Hf and O isotopic signature of 840-730 Ma A-type granites and other felsic magmatisms in southern South America

RAPELA, C.W.; PANKHURST, R.J; CASQUET, C.; BALDO, E.G; COLOMBO, F.; DAHLQUIST, J.A.; GALINDO, C.; FANNING, C.M

Avila

Simposio; 7th International Hutton Symposium on the Origin of Granites and related rocks; 2011

Recent studies of the Pacific and the Atlantic sides of the southern part of the South American plate show evidences of 840-750 Ma magmatism associated with extensional events. In the Pacific side, 840-770 Ma A-tpe granites intruded the 1330-1030 Ma Western Sierras Pampeanas (WSP), that are part of the modern Central Andes basement. On the southeastern side of the continent, the 1300 km long Dom Feliciano Belt, is developed on extensional basins, that include 840-730 Ma felsic volcanism and leucocratic A-type granites. We report here the first Hf and O isotopic determinations in zircons of A-type granites of the WSP, carried out on the same samples and spots of known U-Pb SHRIMP ages.Previous studies of the WSP reveals two contrasting scenarios that evolved separately during the 300 Ma long history: the juvenile oceanic arc and back-arc sector of Sierra de Pie de Palo and Ull¨²n, and the sierras de Maz, Espinal and Umango, which were always part of a continental crust. In the Sierra de Pie de Palo, 774 ¡À 6 Ma mylonitic A-type granites (whole rock: 74-72 % SiO2; ¦ÅNdt = +4.2; 87Sr/86Sr initial = 0.7031-0.7006), show zircons with very primitive Hf and O isotopic mantle signatures (n= 11; ¦ÅHft = 11.2-8.0; ¦Ä18O¡ë = +5.74- +4.72). The average Hf model age (TDM2nd stage = 1025 Ma), suggest derivation from a source of similar age to the youngest, juvenile TTG suite of the basement (1027 ¡À 17 Ma).The A-type granites of the Sierra de Maz (846 ¡À 6 Ma; whole rock: 68.8 % SiO2; ¦ÅNdt = +0.3; 87Sr/86Sr initial = 0.7068), and Sierra del Espinal (842 ¡À 5 Ma; whole rock: 77.5 % SiO2; ¦ÅNdt = -1.5; 87Sr/86Sr initial = 0.7067), show zircons also dominated by juvenile sources, but probably contaminated by variable proportion of Mesoproterozoic continental crustal components, Maz: n= 14; ¦ÅHft = +5.0- +1.7; ¦Ä18O¡ë = +2.83- +2.01 and Espinal: n= 5; ¦ÅHft = +1.25- -0.18; ¦Ä18O¡ë = + 5.11- +4.10. This is also suggested by older average Hf model ages (TDM2nd stage = 1460-1610 Ma). The ¦Ä18O of the sample of Sierra de Maz, is lower than typical mantle values (¦Ä18O¡ë ~ +5.3), suggesting an anomalous mantle source or interaction of the granitic melt with hydrothermal fluids.In the Dom Feliciano Belt, the origin of the 840-740 Ma magmatism was inferred from detrital zircons of sedimentary samples dominated by conspicuous peaks within this age range. Two samples of the Early Neoproterozoic Punta Mogotes Formation (n=28, range:840-720 Ma; peaks at 790-760 Ma) show concordant detrital igneous zircons, with average ¦ÅHft = - 10.49 and ¦Ä18O¡ë = +7.78. These values point to a crustal origin for the Early Neoproterozoic magmatism of the Dom Feliciano Belt, and Hf model ages are dominated by Palaeoproterozoic ages (average TDM2nd stage = 2270 Ma), up to Neoarchaean ages (3150 Ma), that indicate recycling of an old crustal source. These data indicate the Early Neoproterozoic granites of the WSP was not the source of the coeval magmatism in the Dom Feliciano Belt, and that the two regions evolved separately.