Sedimentology (2007) 54, 107–136 doi: 10.1111/j.1365-3091.2006.00820.x Zircon and whole-rock Nd-Pb isotopic provenance of Middle and Upper Ordovician siliciclastic rocks, Argentine Precordillera JAMES D. GLEASON*, STANLEY C. FINNEY ,SILVIOH.PERALTAà, GEORGE E. GEHRELS§ and KATHLEEN M. MARSAGLIA– *Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, USA (E-mail:
[email protected]) Department of Geological Sciences, California State University at Long Beach, Long Beach, CA 90840, USA àCONICET, Universidad Nacional de San Juan, 5400 Rivadavia, San Juan, Argentina §Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA –Department of Geological Sciences, California State University at Northridge, Northridge, CA 91330, USA ABSTRACT Graptolite-bearing Middle and Upper Ordovician siliciclastic facies of the Argentine Precordillera fold-thrust belt record the disintegration of a long- lived Cambro-Mid Ordovician carbonate platform into a series of tectonically partitioned basins. A combination of stratigraphic, petrographic, U-Pb detrital zircon, and Nd-Pb whole-rock isotopic data provide evidence for a variety of clastic sediment sources. Four Upper Ordovician quartzo-lithic sandstones collected in the eastern and central Precordillera yield complex U-Pb zircon age spectra dominated by 1Æ05–1Æ10 Ga zircons, secondary populations of 1Æ22, 1Æ30, and 1Æ46 Ga, rare 2Æ2 and 1Æ8 Ga zircons, and a minor population (<2%) of concordant zircons in the 600–700 Ma range. Archaean-age grains comprise <1% of all zircons analysed from these rocks. In contrast, a feldspathic arenite from the Middle Ordovician Estancia San Isidro Formation of the central Precordillera has two well-defined peaks at 1Æ41 and 1Æ43 Ga, with no grains in the 600–1200 Ma range and none older than 1Æ70 Ga.