) (Fig. I b). In 01 her orogenic seclors., all hough roore scarce, such granites are nOl absenl. ThUs.. in Ihe Spanish C�ntral Region. the La)'os resljlc­rich graniloids plOI in Ibis hP field (Barbera & Villascca 1992). In other Hercynian sectors, the Trois Seigneurs granitcs in Ihe Pyrenees (Wickham 1987). the Tourem cordiente­bearing granites in norrhcm POTlUgal (HoI7. 1989), the Vday anll.lectic granites (Williamson ct al. 1997) and the mOil malic facies of Ihe Roycre granile (S(us.;;; & Cuney 1993), Doth in the French Massif Central. arc examples of th.is hP Iype (Fig.. le). Far from the. Hercyoian orogen. the Kinsman illlrusiv� suite of Ihe eastern USA (Clad� & Lyons 1986) also belongs 10 Ihis group (Fig. le).

All these granilic suites are cnaraetens.ed by the presellO! of aluminous mineral phases other than biolile, which in several cases can be lhe mosl abundanl manc phase. Cordiente or garnet (alm.andin�pyrope series) are the mosl Iypical alumi­nous phases.. but it is nOI uousu31 10 find a=sory sillif('laniLe. In general. twO mica graniles �ensu $rriclo are scarce as mosl

of Iht: liP granilcs are characteristic of bigb-T c�lazonulllreas, equivalent 10 regional migmluitc terrains or regional aureole granitcs (While & ChappeU 1988). CordicrilC (ram Ihis type

o( hP granite is te)llurally and chemically d itfu\!nr (rom that or other pcralumioous granites. ren�ting its peritcclic o rigin in conlrast with the lale mag.rnalic origin of cordieri!, in higher level pcrlllumillous graniles (Williamson el Ill. 19971. A no I he r rem a rk.ab le (;h it racle rist ie of the It P type. e.specia 11 y �n in the most peralumioous suites (rom calazonill areas (Layo� Coomn. Tourem, Trois Seigneurs, Velay). is ils great heterogeneity and t he abundance o( reslitic enclaves.

1.1. Moderately per.9.IUOlU:10US graniloids (mP) These Br�nile� occupy an intermediate field in Ihe A-· B d iagnam generally showing negative slope trends, i.e. increasing peraluminDsiry lowards the more differentiated samples. Most of rhe Australi:m moderately peralumino us S-typcs plol in this field (c.g. Dalgcty suite, White: el (1./. 1977). The Stralhbogic suite from Cenlral Victoria IIlso plots in Ihis field but wirt-. a

positive slope, without reaching men high A ... alue� itS those of the lIP type (Fig. I b). Hercynial) eordicritc-bcaril)8 fIlonzo­gfllnitcs (Hcrcyniao S-Iypc after Pile!)er 1983), such 3S Ihe Margu�ride and Gueret plutons in Ihe French Massif Central (Debon & � Fort 1983) or Mora-Veotas and Hoyo de Pinsres plulons in the Spani�h Cenlral Region, are reprc�nln­live grJlniwids or lh.is moderalely pcraluminou5 type (Fig. la and c). This is Ihe field of pentluminous granodionlcs (Lameyre 1988), also called CPG Iypes (Durharin 1996).

Most o( Ihe g,ranitcs whieh plot iD Ihis Geld arc biclilc' bearing varieties. gene.rally wilh l1llolhcY miner;!l phase more aluO'linous than biotile as on accessory pha�e. Cordierile IS Lhe most common acce�ory minerJl..i, ""hilgl in highly {raC1ion' ated types, muscQvile may be dominanl togclher wilh garnet

Qf the alm<lndine--,spe$S<l rI inc series or other low· temperature AFM minerals sLlch as andiJlllsitc or lourmaline. Micro· granula r enclaves are conspicuous io these granite.!>, usually bcing l� than 1% of the area of OUICrOP,

