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Geological Society of America Bulletin

doi: 10.1130/0016-7606(1974)85<273:GEOTSN>2.0.CO;2 1974;85, no. 2;273-284Geological Society of America Bulletin

GERALD T. CEBULACHARLES M. TSCHANZ, RICHARD F. MARVIN, JAIME CRUZ B., HARALD H. MEHNERT and Geologic Evolution of the Sierra Nevada de Santa Marta, Northeastern Colombia

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} CHARLES M. T S C H A N Z , „ „ , , , , R I C H A R D F M A R V I N ' Geological Survey, Denver, Colorado 80225

JAIME CRUZ B. Instituto Nacional de Investigaciones Geológico-Mineras, Bogotá. Colombia H A R A L D H. M E H N E R T GERALD T. CEBULA f Geological Survey, Denver, Colorado 802 25

Geologic Evolution of the Sierra Nevada de Santa Marta, Northeastern Colombia

ABSTRACT

N e w geo log ic , p e t r o g r a p h i c , and radiometric evidence (52 ages) f rom the Sierra Nevada suggest that plate tectonics controlled the complex Mesozoic evolution of the Caribbean continental margin.

The triangular Sierra Nevada massif is b o u n d e d by the Oca f a u l t , San ta M a r t a - B u c a r a m a n g a f au l t , and Cesa r lineament. During the Tertiary, dextral and sinistral movement of 65 and 110 km, re-spectively, occurred along the Oca and Santa Marta—Bucaramanga faults; subse-quently, several thousand meters of uplift produced the present geomorphic setting.

Three metamorphic terranes are present; they d i f f e r p e t r o g r a p h i c a l l y and geo-chronologically and are separated by the Sevilla and Cesar lineaments (geosutures). The younges t t e r rane consists of three northeast- t rending regional metamorphic belts (Permian-Tr iass ic gneiss, Jurass ic schist, and Cretaceous-Paleocene green schist) that formed in successive subductiori zones northwest of the Sevilla lineament. Tertiary plutons intrude this terrane.

Most of the Sierra Nevada massif consists; of l ,300-m.y.-old granulite terrane overlair by u n m e t a m o r p h o s e d Pa leozo ic and Permian(?)-Triassic rocks and intruded by four northeast-trending belts of plutons that filled successive d i la t iona l r i f t s . These plutonic belts become younger, shallower, and more potassic in a southeastward direc-tion. Extensional disruption, with transform separations up to 46 km, culminated with Middle Jurassic emplacement of two belts of composite batholiths and extensive ignim-britic eruptions. These events are related to the same southeast-dipping subduction zone that produced the Jurassic schist in the youngest metamorphic terrane.

The third metamorphic terrane consists of younger(?) Precambrian amphibolite-grade rocks overlain by Silurian phyllites and un-metamorphosed Paleozoic and Mesozoic rocks that are typical of the Cordil lera Oriental.

INTRODUCTION Publication of the geologic map of the

Sierra Nevada de Santa Marta region (Fig. 1)

by the Instituto Nacional de Investigaciones Geológico-Mineras (Tschanz and others, 1969) culminated 3 years (1964-1967) of geologic mapping of 22,500 sq km by 15 Colombian geologists and C. M. Tschanz. Technical assistance was provided by the U.S. Geological Survey under the auspices of the Republic of Colombia and the Agency for International Development, U.S. De-partment of State. The geologic mapping of the northern Sierra Nevada de Santa Marta partly overlapped independent reconnais-sance studies by W. D. MacDonald of the State University of New York at Bingham-ton, and preceded the detailed mapping and petrographic studies of the Santa Mar ta area by Doolan (1970).

The Sierra Nevada de Santa Mar ta (Fig. 1) is a triangle-shaped mountainous region that covers 13,700 sq km and rises from near sea level to a maximum altitude of 5,700 m (—19,000 ft). The region is isolated from the main mountain ranges of Colombia (Fig. 1) by wide alluvial plains which are almost at sea level. It is bounded on the north by the Caribbean Sea and the plains of the lower Guajira Peninsula, on the west by the broad Magdalena River plain, and on the south-east by the valleys of the Rios Cesar and Ranchería.

Approximately 80 percent of the exposed rocks in the Sierra Nevada are metamorphic or igneous. The preponderance of such rocks, together with the poorly exposed geologic contacts, the rugged, forested terrain, and the extensive faulting, makes the geologic history exceedingly difficult to unravel by field mapping alone. Therefore, many of the major igneous and metamor-phic units were radiometrically dated.

This paper presents the radiometric ages of 36 samples obtained during our study and discusses the geologic evolution on the Sierra Nevada de Santa Marta as deduced from radiometric ages and geologic evi-dence. The regional framework, major faults, and geotectonic provinces are briefly described. Our interpretations of orogenic and structural events are based on rapid regional mapping of very complex geology and on radiometric ages which may reflect modification by subsequent dynamothermal or me tasomat i c events. Some m a j o r

Figure 1. Index map showing location of Sierra Nevada de Santa Marta, Colombia (line pattern), and the principal mountain systems in Colombia and Venezuela.

lithologic units are not well mapped, and many additional age determinations (Rb-Sr, U-Pb) are needed to unravel the early metamorphic history. A comprehensive report on the geology of the Sierra Nevada (to be published in Spanish by the Instituto Nacional de Investigaciones Geológico-Mineras) will present evidence in greater detail for the interpretations outlined in this paper.

REGIONAL GEOLOGIC SETTING The Sierra Nevada de Santa Marta is a

composite, fault-bounded structural ele-ment that lies far northwest of its proper position within the large-scale geotectonic framework of northwestern South America (Fig. 1).

It is bounded by major fault systems and basins in which there are thick deposits of Tertiary sediments. The Oca fault forms the geologic boundary on the north; the Santa Marta—Bucaramanga fault system forms the west boundary; and a structural trough along an inferred northeast-trending fault forms the boundary on the southeast (see Fig. 2).

The basement of the Sierra Nevada consists of segments of four metamorphic belts that, similar to the bounding fault systems (Fig. 2), formerly extended as continuous, linear regional features far beyond this region. These metamorphic belts were offset great distances along the

Geological Society of America Bulletin, v. 85, p. 273-284, 2 figs., February 1974

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2 7 4 TSCHANZ A N D OTHERS

Oca and Santa Mar ta -Bucaramanga fault systems in latest Mesozoic or earliest Tertiary time. The massif was uplifted as a horstlike unit, approximately along these same faults, beginning in earliest Miocene time when the adjacent basins were formed. The maximum structural relief on the metamorphic basement between the uplifted block and the basins is about 12 km.

The nor thwestward movement of the massif relative to the adjacent buried s e g m e n t s of the n o r t h e a s t - t r e n d i n g metamorphic belts is partly revealed by radiometric ages and thin sections of samples f rom a few deep oil wells that penetrated the basement of the Tertiary basins (Fig. 2). The new evidence in this paper suggests 100 to 115 km of left-lateral separation of the Santa Mar ta Schist along the Santa Mar t a -Buca ramanga fault system and at least 65 km of right-lateral separation of these rocks along the Oca fault. West of the Santa Marta—Bucaramanga fault system and south of the massif, oil wells have disclosed the presence of subsurface Pre-cambrian granulite and also gneiss and schist that correlate with the Sevilla metamorphic belt of the Sierra Nevada massif. Therefore, subsurface evidence indicates that all four metamorphic belts are offset about the same distance.

The late Tertiary normal movement of these faults has caused controversy. Camp-bell (1968) suggested about 110 km of left-lateral displacement on the Santa Mar ta -Bucaramanga fault, but he thought that this displacement was post-Miocene. Poison and Henao (1968) disproved this age and did not accept Campbell 's evidence for large left-lateral wrench faulting. Both interpretations were based mainly on the stratigraphy of the Cretaceous and Tertiary rocks, whereas our interpretation is based on the lithology and geochronology of the metamorphic basement.

GEOTECTONIC PROVINCES The Sierra Nevada de Santa Mar ta ,

together with the adjacent part of the Serranía de Perijá, consists of four distinct geotectonic provinces. From southeast to northwest , these are the Perijá, Sierra Nevada, Sevilla, and Santa Mar ta provinces (Figs. 1 and 2). Each province contains a distinctive metamorphic series not found in the other provinces (Fig. 2). Lithologic and stratigraphic differences imply structural discontinuities between the first three provinces. The Sevilla lineament separates the Sevilla and Sierra Nevada provinces; the Cesar lineament separates the Sierra Nevada and Perijá provinces (Fig. 2).

Perijá Province Generally, the Precambrian, Paleozoic,

and Mesozoic rocks of the Perijá province more closely resemble rocks of similar age in

the Cordillera Oriental (Fig. 1) than they do rocks of the Sierra Nevada massif. Precam-brian schist is .inconformably overlain by phyllitic and cor.rse-grained clastic rocks of Cambrian(?) and Ordovician age (Fig. 2) that were weakly metamorphosed by Early Silurian time. These metasedimentary rocks are unconformably overlain by conglomer-ate, sandstone, shale, and limestone of Devonian and l'ennsylvanian age that are generally similar to, but thicker than, equivalent rocks in the Sierra Nevada. Permian conglomerate, red sandstone, and limestone are present in Perija province but absent in the Sierra Nevada.

The Paleozoic rocks are unconformably overlain by Mesozoic red beds that resemble the Guatapuri f o rma t ion (Triassic) in the Sierra Nevada (Fig. 2), but contain fewer volcanic rocks and may be mostly younger in age. These Mesozoic red beds are unconformably overlain by sandstone or, locally, by conglomerate and red beds. These, in turn, are unconformably overlain by the Golero Rhyolite (Cretaceous), which is succeeded by more unconformable red beds (Fig. 2). 'I he Golero Rhyolite is also present in the Sierra Nevada.

Cretaceous limestones were deposited unconformably over the older Mesozoic rocks. Thick sedimentary rocks of latest Cretaceous, Pabocene, and F'ocene age were deposited in a narrow structural trough in the Rancheria Valley (Fig. 2) along the inferred fault separating the Perija and Sierra Nevada provinces.

