2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 14
Presentation Time: 8:00 AM-12:00 PM


SAINI-EIDUKAT, Bernhardt1, BEARD, Brian2, BJERG, Ernesto Alfredo3, GEHRELS, George4, GREGORI, Daniel5, JOHNSON, Clark2, MIGUELES, Nathalia5 and VERVOORT, Jeffery D.6, (1)Department of Geosciences, North Dakota State Univ, Fargo, ND 58105-5517, (2)Geology & Geophysics, Univ of Wisconsin, Madison, WI 53706, (3)Geología, Universidad Nacional del Sur, San Juan 670, Bahia Blanca, B8000ICN, Argentina, (4)Geosciences, Univ of Arizona, Tucson, AZ 85721, (5)Geología, Universidad Nacional del Sur, San Juan 670, Bahia Blanca, B8000ICN, (6)Department of Geology, Washington State Univ, Pullman, WA 99164-2812, bernhardt.saini-eidukat@ndsu.nodak.edu

The Somoncura Magmatic Belt in N Patagonia comprises a suite of intrusive and extrusive silicic units in a region overlapped by two major magmatic provinces: the older Choiyoi to the N and W, and the younger Chon Aike to the S. We are determining age constraints as part of a larger study of the region’s tectonic setting.

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.

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. Rb-Sr isotope measurements were conducted at UW-Madison on whole rocks and mineral separates.

For the AC, a coherent group of 9 of 18 grains gives an age of 223 +/- 6 Ma, significantly older than the Rb-Sr age of 192 +/- 0.21 Ma (MSWD=0.326) based on feldspar, biotite and whole rock analyses. However, a group of 4 grains with age 195 +/- 3.1 Ma is also present. CL images of grains show little or no zoning.

In the foliated unit, 3 zircon age populations can be discerned out of 50 analyses: a coherent group of 6 cores + 1 tip with age 224 ± 5 Ma; out of 21 cores, a coherent group of 9 shows 216.0 ± 8.4 Ma, and 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 AC.

The measured ages could be interpreted as a) both units are contemporaneous at 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 AC, during which the U-Pb system closed at a higher temperature/earlier time and the Rb-Sr system at a lower temperature/later time, or c) older ages in the AC are measured on inherited zircons.