chew+ 2007 nprz glaciation speru

8/14/2019 Chew+ 2007 NPrz Glaciation sPeru

1/4

GEOLOGY, December 2007 1095

INTRODUCTION

The proto-Andean margin of South Amer-

ica has been the subject of recent debate. In

northwestern Argentina (Fig. 1), the early

Palaeozoic history consists of two pre-Silurian

tectono-magmatic episodes (Early Cambrian

Pampean orogeny and Early-Middle Ordovi-

cian Famatinian orogeny), each of which cul-

minated in accretion of continental fragments

against the proto-Andean margin (Ramos and

Aleman, 2000). This latter collisional episode is

noteworthy, because it involved the Ordovician

accretion of the Laurentian Precordillera terrane

with the proto-Andean margin (Ramos, 2004).

It would appear that Famatinian age (Early-

Middle Ordovician) orogenic activity is con-tinuous from Patagonia (Pankhurst et al., 2006)

through northern Argentina (Pankhurst et al.,

2000), Peru (Chew et al., 2007) to Colombia

and Venezuela (Bellizzia and Pimentel, 1994).

Extending our knowledge of the proto-Andean

margin prior to the Cambrian is difficult due to

the paucity of exposed Precambrian basement

rocks. The majority of Precambrian basement

inliers are within the northern (e.g., the Garzn

and Santander inliers of Colombia; Fig. 1) and

central segments of the Andes Orogen (e.g.,

the Arequipa-Antofalla basement; Fig. 1). The

ca. 1 Ga gneissic basement inliers in the Colom-

bian Andes (Restrepo-Pace et al., 1997) are

likely contiguous with the ca. 11.3 Ga Sunsas

Orogen (Fig. 1). This mobile belt is one of many

Grenvillian belts resulting from the assembly of

the Mesoproterozoic supercontinent of Rodinia

(Hoffman, 1991; Kirkland et al., 2007).

The origin of the Arequipa-Antofalla base-

ment (AAB) on the western coast of southern

Peru and northern Chile is debated. The Ama-

zonian craton exhibits a simple pattern of crustal

growth; a Paleoproterozoic core with progres-

sively younger domains toward the southwest

(Fig. 1). The AAB disrupts this simple pattern,

as it exhibits a southward trend of crustal growth(Loewy et al., 2004; Wasteneys et al., 1995), with

Palaeoproterozoic (1.792.02 Ga) components

in the north, Mesoproterozoic rocks in the cen-

tral segment, and Ordovician units in the south

(Loewy et al., 2004). Although a paraautoch-

thonous origin for the AAB (e.g., Tosdal, 1996)

has been postulated, the anomalous position and

crustal growth pattern of the AAB has led most

authors to propose that the AAB is allochtho-

nous to Amazonia (e.g., Ramos, 1988; Dalziel;

1994; Loewy et al., 2004). Ramos (1988) con-

sidered that the AAB accreted to Amazonia

during the early Palaeozoic Pampean orogeny

Dalziel (1994) envisioned the AAB to be trans

ferred to Amazonia from the northeast corne

of Laurentia during fragmentation of Rodinia

Loewy et al. (2003; 2004) refuted this correla

tion using whole-rock Pb isotopes and U-Pb

geochronology, and suggested that the AAB

was derived from the Kalahari craton, probably

colliding with Amazonia at ca. 1.0 Ga.The AAB is locally overlain in southern Peru

by the Chiquero Formation, a tillite deposit o

probable Neoproterozoic age (Caldas, 1979)

This cover sequence, if of proven Neoproterozoic

age, is a key area for elucidating the docking his

tory of the AAB, but there is a paucity of data

(either geochronological, chemostratigraphic

or field based). Because the docking history o

the AAB is critical to reconstructing the evolu

tion of the proto-Andean margin, we presen

stratigraphic sections based on our field work

Geology, December 2007; v. 35; no. 12; p. 10951098; doi: 10.1130/G23768A.1; 4 figures; Data Repository item 2007271. 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].

