Aspden et al., 1992b.pdf

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Journal of South American Earth Sciences Vo l . 6 , No . 1 / 2, p p . 7 7 - 9 6 , 1 9 92 P er g am o n P r es s L t d

P r i n t ed i n Gr ea t Br i t a i n E ar t h S c i en ces Reso u r ces I n s t i tu t e

N e w g e o c h r o n o 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 ,

C o r d i l le r a R e a l a n d E O r e P r o v in c e o f E c u a d o r

J . A . A S P D E N I , S . H . H A R R I S O N2 , a n d C . C . R U N D L E 3

1 M i s i 6n B r i ff m i c a / O D A , F C O ( Q u i t e ) , K i n g C h a r l e s S t r ee t , L o n d o n S W 1 A 2 A H , U K 2 68 G a i m T e r r a ce ,

A b e r d e e n A B 1 6 A T , U K ; 3 N E R C I s o t o p e G e o s c i e n c e L a b o r a t o r y , K e y w o r t h , N o t t in g h a m , N G 1 2 5 G G , U K

R e c e i v e d M a y 1 9 9 2 ; R e v i si o n A c c e p t e d J u l y 1 9 9 2 )

Ab strac t--S om e 150 new isotopic ag e determinations on metamorphic rocks from the Cordillera Real and parts of E1 Ore Pro-

vin ee in Ecuador, u sing K -At , Rb-Sr, and Sm-N d method s, help to clarify a complex succession of magmatic an d tectonic events.

Th e earliest regional m etamorphic/plutonic eve nt recognized, from the Tahuin Group in El O1 o, s dated as between c a . 2 2 0 a n d

200 M a (Late Triassic-Early Jurassic) . Sim ilar but less well constrained ages were also obtained f rom orthogneiases of the

Sabanilla and Tres Lagtmas subdivisions in the Cordillera Real. Major eale-alkaline granitdids were emplaeed

c a .

190-150 Ma

(M iddle -Late Jurassic) in the eastern part of the Cordillera Real, to the north of 2 S, and throughout the sub-Andean zone.

Between

c a .

140 and 120 M a (Early Cretaceous), the Oriente region w as uplifted and eroded and th e Cordillera was affected by an

important shearing (dynam othermai) ev ent which resulted in the resetting of older plutonic ages. From

c a .

120 to 85 Ma, condi-

tions w ere relatively stable, but during

ca .

85 to 65 Ma (Late Cretaceous), the Cordillera and Oriente were again uplifted. This

uplift corresponds to a second w idespread thermal overprinting, w hich produced a regional disturbance in the K- Ar isotopic sys-

tems. Throughout the Co rdillera, a number of generally small, undeformed, dom inantly lowe r Tertiary plutons are also present. A

few older

i .e . ,

pre-Mesozo ic) dates have been obtained but their interpretation remains uncertain.

R e s u m e n - - U n a s 150 nuevas determinaciones de edades isot6picas de rocas metam6rflcas de la Cordillera Real y parte de la pro-

vincia de E1 Ore en el Ecuador, usando los m~todos K -Ar, Rb-Sr, Sm-Nd, ayudan en clarif icar una sucosi6n eompleja de eve ntos

magrn~lticos y tect6nicos. E l evento m etam6rfico/plut6nico regional m ~ temprano reconocido, es el d el grupo Tahufn en E1 Ore;

est~ entre c a . 220 y 200 Ma (TriMico tardfo-JurMico temprano). Edades similares, pete menos definidas, fueron tambi6n obteni-

das de los o r togne ises de Saban i l la y T ros Lagunas en la Cord i l le ra Rea l . Los mayores g ran i to ides ea lco -a lca l inos fue ron

emplazados ca. 190-150 M a (Jur~sico medic a tard(o), en la parte oriental de la Cordillera Real, a l norte de 2 de lati tud S , ye n

toda la zuna subandina. Entre ca . 140 y 120 Ma (CrelAcico em prano) la regi6n Oriental fue levantada y erosionada; y la cordillera

rue afectada pe r un eve nto de cizallamiento muy importante (dinamotermal) , q ue result6 en el reajuste d e 1as antiguas eda des

plut6nicas. Desd e

c a .

120-85 Ma las condiciones fueron relativamente estables, pete durante

c a .

85-65 Ma (Cretatcico tardfo) la

cordillera y el Oriente fueron de nuev o levantadas. Este levantamiento corresponde a una segunda sobre impresi6n termal que

produjo una perturbaci6n regional e n los sistemas isot6picos K -At. Pe r toda la cordillera est,Ln presentes un ndm ero de plutones

generalmente pequetlos, no deformado s; dom inantemente d el Tereiario temprano. H art side obtenidas pocas edades antiguas (Pre-

mesozoicas), pe te la interpertaci6n de 6stes permaneee todavfa incierta.

I N R O D U C T I O N

T H E C O R D I L L E R A R E A L R e s e a r c h P r o j e c t i s a j o i n t T e c h -

n i c a l C o o p e r a t i o n P r o g r A m m e u n d e r t a k e n b y t h e O v e r s e a s

D e v e l o p m e n t A d m i n i s t r a t io n ( O D A ) o f G r ea t B ri ta i n

t h r o u g h t h e B r i t i s h G e o l o g i c a l S u r v e y ( B G S ) i n c o n j u n c -

t i on w i t h t h e C o r p o r a c i 6 n d e D e s a r r o l l o e l n v e s t i g a c i 6 n

G e o l o g i c o - M i m r o M e t a l u r g ic a ( C O D I G E M ) i n E c u a do r .

' S i n c e t h e s t a r t o f t h e p r o j e c t i n 1 9 8 6 , a d v a n c e s h a v e

b e e n m a d e i n t h e u n d e r s t a n d i n ~ o f th e s tr a t i g r a p h y a n d

s t r u c tu r a l e l e m e n t s i n t h e C o r d i l l e r a R e a l a n d p a r t s o f E l

0 r e P r o v i n c e i n s o u th w e s t E c u a d o r ( F i g . 1 ). S o m e o f t h e se

f i n d i n g s h a v e a l r e ad y b e e n p r e s e n t e d e . g . , L i t h e r l a n d e t

l i b .

F i g . 1 . P r i n c i p a l g e o m o r p h o l o g i e a l / g e o l o g i e a l z o n e s o f E c u a d o r.

D o t p a t t e rn i n d i c a te s m e t a m o r p h i c r e e k s o f t h e p r e - C r e t ac e o u s

b a s e m e n t .

/

A d d r e s s a l l c o r r e s p o n d e n c e a n d r e p r i n t r e q u e s t s t o D r . J o h n A . A s p d e a a t B G S I n t e r n a ti o n a l D i v i s i o n , K e y w o r t h , N o t t i n g h a m N G 1 2

5 G G , U K : t e l e p h o ne 4 4 ] 6 0 2 ) 3 6 3 1 0 0 ; f a x [ 4 4 ] 6 0 2 ) 3 6 3 - 2 0 0 ; r ele nt 7 8 1 7 3 B G S K E Y G .

1 9 9 2 C r o w n C o p y r i g h t


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78 J.A . ASPDEN,S. H. HARRISON,and C. C. RUNDLE

a l . ,

1990; Aspdo n and Litherland, 1992). In this contribu -

t ion w e concen t ra t e so le ly on docum ent ing the geochrono-

logical data that have b een obtained. The isotopic

analyses

were carr ied out at the Natural Environment Research

Counc i l s I so tope Geo logy Cen t re in London - - now re-

named the Nat ional Isotope G-eosciences Laboratory

(NIG L) and reloca ted at Ke ywo rth, Nott ingham.

G e o g r a p h i c a l S e t t i n g

The Cordi l lera Real i s the eas tern of two parallel m oun-

tain chains that de fine the Ecuadorian Andes. In the north ,

the Western Cordi l lera is separated from the Cordi l lera

Real by a prominent s t ructural val ley, the Inter-Andean

Depre ss ion, but in the south the And es are represented by a

s ingle cordil lera. To the eas t of the Ecuadorian A ndes l ies

the sub-Andean zone and the Oriente, which form part of

the upper reaches o f the Am azon Bas in . T o the w es t l ies the

flat , low coastal region o f the Costa (Fig. 1).

G e o l o g ic a l B a c k g r o u n d

North o f Guayaqu i l , t he C os ta compr i ses Upper Cre ta -

ceous to C enozo ic fo re-arc sedimentary rocks f loored by

Low er Cre taceous ocean ic basa l t s o f the P i flon Format ion

(Baldock. 1982; Goossens and Rose. 1973). There is no

e v i d e n c e o f

c o n t i n e n t a l

rus t be low these

r o c k s

(Feininger

and Seguin, 1983). This part of Ecua dor is thus thought to

represent oceanic crust that was accreted to the South

Am er ican p la te in the La te Cre taceous o r Pa leocene (Bour-

go i s e t a l . , 1990; Daly, 1989). In contras t, south of Guaya-

qui l , the rock s of El 0ro Provinc e (Fig. 1) consis t mainly of

grani t ic plutons and metam orphic rocks , which include am-

phibol i tes , schism, and gneisses . The Western Cordi l lera

compr i ses

a NNF_,-trevdin

bel t o f Cre taceous to lower Ter -

t iary volcanic, volcaniclas t ic and sedimentary rocks that

h a v e b e e n r e p o r t e d o n b y V a n T h o u mo u t a n d Q u e v e d o

(1990) . Lebra t e t a l . . (1985), and H enderso n (1979).

