GD4.3 | Significance of ophiolites, melanges and blueschist assemblages in probing the crustal anatomy and geodynamic evolution of accretionary and collisional orogenic belts
Significance of ophiolites, melanges and blueschist assemblages in probing the crustal anatomy and geodynamic evolution of accretionary and collisional orogenic belts
Co-organized by GMPV6/TS6
Convener: Yildirim Dilek | Co-conveners: Andrea Festa, Edoardo BarberoECSECS
Orals
| Tue, 25 Apr, 16:15–18:00 (CEST)
 
Room -2.91
Posters on site
| Attendance Tue, 25 Apr, 14:00–15:45 (CEST)
 
Hall X2
Posters virtual
| Attendance Tue, 25 Apr, 14:00–15:45 (CEST)
 
vHall GMPV/G/GD/SM
Orals |
Tue, 16:15
Tue, 14:00
Tue, 14:00
Ophiolites, mélanges, and blueschists (OMB) are significant components of both accretionary and collisional orogenic belts and provide critical quantitative constraints for the timing of rift-drift, seafloor spreading and subduction initiation, ophiolite emplacement and collision events, peak P–T conditions during orogeny, and exhumation within subduction channels and along suture zones. Typical accretionary orogenic belt examples are exposed around the circum-Pacific region and in the Central Asian Orogenic Belt, whereas characteristic collisional orogenic belts occur in the circum-Mediterranean region, Alpine–Himalayan–Tibetan belt, Uralides, Taiwan and Papuan belts, Tasmanides, and Appalachians. Ophiolites and mélanges in these two major types of orogenic belts may show major differences in their crustal anatomies and geochemical fingerprints.

Collectively, OMB complexes and ocean plate stratigraphy (OPS) assemblages display the archives of ocean basin development, subduction initiation, crustal growth via accretionary processes (i.e., offscraping–shallow underplating) and volcanic arc formation at convergent margins, deep tectonic underplating and exhumation within subduction channels, and thermal evolution of subducting slabs. Therefore, systematic documentation of the tectonomagmatic settings of ophiolite formation, mechanisms and processes of mélange development (including non-metamorphosed ones), and P-T-t paths of both blueschist assemblages and high–temperature metamorphic belts in orogenic belts provide significant constraints for a quantitative establishment of the Wilson Cycle evolution of ancient ocean basins and the geodynamics of accretionary and collisional orogenic belt development.

This session will provide an international forum for interdisciplinary presentations and discussions on the diverse origins of OMB terrains and OPS assemblages, and their significance in probing the crustal anatomy and geodynamic evolution of accretionary and collisional belts around the world in a multi-scale approach. We welcome contributions dealing with the structural geology–tectonics, geochemistry–petrology, geochronology, and geophysics–geodynamics of OMB and OPS terrains, as well as numerical and analogue modelling of divergent and convergent margin processes that involve OMB evolution.

Orals: Tue, 25 Apr | Room -2.91

Chairpersons: Yildirim Dilek, Andrea Festa, Edoardo Barbero
16:15–16:20
16:20–16:40
|
EGU23-17224
|
GD4.3
|
solicited
|
Highlight
|
On-site presentation
Emilio Saccani

