New geochronological control for the tectono-magmatic evolution of the metamorphic basement, Cordillera Real and El Oro Province of Ecuador

Journal of South American Earth Sciences, Vol. 6, No. 1/2, pp. 77-96, 1992 Printed in Great Britain

Pergamon Press Ltd & Earth Sciences & Resources Institute

New geochronological control for the tectono-magmatic evolution of the metamorphic basement, Cordillera Real and E! Ore Province of Ecuador J. A. ASPDEN I, S. H. HARRISON2, and C. C. RUNDLE3 1Misi6n Briffmica/ODA, FCO (Quite), King Charles Street, London SW1A 2AH, UK 268 Gaim Terrace, Aberdeen AB1 6AT, UK; 3NERC Isotope Geoscience Laboratory, Keyworth, Nottingham, NG12 5GG, U K (Received May 1992; Revision Accepted July 1992)

Abstract--Some 150 new isotopic age determinations on metamorphic rocks from the Cordillera Real and parts of E1 Ore Provinee in Ecuador, using K-At, Rb-Sr, and Sm-Nd methods, help to clarify a complex succession of magmatic and tectonic events. The earliest regional metamorphic/plutonic event recognized, from the Tahuin Group in El O1"o,is dated as between ca. 220 and 200 Ma (Late Triassic-Early Jurassic). Similar but less well constrained ages were also obtained from orthogneiases of the Sabanilla and Tres Lagtmas subdivisions in the Cordillera Real. Major eale-alkaline granitdids were emplaeed ca. 190-150 Ma (Middle-Late Jurassic) in the eastern part of the Cordillera Real, to the north of 2"S, and throughout the sub-Andean zone. Between ca. 140 and 120 Ma (Early Cretaceous), the Oriente region was uplifted and eroded and the Cordillera was affected by an important shearing (dynamothermai) event which resulted in the resetting of older plutonic ages. From ca. 120 to 85 Ma, conditions were relatively stable, but during ca. 85 to 65 Ma (Late Cretaceous), the Cordillera and Oriente were again uplifted. This uplift corresponds to a second widespread thermal overprinting, which produced a regional disturbance in the K-Ar isotopic systems. Throughout the Cordillera, a number of generally small, undeformed, dominantly lower Tertiary plutons are also present. A few older (i.e., pre-Mesozoic) 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 clarificar una sucosi6n eompleja de eventos magrn~lticos y tect6nicos. El evento metam6rfico/plut6nico regional m ~ temprano reconocido, es el del grupo Tahufn en E1 Ore; est~ entre ca. 220 y 200 Ma (TriMico tardfo-JurMico temprano). Edades similares, pete menos definidas, fueron tambi6n obtenidas de los ortogneises de Sabanilla y Tros Lagunas en la Cordillera Real. Los mayores granitoides ealco-alcalinos fueron emplazados ca. 190-150 Ma (Jur~sico medic a tard(o), en la parte oriental de la Cordillera Real, al norte de 2" de latitud S, yen toda la zuna subandina. Entre ca. 140 y 120 Ma (CrelAcicotemprano) la regi6n Oriental fue levantada y erosionada; y la cordillera rue afectada per un evento de cizallamiento muy importante (dinamotermal), que result6 en el reajuste de 1as antiguas edades plut6nicas. Desde ca. 120-85 Ma las condiciones fueron relativamente estables, pete durante ca. 85-65 Ma (Cretatcico tardfo) la cordillera y el Oriente fueron de nuevo levantadas. Este levantamiento corresponde a una segunda sobre impresi6n termal que produjo una perturbaci6n regional en los sistemas isot6picos K-At. Per toda la cordillera est,Lnpresentes un ndmero de plutones generalmente pequetlos, no deformados; dominantemente del Tereiario temprano. Hart side obtenidas pocas edades antiguas (Premesozoicas), pete la interpertaci6n de 6stes permaneee todavfa incierta. INRODUCTION 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 Project is a j o i n t T e c h nical C o o p e r a t i o n P r o g r A m m e undertaken by the Overseas D e v e l o p m e n t A d m i n i s t r a t i o n ( O D A ) o f Great Britain through the British G e o l o g i c a l S u r v e y ( B G S ) in c o n j u n c tion w i t h the C o r p o r a c i 6 n de 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 i c a ( C O D I G E M ) in Ecuador. ' S i n c e the start o f the project in 1986, advances h a v e b e e n m a d e in the understandin~ o f the stratigraphy and structural e l e m e n t s in the Cordillera R e a l and parts o f E l 0 r e P r o v i n c e in s o u t h w e s t E c u a d o r (Fig. 1). S o m e o f these f i n d i n g s h a v e already b e e n p r e s e n t e d ( e . g . , Litherland e t

\ / / /

lib.

Fig. 1. Principal geomorphologieal/geologieal zones of Ecuador. Dot pattern indicates metamorphic reeks of the pre-Cretaceous basement.

Address all correspondence and reprint requests to Dr. John A. Aspdea at BGS International Division, Keyworth, Nottingham NG12

5GG, UK: telephone[44](602) 363100; fax [44] (602) 363-200; relent378173 B G S K E Y G. ©1992 Crown Copyright 77

78

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

al., 1990; Aspdon and Litherland, 1992). In this contribution we concentrate solely on documenting the geochronological data that have been obtained. The isotopic analyses were carried out at the Natural Environment Research Council's Isotope Geology Centre in London - - now renamed the National Isotope G-eosciences Laboratory (NIGL) and relocated at Keyworth, Nottingham.

Geographical Setting The Cordillera Real is the eastern of two parallel mountain chains that define the Ecuadorian Andes. In the north, the Western Cordillera is separated from the Cordillera Real by a prominent structural valley, the Inter-Andean Depression, but in the south the Andes are represented by a single cordillera. To the east of the Ecuadorian Andes lies the sub-Andean zone and the Oriente, which form part of the upper reaches of the Amazon Basin. To the west lies the flat, low coastal region of the Costa (Fig. 1).

Geological Background North of Guayaquil, the Costa comprises Upper Cretaceous to Cenozoic fore-arc sedimentary rocks floored by Lower Cretaceous oceanic basalts of the Piflon Formation (Baldock. 1982; Goossens and Rose. 1973). There is no evidence of continentalcrust below these rocks (Feininger and Seguin, 1983). This part of Ecuador is thus thought to represent oceanic crust that was accreted to the South American plate in the Late Cretaceous or Paleocene (Bourgois et al., 1990; Daly, 1989). In contrast, south of Guayaquil, the rocks of El 0ro Province (Fig. 1) consist mainly of granitic plutons and metamorphic rocks, which include amphibolites, schism, and gneisses. The Western Cordillera comprises a NNF_,-trevdinS belt of Cretaceous to lower Tertiary volcanic, volcaniclastic and sedimentary rocks that have been reported on by Van Thoumout and Quevedo (1990). Lebrat et al.. (1985), and Henderson (1979). The inhospitable nature of the CcediUera Real, with its high altitude and abundant rainfall, together with limited road access, has hindered study of the geology of thisregion of Ecuador. The Cordillera Real forms a continuous belt of variably deformed and metamorphosed rocks that extends the length of the Ecuadorian Andes and COnSiStsof schists, quartzites,calc-schists,marbles, and ortho- and paragneisses (Aspden and Litherland, 1992). A number of late,undeformed plutons cut the metamorphic rocks, and a series of major Plio-Pleistocene stratovolcanoes dot the Cordillera. Overlying the basement of the Oriente,which comprises rocks belonging to the Amazmic cratou (Almeida et aI., 1981), are epi-platform Paleozoic and lower Mesozoic sedimentary strata. These are overlain by Upper Jurassic volc~n~ and Upper Cretaceous marine miogeosynclinal sedlmantary rOCks. Following the Andean uplift, back-arc sedimantation occurred during the Cenozoic (Jalliard et al., 1990; Baldock, 1982; Tschopp, 1953).

PREVIOUS GEOCHRONOLOGICAL STUDIES The apparent correlation of the predominantly Paleozoic metamorphic rocks of the Cordillera Central of southern Colombia with those of the Cordillera Real has persjaded some to suggest a similar age for the latter (e.g., BaldocL 1982). Equally, in the south, the CordilleraReal metamorphic belt has been correlated lithologically with the basement rocks of northern Peru (Kennerly. 1980). which are overlain by Triassic and possibly Devonian sedimentary rocks (JaUiard et al., 1990; Cobbing et al., 1981).

Metamorphic Rocks of the Cordillera Real Previously published geochronological studies of the CordilleraReal metamorphic rocks have reliedentirelyon the K-At technique.Results from some of the more importantlocalities(Fig.2) are noted below: a) Herbert and Pichler (1983) presented K-At dates of 59 + 2 M a from muscovite and biotite separates from schistswhich crop out along the Papallacta-Baezaroad. A ~imilar K-At biotite date of 54 + 2 Ma was recorded by Feinlnser and Silberman (1982) from the same area, and a slightly older age of 82 + 3 Ma was reported bv Kennedy (1980) for a muscovite sample. b) Between Baflos and Puyo. garnet muscovite biotite para- and orthogneisses are exposed near Agoy(m. Herbert and Pichler (1983) analyzed muscovite separates from both of these rock types and obtained ages of 56.5 + 2 and 60 + 2 Ma, respectively. Hall and Calle (1982) reported six K-Ar ages from gneisses along this road section, ranch S from 54 to 79 Ma. c) Hall and Calle (1982) quoted K-Ar ages of between 61 and 90 Ma (three determinations) for metamorphic rocks from the Cuenca area and ages between 51 and 79 Ma (three determinations) for rocks in the Zamora area. Baldock (1982) also reported a K-Ar biotite age of 52 + 2 Ma for gneiss collected from the Loja-Zamora road section. d) Kennerly (1980) recorded two K-At ages of 72 + 2 and 81 + 3 Ma from biotite gneisses near Palanda and Zumba in the extreme south of Ecuador. Based on these data, Feininser (1982) and Hall and CaUe (1982) interpreted the Cordillera Real metamorphic belt as predominantly of Late Cretaceous to early Tertiary age.

