• Author / Uploaded
• Liz Nv

Citation preview

-'.: ::". t;i'",.,...

i;t , r:1r{..:.':

lr*b::q " q-

The metamorphic belts of Ecuador

Couer photograph

Cerro Hermoso in the Llanganates mountains. a lr¡lrlecl ancl metamorphosed carbonate sequence, recenth glact.rre ri

Cerro Hermoso (4571 m) from the south-west, a folded carbonate sequence and the highest point in the Llanganares mountains (photo: ML).

:.€;!?+= .

-&{-1

,a.1

"# i$ ;".

::=:'::' '::,a

ii:t{,ii..:1!: il:r,.:-r. .. :.1r l4iiE:,:.n ':ri. I i:-:tit

ir

Project field party negociatinq the Rio Parcar aclr tributary along the Rio Mulatos tra\¡erse

(photo:ML¡.

'

.

'

::::l

á

dtiñ

BRITISH GEOLOCICAL SUR\IEY

O\'ERSEAS MEMOIR 11

The metamorphic belts of Ecuador M Litherland, J A Aspden and R AJemielita

Ke1"r,vorth,

Nottingham British Geological Suner 1g9i

O ¡/El?C copyright 1991 Firstpublished 1994

BRTTISH GEOLOGICAL SURVEY Kepvorth, Nottingham NG12 5GG (0115) 936 3100

Murchison House, West Mains Road, Edinburgh Bib li o grap hi c r eferen

ce

Lrrurnr-wo, M, AsrooN,.f A, andJnrumura, RA. 1994. The metamorphic belts of Ecuador. Ouerseas Memoir of the British Geological Sut'uq, No. 11. Authors

Martin Litherland, OBE, BSc, PhD, CGeol, MIMM John A Aspden, BSc, PhD Bril i sh Geologt

ca

I Su rue'1

Richard AJemielita, BSc, PhD, MIMM fornerly of the British Geological Suruey

EHg

3r.{

0131-667 1000

London Information Office, Natural History Museum Earth Galleries, Exhibition Road, London SW7 ZDE 0171-589 4090

The full range of Sut-vey publications is available from the BGS Sales Desk at the Sur-vey headquarters, Ke1'l'vorth, Nottingham. The more popular maps and books may be purchased from BGS-approved stockists and agents and over the counter at the Bookshop, Gallery 37, Natural History Museum, Cromwell Road, (Earth.Galleries), London. Sales Desks are also located at the BGS London Information Office, and at Murchison House, Edinburgh. The London Information Office maintains a reference collection of BGS publications including maps fbr consultation. Some BGS books and rePorts may also be obtained from HMSO Publications Centre or from HMSO bookshops and agents. The Rritish Geolo$cal Suruey canies out the geological suruq oJ Great Britain and Noñhetn lrel.a.nd (the h,tter as (r'n agenq sentice for the gorernment of Norfhem I'reland), and oJ the sunounding continenlal

shelf, as zuell as its basic resenrth projects. It also undeñakes ltrogrammes of British technical aid in geolog¡ in deaeloping countries as ananged try the Ouerseas Dnelopment Admi.nistration.

I'he British Geological Sut-uq is a com|onent bodl of the Enu iro nment R¿s e at ch Coun til.

cl3 ISBN

12/94 O

8Ú272 239 7

Printed in England by Linney Colour Print Ltd

Natural

CONTEI{TS

One Introduction Fluman

aspect

Access

3

history

Five Cordillera rocks 48

1

3

work

Previous geological

Mining

Peltetec ophiolitic belt 42 Other Cordillera Real serpentinites 47

1

Physiography 1 Climate and vegetation

Sedimentary and volcanic formations 48 Granitoids and porphlries 51 Mafic and ultramafic intrusives 52 Six Cordillera Real: tectonometamorphic

3

4

Project operations Reports and maps

5 5

^\cknowledgements

Geological setting -\ndean plate tectonics 7

7

Guamote

.\rdean metamorphic rocks 7 Growth of South America 7 Terranes in the Northern Andes 7 -\ndean metallogenesis 9 \Ietamorphic rocks of Ecuador 10 Stratigraphicalnomenclature l0 \Ietamorphicterranes 10 Three Cordillera Real: Precambrian, Palaeozoic and Triassic rocks 12 -\mazonic craton 12

