U–Pb geochronology, geochemistry, and provenance of the Grenvillian Huiznopala Gneiss of Eastern...

Precambrian Research 94 (1999) 73–99

U–Pb geochronology, geochemistry, and provenance of theGrenvillian Huiznopala Gneiss of Eastern Mexico

P.J. Lawlor a, F. Ortega-Gutierrez b, K.L. Cameron a,*, H. Ochoa-Camarillo b,R. Lopez a, D.E. Sampson a

a Department of Earth Sciences, University of California, Santa Cruz, CA 95064, USAb Departamento de Geologia, Instituto de Geologia, UNAM, 04510 Mexico, D.F., Mexico

Received 15 December 1997; accepted 29 September 1998

Abstract

The Huiznopala Gneiss is the smallest (~25 km2) of four exposures of Grenvillian granulites in eastern andsouthern Mexico. The Gneiss comprises three major lithologic units: (1) a main series of orthogneisses; (2) ananorthosite–gabbro complex; and (3) a layered paragneiss sequence. Thirty-one U–Pb zircon ages from seven samplesall lie within a restricted range of ~200 million years, and the geochronological results are interpreted to reflectprincipally a two-stage history. The earliest stage was arc magmatism that extended from ~1200 to ~1150 Ma. Thesecond stage included granulite facies metamorphism, which peaked at about 725±50°C and 7.2±1.0 kbar, andperhaps emplacement of the anorthosite–gabbro complex at ~1000 Ma. A pegmatite with an age of 988±3 Ma post-dates ductile deformation. The timing of ductile deformation and granulite facies metamorphism of the HuiznopalaGneiss corresponds closely to that of final thrusting and deformation in the Grenville Province. Thus, the HuiznopalaGneiss may be in the core of the orogen formed during the final thrusting event recognized in the Grenville orogeny.

Gondwanan faunal assemblages are found in lower Paleozoic sedimentary rocks that overlie Grenvillian granuliteselsewhere in Mexico. However, the Pb isotope compositions of samples from the Huiznopala Gneiss overlap withthose of the basement from Laurentia (Adirondack Highlands and Lowlands, Texas), and they are distinct fromthose of the Gondwanan Arequipa–Antofalla craton of southern Bolivia and northern Chile. The magmas parentalto the Huiznopala arc and those parental to the Laurentian crust may have shared a common isotope reservoir, andthe Pb isotope compositions are consistent with the paleomagnetic results that place southern Mexico adjacent toOntario/Quebec at about 950 Ma. © 1999 Elsevier Science B.V. All rights reserved.

Keywords: Granulites; Grenville orogeny; Mesoproterozic; Mexico; U–Pb geochronology

1. Introduction Rodinia (Moores, 1991; Hoffman, 1991; Dalziel,1994; Windley, 1995; Rogers, 1996). Rodinia was

The worldwide Grenville orogeny, which culmi- fragmented by about 0.7 Ga (Hoffman, 1991), andnated at ~1.0 Ga, involved the collision of many segments of the formally continuous Grenvilliancontinental blocks to form the supercontinent of orogenic belt are now found attached to and

embedded within continents scattered around the* Corresponding author. E-mail: [email protected] globe. One segment of the orogen forms the base-

0301-9268/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved.PII S0301-9268 ( 98 ) 00108-9

74 P.J. Lawlor et al. / Precambrian Research 94 (1999) 73–99

ment of eastern and southern Mexico. Ortega- Although these outcrops are separated by as muchas 700 km, they have a number of characteristicsGutierrez et al. (1995) interpreted this basement

to be a portion of a microcontinent they termed in common, including pervasive granulite faciesmetamorphism, abundant anorthosite complexes,‘Oaxaquia’ (wa ha∞ ke a). This microcontinent,

which was accreted to Laurentia in the late and a northwestern trending structural grain(Ortega-Gutierrez et al., 1995).Paleozoic, is inferred to extend from the Ouachita

suture south for ~2000 km into Central America This paper addresses a series of fundamentalquestions regarding the formation, deformation,(Fig. 1). It underlies an area of approxim-

ately 1×106 km2, four times the size of the and provenance of the Grenvillian HuiznopalaGneiss, which is exposed near the town of MolangoSveconorwegian orogen of southern Scandinavia,

and thus is a major segment of the worldwide in the state of Hidalgo (Figs. 1 and 2). Thesequestions include the following: (1) when didGrenvillian orogenic belt. Given the large inferred

size of Oaxaquia, it is a vital piece in any recon- igneous protoliths crystallize; (2) what was thetectonic environment of crust formation; (3) whatstruction of Rodinia, and it should shed new light

on the worldwide Grenville orogeny. The micro- was the age of granulite facies metamorphism; (4)what were the pressure–temperature conditionscontinent is exposed in four areas of eastern

and southern Mexico (Fig. 1, Ciudad Victoria, near the peak of metamorphism; and (5) what wasthe provenance of the Huiznopala Gneiss withMolango, Oaxacan Complex, La Mixtequita); else-

where in Mexico it is concealed beneath a blanket respect to Laurentia and Gondwana? Althoughthe Huiznopala Gneiss has the smallest outcropof Phanerozoic sedimentary and volcanic rocks.

Fig. 1. Location map. The four areas where Grenvillian granulites are exposed in eastern and southern Mexico are Ciudad Victoria,Molango, the Oaxacan Complex, and La Mixtequita.

75P.J. Lawlor et al. / Precambrian Research 94 (1999) 73–99

Fig. 2. Geological map of the Proterozoic basement rocks, the Huiznopala Gneiss, of the Molango area (Ochoa-Camarillo, 1996).Sample numbers for geochronology samples are underscored. Prefix not shown for the PJL samples. See Fig. 1 for the locationof Molango.

area (~25 km2) of the four Grenvillian localities Huiznopala Gneiss is exposed beneath a cover ofPermian and Early Jurassic volcanic and sedi-of eastern and southern Mexico, this report is of

special significance because it is the first published mentary rocks in six separate areas (Fig. 2). Eachexposure consists of a dominant rock type orstudy to focus on the U–Pb geochronology of the

Grenvillian rocks of Mexico, as well as the first to association. Paragneiss forms the largest exposure,and these rocks are best exposed along the Aguaintegrate geochronological, geochemical, thermo-

barometric, and Nd–Pb isotopic results. Salada river west of the village of Huiznopala(Fig. 2). This gneissic sequence is characterized by1–2 m thick strata which are banded on a scale ofmillimeters to centimeters. Common paragneiss2. Geology and structuretypes include feldspathic, light gray, garnetiferousgneisses, and calc-silicate gneisses that grade intoRegional mapping in the Molango district by

Ochoa-Camarillo (1996) established that the silicic marbles rich in clinopyroxene. Along the


Rio Claro (Fig. 2), the paragneiss has been exam- in nature. Although some lithologies, especiallythe paragneisses, are conspicuously banded, foldined only at the southern edge of the exposure,

but here too the rocks are mainly impure marbles closures are rarely observed. The lithologic band-ing in the Agua Salada and Rio Claro sections isand graphite-rich calc-silicate gneisses.

Andesine meta-anorthosites and associated subhorizontal, with rare fold hinges plunging NW.An intense cataclastic fabric is superimposed onmafic and leucocratic gabbroic gneisses crop out

northwest of the village of Chipoco (Fig. 2). the entire Huiznopala Gneiss. This fabric is olderthan the lower Jurassic unconformity, and appa-Although fresh garnet is found in the more Fe-rich

rocks, the mafic minerals in most specimens have rently was the main cause of the penetrative retro-gression of the mafic minerals. Overall, thebeen replaced by retrograde chlorite, amphibole,

and calcite. The two principal rock types exposed foliation trends NW and dips 0–45° E. Poles tofoliation (Fig. 3) define a great circle at 27° 45∞ E,southeast of the town of Otongo are metatonalite

and charnockite, and these two types appeared to with a pole at 297/45 indicating a secondary fold-ing of the foliation along regional folds trendinggrade into one another. Mafic granulites are also

present, but are less common. The only clear WNW with a rather steep plunge. Lineations,although inconspicuous and therefore poorly mea-contact between different rock types was found in

the Pilapa River, where a band of garnetiferous sured, trend roughly NW.paragneiss and quartzite, about 100 m thick,occurs within a section of orthogneiss. The con-tact relationships between the paragneiss and 3. Geochemistryorthogneiss are sharp and locally concordant. Theremaining three small exposures of Huiznopala 3.1. Major and trace elementsGneiss, all east of Highway 105, are garnet-bearinggranitic gneiss and metatonalite–charnockite Thirteen orthogneisses and one paragneiss were

analyzed for major elements, 25 trace elements,(Fig. 2).Thus far, the structural studies of the and Nd–Pb isotopes (see Fig. 2 for sample loca-

tions). We have divided the metaigneous rocks ofHuiznopala Gneiss have only been reconnaissancethe Huiznopala Gneiss into two geochemicalgroups, the ‘main series’ and the gabbro–anortho-site series. The latter is restricted to the exposurenorthwest of Chipoco (Fig. 2), whereas the formerincludes all the remaining orthogneisses.

The main series is interpreted to represent arcmagmatism based on its broad range in SiO2contents [54 to 78 wt%, Fig. 4(A)] and its traceelement characteristics. The trace element discrimi-nation diagrams of Pearce et al. (1984) havecommonly been used in Grenvillian studies todistinguish volcanic arc from within plate granites(e.g. McLelland and Chiarenzelli, 1990; Goweret al., 1991; Dickin and Higgins, 1992; Higginsand van Breemen, 1996), and the main series rocksdo plot within the field of volcanic arc granites onthe Nb–Y discrimination diagram (Fig. 5). We,however, do not consider this the best discrimina-tion diagram in general for arc rocks for tworeasons. First, it is not applicable to mafic rocks,Fig. 3. Equal area great circle defined by poles on foliation

planes measured in the Chipoco and Agua de Sal River. which are petrogenetically important in many


Fig. 5. Nb–Y discrimination diagram of Pearce et al. (1984).Data points are the Huiznopala main series. VGA=volcanicarc granitoids, WPG=within-plate granitoids, ORG=ocean-ridge granitoids.

entiation processes, whereas ratios of elements thathave similar bulk distribution coefficients, especi-ally strongly incompatible elements, remain nearlyconstant during differentiation. Niobium and La,a light rare earth element (REE), are usuallystrongly incompatible elements during igneousprocesses, and both are relatively immobile duringgranulite facies metamorphism, provided that therocks do not experience melting (see below). TheLa/Nb ratio is a measure of depletion of Nbrelative to large ion lithophile (LIL) elements suchas La, and this depletion (e.g. ‘negative Nb anoma-lies’ on spidergrams) is the most characteristictrace element feature of volcanic arc rocks (e.g.Pearce, 1983). Most of the main series samplesFig. 4. Silica variation diagrams for the Huiznopala Gneiss.have La/Nb ratios >1.5, and thus have a clearSample numbers are shown for the main series samples

(squares) without the PJL prefix. Chipoco anorthosite–gabbro arc signature. Three samples (PJL8, 10, 12),complex (+). Field of Adirondack metatonalites from referred to as the ‘H-type’ of the main series, haveMcLelland and Chiarenzelli (1991). La/Nb between 1.0 and 1.5 [Fig. 4(B)] and, as

discussed below, they also share other distinctivetrace element characteristics. The mafic to interme-modern arcs. Second, it is based on trace element

concentrations rather than ratios. Trace element diate composition main series samples generallylie in the ‘medium-K’ field of orogenic volcanicconcentrations can be strongly affected by differ-

78 P.J. Lawlor et al. / Precambrian Research 94 (1999) 73–99T

able

1M

ajor

elem

ent,

trac

eel

emen

t,an

dis

otop

eco

mpo

siti

ons

ofth

eH

uizn

opal

aG

neis

s

Ano

rtho

site

–Gab

bro

Seri

esM

ain

Seri

esP

arag

neis

s

PJL

1P

JL4

PJL

5P

JL6

PJL

1441

96P

JL13

PJL

10P

JL11

PJL

7P

JL12

PJL

8P

JL3

PJL

9P

JL2

SiO2

54.2

954

.96

55.1

255

.82

37.4

548

.06

51.6

252

.70

56.1

562

.16

62.4

666

.26

70.7

377

.28

88.9

6T

iO2

0.55

0.22

0.15

0.34

8.70

2.25

0.84

1.68

0.42

0.68

1.01

0.82

0.65

0.07

0.11

Al 2

O3

17.6

925

.57

26.8

921

.76

9.23

17.8

817

.24

17.2

813

.84

15.9

417

.23

15.2

713

.79

12.4

22.