Neoproterozoic glaciation in the Proto-Andes: Tectonic implications

and global correlation

David Chew Department of Geology, Trinity College Dublin, Dublin 2, IrelandChristopher Kirkland Laboratory for Isotope Geology, Swedish Museum of Natural History, S-104 05 Stockholm, SwedenUrs Schaltegger Department of Mineralogy, University of Geneva, Rue des Marachers 13, CH-1205 Geneva, SwitzerlandRobbie Goodhue Department of Geology, Trinity College Dublin, Dublin 2, Ireland

ABSTRACT

The Chiquero Formation in southern Peru records the only documented Neoproterozoic

glacial episode in the entire Andean Belt. We present U-Th-Pb secondary ion mass spec-

trometry (SIMS) detrital zircon ages and C isotopic data from the tillite and its overlying

dolomite cap, the San Juan Formation. Two prominent negative C isotopic excursions are

documented: an older excursion (13C = 2) in the cap-carbonate unit overlying the tillite,and a younger excursion (13C = 8) in a laminated limestone unit 700 m up sequence. Inboth cases, 13C values recover to 2. U-Th-Pb SIMS detrital zircon results from the tillite

(both matrix and interbedded turbiditic sandstones) indicate a restricted age distribution of

9501300 Ma. Turbiditic dolomitic sandstones overlying the younger (8) carbon isotope

excursion yield a similar 9501300 Ma peak, but also contain grains dated as 16002000 Ma

and 700820 Ma. The detrital zircon geochronology and C isotope chemostratigraphy are

consistent with the Chiquero Formation being equivalent to the ca. 700 Ma Sturtian gla-

cial. The younger negative C isotope excursion is delimited by the youngest detrital zircon(697 11 Ma) in overlying strata. A correlation with the 635 Ma Marinoan glacial is inferred,

although no unequivocal glaciogenic strata have been identified. The detrital zircon data are

consistent with derivation from the Proto-Andean margin, despite the Chiquero Formation

unconformably overlying basement gneisses of the 18002000 Ma Arequipa-Antofalla base-

ment (AAB), which is exotic to Amazonia. This implies the Chiquero Formation and AAB

were proximal to the proto-Andean margin during Neoproterozoic glaciation, and supports

paleogeographic reconstructions that favor AAB accretion to the Amazonian craton during

the 10001300 Ma Grenville-Sunsas orogeny.

Keywords: Neoproterozoic, Proto-Andes, cap carbonate, glacial, Amazonia, provenance.

SF

80 W

0

60 W 40 W

20S

40 S

N

SoFranciscoCraton(SF)

CentralAmazonianProvince(>2.3 Ga)

Maroni-ItacainasProvince (2.2 - 1.9 Ga)

Ventuari-TapajsProvince (2 - 1. 8 Ga)

MI

VT

CA

Rio Negro-JurenaProvince (1.8 - 1.5 Ga

Rondonia-San IgnacioProvince (1.5 - 1.3 Ga)

SunssProvince (1.3 - 1.0 Ga)

RNJ

RO

SS

Neoproterozoictectonic provincesin Amazonian craton

Rio

de la

Plata

Craton

So LuisCraton

Garzn

ArequipaAntofallaBasement

Sierra

PampeanasFamatinaArc

PrecordilleraTerrane

Precambrianbasement

AndeanBelt

Paleozoicsequences

Santander

ROSS

VT

VT MI

MI

CA

CA

RNJ

RNJ

Brasliabelt

ChiqueroTillite

MaraonComplex

Figure 1. Major tectonic provinces of SouthAmerica and the ages of their most recenmetamorphic events (from Cordani et al.2000). Precambrian and Paleozoic inliers in

the Andean belt are shown in black and lighgray, respectively; terrane boundaries aredenoted by solid lines.


2/4

1096 GEOLOGY, December 2007

U-Th-Pb secondary ion mass spectrometry

(SIMS) detrital zircon results, and C isotopic data

from the tillite and its thick dolostone cover.

