P-T-d PATH OF A GARNET-BEARING METAGRANITE FROM THE PALEOPROTEROZOIC BASEMENT OF THE TANDILIA TERRANE, RIO DE LA PLATA CRATON, ARGENTINA

ANGELETTI, MELISA; MASSONE, HANS-JOACHIM; MARTÍNEZ, JUAN C.; FRISICALE, M. CRISTINA

Hamburgo

Congreso; Latin-American Colloquium of Geosciences; 2019

Universität Hamburg / German Science Foundation

The Tandilia Terrane (TT), in central-eastern Argentina and southeastern Uruguay (Pamoukaghlian et al.,2017) is a Paleoproterozoic igneous-metamorphic complex where the southernmost portions of the Rio de la Plata craton are exposed. The study area includes small outcrops of garnet-bearing metagranite at the LaVirgen hill, western TT. To constrain the P-T-d evolution of this rock, we considered microstructures and a PT pseudosection contoured with chemical parameters of interest. This pseudosection was constructed with the PERPLE_X software for a selected bulk-rock composition in the 11-component system Si-Ti-Al-Fe-Mn-Mg-Ca-Na-K-O-H. The analyzed rock is a slightly deformed, peraluminous (ASI= 1.07), grey granite composed of (~ vol.%): K-feldspar (Kfs, 37), quartz (Qtz, 30), plagioclase (Pl, 18), biotite (Bt, 7), garnet (Grt, 5), muscovite (Ms, 2), and zircon, apatite, monazite, ilmenite (Ilm), and magnetite (Ma) as accessory phases (1). The porphyroclastic texture is characterized by 15 vol.% of a fine-grained (20-100 mm)granoblastic matrix.The derived P-T path is based on garnet zoning. The garnet core composition pyr4(gro+andr)12spes10alm74, which equilibrated with andesine (XAn = 0.37), yielded P-T conditions of 6 kbar and 650°C (Fig. 1, point A) above the solidus (~7 vol.% of melt). The stable assemblage at these conditions is Ms+Pl+Kfs+Ilm+Ma+Grt+Bt+Qtz+melt, as in the natural rock. Garnet recorded a rimward decreased in pyrope from 4 to 3 mol% by slow cation diffusion to its stability limit which is reached at 5.5 kbar and 620°C (Fig. 1, point B). The subsequent path is constrained by metamorphic muscovite with Si contents of3.09-3.17 Si per formula unit and XAn= 0.36 in plagioclase.The deciphered P-T path in the studied rock demonstrates the cooling of granitic material at near solidus conditions and the exhumation from the middle crust (22 km), under an apparent geotherm of ~ 30 ºCkm-1, to upper crustal depths of ~7 km. The ductile microstructures of deformation in the protomylonitic graniteare consistent with microstructures in feldspars (deformation twins, myrmekites, and flame perthites) from a 1.5 km distant granitic rock of the Siempre Amigos area which were restricted between 650º and 430º C (Angeletti et al., 2016). The exhumation path suggested for the studied metagranite is similar to that of thegarnet-bearing postcollisional leucogranite of San Veran hill, asigned to Trans-Amazonian orogeny cycle (Martínez et al. 2017).