Sierra Nevada Province The Sierra Nevada province is the oldest

and largest p-ovince of the massif. Its geologic history for late Precambrian and much of Paleozoic times is obscure. Mar ine Devonian (?) and Pennsylvanian sediments were depositee and then were partially eroded. During the latest Paleozoic and early Mesozoic, the province was a marine(?) platform on which spilitic rocks and graywackes were deposited. It was then uplifted and intruded by numerous plutons and was covered by contemporaneous ignimbritic rocks (Fig. 2). Volcanic erup-tions continued almost until the Cretaceous marine invasion, when limestone and shale beds were deposited.

The Precar-.brian basement (the Los Mangos Grarul i te ; Fig. 2) consists of alternating layers of many types of felsic and mafic rocks of the granulite fades , many of which contain orthopyroxene. The nearest known occurrence of granulite, outside the Sierra Nevada, is 92 km to the south in t he s u b s u r f a c e w e s t of the S a n t a Mar ta -Bucaramanga fault system.

The differences in the metamorphic basement and the Permian, Triassic, Juras-sic, and Lower Cretaceous volcanic and sedimentary rocks between the Sierra

Nevada and Perija provinces suggest that a major northsast-trending fault system, the Cesar lineament, lies buried beneath the Cesar Valley (Fig. 2). The regional structural pattern and the great lithological differences across the inferred lineament suggest thai: the Cesar lineament may be comparable in importance and may be related to the Sevilla lineament.

Sevilla and Santa Marta Provinces The Sevilla and Santa Mar ta provinces are

parallel, northeast-trending metamorphic belts that ate intruded by Tertiary plutons (Fig. 2). The Sevilla province consists mainly of a mafic basement gneiss and a possibly younger schist which are not distinguished on Figure 2. The Santa Mar ta metamorphic bel t cons i s t s of an i nne r bel t of amphibolite-grade schist and a structurally complex outer belt of greenschist and phyllite (Fig. 2). The geological and structural relations between the rocks in these three belts are complex, but are thought to result f rom two distinct events — the Sevilla and Santa Mar ta orogenies.

RADIOMETRIC AGES A total of 48 K-Ar ages (Table 1) and four

Rb-Sr ages (Table 2) were determined for 36 samples. K-Ar and Rb-Sr ages were determined by the techniques described, respectively, by Evernden and Curtis (1965) and Peterman and others (1967).

Most ages were determined by the K-Ar age method, because specimens of the major metamorphic and igneous units were generally not suitable for Rb-Sr dating. The K-Ar ages are minimum ages and thus may not signify the time of mineral crystalliza-tion, but, instead, may signify the latest thermal event, such as nearby igneous intrusions, dynamic metamorphism, fault-ing, or deep burial.

INTERPRETATION OF RADIOMETRIC AGES

Precambrian Rocks A Precambrian age for the granulite

basement complex in the Sierra Nevada massif was established by radioactive datin g (Tschanz arid others, 1969), but no a t tempt was made to work out the Precambrian history of these rocks. T w o widely separated samples of similar quartz-perthite granulite (Iocs. 34 and 35, Fig. 2) gave Rb-Sr whole-rock ages of 752 and 1,300 m.y. (Table 2). The older age is a minimum for the Precambrian granulite and agrees, within analytical uncertainty, with a Rb-Sr isochron age of 1,400 m.y. for the Dibulla Gneiss (same as Los Mangos Granulite) (B Iocs., Fig. 2; MacDonald and Hurley, 1969). Similar granulite in the fault-bounded Imataca complex in the northern part of the Guayana Shield of Venezuela has given



GEOLOGIC EVOLUTION OF THE SIERRA NEVADA DE SANTA MART A, NORTHEASTERN COLOMBIA 2 7 5

radiometric ages as old as 3,136 m.y. (Martin-Bellizzia, 1968, map). It is likely that the granulite in Colombia and Ven-ezuela is all approximately the same age and is older than the lower grade Precambrian rocks between the Guayana Shield and the Sierra Nevada massif.

The available Precambrian radiometric ages f rom the Sierra Nevada massif and from the broad region between it and the Guayana Shield suggest that the granulite ages in the Sierra Nevada massif were reset by two or three subsequent Precambrian metamorphic events about 1,200 to 1,400, 940, and possibly about 752 m.y. ago. The geo log ic a n d r a d i o m e t r i c d a t a f u r t h e r suggest that two successive Precambrian metamorphic belts developed during these orogenies around the margin of the granulite nucleus of the ancestral Guayana Shield.

Rocks giving radiometric ages betweer. 1,200 and 1,400 m.y. are widespread near the western and northern margins of the Guayana Shield in Colombia (Pinson and others, 1962) and in Venezuela (Kal-liokoski, 1965, p. 1047), but rocks with radiometric ages older than 945 m.y. are not known in the isolated Precambrian expo-sures nearer the Sierra Nevada massif. The dated rocks are intrusive rocks and amphibolite-grade gneiss and schist.

The Precambrian metamorphic basement consists of gneiss and schist with radiomet-ric ages ranging from 660 to 800 m.y. in the Serrania de Perija and adjacent regions of Venezuela (Martin-Bellizzia, 1968, p. 369, Table 3) and f rom 680 to 945 m.y. in the Santander massif in the Cordillera Oriental (Goldsmith and others, 1971). The Precam-brian schists in the Colombian part of the Serrania de Perija have not been dated.

The K-Ar hornblende age of 940 m.y. for a mafic granulite (loc. 31, Fig. 2 and Table 1) may represent a me tamorph ic event loca l i zed a l o n g an a n c i e n t S a n t a Marta—Bucaramanga lineament. A similar K-Ar hornblende age for an amphibolite-grade gneiss near this lineament 275 km to the south (Goldsmith and others, 1971, p. D46) and K-Ar mica ages of 930 to 960 m.y. for anorogenic tin-bearing granite in Ron-donia, northwestern Brazil (Priem and others, 1966), provide some support for inferring a possible 940-m.y.-old postgranu-lite geologic event along this trend. The 940-m.y. ages are confined to this trend.

Granulite is not exposed or easily inferred north of the Oca fault or between the Sierra Nevada and the Guayana Shield. In all this region, the Precambrian rocks are either intrusive rocks or amphibolite-grade gneiss and schist. The granulite in the Sierra Nevada massif appears to be an isolated tectonic block within a younger metamor-phic basement. To the south, similar granulite has been reported in the Cordillera Central (Fig. 1) by T. Feininger and others

(1970) and by Dario Barrero (1968, oral commun.) and in the Garzon massif by T. Feininger (1970, written commun.) . The known granulite areas are separated f rom younger and lower grade metamorphic rocks by faults trending N. 15° E., N. 70° E., and N. 15° W. Thus, the granulite in Colombia appears to be confined to a discontinuous(P), faulted, narrow belt that extends f rom the Sierra Nevada southward along the eastern margin of the Cordillera Central to the Garzon massif in the Cordillera Oriental, near the divergence of the two Andean ranges (Fig. 1). Precambrian rocks probably are absent west of this belt; those in the Andean ranges east of the belt are younger, lower grade Precambrian gneiss and schist.

The widely separated granulite areas in Colombia are probably tectonic fragments that were formerly contiguous and are possibly part of the Imataca complex in the Guayana Shield. Martin-Bellizzia (1968, map) has shown that the Imataca complex is bounded on the south by a major N. 70° E. fault that separates it from other Precam-brian rocks. A parallel fault may extend along Rio Meta into Colombia and form the north boundary of the granulite in the Garzon massif. The data suggest large-scale tectonic t ransport of microcontinental fragments of an Archean craton with a granulite basement, but several subsequent major periods of metamorphism and transcurrent faulting have produced a complex regional tectonic mosaic.

Syntectonic Plutons We infer from geologic and petrographic

evidence that the syntectonic plutons (Fig. 2) were emplaced after the Sevilla orogeny, but before the Jurassic batholiths and the Santa Mar ta orogeny. Because of fault contacts, the age of the metadiorite relative to the Sevilla metamorphic complex cannot be determined. A minimum age of 170 m.y. (Middle Jurassic) for the metadiorite is taken from a sample of hornblende from near the western border (loc. 12, Fig. 2 and Table 1), which may have been affected by the Santa Mar ta orogeny (Jurassic). A maximum age is given by a 250-m.y. K-Ar hornblende age for a dioritic rock of doubtful origin and correlation (loc. A, Fig. 2; MacDonald and Hurley, 1969). Some dioritic rocks east of the Aracataca batholith and within the western border zone (Fig. 2) are tentatively correlated with the metadio-rite pluton to the north and are inferred to be older than the surrounding rocks.

The western border zone is older than the Jurassic batholiths and is probably younger than the metadiorite. A hornblende concen-trate f rom a pegmatitic biotite diorite gave a late Middle Triassic age of 202 m.y. (loc. 14, Fig. 2, and Table 1). This is a minimum age for the western border zone, and yet the

dated rock typifies neither the western border zone nor the nearby Tucurinca plutonic complex.

Although the Rio Piedras complex and the Tres Puntas pluton have not been dated, the petrographic evidence suggests that Rio Piedras complex may be the age of the western border zone and that the Tres Puntas pluton may be slightly(?) younger.

Triassic Volcanic Rocks Spilitic volcanic rocks predate the Jurassic

batholiths. The dark-gray spilites (Corual Formation) are inferred geologically to range in age f rom Late Permian(?) to Early Triassic (approximately 240 to 210 m.y.); the red spilites of the Guatapuri Formation are inferred to be Early Triassic to possibly earliest Jurassic in age (—200 to 185 m.y.). An impure plagioclase concentrate f rom a basalt in or on the Guatapuri Formation (loc. 20, Fig. 2 and Table 1) gave a K-Ar age of 138 m.y. Another basalt (loc. 21, Fig. 2 and Table 1), underlying the lowest red beds in the Guatapuri Formation, gave whole-rock K-Ar ages of 165 and 181 m.y. Obviously, these three K-Ar ages fail to substantiate the geologically inferred ages.