Ophiolites are interpreted to form in a variety of plate tectonic settings including oceanic spreading ridge, ocean island, oceanic plateau, intra-oceanic volcanic arc, continental volcanic arc, forearc, and back-arc. Therefore, ophiolites preserve records of tectono-magmatic events that occurred during distinct phases of development of an oceanic basin and its conjugated continental margins. Recognition of the tectono-magmatic setting of formation of ophiolites is fundamental to resolve major questions of Earth evolution through time, such as how, when, and where ancient oceanic basins formed and consumed. Geochemical fingerprinting of ophiolitic basalts was a fundamental tool in reconstructing ancient oceans as they represent the best record of the Earth's mantle composition and evolution. Since the 1970s, many methods of fingerprinting ophiolitic basalts have been proposed. At the beginning, fingerprinting was mainly performed using triangular diagrams based on immobile elements. Subsequently, there has been a trend towards using binary diagrams plotting elemental ratios, (e.g., Th/Yb, Ta/Yb, Nb/Yb, Zr/Y, Nb/Y); though the use of absolute concentrations (e.g., Ti, V, Y, Cr) has also been proposed. Despite the wide range of fingerprinting methods, most methodologies are not entirely satisfactory either because often failing to correctly classify data, or because considering a restricted number of all possible basaltic types. Some authors proposed basalt fingerprinting based on statistical calculation, which, though very effective, but difficult to be used because of complex calculations. Saccani (2015; http://dx.doi.org/10.1016/j.gsf.2014.03.006) proposed a very simple binary diagram for discriminating ten different ophiolitic basaltic types based on absolute contents of Th and Nb. This diagram was obtained using >2000 ophiolitic basalts (from Proterozoic to Cenozoic) and was tested using ~560 modern rocks from known tectonic settings. Two types of basaltic varieties that have never been considered before were included: a) medium-Ti basalts (MTB) generated at nascent forearc settings; b) Alpine-type mid-ocean ridge basalts showing garnet signature (G-MORB). In this diagram, basalts generated in subduction-unrelated settings can be distinguished from subduction-related basalts with a misclassification rate <1%. Subduction-unrelated basalts show a continuous chemical variation from depleted compositions to progressively more enriched compositions reflecting, in turn, the degree of enrichment of mantle source by plume-type components. Enrichment in Th relative to Nb is dependent on crustal input via subduction slab contamination. Basalts formed at continental margin volcanic arcs can be distinguished from those generated in intra-oceanic arcs (SSZ) with a misclassification rate <1%. SSZ basalts characterized by chemical contribution from subduction-derived components (forearc and island arc tholeiite and boninite) can be distinguished from those with no contribution from subduction-derived components (nascent forearc MTB and depleted-MORB). Since 2015 many geologists effectively used this diagram; however, since that time the dataset of ophiolitic basalts has increased significantly. Therefore, after eight years a check-up for testing its validity with new data would be certainly welcome. The aim of this contribution is, therefore, to present an eight-years check-up of the Saccani (2005) Th-Nb discrimination diagram.

 

How to cite: Saccani, E.: Discriminating ophiolitic basalts and their tectonic setting of formation using Th-Nb systematics: The eight-years check-up, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17224, https://doi.org/10.5194/egusphere-egu23-17224, 2023.

16:40–16:50
|
EGU23-17195
|
GD4.3
|
ECS
|
On-site presentation
Marshall Palmer, James Scott, Steven Smith, Petrus le Roix, Chris Harris, Marianne Negrini, and Matthew Tarling

Juxtaposition of oceanic and continental lithosphere along terrane boundary faults is an important tectonic process that can occur during closure of an ocean basin; however, the timing of faulting can be difficult to constrain. Here, we show that a spectacular exposure of the basal fault (Livingstone Fault) to the Dun Mountain Ophiolite in New Zealand may be dated using 87Sr/86Sr isotopes. At this boundary, quartzofeldspathic schist is faulted against the ultramafic base (peridotites and serpentinites) of the ophiolite and has resulted in metasomatic alteration of the schist, driven by the significant geochemical contrast between the contrasting rock types. We show that metasomatic alteration of the schist resulted in near complete removal of Rb due to the loss of mica, an increased modal abundance of metasomatic actinolite and appearance of metasomatic garnet and hedenbergite. Because Rb was removed from the metasomatized schist, its 87Sr/86Sr composition was essentially frozen at the time of metasomatism, while the 87Sr/86Sr composition of unaltered schist evolved due to the radioactive decay of 87Rb. Back calculating the present day 87Sr/86Sr composition of the unaltered schist to the frozen 87Sr/86Sr composition of the metasomatized schist yields a date of 170 Ma + 5 Ma. This date is broadly consistent with geological reconstructions of the Triassic-Jurassic Zealandia margin and provides a minimum age constraint on the timing of juxtaposition of the Dun Mountain Ophiolite against the crustal rocks and therefore the closure of the vast ocean basin along the eastern margin of Gondwana. Similar metasomatic reactions are described in similar settings elsewhere and so this method may be applied outside of this example.

How to cite: Palmer, M., Scott, J., Smith, S., le Roix, P., Harris, C., Negrini, M., and Tarling, M.: The timing of Dun Mountain Ophiolite emplacement via Rb-Sr isotope dating of metasomatic reactions along the basal Livingstone Fault in New Zealand, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17195, https://doi.org/10.5194/egusphere-egu23-17195, 2023.

