New mapping, 40Ar/39Ar ages and thermobarometric constraints for the Late Triassic Central Patagonian Batholith, Argentinean Patagonia

• Autores: Zaffarana, Claudia Beatriz; Somoza, Rubén; López de Luchi, Mónica
• Año de publicación: 2015
• Idioma: inglés
• Tipo de recurso: documento de conferencia
• Materia: Ciencias Exactas y Naturales; New mapping; thermobarometric constraints; Patagonian Batholith; Ciencias Exactas y Naturales
• Institución: Universidad Nacional de Río Negro

Fil: Zaffarana, Claudia Beatriz. IIPG-Consejo Nacional de Investigaciones Científicas (CONICET). Argentina.
Fil: Somoza, Rubén. IGEBA-CONICET. Argentina.
Fil: López de Luchi, Mónica. Instituto de Geocronología y Geología Isotópica, INGEIS, Facultad de Ciencias Exactas y Naturales- Universidad de Buenos Aires Pabellón INGEIS. Argentina.
This contribution reports a reassessment of the units of the Late Triassic Central Patagonian Batholith (CPB, North-Patagonian Massif, argentinean Argentina) based on a detailed field mapping and petrographic studies complemented by new 40Ar/39Ar biotite ages. This batholith bears magmatic to solid-state foliations, and its emplacement was interpreted as due to the activity of a major transcurrent fault during the Early Mesozoic. The CPB consists of the Gastre and the Lipetrén superunits. The Gastre Superunit is composed of four units, where the oldest one is the Equigranular Hornblende-Biotite Granodiorites. This unit is characterized by medium to coarse-grained grey equigranular granodiorites and monzogranites with a modal composition given by plagioclase (40- 60%), quartz (15-35%), K-feldspar (15-20%), hornblende (5-8%), biotite (2-5%), titanite, apatite and opaque minerals (1-5%). This unit is in transitional contact with the Porphyritic Biotite-Hornblende Monzogranites, which are a suite of monzogranites and granodiorites composed of euhedral Kfeldspar megacrysts immersed in a medium-grained groundmass with hypidiomorphic texture. A third unit, the Equigranular Biotitic Monzogranites, forms stocks of light pink, medium-to coarsegrained equigranular monzogranites which intrude the previously mentioned units. The stocks are constituted by plagioclase (30%), microcline (35-20%), quartz (35-25%), biotite (10%), hornblende (5% or absent) and titanite, apatite and titanomagnetite (5%). The Gastre Superunit of the CPB is finally composed of the The Hornblende Quartz-Diorites, a unit characterized by three stocks of dioritic to quartz-monzodioritic composition and by many dioritic to quartz-dioritic dikes. The stocks intrude and are, in turn, locally intruded by the other units (intrusion-reintrusion phenomena). The dikes intrude all the other units of the Gastre Superunit. Data of mineral chemistry from a granodiorite belonging to the Porphyritic Biotite-Hornblende Monzogranites and from a tonalite belonging to a stock of the Hornblende Quartz-Diorites allowed us to constrain emplacement depth and crystallization temperature in these units. Emplacement depth is bracketed between 6 and 11 kilometers (1.8 to 3 kbar using the calibration of Schmidt 1992), with maximum recorded temperatures of emplacement comprised between 760 and 800⁰C (Holland and Blundy 1994 geothermometer). The Porphyritic Biotite-Hornblende Monzogranites yield a 40Ar/39Ar age in biotite of 213±5 Ma.