RB-SR AND U-PB AGE SYSTEMATICS OF THE ALESSANDRINI SILICIC COMPLEX AND RELATED MYLONITES, PATAGONIA, ARGENTINA

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.