16:50–17:00
|
EGU23-4669
|
GD4.3
|
ECS
|
Virtual presentation
Alexander Edgar, Ioan Sanislav, Paul Dirks, and Carl Spandler

The Tasmanides of eastern Australia record a complex geological history. The central, and southern Tasmanides have been widely interpreted to reflect long-lived, accretionary-style convergent tectonics. The northernmost Tasmanides, which extend into north Queensland, are more poorly understood, but considered highly prospective for numerous styles of mineralization. The region contains several slices of mafic-ultramafic rocks, situated along major regional structures. The mafic-ultramafic complexes record strong, oceanic geochemical signatures, and are structurally interleaved within high grade, strongly deformed, Paleozoic basement metamorphic assemblages. Along the Clarke River Fault, the Running River Metamorphics, which host ophiolitic mafic-ultramafic rocks, also record evidence of diamond facies, ultra-high pressure (UHP) metamorphism. The discovery of diamond facies metamorphism, in conjunction with convergent margin ophiolites, suggests that the Clarke River Fault may represent a continental suture zone. This is the first indication of continent suturing in the Tasmanides, and challenges the idea that the Tasmanides, and greater Terra Australis Orogen, represent a simple accretionary system.

How to cite: Edgar, A., Sanislav, I., Dirks, P., and Spandler, C.: Neoproterozoic-Paleozoic Convergent Margins in Northeast Queensland, Australia - New Ideas from the Discovery of Metamorphic Diamonds and Ophiolites., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4669, https://doi.org/10.5194/egusphere-egu23-4669, 2023.

17:00–17:10
|
EGU23-9587
|
GD4.3
|
Highlight
|
On-site presentation
Philippe Agard, Mathieu Soret, Guillaume Bonnet, Dia Ninkabou, Alexis Plunder, Cécile Prigent, and Philippe Yamato

Fragments of ancient oceanic lithosphere preserved in mountain belts, though volumetrically subordinate, provide essential insights into past geodynamics and formation and destruction of oceanic lithosphere. This contribution shows how the two types of oceanic fragments, blueschists and eclogites, on one hand, and ophiolites on the other, preserve crucial information on the dynamics of oceanic convergence, i.e. subduction and obduction.

Their mutual relationships, as well as the similarities and differences in the mechanisms leading to their preservation, allow tracking the evolution of the subduction process through time, from the onset of intra-oceanic subduction to the cessation of continental subduction – and, in some cases, to the obduction of ophiolites.

Fragments located at the base and immediately below unmetamorphosed (true) ophiolites represent witnesses of intra-oceanic subduction initiation and reveal, in particular, initial mechanical resistance to subduction, subsequent cooling and gradual strain localization. Subducted fragments of oceanic lithosphere metamorphosed as blueschists and eclogites, scraped off the downgoing slab episodically, at shallow or great depths, provide direct access to the composition, structure and rheology of rocks at the plate interface.

Both types reflect the mechanical behavior and 'hiccups' of the subduction plate boundary, during subduction initiation and mature subduction respectively.

How to cite: Agard, P., Soret, M., Bonnet, G., Ninkabou, D., Plunder, A., Prigent, C., and Yamato, P.: Subobduction: subduction plate boundary hiccups revealed by blueschists, eclogites and ophiolites, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9587, https://doi.org/10.5194/egusphere-egu23-9587, 2023.

17:10–17:20
|
EGU23-7405
|
GD4.3
|
ECS
|
On-site presentation
Iskander Ibragimov and Evangelos Moulas

Ophiolite obduction, the process by which part of the oceanic crust overlaps the continental margin, is challenging when it comes to the geodynamic reconstruction of lithospheric processes. The oceanic crust is, on average, denser than the upper continental lithosphere. This density difference makes the obduction of the oceanic crust difficult, if not impossible, when only buoyancy forces are considered. To overcome the difficulties posed by the negative buoyancy, the initial configuration of the oceanic basins must have specific thermal and geometric constraints. Here we present a systematic investigation of the geometrical/geodynamical parameters which control the ophiolite emplacement process. We used the LaMEM finite-difference code and acounted for petrologically consistent density structure of the oceanic and continental regions. Our study reveals which parameters are the most important during ophiolite emplacement and which are the most optimal geometries that favor ophiolite emplacement.