01F

eO*

6.75

2.12

1.22

4.13

23.7

610

.01

9.94

12.9

69.

096.

416.

105.

964.

230.

591.

94M

nO0.

110.

040.

030.

080.

400.

190.

150.

220.

160.

090.

080.

120.

080.

010.

05M

gO4.

861.

961.

323.

083.

182.

285.

072.

987.

722.

991.

881.

760.

660.

183.

15C

aO11

.36

9.34

7.48

8.59

13.1

414

.84

9.50

7.17

8.08

6.18

4.18

3.62

1.30

1.12

3.06

Na 2

O3.

625.

245.

584.

800.

293.

374.

444.

093.

593.

315.

833.

922.

693.

480.

15K2O

0.74

0.51

2.18

1.36

0.00

0.66

1.01

0.51

0.85

2.09

1.02

2.10

5.66

4.83

0.50

P2O

50.

040.

040.

030.

053.

860.

450.

190.

410.

120.

150.

220.

190.

210.

020.

06L

OIa

1.99

2.16

3.84

2.66

4.25

7.90

2.54

0.12

1.52

1.90

1.82

2.08

1.40

1.14

2.72

Tot

ala

100.

4610

0.01

99.2

297

.64

100.

0410

1.69

99.8

510

0.68

100.

3910

1.20

99.9

610

1.51

100.

0410

1.06

100.

47

Mg#

58.8

64.7

68.2

59.6

21.0

31.1

50.3

31.3

62.7

48.0

37.9

36.9

23.6

37.7

76.3

Sc(p

pm)

214

110

4522

3027

2819

1613

71

3R

b5

525

103

910

315

248

2395

619

Sr60

470

812

6676

316

838

558

560

948

447

371

937

522

416

917

Zr

3614

924

138

5710

880

951

142

436

291

453

3630

Nb

21

12

4913

617

57

1610

112

4Y

153

28

104

3826

2931

1924

1729

17

Ba

311

198

652

380

723

823

628

233

890

743

213

9910

8512

2839

La

4.18

2.42

1.54

3.13

72.2

627

.57

10.9

819

.99

13.6

215

.95

15.0

512

.16

24.5

83.

043.

07C

e9.

814.

272.

506.

3617

6.6

63.5

726

.70

44.8

333

.32

33.7

134

.39

25.5

351

.45

2.58

8.25

Nd

7.72

2.23

1.76

4.40

120.

338

.16

18.6

130

.16

24.1

820

.28

22.3

215

.83

29.4

01.

034.

41P

r1.

560.

570.

420.

9425

.65

8.67

3.95

6.48

5.00

4.55

4.82

3.46

6.87

0.28

1.03

Sm2.

230.

460.

391.

1425

.98

8.46

4.59

6.66

6.41

4.40

5.02

3.58

6.24

0.19

1.04

Eu

1.06

0.74

0.58

1.01

3.66

1.99

1.74

2.65

0.86

1.40

2.16

2.11

2.76

0.74

0.16

Gd

2.23

0.47

0.50

1.19

22.7

66.

984.

145.

775.

403.

744.

323.

385.

520.

260.

95T

b0.

384

0.06

60.

052

0.19

53.

193

1.01

10.

652

0.85

70.

855

0.54

20.

629

0.50

60.

851

0.02

40.

158

Dy

2.46

0.42

0.32

1.24

18.4

26.

024.

095.

145.

273.

203.

753.

105.

120.

141.

00H

o0.

506

0.08

40.

063

0.25

73.

551.

210.

851

1.04

1.04

0.62

50.

757

0.63

51.

004

0.02

80.

204

Er

1.41

0.24

0.18

0.71

9.37

3.33

2.41

2.94

2.91

1.73

2.08

1.76

2.78

0.08

0.58

Tm

0.20

30.

036

0.02

40.

100

1.23

0.47

50.

342

0.42

50.

421

0.24

50.

295

0.24

20.

392

0.01

30.

088

Yb

1.29

0.24

0.16

0.64

7.58

3.10

2.20

2.88

2.73

1.59

1.94

1.64

2.50

0.08

0.58

Lu

0.21

50.

040

0.02

60.

104

1.22

0.51

00.

368

0.51

60.

452

0.26

20.

336

0.28

70.

405

0.01

50.

096

Hf

1.42

0.84

0.56

1.05

3.06

1.50

2.98

18.6

42.

175.

2211

.43

8.15

7.78

1.75

1.64

Pb

0.19

0.11

0.07

0.16

0.54

0.42

0.44

0.27

0.53

0.58

0.34

0.40

0.94

0.37

0.32

Th

0.22

0.10

0.04

0.06

0.63

1.71

0.27

0.17

0.12

0.19

0.07

0.15

0.90

0.02

0.05

U0.

049

0.04

70.

016

0.03

00.

230.

470.

1305

0.33

40.

068

0.12

60.

192

0.31

10.

384

0.03

10.

169


Tab

le1

(con

tinu

ed)

Maj

orel

emen

t,tr

ace

elem

ent,

and

isot

ope

com

posi

tion

sof

the

Hui

znop

ala

Gne

iss

Ano

rtho

site

–Gab

bro

Seri

esM

ain

Seri

esP

arag

neis

s

PJL

1P

JL4

PJL

5P

JL6

PJL

1441

96P

JL13

PJL

10P

JL11

PJL

7P

JL12

PJL

8P

JL3

PJL

9P

JL2

147S

m/144

Ndb

0.17

30.

125

0.11

40.

157

0.13

10.

134

0.14

70.

133

0.16

20.

132

0.13

60.

136

0.12

60.

108

0.13

6143N

d/144N

d0.

5125

670.

5122

080.

5121

680.

5124

260.

5122

730.

5122

220.

5124

050.

5122

500.

5125

020.

5122

230.

5122

630.

5122

560.

5122

180.

5120

140.

5122

14ENd

(0)

−1.

38−

8.39

−9.

16−

4.14

−7.

12−

8.11

−4.

55−

7.57

−2.

65−

8.10

−7.

32−

7.45

−8.

19−

12.1

7−

8.27

ENd

(T)

1.67

0.83

1.36

0.98

1.26

1.47

3.18

2.07

2.65

1.84

1.96

1.86

1.93

1.32

1.01

TDM

(Ga)

1.69

1.42

1.34

1.61

1.41

1.55

1.43

1.48

1.56

1.51

1.51

1.52

1.42

1.47

1.60

206P

b/204P

b-A

L17

.325

17.1

4417

.324

17.2

3217

.565

17.6

5217

.271

17.3

8216

.907

16.9

6717

.207

17.3

4617

.531

16.9

3918

.923

207P

b/204P

b-A

L15

.476

15.4

7515

.481

15.4

7415

.494

15.5

1915

.463

15.4

7815

.440

15.4

2315

.463

15.4

7815

.509

15.4

3815

.633

208P

b/204P

b−A

L36

.593

36.4

8436

.695

36.5

7037

.066

37.0

9736

.476

36.3

0236

.159

36.2

1136

.306

36.3

5836

.927

36.2

5937

.535

206P

b/204P

b-N

AL

17.3

8117

.291

17.4

6517

.450

17.7

9218

.468

17.2

9617

.724

16.9

6416

.940

17.3

4317

.603

17.7

5917

.016

19.0

08207P

b/204P

b-N

AL

15.5

0115

.473

15.5

1515

.504

15.5

2615

.587

15.4

7515

.512

15.4

5515

.436

15.4

8015

.517

15.5

3415

.463

15.6

57208P

b/204P

b-N

AL

36.6

8936

.569

36.9

0136

.843

37.0

9337

.876

36.4

6636

.436

36.2

2136

.253

36.3

7736

.494

37.1

9836

.394

37.9

96206P

b/204P

b-N

AL

15.4

0714

.145

15.7

2616

.010

14.5

949.

795

15.0

708.

667

16.0

1415

.331

13.1

9411

.866

14.6

6816

.403

14.8

79(i

nit)

207P

b/204P

b-N

AL

15.3

4315

.222

15.3

7615

.388

15.2

7014

.892

15.2

9714

.787

15.3

7915

.307

15.1

4815

.057

15.2

8615

.414

15.3

27(i

nit)

208P

b/204P

b-N

AL

34.0

5734

.594

35.5

2736

.019

34.4

6428

.403

35.0

6435

.024

35.7

0835

.530

35.9

0835

.667

35.0

0736

.281

37.6

06(i

nit)

206P

b/204P

b-F

EL

D—

——

17.0

90—

——

17.3

06—

——

17.2

5117

.361

16.9

33—

207P

b/204P

b-F

EL

D—

——

15.4

65—

——

15.4

85—

——

15.4

9415

.498

15.4

42—

208P

b/204P

b-F

EL

D—

——

36.4

11—

——

36.3

09—

——

36.4

2036

.755

36.2

66—

Maj

orel

emen

tsno

rmal

ized

to10

0%an

hydo

us.

aPre

norm

aliz

edva

lues

.F

eO*=

Tot

alF

eas

FeO

.M

g#=

atom

icM

gO/(

MgO

+0.

9FeO*).

Maj

orel

emen

tsan

dZ

r,Sr

,R

b,Y

,an

dN

ban

alyz

edby

XR

F.

Oth

ertr

ace

elem

ents

byIC

PM

S.b

Rat

ioba

sed

onID

TIM

San

alys

es.

AL=

acid

-lea

ched

,N

AL=

non-

acid

-lea

ched

.F

elds

par

sepa

rate

s:P

JL3,

9=K

-fel

dspa

r;P

JL6,

8,10

=pl

agio

clas

e.


rocks, but the two silicic samples are much more (Appendix 1) yet has a strong negative slope toheavy REE pattern [Fig. 6(B)]. Thus, there is nopotassic [Fig. 4(A)].

The REE have proven to be useful in identifying evidence that any of the analyzed Huiznopalasamples experienced melt-extraction to such apetrologic processes by which some orthogneisses

formed (Fig. 6). A principal advantage of using degree that the REE patterns were stronglymodified.the REE to interpret the petrogenesis rocks such

as the Huiznopala Gneiss is that the REE are little Four of the five analyzed samples from Chipocogabbro–anorthosite complex have moderate tomodified by granulite facies metamorphism, pro-

vided the rocks have escaped melting (e.g. Weaver, large positive Eu anomalies [Fig. 6(A)] typical ofplagioclase-rich cumulate rocks (e.g. Ashwal,1980). Most of the Huiznopala samples contain

garnet, except for those Chipoco gabbros and 1993). The fifth Chipoco sample (PJL14) has anunusually low silica content and very high concen-anorthosites with relatively high Mg# values. The

REE patterns of garnet-bearing restites have dis- trations of TiO2 and FeO [Table 1, Fig. 6(B)], andits protolith is interpreted to have been a gabbroictinctive shapes with positive slopes in the heavy

REE from Gd to Lu (Cameron et al., 1992; Scherer rock rich in cumulate Fe–Ti oxide mineral(s).The most primitive of the Rio Pilapa main serieset al., 1998). None of the Huiznopala samples

have restite-like REE patterns. This is emphasized samples is PJL13. Its Mg# value (50; Table 1) istoo low for it to be an unfractionated, mantleby PJL14, which contains about 50% modal garnet

Fig. 6. REE diagrams for samples of the Huiznopala Gneiss.