The two most likely explanations for the above ages are (1) either these dated basalts are intrusive sills (loc. 21) or unconformable flows (loc. 20) related to numerous postbatholithic mafic dikes, or (2) argon has diffused f rom the samples, because of heating from the recurrent igneous activity during the Jurassic and (or) Early Creta-ceous.

Jurassic Batholiths The radiometric ages indicate that the

major plutons in the central and southeast-ern batholithic belts (Fig. 2) are Middle Jurassic, ~ 1 6 5 to 175 m.y. old, bu t thesmal l Don Diego pluton in the central belt apparently is Late Jurassic. In the southeast-ern belt, both the Patillal and Pueblo Bello batholiths have average K-Ar ages of ~ 175 m.y. (Iocs. 28, 29, and 30, Fig. 2 and Table 1). The fair agreement between the K-Ar ages for coexisting biotite and hornblende, combined with the Rb-Sr whole-rock ages (Iocs. 32 and 33, Fig. 2 and Table 2), strongly suggests that these batholiths are early Middle Jurassic. The Rb-Sr ages were determined on late granophyre (162 to 167 ± 1 8 m.y.) of the Patillal batholith that clearly intrudes the older quartz monzonite (174 m.y.). The radiometric ages of the Central and Ojeda batholiths (Gansser, 1955), of the central belt, and of the aforementioned batholiths are in agreement.

The biotite f rom the Don Diego trondh-jemite pluton (loc. 26, Fig. 2 and Table 1) in the Central batholith gave a K-Ar biotite age of 138 m.y. (Late Jurassic). This age and the 137-m.y. biotite age (loc. 14, Fig. 2 and Table 1) of the pegmatitic diorite may



2 7 6 T S C H A N Z A N D OTHERS

~> ' • . / / ! B u r i e d San ta

/ N a r t o S c h i s t / / . ' ! / / ? CARIBBEAN

7 ' , / / , Perico w e l l —^ / / / ' ' * 6 / '

20 KILOMETERS

Figure 2. Simplified geologic map of the Sierra Nevada de Santa Marta showing collection localities for radiomctrically dated samples (after Tschanz and others, 1969).

i n d i c a t e l imi ted p l u t o n i c and (or) metasomatic activity during the Late Juras-sic. Possibly, one or both these K-Ar biotite ages are reset ages.

The K-Ar ages suggest that the Atanques and Aracataca batholiths (Iocs. 23 and 22, Fig. 2 and Table 1) are younger than the other dated Middle Jurassic batholiths. However, the 57-rn.y. biotite age (loc. 23, Table 1) indicates that some geologic event, such as emplacement of the nearby lac-col i th^) (Fig. 2), caused loss of argon f rom the Atanques batholith. The same event may also have affected the 162-m.y. hornblende age (loc. 23, Fig. 2 and Table 1); the Atanques batholith may, then, be somewhat older than 162 m.y.

It seems reasonable to assume that the

Bolivar batholith (Gansser, 1955) is also Jurassic.

Before we could state the sequence of emplacement of the batholiths, the K-Ar biotite ages would have to differ by at least 5 m.y., and the ~ornblende ages must differ by at least 10 m.y. (Dalrymple and Lanphere, 1969, p. 120). Under these constraints, the Aracataca batholith (and possibly the Atan-ques) appear to be the youngest Jurassic batholiths.

A small stock of gray granodiorite f rom locality 27 (?ig. 2 and Table 1), which intrudes the Los Clavos Ignimbrite, gave ari age of 176 m.y., making the stock the same age as most of the batholiths and almost contemporaneous with the surrounding ignimbrite.

The youngest pluton in the Sierra Nevada province is probably the Atanques lac-coIith(?) (Fig. 2) of Paleocene(P) age, which is inferred to intrude the subhorizor ta l contact between the Atanques batholith and the volcanic rocks. A maximum age for this pluton may be 57 m.y., as indicated by the K-Ar biotite age determination (loc. 23, Fig. 2 and Table 1) f rom the Atanques batholith.

Jurassic and Cretaceous Volcanic Rocks Although volcanic specimens suitable for

isotopic dating were difficult to find, three samples were dated. The sample f rom locality 19 (Fig. 2 and Table 1) is a typical, unweathered, dark-gray, crystal-vitric ig-nimbrite from the Los Clavos Ignimbrite. T w o analyses of hornblende from this



GEOLOGIC EVOLUTION OF THE SIERRA N E V A D A DE SANTA M A R T A, N O R T H E A S T E R N C O L O M B I A 2 7 7

S A N T A M A R T A P R O V I N C E

POST-METAMORPHIC INTRUSIVE ROCKS

E X

S E V I L L A P R O V I N C E

POST-METAMOIÏPHIC INTRUSIVE ROCKS

P L A N A T I 0 N

S I E R R A N E V A D A

POST-METAMORPHIC INTRUSIVE ROCKS

' Santo Marta bot hol i t h . Quarti diorite, diorite, and pegmatite

J

SS

Buritoco pluton Quarti diorite, muscovite granite, ond

pegmatite

l.atal pluton Diorite, hornölendite, pegmatite. Includes

TuCurinquitc pluton of grcnodiorite

ûtonques loccolith(?) Gronodiorite porphyry

Socorro pluton Gronodiorite

Galilea pluton Granite porphyry

CENTRAL PLUTONIC BELT

SOUTHEASTERN PLUTONIC BELT

SANTA MARTA METAMORPHIC BELT /

Santo Morto Schist p, green schists ond phyllites s, mica and amphibolite schists

Metamorphosed between Paleocene and Late Jurassic

FOLDED AND METAMORPHOSED UNCONFORMITY OR FAULT SEVILLA METAMORPHIC

Contoct BELT

Gray gronodiorite Pink quartz mon-ond quartz monzonite, qronite, zonite and granophyre

SYNTECTONIC PLUTONIC BELT

E3 Tres Puntas pluton

Thrust fault Sawteeth on upper plate

•f U —=" D

Major fault, showing direction of movement

Dashed where approximately located or inferred; dotted where hidden by younger rocks. U, upthrown side; D, downthrown side

Sevilla metomorpt-ic complex

Gneisses and schists, poly-metamorphic ¡n part. Metamorphosed in Per-mian to Triassic time and ogain in Juros.vc time

Western border zone ond Rio Piedros complex

Quartz diorite and gronodiorite

Mefodiorite or epidionfe

Localities of samples for which radiometric oge were determined W,-W 6 , Segments of disrupted and

dilated east wall of pre-Triassic Sevilla Lineament

X - X ' Lateral offset

Aiiochthonous(?) Les Mangos Granulite and anorthosite

Figure 2. Continued.

P R O V I N C E P E R I J A P R O V I N C E

S T R A T I F I E D R O C K S

Cool-bearing clastic rocks

Shale ond limestone

Limestone ond shale of Maesinchtian to Aption oge Limestone of Aptian age

Red beds, sandstones, ond conglomerates

Golero Rhyolite Ignimbrite

Volconic rocks Includes Los Clavos Ignimbrite

and porphyries

Golero Rhyol ite

Mzr

S

Mesozoic red beds and volcanic rocks

Chiefly Jurassic, may include some Guata-puri Formation

Guatapuri Formation Red beds and red spilitic

volcanic rocks

Coruol Formotion Dork-gray spinile volconic

rocks ond groywockes

Limestone, sandstone, and shale

.¡mestone, sandstone, shole, conglomerate, ond red beds

Los Mangos Granulite A/ternafing loyers of mafic

and felsic gronuh'tes

Metasedimentary rocks

NOTE • Gneisses and schists of amphibolite focies outside mopped dreds are younger than the Los Maggos Granulite -

zQcn o z z ><2

Ü O Ü

£D<I O

UJCC S.Z

sample gave K-Ar ages of 175 and 180 m.y. These ages indicate that the Los Clavos could have been contemporaneous with i:he emplacement of the batholiths, but do not preclude its being older. Thus, as suggested by field relations, the possibility that the Los Clavos Ignimbrite formed by vesiculation of granodiorite or quartz monzonite in the upper part of the Pueblo Bello and Patillal batholiths is compatible with its apparent age.

An obsidian sample (loc. 18, Fig. 2 and Table 1) is from a flow-banded felsic dike, one of many intrusive domes and dikes and extrusive viscous flows of felsite of proba ble rhyodacitic or rhyolitic composition found near the southern tip of the Sierra Nevada. A concentrate of anorthoclase(P) phenocrysts gave a K-Ar age of 142 m.y. A Late Jurassic age (142 m.y.) for these felsite domes, dikes, and flows seems geologically reasonable

The Golero Rhyolite, formed by the last major volcanic activity in the Sierra Nevada province, has a minimum age of 129 rn.y.

(Early Cretaceous), according to a single whole-rock K-Ar analysis (loc. 17, Fig. 2 and Table 1).

Volcanic and intrusive activity during the Jurassic extended into the Early Cretaceous. The volcanic activity between 142 and 129 m.y. ago may be related to the intrusive activity that also occurred at almost the same time, as suggested by the following ages: the 138-m.y. Don Diego pluton (loc. 26, Fig. 2 and Table 1); the 137-m.y. biotite in pegmatitic diorite (loc. 14); the 138-m.y. basalt intrusion(P) in the Guatapuri Forma-tion (loc. 20); the 133-m.y. alteration age of slightly saussuritized plagioclase from the Los Clavos Ignimbrite (loc. 19); and the 131-m.y. Socorro stock (loc. 15).

The postbathol i thic intrusive and vol-canic activity southeast of the Sevilla linea-m e n t w a s a l m o s t s y n c h r o n o u s wi th metamorphism northwest of the lineament, as shown by two amphibole ages: (1) the 147-m.y. age f rom amphibolite (loc. 13, Fig. 2 and Table 1) in the Sevilla metamorphic

complex, and (2) the 138-m.y. age from the basement schist in the Perico well (loc. 6, Fig. 2 and Table 1). These ages overlap the probable age of the Mesozoic red beds in the Perija province.

