Migmatite Dome Research Articles

Migmatite dome as a result of multi-fold interference pattern, in the Damara Belt, Namibia

The western part of the Pan-African Damara Orogen in Namibia is located at the intersection of two highly oblique mobile belts: the coastal NNW-trending Kaoko-Gariep belts and the inland NE-trending Damara Belt. The Namibfontein-Vergenoeg (NV) domes are two, basement-cored, migmatite domes exposed in this orogenic intersection. These domes formed through the superposition of four deformation phases in the presence of melt, giving rise to syn-migmatitic multi-fold interference patterns. Early E-W shortening produced steep N–S striking S1 fabrics linked to km-scale steeply E-inclined F1 folds, likely reflecting the southern continuity of the Kaoko Belt fabrics. The superposition of NE-plunging, NNW-inclined F2 folds formed during NNW-SSE, orogen-perpendicular shortening and produced a Type 1 dome-and-basin fold interference pattern. Progressive with D2, D3 deformation caused localised m-scale folding of steep fabrics into inclined to recumbent, NW-plunging F3 folds, and local top-to-the-S or -SE thrusting. NNW-SSE shortening is correlated with the collision of the Congo and Kalahari cratons during Damara Belt formation. Finally, orogen-parallel NE-SW shortening, produced localised NE-inclined and NE-plunging F4 folds. This weaker shortening phase in the Damara Belt is enigmatic and may reflect tectonic activities further in eastern and western Gondwana. The superposition of F4 folds on F2 folds defines a Type 3 fold interference pattern that established the final 3D architecture of the NV domes. The deformation fabrics at the NV domes document three distinct regional shortening events related to large-scale tectonic switches involved in the western Gondwana Supercontinent assembly.

Formation of the Naxos nested domes and crustal differentiation by convection and diapirism

The Naxos dome, in the middle of the Aegean domain, exposes the former root of the Alpine orogenic belt and represents a key natural example to investigate the development of gravitational instabilities during orogenic evolution and their impact on crustal differentiation. The Naxos dome is cored by migmatites with structures depicting second order domes with a diameter of 1–2 km nested in the first order deca-kilometer scale dome that formed at the onset of orogenic collapse. Zircon grains from the migmatites record a succession of crystallization-dissolution cycles with a period of 1–2 Myr. These features have been attributed to the development of convective and diapiric gravitational instabilities, related to thermally induced and compositional buoyancy. In this paper, we test the pertinence of this model with a thermal-mechanical numerical experiment performed with a volume of fluid method (VOF) known to preserve material phase interfaces during large deformation of viscous layers. Partial melting of the crust is modeled by strain-rate and temperature dependent viscosity and temperature dependent density. Moreover, horizontal layers with density, viscosity and heat production variations mimic more felsic or more mafic lithologies in a crust of intermediate composition. With basal heating, gravitational instabilities initiate with local segregation of the buoyant versus heavier layers, followed by diapiric upwelling of buoyant pockets of aggregated less dense material. Convection starts after 5 Myr, approximately when half of the crust has a viscosity lower than 1019 Pa s. The size of the convection cells increases as the temperature rises in the crust and reaches ∼25 km in diameter after ca. 20 Myr, which defines the size of first order domes. Some of the heterogeneous material is entrained in the convection cells with a revolution period of 1 to 3 Myr. However, most of the denser material accumulates in the lower crust, while the buoyant material segregates at the top of the convection cells and forms diapirs that correspond to second order domes, of several kilometers in diameter and nested within the first order domes. This model, which reproduces the first order characteristic dimensions of the Naxos nested domes and the periodicity of their zircon geochronological record, demonstrates the efficiency of gravitational instabilities in the formation of migmatite domes and, more generally, in the multi-scale dynamics of crustal differentiation leading to a felsic upper crust, an intermediate middle crust and a mafic lower crust.

Convection and segregation in heterogeneous orogenic crust with a VOF method – II: how to form migmatite domes

SUMMARY Migmatites and granitic-gneisses exhumed in Archean to Phanerozoic segments are former partially molten crustal roots, display typical domes structures ranging in size from kilometres to decakilometres, and are often interpreted as resulting from the development of diapiric or convective gravitational instabilities. In previous work (part I), we determined various regimes of gravity-driven segregation, by considering a thick continental crust heated from below and containing melt related heterogeneities. These heterogeneities, represented by inclusions of distinct densities and viscosities with respect to the ambient partially molten material, can be entrained into convection cells (in the ‘suspension’ and ‘layering’ regimes) and/or accumulate as clusters (in the ‘layering’ and ‘diapirism’ regimes). Here we further investigate the specific conditions that allow for the formation and preservation of domes resulting from diapirism at the top of convective cells. We show that both the cessation of basal heating and the freezing of the buoyant inclusions density favour their stacking and preservation at ca. 15 km depth, within about 10 Myr. The buoyant inclusions form domes, 5–20 km in size, that also record several convective cycles at velocities ranging from 0.5–4 cm yr−1. 3-D models demonstrate their radial geometrical nature. The influence of the size and concentration of the inclusions is also assessed, complementing the characteristics of crustal heterogeneity in driving its differentiation and the formation of migmatite domes.