Fig. 7. Sm–Nd isochron diagram. Main series samples (squares). Chipoco anorthosite–gabbro complex (+). Paragneisses (soliddiamonds). The plot includes data from Ruiz et al. (1988).

derived melt, which is required to have a Mg# upward; and (3) Eu anomalies that become morepositive. The Eu anomaly will remain constant invalue >65 to be in equilibrium with mantle perido-

tite. However, its primitiveness compared with the size if amphibole and plagioclase are removed inratios of about 3:1. The spoon-shaped REE pat-other arc suite samples it demonstrated by a combi-

nation of low SiO2 content, relatively high Mg# terns of the ‘H-type’ of the main series stronglysuggest that these rocks evolved by fractionationvalue compared with most other main series

samples, and relatively high concentration of Sc, of amphibole [Fig. 6(C)]. However, within thisgroup the concentrations of all the REE, even thea compatible element in basaltic melts (Table 1).

Most of the other Rio Pilapa main series samples light REE, decrease with increasing silica[Fig. 6(C)]. This cannot be explained by fraction-can be interpreted qualitatively as having evolved

from a parental melt like PJL13 by fractionation ation of amphibole from one line of liquid descent,because in that case the REE patterns would crossof plagioclase, pyroxene, and/or amphibole. With

fractionation of plagioclase plus pyroxene, the in the light REE. Nevertheless, the REE patternsof these rocks are so similar that it is strongconcentrations of all the REE except Eu increase

and the depth of the negative Eu anomaly grows. evidence that the rocks are consanguineous; thatis, the protoliths formed by similar processes fromAmong the main series samples, only PJL11 and

4196 have negative Eu anomalies indicating frac- similar parental magmas.Surprisingly, the two silicic samples analyzedtionation of plagioclase, probably accompanied by

pyroxene [Fig. 6(B)]. both have positive Eu anomalies [Fig. 6(D)]. Thegranitic gneiss, PJL3, probably contains a signifi-Amphibole fractionation results in the following:

(1) REE patterns that cross with concentrations cant component of cumulate feldspar. The protol-ith of PJL9, the only analyzed charnockite, wasof the middle and heavy REE decreasing and light

REE increasing; (2) heavy REE (Gd to Lu) pat- composed mostly of cumulate K-spar, and thusthis sample is probably not representative of theterns that become spoon-shaped and concave


Tab

le2

U–P

bzi

rcon

resu

lts

from

the

Hui

znop

ala

Gne

iss,

Hid

algo

,M

exic

o

Sam

ple

Fra

ctio

nde

scri

ptio

nW

eigh

tT

otal

Tot

alT

h/U

Tot

alco

m.206P

b/208P

b/206P

b/207P

b/207P

b/A

ge(M

a)P

erce

ntnu

mbe

r(m

g)P

b(p

pm)

U(p

pm)

Pb

(pg)

204P

ba206P

bb238U

c235U

c206P

bcdi

scor

danc

e206P

b/207P

b/207P

b/238U

235U

206P

b

PJL

3G

rani

tic

Gne

iss

Apr

(4:1

),eu

,(1

5)

9214

710.

4967

982

0.14

00.

1850

41.

9560

40.

0766

610

9511

0111

12±

61.

5%B

pr(3

:1),

eu,

(13)

111

2010

30.

3625

3792

0.10

40.

1890

22.

0130

10.

0772

411

1611

2011

27±

21.

0%C

pr(3

:1),

eu,

(22)

4315

770.

403

1744

0.11

30.

1878

81.

9831

50.

0765

611

1011

1011

10±

50.

0%D

pr(4

:1),

eu,

(23)

43d

1573

0.44

134

930.

127

0.19

584

2.11

822

0.07

845

1153

1155

1158±

40.

4%E

pr(5

:1),

eu,

(27)

50d

3014

90.

458

3402

0.12

80.

1943

62.

0967

80.

0782

411

4511

4811

53±

20.

7%re

sidu

e65

502

0.10

60.

1955

32.

1069

20.

0781

511

5111

5111

51±

10.

0%

PJL

6A

nort

hosi

tic

Gab

bro

Aeq

toel

(1.5

:1),

sub,

mf,

nm,

(9)

3527

146

0.39

3213

560.

112

0.17

763

1.83

094

0.07

476

1054

1057

1062±

50.

8%B

eq,

an,

rd,

70–1

00m

u,nm

,(6

)24

1589

0.32

317

780.

092

0.16

868

1.69

164

0.07

274

1005

1005

1007±

30.

2%E

eqto

el(1

.5:1

),su

bh,

mf,

9328

166

0.30

411

479

0.08

70.

1708

11.

7325

20.

0735

610

1710

2110

30±

31.

3%70

–100

mu,

nm,

unab

r,(4

5)

PJL

8M

etat

onal

ite

Bpr

(4:1

),su

b,rd

,nm

,(1

5)50

3517

90.

2815

2809

0.08

20.

1918

72.

0627

60.

0779

811

3211

3711

46±

21.

2%C

pr(3

:1),

sub,

rd,n

m,

(10)

120

2312

20.

246

6815

0.07

00.

1908

42.

0467

10.

0777

811

2611

3111

41±

11.

3%D

eq,

mf,

rd,n

m,

(14)

7617

820.

258

2799

0.07

00.

2049

72.

2674

60.

0802

312

0212

0212

03±

10.

1%E

pr(4

:1),

sub,

rd,n

m,

unab

r,(1

2)12

150

262

0.25

234

800

0.07

20.

1925

62.

0783

70.

0782

811

3511

4211

54±

11.

6%F

eqto

o,m

f,nm

,un

abr,

(10)

9111

610.

185

4484

0.05

00.

1914

32.

0461

30.

0775

211

2911

3111

35±

10.

5%H

pr(4

:1),

sub,

rd,

(4)

15d

2211

80.

3311

975

0.09

40.

1860

71.

9689

50.

0767

511

0111

0511

15±

41.

3%J

eqto

o,m

f,(2

)19d

1690

0.20

810

910.

055

0.18

364

1.92

089

0.07

586

1087

1088

1091±

40.

4%


PJL

9C

harn

ocki

teB

pr(4

:1),

sub,

(9)

1549

244

0.36

1418

680.

103

0.19

689

2.13

247

0.07

855

1159

1159

1161±

40.

2%E

eqto

el(1

.5:1

),su

b,m

f,nm

,(2

7)70d

1466

0.46

524

710.

130

0.20

366

2.24

480

0.07

994

1195

1195

1196±

60.

1%F

pr(4

:1),

sub,

rd,

(31)

60d

2612

50.

412

7140

0.11

60.

2013

52.

2227

40.

0800

611

8311

8811

99±

21.

3%G

eqto

el(1

.5:1

),su

b,m

f,nm

,(5

)65

1988

0.48

4114

130.

136

0.20

263

2.23

249

0.07

991

1189

1191

1195±

30.

5%J

eqto

el(1

.5:1

),su

b,m

f,nm

,(5

)69

3717

00.

4899

1361

0.13

40.

2023

92.

2142

10.

0793

511

8811

8611

81±

4−

0.6%

PJL

10M

afic

Gra

nulit

eB

pr(2

:1),

sub,

unab

r,(7

)36

2110

00.

5997

4186

0.16

40.

1861

51.

9821

60.

0772

311

0111

1011

27±

142.

3%C

eqto

o,m

f96

2312

00.

3133

4196

0.08

90.

1933

82.

0884

00.

0783

311

4011

4511

55±

31.

3%

4196

Gar

net

Gra

nulit

e1A

eqto

el(3

:1),

an,

rd,n

m22

040

206

0.58

111

5773

0.16

60.

1824

91.

9054

30.

0757

310

8110

8310

88±

10.

6%1B

eqto

el(3

:1),

an,

rd,n

m88

601

3145

0.52

4141

618

0.14

80.

1813

21.

8921

80.

0756

910

7410

7810

87±

81.

2%2A

eq,

an,

rd,n

m77

398

2071

0.60

4422

459

0.17

40.

1787

51.

8564

00.

0753

210

6010

6610

77±

91.

6%2B

eq,

an,

rd,n

m45

141

756

0.46

3358

570.

136

0.17

827

1.83

648

0.07

472

1058

1059

1061±

20.

3%

1395

Peg

mat

ite

Asp

toel

(2:1

),an

,rd

,pi

nk,

(3)

678

2413

90.

447

4035

10.

126

0.16

531

1.64

085

0.07

199

986

986

987±

10.

1%B

spto

o,an

,rd

,pu

rple

,cr

,(5

)18

035

198

0.55

429

409

0.15

60.

1676

51.

6733

70.

0723

999

999

899

7±1

−0.

2%C

o,an

,rd

,pi

nk,

(1)

273

5029

50.

3833

1915

80.

108

0.16

633

1.65

540

0.07

218

992

992

991±

1−

0.1%

Fo,

an,

rd,

pink

,(1

)19

946

267

0.49

3763

880.

140

0.16

550

1.64

446

0.07

207

987

987

988±

10.

1%

Init

ial

com

mon

lead

com

posi

tion

calc

ulat

edfr

omSt

acey

and

Kra

mer

s(1

975)

orfe

ldsp

aran

alys

is.

Fra

ctio

nati

onco

rrec

tion

for

Pb

and

Uw

as0.

1±0.

04%

per

amu.

Ana

lyse

sw

ere

redu

ced

usin

g5–

10pg

ofbl

ank

lead

.aM

easu

red

rati

o.b

Rat

ios

corr

ecte

dfo

rbl

ank,

frac

tion

atio

n,an

dsp

ike.

Err

ors

on207P

b/206P

bag

esar

e±

2si

gma.

cRat

ios

corr

ecte

dfo

rbl

ank,

frac

tion

atio

n,co

mm

onle

ad,

and

spik

e.E

rror

son

207P

b/206P

bag

esw

ere

esti

mat

edus

ing

the

prog

ram

ofL

udw

ig(1

991

).U

ncer

tain

tyon

the

U/P

bra

tios

is0.

5%.

dSa

mpl

ew

eigh

tses

tim

ated

from

phot

ogra

phs.

Unl

ess

othe

rwis

eno

ted,

all

grai

nsw

ere>

100

nm,

colo

rles

s,cr

ack-

free

,di

amag

neti

c,an

dab

rade

d.A

bbre

viat

ions

:an

=an

hedr

al;

cr=

frac

ture

d;el=

elon

gate

;eq=

equa

nt;

eu=

euhe

dral

;m

f=m

ulti

face

ted;

nm=

non-

mag

neti

c;pr=

pris

mat

ic;

rd=

roun

ded;

sub=

subh

edra

l;sp=

sphe

rica

l;un

abr=

not

abra

ded;

(len

gth:

wid

th);

(num

ber

ofgr

ains

).


charnockite that is widespread along the Rio ‘age’ of 284 Ma, and a 208Pb/232Th ‘age’ of 213 Ma.The discrepancies between the Pb–Pb, U–Pb, andPilapa (Fig. 2). Evidence supporting the cumu-

late interpretation includes a large positive Eu Th/Pb ages demonstrate that the U–Th–Pb system-atics have been disturbed. Calculated initial Pb[Fig. 6(D)], low concentrations of the other REE

as well as Y, Zr, Nb, Th and U, and a high isotope ratios based on the NAL samples and themeasured U, Th, and Pb concentrations are inconcentration of K2O (Table 1).many cases unreasonably low (Table 1). The ALpowders yielded far more consistent Pb isotope3.2. Nd and Pb isotopesratios than the NAL ones, and for the five sampleswith feldspar separates, the AL powders have PbWhole rock Sm–Nd systematics suggest that the

main series and anorthosite–gabbro series differ in isotope ratios almost identical to those of thefeldspars (Table 1).age, although the ages are not well constrained. A

dozen samples of the main series, including ninefrom this study and three from Ruiz et al. (1988),yield a Sm–Nd ‘errorchron’ age of 1403±140 Ma 4. U–Pb zircon geochronology(Fig. 7), whereas the five samples of the Chipocogabbro–anorthosite complex have an ‘errorchron’ 4.1. Overview of resultsage of 1040±130 Ma. ( We use the term‘errorchron’ to emphasize the large two-sigma Thirty-one moderately small fractions, usually

less than 20 grains, of generally high quality zirconsuncertainties in the ages.) Paragneiss PJL2 fromRio Claro (Fig. 2) has a Nd isotope composition were analyzed from seven different rocks (Table 2).

