Garnet Systematics of Subducted Continental Margins

Four case studies were performed on former continental margins and on a continental fragment inside an ophiolitic unit. The studies include Lu-Hf garnet dating, and garnet characterization by its chemical zoning. Lu zoning inside garnet was monitored by LA-ICPMS analyses in order to assign the obtained garnet ages to metamorphic stages of the individual subduction cycle. The Adula Nappe in the Central Alps represents a coherent part of pre-Mesozoic continental basement with minor amounts of Mesozoic cover rocks from the distal European continental margin. This unit was subducted to mantle depth during the Eocene orogenic cycle but already recorded Variscan high-pressure metamorphic conditions. It shows a continuous gradient of Alpine metamorphic conditions. Variscan eclogite-facies relics are well preserved in the in the North but mainly obliterated in the South. The Adula Nappe was subducted as a cohesive body in a single event between c. 38 to 36 Ma. Subduction was short-lived as the entire nappe pile was established before 32 Ma. The high grade metamorphic series of southeastern Pohorje in Slovenia was subjected to the highest pressure and temperature conditions during the Cretaceous Eo-Alpine metamorphic cycle. Eclogite-facies metamorphism in the ultramafic complex at the southeastern border of the Pohorje Mountains is contemporaneous with its surrounding country rocks. These rocks were subjected to eclogite facies conditions as a cohesive terrane in a single tectonic event between c. 97 and 90 Ma. The new Lu-Hf garnet age of 5.1 ± 1.7 Ma obtained from a blueschist-facies garnet-amphibole schist, reveals that the Yuli Belt of Taiwan’s Central Range was subducted after the beginning of arc-continent collision at about 6.5 Ma. Rocks from the Yuli belt remained at their maximum depth only for a very short period of time and were exhumed rapidly during ongoing convergence. Fast exhumation of these rocks can be explained by downward removal of the overlying forearc lithosphere and associated rise of deeply subducted material. The dated eclogite samples from the Theodul Glacier Unit in the western Central Alps revealed garnet growth ages of c. 57 Ma. The unit represents a continental outlier inside the ophiolotic Zermatt-Saas Zone. Subduction was controlled by gravity acting on the downward moving slab in a setting of rollback. The differing metamorphic ages of continental occurrences inside the Zermatt-Saas Zone reveal that subduction was continuous in this unit. Thus, the Zermatt-Saas Zone was not subducted in one coherent slab but rather represents slivers subducted at different times. The studies of the Adula Nappe, the Pohorje Nappe, and the Yuli Belt demonstrate that continental margins are subducted and exhumed to mid-crustal levels or even to the surface within short periods of time of clearly less than 10 Ma. In the case of the Adula Nappe this timeframe is evidently just 2 Ma. The results received from investigating the Adula Nappe and the high-grade metamorphic rocks of the Pohorje Mountains further demonstrate that these continental terranes underwent subduction cycles as coherent units. In contrast, garnet dating of a continental fragment from the Zermatt-Saas Zone and its comparison with literature data lead to the conclusion that this ophiolotic unit was assembled in a long-lasting subduction event

Late Miocene to Early Pliocene blueschist from Taiwan and its exhumation via forearc extraction

Blueschist facies rocks in the Yuli Belt of Taiwan's Central Range record ongoing subduction of the Eurasian plate. We present a prograde Lu-Hf garnet-whole-rock age of 5.1 +/- 1.7Ma from a retrogressed blueschist in the Yuli Belt. This age is considerably younger than the previously assumed age of 14-8Ma for high-pressure metamorphism in the Yuli Belt and represents the youngest Lu-Hf garnet age ever recorded for blueschist facies metamorphism. The age sheds new light on the palaeogeographic origin and exhumation scenario of the Yuli Belt. We propose that the Yuli Belt originated from the ocean-continent boundary of the Chinese passive margin. It was subducted eastward during collision with the Luzon island arc and rapidly exhumed when the forearc lithosphere was removed from above the continental slab by discrete subduction (extraction). This process reduces the pressure above the continental slab and may prompt the ascent of subducted crust into the opening gap. Thus, it can control the exhumation of high-pressure rocks

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

Lu–Hf garnet systematics of a polymetamorphic basement unit: new evidence for coherent exhumation of the Adula Nappe (Central Alps) from eclogite-facies conditions

The Adula Nappe in the Central Alps is a mixture of various pre-Mesozoic continental basement rocks, metabasics, ultrabasics, and Mesozoic cover rocks, which were pervasively deformed during Alpine orogeny. Metabasics, ultrabasics, and locally garnet–mica schists preserve eclogite-facies assemblages while the bulk of the nappe lacks such evidence. We provide garnet major-element data, Lu profiles, and Lu–Hf garnet geochronology from eclogites sampled along a north–south traverse. A southward increasing Alpine overprint over pre-Alpine garnets is observed throughout the nappe. Garnets in a sample from the northern Adula Nappe display a single growth cycle and yield a Variscan age of 323.8 ± 6.9 Ma. In contrast, a sample from Alpe Arami in the southernmost part contains unzoned garnets that fully equilibrated to Alpine high-pressure (HP) metamorphic conditions with temperatures exceeding 800 °C. We suggest that the respective Eocene Lu–Hf age of 34.1 ± 2.8 Ma is affected by partial re-equilibration after the Alpine pressure peak. A third sample from the central part of the nappe contains separable Alpine and Variscan garnet populations. The Alpine population yields a maximum age of 38.8 ± 4.3 Ma in line with a previously published garnet maximum age from the central nappe of 37.1 ± 0.9 Ma. The Adula Nappe represents a coherent basement unit, which preserves a continuous Alpine high-pressure metamorphic gradient. It was subducted as a whole in a single, short-lived event in the upper Eocene. Controversial HP ages and conditions in the Adula Nappe may result from partly preserved Variscan assemblages in Alpine metamorphic rocks

Timing of eclogite-facies metamorphism of mafic and ultramafic rocks from the Pohorje Mountains (Eastern Alps, Slovenia) based on Lu-Hf garnet geochronometry

The metamorphic series of the Pohorje Mountains represents a part of the Eastern Alpine realm that was subjected to ultrahigh-pressure conditions during the Cretaceous Eo-Alpine orogenic cycle. The Slovenska Bistrica Ultramafic Complex located in the south-eastern Pohorje Mountains is an 8 km wide serpentinite body that contains lenses of garnet-bearing ultramafites and eclogites. It is embedded in and part of a mixed continental unit of metapelitic gneisses, orthogneisses, and eclogites. We present Lu-Hf garnet chronometry coupled with geochemical and petrological data from three samples: one garnet lherzolite, one eclogite from within the ultramafic complex, and one eclogite from the surrounding mixed unit. All obtained ages are identical within error, i.e. 96.6 +/- 1.2 Ma and 94.8 +/- 5.1 Ma, respectively, for the two eclogites and 91.6 +/- 4.1 Ma for the garnet lherzolite. Garnet of all samples shows homogeneous concentrations of major bivalent elements due to high temperature re-equilibration. It does, however, preserve growth-related zoning with respect to Lu in all three samples implying that Lu-Hf ages still record garnet growth. The coincidence of ages suggests that the ultramafic complex and the surrounding continental mixed unit share the same subduction history, i.e. the complex was part of the subducting plate during and after the garnet growth stages. (C) 2016 Elsevier B.V. All rights reserved

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

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