Late Precambrian to Early Mesozoic tectonic evolution of the Colombian Andes, based on new geochronological geochemical and isotopic data.

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LATE PRECAMBRIAN TO EARLY MESOZOIC TECTONIC EVOLUTION OF THE COLOMBIAN ANDES, BASED ON NEW GEOCHRONOLOGICAL GEOCHEMICAL AND ISOTOPIC DATA

by Pedro A. Restrepo

Copyright © Pedro A. Restrepo 19~5

A Dissertation submitted to the faculty of the

DEPARTMENT OF GEOSCIENCES

In Partial Fulfillment of the Requirements For the Degree of

DOCTOR OF PHILOSOPHY

In the Graduate College

THE UNNERSITY OF ARIZONA

1995 UM! Number: 9624154

Copyright 1995 by Restrepo, Pedro Alonso

All rights reserved.

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UMI 300 North Zeeb Road Ann Arbor, MI 48103 2 THE UNIVERSITY OF ARIZONA ® GRADUATE COLLEGE

As members of the Final Examination Committee, we certify that we have read the dissertation prepared by Pedro Alonso Restrepo-Pace ------~------entitled ------LATE PRECAMBRIAN TO EARLY MESOZOIC TECTONIC EVOLUTION OF THE COLOMBIAN ANDES, BASED ON NEW GEOCHRONOLOGICAL GEOCHEMICAL AND ISOTOPIC DATA

it be accepted as fulfilling the dissertation Doctor of Philosophy

Date IS lAt;U 'is Date

Suzanne Baldwin Date ~cJ (4 · tts AG~ Date (ri'Vb\f (f Jr- Date

Final approval and acceptance of this dissertation is contingent upon the candidate's submission of the final copy of the dissertation to the Graduate College.

I her I have read this dissertation prepared under my direc that it be accepted as fulfilling the dissertation

tor Date 3

STATEMENT BY THE AUTHOR

This thesis has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to made available to borrowers under rules of the Library.

Brief quotations from this dissertation are allowable without special permission, provided that accurate acknowledgement of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his interests of scholarship. In all other instances, however, permission must be obtained from the author.

SIGNED: ~_~_ 4 ACKNOWLEDGMENTS

During 1993's summer field session a farmer approached me one day and asked the obvious question: II what could possibly intrigue you so much that makes you come to such remote areas of Colombia to hammer rocks every single day???". After I informed this man that I was not searching for gold or emeralds and going out of my way to explain what I was attempting to

elucidate, he simply giggled and replied : II I have no time for such nonsense, I have to feed my family ..... ". I could not be more grateful to life in general. Circumstances have permitted me to pursue and fulfill my intellectual curiosity in the most ideal of settings. I am greatly indebted to my parents who have provided me with the highest level of education and have raised me in a highly stimulating environment. I also have to be grateful to my brother who has and continues to be a motivating force and an example. Now, the single best thing that happened to me in the course of these six years was that, during the 1994 field session I met my wife Juliana. She should be granted at least a M. Sc. in Geology ( honoris CQusa- pazienza ). She became my right hand, rock crusher and mineral separator. Without her help, support and patience I would not been able to finish this dissertation. Moreover she constantly stimulated me with her endless happiness and positive attitude. Now, speaking of luxuries: Joaquin Ruiz as an advisor. Not only did he cover the fundamentals of an excellent academic advisor. He unconditionally supported me during my 5 studies in Arizona. Most important is that he became a friend, together with his family Bernadette and Peter. Special thanks go to the 'Bogota Cartel' : Diego, Emesto and Guillermo for their logistical support in Colombia. To Peter Coney I will always be grateful for personally 'opening the door' to me in Arizona ( and to graduate school in the States for that matter). To Mike Cosca for generously helping me gather the bulk of my data and for all the help in Lausanne. To Susanne Baldwin for permitting me to use the mineral separation lab and for having mercy on my Ar / Ar data. To George Gehrels for his field geology teachings and for his help in providing the U/Pb ages. To John Patchett for his harsh short-term yet positive long-term critique. To James Gleason for the Sm/Nd analyses. Helena and Bo for their academic logistical support. Finally I want to thank all of my friends of the 'Gran Combo' Lukas, Helge , Patricia, Marta, Andres, Elena, Joel, Diana and Sergio. We certainly had the best of times together. 6

A Juliana, mi Madre, mi Padre y mi Hermano 7

TABLE OF CONTENTS

I. LIST FIGURES ...... 11

II. LIST OF TABLES ...... 14

III. ABSTRACT/RESUMEN ...... 15

IV. INTRODUCTION ...... 19

Results from this study...... 22

General background geology...... 23

Previous work:

Precambrian...... 25

Lower Paleozoic sediments...... 26

Lower Paleozoic metamorphics...... 27

Lower Paleozoic intrusives...... 28

Middle Paleozoic sediments...... 29

Middle Paleozoic intrusives...... 30

Upper Paleozoic sediments...... 30

Upper Paleozoic intrusives...... 31

Figure captions...... 32 8

TABLE OF CONTENTS-continued

V. DISTRIBUTION AND PETRO-TECTONIC CHARACTER

OF THEGRENVILLE AGE BASEMENT IN

THE COLOMBIAN ANDES...... 45

Abstract...... 45

Introduction..... , ...... _... _. _...... 46

Analytical methods...... _...... _...... 47

Garzon massif.... _...... _.... _. _...... _ .... " ...... 47

Santander massif.... _...... _... _. _...... 51

Santa Marta massif.... " ... _...... _ ...... 53

Tectonic implications ...... _...... 54

Figure captions...... _.. _...... 57

~.GEOLOGYANDGEOCHRONOLOGYOFTHE

SANTANDER MASSIF...... 83

Abstract...... _...... 83

Introduction...... _...... _.... , .... 84

Stratigraphy..... , .. _...... _...... 86

Structure...... 88

Petrologic character of basement...... 91 9

TABLE OF CONTENTS-continued

Geochronology..... , ...... 94

Discussion of geochronological data...... 95

Regional time-space basement correlatives...... 100

Figure captions.... " ...... 102

VII. SUMMARY OF RESULTS FROM THIS STUDY...... 143

VIII. TECTONIC EVOLUTION OF NW ANDES FROM LATE

PRECAMBRIAN TO EARLY MESOZOIC TIME ...... 144

Time slices:

1.3 - 1.1 Ga ...... 145

1.1 - 0.9 Ga ...... 145

0.75 - 0.48 Ga ...... 146

0.48 - 0.47 Ga ...... 148

0.47 - 0.36 Ga ...... 148

0.25 - 0.23 Ga ...... 150

0.23 - 0.16 Ga ...... 150

Cyclic events (?) ...... 152

Figure captions ...... 153

IX. REFERENCES ...... 159 10

TABLE OF CONTENTS-continued

x. APPENDIX ...... 183

Logistics and methods ...... , 184

Analytical methods:

Rare earth element ...... 185

4()Ar/39Ar ...... " ...... '" ..185

U/Pb ...... 187

Sm/Nd ...... 188

Additional data ( some not incorporated directly to this study) :

Sm/Nd data for the basement and Paleozoic low grade

metamorphic cover ...... 190

40 Ar /39 Ar data for two samples of the Central Cordillera. 191

40Ar/39Ar spectra of Central Cordillera samples ...... 193

Complete list of sample location and description...... 195 11

I. LIST OF FIGURES

Figure 1. Physiographic provinces of the northern Andes...... 34

Figure 2. Location of basement uplits ...... " 35

Figure 3. Simplified regional stratigraphic columns...... 36

Figure 4. Garz6n Massif -1.1 Ga age gneiss...... 37

Figure 5. Garz6n Group -1.1 Ga gneiss...... 38

Figure 6. Santander Massif -1.0 Ga gneisses...... 39

Figure 7. Santander Massif -1.0 Ga gneisses ...... 40

Figure 8. Orthogneiss, Santander Massif...... 41

Figure 9. Intrusive contact Jurassic I Lower Paleozoic...... 42

Figure 10. Santa Marta massif -1.0 Ga gneises ...... " 43

Figure 11. Intrusive rocks of the Santander Massif...... 44

Figure 12. Location of basement exposures...... 59

Figure 13. Simplified geologic maps ...... " ...... 60

Figure 14. Chondrite normalized REE patterns ... ' ... " ...... 61

Figure 15. 40Ar /39Ar apparent age spectra, basement ...... 62

Figure 16. U IPb zircon ages for the basement...... 66

Figure 17. Nd crustal residence ages...... 67 12

LIST OF FIGURES-continued

Figu:re 18. Distribution of Nd crustal residence ages ...... 68

Figure 19. Temperature-time curve ...... 69

Figure 20. Paleogeographic reconstruction ...... 70

Figure 21. Location Map of basement uplifts...... 106

Figure 22. Geologic map of the Santander ...... 107

Figure 23. n diagrams and S1 Foliation ...... 108

Figure 24. Simplified multiple fold diagram ...... 109

Figure 25. Map of brittle structures ...... " 110

Figure 26. Schematic cross section...... 111

Figure 27. Normalized REE patterns Santander...... 112

Figure 28. Trace element discrimination plots ...... 113

Figure 29. 4OAr/39Ar apparent age spectra Santander ...... 114

Figure 30. Temperature-time curve...... 120

Figure 31. U / Ph age, orthogneiss Santander Massif...... 121

Figure 32. Distribution geochronological data ...... 122

Figure 33. Orogenic phases in the Santander Massif ...... 123

Figure 34. Regional paleogeographic reconstructions...... 154 13

LIST OF FIGURES - continued

Figure 35. Cross-sections depicting the evolution...... 156

Figure 36. Histogram of pre-Cretaceous ages...... 158 14

III. LIST OF TABLES

Table 1. Rare Earth Element data - basement ...... 71

Table 2. 40 Ar /39Ar data - basement...... 72

Table 3. U/Ph zircon age data- basement ...... 79

Table 4. Previously reported Grenvillian ages ...... 80

TableS. Neodymium isotope data - basement ...... 81

Table 6. Sample location and description...... 82

Table 7. REE composition- Santander Massif...... 124

Table 8.40Ar /39 Ar data Santander Massif...... 127

Table 9. U /Ph isotope data for sample BP-2 ...... , ..... '" 140

Table 10. Location and sample description ...... 141 15 m.ABSTRACT

40 Ar1 39Ar and U 1Pb geochronology of the basement rocks in the Colombian Andes confirm the presence of the Grenvillian age high metamorphic grade belt . The Grenvillian, or locally known as Nickerie­ Orinoquiense orogenic belt, is exposed within basements uplifts along the Eastern Cordillera of Colombia and the Sierra Nevada of Santa Marta in the Caribbean coast. Rare Earth element geochemistry and petrology indicate that the Nickerie-Orinoquiense basement rock's protoliths are dominantly of continental affinity, now consisting mainly of metapsammites, metavolcanics and metaplutonic rocks metamorphosed to granulite facies PI' conditions. Nd crustal residence ages and U/Pb zircon data indicate variable involvement of 'older' Late Archean - Early Proterozoic components and 'younger' - 1.1 Ga additions, which were tectonically mixed during the Nickerie-Orinoquian collisional metamorphic episode. Low metamorphic grade rocks that overlie the Nickerie-Orinoquian basement are exposed along the Eastern Cordillera of Colombia at the Quetame-Floresta-Santander massifs, Perij a Range and Merida Andes. A U 1Pb zircon age obtained from a synkinematic pluton structurally concordant with the low metamorphic grade belt from the Santander Massif, yielded a 477 ± 16 Ma, indicating a Mid-Ordovician regional greenschist to amphibolite

facies metamorphic event for these rocks. The latter is referred-to as the 'Caparonensis Orogeny' in the Venezuelan Andes. Rare Earth Element 16 geochemistry and petrologic data indicate that the low metamorphic grade belt consists of a thick supracrustal sequence i.e. metapelitic-metapsammitic sequence with minor crosscutting mafic dikes. Additional trace element discrimination plots indicate that the Caparonensis synkinematic plutons are of continental arc affinity. 40Ar/39Ar geochronology, petrology and field observations in Santander Massif, indicate a widespread regional metamorphic overprint took place in Late Triassic-Early Jurassic time. This event was the result of a thermal welt associated with back-arc extension and concomitant intrusion of a high volume of calk-alkalic plutons. Deposition of a thick molassic sequence (2000-4000 m) followed, flanking the uplifted region. The lower Paleozoic metamorphic rocks were elevated from greenschist to sillimanite (locally kyanite) PT metamorphic conditions and the Mid-Upper Paleozoic sedimentary cover was locally metamorphosed from greenschist to lower PT metamorphic conditions, as a function of relative distance to the plutonic centers at time of metamorphism.

RESUMEN

Geocronologia del sistema 40 Ar/39 Ar Y U /Pb en rocas del basamento de los Andes Colombianos, confirman la presencia de un cintur6n de alto grado metam6rfico de edad Grenvilliana (1.1 Ga). El cinturon Grenvilliano, conocido localmente como Nickeriense u Orinoquiense, esta expuesto como 17 levantamentos de basamento a 10 largo de la Cordillera Oriental de Colombia y la Sierra Nevada de Santa Marta en la costa del Caribe. Geoquimica de elementos de tierras raras junto con observaciones petrologicas indican que el basamento Nickeriense tiene un prototolito de afinidad continental, que consiste en metasamitas, metavolcanitas y metaplutonitas metamorfoseados bajo condiciones de P-T facies granulita. Edades de residencia cortical y edades U /Pb indican adicionalmente un aporte variable de materiales corticales antiguos Proterozoicos tempranos y materiales mas jovenes generados durante el evento -1.1 Ga, tectonicamente mezclados durante el episodio de colision continental Nickeriense - Orinoquiense . Rocas metamorficas de bajo grado suprayacen el basamento Nickeriense a 10 largo de la Cordillera Oriental en los Macizos de Quetame, Floresta y Santander,la Serrania de Perija y Andes de Merida. Una edad U/Pb en zircon obtenida de un granitoide sin-cinematico emplazado en concordancia estructural con el cintur6n metamorfico de bajo grado del Macizo de Santander, arroj6 una edad de 477 ± 16 Ma, indicando un evento metamorfico regional de facies esquisto verde a anfibolita baja para estas rocas a mediados del Ordovicico. Este evento se Ie conoce como "orogenia Caparonensis" en los Andes de Merida de Venezuela. Geoquimica de elementos de tierras raras e informaci6n petrol6gica indican que el cintur6n de bajo grado consiste de una secuencia gruesa de naturaleza supracortical i.e. metapelitica-metasamitica con menor proporci6n de diques mcificos que la cortan. Diagramas de discriminacion tect6nica basados en quimica de elementos traza indican que los plutones sin-cinematicos son de afinidad de 18 arco continental. Geocronologia del sistema 40Ar/39Ar, petrologia y observaciones de campo en el Macizo de Santander, ponen en evidencia un evento metam6rfico regional en el Triasico-Jurasico superimpuesto a los anteriormente mencionados. Este evento es el resultado de una anomalia termica regional producida por extensi6n cortical en el area posterior al arco y el gran volumen de intrusivos calcalcalinos que tuvo lugar simultaneamente. El levantamiento del area del Macizo de Santander gener6 la depositaci6n de 2000-4000 m de materiales moIasicos en sus flancos. Las rocas metam6rficas del Paleozoico inferior fueron elevadas de grado metam6rfico de facies esquisto verde a silimanita ( 10ca1mente cianita) y las secuencias del Paleozoico medio a superior se metamorfosearon variablemente, facies esquisto verde a muy bajo grado dependiendo de su distancia relativa a los centros intrusivos. 19 IV. INTRODUCTION

Knowledge of the pre-Mesozoic history of the northern Andes is sparse. My experience, and perhaps that of those interested in regional Andean tectonics was that, reading northern Andean literature was analogous to reviewing an engineering plan for a fifty story building, in which the basement and initial thirty floors have barely been sketched-out. This is partly due to the fact that the economic basement is situated at the base of the Lower Cretaceous. Nonetheless, geologists in the 1940's had recognized the importance of basement geology in oil exploration. Comprehensive regional studies were carried out by Trtimpy (1943), based on his own work together with research conducted by O. Renz, H. Hubach and A. Gansser.Results of their extensive field work are still today the mainframe for research pertaining basement rocks of the Colombian Andes. Regional geologie studies of the Colombian Andes were later conducted by BUrgI ( 1967), Radelli (1967), Julivert ( 1968 ), Irving (1971, 1975), Campbell ( 1974), Toussaint et al ( 1978 ) and Forero-Suarez (1989). Regional stratigraphic compilations were carried out by Julivert (1968), Etayo - Serna et aI, (1983), Mojica et aI, (1990) and Toussaint, (1993). Geological uncertainties regarding pre-Mesozoic orogenic events stand-out in all of these studies . Especially with regard to crystalline-metamorphic basement units. The main contributing factors toward these uncertainties include the strong Mesozoic-Cenozoic deformational overprint which masks the earlier history of the basement rocks together with insufficient detailed 20 geochronologic investigations. Moreover, the geochronologic methods utilized in some investigations were applied not knowing specifically what was being determined e.g. crystallization age, cooling age or metamorphic age etc. Given the impossibility of assessing the validity of the available geochronological data for the Colombian Andes, regional geologists have been finding "an event for every occasion" (paraphrasing Miall ,1992) and thus, the northern Andes have been turned into a geological 'wild card' in regional tectonic-paleogeographic reconstructions of the Americas. In recent years a highly controversial issue in tectonics has surfaced, regarding the possible interaction between the Andean and Appalachian orogens in early Paleozoic time e.g. Bond et aI, ( 1984 ) : Forero-Suarez, (1989 ); Kent et al, ( 1990); Dalziel, ( 1991) ; Hoffmann, ( 1991 ); Dalla Salda et al. ( 1992 ) ; Dalziel et al, (1994) and others. Geochronology performed on granulitic rocks of the Garz6n Massif, Eastern Cordillera of Colombia by Alvarez et al (1981), Kroonenberg (1982) and Priem et aI (1989) determined the existence of a Grenville-age (.... 1.0 Ga) orogenic event in the eastern Andes of Colombia. Locally, the Grenvillian event is known as the Nickerie Metamorphic Episode (Kroonenberg, 1982 ) or Orinoquiense Orogenic Event (Martin, 1974 ). Along the same Andean range, Lower Devonian fossiliferous sedimentary rocks rest on top of low to medium metamorphic-grade rocks. These stratigraphic relationships indicated that an orogenic event may have occurred between -1.0 and -0.4 Ga in the Colombian-Venezuelan Andes. The latter also was suggested in some paleogeographic reconstructions e.g. Forero­ Suarez, (1989); Kent et al, ( 1990); Dalla Salda et ai, (1992); Dalziel et al, ( 1994). 21 Additionally, based on the age-polarity and petrological character of the Oaxacan-Acatlan metamorphic complexes of southern Mexico, Yanez et al, (1991) postulated that part of the Mexican basement rocks may have been transferred from northern South America in Late Paleozoic time. The viability of these paleogeographic models required a closer look at the early history of the northern Andes. Specifically, the present study sought to establish the deformational history of the Colombian Andean basement, by addressing the following objectives: -To confirm and tectonically charecterize the previously reported Grenville - age orogenic event in the Colombian Andes ( McDonald et al, 1969; Tschantz et al, 1971 ; Ward et al1973; Alvarez et al, 1980; Kroonenberg et al, 1982 and Priem et al, 1989) (1) to confirm its existence and characterize it tectonically. -To establish the Paleozoic deformational history of the Colombian Andes since the precise chonology of events is poorly constrained (c.f. TIiimpy, 1943; Ward et al1973; Irving, 1975; Shagam, 1975 and others) (2) to establish their precise age and characterize such event(s) tectonically. -Provide viable tectonic-paleogeographic models by selecting from the most widely accepted paleogeographic reconstructions (e.g. Bond et al, 1984 ; Forero-Suarez, 1989; Kent et aI, 1990 ; Dalziel, 1991 ; Hoffmann, 1991 ; Moores, 1991; Dalla Salda et al. 1992 ; Dalziel et al, 1994) those which suit the results from objectives ( 1, 2 ). This was conducted as a test for their validity. 22 Results from this study Contributions to the knowledge of pre-Mesozoic geology of the northern Andes which resulted from this investigation are outlined below: 1- 40Ar/39Ar and U/Pb zircon ages are reported for the basement of the

Colombian Andes which confirm the existence of a Grenvillian age metamorphic event. Together with the Sunsas Belt in southern Bolivia, the Colombian Andean basement constitute the largest exposures of Grenvillian age rocks in South America. 2- A reliable Grenvillian cooling age is reported for the Central Cordillera of Colombia, which places a western limit for the Grenvillian basement. 3- ALl Ga U/Pb zircon crystallization age obtained for an augen gneiss in the Garz6n Massif, rules out the previously reported 1.6 Ga Rb/Sr for the same unit (Priem, et al1989). No crust older than -1.3 Ga could be accounted for in the Andes of Colombia. 4- Nd crustal ages reported for the Grenvillian basement rocks of Colombia, suggest a possible provenance from the Guyana Shield. S- U /Pb zircon crystallization age for a Paleozoic intrusive of the Santander Ivlassif, indicating that the metapelitic rocks covering the Grenvillian basement were deformed in Middle Ordovician time, settling a long standing debate in Colombian geology concerning Early Paleozoic orogenic cycles.

6- Evidence for Jurassic uplift and metamorp~sm related to back-arc

extension and calc-alkaline intrusion, backed by over 20 4OAr/39Ar cooling ages, petrologic and stratigraphic data . This new dynamic view of Mesozoic tectonics in NW South America contrasts with the prevailing 'static' view i.e. 23 merely a gradual sedimentary filling of extensional grabben structures. 7- Field and regional geology , structural geology , geochronology , isotope geochemistry, trace element geochemistry stratigraphy and paleontology have been used in an integrated fashion to resolve regional tectonic questions.

General background geology The northern Andes are divided into two distinct geologic and physiographic provinces: the Andean region to the west and the flat foreland plains of the Llanos extending the entire Orinoco drainage basic (Fig. 1). The Andean region is thrust over the Llanos by the Borde-Llanero thrust system. Three main ranges make up the Andean domain, namely the Eastern, Central and Western Cordilleras. The Eastern Cordillera branches into the Perija Range to the north and the Merida Andes to the northeast. The Santa Marta Massif constitutes an isolated triangular shaped range on the Caribbean coast of Colombia. The Llanos domain consists of an Early Paleozoic through Tertiary sedimentary succession with various stratigraphic hiatuses, thicker toward the Andean front and gradually thinning eastward ( cf. Etayo-Serna et aI, 1983 ). The sedimentary wedge is underlain by the Guyana Shield (Priem et al, 1982). In the Andean domain, the Eastern Cordillera and the eastern flank of the Central Cordillera are underlain by - 1.0 Ga continental crust (Alvarez et aI, 1981; Kroonenberg, 1982) . Along the Eastern Cordillera of 24 Colombia, the 1.0 Ga granulitic basement is covered by Lower Paleozoic metamorphic rocks and Mid-Paleozoic through Tertiary sedimentary rocks ( Etayo-Serna, et al1983 ). The eastern flank of the Central Cordillera is made of metavolcanoclastic-metapelitic rocks of pre-Jurassic age ( Restrepo-Pace, 1992). The remaining Andean region to the west is underlain by Jurassic - Cretaceous oceanic crust and trench assemblage rocks, accreted at different periods ranging from Late Cretaceous to Early Tertiary time ( Barrero, 1979; Millward et aI., 1984; Aspden et aI, 1986 and others). Suites of calk-alkaline plutonic rocks are exposed throughout the Colombian Andes with ages ranging from Early Paleozoic to Tertiary. In broad terms the plutonic-magmatic activity has migrated westward through time (Toussaint et aI, 1982 ).