Several general points should be noted regardingsimilar to the orthogneisses. A second paragneiss,MOL-5-86, analyzed by Ruiz et al. (1988) and the results. First, ages of 60% of the fractions are

less than 1% discordant analytically, and only onefrom the Rio Pilapa area, has a less radiogenic Ndisotope composition than any of the orthogneisses, is as much as 2% discordant. Thus, there is little

or no evidence of recent Pb loss, and this attestsindicating that it contains a significant componentof older crust. to the high quality of the analyzed zircons (Fig. 8).

Second, although most ages are nearly concordantCommon Pb isotopes can potentially provideboth geochronological and provenance informa- analytically, as will be discussed below, we inter-tion; however, results of common Pb isotopestudies must be interpreted cautiously and criticallybecause U–Th–Pb isotopic systematics of wholerock samples are easily disturbed. In order todetermine the effects, if any, of U/Th/Pb opensystem behavior, the following three types of mate-rials were analyzed: (1) acid-leached (AL) wholerock powders; (2) non-acid-leached (NAL) wholerock powders; and (3) feldspars from five of thegeochronology samples. The AL powders wereleached in 6 M HCl for about 1 h at approximately80°C, and then rinsed thoroughly at least threetimes with Mill-Q, ultra-clean water. The feldsparswere leached in warm HF (15%) for about 10 min.

On a plot of 207Pb/204Pb vs. 206Pb/204Pb (notshown), the NAL main series samples scatter alongan errorchron with an apparent age of1488±260 Ma. However, the same set of samples Fig. 8. Concordia diagram showing all the zircon results from

this study.yield a 206Pb/238U ‘age’ of 170 Ma, a 206Pb/238U


pret most fractions to be highly discordant with igneous zoning, and they have relatively thin meta-morphic rims that are more luminescent than therespect to inferred upper and lower intercepts.

Third, all zircon ages lie in a restricted range of cores [Fig. 10(B), upper grain]. The zoning in the‘core’ area of the lower grain in Fig. 10(B) is more~200 million years. The first order interpretation

of the zircon results is that the main series plutons diffuse than that in the upper grain, and this maybe due to the polished section passing through thewere emplaced at ~1200–1150 Ma, and they were

then overprinted by granulite facies metamorphism outer portion of the zircon rather than the center.The three analyzed equant fractions from PJL8at ~1000 Ma.

had slightly lower U concentrations (D, F, J,61–90 ppm U, Table 2) than the prismatic ones4.2. Metatonalite PJL8 and mafic granulite PJL10(B, C, E, H, 118–262 ppm), but no relationshipwas found between age and morphology. TheThese two samples are from the ‘H-type’ of

main series. They are interpreted as being consan- oldest fraction, D [Fig. 11(A)], consisted of multi-faceted, equant crystals. Surprisingly, there is noguineous because they have REE patterns that are

both unusual and identical in shape [Fig. 6(C )], evidence that metamorphic effects are removed ordiminished by intense air abrasion. Fractionsbecause their initial eNd values are indistinguishable

(Table 1), and because they are from the same J (equant) and H (prismatic) were abraded about10 h each and all crystal faces were removed, yetgeographic area (Fig. 2). PJL8 is a metatonalite,

and it represents one of the two principle rock they have younger U–Pb and 207Pb/206Pb agesthan fractions F (equant) and E (prismatic) thattypes exposed along Rio Pilapa, with the second

being charnockite. PJL10 is a mafic granulite with were not abraded [Fig. 11(A)]. The zircons forPJL8 and probably other samples from thea very high Zr concentration (809 ppm).

Seven zircon fractions from PJL8 were analyzed, Huiznopala Gneiss may be similar to the ‘dis-turbed’ zircons of Chiarenzelli and McLellandand the results illustrate the complexities found in

many of the Huiznopala samples. Two morpholog- (1993) and those with ‘unusual discordance’described by Wasteneys et al. (1997). Those zirconsical varieties of zircons occur in PJL8: (1) subhe-

dral prisms with rounded interfacial edges and were interpreted as igneous zircons that accumu-lated significant radiation damage at temperatureswith length to width (l/w) ratios of 3:1 to 5:1; and

(2) multifaceted equant to ovoid grains (Fig. 9). <650°C (Mezger and Krogstad, 1997), perhapsat near surface conditions ( Wasteneys et al., 1997).Both populations contain clear, colorless crystals

free of inclusions and cracks, and neither cores They were then subjected to granulite facies condi-tions when they were partially reset and lost bothnor growth zoning were observed under the binoc-

ular microscope. Granulite facies zircons that have Pb and some U (Chiarenzelli and McLelland,1993).grown in a solid state are usually characterized as

multifaceted and equant (e.g. van Breemen et al., Regression of all seven fractions from PJL8yields imprecise intercepts of 1227±78 Ma and1986; Chiarenzelli and McLelland, 1993; Scoates

and Chamberlain, 1997); but those metamorphic 1025±92 Ma [Fig. 11(A)] that presumably reflectigneous crystallization and granulite facies meta-zircons are commonly described as ‘small’, which

is not necessarily the case for the equant var- morphism, respectively. The oldest fraction, D, isimportant for constraining the age of protolithiety from PJL8 (Fig. 9). Cathodoluminescence

revealed that equant to ovoid grains typically have crystallization [Fig. 11(A)]; however, its inter-pretation is complicated by the morphology of thecores that show igneous growth zoning, and these

cores are rimmed by more luminescent zones inter- zircons, equant and multifaceted. The age of frac-tion D, 1203±1 Ma (Table 2), is strictly a mini-preted as metamorphic overgrowths. The meta-

morphic rims vary in thickness from grain to grain, mum for igneous crystallization because thezircons may have retained a metamorphic compo-and in some but not all cases they form the greatest

volume of the zircon [Fig. 10(A)]. The prismatic nent even after intense abrasion. Nevertheless, weinterpret the age of fraction D as being very nearzircons are usually dominated by cores that shows


Fig. 9. Photomicrograph of equant and elongate zircons from PJL8. Scale bar is 100 mm long.

that of igneous crystallization, because the fraction from PJL9, but again, no relationship was foundbetween morphology and age. Three of the fiveis concordant, because it plots near the upper

intercept of the chord for the sample [Fig. 11(A)], analyzed fractions have 207Pb/206Pb ages that liein the range 1196±2 Ma, two of which are concor-and because it is the oldest zircon fraction analyzed

from any rock in this study (Fig. 8). dant (Table 2). These three fractions have agessimilar to the inferred crystallization age of metato-The two analyzed fractions from PJL10 were

discordant, but the zircons appear to have experi- nalite PJL8 previously discussed, and they mayestablish the crystallization age of the protolithenced a history similar to those of PJL8. Their

error ellipses intersect the chord defined by the of PJL9.Zircon fraction B, however, complicates thezircons from PJL8 [Fig. 11(A)], and regression of

all nine fractions from the two rocks yields almost interpretation of the results from PJL9. This frac-tion is concordant with a 207Pb/206Pb age ofidentical intercepts to those defined by PJL8

alone, but with somewhat smaller uncertainties 1161±4 Ma, which is considerably younger thanthose of the other four zircon fractions from this[1227±63 Ma and 1018±79 Ma, cf. Fig. 11(A)].sample (Table 2). There are at least three possibleinterpretations of these results. First, the crystalli-4.3. Charnockite PJL9zation age of the protolith could be about1161 Ma, and in this case, those zircons with olderCharnockite is the second principal rock type

exposed along Rio Pilapa (Fig. 2). However, this ages would be inherited. This interpretation mustbe seriously considered because the abundantparticular sample is probably not representative of

all the charnockites because it is dominated K-spar in PJL9 indicates that this rock is from amore potassic suite than PJL8 and PJL10, whichby cumulate potassium feldspar [Fig. 6(D)].

Determining the crystallization age of this sample are medium-K and more tonalitic in character[Fig. 4(A)]. Furthermore, the other potassicis particularly challenging, because it contains only

36 ppm Zr and because inherited, metamorphic, sample, PJL3 (discussed below), is inferred to havea crystallization age of 1153±4 Ma, which isand igneous (cumulate or crystallized from intercu-

mulus melt) zircon components may all be present. within the uncertainty of the age of fraction PJL9B,1161±4 Ma. This interpretation is petrologicallyBoth equant and prismatic zircons were recovered


Fig. 10. Cathodoluminescence images of Huiznopala zircons. (A) Equant grain from metatonalite PJL8. Longest dimension of grainis 195 mm. (B) Elongate grains from PJL8. Longest dimension of upper grain is 255 mm. (C, D) Grains from gabbroic anorthositePJL6. Longest dimensions of grains in C and D are 325 and 130 mm, respectively.

attractive, but an argument against it is that high 4.4. Granitic gneiss PJL3quality inherited zircons would be required to bemore abundant in this sample than igneous zircons. This is a representative sample of the potassium-

rich granitic orthogneiss exposed near ChapulaThis possibility cannot be ruled out because of thevery low Zr concentration of this cumulate rock. [Figs. 2 and 4(A)]. Because of its high Zr concen-

tration (453 ppm), the protolith should have con-A second possible interpretation of fraction B isthat it could represent a granulite metamorphic tained abundant igneous zircon, and all the

analyzed fractions were prismatic and euhedralevent at 1161 Ma. The argument against this inter-pretation is that the zircons from this fraction are (Table 2). Five conventional analyses were made

of zircon fractions from PJL3 and, as for theprismatic, a morphology that is usually interpretedas igneous. Third, fraction B could include a previously described samples, the data were spread

along and very near concordia. The two oldestcomponent of protolith zircon that crystallized~1200 Ma, and these zircons were affected by Pb fractions have 207Pb/206Pb ages of 1158±4 and

1153±2, but the significance of these ages wasloss and growth of new zircon at ~1000 Ma. Theerror ellipse of fraction B intersects a reference uncertain because a reference chord with intercepts

of 1200 and 1000 Ma passed through the errorchord with intercepts of 1200 and 1000 Ma[Fig. 11(B)]. Our preferred interpretation is the ellipses for these points. However, results of a

stepwise dissolution experiment (Mattinson, 1994;third possibility.


Fig. 11. Concordia diagram showing zircon results.


