Transcript GSA2004poster_modified

Rb-Sr and U-Pb Age Systematics of the Alessandrini Silicic Complex
and Related Mylonites, Patagonia, Argentina
Bernhardt Saini-Eidukat, Dept. of Geosciences, North Dakota State University, Fargo, ND 58105 USA
Brian 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 USA
3CONICET-Depto. de Geología, Universidad Nacional del Sur, San Juan 670, B8000ICN Bahía Blanca, Argentina
4Department of Geosciences, University of Arizona, Tucson, AZ 85721 USA
5Department of Geology, Washington State University, Pullman, WA 99164 USA
Location
Introduction
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 Alessandrini Granitic Complex
The Foliated Unit
PRE-ANDEAN
(Southern Coastal Batholith & North Patagonian Massif)
ANDEAN: Mesozoic-Cenozoic Patagonian Batholith
Jurassic Volcanic Rocks
(Chon Aike, Lonco Trapial, Marafil)
35° S
M
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.
40° S
T
P
View of the Alessandrini granite quarry and hand specimen.
45° S
Left: showing multiple generations of crosscutting rhyolitic dikes.
70° W
Figure 1. Sketch map of southern South America,
showing the location of the Somoncura Magmatic
Belt. Modified from Rapela et al. (1992).
Ages - LA-ICP-MS Zircons
Ages - Rb-Sr
Right: Mylonitic fabric with rotated clast.
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 - LA-ICP-MS Zircons
0.7100
Sr/
86
Sr
In the foliated unit, 3 zircon age populations can be
discerned out of 50 analyses:
0.7090
QA-5: Feldspar and
biotite mineral
separate + whole
rock
0.7085
1) a coherent group of 6 cores + 1 tip with age 224 ± 5
Ma;
0.7080
0.9
1.0
1.1
1.2
87
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)
Rb/
86
Sr
1.3
1.4
Saini-Eidukat et al., 1999
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.
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.
data-point error ellipses are 2 
0.719
data-point error ellipses are 68.3% conf
0.717
0.715
87
Sr
Sr
86
500
0.08
0.713
400
0.709
Age = 195 ± 11 Ma
0.707
Initial
Sr/ Sr =0.70591 ± 0.00038
MSWD = 7.3
87
86
0.06
0
1
2
3
87
Rb/86Sr
4
Pb/238U
0.705
5
Saini-Eidukat et al., 2002
Cathodoluminescence
images of zircons
300
0.04
200
0.02
0.000
0.0
100
CONCLUSIONS
Nine zircons with
age 223 ± 6 Ma
0.2
0.4
207
.
Pb/235U
The measured ages could be interpreted as
0.6
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
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.
Ma
Methods
Figure 3. Geologic map of the El Cuy area (this study).
Rb-Sr isotope measurements on whole rocks and mineral
separates were carried out at the University of WisconsinMadison 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.
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.
0.711
206
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.
0.7095
87
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.
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.
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!