Previous work In the following paragraphs I will attempt to provide sufficient Pecambrian - Paleozoic descriptive stratigraphic background for a better understanding of this study . To follow on some of the localities here mentioned refer to figures 2 and 3 . Geologic times discussed here are in reference to Palmer's DNAG ( 1983 ) time scale. Many details and formational names will be omitted to keep focused on important issues and the discussion is limited to important exposures that show key stratigraphic relationships relevant to regional tectonic events. 25 Precambrian In Colombia, the metamorphic basement of the Guyana shield is exposed at the border with Brasil and Venezuela. Exposures consist of felsic and metapelitic-metarenaceous gneisses, migmatites and homogeneous granites of the -1.5 Ga MitU Migmatitic Complex, the -1.5 Parguaza rapakivi granites and overlying this crystalline basement , a post -1.45 Ga pelitic psammitic sedimentary sequence about 1000 m thick (Huguet et, 1979 ; Priem et al, 1982). The basement units are cut by -1.2 Ga mafic dikes. 0.95 Ga felsic volcanics of the Piraparana Fm constitutes the youngest unit (Priem et al, 1982). In the Andean region, granulitic and migmatitic Precambrian basement rocks are exposed as upthrust blocks, along the Eastern Cordillera and the southern portion of the Sierra Nevada of Santa Marta on the Caribbean coast (Fig 1-2 and Figs 4-7) . Another exposure of probable Precambrian rocks, yet to be proven on geochronological grounds, is located in the northern termination of the Central Cordillera of Colombia known as the Puqw Complex (Etayo-Serna et al, 1983). K/ Ar and Rb/Sr geochronology and petrologic work was carried-out mainly in the Garz6n Massif by Alvarez (1981), Kroonenberg ( 1982, 1983) and Priem et al (1989). Here ages range from 1.6 to 0.6 Ga though most ages duster - 1.0 Ga. Other Grenvillian K/ Ar and/ or Rb/Sr ages have been reported in the northern portion of the Santander Massif ( Eastern Cordillera) ( Ward et aI, 1973 ) , in the Sierra Nevada of Santa Marta (Tschanz et al, 1974) and the Guajira Penninsula (Irving, 1975). These ages altogether range from 0.8 to 1.3 Ga and led to the 26 postulation of a Grenville - age granulite belt running along the Eastern Cordillera of Colombia (Alvarez, 1981; Kroonenberg, 1982). Precambrian tectonothermal events recognized in northern South America have been summarized by Martin ( 1974) and Toussaint (1993) and include: a 3.4 Ga Guriense metamorphic event, a 2.7-2.6 Ga Aronensis metamorphic event, a 2.1-1.8 Ga Trans-Amazonian metamorphic cycle, a1.56- 1.45 Ga Parguazan high-grade metamorphism and granitic magmatism, and the 1.0 Ga Nickerie/Orinoquiense Metamorphic Event.

Lower Paleozoic-sediments Lower Paleozoic sedimentary rocks are distributed throughout the Llanos area and mainly along the Eastern Andean Cordillera(Fig.l). Fossiliferous Cambrian and Ordovician rocks in the northern Andes range from Middle Cambrian to Llanvirnian in age. In the Merida Andes of Venezuela a Caradocian section is exposed (the Caparo Fm ) ( GonzaIez de Juana, 1980) (Fig.3). The Cambrian sequence exposed at la Uribe in the eastern foothills of the Eastern Cordillera-Colombia , consisting of a marine limestone sequence containing stromatolitic horizons, followed by interlayered basaltic-doleritic rocks and graywackes and capped by a turbiditic sequence with a total thickness of -2000 m (Bridger, 1982 ). The upper unit yielded Mid-Cambrian fauna (Harrington et aI, 1951; Rushton, 1962). Tremadocian to Llanvirnian fossiliferous sedimentary rocks in northwestern South America are dominantly clastic turbidites consisting of interlayered sandstone, siltstone, marls and shale with occasional limestone beds. 27 Thicknesses for these sequences average 500 m and may reach 1000 + m ( e.g. Giiejar Gp in La Macarena overlying unconformably Precambrian basement, Triimpy, 1943). In broad terms the sections exposed to the east on the Llanos area are dominantly psammitic and become more pelitic to the west ( Mojica et aI, 1992). In the Colorado Massif, Merida Andes of Venezuela the exposed Caradocian elastic marine sequence is -200 m thick. Its stratigraphic contacts are hindered by structural complications and granitic intrusions ( Gonzalez de Juana, 1980 ).

Lower Paleozoic - metamorphics Constrained to be pre-Devonian on stratigraphic grounds ( Triimpy, 1943; Renzoni, 1968; Cediel, 1969; Forero-Suarez, 1972, 1989; Boinet al, 1986 and others ), rocks of this group constitute a metapelitic belt running along the Eastern Cordillera of Colombia and Merida Andes of Venezuela. Inlicus may be found along the Cordillera Real of Ecuador. In Colombia the metapelites are exposed mainly in the Quetame Massif, Floresta Massif, Santander Massif and Perija Range (Fig.2) known locally as the Quetame series, Floresta Group, Silgara Group. or Perija Series. In Venezuela these units are referred to as the Iglesias Group and Bella Vista Formation (Gonzalez de Juana, 1980). Lithologically they consist of a thick pile of quartzose schists which attain low metamorphic grade, chlorite to lower amphibolite facies metamorphism. The precise age of this low grade metamorphic metapelitic is one of the central subjects of the present work. As stated previously, it is known to be pre-Devonian on stratigraphic grounds 28 and is intruded by various Paleozoic plutons with ages ranging from - 480 to 350 Ma. As I will show from the results of this investigation, many of the ages reported for these intrusives may represent 'mixed' ages. A similar belt of low grade metamorphic rocks but with a major component of metavolcanoclastics is exposed along the Central Cordillera of Colombia. It is variously referred to as the Central Andean Terrane (Restrepo et al, 1978; Restrepo-Pace, 1992) or as the Zamora Terrane (Richards et al, 1990 ). Constrained to be pre-Jurassic from numerous plutonic bodies that· intrude it, its deformational age is one of the most controversial issues in northern Andean geology. It has been considered by many to be correlative, in terms of age of deformation, to the metapelitic belt exposed along the Eastern Cordillera of Colombia and the southern Merida Andes, on the basis of similar lithology and metamorphic grade. Some authors suggested that it is a separate and distinct unit deformed at the end of the Paleozoic ( e.g. Irving, 1971 ; Hall, 1972 ; Campbell, 1976 ). Based on the presence of Devonian unmetamorphosed rocks on the eastern edge of the Central Cordillera (Forero-Suarez, 1970b), some argue that the Central Andean Terrane rocks were metamorphosed in pre-Devonian time. The general consensus is that this unit is of Cambro-Ordovician metamorphic age (d. Cediel et al, 1988 ; Gonzalez et al, 1988).

Lower Paleozoic intrusives Dated Lower Paleozoic intrusives outcrop along the Eastern Cordillera

of Colombia ( compiled in Etayo-&~rna, 1983; Forero-Suarez, 1989 and Maya, 29 1993 ), the Perija range and the Merida Andes of Venezuela (compiled in Shagam, 1975 and Gonzalez de Juana, 1980). Most of the reported ages for the Colombian Andes are either K/ Ar or Rb/Sr whereas in Venezuela many U/Pb zircon ages are available ranging from - 460-490 Ma (Burkley, 1986 ; Gonzalez de Juana, 1980, p. 117 ). These intrusives crosscut the metapelitic units and are overlain by Devonian sedimentary rocks (Boinet et al, 1985). Most of the intrusive bodies are granitic (Fig 11) and few are gabbroid in composition.

Middle Paleozoic sediments Silurian fossiliferous sedimentary rocks are very scarce in the northern Andes. The best preserved Silurian outcrops, namely the Homo formation, are found in the Merida Andes. Fossilsindicate a Ludlovian age for the section (Boucot et al, 1972). Two localities in the Eastern Cordillera of Colombia have yielded Silurian-Ludlovian fossils, one at the Quetame Massif (Grosser et aI, 1991) and another in the Santander Massif ( Forero-Suarez, 1989). Devonian fossiliferous sediments are quite abundant along the Eastern Cordillera of Colombia but are absent in the Sierra Nevada of Santa Marta or Merida Andes of Venezuela. The best exposures are found in the Perija Range, Santander, Floresta and Quetame Massifs and in southeastern foothils of the Central Cordillera. Most of the sections begin with a basal conglomerate overlying metamorphic rocks, and consist of interlayered marine sandstones, siltstones and shales with few minor limestones. 30 Thicknesses vary from a couple of hundred meters up to approximately eight hundred meters. The upper contact depicts a paraconformable relationship with Carboniferous red-beds in some of the localities and in others, for example in Manaure, Perij a Range and northern Santander Massif, an angular and! or discordant relationship has been described Forero-Suarez, 1972; Bayer et al, 1973).

Middle-Paleozoic intrusives Middle Paleozoic ages have been reported for some intrusive bodies along the Eastern Cordillera (Maya et al, 1993) the Perij a Range (Dasch et al, 1984) and the Merida Andes ( Burkley, 1976; Gonzales de Juana, 1980 ). All the ages reported with the exception of those in the Perija Range and Merida Andes are KI Ar or RbISr ages. Of these, thirty ages total, six are whole rock­ mineral isochrons and the rest are whole rock isochrons. Many have very poorly defined isochrons. Three U IPb zircon ages are reported ( Dasch et al, 1984; Burkley, 1976) two of which are unequivocally Devonian.

Upper Paleozoic sediments Mississippian sedimentary rocks could rarely be accounted for, since very little fauna have been collected representing this period. For the most part it is considered to be a stratigraphic hiatus in the northern Andes. Upper Carboniferous - Middle Permian sedimentary rocks are generally exposed in the same localities were Devonian sections have been measured, with the exception of the Floresta Massif (Mojica et aI, 1984 ). Important outcrops are 31 found in the Sierra Nevada of Santa Marta ( Forero-Suarez, 1970 ) , the Perija Range ( Forero-Suarez, 1972) , along the Eastern Cordillera at the Santander Massif ( Rabe, 1977) and the Quetame Massif ( de la Espriella et al, 1983) and in the Merida Andes (Gonzalez de Juana, 1980) . Red-beds lay at the base of the known Carboniferous sedimentary rocks which gradually turn into a marine flysch sequence consisting of interlayered sandstones, marls and shales. Occasionally limestone layers are present. Conformable on the top continues an Early-Middle Permian marine limestone sequence. Late Paleozoic sections are several hundred meters thick but may attain up to 2500 m thickness in the Eastern Cordillera and up to 5000 m in the Venezuelan Andes. The Devonian to Permian sequences are regarded as a single transgressive unit, containing local unconformities (Cediel, 1972 ) . In the Merida Andes lower Carbonniferous rocks include locally tectonic breccia horizons, referred to as the Merida facies of the Sabaneta Fm (Hargraves et al, 1970).

Upper Paleozoic intrusives The majority of Late Paleozoic ages so far recorded come from rocks in the northern portion of the Central Cordillera of Colombia ( 17 ages according to Toussaint, 1993; p. 172 ). Many of the sampled units correspond to the 'gneissic intrusives' of Feininger et al ( 1972 ) which form part of the Puqui Complex, northern Central Cordillera . K/ Ar and Rb / Sr ages for the Puqui Complex gneisses and of other foliated intrusives sampled along the Central Cordillera, range from 242 - 285 Ma (Toussaint, 1993 ). In the Merida Andes a 32 few Late Paleozoic K/ Ar and Rb / Sr ages have been reported, ranging from 252 - 332 Ma (Gonzalez de Juana, 1980; p. 117 ). The same problems outlined for the Mid-Paleozoic intrusive radiometric data applies here.Figures 4 to 11 show some of the exposures of the units previously described.

Figure captions Figure 1. Physiographic provinces of the northern Andes. (1) Eastern Cordillera, (2) Magdalena Valley, (3) Central Cordillera, (4) Western Cordillera, (5) Perija Range, (6) Sierra Nevada of Santa Marta and (7) Merida Andes. Figure 2. Location of basement uplits in the northern Andes. (1) Borde Llanero fault system, (2) Santa Marta-Bucaramanga fault, (3) Tachira depression, (4) Oca fault. Figure 3. Simplified key Precambrian-Paleozoic stratigraphic columns for the northern Andes. (1) Migmatites and granulites, (2) low metamorphic grade metapelitic rocks, (3) foliated intrusive rocks, (4) non-foliated intrusive rocks, (5) felsic volcanics, (6) mafic volcanics, (7) red-beds and conglomerates, (8) sandstones, (9) mudstones, (10) limestones. Figure 4. Garz6n Massif -1.1 Ga age gneiss displaying interference folds. Taken at Pescado creek, near the town of Guadalupe, Huila Province. Figure 5. Garz6n Group -1.1 Ga gneiss with boudinage structure. Photo taken in EI Higado creek, near the town of Tarqui- Serranfa de las Minas, Huila Province. Figure 6. Santander Massif -1.0 Ga gneisses, along the road Totumal-Ocafta, 33 Santander Province. Figure 7. Santander Massif -1.0 Ga gneisses displaying inharmonious intrafolial folds, along the road Totumal-Ocafia, Santander Province. Figure 8. Orthogneiss with isoclinally folded amphibolitic layers. Taken along the road Berlin-Pamplona, Santander del Sur province ( my friend and colleague Camilo Montes for scale ). Figure 9. Intrusive contact between the Pescadero granitoid ( light colored) and Lower Paleozoic metapelites. Aratoca-Bucaramanga road, Santander del Sur province ( myself for scale). Figure 10. Santa Marta massif -1.0 Ga gneises along the Guatapuri River, north of Valledupar, Cesar province. Figure 11. Intrusive rocks of the Santander Massif. A - 200 Ma granite sampled along the Curos-Santa Barbara road versus a -470 Ma foliated granitoid sampled along Berlin-Vetas road- Santander del Sur province. 34

--~; ' ,.-_ .. , ----.... 4. • .I'" , ... ,,,,, Perija \ r r' Santa Marta I " II' I"'~ Range .' 1 ( ••' Massif ,. il ~.! .. ,... t" ... ~7 --_ .... ' ! ~ \ ~\ J-' / "'I~~\ /' 'J\\a ~ ... ,,,... \ l'l. --..... - ... " ~'!.~ \ \ , .....,"...... J \ \ ... , ~ / < 1000 Km. f

" "J ) I.J\~ rtf:)}, Foresta ( I Massif ! \ \ '.

Garz6n­ Massif

-;"J Figure 2. ------,... _... _-- ._. '.J1 ~l-·:,~UlhAIllCri<., MERIDA ANDES \ "'; \--)/ f / ,-' l)rd 1\ "Cilfddodiln 500-460 1\1<1 U / Pb ;-I r;'"_ I. pC \' 50111/11'1'11 SANTA MARTA MASSIF

SANT ANDER-FLO/{EST.4 AJ.4SSIfS -200 l\'la • .-200 I\lil. K/ AI' K/Al' 11\ - .Iur 1\.'1lI1 - E Pt.'nn . .:~ 1.3 Gil Rb/SI' .• ~D~ - 0.94 Ga K/ AI' M-L Dev 360-470 Mil K/ AI' GARZON AI.4SSIF -150 Ma Rb/Sr • A1ACARENA UPLIFT r ----.--:? ijw "'" ·0.95 Gil K/ AI' ~.,.., .·,tt, pC '~~-. .~'.~ g: m~' ~ ~

...... ~ ~~ §2g~~~ Ilt~~~~~ t:::~:::. m~~m~: ~l!tl!~!lr!l!; _ H '.,J 0' L--Figure______3. __ 37

Figure 4. 38

Figure 5. 39

Figure 6. 40

Figure 7. 41 42 43 Figure 11. 45 v. DISTRIBUTION AND PETRO-TECTONIC CHARACTER OF THE GRENVILLE-AGE BASEMENT IN THE COLOMBIAN ANDES, BASED ON NEWGEOCHRONOLOGICAL,GEOCHEMICALANDISOTOPICDATA

Abstract

New U-Pb zircon crystallization ages and 40Ar/39Ar cooling ages confirm the existence of a Grenville-age orogenic belt along the eastern Colombian Andes, including the first reliable -1.0 Ga age reported for the eastern margin of the Central Cordillera. Our data also indicates that previously reported Rb-Sr isochron 1.6 Ga age assigned for an augen-gneiss of the Garz6n Massif is more accurately 1.1 Ga age. The former age is considered to be a mixing or inherited age. No rocks older than -1.3 Ga could be accounted for in the Andean basement of Colombia. The first neodymium model ages for the Grenvillian basement of the northern Andes may be interpreted to suggest that these consist of "new" crustal additions together with Early Proterozoic - Late Archean rocks tectonically remobilized during the -1.0 Ga age event. Nd crustal residence ages obtained here are similar to those obtained in the Grenville Province eastern U.s. and Southern Mexico Oaxaca Terrane basement rocks. The Grenvillian or locally known as Nickerie or Orinoquian belt of Colombia passes through the western-central Peruvian Andes extending into southern Bolivia, and perhaps even to southern South America. Altogether, the geological evidence indicate that the Grenvillian belt of northern South America is of collisional character. 46 The data here presented is incorporated to the most viable paleogeographic reconstructions .

Introduction The Andes of Colombia branch out into three main ranges namely the Eastern, Central and Western Cordilleras. The Borde-LLanero fault system constitutes the structural boundary between the Colombian Andean domain and the stable westernmost portion of the Guyana shield . Guyana shield rocks are mainly exposed near the Colombian-Venezuelan-Brazilian border were the Andean foreland clastic wedge thins-out. McDonald et al ( 1969 ) , Goldsmith et al ( 1973 ) and Tschantz et al, (1974) first reported geochronologically the existence of Precambrian rocks in the Andes of Colombia. Subsequent K/ Ar and RblSr geochronology on basement rocks the Garz6n Massif southeastern Andes of Colombia led Alvarez et al (1980), Alvarez (1981,1983) and Kroonenberg (1982) to suggest that the basement exposures in the Colombian Andes constituted sporadic exposures of a Grenville-age granulite belt. They refered to the -1.0 Ga tectonothermal event as the Nickerie Metamorphic Episode. It is also locally referred to as the Orinoquiense Orogenic Event (Martin, 1974) Basement rocks crop-out along the Eastern Cordillera of Colombia namely at the Garz6n Massif, northernmost Santander Massif and on the Santa Marta Massif near the Caribbean coast ( Fig. 12 ) . Cover rocks consist of Lower Paleozoic metamorphic and unmetamorphosed sedimentary rocks and Mid-Paleozoic through Tertiary sedimentary rocks. Ordovician and more 47 significantly, Triassic - Jurassic plutonic rocks were emplaced throughout the eastern range.

Analytical methods Sampling was carried out according to guidelines described in Yanez et al ( 1989 ) and analytical methods are described in detail in Restrepo-Pace (1995 ) . Mineral separations were done at the University of Arizona, using heavy liquids only for the zircon separates. All minerals were hand-picked and inspected to ensure 99.9 % purity. Trace element geochemistry was performed in an ICP mass spectrometer at the University of Arizona. U/Pb analyses were conducted by Dr. George Gehrels and Sm/Nd analyses by Dr. James Gleason at the University of Arizona. 40Ar/39Ar analyses were carried out at the University of Lausanne, Switzerland . Refer to table 6 for sample location and description .

The Garzon Massif The Garz6n Massif is the most extensive exposure of Andean basement rocks in Colombia (Fig. 13c ). It is bounded on the east by the east verging Borde Llanero fault-system which places the massif over Tertiary clastics of the Andean foreland. On the west it is bounded by the west verging Garz6n­ Suaza thrust-fault, placing the massif basement over either Triassic-Jurassic plutons e.g. Suaza-Altamira syenite or Meso-Cenozoic sediments of the Upper Magdalena Valley. Kroonenberg (1982) divided the massif's metamorphic rocks into two 48 petrotectonic units: the Garz6n Gp banded granulites and the homogeneous Guapot6n and Mancagua Gneisses augen-gneisses. Lithologically these units comprise charnockitic-enderbitic granulites, garnetiferous charnockitic granulites, pegmatitic augen - orthogneises, pelitic gneisses, metacalcsilicate rocks, mafic granulites , hornblende-biotite augen gneisses, amphibolites and locally othopyroxene hornblendite and meta-ultramafic lenses . The ubiquitous presence of orthopyroxene and strongly perthitic and/ or anthiperthitic feldspars are indicative of granulite grade P-T conditions (Kroonenberg, 1983). A wide spectrum of Rare Earth Element (REE) patterns characterize the Garz6n metamorphics (Fig. 14, Table 1 ) : from enriched to flat patterns with LREE 0.5 - 300 x chondrite, (La/LU)N from 1.4 -18.24 and I

REE 14 - 399. Eu anomalies range from highly negative for the felsic gneisses and paragneisses to highly positive for the mafic orthogneises, with Eu/Eu* 0.29 - 2.59. The rock assemblages of the Garzon Massif basement are dominantly ensialic in character including metavolcanics and paragneisses , feldspar-rich metaplutonic rocks. Subordinate mafic orthogneisses and cumulates are lodged within the felsic ensialic suite. 40Ar/39Ar geochronology performed on a hornblende separate of sample SnAnKr-l, a biotite-hornblende augen-gneiss from the Guapot6n unit, yielded a staircase-like apparent age spectra (Fig. 15 , Table 2 ). In thin section, dark green hornblendes in most cases exhibit twining. Approximately 45 % of 39 Ar is released at -180 Ma apparent age in the first steps of the heating experiment. During the following steps the gas was released gradually, with the final gas discharged at -890 Ma apparent age. 49 K/ Ca ratio pattern which mimics age spectra, suggests that two mineral phases may have been present in the sample. Using the criteria of McDougall et al (1988), the SnAnKr-1 40Ar/39Ar spectra could be interpreted as having an older -890 Ma cooling age which approximates the crystallization age for the mineral and a younger thermal overprint at -180 Ma. The older age is interpreted to be a cooling age related the initial metamorphic event (at peak T) and the younger related to partial resetting due to the intrusion of the Suaza-Altamira granitoid, exposed along the western margin of the massif. A U-Pb age on four abraded single zircon grains for the same sample, SnAnKr-1, yields a 1098 ± 9 Ma crystallization age and a Pb - loss age of 600 ± 120 Ma (MSWD= 1.3) (Fig. 16, Table 3). 40Ar/39Ar apparent age spectra for a biotite separate from sample G-17 also seems to register the - 600 Ma event and Alvarez (1981) reported a 601 ± 56 Ma Rb-Sr age for a similar sample. The interpretation of this - 600 Ma perturbation is hampered by the large errors associated with these ages . In southwestern Venezuela's Merida Andes the -600 Ma event is represented by several U-Pb zircon ages obtained from its basement rocks (Burkley, 1976; Marechal, 1983, p. 128 ). A 1.6 Ga whole rock Rb - Sr isochron had been reported for the Guapot6n Augen gneisses (same unit of sample SnAnKr-1) by Priem et al (1989). The 1.6 Ga age could be considered to represent a 'mixed' age between the -1.0 Ga age component and some older inherited component. Similar differences in Rb/Sr and U-Pb ages were reported for the Arequipa Massif­ Peru ( Cobbing et all 1977), the older Rb/Sr age interpreted to be an inherited component (Wasteneys, 1994). 50 40Ar/39Ar age saddle shaped spectra for a hornblende separate, sample G - 20 an orthopyroxene bearing hornblendite, indicates that this sample most likely contained excess argon. We were unable to confirm the presence of exess argon given that inverse isochoron plots yielded poor correlations. Clustering of data was common due to the overwhelming 40Ar radiogenic component in the samples . Approximately 50 % of the 39 Ar was released in the middle stages of the heating experiment . The central portion of the spectra defines a plateau age at -1028 Ma interpreted here as the cooling age related to metamorphism (cf. McDougall et al, 1988 ). Feldspar separates for samples G-2 and G-17 charnockites, display complex 40Ar/39Ar age spectra, yielding integrated ages of 188 ± 3 Ma and 174 ± 3 Ma respectively. Both samples seem to imply a Jurassic thermal event as in sample SnAnKr-l. The complexity of the spectra is aided by the strongly perthitic nature of the feldspar separates . Sample HP-3, an amphibolitic orthogneiss, was collected on the eastern edge of the Central Cordillera. 40Ar/39Ar apparent age spectra for a hornblende separate of sample HP-3, an amphibolitic gneiss, yielded a simple plateau age of 911 ± 2 Ma indicating that the Grenvillian basement extends to the eastern edge of the Central Cordillera. Previous K-Ar whole rock ages of 1360 ± 270 Ma and 1670 ± 500 Ma had been reported for similar rocks, north of sample HP-3 along the eastern edge of the Central Cordillera ( Vesga et al, 1978; Restrepo et aI, 1978 in Maya, 1992). The latter mentioned ages are of uncertain geological significance given the large errors associated with them. We consider the above -1.3 and -1.6 ages as suspect and warranting 51 independent corroboration. Further geochronologic work is needed to determine if outcrops along the eastern edge of the Central Cordillera, of similar metamorphic rocks to the unit from which sample HP-3 was collected, form part of the Grenvillian - age belt.