The inhospi table nature of the CcediUera Real , wi th i t s

high al t i tude and abundant

rainfall,

together wi th l imi ted

r o a d a c c e ss , h a s h i n d e r e d

s tudy o f the geo logy o f

t h i s e -

g ion o f Ecuador . The Cord i ll e ra Rea l fo rms a con t inuous

be l t o f var i ab ly defo rme d and m etamorphosed rocks tha t

ex tends the l eng th o f the Ecuador i an A ndes

and

COnSiSts of

s c hi s ts , q u a rt z i te s , a l c -s c h is t s , a r b l e s , a n d o r t h o - a n d

p a r a g n e i ss e s A s p d e n a n d L i t he r l an d , 1 9 9 2 ) . A n u m b e r o f

l at e, n d e f o r m e d p l u t o n s c u t t h e m e t a m o r p h i c r o c k s , a n d a

s e r i e s o f m a j o r P l i o - P l e i s t o c e n e s t r a t o v o l c a n o e s d o t t h e

C o r d i l l e r a .

O v e r l y i n g t h e b a s e m e n t o f t h e O r i en t e , h i c h c o m p r i s e s

r o c k s b e l o n g i n g to t h e A m a z m i c c r a t o u A l m e i d a e t a I .

1 9 8 1 ) , a r e e p i - p l a t f o r m P a l e o z o i c a n d l o w e r M e s o z o i c

sedimentary s t rata. These are overlain by Upper Jurass ic

vo lc~n~ and Upper Cre taceous mar ine miogeosyncl ina l

sedlmantary rOCks. Fol low ing the A ndean upl i f t , back-arc

sedimantat ion occurre d during the

C e n o z o i c

(Jalliard

e t a l . ,

1990; Ba ldock, 1982; Tscho pp, 1953).

P R E V IO U S G E O C H R O N O L O G I C A L S T U D IE S

The apparent correlation of the predominantly Pale o-

zoic metamorphic rocks o f the Cordi llera Central of south-

ern Colombia wi th those of the Cordi l lera Real has

per s jad ed so me to suggest a s imilar age for the latter e . g . ,

B a l d o c L

1982).

E q u a l l y , i n t h e s o u t h , t h e C o r d i l l e r a e a l

metamorphic be lt has been correlated li thologically with

the basemen t rocks of northern Peru (Kennerly . 1980).

wh ich are overlain

b y

Triass ic and possibly Dev onian sedi-

men tary rocks (JaUiard e t al., 1990; Cobbing e t a l . , 1981).

M etamorphic Rocks of he Cordi l lera Real

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

C o rd i ll e ra e a l m e t a m o r p h i c r o c k s h a v e r e li e d n t ir e ly n

t h e K - A t t e ch n iq u e. e s u lt s r o m s o m e o f t h e m o r e i m p o r -

t a n t o c al i t ie s F i g . ) a r e n o t e d b e l o w :

a ) H e r b e r t a n d P i c h l e r 1 9 8 3 ) p r e s e nt e d K - A t d a t e s o f 5 9

+ 2 M a f r o m m u s c o v i t e a n d b i o t i te s e p ar a t e s f r o m

s ch is ts h i c h c r o p o u t a l o n g t h e P a p a l l a c t a - B a e za o a d .

A ~imilar K-A t b io ti t e da te o f 54 + 2 M a w as recorded

by F e in ln se r

and Silberman (1982) from the sam e area,

and a s l igh t ly o lder age o f 82 + 3 M a was rep or t ed bv

Ke nne dy (1980) for a mu scovi te sample.

b ) Be t w e e n Ba f l o s a n d Pu y o . g a r n e t mu s c o v i t e b i o t i t e

para- and orthogneisses are exposed near Agoy(m. Her-

ber t and Pich ler (1983) ana lyzed muscov i t e separa t es

f rom bo th o f these rock types and ob ta ined ages o f 56 .5

+ 2 and 60 + 2 M a, respect ively. Hal l and C al le (1982)

repor t ed s ix K-A r ages f rom gnei s ses a long th i s road

sec tion , r an ch S f rom 54 to 79 Ma.

c) Hal l and Cal le (1982) quo ted K-Ar ages o f be tween 61

a n d 9 0 Ma ( t h r e e d e t e r mi n a t i o n s ) f o r me t a mo r p h i c

r o c k s f ro m t h e Cu e n c a a r e a a n d a g e s b e t w e e n 5 1 and

79 M a ( th ree de te rmina tions ) fo r rocks in the Zam ora

area . Ba ldock (1982) a l so repor t ed a K-A r b io t i t e age

of 52 + 2 M a fo r gne i ss co l l ec ted from the Lo ja-Zamora

road section.

d ) Kenner ly (1980) recorded two K-A t ages o f 72 + 2 and

81 3 M a f r o m b i o t it e g n e i s s e s n e a r P a l a n d a a n d

Zum ba in the ex t reme sou th o f Ecuador.

Ba sed on these data, Feininser (1982) and Hal l and CaUe

(1982) interpreted the Cordillera R eal m etam orph ic bel t as

predominantly of Late Cretaceous to early Tert iary age.

M etamorph i c Rocks o f E1 ro Prov ince

The metamorph ic rocks o f E l O ro Prov ince (Figs . 1 and

2b). which s tr ike E-W , are obl ique to the NN E trend of the

C o r di l l er a e a l , a n d t h e c on t a c t b e t w e e n t h e t w o b e l t s i s

h i d d e n b y y o u n g e r s e q ue n c es . T h e r o c k s o f E l O r o P r o v .

i n c e i n c l u d e a c e n t ra l c o r e o f a m p h l b o li t e , t h e P i e d r a s

Group r-,eininee.r,1978). dated as Precamb rian by a s ingle

K-At age obtained from a hornblende separate (743 + 14

Ma; Kennerly , 1980). However, hornblende determina-

tkms from similar amphiboli tes in the province have yield-

ed ages o f 196 + 8 Ma and 74 + 1 Ma r-,einin~,er and


3/20

N e w

g e o c h r c m o l o g i c l

ont rol fox the tectono-mac,m~t ic evolut ion of the metamorphic basement , Ecuado r 79

a )

b )

--OoO0

. \

oo w -.COL O~

Pimompiro . ~ . ~ . ~ 7 j /

| o u o j i

~7 :..:-....-:.- :-f w B a eza

IqO0 S

Teno

Arnobato

eRiobamba

M a r e

e P u y a

~ o o w

]

Metamorphic rocks ( f f

CordHlera Real

I T : : Lag . . . g ra ni.

U N D EFO R M ED PLU TO N S

(~ imampiro

Condue

~) Azuela

(~ Pungala

..

Fig. 2. L ocation and simplifiedgeological map of the Cordillera Real/sub-Andeanzone: a) between he Colombian frontier and 2 S

(based on L itherland

e t a l .

1990); b) between 2S and the P eruvian frontier (based on L itherland

e t a l . .

1990), inset map of E10 ro

metamorphicprovin ce after Baldock (1982).

Silberman, 1982), throwing the Precam briau age assigned

to these rocks in to considerable doubt. Th e Hed ras e0nve-

l op s t h e R a s pa s b l ~ t c o m p le x ~ i n i n ~ , 1980,

1978) (Fig. 2b) , fro wh ich a K -A t age of 132 5 M a (phen-

gi te ) has been obta ined Feininoer and Silbermsn; 1982).

Both the Pie&as G roup and the Raspas complex a re bound-

ed to the n or th and south by low- to high-grade semi-pe li fic

r oc ks a nd va r ia b ly de f o m ~ gr a n it e s o f t he T a hu in Gr oup

(Fig. 2b) . Fein inge r (1982) has interpreted this grou p to be

D~ cm isn in age , based on a brac .hiopod foLmd n a weak ly

metam orphosed quar tz i te in nor thwest Peru. Howev er ,

F e in in ~ a nd S ilbe ~m an (1982) ob t a ined a n a ge o f 210 + 8

M a h em biot ite separa ted f rom a pe l i tic gne iss of the

T a buin Gr oup , whic h t he y c ons ide r e d t o be t he a ge o f up-

lift.

SAES 6:1/2-F

r a n i ti c o c k s

Three majm NNE- t rendins ekmsate grani toid ba tho-

l iths occur in the sub-Andean zone a l to 8 the eas te rn tec-

tonic margin of the C ordil lera Real. F rom n orth to south,

these a re the Rosa Flor ida , Abi tagua , and Zam~a

batholiths (Fig. 2).

The A bitasua batholith h as rece ived the mos t at tenticm.

Kenn er ly (1980) repor ted a K-Ar age of 87 + 7 M a co a bio-

r ite separate, w hereas Herbert and Pich ler (1983) record ed

178 + 7 M a from a bioti te separate at a nea rby locali ty (both

samples we re collected aloe8 the Bsflos-Pu yo road) 0r18.

2 a .