Orogenic eclogites record relative magnitude of deep crustal flow and extent of migmatite-eclogite interaction

In exhumed orogens, refractory mafic rocks have the potential to preserve a record of petrogenesis and high-pressure (H P ) metamorphism that is commonly obliterated in quartzofeldspathic rocks owing to re-equilibration at high-temperature, low-pressure (L P –H T ) conditions. In the Montagne Noire (France) migmatite dome, located in the foreland of the Variscan orogen, eclogite is exposed in both the core and margin of the dome. In this study, we combine in situ U Pb petrochronology and oxygen-isotope analyses of key eclogite phases to demonstrate that eclogites from the two distinct domains had different protoliths and source regions, traveled relatively variable distances in the deep crust, and differentially interacted with surrounding migmatite prior to exhumation. Dome-margin eclogite zircons are small (~40 μm) with well-preserved inherited cores and thin (<15 μm) rims, compared to larger (40–120 μm) neo- and recrystallized dome-core zircons with small relict cores and wide (15–30 μm) recrystallized rims. Protolith and H P metamorphism ages were determined using in situ zircon and rutile petrochronology (LASS-ICP-MS). Both eclogites formed in a continental setting; dome margin protolith zircon cores formed at 442.5 ± 3.4 Ma (steep HREE slope, no Eu-anomaly) whereas zircon cores of the dome-core eclogites yielded scattered dates suggesting protolith crystallization between ~500–400 Ma (steep HREE slope, pronounced Eu-anomaly). Both eclogites experienced H P metamorphism at c. 320–310 Ma in garnet-stable, plagioclase-absent conditions. Most analyzed rutile yielded dates of 307–304 Ma associated with cooling. The record of H P fluid conditions was determined by O-isotope (SIMS) analyses of garnet and zircon. Dome-margin zircon cores and rims have δ 18 O of ~8.2–8.5 ‰, indistinguishable within uncertainty, in isotopic equilibrium with isotopically unzoned garnet (δ 18 O ~ 8.0–8.2 ‰). In contrast, zircons in dome-core eclogites have systematically lower zircon-core δ 18 O values compared to their rims and neocrystallized grains, and zircon cores were in equilibrium with major-cation zoned garnet with respect to oxygen. The two dome-core eclogite samples yielded zircon and garnet δ 18 O values of ~8.6–9.5 ‰ and ~ 9.7–10.5 ‰. Based on these results and existing H P fabric data for these eclogites, we propose that (1) gabbro protoliths for the two eclogites were emplaced at different depths in a Cambro-Ordovician continental crustal package; and (2) dome-core eclogites interacted extensively with surrounding gneiss during burial and foreland-vergent crustal flow, whereas the dome-margin eclogite was sourced proximally to the dome-emplacement location and had minimal chemical interaction with surrounding gneiss. At least parts of the Montagne Noire migmatite dome were deeply sourced, but rocks exhumed in the core had a more extensive and protracted history of deep-crustal flow than deep-crustal rocks exhumed at the margin. • Eclogites in migmatite dome retain memory of deep crust history. • Zircon (re)crystallization extent reveals distinct eclogite source, flow paths. • Petrochronology and O-isotopes record migmatite-eclogite interaction at depth. • Integrated garnet, zircon datasets reveal protolith, peak, exhumation histories. • Domes may exhume crust laterally-sourced >100 s of km from emplacement location.

Models of convection and segregation in heterogeneous partially molten crustal roots with a VOF method – I: flow regimes

SUMMARYWe investigate numerically some thermomechanical conditions for the development of crustal scale diapirism and convection in a heterogeneous continental crust independently from the action of regional tectonics. Here, we consider a hot crust, with unmolten and partially molten domains of specific temperature and strain-rate dependent power-law rheologies. We take advantage of the volume of fluid (VOF) method to capture the coalescence and separation of deformable inclusions in the partially molten domain. The inclusions, of several hundred metres in size, are more or less dense and viscous with respect to the ambient medium (they also behave with a power-law rheology). We restrict our study to a 20 Myr time scale, during which gravitational dynamics may dominate over lateral tectonics and lithospheric thermal re-equilibration. The motion of these inclusions during the development of gravitational instabilities displays distinct flow regimes that depend on two Rayleigh numbers denoted RaUM and RaPM, for the unmolten and partially molten rock properties, respectively. A ‘suspension’ regime occurs at high RaUM and high RaPM, in which most of the light compositional heterogeneities remain entrained in the convective cells. In contrast at low RaUM and high RaPM, a ‘layering’ regime is characterized by merging of the light inclusions as floating clusters below the rigid upper crustal lid. This regime occurs in association with a sharp viscosity gradient at upper-to-middle crust transitional depths. In these two regimes, the dense inclusions accumulate at the bottom of the partially molten zone. Finally at moderate RaPM, a ‘diapiric’ regime reflects the segregation of the heavy and the light inclusions, respectively downward and upward, without global convection. These numerical experiments lead to a first order evaluation of the physical parameters required for the segregation of deformable inclusions of variable densities and convection, in a partially molten crust, and provide insights on the conditions for the development of migmatite domes. Geological data indicate that these processes likely occur in a large number of settings from Archean to Phanerozoic times, and contribute to the differentiation of the continental crust.

