TITANIUM IN QUARTZ THERMOMETRY: BEYOND SHEAR SENSE, ARE RECRYSTALLIZED QUARTZ TAILS ON PORPHYROCLASTS USEFUL FOR EVALUATING DEFORMATION CONDITIONS?

In the internal portion of fold-thrust belts, there are locations where P-T conditions are right for recrystallized quartz tails to form on feldspar porphyroclasts. Various geometries of these mantled clasts can arise and it is well established that the asymmetric geometry of the tails can be used to determine the sense of shear locally within the rock. But these tails may hold more information than previously thought. The quartz in the tails is formed/deformed syntectonically and thus records the most recent deformation event together with associated P-T conditions. Several gneiss samples, with abundant feldspar porphyroclasts and associated quartz tails, from the Korean Collision Belt were analyzed using LA-ICP-MS to obtain titanium concentrations within quartz grains comprising the tails. In this study, porphyroclasts predominantly had an asymmetric sigma geometry. For each clast, 10-25 spots were measured per tail along transects extending from the clast to the tail extremities. Then the recalibrated “TitaniQ” titanium in quartz thermometer (Thomas et al., 2010) was used to determine the temperature in quartz grains at various points within the tails. Mean temperatures in the tails range from 550 to 415 °C from hinterland towards the foreland. Tails on individual clasts show a systematic decrease in temperature with increased distance from the clast, with a slight increase at the ends of the tails. Grain size and other microstructural analyses were conducted to assess the tails’ mechanism of formation. The temperature data when paired with microstructures give insight into the deformation conditions and kinematics occurring at depth within the hinterland of the fold-thrust belt.