1.3, l,o,,' pcrsluminous graoiloids (IP) The lowest part of the pcraluminous field is occupied by the low pcraluminosity seri� and by the most evolved tenus of those series wrucb evolve from metalumino us (0 slightly peraluminous compositions... in either C� defining negali\c slopt! trends in the A-B diagram (Fig.. I). fcJsic I-types from the Lachlac Fold Belt such as the Moru),B &nd J indabyne su persuilts (H i ne el al. 1978; Cha ppel1 er ul 1'191). t ogel her with several differentiated ca1c-albline suites. (for example Ihe Querigut in F'rance and Mont Givens in California, as referred to by [kban & Le rort 1983). plot witbin ,hiS JP field. In Ihe

Hcrcynian area from Central Spain, several plutons wilh characteristic acce.s.�or)' a mph ibolc and allsn;le In Ihe I�s (vol ved gr:lnodiorite facies plot in Ihis narrow field 01 low peraluminous granitcs (sce for cx.ample La Alala)'3 Real. La Cabre(a and Las Na vas del Marques plutons:, Fi&. I l\). SOll'lo! Iransitianal r ·types of the Moldlloubiao Bat hol.ilh of A US! "'1 (Licw t!1 aI, 19891 and some Caledonian I-types (Pitcher 198}) are among granites (het plot in [his JP field. It is noticeable that the more evolved [acies of this Iypc of series can be markedly more peruluminous than the rest o{ [he series and thus th�y sometimes be!!r aluminium-rich minerals (see., ror cxample. Ihe gamel peralllrninous I-Iypes and &arnct-andalus. ile I.IYpes from the Namungo relsic phHon. Chappcll et 01. 1991. or the ga rnel-cordierilt leucocralic (acics o( La Cabrern I·type plulon. Vlllascca & Barbero 1994).

Mineral chemistry of A F M ph ases common 10 the In P a od JP gr<,niles is :I powerlul 1001 fc;r dislinguishing both Iyp.es (\o( granile!:, esp-..cially when characteristic accessory minerals such as cordicrile, garnet or alumiruum silicate io the mP types, or amphibole (or allanite) ;n the I P lypes are not pr�ent in 3.

p:lMicular varielY. Biotitcs of IP type have characteristically lQwer �rahlJninosity w1lb respect to those of the }/)P type!. Ihe laller plollin� wit hin L"e illumino-pota5Sic field in the Mg versus total Al diagram (fig. 2 (rom Nacbil et al. 1985).

When rordierilC appears in both mP and IP types, it usually has textures and rompositions which indicate a magmatic origin, and wnich are quite different (rom Ihose of the hP types. ThO.ie eordieritcs from I P differenlialed Iypcs (La ClIbrera pillIon in the Spanish Cenlral Region) have lower AI10} and alkali contents, especially Na, than cordierites from the mP types with similar XFc compo�tion (Villase.ca &: Barbero 1994). As io the ",p types, malic microgranular enclaves are widespread.

t .4. Highly fclsic peralurninous grani(oids (JP) In this region or (he A-8 diagram, all the above-described peraltJrninous series converge. Seve(al granitic and volcanic series (sce. for e,xample. the MaCllsani rhyoliles.. Picbav<lnt el ai, 19&&) lire exclusively composed or very acidic members and Ihus projecl within theJP type field, generally not showing a typical varialion trend. Ncvenheless, some tend to sh(llll verlical trends and correspond 10 the traditionally (ermed an3 teClic Je\loogranites (umeyre 1988) or M PG (ypes (Barbarin 1996). The mosl rep�nlative o( this JP type are Ihe H imalayan.type leucograniles (Le Fori et al. 1987), and also the Limousin leucog.ranitt'S in (he European Hercynid� of France (Leger et al. 1990) and some of tbe Cervatos IlnLllCCllC ICllcogr!\nilcs in the Span;$n Cenl'al Region (Darbcro & Vlllaseca 1992).

