DELINEATING THE P-T-T HISTORY OF BLUESCHIST ROCKS IN THE RUBY TERRANE, ALASKA USING 40AR/39AR AND LU/HF THERMOCHRONOLOGIC TECHNIQUES

The combination of in-situ ⁴⁰Ar/³⁹Ar UV laser ablation technique on white mica, infrared laser step-heating of white mica crystals, and Lu/Hf dating of co-occurring garnet are used to better understand whether single crystals of chemically heterogeneous white mica can provide reliable ages of metamorphic events. Three different thermochronologic techniques are combined to constrain the timing of burial and exhumation of blueschist rocks in the Ruby Terrane region of west-central Alaska. Use of these three techniques permits discrimination of P-T-t paths by comparing garnet (highest temperature) ages to the ages of the cores and rims in white mica crystals.

Blueschist facies rocks, particularly those generated in continental subduction zone systems, are common sources for assemblages that include both abundant white mica and garnets. The correlation of the garnet age to either the core or rim age from the mica can help determine whether the retrograde path of the crystal is the result of isothermal decompression or isobaric heating. These two possible paths are significant for understanding geodynamic processes of subduction zones that evolve to continental collision, such as recorded in west-central Alaska. Continental crust in this part of Alaska experienced extreme shortening and widespread blueschist facies metamorphism, during the Brookean orogeny, broadly from the Mid-Jurassic to the Mid-Cretaceous. However, the timing of the onset of subduction and collision between the continental crust of Alaska and an island arc is not well constrained.

Garnet Lu/Hf ages have been acquired using column chemistry at Washington State University. Eight fractions of garnet were used to calculate a date, with two fractions showing significantly older ages than the rest. These two fractions, when isolated, yield an age of 159 ± 9 Ma. In contrast, the other six fractions yield an isochron of 132 ± 8 Ma date. Since the majority of the fractions cluster near ~132 Ma, we interpret this age as the main time of garnet growth. If the two possible ages represent episodes of garnet growth, this would require a protracted time interval (~ 30+ my) between initial subduction underplating of continental material and initiation of exhumation. The ⁴⁰Ar/³⁹Ar data are currently being acquired and should shed light on permissible tectonic models.