Our current study focuses on “Tethyan” ophiolites which are characterized by relatively small inferred basin size and are commonly found in Mediterranean region. Based on a combination of various parameters, our study identified the most susceptible configurations for ophiolite obduction. Our models demonstrate, in agreement to geological data, that the obducted lithosphere must be young (<10Myr) and the length of the nature of Ocean-Continent-Transition (OCT) must be relatively sharp (length of initial OCT zone < 60 km) in order to achieve ophiolite obduction. In addition, our results show that the presence of a weak zone separating two parts of the oceanic lithosphere has a profound influence on the subduction initialization and final ophiolite obduction.

How to cite: Ibragimov, I. and Moulas, E.: A systematic investigation of ophiolite obduction resulting from the closure of small oceanic basins., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7405, https://doi.org/10.5194/egusphere-egu23-7405, 2023.

17:20–17:30
|
EGU23-9188
|
GD4.3
|
On-site presentation
Carl-Heinz Friedel

Blocks of fault-bounded imbricate stacks of Devonian limestones, a diagnostic feature for a tectonic origin of chaotic rock fabrics in the Harz Mountains (Eastern Rhenohercynian Belt, Germany).

Friedel, C.-H.1, Cunäus, E.L.2, Kreitz, J.3, Leiss, B.4, Stipp, M.5

1) Karl-Marx-Str. 56, 04158 Leipzig, chfriedel@gmx.de; https://orcid.org/0000-0002-3380-5193
2) Baugrunduntersuchung Naumburg GmbH, Wilhelm-Franke-Str. 11, 06618 Naumburg, info@baugrunduntersuchung-naumburg.de
3) Smart Asphalt Solutions GmbH, Goethestraße 2, 37120 Bovenden, j.kreitz@smart-asphalt- solutions.de
4) Geowissenschaftliches Zentrum der Universität Göttingen, Strukturgeologie und Geodynamik,     Goldschmidtstr. 3, 37077 Göttingen, bleiss1@gwdg.de
5) Institut für Geowissenschaften und Geographie, Martin-Luther-Universität Halle-Wittenberg, Von‑Seckendorff‑Platz 3, 06120 Halle,  michael.stipp@geo.uni-halle.de

 

The distinction between sedimentary and tectonic processes in the formation of chaotic rock units (mélanges, broken formation) is especially difficult in ancient orogenic belts, where sedimentary structures are usually overprinted by tectonic deformation (e.g. Fiesta et al. 2019). This also applies to the chaotic rock units, which are widespread in the allochthonous domain of the Harz Mountains, an exposed part of the Eastern Rhenohercynian Belt in Germany. For these units, it has been previously assumed that their chaotic rock fabric was initially sedimentary in origin and was merely tectonically overprinted by subsequent Variscan deformation. In contrast, it could be shown, that tectonic deformation is crucial for the formation of the "chaotic" texture (Friedel et al. 2019). This is particularly evident in the structural characteristics of Devonian limestone blocks.

Within the allochthonous domain of the Harz Mountains, blocks of predominantly hemipelagic, condensed limestone of different ages and up to several tens of metres in size are widespread incorporated in a slaty clayey matrix. So far, the blocks were mostly regarded as olistholites and thus considered as clear evidence for a sedimentary origin of the chaotic rock units (olistostromes). However, our investigations show that the limestone blocks are fault-bounded, folded and internally imbricated stacks of limestone strata, i.e. tectonically sheared blocks formed during Variscan collisional deformation whose final fragmentation and isolation occurred subsequently to folding.  Like rootless folds, also blocks of fault-bounded imbricate stacks of rock strata are a diagnostic feature to identify a strong tectonic overprint or even a tectonic origin of chaotic rock fabrics, provided that the tectonic character of folding and faulting is sufficiently proven (Blanc et al. 2010, Friedel et al. 2022). Since such blocks of imbricated limestone stacks are regionally widespread, they support, together with other criteria, a predominantly tectonic origin of the chaotic rock units in the Harz Mountains.

