The subduction of continental crust - insights from eclogite geochronology and petrology

When continents collide slices of continental crust may be dragged down into the subduction zone to depths of up to 200 km. The most straightforward evidence for continental subduction refers to the presence of high-pressure (HP) and ultrahigh-pressure (UHP) eclogite terranes in many collisional mountain systems. In addition, as inferred from numerical modelling continental subduction provides a physically most consistent explanation for the formation and exhumation of these terranes. Reconstruction of the P-T-t paths of HP and UHP rocks can provide direct constraints on the thermo-mechanical processes in subduction zones. For that, a multidisciplinary approach is the most promising one for understanding the subduction dynamics as well as the mechanisms of continental convergence. Three case studies were performed on HP and UHP eclogite terranes following similar procedures: the Eoalpine high-pressure belt in the Eastern Alps (Austria and Italy), the Byala Reka Dome in the Eastern Rhodopes (Bulgaria) and the Lofoten basement in the Scandinavian Caledonides (Norway). High precision Lu-Hf geochronology was applied on eclogites and combined with characterization of the chemical zoning in garnet. Additionally, thermodynamic modelling was performed on each of the dated samples for constraining the metamorphic conditions. The Eoalpine belt includes (ultra)high-pressure rocks that crop out along a northwest-southeast trending line extending from the Texel Complex in Italy to the Pohorje Mountains in Slovenia. Dating of garnet growth during pressure increase was achieved by using Lu-Hf chronometry that yielded results between c. 100 and c. 90 Ma. This time span of c. 10 Ma suggests short-lived period of subduction. Combined with the already published data the estimated metamorphic conditions indicate a field gradient with increasing P and T from northwest to southeast where the rocks experienced UHP Eoalpine metamorphism. The oldest Cretaceous eclogites are found in the Saualpe-Koralpe area that comprises widespread Permian gabbros formed along rift zones within a thinned continental margin during Permian-Triassic time. This supports the hypothesis that subduction initiation was intracontinental and localized by e Permian rift. In Texel Complex Lu-Hf dating of two-phased garnets yielded a Variscan-Eoalpine mixed age indicating re-subduction and eclogitization of Variscan eclogites during the Eoalpine orogeny. In eclogite from the Byala Reka Dome in the Eastern Rhodopes, garnet growth was dated at 81.6 ± 3.5 Ma by Lu–Hf chronometry. Petrological data and modelling suggest peak-pressure conditions of 1.2–1.6 GPa and 570–620 °C. The eclogite-facies metamorphism coincides with the main phase of granitoid intrusions in the Sredna Gora Zone and, thus, suggests that metamorphism took place in a subduction zone dipping towards north under this section of the Apuseni–Banat–Timok–Sredna Gora continental magmatic arc. During the Late Cretaceous, the site of magmatic activity shifted southward and arrived in the Eastern Rhodopes at ~69 Ma, as evident by granite intrusions of that age near the locality of the dated eclogite sample. This proximity may be explained by south-directed rollback of the subduction zone, although also post-69 Ma tectonic displacement has to be considered. Together with published age data from other parts of the Rhodopes, the new data confirm that multiple subduction/exhumation cycles took place between ~200 and ~40 Ma along this section of the southern European plate boundary. The Proterozoic basement of the Lofoten Islands contains Caledonian eclogite, although Caledonian deformation is only minor. Previous dating suggested that HP metamorphism in Lofoten occurred ca. 480 Ma, i.e., ~50 Ma before the collision between the major continents Baltica and Laurentia. Therefore, the Lofoten basement was considered not to originate from Baltica but rather to represent a stranded microcontinent. Newly discovered kyanite-bearing eclogites from the Lofoten Islands record deep subduction of continental crust during the main (Scandian) stage of Baltica-Laurentia collision ca. 400 Ma. Conventional geothermobarometry and thermodynamic modelling yield metamorphic conditions of 2.5-2.8 GPa and ~650 °C. Lu-Hf dating results in prograde garnet growth age of 399 ± 10 Ma. These results demonstrate that the Lofoten basement belonged to Baltica, was subducted to ~90 km depth during the collision with Laurentia, and was exhumed at an intermediate to high rate (>6 mm/yr) while thrusting of a Caledonian allochthon (Leknes Group) was still ongoing. This supports the challenging conclusions that (1) subducted continental crust may stay rigid down to a depth of ~90 km, and (2) it may be exhumed during ongoing collision and crustal shortening