Metamorphic Rocks of E10ro Province The metamorphic rocks of El Oro Province (Figs. 1 and 2b). which strike E-W, are oblique to the NNE trend of the Cordillera Real, and the contact between the two belts is hidden by younger sequences. The rocks of El Oro Prov. ince include a central core of amphlbolite, the Piedras Group (r-,eininee.r,1978). dated as Precambrian by a single K-At age obtained from a hornblende separate (743 + 14 Ma; Kennerly, 1980). However, hornblende determinatkms from similar amphibolites in the province have yielded ages of 196 + 8 Ma and 74 + 1 Ma (r-,einin~,er and

New geochrcmologicalcontrol fox the tectono-mac,m~tic evolution of the metamorphic basement, Ecuador

(a)

79

(b) .'

\

oo'w

-.COLO~

Pimompiro~ . ~ . ~ . ~ 7

j/

--OoO0 '

|ou,,o

j

Ni

~7": ...:-....-:.-;:-f w Baeza

IqO0'S Teno•

Arnobato

[]

Metamorphic rocks (ff CordHlera Real

I T : :

Lag . . . .

grani.

•Mare ePuya

UNDEFORMED PLUTONS (~ ® ~) (~

eRiobamba

Pimampiro Condue Azuela Pungala

~oo'w

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Fig. 2. Location and simplifiedgeological map of the CordilleraReal/sub-Andeanzone: a) betweenthe Colombianfrontier and 2"S (based on Litherland et al., 1990); b) between 2°S and the Peruvian frontier (based on Litherlandet al.. 1990), inset map of E10ro metamorphicprovinceafter Baldock(1982). Silberman, 1982), throwing the Precambriau age assigned to these rocks into considerable doubt. The Hedras e0nvelops the Raspas b l ~ t complex (~inin~, 1980, 1978) (Fig. 2b), fro"which a K-At age of 132 ± 5 Ma (phengite) has been obtained (Feininoer and Silbermsn; 1982). Both the Pie&as Group and the Raspas complex are bounded to the north and south by low- to high-grade semi-pelific rocks and variably d e f o m ~ granites of the Tahuin Group (Fig. 2b). Feininger (1982) has interpreted this group to be D~cmisn in age, based on a brac.hiopod foLmdin a weakly metamorphosed quartzite in northwest Peru. However, F e i n i n ~ and Silbe~man (1982) obtained an age of 210 + 8 Ma hem biotite separated from a pelitic gneiss of the Tabuin Group, which they considered to be the age of uplift. SAES6:1/2-F

GraniticRocks

Three majm" NNE-trendins ekmsate granitoid batholiths occur in the sub-Andean zone alto8 the eastern tectonic margin of the Cordillera Real. From north to south, these are the Rosa Florida, Abitagua, and Zam~a batholiths (Fig. 2). The Abitasua batholith has received the most attenticm. Kennerly (1980) reported a K-Ar age of 87 + 7 Ma co a biorite separate, whereas Herbert and Pichler (1983) recorded 178 + 7 Ma from a biotite separate at a nearby locality (both samples were collected aloe8 the Bsflos-Puyo road) 0r18.

2a). A K-Ar age of 171 ± 6 Ma was obtain_ by Aly (1980) from the Zamcca batholith, and K-At dates of 173 ± 5 Ma

80

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

(hornblende) and 180 + 5 Ma (biotite) have been published by Kennerly (1980) for a single sample collected to the east of Palanda (Fig. 2b). Originally, this intrusion was thought to be a separate pluton (Baldock, 1982), but more recent work has shown that it forms part of the regionally extensive Zamora batholith (Aspden and Litherland, 1992). Within the eastern part of the Cordillera Real north of 2°S, are the variably deformed, often gneissic, Azafran and Chingual batholiths. To the west lies a distinctive suite of generally foliate& garnet biotite + muscovite granites of the Tres Lagnnas subdivision. In the southeastern part of the Cordillera is the Sabanilla subdivision, a mixed unit (Fig. 2) which is dominated by biotite + muscovite + garnet orthogneiss but also includes migmatites, paragneisses/ schists and amphibolites. Two samples from the Tres Lagunas subdivision east of Saragnro (Fig. 2b) gave K-Ar ages of 76 + 1 Ma (biotite) and 173 + 4 Ma (plagioclase) (Kennerly, 1980). K-Ar dates obtained from the Sabanilla subdivision west of Zamora and from the Palanda and Zumba areas (Fig. 2b) have been referred to earlier. Other smaller plutonic bodies in the Cordillera Real appear as essentially undeformed granitoids, many of which show intrusive relationships with the metamorphic rocks. They include the Pimampiro, Magtayan, Amaluza, and San Lucas plutons (Fig. 2). Various K-Ar ages. ranging from Late Cretaceous to Tertiary, have been previously published for these plutons: 72 + ? Ma from Pimampiro (Evemden, 1961); 85 + 3 Ma (hornblende), 75 + 3 Ma Oaomblende). and 54 + 2 Ma (K-feldspar) from Magtaygm (Kennerly. 1980); 34 + 1 Ma to 49 + 2 Ma from Amaluza and 70 + 2 Ma to 50 + 2 Ma from San Lucas (Herbert and Pichler, 1983; Kennerley, 1980). The above suggests that although isotopic data exist for the metamorphic and plutonic rocks of the Cordillera Real and El Oro Province, the actual ages of the main tectonomagmatic events remain poorly defined. The aim of this investigation was thus. first, to clarify the age of metamorphism, using a combination of K-Ar (hornblende. biotite, and muscovite) and Sm-Nd (garnet/whole-rock pairs) methods, and second, to date the main plutous by the Rb-Sr whole-rock isochron method.

ANALYTICAL TECHNIQUES

Sampling and Rock Preparation Sampling was restricted largely to road cuts and incised fiver beds. Wherever possible, samples were taken from in situ outcrops, but some of the least altered samples were from large boulders. For the Rb-Sr whole-rock analyses, after initial jawc~l~ing _and roller milling, representative 200 g subsamples were removed using a riffle splitter and powdered in a tungsten carbide Swing-mill. For samples requiring mineral separation, the roller-milled material was sieved and the + 65 to -200 mesh fraction was washed in distilled water to remove any free powder. Some initial coarse mineral separation was carried out in Ecuador. using heavy

liquids, but most of the purification was completed in the UK, using a super-panner and Frantz magnetic separator.

Rb-Sr Amllysis Rb-Sr analyses were carried out on whole-rock powder samples from meta-plutonic rocks and orthogneisses. Rb/ Sr ratios were determined by X-ray fluorescence using an automated Philips spectrometer. For the isotope ratio determinations, strontium was extracted from the samples using acid dissolution and ion exchange methods in a cleanchemistry laboratory and analyzed with an automated multi-collector VG354 mass spectrometer. The Rb/Sr ratios are quoted with a blanket error of :L-0.5% (1-sigma). Replicate analyses of samples and standards suggest that a reproducibility of _+0,005%is appropilate for the strontium isotope measurements. Replicate analyses of international standards indicate that the results are accurate within the precision estimates. The errors on age and initial ratio (Ri) are quoted as 2sigma (95% confidence level) and refer to the last significant figure. Best-fit lines on the isochron diagrams were calculated using a least-squares fitting program. An MSWD (mean square of weighted deviates) exceeding 3.0 means that the points do not all fit the line within the limits of analytical error and, following conventional practice, the errors on age and intercept have been enhanced by multiplying by the square root of the MSWD. All ages were calculated using a decay constant for 87Rb of 1.42 × 10 -11 a--l.

Sm-Nd Analysis This technique was used on whole-rock and garnet pairs, relying on the fractionation of the rare earth elements in garnet relative to the host whole-rock. Sm and Nd were analyzed by a double isotope dilution method. Powdered whole-rock and garnet samples were dissolved in acid with an added mount of a Sm-Nd mixed spike. Both the Sin and the Nd were then exlracted using ion exchange methods and separately analysed on the mass spectrometer. Errors in the Sm/Nd and the 143/144 Nd Analyses are quoted as 0.2% and 0.005% (1-sigma), respectively, again based on replicate analyses of international standards. The results are presented in the form of isochron diagrams similar to Rb-Sr, and the techniques used in calculating the best-fit lines, ages. and errors are the same.

K-Ar Analysis K-At analyses were carried out predominantly on biotite, muscovite, and hornblende separates and only rarely ou whole-rock samples. This technique was used on all suite,s collected where the appropriate unaltered minerals were present to support either the Rb-Sr or the Sm-Nd resuits.

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

81

Potassium was determinod, at least in duplicate, using an Instrumentation Laboratories IL543 flame photometer with lithium as internal standard. Argon was extracted by fusion under vacuum using external radio-frequency induetion heating and analyzed by the isotope dilution method in a VG Isotopes MM1200 mass spectrometer.

against international standards, so the results can be expected to be accurate within the limits of analytical error. The ages were calculate~l using the constants recommended by Steiger and Jaeger (1977), and the error on the age is quoted at the 95% confidence level.