References:
Blanc et al. 2010, Geogazeta, 48, 187-190,
Fiesta et al. 2019, Gondwana Research, 74, 7-30
Friedel et al. 2019, Intern. Journal of Earth Science, 108, 2295-2323
Friedel et al. 2022, Hallesches Jahrb. f. Geowissenschaften, Beiheft 51, 47-53 

 

How to cite: Friedel, C.-H.: Blocks of fault-bounded imbricate stacks of Devonian limestones, a diagnostic feature for a tectonic origin of chaotic rock fabrics in the Harz Mountains (Eastern Rhenohercynian Belt, Germany)., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9188, https://doi.org/10.5194/egusphere-egu23-9188, 2023.

17:30–17:40
|
EGU23-10113
|
GD4.3
|
ECS
|
Virtual presentation
Yunus Can Paksoy, Nefise Paksoy, and Gültekin Topuz

Orhaneli ophiolite is a Late Cretaceous ophiolitic suite, obducted over the Late Cretaceous high-pressure rocks of the Tavşanlı Zone that represents the subducted part of the southern passive continental margin. It is part of the Neotethyan ophiolites related to the Izmir-Ankara-Erzincan Suture. The present work aims to decipher the inner structure of the Orhaneli ophiolite. This implies constructing the geometrical relationships between structural elements and to evaluate their original positions relative to the paleo-horizontal and paleo-ridge axis.

The Orhaneli ophiolite comprises three tectonic domains separated from each other by N-S trending east-vergent thrusts. The middle domain comprises mantle harzburgite, dunite, pyroxenite, and crustal layered gabbro and cumulate peridotite. The Moho transition zone is represented by a 1 km thick, highly sheared zone that consists of serpentinite and mylonitic gabbro. Mylonitic gabbro has a layered-laminated structure and is very well lineated. Mantle structures (compositional layerings and foliations) are dominantly sub-vertically dipping with the N-S trend. While the layered gabbros are dipping to the east with 65° near the Moho, the dip direction progressively changes to the west stratigraphically upward. The eastern domain is the tectonic repetition of the mantle section of the middle domain. Foliations and compositional layerings strike N-S and sub-vertical dips. The western domain corresponds to relatively lower parts of the mantle which consists of harzburgite and dunite. The absence of pyroxenites distinguishes the mantle rocks of this domain from the others.

It is observed that (1) there is a low-angle relationship between the mantle structures and the lower parts of the layered gabbro, (2) layered gabbros are progressively steepening stratigraphically upward, (3) the boundary between the lithospheric mantle and the crust is strongly sheared.

How to cite: Paksoy, Y. C., Paksoy, N., and Topuz, G.: Establishing the structure of the Cretaceous Neotethyan Orhaneli ophiolite, NW Turkey, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10113, https://doi.org/10.5194/egusphere-egu23-10113, 2023.

17:40–17:50
|
EGU23-2098
|
GD4.3
|
ECS
|
Virtual presentation
|
Valentina Brombin, Emilio Saccani, and Gianluca Bianchini

Sulfur (S) is one of the key volatiles in Earth’s chemical cycles as it affects biological, climate, ore-deposits, and redox processes. It is known that S stored in the crust is recycled into the mantle at subduction zones. However, some aspects of the S cycle in the deep Earth such as S speciation, flux, and isotope composition and fractionation processes still remain unclear. Most of the S isotopic studies investigate the melt inclusions, which potentially preserve the original budget and isotopic signature of the magma. However, these researches are limited, as melt inclusions are rare. Studying ophiolites represent a valid alternative to investigate contents and isotopic features of S with the aim to reconstruct its cycle in different geodynamic settings. Ophiolites are fragments of ancient oceanic lithosphere that were tectonically emplaced into orogenic belts and, according to the Dilek and Furnes (2014) classification, they can be discriminated as subduction-unrelated and subduction-related magmatic rocks. In this work we compiled a global dataset of both subduction-unrelated and subduction-related ophiolitic basalts, and we measured their whole rock S contents and the relative S isotopic ratio (34S/32S) through using an elemental analyzer coupled with a mass spectrometer (EA-IRMS). The considered samples are Mid-Ocean Ridge Basalts (MORBs) from Corsica, Romania, Albania, and North Macedonia; ii) Island Arc Tholeiites (IAT) from Albania and Greece; iii) Calc-Alkaline Basalts (CAB) from Greece, Romania, North Macedonia, and Iran already constrained from a petrological and geochemical point of view by different studies (Moberly et al., 2006; Saccani et al., 2011; Brombin et al., 2022). In the studied basalts, the S contents range from 200 and 300 ppm. Despite the different areas of provenance, for most of the samples the S isotopic signatures are similar in rocks having similar geochemical affinity. The average S isotopic ratios are –0.7‰, +5.8, and +7.4‰, for MORBs, IATs, and CABs, respectively. It is evident that only MORBs preserved the typical S signature of the Earth mantle (i.e., from –2‰ to 0‰). The subduction related magmatic rocks (i.e., IATs and CABs) show positive S isotopic values, probably due to the contamination of i) enriched-34S subducting sediments in the magma sources or ii) fluids released by serpentinized rocks of the slab, which typically have comparatively more positive S signature.