McLelland and Mattinson, 1996) greatly increased removed from concordia [Fig. 11(D), Table 2].Although the protolith of 4196 is problematic,our confidence in interpreting the results from

PJL3. The residue from a four-step dissolution there is no good evidence for it containing a zirconcomponent significantly older than that found inproved to have a very high 204Pb/206Pb and was

concordant at 1151±1 Ma (Table 2), clustering the main series samples.with the two oldest conventional analyses[Fig. 11(C)]. We think that it is more than a 4.6. Anorthositic gabbro PJL6coincidence that three data points produced bytwo very different techniques cluster, and we con- Anorthositic gabbro PJL6 is the only sample we

attempted to date from the Chipoco anortho-sider the best estimate of the crystallization age ofthis sample to be 1153±4 Ma. site–gabbro complex. Two of the three dated zir-

cons fractions (A, E) consisted of equant to slightlyAnalytically, fraction C is concordant with anage of 1110 Ma (Table 2). However, because there elongate, subhedral, multifaceted crystals, and

those were discordant [Fig. 11(E)]. The remainingis no other evidence for an event at about 1110 Ma,and because the error ellipse of this fraction fraction, B, was concordant within error with a

207Pb/206Pb age of 1007±3 Ma (Table 2).intersects a reference chord with intercepts of 1150and 1000 Ma [Fig. 11(C)], we interpret this frac- Zircons of fraction B were morphologically dis-

tinct from those of fractions A and E; they weretion to represent protolith zircons that crystallizedat ~1153 Ma and then experienced Pb loss and equant, anhedral, rounded, cracked and possibly

resorbed. Cathodoluminescence revealed coresgrowth of metamorphic zircon at ~1000 Ma.with faint, indistinct structures that were rimmedby ≤30 mm thick, highly luminescent metamorphic4.5. Granulite 4196overgrowths [Fig. 10(C,D)]. Fraction B must havebeen dominated by the core material, because theSample 4196 is a dark garnet granulite collected

from the relatively large exposure of paragneiss overgrowths are relatively thin and because theyshould have been removed by the intense abrasion.along Agua de Sal (Fig. 2). Mineralogically, the

rock contains plagioclase, garnet, hornblende, and Thus, the problem is to interpret the concordantage of the cores. We will consider the followingpseudomorphs after clinopyroxene (?). The sample

was from a typical decimeter scale layer in the three possibilities: (1) the cores are metamorphic;(2) the cores are igneous zircons precipitated fromparagneiss sequence, and it was interpreted in the

field to be a metasedimentary rock. However, its the Chipoco gabbroic magma; or (3) the cores areinherited zircons entrained in the gabbroic magma.major element composition is similar to that of a

basalt (Table 1), and its REE pattern is almost The first possibility requires two periods of growth(cores and rims) of metamorphic zircons, and itidentical in shape to that of mafic orthogneiss

PJL7 from the Rio Pilapa area [Fig. 6(B)]. The cannot be ruled out, although there is no evidenceof zircon growth in the region after ~1000 Maprotolith could have been a basaltic ash or epiclas-

tic deposit. If the protolith was basaltic, then the (Fig. 8). We consider the second possibilityunlikely because the cores are anhedral and lackZr content (57 ppm) of the rock is far too low for

the parental melt to have been saturated with oscillatory growth zoning similar to that found inthe upper grain of Fig. 10(B) [cf. Fig. 10(C,D)].zircon. Zircons recovered from this sample were

anhedral, rounded and perhaps resorbed. They are The morphologies of the cores seem consistentwith the third possibility. The dark core shown inmost likely xenocrystic or detrital and not purely

metamorphic, because cores with growth zoning Fig. 10(C) appears embayed along the left side.Linear streaks that may be igneous growth zoningcould be seen under cathodoluminescence.

The four analyzed fractions range from only can be seen in the left portion of the core shownin Fig. 10(D). This core appears to be a crystal0.3% to 1.6% discordant, but their 207Pb/206Pb

ages differ by as much as 26 million years, and the fragment because the linear streaks are truncatedby the margins of the core.data points are spread along a chord only slightly


If the cores analyzed as fraction B are xenocrys- fractions (A, C, F) consisted of pale pink, crack-free grains, and these all have 207Pb/206Pb ages intic [possibility (3) above], then the following ques-

tion arises: does the concordant age reflect the range 988±3 Ma. We interpret this as thecrystallization age of the pegmatite. Zircons of thecomplete resetting of the U–Pb systematics by a

magmatic and/or metamorphic event at remaining fraction, B, were dark purple andcracked, and they gave a significantly older age1007±3 Ma? We find no support for the complete

resetting hypothesis. SHRIMP studies of zircons (997 Ma, Table 2). Given the abundance of zirconin the pegmatite, the presence of significance inher-from deep crustal granulite xenoliths strongly sug-

gest that zircons are not completely reset under itance seems unlikely, and this slightly older ageremains unexplained.crustal metamorphic conditions (e.g. Rudnick and

Williams, 1987; Rudnick and Cameron, 1991).Furthermore, Mezger and Krogstad (1997) con-cluded that complete resetting of zircons under 5. Metamorphism and thermobarometrycrustal conditions is possible only through dissolu-tion and reprecipitation. Anhydrous mineral assemblages were probably

stable in most orthogneisses at the peak of meta-The interpretation that the cores analyzed asfraction B are xenocrysts that may have been morphism, although titaniferous hornblende may

have been present in some. Pseudomorphs inter-partially but not completely reset by the gabbromagmatism and granulite facies metamorphism preted to be after orthopyroxene are found in most

orthogneisses. The presence of garnet is closelyleads to the speculative suggestion that threeevents, crystallization of the protolith from which related to the Mg# value of the rock. No sample

with Mg# values >28 contains garnet or pseudo-the xenocrysts were derived, Chipoco gabbroicmagmatism, and granulite facies metamorphism, morphs after garnet, whereas all those with Mg#

values <28, except one, contain garnet. The onewere all closely spaced in time (e.g. perhapsbetween ~1020 and ~1000 Ma). If this were not exception is the K-feldspar cumulate, PJL9, and

garnet may be absent because of the exceptionallythe case, then we would expect fraction B to besignificantly discordant in a manner similar to low concentration of FeO+MgO (<0.75 wt%) in

the sample. The anhydrous assemblages have com-fractions A and E (Table 2). This speculation isconsistent with the Sm–Nd results that suggest the monly been overprinted by at least two retrograde

events. First, the pyroxenes were partially replacedgabbro–anorthosite suite is significantly youngerthan the arc suite (Fig. 7). by titaniferous biotite/phlogopite and amphiboles

of the anthophyllite–gedrite–cummingtonitefamily. This was followed by low grade, hydrother-4.7. Pegmatite 1395mal alteration resulting in assemblages containingcalcite–chlorite–sericite–saussurite. Local butThis sample is from a small, decimeter scale

pegmatite that cuts the ductile fabric of the parag- intense mylonitization produced zoisite–amphiboleassemblages.neiss exposed along the Agua Salada (Fig. 2). The

rock has a most unusual mineralogy, consisting of Sample JPL10 is an exceptionally fresh maficgranulite (the whole rock LOI is only 0.12%,approximately 65% ilmenite, 20% plagioclase and

10% zircon, with the remaining phases being rutile Table 1), and its mineral assemblage is probablytypical of the more Fe-rich orthogneisses atand an altered mafic phase. This sample is appa-

rently not a nelsonite, which is a Fe–Ti oxide- the peak of metamorphism. The assemblageis plagioclase+orthopyroxene+clinopyroxene+apatite rock usually associated with anorthosites

(Ashwal, 1993; Aleinikoff et al., 1996), because it quartz+garnet+ilmenite. Garnet occurs both aslarge (e.g. 1 mm) crystals and as narrow (e.g. aboutlacks apatite. The analyzed zircons were very large,

spherical to ovoid, anhedral, rounded grains, and 20 mm thick) reaction coronas on ilmenite grainsat contacts with either plagioclase or clinopyro-fractions C and F were single crystals. All four

fractions were within 0.2% of concordance. Three xene. Electron microprobe analyses established


that the phases are very homogeneous; that is, no Grenville Province with the results of this projectand other studies of the Mexican Grenvillian base-significant compositional zoning was found from

core to rim or from grain to grain (Table 3). The ment should accelerate the process of explainingthe formation and tectonic evolution of Oaxaquiagarnet of the reaction coronas is the same composi-

tion as that from the cores of the larger crystals. (Fig. 1; Ortega-Gutierrez et al., 1995). A first orderconclusion regarding the Grenville Province is thatPressure and temperature conditions were calcu-

lated using the TWQ program of Berman (1991). most of the rocks are polycyclic, that is, theyexperienced older major orogenies (e.g. RiversBerman’s method has two important advantages

over the conventional approach of using indepen- et al., 1989). At least three major orogenic eventshave been recognized and named in the Grenvilledently calibrated geothermometers and geobaro-

meters. First, the P–T calculations are made using Province. These are the Labradorian(~1.71–1.62 Ma), Elzevirian (~1.25 Ma), andall possible equilibria pertinent to the mineral

assemblage using an internally consistent set of Ottawan (~1.09–1.00 Ga) (Gower et al., 1991).But the recognized deformation events are notthermodynamic data for end members. Second,

compositional disequilibrium among the phases limited to these; for example, Corrigan and vanBreemen (1997) reported evidence of thrusting andcan be detected. The calculated temperature

and pressure for PJL10 were 725±23°C and granulite facies metamorphism between 1.15 and1.09 Ga in Quebec. Rivers et al. (1989) consider7.2±0.3 kbar, respectively. Those calculated

uncertainties are small enough that the assemblage the Grenvillian orogenic cycle in the Province tocomprise post-Elzevirian deformation, metamor-can be considered in equilibrium according to

Berman’s criteria (Berman, 1991, p. 840). Actual phic, and pluton events that extended over a periodof 200 million years. In the following discussion,uncertainties are more likely ±50°C and ±1 kbar

(Essene, 1989). we will present evidence that the basement ofsouthern Mexico was affected by only theGrenvillian orogenic cycle, and at least in theHuiznopala area, the deformation and metamor-6. Discussionphism were restricted to the latest stages of theOttawan orogeny. This and other evidence willThe Grenville Province of eastern Canada and

the adjacent Adirondack Mountains of New York lead to the conclusion that the Huiznopala Gneissmay have formed outboard of the GrenvilleState is the best studied, large exposure of the

worldwide Grenvillian orogenic belt. Comparing Province, and it may have experienced ductiledeformation and metamorphism within the coreand contrasting the tectonic history of theof the orogen formed during the final thrustingevent of the Grenville orogeny.Table 3

Electron microprobe analyses of minerals from mafic granu-lite PJL10 6.1. Geochronology

Garnet Cpx Opx PlagioclaseWe interpret the U–Pb zircon results of the

SiO2 37.30 50.28 49.14 60.15 Huiznopala Gneiss to reflect principally a two-TiO2 0.00 0.08 0.04 0.00 stage history. The first stage began with medium-KAl2O3 20.60 2.02 1.13 25.60 arc magmatism at ~1200 Ma, and this is bestFeO 30.14 14.22 34.40 0.00

documented by samples PJL8 and PJL10MnO 1.32 n/a n/a n/a[Fig. 9(A)]. Whether or not potassium magmatismMgO 3.74 10.77 14.27 0.00

CaO 7.06 21.72 0.60 7.46 was coeval with the medium-K activity or simplyNa2O 0.10 0.65 0.09 7.57 followed it is problematic, because of the uncer-K2O 0.03 0.00 0.02 0.35 tainty in the interpretation of the geochronologicalTotal 100.25 99.73 99.67 100.78

results from potassic sample PJL9 [Fig. 11(C)].Nevertheless, we consider the first stage to includen/a=not analyzed.


the crystallization of the potassic granitic gneiss Mixtequita (Fig. 1). Those ages were interpretedto date granulite facies metamorphism.PJL3 at 1153±4 Ma [Fig. 11(B)].

