COUPLED U-PB AGES AND HF ISOTOPE COMPOSITIONS OF SINGLE DETRITAL ZIRCONS: A NEW PROVENANCE TOOL APPLIED TO SEDIMENTS FROM THE SW SOUTH AMERICAN GONDWANA MARGIN

Several analytical techniques can be applied to detrital zircon grains in order to obtain clues on the provenance of sediments. In this study we combined U-Pb dating of single detrital zircons with the initial Hf isotope composition of the same grains. This new technique provides new insights into the history of the source rocks prior to crystallisation of the zircons. We applied this tool to Late Paleozoic sedimentary rocks from southern Chile and Argentina. U-Pb SHRIMP analyses of detrital zircons from four localities gave maximum depositional ages of 328-387 Ma, as inferred from the youngest grains. Of the 240 analysed zircons of both metamorphic and magmatic origin, ca. 90 % are younger than 1500 Ma. U-Pb age maxima at 400-600 Ma and 1000-1500 Ma represent the predominance of rocks of Panafrican and Grenvillian ages in the source areas. Hf isotope compositions in single zircon grains as small as 50 mm were measured using the MC-ICP-MS at ZLG, Münster. Only grains with one apparent crystallization phase were selected. Our Hf isotope results, from zircons ranging in age from 0.3-3.5 Ga, indicate the predominance of recycled crustal sources. For the zircons younger than 1.5 Ga, two-stage Lu-Hf model ages indicate crustal residence times of the protoliths from 600-1400 Myrs prior to zircon crystallization (T_DM(Hf)=ca. 1.0-2.2 Ga). However, positive initial Hf (e_Hf(T)=+7 to +12) in a zircon population of Grenvillian (1.0-1.3 Ga) age indicates some juvenile sources. The sediments, deposited in a passive margin environment, are suggested to mainly have been fed by rocks from East Antarctica and southern Africa, as well as locally from Patagonia. The results tentatively suggest, that the source rocks of the juvenile zircons could be located to the present north and to the east of the study area, e. g. in the Precordillera or on the Falkland Islands.