The Santander Massif The Santander Massif is located in the northern portion of the Eastern Cordillera (Fig. 13b ). Bounded on the west by the left-lateral Bucaramanga - Santa Marta fault. On the east it is bounded by the east vergent Pamplona­ Cubug6n- Mercedes thrust-system that places the Santander Massif over to the Tcichira depression, southern Merida Andes and the Catatumbo basin to the north. Basement rocks are best exposed in the northern portion of the massif. A greater part of the massif's core is made up of a thick supracrustal sequence of rocks metamorphosed during an Early-Middle Ordovician orogenic cycle and a Jurassic thermal event (Restrepo-Pace, 1995). The sedimentary cover rocks comprise, with several stratigraphic hiatuses, Silurian (?) to Tertiary sedimentary sequences. Many large calk-alkaline plutons of Triassic-Jurassic age are exposed. A thick section of migmatites crops-out along the road between Totumal and Ocana. Lithologically, the dominantly quartz-feldspatic gneisses are interlayered with subordinate amphibolitic gneisess. 40Ar/39Ar apparent age spectra for two hornblende separates of samples OT-l and OT-2 (amphibolitic gneisses) produced complex patterns (Fig. 15) . Both samples exhibit partial argon loss in the initial stages of the heating 52 experiment, due to a thermal perturbation at - 200 Ma . This is interpreted to be the result of heat transferred following the development of an extensive plutonic arc along the Eastern Cordillera in Triassic-Jurassic time (Ward et al,

1973 ; Restrepo-Pace, 1995 ). Both spectra exhibit a stair-case 39Ar release pattern with increasing apparent ages up to -800 Ma for sample OT-1 and -850

Ma for sample OT-2 at 1100 0 C . During the last stages of the heating experiment apparent ages decrease gradually. The K/O ratio plot mimics that of the age spectra, implying that more than one mineral phase was present in the degassed sample. In thin section, a few hornblendes were observed to be associated with retrogressive chlorite. The resulting age spectra therefore could represent a mixing age between the older hornblende phase and the younger phynosilicate phase (McDougall et al, 1988 p. to0-101 ). Nonetheless these samples reveal an 'old' argon component with an apparent initial cooling age of -800 Ma, which added to simmilar lithologies, metamorphic grade and stratigraphic relationships suggests that these rocks represent a continuation that these of the Grenvillian-age rocks exposed in the Garz6n Massif. Moreover, two granitoids in this region have been reported to have inherited Grenvillian U /Pb zircon ages: The Lajas Granite, located approximately 150 Km north of samples OT-l,2 in the Perija range, yielded a -1.0 Ga inherited age (Dasch et al, 1981 ) and the Paramo Rico pluton, ... 30 Km northeast of Bucaramanga which yielded -1.3 Ga age (Grosser, written comm, 1994). Additionally the Jojoncito granitic gneiSS in the Guajira Peninsula of Colombia, gave a 1250 Ma U/Pb zircon age (Irving, 1975; Etayo­ Serna et al, 1983 ). 53 The Santa Marta Massif The Santa Marta Massif is an isolated triangular-based basement uplift located at the Caribbean coast (Fig. 13 a ) . Bounded north by right-lateral Oca fault, west by the left-lateral Santa Marta- Bucaramanga fault and on the southeast by the Cesar Valley. The massif rises to 5800 m elevation from 0 m elevation at the Caribbean coast, in an horizontal distance of -45 Km. Geologically it consists of three northeast-southwest trending tectonostratigraphic belts ( Tschantz et aI, 1969, 1974 ). The two younger northwestern belts consist low metamorphic grade schists intruded by Triassic - Jurassic and Tertiary plutonic rocks. Granulite metamorphic grade migmatites are exposed along the southeastern belt. These consist of an interlayered sequence of pelitic and quartzfeldspatic gneisses, orthopyroxene - clinopyroxene metabasites , hornblende - clinopyroxene mafic gneisses and anorthosites . Samples were collected along the Guatapuri river-northwest of VaUedupar and south of Dibulla. REE patterns for basement rocks of the Santa Marta Massif (Fig. 14 ) are typical of highly evolved rocks. LREE

enriched 37x -197x chondrite, (La/Lu) N ratios from 9.17 to 19.86 I ~ REE 56-

281 and a large negative Eu anomaly with Eu/Eu* between 0.18 - 0.49. Samples RG-3 and RG-6 , quartz - pyroxene-gamet-biotite gneisses, yield complex 40Ar/39Ar apparent age spectra for two biotite separates ( Fig. 15) . Both samples have saddle-shaped age spectra that "sag" during the intermediate stages of the heating experiment. Interlayered or included dorite could account for the sample's degassing behavior. SEM analyses are needed to corroborate the presence of inclusions since these were not evident in thin 54 section. Additional U IPb zircon ages were conducted for two other samples. Of the seven zircon grains analyzed from sample RG-1, a quartz - pyroxene­ garnet-biotite gneiss, three were approximately 1.5 Ga old. Two of these older grains lie above concordia which may be due high metamorphic grade mobilization of Ph or U (Gehrels, pers. com., 1995) . The other four grains duster below concordia and yielded ages between 1.0 - 1.3 Ga . Zircons for sample SMR-4, a quartz - plagioclase - hornblende gneiss, contained zircons with ages between 1.0-1.3 Ga . The detrital character of the zircons from samples RG-1 and SMR-4 precluded a unique crystaIization or Pb-Ioss age . The geochronological data and lithological associations for the Santa Marta Massif basement rocks suggests that these are also part of the Grenvillian-age belt that is exposed along the Eastern Cordillera of Colombia. In Table 4 we present a summary of previously reported geochronologic data for the Grenvillian-age basement of the Colombian Andes.

Tectonic implications Four neodymium crustal residence ages for basement rocks of the Garz6n and Santa Marta massifs range from 2.7 - 1.5 Ga (Table 5 ). Sample G-20 is considered unrelaible in terms of its model age due to the low Nd

fractionation of the sample, resulting in large errors in the estimated crustal residence age. Samples HP-3, SnAnKr-1 and RG-6 yieled 2.0, 1.7 and 1.5 Ga respectively. These preliminary ages are comparable to ages obtained for 55

Grenvillian rocks in eastern U. S. and sourthern Mexico (Fig. 17). £Nd(t) range from -3.9 to + 0.6 implying variable involvement of older crustal materials which were 'mixed' with newer crustal materials produced during the -1.0 Ga orogenic cycle . REE geocheinistry and petrology and zircon data indicate that most of the older crustal material involved must have been highly evolved sedimentary and/ or volcano sedimentary rocks, partly derived from some Early Proterozoic source. This is indirectly implied by the overlap between crustal residence ages for the Andean Grenvillian-age rocks, dated detrital zircons with the ages for the crustal provinces in the Guyana shield ( Fig. 18 ). It seems reasonable to suggest that the older source may have been Guyana Shield rocks, therefore the protoliths of the Grenvillian age rocks for the most part were laid down as a pericratonal sequence . Other lithological components include synkinematic plutons e.g. the Guapot6n augen gneisses from the Garz6n Massif with a U-Pb zircon crysta1Iization age of 1.1 Ga and a crustal residence age of 1.7 Ga. The near flat REE patterns for the mafic - ultramafic suite- of the Garz6n Massif- , 1 to 2 x chondrite, L REE from 13 to

23 and a positive Eu anomaly may indicate that these constitute restites. Nonetheless, their apparently older crustal residence age may imply that these represent older crust incorporated tectonically during the -1.0 Ga event. For the Garzon Massif were the most reliable data has been obtained thus far, a temperature-time curve has been constructed ( Fig. 19 ). From this curve a complex cooling history is derived which begun with a relatively slow cooling period related to the Nikerie metamorphic episode ( -1.0 Ga ), a rapid 56 cooling period following a Jurassic thermal event related to multiple intrusions and back-arc extension and finally by an accelerated rate since - 10 Ma related to the final uplift of the massif during the Andean orogeny. The Colombian Grenville-age belt shares many characteristics of the Grenville Province sensu stricto exposed in eastern Ontario, Canada and southern Mexico's Oaxaca terrane rocks. Common lithologies and metamorphic grade, common peak age of metamorphism and crustal residence ages . Additionally, there are striking biostratigraphic affinities between the Ordovician sedimentary rocks that immediately overlie the Grenvillian-age basement of southern Mexico and eastern Colombia. These similarities have led some researchers to postulate that the basement rocks of southern Mexico and northeastern South America, had been continuous in an earlier time (Yanez et al, 1991; Restrepo-Pace et al, 1994). In South America the Orinoquiense-Nickerie belt is exposed discontinuously along the entire Andes from Colombia, continuing in Bolivia's Sunsas Belt (Litherland et al, 1989), the Arequipa massif ( Westeneys, 1994 ), Eastern Cordillera of Peru ( Megard, 1987) and northwestern Argentinean Precordillera and the Malvinas plateau ( Ramos, 1988 ). The thermal perturbation associated with the Grenville-Nickerie­ Orinoquiense collisional event could be perceived isotopically well into the craton, evidenced by the 0.9 -1.2 Ga cooling ages obtained along eastern edge of the Guyana shield ( Kroonemberg, 1983; Onstott, 1989). The magnitude of its tectonic and thermal effects and the extent of the Grenville age basement in South America is suggestive of a collisional event that fits well in the 57 paleogeographic scheme of Hoffman ( 1991) and Park ( 1990) (Fig. 20 ).

Figure Captions Figure 12. Location of basement exposures in the northern Andes. (1) Borde­ Llanero fault system, (2) Santa Marta-Bucaramanga fault, (3) Oca fault. Figure 13. Simplified geologic maps of the Garz6n (C), Santander (B) and Santa Marta (A) Massifs, Colombia. 1. Grenvillian granulite basement, 2. Lower Paleozoic metapelitic belt, 3. Middle-Upper Paleozoic sedimentary rocks, 4. Triassic-Jurassic intrusives, 6. Jurassic sedimentary rocks. Figure 14. Chondrite normalized Rare Earth Element patterns for representative Grenvillian basement rocks of the Colombian Andes. Shaded area depicts spectrum of REE (ppm) normalized compositions.

Figure 15. 40 Ar/39 Ar apparent age spectra for basement rocks of the Colombian Andes. Figure 16. U/Pb zircon ages for selected basement rocks of the Colombian Andes. Figure 17. a. Nd crustal residence ages for selected - 1.0 Ga age rocks of Grenvillian age - Colombia and b. comparison with crustal residence ages for the Grenville of North America compiled by Emslie et al, (1994) Figure 18. Distribution of N d crustal residence ages for the Grenvillian basement of Colombia in relation to age provinces from western Guyana shield (from Texeira et ai, 1989) : (1) Grenvillian out-crops, (2) Early - Paleozoic metapelitic rocks, (3) Early - Paleozoic intrusives, (4) Rio Negro - Juarena mobile belt, (5) Central Amazonian Province (6) Maroni - Itacauinas 58 mobile belt (7) Phanerozoic cover. Figure 19. Temperature-time curve for the Garzon massif. Data from Maya 1992 ; van der Wiel, 1991 and this work. Vertical error bars relate to range of closure temperatures: U-Pb zircon from Ghent et al, 1988 ; all others from Geyh et al , 1990 . Continuous curve represents best contrained trayectory, broken curve represents warming associated with a possible 0.6 Ga event. Figure 20. Paleogeographic reconstruction for the 1.2-0.6 Ga period, showing the position of A=Amazonia (South America) with respect to L=Laurentia (North America) and B=Baltica (Baltic Plate) according to Hoffman (1991) and Park (1992).

Table 1. Rare Earth Element data for representative basement rocks of the Colombian Andes. Table 2. 4OAr/39Ar step heating experiment data for selected basement

samples of the Colombian Andes. Table3. U/Pb zircon age data for selected basement samples of the Colombian Andes. Table 4. Previously reported Grenvillian ages in the Colombian Andes ( from Maya, 1992). Table 5. Neodymium isotope data for selected basement rocks of the Colombian Andes. Table6. Sample location and description. 'V''-'~:::''~=:~-I- San taM ar t a Perija . /,..1 M of ,/"!\ (c:-/ ass! Range I. c.... "L'.. -.J ~ r,\ \' .,." ~ .. v" ~~--~~'\ . \~~~l c, .....{.f 1-;;;;:::='~.....--"'>'"-- 3 . ---'». '_. "1.. , r .... ' .--_... ~.-...... -... "1...... " r:::::'

1000Km. ')'··'("5. a~' der ) \~ r::.. ~~~!( r'l)'la: sif " ' .. ec~' I I

\ , ANDES

L-----______.)1 Figure 12. .G Caribbean Sea

___---l. SOKm

t N

o__ --I SOKm

I 750W c

5

4

3

!·:\":/;J·;·::';~·i?';::·i:1 1 Figure 13. Rock / Chondrite Rock / Chondrite ...... 'T.I ...... 0 crq ..... <::> ..... <::> 0 c: ..... <::> <::> ..... 0 0 0 '"t ..... 0 <::> 0 II) )000\ ~ La La CJ (f) Ce > Ce > Pr Pr Nd ~ Nd ~ Sm Z Sm > Eu Eu ~ ~ Gd > Gd (f) ~ Cl § Tb tI1 Th tI1 g Dy > Dy ~ Ho Ho > Er Er (f) Tm Tm ~ Yb Yb Lu Lu

0' ~ 62

~ ~ 950- ~ ------'911±2 Ma------..~ F 900- =~ ~ HP-3 Hornblende Total fusion age = 905.6 Ma ~ 850- Amphibolite gneiss I I I I I I I I I

1.50 CIS U 1.00 -~ 0.50

~ SnAnKr-l Hornblende ~ 700 ..ca Augen gneiss ~ 500 Age min = 180 Ma Age max = 890 Ma ~ 300 <

~ G - 20 Hornblende ~ 1150 Amphibolite ~ ~ Total fusion age = 1117Ma =1050 Age min = 1028 Ma ~ Page = 1073.9 Ma ~ ~ 1000 < 50 Cumulative % 39 Ar Released Figure 15. 63

ftI 300 G-2 Feldspar

~ Charnockitic gneiss .. ' ",1: .. " ~ QI be ftI 200 ....c:: ~ ftI 100 Integrated Age = 188 ± 3 Ma ~

ftI 300 G-17 Feldspar ::E Chamockitic gneiss QI ~ 200 ....ftI =~ ftI 100 CI. Integrated Age = 174 ± 3 Ma

ftI 1000~------~ ::E QI 800 ~ ftI -= 600 --I~"'" G-17 Biotite ~ Chamockitic Gneiss ~ 400 Integrated Age = 765 ± 7 Ma ~ 50 Cumulative %39 Ar Released 64

~ 20 ..... a o 20 0 ..... ~ i1000 ~------~o :0 800 t'II 1: 600 t t'II OT-l Hornblende =- 400 =­ Amphibolitic gneiss < 200 Integrated Age = 574 ± 8 Ma

'0 2 ..... ~ t'II10OO ~------;O ::2 800 III f 600 .. OT-2 Hornblende ~ 400 Amphibolitic gneiss ~ ~200 < 0 50

Cumulative %39Ar Released 65

RG-3 Biotite ~ 800 fa Granulite ....~ 600 C 400 efII a. Integrated Age = 561 ± 6 Ma ~ 200

~ 1000 RG-6 Biotite :0 950 Granulite ..ftI ! 900 a.Q. 850 <

50 Cumulative %39 AI Released 0.20 I" 1160 SMR4 /, 1120 0.19 t- 1080 ./ , 0.190. .. 206Pb* 0.18 [ 1040 238 U 1000 SnAnK-l 1100 0.17 , 0.186 960 , , 0.161- 1080 920 , 206Pb* 0.182 238 U 0.15 ,

0.182 0.14~'--~--~--~--~--~--~ __~~~~~ 1.4 1.6 1.8 1040 0.32 • , 0.1741. /, Upper inll'rccpl = 111911 ± 9 !VI.! RGl 1600 , LOlVl'r inll'rcepl = 6(1(1 ± 12(1 Mil 0.26

0.170~'·~ __~~-L~~~-L~ __L-~-L~~L-~ 1400 1.74 1.78 1.82 1.66 1.90 1.94 U6 2.02 0.24 , 1200 207Pb* /235 U 206~ 0.20 238 U

0.16

0.12

0.08 • J o z 3 4 207Pb*/235 U Figlll'e 16.

0' ::1' 50 _DM ID 40 : f-~~:: ~B- ....~ ~ 30 ENd -4 ~ -8 E20 ] -12 ~ 10 -14 I I I o 2.5 2.0 1.5 1.0 0.5 o o 0.5 1.0 1.5 2.0 2.5 Nd Crustal Residence Age (Ga) T DM model ages, Ga

GB= Andean Grenvillian basement • Southwest U.S. & Mexico A N d crustal age range • Eastern U.S. • Grenville Province til Andes - Colombia B' Figure 17.

0' '-l ~·'.'·"·',··'J I~}: 2 3 4 5 6 7 *=crustal residence age1

Figure 18. 0'- 00 1000 69

U-l'b"lS.~ Zircon 800

Ar-Ar Hnmblende ~ .. min age \ U 600 I" 0 E-4 I~b-Sr Biutite 400

200

. F-TR ,. AP'ltite

O~~~~T-T-T-~~~r-~r o 200 400 600 800 1000 1200 AgeMa

Andean Uplift

Partial resetting related to intrusions and back-arc extension

Pan-African (?)

Nickerie orogenic event

Figure 19. 70

0----

-1.2 Ga M. Proterozoic

L

-1.05 Ga M. Proterozoic

-0.6 Ga L. Proterozoic ~) I

Arc magmatism

Grenvillian-Orinoquiense orogen Figure 20. -1.0 Ga 8J\SEMllNT ROCKS-ANDES OF COLOMBIA Sample G-I G-20 G-2I HP-3 SnAnK- RG-I RG-3 RG-4 RG-6 RG-8 OT-} I

I.a 13.33 1.22 2.06 23.29 91.89 13.37 32.33 52.33 69.62 22.85 11.89 Ce 29.17 3.5~ 7.53 61.35 151.65 23.01 82.34 77.03 113.22 45.08 28.92 Pr 3,(>8 0.52 0.79 8.00 23.21 2.23 8.66 7.60 11.83 4.95 3.53 Nd 20.-K1 2.-18 4.36 41.07 77.66 10.28 37.63 34.17 51.01 23.% 18.68 Sm ~.67 0.87 1.28 9.87 15.33 2.50 7.f1J 5.69 9.03 5.09 4.17 Eu 1.00 0_~2 0.90 1.97 1.42 0.23 1.16 0.54 0.52 0.88 1.00 Gd ~.87 0.65 0.76 7.99 1~i.86 2.13 8.81 7.74 7.89 5.86 5.09 'Ib 0.75 0.19 0.30 2.18 2.12 0.22 0.94 0.79 1.29 0.76 0.63 D,r 4.37 1...10 1.81 14.21 10.17 1.01 4.83 4.08 6.92 4.07 4.05 110 0.93 0.33 0.37 3.14 1.91 0.21 1.09 0.85 1.49 0.84 0.99 Er 2.36 0.86 1.11 8.21 4.99 0.55 3.05 2.33 4.04 2.29 2.84 'lin 0.29 0.14 0.15 1.05 0.53 0.07 0.36 0.27 0.48 0.27 0.29 Yb 2.2~ 1.05 1.18 8.43 4.10 0.51 2.76 2.25 3.81 2.09 2.38 I.u 0.28 0.15 0.15 1.()4 0.52 0.07 0.36 0.29 0.46 0.26 0.27 \If 1.72 0.53 0.69 3.85 14.79 2.35 4.64 4.01 4.83 3.70 1.84 Ta 2.14 1.93 6.79 1.15 1.46 0.28 1.01 0.57 0.72 1.12 1.69

r REI: 88.38 13.72 22.75 191.79 399.36 56.40 191.42 195.95 281.60 119.23 84.73 Eu/Eu* O.fH 1.06 2.59 0.66 0.29 0.30 0.45 0.25 0.18 0.49 0.66 (La/Lu) 5.03 0.86 1...10 2.32 18.24 19.86 9.21 18.69 15.84 9.17 4.54 N Concentrations in ppm Tablet.

'1 I-' GARZON MASSIF

I = Integrated age TF= Total fusion age P = Plateau age

Sample G-2 K-spar I age = 188 ± 3 Ma J=O.004768±0.000024

TOc CalK 36Ar/39Ar *40Ar/39Ar(K) Moles %Radiogenic % 39Ar Age 40Ar.l0-14 Release 850 0.0000 0.02236 22.022 9.5 76.9 5.0 180±5 950 0.0355 0.01325 12.021 10.2 75.4 9.7 101±2 1000 0.0000 0.01160 14.166 10.8 80.5 9.3 118±2 1050 0.0000 0.00652 21.229 25.1 91.7 16.5 174±2 1100 0.0000 0.00443 23.291 33.9 94.7 21.0 190±3 1150 0.0000 0.00412 28.687 34.6 95.9 17.6 231±4 1200 0.0000 0.00898 31.784 22.3 92.3 9.9 255±4 1251 0.0000 0.01782 25.670 10.3 83.0 5.1 208±3 1300 0.0000 0.01069 27.813 12.1 89.8 5.9 225±7

Sample G-17 Biotite I age = 765 ± 7 Ma J=0.004780±0.000041

TOe 36Ar/39Ar *40Ar/39Ar(K) Moles %Radiogenic %39Ar Age 40Ar.l0-14 Release 700 0.20401 58.652 19.9 49.3 2.5 446±31 800 0.00455 81.731 46.3 98.4 8.4 595±6 850 0.00241 82.958 55.0 99.2 9.9 603±6 Table 2. ;:j 900 0.00228 81.028 58.7 99.2 10.9 591±4 950 0.00307 77.367 54.3 98.8 10.5 568±4 975 0.00563 70.826 41.7 97.7 8.7 526±5 1000 0.00789 69.519 36.4 %.8 7.7 518±6 1050 0.00599 64.160 39.6 97.3 9.1 483±5 1100 0.00335 65.909 47.2 98.5 10.7 495±4 1200 0.00051 86.599 99.5 99.8 17.3 625±4 1351 0.02259 84.470 24.0 92.7 4.3 612±8

Sample G-17 K-spar Iage=174±3Ma J=0.00470±O.000024

'fOe CalK 36Ar/39Ar • 40Ar/ 39 Ar(K) Moles % Radiogenic %39Ar Age 40Ar.10-14 Release 750 0.1311 0.02358 1.261 2.9 15.3 4.2 11±6 850 0.0203 0.00922 17.665 9.3 86.6 5.4 146±4 950 0.0505 0.00500 16.613 12.9 91.8 8.5 138±3 1000 0.0197 0.00667 14.577 10.7 88.1 7.7 121±2 1050 0.0009 0.00377 21.161 20.0 95.0 10.7 174±3 1100 0.0000 0.00203 22.944 31.3 97.5 15.8 187±2 1150 0.0000 0.00053 22.942 35.0 99.3 18.0 187±2 1200 0.0000 0.00035 24.233 34.8 99.6 17.0 197±3 1251 0.0000 0.00333 23.002 17.6 95.9 8.7 189±3 1300 0.0000 0.00777 25.279 9.4 91.7 4.1 205±7

w'-J Sample G·20 Hornblende TF age = 1117 Ma J=0.001602±O.000041 Page = 1073.9 Ma

TOC Ca/I< 36Ar/39Ar *40Ar/39Ar(K) Moles % Radiogenic %39Ar Age 40Ar.10-14 Release 850 0.038 1.24458 832.265 243.1 69.2 0.2 4803±243 950 0.034 0.16787 260.027 37.5 84.2 0.1 2962±139 1000 0.028 0.02808 204.873 30.0 96.7 0.2 2624±182 1050 0.040 0.02975 67.306 35.5 89.5 0.5 1320±112 1075 0.045 0.01304 61.369 30.2 95.3 0.5 1235±49 1100 0.058 0.00309 56.291 143.9 99.5 2.8 1160±10 1125 0.060 0.00392 53.219 929.4 99.0 19.1 1112±4 1150 0.059 0.00307 49.512 1980.4 99.4 44.0 1054±3 1175 0.059 0.00293 47.959 432.3 99.5 9.9 1028±4 1200 0.061 0.00380 51.342 544.6 99.0 11.6 1083±4 1225 0.060 0.02276 52.638 324.3 98.2 6.7 1103±8 1250 0.058 0.00539 57,243 182.6 98.4 3.5 1174±16