BOlb the paucity of mafic minerals in these highly felsic types �nd their emplacement level influence the petrographic

charo.cteristlcs of t heS<! rocks. Thus, the parautochthonous leucograniles. poorly sei:(c�atcd (rom tbeir source region. can present a. high·temperalUre mineral parageoesis with typical Mg-cordieri(e. almaodi ne-pyropc garnet or sillimani(t. Thi� ki.nd of high-temperature mineral parage"esi� is found in most o( the lIP Iypes from anatectic areas, 0,.. tbe ot her band. in lbe more Dllochlhonous epi.2onal leucograni(cs (H i mal&ya or Limousin). Ihe AFM mineral parageQe5eS are of lower temperature and pressure, and minerals tend to show aplopeg­roat;t;c lext urcs and a Iso higher volatile contents, typically being represented by two micn iind tourmaline-bearing &101 niloids.

2_ Discussion

2.1. RclQlionship with other classification scbemes On Figure 1 b an 1/5 boundary line has hoen drawn U$log I be data of ) . and S-IYPCS from the Lachlan Fold Belt where (his nomenclaturc originated. It can be observed thal lrus IJS line coincides with the boundary linc between mP and JP types proposed In Ihis work, BOlh lines �how a negative slope in the diagram and do not coincide with the peraluminous-metalumi­nous division. There is not an ab�olute limit in peraluminOsity dividing tbe fields of mP and IP types and Ihe boundary is a

(unelion of I he degree of differentiation of the granite. 3S

slated by Chappell & White (1992). The Sood discrimination of Ihis mP-Jf' line between accessory cordicrite-beariog and amphibole-�aring granitcs reinforces tbis divisiol1.

The ]Jctrogenetic connotation o( the I-S terminology has made ils adoption in olher orogenic bellS debatable,

Furthermore. al(hou2b there is. as noted above. a Stfoni similanty between tbe composiuons of the l:tchlan Fold Belt Bnd the e.uropeno Hercyni&n Bell granitic series. the volumes of I he several granitiC series found in each orogen are different r Fig. 2). In Ihe LacMan Fold Belt, the hP types C<lns(itu(e ;,round onc-third o( the cxposed granites (the Bullenballong suite alone; hss 1\ .200 kml, Chappell i!1 01. J 9':11) and were eroplncxd al epizooal levels (sometimes even witb associated volcanic roch) in contrast 10 the less than 5% of ilP types in (ht European Hercynid� where they are (ypically catazonal plulons rclated to migmatil)c areas. In further contrast, ill tbe European Hercyoian B�ll, Ihe most abundanl Iypes fall \n tbe mP and IP fields., some of Ibero close to Ihe boundary betwten lhe I-types and S-types from Ihe UlchJan Fold BelL This has led 10 t he classification of some European Htrcynian granites as 8 lransitionDI sencs (Liew et 01, 19&9).

Apart rrom ;ntracontinenLaI orogenic wnes. where peralumi. oous granites 8rc rc:.a lly dominanl, griUlilic balholith>: also appear above subduction 7.ones al continenlal marglns.

Ncvertheless, in subduction sellings peraluminous serics are very s�rcc. &Ithough some peraluminous granites can be geoer,ued as a resull of differentiation from <:alc-alkaline series C<lmposed 0'111101)' of gabbros to granodiori(e$. The pro�lion of [he rontinenlal margin ealc-alkaline batholiths in the A-B diagram would show Iha! Ihe majority of The dua plot in t� metalummous field, with the most acid vaneties plolling in the IP field (set:! examples in Dcbon & Le Fon (983), This is a ronsequencc of the higher abundance of in!ennediale cOCKS in (he conllocntlll place margin orogens (Fig 2). Contrasts in {he abundance of petrographiC types belween differenl orogenic batholiths have led lO the difficulty o( reconciling (he differeot nomenc\alUn:.<. Several altempLS to roc.onciJc: tbc.>C different nomenclatures bave been made, as {oc ell am pie the distinction of PilCher (19&J) between Cordilleran {·types, equivalent to tbe calc-a.Lkaline SUItes (rom contineo(al margins., liind