Isimanchi unit 12 Piuntza unit 12 Loja terrane 14

unit -\goyán unit 15 Chiguinda

12 72

unit

16

22

64

69

complex

69

Palenque event 73 Structural limits of the El Oro

Cretaceous

belt

74

Other metamorphic occurrences 74 Eight Correlation and interpretation 76 Palaeozoic and Triassic

history

Jurassic-Lower Cretaceous

76

history

77

Correlation 77 Interpretation 77

Upano unit 27 Curuja unit 30 Cerro Hermoso unit 30

Later Cretaceous and Cenozoic

Nine

-Vafrán granitoids 32

skarns

rocks

Correlation 76 Interpretation 76

Zarrrora granitoids 24 Salado terrane 27

33

-\lao terrane 36 Alao-Paute unit 36

unit 38 \Iaguazo unit 39 El Pan

Guamote

55

Tectonometamorphicevents 7l Moromoro event 7\

t9

Contact rocks and

unit

EI Oro ophiolitic

Santiago Formation 22 Chapiza unit 22

unit

mélange

65

Baños fault or shear zone 56 Salado terrane 58 Cuyuja nappes 58 Upper Cretaceous and Cenozoic events 60 Sub-Andean thrust belt and associated structures 60 Peltetec fault and associated strlrctures 62 Tectonics and K-Ar resetting 62

Palenque

Four Cordillera Real:Jurassic-Lower rocks 22 ,\¡razonic craton 22 \fisahuallí

terrane

Piedrasamphibolites 68 Jurassic-Lower Cretaceous rocks 69

Tres Lagunas

Sabanilla

Alao

Moromoro granites 66

15

\Ionte Olivo amphibolites granites 16

53

55

Seven Geology of the El Oro metamorphic

Formation

\Iacuma Formation

terrane

Peltetec fault and ophiolitic

Palaeozoic and Triassic El Tigre unit 64 La Victoria unit 66

Precambrianbasement )2 Pumbuiza

events

Tres Lagunas event 53 Peltetec event 54

5

Two

Real: Later Cretaceous and Cenozoic

terrane 4l

Ophiolitic rocks and melanses 42

geology Project mineral survey 81 Precious metals 81 Gold (primary) 81 Gold (secondary) 86 Economic

Gold analysis 87

Silver 91 metals

Base

94

Copper. lead and

Cadmium

97

zinc

95

81

history

79

belt

64

V1

mercury Antimony 98 Arsenic 98 Bismuth 98 Mercury 98 Tellurium 98

Metalloids and

PI-{TES

97

Metals related to mafic-ultramafic

intrusions

1 Cordillera Real scenery 2 2 Triassic fossil 14 3 Tres Lagunas granites 18 4 Salado terrane rocks 28 5 Skarn rocks 35 6 Alao valley 37 7 Jurassic fossils 40 8 Cuamote terrane rocks 43 9 Serpentinites 46

98

Platinum-group metals 98

Chromium

Nickel 99 Cobalt 99

99

Vanadium 99 Titanium 99 Ferrous metals

Iron

10 Lower Cretaceous ammonites 49 11 Cretaceous and Cenozoic fossils 50 12 Condue granite 51 13 Amphibolite xenoliths 53 14 Baños fault or shear zone 57 15 Culuja nappe complex 61 16 Progressive metamorphism in El Oro 66 17 Progressive deformation in El Oro 72 18 Massive sulphides 82 19 Skarn mineralisation 85 20 Secondary gold in the highlands 88 21 Nluvial gold in the lowlands 89 22 Gold grains 92-93 23 High-level mineralisation 96