Sample HP-3 Hornblende TF age = 905.6 Ma J=0.001602±O.OOO041 Page = 911±2Ma

-roC K/Ca 36Ar/39Ar *4°Ar/39Ar(K) Moles %Radiogenic % 39Ar Age 40Ar.10-14 Release 850 0.151 0.22696 24.931 158.2 27.13 1.1 606±51 1000 0.163 0.00883 32.710 113.4 93.2 2.1 760±20 1075 0.139 0.00236 40.867 1249.6 99.0 19.5 909±4 1100 0.140 0.00144 41.102 761.8 99.6 11.9 912±3 1125 0.106 0.00177 41.194 701.7 99.4 10.9 911±3 1150 0.139 0.00123 40.987 1921.8 99.8 30.1 910±5 1175 0.139 0.00130 41.236 1200.5 99.7 18.7 915±4 ~ 1200 0.141 0.00220 40.871 187.7 99.1 2.9 909±6 1250 0.141 0.00093 42.007 152.7 99.9 2.3 928±5 1400 0.153 0.00744 43.317 30.0 99.7 0.4 951±22

Sample SnAnKr-l Hornblende Age minimum = 180 J=0.OO1602±O.OOO041 Age maximum = 890

'fOC K/Ca 36Ar/39Ar *40 Ar/39Ar(K) Moles %Radiogenic %39Ar Age 40Ar.1O-14 Release 850 1.307 0.00279 6.637 585.1 89.2 25.5 182±2 950 1.927 0.00162 6.537 419.8 93.4 19.4 180±1 1000 1.608 0.00144 8.315 147.9 95.3 5.5 226±2 1050 0.525 0.00169 18.219 793.3 97.7 13.8 462±2 1075 0.332 0.00173 25.116 691.1 98.4 8.8 610±2 1100 0.248 0.00107 32.241 635.8 99.5 6.4 751±2 1125 0.203 0.00095 38.276 1307.4 99.7 11.0 863±3 1150 0.184 0.00161 39.759 926.5 99.3 7.5 889±3 1175 0.124 0.00145 38.439 192.5 99.7 1.6 866±4

SANTANDER MASSIF

Sample OT-l Hornblende Iage=574±8Ma J=0.004783±O.000024

TOC CalK 36Ar/39Ar *40Ar/39Ar(K) Moles % Radiogenic %39Ar Age 40Ar.l0·14 Release 850 0.500 0.06991 40.704 20.3 66.4 6.8 321±10 950 0.071 0.00361 30.399 19.5 97.7 12.8 245±4

'1 rJ1 975 0.071 0.00662 35.177 15.4 96.2 8.6 281±5 1000 0.072 0.00534 46.490 19.7 97.8 8.5 362±6 1026 0.095 0.00440 60.296 22.3 98.6 7.4 457±5 1051 0.139 0.00579 83.403 33.0 98.3 7.9 606±6 0.0 0.172 0.00415 133.545 120.8 99.3 18.3 891±11 1126 0.160 0.00209 104.069 49.5 99.6 9.7 729±13 1150 0.155 0.00125 115.768 53.2 99.9 9.3 795±14 1200 0.15 0.01281 112.821 50.3 97.0 9.1 779±14 1251 0.12 0.02195 81.079 12.4 92.9 0.8 591±8 1400 0.04 0.06082 74.094 3.4 81.2 0.8 547±29

Sample OT·2 Hornblende I age =668±9 Ma J=0.004770±0.000024

TOC CalK 36Ar/39Ar *40Ar/39Ar(K) Moles %Radiogenic %39Ar Age 40Ar.l0-14 Release 850 0.202 0.14573 19.276 14.4 31.0 5.3 159±22 950 0.062 0.01058 56.292 25.5 96.1 10.0 429±10 975 0.062 0.00855 71.543 29.0 97.4 9.1 530±9 1000 0.092 0.00281 89.632 36.9 99.6 9.4 642±9 1026 0.124 0.00165 106.017 46.6 99.8 10.1 738±6 1051 0.145 0.01465 114.971 59.2 96.6 11.4 789±6 1100 0.159 0.00527 120.667 110.2 98.9 20.8 820±6 1126 0.129 0.01516 114.933 60.4 96.5 11.7 789±5 1150 0.110 0.02088 104.332 28.9 94.7 6.0 729±7 1200 0.08 0.01871 93.641 23.1 94.9 5.4 666±6 1251 0.06 0.15437 38.732 3.1 46.2 0.9 306±34

d! SANTA MARTA MASSIF

Sample RG·3 Biotite I age = 561 ± 6 Ma }=0.004783±O.000041

"fOe 36Ar/39Ar *40Ar/39Ar(K) Moles % Radiogenic %39Ar Age 40Ar.10-14 Release 700 0.19181 88.211 40.2 60.9 6.5 635±18 800 0.00097 116.583 64.4 99.8 12.9 795±5 850 0.00021 117.949 63.6 99.9 12.6 S07±S 900 0.00105 117.398 57.4 99.7 11.4 804±5 950 0.00348 105.772 46.4 99.0 10.1 739±7 975 0.01034 92.276 35.0 96.8 8.6 660±7 1000 0.01203 91.917 34.5 96.3 8.4 657±7 1050 0.00434 104.549 40.4 98.8 8.9 732±7 1100 0.00448 117.552 43.8 98.9 8.6 805±8 1200 0.00076 132.823 69.0 99.8 12.1 887±5

Sample RG-6 Biotite I age =845Ma J=0.001602±O.000041

TOe 36Ar/39Ar *40 Ar/39Ar(K) Moles %Radiogenic %39Ar Age 40Ar.10-14 Release 750 0.00412 25.759 185.2 95.2 2.5 623±11 800 0.00334 36.720 406.8 97.4 4.0 835±2 850 0.00059 3S.809 1362.4 99.5 13.0 869±2 900 0.00054 38.885 1365.2 99.6 12.5 871±2 950 0.00037 37.625 874.2 99.7 8.6 851±2 1000 0.00072 35.232 1459.9 99.4 15.4 803±3

'.J '.J 1050 0.00130 32.656 485.8 98.8 5.5 759±7 1150 0.00030 36.525 1813.8 99.8 18.4 831±5 1200 0.00042 41.582 2052.6 99.7 18.3 921±2

Corrections for background, 37 Ar decay, interferences and mass discrimination were made. Errors on indivi­ dual ages are one standard deviation and do not include the uncertainty in}. Ar*=Radiogenic.

'-l 00 ------MEASURED RATIOS APPARENT AGES

Sample WT Pb U 206 206 206 206- 207* 207- (mg) (ppm)(ppm) 204 207 2011 2311 235 206------SnAnKr-l 25 104.3 600 4200 12.79 12.5 1036:1:4 1046:1:4 1067:1:4 21 21.5 112 700 10.53 7.6 1062±6 1066:1:8 1074±1 0 34 35.9 195 10600 12.99 11.3 1088:1:4 1089:1:5 1092:1:4 23 72.9 385 990 11.09 11.5 1088:1:4 1091±6 10911:1:8

SMR-4 53 82.8 567 7870 13.44 19.7 906±3 936:1:3 1007:1:3 14 107.5 690 4200 12.42 14.6 942:1:3 1000:1:4 1130:1:5 24 91.11 584 5500 12.39 18.8 963:1:3 1023:1:4 1155:1:5 9 120.1 723 1580 11.29 11.7 975:1:4 1045±5 1194±5 24 205.8 1290 13600 13.00 23.5 989:1:3 1023:1:4 1096:1:4 9 291.3 1600 8800 12.11 13.3 1082:1:4 1130:1:4 1224:1:3 33 27.6 145 6100 12.19 16.9 1146±5 1164:1:5 1197:1:4

RG-l 19 190.6 1750 1850 11.99 32.9 691:1:3 793:1:4 1092:1:5 23 117.11 1550 3060 12.11 12.9 697:1:3 815:1:4 1150:1:3 26 235.2 2030 3700 12.119 46.5 743:1:4 820:1:5 1037:1:3 26 301.5 2214 1290 1131 23.9 834:1:4 921:1:6 1137:1:8 13 40.0 146 450 8.10 6.1 1384:1:13 1427:1:14 1492:1:15 36 106.2 377 7490 10.26 20.3 1620:1:7 1587:1:8 1543:1:2 51 129.8 416 1197 9.37 13.5 1707:1:7 1631:1:8 1534:1:3

- = radiogenic Pb Measured ratios are uncorrected for blank, spike, or initial Pb. Constanl~ used: >-.235=9.8485xl0-10,>-.238= t.55125xl 0-10, 238/235=137.88. Data reduction from Ludwig (199101), concordia diagrams from LudWig (1991b). Analytical methods described by Gehrels (1990). Samples corrL'Cted for: (1) fractionation faclors of 0.14:1: 0.06 ~:./ amu for Pb and 0.04:1: 0.06 %/ amu for U, (2) blank values of 5 pg for Pb and 1 pg for U, And (3) initial Pb values interpreted from Stacey and Kramers (1975). Table3.

".J \0 80

Lithology Location Method Type AgeMa Author

CHARNOCI

WR = Whole Rock KF = Polasium Feldspar PHLOG = l'hIogopite HB = Hornblende ZR=Zircon n.s. = not specified

Table 4 Sample Unit Age Sm Nd 147:m 143N.ll b eNd C eNd C ToM (Mil) (ppm) (ppm) 144Nd 144Nd present initial d measured (Ga)

G-20 pC-Garzon 1000 1.14 3.62 0.1896 0.512626±7 -0.24 +0.6 2.71 Massif Hr-3 pC- Garzon 1000 9.25 32.95 0.1697 0.512472±10 -3.23 +0.2 1.97 Massif SnAnKr-l pC- Garzon 1000 14.93 78.97 0.1143 0.S12062±7 -11.2 -0.7 1.5 Massif RG-3 pC- S. Marla 1000 7.38 4D.42 0.1104 0.511872±8 -14.9 -3.9 1.72 Massif

Table5. a Uncertainties at 2-sigma are ± 0.5 % bRatios normalized to 146Nd/144Nd = 0.7219 (2-sigma errors reflect in-run percision) 4 I44 (eNd = 10 [(143Nd/144Nd(t)CHUR) • 1)), using 143Nd/144Nd = 0.512638 as present day CHUR value, and 147Sm/ NdCHUR = 0.1966 d model ages calculated using equation of DePaolo (1981)

.....O'J SAMPI.E If) IInll Lllhology ··I.aealion of daled Precision: Commenl: samples Map SClIle latitude N I.ongltudeW ------SANTANIlER MASSif

01'-1 Basemelll Q/!-hnb-plag-gnel5S S<>tS'14" 73"2&'n" 1:100000 Ocafta-TotumaJ ro.,d 0'1'-2 Bascnumt Q/!-hnb-plag-gnel5S 80 18'14" 73"2&'22" 1:100000 Ocnlla-Tolumnl road

GARlON MASSif

G-I Ba.wmenl ",,-gnel granulite G-2 Basement ",,-gnel granulite 1"58'38" 75"44'03" GUadalupe-Borenel., Vlelosa creek G-17 Basement Q/!-hornb-plag-blol gneiss 1"56'00" 75"41'00" I: 100000 Guadalupe-Florenrla, Vlelosa creek GZO Basemenl p.-homblendlle 1<\10'00" 75"4Z'OO" I: 100000 Guadalupe-Florenrla road C-21 Basement px-homblendlle 1<\10'00" 75"42'00" I: 100000 Guadalupe-Flonmcla road SnAnK-1 Basemelll Plag-Q/!-homb- augen gneiss 2'b7'30" 75"42'37" I: 100000 San Antonio-Guadalupe. 'ft-est of road IIP-3 8a.scment Homb-plag-OJ gneiss 2<>t 1'25" 75"40'00" I: 200000 Serranra de Minas, IUg.do creek

SANTA MARTA MASSIF

RG-I Basement Gnet-p.-qz-plag granUlite 10"3&-201" 73"201'19" I: 200000 Gunlapurl river RG·.~ Basement Gnet-p.-qz-plag granulite 10036'201" 73"201' 19" I: 200000 Guotnpurf n\'cr RG ... Basement Gnet-p.-qz-plag granUlite 10036'2.. " 73"24'19" I: ZOOOOO Guatapuri river RG-6 Ba.'Wment IInb-p.-qz-plag gnel ... 10036'2 .." 73"2,,'19" I: 200000 Guatapurl river RG-S Basement Hnb-px-qz-plag gneiss 1003&'201" 73"2"'19" I: 200000 Guatapurf river SMR-4 Basemelll ~-hnb-plag gneiSS I: 200000 Santa-Marta R10hacha road

Table 6

00 N 83 VI. GEOLOGY AND GEOCHRONOLOGY OF THE SANTANDER MASSIF, COLOMBIA: A UNIQUELY PRESERVED PHANEROZOIC TECTONIC RECORD IN THE NORTHERN ANDES

Abstract New field and geochronological data of the Santander Massif­ Colombia indicates that this uplifted basement block is the site of one of the most complete geological records for the northern Andes. The pelitic core of the massif which may have tectonically incorporated slivers of Grenvillian­ age basement, was regionally deformed during an Early Paleozoic tectonothermal event. This event was characterized by regional metamorphism accompanied by subduction related intrusion of synkinematic granitoids for which a U I Pb zircon crystallization age of 477 ± 16 Ma was obtained. The Early Paleozoic orogen was partially covered by a transgressive marine clastic sequence in late Silurian (?)-Devonian to Permian time. Late Mississippian ( ? )- Permian rocks consist of a basal red­ bed sequence followed by turbidites and capped by Permian shelf limestones. By Late Triassic and mainly in Jurassic time, extensional tectonics were dominant. Sediments from this period consist of two distinct lithofacies, namely molassic and intracontinental rift related sequences, which occasionally interfinger. The molassic sequence in Santander comprises two units, the first deposited following a regional folding event at the end of the Paleozoic and the second of Early Jurassic age, deposited following thermal 84 uplift of the Santander Massif. Back - arc crustal thinning accompanied by an enormous volume calc-alkaline intrusions caused uplift and igneous related low-medium pressure regional metamorphism. Petrologic, structural and

40 Ar /39Ar cooling data clearly attest for this regional thermal perturbation.

The back arc extensional phase concluded with the deposition a thick (-10 Km) Late Jurassic - Cretaceous marine sedimentary pile in the subsiding basins flanking the massif. Structural inversion and final uplift, in a pop-up structural style, took place during the Late Neogene 'Andean Orogeny' yielding more than 10 Km of structural relief. The Andean deformation was characteristically brittle. The sequence of events that took place in the Santander Massif are typical of the inner Adean chains of the northern Andes, from northern Peru to Venezuela. The Early to Middle Ordovician orogenic cycle extends north and south of the massif along the Eastern Cordillera of Colombia and eastward into the Merida Andes of Venezuela. It appears again further south in the northern Argentine-southern Bolivian Andes.

Introduction The Santander Massif is an upthrusted block of basement rocks exposed in the northern portion of the Eastern Cordillera of Colombia (Fig. 21). Here the Eastern Cordillera shifts from a northeasterly to a north-northwesterly structural trend, and continues north into the Perij a range and east into the Merida Andes of Venezuela. The metamorphic basement of the Santander Massif consists a thick sequence of low to medium metamorphic-grade 85 metapelites and a suite of granitic orthogneises of Early Paleozoic age. Inlays of older Grenvillian-age ( -1.0 Ga ) are exposed in the northern portion of the massif ( Ward et al, 1973; Restrepo-Pace, 1995 ). The cover rocks consists of a fossiliferous , partly metamorphic, shallow marine clastics of late Silurian (?) - mid Devonian age, a Permo-Carboniferous mainly marine flysch-shelf unit followed by Triassic-Jurassic molasse and rift continental sediments and Late Jurassic- Cretaceous- marine clastics. The massif is extensively intruded by Triassic-Jurassic calc-alkaline granitic plutons. Low to medium metamorphic grade metapelitic rocks of the Santander massif form part of an extensive belt exposed along the Eastern Cordillera of Colombia, from the Quetame-Floresta Massifs to the Perija range . Field relationships show that the metapelitic suite is covered unconformably by unmetamorphosed Lower to Middle Devonian marine clastic rocks ( Forero­ Suarez, 1972 ; Courts et ai, 1983; Boinet et al, 1986; Forero-Suarez, 1986 and others). Plutonic rocks crosscutting the metapelitic suite yield K-Ar and Rb-Sr ages between 471 - 350 Ma (Ward et al, 1973; Ulloa et al, 1982; Boinet et al, 1985) . Boinet et aI, 1985 noted the 'imprecision' of some of these ages. These authors obtained a 350 ± 17.5 Ma K-Ar age for gabbroic intrusive that is overlain unconformably by Middle Devonian sedimentary rocks. Until now, the precise age of metamorphism for the basement rocks of the Santander Massif was unknown, constrained to be pre-Devonian on stratigraphic grounds and some ambiguous geochronologic results. This relationship led some researchers to believe that a Siluro-Devonian ( - Acadian age ) orogenic event was responsible for the metamorphism of the pelitic basement (e.g. 86 Forero-Suarez, 1989 ) or a protracted metamorphic event spanning from late Cambrian to Late Silurian ( Ward et aI, 1973 ). Yet others believed that this event was older, due to the presence of unmetamorphosed Ordovician rocks in other localities within the Andean domain. A consequence of such ambiguity was the impossibility of integrating the northern Andes into regional early Paleozoic paleogeographic reconstructions . In addition to the Paleozoic geologic uncertainties, the Mesozoic geology of the Santander Massif has been regarded as tectonically 'passive', with the exception perhaps of Cediel (1968) who first interpreted the Triassic - Jurassic Gir6n group. as a tectonic molasse and Shagam (1975) with his extensive field - metamorphic studies of the Merida Andes of Venezuela. The present paper summarizes the geology of the Santander Massif in a local and regional context, including new geochronological data that indicates that the basement rocks were involved initially in a tectonothermal event in Early-Mid Ordovician time (-470-480 Ma). This event is recognized in the Merida Andes of Venezuela, southern Bolivian and northwestern Argentinian Andes . In the northern Andes, the Ordovician orogenic cycle has been 'masked' by a pervasive early Mesozoic thermal event and structurally by Neogene'Andean' deformation.

Stratigraphy A thick Early Paleozoic metapelitic and metaplutonic suite of rocks make up the core of the massif. NS elongated slivers of migmatitic gneisses of Grenvillian age (Ward et aI, 1973 ; Restrepo et al, 1995 ) are exposed 87 througout the massif ( Fig. 22). The Devonian marine sequence in the Santander massif consists of slaty gray shales with interlayered sandstones, marls and occasional limestones with a total thickness between 600 and 1000 m. The contact with the underlying metapelitic basement is obscured due to metamorphic and structural complexities, but at the Perij a Range and the Floresta massif it is clearly unconformable ( Triimpy, 1943; Forero-Suarez, 1969, 1972 ; Cediel, 1969) and it is marked by a basal conglomeratic section. The contact with the overlying Permo-Carbonniferous clastic rocks is not exposed in Santander but in the Floresta Massif appears to be transitional ( Cediel, 1968 ; Villarroel et al; 1988). In the Santander Massif, the best Devonian exposures are located near Guaca and San Andres, Mogotes, north of Bucaramanga and east of Pamplona. A rich collection of briozoans, brachiopods, corals, trilobites, echinoids, spores and plant remains recovered at these sites indicate a late Early to Middle Devonian age for the unit ( Ward et aI, 1973 ; Boinet et al, 1986). At one site north of Guaca, within this sequence, Late Silurian fossils have been reported ( Forero-Suarez, 1986). The Devonian sediments apparently become continental toward the top as observed south in the Floresta Massif ( Mojica et aI, 1984). The Permo-Carbonniferous clastic sequence which follows is made up of two distinct members ; the basal member of Late Mississippian-early Pennsylvanian age is a red-bed sequence, here -150 m thick. The remaining section consists of a predominantly turbidite-flysch sequence made up of marls, shale and sandstone intercalations capped by limestones of Permian age, with a total thickness of 88 -500 -2400 m ( Triimpy, 1943 ; Navas, 1962; Ward et al 1973; Rabe, 1977 ). Permian limestones have been locally recrystalized to marbles in the massif's interior . The contact with the underlying Devonian sequence is not exposed in Santander, but north in the Perij a range is unconformable (Forero-Suarez, 1972). The following unit up-section consists of 4700 m of post-orogenic clastic sediments of Late Triassic-Jurassic age (Cediel, 1968; Rabe, 1977 ). The Triassic-Jurassic sequences are widely exposed west of the Bucaramanga fault, in the central portion and sporadically on the eastern flank of the massif. These consist of continental fluvial shales followed by a thick molassic section of red-beds with interlayered oligomictic limestone-clast conglomerates, in tum by polymictic conglomerates toward the top of the section, containing clasts of intrusive and metamorphic rocks ( "kalk­ konglomerat" and ''buntes-konglomerat'' respectively of Rabe, 1977 ) . Nearly 50% of the map area comprises Late Triassic and mainly early Jurassic calk­ alkaline batholitic intrusions. Finally, the Santander Massif is flanked by thick sequences of Cretaceous to Early Tertiary marine clastics along the Middle Magdalena Valley to the west and Tachira Depression to the east. NS elongated inlays of the Late Mesozoic- Early Cenozoic sediments are preserved in the central portions of the massif.

Structure The Santander Massif is bounded on the west by the Santa Marta­ Bucaramanga fault and on the east by the of east-vergent Pamplona-Cubug6n­ Mercedes thrust front, which in turn constitutes the western boundary of the 89 Tachira structural depression and Catatumbo basin respectively . The metamorphic core of the massif rises to 4000 meters above sea level . Metamorphic rocks are poly-deformed in a ductile fashion . Deformation increases with metamorphic grade, with the higher metamorphic grade suite exhibiting complex interference folds. Lower metamorphic grade rocks have an So defined by internal mineral layering e.g. micas and elongated quartz, generally parallel to schistosity S1. Garnet porphyroclasts from samples obtained near the Bucaramanga fault, display

o-type rotation and 'snowball' structures as described h~ Passchier et al, 1986. 3t

diagrams of S1 foliation define tight F 1 isoclinal folds ( Fig. 24 ) , with axis

generally oriented to the northwest and with dips lesser than 300 either north

or south. F2 folds are defined by the folding of Fl isoclinal folds, into open folds with a WNW-ESE axis dipping .... 30 to the SE (e.g. north of Berlin on geologic map figs 23 and 24). Paleozoic granitic gneisses are emplaced in structural concordance in the metapelitic basement. Fossiliferous Devonian gray shales and sandstones have been partly metamorphosed and the contact with the underlying units is obscure. Generally these strata tum into silver colored schists or gray slates which generally preserve their sedimentary layering and in some cases highly deformed fossil fragments. As a rule, these display S1 deformation which parallels So sedimentary layering, though

locally crenulation cleavage was observed . Mesozoic intrusives are not foliated with the exception of localized sheared areas related to the Neogene fault system described below. 90 The Bucaramanga fault, which is nearly vertical at the surface as evidenced by its straight trace west of the massif, is considered to have at least 100 I

22 ), the Triassic-Cretaceous sedimentation in the massif was fault confined within elongated NS extensional basins. Additionally, Triassic-Jurassic plutons are elongated north to south suggesting that these intruded along the principal Mesozoic extensional structures. Inversion of Triassic-Cretaceous extensional basins produced a concave upward arching of the basement units and the Mesozoic cover exhibits either drape folds on both flanks of the massif's core or was passively uplifted forming monoclinal flexures . An east-west section across the Santander Massif reveals its "pop-up" , fanned 91 block, structural geometry ( Fig. 26). Specifically, it resembles the "inversion­ fan structure of back-thrusts and footwall shortcut thrusts" geometry described by McKlay et al, 1992; p. 103. Faults perhaps flatten out with depth and feed into an east vergent basal decollement at the brittle-ductile transition zone (-10-15 Km depth) as proposed for the Merida Andes (Kellog and Bonini, 1985) . Gravity data indicates that the massif is bounded on either side by thick sedimentary basins. Vertical structural offset at the base of Cretaceous section on either side of the Bucaramanga fault amounts to about 9-12 Km. Brittle structures of the Santander Massif form part of a regional and complex network of faults which resulted from Neogene relative motions between the Nazca plate, the Panama-arc the Caribbean plate and northwestern South America (Fig. 25c ) . Fission-track data indicates slow differential uplift of rigid fault bounded blocks begun in early Miocene time and at substantially increasing rate, in late Pliocene-Pleistocene time i.e. during the Andean Orogeny (Shagam et al, 1984; Kroonenberg et al, 1990).