In summary, this work allowed the definition of: i) the S isotope compositions in both subduction-unrelated and subduction-related magmatic rocks; ii) the possible causes which modify the original S signature (e.g., contamination by subducting sediments). This research is therefore essential to understand the global S cycle.

 

References

Dilek Y., Furnes H., 2014. Elements, 10: 93-100.

Moberly R., et al., 2006. Proc. ODP, Sci. Results, 203: 1-36.

Saccani E., et al., 2011. Lithos, 124: 227-242.

Brombin V., et al., 2022. Ofioliti, 47: 85-102.

How to cite: Brombin, V., Saccani, E., and Bianchini, G.: Unravelling the deep sulfur cycle: isotopic signatures of ophiolitic rocks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2098, https://doi.org/10.5194/egusphere-egu23-2098, 2023.

17:50–18:00
|
EGU23-4786
|
GD4.3
|
Virtual presentation
Ashima Saikia and Eyozele KIso

Basaltic samples from Nagaland-Manipur Hill Ophiolite (NHMO) complex in north-eastern India comprise predominantly of plagioclase with small amounts of pyroxene and exhibit porphyritic texture. In whole rock Zr/Ti vs. Nb/Y discrimination diagram, these rocks are classified as basalt (TiO2 < 2 wt.%) and alkali basalt (TiO2 < 2 wt.%). Based on whole rock and clinopyroxene composition, basalt and alkali basalt show tectonic affinities to MORB and WPB, respectively. In N-MORB normalized trace element plot, basalt display near-horizontal trend at rock/N-MORB = ~1 and show positive anomalies at Pb, Th and Sr, whereas alkali basalt display increasing enrichment from left to right with marked negative anomalies at Ti and Sr. In chondrite normalized REE plot, basalt display near-parallel horizontal pattern similar to average N-MORB, whereas alkali basalt show parallel but increasing enrichment pattern from HREE to LREE similar to average OIB. Incompatible trace element ratios Sm/Yb, La/Sm, TiO2/Yb and Nb/Yb suggest N-MORB- and OIB-type parental magma for basalt and alkali basalt, respectively.

Dynamic melting inversion model for alkali basalt suggests melting of OIB-like spinel lherzolite composition (S1) at F = ~5%, with S1 being more enriched in MREE, LREE, Nb and Zr as compared to DMM. Non-modal batch melting model for basalt suggests melting of N-MORB-like spinel lherzolite composition (S2) at F = ~5 - 10%, with S2 being very similar to DMM. Constraints from trace elements indicate that basalt with N-MORB signatures is believed to be part of an ophiolite suit, whereas the alkali basalt with OIB signatures is likely due to some localized plume activity.

How to cite: Saikia, A. and KIso, E.: Origin of basaltic rocks of Nagaland-Manipur Hill Ophiolite (NMHO) complex in North-Eastern India: Inferences from mantle melting models., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4786, https://doi.org/10.5194/egusphere-egu23-4786, 2023.