LITHO- AND CHEMOSTRATIGRAPHY

The Chiquero and San Juan Formations crop

out on the coast of southern Peru over a strike

length of 100 km, but are best exposed near San

Juan (1524 S, 758 W). Here the Chiquero

Formation rests unconformably on gneisses of

the AAB. The Chiquero and San Juan Forma-tions are weakly deformed and metamorphosed

to low greenschist facies (Shackleton et al.,

1979). No unconformities have been observed

in either formation, but minor time gaps can-

not be discounted. The basal siliciclastic sec-

tion of the Chiquero Formation is 348 m thick

(Fig. 2A), and consists chiefly of massive dia-

mictite with poorly developed internal stratifica-

tion. The majority of clasts are granitic gneiss

that superficially resemble the underlying AAB.

Stratified beds (thin siltstone and graded tur-

biditic sandstone beds) are present between 76

and 152 m above the basement contact, andcontain abundant dropstones of granitic gneiss

(Figs. 2A and 2C). The clasts in the massive

diamictite units straddling the stratified interval

may represent ice-rafted debris, or alternatively

may have been produced by submarine debris

flows. A melt-out till origin (from grounded ice)

for these massive units is unlikely, as there is no

evidence of a transition to shallow water condi-

tions (e.g., wave ripples, cross-bedding).

The upper part of the Chiquero Formation

and the overlying San Juan Formation are pre-

dominantly carbonate. We present C isotope data

for this portion of the section, as C isotope

chemostratigraphy is a particularly useful late

Precambrian chronostratigraphic correlation

tool due to the large fluctuations of C isotope

composition in the Neoproterozoic oceans (e.g.,

Halverson et al., 2005). C and O isotope data

(Table DR1), sampling methodology, detailed

stratigraphic sections (Fig. DR1) and analytical

techniques (Appendix) are provided in the GSA

Data Repository.1 Powders were microdrilled

from fresh hand samples. Diagenetic overprint-

ing of original seawater isotopic signatures isa concern in Neoproterozoic carbonates. How-

ever, due to the high concentration of C, relative

to meteoric fluids, the 13C composition of car-

bonate rocks is more resistant to chemical over-

printing than 18O. The samples are interpreted

to record primary seawater isotopic signatures

(see the Appendix).

The abrupt switch in the upper part of the

Chiquero Formation to carbonate-dominated

sedimentation occurs at 348 m (Fig. 2A). The

dominant lithology is a calcareous diamictite with

white dolostone and limestone clasts (Fig. 2D),

and only minor amounts of granitic gneiss. Boththe clasts and limestone beds yield 13C values

between 0 and +2 (Fig. 3). Overlying the

carbonate diamictite are 11 m of finely laminated

(0.25 cm) pink dolostones and dark dolomicrites

(Fig. 2B, Fig. DR1). This dolomite unit shows

little internal structure apart from prominent

lamination, and yields consistent negative 13C

values of 2 (Fig. 3). Cap carbonates, distinc-

tive laminated dolostone units that immediately

overlie glacial rocks, are found in many Neo-

proterozoic successions and exhibit characteristic

C isotopic profiles (e.g., Hoffman and Schrag,

2002). The lithologies and the negative 13C

excursion in the laminated dolostone-dolomicrite

unit are consistent with a cap carbonate origin.

The overlying San Juan Formation (Caldas,

1978) exhibits a recovery in 13C values tobetween +1 and +2 (Fig. 3). The basal por-

tions of the formation consist of several hundred

meters of predominantly massive beige dolo-

mite. The sampling density for C isotopic analy-

sis is lower in this lithologically monotonous

package and it is unlikely that excursions have

B C

ED

San Juan Fm. -Chiquero Fm.contact

c s s cfmc

100

200

300

400

Massive diamictite,

internal stratification

poorly developed

Stratified diamictite,

thin siltstone beds andgraded sandstone beds

Poorly stratified

diamictite,

abundant dropstones

and lenses of boulderconglomerates

Buff-colored, massive dolomite

Cap carbonate

BThinly bedded dol.

