U-PB DATING OF LEUCOGRANITE INTRUSIONS IN OROGENIC BELTS: CHALLENGES AND APPROACHES TO DEFINING A CHRONOLOGY OF DEFORMATION

Leucogranite dikes, sheets, and bulbous intrusions are ubiquitous in orogenic belts. They result from partial melting of crustal rocks as a product of prograde metamorphism, decompression, and intrusion of higher temperature magmas. The presence of leucogranites in metamorphic environments and their cross-cutting relationships with metamorphic fabrics can be used to define deformational and thermal events, by the absolute dating of their intrusions. The formation of leucogranite magma, however, typically occurs near minimum melt conditions with high fluid contents. As a result magmatic zircons are not abundant and those that do occur have large pre-magmatic cores with or without thin overgrowths, high U concentrations, are metamict or show patchy zoning in CL images, high common Pb, and high U/Th ratios on rims typical of metamorphic growth. These factors complicate the U-Pb dating of zircons separated from leucogranites even with in-situ methods.

We performed LA-MC-ICP-MS analyses on zircons from ~15 leucogranites from the Eocene Pioneer Mountains Metamorphic Core Complex in south-central Idaho, and the Cambrian Damara belt in Namibia, in order to constrain the age of deformational events. Inheritance, patchy zoning, metamictization, and dark CL emissions are all characteristics on the CL images, as well as U and Th silicates as inclusions.

The U-Pb geochronology data shows high discordance values related to radiogenic Pb-loss and high common Pb concentrations, as well as high U/Th values, normally attributed to metamorphic zircons.

In order to accurately determine the age of these leucogranites we attempted to reduce the impact of these factors by: 1) taking into account the relationship of age-vs. discordance and consider grains that fall away from trend lines, 2) plotting T-W diagrams for samples with high discordance, to identify and constrain the 206Pb/204Pb ratio of common lead, 3) selecting, if possible, grains that fall below the 204 cps from the FC1 standard value, 4) calculating weighted mean ages for data, sometimes above the generally accepted 3% discordance, 5) taking into account the relationship of age vs. 204Pb and calculating an age intercept whenever there is a correlation to compare with weighted mean ages, and 6) attempting to analyze the U-Pb age of comagmatic apatite to define a minimum age.