Replicate determinations of in-house standards suggest that an overall precision of +1% (1-sigma) is realistic for the potassium analyses. The error in the radiogenic argon determination is partly dependent on the amount of contaminating atmospheric argon, which often reflects the degree of deuteric alteration and hence varies considerably between samples. The argon spike system was calibrated

RESULTS The samples collected for dating axe listed in Table 1. The Rb-Sr and Sm-Nd analyses are given in Tables 2 and 3, respectively, and the K-Ar data. with the calculated ages. are presented in Table 4. These results are discussed below. and the localities mentioned in the text are shown in Fig. 2.

Table 1. Location and description of samples collected for isotopic analysis. Sample No.

Rock type(s)

Grid Reference

Area

Map Sheet*

Bafios-Puyo road

Batios (e)

7939-8458/ 7918-8457

Garnet Gnetsses, Agoydn CCR/87/11A-E

Garnet biotite muscovite sehists/gneisses

Garnet Gneisses/Amphibolite, Papallacta CCR/87/4

Biotitic amphibolite

Papallacta village

Papallacta (c)

8184-99596

CRSH/89/1A

Garnet amphibolite

Float block, Rio Chalpi Grande

Papallacta (c)

8246-99608

CRSH/89/1B-C

Garnet biotite 4-muscovite gneisses

Float blocks, Rto Chalpi Grande

Papallacta (c)

8246-99608

Sabantlla Subdivision Garnet Gnetsses/Amphtbolite, Valladolid CCR/87/24A

Amphibolite

North of Valladolid

Valladolid

7079-94983

CCR/87/24B

Muscovite pe~aatite

North of Valladolid

Valladolid

7079-94984

CCR/87/24C

Muscovite pegmatite

South of Valladolid

Valladolid

7075-94935

CCR/87/24D

Biotite pegmatite

Near Palanda

Valladolid

7074-94868

CRSH/89/10A-D

Garnet-bearing gneisses/migmatites

Float blocks, Rio Valladolid

Valladolid

7075-94976

Sabantlla Subdivision Orthognetsses, Lo]a-Zamora Road CCR/87/23A-H

Biotitic orthogneisses

East of Sabanilla

Loja Norte

7199-9562/ 7199-95588

CRSH/89/12A-C

Biotite orthogneisses

East of Sabanilla

Loja Notre

7199-95587

CRSH/89/12D-J

Migmatitic biotite orLhogneisses

East of Sabanilla

Loja Norte

7197-95600

FV57/FV58

Biotitic orthogneisses

East of SabaniHa

Loja Norte

7194-95614

Float blocks, Rfo Santa B/u'bara, Peggy Mine

Sigsig

7476-96578

Tres Lagunas Subdivision Orthogneisses, South o f Sigsie CCR/87/14A-D

Biotite orthogneisses + igneous xenolith (14C)

Tres Lagunas Subdivision Orthognetsses, North Edge of Malaeatus Basin CRSH/89/11A-F

Biotite + muscovite ± tourmaline granitic orthogneisses

Qda. La Pieota

Nambacola

6917-95396

CRSH/89/11G-J

Biotite ± muscovite + tourmaline granitic orthogneisses

Qda. Cobalera

Nambacola

6914-95399

Saraguro

ca. 712-9604

Tres Lagunas Subdivision Orthogneisses, Tree Lagunas, East of Saraguro CRSH/89/14A-K

Biotite 4-muscovite orthogneisses + aplitie variant (14K)

Rfo Negro

* 1:50,000 Topographic Sheet, published by Instimto Geogr~co MAlitarQuito; (c) indicates uncontrolled topographic base map without contours (censal).

(continued)

82

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

Table 1 (continued) Sample No.

Rock type(s)

Area

Map Sheet*

Grid Reference

Arenillas bridge

Arenillas

6049-96072

West of Poctovela

Zamma

8519-95882

Rio Piedras north of La Bocana

La Avanzada

6213-95955

Ptedras Group ArenUlaz Amphtbotit¢ CRSH/89/5A-B

Amphibolites

Ptedras Group Portovelo AmpMboUte CRSH/89/SA-B

Amphibolites

Tahuin Group Garnet Gneines CRSH/89/6A-E

Garnet biotite 8neisses and felsic peEmatites

Tahuin Group Pesma~t~ Gnclsses CRSH/89/TA-B

Biotite muscovite granite and muscovite tourmaline pegrnatite

Rfo El Negro south of La Bocana

Marcabefi

6218-95911

CRSH/89/19

Muscovite tourmaline pegmatite

Float block, Rio Pie.Areas at La Bocana

Maroabeli

6219-95927

Tahuin Group MarcabeU Pluton CRSH/89/4A-E

Biotite muscovite granites

Balsas quarry

Marcabeli

6308-95837

CRSH/89/4F-J

Biotite muscovite granites

Southwest of Marcabeli

Marcabeli

6188-95775

CCR/gT/16A-H

Hornblende granodiorites/homblende diorires + felsic vein (16D)

La Pax area

Yantzaxa

7362-95864/ 7369-95845

CCR/87/17

Hornblende diorite

Float block, Qda. Curishp¢, south of La Pax

Yantzaxa

7368-95845

CCR/87/18

Porphyritic hornblende feldspar andesite

Float block, Qda. Curishpe, south of La Paz

Yantzaxa

7368-95845

CCR/87/19

Hornblende granodiodte

Qda. Maycunantza, south of La Paz

Yantzaxa

7351-95830

CCR/87/20

Hornblende biotite granodiorite

South of La Pax

Yantzaxa

7340-95783

CCR]87/21A-J

Hornblende biotite granodiorites + felsic vein (21C) + partially digested xenolith (21D)

South of Qda. Chapintza, Paquisha area

Guaysimi

7660-95530/ 7652-95540

CCR/87/22A-F

Pink porphyritic biotite hornblende (?)monzogranites + hornblende microgranodiorite (22F)

Rio Pituca area and Rio Jambue

Zamora

7294-95428 7288-95432 (22F)

CCR/87/25

Porphyritic hornblende andesite dike

Palanda-Zumba road

Rio Mayo

7074-94804

CCR/87/26A-E

Hornblende biotite granodiorites/diorites

Palaada-Zumba road

Rfo Mayo

7074-94809/ 7075-94781

CRSH/89/13A-B

Hornblende diorites

Rfo Chicana east of La Pax

Yantzaza

7432-95930

FV60

Porphyritic hornblende granodiorite/diodte

Float block from Guaysimi south of Paquisha

Guaysimi

7575-95527

RM1

Hornblende biotite granodiorite

Rio Mayo

Zumba

7144-94536

FV681

Hornblende biotite granodiroite

East of palanda

Vailadolid

7218-94880

FV485

Hornblende biotite granodiorite

Qda. de Loa Derrumbes east of Valladolid

Valladolid

7175-94972

CCR/87/5A-I

Hornblende biotite granodiorite + felsic vein material

Cosanga-'l~na road (ca. 55 km north of Tena)

Cosanga (c)

CCR/87/6A,B,D, G-K

Hornblende biotite granodiorites + felsic vein material

Bafioa-Puyo road

Mera (c)

8131-98442/ 812%98444

CCR/87/6C,E,F

Pink porphyritic hornblende biotite granodiorites

Bafloa-Puyo road

Mera (c)

8148-98405/ 812%98444

CCR/87/7

Hornblende andesite dike

Bafloa-Puyo mad

Mera (c)

8147-98404

ADML5

Hornblende granodiorite

Float block, Rio Zuflag, BaflosPuyo road

Mera (c)

8127-98444

Zamora Bathollth

Abitagua Batholtlk

* 1:50,000 Topographic Sheet, published by Instituto Geogr~ico Militar Quito; (c) indicates uncontrolled topographic base map without contours (censal).

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

83

Table 1 (continued) Sample No.

Grid Reference

Rock type(s)

Area

Map Sheet*

CCR/87/SA-I

Leucogranites + aplite vein (SD + quartzfeldspar pegmatite (8C)

Baflca-Puyo road

Baflos (c)

8058-98448/ 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, BaflosPuyo road

Bafios(c)

8009-8452

Biotite orthogneisses

Northwest of Pimampiro

Huaca (c)

8869-100605/ 8871 - 100595

Hornblende biotite diorite

Qda. Tungurahua

Huaca (c)

8834-100690

CCR/87/1A

Hornblende granodiorite

Near Mataqui

Pimampiro

1744-00420

CCR/87/1C

Hornblende granodiorite

Qda. Manzanal

Pimampiro

1785-00438

Hornblende biotite diorites and hornblende gabbro (13B)

Osogochi area

Totoras

7678-97580/ 7621-97520

Principal (c)

7650-96663

Azafl~n Bathollth

CIiiagual Batholith, San~ Bdrbara.La Bonita Road CCR/87/2A-J

Sacha Pluton CCR/87/3

Plmampb'o Pluton

Ma~taydn Pluton CCR/87/13A-C

Unnamed Pluton, Cuenca-Ltm6n Road

FV83

Biotite granodiorite

San Lueas Pluton CCR/87/28A-C

Pink porphyritic biotite granediorites

Qda. Tunttln

Santiago

6933-95849

FVI1

Hornblende granodiorite

Qda. Bucashi

Santiago

6928-95857

FV15

Hornblende biotite granodiorite

FV34

Biotite granodiorite

Juntas Qda. E1Gallo

6948-95785

Loja Norte

6985-95740

Cola de San Pablo

7625-97080

Tampanchi Marie lgneous Complex CRSH/89/17A-C

Hornblende gabbro, pegmatitic hornblendites and hornblende basalt

Catamayo Pluton CCR/87/29A-B

Biotite granodiorite

Loja-La Toma road

Catamayo (La Toma)

Biotite granodiorite

Rfo Pinchinal

Saraguro

7045-95999

Guamote and Riobamba

7680-97965/ 7680-98000

PtcMnal Pluton CRSH/89/15

Pungald Pluton CCR/87/12A-C

Hornblende biotite granodiorites

P o r ~ h u e l a Batholt.~, Track.from Jtmbura to Zumba CCR/87]27A-B

Biotitic felsic porphyry

..........

