Geochemical constraints for the Permian Tunnel Tonalite: Implications for the evolution of the SW margin of the Northpatagonian Massif

LÓPEZ DE LUCHI, M. G.; GRILLO VIDAL CAROLINA; MARTÍNEZ DOPICO, C.

Pto Madryn, Chubut

Congreso; XXI Congreso Geologico Argentino; 2022

Asociacion Geologica Argentina

The Tunnel Tonalite (TT, Cerredo and López de Luchi 1997) that crops out NW of Río Chico town is composed of mesocratic biotite-amphibole tonalites and minor diorites in sharp contact with the Cushamen Formation. The crystallization of magmatic epidote+allanite after plagioclase and before biotite suggests pressures ca. 8.5-9.5 kbar which would correspond to the intrusion depth. The high modal (> 10%) amount of epidote indicates that it was formed near the solidus (Cerredo and López de Luchi 1997). SHRIMP zircon age is 295±2 Ma (CUS 131, Pankhurst et al. 2006) whereas a zircon age on magnetic fractions is 286± 13 Ma (AB 70, Varela et al. 2005). TT as well as the 294± 3 Ma Laguna del Toro only available chemical data would support a thick crust at ca. 295 Ma. The first evidence of a thick crust is indicated by the 314 Ma tonalite of Paso del Sapo as proposed Renda et al. (2021) and perhaps by the 318 Ma biotite-garnet 62% SiO2 granitoid of Sierra de Pichiñanes (Pankhurst et al. 2006). Crustal thickness might remain stable or slightly increase from 314 to 295 Ma.