Stratified diamictite,dol. dropstones

76 m

152 m

348 m

398 m

359 m

387 m

0Basement gneiss

SJ-11

SJ-16

D

C

A

Figure 2. A: Stratigraphic section of Chiquero Formation and basal portion of San JuanFormation; doldolomite. B: Finely laminated pink dolostone and dark dolomicrite (cap car-bonate) at top of Chiquero Formation. C: Dropstone of granitic gneiss on graded turbiditicsandstone bed (younging direction indicated). D: Deformed white dolostone clasts in thecarbonate-dominated upper portion of Chiquero Formation. E: Thinly bedded limestone anddark micrite of San Juan Formation. This unit exhibits strongly negative 13C values, and itsposition is marked in Figure 3. Lens cap in BE is 6 cm diameter.

1GSA Data Repository item 2007271, Appendix(sampling methodology and analytical techniques),Table DR1 (C and O isotope data), and Figure DR1(detailed stratigraphic sections), is available online atwww.geosociety.org/pubs/ft2007.htm, or on requestfrom [email protected] or Documents Secre-tary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

400

700

800

900

1000

1100

1200

1300

1400

1500

1600

2000

380

400

420

-5 0 5

1000

1500

2000

-10 -5 0 5

DolomiticturbiditePhyllitic slateand mudstone

Massive beigedolomite

Thinly-beddedlimestone

Black shale

13C()

San Juan Fm.

Chiquero Fm.

13C()

SJ-57

Fig. 2E

Figure 3. Stratigraphic section and C iso-topic trends through upper part of ChiqueroFormation and San Juan Formation. Strati-graphic heights are in meters above Chi-quero FormationArequipa-Antofalla base-ment contact.


3/4

GEOLOGY, December 2007 1097

been omitted. This is overlain by a lithologically

varied unit of black shale, massive dolomite, and

thinly bedded dolomitic turbidite (9501093 m;

Fig. 3). The overlying unit is 170 m thick, and

consists of thinly bedded limestone and dark

micrite (Fig. 2E). This unit exhibits strongly

negative 13C values, from between 5 to

8 (Fig. 3). Above this unit, there is massive

dolomite with 13C values between +1 and

+2.5. This portion of the sequence is nearly

1 km thick, and is only briefly interrupted bydeposition of a thin package of dolomitic tur-

bidite and mud (13951487 m; Fig. 3). We have

not identified unequivocal glaciogenic strata

associated with this younger, strongly negative

13C excursion.

U-Th-Pb SIMS DETRITAL ZIRCON

GEOCHRONOLOGY

In order to assess the setting of the AAB during

Late Neoproterozoic time (the inferred depo-

sitional age of its cover sequence), we under-

took U-Th-Pb SIMS analyses of detrital zircons

from three samples from the Chiquero and SanJuan Formations using a Cameca IMS 1270

ion microprobe. Combined age (probability-

density-distribution) plots and histograms for the

three samples are illustrated in Figure 4, along

with detrital zircon data from the Maraon Com-

plex in the Eastern Cordillera of Peru (sample

DC 5/54; Chew et al., 2007). This sample is

included for comparison, as it is a Gondwanan

margin sequence (Chew et al., 2007) that over-

laps with the assumed depositional age range of

the Chiquero and San Juan Formations. This

sample is cut by ca. 480 Ma leucosomes (Chew

et al., 2007) and yields a youngest detrital zircon

of 869 18 Ma. (U-Th-Pb SIMS data are in

Table DR2; see footnote 1).

