Associated middle to late Jurassic volcanism and extension in southern South America

223

Tectonophysics, 116 (1985) 223-253 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

ASSOCIATED MIDDLE TO L,ATEJURASSIC VOLCANISM AND EXTElNSION IN SOUTHERN SOUTH AMERICA

D.A. GUST I*, K.T, BIDDLE *, D.W. PHELPS * and M.A. ULIANA ‘** ‘. SN4

NASA,

Johnson Space Center, Houston, TX 77058 (U.S.A.)

’ Exxon Production Research Company, Box 2189, Houston, TX 77252.2189 J Esso SAPA,

(U.S.A.)

Deiia Polera 297, 1001 Buenos Aires (Argentina)

(Received July 8, 1984; revised version accepted January 24, 1985)

ABSTRACT Gust, D.A., Biddle, K.T., Phelps, D.W. and Uliana, M.A., 1985. Associated Middle to Late Jurassic volcanism and extension in southern South America. Tectonophysics, 116: 223-253. The extrusive products of a Middle to Late Jurassic volcanic event occur throughout a wide area of southern South America. These volcanic rocks are associated in time and space with a series of NNW-trending grabens. The extension that produced the grabens began perhaps in the latest Triassic and continued throughout most of the Jurassic. The Middle to Late Jurassic volcanic rocks represent the culminating event of this period of extension. The Jurassic volcanic rocks described here are dominantly rhyolites and basal&, but flows of intermediate composition are also present. Major element gmhemistry on a suite of samples taken from a west-east transect near 44*S latitude shows that these rocks are not related directly to convergent arc volcanism along Ihe margin of South America, but are the products of a separate tectonic/magmaric event that involved significant crustal anatexis. The extension and related vokanism directly preceded the opening of the Rotas Verdes marginal basin along the western margin of Chile and may have led to the initial separation of South America and South Africa. As such, the Middle-to-Late Jurassic extension and volcanism heralded the breakup of part of Gondwanaland. INTRODUCTION

During the Middle and Late Jurassic, a major igneous event affected most of southern South America. The extrusive products of this event occur throughout an area of over l,OOO,OOOkm2 (Fig. l), and range in composition from basalt to rhyolite. Although these rocks form a major volcanic province, comparable in size to major ignimbrite provinces, little is known of their field reIations or geochemistry outside of Argentina.

* Currentaddress: Lunar and Planetary Institute, 3303 NASA Road 1, Houston, TX 77058 {U.S.A.). ** Currentaddress:ExxonProduction Research Company, Box 2189, Houston, TX 77252.2189 (U.S.A.).

OO4O-1951/85/$03.3O

0 1985 Elsevier Science Publishers B.V.

224

70”

‘40

1

66” /

I

', ,_i i

62”

:OMPLEJO

1”

NERERO’B’.#&CHON

AIKE

CHON

’

;A

AIKE

MATILDE

OUTCROP LATE JUR P VOLCANI’ C LOCATION OF WELL THATENCOUNTERED MID TO LATE VOLCANIC ROCKS IN THE SUBSURFACE

r

GROUPS FORMATIONS

SERIE TOBIFERA .

KILOMETERS

Fig. 1. Map of southern of Middle-to-Late related provides

South America

Jurassic

volcanic

to Late Triassic-Jurassic an indication

Oi;250

I 70”

1 740

showing outcrop

distribution

and selected subsurface

rocks. Also shown are zones of numerous

extension.

The distribution

of the total area affected

of the Middle-to-Late

by the extensional

I 58”

62”

66”

event.

grabens

occurrences

and half grabens

Jurassic

volcanic

rocks

225

This

volcanic

extensional

event

represents

deformation

of a number

the volcanism

were soon followed

the western

margin

of a widespread

that began in the Late Triassic

the formation

oceanic

the culmination

of basins

in southern

of Gondwanaland

crust in the South Atlantic

or Early Jurassic

South America.

by the formation (Dalziel,

(Rabinowitz

episode

and led to

The extension

of a small oceanic 1981)

and

of

basin

and along

by the formation

and La Brecque,

of

1979).

An understanding of the Middle to Late Jurassic extension and volcanism is critical for several reasons. First, these events heralded the breakup of Gondwanaland. Second, they initiated the formation of basins such as the Magallanes and Malvinas of Argentina and Chile, and, third, the associated thermal event set the stage for hydrocarbon maturation in those basins. In this paper, the Middle and present

we will summarize the complicated stratigraphic nomenclature Upper Jurassic volcanic rocks of southern South America,

new petrographic

traverse

near

44”s

and geochemical

latitude.

Using

analyses

of samples

this information,

from a west-east

in concert

interpretations based on previous work and subsurface data, tectonic significance of this period of extension and volcanism.

of and

with

structural

we will discuss

the

STFUTIGRAPHY

Pre-Lower

Triassic stratigraphy

The volcanic rocks discussed here overlie a heterogeneous assemblage of sedimentary, metamorphic and igneous rocks which range from possibly Precambrian to Early Jurassic. Basement rocks include Precambrian(?) to Permian gneisses and amphibolites; pre-Silurian phyllites, slates, and metamorphosed volcanic rocks; and Silurian-Devonian to Lower Carboniferous schists (Gonzalez Bororino and Aguirre, 1970; Halpern, Riccardi represent

and

1973; Natland Rolleri,

1980;

the products

margin (present-day Permian. In limited

areas

et al., 1974; Miller, Herve

of episodic

coordinates)

subduction

and

of Gondwanaland

west of 69”W

1976; de Guisto

et al., 1981). The

longitude

basement

accretion

et al., 1980;

rocks along

probably

the western

from the latest Precambrian

the crystalline

to

basement rocks are elastics, tillites and Rio Genoa

overlain by a series of Upper Carboniferous and Permian continental and rhythmically bedded marine sedimentary rocks (e.g., the Tepuel

Groups in Argentina and the Panguipulli Formation in Chile). The erosional remnants of these rocks define a narrow basin that trends obliquely across Patagonia (Suero, 1961; Lesta and Ferello, 1972). This basin has been recently interpreted as a forearc basin by Herve et al. (1981) and Forsythe (1982). Permian to Lower Triassic plutonic and volcaniclastic rocks east of 70°W longitude document the southern extension of the well-known late Paleozoic magmatic arc of western Argentina and Chile (Polanski, 1970; Herve et al., 1981;

LATE PALEOZOIC FORE-ARC BASIN OF

-

Ml”

TO

LATt J”HA>blL VOLCANIC ROCKS

G&p?~

-PALEOZOIC TRENDS

STRUCTURAL WHEMATICI

KILOMETERS I 74

Fig. 2. Distribution Jurassic

volcanic

of Late Paleozoic-Early rocks.

o-

/ 70”

66”

Mesozoic

tectonic

250 62”

belts and superimposed

I 580

Middle

to Late

221

Forsythe,

1982). The locations

belt of igneous

activity

in the central

at least 500 km wide (Nunez and Methol,

1980; Cortes,

The distribution superimposed

1981; Forsythe,

of Paleozoic

Patagonia,

regional

cini, 1968; Stipanicic, occurrences

and eastern

and

rocks define

parts of northern

Upper

Patagonia

that is

1975; Lesta et al., 1980; Stipanicic

trends

and

the known

extent

rocks are illustrated

Triassic

and

Lower

Jurassic

of the

in Fig. 2.