A K- Ar a ge o f 171 6 M a wa s ob t a i n_ by A ly ( 1980)

f rom the Zamcca ba tholi th , and K-A t da tes of 173 5 M a


4/20

80 J.A . ASPDEN,S. H. HARRISON,and C. C. RUNDLE

(hornblende) and 180 + 5 M a (biotite) hav e bee n publishe d

by Kenn erly (1980) for a s ingle samp le col lected to the eas t

of Paland a (Fig. 2b). Originally, this intrusion wa s thought

to be a separate pluton (Baldock, 1982), but more recent

wo rk has show n that i t form s part of the regional ly exten-

s ive Zam ora bathol i th (Aspd en and Li therland, 1992).

Within the eas tern part of the Cordi llera Re al north of

2S, are the variably deform ed, often gneiss ic, Azafran and

Chingual batholiths. To the west l ies a distinctive suite of

general ly fol iate& garnet biot i te + muscovi te grani tes of

the Tres Lagnnas subdivision. In the southeastern part of

the Cordillera is the Sabanilla subdivision, a mixed unit

(Fig. 2) w hich is dom inated by b iot i te + mu scovi te + garnet

orthogneiss but also includes migmat i tes , paragneisses /

schists and amphibolites.

Tw o sam ples from the Tres Lagunas subd iv i sion eas t o f

Saragnro (Fig. 2b ) gave K- Ar a ges of 76 + 1 M a (bioti te)

and 173 + 4 M a (plagioclase) (Kennerly , 1980). K-A r dates

obtained from the Sabanil la subdivis ion west of Zamo ra

and f rom the Pa landa and Zum ba areas (F ig . 2b ) have been

referre d to earlier.

Other sm al ler plutonic bodies in the Cordil lera Rea l ap-

pear as essent ial ly unde form ed grani toids, ma ny of which

show int rusive relat ionships wi th the metamorphic rocks .

Th ey include the Pimam piro, Magtayan, Am aluza, and San

Lucas plutons (Fig. 2) . Various K-Ar ages . ranging from

Late Cre taceous to Ter t i a ry , have been p rev ious ly pub-

l ished for these plutons: 72 + ? Ma from Pimampiro

(Evemden, 1961); 85 + 3 Ma (hornblende), 75 + 3 Ma

Oaomblende). and 54 + 2 M a (K-feldspar) from M agtaygm

(Kenner ly . 1980) ; 34 + 1 M a to 49 + 2 Ma f rom A maluza

and 70 + 2 M a to 50 + 2 M a f rom San Lucas (Herber t and

Pichler , 1983; Kennerley, 1980).

The abo ve suggests that although isotopic data exis t for

the metam orphic and plutonic rocks of the Cordil lera R eal

and E l Oro P rov ince , t he ac tua l ages o f the main t ec tono-

magm at i c even t s remain po or ly def ined . The a im o f thi s in -

vest igat ion w as thus . f i rs t, to clari fy the age o f metamor-

phism, us ing a combinat ion o f K-A r (hornblende. biot i te ,

and mu scovi te) and Sm -Nd (garnet/whole-rock pairs)

methods , and second , to da te the main p lu tous by the Rb-Sr

whole- rock i sochron m ethod .

A N A L Y T IC A L T E C H N I Q U E S

Sampling and Rock reparat ion

Sampling wa s res t r icted largely to road cuts and incised

f iver beds . Wherever poss ib le , s amples were t aken f rom

in

situ

outcrops , but some of the leas t al tered samples were

from large boulders .

For the Rb-Sr whole-rock analyses , af ter in i t ial jaw-

c~ l~ in g _and rol ler mi l ling, representat ive 200 g sub-

samples were rem oved us ing a r i ff l e sp li tt er and powdered

in a tungsten carbide Swing-mil l . For samples requir ing

mineral separation, the roller-milled material was sieved

and the + 65 to -20 0 mesh f rac tion was w ashed in d i st il led

water to rem ove any f ree pow der . Som e in it ia l coarse min-

eral separat ion wa s carr ied out in Ecuado r. us ing heav y

l iquids , b ut most o f the puri ficat ion was co mp leted in the

U K, using a super-panner and Frantz m agne tic separator.

Rb Sr Amllysis

Rb-S r analyses we re carried out on whole-ro ck pow der

samples from meta-plutonic rocks and orthogneisses. Rb/

Sr rat ios were determined by X-ray f luoresce nce using an

automated Philips spectrometer. Fo r the isotop e ratio deter-

minations, s trontium was extracted from the samples us ing

acid dissolut ion and ion exchange methods in a clean-

chemistry laboratory and analyzed with an automated

multi-collector VG354 mass spectrometer.

The Rb/Sr rat ios are quoted wi th a blanket error of

:L-0.5 (1-sigma). R eplica te analy ses of samp les and stan-

dards sugg est that a reproducibili ty of _+0,005 s appro-

pi late for the s trontium isotope measurements. Rep l icate

analyses o f international standards ind icate that the results

are accurate w ithin the precision estimates.

The errors on age and initial ratio (Ri) are quoted as 2-

s igma (95 confidence level) and refer to the las t s ignifi -

cant figure. B est-fit l ines on the isochron diagrams we re

calcu lated using a least-squares fi tt ing program. An

M SW D (mean square o f weighted deviates) excee ding 3.0

me ans that the points do no t all fi t the line w ithin the limits

of analytical error and, fol low ing convent ional pract ice, the

errors on age and intercept h ave bee n enhanced b y mul t i -

p ly ing by the square roo t o f the MS WD . Al l ages w ere ca l -

culated using a dec ay constant for 87R b of 1 .42 10 -11

a--l.

Sm Nd Ana lys i s

This technique was used on whole-rock and garnet

pairs, relying on the fractionation of the rare earth elem ents

in garnet relative to the ho st whole-rock.

Sm and N d were analyzed b y a double isotope di lut ion

method. Powdered whole-rock and garnet samples were

d i s so lved in ac id wi th an added m ou nt o f a Sm-Nd mixed

spike. Both the Sin and the Nd were then exlracted using

ion exchange methods and separa te ly ana lysed on the mass

spectrometer.

Errors in the Sm /Nd and the 143/144 Nd An alyses are

quo ted as 0.2 and 0.005 (1-sigma), respe ctive ly, again

bas ed on replicate analy ses of international standards. Th e

resul ts are presented in the form o f isochron diagrams s imi-

lar to Rb -Sr, and the techniques use d in calculating the

b e s t f i t

l ines, ages. an d errors are the sam e.

K Ar Analys is

K- At analyses were carr ied out predominantly on bio-

ti te, mu scov ite, and hornblende separates an d only rarely

ou whole-rock samples . This technique was used on al l

suite, colle cted wh ere the appropriate unaltere d minerals

were p resen t to suppor t e ither the Rb-Sr o r the S m-N d re-

suits.


5/20

Ne w geoch rono log ica l contro l for the tec tono-m agm at ic evo lut ion o f the m etam orphic basem ent , Ecuad or 81

P otas s ium was de term inod, a t l eas t in dupl icate, u s ing

an Instrumentation Laboratories IL543 f lame photometer

wi th l i th ium as interna l s tandard. Argon w as extrac ted by

fus io n under vac uu m us ing externa l radio - frequency indue-

t ion heat ing and ana lyzed by the i s o tope d i lut ion m ethod in

a V G I s o t o p e s M M 1 2 0 0 m a s s s p ec tr o m et er .

Repl ica te de term inations o f in -hou s e s tandards s ugg es t

that an overa l l prec i s ion o f +1 (1 -s igm a ) is rea li st ic for

the potass ium analyses . The error in the radiogenic argon

determ ination i s par tly dependen t on the am ount o f con-

taminating atmospheric argon, whic h often reflects the de-

g r e e o f d e u t e r i alteration and hence varies considerably

be tween s am ples . The argon s pike s ys tem was ca l ibrated

against international s tandards , so the results can be ex-

pected t o be accurate within th e l imits o f a n a l y t i c a l e r r o r

The ages were ca lcula te~ l us ing the cons tants recom -

m en ded by Ste iger and Jaeger (1977) , and the error on the

age i s quoted a t the 95 conf id ence l eve l .

R E S U L T S

The s am ples co l l ec ted for dat ing axe l is ted in Table 1 .

The Rb-Sr and Sm -N d analyses are g iven in Tables 2 and

3, respectively, and the K -Ar data. with the calculated a ges .

are presented in Table 4 . T hes e re s ul ts are d i s cus s ed be lo w.

and the loca l i t i e s m ent ioned in the text are s how n in F ig . 2 .

T a b l e 1 . L o c a t i o n a n d d e s c r i p t i o n o f s a m p l e s c o ll e c t e d f o r i s o t o p i c a n a l y s is .

