collaborating graduate student: Fawna Korhonen
advising professor: Jim Stout
University of Minnesota, Minneapolis
NSF Geology REU 2002

Zircon zonation patterns as revealed by cathodoluminescence and backscattered electron images: implications for the interpretation of the thermotectonic evolution of Western Labrador

The metamorphic events that occurred in western Labrador remain foggy. It can be stated with certainty that these rocks experienced high grade metamorphism, displaying amphibolite and granulite grade assemblages. What event, however, do these assemblages represent? K-Ar dates at 950+/-150 Ma (Moore, 1986). U-Pb ages range from 1.8-1.6 Ga (Moore, 1986). Do these ages represent two distinct orogenic events, or could the earlier age in fact record an Archean date that was partially reset by the metamorphic event at 950 Ma?

Using cathodoluminescence and back-scattered electron imaging techniques we investigated the growth structures of zircons. Zircon retains a high-quality record of its crystallization history due to sluggish kinetic properties for U, Th, and Pb, and its resistant character during most metamorphic reactions up to and surpassing anatectic conditions. Thus for most rocks in the earth's crust, it can be assumed that abundant zircon growth/formation during high-grade metamorphism occurs only in connection with partial melting and subsequent megmatic crystallization. For each event, a variety of growth rim occurs, the characteristics of which can be indicative a specific pressure, temperature, and chemical conditions.

Preliminary imaging shows that zircons from the Wilson Lake reveals that these zircons have experienced at least two metamorphic events subsequent to original crystalization. Do these rims record high-grade metamorphism of a Labradorian orogeny at approximately 1.7 Ga and a Grenvillian orogeny at 990 ma?

To better answer this question greater understanding of field relationships and ion microprobe dating is necessary. August 9 to 23, myself and graduate student Fawna Korhonen will be in the field, mapping and sample collecting, to increase knowledge of regional geological relationships. 1 am going to continue to work on this project at Mount Holyoke College over the course of the 2002/2003 academic year with the help of Korhonen. In the future, we hope to work with the Canadian Geological Association to date these zircons using ion microprobe techniques.