99

99

Manganese 100 Cranite-relatedmetals 100 Molybdenum 100

Tin

100

Tungsten 101 Rare metals I0l Rare-earth elements

Niobium

101

101

Non-metallic and industrial minerals 10i

Asbestos 101 Barytes/barium Carbonate 102

101

Corundum 102 Diatomite 102 Feldspar I02

FIGURE,S

1 2 3 4

Fluorite \02 Carnet I02

Cemstones and mineral

Graphite

curiosities 102

103 Gypsum 103 Hot springs 103

5 6

Kaolin and other clay minerals 103

Magnesite

Mica

103

7

stone 103

B 9

103

Ornamental

Phosphate rock 103 Quartz and silica minerals 103

Sillimaniteminerals

Sulphur

Talc Energy

103

Coal 104 oil 104

Cerro

31

32

skarnfield

belts

106

References I07 Number references of Project open-file reports Unpublished project-related reports 107

Project-relatedpublications 108 Other references 109 Appendices 115 Geochronological data 115 Geochemical data 121 Ceological index I+4

Hermoso

plutons

conclusions 104 Mineral potential and metallogenesis I04 Structural controls and mineral

23

13 Location map and stream section of the El Placer

Uranium I04

1 2

Ecuador

12 Geochrolological plots of the Azafrán and Chingual

104

Discussion and

1 2 3 4

Pacific area 9 Provisional terrane map of Ecuador 10 Precambrian, Palaeozoic and Triassic rocks and terranes ofEcuador 13 Geochronological plots for the Tres Lagunas granite and Sabanilla unit I7 Ceochemical plots for granitoid rocks 20 Jurassic-Lower Cretaceous rocks and terranes of

10 Geochronology of the Zamora granitoids 25 11 Geological map and section of the area around

103

104

minerals

Geomorphological framework of Ecuador I Location of the metamorphic/plutonic belts 4 The geotectonic growth of South America 8 Present-day plate tectonics of the equatorial east

r07

34

14 Basic rocks of the Ecuador metamorphic belts plotted on Ti v. Zr diagram 38 15 Mafic rocks of the Cordillera Real plotted on MORB normalised'spiderdiagrams' 39 16 Geological map and section of the lower Alao Valley and

environs

44

17 Llltramafic rocks from ophiolitic units plotted on Cr v. TiO, diagram 45 18 Schematic E-W section across the Cordillera Real metamorphic complex 56 19 Geological section across the Cu¡rja nappe

complex

59

20 Reset K-Ar ages and geological events over the Cordillera Real 63 21 Location map fc-rr the El Oro metamorphic

complex

67

23 U-Pb zircon data for the Piedras amphibolite 24 Geochemical plots of El Oro mafic rocks 70 25 An evolutionary model fbr the Ecuadorian

terranes

78

26 A two-dimensional evolutionary model for the northern Cordillera Real 80 27 Approximate gold production figures 81 TARI,ES

1 2

Summary of alluvial gold deposits 90 Variations in gold composition 91

65

22 Geochronological plots for the Moromoro

granites

3 4

Average monthly rainfall figures 3 Summary of geological history 11

N,tr{PS

1

Geological and metal occurrence maps of the

northern Cordillera Real metamorphic belt

2

(in pocket) Geological and metal occurrence maps of the southern Cordillera Real and Ei Oro metamorphic belts (zn pocket)

PREFACE

This overseas Memoir reports on geological and mineral studies of the metamorphic rocks of Ecuador which are essentially confined to the cordillera Real and El oro metamorphic belts. The objectives were the elucidation of their nature, structure, relationships, genesis and mineralisation, in response to the need for a geological and mineral database to support and promote a viable national mining industry. The work was carried out under the Cordillera Real Geological Research Project (1986-1993), a bilateral Technical Cooperarion projecr berween the Governments of Ecuador (Ministry of Energy ánd Mines) and the United Kingdom (Overseas Developmenr Administration ODA), using geoscientists from the Ecuadorian Mining Institute (INEMIN), replaced in 1991 by rhe Geological and Mining Corporation (CODIGEM); and the British Geological Survey (BGS). ODA and BGS participation in the geological and mineral development of Ecuador began in lg69 and during the 1970s British and Ecuadorian geologists carried out the primary mapping and mineral exploration of the coasral area and the western cór¿lttéra. The present work thus completes the reconnaissance studies of the Ecuadórian Andes by Anglo-Ecuadorian teams, and provides the springboard for the regional plate tectonic interpretations outlined in this téxt and on ,l: r?Ir9iul maps (Geologic and Tecronometallogenic) prepared by ODA/BGS in conjunction with CODIGEM BGS staff were contracted by ODA to serve on the project. Dr M Litherland (Project Leader) and Dr J A Aspden (Seni,or Geologist) were resident in Quito for the duration of the project and'underiook the reconnaissance studies of the cordillera Réd lrom 1986 to 1990. From mid-1990 to 1993 Dr Aspden covered the El oro meramorphic district whilst Dr R A Jemielita (Economic Geologisr) carried our follow-up mineral studies over the cordillera Real. -During the same period Dr Litherland compiled the national maps. In addition ro rhe residential staff and rheir counrerparrs, orher-INEMIN/CODIGEM and BCS geoscientists carried out geochemical, petrographic, geochronological and palaeontological studies on pioject-samples."Dt Litherland was responsible for the compilation óf ihir Me-oir and accompanying maps.

