Hf AND O ISOTOPIC SIGNATURE OF 840–730 MA A-TYPE GRANITES AND FELSIC IGNEOUS ROCKS IN SOUTHERN SOUTH AMERICA.

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

Ávila

Simposio; Seventh Hutton Symposium on Granites and Related Rocks.; 2011

Universidad de Salamanca. Universidad de Granada. Escuela Politécnica Superior de Ávila.

Extension-related magmatism occurred at 840–730 Ma on both ‘Pacific’ and ‘Atlantic’ margins of southern South America. On the Pacific side, 840–770 Ma A-type granites intruded the 1330–1030 Ma Western Sierras Pampeanas (WSP) basement rocks of the Central Andes; on the Atlantic side the 1300 km long Dom Feliciano Belt developed on extensional basins with 840-730 Ma felsic volcanism and leucocratic A-type granites. We report Hf and O isotopic data for SHRIMP-dated zircons from these regions. The Mesoproterozoic WSP basement presents two contrasting tectonic regimes: a juvenile oceanic arc and back-arc was located in the sierras de Pie de Palo and Ullún, whereas the sierras de Maz, Espinal and Umango consisted of older continental crust. Mylonitic A-type granites in the Sierra de Pie de Palo (774 ± 6 Ma, 74–72 % SiO2, εNdt = +4, 87Sr/86Sr initial < 0.703) contain zircons with very primitive Hf and O isotopic mantle signatures (n= 11, εHft = +8 to +11, δ18O = +4.7 to +5.7 ‰). An average two-stage Hf model age of 1025 Ma suggests derivation from a source such as the juvenile 1027 ± 17 Ma TTG suite of the basement. The A-type granites of the Sierra de Maz (846 ± 6 Ma, 69 % SiO2, εNdt = +0.3, 87Sr/86Sr initial = 0.7068) and Sierra del Espinal (842 ± 5 Ma, 78 % SiO2, εNdt = -1.5, 87Sr/86Sr initial = 0.7067) also have zircons reflecting juvenile sources, but probably contaminated by variable proportions of Mesoproterozoic continental crust (Maz, n= 14, εHft = +2 to +5, δ18O = +2.0 to +2.8 ‰ and Espinal, n= 5, εHft = -0.2 to +1.3, δ18O = +4.1 to +5.1‰). This is consistent with older average Hf model ages of 1460–1610 Ma. The δ18O of the Sierra de Maz sample is lower than typical mantle values, possibly suggesting an anomalous mantle source. The origin of the 840–740 Ma magmatism in the Dom Feliciano Belt was inferred from two samples of the Early Neoproterozoic Punta Mogotes Formation, whose detrital age patterns are dominated by conspicuous peaks within this range. 28 grains with concordant U–Pb ages of 840–720 Ma (peaks at 790–760 Ma) yielded an average εHft of -10 and δ18O = +7.8 ‰, pointing to a crustal origin for the magmatic source rocks. Hf model ages are dominantly Palaeoproterozoic (average 2270 Ma), up to Neoarchaean (3150 Ma), indicating recycling of old crust. These data indicate that sources of the Early Neoproterozoic granites of the WSP were different from those of the coeval magmatism in the Dom Feliciano Belt, and that the two regions evolved separately.