New Mesoproterozoic ages in the Bloques de San Rafael and Las Matras, western Argentina

GREGORI DANIEL; GERALDES MAURO; BENEDINI LEONARDO; STRAZZERE LEONARDO; STREMEL AGUSTÍN ; DIAZ PABLO ESTEBAN

Armação dos Búzios, Rio de Janeiro, Brazil

Simposio; XIII South American Symposium on Isotope Geology; 2024

SSAGI Buzios Secretariat

In order to improve the age of the Mesoproterozoic units of western Argentina, rocks from the Cerro Ventana Formation, in the Mendoza province and the Las Matras Complex in La Pampa province were dated using LA-ICP-MS U-Pb in zircon methodology at the Multilab, Universidad Estatal de Río de JaneiroIn the first unit, located in the San Rafael Block, two samples were analized. CV 02 is located in the Arroyo Ponón Trehue (35°10´17.45"S- 68°18´27.15"W). There, the coarse-grained granitic rocks are notably deformed. Discarding all zircons located outside the Concordia curve, 7 zircons were used for calculation and yielded a U-Pb age of 1270 ± 6.9 Ma with a MSWD of 6.Another sample (CV05) was collected at the Río Seco de los Potrillos, located 7 km south of the first locality (35°14´24.60"S- 68°17´12.63"W). It is a dark-colored tonalite-granodiorite, covered by fault by stratified Carboniferous sedimentary rocks. This is the first time that results have been obtained for this location. Twelve zircons yielded an age of 1125.3 ± 4.5 with a MSWD of 0.23.Samples from Arroyo Ponón Trehue and others from Río Seco de los Leones had previously been analyzed by U-Pb in zircons by Cingolani et al (2017), obtaining an age of 1214.7 ± 6.5 Ma.Our sample is located 3 km south of those of the Cingolani et al. (2017) from the Río Seco de los Leones, which allows us to confirm the extension and age of the Cerro Ventana Formation.Cingolani et al. (2017) dated orthogneiss from Lomas Orientales region (48 km NW of Ponón Trehue) which yield a Concordia age of 1256 ± 10 Ma, our results agree with the result of these authors.In the case of the Las Matras Complex, two samples were analyzed (LM 02 and LM 04) and are located at 36°47´36.75"S- 67° 5´40.79"W. One corresponds to a granite and the other a tonalite. As above indicated only zircons that fall in the Concordia curve were used to determine the age. Using these results, an age of 1216 ± 8.9 Ma was obtained, with a MSWD of 0.58. In the case of LM 4, the result, using 7 zircons, is 1247 ± 6.6 with a MSDW of 1.4.Compared with the Rb-Sr age of 1212 ± 47 Ma, the Sm-Nd age of 1178 ± 47 Ma and the U-Pb age in zircons of 1244 ± 42 Ma published by Sato et al. (2004) it can be recognized that the U-Pb results are concordant considering the error limits.It should be noted that our sample CV05 is about 90 Ma younger than the samples located in Río Seco de los Leones and 145 Ma younger than those from Ponón Trehue, indicating the existence at less of two magmatic cycles.These results confirm the presence of Mesoptoterozoic rocks in both regions, San Rafael and Chadileuvú blocks, indicating the presence of an orogen of such age more than 250 km in a NW direction, although the deformation of the rocks varies widely. This segment can be extended to the Sierras de Pie de Palo, Umango and Maz, in the San Juan and La Rioja provinces. Together they constitute a strip of more than 850 km in an N-S direction, suggesting an extension of Grenvillian-age basement rocks in the San Rafael and Las Matras blocks.