PeterJ Cook, DSc Director

Kingslq Dunham Centre Ge olo gi cal Sur u ey Kqworth Nottingham IlGl2 5GG Britis h

October 1994

ONE,

Introduction PHYSIOGRAPITY

Considering its smail size (283520kmz), mainlancl Ecuador is physiographically very varied. The Anclean range, or Sietra, is at its narrowest here (120 km), beine bounded to the wesr by the coasral lowlands (Costa) anil to the east by the upper Amazon basin ( Oriente). Tine Sierra comprises trvo parallel ranges, the Cc¡rdille¡a Occid,en,tal (Western Cordillera) and the Cordillent, Rea,l or Cord,illera Oriental (Eastern Cordillera) (Fieure 1). The

highest peak is Chimborazo (6310 m). The

rwo cordilleras trend approximately NNE-SSW, separared in central and northern Ecuador brr the Inter-Andean vallev rvhich contains most of the major cities.

The Cordillera Real, the main subjecr of this memoir, is about 650 km long and 60 krn wide within Ecuador. Elevations along its watershed are controllecl in the north by a chain of Plio-Quaternary volcanoes (Piate l). From north to south the major peaks are Cayambe (5790m), Antisana (5704m). Cotopaxi (5897m), Tunsurahua (5016 m), Altar (5319 m) and Sangay (b230 m). To the west the floor of the Inter-Andean Vailey occurs at 2000-3500 m, while the sub-A¡dean foothills of the eastern slope are at altitudes of about 1000 m. The highest peaks of metamorphic rock are Cerro Soroche (4730m), Sara Urcu (4670rn) and Cerro Hermoso

(4571m). Prcrject expedirions visited rhese areas; Hermoso was ciimbed (Cover and Frontispiece).

In the south of the Cordillera

Real, the watershed fall to 3000-3500 m; the Inrer-Andean valley clisappears and the two cordilleras merge. West of this massif, and trending east-west, at risht anqles to the Andt s. is tlre coastal loorhills range olrhe El Or 1.1). Equally on the ACF plot (Aspden et al., 1992b), the granites straddle the plaeioclase-biotite tie line and exrend into the Al-rich part of the diagram. An Stype parentage was also suggested on a petrographic basis by Atherton (1987) and by Clarke (1989), based on plots of preliminary analytical results.

The Chappell and \¡Vhite (1974) classification of granites into S- and I-types is broadly similar to the ilmeniteand magnetite-series of Ishihara (1977) in rhat ail S-tvpes belong to the ilmenite-series, and the majoriq' of I-n'pes correspond with the magnetite series. The FeoO*,/FeO r-.

SiO, plot (Lehmann and Harmanto, 1990) shorvs the separation of the Tres Lagunas granites and Zamora granitoids into the ilmenite- and magnetite-series re\pectively (Figure 8e).