Laguna Cox

6773-94723

CCR/87/27C-G

Hornblende biotite granodiofites and diorites

..........

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).

Metamorphic Rocks of the Cordillera Real Metamorphic rocks from the Cordillera Real were collected from four separate localities: Papallacta (on the road between Quito and Baeza). Asoy~n (between Bafios and Puyo), east of Sabaniila (between Loja and Zamoxa), and

the Valladolid area in southern Ecuador. Although a combination of K-At, Rb-Sr, and Sm-Nd data has been ohmined frown these rocks, the resultsare far from conclusive. T w o suitesof orthogmiss from the Sabanilla and VaUadolid areas were dated by the Rb-Sr method, but both data sets show a wide scattexon the isochrcm diagrams. Never-

84

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

Table 2. Rb-Sr analytical data.

Sample No.

Rb

Sr

gTRb

$7Sr

~Sr

S6Sr

Sabanilla Subdivision Orthogneisses, near Zamora

Sample No.

Rb

8/Rb

gTSr

Sr

S6Sr

SeSr

23.4

19. 85

Abitafua BathoHOt (continued)

CRSH/89/12A

106.2

188.7

1.6714

0.71801

CCR/87/5E

CRSH/89/12B

97.8

207.8

13973

0.71690

CCR/87/5F

66.5

421

0.4574

0.70560

CRSH/89/12C

83.7

182.5

13628

0.71686

CCR/87/5G

92.3

389

0.6868

0.70620

327

0.9044

0.70664

160

CRSH/89/12D

82.8

178.6

13767

0.71671

CCR/87/SH

CRSH/89/12E

104.0

204.9

1.5065

0.71717

CCR/87/5I

150

CRSH/89/12F

100.3

191.3

1.5575

0.71740

CCR/87/6B

132

CRSH/89/12G

117.5

209.9

1.6629

0.71742

CCR/87/6D

130

98.5

CRSH/89/12H

87.9

188.6

13848

0.71671

CCR/87/6G

225

15.2

CRSH/89/I 2I

82.7

176.2

13946

0.71670

CCR/87/6H

102

CRSH/89/12J

73.5

214.5

1.0175

0.71596

CCR/87/61

103

93.1

CCR/87/23A

123

204

1.747

0.71788

CCR/87/6J

CCR/87/23B

110

197

1.601

0.71774

CCR/87/6K

201

0.6521

0.71436

Zamora Batholith, La Paz Area

CCR/87,r23c

45.2

235 54.9

54.1 428

0.74963

8.016

0.72298

0.8886

0.70667

3.821

0.71348

43.31

0.80394

355

0.8319

0.70670

382

0.7041

0.70615

10.3 959

67.85

0.86170

0.1659

0.70494

5.190

0.71840

CCR/87/23D

128

208

1.768

0.71776

CCR/87/16D

82.6

CCR/87/23E

119

210

1.633

0.71682

CCR/87/16E

46.7

247

0.5469

0.70609

CCR/87/23F

128

192

1.931

0.71716

CCRf87/16F

51.1

238

0.6231

0.70622

231

1.205

0.71546

CCR/87/16G

14.9

374

0.1160

0.70499

124

2.833

0.72173

CCR/87/16H

26.1

270

0.2802

0.70530

391

0.4904

0.70631

CCR/87/23G CCR/87/23H

96.3 121

Tres Lagunas Subdivision Orthogneisses

46.2

Zamora Batholtth, Paqutsha Area

CRSH/89/11A

124.5

142.0

2.6054

0.71922

CCR/87~1A

66.2

CRSH/89/I 1B

124.6

138.3

26795

0.71994

CCR/87/'21B

70.7

367

0.5582

0.70665

CRSH/89/11C

129.5

133.8

2.8755

0.71967

CCR/87/21D

63.9

432

0.4281

0.70629

CRSH/89/11D

117.5

144.6

2.4150

0.71883

CCR/87/21E

79.1

391

0.5844

0.~

CRSH/89/11E

126.1

137.1

27307

0.71975

CCR/87/21F

96.7

339

0.8275

0.70734

CRSH/89/11F

131.9

168.0

23324

0.71871

CCR/87/21G

62.8

364

0.4992

0.70635

CRSH/89/11G

138.7

99.5

4.1415

0.72156

Zamora Batholith, R fo Pituca Area

CRSH/89/11H

134.3

131.1

3$)438

0.72075

CCR/87/22A

373

0.8170

0.70660

CRSH/89/11I

135.1

129.4

3.0908

0.72082

CCR/87/22B

181

1.139

0.70770

CRSH/89/14A

189.7

95.0

5.9439

0.72867

CCR/87/22C

103

387

0.7701

0.70645

CRSH/89/14B

174.8

106.9

4.8(~6

0.72590

CCR/87/22D

107

385

0.8054

0.70649

CRSH/89/14C

186.7

93.3

5.9499

0.72893

CCR/87/22E

96.8

388

0.7213

0.70640

CRSH/B9/14D

182.7

102.0

53283

0.72839

CCR/87/22F

59.2

674

0.2545

0.70460

CRSH/89/14E

175.1

97.3

53507

0.72684

Zamora Batholith, Palanda Area

CRSH/89/14G

186.3

97.7

5.6710

0.72905

CCR/87/26A

58.6

329

0.5161

0.70617

CRSH/89/14H

197.0

85.8

6.8323

0.73043

CCR/87/26B

56.0

335

0.4848

0.70612

105 71.8

CRSH/89/14I

173.7

103.2

5.0067

0.72579

CCR/87/26C

53.5

325

0.4763

0.70599

CRSH/89/14J

169.8

109.9

4.5905

0.72520

CCR/87/'26D

47.3

359

0.3821

0.70592

CRSH/89/14K

144.7

102.1

4.1989

0.72379

CCR/87/26E

42.4

353

0.3476

0.70578

A ldtagua Batholtth CCR/87/SA CCR/87/5B CCRhl7/5C CCR/87/5D

Azafran Batholith 159 85.9 156 87.0

22.7 285 26.3 259

20.42 0.8717 17.29 0.9730

0.75183

CCR/87/8A

100

86.5

3.345

0.71029

0.70652

CCR/87/SB

127

70.8

4.817

0.71291

0.74410

CtI{/87/SD

109

77.1

4.074

0.71160

0.70677

CCR/87/SE

110

75.5

4.229

0.71171

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

Table 3. Sm-Nd analytical data for the Tahuin Group garnet orthogneiss (219 + 22 Ma)

Table 2 (continued)

Sample No.

Rb

85

87Rb

STSr

Sr

SSSr

S6Sr

CCR/87/SF

111

71.0

4.495

0.71223

CCR/87/8G

111

80.5

3.984

0.71147

CCR/87/SH

104

60.3

5.004

0.71309

Chingual Batholith CCR/87/2B

38.2

467

0.2368

0.70414

CCR/87/2C

44.4

353

0.3640

0.70450

CCR/87/2D

46.3

335

0.3999

0.70460

CCR/87/2E

31.6

517

0.1766

0.70406

CCR,t87/2F

30.6

507

0.1748

0.70402

CCR/87/2G

31.2

507

0.1779

0.70413

CCR/87/2I

44.7

417

0.3106

0.70428

CCR/87/2J

46.6

405

0.3326

0.70433

123

3.049

0.70703

5.561

0.70887

0.9165

0.70536

143Nd

Sm (ppm)

Nd (ppm)

144Nd

144Nd

CRSH/89/6A (wr)

5.59

30.63

0.1102

0.512075

CRSH/89/tA (gt)

4.56

14.52

0.1898

0.512220

CRSH/89/6B (wr)

7.63

37.41

0.1232

0.512132

CRSH/89/tB (gt)

6.62

23.54

0.1700

0.512170

CRSH/89/6C (wr)

6.33

34.64

0.1105

0.512074

CRSH/89/6C (gt)

4.22

10.73

0.2377

0.512280

CRSH/89/6D (wr)

7.92

40.13

0.1193

0.512111

CRSH/89/6D (gt)

5.53

16.74

0.1997

0.512245

CRSH/89/6E (wr)

7.23

38.83

0.1126

0.512099

CRSH/89/6E (gt)

4.83

14.93

0.1956

0.512237

Sample No.

Azafran BaOtoliOt (continued)

147Sm

Key: wr, whole rock; gt, garnet.