Comparative analysis of different nuclear medicine techniques in evaluation of renal function

Introduction: Nuclear medicine (NM) methods play an important role in the evaluation of renal function in a wide range of clinical indications. The aim of our study was to evaluate the correlation between measured GFR (mGFR) obtained by the three-plasma sample slope-intercept NM method (TPSM) — reference method vs. estimated GFR (eGFR) using Fleming’s single plasma sample method (SPSM) at 120 min, 180 min, and 240 min and correlation of reference method with eGFR with camera-based Gates’ protocol. Material and methods: A total of 82 subjects (33 male/49 female) with a mean age of 54.87 ± 15.65 years were included and mGFR value was obtained by the three-plasma sample slope-intercept NM method and eGFR was obtained with Fleming’s single sample method. eGFR was also quantified with the camera-based Gates’ protocol after i.v. application of [99mTc]Tc-DTPA. Results: Our study revealed a very strong positive significant correlation between all three SPSMs with the TPSM as the reference method. Between the Gates’ method and the TPSM in the group of patients with mGFR ≥ 61–84 mL/min/1.73 m2 and mGFR ≥ 84 mL/min/1.73 m2, a moderate positive statistically significant correlation was obtained. Conclusions: The SPSM method shows a very strong correlation with the reference and low bias in all three groups of patients and can be routinely used for GFR estimation

Fluid‐Mantle Interaction Along the Mariana Convergent Margin

Abstract Active serpentinite mud volcanoes in the forearc region of the Izu‐Bonin‐Mariana system represent an excellent natural laboratory for studying the geochemical processes along convergent plate margins and the associated forearc. During IODP Expedition 366, serpentinite mud with lithic clasts from the underlying forearc crust and mantle as well as from the subducting Pacific Plate was recovered. Ultramafic clasts from Fantangisña Seamount reveal very high degrees of serpentinization with mesh and bastite textures as well as development of late lizardite and chrysotile veins, which suggests serpentinization temperatures below 200°C. On the other hand, recovered harzburgites and, on occasion, dunites from Asùt Tesoru Seamount show a well‐preserved primary assemblage with low degrees of serpentinization and forearc peridotite characteristics. Fine‐grained antigorite associating with lizardite has been identified throughout the serpentine mud matrix, suggesting an alteration temperature of c. 340°C. Furthermore, alteration conditions during rodingitization point to temperatures of at least 228°C, estimated via chlorite geothermometry. Additionally, a rare ophicarbonate clast containing andraditic as well as Cr‐rich hydrogarnets from Asút Tesoru Seamount indicates crystallization temperatures of at least 230°C. Hence, a trend of lower temperature of serpentinization and higher degree of alteration closer to the trench. The detailed characterization of the fluid‐rock alteration conditions as well as fluids composition and transport permits a better constraining of the fluid–rock interactions and related mass transfers within subduction zones and during ascent of serpentinite fault gouge within mud volcano conduits and in mudflows after their emplacement on the flanks of the edifices

Constraining the process of intracontinental subduction in the Austroalpine Nappes: Implications from petrology and Lu‐Hf geochronology of eclogites