ACTrvE CONTINENTAL MARGINS fNTRACONTINENTAL OROG£NIC BELTS UPPER CONTlNEWTAL CRUST

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Figure 2 Compa rison of rclul ive vol umti or i�bbfo·q ua.-udiocitG, IOnal i(� srana.diorite.s snd gr anil � bel ween aCI jvc continental margi ns 0 nd int raeon I i n�otaJ OfCl!,en.ic ba Ihol; Ihs. EstimAtes a re baseO on goologica 1 ma pi and pelrograpbic descriplion of Ihe batboliths (I. 3. 4, 5J. other rehilivc volumes are d�taiJed by tbe BUlhors 12. 6, 7. SI. 1 =New Srilain lslund arc (W1u.len 198$). 2= Pcruvian Coastal Balholilh (?ilcher �I 01. 1985). J - Sierra Nevada BOlholilh (Blllcman rr al. 196J). h, Cor<!illef3 81anca Balholit h. (PClford &. Alher1Dn 1996). 5 = Ikga BlIlholilh, Lachlan Fold Bdt (Chappell ('I "I, 1 99 1 J. 6= Cerursl Spain Hercynill.n Ikll (VillosCG3 el of. 1998). 7 .. Higher Hinlalaya (� Fori et Ill. (981), 8 - Avt.f'lI.ged upper �nlineotal crU�1 (Wcd(pohl 199 I). Tooo.liles lire t.he most ljbund�nl rock Iy,,� in coas t2.1 b&lholilhs (Pilciler et 01, 1985: Barker & Anh 19<JOI bUl granodiorilC:; dam;""le in inbonrd bntnolilh. o( �eti\'e eon,;ocn,ol n'''<&ins (Botemon �I al. 1963; Soloeby '" "I. Im), OlrdiUere BlanCll B�lbolilh lies inbOArd of rhe Peruvian COAstal Batbolilb, dift,clly over (he mlWil'�\y l bichn,d A ndta n coni incnlal cruSI (Pelf orcl & A thellon 1996 I. ,n transition 10 inl "'�conli"cntal :>ell ing.�. DOlled line ><:p�rales felsie (�65% in volume or rcl�ie minuals) from more maF.c rock •.

Calcdoni'!n I-types.. more typiesJ of iOI racontinenlru wne!;, which usually plOI in the JP field of Figure (.

2.2. Protolirh type lInd the origin of �ralJUl1inous suie:'l It ha5 become progressively l:Ippareot that the main mecbanism for generating important volumes of acid pcr:alum.iJJous granitoids is vie melting of crust!!1 rocks. including basic ma n t le-deri \led proto1it hs ( i .e. M iller 1985; Pet( o(d & A then 00 1996). In continental crustal settings, [he three main prol olil t!s

which caD prod uoe important volumes o( acid peraluminous magmas include mewsedimcnls (metepeliles), quanz· fcldsp!lthic metaignoous rocks (greywackes and orrhogneisses) and bssic metaigneous (amphibolitcs). On Figure) different composiliona I fields for these protol i lhs and Ihe eompositional

paths of .cvera) melting experiments have been represented. When melling OCCurs st low melt fractions. it is possible (or the composilion or rhe melts to pIal in thefP field irrespcctive of Ihe nature of thc �ourcc proto/ith. Even melllluminous

melaig,neous rocks such as reporled by Conrad n al. (1988) and Beard ee ill. (1993) and amphibo)i!es as reponed by Ellis & Thompson (1986). Bcard & Lofgren (1991). Patiflo Douee & Beard (J 995) and Springer & Sed:. (1997), arc able to produce peraluminous (elsle melts at low melting fractions and in v.. a te r-deftcien I condilion�. In general, all the mells produced at low mchiog fractions are indislingui�hablt. although it is possible 10 observe a lcodency of pelite-dcrilled Jnch� 10 plol on the upper pori oC the f P field (Fit-)). Ana-lcelic granitic series sen.su stTicU) have few opponunitics to becotTle more differentiate<! by erystal fractionation and usually do not define clear paths i.n the/P field. On the Other

hand. as stated earlier.!? granites could be highly fractionated magmas of the mP or IP series inslead of primary anatectic leucograDiles. So, lhe assumption that IWO mica leucogranites form a differenl magma I ie suilc should be carerully studied in c!.8ch panicular case, as Ihe /hP and IP peraluminous granitoid

types Cl)uld grade ioto highly (els;c peraluminous leucogranites ( Barbs rin 1996).

