Rb-Sr and U-Pb Age Systematics of the Alessandrini Silicic Complex and Related Mylonites, Patagonia, Argentina.

SAINI-EIDUKAT, B; BEARD, B; BJERG, E A; GEHRELS, G; GREGORI, D; JOHNSON, C; MIGUELES, N; VERVOORT, J D

Denver

Congreso; Geological Society of America, Denver Annual Meeting; 2004

The hills of La Huerta and Valle Fertil, wich belong to the Western Pampean Ranges in the Province of San Juan, NW Argentina, mainly consist of metagabboic to metadioritic rocks that intruded the Proterozoic basement in the Early Middle Ordovician (Famat <!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman";} @page Section1 {size:21.0cm 842.0pt; margin:70.9pt 3.0cm 70.9pt 3.0cm; mso-header-margin:35.45pt; mso-footer-margin:35.45pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> 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 coarsegrained, 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.