Petrologic character of basement and crystalline roeks Basement rocks comprise two distinct petrotectonic units: a suite of low to medium metamorphic grade metapelite metapsamitic suite and a suite of syndeformational granitic rocks. Slivers of underlying Precambrian age basement ( Ward et aI, 1973; Restrepo et al, 1995) have been tectonically incorporated into the metapelitic suite. The first group comprises, for the most part, paragneisses and schists with minor interlayeted amphibolitic 92 gneisses. The second group comprises foliated-gneissic granitic rocks. Quartz ± muscovite ± sillimanite ± cordierite ± garnet ± K feldspar and quartz ± staurolite ± sillimanite ± biotite are typical assemblages of the higher metamorphic grade suite. Quartz ± plagioclase ± muscovite ± dorite, quartz ± plagioclase ± biotite ± muscovite ± staurolite ± garnet and quartz ± muscovite ± biotite ± andalusite ± cordierite ± garnet are common mineral assemblages for the lower metamorphic grade suite. Interfoliated amphibolitic dykes are found sporadically within the lower metamorphic grade suite ( e.g. near Pescadero and north of Pamplona ) and have tholeiitic chemical signatures ( Grosser et al , 1994 ). The mineral paragenesis indicate low-medium pressure ( - 1 - 9 Kbar) - high temperature ( 300 - 700 0 C) , greenschist to upper amphibolite facies metamorphic PT conditions. Kyanite is locally present ( Rodriguez, personal comm., 1993), implying medium pressure conditions were attained in restricted areas. K - feldspar + sillimanite indicates granulite facies conditions, but this paragenesis seems to be restricted to the Precambrian basement inlays. Rare Earth Element (REE) patterns for the high-grade suite are LREE enriched 20x-200x chondrite with (La/LU)N 5.51- 11.59, I REE from 33-237 , and variable europium negative

anomaly with Eu/Eu* between 0.20-0.73 ( Fig. 27 , Table 7). Patterns for the low-grade suite are LREE enriched 20x to 500x chondrite with (La/Lu)N 2.4-

22.6, I REE from 110-470 ppm and a conspicuous europium anomaly with Eu/Eu* between 0.24-0.49. REE patterns for the metamorphic units of the massif are typical of supracrustal rocks (d. Taylor et al, 1985). A thick clastic 93 sedimentary sequence could be inferred as the dominant protolith of the metamorphic pile, based on the mineral assemblages, clastic relict features and REE data . Granitic orthogneisses are lodged in structural concordance within the metapelitic basement. Tectonically, these constitute a suite of synkinematic granitoids. The largest exposure of this unit is located north of Berlin . The orthogneisses have a well defined gneissic texture and a compositional range from granitic-tonalitic to granodioritic. Principal mineral components are quartz, oligoclase, micro cline, biotite, hornblende and occasionally muscovite. REE patterns for the suite display LREE enrichment from 20x to 200x chondrite, from steep to nearly flat patterns, (La/Lu)N from 1.6-15 , L REE

from 64- 370 ppm and a negative europium anomaly, Eu/Eu* between 0.25- 0.87 . Triassic-Jurassic granitoids have wide compositional range, though the felsic types dominate. These include granite, granodiorite, quartzmonzonite, tonalite and diorite. Textures are varied as well, from very fine to coarse or porphyritic. The intrusives shown in the geological map correspond to composite batholitic bodies comprising several plutons of different . compositions and generated perhaps at different crustal depths. REE patterns for the Triassic-Jurassic suite display LREE enrichment -100x chondrite, (La/LU)N from 8-12, L REE from 87-309 ppm, and a negative europium

anomaly with Eu/Eu* between 0.23-0.56 . The Paleozoic and Mesozoic granitoid rocks plot in the Volcanic Arc Granite field in trace element discrimination diagrams ( Fig. 28 ). Petrography, REE data and trace element

... ------...... :.--;-=.;.. -. 94 geochemistry indicate that plutonic rocks in the Santander Massif are likely to be subduction related.

Geochronology A total of 20 40Ar/39Ar cooling ages and one U/Pb zircon age were carried out for the Santander massif basement units (Tables 8 , 10 ). Precambrian basement and Lower-Mid Paleozoic units were sampled. In many localities the sampling was conducted where previous K/ Ar and Rb / Sr ages had been reported as to further evaluate them. A published 1:100000 geological map for the southern portion of the massif was modified according to field observations and more importantly, by the results of this research. Mineral separations were done at the Department of Geosciences, University of Arizona. No heavy liquids were used, with the exception of the zircon separate. 4oAr/39Ar analyses were undertaken at the University of Lausanne, Switzerland, according to the techniques described by Cosca et al, (1994). The U/Pb age was conducted by Dr. George Gehrels at the University of Arizona. Additionally, four Neodymium crustal residence ages were obtained for the metapelitic basement; these were performed by Dr. James Gleason in Arizona. 40Ar/39Ar apparent age spectra for the metapelitic and metaplutonic basement samples are shown in Figure 29. All but one of the mica separates yielded cooling ages that cluster around -200 Ma cooling age. Sample PC-l was collected in the northeastern portion of the massif yielding an age of -383

Ma. 40Ar/39Ar spectra for a hornblende separate of sample CB-7, yielded a 95 staircase pattern with a minimum age of 180 Ma and a maximum age at 350 Ma, using the criteria of McDougall et al, 1988. K-Ar cooling ages of many of the calk-alkaline intrusives in the Santander Massif also fall in the Jurassic period (Ward et al, 1973 ). The Paramo Rico intrusive, northeast of Berlin, yielded a 210 Ma U /Pb concordant zircon crystallization age (Grosser, personal communication ), and 4oAr/39Ar cooling ages of 196 ± 1.5 Ma and 191± 1.8 Ma for a hornblende and biotite separates respectively. A temperature- time curve for the Paramo Rico pluton ( Fig. 30 ), indicates high cooling rates immediately following emplacement -210 Ma ago. An average initial -200 C/Ma cooling rate during for the Paramo Rico is derived from this curve. Uplift of the same pluton begun at an increased rate since - 16.0 Ma. Synkinematic pluton samples collected near the Paramo Rico pluton ( north of Berlin ), also yield -200 Ma cooling ages for the biotite and hornblende separates . The precise age of emplacement of all other non - foliated calc-alkaline intrusives unknown but all have cooling ages between - 170 Ma to 200 Ma (Ward et aI, 1973). Sample BP-2 syndeformational granitoid, yielded a 477 ± 16 Ma U /Pb zircon crystallization age and a 254 ± 60 Ma Pb-loss age, of based on tree discordant abraded single grains (MWSD=OA) (Fig. 31, Table 9). Plotted in a map, most of the older ages occur in along the eastern flank of the massif ( Fig. 32 ). The younger ages occur on the western flank, where the exposed area of Mesozoic intrusives is greatest.

Discussion of geochronological data and tectonic implications Geochronologica11y two discrete events could be recognized in the

... -... -----.-~;;.;:: .. -. -. 96 Santander Massif : an initial Early - Middle Ordovician (-470-480 Ma) tectonothermal event during which the protolith of the metapelitic basement and a suite of contemporary granitic intrusions were metamorphosed to greenschist facies, followed by a strong thermal overprint of Triassic and most importantly Jurassic age ( Fig. 33 ) . The heat transferred by the enormous volume of Mesozoic intrusions led to the development of a low-medium pressure regional metamorphic overprint on the Early Paleozoic rocks and locally metamorphosed the Mid-Upper Paleozoic rocks. Additionally, it caused total or partial resetting of isotopic geochronometers as evidenced by our 40Ar/39Ar data. Many samples of the metapelitic basement display two stage mineral growth. Mica crystals were observed to grow at high angle to the main metamorphic foliation ( in samples collected east of Aratoca), arranged in a trellis texture. Also, crystals of staurolite, andalusite and sillimanite ( with the exception of the fibrolitic type), were observed to grow at varying angles with respect to 51 foliation. In other cases garnet porphyroblasts had grown cordierite rims as observed from samples NE of Rionegro, in proximity to a Jurassic pluton. Retrograde metamorphism is extensive indicated by the presence of sericitized andalusite and sillimanite, and pinitized cordierite. These observations indicate, as mentioned previously by Ward et al, (1973), a two stage metamorphic history. During the first event, which was dynamic in character and tectonically driven, the Early Paleozoic pericratonal marine clastic sequences were metamorphosed to greenschist grade and developed a regional metamorphic 97 fabric. This event took place - 470 - 480 Ma ago (Caparonensis Orogeny ) . Magmatism continued and decreased until Silurian time. The Caparonensis orogenic pulse appears to have been subduction related i.e. Andean-type, as implied by the trace element geochemistry of the synkinematic plutons. A topographically irregular landscape controlled the deposition of the overlaying sedimentary sequences, following the Early Paleozoic event. Late Ordovician and Late Silurian sedimentary rocks were deposited locally in the Merida Andes though Devonian sediments are absent. Devonian sediments are widespread along the Eastern Cordillera of Colombia. Only until latest Mississippian (?)- Carboniferous is flysch sedimentation widespread in thenorthwestern Andes. The Mississippian, for the most part, corresponds to a regional stratigraphic hiatus . Late Mississippian (?) - Early Carboniferous clastics consist of basal red-beds though exposed in the Merida Andes Imolassic-facies' made up of conglomeratic and brechoid deposits ( Merida facies - Sabaneta Fm of Shagam et aI, 1970 ). These coarse clastics were deposited following compressive pulses related to the initial convergence between South and North America. While Late Paleozoic deformation is well represented in the Merida Andes ( d. Marechal, 1983) it is not as evident in the Colombian Andes. The final expression of this event is marked by the deposition of mollasic sediments containing calcareous clasts from the Permo-Carboniferous sequence ( "kalk-conglomerate" of Rabe, 1977), along the margins of the uplifted blocks. The Pb-Ioss age of 254 ± 60 Ma on sample BP-2 from the Santander Massif may be related to this Late Paleozoic folding event, though the large error precludes a definite linkage. Many ages U /Pb in 98 the northern Andes between 230-225 Ma have been related to the terminal phases of the ocean closure between Laurentia and Gondwana ( e.g. Shagam, 1975). We consider these to indicate emplacement of intrusions related the initiation of back-arc extension. The second event recognizable in the Santander Massif, on geochronological grounds, is largely static and thermally driven . During this event not only was the Early Paleozoic metamorphosed basement involved, but locally parts of the overlying Middle-Late Paleozoic cover. By Late­ Triassic time but mainly in Jurassic time back-arc extensional tectonics were operating full scale in north western South America. This NE-SW directed extension was related to the breakup of Pangaea and the establishment of a subduction zone along the western continental margin. Thick rift related sediments accumulated in the depressed areas. Major crustal thinning took place and large volumes of intrusive plutonic rocks were emplaced along the Eastern and Central Cordilleras of Colombia and the Merida Andes. U-Pb zircon ages, mainly conducted for the Merida suite, indicate that these begun to be emplaced from -230 - 210 Ma (Gonzalez de Juana, 1980 p. 117, Grosser, 1993, pers. comm). K-Ar cooling ages range between - 170 Ma to 200 Ma in Santander (Ward et al, 1973). For the Paramo Rico intrusive, north of Berlin, the differences between the U-Pb zircon age and 40Arj39Ar hornblende-biotite cooling ages are from 14 to 19 Ma. In the Santander Massif the large thermal welt associated with extension and the large volume of intrusives caused rapid uplift in Early Jurassic time as evidence by the thick accumulation molassic sediments along its western 99 flank containing clasts of the Late Triassic-Early Jurassic igneous intrusives. Additionally, the metamorphic basement was 'pushed' into sillimanite zone and locally kyanite zone, amphibolite facies metamorphic grade metamorphism, at low pressure conditions. The Devonian - Permian cover rocks, at shallow crustal depth, were metamorphosed locally to greenschist facies. Qualitatively peak metamorphic temperatures may have reached - 700 o C, well above K-Ar and Rb-Sr biotite (-250-350 0 C), muscovite (,.. 300-450 0

C ) and near to hornblende ( - 550 0 C ) ( Geyh et al , 1990) , yet bellow zircon

U-Pb closure temperature ( - 900-1000 0 C, Ghent et al, 1988). Widespread

resetting of K-Ar and Rb-Sr geochronometers took place. The latter statement is well corroborated by our data and suggests that many Mid-Late Paleozoic K­ Ar and ages reported previously for the eastern portion of the Santander Massif ( Ward et al, 1973; Boinet et aI, 1985) represent partially reset ages. Igneous related low pressure regional metamorphism has been modeled for the western U.S. by Barton, et al (1989). According to their model, the integrated effects of numerous shallow plutonic intrusives may be sufficient to cause regional low pressure metamorphism, in areas where the volume of intrusives is higher than 50%. Hom£elsic textures may be found in the immediate contact with the intrusive but regional penetrative textures are well developed as a result from the protracted thermal event (Barton et al, 1989; p. 1063). The Santander Massif fits well in this scheme: plutonic activity may have lasted over 30 m.y.; field evidence indicate that the various batholitic bodies are composite made of various individual intrusives. The 100 exposed area of intrusives surpasses the 50 % and, a geothermal gradient of - 35 0 C / Km required for low pressure regional metamorphism could have been achieved aided by contemporaneous back-arc extension. The metamorphic grade generally drops down away from the plutonic centers, though this observation is limited by the fact that the shape of the plutons at depth is unknown . Additionally, isograd tracing is complex, resulting of the overprinting of metamorphic events. Shagam (1975) had discussed igneous related metamorphism in the Merida Andes, relating it to a compressive event at the end of the Permian period. Our data indicates igneous related metamorphism took place in the Santander Massif in Late Triassic- Early Jurassic time . Final uplift and reactivation of deeply rooted structures occurred in Pliocene-Pleistocene time associated with the Andean Orogeny.

Regional time-space basement correlatives The metamorphic metapelitic basement of the Santander massif continues north into the Perij a Range and South into the Floresta-Quetame Massifs and inlays in the Cordillera Real in Ecuador. - 1.5 Ga crustal residence ages obtained for the metapelitic basement units of the Santander Massif (Restrepo-Pace, 1995) are identical to equivalent units in the eastern Andes of Ecuador ( Hegner et all 1994 ) ; here rocks have also been extensively affected by the Jurassic magmatic arc ( Aspen, et al 1992 ). The basement geology of the Santander Massif has a strong correlation with the basement of the Colorado Massif in the Merida Andes of Venezuela i.e. Iglesias Gp and Bella Vista Fm. These basement units have the same lithologic associations 101 and metamorphic grade and also have been intruded by numerous granitic bodies with U /Ph zircon crystallization ages between 460-500 Ma ( Burkley, 1976 in GonzaIez de Juana, 198; pp. 92-97) . The Early-Mid Ordovician (-470- 480 Ma) tectonothermal event described for the Santander Massif is well recognized in the Merida Andes (Shagam, 1975, Bennedetto et al, 1982, Case et al, 1990 ) locally named Caparonensis Orogenic Cycle ( Gonzalez de Juana, 1980; p. 110 ). The presence of Silurian fossiliferous marine clastic rocks and the absence of Devonian-Mississippian sediments in the Merida Andes led some investigators to believe that another tectonothermal event followed, taking place in Early Devonian time (e.g. Forero-Suarez, 1986 ). The postulation of this "Late Caledonian" tectonothermal pulse, was backed by two U/Pb zircon ages of 390 ± 30 Ma for a granitic intrusion in the Merida Andes ( Burkley, 1976 in Gonzalez de Juana, 1980 ) and a couple of K-Ar ages on plutonic rocks along the Eastern Cordillera of Colombia. We argue that this latter pulse is difficult to justify with the current available geologic­ geochronologic data. Deposition of the Mid-Late Paleozoic sequences was controlled mainly by paleotopography and structures. It seems viable that .the Merida Andes remained a positive feature from Late Ordovician through

Mississippian time, only to be I submerged' structurally and locally in Silurian time by the marine transgressive clastic sequences (e.g. EI Homo Fm). Greater portions of the Eastern Cordillera of Colombia and Perij a range had been leveled or brought down structurally, providing the environment of a continental marginal shelf-slope sequence to develop in Devonian time. Further south, the -470 Ma tectonothermal event is also recognized in 102 the Puna of northern Argentina - southern Bolivian Andes , named locally the Ocloyic - Famatinian orogenic phase. Here it is marked by numerous syntectonic intrusions with ages ranging from 480 to 460 Ma and low to medium pressure high- temperature metamorphism ( Aceftolaza, 1982 ; Rapela et al, 1990 and others) . Aditionally, a regional unconformity at the base of the Late Ordovician marine clastic sequences is observed in the San Juan region as a result of the Mid-Ordovician orogenic cycle (Baldis et al, 1992 p.348).

Figure captions Figure 21. Location Map of the various Late - Precambrian to Early Paleozoic basement uplifts in the Northern Andes. (1) Borde - Llanero fault system, (2) Santa Marta - Bucaramanga fault , (3) Tachira depression, southwestern Merida Andes, (4) Oca fault. Figure 22. A. Simplified geologic map of the Santander Massif. Map : (1) Lower Paleozoic- metapelitic gneisses including inlays of Precambrian rocks, (2) Lower Paleozoic- metapelitic schists , (3) Lower Paleozoic synkinematic intrusive rocks, (4) Middle Devonian marine sedimentary rocks, (5) Permo­ Carboniferous sedimentary rocks, (6) Triassic- Jurassic intrusive rocks, (7) Upper Triassic - Lower Jurassic rift-molassic sedimentary rocks, (8) Upper Jurassic to Tertiary sedimentary rocks. A-A' line of section. B. Simplified lithostratigraphic column of the Santander Massif: (1) migmatites, (2) a.

Foliated granitoids and b. metapelites; (3) a. sandstones, b. shale-marls, Co siltstones, d. limestones (4) a. Red-beds and/ or conglomerates, b. 103 volcanoclastics and/ or dolerite sills; (5) non foliated granitoids.

Figure 2l. 1t diagrams and S1 Foliation domains for the Santander Massif.

Early Paleozoic units: (1) Lower metamorphic grade suite, (2) higher metamorphic grade suite and (3) synkinematic intrusives. Figure 24. Simplified diagram illustrating multiple fold generation of the metamorphic core of the Santander Massif. Refer to text for description. Km = kilometric , m= metric and em= centimetric scale features. S refers to axial surfaces and F to fold axes. Figure 25. A. Map of brittle structures of the Santander Massif, faults : 1. Bucaramanga, 2. Surata, 3. Tona, 4. Rio Charta, 5. Rio UmpaIa, 6. Rio Perchiquez, 7. Servita, 8. Chitaga, 9. Morro Negro, 10. Pamplona . B. Simplified kinematic model for the development of brittle structures (a) Early Mesozoic extension, (b) Early-Mid Tertiary compression, (c) post Miocene shear-couple: 1- Bucaramanga fault, 11. Pamplona- Cubug6n- Mercedes thrust front. C Simplified regional model : Pa; Panama arc, N a: Nazca Plate, Ca: Caribbean Plate, Cc : central Colombia block, Ba: Barranquilla block, Cu:

Cura~ao block, Sa : Santa Marta block, Pe: Perija block, Ma : Maracaibo block, Me : Merida block, Sn : Santander Massif, SA : South American Plate. Arrow depicts direction of transport of the Merida-Maracaibo- Santa Marta-Perija blocks ( Modified from Bonini et aI, 1982 and Laubscher, 1987). Shallow earthquake solutions from Dewey (1972) and Pennington (1981).

Figure 26. Schematic cross section of the Santander Massif at - 7 0 13 ' latitude north. Gravity data from Bermudez et aI, 1985. (1) Shield Precambrian basement; (2) Andean Precambrian-E. Paleozoic basement; (3) E. Paleozoic 104 synkinematic intrusives; (4) Mid-Late Paleozoic sediments; (5) Jurassic intrusives; (6) Jurassic molasse; (7) Cretaceous - Tertiary sediments. Figure 27. Chondrite normalized Rare Earth Element patterns for representative samples of the metamorphic and crystalline rocks of the Santander Massif. Shaded area represents total spectra of patterns. Color code according to geologic map. Figure 28. Santander selected granitoids plotted in tectonic environment discrimination diagrams based on trace element data. Ta vs. Yb and Rb vs. Y + Nb concentrations (ppm) of Pearce et al, 1984. Rb/10-Hf-Tax3 diagram from Harris et aI, 1986. Fields for granites : syn-COLG= syncollisional, WPG=within plate, ORG=orogenic and VAG=volcanic arc. Figure 29. 4OAr/39Ar apparent age spectra for selected samples of the Santander Massif. For location and description of samples refer to table 10. Figure 30. Temperature-time curve for the Paramo Rico pluton, Santander Massif. Data from Case et al, 1990; Grosser, 1993; Kroonenberg et al, 1990, and this work. Vertical error bars relate to range of closure temperatures: U­ Pb zircon from Ghent et aI, 1988 ; all others from Geyh et al , 1990 . Continuous curve represents best contrained trayectory. Shaded areas represent dicrete tectonic events. Figure 31. U/Pb age of a synkinematic intrusive north of Berlin, Santander Massif. Figure 32. Distribution of relevant geochronological data in the Santander Massif. Data from Ward et al (1973), Boinet et al (1985), Grosser (1993) and this work. 105 Figure 33. Schematic representation of orogenic phases in the Santander Massif, from Early Paleozoic to Middle Mesozoic time.

Table7. Rare Earth Element composition of selected samples in the Santander Massif.

Table 8. 40Ar/39Ar step heating experiment data of selected samples in the

Santander Massif. Table 9. U!Pb isotope data for sample BP-2. Table 10. Location and sample description of selected samples in the Santander Massif. ,--_.----: .. ,. ' .- \ r~ " f-' Santa Marta t " Massif //\ <~:~F5 .. '{L,•. I .~\ ~. ~ /,,, I·'~, ;J\ i.~::~ '--" ~~~ , 0 '\ .\

, .....)' ..... -... I '. \.~ i 1000 Km. j .:' .. iO~", f~~1f Foresta { I Massif , \ i \.

Garz6n­ Massif

,..., Figure 21. o a- 73010' 73000' W 72 050' 72040 . I I I I 1N

-7020' lR-Jur

-- A AI Penn - E.Perm 7 () 10'

to.-I - L. Dev I""

(:-0

p€

---d. _

Ie ~ :..-.:>:» ~ ~---~ "; ~~~?~~:~~~~:4 1 2 - 3 5 B

IO 20 JO .w :;0 Kin 12345678 Figure 22. A :: __ J ___ =-~~~~}~- o '·1 lns . • /" •

[J "

•o e· • •• J

o 50Km

Metamorphic facies

(hhlrih..' 1

I) ,~2 . \nd.l1usilc· St..llUlllitc -.--­ .3 Sillim.1I1ltl! -.--­ G.Ullcl

Figure 23. 109

L ...... ~ ...... J

...... ~....

