TITANITE PETROCHRONOLOGY OF RETROGRESSED ECLOGITE AND GNEISS IN HIGH-STRAIN DOMAINS OF THE WESTERN GNEISS REGION, NORWAY

The deformation, metamorphism, and melting behavior of exhuming UHP terranes is highly relevant to our understanding of how collisional orogens evolve. These processes can be explored with outcrop observations and petrochronology of minerals that (re)crystallize along the exhumation path from mantle to mid-crustal levels. Titanite is one such chronometer that incorporates many trace elements during (re)crystallization, making it a sensitive monitor of co-crystallizing phases. Titanite petrochronology has been previously applied to Norway’s Western Gneiss Region (WGR), a large UHP terrane formed during the Caledonian orogeny. Common preservation of pre-UHP U-Pb titanite ages suggests that much of the WGR was unreactive during subduction and exhumation, interpreted to indicate limited Caledonian deformation (Spencer et al., 2013). We use titanite from localized high-strain domains active during WGR exhumation to explore the mineral’s behavior in retrogressed eclogite and gneiss to better understand how UHP terranes deform during exhumation.

We present a study of titanite behavior during WGR exhumation using the Tangen high-strain zone on Otrøya. Tangen contains eclogite pods in migmatitic grt-hbl gneiss in higher-strain quartzofeldspathic host gneiss. Titanite in weakly-deformed grt-hbl gneiss yields U-Pb ages between 389 and 377 Ma, depleted HREE, and no negative Eu anomaly, consistent with grt-present (re)crystallization. Mafic gneiss is transposed into cm-scale shear zones compatible with host gneiss sinistral shearing, indicating that deformation outlasted titanite (re)crystallization. High-strain host gneiss titanite yields many pre-UHP, inherited ages. Grains display progressive depletion in HREE and a transition from negative to flat Eu anomaly with decreasing age from 680 to 360 Ma. Titanite is pre-kinematic in host gneiss, indicating incomplete recrystallization during deformation. High-strain gneisses have subhorizontal lineations parallel to fold axes, consistent with transtension. WGR high-strain domains contain multiple titanite populations, from inherited titanite to grains formed during terrane exhumation. Titanite petrochronology and outcrop-scale mapping show that deformation at Tangen is Caledonian, and is compatible with exhumation via transtension.