G r i d

S a m p l e N o . R o c k t y p e s ) A r e a M a p S h e e t * R e f e r e n c e

Garnet Gnetsses, Agoydn

CCR/87/11A-E Garnet biotite mu s co v i t e s eh i s t s / g n e i s s e s

Bafios-Puyo road Batios (e) 7939-8458/

7918-8457

Garnet Gneisses/Amphibolite, Papal lac ta

CCR/87/4 Biotitic amphibolite

CRSH/89/1A Garnet amphibolite

CRSH/89/1B-C Garnet biotite 4-

mu s co v i t e g n e i s s e s

Sabant l la Subdiv i s ion Garnet Gnetsses/Amphtboli te, Valladolid

CCR/87/24A

CCR/87/24B

CCR/87/24C

CCR/87/24D

CRSH/89/10A-D

Amphibolite

M u s c o v i t e p e ~ a a t i t e

Muscovite p eg ma t i t e

Biotite p eg ma t i t e

Garnet-bearing g n e i s s e s / mi g ma t i t e s

Saba nt l la Subdiv i s ion Or thogne t s se s Lo]a-Zamora Road

CCR/87/23A-H Biotitic o r th o g n e i s s e s

Papallacta village

Float block, Rio Chalpi Grande

Float blocks, Rto Chalpi Grande

Papallacta (c) 818 4-9 9596

Papallacta (c) 824 6-9 960 8

Papallacta (c) 824 6-9 960 8

North of Valladolid Valladolid 7079-94983

North of Valladolid Valladolid 7079-94984

S o u th of Valladolid Valladolid 7075-94935

Near Palanda Valladolid 7074-94868

Float blocks, Rio Va ll ad ol id Val la do lid 7075-94976

CRSH/89/12A-C Biotite o r th o g n e i s s e s

CRSH/89/12D-J Migmatitic biotite

orLhogneisses

FV57/FV58 Biotitic

o r th o g n e i s s e s

Tres a g u n a s S u b d i v i s io n Orthogneisses, South f S i g s i

CCR/87/14A-D Biotite orthogneisses +

i g n eo u s x en o l i th

(14C)

East of Sabanilla

East of Sabanilla

East of Sabanilla

East of SabaniHa

Float blocks, Rfo Santa B/u bara,

Peggy Mine

Tres a g u n a s S u b d i v i s io n Orthognetsses, Nor th Edge of Malaeatus Basin

CRSH/89/11A-F Biotite + muscovite tourmaline granitic Qda. La Pieota


CRSH/89/11G-J Biotite muscovite + tourmaline granitic Qda. Cobalera


Tres a g u n a s S u b d i v i s io n Orthogneisses, Tree agunas, East of Saraguro

CRSH/89/14A-K Biotite 4-mu s co v i t e o r th o g n e i s s e s + a p l i t ie R fo Negro

variant (14K)

Loja Norte 7199-9562/

7199-95588

Loja Notre 71 99 -9 55 87

Loja Norte 71 97 -9 56 00

Loja Norte 71 94 -9 56 14

Sigsig 7476-96578

Nambacola 6917-95396

Nambacola 6914-95399

Saraguro ca. 712-9604

* 1:50,000 Topographic Sheet, published by Instimto Ge ogr~co MAlitarQuito;

(c) ind icates uncontrol led topographic base m ap wi thout contours (censal).

continued)


6/20

82 J. A . ASPDEN, S. H. HARRISON, and C. C. RUNDLE

T a b l e 1 c o n t i n u e d )

G r i d

S a m p l e N o . R o c k t y p e s ) A r e a M a p S h e e t* R e f e r e n ce

Pte dras Group Are nUlaz Amphtbot i t

C R S H / 8 9 / 5 A - B A m p h i b o l i t e s

P t e d r a s G r o u p P o r t o v el o A m p M b o U t e

C R S H / 8 9 / S A - B A m p h i b o l i t e s

T a h u i n G r o u p G a r n e t G n e i n e s

C R S H / 8 9 / 6 A - E G a r n e t b i o t i te 8 n e i s s e s a n d f e l s ic p e E m a t i te s

T a h u i n G r o u p P e s m a ~ t ~ G n c l ss e s

C R S H / 8 9 / T A - B B i o t i t e m u s c o v i t e g r a n i t e a n d m u s c o v i t e

tourm a l ine pe grna t i t e

C R S H / 8 9 / 1 9 M u s c o v i t e t o u r m a l i n e p e g m a t i t e

T a h u i n G r o u p M a r c a b eU P l u t o n

C R S H / 8 9 / 4 A - E

C R S H / 8 9 / 4 F - J

Z a m o r a B a t h o l l t h

C C R / g T / 1 6 A - H

C C R / 8 7 / 1 7

B i o t i t e m u s c o v i t e g r a n i t es

Bio t i t e m us c ov i te g ra n i te s

H o r n b l e n d e g r a n o d i o r i te s / h o m b l e n d e d i o r -

i r e s + f e l s ic ve in (16 D)

H o r n b l e n d e d i o r it e

C C R / 8 7 / 1 8 P o r p h y r i t i c h o r n b l e n d e f e l d s p a r a n d e s i te

C C R / 8 7 / 1 9

H o r n b l e n d e g r a n o d i o d t e

C C R / 8 7 / 2 0

C C R ] 8 7 / 2 1 A - J

C C R / 8 7 / 2 2 A - F

C C R / 8 7 / 2 5

C C R / 8 7 / 2 6 A - E

Hornb le nde b io t i t e g ra nod io r i t e

Hor nb le n de b io t i t e g ra nod io r i t e s + f e l s ic

ve in (21C) + pa r t i a l ly d ige s te d xe no l i th

(21D)

P i n k p o r p h y r i t i c b i o t it e h o r n b l e n d e

( ? ) m o n z o g r a n i te s + h o r n b l e n d e m i c r o g r a n o -

d io r i t e (22F)

Porphy r i t i c ho rnb le nde a nde s i te d ike

Hor nb le n de b io t i t e g ra nod io r i t e s /d io r i t e s

C R S H / 8 9 / 1 3 A - B

F V 6 0

H o r n b l e n d e d i o r i t e s

P o r p h y r i t i c h o r n b l e n d e g r a n o d i o r it e / d i o d te

R M 1

F V 6 8 1

F V 4 8 5

H o r n b l e n d e b i o t it e g r a n o d i o r i t e

H o r n b l e n d e b i o t i te g r a n o d i ro i t e

Hornb le nde b io t i t e g ra nod io r i t e

A b i t a g u a B a t h o l t l k

C C R / 8 7 / 5 A - I

C C R / 8 7 / 6 A , B , D ,

G - K

C C R / 8 7 / 6 C , E , F

C C R / 8 7 / 7

A D M L 5

Ho rnb le n de b io t i t e g ra nod io r i t e + f e l s ic ve in

m a te r ia l

Ho rnb le n de b io t i t e g ra nod io r i t e s + f e l s ic

ve in m a te r ia l

P ink porph yr i t i c ho rnb le nde b io t i t e g ra no-

d io r i t e s

H o r n b l e n d e a n d e s i t e d i k e

H o r n b l e n d e g r a n o d i o r it e

Are n i l l a s b r idge Are n i l l a s 6049-96072

W e s t o f P o c t o v el a Z a m m a 8 5 1 9 - 9 58 8 2

Rio P i e d r a s n o r t h o f L a B o c a n a L a A v a n z a d a 6 2 1 3 - 95 9 5 5

Rfo E l N e g r o s o u t h o f L a B o c a n a M a r c a be f i

Floa t block , Rio Pie .Areas a t La Maro abe li

B o c a n a

6218-95911

6219-95927

Ba ls a s qua r ry Ma rc a be l i 6308-95837

Sou thw e s t o f Ma rc a be l i Ma rc a be l i 6188-95775

L a Pa x a re a Ya n tz a xa 7362-95864 /

7369-95845

F loa t b loc k , Qda . Cur i s hp , s ou th Ya n tz a xa 7368-95845

o f L a P a x

F loa t b loc k , Qda . Cur i s hpe , s ou th Ya n tz a xa 7368-95845

o f L a P a z

Qda . Ma yc un a n tz a , s ou th o f L a Ya n tz a xa 7351-95830

Pa z

Sou th o f L a Pa x Ya n tz axa 7340-95783

S o u t h o f Q d a . C h a p i n t z a, G u a y s i m i 7 6 6 0 - 9 5 5 3 0 /

Pa qu is ha a re a 7652-95540

R i o P i t u c a a r e a a n d R i o J a m b u e Z a m o r a 7 2 9 4 - 9 5 4 28

7288-95432

(22F)

P a l a n d a - Z u m b a r o a d R i o M a y o 7 0 7 4 - 9 48 0 4

P a l a a d a - Z u m b a r o a d R f o M a y o 7 0 7 4 - 9 4 8 0 9 /

7075-94781

Rfo Chic a na e a s t o f L a Pa x Ya n tz a z a 7432-95930

F loa t b loc k f rom Gua ys im i s ou th Gua ys im i 7575-95527

o f P a q u i s h a

Rio M a y o Z u m b a 7 1 4 4 - 9 45 3 6

E a s t o f pa la nda Va ilado lid 7218-94880

Qda . de L oa De r rum be s e a s t o f Va l la do l id 7175-94972

Valladolid

Cos a nga - ' l~na roa d ca. 5 5 k m C o s a n g a ( c )

n o r t h o f T e n a )

Ba f ioa -Puyo roa d Me ra ( c )

Ba f loa -Puyo roa d Me ra ( c )

Ba f loa -Puyo m a d Me ra ( c )

F loa t b loc k ,

Rio Zuflag,

Ba f los - Me ra

c)

P u y o r o a d

8131-98442 /

812 98444

8148-98405/

812 98444

8147-98404

8127-98444

* 1 :50 ,000 T opogra ph ic She e t , pub l i s he d by Ins t i tu to Ge og r~ ic o M i l i t ar Qu i to ;

( c ) ind ic a te s unc on t ro l le d topogra ph ic ba s e m a p wi thou t c on tour s ( c e ns a l) .