I Ian I

Figure 24. 110 B

c

'. ~t, .. •.., , ~,y~)

I _

Ca

Na SA

Figure 25. .------+------_._---; 111

+

+

+

11"''.1 I''';':''.\' IJ.t.:e'I\'

+ I~

11",'.fmlll'11~.'.4_1

1/111:/ 111111./" + I~ + I~ 1M //"U I /lSI//lI/IV,III,"'H + + IN +

+ I~ 112 nl nl I .~ 'lA 'lA ...!:: 0.. ill.L u ill.L :p co .13: .... (I) .13 a t: :>- E (I) oH ~ .... oH ~ ...... '"' CIJ (I) :::; AG 0.0:::; AG ,... CIJ Nl:: (I) 'li.U-l s:: "'0 ca P9 P9§ ....JI;).()o '"' n3: n3: illS illS PN PN old Jd i3J i3J el lq

0 0 0 ...-i 0 0 0 ...-i 0 0 ...-i 0 0 0 ...-i 0 ...-i 0 ...-i 0 .-I ...-i nl nl (" ca 'lA 'lA t: CIJ .:E ill.L o (I) ill.L :.0 > & .13: (!l.U) .13: 0 oH E :::; oH S el) '"'0 .... ca ...... '"'t: AQ AG ClJ- ....(I) -::s "'0 .~ 'li.U-l S CIJ 'li. t:;:: ..eel) P9 >.t: P9 § ._I;).() "'0ca CIJ (!l n3: N '"' n3 ::ebb illS r:l.,C) wS PN PN Jd Jd i3 J i3J l:?1 el

0 0 0 ...-i 0 0 0 .-I 0 0 .-l 0 0 .-l r:-: 0 ...-i 0 0 ...-i 0 M .-l ...-i ...aI :: i3l!JPUOl.IJ / ~oN e.o i3l!.Ipuol.{J / ~oN i;: 10\)."..--______======------A ~ lOJ ~ WPC • ML In,rusiws 1 syn-COLG ~ ~ o Pi' Intrusives .. ...--/. .---' .--.. --'--- --~. ..--/" ~ ~-- ./.,.--- ' -----"--. ~. .• I' ! .1 VAG i ORC

01 j : B , ""I i i ••I • .1 I Yb 1000 I syn-COLG . ,---0. WPG Rb/30 ~:n.. c 100 "L-~ • 1-' 0' ~ ; 0 /' 0; / ORC r' 10 o· VAG ~LG o I ..... o VAG I L-PC'-....·'~ 1t- ro too "'i11oo .' L. / / Y+Nb / i WPG : I \ Figll1'e 28. .... Hf Tax3 '-' w

115 300~------1

------1~±3Ma------

BV-3 Biotite Integrated Age = l~ ± 4 Ma Granitic orthogneiss

tU 300 BV-4 Biotite :a Granitic orthogneiss QI bO tU 200 ...=: ~ tU Q, 100 Integrated Age = 187 ± 4 Ma Q, <

~ 300 BV-5 Biotite cu Granitic Orthogneiss tID -«I 200 «IE Q, 100 Integrated Age = 175 ± 3 Ma Q, <

«I ::E 225 ~ ~~~------2~±ODMa------+~ ....ca e= 200

8: Total fusion Age = 203.4 Ma CB - 7 Hornblende < 175 Granitic Orthogneiss

50 Cumulative %39Ar Released 116

!II SBb-l Muscovite ::E 225 - Mica schist QI bD !II .. ...• ----195.4±O.5 Mal-----... QI=... 200 - !II 1:1.. 1:1.. -< 175 - Total fusion age = 193.3 Ma

Ir I

Sbb-2 Muscovite !II 300 ::E Mica schist QI '- ~ ~.' •.... r,:.. • bD 200 .' . ) -'-.; ..!II ...... \" ••• .1 =QI 100 ...!II Integrated Age = 197 :I: 6 Ma 1:1.. -<1:1..

!II ::E QI 200 ' 1· ;V:''l,,\"t.4J;.'.:,: i!.' ':\'1. bD '. , .4). !II .. CSB -2a Biotite =~ 100 !II Paragneiss 1:1.. Integrated Age = 177 :I: 4 Ma -<1:1..

!II ~ QI ..f 200 =...QI !II CSB -3a Biotite 1:1.. 100 1:1.. Integrated Age = 175.4 ± 1.7 Ma Paragneiss -<

50 Cumulative %39Ar Released 117

191.2± 1.8 Ma------~ Q.I btl !U 150 ~ :; 100 Q. BV-1 Biotite Q. Granitic intrusive Integrated Age = 187 ± 3 Ma « 50

240 eo 196.3 ± 1.5 Ma ------~ ~ 200 ....eo 160 Eeo Q. Q. « 120

~ 200 RSC-l Biotite QJ Granitic intrusive i ::: --t=~r~~~~'···Si"'~~"~':'~'::::::...J ".... -.' .. " Integrated Age = 133 ± 3 Ma ~ ~50 < 50 Cumulative %39 Ar Released 118

AB-6 Muscovite ~ 300 ...-----'213-±-3-Maa------Mica schist

:0 • ,_, ~~.I :~"" . '-. . . -. til 200 ..... \ ,'. 1 ~ 100 Integrated Age =195 ± 11 Ma <

400

3SO

til AB-7 Hornblende ::e 300 Foliated basaltic dike CII 110 ..til 250 =~ til 200 !:). !:). < 150

100 Integrated Age = 242 ± 5 Ma

50

50 Cumulative %39Ar Released 119

BP-2 Biotite Granitic gneiss

....~----198.3±O.5 Ma------+-_

Total fusion age = 196.6 Ma 175 -111='='=

til 240 :2 CII ------194±2Ma------f200 1: ~ til 160 BV-2 Hornblende c.. Integrated Age = 190.6 ± 1.2 Ma c.. Granitic orthogneiss < Orthogneiss

~ 400 f ~ 350 PC-l Biotite c.. c.. Total fusion Age = 383.4 Ma Mica schist <

50 Cumulative %39Ar Released 120

0.080 BP2

0.076 20SPb* 238 U 0.072

0.068

Upper intercept = 477± 16 Ma Lower intercept = 254 ± 60 !VIa 0.064

0.060 ...... :;-"'""-...... --'--"--...... "----'---'--..&--"-- ...... 0.44 0.48 0.52 0.56 0.60 0.64

207Pb* /235 U

Figure 31. 121

1000 900 Triassic-Jurassic back arc extension 800 U-Pb Zircon 700 ~ 600 U a f-4 500 400 Andean Orogeny 300 200 J 100 a 0 50 100 150 200 250 300 AgeMa

Figure 30. 122

o 456±22.5 1i1210 0383.4 (Paramo Rico pluton) 730~O'W r 72~50'

0185±2 o 177±6 0172±2 -~"- -7 °20' A 450±80 :;.::;;=p.\-~t • 477±16 o 199.4±0.5

0192±7

o 213±3 180 min o 350 max

o K/Ar o Ar/Ar A Rb/Sr ~B •••~. • U/Pb Figure 32. Caparolleltsis Orogeny -. .- -- -- ..... O. 47 Ga • Grenvillian age basement t Salltallder 1l'1assij ~w Molassic sedimentation .f:: ~;.,;-.__ ~ ~:...:J Triass-]ul' intrusives !:~l Unmetamorphosed rocks -<10 Km " :",1f-' Ill. Low pressure facies series metamorphism ~10-20 Km f' • Medium-pressure fades series metamorphism -20+ Km

Late Triassic-Early Jurassic -. .- -- -- ..... O. 22 Ga

figme 33.

.--I t-.) VJ

I I i ! Ii SANTANDER MASSIF

Sample AB-l AB-2 AB-2a AB-3 AB-6 AB-7 AB-8 BM-l BM-3 BP-3 CB-7

I.a 32.55 53.63 119.40 30.30 33.92 43.90 58.40 10.52 32.33 18.69 33.88 Ce 90.95 123.23 234.23 78.78 76.40 92.20 118.97 21.72 72.91 45.80 74.50 Pr 9.24 12.Q9 27.53 7.12 7.64 10.04 12.71 2.22 7.95 4.54 9.26 Nd 39.19 49.01 124.98 30.82 31.24 45.89 53.33 9.21 36.66 21.23 48.05 Sm 7.55 8.39 19.48 5.75 6.46 6.71 956 1.96 5.68 4.58 11.38 Eu 1.23 1.05 2.74 0.48 0.74 1.16 1.29 0.25 0.86 0.75 2.84 Gd 7.66 9.43 22.13 6.19 6.42 6.08 954 2.48 6.88 4.71 8.84 'Ib 1.13 1.28 2.56 0.S2 0.99 0.75 1.37 0.39 0.96 0.76 1.73 Dy 6.15 6.08 14.45 3.76 5.31 4.31 7.21 3.56 5.71 4.73 10.73 110 1.22 1.30 3.17 0.72 1.16 0.% 1.50 0.84 1.45 0.99 2.04 Er 3.03 3.66 8.97 1.92 2.n 2.66 3.95 2.46 4.35 2.40 5.16 Tm 0.37 0.50 0.90 0.21 0.34 0.24 0.47 0.35 0.49 0.31 0.61 Yb 2.70 3.61 6.S6 1.75 2.74 2.25 3.69 3.07 3.99 2.51 4.17 l.u 0.33 0.50 0.84 0.22 0.33 0.31 0.46 0.45 0.55 0.33 0.57 IIf 5.07 9.17 5.36 3.69 5~~2 4.71 4.64 2.37 9.14 8.38 4.92 'Ht 1.53 2.73 4.31 1.41 1.42 1.98 1.55 2.54 3.25 O.S9 1.30 Y Nb

r REE 203.31 273.75 588.25 168.82 176.45 217.46 282.46 59.48 180.76 112.31 213.75 Eu/Eu* 0.49 0.36 0.40 0.24 0.35 0.55 0.49 0.35 0.42 0.41 0.83 (l.a/l.u) 7.30 9.93 14.69 10.74 8.30 14.49 5.29 2.41 6.0S 10.04 4.45 N

Concentrations in ppm. -= not measured Table 7.

I-' ~ Sample PC-J SBb-J SBb-S CSB-2 CSB-3 CSB-4 RSC-l RSC-3 BP-2 BV-2 BV-3

I.a 93.65 26.82 31.85 25.25 6,42 20.37 59.73 39.68 30.14 6.49 42.60 Ce 191.39 58.82 79.28 51.18 13.82 41.36 153.05 82.06 95.37 17~~7 94.51 Pr 20.63 6.\0 7.93 5.52 1.38 4.45 13.58 8.87 8.61 2.30 10.46 Nd 90.84 28.0" 37.50 24.28 6.14 19.63 59.66 40.14 35.46 14.]8 46.43 Sm 14.37 5.55 6.92 5.40 1.55 6.66 10.27 6.97 7.44 3.46 8.87 Eu 1.20 0.63 0.82 0.76 0.32 1.20 0.74 0.86 0.85 1.21 0.97 ((1 16.96 4.88 7.06 4.23 1.05 3.52 12.28 7.34 6.33 S.B 8.92 'Ib 2.67 0.70 1.10 0.64 0.15 0.61 1.45 0.99 1.14 0.71 1.16 l1y 15.67 3.98 6.27 3.23 0.87 3.72 7.46 5.31 6.91 4.94 7.19 flo 3.31 0.81 111 0.61 0.20 0.75 1.41 1.04 159 1.28 1.71 Er 9.47 2.38 3.29 1.62 0.62 2.30 3.78 2.75 4.06 3.58 4.82 '1m 1.26 0.33 0.43 0.21 0.10 0.35 0.41 0.33 0.49 o.·n 0.50 Yb 8.W 2.51 3.05 1.48 0.81 2.35 3.1 1 2.-18 3.83 2.98 3.76 Lu 1.11 0.34 0.40 0.23 0.13 0.32 0.38 0.32 0.46 0.42 0.51 /If 5.36 5.92 5.84 6.25 2.84 3.22 4.27 5.55 4.97 2.74 8.35 111 1.87 1.44 4.01 0.77 1.08 1.73 1.57 1.85 (J.9S 0.74 0.71 Y 39.28 29.94' 53.05 Nb 10.21 3.29 16.16 Rb 16 50 140

l: REt: 471.19 141.89 187.12 124.62 33.55 107.60 327.33 199.14 202.69 64.46 232.40 Eu/Eu* 0.29 0.36 0,35 0.47 0.73 0.68 0.20 0.36 0.37 0.87 0.33 (La/tu) 22.63 8.22 8.19 11.59 5.21 6.56 10.71 9.79 (,.81 1.(,2 8.69 N

Conrenlralions in ppm. - = Not measured.

I-l N r.J1 Sample BV-4 BV-S BV-l CB-3 CB-S CSB-l SJO-l SJ-0-2 RSC-O

J.a 43.27 36.04 75.79 36.45 39.M 68.82 23.46 44.99 70.44 Ce 91.17 78.87 156.74 82.60 74.80 133.93 38.54 82.58 144.70 Pr 9.04 8,40 17.21 9.43 7.75 13.81 3.24 8.46 14.98 Nd ..10.44 38.52 75.87 42.43 30.25 58.18 13.34 36.92 61.22 Sm 6.12 7.30 10.90 7.85 4.91 9.61 1.67 6.80 10.54 Eu 0.66 0.66 1.65 1.14 0.66 0.92 0.16 1.03 2.02 <41 6.88 8.68 10.49 7.42 6.02 9.95 2.47 6.43 11.50 'Ib 0.95 1.16 1.26 1.02 0.76 1.19 0.21 1.04 1.48 Dy 6.30 7.56 7.93 5.25 3.84 5.57 1.16 3.98 7.40 11o 1.53 1.73 1.92 0.98 0.84 0.% 0.29 1.09 1.48 Er 4.16 4.98 5.12 2.63 2.30 2.87 0.95 2.46 4.21 'lin 0.49 0.57 0.52 0.28 0.27 0.31 0.17 0.64 0.47 Yb 4.01 4.09 3.28 2.03 2.36 2.26 1.80 2.20 3.81 I.u 0.57 0.60 0.54 0.26 0.30 0.32 0.22 0.51 0.46 !If 6.75 6.91 12.98 4.59 2.91 6.13 3.22 5.46 7.22 Ta 1.08 1.34 5.01 0.96 0,45 0,48 2.03 0.68 1.53 Y 33.02 28.46 22.37 16.18 25.72 22.45 8.73 23.93 39.19 Nb 11.15 12.8 12,47 16.16 11.46 15.88 18.17 15.97 18.41 Rb 140.0 140.0 120.0 89.0 250.0 240.0 240.0 170.0 130.0

r REI: 215.60 199.17 369.92 199.76 174.70 308.70 87.19 1<)9.13 334.70 Eu/Eu* 0.31 0.25 0.47 OA5 0.37 0.23 0.26 0.47 0.56 (I.a/J.u) 7.91 6.25 14.69 9.39 11.68 8.76 10.98 9.24 11.34 N Concentrations in ppm

...... ~ SANTANDER MASSIF

I = Calculated total Integrated age TF= Total fusion age P = Plateau age

Sample AB-2a Muscovite I age = 208±4 Ma J=0.004770±O.000024

TOC 36Ar/39Ar *40Ar/39Ar(K) Moles %Radiogenic %39Ar Age 40Ar.l0_14 Release 700 0.12161 15.988 15.3 30.8 2.8 133±22 800 0.00116 27.120 41.8 98.8 14.3 220±3 850 0.00047 27.410 44.8 99.5 15.2 222±3 900 0.00251 26.267 32.8 97.3 11.4 213±3 950 0.00505 25.574 24.0 94.5 8.3 208±3 975 0.00810 24.512 20.9 91.1 7.3 200±4 1000 0.00540 27.584 27.7 94.5 8.9 223±3 1050 0.00214 26.211 42.1 97.6 14.7 213±3 1100 0.00230 26.796 38.9 97.5 13.3 217±2 1200 0.01845 22.678 11.9 80.6 3.4 185±3

TableS...... t::l Sample AB-6 Muscovite I age = 195 ± 11 Ma J=0.004778±O.OOO024 Page= 213±3 Ma

'fOC 36Ar/39Ar *40Ar/ 39Ar(K) Moles %Radiogenic %39Ar Age 40Ar.10-14 Release 700 0.16073 13.038 19.4 21.5 5.7 109±21 800 0.00863 26.056 22.3 91.1 13.9 212±7 850 0.00450 24.862 21.0 94.9 14.3 203±7 900 0.00700 26.377 20.5 92.7 12.9 214±3 950 0.00988 26.583 16.1 90.1 9.8 216±5 975 0.01178 22.430 12.8 86.6 8.8 184±5 1000 0.01164 25.041 17.3 87.9 10.9 204±5 1050 0.00687 25.850 21.1 92.7 13.5 21O±3 1100 0.01917 23.767 13.2 80.8 8.0 195±5 1200 0.07032 18.163 4.6 46.7 2.1 150±7

Sample AB-7 Hornblende Age minimum = 180 Ma J=0.OO4782±O.OOO024 Age maximum = 350 Ma

TOC K/Ca 36Ar/39Ar *40Ar/39Ar{K) Moles % Radiogenic %39Ar Age 4OAr.10-14 Release 850 0.330 0.10804 19.142 10.2 37.5 9.57 159±9 950 0.056 0.01875 22.236 7.9 81.5 13.86 183±4 975 0.056 0.01946 28.002 4.4 84.7 6.37 228±6 1000 0.047 0.01596 30.421 4.9 88.6 6.83 246±6 1050 0.035 0.00886 30.353 19.2 95.3 28.86 245±4 1100 0.047 0.01752 28.947 5.2 86.9 7.47 235±5 1125 0.033 0.01778 33.641 10.4 89.1 13.19 270±4 1150 0.027 0.02356 40.228 8.5 87.8 8.88 319±5 1250 0.030 0.02443 43.212 5.1 87.9 4.97 340±7 ...... N 00 Sample BM-l Muscovite I age = 182±3 Ma J=0.004779±O.OOO024 Page= 185±2 Ma

Toe 36Ar/39Ar *40 Ar/ 39 Ar(K) Moles % Radiogenic %39Ar Age 40Ar.l0-14 Release 900 0.00124 23.363 33.5 98.5 14.33 192±3 950 0.00456 21.485 26.0 94.1 11.55 177±4 975 0.00711 21.773 23.5 91.2 9.98 179±2 1000 0.00461 22.236 30.3 94.2 13.02 183±2 1050 0.00197 22.937 44.0 97.5 18.97 188±2 1100 0.00113 23.227 51.4 98.6 22.13 191±2 1200 0.00663 21.836 23.5 91.8 10.02 180±2

Sample BM-4 Muscovite lage=184±2Ma J=0.004781±0.000041

TOe 36Ar/39Ar *4°Ar/39Ar(K) Moles %Radiogenic %39Ar Age 40Ar.l0-14 Release 700 0.06024 17.857 13.1 50.1 2.6 148±14 800 0.00391 22.557 38.0 95.1 11.5 185±3 850 0.00102 23.279 54.8 98.7 16.7 190±3 900 0.00188 22.729 42.5 97.6 13.1 186±2 950 0.00447 22.104 32.3 94.4 9.9 181±2 975 0.00527 21.915 32.0 93.4 9.8 180±2 1000 0.00398 22.105 36.1 94.9 11.1 181±1 1050 0.00224 22.821 46.1 97.2 14.1 187±1 1100 0.00357 22.453 37.0 95.5 11.3 184±1

..... N \,Q Sample BP-2 Biotite TF age = 196.6 Ma J=0.001602±O.OOO041 Page= 198.3 ±0.5 Ma

'roc 36Ar/39Ar *40Ar/ 39A r(K) Moles %Radiogenic %39Ar Age 40Ar.l0-14 Release 750 0.01964 6.636 261.5 66.9 7.8 175±1 800 0.00111 7.184 136.6 95.6 5.2 197±1 850 0.00052 7.256 289.8 97.9 11.1 198±1 900 0.00047 7.257 329.2 98.0 12.5 198±1 950 0.00072 7.253 237.8 97.1 9.1 198±1 1000 0.00079 7.251 270.2 96.8 10.2 198±1 1050 0.00042 7.279 344.9 98.3 13.3 199±1 1100 0.00026 7.281 353.5 99.0 13.7 199±1 1150 0.00024 7.340 343.6 98.1 13.3 199±1 1200 0.00046 7.267 80.2 98.1 3.1 199±1

Sample BP-3 Muscovite TF age = 197.6 Ma J=0.001602±O.OOO041 Page = 199.3 ± 0.5 Ma

TOC 36Ar/39Ar *4°Ar/39Ar(K) Moles %Radiogenic %39Ar Age 40Ar.10.14 Release 750 0.00531 6.488 42.1 80.4 1.9 178±14 850 0.00345 7.479 220.0 98.6 10.9 201±1 900 0.00008 7.327 663.9 99.6 33.6 200±1 950 0.00008 7.293 408.8 96.6 20.1 199±1 1000 0.00037 7.255 163.8 98.4 8.2 198±1 1050 0.00034 7.300 75.6 98.6 3.9 200±2 1100 0.00036 7.288 137.4 98.5 6.9 199±2 1150 0.00042 7.239 177.1 98.2 8.9 198±1 191±5 1250 0.00151 7.983 80.1 93.9 4.0 ...... {.oJ 0 Sample BV-l Biotite Iage=187±3Ma J=0.004788±O.000024 Page = 191 ± 1.8 Ma

TOC 36Ar/39Ar *4°Ar/39Ar(K) Moles %Radiogenic %39Ar Age 40Ar.l0-14 Release 850 0.33575 18.081 7.9 15.4 1.6 150±37 950 0.03798 20.569 4.0 65.5 3.0 169±9 975 0.03481 21.078 3.0 68.1 2.3 173±10 1000 0.01723 21.591 8.2 82.2 7.5 177±4 1050 0.00167 23.481 44.6 100.0 45.4 192±2 1100 0.00800 23.075 15.3 92.3 14.6 189±3 1125 0.00938 23.134 16.5 91.4 15.6 190±3 1150 0.03216 20.374 4.0 69.2 3.2 168±7 1250 0.01429 22.286 9.0 85.6 6.7 183±4

Sample BV-l Hornblende Jage=195±1 Ma J =0.0()4.783±O.OOO024 Page = 196±1.SMa

TOC K/Ca 36Ar/39Ar *4°Ar/39Ar(K) Moles %Radiogenic %39Ar Age 40Ar.10-14 Release 700 0.157 0.10214 20.777 14.7 40.8 4.4 171±16 800 0.089 0.00072 23.853 24.4 99.1 10.6 195±4 850 0.089 0.00052 23.834 25.1 99.4 10.7 193±2 900 0.089 0.00103 23.848 21.7 98.7 10.6 195±2 950 0.082 0.00120 24.150 17.8 98.6 10.6 197±2 ...... w TOC K/Ca 36Ar/39Ar *40Ar/39 Ar(K) Moles %Radiogenic %39Ar Age 4OAr.l0-14 Release 975 0.087 0.00080 24.497 11.4 99.1 10.6 200±2 1000 0.066 0.00122 24.913 9.4 98.6 10.6 203±2 1050 0.082 0.00120 23.958 16.4 98.6 10.6 196±2 1100 0.081 0.00021 23.814 30.6 99.8 10.6 195±2 1351 0.081 0.00120 23.840 21.0 98.6 10.7 195±2

Sample BV-2 Hornblende I age = 190.6 ± 1.2 Ma J=0.004782±O.OOO024 Page = 194±2 Ma

TOC K/Ca 36Ar/39Ar *40Ar/39Ar(K) Moles %Radiogenic %39Ar Age 40Ar.10-14 Release 850 0.279 0.37899 1.911 12.6 1.7 2.2 16±33 950 0.120 0.03141 20.462 6.2 69.4 4.1 168±5 975 0.120 0.01648 22.428 6.7 83.1 4.9 184±4 1000 0.106 0.00518 23.725 19.2 95.4 15.1 194±3 1050 0.097 0.00137 23.882 47.9 100.0 39.2 195±2 1100 0.107 0.00323 23.160 19.5 97.5 16.0 189±3 1126 0.099 0.00756 24.199 10.4 93.0 7.8 198±3 1150 0.094 0.00861 25.267 7.4 92.2 5.3 206±4 1250 0.108 0.00763 24.960 7.5 93.0 5.5 203±4

...... ~ Sample BV·3 Biotite lage=184±4Ma J=0.004765±0.000041 Page= 184±3Ma

TOe 36Ar/39Ar *40Ar/39 Ar(K) Moles %Radiogenic %39Ar Age 40Ar.10.14 Release 700 0.01318 22.873 26.1 85.4 12.3 187±5 800 0.00163 23.272 24.4 98.0 13.0 190±6 850 0.00754 21.558 15.8 90.6 8.4 177±4 900 0.01057 20.771 14.7 86.9 7.8 170±3 950 0.00692 21.912 19.9 91.5 10.5 179±3 975 0.00890 21.215 17.9 89.0 9.5 174±3 1000 0.00840 21.382 19.4 89.6 10.3 175±2 1050 0.00388 22.076 29.4 95.1 16.0 180:1-2 1100 0.00722 21.751 23.0 91.1 12.2 180±2

Sample BV·4 Biotite I age = 187 ± 4 Ma J=0.004777 ±O.000041

'fOe 36Ar/39Ar *40Ar/39Ar(K) Moles % Radiogenic %39Ar Age 40Ar.10-14 Release 700 0.32580 17.524 35.5 15.4 5.5 145±26 800 0.00064 22.843 20.9 99.2 16.1 187±4 850 0.00175 21.440 14.3 97.6 11.6 180±4 900 0.00362 24.156 13.0 95.8 9.2 197±3 950 0.00247 23.065 14.8 96.9 11.0 189±3 975 0.00535 22.489 11.2 93.4 8.3 185±2 1000 0.00380 22.952 10.1 95.3 7.5 188±2 1050 0.00241 23.508 18.8 97.1 13.8 192±2 1100 0.00350 24.827 23.0 96.0 17.1 202±2 ...... VJ VJ Sample DV-5 Biotite I age =175 ± 3 Ma J=0.004777±0.000041

