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

CLAUDIA BEATRIZ ZAFFARANA; RUBÉN SOMOZA; LÓPEZ DE LUCHI, M.

Florianopolis

Simposio; 8th Hutton Symposium on Granites and Related Rocks; 2015

New mapping, 40Ar/39Ar ages and thermobarometric constraints for the Late Triassic Central Patagonian Batholith, Argentinean PatagoniaZaffarana CB1, Somoza R2, López de Luchi MG3 - 1IIPG-Consejo Nacional de Investigaciones Científicas (CONICET), General Roca, Argentina, 2IGEBA-CONICET, Buenos Aires, Argentina, 3INGEIS-CONICET , Buenos Aires, 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 K-feldspar megacrysts immersed in a medium-grained groundmass with hypidiomorphic texture. A third unit, the Equigranular Biotitic Monzogranites, forms stocks of light pink, medium-to coarse-grained 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.The Lipetren Superunit is constituted by fine-grained biotite monzo- and syenogranites which intrude all the units of the Gastre Superunit. The monzogranites yielded a 40Ar/39Ar age in biotite of 206 ± 4 Ma. The fine-grained and even granophyric textures in the Lipetrén Superunit suggest a rapid cooling in a subvolcanic environment; therefore its biotite 40Ar/39Ar age would be close to its real emplacement crystallization age. The new 40Ar/39Ar ages, together with previously reported Rb/Sr ages, indicate that a time span ≥7 Ma elapsed between the intrusion of the Gastre and the Lipetren superunits. Unlinke the Gastre Superunit, the Lipetren Superunit does not bear magma hybridization textures.Holland, T., Blundy, J., 1994. Non-ideal interactions in calcic amphiboles and their bearing on amphibole-plagioclase thermometry. Contributions to Mineralogy and Petrology, 116, 433?447.Schmidt, M.W., 1992. Amphibole composition in tonalite as a function of pressure: an experimental calibration of the Al-in-hornblende barometer. Contributions to Mineralogy and Petrology, 110, 304?310.