Posters on site: Tue, 25 Apr, 14:00–15:45 | Hall X2

Chairpersons: Yildirim Dilek, Andrea Festa, Edoardo Barbero
X2.201
|
EGU23-14738
|
GD4.3
|
ECS
The Jurassic oceanic tectonostratigraphy of the high-pressure Lanzo Valleys Ophiolites (Western Alps)
(withdrawn)
Marcello De Togni, Marco Gattiglio, and Gianni Balestro
X2.202
|
EGU23-12006
|
GD4.3
|
ECS
Michele Locatelli, Laura Federico, Paola Cianfarra, Danilo Morelli, and Laura Crispini

Mélanges are abundant in both accretionary and collisional orogenic belts. Their chaotic, block-in-matrix structure can have different origins: sedimentary mélanges can be overprinted by later metamorphic and deformative events or, conversely, tectonic mélanges can form directly at the plate interface, at different tectonic levels and either in prograde (i.e. during underplating) or retrograde (i.e. during exhumation) conditions.

The HP-metaophiolitic Voltri Massif (W Alps, Italy), considered as an exhumed piece of the plate interface of the Alpine orogen, includes various, well-preserved examples of tectonic mélanges at different scales (from m- to km-scale). Here, we investigate a 100 meters-thick tectonic mélange, where blocks of various metamorphic lithologies (e.g. metagabbro, eclogite, serpentinite, calchschist and qtz-micaschist) and sizes (0,1-m- to 10-m scale) are dispersed within an intensely foliated, lithologically heterogeneous matrix made of a mixture among serpentinite-schist, chlorite-actinolite schist and graphitic schist, predominantly equilibrated at grenschist facies conditions.

Preliminary field investigations reveal a pronounced strain and metamorphic partitioning between the matrix and the blocks. These latter show internal metamorphic layering, shear zones and extensional veins discordant to the pervasive s-c-fabric and folding that characterize the enclosing matrix. Locally, eclogitic blocks show progressive internal fragmentation (e.g., fracturing/veining) up to pervasive brecciation. Petrographic/microanalytical investigations on the most preserved (Fe-Ti-bearing) metagabbro and metabasalt blocks indicate prograde peak metamorphism either in eclogite (grt + omp + rt ± Na-amp ± ph assemblage) or blueschist-facies (Na-amp + ttn + chl ± ep ± ph assemblage); some eclogites show either a retrograde syn-tectonic stage in blueschist facies or a static greenschist overprint. PT estimates on eclogitic blocks indicate a peak stage at P = 18,6 ± 1,0 Kbar (gnt-ph-cpx geobarometer) and T = 530 ± 10°C. The block-matrix transition is characterized by dm- to cm-thick metasomatic rinds rich in hydrous minerals, such as tremolitic amphiboles, biotite, chlorite and minor titanite, tourmaline, adularia and sulphides. Locally, tensile fractures filled by a polymineralic gouge material with the same mineral composition (±biotite) and syntectonic extensional veins with fibrous amphibole depart from the rinds and intrude the prisitne blocks. Abundant hydrothermal fluid circulation is suggested, among other, by peculiar microstructures, i.e. the growth of chlorite in vermicular form.

The block-in-matrix structures and microstructures (shear zones and extensional cracks repeatedly crosscuting eachother) point to the occurrence of a cyclic deformation characterised by episodic switch between brittle and ductile regimes and changes in the rehological properties of blocks and matrix. The occurrence of (i) abundant mélange matrix, (ii) metasomatic rinds digesting blocks with (iii) sets of veins/cracks irradiating inside the intact rocks suggest the key-role played by fluids in the evolution of the Piota River mélange.

The evidence recorded in the studied lithologies, such as episodic switch between deformation regimes assisted by transient exceed of the rock tensile strenght by pore fluids overpressure, would permit to better understand the mechanisms controlling slow earthquake generation at shallow plate interface. Morover, this study, combined with studies of other melange occurrences of the Voltri Massif, will help to better understand the complex geodynamic phenomena acting on collisional orogens.

How to cite: Locatelli, M., Federico, L., Cianfarra, P., Morelli, D., and Crispini, L.: Study of tectonic mélanges from a fossil plate interface: probing geodynamic phenomena, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12006, https://doi.org/10.5194/egusphere-egu23-12006, 2023.