TOC 36Ar/39Ar *40Ar/39Ar(K) Moles % Radiogenic %39Ar Age 40Ar.l0-14 Release 700 0.07511 18.828 27.1 45.9 9.8 155±7 800 0.00211 22.103 23.0 97.3 15.0 181±4 850 0.00582 21.871 14.5 92.7 9.1 179±4 900 0.01247 19.571 11.0 84.2 7.0 161±3 950 0.00895 20.657 12.8 88.7 8.1 170±3 975 0.00854 21.702 15.0 89.6 9.1 178±2 1000 0.00799 21.348 15.4 90.0 9.6 175±2 1050 0.00356 22.553 23.6 95.5 14.7 185±2 1100 0.00604 22.355 20.5 92.6 12.5 183±2 1200 0.01413 20.174 9.7 82.8 5.1 166±2

Sample CD-7 Hornblende TF age =203.4 Ma J=0.001602±O.OOO041 Page = 204 ± 0.6 Ma

TOC K/Ca 36Ar/39Ar *40Ar/39Ar(K) Moles %Radiogenic %39Ar Age 40Ar.10.14 Release 850 0.496 0.98617 5.894 134.7 5.6 2.7 163±9 950 0.268 0.00394 7.596 9.9 88.1 2.4 207±t 1000 0.140 0.00255 7.824 15.1 94.1 3.8 213±2 1050 0.091 0.00199 7.512 48.6 97.7 13.2 205±1 1075 0.088 0.00191 7.440 41.4 98.2 11.4 203±1 1100 0.091 0.00180 7.528 100.4 98.5 27.7 203±t 1125 0.095 0.00162 7.459 18.3 98.9 5.0 204±1 1150 0.087 0.00186 7.462 61.6 98.4 16.9 204±1 1175 0.087 0.00168 7.506 52.0 99.1 14.3 205±1 1200 0.094 0.00134 7.926 4.6 100.0 1.2 216±3 ..... Vl ~ Sample CSB-2a Biotite I age = 177 ±4 Ma J=0.004770±O.000041

TOe 36Ar/39Ar *40Ar/39Ar(K) Moles %Radiogenic %39Ar Age 40Ar.1O-14 Release 700 0.16554 16.734 40.3 25.5 7.7 139±10 800 0.00039 23.115 28.1 99.9 1S.2 l88±S 850 0.00147 22.035 16.0 98.1 8.9 180±4 900 0.00394 20.483 14.3 94.6 8.3 168±3 950 0.00156 21.717 17.8 97.9 10.0 178±4 975 0.00315 20.801 16.1 9S.7 9.3 171±2 1000 0.00265 22.689 17.5 96.7 9.3 18S±2 1050 0.00038 22.912 24.5 99.5 13.3 187±2 1100 0.00049 22.264 24.8 99.4 13.8 182±2 1200 0.00588 20.468 7.5 92.2 4.2 168±3

Sample CSB3a Biotite I age =175.4 ± 1.7 Ma J=0.004778±O.000024

Toe 36Ar/39Ar *40Ar/39Ar(K) Moles %Radiogenic %39Ar Age 40Ar.l0·14 Release 850 0.13370 4.404 11.5 10.1 4.4 38±10 950 0.01683 19.170 9.3 82.3 6.7 158±4 975 0.01732 21.395 7.1 83.1 4.6 176±4 1000 0.01211 22.797 8.8 88.6 5.8 187±3 1025 0.01099 21.767 10.2 89.0 7.0 179±3 1051 0.00633 23.292 15.0 94.6 10.2 190±2 1100 0.00291 23.628 27.8 98.4 19.5 193±2 wI-l U1 TOC 36Ar/39Ar *40Ar/39Ar(K) Moles %Radiogenic % 39Ar Age 40Ar.10-14 Release 1126 0.00650 21.116 16.2 93.6 12.1 173±2 1150 0.00554 20.624 18.4 94.9 14.2 170±2 1200 0.00626 22.254 19.1 94.7 7.9 182±2 1400 0.00590 23.800 11.1 95.6 7.5 194±3

Sample OT-l Hornblende lage=574±8Ma J=0.004783±O.000024

TOC CalK 36Ar/39Ar *40Ar/39Ar(K) Moles % Radiogenic %39Ar Age 40Ar.l0-14 Release 850 0.500 0.06991 40.704 20.3 66.4 6.8 321±1 0 950 0.071 0.00361 30.399 19.5 97.7 12.8 245±4 975 0.071 0.00662 35.177 15.4 96.2 8.6 281±5 1000 0.072 0.00534 46.490 19.7 97.8 8.5 362±6 1026 0.095 0.00440 60.296 22.3 98.6 7.4 457±5 1051 0.139 0.00579 83.403 33.0 98.3 7.9 606±6 0.0 0.172 0.00415 133.545 120.8 99.3 18.3 891±11 1126 0.160 0.00209 104.069 49.5 99.6 9.7 729±13 1150 0.155 0.00125 115.768 53.2 99.9 9.3 795±14 1200 0.15 0.01281 112.821 50.3 97.0 9.1 779±14 1251 0.12 0.02195 81.079 12.4 92.9 0.8 591±8 1400 0.04 0.06082 74.094 3.4 81.2 0.8 547±29

..... UJ 0' Sample OT-2 Hornblende I age = 668 ± 9 Ma J=0.004770±O.000024

TOC CalK 36Ar/39Ar *40Ar/39Ar(K) Moles %Radiogenic %39Ar Age 40Ar.10·14 Release 850 0.202 0.14573 19.276 14.4 31.0 5.3 159±22 950 0.062 0.01058 56.292 25.5 96.1 10.0 429±10 975 0.062 0.00855 71.543 29.0 97.4 9.1 530±9 1000 0.092 0.00281 89.632 36.9 99.6 9.4 642±9 1026 0.124 0.00165 106.017 46.6 99.8 10.1 738±6 1051 0.145 0.01465 114.971 59.2 96.6 11.4 789±6 1100 0.159 0.00527 120.667 110.2 98.9 20.8 820±6 1126 0.129 0.01516 114.933 60.4 96.5 11.7 789±5 1150 0.110 0.02088 104.332 28.9 94.7 6.0 729±7 1200 0.08 0.01871 93.641 23.1 94.9 5.4 666±6 1251 0.06 0.15437 38.732 3.1 46.2 0.9 306±34

Sample PC-l Biotite TF age = 383.4 Ma J=0.001602±O.000041

TOC 36Ar/39Ar *40Ar/39Ar(K) Moles %Radiogenic %39Ar Age 40Ar.10·14 Release 650 0.06519 9.232 138.8 32.4 2.5 249±2 750 0.00201 13.238 289.9 95.5 10.5 332±1 800 0.00113 15.040 283.0 97.8 9.2 389±1 850 0.00069 15.371 355.0 98.7 11.5 39S±1 900 0.00086 15.278 383.8 97.9 12.4 397±1 950 0.00104 15.089 367.4 97.8 12.0 390±1 1000 0.00111 14.805 380.3 98.4 12.7 384±1 1050 0.00082 14.929 278.9 98.6 9.3 386±1 ~ \,;J 'I Toe 36Ar/39Ar *40 Ar/39 Ar(K} Moles %Radiogenic %39Ar Age 40Ar.l0-14 Release 1100 0.00069 14.898 318.5 99.0 to.6 386±t 1150 0.00048 15.242 204.8 98.9 6.7 394±1 1200 0.00057 15.862 90.6 97.5 2.8 408±1

Sample RSC-l Biotite I age = 133 ± 3 Ma J=0.004780±O.000041

Toe 36Ar/39Ar *40Ar/39Ar(K) Moles %Radiogenic %39Ar Age 40Ar.l0-14 Release 700 0.03024 14.145 17.3 61.4 8.4 118±9 800 0.00133 18.615 23.7 97.9 14.0 154±3 850 0.00382 16.438 16.2 93.6 10.4 136±3 900 0.00613 15.539 14.6 89.6 9.4 129±2 950 0.00509 15.361 15.7 91.1 10.4 128±2 975 0.00668 14.995 13.0 88.4 8.6 125±1 1000 0.00581 16.350 14.4 90.5 8.9 136±1 1050 0.00380 18.755 22.4 94.3 12.6 155±2 1100 0.00462 18.617 19.8 93.2 11.1 155±2 1200 0.01223 15.596 10.4 81.2 6.1 130±2

Sample SBb-l Muscovite TF age = 193.3 Ma J=0.001602±O.000041 Page = 195 ± 0.5 Ma TOe 36Ar/39Ar *40Ar/39Ar(K) Moles % Radiogenic %39Ar Age 40Ar.l0-14 Release 650 0.00534 4.129 74.7 20.7 1.1 116±2 ..... w 00 TOC 36Ar/39Ar *40Ar/39 Ar(K) Moles %Radiogenic % 39Ar Age 40Ar.10-14 Release 750 0.00098 5.654 51.6 95.0 2.5 156±1 850 0.00046 6.471 124.9 97.9 5.5 178±1 900 0.00028 7.119 556.4 98.8 22.3 195±1 950 0.00019 7.132 620.2 99.1 24.9 195±1 1000 0.00030 7.132 304.3 98.7 12.1 195±1 1050 0.00041 7.263 164.2 99.3 6.6 195±1 1100 0.00022 7.192 144.9 99.0 5.8 197±1 1150 0.00032 7.275 294.7 98.7 11.6 199±1 1200 0.00009 7.301 196.4 99.6 7.8 200±1

Sample SBb-2 Biotite I age = 197±6 Ma J=0.004770±O.000041

TOC 36Ar/39Ar *40Ar/39Ar(K) Moles %Radiogenic %39Ar Age 40Ar.10.14 Release 800 0.00745 28.652 10.8 92.9 21.2 231±7 850 0.00051 23.663 9.5 99.4 24.2 193±6 900 0.00694 24.886 5.8 92.4 13.0 202±3 950 0.01420 21.442 3.0 83.6 7.1 177±6 975 0.02763 18.482 2.0 69.4 4.5 152±9 1000 0.01333 22.608 2.5 85.2 5.7 185±7 1050 0.00675 23.146 6.0 92.1 14.5 189±3 1100 0.01446 22.110 4.3 83.8 9.9 181±5

Corrections for background, 37 Ar decay ,interferences and mass discrimination were made. Errors on individual ages are one standard deviation and do not include the uncertainty in J. Ar*=Radiogenic.

I-' VJ \0 MEASURED RATIOS APPARENT AGES

Sample WT I'b U 206 206 206 206" 20r 207" (mg) (ppm)(ppm) 204 207 208 238 235 206"

BP-2 21 16.6 252 1710 15.76 7.1 397±2 401±3 428±9 11 16.9 231 1540 15.40 7.3 444±4 447±5 462±11 23 78.9 993 3000 16.41 5.2 457±2 458±2 465±6

• =radiogl'nic Pb Ml'asurcd ratios arc uncorrl'cted for blank, spike, or initial Pb. Constants used: 1-.235=9.8485x10-1 0,1-.23R=I.55125xl0-10, 238/235=137.88. Data reduction from Ludwig (1991 a), concordia diagrams from Ludwig (1991b). Analylical methods described by Gehrels (1990). Samples corrected for: (1) fractionation factorsofO.14± 0.06 %/amu for Pb and 0.04 ± 0.06 %/amu for U, (2) blank values of 5 pg for Pb and 1 pg for U, and (3) initial Pb values interpreted from Stacey and Kramers (1975). Table 9.

.... ~o SAMPLE Unit I.Ithology **LocatJon of Precision: comment: II) dated samples

latitude N longItude W Map scale

SANTANDER MASSIF

All-I E.Pz metapelltlc belt Q.z-chlo-musc schist Aratoca-Bucaramanga road AB-2a E.Pz meta pelitic belt Q.z-chlo-musc-gnet schIst 60 49'4r 73000'40" 1:100000 Aratoca-Bucaramanga road AB-Z E.I'1. metapelltlr belt Q.z-chlo-musr-gnet schIst Aratoca-Bucaramanga road AB-;:! E.Pz metapelltlc belt Q.z-chlo-musc-gnet schIst Aratoca-Buraramanga road AB-6 E.Pz metapellt1r belt Q.z-chlo-musc-gnet-stau schIst 6°49'47" 730(10'40" 1:100000 Aratoca-Bucaramanga road AB-7 E.Pz meta pelitic helt Hornb-qz foliated dyke 6°49'49" 73000'40" 1: I 00000 Arato·ca-Bucaramanga road A\l-S E.Pz meta pelitic belt Q.z-chlo-musc- schIst Aratoca-Bucaramanga road BM-l E.Pz meta pelitic belt Q.z-plag-gnet-musc gneIss schlst7ll6'30" 73003'33" 1: 100000 Bucaramanga-Matanza road BM-3 1:.Pz meta pelitic belt Q.z-blot-musc schIst Bucaramanga-Matanza road BM-4 E.Pz metapelltlc belt Q.z-blot-musc schIst 70(;'45" 73003'33" 1: 100000 Bucaramanga-Matanza road BP-3 E.l'z meta pelitic belt Q.z-musc schIst 7°14'06" 7204S'4Z" 1:100000 Berlln-Pamplona road PC-I E.Pz metapelltlc belt Q.z-blot-musc schIst 70Z8'14" 7Z04Z'OO" 1: 100000 Pamplona-Cucutllla road SBb-l Dev. metapelite Q.z-musc schIst 7°11'36" 7Z045'50" I: 100000 Silos-sabega road SBb-Z E.Pz metapelltlc belt Q.z-musc schIst 7°11'36" 72°45'50" 1:100000 Silos-Babega road SBb-5 E.Pz meta pelitic belt Q.z-musc-clo-stau-gnet schIst Silos-Babega road CSB-2a PC Basement Q.z-plag-blot paragnelss 6°57'17" 72 0 58'43" 1:100000 Curos-Santa Barbara road CSB-3a PC Basement Q.z-plag-blot paragnelss 6°57'11" 72°55'56" 1:100000 Curos-Santa B:irbara road CSB-4 PC Basement Q.z-plag-blot paragnelss Curos-Santa Barbara road RSC-I 1:.l'z metapelltlc belt Q.z-blot-sll\-gnet paragnelss 7019'ZS" 73007'53" 1: 100000 Rio Negro-Santa Cruz road RSC-3 E.Pz metapelltlc belt Q.z-blot-sll\-cord paragnelss Rio Negro-Sllnta Cru1. road OT-I PC Basement Q.z-hnb-plag gneIss S018'14" 73oZ6'22" 1:100000 Ocaila-Totumal road OT-2 PC Basement Q.z-hnb-plag gneiss S018'14" 73 OZ6'22" 1: \00000 Ocaila-Totumal road BP-2 I:.Pz Syndeformatlonal Granitoid Q.z-blot-hnbl-plag orthogneiss 7°24'23" 72049'24" 1:100000 Berl\n-Pamplona road BV-2 F..pz Syndeformatlonal GranitoId Hnbl-plag-Q.z orthognelsss 7°14'30" 72054'21 " 1:100000 Berlin-Vetas road BV-3 E.Pz Syndeformatlonal GranitOid Q.z-blot-plag orthogneiss 7°14'58" 72°54'58" 1:100000 Berlin-Vetas road BV-4 E.Pz Syndeformatlonal Granitoid Q.z-blot-plag orthogneiss 7006'IZ" 7Z054'07" 1:100000 Berlin-Vetas road Tablel0. .-. "'".-. S/\MPLE lInlt I.Ithology **LocatJon of PrecIsIon: Comment: II) dated samples

latitude N Longitude W Map scale

BV-5 E.l'z Syndeformatlonal GranItoId cu.-blot-plag orthogneiss 70 14'39" 720 54'17" 1:100000 Berlin-Vetas road CB-7 E.l'z Syndeformatlonal GranItoId IInhl-plag-Qr. orthogneIss 7~&'30" 72042'00" I: 100000 Corcova-Berlin road CB-3 M7.. non-foliated GranitoId Diorite Corcova-Berlin road CB-5 M7.. non-foliated GranitoId Qr.-monzonlte Corcova-Berlin road CSB-I Mz. non-foliated GranitoId Qr.-monzonllC Curos-Santa Barbara road MSJ-5 Mz. non-foliated GranitoId Alasklte Mogotes-San Joaquin road SJ-O-I Mz. non-foliated GranItoid Qr.-monzonite San joaquin-Onzaga road SJ-0-2 Mz. non-foliated GranitoId GranodiorIte San Joaquin-Onzaga road RSC-O Mz. non-foliated GranitoId GranodiorIte Rio Negro-Santa Cruz road Table to_continuation_

.... """N 143 VII. SUMMARY OF RESULTS FROM THIS STUDY

Granulitic and migmatitic basement rocks from the Andes of Colombia, with dominantly sedimentary and igneous protoliths of continental affinity, are the product of a regional high-grade metamorphic event which took place -1.0 Ga ago, locally referred to as the Nickerie­

Orinoquiense metamorphic event. U I Ph and 40 Ar1 39 Ar geochronology from this study confirm previously reported Rb/Sr and KI Ar Grenvillian ages. Nd crustal residence ages and U IPb zircon indicate that 'young' crustal additions were tectonically mixed with rocks with Early Proterozoic-Late Archean sources. These older sources are likely to be located east on the Guyana Shield basement. Data from this study fits well within the paleogeographic reconstructions of Hoffman ( 1991 ) and Park ( 1992) . The regional extent of this belt, running from the Santa Marta Massif in Colombia to the Sunsas Belt in the Bolivian Andes and perhaps even further south along the Andean system together with the characteristic high metamorphic grade and peak metamorphic age at -1.0 Ga, attest in favor of a continent-continent collisional type event. Metapelitic rocks exposed along the Eastern Cordillera of Colombia and southern Merida Andes were regionally metamorphosed in Mid-Ordovician time, during the Caparonensis Orogeny. This regional low metamorphic grade event developed penetrative foliation and was accompanied by continental arc related synkinematic intrusions dated at 477 ± 16 Ma by U IPh on zircons. The Caparonensis orogeny forms a discontinuously exposed belt 144 along the Andes. In the Argentinean Andes it is locally known as the Famatinian Orogenic cycle. The following regional deformational pulse in the northern Andes occurred in Permo-Carboniferous time, was characterized by folding of the Devonian to Permian marine elastic sequences. A Triassic - Jurassic metamorphic event ( evident in the Santander Massif) related to back-arc extension and concomitant intrusions caused uplift and low-medium pressure regional metamorphism . A thick (-2000-4000 m) molassic sequence was deposited flanking the uplifted areas. Widespread resetting of K/ Ar and partial resetting of Rb / Sr and U / Pb geochronometers occurred due to the elevated geothermal gradients . High volumes of intrusive bodies ( > 50 % of the exposed area in the Santander

Massif ), numerous 4OAr/39Ar cooling ages at - 200 Ma for the various basement units and petrologic observations, together clearly indicate an igneous related regional metamorphic overprint. 145 VIII. TECTONIC EVOLUTION OF NW ANDES FROM LATE PRECAMBRIAN TO EARLY MESOZOIC TIME

Time slices: * * Refer to figures 34 and 35

1.3 - 1.1 Ga. Rifting, continental margin subsidence and development of a pericratonic clastic sequence.

The Guyana shield began to experience extensional block faulting. Bimodal magmatism took place consisting of a 1.2 - 1.1 Ga basic dike swarm and 0.95 Ga rhyodacitic volcanism (Priem et al, 1989) .

1.1 - 0.9 Ga. Continent-continent collision. Grenvillian- Orinoquiense orogenic event.

A collision of western South America , eastern North America and southern Baltica occurs, giving rise to the Grenville-Nickerie-Orinoquiense

Orogeny (Hoffmann, 1990 i Park, 1991) . This collision involved deep tectonic reworking of the pre 1.1 Ga age continental passive margin sequences which were metamorphosed to granulite grade. The Grenvillian belt of South America extends from northern Colombia to southern Bolivia and perhaps continues to southern South America (Kroonenberg, 1982; Priem et al, 1989; 146 Litherland et al1986; Ramos, 1988; Wasteneys, 1994 and Restrepo-Pace, 1995).

0.75 - 0.48 Ga. Continental breakup. Rift to drift transition is estimated to have taken place between 0.62 Ga and 0.56 Ga (Bond et al, 1984 ; Hoffmann, 1991). A carbonate-clastic shelf sequence developed along the continental margin . Sections resting at present on Guyana shield rocks are undeformed whereas correlative sections within the Andean domain display varying degree of regional metamorphism, from 'anchizone' to lower greenschist facies. The Upper Precambrian-Cambrian is exposed at three, widely separated areas, in eastern Venezuela shield close to EI BaUl and at Rio Duda eastern Andes of Colombia . In Venezuela the sequences consist mainly of marine clastics. The 150 - 300 m unfossiliferous basal section consists of quartzites, arkosic sands and metaconglomerates, followed by a 900 m thick pelitic section of Middle Cambrian to Tremadocian age. Tremadocian rocks contain acritarchs and more importantly, the trilobite Parabolina Argentina (Frederickson, 1958 ) an olenid trilobite index for the Lower Tremadocian of northern Argentina - southern Bolivia (Harrington et aI, 1951 ; Aceflolaza, 1982). The Cambrian of Colombia consists of a basal section made of stromatolitic limestones, dolomite and limestone; a middle section with basic tuffs, chloritic-amphibolitic schists, dolerite flows and basalts and an upper turbiditic section made up of interlayered dark shales, sandstones, conglomeratic sandstones and calcareous sandstones displaying slump deposits. The section is 2750 m thick (Bridger, 147 1982). Faunules collected from this section include Peronopsis or Hypagnostus sp Ehmania Akanthophora n. sp and Ehmania amphiboLe together with Paradoxides sp ( Harrington et aI, 1951; Rushton, 1963). Ehmania is represented by two species in the Amecephalina zone, San Juan province western Argentina ( Harrington et al, 1951) . Paradoxides is a typical Acado - Baltic trilobite, best represented in western Europe ( Dean, 1985 ). The Tremadocian to Llanvirnian marine sequences are largely clastic with occasional marls and limestone layers. Coarser clastic sections are located on the Guyana shield area and grade into shale dominated turbidites toward the Andean domain ( i.e, in a westerly direction Mojica et aI, 1990) . The faunal assemblages contained in the latter sequences are closely related to the faunal assemblages of the San Juan region, Jujuy and Salta Provinces e.g. Kaianella fauna of the Macarena region ( Triimpy, 1944 ), the presence of Dictyonema and Jujuyaspis (Ulloa et aI, 1982 ). Similar lithofacies and faunules exist in the Tinu Fm. (Robinson et al, 1968 ), southern Mexico ( Oaxaca Terrain of Campa Coney, 1983 ). In summary, during this period a cratonal-pericratonal shelf-slope, marine clastic sequence was developed. Thickness amounts to ... 3500 m for a composite section. The Cambrian of Colombia has Precordilleran (Argentina) and Baltica faunal affinities. The Early Ordovician faunal assemblages are of Gondwanan South America affinity, akin to those of western Argentina-southern Bolivia and Mexico, all forming part of the Olenid-Ceratopygid Province of Whittington et al, 1974.These faunal affinities may have resulted from mixing lithofacies dependent trilobite genera within equatorial warm water currents in a shallow intervening ocean (cf. 148 Ross, 1975). By the end of Early Ordovician time; a subduction zone/ continental magmatic-arc couple was well established as evidenced by the U/Pb zircon ages of intrusive rocks in the Merida Andes (Burkley, 1976 in Gonzalez de Juana et al, 1980 , p. 117).