Sevilla Province

The basement of the Sevilla province is a polymetamorphic mafic gneiss that is overlain(P) structurally or stratigraphically by schist of uncertain metamorphic age and by allochthonous(?) blocks of Precambrian granulite (Fig. 2). Igneous activity produced the synmetamorphic(P) metadiorite pluton along the Sevilla lineament and also produced several postmetamorphic plutons (Fig. 2). A 170-m.y. hornblende age (loc. 12, Fig. 2 and Table 1) establishes a minimum Jurassic age for the metadiorite, and the 147-m.y. hornblende age (loc. 13, Fig. 2 and Table 1) is a minimum Jurassic age for the ensimatic(P) basement gneiss. Nevertheless, these are most likely reset ages.

A Permian K-Ar age (250 m.y.) was




TABLE 1. ANALYTICAL DATA, K-Ar AGES, AND SAMPLE DESCRIPTIONS AND C00RDINA1 ES FOR DATED SPECIMENS FROM THE

SIER-A NEVADA DE SANTA MARTA

Sample loc.

(F ig. 2)

Mater ial analyzed

K¡0+

(wt %) r ~ T ~ T T ••»Ar"

Total A r * 1 • A r - V K " Age (m.y.)

(±2d) Rock type

Geologic u n i t Sample coordinates

Hocks from Santa Marta r tamorphic belt and their equivalents

I B lot He 9.27s 5.897 X 10"10 0.90 0.00252 42.6 ± 1.7 B i o t i t e sch is t High-grade sch is t 10°59'37" N., 74°08'07" W.

2 Amphibole S0.197 0.1503 X I O " " 0.70 0.00302 51.0 ± 3. l i Amphibolite sch is t High-grade s c i i s t 10°59'43" N., 74°07'43" W.

3 Amphibole 50.260 0.1840 X IO"1» 0.59 0.00281 47.4 ± 2.4 Amphibolite sch is t High-grade sch is t 11°06' N., 74°03' W.

4 Amphibole 50.325S 0.2143 X 1 0 " " 0.69 0.00261 44.1 ± 2.7 Amphibolite sch is t High-grade sch is t 11°13'20" N., 73°53'30"W.

5 Heavy f r a c t i o n of rock, c h i e f l y amphibole

50.191 0.1029 X 1 0 " " 0.35 0.00213 36.2 ± 5.1 Amphibolite sch is t High-grade sch is t 11016130" N., 74°09'00" U.

6 Amphibole 50.157s 0.3329 X 1 0 " " 0.66 0.00838 138 ± 7 Amphibolite sch is t Core, 11,170- f t depth in

Perico wel l 11-17' N., 73"09' W.

E. Phyl l i t e {whole rock)

2.45 3.19 0 X 10-" 0.87 0.00576 86.3 ± 4 Fine-grained p h y l 1 i t i c sch is t Cut t ings, 8 ,327- f t depth i n

Algarrobo No. 1 wel l 10°08'08" N., 74°09'20" W.

TerHzry intrusive rockts

7 B i o t i t e Hornblende s 7 ' 5 1

0.852 5.430 X IO"111

0.6246 X 1 0 - " 0.92 0.80

0.00286 0.00290

48.4 ± 1.8 49.0 ± 2.0

Quartz d i o r i t e Buritaca pluton I T U ' N., 73°44'00" W.

8 B i o t i t e Hornblende

9.38 1.328

6.185 X 1 0 - " 0.9680 X 1 0 - "

0.93 0.84

0.00261 0.00289

44.1 ± 1.6 48.8 ± 1.7

Quartz d i o r i t e Santa Marta ba tho l i t h I T I ? ' N., 73°56'30" W.

9 Muscovite 9.82 7.010 X 1 0 - " 0.78 0.00283 47.8 ± 1.9 Pegmatite i n schis t 11°14'20" N. , 74°10'30" W.

10 Hornblende 50.354 0.3070 X 1 0 - " 0.66 0.00344 57.9 ± 2.5 Hornblendite Latal pluton 10°47'39" N., 74°05'46" W.

11 B i o t i t e 8.62 6.967 X 1 0 - " 0.90 0.00320 54.0 ± 2.2 Granodior i te Tucurinquita pluton 10°40'10" N., 74°04'40" U.

Rocke from Sevilla metamorphic belt

12 Hornblende S0.492 1.293 X 1 0 - " 0.87 0.0104 170 ± 6 Metadior i te along Sev i l l a 1i neament

10°53'20" N., 73°59'30" W.

13 Hornblende S0.382s 0.8624 X I O " " 0.85 0.00893 147 ± 6 Amphibolite Buritaca Gneiss 10°53'10" N. , 74°08'50" W.


9.03 0.80

18.89 X I O " " 2.502 X 1 0 " "

0.90 0.94

0.00828 0.0125

137 í 5 202 ± 13

Pegmatit ic d i o r i t e intrudes Tucurinca p lu ton io complex

10-45' N., 73°57' «.

15 B i o t i t e 8.84s 17.68 X TO" " 0.96 0.00792 131 ± 5 Granodior i te

Socorro stock 10°48'25" N., 74°01'43" W.

16 B i o t i t e 9.19s 5.187 X I O " " 0.68 0.00223 37.8 ± 1.7 B i o t i t e c l o t s in g ranod ior i te

Socorro stock 10°56'53" N. , 74°01'40" W.

reported by MacDonald and Hurley (1969) for a poorly foliated amphibolite or metadiorite (loc. A, Fig. 2). Pre-Triassic metamorphic rocks ( > 1 9 5 m.y.) have been reported by MacDonald (1968) in a possibly correlative metamorphic belt in the Guajira Peninsula (Fig. 1). Metaigneous rocks with Permian radiometric ages are present in a possibly equivalent metamorphic complex in the western Cordillera Central (Fig. 1), according to R. B. Hall (1968, written comraun.). Although there is strong regional evidence for the presence of Permian

metamorphic rocks, the age evaluation of the Sevilla metamorphic complex is not conclusive, because of the uncertainty of the 250-m.y. age (iMacDonald and Hurley, 1969). Even a postgranulite, Precambrian age for the basement gneiss is not excluded, but, if true, a subsequent Permian to Triassic(?) metamorphism seems very prob-able.

MacDonald and Hurley's (1969) dated Permian rock is float f rom a small drainage area within the block of Precambrian granulite between branches of the Sevilla

lineament (loc. A, Fig. 2). The origin and correlation of the float are uncertain, as is the reliability of this age determination. The sample probably is an epidiorite that intruded the Precambrian granulite or a fault block of metadiorite. To consider the float to be retrograded granulite seems unreasonable, as similar retrograded granu-lite has not been found elsewhere. If the dated sample is the same age as the metadiorite (Fig. 2), it would indicate an important Permian intrusive event that possibly occurred during or shortly after the



GEOLOGIC EVOLUTION OF THE SIERRA N E V A D A DE SANTA M A R T A, N O R T H E A S T E R N COLOMBIA 2 7 9

TABLE 1 (conti

Sample Mater ia l M + *Ar " 0 *AR"° • A r ' V K " Age (m.y. ) Rock type

loc . (F ig . 2)

analyzed (Wt %) (moles/g) Total A r ' " (±2o) Geologic u n i t Sample coordinates

Mesozoic volcanic rocks, Sierra Nevada Province

17 Rhyoli te (whole rock)

5.31 10.48 x 1 0 " " 0.94 0.00781 129 ± 5 Rhyo l i te i gn imbr i te Golero Rhyo l i te 10°37'35" N. , 73°08'55" W.

18 Anor thoc lase( î ) Glass

2.46 1.90

5.359 0.2805

x 1 0 - " x 10 - ' °

0.90 0.24

0.00863 0.000584

142 Í10 .0

± 5 ì 1.2

Obsidian 9°55'30" N. , 73°52'20" W.

19 Plagioclase Hornblende Hornblende + epidote

S1.24 0.60 0.56

2.277 1.626 1.540

X 10 - ' ° X 10 " ' ° x 1 0 - "

0.59 0.77 0.77

0.00791 0.0107 0.0110

133 175 180

± 5 i 13 i 12

Rhyodacite i gn imbr i t e Los Clavos Formation 10°31'07" N. , 73°18'30" U.

20 Impure p lag ioc lase 0.187s 0.3972 X I O ' " 0.49 0.00839 138 i 9 Basa l t , in t rudes (?) Guatapuri Formation 10°04'15" N. , 73°44'45" W.

21 Basalt Basalt«*

S0.794 0.746

2.020 2.088

x 1 0 - " x I O " "

0.79 0.74

0.0101 0.0111

165 181

± 6 ± 10

Basa l t , below red beds co r re la ted w i t h Guatapuri

10°50'10" N. , 73°02'00" W.

Rocks from central belt of gray mesozonal Jurassic batholiths

22 B i o t i t e 7.54 19.29 x IO"1 0 0.98 0.0101 166 ± 6 Granodior i te Aracataca bathol i t h 10°341 N. , 73°58' U.


7.90 0.50

6.784 1.245

x 1 0 - " X I O ' "

0.92 0.84

0.00340 0.00986

57.3 162

± 2.4 ± 12

Granodior i te Atanques bathol i t h 10°42'10" N. , 73°21'15" W.

24 B i o t i t e 7.06 18.76 X 1 0 - " 0.95 0.0105 172 ± 6 Quartz monzonite Ojeda bathol i t h 10°40' N. , 73°45' W.


8.72 0.74

23.24 2.024

x IO"1 0

X I O " " 0.94 0.75

0.0106 0.0108

172 177

± 7 ± 13

Granod ior i te Central bathol i t h 11°03'07" N. , 73°27'27" W.

26 B i o t i t e 9.23 s 19.55 x 1 0 - " 0.97 0.00837 138 + 5 Trondhjemite Don Diego p lu ton l l°0r20" N., 73°47'40" W.

27 B i o t i t e 8.18 22.28 X I O " " 0.95 0.0108 176 ± 7 Granodior i te stock i n Los Clavos Formation

10°23120" N., 73°21'35" U.