rocks

that cuts into the rocks described

1969), and are mostly confined

in fault-bounded

trending

stratigraphy

unconformity

or half grabens.

a northwest

1982).

older

Middle and Upper Jurassic volcanic

In northern

grabens

granitic

et al., 1975; Ramos,

Upper Triassic- Lower Jurassic

pronounced

of dated

depressions.

rest

on a

above (Bracac-

to thick, but areally restricted

Seismic data show these depressions

They are filled with continental

sedimentary

to be

rocks, local

marine Liassic rocks and volcanic rocks of various types. The Upper Triassic and Lower Jurassic stratigraphy of Patagonia has been summarized by Stipanicic (1969), Stipanicic and Bonetti (1969, 1970a, b), Stipanicic and Rodrigo (1970a, b), Lesta et al. (1980), de Giusto et al. (1980), and Cortes (1981). Upper Triassic and Lower to Middle Jurassic

volcanic

rocks

are

common

throughout much of southern South America, and are related to the slightly younger volcanic rocks discussed here. In northern Patagonia the Upper Triassic Los Menucos and Garamilla Formations include rhyolitic, rhyodacitic, and dacitic ignimbrites, tuffs and lapillites (Stipanicic and Methol, 1980; Pesce, 1976; Nullo et al., 1978; Coira, 1979). To the south units of the same general stratigraphic position yield Late Triassic floras and locally include basaltic flows and sills (de Giusto et al., 1980). In the eastern Formation

part of northern

is composed

Patagonia,

of tuffs, volcanic

the Lower Jurassic

agglomerates

and

rhyolitic

Puesto

Piris

ignimbrites

(Nunez et al., 1975), which have been dated by the K-Ar method as 189 + 5 Ma (Cortes, 1981). Other Lower Jurassic, volcanic-rich units include the El Cordoba Formation (Panza and Zaccomani, 1981), the Puntudo Alto Formation 1981) and the Sanico Formation (Gulisano and Pando, 1981). Pliensbachian to Bajocian (Lower to Middle Jurassic) tuffs,

(Musacchio, volcanic

ag-

glomerates, and rhyolitic ignimbrites of the eastern Somoncura massif have been described as the La Porfia Formation (Cortes, 1981). The equivalent Los Martires Formation to the south has yielded radiometric dates (K-Ar) of 176 it 10 Ma and 172 f 10 Ma from andesites and rhyolites (Pesce, 1978). To the west, close to the Andes, lower Middle Jurassic rocks of the Carnerero Formation are dominantly andesitic (Musacchio, 1981). Unfortunately, the paucity of diagnostic fossils and marker beds makes correlation of the older Mesozoic units between isolated outcrops difficult. As a result, in poorly exposed areas where massive volcanic rocks are dominant, these units and the younger Jurassic volcanics cannot be easily distinguished.

228

Middle and Upper Jurassic volcanic rocks The stratigraphic terminology South America is complicated. between

the volcanic

la Patagonia, named

designation, describe

Cuarciferos

and ignimbritic

which he called the Complejo volcanic

Tobifera

volcanic

An informal rocks,

broadly

rocks of Middle

used

by many

to Late Jurassic

de

which he

stratigraphic

has been used to

of these rocks in the Magallanes

has been

Porfirico

rocks of the Andes

de la Cordillera.

or simply

occurrence term

to the Jurassic volcanic rocks of southern (1949, 1950) initially drew a distinction

time-equivalent y Porfiritas

Serie Tobifera

the subsurface

1949). The informal rhyolitic

rocks of Patagonia,

and the roughly

Porfiros

applied Feruglio

Basin (Thomas,

workers

to describe

age throughout

southern

South America. In the recent literature, Middle and Upper Jurassic volcanic rocks are commonly referred to as lithostratigraphic units of group rank (Bahia Laura Group, Marifil

Lonco Trapial Group, Lago la Plata Group, El Quemado Complex). Numerous formational names have been proposed

Complex, and to account for

area1 changes in the dominant composition of the volcanic pile. The current stratigraphic nomenclature is summarized in Fig. 3. We will use the term mid-Jurassic volcanic rocks when referring to these rocks in a general sense. However, when referring to a specific locality, we will use the appropriate group name. The relationships between the various units shown on Fig. 3 are poorly understood primarily because of complexities caused by rapid lateral changes in thickness and facies (Bruhn et al., 1978; de Giusto et al., 1980). In some areas the combinations of eruptive and sedimentary processes produced unconformable relationships with only limited chronologic

significance

(Lesta and Ferello,

1972). These peculiari-

ties and the general lack of detailed studies limit many of the stratigraphic subdivisions to only local importance (Riccardi and Rolleri, 1980). As a consequence, ideas on regional

trends

of lithologic

and petrographic

composition

obtained

ture surveys are necessarily a crude first approximation. Radiometric ages obtained by K-Ar dating of the mid-Jurassic cluster at 160 Ma and outline

an episode of dominant

activity

between

from literavolcanic 155-165

suite Ma

(Stipanicic and Bonetti, 1970a; Creer et al., 1972; Codignotto et al., 1978; Pesce, 1978; Lesta et al., 1980; Cortes, 1981). Along the North Patagonian-Andean belt the complex is thought to include some younger, Late Jurassic rocks (Haller et al., 1981; Haller and Lapido, 1982) although Late Cretaceous granitic intrusives preclude reliable K-Ar dating of the volcanic rocks in this area. The available stratigraphic and radiometric ages indicate a pronounced increase in eruptive activity in Patagonia both in volume and in area1 extent in the late Bajocian (Lesta and Ferello, 1972). This phase lasted about lo-15 Ma with volcanism persisting until early Callovian-Oxfordian times (Tithonian?) to the west in Leanza, 1968; Ramos et al., During the accumulation

in eastern Patagonia and into the latest Jurassic some segments of the Andean belt (Feruglio, 1936; 1982). of the mid-Jurassic volcanic rocks, depositional sites

229

r

40’

PI N

44

,,ZOl+ODORO

_

RIVADAVIA

PUERTO

DESEADO

AND JURRASSIC 4f

5:

KILOMETERS I

Fig. 3. Stratigraphic

nomenclature

I

for Middle to Late Jurassic

01

y-250 _~

volcanic

rocks of southern

I

South America.

230

covered,

without

accumulation mid-Jurassic Paleozoic

major

discontinuities,

occurs within volcanic

basement.

Within

the troughs,

Jurassic

volcanic

volcanic-rich

between

the Triassic

and

volcanic

rocks is disputed

Lower

however,

the boundary

early

(Lesta and Ferello,

chio, 1981; Franchi and Page, 1980). Prevailing ideas on the composition

graben

location fill and

1972; de Giusto

of the mid-Jurassic

Thickest

the grabens,

the

discontinuity

on

is not always clear

conformable

In some instances, Jurassic

Outside

and structural

rocks are nearly

units.