0.48 - 0.47 Ga. Caparonensis orogeny The Upper Precambrian- Lower Ordovician sequences were regionally metamorphosed to upper amphibolite grade . In the shield area only minor folding affected the Early Paleozoic sequences (Bogota, 1983 ). Deformation was accompanied by the intrusion of subduction related granitoids with U /Pb crystallization ages _ 460 - 490 (Pinz6n et al, 1962; Burkley, 1976 in Gonzalez de Juana et al, 1980 , p. 117; Ulloa et al, 1982 and Restrepo-Pace, 1995 ). Characteristic low metamorphic grade deformation, lack of recognizable sutures, the subduction affinity of the granitic rocks as derived from trace element geochemistry and the paleogeographic position of South America based on paleomagnetic data, indicate an Andean-type orogenic event for this period. Nonetheless, a collisional type event could not be ruled out entirely on the basis of current data. The timing of deformation is inscribed at the base of the Caparo Fm. (Caradocian) devoid of metamorphism, exposed in the southern Merida Andes. 149

_0.47- 0.36 Ga. Erosion and development of a marine transgressive sedimentary cycle. During this period the Caparonensis orogen was differentially exposed as evidenced by the lateral discontinuity of the mid-Paleozoic sedimentary cover . Silurian sequences in the Colombian Andes, are limited to only two localities and are confined to the Ludlovian stage (Forero-Suarez, 1986 ; PrOssl et al , 1994 ). The best record of Silurian rocks is preserved in the Merida Andes i.e. EI Homo Fm ( Llandoverian - Wenlokian) Boucot et al ( 1973 ). This marine sedimentary sequence consists of limestones, sandstones and interlayered shales . The very localized exposures of Silurian sediments indicates that small structurally controlled basins must have existed at this time which where rapidly filled with shallow Silurian transgressive sediments. No Devonian rocks are reported for the Merida Andes, implying that the Caparonensis orogen in Merida for the most part, remained a positive topographical feature throughout Devonian time. Devonian sediments are distributed along the eastern Andes of Colombia and Perij a range. This fining upward transgressive marine sequence begins with, at many localities, a basal conglomerate followed by sandstones marls and shale averaging 600 to 800 m in thickness (Forero-Suarez 1972; Forero-Suarez, 1986 ). Benthic brachiopod fauna of the northern Andean Devonian rocks are of Appalachian affinity and not related to the Mavinokaffric realm ( Johnson et al, 1985; Barrett, 1988 a,b; Forero-Suarez, 1986; Forero-Suarez, 1990 ). These comprise genera such as Cyrtina, Elytha. Atrypa, Nucleospira, Meristella, Cymostrophia, Stropheodonta, Chonostrophia, Leptocoelia, 150 A nphigenia, Platyorthys (Forero-Suarez, 1990 ). This faunal affinity requires proximity of the northwestern South American margin to the eastern margin of North America as illustrated in Kent et aI's, (1990) paleogeographic reconstruction .

0.25- 0.23 Ga. Orogenic event following the assemblage of Pangaea. Folding and Molasse deposition in its terminal stage . Northwestern South America and some elements of Middle America are situated in the region of today occupied by the Gulf of Mexico ( d. Pindell et al, 1982). MissiSSippian (?) and lowermost Pennsylvanian rocks, which locally lay in angular unconformity over the Devonian unit ( Forero­ Suarez, 1972, p. 44 ) are folded together as a single unit. Continental red-bed sequence with breccia and conglomeratic levels ( e.g. in the Merida Andes, Shagam et al, 1970) indicate onset of folding by Early Pennsylvanian time. Middle to Upper Pennsylvanian rocks consist of a flysch sequence capped by a Lower Permian limestone she1fal rocks ; thicknesses varying from several hundred up to -2000 meters. The 'closure' of this folding event is marked in Colombia by molassic coarse deposits containing Permian calcareous rock fragments (Rabe,1977) .

0.23 - 0.16 Ga. Breakup of Pangaea. Back-arc extension and development of an extensive magmatic arc. Rift sediments were deposited in basins formed and associated with the breakup of Pangea and to the development of a subduction zone-arc 151 complex . Triassic- Jurassic rift valleys in northwestern South America were filled initially by red-beds, felsic volcano clastics and red-beds and toward the top by marine limestones and evaporites, totaling between 1000 and 3000 m in thickness (Maze, 1984; Mojica, 1984) . The thermal welt associated with back­ arc rifting and intrusion of many plutonic bodies in Late Triassic-Early Jurassic time, led to uplift and subsequent deposition of thick rift and molassic sequences flanking the positive features . The molasse reaches 2000- 4600 m thickness at the Santander Massif (Cediel, 1968 ). Many crystallization and cooling ages are linked to this regional thermal event ( d. Gonzci1ez de Juana, 1980 ; Maya, 1992; Restrepo-Pace, 1995 and others). Early Paleozoic metapelites were overprinted by igneous related low pressure regional metamorphism and Late Paleozoic sedimentary sequences were metamorphosed locally by Mesozoic intrusives e.g. in the Santander Massif and Merida Andes (Restrepo-Pace, 1995 ) . The metavolcanoclastic-metapelitic rocks that make up the bulk of the Cordillera Real of Ecuador and Central Cordillera of Colombia i.e. Central Andean Terrane or Zamora Terrane ( Restrepo et al, 1988 ; Restrepo-Pace, 1992; Richards and Coney, 1990), the Sevilla Terrane in the Santa Marta block and the Cosinas Terrane in the Guajira Penninsula (Btaye-Serna et al, 1983 ), may represent metamorphosed flysch deposits related to the development of the Triassic-Jurassic subduction zone-magmatic arc couple. Evidence that may be considered in support of this proposition include the arc related geochemical signature for the dominant ( 70 %) volcanoclastic character of the Central Andean Terrane ( Restrepo-Pace, 1992). The volume of the Central 152 Andean Terrane rocks may require an equally extensive development of an arc to provide a sufficient supply of volcano clastics . With no doubt the Triassic - Jurassic plutonic arc is the largest in extension and volume that could be recognized in the northern Andes. Additionally, the high degree of preservation of these rocks compared to the so called equivalent 'Cambro­ Ordovician' units further inland along the Eastern Cordillera is suspect. The notorious absence of Paleozoic fossiliferous rocks above the Central Andean Terrane rocks (the oldest being Cretaceous in age)., the generally younger (by -20-100 m.y.) cooling ages for the plutonic rocks in the Central Cordillera as compared to the Eastern Cordillera and the fact that Precambrian basement along the eastern margin of the Central Andean Terrane is always in fault contact indirectly points to a younger age for the Central Andean Terrane rocks . Nonetheless, an older history for the Central Andean Terrane cannot be ruled out with the available data and will continue to be the most puzzling feature of the northern Andes.

Cyclic events ? The tectonic history outlined for the northern Andes Discrete orogenic pulse are shown to be may be represented by peak number of ages, shown in a compilation of the Colombian- Venezuelan geochronologic data base. Data from this study is incorporated . Further refinement is needed, in terms of separating mineral from whole rock data. Nonetheless a pattern of discrete 'events' and perhaps a kind of cyclicity was established ( Fig. 36). These events were accompanied by widespread thermal perturbations. The cycles 153 established which begin with an initial transgressive sequence deposition, followed by a peak thermal perturbation associated. with increased tectonic activity - which I chose to tie preferably to U /Ph zircon crystallization events - and closing with ( more clearly in Late Paleozoic-Mesozoic events) red-bed molasse deposition. As far as the available data could resolve thus far, from Grenville ( -1.0 Ga) to late Paleozoic time, peak deformation episodes were separated initially by a 500 Ma quiescent period and then followed by two other periods spanning 250 Ma and 50 Ma.

Figure Captions

Figure 34. Paleogeographic reconstructions for Amazonia, Laurentia and Baltica from -1.2 Ga to -0.32 Ga ago. Compiled from reconstructions by

Hofmann, 1991; Park, 1992; Vander Voo, 1993. A = Amazonia, L = Laurentia and BA = Baltica. Figure 35. Schematic cross-sections depicting the evolution of the northwestern Andean margin from -1.2 to 0.22 Ga. Figure 36. Histogram of pre-Cretaceous radiometric ages for the Colombian - Venezuelan Andes. Data from this study and compilations by Maya (1993),

Gonzalez de Juana ( 1980) and Case et al ( 1990). Tm = transgressive marine sequence. Rbd = Red-bed sequence. 154

0---- L

-1.2 Ga M. Proterozoic

. --.. - .' L ___ . ,:; -".--,,--- 0

-1.05 Ga M. Proterozoic

-0.6 Ga L. Proterozoic

Arc magmatism

Grenvillian-Orinoquiense orogen Figure 34. ... ------0 -0.53 - 0.5 Ga L. Cambrian - E. Ordovician

-O.39Ga E. Devonian

-0.32 Ga E. Late Carbonniferous

".!'

--r ..~------0

~ Gondwanan South American (GSA) trilobites

~ Acado-Baltic (AB) trilobites

~ Mixed GSA-AB 1111111111 .... ---. Malvinokaffric realm faunal province

Appalachian realm faunal province 156

Laurentia Amazonia

-1.2 Ga

Grenvillian-Orinoqlliense orogeny -1.0SGa

~\':,...-, • ':7~ 111111111111' '111111111111 .. -O,6Ga

Caparonensis Orogeny • -0.47Ga

Figure 35. 157

-•O.39Ga

Late Pz. folding -O.32Ga

Santandert Massif r-:::------=---~

CAT? ......

11 '111 ...... Late Triassic-Early Jurassic 111111111111111 111 ' - '\ , Rifting-igneous related metamorphism -O.22Ga

Orosenic assembJases

---Nickerie Caparonensis Triass-Jur

Molassic sediments ContientaI-margin sediments Number obtained Ma 0 20 40 60 80 144-163 ~~~r-~--L--""""'--...1.-_.o....----lL...-______--,

Jur 163-187 }} Dismembering of Pangaea. 187-206 Back - arc extensional tectonics . 206-230 Rbd Igneolls related metamorphism Trlass 230-240 240-245 245-258 }} Pangaea amalgamation and Perm , Tm collisional event 258-286, folding and tim/sting C b 286-320 ar 320-360 • U/Pb • Rb/Sr cu 360-374 Q K/Ar bJ:) Dev 374-387 **From this } ffi] Arl Ar <: 387-408 investigation • U/Pb 408-421 Sil 421-438 } Tm (Iocal- Merida Andes) 438-458 Caparonensis Andean - type Ord 458-478 ~ orogenic event -low to lIledil/lIl grade 478-505 metalllorpi/ism 505-532 c 523-540 540-570.1 } 570-700 ,;;~~ Rifting ( ? ) LP 700-900 ~ Orinoquiense-Grenvillian 900-1300 •••• } continental collision- IIigll gmde ...... MP 1300-1600 ~~:::::;,; metamorphism Ul \1J Figure 36. 159 IX. REFERENCES

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Logistics and methods Two field sessions totaling five months were devoted to this research in the Garzon, Santander and Santa-Marta massifs. Fieldwork was aimed at the following : - Check key stratigraphic relationships portrayed on the available maps and/ or literature and conduct detailed structural observations. Also to make the pertinent corrections on the maps by mapping specific contacts. - Sampling the various units pertaining this investigation including many of the same sites for which previous radiometric ages had been reported. ,,york was limited to road cuts, creek and river banks. All other areas are of very limited access. A latitude-longitude sample location list has been included in the Appendix . Basic sampling rules were followed in order to obtain fresh, uncontaminated and representative rocks. Their size and type would depend on grain size, mineral assemblages, field relations and type of analyses to be conducted. The greater part of field and analytical work was devoted to the Santander Massif area and its basement units. Most, if not all, of the Phanerozoic rock record of the northern Andes is exposed in the Santander Massif. A good base map and accompanying report on the geology of the Santander Massif ( Goldsmith et aI, 1971 ; Ward et al, 1973 ), was available at the beginning of the field session . The latter was elaborated jointly between USGS geologists and nineteen Colombian geologists from INGEOMINAS 184 (=Colombian Geological Survey) during a three year mapping project. Additionally very informative discussions were held with Geologist­ Petrologist Gloria I. Rodriguez (INGEOMINAS) who at the time, was involved in an intensive petrological characterization of the metamorphic basement of the Santander Massif. A very extensive network of roads provided relatively good exposure, though the best outcrops are located above

3500 m elevation where vegetation cover decreases. As a compliment to petrographic studies, trace element analyses were conducted to obtain information concerning the protolith of metamorphic rocks and to discriminate tectonically the granitic rocks. Four U /Pb zircon ages and thirty three 40Ar/39Ar cooling ages were obtained aimed at determining the age(s) of metamorphism and to certain degree, obtain an understanding of the thermal history of the basement rocks. Neodymium model ages were conducted to obtain some preliminary data concerning the basement protolith's source terranes. Rock crushing, mineral separation, trace element geochemistry, Neodymium isotope analyses and U/Pb zircon ages were all performed at facilities of the Department of Geosciences, University of Arizona. 4oAr/39Ar analyses were conducted at the Department of Geology-University of Lausanne, Switzerland. 185 Analytical methods:

REE ( Rare Earth Element data) Samples were crushed in a jaw crusher and divided to maintain a representative sample, then pulverized in a porcelain capsule. Five batches of twelve samples were run, including USGS standard SDC-l and two sample duplicates. Reproducibility was found to be better than 10 % for all elements during the five runs standard SDC-l. Approximately 50 mg of sample was dissolved for a week at 160 0 C in HF /HN03 9:1 solution at high-pressure in

Teflon containers enclosed in steel cases. After dissolution the sample was treated with perchioric acid to remove fluorides and organic compounds. Samples were spiked with 10 microlitters of 250 ppm Re-In solution and taken in 2 % HN03 and analyzed for rare earth element concentration in a

Finnigan ® TS Sola quadrupole mass spectrometer coupled to an argon plasma source. A Ippm stock solution of all elements analyzed was prepared and diluted to 50 ppb 2% HN03 and run simultaneously with unknowns. Pure solutions of Ba, La, Nd and Sm were run prior to each batch, for oxide interference corrections.

40Ar/39Ar

Samples were crushed and sieved at the University of Arizona. No heavy liquids were used during separation of minerals. Hand-picking was conducted to ensure> 99 % purity. A1140Ar/39Ar analyses were performed at 186 the University of Laussane, Switzerland under the supervision of Dr. Michael Cosca. The following procedures were extracted from Cosca et al (1994). Samples and standards enclosed in numbered tin capsules were placed in evacuated quartz vials for irradiation at the TRIGA reactor ( U.S. Geological Survey - Denver, Colorado) . These were then heated incrementally in a low blank, double-vacuum resistance furnace. High-temperature (> 1600 0 C ) blanks were measured between samples. The noble gases were purified by exposure to activated getters and a cold finger cooled with liquid nitrogen.

Argon extracted was analyzed statically with a Mass Analyzer Products ® 215-50 mass spectrometer. Five cycles were measured per analysis over the mass range 40 to 35. Raw isotopic abundances were determined by zero linear regressions of peak heights above backgrounds. This data was corrected for

backgrounds, 37 Ar decay and mass discrimination. Ages were calculated by correcting for isotope decay, inferring Ca, K and CI derived isotopes of argon and neutron flux gradients. The isotope production ratios given by Dalrymple

et al. (1981) for TRIGA reactor have been employed for the calculations. Mass discrimination was corrected was corrected for by periodic on-line measurement of air. The neutron flux (J ) was monitored using MMhb-1 hornblende assuming an age of 520.4 Ma (Samson et aI, 1987). Errors in value of J were determined with an in laboratory precision of better than ± 0.25 %. The 4OAr/39Ar ages were calculated incorporating the decay constants tabulated in Steiger et al. (1977). 187 U/Pb Initial sample crushing and separation conducted by the author, final picking and actual analyses perfonned by Dr. George Gehrels at the University of Arizona . The following procedures were extracted from Gehrels ( 1990 ). Samples were crushed in a jaw and roller crushers and then sieved . Zircons were separated initially with the Franz magnetic separator and heavy liquids. Hand picking under transmitted-light petrographic microscope was conducted to ensure that the zircon separate was 100 % pure. Zircons were loaded into a 0.3 ml Teflon microcapsule itself enclosed in a Teflon lined 125 ml digestion bomb. The zircons were dissolved for 30 hr in HF » HN03 at

240 0 C , evaporated to dryness on a hot plate and then dissolved for 12 hr in

6N HO at 215 0 C. Mter Hel dilution the zircon solution is aliquoted and

spiked with a 208fb/235U tracer. The samples are loaded with H3P04 and silica

gel ( for Pb and most U ) or graphite ( for some U ) onto degassed Re filaments. U was run with graphite as V metal initially, and later with silica gel as V02 is run together with the spiked Pb aliquot on the same filament,

with Pb ionizing at a slightly lower temperature than that for U. A VG-354 mass spectrometer was utilized equipped with 6 Faraday collectors and a Daly detector, while 206,207, and 208 were measured simultaneously in the Faraday collectors. The gain factor ( approximately 110) for the Daly detector was determined just prior to data collection by comparing 206/207 ( 206 in the L1 collector and 207 in the axial collector) measured with the Daly off and on. Data collection begun only when 188 successive measurements showed that the gain factor is stable or better than 0.2%. Measured isotopic ratios were adjusted using mass dependent correction factors described in Gehrels ( 1990 ) and also corrected for 50 ± 25 pg of blank Pb and 5 ± 3 pg of blank U. Additional non radiogenic Pb in the sample is assumed to be the initial Pb and is assigned a composition consistent with the model of Stacey et al. (1975). Age calculations and error analyses have been adapted from programs written by Ludwing (1982,1983).

Sm/Nd Analyses were performed by Dr. James Gleason. The following procedures were extracted from Gleason ( 1994 ). Approximately 400 mg of sample was dissolved according to the same procedures outlined for the REE analyses. A 149Sm/15ONd spike was combined with the dissolved sample solution. The concentrated and equilibrated spiked sample solutions were diluted to several ml's of 2.5M Hel, centrifuged, divided into two aliquots and loaded on 8.9 ml quartz ion exchange columns using AG50W-XI2 cation exchange resin. REE were collected as a group using 2.5M HO and 6.0M HO as eluents. The REE fraction was dried down and treated with HCI04 any organics picked from the column. A second stage chemical separation of REE was performed by reverse-phase chromatography using HDEHP ( hydrogen di-2-ethylhexyl phosphate) - coated PTFE powder as the column resin in 1.7 ml quartz ion-exchange columns, and Nd and Sm eluted with 0.18 M HO for Nd and 0.5M HCI. 189 Between 1 and 5 micrograms of sample were loaded as chlorides on two Ta side filaments in a Ta-Re-Ta triple filament assembly and run on a solid source VG 354 thermal ionization mass spectrometer equipped with six Faraday collectors. Nd isotopic ratios were normalized for isotopic fractionation to 146Sm/144Nd = 0.7219 and fractionation-corrected values computed on line. ------

Sample Unit Age Sm Nd 147Sm 143N!t b eNd C eNd C TDI\f (Ma) (ppm) (ppm) 144Nd 144Nd present initial d measured (Gal ------ABI.l 1'2 -S.101.1",I"r -t70 11.31 -t2A3 0.11114 0.512027t6 -12.7 -i\'(; 1.06 Massif AB2a PZ - Santander 470 20.48 110.3 0.1122 0.512970±7 -13.0 -6.9 1.01 Massif BM-) PZ - Sanlandt'r -t70 5.04 28.311 0.1201 O.512007±6 -12.3 -7.4 1.68 Massif BM-of PZ- Sanland .. r 470 0.77 34.55 0.1185 0.512009±8 -12.3 -7.3 1.65 Massif G-20 pE: - Gar",;n WOO 1.14 3.62 0.18% 0.51 2626±7 -0.24 +0.6 2.71 Massif HP·) p€ - Gaul;n WllO 9.25 32.95 0.1697 O.512472±1O -3.23 +0.2 1.97 Massif $nAnKr-1 r€' Gamin lOOO 14.93 78.117 0.1143 0.512062±7 -11.2 -0.7 1.5 Massif RG-) pE:- S. Marla 1000 7.38 40.42 0.1104 0.511872±8 -14.9 -3.9 1.72 Massif a Unct'rlainli". 011 2-sigma art' ± 0.5 % bR.llins nnrmali"ed to 146Nd/144Nd = 0.721<1 (2-sigma "reors rt'flecl in-run percisionl C t' 4 143 144 . 143 144 147 144 Nd = III I< Ndl Nd(I)CHUR) - 1»), usmg Ndl Nd =0.512638 as present day CHUR value, and Sm/ NdCHUR = 0.1966 d mndd agt's calculal.. d using .. qualion of DePaolo (1981)

..... ~ Additional 40Ar/39Ar data :CENffiALCORDlLLERA

I = Integrated age TF= Total fusion age P ,~ Platl'

Sample F1or-l Hornblende I age = 82 ± 4 Ma .1=0.00478110.000024 Page = 83.8 ± 1.3 Mil Isochron age = 85 ± 1 Ma

TOC CalK 36Ar/39M *40Ar/39Ar Moles % Radiogenic %39Ar Age 40Ar.l014 Release 950 0.041 0.02088 9.042 7.8 63.3 10.7 76 ± 4 975 0.061 0.01629 9.240 6.6 69.0 9.7 78 ± 4 1000 0.061 0.00758 10.300 13.3 86.6 22.0 87 ± 2 1025 0.063 0.01899 9.501 4.8 65.5 6.5 80 ± 5 1050 0.060 0.02818 9.756 2.9 55.9 3.3 82 ± 11 1075 0.057 0.02052 9.664 4.1 64.2 5.4 81 ± 7 1100 0.063 0.01090 9.958 8.4 79.3 13.2 84 ± 2 1150 0.060 0.00715 9.886 14.4 87.2 25.0 83 ± 2 1175 0.041 0.02274 9.171 2.4 61.3 3.2 77 ± 6 1200 0.02 0.04799 9.573 1.3 44.3 1.2 80 ± 13

Sample Flor-2 Hornblende 1age = 80.2 ± 1.7 Ma .1=0.004781 ±0.000024

TOe K/Ca 36Ar/39Ar *40Ar/39Ar Moles %Radiogenic %39Ar Age 40Ar.l014 Release 950 0.100 0.01576 8.524 6.0 66.6 7.0 72 ± 3 975 0.079 0.01059 9.498 5.3 77.9 6.2 80 ± 3 1000 0.079 0.00417 9.854 19.4 93.1 22.8 83 ± 1 ...... \0 ~~ TOC K/Ca 36Ar/39Ar *40Ar/ 39Ar Moles %Radiogenic %39Ar Age 40Ar.l014 Release 1050 0.081 0.00176 9.206 19.7 99.6 23.1 78 ± 1 1100 0.079 0.00284 9.741 28.3 96.6 33.2 82 ± 1 1125 0.076 0.04923 3.603 0.7 20.4 0.8 31 ± 28 1150 0.075 0.01086 9.123 4.3 77.2 5.0 77 ± 3 1250 0.074 0.01288 11.028 1.5 77.1 1.8 93 ± 11

Sample Flor-3 Biotite I age = 45.8 -.I: 1,7 Ma J=0,004774±O.OOO041 Page= Ma

'roc 36Ar/39Ar *40Ar/39Ar Moles %Radiogenic %39Ar Age 40Ar.l014 Release 700 0.07761 2.476 36.9 9.7 6.4 21 ± 5 800 0.00100 5,392 15.9 94.8 12.3 46 ± 2 850 0.00029 5,634 13.5 98.5 10.4 48 ± 1 900 0,00123 6.411 14.1 94.6 9.2 54 ± 1 950 0.00108 5.764 14.9 94.8 10.8 49 ± 1 975 0.00118 5.913 14.3 94.4 10.0 50 ± 2 1000 0,00158 4.966 12.4 91.4 10.0 42 ± 1 1050 0.00068 5,386 14.4 96.5 11.4 46 ± 1 1100 0.00121 6.037 15.6 94.6 10.8 51 ± 1 1200 0,00195 4.835 10.8 89.9 8.8 41 ± 1

>-' tS IllU Flor-1 Hornb. HU Amphibolic schist Flor-l Hornb. nI 0.0035 Amphibolic schist ~ 120 ------113.8 ± 1.3 Mil ------Qj ... 0.0030 Age = 85 ± 1 CO 100 nI < MSWD = 0.76 ~ 0.0025 HO (40/36)tT= 271 ± 16 -=...III --.... ~ nI ... 0.0020 c.. bO c.. \0« rf) 0.0015 ~ ·1Il Inll·llr.,,,,.! All'" = H2 ±·I Ma 0.0010 20 0.0005 0 120 0.0000 0 0 0 0 0 0 Flor-2 Hornb. R 8 nI ~ q § 0 2! ... ~ HlO Amphibolitic schist g ci ~ 0 ~ 0 ci ~ 0 ci Qj eo 110 :~::-(f···· 39Ar / 40Ar

'" (,() -=Qj ... Inlcgrntcd Age = 80.2 ± 1.7 Mil c.. 40 c..'" ~ 20

0 !1I JOO ~ Qj Inlcllr~kd Aile = 45.8 ± 1.7 Mil Flor-3 Biotite eo nI Granitic Intrusion 50 -=...aJ !1I c.. c.. 0 ~ 50 ..-.. (;3 Cumulative 0/039Ar Released SAMPI.I' II> !Jnl! lithology ··I.ocallon of dated Precision! comment: samples Map srnJe

l.ntlludc \I Illl1glludel\" SAN TAN Ill'R MASSII'

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