Abstract

ZAPPETINI, Eduardo O; CHERNICOFF, Carlos J; SANTOS, João O. S  and  MC NAUGHTON, Neal J. The Neoproterozoic Santa Elena Schists, La Pampa Province, Argentina: SHRIMP U-Pb ages, Hf isotope composition and geodynamic implications. Rev. Asoc. Geol. Argent. [online]. 2010, vol.66, n.1-2, pp.21-37. ISSN 0004-4822.

The Santa Helena Schists comprise biotite-garnet schists and minor sillimanite-garnet gneissoid lenses, reaching the higher amphibolite to lower granulite facies metamorphism . Suboutcrops of the schists occur at Estancia Santa Helena. U-Pb SHRIMP dating of the Santa Elena Schists detrital zircon yielded mostly Mesoproterozoic and Neoproterozoic ages, with a small number of Paleoproterozoic grains. Almost all the Mesoproterozoic and Neoproterozoic grains are magmatic and crystallized from a juvenile source (positive εHf), with TDM (Hf) ranging 1127 to 1625 Ma, and 948 to 1274 Ma, respectively. The Paleoproterozoic zircons are also magmatic, with a TDM (Hf) of 2310 Ma. The age of the youngest detrital zircon is 556 Ma, which corresponds to the maximum age for the onset of sedimentation, indicating that deposition probably started by Neoproterozoic times. The absolute absence of metamorphic zircons of Pampean age suggests that during the time of deposition of the schists, the metamorphic nucleous of the Pampean orogen was still not formed and exposed. This implies a great contrast with the provenance pattern of the nearby Green Schists, dominated by Cambrian metamorphic zircons, deposited between ca. 500 Ma and ca. 465 Ma, when the Pampean orogen was already exhumed and subject to denudation. It is interpreted that the Santa Elena Schists would pertain to a supracrustal sequence deposited between 556 and ca. 530-520 Ma in the southwestern Gondwana margin, in a foreland basin equivalent to the later stage of the Puncoviscana basin. It is also interpreted that the main source of sediments was derived from the denudation of a Mesoproterozoic magmatic arc, located to the East of the schists. This arc, equivalent to the precollisional magmatism of the Juruena and Sunsás orogens, would have been accreted to the southwestern margin of the Rio de la Plata craton and, at present, would form part of the substratum of its southwestern Paleoproterozoic ´rim´ and the substratum of the para-authochtonous Pampia terrane. This mechanism of accretion would be similar to that of other Archean-Paleoproterozoic cratonic nuclei, such as the Amazon, Kalahari and Congo. Finally, the other important source of provenance of the Santa Elena Schists indicates the existence of a belt of Neoproterozoic magmatic rocks that would pertain to an arc developed on the eastern margin of Pampia.

Keywords : Santa Helena Schists; La Pampa; Mesoptoterozoic; Neoproterozoic; Pampia; Río de la Plata craton.

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