Ultrahigh-temperature granulite-facies metamorphism and exhumation of deep crust in a migmatite dome during late- to post-orogenic collapse and extension in the central Adirondack Highlands (New York, USA)

AbstractThis study combines field observations, mineral and whole-rock geochemistry, phase equilibrium modeling, and U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon geochronology to investigate sillimanite-bearing felsic migmatites exposed on Ledge Mountain in the central Adirondack Highlands (New York, USA), part of an extensive belt of mid-crustal rocks comprising the hinterland of the Mesoproterozoic Grenville orogen. Phase equilibrium modeling suggests minimum peak metamorphic conditions of 960–1025 °C and 11–12.5 kbar during the Ottawan orogeny—significantly higher pressure-temperature conditions than previously determined—followed by a period of near-isothermal decompression, then isobaric cooling. Petrography reveals abundant melt-related microstructures, and pseudosection models show the presence of at least ~15%–30% melt during buoyancy-driven exhumation and decompression. New zircon data document late Ottawan (re)crystallization at ca. 1047 ± 5 to 1035 ± 2 Ma following ultrahigh-temperature (UHT) metamorphism and anatexis on the retrograde cooling path. Inherited zircon cores give a mean date of 1136 ± 5 Ma, which suggests derivation of these felsic granulites by partial melting of older igneous rocks. The ferroan, anhydrous character of the granulites is similar to that of the ca. 1050 Ma Lyon Mountain Granite and consistent with origin in a late- to post-Ottawan extensional environment. We present a model for development of a late Ottawan migmatitic gneiss dome in the central Adirondacks that exhumed deep crustal rocks including the Snowy Mountain and Oregon anorthosite massifs with UHT Ledge Mountain migmatites. Recognition of deep crustal meta-plutonic rocks recording UHT metamorphism in a migmatite gneiss dome has significant implications for crustal behavior in this formerly thickened orogen.

Late Variscan (315 Ma) subduction or deceptive zircon REE patterns and U–Pb dates from migmatite‐hosted eclogites? (Montagne Noire, France)

AbstractEclogites hosted in sillimanite‐bearing migmatites in the Montagne Noire dome (French Massif Central) have an oceanic protolith and recorded a prograde P–T path from ~19.5 kbar, 700°C to the pressure peak at ~21 kbar, 750°C (pseudosection modelling), suggesting metamorphism in a subduction setting. Subsequent exhumation to low‐P high‐T (LP–HT) conditions (~6 kbar, 730°C) is constrained by the equilibration conditions of the embedding migmatite. In samples of a fresh and a retrogressed eclogite, all zircon crystals but one display similar REE patterns (no Eu anomaly, flat HREE), usually ascribed to crystallization under eclogite‐facies conditions. Yet, the U–Pb apparent ages of zircon crystals from both eclogites spread from c. 360 Ma to a dominant data cluster at c. 315 Ma. The c. 315–310 Ma zircon U–Pb dates obtained from the embedding migmatite are interpreted as the age of crystallization of the partial melt during the LP–HT metamorphic stage. First‐order geological evidence, in particular the sedimentary record, excludes the existence of a subduction zone in the region at this period. Unless calling upon a major reappraisal of the tectonics of the European Variscan orogen, this suggests an ambiguous relation between REE patterns and U–Pb dates in the zircon population. Various scenarios that could account for the observations are discussed. Combining our data with the results of a previously published Sm–Nd dating of garnet, and regional considerations, we consider that 360 Ma is the best approximation of the minimum age of the eclogite‐facies event. We hypothesize that the eclogites formed farther north and were transferred to their present location by lower‐crustal flow. It is inferred that during the subsequent exhumation, eclogite‐facies zircon grains recrystallized and underwent partial to total resetting of their U–Pb system, whereas the REE system remained mostly unmodified. These results caution against the use of REE patterns as the only criterion to associate a specific zircon age with HP metamorphism in eclogites occurring in migmatitic domes.

