500

400

300

200

100

00.00

0.02

0.04

0.06

0.08

0.0 0.2 0.4 0.6

207Pb/235U

206

Pb/

238

U

data-point error ellipses are 68.3% conf .

Rb-Sr and U-Pb Age Systematics of the Alessandrini Silicic Complex

and Related Mylonites, Patagonia, ArgentinaBernhardt Saini-Eidukat, Dept. of Geosciences, North Dakota State University, Fargo, ND 58105 USABrian Beard2, Ernesto Bjerg3, George Gehrels4, Daniel Gregori3, Clark Johnson2, Nathalia Migueles3, and Jeffery D.Vervoort5

2Department of Geology & Geophysics, University of Wisconsin - Madison, Madison, WI 53706 USA3CONICET-Depto. de Geología, Universidad Nacional del Sur, San Juan 670, B8000ICN Bahía Blanca, Argentina4Department of Geosciences, University of Arizona, Tucson, AZ 85721 USA5Department of Geology, Washington State University, Pullman, WA 99164 USA

Suites of extensive but poorly documented granitoids and related rhyolitic dikes exist in northern Patagonia, significantly inboard of the current continental margin. These magmatic complexes have uncertain origin, but may be the result of continent-scale tectonic events related to the assembly or break-up of Gondwana.

The Somoncura Magmatic Belt in northern Patagonia,

Argentina, is an example of such an area with an equivocal genesis. It comprises a suite of intrusive and extrusive silicic units located in a region overlapped by two major South American magmatic provinces: the older Choiyoi to the north and west, and the younger Chon Aike to the south. Here we provide age constraints for these units, as part of a larger study of the tectonic setting of this region. Rb-Sr and U-Pb radiogenic isotope data are presented for the Alessandrini Complex, and for associated units.

One of these, the Alessandrini Complex (AC) (39º43.5' S; 67º42.3' W) is an equigranular to porphyritic monzogranite to granodiorite. Porphyritic phases carry K-spar, hornblende and plagioclase phenocrysts in a groundmass of bio, hbld, qtz, epidote, allanite, titanite and zircon. Granodioritic, aplitic, pegmatitic and basic dikes cut the coarser facies. A coarse-grained, locally highly deformed to mylonitic, foliated (strike 315º) granite is associated with the AC and may be part of a major shear zone. It is crosscut by AC rhyolite dikes, and thus has been inferred to be older.

The following units have been mapped in the area:a) a unit consisting mainly of foliated granites that is

previously had been interpreted to be the Mamil Choique Formation (Ordovician-Devonian),

b) the Alessandrini Complex comprising granites and related rhyolite dikes (Saini-Eidukat et al., 2002),

c) mafic dikes that crosscut both the granitoids and rhyolitic dikes, and

d) locally highly deformed rocks (mylonites and protomylonites) adjacent to the foliated unit. The mylonitic units surround the foliated unit on the east and west, indicating that it may have been emplaced along a major shear zone, oriented N 310-340. .

The Alessandrini Complex itself can be subdivided into an intrusive episode with four facies and a volcanic episode with 2 facies. In addition, a few basaltic and andesitic dikes cut the granitic facies, typically along its south and north borders.

PRE-ANDEAN(Southern Coastal Batholith & North Patagonian Massif)

ANDEAN: Mesozoic-Cenozoic Patagonian Batholith

Jurassic Volcanic Rocks (Chon Aike, Lonco Trapial, Marafil)

TM

P40° S

45° S

70° W

35° S

Figure 1. Sketch map of southern South America, showing the location of the Somoncura Magmatic Belt. Modified from Rapela et al. (1992).

Figure 2. Geologic map of Rio Negro Province, showing outcrops of Jurassic age granitoids in red: (1) indicates the location of the Somoncura Batholith and the El Cuy area. (Nullo and Franchi, 1994)

Figure 3. Geologic map of the El Cuy area (this study).

Left: showing multiple generations of crosscutting rhyolitic dikes.

Right: Mylonitic fabric with rotated clast.

View of the Alessandrini granite quarry and hand specimen.

The Alessandrini Granitic Complex

200

210

220

230

240

250

Mean = 223 ± 6 Ma (2s)

data-point error symbols are 1σ

Ma

Ages - LA-ICP-MS Zircons

The Foliated Unit

Rb-Sr mineral separate age of 192 +/- 0.21 Ma (MSWD = 0.326) is similar to four zircons with age 195.4 +/- 3.1 Ma

Ages - Rb-Sr

0

1

2

3

4

5

6

7

170190210230250270290310

Location

Cathodoluminescence images of zircons

Rb/Sr isochron of Alessandrini Facies granites, granodiorites and aplitic dikes from the Cantera Alessandrini and Cañadon Soledad areas show ages of 195 ± 11 Ma.