South America.

grabens.

rocks rest with sharp lithologic

cut since the mid-Jurassic Lower

most of southern

Late Triassic-Jurassic

on Triassic

and

of the boundary the mid-Jurassic

et al., 1980; Musac-

volcanic

rocks in northern

Patagonia indicate a dominance of silicic (rhyolitic) extrusives in the eastern outcrops of the Somoncura area (upper part of the Marifil Complex, Cortes, 1981) with a gradual change to andesites, basaltic andesites and basalts to the west (Lesta et al., 1980). In most areas, the mid-Jurassic suite is composed of a complex mixture of flows, pyroclastics. ash-flow tuffs and reworked volcanic and non-volcanic detritus (e.g., Mazzoni et al.. 1981). Close to the Atlantic Coast

ignimbrites

massive

and Llambias,

units (Fig. 4) (Malvicini

form

a broad

plateau

consisting

1974). This suite includes

of

rocks of

trachytic, rhyodacitic and rhyolitic composition and is known to occur in the eastern Rio Negro area (Malvicini and Llambias, 1974; Cortes, 1981) eastern Chubut (Lesta

Fig. 4. Massive ignimbrite flows of the Lonco Trapial Group exposed in the valley of the Chubut River.

231

et al., 1980; Creer et al., 1972), eastern et al., 1981) and the southernmost Sparse subsurface

control

1973), Magallanes

(Natland

support In

the regional

in the eastern

Chubut,

south

complex

is referred

Canadon

Puelman

Formations,

(Lesta rhyolitic

San Jorge (Lesta,

of the

to as Lonco

Lesta and Ferello,

by olivine

ignimbrites

the Chubut

andesite (Panza

Massif,

the

(Cajon

with cogenetic

the mid-Jurassic

1981)

tends to

mid-Jurassic

de Ginebra

1972) and is reported

flows and andesitic-dacitic

and Zaccomani,

basins

trend.

Group

and basalts River,

et al., 1978).

1970; Lesta and Bianchi,

Somoncura Trapial

to basic suite of andesites

et al., 1980). Near

represented

et al., 1980; Mazzoni

et al., 1974; Bruhn

of this eastern compositional

volcanic

an intermediate

(Dalziel

et al., 1974; Riggi, 1969) and Malvinas

continuity

north-central

Santa Cruz (de Giusto

Andes

and

to consist of volcaniclastics

volcanic

ignimbrites,

suite

overlain

and locally covered

is by

by a group

of olivine basalts (Null0 and Proserpio, 1975; Pesce, 1978). The compositional change from the silicic ignimbrites in the east to the intermediate and basic members of the Lonco Trapial occurs gradually around 67”3O’W (Lesta et al., 1980). Farther west, the central Chubut suite can be followed to 70”3O’W into the andesites and basaltic-andesites of Sierra de Tepuel (Haller et al., 1981). In outcrops along the Andean belt located north of 44’S, the Middle to Late Jurassic event is represented by andesites overlain by dacites and rhyodacites (Lago la Plata Formation, Haller and Lapido, 1982). At about 46”S, the volcanic complex is described (Ibanez

as a bimodal

Formation-Baker

suite of basaltic-andesite

lavas and rhyolitic

et al., 1981). South of 46”s

ash-flow

the proportion

tuffs

of andesite

decreases (Ramos et al., 1982). In the Andean outcrops from 47”s to Tierra de1 Fuego, the volcanic package is a series of rhyolitic to rhyodacitic rocks (Lemaire Formation,

Tobifera

flows and volcanic complex

volcanics-Bruhn agglomerates

et al., 1978; Ramos

have been described

north of Lago San Martin

at 48”3O’S (El Quemado

1978); the upper part of the complex Plutonic mid-Jurassic 41’S

(granitoid) bodies volcanic complex

and usually

is a mixture

et al., 1982). Andesitic

only in the lower part of the

of dacitic

Complex-Nullo and rhyolitic

with radiometric ages roughly are reported along the Andean

lie west of the line of volcanic

outcrops

et al., tuffs.

equivalent to the belt from 52”s to

(Nelson

et al., 1980).

However, away from the Andes, most of the studied and dated plutonic rocks of Patagonia seem to be either older or younger than the mid-Jurassic volcanic episode (Lesta et al., 1980). PETROLOGY

A suite of 35 samples of the mid-Jurassic volcanic suite was acquired during this study. Twenty-eight of the samples come from a west-east traverse that runs from the crest of the Andes across the Somoncura Massif to the Atlantic coast of Argentina (Fig. 5). Seven samples come from the Deseado Massif near the town of Puerto Deseado (Fig. 5).

this study.

Fig. 5. Location

map showing

740 w

outcrop

distribution

of Middle

to Late Jurassic

70” w

volcanic

rock> in north-central

Patagonia

660 w

of samples

analyzed

KILOMETERS

DESEADO

and location

PUERTO

MID JURASSIC VOLCANIC ROCKS

in

233

Petrography The basalts and morphosed

andesites

to lower greenschist

character.

of the sampled

facies but preserve

The effects of this metamorphism

ment of olivine mineral

basaltic

and

phenocrysts

by chlorite,

of the mafic

phenocrysts

are occasionally

saussuritized

groundmass

of some samples. Alteration

phases

rocks

are meta-

much of their original

are generally

serpentine

chloritization

volcanic restricted

pseudomorphing

of the original

in the groundmass.

and small patches

igneous

to the replacePlagioclase

of zoisite occur in the

of these rocks in near-surface

environments

is recorded by the presence of calcite and zeolites. The samples retain igneous textures that range from aphyric and intersertal (glass replaced by chlorite) to porphyritic and intergranular to sub-ophitic (Fig. 6). Olivine, clinopyroxene and plagioclase typify the phenocryst assemblages. Olivine phenotrysts, when not replaced by metamorphic minerals, are euhedral to subhedral with thick iddingsite rims. Clinopyroxene phenocrysts are complexly zoned and twinned. Basaltic Plagioclase phenocrysts have sieve textures, and are usually corroded. andesites

have more plagioclase

than the basalts

and contain

olivine

quartz xenocrysts with clinopyroxene reaction rims (Fig. 6). The few andesites that were sampled possess similar metamorphic

and embayed characteristics

as observed in the mafic rocks. Original igneous textures are somewhat obscured by metamorphism (probably due to the high glass content of the original andesite) and are restricted primarily to phenocryst mineralogy and morphology. Plagioclase, clinopyroxene and amphibole are common phenocryst phases in the mafic andesites and are joined by quartz and alkali feldspar in high-silica andesites. High-silica andesites also contain lithic fragments of volcanic origin, suggesting a pyroclastic origin. Rhyolites and dacites are petrographically similar and include all varieties of crystal and lithic tuffs (Fig. 6). Original igneous textures are preserved in all samples to some degree-if banding,

flattened

only by pseudomorphs glass shards

(Fig. 6). In many samples

and welded

of phenocrysts. textures

Trachytic

are observed

these textures have been obscured

textures,

flow

in some samples

by devitrification

of the

groundmass glass to a dense, cryptocrystalline assemblage of quartz and alkali feldspar. Most samples exhibit a pervasive iron stain due to alteration of iron oxides and some also contain veins of quartz and patches of calcite. Broken,

euhedral

to subhedral

phenocrysts

of alkali feldspar,

plagioclase

feldspar

and quartz are present in varying proportions in the rhyolites and dacites. Quartz phenocrysts are often embayed and feldspars are sericitized or saussuritized but preserve optical evidence of oscillatory zoning, complex twinning and perthitic textures. Biotite and amphibole occur in only minor amounts and are rimmed to variable extents by fine-grained aggregates of opaque oxides and silicate minerals. Muscovite, apatite and zircon are rare phenocryst phases. Lithic fragments which are included in some rhyolites generally possess volcanic textures and are probably rhyolitic, although some fragments are pieces of coarse-grained granitoids.

