DEEP CRUSTAL METAMORPHIC P-T EVOLUTION OF A METADACITIC DYKE FROM THE PALEOPROTEROZOIC BASEMENT OF THE TANDILIA BELT, RIO DE LA PLATA CRATON, ARGENTINA

MARTINEZ JUAN CRUZ; MASSONNE, HANS -JOACHIM; DRISTAS, JORGE A.

Heidelberg

Congreso; 24th International Colloquium on Latin American Earth Sciences; 2017

Heidelberg Center for the Environment

The Rio de la Plata craton (RPC), exposed in theTandilia belt, central-eastern Argentina, includes a complex association ofigneous and metamorphic rocks mainly of a Paleoproterozoic orogenic cycle.Among these rocks, those from a single outcrop at El Quebracho hill, located at road 226 c. 60 km SE of Tandil, turned out to besuitable to decipher the metamorphic P-T path of a portion of the RPC.For this purpose, we applied the electron microprobeto analyze minerals and the PERPLE_X software to calculate P-T pseudosectionsfor a bulk-rock composition in the 11-component system Si-Ti-Al-Fe-Mn-Mg-Ca-Na-K-O-H and to contour them by various chemical parameters. We also used the geothermobarometricapproach by Anderson and Smith (1995) on amphibole-plagioclase pairs andvarious thermometers (Anderson 1996).The studied rockis a dark, foliated porphyritic metadacite with rounded plagioclase phenocrystsin a planar-linear metamorphic fabric. The original igneous rock had intruded agranodioritic gneiss, forming a ~30 m thick main body and branches of smalldykes, being up to 8 m thick. Both rock types display local intense shearing.The metadacite is composed of (vol.%): quartz (25), plagioclase (20), biotite (18),amphibole (10), epidote (10), titanite (8), garnet (5), and K-feldspar (4).Clearly zoned porphyroblasts of garnet allowed us to derive two prograde metamorphicstages. The core composition of pyr0.5(gro+andr)36spes27.5alm36yielded P-T conditions of 0.95 GPa and 540 °C with the calculated mineralassemblage epidote + phengite + Ca-amphibole + Na-Ca-amphibole + plagioclase +K-feldspar + garnet + titanite + quartz and H2O. The outermost rimcomposition pyr0.8(gro+andr)41.5spes16.2alm41.5resulted in conditions of 1.15 GPa and 590 °C, at the onset of melt. Theaforementioned mineral assemblage also occurs at these P-T conditions withoutfree H2O. The unique Ca-amphibole (magnesiohastingsite)analyzed in the natural rock and its associated plagioclase (Ab81)yielded conditions of 1.07 GPa and 575 ºC near the solidus, on the earlyretrograde P-T path. The geotectonic setting for this metadacite isconsistent with a relatively cold subduction zone where a low geothermalgradient of ~15 Cº km-1 was reached at depths of 38-40 km in thelower crust. A similar geotectonic setting was deduced for an amphibole-biotitegneiss of El Cristo hill (Martínez et al. 2016) located 15 km SW of the studiedarea.