San Lucas Pluton CCR/87/28A

130

CCR/87/28B

154

CCR/87,r28c

82.2

79.9 263

theless, data from the Sabanilla orthogneiss yield the best linear correlation and are considered to give the more reliable age of 224 + 37 Ma (MSWD = 108; Fig. 3a). The relatively high initial 87Sr/86Srratio of 0.7123, together with the strongly gneissose character of the rocks, suggests that this is probably the age of metamorphism; thus it is postulated that this took place in Late Triassic-Early Jurassic times. The Valladolid orthogneiss suite, which is more massive in texture but notably weathered, gave a very poorly constrained age of 359 + 99 Ma, with an MSWD of 1877. The wide scatter of these data indicates considerable disturbance of the isotopic systems, and little reliance can be placed on this age. K-Ar data from amphibolites and gneisses from the Papallacta area have given extremely variable ages. Biotite from one sample of garnet biotite gneiss yielded a late Precambrian age (ca. 850 Ma), whereas muscovite from an adjacent block of similar material recorded a Late Cretaceous event (74:1:3 Ma). In contrast, two samples of hornblende from amphibolitic material gave poorly reproducible results, with a mean of 345 :t: 29 Ma, suggesting Devonian to Carboniferous activity. Due to the paucity of exposure, however, most of these analyses were carried out on samples from relatively small, rounded, loose blocks from a river bed, which yielded conflicting data. It is thus difficult to extract any useful information from these. More detailed geological mapping and more specific sampling are required before the presence of pre-Mesozoic rocks in this area can be confirmed. All the K-Ar data from para- and ortho-gneisses of the Sabanilla subdivision near Zamora and VaUadolid gave Late Cretaceous ages, ranging from 85 :l: 2 to 65 + 2 Ma. However, in view of the Rb-Sr data discussed above, which

suggest a Late Triassic-Early Jurassic metamorphism, these are all considered to reflect a major "isotopic event," significantly later than the main gneissification. This interpretation is supported by K-At data from hornblende from an amphibolite dike cutting the garnet gneiss at Valladolid, which has preserved an age of 132 + 5 Ma, presumably reflecting only partial resetting during the Late Cretaceous event. Furthermore, where coexisting pairs of micas were dated (samples CRSH/89/10A,/10C,/12A,/12C), the muscovites (average age 69 + 3 Ma) consistently gave sisnificantly younger ages thnn the biotites (average age 84 + 2 Ma) and also had sionificantly lower K-contents (the reverse of what is normally expected). This pattern may suggest that the muscovites formed at a later stage than the biotites, but this would imply that the event did not result in sit,nif'tcant argon loss from the biotites. Alternatively, and possibly mote likely, it may be that these low-K muscovites have an abnormally low blocking temperature to argon diffusion; this suggestion is supported by the significantly older age (77 + 3 Ma) given by the muscovite from sample CCR/87/24B, which has a more normal K-content. K-Ar dating of garnet gneiss from Agoyan was also rather unsatisfactory. The white mica separated from these rocks proved to be an unusually low-K variety (probably Na-rich paragonite), and hence there are relatively high errors on individual age determinations. Nevertheless, three samples gave concordant results, with a mean of 76 + 3 Ma, in good agreement with the data from the Sabanilla subdivision and presumably reflecting the same Late Cretaceous event. Garnet was separated from samples collected from the Papallacta area and from the Sabanilla subdivision gametiferous gneisses for Sm-Nd analysis. However, in all eight analyzed samples there was little or no fractionatiou of the rare earth isotopes between the garnet and associated whole-rock, and hence the data were of no value for dating pm'lXX~s.

86

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

I

I

I

T

87Sr/86Sr

I

]

I

O. 514

(o)

O. 725

. ....?~, ..... • • """ '"" .....,...""

O. 720

+

I

(b)

........... 4-..--Hl-"-l-

0.5t2

...............

.....

0,5i0

O. 7 t 5 0.710

AGE Intercept

O. 705

I

L

[

I " 2.0

l

O. 508

224 +-. 37 Ma (2s) 0.74.23 4- 0.0008

MSWD 108.6 Enhanced J 4.. 0

0.735

I

~3Nd / 144Nd

l

I

Errors 87Rb/86Sr

AGE 2 t 9 ¢ 22 Me (2s) I n t e r c e p t 0 . 5 1 i 9 • 0.0000 MSWD 0.4 t47Sm/144N(

0.506 0.4,

5.0 [

I

I

I

I

I

l

J

i

0.5

0-5 I

I

[

I

I

I

87 Sr / 86 Sr

87 Sr/86 Sr

(C)

(d)

~ ..........

........

0.850

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

. .....

,....."'"*

0.725 0.800

..."

0.7t5 AGE 200 _+ 12 Ma 12s) Zntercept 0.7t20 + 0.0007 MSWD|69.1 Enhanced Errors 87Rb/86Sr

0.705

"" """'"'"'"" L

I

J

t

I

I

3

I

5

I

""' ~. 750

• ..,¥'tF'"Int ercept .:,+: ....

I

I

I

I

0.7046 -+ 0 . 0 0 0 0

t

7

I

t Mal2s)

MSWD 2.5 [

I

40 87Sr/86Sr

AGE t62 ~

I

3O

87Rb/86Sr I

I

50

I

70

I I

(e) . .-"-"

0.720

0.708

I

[

I

I

87Sr/86Sr

(f)

..... .+ ..............

., .-"1"'" ...."

0.715

0.706

..,.'""

. ...'" . ..'•"

0.710

..." 0.704

. .-"" AGE 187 + Intercept

0.705

0.7046

2 Ma 12s) ! 0.0000

AGE 198 + 34 Ma (2s) I n t e r c e p t 0 . 7 0 5 0 _+ 0 . 0 0 0 3 MSWD 4 . 2 Enhanced E r r o r s 87Rb/86Sr

0 702

MSWD 2 . 9 87b/86Sr

O. 709

1

J

I

i

2

:5

4

5

]

1

I

I

87Sr/86Sr

1

0.1

I

L

L

I

0.3

0.5

0.7

0.9

[

[

I

I

87Sr/86Sr

(cj)

0.707

(h) ..... + . . - ~ ........... , ~ . e ...........

.~..,p~. ......................

O. 707

JP"

0 705 .... ..,'"" •.,..'

O. 705 .....,'•"

0.703 AGE 144 +_ 35 Ma (2s)

AGE 246 +- 17 Ma (2s)

O. 703

Intercept

0.7037

*0-0002

MSWD 4 . 4 Enhanced E r r o r s 87Rb/86Sr

0.701 •

I 0.2

~ntercept

I

I

1

J

0.4

0.6

0.8

1.0

0.7051

+ 0.0002

MSWD 2.7

O. 701

87Rb/86Sr

I

0.1

0.2

0.3

0.4

0.5

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

I

I

O. 714

I

I

I

I .." °..."

(i)

87Sr/86 Sr

I

1

I

87Sr/86Sr

.... "

1

(J)

O. 705

...~.4..++....

87

.#

....

+ ........... ~ , v . . ~

' 4 ...............

0.704

O. 710 ...°'"

O. 706

0.703

.•.•"

.,'" .,.'"

Intercept

AGE 120 *- 5 Mo (25) 0 . 7 0 4 6 _* 0.0003

MSWD 2 . 4

O. 702

0.702

AGE 156 -~ 21 Mo ( I s ) Intercept

0,701 87Rb/86Sr

I

I

2

I

I

I

I

I

3

4

5

6

7

0.7037

*_ O, O00l

MSWD 2 . 8 87Rb/86Sr I

I

I

I

O I

0.2

0.3

0.4

I

87Sr/86Sr

(k)

..,.--"

..÷.-

.o.-'

0.708 ...o"

..,"

,,..~""'" ......,"

0.706

....."" ...-

AGE Intercept

0.704

53 -~ 2 Mo (2s) 0 . 7 0 4 7 +_0.0001

I .6

MSWD i

I

I

2

3

4

87Rb/86 Sr I

I

5

6

Metamorphic Rocks from El Oro Province The data from the metamorphic rocks of El Oro Province proved to be more rewarding. Sm-Nd analysis on garnet/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 lithologics 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 significantly 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, alternatively, the coarse muscovite may have been more susceptible to subsequent argon loss. Thus, the T ahuin Group gneisses probably formed at around 220-210 Ma (Late Triassic), cooled relatively rapidly after this event, and were largely unaffected by the subsequent Late Cretaceous resetting.

Fig. 3. Isochron diagrams for the Cordillera Real and E10ro Province: a) Sabanilla subdivisionorthogneiss. CordilleraReal; b) Tahuin Group garnet orthogneiss, El Oro Province; c) Tres Lagunas granitic subdivision, Cordillera Real; d) Abitagua batholith, sub-Andeanzone; e) Zamora batholith, La Paz area, sub-Andean zone; f) Zamora batholith, Paquisha area, subAndean 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 Lucaspluton south of Saraguro, CordilleraReal.

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 perplexing. 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 of 743 :t: 14 Ma reported by Kennerly (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 geochronometers. Moreover. with such low K-contents, they are likely to be extremely susceptible to the presence of excess argon, which would cause the calculated ages to be spuriously old. Hence, none of these ages can be considered reliable, 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

76.51

44.89

27.13

9.29

0.294

5.817

5.741

6.965

CRSH/g9/IA Cob)

~:

CRSH/89/I B Cot)

.'[

CRSH/89/1C (rose)

9.794

10

6.46

5.96

7.78

5.72

24.6

27.3

15.23

50.44

CCR/87/14A COt)

5.38

8.36

7.39

7.09

6.54

7.02

6.50

7.20

7.41

CCR/87/24A COt)

CCR/87/24B (msc)

CCR/87/24C (rose)

CCR/87/24D COt)

CRSH/89/10A (msc)

CRSH]89/10A COt)

CRSH/89/10C (msc)

CRSH/89/10C COt)

CRSH/89/IOD Cot)

16.30

24.085

23.274

18.926

23.672

17.809

20.35

21.6

22.48

82 -+

81 +

73 +

85 +

69 +

72 +

65 _+

77 +

76 _+

135 +

2

2

3

2

2

2

2

3

3

8

7.26

7.75

7.43

7.32

7.43

7.45

46.30

32.59

67.56

18.07

85.80

44.87

CRSH/89/14E (rose)

CRSH/89/14D COt)

CRSH/89/14D (rose)

8.13

7.22

6.29

36.11

25.96

61.01

Tres Laguna8 Subdivision, Tres Lagunas, Saraguro

CRSH/89/1 IF COt)

CRSH/89/I IF (msc)

CRSH/89/I1B COt)

CRSH/89/llB (msc)

CRSH/89/llA COt)

CRSH/89/11A (rose)

55.0

41.6

:~ Duplicate analysis. Key: arm, atmospheric; bt, biotite; hb, hornblende; msc, muscovite; rad, radiogenic; wr, whole rock.