234

235

Geochemistry Major elements, Assay Laboratory

Rb, Sr, and Zr were analyzed by X-ray fluorescence by X-ray of Canada. In addition, loss on ignition (LOI) was also de-

termined.

Selected analyses

calculated

on a volatile-free

contain

less than

are presented

in Tables

1 and 2 along with CIPW norms

basis with Fe0 adjusted

2.0% LO1 and

appear

to 0.85 total Fe. Most samples

to be relatively

fresh;

however,

several

samples, mostly basalts, contain up to 10.0% LOI. Samples with LO1 greater than 7.0% have not been included in Tables 1 and 2. Several rhyolites were significantly altered by silicification resulting in greater than 80 wt.% SiO,. The samples are classified on a chemical basis (volatile-free) Peccerillo basaltic

and Taylor andesites,

(1976) (Fig. 7). The rocks comprise

potassic

rhyolites,

and high-Si

abundant, are high-K varieties. The analyzed basalts are hypersthene

a bimodal

dacites.

normative

using the scheme of suite of basalts,

Andesites,

tholeiites.

which are not

One basalt

is slightly

quartz normative (possibly an artifact of the Fe0 recalculation procedureSchwarzer and Rogers, 1974) (Table 1). Examination of the normative An (100 An/An + Ab) of the basalts with respect to their normative hypersthene contents suggests that sodium has been enriched through low-grade greenschist metamorphism (Lipman and Mehnert, 1975). Similar sodium enrichment has been reported from altered basalts and spilites (Smith, 1968; Valiance, 1969). Thus, the alkali contents

of these rocks probably

do not reflect the original

chemistry

of the magma.

In a similar sense, burial metamorphism has altered CaO and MgO contents, so that they exhibit an inverse relationship (Smith, 1968). TiO, contents are relatively low and exceed

1.2 wt.% only in rocks with low Mg numbers,

indicating the evolved character than 16.0 wt.%) in all basalts. The basaltic tions varying

andesites between

of those magmas.

(52-55

wt.% SiO,)

Al,O,

(Mg/Mg

contents

are fairly mafic with MgO concentra-

3.5 and 6.5 wt.% and Mg numbers

of 50-63.

are highly variable suggesting that these rocks, like the basalts, metasomatism accompanying burial. Abundances

of Rb, Sr and Zr in the basalts and basaltic

Fig. 6. Photomicrographs photographs A. Basaltic trysts

of selected samples

illustrating

well-preserved

Alkali contents were affected

andesites

volcanic

by

are comparable

textures.

Scale bar in all

is equal to 5 mm. andesite

with fine-grained

of sieve-textured

plagioclase,

B. Crystal-rich

welded rhyolitic

alkali-feldspar.

Small lithic fragments

talline mixture

of quartz

C. Crystal-rich

rhyolitic

embayed

+ Fe x loo),

are high (greater

groundmass

and embayed

of plagioclase, xenocrysts

tuff with phenocrysts

clinopyroxene,

of quartz

of biotite,

quartz,

are also visible. Groundmass

and opaques,

with pyroxene

reaction

zoned plagioclase

glass has been altered

and embayed to a cryptocrys-

and feldspar. tuff with

large

altered

glass

shards

and

phenocrysts

of alkali

feldspar

quartz.

D. Dense rhyolite,

pheno-

rims,

possibly

obsidian

with large euhedral

phenocrysts

of alkali feldspar

and biotite.

and

CIP w “Otm (

4.12 31.70 33.12 7.60 15.99 _ 2.17 0.55 _

2.25 0.56 3.16

12.18 33.43 15.44 11.19 18.17 _

10.53 2.24 0.48

0.99 6.09 19.86 33.22 23.43 _

_ 2.47 49.67 22.73 18.43 1.55 0.26 0.74

_ 13.32 28.69 27.26 11.88 1.88 8.75 3.72 1.50 _

_

59 31 20 1600 70 40

99.95

49.5 0.76 18.4 8.35 0.14 5.57 4.41 5.78 0.39 0.11 6.54

14

11.08 26.67 27.17 17.01 3.17 7.23 3.30 1.33

45 49 60 700 180 80

99.04

49.0 1.86 17.1 9.40 0.09 3.49 8.01 3.22 2.14 0.65 4.08

23

2.81 12.52 28.11 28.97 13.07 8.37 2.48 1.27

14.58 1.36 0.25

50 51 50 840 200 90

99.55

52.1 1.26 17.7 7.62 0.06 3.52 8.26 3.20 2.04 0.56 3.23

21

6.18 6.88 15.21 31.70 20.88 _

0.85XFe.

6.64 34.93 30.01 6.05 6.30 11.61 1.88 0.49

_

60 67 50 310 130 150

100.19

99.86 63 46 40 370 120 40

52.6 0.70 15.5 9.59 0.15 6.43 9.90 1.76 1.14 0.11 2.31

13

52.0 0.97 18.6 6.72 0.09 5.12 9.00 4.04 1.10 0.22 2.00

33

a Mg number = 100 Mg/Mg + 0.85 Fe b An = 100 An/An + Ab ’ Norms are calculated using analyses summed to 100%. volatile free. with Fe’+=

AP C

HYP 01 Di 11

Ab An

Qtz Qr

_

59 63 20 360 110 120

69 32 50 520 60 140

56 50 50 670 160 140

98.89

99.66

98.81

99.83

Mg number ’ Anb Bb (ppm) Sr Zr Cr

Sum

LO1

p205

K2O

MS0 CaO Na,O

MnO

Fe203

57 51 10 290 60 130

1.66 16.4 9.61 0.12 5.61 7.62 3.01 1.79 0.58 3.39

1.12 16.3 9.93 0.16 6.42 9.08 2.23 0.98 0.21 4.62

1.11 17.0 9.35 0.14 9.48 3.20 3.70 1.93 0.24 5.16

1.05 17.5 10.3 0.21 6.35 6.71 3.44 0.64 0.23 8.00

-’

A1203

49.1

25

TiO;

4

48.6

1

41.5

(wt.%\ 45.4

SiO,

Sampie number: 16

7.94 2.69 0.86 _

3.15 0.81 36.69 26.88 18.49 _

60 42 20 730 180 110

99.94

51.8 1.34 16.2 7.79 0.13 5.27 7.42 4.10 0.13 0.37 5.39

2

11.21 1.89 1.06 _

5.44 14.08 22.79 23.27 17.50 _

55 50 80 600 120 190

99.15

53.6 0.96 15.0 8.81 0.12 4.93 7.74 2.60 2.30 0.47 2.62

22

Major and trace element analyses and CIPW norms of basal& and andesites from the Lonco Trapial and Bahia Laura groups

TABLE 1

6.93 2.53 1.Ol

8.16 12.80 29.32 23.92 12.98 _

49 45 70 730 170 90

99.72

55.7 1.29 16.3 7.49 0.09 3.32 6.85 3.36 2.10 0.45 2.17

20

1.90 0.52 3.24

10.03 19.81 31.82 10.56 19.84 _

57 25 170 230 210 30

99.12

57.4 0.96 16.2 7.21 0.24 4.37 2.31 3.60 3.21 0.23 3.39

3 18

14.91 23.98 29.44 14.09 8.67 _ _ 2.84 1 .Oh

25 32 170 410 330 20

98.86

60.5 1.45 14.8 7.21 0.09 1.10 3.94 3.37 3.93 0.47 2.00

237 TABLE 2 Major and trace element analyses and CIPW norms of rhyolites from the Lonco Trapial and Bahia Laura groups Sample Number: SiO, (wt .%I TiO, A’ ~0s Pa@, MnO