Rocks from southeastern belt of pink epizonal Jurassic batholiths

28 B i o t i t e 7.90s 21.50 x 1 0 " " 0.95 0.0108 176 ± 7 Granodior i te P a t i l i a i bathol i t h 10°51'33" N. , 73°13'33" W.

29 B i o t i t e B i o t i t e * « Hornblende

7.81s 7.76 0.484

21.09 21.49

1.259

x 1 0 - "

x 1 0 - "

x 1 0 - "

0.94 0.92 0.92

0.0107 0.0109 0.0103

175 178 168

± 6 ± 7 ± 5

Quartz monzonite P a t i l l a l b a t h o l i t h H ° 0 8 ' N., 72°59' W.

30 B i o t i t e

B i o t i t e * * B i o t i t e « * Hornblende

5.80

5.86 5.51s 0.40

15.41 15.56 16.46 15.44

1.166

x 1 0 - " x 1 0 - "

x I O " " x I O " " x IO"1 0

0.97 0.95 0.86 0.91 0.72

0.0105 0.0106 0.0111 0.0110 0.0116

172 174 181 179 189

± 6 ± 7 T 6 Í 8 ± 19

Quartz monzonite Pueblo Be l lo b a t h o l i t h 10°28' N., 73°35'25"

Precambrian granuli te

31 Hornblende 2.06 37.06 x 1 0 - " 0.99 0.0712 940 ± 30 Hornblende-pyroxene-garnet-p lag ioc lase gneiss

Los Mangos Granu l i t e 10°40'40" N. , 74°08' W.

Analysts: R. F. Marvin, H. H. Mehnert, violet Merritt, and w. T. Henderson, U.S. Geological Survey. notes: Decay constants for K1*0: X = 0.584 x 10~ 1 ° / y r .

4.72 x 1CT1 °/yr. p Atomic abundance: K*°/K = 1.19 X 10~w.

*Radiogenic argon. tAverage of two or more potassium analyses by flame photometry except where noted. § Average of two potassium analyses by Isotope dilution. ItSpurious age caused by argon loss from glass.

"Analysis ¿>y Ceochron, Inc., Cambridge, Mass.

format ion of the Sevilla metamorphic complex.

The schist overlying(?) the basement gneiss possibly formed f rom marine sedi-ment tha t was deposited on the basement gneiss following the Sevilla orogeny and was me tamorphosed in Jurassic time at the same time as the Santa M a r t a Schist.

O u r K-Ar ages may not indicate the correct ages of the Sevilla metamorphic complex and the metadiori te, but they do fo rm a pat tern that might reflect thermal g r ad i en t s du r ing a Ju rass ic reg ional

me tamorph i sm (the Santa M a r t a orogeny). The K-Ar ages (250, 170, and 147 m.y.) decrease nor thwes tward toward the Santa M a r t a Schist at approximately right angles to the nor theast trend of the Sevilla metamorphic belt (Fig. 2). The 138-m.y. age and the inferred original posit ion of the basement schist in the Perico well (loc. 6, Fig. 2) are consistent with this age pat tern (250, 170, and 147 m.y.). The Cretaceous ages and inferred original posit ions of the schists in the Santa Mar t a province (MacDonald and others, 1971) and of

phyllite f rom the Algarrobo N o . 1 well (loc. E, Fig. 2 and Table 1) generally display the same pat tern .

A similar pat tern is shown by the metamorphic ages in correlative metamor-phic belts in the Guaj i ra Peninsula (Fig. 1). There, the K-Ar ages decrease westward, obliquely crossing the northeast- t rending metamorphic belts. Ages range f rom 195 + m.y . ( M a c D o n a l d , 1968) to 1 7 2 m.y . (Lockwood, 1965) in the older belt, and then to 56 m.y. in the younger(?) belt nearer the coast (Alvarez, 1967).




Sample Rb loc . (ppm)

(F ig . 2)

TABLE 2. ANALYTICAL DATA, Rb-Sr AGES, AND SAMPLE DESCRIPTIONS AND COORDINATES FOR DATED SPECIMENS FROM

THE SIERRA NEVADA DE SANTA MARTA

Sr Rb /Sr (ppm)

Sr /Sr Radiogenic Sr Total Sr»7

Age (m.y. )

Rock type Geologic u n i t

Sample coordinates

Rooks from southeastern belt of pink. Jurassic batholiths

Granophyre P a t i l l a l b a t h o l i t h 10°51'30" N. , 73°16'57" W.

Granophyre P a t i l l a l b a t h o l i t h 10°51'05" N. , 73°17'33" W.

Preoambrian granulites

34 182 101

35 240 128

5.19 0.7575 752 ± 73+

1,300 ± 100+

Quar tz -pe r th i t e gneiss Los Mangos Granu l i t e 10°34'20" N. , 73°24'53" W.

Quar t z -pe r th i t e gneiss Los Mangos Granu l i te 1 0 o 5 r 0 5 " N. , 73°04'27" W.

Analysts: Z. E. Peternian, R. Ji. Hildreth, and W. T. Henderson, U.S. Geological purvey. Notes: Decay constants for RbB7: Xg = 1.39 x 10~1'/yr. Isotopic abundance: Rb17/Rb - 0.2B3 g/g. *hge calculated with initial SrB7/SrBB ratio = 0.705. iAge calculated with initial SrB7/SrB6 ratio = 0.703.

This pattern of decreasing ages in the Sevilla province can be explained in two ways: (1) the decreasing ages reflect increasing argon loss toward the zone of greatest metamorphic intensity in the Santa Mar ta metamorphic belt; or (2) the zone of greatest metamorphic intensity in the Sevilla metamorphic belt slowly migrated during the Jurassic from southeast (near the Sevilla lineament) to northwest .

Santa Marta Province Seven samples of metamorphic rocks

(Iocs. 1 to 6 and E, Fig. 2 and Table 1) f rom the parallel belts of greenschist and amphibolite-grade schists and three samples of intrusive rocks (Iocs. 7 to 9, Fig. 2 and Table 1) were dated radiometrically. Of these, five metamorphic rocks gave Eocene or Oligocene ages that probably date Ter-tiary plutonic or tectonic activity and not the metamorphism. This conclusion is sup-ported by several Cretaceous and Paleocene ages (Iocs. C and D, Fig. 2), in addition to Eocene and Oligocene ages, reported from the metamorphic belts northwest of the Santa Mar ta batholi th by MacDona ld and others (1971).

New evidence for the age of metamor-phism comes f rom drill cores and cuttings f rom the bot tom of two oil wells drilled through the Tertiary sediments in the basins north and west of the Sierra Nevada. A drill core of amphibolite schist f rom the bot tom of the Perico well (loc. 6, Fig. 2) is petro-graphically similar to the predominant schist in the inner, higher grade belt of schist (Iocs. 2, 3, and 4, Fig. 2) with which it is correlated. An amphibole concentrate f rom the Perico sample gave a K-Ar age of 138 m.y. (Late Jurassic). This indicates that the metamorphism of the higher grade Santa Mar ta Schist (inner belt) is also Late Jurassic. The Santa Mar ta metamorphism could, however, have begun as early as Middle Jurassic in the Sevilla province.

Drill cuttings f rom a depth of 8 ,327 ft in the Algarrobo No. 1 well (loc. E, Fig. 2 and Table 1) consist of several types of phyllite that are correlated with the autochthonous rocks beneath the thrusts in the outer belt of greenschist and phyllite in the Santa Mar ta province (Fig. 2). According to T. V. Tolleson of Superior Oil Co. of Colombia, these cuttings are from a pre-lower Miocene boulder conglomerate (E. M . Irving, 1971, written commun.). The conglomerate in nearby wells contains huge blocks of metamorphic rock and may be a lower Tertiary sedimentary or tectonic breccia. A heavy-mineral concentrate f rom the drill cuttings gave a K-Ar age of 86 m.y. (Late Cretaceous), an age that strongly suggests a pre-Paleocene metamorphism for the phyl-lite and greenschist.

The above ages f rom offset subsurface segments of the Santa Mar ta metamorphic belt suggest a Cretaceous minimum age for the metamorph 'sm of the rocks in the outer belt and a Late Jurassic minimum age for the rocks in the inner belt. The metamorphic rocks in these belts are probably of different ages, based on the correlation of the greenschist with similar Late Cretaceous greenschist in :he Guajira Peninsula, but they are not necessarily so.

The age of the quartz-diorite plutons in the Santa Ma _ t a and Sevilla provinces is thought to be 49 m.y., based on the radiometric ages of amphibole f rom the Santa Mar ta batholith and the Buritaca pluton (Fig. 2). The batholith at locality 8 (Fig. 2 and Table 1) has discordant ages (48.8 m.y. for hornblende and 44.1 m.y. for biotite) compared to concordant ages (49.0 and 48.4 m.y.) for the same minerals in the Buritaca pluton (loc. 7, Fig. 2 and Table 1). The 44.1-m.y. biotite age is probably due to shearing, accompanied by increased tem-peratures, w'thin a prominent east-west zone of fractures (not shown on Fig. 2).

Quartz-diorite magma was probably

generated deep within the metamorphic belt during the last metamorphism (Paleocene?) and then rose slowly to higher levels. The postulated genetic relation of the quartz diorite to the Santa Mar t a metamorphic belt is supported by a regional association of qu artz diorite with this metamorphic belt. In the Guajira Peninsula (Fig. 1), a similar quartz-diorite pluton (Parashi pluton) of the same age (48 ± 4 m.y., Lockwood, 1965) intruded a belt of phyllite and greenschist th at we and Doolan and MacDonald (1974) correlate with similar rocks in the Santa Mar t a metamorphic belt.