32.83

23.56

58.22

28.51

43.85

12.6

51.4

54.8

40.8

3.178

71.4

6.10

6.03

Tres Lagunas Subdivision Orthogneisses, South of Sigsig

FV58 COt)

FV57 COt)

CRSH/89/12C COt)

CRSH/89/12C (rose)

Tres Lagunas Subdidsion Orthognetsses, Malacatus Basin

3.012

64.6

6

3

44

20

10

23

32.67

33.14

CCR/g7/14D COt)

0.584

128 :t:

74 +

881 +

844 +

306 +

342 +

371 +

6.09 7.83

31.3

22.9

CCR/87/24A (hb)

20.373

253.597

243.146

3.815

4.301

9

7.58

7.64

Sabanilla Subdivision Garnet Gneisses, Valladolid

50.93

39.0

0.294

:~

363 +

CCR/87f23F COt)

CCR/87]23E COt)

CRSH/g9/12A COt)

9.563

7

27

CRSH/89/12A (mse)

0.612

19.3

74 + 75 +

CCR/87/4 (hb)

1.413

0.457

Garnet Gneisses, Amphibolite, Papallacta

79.1

94.6

0.156

25.7

0.472

7.81

CCR/87/11E (mica)

33

CCR/87/llD (mica)

78 +

4°Ara~ (%)

CCR/87/23D COt)

0.421

Sample No.

K (%)

Sabanilla Subdivision Orthogneisses, Loja-Zamora Road

95.3

Age (Ma)

0.135

4OArmd (hi/g)

CCR/87/11B (mica)

(%)

Ga~et Gneisges, Agoydn

Sample No.

°Aram

K

(%)

Table 4. K-Ar analytical data and calculated ages.

23.584

17.619

16.812

18.827

30.615

18.531

28.793

18.996

29.642

20.83

19.5

24.9

19.7

26.784

14.86

26.348

15.841

24.69

24.93

25.44

40Arr.d (nl/g)

73 +

62 +

68 +

65 ::l:

99 +

63 -

99 +

65 +

100 +

86 +

81 -

97 +

83 +

86 +

66 +

84 _+

66 +

82 -+

82 +

82 +

Age OVta)

O

.z

.:z

.>

oo oo

61.822

8.67

8.51

70.042

55.487

193 + 221 +

6

13

CRSH/89/13B (lab) FV60 (hb)

0.570

0.16

0.17

2.13

0.375

5.21

0.549

4.19

0.499

0.382

0.32

4.7

5.16

0. 971

3.93

0.99

2.34

0.591

0.205

0.168

0.227

0.289

35.7

81.95

63.96

34.1

78

38.7

78.6

24.1

78.8

34.7

64.2

20.8

25.3

45.6

25.7

64.0

37.3

40.1

50.2

71.9

40.6

71.8

88.5

(%)

0.371

4°Ara~n

K (%)

(continued)

:[: Duplicate analysis. Key: atm, atmospheric; bt, biotite; hb, hornblende; msc, muscovite; rad, radiogenic; wr, whole rock.

7.06

74.92

CRSH/89/13A (hb)

7.651

12

6.997

201 ±

CRSH/gg/4H (msc)

61.798

CRSH/89/4H (bt)

72.05

7.497

6

CRSH/89/4A (bt)

214 +

CCR/87/26E (bt)

74.353

CCR/87/26C (bt)

CCR/87/26C (hb)

CCR/87/26E (hb)

9.72

5

6

8.405

189 ±

216 ±

CRSI-I/89/4A (msc)

65.994

66.548

Tahuin Group Marcabeli Pluton

15.30

7.47

CRSH/89/'TA (bt)

CRSH/89/19 (msc)

6

CCR/87/25 (hb)

CCR/87/22E (hb)

CCR/87/26B (bt)

220 -I-

6

6

CCR/87/26B (hb)

32.94

8.45

76.941

213 ± 207 _+

CRSH/g9/7A (msc)

48.492

Tahuin Group Pegmatitic Gneisses

14.82

11.08

7.04

5.68

6

CRSI-I/89]6D (msc)

211 +

CRSI-I/89/6C (msc)

54.375 CCR/87/22B (bt)

28.24

CCR/87/21G (hb)

6.26

647 _.+ 37

CRSH/89/6B (bt)

1.389 CCR/87/21G (bt)

75.43

0.05

Piedras Group Garnet Gneisses

CRSH/89/SB (hb)

34

CCR/87/20 (bt)

CCR/87/20 (hb)

CCR/87/21A (bt)

224 +

7

6

CCR/87/21A (lab)

88.72

0.07

0.602

74 + 76 +

CRSH/89]SA (hb)

1.080

Piedras Group Portovelo Ampkibolites

81.36

1.051

0.358

76.17

0.358

CRSH/89/5B (hb)

15

:[:

72 + CCR/87/19 (hb)

1.062

CCR/87/17 (hb) CCR/87/18 (hb)

91.23

2

0.370

62 +

CRSI-I/89/5A (lab)

17.725

CCR/87/16H (hb)

Takuin Group Arentllas Amphibolltes

43.42

2

7.26

69 _+

CRSH/89/14F (bt)

47.33

21.186

7.77

CRSH/89/14F (msc)

2 CCR/87/16C (hb)

50 _+

Sample No. Zamora BathoUth

33.25

14.109

Age (Ma)

7.09

40Arrad (nl/g)

CRSH/89/14E (bt)

4°Araan (%)

Tres Lagunas Subdivision, Ires Lagunas, Saraguro (continued)

Sample No.

K (%)

Table 4 (continued)

3.49

1.230

1.331

15.53

2.688

39.87

4.172

29.34

3.562

2.208

2.363

34.2

32.78

6.036

24.32

6.134

15.77

4.356

1.581

1.602

1.776

2.107

2.009

(nl/g)

4SArrad

Age

5

4

6

14

10

10

21

151 +

187 +

193 :t:

178 +

176 ±

187 +

186 ±

172 +

175 ±

143 -+

181 +

178 +

156 +

153 +

153 +

5

17

9

5

13

6

14

5

14

7

14

5

5

12

4

153 -+ 10

166 +

126 +

188 -+

230 +

191 +

178 +

134 +

(Ma)

O0 ~D

~r

i

23.5

37.6

5.67

0.481

4.04

FV68 1 COt)

FV485 COb)

FV485

3.512 3.806 2.724 2.206 2.298 5.21

56.6

65.3

73.1

57.8

59.8

47.6

0.569

0.537

0.323

0.755

CCR/g7/6A (hb)

CC]R/B7/6A (bt)

CCR/g7f7 COb)

ADlVlL5 COb)

11.38 5.145 25.96 5.777 38.02 5.26

50.3

64.9

27.9

40.0

14.5

26.9

0.995

5.02

0.827

5.296

0.757

CCR/g7/IOA COb)

CCR/'87/10A COt)

CCR/87/IOB (hb)

CCR/87/10B COt)

ADMIA COb)

171 ±

176 ±

171 ±

128 ±

128 +

49 ±

51 ±

47 ±

169 ±

174 ±

168 ±

126 ±

164 ±

152 ±

135 ±

174 ±

166 +

150 ±

153 ±

104 ±

132 ±

(Ma)

Age

5

5

5

4

7

2

2

2

6

8

8

12

10

7

8

6

5

4

5

3

5

0.996

0.409

4.08

1.04

4.72

0.371

4.27

0.363

FV83 COt)

2.31

UnnamcdPluton, Cuenca-Limdn Road

CCR/87/13C (hb)

:~

CCR/g7/13B (hb)

CCR/gT/13A Cot)

CCR/87/13A COb)

Magtaydn Pluton

CCR/87/IC COt)

:1:

CCR/87/IC COb)

CCR/87/IA Cot)

:~

CCR/87/IA COb)

PtmamptroPluton

1 1

19 ± 19 +

5.672 4.821

46.4

47.3

63.5

60.1

76.8

51.2

5

4

3.139

39 ±

3 79 ±

1.446

3.52

4

86 + 89 ±

1.397

5

3 74 + 68 ±

3.066

4

10.95

73 +

13.73

2

94 +

58.2

3 4

91 + 1.341

1.389

73 +

3

84 +

13.71

6

79 + 1.142 14.18

3

1

25 + 6.536

81 +

3 31 + 0.816

1.172

4 34 ±

0.904

l

2O±

Age (Ma)

5.399

(hUg)

4eArmd

21.6

62.3

54.4

46.9

77.8

59.8

63.8 60.4

73.4

69.1

54.1

64.5

(%)

CCR/87/3 Cot)

6.629

0.679

6.41

7.81

6.95

(%)

4eArah.