The production of granitic magmas through crustal anatexis at convergent plate boundaries

Granite petrogenesis is associated with the formation and evolution of continental crust along convergent plate boundaries. While magmatic processes such as partial melting and fractional crystallization result in the compositional diversity of granites, the nature of magma sources is related to tectonic settings of granite generation. Although granites consist predominantly of magmatic minerals, they generally contain small amounts of peritectic and residual minerals. This results in the compositional difference between natural granites and experimental melts. In addition to the fractional crystallization of mantle-derived mafic magmas for the generation of granites during accretionary orogeny, a wide range of crustal rocks can be partially melted to form granites in both accretionary and collisional orogens. In the latter case, metasedimentary and metaigneous crustal sources lead to the dichotomy of S- and I-type granites. Geochemically, granites with negative εNd(t) and εHf(t) values are certainly derived from reworking of the ancient crust. On the other hand, granites with positive εNd(t) and εHf(t) values may be produced either by reworking of the juvenile crust or by fractional crystallization of asthenospheric mantle-derived magmas at actively convergent plate boundaries. In this case, major and trace elements may be of use in distinguishing between the crustal and mantle contributions. Granite, migmatite and granulite generally form lithological associations at previously converged plate boundaries, with migmatitic domes as the major component of metamorphic core complexes. Their formation is related to extensional heating subsequent to thinning of the thickened mantle lithosphere. As soon as the thickened lithospheric mantle was foundered into the asthenospheric mantle, it makes the space for asthenospheric upwelling to induce active rifting along the thinned lithosphere. This provides an external heat source for anatectic metamorphism at high thermal gradients, marking the tectonic transition in the property of orogenic systems into the rifting stage. Therefore, the active rifting is spatially and temporally dictated by thinning of the orogenic mantle lithosphere, and its operation is a geodynamic mechanism for crustal anatexis at convergent plate boundaries. Consequently, the lithospheric thickness has a profound effect on crustal anatexis for granitic magmatism, which is considerably later than plate convergence.

Geophysical evidence for doming during the Pan-African/Brasiliano orogeny in the Seridó belt, Borborema Province, Brazil

The Neoproterozoic Seridó belt located in the Borborema Province, NE Brazil, was deformed and metamorphosed in the Pan-African/Brasiliano orogeny. The basement rocks of the belt were submitted to intense partial melting to form dome-like structures as documented in the Santa Luzia anatetic dome. The aeromagnetic map of the Seridó belt shows a long-wavelength anomaly with elongated sigmoidal shape, whose limits coincide with shear zones. Comparison with gravity data shows that both anomalies share common sources, which are felsic rocks more magnetic and less dense than the surrounding crust. Correlation with geology shows that the geophysical anomalies are associated with magnetite-rich migmatites and granites. We modelled the geophysical datasets using magnetic susceptibility and density values from measurements in cores of a stratigraphic borehole. The results are consistent with a migmatite-dome system formed in the south-central part of the belt. This large-scale crustal structure is composed of elongated internal domes, which were geophysically modelled as four anomalous bodies, that resulted from partial melting of rocks mostly of the basement. The anomalies define an elongated migmatite-dome system with the main axis in the NNE-SSW direction that progressively bends to E-W approaching to the Patos shear zone, thus indicating that this shear zone had a key role in shaping the domic structure. The migmatitic domes of the Seridó belt compose a fault-related dome system, formed in association with the broad strike-slip shear zone array of the Patos-Seridó system, which evolved through the crustal extrusion resulting from the continental collisions that amalgamated West Gondwana in the Ediacaran.

Ordovician metamorphism and magmatism preserved in the Ossa Morena Complex: SHRIMP geochronology, geochemistry and Sr[sbnd]Nd isotopic signatures of the Sierra Albarrana Domain (SW Iberian Massif)

This study integrates SHRIMP UPb zircon geochronology and whole-rock and SrNd isotope geochemistry data from migmatites and granitic rocks of the Sierra Albarrana Domain. We provide new information on the magmatic and metamorphic events of this domain of the Ossa-Morena Complex. The metasedimentary succession, nowadays corresponding to a migmatite dome, was probably deposited during the Ediacaran and Cambrian. SHRIMP UPb ages of magmatic and zoned metamorphic zircon grains indicate that the main tectono-magmatic event in the Sierra Albarrana Domain is Late Cambrian (granite emplacement; 481.2 ± 2.3 Ma) to early Ordovician (migmatization; 477.8 ± 2.4 Ma), supported by the intrusion of La Cardenchosa Pluton, by the low-pressure partial melting and by the ductile deformation of the host metasedimentary rocks. We suggest that the granitic magmatism, the migmatization and the main regional fabrics are part of a single Ordovician event, only slightly modified by Variscan reworking.Geochemical and isotopic data are compatible with an extensional environment in the middle-upper crustal levels, related to a continental rift development. This setting and timing are compatible with the latest magmatic event of the Early Paleozoic rifting (535–460 Ma), well documented in the Ossa Morena Complex. The geochemical characteristics of the subalkaline rift-related rocks of the Ossa Morena Complex and of the Central Iberian Zone partly coincide. As a result, it is not possible to discriminate between them and favor a correlation between both terranes based on the geochemical affinities of magmatic rocks of similar age.