0.7080

0.7085

0.7090

0.7095

0.7100

0.9 1.0 1.1 1.2 1.3 1.4

87

Sr/

86Sr

87Rb/ 86Sr

QA-5: Feldspar and biotite mineral

separate + whole rock

Saini-Eidukat et al., 1999

0.705

0.707

0.709

0.711

0.713

0.715

0.717

0.719

0 1 2 3 4 5

87Rb/86Sr

87Sr86Sr

Age = 195 ± 11 Ma

Initial 87Sr/86Sr =0.70591 ± 0.00038MSWD = 7.3

data-point error ellipses are 2σ

Saini-Eidukat et al., 2002

CONCLUSIONS

The measured ages could be interpreted as

a) both units were contemporaneously emplaced, at approximately 224 Ma, with resetting of the Rb-Sr system at 192 Ma due to thermal effects, perhaps by younger intrusions (dikes),

b) a long cooling period for the Alessandrini Complex, during which the U-Pb system closed at a higher temperature and/or earlier time, and the Rb-Sr system at a lower temperature and/or later time, or

c) older ages in the Alessandrini are measured on inherited zircons.

Although field evidence shows some facies of the Alessandrini Complex intruding the foliated unit, the interpreted age dates are not distinguishable within error.

170

175

180

185

190

195

200

205

210

215

220

225

230

235

240

Age = 216.0 ± 8.4 Ma

Mean = 216.0 ± 5.3 Ma

Age = 206.8 ± 6.7 Ma

Mean = 206.8 ± 2.6 Ma

ZirconCores

ZirconTips

Interpreted Age 224 ± 5 Ma

Titanite

Age = 189.1 ± 6.5 Ma

Mean = 189.1 ± 3.3 Ma

206

Pb*/

238

U age (Ma)

Ages - LA-ICP-MS Zircons

References Cited

Rapela, C.W., Pankhurst, R.J., and Harrison, S.M., 1992. Triassic "Gondwana" granites of the Gastre district, North Patagonian Massif: Trans. Roy. Soc. Edinburgh, v. 83, p. 291-304.

Saini-Eidukat, B., Bjerg, E., Gregori, D., Beard, B.L. and Johnson, Clark M., 1999. Jurassic Granites in the northern portion of the Somoncura Massif, Rio Negro Province, Argentina. Proc. XIV Argentine Geological Congress, v. 2, p. 175-177.

Saini-Eidukat, B., Bjerg, E., Gregori, D., Beard, B.L. and Johnson, Clark M., 2002. The Alessandrini Complex: Early Jurassic plutonism in Northern Patagonia, Argentina. Proc. XV Argentine Geological Congress, v. 2, p. 253-258.

Acknowledgments

BSE gratefully acknowledges NSF INT-0073993 and INT-9724786, NSF ND-EPSCoR IIP, and an NDSU Grant-in-Aid. DAG and EAB gratefully acknowledge the SGCyT (UNS) and CONICET for financial support. Cathodoluminescence images were obtained at the M.I.T. electron microscopy facility. Thanks to Garret Hart, and Nancy Mahlen!

Methods

Rb-Sr isotope measurements on whole rocks and mineral separates were carried out at the University of Wisconsin-Madison Radiogenic Isotope Laboratory on a 16-sample VG Sector 54 six-collector mass spectrometer with Daly multiplier. AC zircons separated at UW-Madison, and zircons and titanite from the deformed granite separated at U of Arizona, were analyzed for single grain U-Pb systematics on Arizona's Micromass Isoprobe multicollector ICPMS.

Nine zircons with age 223 ± 6 Ma

In the foliated unit, 3 zircon age populations can be discerned out of 50 analyses:

1) a coherent group of 6 cores + 1 tip with age 224 ± 5 Ma;

2) out of 21 cores, a coherent group of 9 shows 216.0 ± 8.4 Ma, and

3) out of 20 tips a group of 8 shows 206.8 ± 6.7 Ma.

A group of 5 of 9 titanites shows 189.1 ± 6.5 Ma, which agrees within error with the 192 Ma Rb-Sr age of the Alessandrini Complex.

Introduction