On a number of plots it can also be seen that granites collected from different sectors of the Tres Lagunas out-

crop form individual clusters of compositional subgroups. These subgroups are apparent in diagrams which involve Cr, Ni and SiO, as discriminants (Aspden et al., 1992b) and suggest a lack of resional uniformitv. In conclusion, the Tres L,agunas granites can be classified as S-types and their relatively high 865r/875r rarios also indicate a substantial crustal component in their origin. This value, 0.712 (Figure 7a), is srearer rhan the entire range of Ri values from the Zamora and Azafrán granitoids and is similar to that of crustally contaminated modern andesites in Colombia (ames, 1984). The significance of the discovery of this regional S-type granite belt which is stronglv mylonitised in places, is discussed on p.77. Sabanilla unit (Palaeozoic and Triassic)

This unit is a suite of medium- to high-grade orrho- and paragneisses forming a 10 km-wide belt nclrth from the Peruvian border for 150 km before wedging out. Accessible outcrops occur near Sabanilla, on the Loja-Zamora road, which gives its name to the unit, and alons the Loja-Zumba road and the track to Pico Toledo. Contacts with the adjacent Chiguinda and Isimanchi units, both of lower metamorphic grade, are tectonic. East of

the Isimanchi unit, gneissic rafts in the

Zarnora

batholith, which resemble the Sabanilla unit, have been allocated a Precambrian age.

The geochronology of the metaplutonic phases shows K-Ar muscovite and biotite ages of 60-100 Ma (p.119), and, as in the case of the Tres Lasunas granite, these are

20 THRI,E

C]ORDILLERA REAI: PRECAMBRIAN. PALAT,OZOIC AND TRIASSIC ROC]KS

I

23 4567 -

Quartz-rich granitoids l\¡onzogranite Granodiorite Ouartz-monzon'te Ouadz-monzonite

/ gabblo

Quartz-diorite / gabbro

v

Monzodiorite / gabbro

8-Dlorite/gabbro

W¡thinPlate Granites

2.4

$ t

S{ype

t"

4

'6x

o

O^q

'

o.F/l*' +

* r+O/

+ //

d

c *

*Á

2.4

O

1.8

Y +

16

+

¡

*_-t "*^ ^+

N

z t

14

a

t2

s-Vpe +

+

oB F; v --'.:%-L-

e$f,+é*

xla

o

l-type

l-type

34

65

Na20 (%)

70

si02 (%)

Key to symbols Magnetite

series :

2.00

o r

'1.0c

O

0.50

Áa^ V¡V

o.ofr #

r

bd ¡E

0.20

llmenite series

65

70

.á I trl E O A V * O * + x

Zamora batholith Ab¡tagua granite Azafrán meta-pluton Valladolid sector Malacatus sector Peggy sector Saraguro sector Baños sector Saban¡lla migmatite

L'"" r"o,n". " I

)"

Marcabelí granite El Prado granite

Moromoro migmat¡te

o"n

).,o'"

sio2 (%)

Figure

8

oranite

Geochemical plots for granitoid rocks.

(a) QAP ternary plot for all granitoid rocks after Streckeisen (1976) based on CIPW normative values (Q= quartz; Or = orthoclase; Pl = anorthite + albite); (b) granitoids on Rb v. Y+ Nb discrimination plot of Pearce et al. (1984a); (.) IlO v. NarO diagram fbr all granitoids, I-type and S-qpe fields after Chappell and \4rhite (I97!):' (d) aluminosity index r,. SiOr for all granitoids, I-type and S-type fields after Chappell and \Vhite (W7!); (e) FerO*/FeO r,. SiO, plot fbr Cordillera Real granitoids, fieids after Ishihara et al. (1979) and Lehman and Harmanto (1990).

LOJATERR{NE

regarded

as reset. The hornblende ICAr ages are older at about 130 Ma. The Rb-Sr geochronology (p.118) shows an 8-point errorchron of 233 r 51 (MS\,aD = l7b; Ri = 0.7118) (Rundle, 1987b). A further ser of ten samples collected from a single outcrop (Harrison, 1990) gave 198 t 45 Ma (MSWD = 25.3; Ri = 0.7123). Combiñing the two datasets gave an errorchron of 224 t 37 Má (MSl\,lD = 108) (Figure 7b), an age similar ro thar of the

Tres Lagunas graniles. To the east of Sabanilla and to the south of Yangana the western margin of the unit comprises steeply dipping to vertical gneissic 'quartzites', which contain sillimanite, muscovite, biotite and perthite in places, which pass east-

wards into granodioritic biotite orthogneisses with prominent muscovite. These gneisses may exhibit a streaky biotite foliation along with incipient migmatisation. Further east, towards Zannora, migmatitic gneisses and streaky biotite gneisses carrying sillimanite and kyanite are present (see also Trouw, 1976). High-grade assemblages of sillimanite-biotite-quartz-albite and kyanitebiotite-orthoclase-quartz were noted. Around Valladolid and to the north, amphibolitic gneisses, with or without biotite, are common especially within the metaplutons,