Sample SJ-11 (Fig. 2A, 35 grains) is from a

thin graded turbiditic sandstone bed from the Chi-

quero Formation (similar to Fig. 2C). It yields

a restricted age distribution, 9501300 Ma, with

a prominent peak at ca. 1200 Ma and a sub-

sidiary peak at ca. 1000 Ma. SJ-16 (62 grains)

is a sample of diamictite matrix from the Chi-

quero Formation. It is also characterized by a

restricted age distribution from 950 to 1300 Ma,

with a prominent ca. 1200 Ma peak and a sub-

sidiary ca. 1000 Ma peak. The detrital zircon

data from both samples from the ChiqueroFormation yield very minimal detritus (five

grains between 1800 and 1680 Ma in sample

SJ-16) which could potentially be derived from

the underlying basement, the Palaeoproterozoic

(17902020 Ma) northern domain of the AAB

(Loewy et al., 2004). Three granitic gneiss clasts

from the tillite have yielded U-Pb thermal ion-

ization mass spectrometry zircon upper inter-

cept ages of ca. 11601170 Ma (Loewy et al.,

2004), which correlate well with the prominent

ca. 1200 Ma peak identified in this study.

Sample SJ-57 (55 grains) is from a coarse peb-

bly limestone bed from the San Juan Formation,

1412 m above the Chiquero FormationAAB

contact and 178 m above the second negative C

isotope excursion (Fig. 3). The majority of grains

from this sample also lie in the 9501300 Marange, with peaks ca. 1000 Ma and ca. 1200 Ma.

There are also minor peaks within the ca. 1600

2000 Ma and ca. 700830 Ma intervals.

DISCUSSION: TECTONIC

IMPLICATIONS AND GLOBAL

CORRELATION

The detrital zircon data yield information

concerning the position of the AAB at the time

of deposition of its cover sequences. There-

fore, depositional ages of these sequences (the

Chiquero and San Juan Formations) temporally

constrain its docking history. We concur with

others (Caldas, 1979; Loewy et al., 2004) who

regard the Chiquero Formation as Neoprotero

zoic in age. Existing age constraints yield a

minimum age of 468440 Ma based on a loosely

defined U-Pb zircon lower intercept from the

crosscutting post-tectonic San Juan granite

(Loewy et al. 2004), and maximum ages of

932 28 Ma and 955 18 Ma (the younges

detrital zircons from this study).The laminated cap carbonate facies and it

associated negative 13C excursion are charac

teristic of Neoproterozoic glacials (Hoffman

and Schrag, 2002); in particular, either the Stur

tian or (especially) the Marinoan glacial epoch

(Shields, 2005). There is considerable debate

on the timing and extent of the Sturtian glacia

tion, with existing U-Pb zircon data clustering

between 750 and 685 Ma, and Re-Os ages o

end-Sturtian black shales as young as 643 Ma

(Kendall et al., 2006, and references therein)

Precise age constraints for the Marinoan gla

cial deposits include U-Pb zircon ages o635.51 0.54 Ma for the Upper Ghaub Forma

tion in Namibia (Hoffmann et al., 2004), and

635.23 0.57 Ma for the Doushanto Formation

in southern China (Condon et al., 2005). Given

that cap carbonates appear to be restricted to

the Sturtian and Marinoan glacials (Shields

2005), we consider the Chiquero Formation

and the pronounced negative C isotope excur

sion (8) in the San Juan Formation to repre

sent a Sturtian-Marinoan couplet. Although no

unequivocal glaciogenic strata have been identi

fied with the second negative C isotope excur

sion, it may correlate with the negative Trezon

anomaly, which immediately preceded the

Marinoan glaciation (Halverson et al., 2005). I

this Sturtian-Marinoan correlation is accepted

then the Chiquero and San Juan Formation

were deposited between ca. 750 635 Ma

Alternatively, if they represent the Marinoan

glacial and the Wonoka anomaly (Halverson

et al., 2005) associated with the Gaskiers glacia

tion, then a depositional age of ca. 635580 Ma

is inferred. In either case, a Late Neoproterozoi

age is highly probable.