GEOLOGIC EVOLUTION OF THE SIERRA NEV/UDA MASSIF

The triangular Sierra Nevada de Santa Mar ta massif is divided into two very d i f f e r en t subreg ions by the a r c u a t e northeast-trending Sevilla lineament (Fig. 2). The lineament may have been an important plate boundary f rom Permian to Eocene time. On opposite sides of the l i n e a m e n t , b a s e m e n t r o c k s vary in metamorphic grade and age, and igneous rocks vary in age. Sedimentary and volcanic tocks are found only in the sub region s outheast of the lineament (Fig. 2). Sedimen-tation, volcanism, and plutonism in this subregion were partly contemporaneous with successive metamorphisms northwest of the lineament. The post-Triassic exten-sional tectonic style of the southeastern subregion contrasts with the compressional style of the subregion northwest of the lineament. The metamorphic, igneous, and tectonic evolution of these two subregions f rom Permian to early Tertiary was apparently very different, though perhaps related to the same underlying subcrustal processes. The separate evolution of the two subregions was structurally controlled, at: least indirectly, by the Sevilla lineament during this interval.

Northwest Subregion Northwest of the Sevilla l ineament are the

fo l lowing three ens imat ic n o r t h e a s t -trending Phanerozoic metamorphic belts: (1) the Sevilla metamorphic belt, (2) the inner belt of amphibolite-grade schist, and (3) the outer belt of greenschist and phyllite (Fig. 2), both within the Santa Mar ta metamorphic belt. The first two belts have been intruded by small plutons of Paleocene diorite and large plutons of Eocene quar tz diorite (Fig. 2).

Possibly, the basement gneiss in the Sevilla metamorphic belt is oceanic crust that was metamorphosed during the Sevilla orogeny (Permian?). The gneiss was remetamor -phosed during the Santa Mar t a orogeny (Jurassic) when the overlying(?) schist may have been formed, and when the inner belt of amphibolite-grade schist in the Santa Mar ta province definitely was formed.

The outer belt of greenschist and phyllite




consists of two structurally different metamorphic elements. Near the coast, polymetamorphic greenschist in two thrust plates was metamorphosed first by the mid-Cretaceous, and again, under different physiochemical conditions that destroyed amphibole, during the early Tertiary (Doolan and MacDonald, 1974; Mac-Donald and others, 1971). The autochtho-nous greenschist and phyllite farther inland were metamorphosed by Late Cretaceous, but were not visibly affected by the early Tertiary metamorphism. Within the outer belt, the metamorphic grade increases s o u t h e a s t w a r d in to the a d j a c e n t amphibolite-grade schist of the inner belt. Metamorphic gradients, however, have not been recognized southeast of the Santa Mar ta batholith.

The complex history of the Santa Mar ta province is reflected by the wide range (Late Jurassic to Oligocene) and erratic pattern of the available radiometric ages (Table 1; MacDona ld and others, 1971). T w o Oligocene ages f rom samples within or near the th rus t plates suggest a possible Oligocene age for the thrusting. Another Oligocene age f rom a sample close to an east-trending fault, which is inferred f rom alined headlands on the north coast (Fig. 2), may date a period of movement along the Oca trend.

A few small, poorly exposed masses of ultramafic rocks in both the Sevilla and Santa Marta provinces (Tschanz and others, 1969) include serpentine and harzburgite peridotite that, near their margins, are partly replaced by tremolite, talc, chlorite, and serpentine. These secondary minerals are much lower in grade than the enclosing metamorphic rocks and, if metamorphic, would belong to the greenschist facies. They can not be products of a widespread, younger regional metamorphism.

Southeast Subregion Southeast of the Sevilla lineament, the

basement of the Sierra Nevada province consists of discontinuous blocks of Precam-brian granulite separated by Jurassic intru-sive rocks (Fig. 2). Before the Permian, however, these granulite blocks were probably continuous parts of a Precambrian shield on which rested marine Devonian(P) and Pennsylvanian sedimentary rocks. Dur-ing the Permian(P), the Sevilla lineament came into being. At about the same time, this subregion was covered by dark-gray basaltic and spilitic volcanic rocks and interbedded volcanogenic graywacke of spilitic composi-tion (Corual Formation, Fig. 2). The oldest of these volcanic rocks may be approxi-mately contemporaneous with the forma-tion of the Sevilla lineament, with the intrusion of the metadiorite along it, and with the metamorphism of the basement gneiss in the Sevilla province (Fig. 2). These volcanic rocks were intruded by green

keratophyre porphyry (Tschanz and others, 1969) and then were covered by red volcanic rocks and graywacke of similar composition (Gautapur! Formation, Fig. 2). Slump textures suggest that deposition was suba-queous and probably marine until late Guatapuri time when subaerial trachytic(P) ignimbrite was extruded. The Guatapuri Formation may be contemporaneous with the composite belt of syntectonic intrusive rocks (Fig. 2), which had filled a wide rift zone along the Sevilla lineament by latest Triassic. At this time, the east wall of this rift consisted of Precambrian granulite and was almost parallel to the west wall, which has apparently been stable since the early Mesozoic.

The numerous plutons in the Sierra Nevada province form four northeast-trending intrusive belts: (1) the metadiorite belt of Permian(?) age, (2) the syntectonic plutonic belt of Triassic age, and (3) the central and (4) southeastern belts of Jurassic batholiths (Fig. 2). The rocks in the first three belts were intruded along the Sevilla lineament, and all these intrusive rocks may be genetically related to it, as described below.

The metadiorite probably formed before or during the Sevilla metamorphism, about 250 m.y. ago, when katazonal(P) gabbro or diorite filled the initial(P) rift along the Sevilla lineament. Then, the Triassic syntec-tonic plutons probably crystallized f rom anatectic quartz-diorite magma that formed during the Sevilla orogeny, but were emplaced after it in several stages between the parallel walls of the southern part of the Sevilla lineament. The relative ages of the Triassic plutons — the western border zone, the Rio Piedras complex, the Tres Puntas pluton, and possibly the Tucurinca plutonic complex (Fig. 2) — a r e uncer ta in . Large blocks of metadiorite were engulfed when the syntectonic western border zone (Fig. 2) was emplaced. Then both the metadiorite and syntectonic belts were split apart when the Aracataca batholith (166 m.y.) was emplaced (Fig. 2).

The Triassic syntectonic rocks, in contrast to the Jurassic intrusive rocks, are com-monly metasomatized, recrystallized or protoclastically deformed, and occasionally weakly foliated. Also, in thin section, the plagioclases are not zoned. These plutonic rocks nowhere show compositional layering like that of the regional metamorphic rocks, but contain inclusions of such rocks that probably were derived from the Sevilla metamorphic belt.

The foliated rocks in the western border zone were protoclastically deformed and fractured before they were completely crystallized or metasomatized, presumably by movement along the Sevilla lineament. These distinctive rocks grade in composition f rom quartz diorite in the west to granodiorite in the east. The gradational

contact with the Tucurinca plutonic com-plex (Fig. 2) is drawn on the basis of textural differences. The Tucurinca plutonic com-plex is locally foliated and consequently may be a younger syntectonic pluton or a part of the western border zone that was not protoclastically deformed, instead of a postorogenic pluton as shown in Figure 2.

The Tucurinca plutonic complex and the Tres Puntas pluton seem to have transitional lithologic characteristics and perhaps rep-resent plutons of intermediate age (Late Triassic) between the Triassic syntectonic plutons and the Jurassic batholiths.

The central and southeastern belts of anorogenic Jurassic batholiths are lithologi-cally distinct, but individual plutons in each belt are similar in color and composition. The central belt consists of seven plutons of gray quartz diorite and (or) granodiorite and one pluton (Ojeda batholith, Fig. 2) of quartz monzonite that is transitional in color and composition to the pink, por-phyritic granodiorite and quartz monzonite (177 m.y.), slightly younger(P) undated granite, and still younger granophyre (164 m.y.) in the two composite batholiths of the southeastern belt. These three pink rock types were mapped separately (Tschanz and others, 1969). In the Patillal batholith, the granophyre forms a discontinuous roof facies. The pink rocks contain abundant pink potassic feldspar instead of the microcline that is common in the gray rocks of the central belt. T w o magmas possibly formed simultaneously(?) at different depths along a southeast-dipping structural discon-tinuity whose outcrop may be the Sevilla lineament.

Dilational Emplacement of Jurassic Batholiths

The Mesozoic igneous evolution began with the intrusion of the metadiorite and the syntectonic plutons into rifts along the Sevilla lineament. It culminated with the extensional disruption and dispersion of the pre-Jurassic rocks that accompanied the emplacement of Jurassic batholiths in two or more stages.

During the Jurassic, the granulite base-ment, the overlying Triassic volcanic rocks, and the metadiorite and the syntectonic plutonic belt along the Sevilla lineament were all disrupted by successive plutonic intrusions into dilational rifts, trending N. 15° E. These rifts may be successively dilated segments of the Sevilla lineament. The dilational openings were partly bounded by pre-existing(P) N. 70° E.-trending faults that allowed very large synintrusive transform movements as successive Jurassic batholiths were emplaced. Both of these principal fault systems are parallel to opposite ends of the exposed segment of the arcuate Sevilla lineament. The result was the formation of two composite northeast-trending belts of contemporaneous Jurassic batholiths.



2 8 2 T S C H A N Z A N D OTHERS

The west side of the Triassic rift zone remained continuous; but the east side, originally a cont inuous block of granulite (see segments W r W 2 , W 3-W 4 , W5-W6 , Fig. 2), was subsequently disrupted by a two-stage eastward movement of the central segment (W3-W4). The first stage of this eastward movement is probably explained by synintrusive ( transform) movement along inferred N. 70° E. faults (W2-W3 , W4-W3) that accompanied the emplacement of the Pueblo Bello batholi th into a N. 15° E. rift, which was bounded on the nor th and south by these pre-existing(P) N . 70° E. faults. The second stage accompanied the emplacement of the Aracataca batholi th, which isolated the metadiorite and Triassic syntectonic plutons east of it f rom those west of it.