As the mdting fractiao increases, prima.ry components of Ihe protolith become progressively incorporllted into Ihe melt and thus, in the peraluminolls granitic series. Ihe more mallc rocks or Ihe suite beller image I h eir source (Cbappell & While (992).

When petitic prototili'ts arc melted. Ihe eampos i liono l paths or the resultant series mainly correspond 10 yesl ile unmixiog lines llCld plot inside the hP field (see Cooms, Le.yos or Kinsman series). ro fact, when a large amount of restit.::: is rClained in Ihe crystal-liquid mush. the rtsuhant composition

eau Id be si m i lar to t ha r or r he peLit ie IOU rcc (Fi g . .1). Nevertheless. some hP types do not nece.<>SII rily involve melting or a pelitic prololith. Experimenral data froro HoJ tz & Johannes (1991) demonstrated that particular melaig,neolls prolOlilns (i.e. peraluminous orthogneisses) could also generale

series which plol j n I he 10\\'1!r pa fl of the JJ P field. In ract. t be Tourem granites in northern Portugal nave b�o interpreted in I his way (H oltz & Barbey 1991). The possibility of generating hP types from felsic metaigneOlli !llIurees h!ls bel',n

used as an argurnenl against Ihe equivalence or strongly perl) I urni nous gT3 ni le and S-type (melescdimen la ry deri lied) granite (Miller j 985; Clemens & Wall 1981). Metasedimel118ry

100

250

iii' �OD P-I (.) ('-I 150 + ('(J

MO � +

� 5() --!. � 11

0

q: -50

- 100 I-S boundary line

- 15D

B=Fe +Mg+Ti

F"'lurr J A-B di�l!r:lm �howins Ihe plOjoclioo o f scvcr.U �r\lS\BI prolo\ i t hl !lno the. corrc;;pondint clpcriltlCnlaJ mcll ing da c..a: squal"(:S: p.:\ilc-dcriVL-d mells t Wickh:lm 1 �8); Vi�uI &. H olloway 1 9S5: I'a l i �o DOllcc &. John.loo 1 99 1 1; s l�rs: acid met.a.ignwus. ami gn:)'wacke ·ocrived mcll5 j Holl2 &. Johan� 1 99 1 ; Coo.rad el 01. 1 988: 8CJ1rd rl 01. \ 993: 1':11;00 Oouo: &. Beard 1 995, 1 996: GlIrdicn e/ �/. 1�5); ciJc\CS: mCl ub.:i..<ic-dcrivcd mch� ( Ellis &. Thompson 1 986: 8c;t/d &. Lofg/l:lI \99 1 : wolr &. W)"l Iic 19')41 . FilJeU symbols rcp rcsc o I low md. lr�clion 1< 20% mci l i n a l n:sult$.. Ooued l ines reprcs.ern progTCS)· ive mc\l inJ: (MlClion (01 difTen:(l1 a p.:ritn<:nts.; P-] .. HQ-J6 pelile: (rom I' a lil'\o Doucc & loh !\Slon I 1 99 1 \: \'. H • pcl i le from Vicl2.cu( SI. H ollowOlY ( 1 9118 1: C-N ·W(n) = �ralunoi nou.s greywackc (ram Con r.Jd �J 01. ( I �&8l: C · N ·W(bl= mcl:\JlU11 inollS dacile trom Con r�d �J f)/. ( 1 9gS); B- L - :.mphi bol ilC sample 466 (,(101 �rd &. Lofgrcn ( 1 99 1 \. OI�CI (,chh �. ill Fil!u,,' I .

and/or slrongly pcralu m inous mCl aigDe{)us prololiths a rc Mlh sources (or hP Iype grani tes.

