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Journal of South American Earth Sciences. Vol. 6, No. 3, pp. 123-132. 1992 0895-9811/92 $5.00+.00

Printed in Great Britain 1993 PergamonPress

td

& Earth Sciences & Resources Institute

R e g i o n a l S t y p e g r a n i t e s i n t h e E c u a d o r i a n A n d e s : P o s s i b l e r e m n a n t s

o f th e b r e a k u p o f w e s te r n G o n d w a n a

J. A. ASPDEN1. N. F O R T E Y 2 M. LITHERLAND1. E V I T E R I 3 and S. M. HARRISON

1Misi6n Brit,'tnica (OD A/BG S). FCO (QU ITO).King Charles Street,London, SW1A 2AH . England, UK; 2British

Geological Survey.Keyworth.Nottingham,NG12 5G G. England. UK; 3CODIGEM.CasiUa 17-03-23. Qu ito,

Ecuador; 468 Gaim Terrace,Aberdeen.AB1 6AT. Scotland. UK

Received July 1992; Revision Accepted December 1992)

Abst ract Reconnai ssance geological mappi ng of the Ecuadorian Cordillera Real ha s established the presence of a previous ly

unrecognized region al suite o f variably deformed granitoids for which poorly constrained Rb-Sr whole-rock data indicate a mini-

mum Early Juras sic age of 9.200 + 12 Ma (initial ratio = 0.7120). Th is suite, which is associated with low- to medium-gra de, semi-

pelitic metamorph ic rocks, is dominated b y peraluminous monzogranites containing biotite + garnet + muscovite. Geochemically,

these granites are S-typ es and can be readily distinguished from juxtaposed 1-type granitoids of the Midd le-Upper Juraasie

Zamor a, Abitagna, and AzafrCm bathol iths located immediately to the east. I ntrusio n of these S-type gran ites may be related to the

breakup of western Gondwana

R e s u m e n E 1 reconoc imiento del mapeami ento geol6gico de ia Cordillera Real Ecuatoriana ha establecido la presencia de un

conjunto , previamente no identificado, de granitoides variablemente deformad os para los cuales las dataciones de roea total de

Rb-Sr pobrement e registrados indican una edad minima de Jur~i ca Inferior de 200 + 12 Ma (Ri = 0.7120). E1 conjunto, asociado

con diversas rocas semi-pellticas de bajo a mediano grado de metamorfismo, es dominado pot mon zogranitos peraluminieos con

biotit a + granate + muscovita. G-eoquimicamente, e stos granitos son de tipo S y pueden set f~eilmente distinguido~ de los grani-

toides yuxtapue stos tipo I de los batolitos Zamora, Abitgua y Azafr~in de edad Jur ~ic a Medio-Superior encontrados inmediata-

mente hacia el este. Se considera la posibilidad de que los granitos tipo S puedan ser relacionados a la ruptura de Gondwan a

occidental.

I N T R O D U C T I O N

T HE CORDI LL E RA RE AL r e pr e se n t s t he e a s t e r nm os t o f

two cordi l le ras tha t mak e up the Ecuador ian sec tor of the

Nor the rn Andes . Throu ghou t it s 650-kin length, the Cor -

dil lera Real is crossed by only f ive roads. Lim ited access,

h igh a l ti tude ( t he m a in wa te r she d l ie s be twe e n 5800 a nd

3200 m ) , and he avy ra infa l l com bine to discourage f ie ld-

work, so tha t the geology o f the a rea has remained poor ly

known unti l recently.

In 1986, a bilateral Ecuad orian-Brit ish Techn ical Coop-

eraf iou Program was init iated to carry ou t a regional inves-

t iga t ion into the n a ture and eco nom ic minera l potential of

the metam orphic rocks tha t compr ise the bulk o f the Cor -

d i l l e r a R e a l . B a s e d o n a b o u t 1 0 m a n - y e a r s o f f i e l d -

or iented s tudies, suppor ted by g eochrono logica l (Aspden

e t a l . , 1992) a nd ge o c he m ic a l s tud i e s ( L i the r la nd e t a l . ,

1990), t he p r e - Cr e t a c e ou s r oc ks o f t he C or d i l l e ra ha ve

been divided into a se r ies of informal , r egiona l , li thotec-

tonic divisions (Aspde n and Lither land, 1992).

Lithologically, the Loja d ivision consists pr incipally of

a v a r i a b l y m e t a m o r p h o s e d p e l i t i c -p s a m m i t i c s e q u e n c e

( the Agoy~n and C hiquinda subdivis ions) and metagrani-

toids (Fig. 1). In the w est am the foliated bioti te + garn et +

musc ovi te grani tes of the Tres Lagunas subdivis ion; in the

sou the a s t i s t he e longa t e Sa ba n i l l a subd iv i s ion , a m o r e

heterogeneo us unit that is dom inated by bioti te : : garn et +

muscovite-bearing o rthogneisses an d migmatites.

Al though i solated occur rences of these rocks had been

n o t e d p r e v i o u s ly ( K e n n e r l e y e t a l . , 1973; Ha r r ing ton ,

1957 ; Colony a nd S inc l a i r , 1932) , t he i r t r ue r e g iona l

e x t e n t a nd s ign i f i c a nc e ha d no t be e n a p pr e c i a t e d e . g . ,

Ba ldoc k , 1982). T h i s p r e l im ina r y c on t r i bu t ion g ive s a

br ie f descr iption of the f ie ld and pe t rographic charac te r i s -

t ics of these rocks and presents new geochron logica l and

who le- rock geoc hem ica l da ta . To gether , these da ta indi -

ca te tha t the Loja divis ion grani toids show ma ny fea tures

of S - type g r a n i te s ( Cha ppe l l a nd W hi t e , 1974) a nd a r e

qui te dis tinc t f rom "n orm al" An dean (cordi l le ran) I - type

granitoids (Cobbing, 1990; Pitcher, 1983 ), as exemp lif ied

by the Zamora, Abitagua, and Azafr(m batholiths in Ecua-

do r (Fig. 1).