19

27 78.8 0.12 10.9

28

35

77.5 0.16 10.2

75.9

15.7 0.23

0.15 12.4

11

9

11.4

75.9

74.3 0.30

0.29 11.2

29

12.5

6

15

75.1 0.19 13.3

74.5 0.35 13.0

74.0 0.25 11.8

0.46

0.31

0.66

0.80

1.43

1.23

0.42

1.30

2.25

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.02

0.01

MSG CaO

0.08

0.03

0.17

0.07

0.34

0.23

0.01

0.26

0.23

0.14

0.82

0.21

0.25

0.39

0.36

0.09

1.50

0.36

Na,O

1.79

1.57

0.33

0.23

0.48

3.10

3.76

3.93

2.07

K,G

5.80

6.59

7.45

8.67

7.56

5.57

5.81

3.80

6.98

PsG, LO1

0.02

0.34

0.04

0.02

0.07

0.08

0.02

0.16

0.04

1.23

1.00

1.31

1.39

1.39

1.23

0.62

1.16

1.39

Sum

99.35

98.53

98.63

98.77

99.06

98.91

99.33

99.98

99.38

250

230

340

310

380

220

290

140

Sr

20

50

120

60

10

90

30

290

20

Zr

90

130

90

230

130

160

240

260

410

Bb (ppm)

210

UP w nOrm* Qz

46.50

43.36

45.80

41.36

43.38

34.32

31.12

33.70

33.77

Or

34.93

39.93

45.24

52.61

45.74

33.70

34.78

22.71

42.09 17.87

Ab

15.44

13.62

2.87

2.00

4.16

26.85

32.23

33.71

An

0.59

1.36

0.83

1.15

1.56

1.35

0.33

6.59

1.58

HYP

0.63

0.48

1.08

0.87

2.28

1.73

0.29

2.13

3.14 0.48

I1

0.23

0.08

0.29

0.45

0.56

0.58

0.37

0.31

AP

0.04

0.31

0.09

0.04

0.16

0.18

0.04

0.35

0.09

C

1.49

0.76

3.59

1.26

1.71

0.91

0.71

0.04

0.38

Sample Number: SiO, (wt .%I TiO, A’@,

30

32

73.4 0.12 14.8

73.2 0.15 14.8

26

7

72.5 0.23 13.8

34

71.4 0.40 14.0

5

70.7 0.28 14.6

66.7 0.42 15.7

AGV-1

AGV-lb

58.9

59.0

1.03 16.9

1.04 17.25

Pesos MnO

0.95

0.52

1.65

2.08

2.75

3.26

6.76

6.76

0.02

0.01

0.04

0.02

0.07

0.02

0.09

0.09

MS0 CaO

0.33

0.33

0.18

0.36

1.25

0.79

1.49

1.53

1.20

1.29

0.30

1.31

1.91

2.67

4.93

4.90

Na,O

4.31

4.89

3.36

4.24

4.32

3.77

4.28

4.26

K,G

3.34

3.54

5.98

4.46

2.17

3.65

2.84

2.89

p20,

0.06

0.09

0.03

0.12

0.18

0.30

0.47

0.49

LOI

1.08

0.93

1.16

1.54

1.77

2.85

1.85

1.27

Sum

99.61

99.75

99.23

99.93

100.00

100.13

99.54

99.48

238 TABLE

2 (continued) ._

_.__._____

Sample Number:

30

32

26

7

34

Bb (ppm)

120

100

220

120

130

5 70

AGV-I 70

AGV-1 h 61

Sr

660

570

110

230

550

560

750

657

Zr

70

110

300

260

90

250

240

225

(‘IP w nor* d

Qt7

32.62

28.30

28.96

26.11

30.85

Or

20.03

21.17

36.03

26.79

13.05

22.17

Ab

37.01

41.87

28.99

36.46

37.21

32.79

An

5.68

5.94

1.34

5.89

x.57

11.80

HYP

1.92

1.29

2.31

3.00

6.43

5.65

II

0.23

0.29

0.45

0.77

0.54

0.82

AP

0.13

0.20

0.07

0.27

0.40

0.67

C

2.07

0.78

1.35

0.08

2.10

1.38

a Norms

are calculated

using analyses

h Values from Flanagan

summed

23.70

to 100%. volatile free with Fe2 ’ = O.RSXFe

(1973).

to some island arc basalts (Perfit et al., 1980) and tholeiites from flood provinces, such as the Karoo and Parana (Basaltic Volcanism Study Project, Characterization of the tectonic setting of the Lonco Trapial and Bahia basalts using the available immobile trace elements data (Ti and Zr-Pearce

basalt 1981). Laura and

Cann, 1973) is ambiguous because continental and oceanic within plate basalts exhibit a broad range of Ti and Zr contents that encompass and extend beyond the ocean floor and talc-alkalic basalt discriminant fields. The andesites are high-K varieties but do not compare kalic high-K (Bailey,

andesites

1981). TiO,,

from erogenic

regions

well with typical

(Gill, 1981) or Andean-type

Pz05, Rb, and Zr contents

are higher

calc-alandesites

and CaO contents

are

much lower in the andesites of the mid-Jurassic suite versus erogenic, talc-alkalic andesites. The high Rb may be an artifact of metamorphism; however the high Zr, TiO,

and P20, are probably

primary

magmatic

characteristics.

The mid-Jurassic silicic suite consists of talc-alkalic and high-potassium rhyolites (Fig. 7). The talc-alkalic rhyolites are restricted to the Deseado Massif, and the high potassium rhyolites were sampled in the Somoncura region. Both are peraluminous, suggesting that both types are equivalent to S-type granites (Chappell and White, 1974). However, the relatively low Zr contents (approximately 100 ppm) and high Sr contents (500 ppm) of the talc-alkalic rhyolites are more similar to I-type granites. The geochemistry of the talc-alkalic rhyolites compares favorably with erogenic talc-alkalic rhyolites (Ewart, 1979). The high-potassium rhyolites are most like rhyolites from extensional environments involving anatexis of continental crust (Hildreth, 1981). Two types of low-temperature burial metamorphism occur in the rhyolites. The

1.0 -

2.0 -

3.0 -

4.0 -

5.0 -

andesites

samples

.

SAMPLES

SAMPLES

TRAPIAL

SHOSHINITE

LAURA

GROUP GROUP

rocks by K,O

AND

Deseado

possess

a talc-alkahc

DACITE

119761

,

,

DACITE

character.

Deseado

, 70 0

proposed

*

.

LOW-K

-

by Peccerillo

+

region. The suite is essentially

using divisions

WT%

65.0

I

LOW-K

(CALC-ALKALICI

DACITE

HIGH-K

& TAYLOR

SO2

contents

from the Puerto

and SO,

.

PECCERILL~

ANOESITE

EANAKITE

HIGH-K

and crosses are samples

volcanic

I-K EAS.

from Puerto

traverse

Samples

from our west-east

and rhyohtes.

