Exhumation of high-pressure, low-temperature (HP-LT) metamorphic rocks may be accomplished by a number of complementary or independent tectonic phenomena (e.g.,

The Flowers River Igneous Suite of north-central Labrador comprises several discrete peralkaline granite ring intrusions and their coeval volcanic succession. The Flowers River Granite was emplaced into Mesoproterozoic-age anorthosite–mangerite–charnockite–granite (AMCG) -affinity rocks at the southernmost extent of the Nain Plutonic Suite coastal lineament batholith. New U–Pb zircon geochronology is presented to clarify the timing and relationships among the igneous associations exposed in the region. Fayalite-bearing AMCG granitoids in the region record ages of 1290 ± 3 Ma, whereas the Flowers River Granite yields an age of 1281 ± 3 Ma. Volcanism occurred in three discrete events, two of which coincided with emplacement of the AMCG and Flowers River suites, respectively. Shared geochemical affinities suggest that each generation of volcanic rocks was derived from its coeval intrusive suite. The third volcanic event occurred at 1271 ± 3 Ma, and its products bear a broad geochemical resemblance to the second phase of volcanism. The surrounding AMCG-affinity ferrodiorites and fayalite-bearing granitoids display moderately enriched major- and trace-element signatures relative to equivalent lithologies found elsewhere in the Nain Plutonic Suite. Trace-element compositions also support a relationship between the Flowers River Granite and its AMCG-affinity host rocks, most likely via delayed partial melting of residual parental material in the lower crust. Enrichment manifested only in the southernmost part of the Nain Plutonic Suite as a result of its relative proximity to multiple Palaeoproterozoic tectonic boundaries. Repeated exposure to subduction-derived metasomatic fluids created a persistent region of enrichment in the underlying lithospheric mantle that was tapped during later melt generation, producing multiple successive moderately to strongly enriched magmatic episodes.

<p>Structures that accommodate extension during orogenic relaxation significantly modify the crustal architecture of mountain belts. Discerning the tectonic significance of superimposed structures relating to extensional overprint of initially compressional features is therefore critical to the reconstruction of an orogen, and is easiest where the large-scale mechanical interactions between different crustal domains are exposed. In the Aegean region of Greece, low-angle detachment faults of early Miocene age were partially responsible for exhuming Eocene high-pressure, low-temperature (HP-LT) metamorphic rocks of the Cycladic Blueschist Unit (CBU). Extension in the Cyclades commonly occurred along multiple detachment branches at the kilometer scale, either due to arrest of older detachment planes by late Miocene plutonism, or because strain partitioning along multiple, simultaneously active structures was rheologically favourable. We document a third plausible mechanism whereby crustal attenuation is accomplished via distributed coaxial strain in the footwall of a major detachment, described previously in the Cyclades primarily for deep crustal fabrics contemporaneous with peak HP-LT conditions. This style of deformation is recorded below the basal contact of the CBU on the island of Evia, which delineates the boundary of a major tectonic window exposing an underthrust external carbonate platform known as the Basal Unit (locally Almyropotamos Unit). New structural observations, complemented by white mica <sup>40</sup>Ar/<sup>39</sup>Ar and zircon (U-Th)/He ages, suggest that the upper structural levels of the Basal Unit accommodated flattening strain that coincided with Oligo-Miocene extension likely related to the overlying North Cycladic Detachment System. Vertical shortening, with extension in both other principal directions, is evinced by symmetric chocolate-tablet foliation boudinage and conjugate shear bands in the Basal Unit, alongside coeval type-3 refold structures in the overlying CBU. Pseudosection modelling results from Evia further corroborate a late greenschist-facies (320 ± 40 °C, 7 ± 1 kbar) paragenesis for the fabric associated with this extension that post-dates HP-LT metamorphism. Our observations indicate extrusion of the CBU and underlying Basal Unit was accomplished at least in part by coaxial vertical shortening, in contrast to the predominantly non-coaxial strain observed in the footwalls of other major Cycladic detachments.</p>

<p>Southern Evia in Greece exposes an inverted high pressure-low temperature (HP-LT) metamorphic sequence that has been loosely correlated with the Cycladic Blueschist Unit (CBU). On the island, the CBU is divided into the metavolcanic and ophiolitic Ochi Nappe and predominantly metacarbonate Styra Nappe. A lower-grade unit, the Almyropotamos Nappe, is exposed in the core of a N-S trending antiform and comprises Eocene platform carbonates overlain by metaflysch. The Almyropotamos Nappe occupies a tectonic window defined by the Evia Thrust, a brittle-ductile fault zone that emplaced the Ochi and Styra nappes atop the Almyropotamos Nappe. New multiple single-grain white mica total fusion <sup>40</sup>Ar/<sup>39</sup>Ar ages indicate that deformation occurred along the Evia Thrust at 25-23 Ma. White mica <sup>40</sup>Ar/<sup>39</sup>Ar data on either side of the tectonic window record Eocene dates between 40 and 32 Ma, consistent with previously published <sup>40</sup>Ar/<sup>39</sup>Ar dates and a single Rb-Sr age of c. 30 Ma. These ages broadly coincide with estimates for the timing of NE-directed thrusting of the Ochi Nappe over the Styra Nappe. Strain associated with thrusting localized as cylindrical folds in Styra marbles, with fold axes parallel to the stretching lineation and a clear strain gradient increasing toward the upper contact with the Ochi Nappe. The most prominent structures in the Ochi Nappe are a strong L-S fabric defined by acicular blue amphibole and type-3 refold structures with fold axes trending parallel to the NE-SW oriented stretching lineation. Whereas the Ochi Nappe and Styra Nappe locally preserve peak blueschist facies mineral assemblages, all three units commonly display evidence only for retrogressed initial HP-LT assemblages in the form of ferroglaucophane inclusions in albite porphyroblasts. Isochemical phase diagrams calculated in the Na<sub>2</sub>O-CaO-K<sub>2</sub>O-FeO-MgO-Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub>-H<sub>2</sub>O-TiO<sub>2</sub>±O<sub>2</sub> system support minimum peak metamorphic conditions of 12.5 ± 1.5 kbar and 465 ± 75 °C for an Ochi Nappe blueschist, and 6.0 ± 0.5 kbar and 315 ± 15 °C for an albite mica schist from the Evia Thrust. Peak P-T conditions for the Ochi Nappe support a metamorphic history more closely resembling that of the Lower Cycladic Blueschist Nappe, indicating that the entire section of the CBU exposed on Evia lies below the Trans-Cycladic Thrust. The Early Miocene ages from the Evia Thrust overlap with the proposed timing for the initiation of bivergent greenschist facies extension in the Cyclades. The remainder of the region, including high-strain corridors within individual nappes such as the Almyropotamos Thrust, uniformly records Eocene deformation ages. The similarity in <sup>40</sup>Ar/<sup>39</sup>Ar ages across the tectonic window contrasts with age relationships observed in similar tectonic packages on Lavrion, and suggests that regional scale deformation persisted until the Late Eocene before strain became localized in brittle-ductile corridors by the Early Miocene. </p>