Here anot her q uesl i o n arise:;: I s il �ssible to generate a hP Iype via e x h a ustive fractionat i o n of any olher l iq u id? Sl ussi & Cllney ( 1 99)) considered I h is possibility for I be p l uton ic compicJ of La R6yen:: ( French Massi f Cen tral ) . Theorctica Uy.

fraclionot !on of pcr!l l uminous m i nerals could prod uce an hP t rend. The main pro blem wi lh i.his by�thesis is that erystall�tion of t he majority of the peraluminous AFM ph8SCS i n grani lic melts (sa met. cordierite, m uscovile.. alu­m i ni u m �i l ica les) is prod uced a t late stages, dose lO the solidus cond i t ions of the magma, as evidenced from petrogra phic studies I V i l lo..scea & Barbcro 1 994) as well as from ex perimenlaI d.ata (Clemcns & Wal l 1 98 1 ; Sh.i m u rn el al. 1 992) . This mak� very un l i ke ly the sepa ralion of peraluminous mi nerals in a

mag:ma1ic syst.;m. which is highly yjscous wnen i t is close 1 0

being (ully cryslol l ised. I n gener:! I, mP and IP types could be generated from any

k Ind of crustal protoli t h. as ded uced from Figure J, or even by m i xtu l't.".; of t hem ( Pat ii'lO Douce & Beard 1 995; Mcunby & PJ t i n o Douce 1 997). Partial mel ting of several lc.if\& oC mcta igneou$ rocks u nder yaria ble \ValeT conditions oould t;cner.!Ic ",P I ypc peralu minou.5 granodiorilie magmas (Canrfld (1 01. 1988: BC11 rd &. Lofsrc n 1 99 1 ). In addition. experiments demonS l ra l e t ha! pe rn l u m i noU!! l iqu ids can be prod uced by dchydration me l ting of some mclaiuminous am phiboliles u nder lower erusw.l cond i l ions ( Beard & IArgfen 1 99 1 ; Wolf & W i l ley 1994; PQ l i r'l o Douce & Beard ( 995; Spri nger & &ck 1 9971. Neve r l hc \es..<:. I' $ocms Ihac o n ly metabasic prol 0li lhs could gcner� l e mct a lumlnous i ntcmrcdiate rocks o( (he IP suites. A� experiment ally demonslrated by Beard & Lofgren ( 1 99 \ ). )0% mdtiog of 3n iim phiboli tic protoljlh under water-

undcrsa t urll tcd condil ions would produce a !onal iLic mel t with values of I he A nnd B pa r�mctcrs of - 36 and 1 8 1 res�tiycly. which a r( vc r y doS<! 10 the original composition of the p:l fent e:t l m 3 gTl\aS of se ye rd l of Ihe IP series re presented in Fisu re I I fi� 3). None of the pc raJ u mlnous eruslal pro loli l h s I pelilcs. grcywaekcs or a ci d mcl aigneolJs rods) C() u ld genera l e such a I'a re n f ll l m c h f M I P I ypes..

Mcch� n isms 01 hcr \ han pure crustal meltins eo uld ge(\(::rnlc pcra l unl inous m s g m::ls. II.S s l atcd in (he reviews o f M i ller ( 1 98 5 ) or BOlrbarin ( 1 9961 , but i l seems un l i kel y I b a l I hey

could gcn.:r:lte large bat ho) i l hs of pcraluOlinou5 fclsic grani les. I 1 is rCilsorl'ablc 10 a s.s u me ( h il I the phygic.al pa rameters o{ the � rl ill l mching process i nfluence the m i neral pa ragcne�is and t he composition of the me l t , bUI ne venheless i t is I ne com posit ion of the sou rce rock w h ich u l t i m ately determ i n<!s to wh ich pc ral ll m i n o u s gra n ite type I he seri� will belo ng.