AbstractHigh‐ and ultrahigh‐pressure rocks occur in the Austroalpine Nappes in a ~400 km long belt from the Texel Complex in the west to the Sieggraben Unit in the east. Garnet growth during pressure increase was dated using Lu‐Hf chronometry. The results range between c. 100 and 90 Ma, indicating a short‐lived period of subduction. Combined with already published data, our estimates of metamorphic conditions indicate a field gradient with increasing pressure and temperature from the northwest to the southeast, where the rocks experienced ultrahigh‐pressure metamorphism. The P‐T conditions of the eclogites generally lie on the ‘warm’ side of the global range of subduction‐zone metamorphic conditions. The oldest Cretaceous eclogites (c. 100 Ma) are found in the Saualpe‐Koralpe area derived from widespread gabbros formed during Permian to Triassic rifting. In the Texel Complex garnets showing two growth phases yielded a Variscan‐Eoalpine mixed age indicating re‐subduction of Variscan eclogite‐bearing continental crust during the Eoalpine orogeny. Jurassic blueschist‐facies metamorphism at Meliata in the Western Carpathians and Cretaceous eclogite‐facies metamorphism in the Austroalpine are separated by a time gap of c. 50 Ma and therefore do not represent a transition from oceanic to continental subduction but rather separate events. Thus, we propose that subduction initiation was intracontinental at the site of a Permian rift.German Science Foundation (DFG)Slovak Research and Development Agency http://dx.doi.org/10.13039/50110000535

Dating the initiation of Piemonte-Liguria Ocean subduction: Lu-Hf garnet chronometry of eclogites from the Theodul Glacier Unit (Zermatt-Saas zone, Switzerland)

The Penninic nappe stack in the Central and Western Alps was formed in a collision zone environment after the closure of the Penninic oceans in the Paleogene. This study reports Lu-Hf garnet-whole rock ages of 56.5 +/- A 2.7 and 58.2 +/- A 1.4 Ma for two eclogite samples from the Theodul Glacier Unit, which is inserted within the structurally uppermost parts of the ophiolitic Zermatt-Saas Zone. The distribution of major elements, Mn, Y, and Lu in garnet, and specifically an enrichment of Lu in the cores, indicate that the ages record prograde growth of garnet during pressure increase. They provide direct evidence for the continuation of subduction during the Paleocene restoration phase, often regarded as a tectonically quiescent period due to a reduction in clastic sediment deposition, lack of folds and thrusts of this age, and a cessation of Africa-Europe convergence as derived from the magnetic anomaly pattern in the Atlantic Ocean. The evidence for ongoing subduction in the absence of Africa-Europe convergence suggests that the subduction system was driven by gravity acting on the downgoing slab in a rollback setting, and that subduction was balanced by extension of the upper plate. The overlap of the Lu-Hf ages of both samples from the Theodul Glacier Unit show that this tectonic element represents a coherent body. The difference with respect to the 48 Ma Lu-Hf age of the Lago di Cignana Unit, another element of the Zermatt-Saas Zone, shows that the Zermatt-Saas Zone consists of tectonic subunits, which reached their respective pressure peaks over a prolonged period of approximately 10 Ma

Constraining the process of intracontinental subduction in the Austroalpine Nappes: Implications from petrology and Lu-Hf geochronology of eclogites

High- and ultrahigh-pressure rocks occur in the Austroalpine Nappes in a similar to 400 km long belt from the Texel Complex in the west to the Sieggraben Unit in the east. Garnet growth during pressure increase was dated using Lu-Hf chronometry. The results range between c. 100 and 90 Ma, indicating a short-lived period of subduction. Combined with already published data, our estimates of metamorphic conditions indicate a field gradient with increasing pressure and temperature from the northwest to the southeast, where the rocks experienced ultrahigh-pressure metamorphism. The P-T conditions of the eclogites generally lie on the 'warm' side of the global range of subduction-zone metamorphic conditions. The oldest Cretaceous eclogites (c. 100 Ma) are found in the Saualpe-Koralpe area derived from widespread gabbros formed during Permian to Triassic rifting. In the Texel Complex garnets showing two growth phases yielded a Variscan-Eoalpine mixed age indicating re-subduction of Variscan eclogite-bearing continental crust during the Eoalpine orogeny. Jurassic blueschist-facies metamorphism at Meliata in the Western Carpathians and Cretaceous eclogite-facies metamorphism in the Austroalpine are separated by a time gap of c. 50 Ma and therefore do not represent a transition from oceanic to continental subduction but rather separate events. Thus, we propose that subduction initiation was intracontinental at the site of a Permian rift

