Abstract
Geochronology of ultra-high-pressure metamorphic rocks is able to constrain the timing and rates of subduction-zone processes. Lu–Hf garnet dating has the potential to yield information about the timing of the prograde evolution of subducting rocks under increasing pressure. In combination with other methods, it thus allows constraining the complete P–T–t path with high precision. Ultra-high-pressure eclogites from the Tromsø Nappe, the structurally highest tectonic unit of the Scandinavian Caledonides in northern Norway, were dated using Lu–Hf geochronology on garnet. A sample from Tromsdalstind yielded an age of 448.3 ± 3.6 Ma, interpreted as dating prograde garnet growth due to preserved zoning in the major-element and Lu contents of garnet grains. A sample from the diamond-bearing locality Tønsvika yielded an identical age of 449.4 ± 3.3 Ma. Garnet from this sample shows a weak zoning in Ca content and near-homogeneous Lu content. These ages are identical within error among each other and with published U–Pb ages of peak-eclogite-facies zircon and rutile/titanite from exhumation-related leucosome veins. Consequently, the entire subduction–exhumation cycle leading to the ultra-high-pressure eclogites lasted only very few millions of years during the Late Ordovician.
Highlights
Ultra-high-pressure (UHP) metamorphic rocks record pressures above the quartz–coesite equilibrium line, i.e., more than ~ 2.7 GPa, reflecting either subduction to mantle depths or, alternatively, strongly non-lithostatic pressure at shallower levels
For the sample from Tromsdalstind, we plotted one bomb-digested and two table-top-digested whole-rock splits, one clinopyroxene separate, and three garnet separates on an isochron which yields an age of 448.3 ± 3.6 Ma (MSWD = 1.19, n = 7)
Resorption of garnet and back-diffusion of Lu into the remaining garnet could lead to a Lu increase at the rim (Kelly et al 2011), but the well-preserved shape and major-element zonations of this garnet grain speak against a significant resorption
Summary
Ultra-high-pressure (UHP) metamorphic rocks record pressures above the quartz–coesite equilibrium line, i.e., more than ~ 2.7 GPa, reflecting either subduction to mantle depths or, alternatively, strongly non-lithostatic pressure at shallower levels. Except for a few cases (e.g., Zermatt-Saas ophiolites in the Western Alps; Reinecke 1998), exposed UHP metamorphic rocks are mostly derived from continental crust of the lower plate in a collisional setting that is entering a subduction zone after the subduction of oceanic lithosphere. The mechanisms leading to the subduction and exhumation of continental crust are a matter of scientific debate (e.g., Kurz and Froitzheim 2002; Michard et al 1993; Warren et al 2008). Like U–Pb dating on zircon and monazite, date either the pressure peak or stages along the retrograde P–T path. As these minerals are accessory and not unambiguously part of the peak-pressure
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