Regional-scale polydiapirism predating the Neoarchean Yilgarn Orogeny

Gneiss domes occur in a wide variety of orogenic and anorogenic domains of the continental crust, and play a major role in lithosphere dynamics by allowing upwards transfer of heat and mass. They commonly contain a core of granites and/or migmatites, overlain by a mantle of lower-grade rocks. Evolutionary models of many gneiss domes are controversial.Here we use new structural and zircon isotopic data to unravel the tectono-magmatic evolution of the Yalgoo dome and surroundings, at the margins of the Neoarchean Yilgarn Orogen, Western Australia. The study area includes at least seven granite–migmatite domes (5–70 km in average diameter), and several subdomes within the larger domes, all showing comparable structural features and age. In each dome, a concentric domal foliation is concordant with granite–greenstone contacts, bearing a radial, outward-plunging lineation. The bulk of the structures in each dome reflect magmatic flow, with pervasive subsolidus fabrics occurring only along the outer dome margins, and reflecting dome-up kinematics. Narrow greenstone keels pinched between domes and subdomes display vertical constriction, in an area regionally dominated by flattening along the steep sides of the deeply-eroded domes, and along the flat-laying dome roofs.The regional structural pattern and the kinematics of the high-strain zones are best explained by sequential emplacement of nested diapirs, which delivered large volumes of granitic magma in a c. 20 Myr time-span. The dome-and-keel regional architecture and the internal structures of each dome resulted therefore from multiscale polydiapirism. These diapirs were later weakly overprinted by a tectonic fabric that developed during the Neoarchean Yilgarn Orogeny. The contrast between the Yalgoo region and the rest of the craton, which was strongly reworked by this orogeny, highlights the dichotomy of tectonic styles in Archean terranes, demonstrating that diapirism dominated in times of tectonic quiescence.

Flow of partially molten crust controlling construction, growth and collapse of the Variscan orogenic belt: the geologic record of the French Massif Central

We present here a tectonic-geodynamic model for the generation and flow of partially molten rocks and for magmatism during the Variscan orogenic evolution from the Silurian to the late Carboniferous based on a synthesis of geological data from the French Massif Central. Eclogite facies metamorphism of mafic and ultramafic rocks records the subduction of the Gondwana hyperextended margin. Part of these eclogites are forming boudins-enclaves in felsic HP granulite facies migmatites partly retrogressed into amphibolite facies attesting for continental subduction followed by thermal relaxation and decompression. We propose that HP partial melting has triggered mechanical decoupling of the partially molten continental rocks from the subducting slab. This would have allowed buoyancy-driven exhumation and entrainment of pieces of oceanic lithosphere and subcontinental mantle. Geochronological data of the eclogite-bearing HP migmatites points to diachronous emplacement of distinct nappes from middle to late Devonian. These nappes were thrusted onto metapelites and orthogneisses affected by MP/MT greenschist to amphibolite facies metamorphism reaching partial melting attributed to the late Devonian to early Carboniferous thickening of the crust. The emplacement of laccoliths rooted into strike-slip transcurrent shear zones capped by low-angle detachments from c. 345 to c. 310 Ma is concomitant with the southward propagation of the Variscan deformation front marked by deposition of clastic sediments in foreland basins. We attribute these features to horizontal growth of the Variscan belt and formation of an orogenic plateau by gravity-driven lateral flow of the partially molten orogenic root. The diversity of the magmatic rocks points to various crustal sources with modest, but systematic mantle-derived input. In the eastern French Massif Central, the southward decrease in age of the mantle- and crustal-derived plutonic rocks from c. 345 Ma to c. 310 Ma suggests southward retreat of a northward subducting slab toward the Paleotethys free boundary. Late Carboniferous destruction of the Variscan belt is dominantly achieved by gravitational collapse accommodated by the activation of low-angle detachments and the exhumation-crystallization of the partially molten orogenic root forming crustal-scale LP migmatite domes from c. 305 Ma to c. 295 Ma, coeval with orogen-parallel flow in the external zone. Laccoliths emplaced along low-angle detachments and intrusive dykes with sharp contacts correspond to the segregation of the last melt fraction leaving behind a thick accumulation of refractory LP felsic and mafic granulites in the lower crust. This model points to the primordial role of partial melting and magmatism in the tectonic-geodynamic evolution of the Variscan orogenic belt. In particular, partial melting and magma transfer (i) triggers mechanical decoupling of subducted units from the downgoing slab and their syn-orogenic exhumation; (ii) the development of an orogenic plateau by lateral flow of the low-viscosity partially molten crust; and, (iii) the formation of metamorphic core complexes and domes that accommodate post-orogenic exhumation during gravitational collapse. All these processes contribute to differentiation and stabilisation of the orogenic crust.