7/20

N ew geochronological control for the tectono-magm atic evolution of the metam orphic basement, Ecuador 83

T ab l e 1 c on t i n u e d )

G r i d

S a m p l e N o . R o c k t y p e s ) A r e a M a p S h e e t* R e f e r en c e

A z a f l ~ n B a t h o l l t h

CCR/87/SA-I Leucogranites + aplite vein SD + quartz- Baflca-Puyo road Baflos c) 8058-98448/

feldspar pegmatite 8C) 8039-98449

CCR/87/9 Biotite granodiorite Baflos-Puyo road Baflos c) 80~9-98450

CCR/87/IOA-B Hornblende biotite diorites Bafios-Puyo road Baflos c) 8009-8452

ADMIA Hornblende biotite diorite Float block in Rio Verde, Baf los - Baf ios c) 8009-8452

Puyo road

C I i i a g u a l B a t h o l i t h , S a n ~ B d r b a r a . L a B o n i t a R o a d

CCR/87/2A-J Biotite orthogneisses

Northwest of Pimampiro Huaca c) 88 69 -1 00 60 5/

8871 - 100595

S a c h a P l u t o n

CCR/87/3

P l m a m p b o P l u t o n

CCR/87/1A

CCR/87/1C

M a ~ t a y d n P l u t o n

CCR/87/13A-C

Hornblende biotite diorite

Hornblende granodiorite


Hornblende biotite diorites and hornblende

gabbro 13B)

Un named Pluton Cuenca-Ltm6n Road

FV83

S a n L u e a s P l u t o n

CCR/87/28A-C

FVI1

FV15

FV34

Biotite granodiorite

Pink porphyritic biotite granediorites


Hornblende biotite granodiorite


T a m p a n c h i M a r i e l g n e o u s C o m p l e x

CRSH/89/17A-C Hornblende gabbro, pegmatitic horn-

blendites and hornblende basalt

a t a m a y o P l u t on

CCR/87/29A-B Biotite granodiorite

Qda. Tungurahua Huaca c) 88 34 -1 00 69 0

Near Mataqui Pimampiro 1744-00420

Qda. Manzanal Pimampiro 1785-00438

Osogochi area Totoras 7678-97580/

7621-97520

Principal c) 7650 -96663

Qda. Tunttln Santiago 6933-95849

Qda. Bucashi Santiago 6928-95857

Juntas 6948-95785

Qda. E1Gallo Loja Norte 69 85 -9 57 40

P t c M n a l P l u t o n

CRSH/89/15

P u n g a l d P l u t o n

CCR/87/12A-C


Hornblende biotite granodiorites

Cola de San Pablo 7625-97080

Loja-La Toma road Catamayo

La Toma)

Rfo Pinchinal Saraguro 7045-95999

P o r ~ h u e l a B a t h o lt .~ , T r a ck . fr o m J t m b u ra t o Z u m b a

CCR/87]27A-B Biotitic felsic porphyry .. .. .. .. ..

CCR/87/27C-G Hornblende biotite granodiofites and diorites .. .. .. .. ..

Guamote and 7680-97965/

Riobamba 7680-98000

Laguna Cox 677 3-9 472 3

Laguna Cox 6755-94744/

6745-94765

* 1:50,000 Topographic Sheet, published by Instituto Geogrifico Mllitar Quito;

) indicates uncontrolled topographic base map without contours censal).

M etamorph ic Rocks o f he o rd i l l er a Rea l

Metam orphic rocks fro m the C ordil lera Real were col-

lected from four separate local it ies: Papallacta on the road

betwee n Qui to and Baeza) . Aso y~n between Baf ios and

Puy o) , eas t of Sabani ila between Loja and Zamoxa) , and

t h e V a l l a d o l i d a r e a i n s o u t h e r n E c u a d o r . A l t h o u g h a c o m -

b i n at i o n o f K - A t R b - S r a n d S m - N d d a t a h a s b e e n o h -

m i n e d f r o w n h e s e o c k s t h e r e s u lt s r e f a r r o m c o n c l u s i v e .

T w o s ui te s f o r t h o g m i s s f r o m t h e S a b an i l la a n d V a U a -

d o l id a r e a s w e r e d a t e d b y t h e R b - S r m e t h o d b u t bo t h d a t a

s e ts s h o w a w i d e s c a tt e x n t h e i s o c h r c m d i a g r a m s . N e v e r -


8/20

84 I.A. ASPDEN, S. H. HARRISON and C. C. RUNDLE

Table 2 . Rb Sr analyt i ca l data .

Sample N o. R b Sr

gTRb

~ S r

S a b a n i l l a S u b d i v i s i o n O r t h o g n e i ss es n e a r Z am o r a

7Sr

S6Sr

CRSH/89/12A 106.2 188.7 1.6714 0.7180 1

CRSH/89/12B 97.8 207.8 139 73 0.716 90

CRSH/89/12C 83.7 182.5 13628 0.71686

CRSH/89/12D 82.8 178.6 13 767 0.71671

CRSH/89/12E 104.0 204.9 1.5065 0.71717

CRSH/89/12F 100.3 191.3 1.5575 0.7174 0

CRSH/89/12G 117.5 209.9 1.6629 0.7174 2

CRSH/89/12H 87.9 188.6 13 848 0.71671

CRSH/89/ I 2I 82.7 176.2 13946 0.7167 0

CRSH/89/12J 73.5 214.5 1.0175 0.715 96

CCR/87/23A 123 204 1.747 0.7178 8

CCR/87/23B 110 197 1. 60 1 0.717 74

CCR/87,r23c 45.2 201 0.6521 0.71436

CCR/87/23D 128 208 1.7 68 0.71776

CCR/87/23E 119 210 1.63 3 0.71682

CCR/87/23F 128 192 1. 931 0.71 716

CCR/87/23G 96.3 231 1.2 05 0.71546

CCR/87/23H 121 124 2.833 0.7 217 3

T r e s L a g u n a s S u b d i v i s i o n O r t h o g n e i s s es

CRSH/89/11A 124.5 142.0 2.6054 0.7192 2

CRSH/89/ I 1B 124.6 138.3 26795 0.7199 4

CRSH/89/11C 129.5 133.8 2.8755 0.7196 7

CRSH/89/11D 117.5 144.6 2.415 0 0.71883

CRSH/89/11E 126.1 137.1 27307 0.71975

CRSH/89/11F 131.9 168.0 23324 0.71871

CRSH/89/11G 138.7 99.5 4.1415 0.721 56

CRSH/89/11H 134.3 131.1 3 )438 0.72 075

CRSH/89/11I 135.1 129.4 3.0908 0.720 82

CRSH/89/14A 189.7 95.0 5.9439 0.72867

CRSH/89/14B 174.8 106.9 4.8(~6 0.72590

CRSH/89/14C 186.7 93.3 5.9499 0.72893

CRSH/B9/14D 182.7 102.0 53283 0.7283 9

CRSH/89/14E 175.1 97.3 53507 0.72 684

CRSH/89/14G 186.3 97.7 5.6710 0.72905

CRSH/89/14H 197.0 85.8 6.8323 0.73043

CRSH/89/14I 173.7 103.2 5.0067 0.725 79

CRSH/89/14J 169.8 109.9 4.5905 0.7252 0

CRSH/89/14K 144.7 102.1 4.19 89 0.7237 9

A d t a g u a B a t h o l t t h

CCR/87/SA 159 22.7 20.42 0.7 518 3

CCR/87/5B 85.9 285 0.871 7 0.7065 2

CCRhl7/5C 156 26.3 17.29 0.7 441 0

CCR/87/5D 87.0 259 0.9730 0.70677

Sample N o. R b

A b i t a f u a B a t h o H Ot c o n t i n u e d )