Although there are no exposures of GrenvillianThe second stage included both magmatic androcks between the Ciudad Victoria area and thethermal events at ~1000 Ma. We have previouslyOuachita suture (Fig. 1), evidence that rocks ofspeculated that the emplacement of the Chipocothis age lie in the subsurface has been found inanorthosite–gabbro complex was near 1007 Ma.studies of deep crustal xenoliths and clasts fromPegmatite 1395, dated at 988±3 Ma, post-datesPaleozoic sedimentary deposits. Rudnick andductile deformation. Furthermore, individual dis-Cameron (1991) reported SHRIMP analyses ofcordia from PJL3, PJL6, PJL8 and 4196 havezircons from granulite xenoliths from the Laimprecise lower intercepts that lie in the age rangeOlivina locality (Fig. 1), and one of these xenoliths982 to 1029 Ma. Although the emplacement of theshowed effects of Grenvillian metamorphism. Thisanorthosite–gabbro complex and the granulitesample was interpreted as having an igneous pro-facies metamorphism appear closely spaced intolith age of ~1400 Ma, and it then experiencedtime, we do not attribute the metamorphism speci-granulite facies metamorphism at 1100±130 Ma.fically to the Chipoco complex, because on theSamples of basement are also found as cobblesregional scale in eastern and southern Mexico,and boulders in late Paleozoic marine strata nearthere is no relationship between the grade ofLas Delicias, about 200 km east of La Olivina.metamorphism and distance from anortho-Lopez et al. (1997) describe the textures of thesite–gabbro complexes (Mora et al., 1986).basement samples as varying from igneous toU–Pb zircon geochronology studies ofgneissic, but none are granulites. These clasts haveGrenvillian granulites from other exposures inU–Pb zircon crystallization ages ranging from 1100eastern and southern Mexico have only been recon-to 1238 Ma, and one sample contains an inheritednaissance in nature, and the results, with twozircon component with a minimum age of aboutexceptions (Herrmann et al., 1994; Ruiz et al., in1800 Ma.

press), are limited to abstracts. In some cases it isThe sketchy geochronological results summa-

not clear if the reported ‘ages’ are those of igneous rized above seem to have the same pattern as those(protolith) crystallization or granulite facies meta- obtained from the Huiznopala Gneiss. That is,morphism. The only U–Pb zircon data from the there is solid evidence for Grenvillian magmatismNovillo Gneiss (Fig. 1) were reported by Silver between about 1240 and 1100, and then renewedet al. (1994), who obtained a concordant age of magmatism and granulite facies metamorphism1018±3 Ma for a sample of garnetiferous char- between about 1020 and 1000 Ma. The onlynockite. For the Oaxacan Complex, Anderson and reported ages that lie between 1100 and 1020 MaSilver (1971), Ortega-Gutierrez et al. (1977) and are those described as ‘apparent’ by Silver et al.Silver et al. (1994) report that the oldest dated (1994). A granulite metamorphic event atrock is a syenitic orthogneiss (1130 Ma) believed ~1000 Ma is well established, although the possi-to be associated with an anorthosite complex. bility of older events cannot be ruled out. ThusCharnockitic gneisses yield ‘apparent ages’ ranging far, evidence for Mexican Grenvillian magmatismfrom 1100 to 1040 Ma, and pegmatites have ages and metamorphism reworking or affecting signifi-of 1000 to 960 Ma (Silver et al., 1994). The only cantly older crust has been found only northwestother data from the Oaxacan Complex are those of Ciudad Victoria (Fig. 1).of Herrmann et al. (1994). They analyzed onlytwo zircon fractions from a ‘gneiss’ and three from 6.2. Arc magmatism, crust formation, anda ‘granite’. The results were highly discordant, significance of Nd model ageswith upper intercepts at 1006 and 982 Ma, respec-tively. Finally, Ruiz et al. (in press) report zircon Perhaps the principal process of crust formation207Pb/206Pb ages of 980 to 990 Ma from two in the Grenville Province was arc magmatism.

Comparable geochemical data to those of thissamples thought to be meta-arkoses from


study are available for two groups of arc rocksfrom the Grenville Province. One is the 1.5 Ga‘gray gneisses’ of the central Province (Dickin andHiggins, 1992), and the other is the 1.35–1.23 Gametatonalites of the Adirondack Highlands(McLelland and Chiarenzelli, 1991; Daly andMcLelland, 1991; McLelland et al., 1993). Thelatter represents one of the youngest, if not theyoungest, arcs within the Province. Both of thesegroups have initial Nd isotope ratios near those ofthe depleted mantle (DM) (Fig. 7), and they areinterpreted as juvenile magmatic arcs that containa minimum contribution from older crust. Becausetheir initial Nd isotopes are close to DM, their Nddepleted mantle model ages (TDM) are similar totheir crystallization ages. The metatonalites have

Fig. 12. Plot of eNd vs. time. Metatonalite data from Daly andTDM ages of 1.38 to 1.40 Ga, and most of the grayMcLelland (1991), and gray gneiss data from Dickin andgneisses have TDM ages of ~1.55 Ga.Higgins (1992). The depleted mantle (DM) evolution curve isThe ‘main series’, which has arc-like trace ele-shown, and the CHUR reference line is horizontal at 0. Main

ment characteristics [Fig. 4(B)Fig. 5], represents series samples (squares). Chipoco anorthosite–gabbro complexthe oldest crust forming event recognized in the (+). Representative evolution lines are shown for two

Huiznopala main series samples.Huiznopala Gneiss (~1200 to ~1150 Ma). Nozircons have been recognized that were inheritedfrom crust significantly older than 1200 Ma. found that the Grenvillian rocks of Texas had

TDM ages (1.6 to 1.3 Ga) similar to those of MexicoFurthermore, initial eNd values of the main seriesvary over about two epsilon units, but they do not and the northeastern US. They suggested the

following two possible interpretations of their Ndshow any systematic variation with silica, as wouldbe expected if the silicic rocks contained a signifi- data: (1) the crust of Texas, Mexico and the

northeastern US is composed of ‘material derivedcant component of substantially older crust[Fig. 4(C)]. The relatively young age of the from depleted mantle at 1.3 to 1.0 Ga, with a

minor admixture (0–20%) of older crustal material;Huiznopala arc compared with those of theGrenville Province is consistent with it forming or (2) all the areas are composed of materials

derived from 1.6–1.3 Ga crustal protoliths, andoutboard of the Province. The Huiznopala mainseries has initial eNd values that are clearly lower these protoliths have been obliterated or remain

to be recognized’ (p. 692). Patchett and Ruizthan those of the Grenville Province arcs, and theylie about midway between the DM and CHUR (1987) stated, ‘We cannot choose between these

alternatives on the basis of existing geochronologyevolution lines (Fig. 12). Consequently, their TDMages, 1.42 to 1.56 Ga, are older than their crystalli- and Nd isotope data’ (p. 692). We can address

this question with the combined U–Pb geochrono-zation ages, and they fall in the same age range asthose of the older Grenville Province arc rocks logical and Nd isotope data of this study. If the

medium-K Huiznopala main series rocks, which(Fig. 12). The only other Nd isotope studies ofthe Mexican Grenville rocks are those of Patchett crystallized ~1200 Ma, formed from mantle

derived magmas that assimilated substantiallyand Ruiz (1987) and Ruiz et al. (1988), who madea reconnaissance analyses of rocks from Novillo, older crust, then they should contain older inher-

ited zircons. No evidence of inherited zircons olderMolango and the Oaxacan Complex (Fig. 1). Theyreported TDM ages (1.6 to 1.35 Ga) like those from than ~1200 Ma has been found (Fig. 8). The

Huiznopala arc was probably similar to modernthe Huiznopala Gneiss.In a related study, Patchett and Ruiz (1987) island and continental magmatic arcs that have


eNd values about midway between DM and CHUR. and western portions of the Province (e.g. Anovitzand Essene, 1990; Nadeau and Hanmer, 1992;Modern examples include the Lesser Antilles

( White and Dupre, 1986) and the Sunda ( White Ketchum et al., 1994; Jamieson et al., 1995;Connelly et al., 1995). These metamorphic condi-and Patchett, 1984) island arcs, and the Lassen

area of the Cascades (Bullen and Clynne, 1990). tions have been universally attributed to crustalthickening resulting from the stacking of thrustThe Nd isotopic compositions of these modern

arcs are usually attributed to a variety of sources, sheets. After metamorphism, the rocks in someand perhaps all these areas experienced nearlyincluding subducted sediments, enriched mantle,

and crustal assimilation. isothermal decompression. In contrast, the rocksof the Adirondack Highlands were metamor-Both of the Huiznopala silicic samples (>70%

SiO2) are anomalously potassic with respect to phosed at lower pressure, 7.0 to 8.0 kbar, at750–800°C, and they cooled under nearly isobaricthe mafic and intermediate compositions rocks

[Fig. 4(A)]. The high K2O concentration of PJL9 conditions (Bohlen et al., 1985; Mezger et al.,1991). The tectonic significance of metamorphismis clearly the result of the accumulation of

K-feldspar [Fig. 6(D)]. Nevertheless, the presence like that found in the Highlands remains contro-versial. Bohlen (1987) cited the Highlands as aof abundant K-feldspar in PJL9 demonstrates that

it is atypical of island arc plutonic rocks, which typical example of metamorphism that forms inresponse to magmatic underplating. However,usually have tonalitic affinities. Granitic gneiss

PJL3 has a modest positive Eu anomaly, but the underplating results in model T–t (temper-ature–time) paths that disagree strongly with mea-chemical composition of the sample approaches

the composition of a melt much more closely than sured thermochronometry according to Anovitzand Chase (1990). They proposed a model of post-does the composition of PJL9. It has a K2O

content similar to granites from the AMGC suite thrusting, tectonic denudation near the peak ofmetamorphism to explain the P–T–t paths of the(anorthosite–mangerite–charnockite–granite suite,

Emslie, 1978) found in the Adirondack Highlands Grenville orogen of Ontario. They suggest thatevidence for an early stage of isothermal decom-and elsewhere in the Grenville Province. In the

Adirondack Highlands the AMGC granites post- pression is commonly lost in granulite terranes,and the only evidence preserved is that for a laterdate the arc suite by approximately 20 million

years, and Daly and McLelland (1991) proposed stage of isobaric cooling.The calculated P–T conditions (7.2±0.5 kbar,that the AMGC granites were derived by crustal

melting of the arc basement. A crustal melting 725±50°C) of metamorphism for HuiznopalaGneiss must be viewed with caution because theyorigin could explain the high K2O contents of

Huiznopala rocks like PJL3. Although AMGC are based on mineral compositions from a singlesample; nevertheless, they are similar to thosegranites are typically interpreted as being anoro-

genic (e.g. Emslie and Hunt, 1990; Daly and reported from granulites from Oaxaca and LaMixtequita. Mora and Valley (1985) and MoraMcLelland, 1991; Higgins and van Breemen,

1996), if they are derived by partial melting of arc et al. (1986) determined temperatures from theOaxacan Complex of about 730±50°C based onrocks, then they should have arc-like ratios of

incompatible trace elements such as Nb and La, feldspar thermometry and pressures of 7.0±1 kbarbased on garnet–plagioclase barometers. Murillo-and this is the case for PJL3 [Fig. 4(B)].Muneton and Anderson (1994) estimated that thepeak temperature for the La Mixtequita massif6.3. Metamorphism and the Grenvillian orogenywas as high as 850°C based on two pyroxenesolvus and exchange temperatures at a pressure ofTwo end-member styles of metamorphism devel-

oped during the Grenvillian orogenic cycle in the 7.4±0.1 kbar. However, garnet–clinopyroxenetemperatures from the La Mixtequita massifGrenville Province. Much evidence has been

reported for metamorphic pressures reaching (718±13°C) are indistinguishable from those ofthe Oaxacan Complex and the Huiznopala Gneiss.>10 kbar, commonly 12 to 14 kbar, in the central


Evidence for somewhat higher pressures has been Wasteneys et al. (1997) report evidence forGrenvillian metamorphism between about 1050found in the Novillo Gneiss granulites. Orozco

(1991) reported strongly zoned garnets yield- and 1000 Ma, but they note that the intensityvaried considerably from place to place. In theing higher temperatures and pressures in the

cores (729–773°C, 8.9–9.7 kbar) than the rims Adirondack Highlands the main phase of granulitemetamorphism peaked significantly earlier, about(642±33°C, 7.9±0.5 kbar).