Deep Crustal Flow Within Postorogenic Metamorphic Core Complexes: Insights From the Southern Western Gneiss Region of Norway

AbstractViscous crustal flow can exhume once deeply buried rocks in postorogenic metamorphic core complexes (MCCs). While migmatite domes record the flow dynamics of anatectic crust, the mechanics and kinematics of solid‐state flow in the deep crust are poorly constrained. To address this issue, we studied a deeply eroded and particularly well‐exposed MCC in the southern Western Gneiss Region of Norway. The Gulen MCC formed during Devonian transtensional collapse of the Caledonian orogeny in the footwall of the Nordfjord‐Sogn detachment zone. We developed a semiquantitative mapping scheme for ductile strain to constrain micro‐ to megascale processes, which brought eclogite‐bearing crust from the orogenic root into direct contact with Devonian supradetachment basins. The Gulen MCC comprises different structural levels with distinct metamorphic evolutions. In the high‐grade core, amphibolite‐facies structures record fluid‐controlled eclogite retrogression and coaxial flow involving vast extension‐perpendicular shortening. Detachment mylonites formed during ductile‐to‐brittle noncoaxial deformation and wrap around the core. We present a sequential 3‐D reconstruction of MCC formation. In the detachment zone, the combined effects of simple shearing, incision/excision, and erosion thinned the upper crust. Internal necking of the ductile crust was compensated by extension‐perpendicular shortening within the deep crust and resulted in differential folding of distinct crustal levels. We identify this differential folding as the main mechanism that can redistribute material within solid‐state MCCs. Our interpretation suggests a continuum of processes from migmatite‐cored to solid‐state MCCs and has implications for postorogenic exhumation of (ultra‐)high‐pressure rocks.

Compressional origin of the Naxos metamorphic core complex, Greece: Structure, petrography, and thermobarometry

AbstractThe island of Naxos, Greece, has been previously considered to represent a Cordilleran-style metamorphic core complex that formed during Cenozoic extension of the Aegean Sea. Although lithospheric extension has undoubtedly occurred in the region since 10 Ma, the geodynamic history of older, regional-scale, kyanite- and sillimanite-grade metamorphic rocks exposed within the core of the Naxos dome is controversial. Specifically, little is known about the pre-extensional prograde evolution and the relative timing of peak metamorphism in relation to the onset of extension. In this work, new structural mapping is presented and integrated with petrographic analyses and phase equilibrium modeling of blueschists, kyanite gneisses, and anatectic sillimanite migmatites. The kyanite-sillimanite–grade rocks within the core complex record a complex history of burial and compression and did not form under crustal extension. Deformation and metamorphism were diachronous and advanced down the structural section, resulting in the juxtaposition of several distinct tectono-stratigraphic nappes that experienced contrasting metamorphic histories. The Cycladic Blueschists attained ∼14.5 kbar and 470 °C during attempted northeast-directed subduction of the continental margin. These were subsequently thrusted onto the more proximal continental margin, resulting in crustal thickening and regional metamorphism associated with kyanite-grade conditions of ∼10 kbar and 600–670 °C. With continued shortening, the deepest structural levels underwent kyanite-grade hydrous melting at ∼8–10 kbar and 680–750 °C, followed by isothermal decompression through the muscovite dehydration melting reaction to sillimanite-grade conditions of ∼5–6 kbar and 730 °C. This decompression process was associated with top-to-the-NNE shearing along passive-roof faults that formed because of SW-directed extrusion. These shear zones predated crustal extension, because they are folded around the migmatite dome and are crosscut by leucogranites and low-angle normal faults. The migmatite dome formed at lower-pressure conditions under horizontal constriction that caused vertical boudinage and upright isoclinal folds. The switch from compression to extension occurred immediately following doming and was associated with NNE-SSW horizontal boudinage and top-to-the-NNE brittle-ductile normal faults that truncate the internal shear zones and earlier collisional features. The Naxos metamorphic core complex is interpreted to have formed via crustal thickening, regional metamorphism, and partial melting in a compressional setting, here termed the Aegean orogeny, and it was exhumed from the midcrust due to the switch from compression to extension at ca. 15 Ma.