G E O L O G Y A N D P E T R O G R A P H Y O F L O J A

D I V I S IO N G R A N I T O I D S

I res L agunas rani tes

Outside of shear zon es the Tres Lagun as granites are

p e t r o g r a p h i c a l ly d i s ti n c t i v e n o r m a l l y c o n s i s t i n g o f

m e d i u m - t o c o a r s e - g r a i n e d g r a n i t e s w i t h p r o m i n e n t .

Address all correspondenceand reprint requests o the British GeologicalSurvey,Keyworth,UK:

telephone [44] (602) 363100; fax [44] (602) 363200; tele x378173 BGSKEY G.

SAES--6/3-B 123

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124 J . A . ASP DEN , N. F ORT EY, M. L1THERLAND, F . V1TERI , and S . M. HARR I SON

A

i

o o~- -

PO0' -

g u i n d a

o

~ o r p h i c

40Km

Fig. 1. Pre-Cretaeeous geolog y of the Cordillera Real (af ter

Lithedand

e l a l .

1990): A) north of 2 S, showing disl~ibution of

the L oja division and the A zafr~ln, Abitagua, and Rosa Florida

batholiths; B) south of 2 S, showing distribution of the Loja

division and the Zam ora batholith.

71)ooow

c )

)o' s

co

o

~u

o

Lu

q :

o

S a r o g u r o

co

0

t v

0

to

J

C U E N C A

S i g s i g

/

)

0

JC ~

o.

~ o

0

~0

z

3:0f fS

~-- ~

4O0*S

I s Q,

11eOO'W

C h i g u i n d o / A g o y o n

s u b d i v i s i o n s

m r s s L a g u n o s

s u b d i v i s i o n

~ S o b o n i l l a

s u b d i v i s i o n

~Mr

met morphic

d i v i s i o n s

I 0 ZO 30 40K i n

I I I

/

s m o k y - b l u e t o g r e y a l k a l i f e l d s p a r m e g a c r y s t s , u p t o 1 4

cm in l ength . M any s a m p les conta in pa le b lue quar tz crys -

ta l s o f un cer ta in or ig in . Th e m ajor m e .t ic m inera l , b io t i te ,

i s typica l ly reddis h-brow n in th in s ec t ion and up to 1 cm in

diam eter ; it m a y cons t i tute up to 10 o f the m o de . Horn-

b l e n d e h a s n o t b e e n f o u n d i n t h e s e r o ck s .

Th e alkal i fe ldspa r is nor ma lly perthit ic , and the pla gio-

c l a s e r a n g e s f r o m a l b i te t o o l i g o c l a s e . N a r r o w r i m s o f

blue-g rey a lk ali fe ldspar are re la t ively commodtl on e , r l ier-

f o r me d , o f t e n e u h e d r a l p l a g io c l a s e , b u t r a p a k iv i o v e r -

growth s a l so occur. The a lka l i f e ldspa r megacrys ts con ta in

in c lu s io n s o f c r e a m- c o lo r e d p l a g io c l a s e a n d /o r b io t i t e +

quar tz .

G a r n e t i s a c o mm o n a c c e s s o r y min e r a l , a tt a i n in g 3 0 %

o f t h e mo d e i n a b e l t o f g a r n e t g r a n i t e s b e tw e e n P a p a l l a -

c a t a a n d O y a c h a c h i ( F i g . 1 A ) . C o r d i e r i t e h a s b e e n

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Regional S- type grani tes in the Ecuador ian Andes: Poss ible remnants of the breakup of weste rn Gond wana 125

r e c or de d bu t i s r a r e . M usc o v i t e i s f a i rl y c om m o n bu t i s

mo stly sub ordinate to bioti te , w hich i t gen erally replaces.

Second ary granoblas t ic quar tz fabr ics a re widespread and

a r e a c c om p a nie d by pa r t i a l r e p l a c e m e nt o f f e ldspa r a nd

mica by quar tz . Other la te fea tures inc lude the format ion

of epido te , the se r ic i te -zois i te a l te ra t ion of fe ldspars , the

recrys taUiza t ion and/or chlor i t iza t ion of biot i te , and the

gr ow th o f b r own tour m a l ine . M inor a m ount s o f opa que

minera ls a re a lso present , but the i r composi t ions have not

been d eterm ined directly. How ever , outcrops show consis-

tently low ma gn etic susceptibili ty readings, suggesting the

absence of m agnet i te .

Re l a t i ve ly unde r f o r m e d gr a n i t e s c a n o f t e n be t r a c e d

into gne iss ic be l t s r e la ted to ver t ica l or s teep, westw ard-

dipping, Andean - t rending shear zones in which S-C m ylo-

n i t e s ( Be r th6 e t a l . , 1979; L is te r and Snoke , 1984) a re

w i d e l y d e v e l o p e d . I n p l a c e s , t h e m y l o n i t e s a r e c u t b y

youn ger , und efform ed pegm atite veins o f quartz +_. ourma-

l ine + fe ldspar + m uscovi te .

Xe nol i t hs a r e r e l a t ive ly unc o m m on bu t i nc lude bo th

meta-sedimentary and m eta- igneous material . Large xeno-

crys ts of whi te ve in quar tz (up to 5 cm ) a re comm on in the

Malaca tus a rea , and synplutonic amphibol i tes a re present

eas t of Bafios (Fig. 1).

Con tacts of the Tres Lagun as subdivision are tectonic.