The Rio Piedras complex consists mainly of partly foliated syntectonic or cataclastic q u a r t z d ior i te but includes regional metamorphic rocks that are correlated with those in the Sevilla province. From this we conclude that the Rio Piedras complex and the metadiorite far ther south may have formed near the Sevilla l ineament (Fig. 2), and they were apparently displaced east-ward by the intrusion of the Aracataca batholi th.

Although left-lateral wrench faulting along W 4-W 5 (Fig. 2) might explain the nearly equal eas tward displacements of both the Rio Piedras complex and the granulite block (W3-W4), wrench faulting alone cannot explain the apparent unequal displacements of both the metadiori te and granulite in opposite directions on a parallel fault (W2-W3 , Fig. 2). Such faulting would require a left-lateral displacement of the metadiorite now on opposite sides of the Aracataca batholi th, assuming their former continuity, and a much larger right-lateral displacement of the granulite segments W r W 2 and W3-W4 . This type of movement is no t characteristic of wrench faults. The displacements on both N. 70° E. faults can be explained by a purely dilational model if the order of intrusion were as follows: metadiorites (250 m.y.?); western border zone (pre-202 m.y.), Rio Piedras pluton, Tres Puntas p lu ton; Pueblo Bello batholi th (177 m.y. average); and Aracataca batholi th (166 m.y.). This order of intrusion has not been established.

Al though speculative, the dilat ional model for the emplacement of the Jurassic plutons in the belts described above is compatible with the available radiometric, geologic, and gravity data . The large positive Bouguer gravity values over the entire Sierra Nevada massif require that the continental rocks exposed at the surface be underlain by very dense mantle or ultramafic rocks at depths of less than 15 km (Case and MacDona ld , 1973). Apparently, the Jurassic batholi ths are rootless, platelike masses that

are underlain by subcrustal rocks rather than by Precambrian granulites. The greater than + 7 5 mga' (Case and MacDona ld , 1973) of the only station on a Jurassic batholi th shows this is true for the Atanques batholith. The required platelike form of the b a t h o l i t h s is n a g r e e m e n t w i t h t h e batholithic model proposed by Hamil ton and Myers (1967, p. C I ) . Field evidence suggests that the Pueblo Bello and Patillal batholiths formed their own roofs of contemporaneous ignimbrites and have generally hor izor ta l upper contacts.

The larger Precambrian granulite blocks also probably rest on underlying mantle(?) rocks. The average density of the granulite is greater than that of the batholithic rocks. Therefore, if the granulite masses within the batholi ths did not rest on the mantle(?) rocks, they would have sunk into the magma chamber; conceivably, small blocks consist-ing wholly of quartz-perthite granulite, similar to two dated samples, might float.

The thin crust indicated by the regional gravity data makes plausible the existence of a shallow structural discontinuity beneath the Sierra Nevada province. Such a discontinuity would probably be a zone of weakness that separates the crustal and subcrustal rocks and allows the dilational separa t ion of l au l t -bounded granul i te blocks that is required by the proposed model of dilational emplacement.

Possible Lateral Displacement on the Sevilla Lineament

The Sevilla lineament is the oldest and most impor tant structural discontinuity in the Sierra Nevada massif, and it structural-ly controlled the Permian and Mesozoic igneous, metamorphic, and tectonic evolu-tion. The Sevilla lineament was intruded by plutons of Permian (250 m.y.?), Triassic (pre-202 m.y.), Middle Jurassic (166 to 177 m.y.), Late Jurassic (138 m.y.), Early Cretaceous (131 m.y.), Paleocene (54 m.y.), and Eocene (49 m.y.) ages. Protoclastic deformat ion apparent in thin sections f rom the western border zone (pre-202 m.y.), the western margin of the Aracataca batholi th (166 m.y.), and from the Socorro pluton (131 m.y.) (Fig. 2), indicates movement along this lineament during the crystalliza-tion of some adjacsnt Mesozoic plutons.

Large left-lateral displacements might be inferred along the northern parts of both principal branches of the Sevilla lineament (Fig. 2), but other explanations are plausible for each of four or five possible offset rock units. Left-lateral cisplacements totaling 46 to 5 0 km can be inferred on the southern branch by assuming that the Precambrian granulites (X' , Fig. 2) and Triassic and Jurassic stratified rocks (Y', Fig. 2) were dragged f rom original positions opposite X and Y, respectively. The Triassic rocks undoubtedly overlie Precambrian granulite,

but vertical movements do not readily explain the distribution of the separate granulite blocks.

Synintrusive dilational displacement dur-ing the emplacement of the Central batholith (Fig. 2) rather than postintrusive wrench faulting might explain the apparent displacements on the southern branch. This interpretation is consistent with the pos-tulated process of extensional d isrupt ion farther south and has several advantages. First, left-lateral displacement of the Per-mian(?) metadiorites is not required along the southern par t of the Sevilla l ineament. Second, the only required active block would be the granulite block and the overlying pre-Jurassic rocks at the northeast end of the Central batholi th (loc. B, Fig. 2). Third, the required displacement on the southern branch is similar to the dilational separation of similar granulite blocks that apparently accompanied the simultaneous emplacement of the Pueblo Bello batholi th.

The above interpretation would not explain approximately 28 km of a doubtful total 76 km of left-lateral displacement of the undated Palomino pluton (Z ' , Fig. 2) f rom a possible former position at the nor th end of the Patillal batholi th (Z, Fig. 2). This d i s p l a c e m e n t is sugges ted only by the litholcigic similarity of the quartz monzonite in the Palomino pluton to that in the Pueblo Bello and Patillal batholiths. This displace-ment a long a pre-Eocene N. 70° E. fault would require a Middle Jurassic or younger displacement opposite to the larger early Tertiary displacement on the nearly parallel Oca fault ; therefore it seems very doubtful .

The N. 70° E. and N. 15° E. trends of the opposite ends; of the Sevilla l ineament controlled the separation of the fault-bounded granulite blocks as magma filled opening rifts. These trends might be ma jo r zones of weakness inherited f rom the original Precambrian shield.

The left-lateral(P) Sevilla lineament curves eastward into the east-west trend of the younger right-lateral Oca fault and south-ward into the N. 15° W. trend of the left-lateral Santa M a r t a - B u c a r a m a n g a linea-ment . Because of covering Tert iary sedi-mentary rocks, it is uncertain but likely that the Sevilla l ineament has been cut by these younger faults.

Relation to Plate Tectonics

Geologic features of the Sierra Nevada massif and of the surrounding region suggest that its geologic evolution f rom Permian to Eocene or Oligocene time was determined by subduction sind faulting along the former continental margin as a consequence of sea-floor spreading and continental drif t during the opening of first the Car ibbean and then the South Altlantic, and finally the eastward movement of the Caribbean sea




floor. The three parallel, successively younger Phanerozoic metamorphic belts and the compositional gradient in the two parallel contemporaneous belts of Jurassic batholiths would be very difficult to explain otherwise. The extensional disruption of the Sierra Nevada province and the dilational emplacement of the Mesozoic plutons do not readily fit the accepted simple models of plate tectonics, but might be explained by the tectonic transport and reassembly of microcontinental cratonic fragments along t ransform fault systems that linked intersect-ing subduction systems. The emplacement of two coexisting granitic magmas in successive stages into extensional rifts in the Sierra Nevada province is analogous to t'ae emplacement of basaltic magmas into spreading rifts along mid-oceanic rifts. This suggests a possible extension of the sea-floor—spreading hypothesis to a conti-nental environment — rather than an oceanic environment.

Since the Paleocene, the geologic evolu-tion seems to have been controlled by major lateral and vertical movements along the t ransform(?) Oca and Santa Marta— Bucaramanga fault systems. This movement probably was a consequence of continued subduction along the Pacific continental margin and the eastward movement of the Caribbean plate relative to Nor th and South America toward a subduction zone in the trench east of the Antilles Arc, as outline d by Molnar and Sykes (1969, p. 1649).

In o u r i n t e r p r e t a t i o n , t he t h r ee Phanerozoic metamorphic belts developed in the Sevilla and Santa Mar ta provinces in successive subduction zones along the former Caribbean continental margin be-tween Late Permian(?) or Early Triassic and Paleocene time. The postulated subduction during the Sevilla orogeny may be related to the original opening and subsequent de-velopment of the ancestral Caribbean as the oldest part of the Nor th Atlantic rift; the subduction during the Santa Mar ta orogeny may be related to the opening of the South Atlantic about 150 m.y. ago and the resultant westward drift of South America, as outlined by Dietz and Holden (1970).

We further suggest that not only was the greenschist in the outer part of the Santa Mar ta metamorphic belt formerly continu-ous with the belt of greenschist exposed in the Guajira Peninsula and Aruba, Nether-lands Antilles, as proposed by Doolan and MacDonald (1974), but also that the other rocks in both the Sevilla and Santa Mar ta metamorphic belts are exposed in the Guajira Peninsula and that they formerly extended east along the former continental margin to Trinidad. Rocks of both belts are present just southwest of the Sierra Nevada (Fig. 2) and equivalent rocks are probably also present in the Cordilleras Central and

Occidental (Fig. 1). The correlation of the latter rocks, however, is uncertain, because of lithologic differences and because of their probable formation in subduction zones along the north-trending Pacific margin instead of along the east-trending Caribbean margin.

Ma jo r questions in thus relating the Phanerozoic metamorphic belts in the Sierra Nevada to subduction along the former Caribbean continental margin are (1) the lack of h igh-pressure rocks of the lawsonite-glaucophane-jadeite facies that are characteristic of subduction systems along the margins of the Pacific Ocean, and (2) the uncertain original orientation of the N . 30° E.-trending segments exposed in the Sierra Nevada as compared to the N. 70° E. t rend in the Gua j i r a Peninsula and the eas t -wes t t r end f r o m A r u b a eas t . T h e arcuate pattern could be original or could reflect drag or rotation caused by left-lateral d i s p l a c e m e n t on the San ta Marta— Bucaramanga fault system. This fault could have existed during the Sevilla and Santa Mar ta orogenies as a t ransform fault that linked penecontemporaneous subduction systems along the Pacific and Caribbean continental margins. The prob-lem is not easily resolved, because of the apparent fragmentation of northwestern South America by several periods of transcurrent faulting on northeast-, east-, N. 15° W.-, and N. 15° E.-trending faults (Campbell, 1968; Feininger, 1970). Because of its arcuate pattern, left-lateral displace-ment on the Sevilla lineament alone might explain a possible 40° counterclockwise rotation of the metamorphic belts northwest of it.