LONCO

BAHIA

_+.s

FROM

FROM

of mid Jurassic

ABSAROKITE

+

Fig. 7. Classification

:

f

6.0 -

7.0 -

6.0 -

g’oc

l-

75 0

I

,

bimodal,

I

60 0

I

.

2

I

into basalts

Filled circles

I

being divisible

(1976).

.

RHYOLITE

ICALC-ALKALICI

LOW-K

l+

RHYOLITE

RHYOLlTE

HIGH-K

. .

and Taylor

.

.

to

are

6.00

-

5.00

-

4.00

-

3.00

-

0

LONCO

TRAPIAL

+

BAHIA

LAURA

!

WITH

Si02

> 65 WEIGHT

?+

GROUP

+

+ ae

SAMPLES

GROUP

+ 0

N

s

l

2 .oo

1 .oo

I 0.00

I

I

I

0 00

1 .oo

I

2 .oo

3 .oo

I

I 4.00

5.00

K20

Fig. 8. Plot of Na,O

versus

these alkali abundances

K,O

suggests

for samples potassium

I

I

I

6 .oo

7.00

8 .oo

0 9.00

WT%

with

> 65 wt.% SO,.

metasomatism.

Symbols

The negative

correlation

between

as in Fig. 7.

most prevalent type is not petrographically apparent and involves the replacement of sodic plagioclase by potassic feldspar, resulting in an increase in K ,O and a decrease in Na,O contents (Fig. 8) (Izett, 1981). Similar alteration has been noted in Tertiary rhyolites from the Mogollon-Datil volcanic province in New Mexico where K,O may be as high as 13.5 wt.% (Chapin et al., 1978; Chapin and Glazner, 1983). Chapin

and Glazner

(1983) have shown that unaltered

rhyolites

from the Mogollon-

Datil field rarely have K,O contents greater than 6.0 wt.% and K,O/Na,O values greater than 3.0. Bahia Laura and Lonco Trapial rhyolites, on the other hand, have K,O/Na,O

values which range from 0.5 to 27, indicating

potassium

metasomatism

in the rhyolites with K,O/Na,O greater than about 13. The positive correlation of Rb and K,O indicates that Rb has also been affected by this metasomatic event. The second type of alteration involves leaching of K,O from the rock. Thus, rhyolites that have high LOI totals (greater than 7.0 wt.‘%) also have low K,O contents. CaO and Sr contents are enriched in these samples which also contain significant modal calcite. Petrogenesis It is difficult to propose a detailed petrogenetic scenario for the mid-Jurassic suite of basalts and basaltic andesites on the basis of the data we have collected.

241

Alteration of the basalts has been shown to modify the original magmatic character of the rocks. It is also likely that these basalts experienced fractional crystallization prior to eruption, further obscuring information about their source. Geochemical similarities between the suite of mid-Jurassic basalts and basalts from flood basalt provinces suggest that their petrogeneses may also be similar. We cannot however evaluate whether these basalts are derived from tholeiitic or picritic primary magmas (Cox, 1980) or to what extent their chemistry has been influenced by crustal contamination. We believe that the geochemistry of the Lonco Trapial and Bahia Laura basalts is consistent with that expected within extensional environments. The whole-rock chemical analyses of the mid-Jurassic rhyolites suggests anatexis of crustal material. The least altered of the mid-Jurassic rhyolites are plotted in Fig.

WT.

PERCENT

ALBITE

ORTHOCLASE

Fig. 9. Experimentally-determined

phase relations in the quartz-albite-orthoclase

water pressure (from Tuttle and Bowen, 1958) with normative compositions rhyolites points.

superimposed.

Calc-alkalic

rhyolites

from the Puerto Deseado

ternary at 1 kb and 5 kb

of least-altered

mid-Jurassic

region are the three leftmost

242

9 on the normative Qz-Ab-Or ternary (weight percent). The talc-alkalic rhyolites from the Deseado Massif and some of the potassic rhyolites lie near the minimum melt composition

expected

for pressures

19.58). A number

of high potassium

may reflect a shift in the minimum Ah/An

ratio

Jurassic

rhyolites

of the parent

in this diagram

anatexis

of crustal

materials

between

rhyolites

1 kb and 5 kb (Tuttle

also fall along the Qz-Or

melt composition

decreases

(Winkler,

is consistent

and Bowen, cotectic

and

from the left to the right as the 1979). The behavior

with a petrogenesis

whose major components

are quartz

of the midinvolving

the

and feldspar.

The

S-type characteristics of these rhyolites suggests anatexis of sedimentary source material (Chappell and White, 1974) and is consistent with our structural interpretations (see following section). This inference is also supported by the field observation of a lack of any parental material from which the rhyolites may have been derived by fractional crystallization. The differences observed between the calc-alkalic rhyolites of the Deseado Massif and the high-K rhyolites of the Somoncura region may reflect differences in their crustal sources. A possible scenario of the petrogenesis of the mid-Jurassic volcanics rocks (i.e., large volumes of silicic pyroclastics associated with small volumes of basalt and intermediate rock types) is one of an extremely energetic magma system under stress conditions favoring marked crustal extension (Hildreth, 1981). In this model, basalts generated by partial melting of the mantle are ponded at the base of the crust erupting only infrequently. This input of hot material into the crust causes melting, which in an extensional

regime, develops

large silicic magma

chambers.

Eventually,

these chambers rupture and large volumes of pyroclastic material are ejected. chemical or petrologic relationship exists between the silicic and basaltic rocks. EXTENSIONAL

DEFORMATION

Both stratigraphic preceded

No

and

and accompanied

structural

evidence

the volcanism

show that

described

above.

extensional

deformation

This extensional

event

produced numerous north-northwest-trending fault-bounded troughs in Patagonia and Tikrra de1 Fuego (Fig. 1). Most of these are half grabens with the faults on the eastern sides generally showing the largest displacements. The half grabens are best seen on seismic data from the Magallanes, Malvinas and San Jorge basins, but also have surface expression on the Deseado Massif. Here, a northwest-trending narrow depression is filled with Lower Jurassic and uppermost Triassic sedimentary and volcanic rocks (Bracaccini, 1968; de Giusto et al., 1980). In the Magallanes, western San Jorge and Malvinas basins, thick, areally restricted graben fill underlies well dated Upper Jurassic marine sedimentary rocks. The faults that form the grabens are steeply dipping normal faults, some of which have listric profiles at depth. These faults display several different relationships with the mid-Jurassic volcanic rocks that, together with the stratigraphic data, provide information on the timing of extension.