Nor th of la t i tude 2S, the count ry rocks (Agoyan subdivi-

s ion , F ig . 1A) a r e t yp i c a l l y m e dium - gr a de , a lum inou s

schis t s and paragn eisses , wi th ra re inc ipient migmat iza-

t i on , whe r e a s i n t he sou th t he y nor m a l ly c om pr i se l ow-

gr a de , s e m i - pe l i t i c phy l l i t e s a nd qua r t z i t e s ( Chigu inda

subdivision, Fig. 1B).

S a b a n i U a O r t h o g n e i s s e s

F e w e r p e t r o g r a p h i c d e t a i l s a r e a v a i la b l e f o r th e

Sabani l la sub divis ion or thogneisses , which co nsis t essen-

t i a ll y o f m e d ium - gr a ine d , f o l i a te d b io ti te + m usc ov i t e +

garn et granites. In con trast to Tres Lagunas, th ese granites

do n o t c on t a in b lue qua rt z , nor a r e t he y c om m only m e ga -

c r ys t i c . T h e y a r e a l so m o r e hom o ge ne o us ly f o l ia t e & so

t h e y m a y h a v e b e e n d e f o r m e d a t s o m e w h a t h ig h e r t em -

pera tures and poss ibly deeper leve ls (Gapais , 1989) - - an

i n f er e n c e s u p p o rt e d b y t h e m o r e c o m m o n o c c u r r e n c e o f

m igm a t i t e s a nd t he oc c a s iona l p r e se nc e o f kya n i t e - a nd

s i l l im a n i t e - b e a r i n g a s s e m b l a g e s i n a s s o c i a t e d p a ra -

gne isses . Eve n aw ay f rom the mo re obviously migmat i tic

pa r ts , t he Sa ba n i l l a o r thogne i s se s a r e t e x tu r a l ly he t e r o -

ge ne ous a nd o f t e n c on t a in m e ta - se d im e nta r y xeno l it hs i n

various stages o f digestion. Bioti te schlieren and clots are

e x t r e m e ly c om m on. I n som e ou t c r ops , r andom ly o r i e n t ed

blocks o f orthogneiss, apparen tly similar in com posit ion to

the host , have been no ted. As wi th the Tres Lagunas subdi-

vis ion, r e la t ive ly smal l maf ic bodies , of am phibol i te , a re

present in some areas

e .g . ,

north and ea st o f Valladolid,

Fig. 1B).

A g e o f L o j a D i v i s io n G r a n i te s

T he a ge o f t he L o ja d iv i s ion g r a n i t e s is no t p r e c i se ly

known, bu t b lue qua r t z c l a s t s , p r e sum e d to be de r ive d

f or m the T r e s L a guna s g r a n i t e s , oc c u r i n f os s i l i f e r ous

Low er Jurass ic meta-sedimen tary rocks expo sed south o f

Ba f ios ( Ho wa r th a nd I v im e y- C oo k , 1991) (F ig . 1A) .

Attempts to date both the Tres Lagunas an d SabaniUa sub-

divisions, using K -Ar (biotite , musco vite) , Sm-Nd (garnet/

whole- rock), and Rb-Sr (whole- rock) methods , hav e been

genera l ly unsuccessful . At present, the most re l iable da ta

com e from the Tres Lagunas subdivision, whe re the com -

bined Rb-Sr w hole- rock da ta (17 points) give an i sochron

age of 200 + 12 Ma (MSW D = 169, Ri = 0.7120; Fig. 2) .

Desp ite the high degree of scatter, this is consid ered to be

the minimum age for emp lacement (Aspden

et a l . ,

1992).

A N A L Y T I C A L D A T A

The m ater ia l used in this s tudy was col lec ted us ing a

hand-he ld rock dr il l and dynamite . In the case o f the I res

Lagunas grani tes , only the mo re massive i .e . , least foli-

ated) outcrops w ere sampled. In al l, 24 w hole-ro ck analy-

ses representing each o f the Loja divis ion grani tes (LDG )

a n d t h e Z a m o r a , A b i t a g u a , a n d A z a f r~ in b a t h o l i t h s

(ZAA G) are currently available (Asp den

et a l . ,

1990). The

Tres Lagtmas samples w ere collected from areas so uth of

S i g s ig , e a s t o f S a r a g u r o , a n d n o r t h o f M a l a c a t u s ; t h e

Sabanilla subdivision was samp led north o f Valladolid and

east of Sabanil la (Fig. 1 B). Rep resentative analyse s from

the LDG and ZA AG suites are l isted in Table 1.

Th e Zam ora, Abitagua, and Azafran batholiths, an alyti-

ca l da ta for w hich a re inc luded here for compara t ive pur -

pose s, f o r m the sou the rn pa r t o f a be l t o f p r e dom ina n t ly

M idd le t o Up pe r Ju r a s s i c ba tho l i t hs t ha t c a n be t r a c e d

t h r o u g h o u t t h e N o r t h e r n A n d e s ( A s p d e n e t a l . , 1991,

1990, 1987; McCour t e t a l . , 1984) . They are typica l of

An dean I- type granitoids, h aving a w ide range in SiO 2 and

high NazO values, and are comm on ly hornb lende bearing.

O . 7 3 5

0 . 7 2 5

0 . 7 4 5

0 . 7 0 5

i

i

8 7 S t / e 6 S r

..V. +..#- ..... 4: .......

. . o o ~ 1 ~ o

. . 4 # , . ~ 4 g

, . .

. . , . '

A GE 2 0 0 ~ 1 2 M o 1 2 s )

Z n t e r c e p t 0 . 7 t 2 0 0 . 0 0 0 7

M S Wl ) q 69 . t E n h o n ced E r r o r s

8 7 R b / M S r

i

q 3 5 7

Fig. 2. Rb-Sr whole-rock sochron diagram for the Tres Lagunas

subdivision (Aspd enet a l . , 1992).

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126 J.A . ASPDEN N. FORTEY M. L1THERLAND F. V1TERI and S. M. HARRISON

0

= ~ = ~ ~ = ~

~

~

=.

