U-Pb ages of komatiitic rocks from Sierra de San Luis, Argentina

SATO, A.M.; GONZÁLEZ, P.D.; LLAMBÍAS, E.J.; BASEI, M.A.S.

Punta del Este (Uruguay)

Simposio; 5th South American Symposium on Isotope Geology; 2006

Facultad de Ciencias (Universidad de la Republica)

In western Sierra de San Luis (Sierras Pampeanas, Argentina), komatiitic rocks appear as part of the volcano-sedimentary protolith of the Nogol¨ª Metamorphic Complex, which underwent two major tectonic events associatied with high-grade metamorphism The second event is the best known and results from the Early Paleozoic Famatinian Orogeny, while available data constrains the first event as pre-507Ma. Although mainly transformed into amphibolites, mafic-ultramafic rocks preserve locally their primary spinifex texture and geochemical features that allow their classification as komatiites, komatiitic basalts and high-Fe basalts. One sample of komatiitic basalt (RQ-25-1) and another of komatiite (RQ-25-2) were studied by conventional U-Pb analysis, with additional SHRIMP dating on the second one. Zircon crystals from sample RQ-25-1 are mainly subrounded, ovoid to multifaceted, as well as resorbed grains, probably formed by metamorphism; they align with a lower intercept of 461 ¡À 5 Ma in the concordia diagram. Sample RQ-25-2 yielded in addition minor prismatic, high-U zircons that show recent Pb loss. Its conventional dating result is less clear, with two zircon fractions tending to an old age of 1204 Ma, and three fractions aligned with a lower intercept of 444 ¡À 14 Ma. Ten points of SHRIMP dating on five zircon crystals þuone of them showing polygonal to oscillatory zoningþu yielded 206Pb/238U ages between 516 and 428 Ma, with a mean value of 483 ¡À 11 Ma. Since crystal morphology and internal structure, U contents and Th/U ratios do not allow a conclusive interpretation, and considering that at least some of the crystals experienced Pb loss, we suggest that the conventional age of 461 ¡À 5 Ma represents the best age for the Famatinian metamorphism, and that an age older than 516 Ma must be related to primary crystallization, which might be in accordance with previous Sm-Nd isochrone age of 1502 Ma.