2.3. The role of the tectonic s.etlrng in the partial mcl riog

pro<:es.5 Assignmenl of different peralumino us gJ<I n ill! lypeS 10 spcxtf'ic loctonic s<!l t ings as somc researchers suggest., empha sising l he preyail ing rolc of Lhe pnyslcn.l pa rameter:( of parl isl melong oye r t he n atu re of I he sources on t beir genesis ( Pitcher 1 983; Barbarin 1 996 ), h a:< 10 lake i nl o account tbe common prc�nu of diffe rent coeval p.:raJ unUnou5 graruloids il\ some inl taconti ·

nenta l o rogenic bel ls as e�empl ifiul by EUTOJX'oIn Hc:rcynilrt

segmenls l i kc Ihe Spa nish Central Region. As previously

Sl3 led . Ihe main faclorJ; con t rolling t he t ypol ogy o( peral u mi· n o us sran i l ic series a rc t he nat u re of lhc source roclc and the physical pa ra melers of 111<: mch iog p rocess. BOlh (acl ors Yliry depending on I he I ccl onic sel l i ng in which 1 he granilcs arc: gc ncr;Hcd. Thi� i� lh� fOll ndano[1 fM rh� l ecl "n;c-h�1i classificalion schemes m a de by sevcra l a u t h o rs; AS Pil� hcr ( 1983), M ania, & Piccoli ( 1 9B9) a Dd Rogers & G rec n oc rs ( 1 990), a mong Olhers..

In �ctivc conlinenlal ma rgins, b-asic prol ol i l hs an: more

� bu nda n I i n CONI ps rison to ;1) OeT con t inenta I crusl� I segmcn Is. Th.is is Ihe resu lt of Ihe accum ula l ion of basic rock due 1 0

u nderplal ing or basic mBtenols a l 1 he base o r Ihe c r us t or 1 0

accrel illn o f I he �llbd uCled oceanic slab. The origin o f t he Cord i l lera n ba t holilhs i� usually cxpla ined as a tWD-5 lagc

model. in which I he fi rs I s I a ge is I he c reation of a malic crus! a I Il nderphuoo layer fol lowed by a second sla ge of extensive lower c rusta l mtlt iog ( Pitcher 1 993; Pcdord & A l he-re on 1996 ). The presence o( important volum� of basic pro lo l i !hs al COn l i nen l.:! 1 margi ns implies thal I he amou n l of pcralll m i nous m�ltx genera led m u�J be: of minor i m pon:ance.

Tbe mol'\! rcls;c nat ure of t he con tincnlal crust i ovolved in inl r.leont.ine n l a l coll�io nil\ orogens. bet.ng mainly composed of pcJi\t::S.. srcywackc.s and acid-iotermediate mCla-igneous

l a yers. implies a h i sher fenililY for producing peraluminous melts vln dehydrat ion mcl liog reacl ions. For i nstance. cx�r· i m e n l a l rl::Su l ( � �how I hal III a ro und 950'( at I he Mono. a

pclilic p rotolith co uld produce as much as 50-60% of pcral u m i nous mch, D nd a met3 greywackc or mCl atOnruil ic gneiss could produ� a round 20-35 % of melt . Under (he sa m e

oondi(ions, mctl:lbasic pro to l i ths cou ld only prod uce a round

1 0 ';' o( granitic mel l (scc I1g. I i n Gsrdien e/ 01. 1 9951. TcclonOlhenn:l1 models for crustal thickening ind icate Ihal

al lhe base of the crUSt In<: temperlll Ures reached arc not usual ly h igher th:.rn 9 SO°C, which makes t he melling of a basic pro(olith. i f presenc . difficult (ThompSO rt & Connolly 1 995). The p,od ucl i o rt of gT'Cs ler volumes and a complexity of

pc r.a l u m i nou� grani l lc series is eohanced in such a ICClonic sel l i ng.

3. AcknowledgemeDls

Revisioo and commtots made by Bemard 80nin. Wallacc

Pi lcher and Ed S tephcns on a pre vious version have greal \y

i ncreased t bc q ualily of this work. This paper h as al�o

be oefitcd gre a t l y from der.ailed CQmmenlS by Peler E. Brown

a nd an anonymous reviewer. This resea rcb was financia l l y

supported b y l he P\i96-066 ! Projl:Cl of I he M i nisll:rio d e Educacion y Cult ure of Spain .

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