8

~ ~

e ~

1 ~ t m

~ A

~

g

A

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Reg ional S -type granites in the Ecu adofian Andes: Possible

r e m n a n t s o f t h e

breakup of weste rn Gond wana 127

I n i t i a l 87Sr /86Sr r a t i os f o r t he Abi t a gua a nd Z a m or a

ba tho l i t hs ra nge f r om 0 .7037 to 0 .7056 ( Aspd e n e t a l .

1992) and are similar to values of 0.7034-0.7048 obtained

by Bro ok (1984) f rom the Ibague ba thol i th in Colombia .

These da ta suggest der iva t ion f rom a fa i r ly pr imi t ive iso-

topic source. Consequently, the entire belt is interpreted to

represen t the pr incipal mag ma tic produc ts of Jurassic sub-

duct ion a long the pa leocont inenta l margin o f nor thweste rn

South A mer ica (Aspdou e t a l . 1987).

G e o c h e m i s t r y

B a s e d o n c a l c u l a t e d C I P W n o r m a t i v e v a l u e s , t h e

m a jor i t y o f t he L DG p lo t i n t he qua r t z - r ic h ( nor m a t ive

quar tz > 38 ) par t of the mo nzog rani te f ie ld in the QA P

te r na r y d ia gr a m . I n c on t r a s t , t he Z AA G show a g r e a te r

c om p os i t i ona l ra nge , i nc lud ing no t on ly m onz ogr a n i t e s

bu t a l so g r a nod ior i t e s a nd m or e ba s i c d io r i t i c va r i a n t s

F i g . 3 ) .

Fur the r m or e , va r i a t i on d i a gr a m s r e ve a l c l e a r d i f fe r -

e nc e s be twe e n the L DG a nd Z A AG sui te s. W he n p lo t te d

aga ins t S iO2, the LD G suite stands out as enrich ed in vari-

ous e lem ents , inc luding TiO2, PxOs, Cr , and Zn (Fig. 4) .

S o m e e l e m e n t s t h a t h a v e c o n c e n t r a t i o n s c l o s e t o t h e i r

Z A AG c ounte r pa r ts o f c om pa r a b l e S iO2 c on te n t show a

qui te di f fe re nt var ia t ion t rend. Thus , whe reas Th, Ce , Y,

and Nb gen era l ly r ise wi th increas in g SiO2 in the I -type

Z A AG su i te , t he y a ppe a r t o f a ll w i th i nc r e a s ing S iO2 in

the S- type LDG sui te (Fig. 5) . Based on this , we conclude

tha t t he L DG a r e i nde e d a se pa r a te g r oup a nd c ou ld no t

h a v e f o r m e d b y f r ac t io n a t io n o f Z A A G - t y p e m a g m a s .

This con t ras ts wi th the inte rpre ta t ion of the more evolved

pa r t s o f t he Cor d i l l e r a B la nc a ba tho l i t h i n Pe r u . whic h

exhibi t man y S- type fea tures, as having been der ived f rom

an I- type parent (Atherton and Sanderson, 1987).

D i f f e r en c e s b e t w e e n t h e L D G a n d Z A A G s u it es c a n

a lso be seen o n a n umb er of other plots . In both the 1(20 v s

Na 20 a nd t he /M / ( Na + K+ Ca /2)

v s

SiO2 diagram s (Figs.6

and 7) , the pera luminou s (A/NKC > 1.1) LD G sui te and

the meta l - ruinou s (A/NKC < 1.1) ZA AG sui te fa l l wi thin

the S- and I- type f ields, respectively, with l it tle or n o ov er-

lap be tw een the groups . Equal ly . on the A CT plot (Fig. 8)

the S - type L DG s t r a dd l e t he p l a g ioc l ase - b io t it e t i e l ine

a nd e x t e nd i n to t he /M - r i c h pa r t o f t he d i a gr a m , whe r e a s

the I - t ype Z A AG s t ra dd l e t he p l a g ioc l a se -hom ble nde t i e

l ine , wi th som e points lying on the CaO-r ich s ide .

The Chap pel l and W hi te (1974) c lass i f ica t ion of gra-

nites into S- a nd I- types is bro adly sim ilar to the i lmeuite-

and mag neti te-ser ies of l .~hihara (1977) in that al l S- types

be long t o the i lmenite-ser ies and the m ajori ty (but not al l)

of the I - types cor respond to th e magn et i te - ser ies B e c k i n -

sale, 1979). Magneti te- and i lme nite.ser ies granites can be

dis t inguished us ing an Fe203/FeO vs SiO2 plot (Lekm ana

and Harmanto, 1990) a diagram tha t a l so c lear ly sepa-

ra tes the LDG and the Z AA G sui tes and c lassi fies them as

be long ing t o t he i lm e ni t e - a nd m a gne t i t e -se r i e s , r e spe c -

tively (Fig. 9).

T h e a b o v e d a t a i l l u s t r a t e t h a t t h e L D G a n d Z A A G

suites represe nt two dist inct groups o f granites and that the

Q

O r

PI

Fig. 3. QA P ternary diagram (after Streckeiscn, 1976) based on

CIPW normativevalues (Q = q uartz, Or = orthoclase, H = anor-

thite + albite). O pe n circles, Loja division gran ites (LDG;

Sabani l la , Malaca tus , Peggy, Saraguro, and Val ladol id

areas); closed circles, ZA AG granitoids (Zamora, Abitagua, and

AzafrGmbatholiths). Ke y to numbers: 1, quartz-rich granitoids;

2, monzo granite; 3, grano diorite;4, q uartz-monzonite;5, quartz-

rnonzogranite/gabbro; 6, quartz-diorite/gabbro; 7, monzod io-

dte/gabb ro; 8, diorite/gabbro.

e ~ 0 . 8

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0 . 4

0 . 3

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I I I I

. . . . t . . . . . . . . i . . . . 1 , , 7 , ~ = m = , o ,

, t . . . . . . . . r . . . . 0 ~ 0 ~ 1 I . . . .

