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

1. PRE-ANDEAN (Southern Coastal Batholith & North Patagonian Massif) ANDEAN: Mesozoic-Cenozoic Patagonian Batholith Jurassic Volcanic Rocks (Chon Aike, Lonco Trapial, Marafil) 35° S T M 40° S P 45° S 70° W Saini-Eidukat et al., 2002 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 The Alessandrini Granitic Complex The Foliated Unit 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 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 unit consisting mainly of foliated granites that is previously had been interpreted to be the Mamil Choique Formation (Ordovician-Devonian), the Alessandrini Complex comprising granites and related rhyolite dikes (Saini-Eidukat et al., 2002), mafic dikes that crosscut both the granitoids and rhyolitic dikes, and 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. View of the Alessandrini granite quarry and hand specimen. Left: showing multiple generations of crosscutting rhyolitic dikes. Right: Mylonitic fabric with rotated clast. Ages - LA-ICP-MS Zircons Ages - Rb-Sr 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 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 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. QA-5: Feldspar and biotite mineral separate + whole rock 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. 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) Cathodoluminescence images of zircons CONCLUSIONS The measured ages could be interpreted as 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), 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 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. Nine zircons with age 223 ± 6 Ma Ma 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. Figure 3. Geologic map of the El Cuy area (this study). 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!