The Alessandrini Complex: Early Jurassic plutonism in Northern Patagonia, Argentina.

SAINI-EIDUKAT, B; MIGUELES, N; GREGORI, D; BJERG, E A; BEARD, B L; JOHNSON, C M

Calafate

Congreso; XV Congreso Geológico Argentino; 2002

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;} --> Extensive outcrops of plutonic and volcanic rocks are found in the northwest part of Patagonia, Argentina. In the La Esperanza area, Llambias and Rapela (1984) and Rapela and Llambias (1985) recognized the existence of an event related to the Gondwanan Magmatic cycle. According to these authors, two complexes form part of this event. The first and older, the La Esperanza plutonic complex, comprises granodiorites, granites, pegmatites, and aplites. The upper complex, Dos Lomas Plutonic and Volcanic Complex, is composed of rhyolitic lavas, ignimbrites, rhyolitic domes and granitic bodies. Somewhat further north, in the EI Cuy area, Bjerg et al. (1997), Saini-Eidukat et al. (1999, 2000) and Gregori et al. (2000) mapped and described well-exposed outcrops of plutonic, volcanic, and metamorphic rocks. These rocks trend NE-SW for more than 40 km, following the Canadon Soledad , between the Patu-Co, La Totora and EI Loro lineaments. At least two units can be differentiated stratigraphically. The older, foliated, unit consists of porphyritic granites, aplites and pegmatites. These rocks were assigned by Bjerg et al. (1997) to the Mamil Choique Formation due to their similarity. The younger units had been assigned to the Donosa granite, a component of the La Esperanza Plutonic Complex. However, a Rb/Sr mineral isochron (Saini-Eidukat et al., 1999) indicated that the crystallization age of this unit is 192 ± 0.2 Ma, thus calling into question the previous interpretation.