I a ~ ~ o ~ : : : l o ~

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o

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0

5 5 6 0 6 5 7 0 7 5

L D G S u i t e

o S a b a n i l l a

< ~ M a l a c a t o s

~ P e g g y

v S o r o g u r o

[ ] V o l l o d o l i d

S i 0 2

Z A A G S u i te

Z a m o r a

A b i t o g u a

A z a f r a n

Fig. 4. Hark erdiagrams showingTiO2,P205 Cr and Zn vs SiO2

for LDG and ZAA Ggranitoids.

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128 J , A . A SPD EN , N . FO RT EY , M . ~ D , F . V IT ER I, a nd S. M , H AR RISO N

t -

l - -

2 0

8 O

o 4 O

4 0

3 O

-

2 O

16

.a 12

z 8

, , ~ r 1 i i i 1

. . . . . . . . i j r I&

5 ~ T B w , _

l e

I t

, ', ', ', ~ ', ~ ', ', .. . . I I II ' - ~ ' I ~

t i l t 0

, I , , , , l k I

. . . . I . . . . . . . . I . . . . I . . . . I ~ ' ' ' 4

A l l

r

5 5 6 0 6 5 7 0 7 5

S i 0 2

Fig. 5. Harker diagrams showing

T h

Ce, Y, and Nb

vs

SiO2 for

LDG and ZA AG granitoids.

A I 2 0 3 K 2 0 N o 2 0

~ / ~ M I b l I I ~ o v l f g

P la ' l o la ' ~ f ~ I t .

f \

C o O F e O M g O

Fig. 8. AC F ternary diagram for the LDG and ZAAG granitoids.

Symbols a s in Fig. 5.

o

04v '

5

4 .

3

2

1

2 3 4 5

N o 2 0

Fig. 6. K20

vs

Na20 diagram for the LDG and ZA AG grani-

toids. I-type and S-type granite fields after Chappell and W hite

1974). Symb ols as in Fi g. 5.

2 . 0

N 1 . 8

\

0

O

1 . 6

+

x,

+ 1.,4.

O

Z

,~ , 1 .2

~: 1.o

0 . 8

r

[ ]

S - - t y p e

[ ]

[ ]

I F

~ t y p e

, , , , , , i i I , , , , I , , , i I , , i i I L , ,

5 5 6 0 6 5 7 0 7 5

S i 0 2

Fig. 7. Alum inosity nd ex

vs

SiO2 for the LDG and ZAAG grani-

toids. I-type and S -type granite fields after Chappell and W hite

1974). Symbo ls as in Fig. 5.

o

b .

\

o

h

5 . 0 0

2 . 0 0

1 . 0 0

0 . 5 0

0 . 2 0 L

0 . 1 0 I

0 . 0 5

2 1 1 . . . . I T I . . . . I . . . .

m

~

e A

@

I

, , , . : , J , I , , , i I , i L i I i i i i I , i ,

5 5 6 0 6 5 7 0 7 5

S i 0 2

Fig. 9. Fe,203/FeO

vs

SiO2 plot for the L DG and ZAAG grani-

toids. Fields of magnetite- m) and ilmenite- i) series granites

after Ishihara

et al

1979) and Lehman and Harmanto 1990).

Symbols as in Fig. 5.

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Region al S- type gr : i tes in the Ecuador ian Andes: Possible remn ants of the breakup of weste rn Gond wana 129

L DG ha ve m a ny o f t he ge o c he m ic a l c har a c te r is t ic s o f S -

type grani tes ( see Pi tcher , 1987, 1983; I -l ine e t a l . , 1978;

Cha ppe l l a nd Wh i t e, 1974) . L D G sa m ples c o l l ec t e d f ~om 2 00

dif fe rent a reas appear to form c lose ly c lus te red composi -

t i onal subgr oups on a num be r o f p lo t s. T he se subgr oups

are apparent in diagrams having Cr , Ni , and SiO2 as dis- 15o

crimin ants (Figs.4 and 10) and, al though data are l imited, .a

they suggest a lack of regiona l nni formi ty wi thin the LD G lo o

suite. Ea ch su bgroup appears to be ge och em ically distinct,

a nd we f u r the r sugge s t t ha t t he c ons i s t e nc y o f m o bi l e

e l e m e nt s , suc h a s Rb ( F ig . 11) , a r gue s t ha t, ove r a l l, t he 50

c o m p o s i t io n s o f t h e L D G h a v e n o t u n d e r g o n e m a j o r

c he m ic a l m odi f i c a t i on e i t he r by l a t e -s t a ge a l t e r at i on o r

during region al sh earing (mylonit ization) .

Al though de ta i led pe t rogenet ic mo del ing i s beyon d the

scope of this paper, W hilte and Chappell (1977) sug gested

tha t var ia t ion t rends in grani tes of c rus ta l or igin ma y ari se

by mixing of di f fe rent mel t and res t i te proport ions . In the

c a s e o f t h e L D G s u i te , t h is m o d e l m a y , f o r e x a m p l e , 3 o

a c c oun t f o r t he t r e nds o f de c r e a s ing Y , Nb , Ce , a nd T h

observed in Fig. 5 . These e lem ents could be envisaged as

rem ainin g con centra ted in rest i te phases such as i imenite, ~.z: 2 0

ga r ne t , a nd m o na z i t e a nd f a l l ing t o v e r y l ow c onc e n t r a -

t ions in the separated eutectic me lt (with

c a .

SiO2 = 76% ).

