Abstract We conducted 39 Ar diffusion experiments using potassium feldspar from the South Cyclades Shear Zone on Ios, in the Cyclades, Aegean Sea, Greece. Irradiated samples were step-heated in an ultra-high-vacuum resistance furnace attached to a mass spectrometer, thus also allowing 40 Ar/ 39 Ar geochronology. Conjoint inversion of these datasets allowed estimation of the relevant diffusion parameters, which were then used to forward model the effect of arbitrary temperature–time histories. Simulations used Monte Carlo methods to improve approximations to the observed age spectra. Two periods of rapid cooling could be inferred. The South Cyclades Shear Zone commenced operation during or shortly after the Eocene–Oligocene transition. Episodes of south-directed movement continued into Early Miocene time, however, with the footwall still hot enough to cause biotite ± garnet metamorphic mineral growth at the base of the overlying, already substantially exhumed, eclogite–blueschist unit. Since its footwall continued to cool, the South Cyclades Shear Zone was an extensional shear zone during both episodes of its operation.

Abstract The Lhasa terrane, forming one of the main tectonic components of the Himalayan–Tibetan orogen, has received a lot of attention as it records multiple episodes of plate spreading, subduction and collision within the realm of the Paleo-Tethys Ocean. A review of the mineralogical, petrological, geochemical and geochronological data of eclogites, associated blueschists and garnet-bearing mica schists from the Sumdo high- and ultrahigh-pressure metamorphic (HP/UHP) complex in the central/southern part of the Lhasa terrane, is present here so that the origin and tectono-metamorphic evolution of this important suture can be deduced. By re-evaluating the original published P – T conditions for the metamorphic rocks of the Sumdo Complex, we consider that the Sumdo Complex has experienced low temperature HP/UHP metamorphic conditions, characteristic of fast subduction (and exhumation) in a typical oceanic subduction zone setting. The original wide spread in the maximal peak P – T conditions could be reduced in size due to thus far unknown inconsistencies in the usage of applied geothermobarometric techniques. The remaining spread in the maximal P – T conditions ( c. 200°C/10 kbar) of the HP/UHP regions can be explained by a mechanism that the rocks from individual tectonic slices were subducted to different depths and followed by juxtaposition on their way back to the surface. A re-consideration of the isotopic ages of eclogites from the Sumdo Complex demonstrates that the opening of the Paleo-Tethys Ocean, located in between the two major components of the Lhasa terrane, was initiated prior to c. 280 Ma and the eclogite facies metamorphism is likely to be of late Permian ( c. 260 Ma) to early Triassic age (245–225 Ma), recording different ages of subduction from individual slices of the oceanic crust. The closure of the Paleo-Tethys Ocean resulted, no earlier than 210 Ma, in the final collision between the northern and southern Lhasa blocks. This final collision event may have been triggered by the initial subduction of the Bangong–Nujiang Tethys Ocean in the north.

The Yukon-Tanana terrane lies within the North American Cordilleran accretionary orogen and contains strongly deformed, coherent eclogite-bearing units that are candidates for wholesale subduction erosion of large blocks of crust. The Yukon-Tanana terrane is a composite continental arc built on a peri-Laurentian substrate that experienced subduction on both sides before it was accreted back onto Laurentia in the Mesozoic. Along the present-day eastern margin of the terrane, eclogites are found as layers and lenses in quartzofeldspathic schist derived from both igneous and sedimentary protoliths, all metamorphosed together during the Permian. In the St. Cyr area, coherent slices of eclogite-bearing crust up to 30 km long and 1–2 km thick have been mapped. Phengite with Si = 3.3–3.4 per formula unit from the host schists indicates that they also record eclogite-facies metamorphism. Detrital zircon was recovered from six host-rock samples collected at three high-pressure (HP) localities (St. Cyr, Ross River, and Last Peak), and from two eclogite-free units. Samples from the eclogite-free units and Last Peak have detrital zircon signatures with prominent Mesoproterozoic and Paleoproterozoic peaks typical of the Snowcap assemblage, the peri-Laurentian substrate of the Yukon-Tanana terrane. Detrital spectra from the St. Cyr and Ross River HP localities contain Precambrian, mostly Mesoproterozoic, zircon with significant Paleozoic peaks that match ages of igneous events in the Yukon-Tanana terrane. The coherent slices of crust containing eclogite, meta-tonalite, and arc-derived metasedimentary rocks are interpreted as pieces of Yukon-Tanana terrane that were eroded from the arc during subduction of oceanic lithosphere and reaccreted to the arc prior to Mesozoic emplacement of the Yukon-Tanana terrane onto North America.