T77 Provenance Analysis in Modern and Ancient Sedimentary Systems

Monday, 10 October 2011
8:00 AM - 12:00 PM
Room 211CD

THE GOOD, THE BAD AND THE UGLY: ASSESSING THE UTILITY OF DETRITAL MINERALS IN PROVENANCE ANALYSIS

Paper number: T77-2
Program: Technical Sessions
Time: 8:05 AM – 8:20 AM

Authors

Abstract

There has been an extreme increase in using the ages of detrital zircon (DZ) crystals as a tool to constrain provenance and tectonic history. A common assumption in many detrital zircon studies is that the abundance of zircon grains defining a particular age range directly corresponds to the area of exposed continental crust of that age. This is often not the case, however, as much modern alluvium and Paleozoic sandstone have large 1.3 – 1.0 Ga age peaks (Grenvillian signature), even in material very distal to exposed Grenville crust. Thus ‘zircon fertility’ must be a primary consideration in provenance studies. Further, it must be noted that a single sample is unlikely to characterize an entire formation, or particular time interval of interest. For example, DZ from two samples of alluvium from the French Broad River (TN + NC) collected < 15 km apart have exactly opposite maximum/minimum age abundance peaks. Very different conclusions might be drawn if only one sample was analyzed.

A different approach to provenance analysis is combine DZ ages with age and Nd isotopic composition of detrital monazite (DM). In a comparison of ages of DM versus DZ from modern alluvium and Pennsylvanian-Permian clastic sedimentary rocks in eastern Laurentia, monazite ages consistently recorded the Paleozoic orogenies that are the hallmark of the Appalachians. In contrast, DZ ages are dominated by a Grenville signature, and in several cases did not record any Paleozoic ages. Thus DM ages appear to more accurately reflect the actual nature of eastern Laurentian crust than do DZ ages. The isotopic composition of some DM crystals of the same age from modern alluvium are distinct, allowing for more refined linkage between source and sediment than that based on age alone. Finally, chemical compositions of detrital garnet from modern alluvium also form distinct clusters providing even further resolution of provenance. Using monazite and garnet in conjunction with zircon thus holds the promise for much more robust provenance analysis in both modern and ancient environments.