Given that the detritus in the Chiquero Forma

tion (samples SJ-11 and SJ-16) does not match

the underlying basement (the northern domain othe AAB), an alternative source is required. Late

Neoproterozoicearly Palaeozoic Gondwanan

margin sequences in the Andes (e.g., sample

DC 5/5/4 from the Maraon Complex; Fig. 4A

yield prominent peaks in the range 1.30.9 Ga

with minimal older detritus from the Amazonian

craton (Chew et al., 2007). This closely matche

the observed age spectra (samples SJ-11 and

SJ-16) in the Chiquero Formation. Minor peak

that overlap the younger (830700 Ma) detri

tus in the San Juan Formation (sample SJ-57

1

2

3

4

1

2

3

4

5

6

7

8

9

10

400

500

600

700

800

900

1000

1100

1200

1300

1400

1500

1600

1700

1800

1900

2000

Age (Ma)

SJ-11, n = 35/46

SJ-16, n = 62/71

SJ-57, n = 55/66

DC 5-5-4,n = 54/102

Proterozoic

869 18 Ma

932 28 Ma

955 18 Ma

697 11 Ma

Probability

103

Frequency

c

2

4

6

8

10

12

14

16

0

1

2

3

4

5

6

7

8

9

1

2

3

4

5

6

7

8

1

2

3

4

5

6

7

9

0

1

2

3

4

5

6

7

8

910

10

1

2

3

4

5

6

Turbiditic sandstone

Chiquero Formation

Siltstone (tillite matrix)

Chiquero Formation

Turbiditic sandstone

Maraon Complex

Pebbly limestone,

San Juan Formation

Figure 4. Zircon probability density distri-bution diagrams from Chiquero Formation

(SJ-11, SJ-16), San Juan Formation (SJ-57)and previously published data from MaraonComplex (DC 554; Chew et al., 2007). Lightcurves represent all ages from each sample;darker curves represent ages that are > 90 %concordant. The youngest detrital zirconage in each sample is in a black box.


4/4

1098 GEOLOGY, December 2007

have also been identified in Gondwanan margin

sequences (Chew et al., 2007). Detritus of this

age is typically absent or very restricted in East-

ern Laurentian cover sequences (e.g., Cawood

et al., 2007) and rules out a peri-Laurentian affin-

ity for the San Juan Formation. This alone makes

a Laurentian affinity for the underlying Chi-

quero Formation less likely, and additionally,

Pb isotope data for the Chiquero Formation do

not imply a Laurentian source because the only

Pb isotopic match on Laurentia is the SouthernAppalachian basement, proposed to have been

a piece of Amazonia transferred to Laurentia at

ca. 1 Ga (Tohver et al., 2004). However, Lau-

rentia is not the only possible extracontinental

source of these sediments. Instead, the detrital

zircon results provide a convincing link between

the cover sequences of the AAB and the western

Gondwanan margin during the Late Neoprotero-

zoic, and indicate that accretion of the AAB

did not occur after Late Neoproterozoic time.

A Grenvillian-Sunsas age for AAB accretion is

therefore most likely (cf. Loewy et al., 2004), and

juxtaposition during the early Paleozoic Pampeanand Famatinian orogenies is discounted.

CONCLUSIONS

Detrital zircon populations in the cover

sequences of the Arequipa-Antofalla base-

ment, an exotic crustal block to Amazonia, are

likely derived from the proto-Andean margin.

These cover sequences (the Chiquero and San

Juan Formations in southern Peru) record the

only documented Neoproterozoic glacial epi-

sode in the Andean belt. Based on the presence

of a cap carbonate and two negative C isotope

excursions, these deposits probably represent a

Sturtian-Marinoan couplet (ca. 750635 Ma).

The strong link between the Arequipa-

Antofalla basement cover sequences and the

proto-Andean margin during the Late Neo-

proterozoic rules out accretion of the Arequipa-

Antofalla basement during the early Paleozoic

Pampean and Famatinian orogenies, and strongly

favors accretion to the Amazonian craton during

the 10001300 Ma Grenville-Sunsas orogeny.