How ever , other e lements , inc luding Ti , P , Cr , and Zn, di f - lo

fe r in tha t the i r decreas ing t rends can be ex t rapola ted to

reach zero a t S iO2 > 80% (Fig. 4), w ith signif icant con cen-

t ra tions remaining a t 76% SiO2. This suggests that par t o f

t he se e l em e nt s m a y ha ve e n t e r e d t he m e l t , p r esum a bly by

me l t ing of maf ic s i l ica tes and apa ti te , whi le a propor t ion

was re ta ined in the mo re re f rac tory phases .

D I S C U S S I O N A N D C O N C L U S I O N S

V

o o

4 8

C r

t l l l l l

B

A

v

i l n l l l [ ~ i i l l l l i i i i i i i

1 1 1

N I

Fig. 10. Rb

v s

Cr (A) and Th

v s

Ni (B) diagrams for the LDG

and Z AAG granitoids. Symbolsas in F ig. 5.

Ge oc he m ic a l ly , the L D G su i te c a n be c l a s s i fi e d a s S -

typ e grani tes , and the i r r e la t i ve ly high 875r/g6Sr ra t ios

(Fig. 2) a l so suggest inv olvem ent of a substant ial c rus ta l

com po nen t in their origin. Th is value (0.7120) is consider-

ably grea te r than the ent i re range of R i va lues obm inad for

the Z AA G gr a n i to ids a nd i s s im i l a r t o tha t o f c r us t a l l y

c o n t a m i n a t e d m o d e r n a n d e s ti e s i n C o l o m b i a ( J a m e s ,

1984).

Al thou gh regiona l S- type grani tes have n ot previously

been repor ted f rom the Nor thern Andes , S- l ike grani tes .

pr inc ipa l ly of Permo-Tr iass ic age , a re present in both the

C e n t r a l a n d S o u t h e r n A n d e s , w h e r e t h e y a r e g e n e r a ll y

considered to ha ve been emp laced in extensiona l se tt ings ,

p o s s i b l y r e l a t e d t o c r u s t a l r e l a x a t i o n p r e c e d i n g t h e

break up of Gou dw ana (Avila-Salinas, 1990; Su/trez

e t a l . ,

1990; Rapela e t a l . , 1989; Ken tak e t a l . , 1985). The possi-

bi l i ty therefore exis t s tha t the ~ could represent equiv-

a l e n t s o f t h e S - l i k e g r a n i t e s in P e r u , B o l i v i a , a n d

nor the r n Chi l e . I n m a r ke d c on t r a s t t o t he se oc c ur r e nc e s ,

howev er , the LD G sui te is r egiona l ly deve loped and typi -

caUy has a stron g mylo nit ic fabric. Equally, thes e granites

do n ot appear to be gene t ica l ly re la ted to I - types

i . e . , t h e

Z A AG su i te ) nor , a s f a r a s we c a n t e ll , do t he y oc c ur i n

batholi ths o f mix ed S -I cha racter (cf. Avila-Salinas, 1990;

Cobbing, 1990; Rap ela

e t a l . ,

1989; Atherton and Sander-

son, 1987).

Pi tcher (1987, 1983) suggested tha t di f fe rent types o f

g r a n i t e s c ou ld be r e l a t e d t o d i f f e r e n t te c ton i c e nv i r on-

m e a t s . S p e c if ic a ll y , h e c o n s i d e r e d t h e d e v e l o p m e n t o f

regional S-type granites as character ist ic of zo nes o f coati-

ne n t a l c o l l i son a nd a l so e nc r a ton i c duc t i l e she a r be l t s .

M ore recently, Cobbing (1990), al though agree ing that S-

type grani tes wi th c rus ta l s igna tures a re assoc ia ted wi th

coUisional sett ings, h as em phasized th e diversi ty o f crustal

granites an d pointed out that similar granites can be d eve-

lope d i n a va r i e ty o f t e c ton i c se t t ings . I n h i s v i e w , t he

c o m p o s i t i o n o f t h e c r u s t a l s o u r c e r e g i o n i s o f p r i m e

impo rtance in determ ining granite type and, ultimately, the

re la t ive amo unts of m ant le -der ived and c rus ta l mater ia ls

that a re mobi l ized dur ing mag ma genesis .

P e a r c e e t a l .

(1984; see a lso Brown

e t a l . ,

1984) pro-

pose d t ha t t he t e c ton i c se t ti ng o f m os t g r a n i t e s c ou ld be

de t e r m ine d a c c or d ing t o t he a b unda nc e o f c e r t a in t r a c e

e lements . Usin g a Rb

v s

Nb+ Y p lo t, Pe a r c e

e t a l .

(1984)

dis t inguished be tween grani tes genera ted in volcanic-arc

( V A G ) , s y n c o l li s i o n a l ( S y n - C O L G ) , a n d w i t h i n - p l a te

(W IG ) sett ings. O n their plot , al l the Ecu adorian samp les

wo uld be c lass i f ied as volcanic-arc grani tes . H oweve r , i t

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130 J. A . ASPDEN , N. FORTEY . M. L1THERLAND, F. V1TERI, and S. M. HARRISON

Z

1000

300

100

30

10

3

I I I I I I I I I I I I I I I I I l I I I I I I

Syn COLG

VAQ

ORG

I I I I I I I I I I I I I I I I I I I I I I I

10 100 1000

Y Nb

Fig. 11. Rb v s Y+ Nb d i sc r im ina n t p lo t ( Pe a r c e e t a l . ,

1984): VAG,volcanic-arcgranites; W PG , within-plategranites;

Syn-CO LG, syn-collisionalgranites; OR G, ocean-ridgegranites;

open circles, LDG; closed circles, ZAAG.

