USING MONAZITE PETROCHRONOLOGY TO CONSTRAIN THE METAMORPHIC HISTORY OF THE MOST DEEPLY EXHUMED ROCKS ALONG THE DENALI FAULT

The curved Denali fault has been an active dextral strike slip fault since 50 Ma and exhumation began at a relatively slow rate from 40-25 Ma and an accelerated rate since 25 Ma. The Denali fault broadly marks the boundary between North America and Wrangellia, an accreted terrane that collided in the Cretaceous. A narrow package of deeply exhumed metamorphic rocks lies on the north side of the fault at the apex of the curve.

Previous work in the area has focused on the exhumation history and ongoing work of a collaborator is aimed at better constraining the depositional history; however, there has not been a systematic study of the metamorphic history of these rocks. Detailed electron microprobe imaging, in situ monazite petrochronology, and quantitative petrology will place a suite of these metamorphic rocks in time and P-T space to better constrain their history prior to exhumation.

Field and petrographic observations indicate that these rocks are upper amphibolite facies and the metapelites have an equilibrium assemblage of gar + bio + plag + qtz +/- sill. Four samples along a ~1km long transect perpendicular to the fault have been selected for dating based on their assemblage and petrographic relations. Petrochronology data is hypothesized to show multiple age populations based on petrographic relations and elemental zoning within individual monazite grains. Monazite grains of varying morphology occur within garnet and sillimanite porphyroblasts, in porphyroblast tails, and within or cross-cutting the dominant fabric.

These potential different age populations may correlate with regional and/or local metamorphic events. The oldest age population would be expected between 80-100 Ma—the timeframe of initial collision of Wrangellia and North America in Alaska. This age could imply that the Denali fault has remained localized at what was the original suture between North America and exotic terrane. A population between 55-70 Ma would overlap with known orthogneiss intrusion ages that are associated with the final stages of collision and a population of 50 Ma or younger may be related to Denali fault motion. Better constraining the metamorphic history of these rocks will create a more complete picture of the Eastern Alaska range tectonic history and reveal insight into the vertical component of ling-lived strike slip faults.