CCR/87/5E

CCR/87/5F

CCR/87/5G

CCR/87/SH

CCR/87/5I

CCR/87/6B

CCR/87/6D

CCR/87/6G

CCR/87/6H

CCR/87/61

CCR/87/6J

CCR/87/6K

8/Rb gTSr

Sr S6Sr SeSr

160 23.4 19. 85 0. 74 96 3

66.5 421 0.45 74 0.705 60

92.3 389 0.68 68 0.706 20

103 327 0.904 4 0.706 64

150 54.1 8.016 0.7 229 8

132 428 0.8886 0.70667

130 98.5 3.821 0.7 1348

225 15.2 43.31 0.80 39 4

102 355 0.83 19 0.706 70

93.1 382 0.7041 0.706 15

235 10.3 67.85 0. 86 17 0

54.9 959 0.1659 0.70494

Z am o r a B a t h o l i th L a P a z A r e a

CCR/87/16D

CCR/87/16E

CCRf87/16F

CCR/87/16G

CCR/87/16H

82.6 46.2 5.190 0.71 840

46.7 247 0.54 69 0.706 09

51.1 238 0.6231 0.70 622

14.9 374 0.1 160 0.704 99

26.1 270 0.2 802 0.705 30

Z am o r a B a t h o l tt h P a q u t sh a A r e a

CCR/87~1A

CCR/87/'21B

CCR/87/21D

CCR/87/21E

CCR/87/21F

CCR/87/21G

66.2 391 0.490 4 0.70631

70.7 367 0.55 82 0.7066 5

63.9 432 0.4281 0.70 629

79.1 391 0.5844 0. ~

96.7 339 0.827 5 0.7073 4

62.8 364 0.49 92 0.70635

Z am o r a B a t h o l i t h R o P i t u c a A r e a

CCR/87/22A 105 373 0.8 170 0.7 066 0

CCR/87/22B 71.8 181 1.139 0.70770

CCR/87/22C 103 387 0.7701 0.70645

CCR/87/22D 107 385 0.805 4 0.70649

CCR/87/22E 96.8 388 0.7213 0.7 064 0

CCR/87/22F 59.2 674 0.254 5 0.70 460

Z am o r a B a th o l i t h P a l a n d a A r e a

58.6 329 0.5161 0.706 17

56.0 335 0.484 8 0.706 12

53.5 325 0.476 3 0.705 99

47.3 359 0.3821 0.705 92

42.4 353 0.3 476 0.705 78

CCR/87/26A

CCR/87/26B

CCR/87/26C

CCR/87/'26D

CCR/87/26E

zafranB a t h o l i t h

CCR/87/8A

C C R 8 7 S B

C t I { 8 7 S D

C C R 8 7 S E

100 86.5 3.345 0.710 29

127 70.8 4.817 0.71 291

109 77.1 4.074 0.7 11 60

110 75.5 4.229 0.71 171


9/20

New geochrono log ica l con t ro l fo r the tec tono-magm at ic evo lu t ion o f the metamorphic basement , Ecuador 85

Table 2 continued)

S a m p l e N o .

R b S r

Table 3 . Sm -Nd analyt ical data for the Tahuin G roup

garnet orthogneiss 219 + 22 Ma)

A z a f r a n B a O t o l i O t c o n t i n u e d)

87R b STSr 147Sm 143Nd

S S S r S 6S r S m N d

S a m p l e N o . p p m ) p p m ) 1 44 N d 1 4 4N d

CC R/ 87 / S F 111 71 . 0 4 . 495 0 . 71223

CC R/ 8 7 / 8G 111 80 . 5 3 . 984 0 . 71147

CC R/ 87 / S H 104 60 . 3 5 . 004 0 . 71309

Chingual atholith

C C R / 8 7 / 2 B 3 8 . 2 4 6 7 0 . 2 3 6 8 0 .7 0 41 4

C C R / 8 7 / 2 C 4 4 . 4 3 5 3 0 . 3 6 4 0 0 .7 0 45 0

C C R / 8 7 / 2 D 4 6 . 3 3 3 5 0 . 3 9 9 9 0 .7 0 46 0

C C R / 8 7 / 2 E 3 1 . 6 5 1 7 0 . 1 7 6 6 0 .7 0 40 6

CCR , t 87 / 2F 30 . 6 507 0 . 174 8 0 . 70402

C C R / 8 7 / 2 G 3 1 . 2 5 0 7 0 . 1 7 7 9 0 .7 0 41 3

C C R / 8 7 / 2 I 4 4 . 7 4 1 7 0 . 3 1 0 6 0 .7 0 42 8

C C R / 8 7 / 2 J 4 6 . 6 4 0 5 0 . 3 3 2 6 0 .7 0 43 3

San Lucas Pluton

CC R/ 8 7 / 28 A 130 123 3 . 049 0 . 70703

CC R/ 87 / 28B 154 79 . 9 5 . 561 0 .70887

CCR / 87 , r 28c 82 . 2 263 0 . 9165 0 . 70536

theless , data from the Sabani l la orthogneiss yield the best

l inear correlat ion and are cons idered to give the mo re reli -

ab le age o f 224 + 37 M a (M SW D = 108 ; F ig . 3a). The re la -

tively high initial

8 7 S r 8 6 S r

rat io of 0 .7123, together wi th

the s t rongly gne issose character o f the rocks , suggests that

this i s proba bly the age o f metamorphism; thus i t i s postu-

lated that this took plac e in La te Triassic-Early Jurassic

t imes. The Val ladol id orthogneiss sui te , which is more

mass ive in t ex tu re bu t no tab ly w eathered , gave a very poor -

ly cons t ra ined age o f 359 + 99 Ma, wi th an MSWD of

1877. The wide scat ter o f these data indicates considerable

dis turbance o f the isotopic systems, and l i t tle rel iance can

be place d on this age.

K-Ar da ta f rom amphibo l i t es and gne i s ses f rom the

Papal lacta area hav e given extrem ely variable ages . Biot i te

f rom one sample o f garne t b io t it e gne i ss y i e lded a l a te Pre-

cambr ian age

c a .

850 M a) , whereas musco v i t e f rom an ad -

jace nt bloc k of s imilar material recorde d a Late Cretaceous

eve nt (74:1:3 Ma). In contras t, two samp les of hornblende

from amphibol i t ic material gave poorly reproducible re-

sults , wi th a m ean of 345 : t: 29 M a, suggesting Devo nian to

Carboniferous act ivi ty . Due to the pauci ty of exposure,

however , mos t o f these ana lyses were car r i ed ou t on

samp les from relat ively smal l, rounded, loose blocks from

a river bed, w hich y ielded confl ict ing data. I t i s thus di ff i -

cul t to extract any usefu l informat ion from these. M ore de-

tai led geolog ical map ping an d more spe cif ic sampling are

requ i red b efo re the p resence o f p re-M esozo ic rocks in th is

area can be conf i rmed .

Al l t he K-A r da ta f rom para- and o r tho-gne is ses o f the

Sabani l la subdivis ion near Zamora and VaUadol id gave

Late Cretace ous ages , ranging from 85 :l: 2 to 65 + 2 M a.

How ever , i n v i ew of the Rb-S r da ta d i scussed above , which

C R S H / 8 9 / 6 A w r ) 5 . 5 9 3 0 . 6 3 0 . 1 1 0 2 0 . 5 1 2 0 7 5

C R S H / 8 9 / t A g t ) 4 . 5 6 1 4 . 52 0 . 1 8 9 8 0 . 5 1 2 2 2 0

C R S H / 8 9 / 6 B w r ) 7 . 6 3 3 7 .4 1 0 . 1 2 3 2 0 . 5 1 2 1 3 2

C R S H / 8 9 / t B g t ) 6 . 6 2 2 3 . 5 4 0 . 1 7 0 0 0 . 5 1 2 1 7 0

C R S H / 8 9 / 6 C w r ) 6 . 3 3 3 4 . 6 4 0 . 1 1 0 5 0 . 5 1 2 0 7 4

C R S H / 8 9 / 6 C g t ) 4 . 2 2 1 0 .7 3 0 . 2 3 7 7 0 . 5 1 2 2 8 0

C R S H / 8 9 / 6 D w r ) 7 . 9 2 4 0 . 1 3 0 . 1 1 9 3 0 . 5 1 2 1 1 1

C R S H / 8 9 / 6 D g t ) 5 . 5 3 1 6 . 74 0 . 1 9 9 7 0 . 5 1 2 2 4 5

C R S H / 8 9 / 6 E w r ) 7 . 2 3 3 8 . 8 3 0 . 1 1 2 6 0 . 5 1 2 0 9 9

C R S H / 8 9 / 6 E g t ) 4 . 8 3 1 4 .9 3 0 . 1 9 5 6 0 . 5 1 2 2 3 7

K e y : w r , w ho l e r oc k ; g t , ga r ne t .

sugge st a Late Triassic-Early Jurass ic metamorphism,

these are al l considered to reflect a ma jor isotopic even t ,

significantly later than the m ain gneissification. Th is inter-

pretat ion is supported by K -A t data from h ornblende from

an amphibolite dike cutting the garnet gneiss at Valladolid,

which has p reserved an age o f 132 + 5 Ma, p resumably re -

flecting on ly partial resetting during th e La te Cretac eou s

event. Furthermore, whe re coexis t ing pairs o f micas w ere

da ted ( samples CR SH /89 /10A, /10C , /12A, /12C ) , t he mus-

covi tes (average ag e 69 + 3 M a) consis tent ly gave s isni f i -

cant ly younge r ages thnn the biot ites (average age 84 + 2

Ma) and also had s ionif icantly lower K-con tents ( the re-

verse of what i s normal ly expected). This pat tern may sug-

gest that the muscovi tes formed at a later s tage than the

bioti tes , but th is w ould imp ly that the eve nt did no t resul t

in sit,nif'tcant argon loss from the biotites. Alternatively,

and possibly m ote l ikely , i t may b e that these low-K mu s-

covi tes have an abnormal ly low blocking temperature to

argon diffusion; this suggestion is supported by the signif-

icant ly older age (77 + 3 M a) given by the musc ovi te from

sample CC R/87/24B, which has a m ore norm al K-content.