Textural relations found in the Huiznopala 1060 Ma, than in Mexico (Mezger et al., 1991;Chiarenzelli and McLelland, 1993). Mezger et al.Gneiss suggest that it experienced nearly isobaric

cooling. Coronas of garnet separate ilmenite and (1991) recognized a later metamorphic episode,1026 to 996 Ma, but they suggested that its effectsplagioclase in sample PJL10, and they suggest that

a reaction of the sort ilmenite+anorthite+ were of only local extent. In contrast to theGrenville Province, where the effects of the latestquartz=garnet+rutile may have taken place

during cooling. That reaction has a low, positive portion of the Grenville orogeny were variable,the metamorphic effects in southern Mexico appearslope on a P–T diagram with the garnet-bearing

assemblage stable at lower temperatures (Bohlen to have been uniformly intense. The intensity andthe timing of metamorphism of the Huiznopalaand Liotta, 1986). The formation of garnet at the

expense of ilmenite requires nearly isobaric cool- Gneiss, the Oaxacan Complex and the LaMixtequita massif suggest that southern Mexicoing. Had the sample experienced near isothermal

decompression, then the garnet should show evi- may represent the core of the orogen formedduring the final thrusting event of the Grenvilledence of reaction to ilmenite and plagioclase. The

overall metamorphic style of the Huiznopala orogeny.Gneiss (i.e. regional granulite facies metamor-phism at moderate pressure followed by nearly 6.4. Provenance of the Huiznopala Gneissisobaric cooling) is more similar to that of theAdirondack Highlands than to those parts of the Pb isotopes have proven to be useful in testing

models for the provenance of Grenvillian terranes.Grenville Province where crustal thickening hasbeen well documented. Tosdal (1996) demonstrated that the Pb isotopes

of the Arequipa–Antofalla craton of westernThe timing of the granulite facies metamorphismin the Huiznopala Gneiss seems to correspond Bolivia and northern Chile are similar to those of

the Gondwanan Amazon craton and are charac-reasonably well with the ‘final stage’ of deforma-tion and metamorphism recognized in the terized by having elevated 207Pb/204Pb ratios at a

given 206Pb/204Pb ratio compared with the StaceyGrenville Province. Krogh (1994) dated rocksalong the entire 2000 km length of the Grenville and Kramers (1975) model growth curve for

average crust (Fig. 13). Initial Pb ratios ofFront, and he found that all showed a nearcommon age of final metamorphism between 995 Grenvillian rocks of eastern Laurentia generally

lie below that reference curve, thus the Pb isotopeand 980 Ma. He interpreted this event to reflectGrenvillian thrusting, crustal loading and downw- data seem to preclude the Arequipa–Antofalla

craton from being a detached fragment ofarping that was followed by rapid uplift. East ofthe Grenville Front, evidence for deformation and Laurentia, as had been suggested by some workers.

It had also been proposed based on stratigraphymetamorphism at about 1000 Ma is not uniformlyevident. For example, in western Labrador the Lac and faunal associations that the Precordillera of

Argentina was rifted from the continental marginJoseph terrane was thrust over the Molson Laketerrane during the final phase of the Grenville of the southern United States. Kay et al. (1996)

found that the Pb isotopes of basement samplesorogeny (989±12 Ma; Connelly and Heaman,1993). The Lac Joseph terrane was not strongly of the Precordillera are consistent with this model

(Fig. 13).metamorphosed by that event. In contrast, theMolson Lake terrane was extensively recrystal- The provenance of the Mexican Grenvillian

orogen is unknown. Lower Ordovician andlized. In another terrane in Labrador, the Pinware,


they are permissive of the magmas parental to theprotoliths of Huiznopala Gneiss, drawing on thesame isotope reservoir as magmas parental tothe Laurentian crust. This is consistent with thepaleomagnetic results that place Oaxaca adjacentto Ontario/Quebec at about 950 Ma (Ballard et al.,1989). On the other hand, it should be emphasizedthat those Pb data do not prove Laurentian affini-ties for the Huiznopala Gneiss because someGondwanan cratons may have Pb isotope com-positions like those of eastern Laurentia.

7. Summary and conclusions

(1) Pb isotope data indicate that the magmasFig. 13. Plot of 207Pb/204Pb vs. 206Pb/204Pb. The Stacey andparental to the Huiznopala arc and those parentalKramers (1975) model growth curve for average crustal Pb is

shown. The Huiznopala data (solid squares) are from the AL to the Laurentian crust could have been derivedrock powders and feldspar separates, and they closely approach from a common isotope reservoir. Furthermore,initial ratios. Those from Texas are acid-leached rock powders they are consistent with the paleomagnetic res-of Grenvillian age granulite xenoliths from west Texas

ults that place southern Mexico adjacent to(Cameron and Ward, 1998) and K-feldspar separates fromOntario/Quebec at about 950 Ma (Ballard et al.,central and west Texas (Zartman and Wasserburg, 1969).

Likewise, those from the Adirondack Mts. are from feldspar 1989).separates (DeWolf and Mezger, 1994). Those from the (2) The initial crust forming event recorded inArequipa–Antofalla craton are whole rock ratios, age corrected the Huiznopala Gneiss was arc magmatism thatusing measured U and Pb concentrations (Tosdal, 1996), and

was active from ~1200 to ~1150 Ma. Thethose from the Precordillera are from non-acid-leached rockHuiznopala arc is younger than any major arcpowders (Kay et al., 1996). The field for each area was drawn

to emphasize the weight of the data, and outliers were recognized in the Grenville Province (Dickin andeliminated. Higgins, 1992; McLelland et al., 1993), and these

age relationships are consistent with theHuiznopala arc lying outboard of the Province.Silurian strata that overlie the Oaxacan Complex

and Novillo Gneiss, respectively, contain (3) The Huiznopala Gneiss experienced ductiledeformation and granulite facies at ~1000 Ma,Gondwanan fauna (Robison and Pantoja-Alor,

1968; Stewart et al., 1998), and consequently, and this age corresponds closely to that of finalthrusting and deformation in the Grenvilleseveral researchers have proposed that the Mexican

Grenvillian rocks were rifted from northwestern Province. Thus, the Huiznopala Gneiss may be inthe core of the orogen formed during the finalSouth America, perhaps from the region of the

Arequipa–Antofalla craton (Dalziel, 1994) or from thrusting event of the Grenville orogeny.the Grenvillian massifs of Colombia (Yanez et al.,1991; Keppie and Ortega-Gutierrez, 1995; Ruizet al., in press). In Fig. 13 the Pb isotope composi- Acknowledgmentstions of the Huiznopala Gneiss can be comparedwith those of the Arequipa–Antofalla craton and We thank Peter Holden for maintaining the

isotope lab at UCSC in good working order, andparts of Laurentia. The Huiznopala Pb isotopedata clearly overlap with those of the Laurentian, Bruce Tanner for supervising the XRF analyses.

This research was supported in part by grantsas represented by Texas and the AdirondackHighlands and Lowlands, and are distinct from from the Committee on Research and the MEXUS

program of the University of California, Santathose of the Arequipa–Antofalla craton. Thus,


Connelly, J.N., Heaman, L.M., 1993. U–Pb geochronologicalCruz and by funds from the Instituto de Geologia,constraints on the tectonic evolution of the GrenvilleUNAM. Compania Minera Autlan in Mexico pro-Province, western Labrador. Precambrian Res. 63, 123–142.vided valuable logistical support during our field

Connelly, J.N., Rivers, T., James, D.T., 1995. Thermotectonicwork. John Aleinikoff and James McLelland pro- evolution of the Grenville Province of western Labrador.vided helpful reviews of the manuscript. Tectonics 14 (1), 202–217.

Corrigan, D., van Breemen, O., 1997. U–Pb age constraints forthe lithotectonic evolution of the Grenville province alongthe Mauricie transect, Quebec. Can. J. Earth Sci. 34, 299–316.

References Daly, J.S., McLelland, J.M., 1991. Juvenile Middle Proterozoiccrust in the Adrindack Highlands, Grenville province, north-eastern North America. Geology 19, 119–122.Aleinikoff, J.N., Horton Jr., J.W., Walter, M., 1996. Middle

Dalziel, I.W.D., 1994. Precambrian Scotland as aProterozoic age for the Montpelier Anorthosite, GoochlandLaurentia–Gondwana link: Origin and significance of cra-terrane, eastern Piedmont, Virginia. Geol. Soc. Am. Bull. 108,tonic promontories. Geology 22, 589–592.1481–1491.

DeWolf, C.P., Mezger, K., 1994. Lead isotope analyses ofAnderson, T.H., Silver, L.T., 1971. Age of granulite metamor-leached feldspars: Constraints on the early crustal history ofphism during the Oaxacan orogeny, Mexico. Geol. Soc. Am.,the Grenville Orogen. Geochim. Cosmochim. Acta 58,Abstr. Prog. 3, 4925537–5550.Anovitz, L.M., Chase, C.G., 1990. Implications of post-thrust-

Dickin, A.P., Higgins, M.D., 1992. Sm–Nd evidence for a majoring extension and underplating for P–T–t paths in granulite1.5 Ga crust-forming event in the central Grenville province.terranes: A Grenville example. Geology 18, 466–469.Geology 20, 137–140.Anovitz, L.M., Essene, E.J., 1990. Thermobarometry and pres-

sure–temperature paths in the Grenville Province of Ontario. Emslie, R.F., 1978. Anorthosite massifs, rapakivi granites, andJ. Petrol. 31 (1), 197 late Precambrian rifting of North America. Precambrian Res.

Ashwal, L.D., 1993. Anorthosites. Springer, New York, 422 pp. 7, 61–98.Ballard, M.M., Van Der Voo, R., Urrutia-Fucugauchi, J., 1989. Emslie, R.F., Hunt, P.A., 1990. Ages and petrogenetic signifi-

Paleomagnetic results from Grenvillian-aged rocks from cance of igneous mangerite–charnockite suites associatedOaxaca, Mexico: evidence for a displaced terrane. with massif anorthosites, Grenville Province. J. Geol. 98,Precambrian Res. 42, 343–352. 213–231.

Berman, R.G., 1991. Thermobarometry using multiequilibrium Essene, E.J., 1989. The current status of the thermobarometrycalculations: A new technique with petrologic applications. in metamorphic rocks. In: Daly, J.S., et al. (Eds.), EvolutionCan. Miner. 29, 833–855. of Metamorphic Belts. Geological Society of London, Special

Bohlen, S.R., 1987. Pressure–temperature–time paths and tec- Publication 43, pp. 1–44.tonic models for the evolution of granulites. J. Geol. 95, Gower, C.F., Heaman, L.M., Loveridge, W.D., Scharer, U.,617–632. Tucker, R.D., 1991. Grenvillian magmatism in the eastern

Bohlen, S.R., Liotta, J.J., 1986. A barometer for garnet amphib- Grenville Province, Canada. Precambrian Res. 51, 315–336.olites and garnet granulites. J. Petrol. 27, 1025–1056. Herrmann, U.R., Nelson, B.K., Ratschbacher, L., 1994. The

Bohlen, S.R., Valley, J.W., Essene, E.J., 1985. Metamorphismorigin of a terrane: U–Pb zircon geochronology and tectonic

in the Adirondacks. I. Petrology, pressure and temperature.evolution of the Xolapa complex (southern Mexico).

J. Petrol. 26 (4), 971–992.Tectonics 13, 455–474.

Bullen, T.D., Clynne, M.A., 1990. Trace element and isotopicHiggins, M.D., van Breemen, O., 1996. Three generations ofconstraints on magmatic evolution at Lassen Volcanic

anorthosite–mangerite–charnockite–granite (AMCG) mag-Center. J. Geophys. Res. 95, 19671–19691.matism, contact metamorphism and tectonism in theCameron, K.L., Ward, R.L., 1998. Xenoliths of GrenvillianSaguenay–Lac-Saint-Jean region of the Grenville Province,granulite basement constraint models for the origin of volu-Canada. Precambrian Res. 79, 327–346.minous Tertiary rhyolites, Davis Mountains, west Texas.