The former Caribbean continental margin was disrupted and reassembled more than once by large-scale transform faulting. This was a result of the complex interaction between partly simultaneous subduction systems along the Pacific and Caribbean continental margins during the westward drift of the South American plate and the separation of North and South America. This interaction has produced a complex tectonic mosaic of large, ancient, exotic cratonic fragments, such as the Sierra Nevada province, which are isolated within younger metamorphic belts; these belts were subsequently disrupted by large transcur-rent movements on t ransform faults. The Sierra Nevada province is a smaller scale mosaic of granulite blocks that were separated by the intrusion of successive Mesozoic plutons into extensional rifts. The complexity of the Sierra Nevada massif results f rom its position near the leading (northwest) corner of the westward-drift ing Mesozoic continent and near the intersec-tion of the subduction systems along Pacific and Caribbean margins. The Sierra Nevada

massif is therefore an extreme, small-scale example of a much larger regional structural mosaic.

Among possible explanations of the poorly dated, pre-Cretaceous(P) fragmenta-tion of the granulite basement in northwest-ern South America and the possibly related extensional disruption of the cratonic fragment in the Sierra Nevada in Jurassic time, is sea-floor spreading, with resultant westward drift of the South American plate over the older ancestral Nor th Atlantic rift which had earlier caused the development of the Sevilla metamorphic belt. Sea-floor spreading during the Sevilla orogeny itself may have caused the initial fragmentation of northwestern South America but does not explain the Jurassic disruption of the Sierra Nevada province or several subsequent periods of transcurrent faulting. Whatever the primary cause, the fragmentation was probably accomplished by large transcur-rent faulting on the Sevilla and Cesar lineaments as well as later large lateral d i sp lacements on the Santa Marta— Bucaramanga, Oca, and several other major faults farther south — the Romeral, Otu, and Palestine faults of Feininger (1970).

A genetic relation between the Sevilla lineament or a related parallel subduction zone and the intrusive rocks is suggested by systematic variations in age, composition, mineralogy or texture, and depth of emplacement of the intrusive rocks relative to distance f rom the lineament. The older, more mafic, deeper seated rocks intruded the Sevilla lineament and are most closely related to the Sevilla orogeny. The Mesozoic intrusive rocks in the old craton generally become younger and more highly differen-tiated with increasing distance southeast of the lineament; in contrast, the Tertiary intrusive rocks are progressively younger and more differentiated with increasing distance northwest of the lineament. For Jurassic and Lower Cretaceous rocks of the same age within the craton, the more highly differentiated and shallower seated rocks are farthest f rom the lineament.

The depth of emplacement of the Mesozoic plutons in the Sierra Nevada province, as inferred from petrographic evidence (primarily textural), decreases southeastward f rom the Sevilla lineament. The porphyritic Jurassic batholiths of the southeastern belt are epizonal, whereas the batholiths in the central belt are mesozonal; the contemporaneous Jurassic volcanic rocks are subaerial ignimbrites.

The strongest evidence for the genetic relation inferred above is given by the mineralogical and textural characteristics of the rocks from the contemporaneous central and southeastern belts of Middle Jurassic batholiths. The gray, equigranular, meso-zonal microcline-bearing rocks of the




c e n t r a l be l t a r e r i c h e r in m a f i c m i n e r a l s a n d re la t ive ly ca lc ic p l a g i o c l a s e t h a n a r e t h e m o r e p o t a s s i c , p i n k , m e s o z o n a l t o e p i z o n a l p e r t h i t i c o r t h o c l a s e - b e a r i n g r o c k s of t h e s o u t h e a s t e r n be l t . T h e s e d i f f e r e n c e s s u g g e s t t h a t t w o m a g m a s of d i f f e r e n t c o m p o s i t i o n f o r m e d s i m u l t a n e o u s l y ^ ) by a n a t e x i s a t d i f f e r e n t d e p t h s a l o n g a s o u t h e a s t - d i p p i n g s t r u c t u r a l d i s c o n t i n u i t y b e n e a t h t h e o l d c r a t o n .

T h e p r o g r e s s i v e s o u t h e a s t w a r d i n c r e a s e in K 2 0 ( p o t a s s i c f e l d s p a r ) in t h e J u r a s s i c b a t h o l i t h s a n d t h e L o w e r C r e t a c e o u s i n t r u s i v e r o c k s is a n a l o g o u s t o t h e i n c r e a s e in t h e K 2 0 c o n t e n t of a n d e s i t e in v o l c a n i c a r c s a b o v e s u b d u c t i o n ( B e n i o f f ) z o n e s a n d in i n t r u s i v e r o c k s b e h i n d s u b d u c t e d c o n t i n e n -ta l m a r g i n s . H a t h e r t o n a n d D i c k i n s o n ( 1 9 6 9 , Figs . 1, 6 , a n d 7) s h o w e d t h a t t h e K 2 0 c o n t e n t o f s u c h a n d e s i t e s i n c r e a s e s p r o p o r t i o n a t e l y as t h e ve r t i c a l d i s t a n c e a b o v e t h e i n c l i n e d B e n i o f f z o n e a n d as t h e h o r i z o n t a l d i s t a n c e f r o m t h e o f f s h o r e t r e n c h t h a t m a r k s t h e o u t c r o p of t h e B e n i o f f z o n e . T h e y c o n c l u d e d t h a t t h e a n d e s i t e ( o r q u a r t z - d i o r i t e ) m a g m a s w e r e g e n e r a t e d a l o n g B e n i o f f z o n e s a t d e p t h s o f 8 0 t o 2 9 0 k m a n d t h a t m a g m a s a r r i v i n g a t t h e s u r f a c e f r o m c o r r e s p o n d i n g l y d e e p e r s i tes h a v e p r o g r e s s i v e l y h i g h e r p o t a s s i u m c o n t e n t s . By a n a l o g y , t h e s i m i l a r c o m p o s i t i o n a l c h a n g e s in t h e i n t r u s i v e r o c k s ( q u a r t z - d i o r i t e l ine) of t h e w e s t e r n U n i t e d S t a t e s ( d e s c r i b e d by M o o r e , 1 9 5 9 ) p r o b a b l y c a n n o w b e r e l a t e d t o f o r m e r s u b d u c t i o n z o n e s a l o n g t h e Pac i f ic c o n t i n e n t a l m a r g i n of N o r t h A m e r i c a . T h u s , t h e q u a r t z - d i o r i t e l ine a n d t h e s i m i l a r c o m p o s i t i o n a l l ines s o u t h e a s t of t h e Sevil la l i n e a m e n t a r e a n a l o g o u s t o t h e a n d e s i t e l ine o f H a t h e r t o n a n d D i c k i n s o n ( 1 9 6 9 ) a n d p r o b a b l y h a v e a s i m i l a r g e n e t i c r e l a t i o n t o s u b d u c t i o n .

T h e a r r a n g e m e n t a n d p a t t e r n of t h e M e s o z o i c p l u t o n s a n d m e t a m o r p h i c be l t s r e l a t i ve t o t h e Sevi l la l i n e a m e n t s u g g e s t t h a t t h e Sevi l la l i n e a m e n t m a y b e t h e o u t c r o p of a foss i l s u b d u c t i o n z o n e . T h e e x a c t n a t u r e of t h e l i n e a m e n t , h o w e v e r , is u n c e r t a i n .

T h e f o r e g o i n g p r e l i m i n a r y i n t e r p r e t a t i o n is h i g h l y s p e c u l a t i v e a n d wil l u n d o u b t e d l y b e s u b s t a n t i a l l y m o d i f i e d as n e w d a t a b e c o m e a v a i l a b l e . B u t a n a t t e m p t t o e x p l a i n t h e p r i n c i p a l p a r a d o x i c a l g e o l o g i c a l re la-t i o n s is c r i t i ca l in u n d e r s t a n d i n g t h e g e o l o g i c a l e v o l u t i o n of t h e n o r t h e r n S o u t h A m e r i c a a n d t h e C a r i b b e a n Sea a n d m a y be u s e f u l t o g e o l o g i s t s a n d g e o p h y s i c i s t s s t u d y i n g t h e r e g i o n .

A C K N O W L E D G M E N T S

T h e s a m p l e f r o m t h e P e r i c o wel l w a s s u p p l i e d by C y r u s J a c o b s of C h e v r o n Oi l C o m p a n y . T h e s a m p l e f r o m t h e A l g a r r o b o N o . 1 we l l w a s s u p p l i e d by T . V . T o l l e s o n of

S u p e r i o r O i l C o . of C o l o m b i a t h r o u g h E. M . I r v i n g of t h e U.S. G e o l o g i c a l S u r v e y . T o m á s F e i n i n g e r , R i c h a r d G o l d s m i t h , R . B. H a l l , a n d D . E. W a r d of t h e U.S. G e o l o g i c a l S u r v e y a n d m a n y C o l o m b i a n g e o l o g i s t s of t h e I n s t i t u t o N a c i o n a l d e I n v e s t i g a c i o n e s G e o l o g i c o - M i n s r a s s u p p l i e d d a t a f r o m t h e f o u r a r e a s in C o l o m b i a t h a t w e r e s t u d i e d u n d e r t h e c o o p e r a t i v e p r o g r a m of t h e I n v e n t a r i o M i n e r o N a c i o n a l .

R E F E R E N C E S C I T E D

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Dalrymple, G. B., and Lanphere, M. A., 1969, Potassium-argon dating — Principles, tech-niques, and applications to geochronology: San Francisco, W. H. Freeman and Co., 258 p.

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