K-A r dating of garnet gneiss from Agoya n was a lso

rather unsatisfactory. The whi te mica separated from these

rocks p roved to be an unusua l ly low-K var i e ty (p robab ly

Na-rich paragonite), and hence there are relatively high

errors on individual age determinations. Ne verthe less,

three samples ga ve concordant resul ts , w i th a mean of 76 +

3 Ma, in goo d agreement w i th the data from the Sabani lla

subdivis ion and presumably reflecting the sam e Late C re-

taceous event .

Garnet was separated from samples col lected from the

Papallacta area and from the Sabanilla su bdiv ision gam eti-

ferous gneisses for Sm-N d analysis . Ho we ver, in all eight

analyzed sam ples there wa s li t tle or no fractionatiou of the

rare earth isotopes between the garnet and associated

whole-rock, and hence the data w ere of no va lue for dat ing

pm'lXX~s.


10/20

8 6 J . A .

A S P D E N

S. H . HARRISON and C. C. RUNDLE

O. 7 2 5

O. 7 2 0

O. 7 t 5

0 . 7 1 0

O. 7 0 5

0 . 7 3 5

0 . 7 2 5

0 . 7 t 5

0 . 7 0 5

0 . 7 2 0

0 . 715

0 . 7 1 0

0 . 7 0 5

O. 709

O . 7 0 7

O . 705

O . 703

0 . 701

I I

8 7 S r / 8 6 S r

I T I I

o)

. . . . . ? ~ , . . . . . '

. . . . . , . . .

.....

AGE 224 + -. 37 Ma (2 s )

I n t e r c e p t 0 . 7 4 . 2 3 4- 0 . 0 0 0 8

MSWD 108.6 Enhanced E r r o rs

87R b / 86S r

J L I I I

4.. 0 2 . 0 5 . 0

I [ l l [ I l

8 7 S r / 8 6 S r

( C ) ~ ..........

....., e ~ : ~ : ~ .......

' ' ' t

AGE 20 0 _+ 12 Ma 12s )

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

MSWD |69.1 Enhanced Erro rs

8 7 R b / 8 6 S r

L I J I I I I

t 3 5 7

I I I

8 7 S r / 8 6 S r

e )

I I

. . - -

. , . - 1 '

. . . .

. . , . '

. . . . '

. . .

. . . '

. . -

AGE 187 + 2 Ma 12s )

I n t e r c e p t 0 . 7 0 4 6 0 . 0 0 0 0

M S W D 2 . 9

8 7 b / 8 6 S r

1 J I i

2 :5 4 5

8 7 S r / 8 6 S r

] 1 I I

I

0 . 2

c j )

. ~ . . , p ~ . . . . . . . . . . . . . . . . . . . . . . . J P

. . . . . . , '

. , . . '

. . . . . , '

AGE 24 6 +- 17 Ma (2s )

I n t e r c e p t

0 . 7 0 3 7

* 0 - 0 0 0 2

M SW D 4 . 4 E n h a n c e d E r r o r s

8 7 R b / 8 6 S r

I I 1 J

0 . 4 0 . 6 0 . 8 1 .0

O.

5 1 4

0 . 5 t 2

0 5i0

O. 50 8

0 . 5 0 6

0 . 8 5 0

0 . 8 0 0

~ . 750

0 . 7 0 8

0 . 7 0 6

0 . 7 0 4

0 7 0 2

0 . 7 0 7

0 705

0 . 7 0 3

O. 701

] I I

~ 3 N d / 144Nd

b )

. . . . .. . . . .. 4 - . . - - H l - - l - . . . .. . . .. . . .. . .

I

0 . 4 ,

A GE 2 t 9 22 M e ( 2s )

I n t e r c e p t 0 . 5 1 i 9

0 . 0 0 0 0

M S W D 0 . 4

t47Sm/144N(

I J i

0 5

0 . 5

I I

87 S r / 86 S r

. , 'tF ' " In t e rcept

. : , + : .... MSW D 2. 5

I [ I I

40 3O

I [ I I I

d ) . . . . . . . .

. . . . . . , . . . . . ' *

. . .

" " ' A GE t 6 2 ~ t M a l 2 s )

0 . 7 0 4 6 -+ 0 . 0 0 0 0

8 7 R b / 8 6 S r

I I I

5 0 7 0

I I [ I I

8 7 S r / 8 6 S r

f ) . . . . . . + . . . . . . .. . . . . . .

1

0.1

AGE 198 + 34 Ma (2s )

I n t e r c e p t 0 . 7 0 5 0 _+ 0 . 0 0 0 3

M SW D 4 . 2 E n h a nc e d E r r o r s

8 7 R b / 8 6 S r

I L L I

0 . 3 0 . 5 0 . 7 0 . 9

[

8 7 S r / 8 6 S r

[ I I

( h )

. . . . . . . - ~ . . .. .. . .. .. , ~ . e . .. .. . .. .. .

I

0.1

AGE 144 +_ 35 Ma (2 s)

~ n t e r c e p t 0 . 7 0 5 1 + 0 . 0 0 0 2

M S W D 2 . 7

8 7 R b / 8 6 S r

0 . 2 0 . 3 0 . 4 0 . 5


11/20

New geochronological conlrol for the tectono-magmatic evolution of the metamorphic basement, Ecuador 87

O . 714

O . 710

O . 7 0 6

O . 7 0 2

0 . 7 0 8

0 . 7 0 6

0 . 7 0 4

I

8 7 S r / 8 6 S r

I I I I I . .

. . .

i ) ....

. . . ~ . 4 . . . . .

. . . '

. .

. , '

. , . '

I

I 2

I

8 7 S r / 8 6 S r

. . . . .

. . . -

AGE 120 * - 5 Mo 25 )

I n t e r c e p t 0 . 7 0 4 6 _* 0 . 0 0 0 3

M S W D 2 . 4

8 7 R b / 8 6 S r

I I I I I

3 4 5 6 7

k )

. . .

. . , . - -

. o . - '

. . ,

. . . o

, , . . ~ '

. . . -

. . . ,

G E 53 -~ 2 Mo 2s )

I n t e r c e p t 0 . 7 0 4 7 +_ 0 . 0 0 0 1

M S W D I 6

8 7 R b / 8 6 S r

i I I I I

2

3

4 5 6

O . 7 0 5

0 . 7 0 4

0 . 7 0 3

0 . 7 0 2

0 , 701

I 1 I 1

8 7 S r / 8 6 S r

J )

. # .. . . . . . .. . . .. . . ~ ,v . . ~ '4 .. . .. . . .. . . .. . .

I

O I

AGE 156 -~ 21 M o I s )

I n t e r c e p t 0 . 7 0 3 7 *_ O , O 0 0 l

M S W D 2 . 8

8 7 R b / 8 6 S r

I I I

0 . 2

0 3

0 . 4

Fig. 3. Isochron diagrams for the Cordillera Real and E10ro

Province: a) Sabanilla subdivision orthogneiss. Cordillera Real;

b) Tahuin Group garnet orthogneiss, El Oro Province; c) Tres

Lagunas granitic subdivision, Cordillera Real; d) Abitagua

batholith, sub-Andean zone; e) Zamora batholith, La Paz area,

sub-Andean zone; f) Zamora batholith, Paquisha area, sub-

Andean zone; g) Zamora batholith, Rio Pituca area, sub-Andean

zone; h) Zamora batholith, Palanda area, sub-Andean zone;

i) Azafran batholith, Ba~os road, Cordillera Real; j) Chingual

batholith, near the Colombian border, Cordillera Real; k) San

Lucas pluton south of Saraguro, CordilleraReal.

Metam orphic Roc ks fro m l Oro Province

The data from the metamorphic rocks of El Oro Prov-

ince proved to be more rewarding. Sm-Nd analysis on gar-

net/whole-rock pairs was carried out on samples of the

Tahuin Group collected from localities near La Bocana.

These rocks included garnetiferous pelitic gneisses and

felsic pegmatites. The combined data from these two litho-

logics form a well-defined isochron with an age of 219 + 22

Ma (Fig. 3b). indicating the date of the garnet growth,

which would have been at the height of metamorphism

within these rocks.

K-At dating of the Tahuin Group gneisses was also

highly successful, with three samples of muscovite and two

of biotite giving concordant ages with a mean of 213 + 5

Ma. in remarkably close agreement with the Sm-Nd age.

Furthermore, these ages are also in good agreement with

the age of 210 Ma reported by Felnlnger and Silberman

(1982). Only one sample (CRSH/89/19 (rose)) gave a sig-

nificantly younger age (189 + 5Ma). but this was from a

late pegmatitic facies from a loose fiver boulder and may

not be so closely related as the other samples, or, alterna-

tively, the coarse muscovite may have been more suscepti-

ble to subsequent argon loss. Thus, the T ahuin Group

gneisses probably formed at around 220-210 Ma (Late Tri-

assic), cooled relatively rapidly after this event, and were

largely unaffected by the subsequent Late Cretaceous re-

setting.

