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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
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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
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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.
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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
o r th o g n e i s s e s
CRSH/89/11G-J Biotite muscovite + tourmaline granitic Qda. Cobalera
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, 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)
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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) .
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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 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 granodiorite
Hornblende biotite granodiorite
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
Biotite granodiorite
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 -
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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
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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.
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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
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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
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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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R E S E T / D I S T U R B E D A G E S
ll 1 2 0 1 3 0 1 4 0 1 5 0 lt SO 2 1 0 2 = 0 2 3 0 2 4 0 M 0i 7 l iJ 19
2
Fig. 4. Histogramof K-Ar mineral ages listed in Table 4 obtainedfrom the Zamora batholith.
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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