\

.Q

It::

30 .00

10.00

3 .00

1.00

0.,50

0.10

0 .03

M . ~ . . . _ . ' . , _ z . . . - . z 2 - _ _ m /

~

,, ~

Ecuador ion Re f o; ' ence Line

I i i i I i i i i I i i

4-0 60 80

D i f f . I ndex

Fig. 12. Rb/Sr v s DI (Thorton and Tuttle differentiation ndex)

diseriminantplot, I- and S-typegranite fields after P itfield (1988)

and Cobbing (1990). Open circles, LDG; closed circles. ZAAG.

ma y be s iLmificant ha t the Ecuador ian S- types plot in the

upper par t o f the VAG f ield , c lose to w here these grani tes

converge wi th W IG and Syn-CO LG (Fig. 11) . Com pared

with othe r S-types. th e LDG suite is relat ively poo r in Rb

a nd , a l t hough no t p r ove n , we a s s um e tha t t h i s is m o r e

l ike ly to re f lec t the com posi t ion of the source a rea ra ther

than be ing diagno st ic of a par t icula r tec tonic se tt ing ( see

a l s o C o b b i n g . 1 9 9 0 ; C h a p p e l l a n d S t e p h e n s . 1 9 8 8 ) .

Indeed. the more pr imi t ive na ture of the LD G suite wi th

respec t to Southeas t Asian S- type grani tes can b e se en in

Fig. 12. On this diagram, m ost Ecua dofian samples plo t in

the I- type f ield ( i . e . , below the M alaysian re fe rence line)

bu t . a s Cobbing ( 1990) po in t e d ou t ( se e a l so P i t f i e ld .

1988). the M alaysian re fe rence l ine i s em pir ica l and m ay

not se rve to dis t inguish I - and S- type grani tes f rom e lse-

whe r e i n t he wor ld . Ne ve r the le s s , a lt hough the L DG do

not ach ieve Rb/Sr rat ios > 1.0 at DI values < c a . 85 (as is

the case in Southeast A sian S-types) . they do h ave consis-

tent ly higher Rb/Sr ra t ios a t the same DI than those of the

Z AAG.

In a review of the ear ly M esozoic his tory of the Nor th-

em and C entral And es. Jai llard e t a l . (1990) suggested that

the Late Tr iass ic -Liass ic separa t ion of Nor th and So uth

Am er ica . be tween the pa leo-M exican marg in and w hat i s

n o w p a r t o f t h e N o r t h e r n A n d e s , i n c l u d e d r i f t- r e l a te d

e x t e ns ion a nd p oss ib l e t r a nsc ur r e n t ( ? tr a nspr e s s iona l )

stress. This mod el accounts for the ear ly Mesozo ic exten -

s i o n a l r e g i m e p r e s e rv e d i n t h e g e o l o g i c a l r e c o r d o f

Colombia and Ecuad or and, in o ur view, m ay a lso expla in

the presence of a regiona l be l t of var iably deform ed S- type

g r a n i t e s a n d m i g m a t i t e s . W e a s s u m e t h a t t h e h i g h l y

obl ique app roach of the pa leo-Pac i f ic pla te resul ted in a

significant am ou nt of transp ressio nal strike-slip (Fig. 13)

a nd poss ib ly l im i te d subduc t ion a long the no r thwe s t e r n

m a r g in o f Sou th Am e r ic a . D ' L e m os e t a l . (1992; see also

Hut ton , 1988 ; Wic kha m a nd Ox bur gh . 1986 ; P i t c he r ,

1983) have re cently propo sed that crustal-scale transpres-

s iona l shear zones provide se t t ings favorable for genera-

t ing and em placing S-type (anatectic) granites. In Ecu ador,

S-C my loni tes (L is ter and Snoke , 1984) a re wide ly deve-

loped throug hout the Cordil lera R eal. and i t has be en sug-

ge s t e d t ha t r e pe a t e d e p i sode s o f de x t r a l t r a nspr e s s ion

affected the Cordillera during M esozoic t ime (Aspden and

Litherland, 1992).

The w este rn ( tec tonic) limi t of the Lo ja divis ion coin-

c ides wi th the Las Aradas-Bal tos faul t (Fig. 1) , a l ine of

i n t e n s e s h e a ri n g a n d r e g i o n a l m y l o n i t e d e v e l o p m e n t .

whic h w e sugge s t r epr e se n ts t he r e m na nt s o f t he z one o f

separa tion be twee n the pa leo-Mexican marg in and no r th-

western Sou th Ame rica. Farther to the no rth in Colombia,

we w ould cor re la te the Las A radas-Bat tos faul t wi th the

Rom eral fault zone (Fig. 13) . I f such a correlat ion is valid,

then one w ould predic t that , as geolog ica l explora t ion of

Hg. 13. Sketchof the p roposed Late Tfiassic-Liassic (dextral)

transpressional regime affecting the m argin of northwestern

South America (modified rom Jaillard e t a l . , 1990), illustrating

the possible tectonic setting of Loja division granitoids.

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Regional S type grani tes in the Ecuador ian Andes: Possible remnants of the breakup of weste rn Gondwan a 131

t h e C o l o m b i a n C e n t r a l C o r d i l l e r a c o n t i n u e s , g r a n i t e s

equivalent to those o f the Loja division w i l l be recognized.

A t p r e s e n t , t h e m o s t p r o b a b l e c o r r e l a t i v e s a r e a s m a l l

g r o u p o f p l u t o n s d e s c r i b e d m a i n l y f r o m A n t i o q u i a a n d

Caldas in the central part of the C ordi llera, immediate ly to

t h e e a s t o f t h e R o m e r a l f a u l t z o n e e . g . , Puqu i , E1 Buey .

and Am ago s tocks) . These p lutons typical ly have faulted

contac ts and a s trong , t ec tonic , b io t i t e fabr i c ; a l though

poor ly dated, they are cons idered to be Tr iass i c in age

(Ja i l l ard e t a l . , 1 9 9 0 ; A s p d e n e t a l . , 1 9 8 7 ; M a c i a a n d

Moj ica . 1981) . According to Hal l e t a l . (1982) . the Puqui

stock, which has K-At muscovite and biot i te ages ranging

from 2 39 _+. 7 to 211 + ? M a (see also Boter o, 1975 ) . has

gradational and, in par t, migm at i t i c contac ts wi th the

surrounding micaceous , gne i ssose , host rock and i s inter-

preted to be syntectonic and of po ssible crustal or igin.