-t

CCR/87/3 (hb)

SackaPluton

t-

CCR/87f2E Cot)

CCR/87/2C COt)

Chingual Batholi~

Sample No.

K

~. Duplicate analysis. Key: arm, atmospheric; bt, biotite; hb, hornblende; msc, muscovite; rad, radiogenic;, wr, whole rock.

4.706 11.79

57.5

37.2

2.54

5.914

CCR/S7/SE COt)

CCR/87/9 Cot)

,'L~0mt Batko//t/t

4.638

58.9

0.849

28.73

3.247

CCR/8715G (hb)

Abt~ua Ba~oU~

52.0

5.708

22.5

0.916

FV681 (hb)

COt)

19.29

15.9

4.64

RMI COt)

34.53

3.161

59.3

(hi/g)

~Armd

0.593

(%)

'teAr,,..

RM1 COb)

Zamora Batkoltth (coRtisuzd

Sample No.

K

(gt)

Table 4 (continued)

O O

.>

6.83

F V ~ CoO

17.57 58 ±

2

CCR/87/27G Cot)

K

6.94

6.38

0.508

6.21

0.839

6.56

5.07

1.25

1.969

0.264

7.11

(91)

61.4

67.2

90.9

77.7

94.3

67.3

81.2

38.2

53.2

51.9

76.3

27.9

(~)

~Arjam

.$Duplicate analysis. Key: aim, atmospheric; bt, biotite;hb, hornblende; msc, muscovite, tad, radiogenic, wr, whole rock.

36.9

7.62

2

CCR/g7/29B (bt)

57 +

CCR/87/27F Cot)

16.08

CCR/87/27F (hb)

13.7

CCR/87/27C (bt)

7.18

61 4- 10

CCR/87/29A (b0

0.276

CCR/87/27C (hb)

Catumzyo P/uto~

88.29

3

4

0.12

61 4-

66 4-

CRSHJ89/I 7C (wr)

1.386 1.229

$

71.69

0.51

63.40

0.53

CRSH/89/17B (hb)

P o r ~ k u s l a Ba~olL~

CCR/87/12C (bt)

CCR/87/12C (hb)

CCR/87/12B (hbfot)

CCR/87/12A (hb/bt)

CCR/87/27A Cot)

2

4

4

2

2

CRSH/89/I 7A (hb)

59 ±

61 +

66 4-

51 4-

52 +

TamFucM Mariel~,oua Complex

15.8

1.70

63.8

35.8

1.82

69.8

37.7

Pungala Plu~n

0.702

9.85 9.71

35.0

2

FVI5 (hb)

2

4.84

58 4-

57 +

F V l l (b0

17.57

16.08 CRSH/89/15 Cot)

13.7

7.62

36.9

7.18

Sample No.

CCP./ST/2SB (b0

(hi/g)

CCR/g7f28A (bt)

Age

(Ma)

OArmd

Pt~tnal Pluton

4mAratm (~)

P/uton

$dm L ~

Sample No.

K (gb)

Table 4 (continued)

4.893

4.645

0.469

4.055

0.670

3.101

3.109

8.298

2.080

3.276

0.471

15.27

(nVg)

~Armd

~C

18 4-

19 4-

24 4-

17 4-

2O 4-

12 4-

12 4-

42 4-

42 4-

42 4-

45 4-

54 ±

(Ma)

1

I

5

I

7

I

I

1

2

2

4

4

l

o"

i

t

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 deformation 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.

Meta-lgneous and Igneous Rocks of the Cordillera Real and Sub-Andean Zone In an attempt to date the garnet biotite + muscovite granites 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 unsuitable 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 reliable (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 Lagunas 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 episo~s (see below). Rb-Sr data (18 samples) from the Abitagua batholhh, located in the sub-Andean zone, define an isochron with a particularlywell constrained age of 161 + I M a ( M S W D = 2.5, Fig 3d). K-At from hornblende and biotite separated from these samples gave more variable results.T w o samples (CCR/87/5Gt~b) and CCR/87/6A(b0) gave younger 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 generally 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 isochron 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 distinctive 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

a.

7-

6-

),(D

5-

Z I~1 0 tkl e,b.

4-

3-

•

RESET / DISTURBED AGES

2-

I

0

llO

120

130

140

150

ltSO

i70

liJ0

190

200

210

2='0

230

I

I Fig. 4. Histogramof K-Ar mineralages listed in Table4 obtainedfrom the Zamorabatholith.

240M0

New geochronological control for the tectono-magmatic evolution of the metamorphic basement. Ecuador

youngest of which (i.e.. < ca. 140 Ma) are probably due to subsequent argon loss during alteration and can therefore be disregarded. Nevertheless, a careful appraisal of these data can help to more closely constrainthe ratherimprecise Rb-Sr results and provide extra insight into the development of the Zamora batholith. In the La Paz area,threehornblende separates(CCR/87/ 16H, 17. and 19) gave ages of 178 :I:10, 188 ± 6. and 191 + 10 Ma, in good agreement with the Rb-Sr age (187:1:2 Ma). A fiver boulder of coarse-grained porphyritic hornblende-feldspar andesite (CCR/87/18) gave an age of around 230 Ma, suggesting the presence of older elements within the batholith. Near Paquisha. two c,o-existing pairs of hornblende and biotite samples (CCR/87/21A and G) gave a remarkably close cluster of ages with a mean of 154 :!: 3 Ma, which must record the age of rapid cooling thro-sh the blocking temperatures for these two rni~rals. This could suggest either that the magma cooled sufficiently to set the Rb-Sr clock at ca. 200 Ma but then remained! above the argon blockin~ temperature for some 45 million years before final cooling, or that emplacement and cooling occurred at ca. 200 Ma, followed by reheating to completely reset the K-At in both hornblende and biotite at ca. 155 Ma, with only minimal disturbance of the Rb-Sr system. Alternatively, and probably far more likely, it may suggest that the true Rb-Sr age must lie at the lower limit of the error bar of the isochron age (198 -4- 34 Ma) and that this intrusion is no older than ca. 165/via. Coexisting biotite and hornblende from samples defming the 246 :!: 17 Ma Rb-Sr age at Rio Pituca (CCR/87/22B and E) gave concordant results, with a mean of 180 + 8 Ma, in good agreement with both the K-Ar and Rb Sr results from La Paz. Samples from the south of Palanda (CCR/87/ 26B, C and E) again yielded good agreement for coexisting mineral pairs, with a mean of 179 + 5 Ma for three pairs. This age is just within the error of the rather p o ~ RbSr date of 144:1:35 Ma and thus, in this case, could be interpreted to suggest that the true Rb-Sr age lies at the upper limit of the error bars. However, the coincidence of this RbSr age with the K-Ar at Paqulsha could alternatively suggest the fairly common and well-documented phenomenon of resetting of the Rb-Sr system by cool hydrothermal circulations during a ca. 150 Ma event that did not disturb the K-At systems. Several other samples from the Zamora batholith gave ages in the ranges ca. 180-190 Ma or 155160 Ma. Thus. whatever the precise reasons for the patterns of ages found in the Zamora batholith, a major isotopic event clearly occurred at around 170-190 Ma, and there is a strong suggestion of some activity as early as 230.250 Ma in the Rio Pituca area and to the east of La Paz (Qda. Cufishpe). A later event, particularly in the Paquisha area and to the east of Palanda (FV681), occurred between ca. 150 and 160 Ma. Along the Baflos-Puyo road, a weakly foliated, late epidote-beafing, leucocratic monzogranite of the Azafran batholith gave a seven-point Rb-Sr isochron indicatingan age of 120+ 5 Ma (Fig. 3i). However, the K-Ar data from this area are more perplexing. Two biotite separates from

93

these samples gave much younger K-Ar ages of 47 + 2 and 51 4- 2 Ma, presumably representing subsequent resetting. whereas two hornblende, biotitedioritesamples collected in the Rio Verde area.a few kilometers east of the monzogranite locality, gave very different ages. These two samples were collected from outcrops some 50 m apart. COt/87/10A came from the margin of a steep-to-vertical, NNE-trending shear zone. and CCR/87/10B was collected away from this zone in a massive and apparently unaffected part of the pluton. Each sample contained co-existing hornblende and biotite which gave concordant ages. indicating that the minerals cooled below their respective blockins temperatures over a relatively short period of time and must have recorded real geological events. Nevertheless, whereas sample CCR/87/1OA gave a mean age of 128 :!: 3 Ma. in reasonable agreement with the isochron age, sample COt/87/10B gave a mean of 174 :!: 3 Ma (see also ADM].~, Table 4). Althongh the interpretationof these data remains uncertain, a possible explanation is that the 128 M a K/Ar ages were resetby shearing and that the older 174 M a dates represent originalmagmatic cooling ages. Ifthisis indeed the case. then the statusof the 120 M a Azafran monzogranite isochron is open to question. However. the absence of major Early Cretaceous plutons elsewhere in the Northern Andes (Aspden et al.,1987) argues againstinterpretingthis date as a magmatic cooling age. Zircon analysis (U/Pb) planned for the future should resolve this problem, but at present we speculate that the Rb-Sr system in the Azafran monzogranite was reset by circulating fluids associated with a 120-130 Ma regional shearing event. The late epidote present in the monzogranite may also have formed at this time. Farther to the north, the Chingual batholith is thought to be the northerly extension of Azafran, and seven samples from thisgranitoidgave a Rb-Sr isochron age of 156 + 21 Ma (Fig 3j) - - the high error being due to the small spread in the Rb-Sr ratios. In contrast to the Rb-Sr result, the K-At data from three biotite analyses from the Chin aqlal batholith gave almost identical apparent ages, with a mean of 19 :l: 1 Ma. These dates most probably reflect a heating event at this time. Young, d i ~ t hornblende and biotite ages of 34:1:4 and 25:1:1 Ma, respectively, were obtained from the Sacha pluton. Based on field evidence (Litherland et o2, 1990), this pluton appears to form part of the Chingual batholith. Samples from this area may have been less affected and not completely reset during the ca. 19 Ma event noted above.