Late Cretaceous eclogite in the Eastern Rhodopes (Bulgaria): evidence for subduction under the Sredna Gora magmatic arc

The Rhodopes in Bulgaria and Greece represent a nappe stack of high-grade units with polymetamorphic history. Constraining the time of metamorphism in individual subunits is essential for unraveling the controversial framework of subduction, exhumation and nappe stacking. Here we present new evidence for Late Cretaceous high-pressure metamorphism in the Eastern Rhodopes. In eclogite from the Byala Reka-Kechros Dome (Kazak eclogite), garnet growth is dated at 81.6 +/- 3.5 Ma by Lu-Hf chronometry, indicating that prograde eclogite-facies metamorphism occurred during the Late Cretaceous. Petrological data and modeling suggest peak-pressure conditions of 1.2-1.6 GPa, 570-620 A degrees C. We propose that metamorphism took place in a subduction zone dipping towards north under the Sredna Gora section of the Apuseni-Banat-Timok-Sredna Gora continental magmatic arc. Eclogite-facies metamorphism coincides with the main phase of granitoid intrusions in the Sredna Gora Zone. The site of magmatic activity in this area shifted southward during the Late Cretaceous and arrived in the Eastern Rhodopes at similar to 69 Ma, as shown by granite intrusions of that age only 4 km north of the locality of the dated eclogite sample. This proximity may be explained by south-directed rollback of the subduction zone, although also post-69 Ma tectonic displacement has to be considered. Together with published age data from other parts of the Rhodopes, the new data confirm that multiple subduction events took place between similar to 200 and similar to 40 Ma along this section of the southern European plate boundary

Two high-pressure metamorphic events, Variscan and Alpine, dated by Lu-Hf in an eclogite complex of the Austroalpine nappes (Schobergruppe, Austria)

The Eo-Alpine high-pressure belt in the Austroalpine nappes consists of pre-Mesozoic basement rocks overprinted by eclogite-facies metamorphism during the Late Cretaceous. Parts of this basement were already eclogitized during the Variscan orogeny. Lu-Hf geochronology allowed to identify two high-pressure events in an eclogite body in the Schobergruppe, an Austroalpine basement complex south of the Tauern Window. Two samples from closely neighboring outcrops were studied. Both contain two garnet generations. In one sample, PRI3, garnet belongs almost exclusively to the younger (Alpine) generation with only rare preservation of relic cores. In the other sample, PRI4, Variscan garnet is merely coated by a thin rim of the second, Alpine generation, which is in equilibrium with the high-pressure matrix assemblage. In PRI3, two-point garnet-whole rock ages scatter between 97 and 104Ma, reflecting Alpine garnet growth with minor contamination by a Variscan component. In PRI4, two-point isochrons yield ages between 300 and 313Ma. The limited spread in these ages suggests minor contamination by Alpine garnet rims. We propose 97Ma as the maximum age for Alpine metamorphism, which is close to previously determined ages from other parts of the high-pressure belt, and 313Ma as a minimum age for Variscan metamorphism. Thermodynamic modelling infers that eclogite-facies conditions were reached in both events; during the Late Cretaceous, these were ca. 1.9GPa/650 degrees C. Variscan high-pressure conditions in PRI4 are inferred from the amount of garnet in the sample, which indicates at least 1.6GPa. We propose that the occurrence of Alpine versus Variscan garnet in eclogites depends on the intensity of Variscan and post-Variscan retrogression: the more prograde, Variscan garnet was removed during retrogression, the more garnet grew during the Alpine cycle