Ediacaran Obduction of a Fore‐Arc Ophiolite in SW Iberia: A Turning Point in the Evolving Geodynamic Setting of Peri‐Gondwana

AbstractThe Calzadilla Ophiolite is an ensemble of mafic and ultramafic rocks that represents the transition between lower crust and upper mantle of a Cadomian (peri‐Gondwanan) fore arc. Mapping and structural analysis of the ophiolite demonstrates that it was obducted in latest Ediacaran times, because the Ediacaran‐Early Cambrian sedimentary series (Malcocinado Formation) discordantly covers it. The ophiolite and emplacement‐related structures are affected by Variscan deformation (Devonian‐Carboniferous), which includes SW verging overturned folds (D1) and thrusts (D2), upright folds (D3), extensional faults (D4), and later faults (D5). These phases of deformation are explained in the context of Variscan tectonics as the result of the progressive collision between Gondwana and Laurussia. Qualitative unstraining of Variscan deformation reveals the primary geometry of Ediacaran‐Cambrian structures and uncovers the generation of east verging thrusts as responsible for the primary obduction of the Calzadilla Ophiolite. Restoration of planar and linear structures associated with this event indicates an Ediacaran, east directed obduction of the ophiolite, that is, emplacement of the Cadomian fore arc onto inner sections of the northern margin of Gondwana. According to regional data, the obduction separates two extension‐dominated stages in the tectonic evolution of the African margin of northern Gondwana preserved in southern Europe. Preobduction extension brought about the onset and widening of fore‐arc and back‐arc basins in the external part of the continent, while postobduction extension facilitated the formation of extensional migmatitic domes, an oceanward migration of back‐arc spreading centers across peri‐Gondwana, and the eventual opening of a major basin such as the Rheic Ocean.

Complex mid‐crustal flow within a growing granite–migmatite dome: An example from the Variscan belt illustrated by the anisotropy of magnetic susceptibility and fabric modelling

AbstractThis study describes anisotropy of magnetic susceptibility (AMS) of migmatites associated with crustally derived granites of the Pelhřimov core complex, Bohemian Massif. In combination with numerical modelling, we use this case example to discuss some of the general complexities related to interpreting flow patterns of anatectic lower and middle crust from magnetic fabric during growth of metamorphic domes. Magnetic lineations, reflecting tiling or zone axis of magnetically oblate grains, commonly tend to develop along the principal stretching direction. Our fabric modelling, however, shows that a lineation may also develop at a high angle to the overall flow (tectonic transport) direction as a relict of a pre‐existing fabric or due to a component of core complex axis‐parallel stretching and may be maintained for a significant period of time. Furthermore, we demonstrate that the finite magnetic fabric orientation is largely independent on the initial migmatite fabric intensity (formed before dome growth) but is rather a function of (a) shear strain, causing magnetic foliation and lineation to rotate towards the shear plane, and (b) magnitude of dome growth, causing their external rotation. Similarly, the degree of anisotropy P is also a function of accumulated shear strain and increases over time. These findings suggest that low fabric intensities may point to rapidly cooling, less sheared domains that are more likely to preserve early formed fabrics. In dome limbs undergoing protracted simple shear, low and high degrees of oblateness (as expressed by the shape parameter T) may discriminate between domains with weak and strong initial fabrics and may thus be used to map out domains with different strain histories and perhaps also with different melt proportions and rheological behaviour within metamorphic domes.

The 3D geometry of the Naxos detachment fault and the three-dimensional tectonic architecture of the Naxos metamorphic core complex, Aegean Sea, Greece

We reconstruct the three-dimensional (3D) geometry of the Naxos detachment fault and quantify E–W shortening associated with strong N–S extensional deformation. In addition, 3D reconstruction of the detachment indicates how it interacted with the underlying metamorphic sequence, as well as a set of concentric metamorphic isograds that formed during extensional deformation. For doing so, we used the software MOVE™ (Midland Valley Ltd.) to develop a 3D model of the Naxos metamorphic core complex (NCC) and the Naxos detachment fault. The model is constrained by structural data, metamorphic isograds, and fission-track ages from the footwall of the NCC. Our analysis shows that greater minimum amounts of E–W shortening correlate with higher metamorphic grade, ranging from 6 to 10% outside the migmatite dome and up to 17% within the migmatite dome. The metamorphic isograds around the migmatite dome are less intensely folded by E–W shortening than the lithologic layering. We conclude that this is because the isograd surfaces “froze-in” after folding had already started. Our model indicates that the isograds are cut by the brittle detachment, demonstrating that the final stages of top-to-the-NNE extension outlasted the formation of the metamorphic dome and the isograds. Despite some limitations of the 3D model, the outlined geometric reconstruction of a major extensional fault system in the central Aegean highlights: (1) the interplay between large-scale extension and temperature-dependant shortening perpendicular to the extension direction, (2) the evolution of metamorphism and migmatization, (3) the formation of metamorphic isograds, and (4) granitoid intrusions during extension and exhumation.