Hoffman, P.F., 1991. Did the breakout of Laurentia turnGeology, in press.Gondwanaland inside-out? Science 252, 1409–1412.Cameron, K.L., Robinson, J.V., Niemeyer, S., Nimz, G.J.,

Jamieson, R.A., Culshaw, N.G., Corrigan, D., 1995.Kuentz, D.C., Harmon, R.S., Bohlen, S.R., Collerson, K.D.,North–west propagation of the Grenville orogen: Grenvillian1992. Contrasting styles of pre-Cenozoic and mid-Tertiarystructure and metamorphism near Key Harbour, Georgiancrustal evolution in northern Mexico: evidence from deepBay, Ontario, Canada. J. Metamorph. Geol. 13, 185–207.crustal xenoliths from La Olivina. J. Geophys. Res. 97,

Kay, S.M., Orrell, S., Abbruzzi, J.M., 1996. Zircon and whole17353–17376.rock Nd–Pb isotopic evidence for a Grenville age and aChiarenzelli, J.R., McLelland, J.M., 1993. Granulite faciesLaurentian origin for the basement of the Precordillera inmetamorphism, palaeo-isotherms and disturbance of theArgentina. J. Geol. 104, 637–648.U–Pb systematics of zircon in anorogenic plutonic rocks from

the Adirondack Highlands. J. Metamorph. Geol. 11, 59–70. Keppie, J.D., Ortega-Gutierrez, E., 1995. Provenance of


Mexican terranes: isotopic constraints. Int. Geol. Rev. 37, of Grenville(?) Granulites of the La Mixtequita Massif,Southern Mexico. Geol. Soc. Am. Abstr. 26, 76813–824.

Ketchum, J.W.F., Jamieson, R.A., Heaman, L.M., Culshaw, Nadeau, L., Hanmer, S., 1992. Deep-crustal break-back stack-ing and slow exhumation of the continental footwall beneathN.G., Krogh, T.E., 1994. 1.45 Ga granulites in the southwest-

ern Grenville province: Geologic setting, P–T conditions, and a thrusted marginal basin, Grenville orogen, Canada.Tectonophysics 210, 215–233.U–Pb geochronology. Geology 22, 215–218.

Krogh, T.E., 1994. Precise U–Pb ages for Grenvillian and pre- Ochoa-Camarillo, H., 1996. Geologıa del Anticlinorio deHuayacocotla en la region de Molango, Estado de Hidalgo,Grenvillian thrusting of Proterozoic and Archean metamor-

phic assemblages in the Grenville Front tectonic zone, Mexico, D.F. Tesis de Maestrıa, Universidad NacionalAutonoma de Mexico, Facultad de Ciencias, 91 pp.Canada. Tectonics 13 (4), 963–982.

Lopez, R.L., Cameron, K.L., Jones, N.W., 1997. Evidence from Orozco, M.T., 1991. Geotermobarometria de granulitasprecam-bricas del basamento de la Sierra Madre Oriental, Pachuca,U–Pb zircon ages for the Grenvillian reworking of 1.8 Ga

crust south of the Ouachita suture in the Coahuila terrane, Hidalgo. Convencion sobre la evolucion geologica de Mexico,Instituto de Geologia, Memoria, pp. 138–141.Northern Mexico. EOS, Fall AGU Meeting.

Ludwig, K.R., 1991. A computer program for processing Ortega-Gutierrez, F., Anderson, T.H., Silver, L.T., 1977.Lithologies and geochronology of the Precambrian craton ofPb–U–Th isotopic data. US Geological Survey Open-File

Report 88-542. southern Mexico. Geol. Soc. Am., Abstr. Prog. 9, 1121–1122.Ortega-Gutierrez, F., Ruiz, J., Centeno-Garcia, E., 1995.Mattinson, J.M., 1994. A study of complex discordance in

Zircons using step-wise dissolution techniques. Contri. Oaxaquia, a Proterozoic microcontinent accreted to NorthAmerican during the late Paleozoic. Geology 23, 1127–1130.Mineral. Petrol. 116, 117–129.

McLelland, J.M., Chiarenzelli, J., 1990. Isotopic constraints on Patchett, P.J., Ruiz, J., 1987. Nd isotopic ages of crust forma-tion and metamorphism in the Precambrian of eastern andemplacement age of anorthositic rocks of the Marcy Masiff,

Adirondack Mts., New York. J. Geol. 98, 19–41. southern Mexico. Contrib. Miner. Petrol. 96, 523–528.Pearce, J.A., 1983. Role of the sub-continental lithosphere inMcLelland, J.M., Chiarenzelli, J.R., 1991. Geochronological

studies in the Adirondack Mountains and the implications of magma genesis at active continental margins. In:Hawkesworth, C.J., Norry, M.J. (Eds.), Continental Basaltsa Middle Proterozoic tonalitic suite. In: Gower, C.F., Rivers,

T., Ryan, B. (Eds.), Mid-Proterozoic Laurentia–Baltica. and Mantle Xenoliths. Shiva, Cambridge, MA, pp. 230–249.Pearce, J.A., Harris, N.B.W., Tindle, A.G., 1984. Trace elementGeological Association of Canada, Special Paper 38,

pp. 175–194. discrimination diagrams for the tectonic interpretation of gra-nitic rocks. J. Petrol. 25, 956–983.McLelland, W.C., Mattinson, J.M., 1996. Resolving high-

precision U–Pb ages from tertiary plutons with complex Rivers, T., Martignole, J., Gover, C.F., Davidson, A., 1989.New tectonics divisions of the Grenville Province, southeastzircon systematics. Geochimica et Cosmochimica Acta 60,

3955–3965. Canada shield. Tectonics 8, 63–84.Robison, R., Pantoja-Alor, J., 1968. Tremadocian trilobitesMcLelland, J.M., Daly, J.S., Chiarenzelli, J.R., 1993. Sm–Nd

and U–Pb isotopic evidence of juvenile crust in the from Nochixtlan region, Oaxaca, Mexico. J. Paleontol. 42,767–800.Adirondack Lowlands and implications for the evolution of

the Adirondack Mts.. J. Geol. 101, 97–105. Rogers, J.J.W., 1996. A history of continents in the past threebillion years. J. Geol. 104, 91–107.Mezger, K., Krogstad, E.J., 1997. Interpretation of discordant

U–Pb zircon ages: an evaluation. J. Metamorph. Geol. 15, Rudnick, R.L., Cameron, K.L., 1991. Age diversity of the deepcrust in Northern Mexico. Geology 19, 1197–1200.127–140.

Mezger, K., Rawnsley, C.M., Bohlen, S.R., Hanson, G.N., Rudnick, R.L., Williams, I.S., 1987. Dating the lower crust byion microprobe. Earth Planet. Sci. Lett. 85 (1–3), 145–161.1991. U–PB garnet, sphene, monazite, and rutile ages: impli-

cations for the duration of high-grade metamorphism and Ruiz, J., Tosdal, R.M., Restrepo, P.A., Murillo-Muneton, G.,in press. Pb isotope evidence for Colombia–southern Mexicocooling histories, Adirondack Mts., New York. J. Geol. 99,

415–428. connection in the Proterozic. In: Ramos, V., Keppie, J.D.(Eds.), Laurentia–Gondwana Connections before Pangea.Moores, E.M., 1991. Southwest US–East Antarctic (SWEAT)

connection: A hypothesis. Geology 19, 425–428. Geological Society of America, Special Paper.Ruiz, J., Patchett, P.J., Ortega-Gutierrez, F., 1988. ProterozoicMora, C.I., Valley, J.W., 1985. Ternary feldspar thermometry

in granulites from the Oaxacan Complex, Mexico. Contrib. and Phanerozoic basement terranes of Mexico from Nd iso-topic studies. Geol. Soc. Am. Bull. 100, 274–281.Miner. Petrol. 89, 215–225.

Mora, C.I., Valley, J.W., Ortega-Gutierrez, F., 1986. The tem- Scherer, E.E., Cameron, K.L., Johnson, C.M., Beard, B.L.,Barovich, K.M., Collerson, K.D., 1997. An improved whole-perature and pressure conditions of Grenville-age granulite-

facies metamorphism of the Oaxacan Complex, southern rock Lu–Hf analytical method applied to dating Cenozicevents affecting lower crustal xenoliths from Kilbourne Hole,Mexico. Univ. Nat. Auton. Mexico, Inst. Geol., Rev. 6,

222–242. New Mexico. Chem. Geol., 142, 63–78.Scoates, J.A., Chamberlain, K.R., 1997. Orogenic to Post-Murillo-Muneton, G., Anderson, J.L., 1994. Thermobarometry


Orogenic Origin for the 1.76 Ga Horse Creek Anorthosite Wasteneys, H.A., Kamo, S.L., Moser, D., Krogh, T.E., Gower,C.F., Owen, J.V., 1997. U–Pb geochronological constraintsComplex, Wyoming, USA. J. Geol. 105, 331–343.

Silver, L.T., Anderson, A.T., Ortega-Gutierrez, F., 1994. The on the geological evolution of the Pinware terrane and adja-cent areas, Grenville Province, southeast Labrador, Canada.‘thousand million year’ orogeny in eastern and southern

Mexico. Geol. Soc. Am., Abstr. Prog. 26 (7), A48 Precambrian Res. 81, 101–128.Weaver, B.L., 1980. Rare-earth element geochemistry ofStacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial

lead isotope evolution by a two-stage model. Earth Planet. Madras granulites. Contrib. Miner. Petrol. 71, 271–279.White, W., Dupre, B., 1986. Sediment subduction and magmaSci. Lett. 26, 207–221.

Stewart, J.H., Blodgett, R.B., Boucot, A.J., Carter, J.L., Lopez, genesis in the Lesser Antilles: Isotopic and trace element con-straints. J. Geophys. Res. 91, 5927–5941.R., 1998. A mid-paleozoic exotic terrane near Ciudad

Victoria, Tamaulipas, Mexico: stratigraphy, paleontology, White, W.M., Patchett, J., 1984. Hf–Nd–Sr isotopes and incom-patible element abundances in island arcs: implications forstructure, and tectonic significance. In: Ramos, V., Keppie,

J.D. (Eds.), Laurentia–Gondwana Connections before magma origins and crust–mantle evolution. Earth Planet. Sci.Lett. 67, 167–185.Pangea. Geological Society of America, Special Paper, in

press. Windley, B.F., 1995. The Evolving Continents, 3rd ed. JohnWiley and Sons Ltd, West Sussex, England, 526 pp.Tosdal, R.M., 1996. The Amazon–Laurentian connection as

viewed from the Middle Paleozoic rocks in the central Andes, Yanez, P., Ruiz, J., Patchett, P.J., Ortega-Gutierrez, F.,Gehrels, G.E., 1991. Isotopic studies of the Acatlan complex,western Bolivia and northern Chile. Tectonics 15, 827–842.

van Breemen, O., Davidson, A., Loveridge, W.D., Sullivan, southern Mexico: Implications for Paleozoic North Americantectonics. Geol. Soc. Am. Bull. 103, 817–828.R.W., 1986. U–Pb zircon geochronology of Grenville tecton-

ites, granulites and igneous precursors, Parry Sound, Ontario. Zartman, R.E., Wasserburg, G.J., 1969. The isotopic composi-tion of lead in potassium feldspars from some 1.0 b.y. oldIn: Moore, J.M., Davidson, A., Baer, A.J. (Eds.), The

Grenville Province. Geological Association of Canada, North American igneous rocks. Geochim. Cosmochim. Acta33, 901–942.Special Paper 31, pp. 191–207.

U–Pb geochronology, geochemistry, and provenance of the Grenvillian Huiznopala Gneiss of Eastern...

Documents

Transcript of U–Pb geochronology, geochemistry, and provenance of the Grenvillian Huiznopala Gneiss of Eastern...