Post-Maamorphic Plutons o f the Cordillera Real

Within the Cordillera Real are a number of relatively small~ essentially undefonned, post-metamorphic plutonic bodies (Fig.2). some of which have been dated in thisand other studies.These plutons are dominantly granodioritic in composition and have given Late Cretaceous and Tertiary K-At ages. In order of decreasing age, they are: Pimampiro (94-73 Ma), Magtayfm (86-68 Ma), San Lucas (66-51 Ma), Catamayo (58 Ma), Pichinal (54 Ma), and

94

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

Amaluza (49-34 Ma) (Kennerly, 1980); Pungala (45-42 Ma); an unnamed granodiorite stock exposed along the Cuenca to Lim6n road (39 Ma); and Portachuela (24-12 Ma). All these granitoid rocks display fresh igneous textures, and their ages are thus assumed to represent times of magmatic cooling. In addition to the K-At data, a single, 3point Rb-Sr whole-rock isochron obtained from the San Lucas pluton gave an age of 53 + 2 Ma (Fig. 3k). The Condue and Azuela plutons (Fig. 2a) have not been dated, but since they are also undeformed they are considered to be of probable Tertiary age. In contrast to the above plutons, the Tampanchi marie complex consists of gabbroic and basaltic rocks. Two gabbroic samples from the complex yielded hornblende K-At ages of 65 --+3 and 61 + 4 Ma, whereas one hornblende-rich basalt gave a whole-rock K-Ar age of 61 + 10 Ma. Thinsection studies of these rocks found no evidence of metamorphism or alteration, and all three dates are therefore interpreted as cooling ages following emplacement.

CONCLUSIONS It is clear that the Cordillera Real has been subjected to a complex succession of magmatic and tectonic events, both localized and widespread, throughout much of Mesozoic and Cenozoic times. Both the Rb-Sr and the K-Ar isotope systems have been affected, and many of the ages are poorly constrained. However, having used a combination of isotopic methods, it has been possible, in most cases, to distinguish between the various tectonic and magmatic events and hence obtain a considerably better -nd_erstanding of the geological hist(xy of the Cordillera Real and El Oro Province. The pre-Mesozoic history remain.¢unknown. A number of pre-Mesozoic K-Ar ages have been recorded, both in previous publications and in this study. However, with the exception of the Portovelo amphibolite, similar samples taken frem the same locality gave very different ages. It may be that some of these older ages are real, but they can also be explained by disturbance of the K-Ar system, with localized argc~ enrichment or preferential loss of potassium leading to spuriously old ages. Until more detailed work is carried out in the respective localities, these ages must remain suspect and should not be quoted in the literature as otherwise. The earliest event recorded, based on reliable data, is the metamorphism and magmatism within the Tahnin Group of El Oro Province dated at 220-200 Ma (Late Triassic to Early Jurassic). This has been clearly defined by both StaNd and K-Ar data. There is a remarkable similarity between the garnet gneisses of El Oro and those from Papallacta, Agoy~n, and the Sabanilla subdivision in the Cordillera Real. Not rely are the rocks all garnet biotite gneisses, but they also conabundant tabular graphite with typical shiny luster. It is therefore possible that El Oro and the Cordillera Real gneisses were ori$inally part of the same metamorphic complex. The garnets from the Cordillera Real have not fractionated the Sm-Nd isotopes, so we cannot prove that

they are of the same age, but the Rb-Sr data fron3 the Sabanilla subdivision orthogneisses (Fig. 3a) do suggest that a similar metamorphism occurred at approximately 220 Ma in the Cordillera Real. The age of the Tres Lagunas subdivision is relatively well-constrained at about 200 Ma. The initial 87Sr/~6Sr ratio of 0.7120 (Fig. 3c) (and also of the Sabanilla orthogneiss, 0.7123) is considerably higher than that of the Abitagua. Azafran, and Zamora granitoids (ca. 0,705) and clearly indicates that the Tres Lagamas granite has had a much greater crustal component involved in its genesis than the other Ecuadorian granitoids. The Tres Lagunas granite probably represents a crustal melt that formed during the 220-200 Ma metamorphic event. The main intrusion of the typical Andean-type granitoids in both Ecuador (i.e., Zamora, Abitagua) and Colombia (Aapden et al., 1987) occurred between ca. 190 and 150 Ma (Middle-Late Jurassic); however, part of what is now included within the Zamora batholith may have been eraplaced at ca. 240 Ma (Early Triassic), With the possible exception of the emplacement of the Azafran monzogranite, only sporadic magmatism occurred between 150 and 90 Ma (Early Cretaceous). Most of the other plutous that have been dated range between 80 and 40 Ma (Late Cretaceous to early Tertiary). One of the chief results of this study is documentation of the Cretaceous to early Tertiary K-Ar reset mineral ages. especially those recorded from the Cordillera Real. These are represented by a histogram (Fig. 5), which also displays the principal sedimentary/tectonic events revealed by the sedimeutary record preserved in the Ofienteand along the flanks of the Cordillera. Following the cessation of phitonism associated with the Zamora and Abitaqua batholith at ca. 150 Ma, the Oriente and sub-Andean zones were deformed, uplifted, and eroded prior to the deposition of the Hollin and Napo Formations (Baldock, 1982). The peak of Early Cretaceous reset ages (ca. 135-125 Ma) (Fig. 5), obtained principally from the Abitagua, Azafran, and Zamora batholiths,is interpretedto relateto this pre-Hollin event, which is of regionalimportance,having also been identified in Colombia where itcorresponds to a period of accretion,widespread dynsmothermic metamorphism, and blueschist emplacement along the Romeral fault zone (Aspden et al., 1987; Aspden and McCourt, 1986). In Ecuador, previously mentioned fieldevidence from Rio Verde suggests thatthisEarly Cretaceous event included an important component of transpressi~ai shearing along steep-to-vertical NN~SW-trending ZOneS(see alSOAapden and Litherland. 1992). The Aptian Hollin Formation (ca. 119-113 Ma) (Bristow and Hoffstetter. 1977) is overlain conformably by the marine shales and limestones of the Napo Formation, and both formations were deposited under relatively stable, epicontinental conditions (Baldock, 1982). After the upper Napo was deposited, a major period of Campanian erosion (ca. 83-73/via) took place in the Oriente (Baldock, 1982) which, together with the subsequent deposition of the Tena and Yunguilla Formations, c . x ~ d e s with the marked peak in reset ages (ca. 85-65 Ma) obtained from the Cordillera Real (Fig. 5). During this period, the Pimampiro and Mag-

New geochronological control for the tectono-magmatic evolution of the metamorphic basement. Ecuador

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Fig. 5. Histogram of Cretaceous-earlyTertiaryreset/disturbedK Ar mineral ages from the Cordillera Real. showingtheir correlation with eventspreservedin the sedimentaryrecord.K-At mineralages are listed in Table4 (see text for furtherexplanation).

tay~n plutons were emplaced (Table 4) but, in general, plutonic activity was apparently restricted. The red bed Tena Formation, conf'med principally to the eastern fl~nk.~of the Cordillera Real/sub-Andean zone, was derived frown the west and is the chronostraligraphic correlative of the marine Maaslrichti~n (ca. 73-65 Ma) Yunguilla Formation Of the Cuenca area (Fig. 2b) (Baldock, 1982; Bristow and Hoffstetter, 1977). It thus seems reasonable to conclude that the widespread Late Q'etaceams disturbance of the K-At mineral ages relates to the uplift of the Cordillexa Real. As with the pre-Hollin event, this disturbance has also been rect~iT~l in Colombia, where reset agesrangmgfmm ca. 75 to 57 Ma recorded from the Central Cordillera have been correlated with the approach and subsequent accretion of the allochthonous, oceanic WestSAES6:1/2-G

ern Cordillera (McConrt et al., 1984). The geological record clearly indicates that conditions of instability continued in Ecuador during Tertiary-Recent time. with the main Andean uplift taking place from the late Neogene onward (Baldock, 1982). These events are also presumably reflected in the continued ~ t m a l overprinting of the older metamorphic rocks in the Cordillera Real and would, for example, explain the existealce of young Miocene K-At nfmeral ages (ca. 20 Ma) obtained fr~n the Jurassic Chingruff batholith (Table 4, Fig la).

Aclmowladgmmnta~This paper is publishedwiththe permissionof the

British GeolosicalSurvey (NERC) and CODIGEM,Quito, Ecuador.

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

96

Work in Ecuador and in the UK was carried out as part of an ongoing bilateral technical cooperation project between the governments of the UK (Overseas Development Administration) and Ecuador. Special thanks are due to Ings. Viteri, Bermudez, and Endara, and Srs. Casanova and Erazo, who carried out much of the initial sample preparation in Quito. The efforts of Srs. Celled and Revelo are gratefully acknowledged. M. Litherland is also thanked for his comments on an earlier draft of this paper.

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