ACKNOWLEDGMENTS

This study was funded by a grant of the SwissNational Science Foundation to Schaltegger. TheNordSIMS facility is operated under an agreement

between the research councils of Denmark, Norway,and Sweden, the Geological Survey of Finland, and theSwedish Museum of Natural History. We thank StaciLoewy, Victor Ramos and Carol Dehler for careful andinsightful reviews. This is NordSIMS Contribution 185.

REFERENCES CITED

Bellizzia, A., and Pimentel, N., 1994, TerrenoMrida: Un cinturn alctono Herciniano en laCordillera de Los Andes de Venezuela: V Sim-

posio Bolivarano Exploracin Petrolera en lasCuencas Subandinas, Memoria, p. 271299.

Caldas, J., 1978, Geologa de los Cuadrngulos de SanJuan, Acar y Yauca, Hojas: 31-m, 31-n, 32-n:Lima, Instituto de Geologa y Minera, 78 p.

Caldas, J., 1979, Evidencias de una glaciacin Pre-cambriana en la costa sur del Per, SegundoCongreso Geolgico Chileno, Volume J: Arica,p. 2937.

Cawood, P.A., Nemchin, A.A., Strachan, R.A.,Prave, T., and Krabbendam, M., 2007, Sedi-mentary basin and detrital zircon record along

East Laurentia and Baltica during assemblyand breakup of Rodinia: Geological Society[London] Journal, v. 167, p. 257275.

Chew, D.M., Schaltegger, U., Koler, J., White-house, M.J., Gutjahr, M., Spikings, R.A., andMikovic, A., 2007, U-Pb geochronologicevidence for the evolution of the Gondwananmargin of the north-central Andes: GeologicalSociety of America Bulletin, v. 119, no. 56,p. 697711, doi: 10.1130/B26080.1.

Condon, D., Zhu, M.Y., Bowring, S., Wang, W.,Yang, A.H., and Jin, Y.G., 2005, U-Pb agesfrom the Neoproterozoic Doushantuo Forma-tion, China: Science, v. 308, p. 9598, doi:10.1126/science.1107765.

Cordani, U.G., Sato, K., Teixeira, W., Tassinari,C.G., and Basei, M.A.S., 2000, Crustal evolu-tion of the South American Platform, inCor-dani, U.G., Milani, E.J., Thomaz-Filho, A.,and Campos, D.A., eds., Tectonic Evolution ofSouth America: 31st International GeologicalCongress, Rio de Janeiro, Brazil, p. 1940.

Dalziel, I.W.D., 1994, Precambrian Scotland as aLaurentia-Gondwana LinkOrigin and Sig-nificance of Cratonic Promontories: Geology,v. 22, p. 589592, doi: 10.1130/00917613-(1994)0222.3.CO;2.

Halverson, G.P., Hoffman, P.F., Schrag, D.P., Maloof,A.C., and Rice, A.H.N., 2005, Toward a Neo-proterozoic composite carbon-isotope record:Geological Society of America Bulletin, v. 117,p. 11811207, doi: 10.1130/B25630.1.

Hoffmann, K.H., Condon, D.J., Bowring, S.A., and

Crowley, J.L., 2004, U-Pb zircon date from theNeoproterozoic Ghaub Formation, Namibia:Constraints on Marinoan glaciation: Geology,v. 32, p. 817820, doi: 10.1130/G20519.1.

Hoffman, P.F., 1991, Did the breakout of Lauren-tia turn Gondwanaland inside-out?: Science,v. 252, p. 891901.

Hoffman, P.F., and Schrag, D.P., 2002, The snowballEarth hypothesis: Testing the limits of globalchange: Terra Nova, v. 14, p. 129155, doi:10.1046/j.13653121.2002.00408.x.

Kendall, B., Creaser, R.A., and Selby, D., 2006,Re-Os geochronology of postglacial blackshales in Australia: Constraints on the timing ofSturtian glaciation: Geology, v. 34, p. 729732, doi: 10.1130/G22775.1.

