THERMAL EVOLUTION OF SOURCE ROCKS OF DETRITAL PRECAMBRIAN ZIRCON REVEALED THROUGH INTEGRATED RADIATION DAMAGE, CHUGACH-PRINCE WILLIAM TERRANE, ALASKA

The thermal evolution of rocks in orogenic settings is recorded by several isotopic systems that rely on the disintegration of radionuclides. Zircon has been the one of the champions of geochronological analyses because detrital grains can be dated by U/Pb, FT, and He techniques. The radioactive disintegration of uranium and thorium drives radiation damage in zircon manifest by disruption of the crystal lattice and in some cases the accumulation of pink color; both of these effects can be thermally annealed and thus can provide a unique thermal record of source rocks. Part of the basement of the volcano-plutonic source terrain that supplied clastic sediment to the flysch of the Chugach-Prince William terrane (CPW) supplied a small but significant fraction of Precambrian zircon. Using a combined approach of U/Pb dating and μ-Raman analysis to measure disorder in single detrital grains, we discovered that the source rocks for the Precambrian grains have two distinct thermal histories. The Precambrian grains fall into two distinct arrays of radiation damage and age: (1) zircon from western units of the CPW have a wide range of grain ages with modes at 1810-1870 Ma and 2520-2680 Ma, similar to a northern Laurentian source, and these grains have considerable disorder and have likely accumulated radiation damage for 500 to 1000 Ma; (2) zircon from the eastern units of the CPW have modes at 1380-1450 Ma and 1710-1740 Ma, but they have very little disorder and have likely only accumulated significant radiation damage since the late Mesozoic (c. <100 Ma). Thus the low levels of disorder as revealed from μ-Raman analysis of this latter suite of Precambrian zircon indicate derivation from an exhumed metamorphic source where the accumulated radiation damage was reset (annealed) in the Cretaceous. Our laboratory experiments and empirical data from samples in thermal aureoles of plutons indicate that amphibolite grade metamorphism is required to fully anneal internal disorder caused by radiation damage.