Re-folded structure of syn-orogenic granitoids (Padrón dome, NW Iberia): Assessing rheological evolution of cooling continental crust in a collisional setting

This contribution discusses about the rheological, kinematic and dynamic frameworks necessary to produce recumbent and upright folds from syn-orogenic granitic massifs that were formed during an early stage of magma genesis related to the onset of a migmatitic dome. Syn-kinematic granitoids occurring within the high-grade infrastructure of the Padrón migmatitic dome (NW Iberia) are deformed into large-scale recumbent folds (D2) that are later affected by upright folds (D3). Petrostructural analysis of a selected area of this dome reveals that after a period of crustal thickening (D1), NNW-directed extensional flow gave way to recumbent folds and penetrative axial plane foliation (S2). Superimposed subhorizontal compression resulted in upright folds (D3). A closer view into the dynamics of the dome allows exploring the factors that may condition the nucleation of folds with contrasting geometries during progressive deformation of molten continental crust. The formation of folds affecting syn-kinematic granitoids suggests a cooling metamorphic path in migmatitic domes. Active and passive folding mechanisms require a crystallizing (cooling) magma to nucleate folds. A more competent metamorphic host inhibits fold nucleation from much less competent magmas. As it crystallizes, magma becomes more rigid (competent), and approaches viscosity values of its host. Passive folding is favored when no significant competence contrast exists between magma and host, so this folding mechanism is more likely shortly after magma genesis and emplacement. In such conditions, and under dominant subhorizontal flow accompanied by flattening (D2), passive folding would produce isoclinal recumbent geometries. Further magma cooling introduces a shift into the rheological behavior of partially molten crust. Thereon, crystallizing magma bodies would represent significant competence contrasts relative to their host. At this point, buckling is a more likely folding mechanism, and more regular, buckle folds re-fold previous structures after significant cooling. The geometry of resulting folds is upright due to dominant subhorizontal compression (D3) at this stage.

Crustal-scale convection and diapiric upwelling of a partially molten orogenic root (Naxos dome, Greece)

The goal of this paper is to use the structural, metamorphic and geochronological record from the migmatitic core of the Naxos dome (Greece) and its associated subdomes to address the internal dynamics of a partially molten orogenic root. U-Pb ages from ca. 24 to 16 Ma and textures of zircon in the migmatites suggest successive dissolution and precipitation cycles with a period of 1 to 2 Ma, interpreted as the timescale of convective instabilities in a ca. 20 km thick partially molten layer. Dimensional analysis indicates that convection of this root requires a viscosity lower than 1018 Pa·s, consistent with viscosity values expected for partially molten felsic rocks. Structural analysis and U-Pb geochronology of deformed granitic dikes rooting in the migmatites record the subsequent development of the Naxos dome by diapirism from ca. 16 to 13 Ma. The size of the first order migmatite dome on Naxos (5 × 12 km) requires that the unstable layer at the onset of diapirism was 5 to 10 km thick and presented a moderate viscosity contrast with its envelope. From this analysis we propose that the Naxos migmatite dome documents a two stage dynamic evolution for the partially molten root of the Aegean belt characterized by (1) crustal scale convection for at least 8 Ma and (2) diapirism for about 3 Ma during progressive thinning of the collapsing orogenic crust.

An Eocene/Oligocene blueschist‐/greenschist facies P–T loop from the Cycladic Blueschist Unit on Naxos Island, Greece: Deformation‐related re‐equilibration vs. thermal relaxation

AbstractGeothermobarometric and geochronological work indicates a complete Eocene/early Oligocene blueschist/greenschist facies metamorphic cycle of the Cycladic Blueschist Unit on Naxos Island in the Aegean Sea region. Using the average pressure–temperature (P–T) method of thermocalc coupled with detailed textural work, we separate an early blueschist facies event at 576 ± 16 to 619 ± 32°C and 15.5 ± 0.5 to 16.3 ± 0.9 kbar from a subsequent greenschist facies overprint at 384 ± 30°C and 3.8 ± 1.1 kbar. Multi‐mineral Rb–Sr isochron dating yields crystallization ages for near peak‐pressure blueschist facies assemblages between 40.5 ± 1.0 and 38.3 ± 0.5 Ma. The greenschist facies overprint commonly did not result in complete resetting of age signatures. Maximum ages for the end of greenschist facies reworking, obtained from disequilibrium patterns, cluster near c. 32 Ma, with one sample showing rejuvenation at c. 27 Ma. We conclude that the high‐P rocks from south Naxos were exhumed to upper mid‐crustal levels in the late Eocene and early Oligocene at rates of 7.4 ± 4.6 km/Ma, completing a full blueschist‐/greenschist facies metamorphic cycle soon after subduction within c. 8 Ma. The greenschist facies overprint of the blueschist facies rocks from south Naxos resulted from rapid exhumation and associated deformation/fluid‐controlled metamorphic re‐equilibration, and is unrelated to the strong high‐T metamorphism associated with the Miocene formation of the Naxos migmatite dome. It follows that the Miocene thermal overprint had no impact on rock textures or Sr isotopic signatures, and that the rocks of south Naxos underwent three metamorphic events, one more than hitherto envisaged.