Amphibolite samples from the Piedras Group, however,

do appear to have been reset during the Late Cretaceous, as

two hornblende separates from the Arenillas area have

given a mean K-Ar age of 74 + 2 Ma. which agrees with

that of 74 + 1 Ma (K-Ar biotite) obtained by Feininger and

Silberman (1982) from the same area.

Other samples from the Piedras Group are more per-

plexing. Two amphibole separates from the same locality

near Portovelo have extremely low K contents (0.07% and

0.05%). yielding very different ages: 224 + 34 and 647 : : 37

Ma. respectively. These may be compared with the widely

quoted Precambrian date o f 743 :t: 14 Ma reported by Ken-

nerly (1980) for a similar amphibole from Portovelo which

also had a very low K-content (0,084%). Clearly these are

not normal hombtendes and may not be reliable geochro-

nometers. Moreover. with such low K-contents, they are

likely to be extremely susceptible to the presence o f excess

argon, which would cause the calculated ages to be spuri-

ously old. Hence, none of these ages can be considered re-

liable, and the presence of Precambrian rocks in this area

cannot be confu'med.

A relatively undeformed granodiofitic intrusion, the

Marcabeli phiton, is exposed within the Tahuin Group of El

Oro Province. K-Ar ages obtained from co-existing biotite

and muscovite separates from this intrusion range from 221

+ 6 to 193 + 13 Ma, with no systematic difference between

the two minerals. The mean age of 207 + 13 Ma is in good

agreement with that of 214 + 7 Ma (biotite) published by

Feininger and Silberman (1982). The Rb-Sr data for this


12/20

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16/20

92 J.A. ASPDEN, S. H. HARRISON,and C. C. RUNDLE

phiton, however, scattered widely on the isochron diagram

and no reliable age could be calculated. Nevertheless, since

there is little evidence of subsequent metamorphism or de-

formation in these rocks, it is suggested that the K-Ar ages

record emplacement and cooling of this pluton at around

220-190 Ma (Late Triassic-Early Jurassic). These data also

provide evidence for the lack of any effects from the Late

Cretaceous event in this area.

Me ta lgn eous a nd Igneous Roc ks o f the Cordi llera Real

a n d S u b A n d e a n Z o n e

In an attempt to date the garnet biotite + muscovite gra-

nites of the Tres Lagunas subdivision in the Cordillera

Real, samples were collected from three areas: east of

Saraguro, north of Malacatus, and south of Sigsig. The

granite at Sigsig is pervasively net-veined by sulfides and

other secondary mineral; hence it unlikely to give an age of

magmatic crystallization. In contrast, the granites from the

other two localities are relatively fresh. Sm-Nd data from

garnet/whole-rock pairs from east of Saraguro were unsuit-

able for dating because there was little variation in isotopic

ratios between the garnet and whole-rock analyses. The

Rb-Sr data for these samples are also rather unsatisfactory

because of the wide scatter on the isochron diagram

(MSWD = 169). Nevertheless, they provide the most reli-

able (minimum) age thus far for the emplacement of the

Tres Lag,mas subdivision at 200 + 12 Ma (MSWD = 169;

Fig. 3c), similar to the age of metamorphism in El Oro

Province. The K-Ar data from all localities for the Tres La-

gunas subdivision give Late Cretaceous and Tertiary ages,

ranging from 100 + 3 to 51+ 2 Ma, and they are interpreted

to have been reset as a result of younger Cretaceous epi-

so~s (see below).

Rb-Sr data (18 samples) from the Abitagua batholhh,

l o c a te d i n t h e s u b - A n d e a n z o n e , d e fi n e a n i s o c h r o n w i t h a

p ar ti cu la rl y e l l c o n s t r ai n e d a g e o f 1 6 1 + I M a M S W D =

2 . 5, F i g 3 d ) . K - A t f r o m h o r n b l e nd e a n d b io t it e s e p a r a t e d

f r o m t h e s e s a m p l e s g a v e m o r e v a r i ab l e r es ul ts . w o s a m -

p l e s C C R / 8 7 / 5 G t ~ b ) a n d C C R / 8 7 / 6 A b 0 ) g a v e y o u n g e r

ages of 135 + 8 Ma and 126 + 2

Ma

which are interpreted

to be reset, but the rest of the samples gave dates ranging

from 152 : : 7 to 174 + 8 Ma. The latter ages are in general

agreement with the Rb-Sr data and confu'm a Middle to

Late Jurassic age for this intrusion.

The Rb-Sr results from five separate suites of samples

from the Zamora batholith all gave reasonably good linear

correlations with low MSWD. However, the calculated

ages are variable, normally with high errors due to the gen-

erally small spread in Rb-Sr ratios and hence are difficult

to interpret. Probably the most reliable data are from a suite

of five samples from the La Paz area which define an iso-

chron with an age of 187 + 2 Ma (MSWD = 2.9; Fig. 3e).

Six samples from the Paq-isha area gave an age of 198 : :

34 Ma (MSWD = 4.2; Fig. 3f), and mother suite of a dis-

tinctive pink, p(xphyritic, K-feldspar, hornblende-biotite

granite (six samples) from the Rio Pituca area yielded an

age of 246 + 17 Ma (MSWD = 4.4; Fig. 3g). A group of five

hornblende-biotite granodi(xites/diorites, collected from

the south of Palanda, define an isochron with an age of 144

- 35 Ma (MSWD = 2.7; Fig. 3h).

In addition to the Rb-Sr data, a considerable number of

K-At ages have been determined on minerals separated

from samples from the Zamora batholith (Fig. 4). These

have also yielded a wide range of results, several of the

) , -

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2

Fig. 4. Histogramof K-Ar mineral ages listed in Table 4 obtainedfrom the Zamora batholith.


17/20

Ne w geoch ronologica l control for the tectono-magmatic evolut ion of the metamorphic basem ent . Ecua dor 93

younges t o f which

i.e.. < ca.

140 Ma) a re p robab ly due to

subseque nt argon loss during al teration and can therefore

b e d i s r e g a r d e d . N e v e r t h e l e s s , a c a r e f u l appraisal of these

d a t a c a n h e l p t o m o r e c l o s e l y o n s t r a i n h e r a t h e r m p r e c i s e

Rb-Sr resul ts and provide extra ins ight in to the develop-

m ent o f the Zam ora batholi th .

I n t h e L a P a z a r e a , h r e e o r n b l e n d e s e p a r a t e s C C R / 8 7 /

1 6 H , 1 7 . a n d 1 9 ) g a v e a g e s o f 1 7 8 :I: 0 , 1 8 8 6 . a n d 1 9 1

+ 10 Ma, in good agreem ent wi th the Rb-S r age (187 :1 :2

Ma). A f iver boulder of coarse-grained porphyri t ic horn-

b lende- fe ldspar andes i t e (CCR/87 /18) gave an age o f

around 230 Ma, sugges t ing the p resence o f o lder e lement s

within the batholith.

Nea r Paquisha. two c,o-exis ting pairs of hornblende and

b io t i t e samples (CCR/87 /21A and G) gave a remarkab ly

c lose c lus te r o f ages wi th a m ean o f 154 : : 3 M a, which

mus t record the age o f rap id coo l ing th ro -sh the b lock ing

tempera tu res fo r these two rn i~ra l s . Th i s cou ld sugges t

e i ther tha t t he m agm a c oo led su f f i c i en tly to se t the Rb-S r

clock at ca. 200 M a but then remained above the argon

b lock in~ t empera tu re fo r som e 45 mi l l ion years befo re f i -

nal cool ing, or that emplacement and cool ing occurred at

ca. 200 Ma, fo l lowed by reheat ing to comple te ly rese t t he

K-At in both hornblende and biot i te at ca. 155 Ma, wi th

only m inimal dis turbance of the Rb -Sr system. Al ternat ive-

ly , and pro bably far mo re l ikely , i t m ay suggest that the t rue

Rb-S r age m us t l i e a t t he lower l imi t o f the e r ro r bar o f the

isoch ron age (198 -4- 34 M a) a nd that this intrusion is no

older than

ca.

165/via.

Co ex i s t ing b io t i te and hornb lende f rom samples defm-

ing the 246 : : 17 Ma R b-S r age at Rio Pi tuca (CC R/87/22B

and E) gave co ncordan t resu l ts , w i th a mean o f 180 + 8 M a,

in good agreement w i th bo th the K-Ar and Rb Sr resul ts

f rom L a Paz . Sam ples from the sou th o f Pa landa (CCR/87 /

26B, C and E) aga in y ie lded good agreement fo r co -

exis t ing mineral pai rs , wi th a mea n of 179 + 5 M a for three

pairs . This a ge is jus t wi thin the error of the rather p o ~ Rb-

Sr date o f 144:1:35 M a and thus , in th is case, could be in-

terpreted to suggest that the t rue R b-S r age l ies at the upper

l imi t o f the e r ro r bars . How ever , t he co inc idence o f th is R b-

Sr age w i th the K- Ar at Paqu lsha could al ternat ively sug-

ges t t he fa i r ly com mo n and w el l -documented phenomenon

of rese t ting o f the Rb -Sr sys t em by co o l hydro thermal c i r -

culat ions during a

ca.

150 Ma e vent that d id not dis turb the

K-At sys t ems . Severa l o th

Aspden et al., 1992b.pdf

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