A c k n o w l e d g m e n t s - - T h i s

p a p e r i s p u b l i s h e d w i t h p e r m i s s i o n o f t h e

Di re c to r o f the Br i t i s h Ge o log ic a l Surve y (NE RC) a nd the Ins t itu to E c u-

a t o r i an o d e M i n e r i a ( I N E M I N ) . W o r k i n E c u a d o r a n d i n t h e U K w a s c a r-

r i e d o u t a s p a r t o f a n o n g o i n g b i l a t e r a l t e c h n i c a l c o o p e r a t i o n p r o j e c t

b e t w e e n t h e g o v e r n m e n t s o f E c u a d o r a n d t h e U K ( v i a t h e O v e r s e a s

De ve lopm e nt Adm in is t r a t ion ) . Spe c ia l tha nks a re due S r s . Ca s a nova a nd

C611er i o f IN E M IN. W e a re g ra te fu l , to R . J. C obb ing a nd R . J . Pa nkhurs t

f o r t h e i r c o m m e n t s o n a n e a r l y d r a f t o f t h i s p ap e r , a n d t o P r o f s . D u q u e

a nd E que z fo r he lp fu l s ugge s t ions .

R E F E R E N C E S

As p de n , J . A . , a nd L i the r la nd , M. , 1992 . T he ge o logy a nd M e s oz o ic c o l -

l i s i o n a l h i s to r y o f t h e C o r d i l l e r a R e a l , E c u a d o r . I n : Ande an Ge ody -

namic s

( e d i te d by R . A . O l ive r ) .

Tectonophysics

205, 187-204.

As p de n , J . A . , Ha r r i s on , S . H . , a nd Rund le , C . C . , 1992. Ne w ge o c hrono -

l o g i c a l c o n t r o l f o r t h e t e c t o n o - m a g m a t i c e v o l u t i o n o f t h e m e t a m o r -

p h i c b a s e m e n t o f t h e C o r d i l l e r a R e a l a n d E 1 0 r o P r o v i n c e , E c u a d o r.

J ournal o f South Ame r ic an Ear th Sc ie nc e s 6 (1/2) , 77-96.

As pd e n , J . A . , L i the r la nd , M. , Be rm t ide z , R . , a nd V i te r i, E , 1990. Four th

Annual Re p or t o f the C ordi l le ra Re al G e ologic al Proje c t . INEMIN-

Br i t i sh Mis s ion .

B G S ( K e y w o r t h ) a n d I N E M I N ( Q u i to ) O p e n - F i l e

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As p de n , J . A . , M c Cour t , W . J . , a nd Bro ok , M. , 1987. Ge o m e t r ic a l c on t ro l

o f s u b d u c t i o n - r e l a le d m a g m a t i s m : T h e M e s o z o i c a n d C e n o z o i c p l u -

t o n i c h i s t o r y o f w e s t e r n C o l o m b i a .

J ournal o f the Ge ologic al Soc ie ty

o f L o n d o n

144 , 893-905 .

A s p d e n , J . A . , R u n d l e , C . , V i t e r i, F ., B e r m d d e z , R . , a n d H a r r i s o n , S . ,

1 9 9 0 . E d a d e s r a d i o m 6 t r i c a s d e l b a t o l i t o d e Z a m o r a - R f o M a y o .

Bole t6n Ge ol6gic o Ec uator iano

1 (1), 85-88.

A t h e r t o n , M . P . , a n d S a n d e r s o n , L . M . , 1 9 8 7 . T h e C o r d i l l e r a B l a n c a

b a t h o l i t h : A s t u d y o f g r a n i t e i n t r u s i o n a n d t h e r e l a t i o n o f c ru s t a l

th ic ke n ing to pe ra lum inos i ty .

Ge ologi sc he Rundsc hau

76 , 213-232 .

A v i l a - S a l i n a s , W . A . , 1 9 9 0 . T i n - b e a r i n g g r a n i t e s f r o m t h e C o r d i l l e r a

Re a l , BoL iv ia : A pe t ro log ic a l a nd ge oc he m ic a l r e v ie w . In : Plutoni sm

from A ntarc t i c a to A lask a ( e d i t e d b y S . M . K a y a n d C . W . R a p e l a ) .

G e o l o g i c a l S o c ie t y o f A m e r i c a , S p e ci a l P a p e r 2 4 1 , 1 4 5 - 1 5 9 .

Ba ldo c k , J . W . , 1982 . Ge ologia d e l Ec uador : Bole t ln de la Ex pl ic ac i6n

de l Ma pa Ge o l6gic o 1:1 ,000,00 0) de la Re pt~ blica de l Ec uador .

M i n i s t e r i o d e R e c u r s o s N a t u r a l e s y E n e r g 6 t i c o s , D i r e c c i 6 n G e n e r a l

de Ge o log fa y Min a s , Qu i to , E c ua dor , 54 p .

Be c k ins a le , R . D . , 1979. Gra n i te m a grna t i s m in the t in be l t o f Sou th -e a s t

As ia . In : Origin o f Grani te Bathol i ths : Ge oc he mic al Ev ide nc e ( e d i te d

b y M . P . A t h e r t o n a n d J . T a m e y ) , p p . 3 4 - 4 4 . S h i v a P u b l i s h i n g L t d ,

N a n t w i e h , C h e s . , E n g l a n d , U K , 1 4 8 p .

Be r thS , D . , Cho ukrone , P . , a nd J e gou z o , P ., 1979 . Or thog ne is s , m ylo n i te

a n d n o n c o a x i a l d e f o r m a t i o n o f g r a n i te s , t h e e x a m p l e o f t h e S o u t h

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