ARC-RELATED PATAGONIAN BASINS: EXTENDING THE LATEST NEOPROTEROZOIC TO CAMBRIAN PERI-GONDWANA REALM INTO CENTRAL NORTHPATAGONIAN MASSIF

RAPALINI, A.; MARTINEZ DOPICO, C.I.; LÓPEZ DE LUCHI, M. G.

Rio de Janeiro

Congreso; 49º Congresso Brasileiro de Geologia; 2018

Sociedad Brasileña de Geologia

The Early Cambrian Tardugno Orthogneiss (528-522 Ma) is a low to moderate peraluminous high-K calc-alkalic magnesian granodiorite-monzogranite (68- 73wt%) that crops out in Nahuel Niyeu area, northeastern Patagonia. The rocks are variably mylonitized under high strain and low-T conditions; No enclaves were observed. Geochemical Sr, Nd and Hf data suggest a greywacke-type source of Mesoproterozoic origin (Nd and Hf TDM -2S model ages ca 1.5 Ga). Random correlations between ASI, alkalis, P2O3 vs SiO2 are observed. In turn, positive linear trends are shown when mafic phase content is function of SiO2, K2O and Ba. Moderate REE slope and low Sr/Y suggest plagioclase retention in the source whereas, biotite and inclusions seemed to have remained as peritectic entrained phases in the magma. Recent datings in leucocratic differentiates within the Early Cambrian metaclastics yielded ca 515 Ma. Even though this age is not sound for the Patagonian framework, detrital zircon grains between 520 and 510 Ma in the metasedimentary rocks are not infrequent. Maximum depositional ages for metaclastic Nahuel Niyeu, El Jagüelito, Mina Gonzalito rocks in the NE and Colo Niyeu Fm. in central northern Patagonia span between 535 and 515 Ma. K-S statistical parameter of likeness among detrital populations does not allow separating in origin these depocenters. WR geochemical analysis and detrital ages cumulative function indicates intra arc to forearc environments extended for more than 300 km (at present) from Atlantic coast until Los Menucos area, across Northern Patagonia. The most conservative depositional age for the depocenters is bracketed between 520 and 480 Ma. Combined detrital spectra (N > 500 detrital grains) indicates highly concordant clusters around Paleoproterozoic (1.8-2.2 Ga -Transamazonian orogeny) and late Mesoproterozoic to early Neoproterozoic (0.9-1.2 Ga Grenville Orogeny) whereas the largest group has late Neoproterozoic- Criogenian- to mid-Cambrian ages (0.7-0.5 Ma Brasiliano- Pampean orogenies). Comparison of whole rock geochemistry of the metasedimentary rocks of north Patagonia prevents considering early Cambrian granites (540-525 Ma) from the Eastern Sierras Pampeanas and Río de la Plata Craton as sources for the Patagonian basins. In turn, a local supply from the early Cambrian synkinematic Tardugno Orthogneiss and mid-Cambrian volcanic arcs (520-510 Ma) in Río de la Plata and Saldaha belt in South Africa are envisaged as the main source based on Nd, Hf- isotope correlation and REE-patterns. Evidence across Northern Patagonia suggest that an active contractional convergent tectonic setting in which Tardugno magmatism (Arc) was active from the latest Neoproterozoic until ca 522 Ma. Arc building produced a first stage of subsidence (intra-arc basins). After ca 515 Ma, extension and decompression in intra-arc and fore-arc domains would have produced a critical erosion of the Tardugno Arc, an increase in the subsidence rate and evolution into backarc basins, such as Mina Gonzalito, and the generation of younger peraluminous melts (