243

Most of the normal thinner

high-side

volcanic-rich

faults separate

accumulations.

section has been sampled

little as 20 m of equivalent few show limited

displacement

rocks are confined

horst block is devoid mid-Jurassic volcanic

low-side

by drilling

section is present

Most of these faults die out upward volcanic

thicker

volcanic-rich

As much as 2000 m of Middle

sections

to Upper

on the low side of a fault, while as

on the high side (Natland

in the mid-Jurassic

higher in the section. to the downthrown

from

Jurassic

volcanic

et al., 1974).

rocks, although

a

In some cases, the mid-Jurassic

side of the fault, and the adjacent

of deposits (Natland et al., 1974). Finally, in some areas, the rocks cover faults with no significant thickness variation on

either the high or low side of the fault. These relationships,show that extension, accommodated by normal faulting at high crustal levels and some volcanism, began considerably before the major mid-Jurassic volcanic event discussed here, but extension continued during mid-Jurassic volcanism and finally died out with the cessation of mid-Jurassic volcanism, or shortly thereafter. We have no direct

evidence

of the amount

of extension

that took place across

southern South America during the Jurassic, but because the extension was associated with significant crustal anatexis, as expressed by the volume of ignimbrites, we feel that it must have been considerable. DKXUSSION

The distribution of the mid-Jurassic volcanic rocks shown on Figs. 1 and 2 provides an indication of the area affected by the extension and volcanism that we have described. This is a minimum estimate, however. Similar volcanic rocks have been described from the Antarctic Peninsula (Saunders and Tarney, 1982), and although siliceous volcanic rocks of the appropriate age are not present on the Falkland

Islands

(Islas Malvinas),

bimodal,

mid-Jurassic

volcanic

rocks have been

reported from the Algoa Basin in South Africa (Marsh et al.,. 1979), and abundant devitrified glass shards and volcanic fragments occur in the Jurassic sedimentary rocks of the Karroo Basin (Elliot and Watts, 1974; Bristow and Saggerson, 1983). This suggests that the area affected present Pacific margin of South crust boundary along Argentina’s north to at least the reconstructed

by the volcanic

event extended

from nearly

the

America east to at least the oceanic-continental eastern edge, and from the Neuquen Basin in the position of the Antarctic Peninsula in the south.

Extensional deformation also began in the Cape Fold Belt of southern about the same time that grabens were forming in southern South America

Africa at (Table 3)

(Lock et al., 1975; Lock, 1978). Although the culminating volcanic episode that produced the Tobifera volcanics, the Bahia Laura and Lonco Trapial Groups, and related volcanic rocks was of limited duration, the associated extension and preceding volcanism occurred (perhaps sporadically) throughout most of the Late Triassic and Jurassic. There is good evidence that the western edge of southern South America was a

paleo-pole

Carboniferous

3

Mid-Jurassic

Late Triassic

to

Permian

mid-Triassic

Early

to Early Permian

Late

Time

Magnetic

TABLE

to

1976; Vilas, 1981)

and Vilas, Bahia

Laura,

Lonco

Trapial

Groups).

event

culminates volcanic

volcanism, mid-Jurassic

silicic to bimodal

widespread

(Tobifera,

in

localized

basalts

Hoachanas

by

Suurberg basalts (Bristow

Basm

1982) and the extensive m the Karroo

and Smith.

Basin indicated

Drakenberg

1983). favas in AC wells (Gerrard

and rhyolites. and Saggerson.

basahs

uolcanrsm

Bank, and Orange $yntectonic

Agulhas

(Valencio

Newark-type

gether

formation,

Extensronai deformatton --produces graben

basins (Lock et al., 1975) in the Cape Fold Belt,

America,

southwestern

S.

Back -arc extension and crustal anatexis to-

America,

Africa

(Beaufort

accommo-

K/Ar

Semr -Stable

Cr.)

Basin up megasequence

deposition - Karroo a shallowing

Foreiand dates

resets

orogeny) S. Africa

(Gondwanide offshore et al., 1978)

event ages (Gentle

Thermal

(?) in S. America

South

Chile.

but and

Argentina

narrower

north of 36’N in central

matic arc was 500 km wide in N. Patagonia,

Eq.) (Forsythe,

arc

basin Mag-

Fm. and

Fm.) and magmatic 1982).

(Choiyoi

(Rio Genoa-Panguipulli

de Dios Fm. ) forearc

accretion-

(Madre

and Vilas,

included:

ary prism

1976; Vilas, 1981)

complex

(Vaiencio

time

onset of Ecca

gether

deformation

or

suggests

Cape Fold Belt (Lock. 1978) and Sierras Auscrales

1978)

polarity in Dwyka

Back -arc compressional

(Lock,

regime

margin

et al..

contractional

Subduction, accretion and arc magmatism

Herve

Carbonifer-

convergent

to-

1976:

to Lower

America,

1981)

(Miller,

Silurian

South

Africa

deformed

ous accumulations

cludes

Semt -stable

1976; Vilas, 1981)

and Vilas.

IOforeland deposition

in depositional

(Valencio

in-

gether

substratum

reversal

edge of Gondwanaland,

Epicratonic

Africa

Subductson, accretion and arc mugmatism

Africa

western

to-

Africa

Southern

and southern

America,

South America

events

South America

South

Africa

Major tectonic-magmatic

events in southern

Rapid shqt

S. America/S.

pole positions

Paleo

and major tectonic/magmatic

Magnetic

positions

Late Early ceous

Greta-

Late Jurassic to Early Cretaceous

South America Rapid Shift Africa (Vale&o and Vilas, 1976; Vitas, 1981)

Semi-stable

-western margin of South America (Hailer and Lapido, 1982; Ramos et al., 1982) Thermally induced subsidence in Magaltanes, and San Jorge basins. Continental to marine sedimentation in broad depressions (Inoceramus shale-Upper Las Heras Group) Subduction and arc magmatism

Unknown Late Jurassic Back-arc extension -mostly confined to opening Rapid Shift (Early Creta- of Rotas Verdes marginal basin by 140 Ma ceous) South America (Daiziel, 1981) Africa together (Valencio Wuning of volcanism and normal faulting in and Was, 1976; ViIas, 1Y81) central eastern Patagonia Onset of thermally driven subsidence in the Magallanes and Malvinas basins

sedimentation -in the Agulhas and Orange basins (early drift sequence-Upper Sundays River, Gerrard and Smith, 1982; Du Toit, X979)--waning of magmatic activity.

Subsidence and pass&e-margin

Extensional deformation -Agulhas Bank (Du Toit, 1979) and Orange basin (Gerrard and Smith, 1982). Onshore faulting associated with silicic and basaltic volcanics (Bristow and Saggerson, 1983). Central Karroo volcanism dies out after 160 Ma. Sedimentary accumulation mostly confined to fault bounded depressions (rift valley sequence = Pre-Sundays River). Opening of the south Atlantic around 130 m.y.

246

subducting

margin

with an associated

(Nelson

et al., 1980). The widespread

indicate

that they represent

activity

associated

margin Jurassic

arc during extension

a tectonic/magmatic

with the marginal

event

The

cause

of the extension

separate

arc. The geochemical

volcanic rocks presented here support this contention silicic volcanic rocks had a crustal magma source. and

volcanism

much

of the Mesozoic

and mid-Jurassic

data on the mid-Jurassic

and indicate

remains

volcanism

from the igneous that most of the

speculative.