21

and their general form suggests they were originally basic dykes or sheets. Staurolite-bearing gneisses occur to the north of Palanda over an area where muscovite and/or biotite pegmatites are common. There are also minor, unfoliated, muscovite-tourmaline-garnet pegmatitic leucogranites associated with two-mica, garnetbearing orthogneisses. Gneissic xenoliths are common and in some outcrops it can be seen that the progressive digestion of this material produces the biotite-rich schlieren which are themselves streaked out. The field observations indicate plutonic granodiorites (orthogneisses) associated with pelitic sediments now metamor-

phosed to medium- to high-grade paragneisses. Geochemically, the Sabanilla unit orthogneisses plot into the S-type granite field along with the Tres Lagunas granites (Figure 8), and are of similar age, with a similar initial strontium ratio of about 0.712 (Figure 7b). In contrast to the Tres Lagunas granites, however, they do not contain blue quartz, are not commonly megacrystic, are more homogeneously foliated, and are associated lvith

high-grade migmatites. It is thus probable thar the Sabanilla unit represents a deeper-level, migmatitic phase of the Tres Lagunas granite.

,,)

FOUR

cordillera Real: Jurassic-Lower cretaceous rocks AMAZONIC CRATON During theJurassic and Lower Cretaceous' the Amazonic Formation craton" rvas the site of deposition (Santiago unit) .r-rá Cnupiru unit), volcanic extrusion (Misahuallí a reover granitoids) ar-rcl mugrnatic intrusion (Zanora tectonometamorsubsequent gion which underwent no ihic elent and thus contrasts with theJurassic metamor,.rrut.s of Salado, A]ao and Guamote further west'

[n;.

Santiago Formation (Lower Jurassic)

This unit, named by K T Goldschmid of Shell Co" in and vol1940, of marine limestones, shales, sandstones of southCutrrcír canic rocks, fbrms much of Cordillera east the to 1982)' east Ecuador (Tschopp, 1953; Baldock, from recovered Ammonites .ri ,f-t" uccompanyitlg'trrup' it'. Arietites (Tschopp, 19ó3), Arnioceras, ancl Cr¡roniceras (C.y.., 1974); aná-I'eptechiocera\ sp' indet' and Pah'echio)t*l "p. inaet. lAspaán and lvimey-Cook' 1992) (Plate 7. ur-rá f), indicate á Sinemurian age' The formation extends southwards into Peru but is absent in boreholes further north and east in Ecuador' The formation, about 2000 m thick, comprises a lowdipping, unmetamorphosed sequence -9f. gt"y siliceous liÁ"stá"s, calcareoús sanclstones, turbiditic in places' and black shales. Westwards, in outcrops along the new road to Santiago, calcareous turbidites of the Santiago Formation gráe laterally into a continental-type sewith quence of tñffaceous grey'siltstones and sandstones in places structures basaltic lava flows con*taining pillow which lithoiÁp¿." and Litherlana, 1oé2), a sequence It thus Formation' Chapiza the to ügi."lfy corresponds Fo^rmation Chapiza the of base the aileast that up-p"urá .t.tt¿ U" the lateral equivalent of the top of the Santiago Formation. A similar transition has been reported in northern Peru (Cobbing et al', 1981;Jaillard et al'' 1990)'

an (Aspden and Ivimey-( look' 1992) ' In the Oriente' dala lrom an oil well indicates Neo"ifrn"l"ni.al io-iu"-Áptian rocks at 33 m below the Hollin uncon-

m formity and the Jurassic'/Cretaceous boundary at 213 1977) ' (Bristow and Hoffstetter, The sedirnentary Chapiza sequence of Cordillera and grey c.ri.^t comprises á¡"rlt ifOo m of red' pinkanhydrite, of shales and sindstones with thin horizons strong dolomite and gypsum (Tschopp, 1953)'There are rs sequencethe of part least at indications (aÉove) that the along Thus Formatiol' Santiago the to grey to "q.riuut"r't, rJacl east to Santiaso, thére are sandstones and basalt with reddish tuflaceous siltstones, intercalated sandstones and flows, and rninor calcareous siltstones similár to those in the Santiago Formation' The volcanic rocks fomporlertt clecreases east of"Rio Und'o where the strict the in Formation Santiago as classified .ut L" are there sense. In both the Chapiza and Santiago facies wide 100m and i.tg. i".rri,itic canals'10-20m deep .oitulr-tirtg poorly sortecl material rich in volcaniclasts'