Kirkland, C.L., Daly, J.S., and Whitehouse, M.J., 2007,Provenance and terrane evolution of the KalakNappe Complex, Norwegian Caledonides: Impli-cations for Neoproterozoic palaeogeography andtectonics: Journal of Geology, v. 115, p. 2141,doi: 10.1086/509247.

Loewy, S.L., Connelly, J.N., Dalziel, I.W.D., andGower, C.F., 2003, Eastern Laurentia in Rodinia:Constraints from whole-rock Pb and U/Pb geo-chronology: Tectonophysics, v. 375, p. 169197,doi: 10.1016/S00401951(03)00338-X.

Loewy, S.L., Connelly, J.N., and Dalziel, I.W.D.,2004, An orphaned basement block: TheArequipa-Antofalla basement of the centralAndean margin of South America: GeologicalSociety of America Bulletin, v. 116, p. 171187, doi: 10.1130/B25226.1.

Pankhurst, R.J., Rapela, C.W., and Fanning, C.M.,2000, Age and origin of coeval TTG, I- andS-type granites in the Famatinian belt of NWArgentina: Royal Society of Edinburgh Trans-actions, Earth Sciences, v. 91, p. 151168.

Pankhurst, R.J., Rapela, C.W., Fanning, C.M., and

Marquez, M., 2006, Gondwanide continentalcollision and the origin of Patagonia: Earth-Science Reviews, v. 76, p. 235257, doi:10.1016/j.earscirev.2006.02.001.

Ramos, V.A., 1988, Late Proterozoic-Early Paleo-zoic of South-AmericaA Collisional His-tory: Episodes, v. 11, p. 168174.

Ramos, V.A., 2004, Cuyania, an exotic block toGondwana: Review of a historical success andthe present problems: Gondwana Research,v. 7, p. 10091026.

Ramos, V.A., and Aleman, A., 2000, Tectonic evolu-tion of the Andes, inCordani, U., Milani, E.J.,Thomaz Filho, A., and Campos Neto, M.C.,eds., Tectonic Evolution of South America:31st International Geological Congress, Rio deJaneiro, Brazil, p. 635685.

Restrepo-Pace, P.A., Ruiz, J., Gehrels, G., and Cosca,M., 1997, Geochronology and Nd isotopicdata of Grenville-age rocks in the ColombianAndes: New constraints for late Proterozoicearly Paleozoic paleocontinental reconstruc-tions of the Americas: Earth and Planetary Sci-ence Letters, v. 150, p. 427441, doi: 10.1016/S0012821X(97)000915.

Shackleton, R.M., Ries, A.C., Coward, M.P., andCobbold, P.R., 1979, Structure, metamorphismand geochronology of the Arequipa Massifof coastal Peru: Geological Society [London]Journal, v. 136, p. 195214.

Shields, G.A., 2005, Neoproterozoic cap carbonates:A critical appraisal of existing models and theplumeworld hypothesis: Terra Nova, v. 17,

p. 299310, doi: 10.1111/j.13653121.2005.00638.x.

Tohver, E., Bettencourt, J.S., Tosdal, R., Mezger, K.,Leite, W.B., and Payolla, B.L., 2004, Terranetransfer during the Grenville orogeny: Tracingthe Amazonian ancestry of southern Appala-chian basement through Pb and Nd isotopes:Earth and Planetary Science Letters, v. 228,p. 161176.

Tosdal, R.M., 1996, The Amazon-Laurentian con-nection as viewed from the Middle Proterozoicrocks in the central Andes, western Bolivia, andnorthern Chile: Tectonics, v. 15, p. 827842,doi: 10.1029/95TC03248.

Wasteneys, H.A., Clark, A.H., Farrar, E., andLangridge, R.J., 1995, Grenvillian granulite-facies metamorphism in the Arequipa Massif,PeruA Laurentia-Gondwana link: Earth andPlanetary Science Letters, v. 132, p. 6373,doi: 10.1016/0012821X(95)00055-H.

Manuscript received 1 February 2007Revised manuscript received 20 July 2007Manuscript accepted 23 July 2007

Printed in USA