U-Pb Ages of Detrital Zircons from the Cambrian MesóN Group in NW Argentina - Source Rocks Implications
The in situ U-Pb dating of detrital zircon grains in provenance studies has the advantage of revealing the age of the protolith rocks of the zircons. Ideally, it is possible to identify the precise sources of the detrital grains. In this study, we use U-Pb ages of detrital zircons from NW Argentina to reconstruct Paleozoic sediment transportation paths. In NW Argentina the Lower to Middle Cambrian Mesón Group includes shallow-marine fine- to middle-grained quartz-cemented sandstones and some granule-sized conglomerates, both of which are dominated by quartz (> 85 %) in the framework mode. The Mesón Group is assumed to have been deposited in an extensional basin along the Gondwana margin. The detrital zircons from the sandstones are mostly oval to elongated 50-150 µm large grains. Commonly, they are slightly abraded to rounded and have oscillatory zoning, as revealed by cathodoluminescence (CL) images. The CL zoning indicates a magmatic origin. Less than 20 % of the grains contain irregular zoning or are homogeneous, indications of a metamorphic origin. The varying high and low degrees of rounding imply varying degrees of abrasion and therefore a possible input from both regional and local source areas. In situ U-Pb measurements were carried out on ca. 350 detrital zircon grains from 4 sandstones of the Mesón Group. The concordant U-Pb ages reveal distinctive age peaks at 520-580 Ma, ca. 1 Ga and ca. 2 Ga for the sandstones at the base of the Mesón Group. At the top, zircons with ages 580-640 Ma and ca. 2 Ga predominate. Ages around 2 Ga are typical for material influenced by the Transamazonian orogeny. Potential source rocks of this age are present on the Amazon Craton to the north and the Río de la Plata Craton to the east of the study area. The Sunsás belt to the north could be a source for the zircons of ca. 1 Ga age (“Grenvillian ages” connected to the formation of Rhodinia). Younger, 500-700 Ma ages can be ascribed to the Pampean and Brazilian orogenies. These zircons may partly have been eroded and transported from crystalline rocks in present-day NW Argentina. The youngest ages (ca. 520 Ma) agree with the age of the Tastil batolith, only 100-200 km from our sampling locations. The ages of the detrital zircons can be explained with river transport from the Brazilian Shield. The river system(s) cut through crystalline rocks of Brazilian age on its / their way towards the coast.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009