High-pressure metamorphic age and significance of eclogite-facies continental fragments associated with oceanic lithosphere in the Western Alps (Etirol-Levaz Slice, Valtournenche, Italy)

The Etirol-Levaz Slice in the Penninic Alps (Valtournenche, Italy) is a piece of eclogite-facies continental basement sandwiched between two oceanic units, the blueschist-facies Combin Zone in the hanging wall and the eclogite-facies Zermatt-Saar Zone in the footwall. It has been interpreted as an extensional allochthon from the continental margin of Adria, emplaced onto ultramafic and mafic basement of the future Zermatt-Saar Zone by Jurassic, rifting-related detachment faulting, and later subducted together with the future Zermatt-Saas Zone. Alternatively, the Etirol-Levaz Slice could be derived from a different paleogeographic domain and be separated from the Zermatt-Saas Zone by an Alpine shear zone. We present Lu-Hf whole rock-garnet ages of two eclogite samples, one from the center of the unit and one from the border to the Zermatt-Saas Zone below. These data are accompanied by a new geological map of the Etirol-Levaz Slice and the surrounding area, as well as detailed petrology of these two samples. Assemblages, mineral compositions and garnet zoning in both samples indicate a clockwise PT-path and peak-metamorphic conditions of about 550-600 degrees C/20-25 kbar, similar to conditions proposed for the underlying Zermatt-Saas Zone. Prograde garnet ages of the two samples are 61.8 +/- 1.8 Ma and 52.4 +/- 2.1 Ma and reflect different timing of subduction. One of these is significantly older than published ages of eclogite-facies metamorphism in the Zermatt-Saar Zone and thus contradicts the hypothesis of Mesozoic emplacement. The occurrence of serpentinite and metagabbro bodies possibly derived from the Zermatt-Saas Zone inside the Etirol-Levaz Slice suggests that the latter is a tectonic composite. The basement slivers forming the Etirol-Levaz Slice and other continental fragments were subducted earlier than the Zermatt-Saar Zone, but nonetheless experienced similar pressure-temperature histories. Our results support the hypothesis that the Zermatt-Saas Zone and the overlying continental slivers do not represent a coherent paleogeographic unit but rather formed by successive, in sequence subduction and accretion of different fragments. (c) 2016 Elsevier B.V. All rights reserved

Devonian subduction and syncollisional exhumation of continental crust in Lofoten, Norway

When continents collide, continental crust of the lower plate may be subducted to mantle depth and return to the surface to form eclogite facies metamorphic terranes, as typified by the Western Gneiss Complex of the Scandinavian Caledonides. Proterozoic basement of the Lofoten Islands, located northeast and along strike of the Western Gneiss Complex, contains Caledonian eclogite, although Caledonian deformation is only minor. Previous dating suggested that Lofoten eclogites formed at ca. 480 Ma, i.e., similar to 50 Ma before the collision between the major continents Baltica and Laurentia, and that the Lofoten basement may not originate from Baltica but rather represents a stranded microcontinent. Newly discovered kyanite eclogites from the Lofoten Islands record deep subduction of continental crust during the main (Scandian) stage of Baltica-Laurentia collision ca. 400 Ma. Thermobarometry and thermodynamic modeling yield metamorphic conditions of 2.5-2.8 GPa and similar to 650 degrees C. Lu-Hf geochronology yields 399 +/- 10 Ma, corresponding to the time of garnet growth during subduction. Our results demonstrate that the Lofoten basement belonged to Baltica, was subducted to similar to 90 km depth during the collision with Laurentia, and was exhumed at an intermediate to high rate (>6 mm/yr) while thrusting of a Caledonian allochthon (Leknes Group) was still ongoing. This supports the challenging conclusions that (1) subducted continental crust may stay rigid down to a depth of similar to 90 km, and (2) it may be exhumed during ongoing collision and crustal shortening