Wed 27th June @ noon, Rm 312.222

Abstract:

An increasing number of tectonic studies rely on U-Pb ages of detrital zircon to identify sediment source regions.  Such studies can be confounded if zircon ages reflect original primary sources rather than final, proximal sources. Such is the case with ~ 1 Ga Grenville zircon in North America – detrital grains are found in abundance thousands of kilometres from known exposed sources. This is likely a result of the incredible zircon fertility of Grenville age plutons (i.e. granitoids with very high Zr abundance).

To test further the utility of detrital accessory minerals we examined zircon and apatite from plutons exposed in a series of inselbergs, i.e. sediment point sources, in SE California. For zircon from granitic bedrock 80% yield dates of 74 Ma; 20% are Proterozoic xenocrysts. Detrital zircon ages overlap those of the bedrock, but show a different age frequency; in all sediment size fractions there is > 50% of Proterozoic grains. To determine if apatite was a more faithful recorder of provenance we analysed grains from both bedrock and alluvium for Sr isotopic composition. The ⁸⁷Sr/⁸⁶Sr ratio of detrital apatite largely overlap those of bedrock apatite but some analyses are significantly more radiogenic implying an exotic component.  Single apatite crystals from both bedrock and alluvium were also analysed for (U-Th)/He.  The average cooling date for bedrock apatite is 21.8 Ma. Approximately 80% the detrital grains overlap the bedrock values demonstrating the value this technique to provenance studies. However, ~ 20% of the cooling dates are ≥ 30 Ma also implying exotic sources.  Because sediment was collected in a single watershed contained entirely within the granite these apatite grains are potentially of aeolian origin. A similar origin could also explain the enigmatic detrital zircon U-Pb age distributions.  These results suggest considerable caution in interpreting provenance, particularly if relying on single mineral studies.

Short bio:

Dr. Scott Samson is a Professor of Earth Sciences at Syracuse University, in Syracuse, NY, USA. His specialty is in radiogenic isotope geochemistry and U-Pb dating.  His interests include continental crustal formation, Neoproterozoic tectonics, sedimentary provenance, and felsic magmatic evolution.  He has published over 90 peer-reviewed papers, is a Fellow of the Geological Society of America, and winner of the Wasserstrom prize for postgraduate teaching.