Any

model

proposed to explain these two phenomena must also explain several other aspects of the history of southern South America and southern Africa. We believe that the major

events

that affected

ceous are related. We postulate

both areas from the Late Paleozoic

Table 3 summarizes that a series of events

and Range extension

and bimodal

to the Early Creta-

these events. similar

volcanism

to those proposed

in the western

United

to explain

Basin

States (Lipman,

1980) provides the best model for the current data from southern South America. Bruhn et al. (1978) have also compared the mid-Jurassic volcanism of southern South America with the Tertiary volcanism of the Basin and Range province. Following Lock (1980) we propose that the dip of the lithospheric slab being subducted beneath southern South America changed from a steeply dipping configuration in the Late Paleozoic to a more shallowly dipping slab in the latest Paleozoic and Early Mesozoic. This caused the magmatic arc in southern South America to expand in width to perhaps 500 km in Patagonia by Middle Triassic time. To the north, in Chile, the Early Mesozoic magmatic arc retained a more conventional width indicating segmentation of the subducted slab. The shallowly dipping slab also allowed back-arc

compressive

range

America

of South

and

deformation the Cape

to take place in the Sierra de la Ventana Fold

Belt of southern

removed from the subducting margin (Lock, 1980). By the Late Jurassic, the subducted slab appears to have returned and the associated southern

arc magmatism

South America.

was limited

As arc magmatism

to a narrow migrated

Africa,

both

far

to a steep dip,

belt along the margin

of

back to the South American

margin, Patagonia, Tierra de1 Fuego, and the Cape Fold Belt area of southern Africa began to extend as the subducted slab returned to a steeply dipping state. The extension resulted in the intrusion of basaltic magma into the lower crust, triggering significant crustal melting and the tremendous outpouring of mid-Jurassic volcanic rocks in southern South America. This is envisioned as the beginning of fragmentation of western Gondwanaland. The extension that preceded and accompanied the mid-Jurassic volcanism culminated in the formation of a back-arc basin, the Rotas Verdes Basin, along the western edge of southern South America (Bruhn et al., 1978; Saunders et al.. 1979; Nelson et al., 1980; Dalziel, 1981). have formed in this basin by 140 event we have discussed here. Once Basin, normal faulting essentially

Dalziel (1981) has shown that oceanic crust must Ma, or near the end of the extensional/volcanic oceanic crust began forming in the Rotas Verdes stopped to the east in Patagonia and Tierra de1

247

2

-

t

CONDOR-l

0

ELFONDO-1

0

EVANS-

ff

MANZANO

z fki 0

-3

1 -1 -4

&I

11

fl

11

11

150

11

TIME. IN MILLIONS

Fig. 10. Subsidence Jurassic during

I

*

I 50

100

curves

and their shapes

8

I

5

OF YEARS

for four wells in the Magallanes

and Early Cretaceous,

I(

suggest

Basin. The curves are similar

that subsidence

was driven

for the Late

by thermal

decay

that time.

Fuego, and the extended areas began to subside, giving rise to the post-rifting phase of subsidence of the Magallanes and Malvinas basins. Results of standard backst~pping procedures on data from wells from the Magallanes Basin (Natland et al., 1974) support the supposition that the early history of these basins is related to a rifting event that ended in Late Jurassic time. Using backstripping techniques similar to those of Steckler and Watts (1978), we constructed total subsidence curves and thermo-Teutonic subsidence curves (subsidence that would have occurred if there was no loading due to sediments) assuming Airy isostasy for four wells from the Magallanes Basin (Fig. 10). Corrections were made for sediment decompaction, water depth changes, and long term eustatic sea level changes. All four curves start with deposition of the Sp~n~ll Sandstone immediately following the Tobifera volcanic episode (151 Ma). The shape of the thermo-tectonic subsidence curves for the Late Jurassic and Early Cretaceous for the Evans-l, Manzano-7, and El Fondo-1 wells all approach the shape expected from thermal decay following a rifting event (McKenzie, 1978; Steckler and Watts, 1978) that ended in the Late Jurassic. Similar styles of subsidence are seen in the San Jorge basin and on the Malvinas Plateau at about the same time. The magnitude of the subsidence reflects position in the basin. For example, the Condor-l well, which is located on a structurally high block, shows little tectonically controlled subsidence (Fig. 10). Finally, the abrupt increase in the rate of subsidence in the Late Cretaceous and Early Tertiary reflects flexural loading in response to contractional

248

deformation in the Andes the mid-Jurassic event. The separation Rotas

Verde basin.

South Atlantic

to the west (Winslow,

of Africa and South America The oldest

known

oceanic

followed

the South African,

anomaly

identified

area of Patagonia

is M4 (Rabinowitz

related

soon after opening

crust on the Argentine

occurs to the east of the Somoncura

the oldest magnetic

to

of the

side of the

(Fig. 1). Here,

and LaBrecque,

1979). On

side, Larsen and Ladd (1973) picked Ml1 as the oldest identified

magnetic anomaly, although recently released multichannel (Jaunich,

1981) and is not directly

1983). The magnetic

this has been called into question by a number of seismic lines from the western margin of South Africa anomalies

show that the creation

of oceanic

crust

began in the southern South Atlantic Ocean certainly before 122 Ma and perhaps as early as about 130 Ma (using the time scale of Van Hinte, 1976). This is about 25-30 Ma after the peak of the ignimbritic America

eruptions

and only 10 Ma after formation

the extension

and volcanism

in the mid-Jurassic

of the Rotas

in southern

South

Verdes basin. Thus, we view

that began in the Late Triassic

and culminated

in the

mid-Jurassic ignimbritic eruption and the opening of the Rotas Verdes basin as the initial stages of crustal extension that led directly to the opening of the south Atlantic. These events also have direct economic importance. Decay of the thermal signature associated with mid-Jurassic extension initiated subsidence in the commercially-producing petroliferous basins of southern Chile and Argentina. In the Magallanes, Malvinas and Neuquen basins, and to some degree the San Jorge Basin, the thermally driven subsidence controlled facies distributions and depositional thicknesses after the initial phase of fault-related sedimentation and volcanism. The heat released into the upper crust by extension and volcanism presumably important role in the maturation of hydrocarbons in these basins.

played

an

CONCLUSIONS

(1) Timing relationships show that extension, accompanied by limited volcanism, began in the Late Triassic-Early Jurassic in southern South America and continued into Late Jurassic time. The mid-Jurassic volcanic rocks described here represent the culmination of that event. (2) The geochemical data on the mid-Jurassic rhyolitic volcanic rocks indicate that they were produced by melting of a crustal source. Differences in geochemical signatures of rhyolites from the Somoncura and Deseado massifs may reflect differences in source areas. (3) Although there was an active margin arc along the South American edge of Gondwanaland during most of the Jurassic, the geochemistry and distribution of the mid-Jurassic volcanic rocks show that they are not directly related to igneous processes associated with that arc, but instead represent a separate tectonic/magmatic event. We propose that these rocks are related to events associated with the

249

transition from a shallowly dipping subducted slab to a steeply dipping configuration. (4) The extension and volcanism were precursors to the opening of the Rotas Verdes marginal basin to the west (present day coordinates) and to the initial separation of South America and Africa. As such, they heralded the breakup of western Gondw~aIand. (5) Decay of the thermal event associated with the extension and volcanism began the post-rifting subsidence of the Magallanes, Malvinas, San Jorge, and Neuquen basins, and presumably set the stage for later maturation of hydrocarbons in these basins. ACKNOWLEDGMENT

We wish to thank G. Chebli of Yacimientos Petroliferos Fiscales, Argentina and R.F.N. Page of Servicio Geologic0 National, Argentina, for assistance in assembling the suite of samples analyzed during this study, and Dr. M.G. Fitzgerald for assistance in preparing the subsidence curves. We thank anonymous reviewers for their efforts and useful comments. Exxon Production Research Company, Esso Exploration Inc., and Esso SAPA approved this work for publication. A portion of this research was done while D.A. Gust was a visiting scientist at the Lunar and Planetary Institute. This paper is Lunar and Planetary Institute Contribution No. 546. REFERENCES

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