Misahualli unit $urassic- ?Cretaceous)

Lou'clipping]avasanclpyroclasticrocksnotedintheRio ""ast of the accompanying map' were named Uisatruatti, the 'Misahualli basalts and tuifs' by Wasson and Sinclair (Ig27); ancl these formed the upper Nlisahuallí Member of ,n.'ónupiza Formation (Tschópp, 1953) ' with a thickterm ness of up to 2000 m. In the present memoir the v9]continental+ype ihe all Misahualií unit inclucles considor known' belt sub-Andean canic rocks of the ered to be, essentially of Jurassic age, overlying the the Lower Jurassic Santiagb Foimation and overlain by presthe Over Hollín Formation ot Áptian-Albian age' ent area these comprise, in the main, the,volcanic rocks associated rvith the major Jr'rrassic batholiths' These rocks are extensively ""p"t"a along the sub-Andean zone of the accompanyin[ map' and' with the exception of the extreme nortír, áre úmited westwards by the

Chapiza unit (Jurassic-Lower Cretaceous)

The Chapiza Formation (named by K T Goldschmid of Shell Co., in 1940) comprised a succession of lowdipping, unmetamorphosed, continental-type',clastic sediments overlain by the volcanic Misahuallí Member Misahuallí rocks 1it.f-t"pp, 1953). In this memoir the ur. ¿...illt.d separately and the sediments are referred to as the ChaPiza unit. The Chapiza unit forms much o.f the Cordillera it Cutucú to tñe east of the accompanving map where of overlies, or is a facies of, the Sántiago Formation Lower Jurassic age. Indeed, the Sinemurian ammonites collectÉd by thJproject from the Santiago Formation boundwere from the SantiJgo road close to this facies

Cosanga f'ault.

Hall" and Calle (1982) quote a K-Ar age of 132 Ma from P Espin (personal coirmunication) of a sample of the Misahuallí Member from a deep well in the Oriente' In the present suruey, rocks assigned. to the Misahuallí cut by thé Abitagua granite of 162 + l Ma

.rnlt uti intruding the io inl, whilst a hornblende ándetit" dyke8 Ma' Further t 168 of age K-Ar a gave ,u-. g.ur-rite south,"the volianic rocks are related to the Lower to

'

MiddíeJurassic Zamora batholith (p'26)' with associated 14 Ma lava ur-tá dyk. samples giving IGAI ages of 230 t (p.119)' and 143 x7 Marespectively basalts' traLithologies comprise green to dark

^grey chytes, grev-green,^violei and pink tuffs ancl tuffaceous

AMAZONICCR{TON

Figure 9

Jurassic-Lower

ffi

Granitoids

Cretaceous rocks

'a/,i

and terranes of ?ALAO

Pelitic schists and marbles

F,crraclor.

TERFANE

ROSA FLORIDA

Il ll

Quartzites / slates

PLUTON

?Peltetec ophiolite

-Y

Ambuquí outcrop

t_--__

W YWffil

CHINGUALLA BONITA PLUTON

tl Ophiolitic complex

I

q a.

Skarnfields

rÍ-:_ h]--T# l-- Lrr+

'3l

Meiavolcanics / metasediments

l

c¡

Northern skarnfields

lz

Unmetamorphosed limestones

r¡l

c c

t- l ll

Unmetamorphosed sandstones

tl

Unmetamorphosed continental volcanics

¿!

Northern skarnfield

:u !a

0

Cerro Hermoso unil

t/ ul ts, uk -fF|{

ABITAGUA GRANITE skarniield

AZAFMN PLUTON

o/