PETROCHRONOLOGY OF A GARNET SILLIMANITE-BEARING MICASCHIST FROM THE RÍO DE LA PLATA CRATON EXPOSED AT THE CAPITAL OF URUGUAY

MARTINEZ JUAN CRUZ; MASQUELÍN HENRY; MASSONNE, HANS -JOACHIM; DRISTAS A. JORGE; BIANCHI FRANCO DENIS

Congreso; XXI Congreso Geológico Argentino; 2022

Asociación Geológica Argentina

Paleoproterozoic basement rocks of the Rio de la Plata Craton in Uruguay (Piedra Alta Terrane) are grouped in a central granitic-gneiss belt (Florida Belt), two supracrustal metamorphosed volcano-sedimentary belts (Arroyo Grande and San José Belts) and several post-orogenic granitoids and gabbros (Oyhantçabal et al., 2018). The Montevideo Formation is the southernmost unit of the San José Belt composed of micaschist, ortho- and paragneisses of the greenschist to amphibolite facies (Oyhantçabal et al., 2018). A maximum sedimentation age of 2145 ± 41 Ma for the protoliths of the San José Belt was constrained by crystallization ages of zircon (U-Pb SHRIMP) in a metarhyolite from the northern volcano-sedimentary unit (Paso Severino Formation) of this belt (Bossi et al., 2001). A regional whole-rock Rb-Sr isochron obtained from 11 samples of metamorphic rocks yielded an age of 2094 ± 28 Ma, which was related to the metamorphic episode recognized in the Piedra Alta Terrane (Preciozzi et al., 1999). As we wanted to gain knowledge on the metamorphic pressure (P) and temperature (T) evolution and age of the Montevideo Formation, we carried out T-H2O and P-T pseudosection modeling and U-Th-Pb in situ monazite dating of a garnet-bearing micaschist intruded by a granodiorite at Punta Sayago (Fig. 1a) in Montevideo city. The studied rock displays garnet porphyroblasts in a lepidogranoblastic matrix composed of biotite + muscovite + plagioclase + quartz + sillimanite and local K-feldspar. Idioblastic pre- to syn-kinematic almandine-rich garnet enclosed quartz and biotite, arranged in an internal foliation (Si) different from the external, NW-SE striking dominant foliation (Se), in its core. A bulk-rock composition of the micaschist was modeled in the 11-components system Si-Ti-Fe-Mn-Mg-Ca-Na-K-OH with the PERPLE_X software package (Connolly, 2005; downloaded in January 2021 from http://www.perplex.ethz.ch/). The P-T pseudosection modeled at H2O saturation conditions as suspected of prograde metamorphic conditions do not fit the observed mineral assemblage in the studied rock. However, calculations with a fixed undersaturated water amount of ~1.2 wt.%, that resulted from a T-H2O pseudosection calculated at 5 kbar, yielded the observed mineral assemblages and the corresponding mineral modes. The modeling, using P-T fields of mineral assemblages, isomodes and isopleths for different chemical parameters in garnet and biotite, and X-ray compositional maps of zoned garnet prepared with the electron microprobe allowed us to decipher the following clockwise P-T path (Fig. 1b): (a) an early garnet core with the composition of Pyr8.5(Gro + Andr)6Spes5.5Alm81 crystallized at 4.8 kbar and 590 °C, (b) subsequent heating to 620 °C at 4.4 kbar is recorded by a rimward decrease of grossular (+andr) to 5 mol%, and (c) nearly isobaric cooling to conditions of 4.2 kbar and 510 °C accompanied by the increase of grossular (+andr) and decrease of pyrope to 8 mol.% and 6.5 mol.% at the outermost garnet rim, respectively, followed. The composition of the latter garnet equilibrated with biotite (XMg ~ 0.42) of the Se foliation. Twelve monazite crystals, hosted in quartz, plagioclase, and muscovite were analyzed with a Cameca SX 100 electron microprobe to provide a total of 19 U-Th-Pb monazite ages. Diagrams of elemental compositions vs single ages show two age populations without significant chemical differences. The U-Th-Pb ages and their associated errors (1σ) were statistically treated with the IsoplotR software (Vermeesch 2018) to yield the weighted mean ages at 2071 ± 8 (2σ) Ma (group I) and 2025 ± 7 (2σ) Ma (group II; Fig. 2). The obtained results suggest subsolidus thermal peak metamorphic conditions for the studied garnet-sillimanite-bearing micaschist after the intrusion of a granodiorite at Punta Sayago in a water-undersaturated country rock at the age of monazite group I. The following exhumation path occurred at progressively younger ages of monazite group II.