X2.203
|
EGU23-4276
|
GD4.3
|
ECS
|
Highlight
Edoardo Barbero, Maria Di Rosa, Luca Pandolfi, Morteza Delavari, Asghar Dolati, Federica Zaccarini, Emilio Saccani, and Michele Marroni

Seamounts are topographic highs of the oceanic plates, and they are passively carried toward convergent margins where they may interact with the frontal part of the subduction complexes, modifying their shape and influencing the operating tectonic processes. In this tectonic setting, seamount fragments can be transferred from the subducting plate to the accretionary prism with different mechanisms, including deformation within the subduction channel, accretion via decapitation of the seamount summit by the basal décollement of the prism, and offscraping and underplating of thrust-bounded assemblages at both shallow (4-8 km) and deep (20-30 km) structural levels of the prism. In this complex tectonic scenario, it is not completely clear which are the factors controlling deformation mechanisms and localization of the basal décollement below, inside, or above the subducting seamount. Detailed geological mapping, stratigraphic-structural analysis and petrological studies are promising tools to better understand the mechanism of seamount materials accretion, providing data to recognize the role of subducting seamounts for the geodynamic evolution of exhumed accretionary and collisional orogenic belts.

We present here new structural and thermobarometric data on the Durkan Complex to discuss how Late Cretaceous seamount materials has been accreted into the Makran accretionary prism (SE Iran) during the Late Cretaceous – Paleocene subduction-accretionary stages. Throughout a map- to micro-scale structural studies of the western part of this Complex, we describe its structural and tectono-metamorphic evolution using crosscutting relationships between structural elements and stratigraphic unconformities.

Our results indicated that seamounts material has been incorporated in the prism as imbricated tectonic units separated by NNW-striking thrust zones. During the accretion, seamounts successions are folded by sub-isoclinal folds, associated with a blueschist facies axial plane foliation and shear zones along the limbs. These shear zones show block-in-matrix fabric and are mainly composed of volcaniclastic material from the seamount slope successions indicating that the seamount stratigraphy play a key role in controlling the position of the basal décollement of the prism during underplating. Thermobarometric estimates indicate that the accretion took place at T = 160-300 °C and P = 0.6-1.2 GPa, corresponding to a depth of 25–40 km. This data indicates the incorporation of seamount materials via underplating at blueschist facies conditions within the Makran subduction complex. The folds and shear zones formed during the accretionary stage are later deformed by open to close folds associated with normal faults, recording the progressive exhumation of the accreted seamount materials at shallower levels of the Makran Accretionary Prism. The unconformable deposition of upper Paleocene – Eocene turbiditic successions onto the exhumed seamount materials of the Durkan Complex constrain the accretionary stages during the Late Cretaceous – early Paleocene evolution of the Makran Accretionary Prism.

How to cite: Barbero, E., Di Rosa, M., Pandolfi, L., Delavari, M., Dolati, A., Zaccarini, F., Saccani, E., and Marroni, M.: Processes of seamount materials accretion in subduction complexes: The example of the Durkan Complex (Makran Accretionary Prism, SE Iran), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4276, https://doi.org/10.5194/egusphere-egu23-4276, 2023.

X2.204
|
EGU23-5270
|
GD4.3
Emilio Saccani, Edoardo Barbero, Luca Pandolfi, Morteza Delavari, Asghar Dolati, Michele Marroni, Rita Catanzariti, and Marco Chiari

Ophiolites may originate in a variety of oceanic settings such as mid-ocean ridges, intra-oceanic and continental margin volcanic arc, marginal basins, and seamounts. Ophiolites from different settings show distinctive lithological features and geochemical fingerprinting, so that they can conversely be used to identify their geodynamic setting of formation. Therefore, ophiolite geochemistry coupled with geochronological data represents an effective tool for tracking the magmatic events occurring during the life of an oceanic basin and surrounding continental margins. The northern part of the Makran Accretionary Prism in south Iran is characterized by extensive occurrence of tectonically imbricated ophiolitic, metaophiolitic, and ophiolitic mélange units, which represent or incorporate remnants of the Neotethys Ocean located between the Lut block and the Arabian Plate and of its northern continental margin. In this contribution we present a review of geochemistry and age data of volcanic rocks from these units with the aim of defining the nature and tectono-magmatic evolution of the Middle East sector of the Neotethys.

The North Makran ophiolitic units are from north to south (from the structural top to bottom): 1) the Ganj Complex; 2) the Northern Ophiolites including Band-e-Zeyarat/Dar Anar, Remeshk-Mokhtarabad, and Fannuj-Maskutan units: 3) the Deyader Complex; 4) the Bajgan Complex; 5) the Durkan Complex; 6) the Sorkhband-Rudan ophiolites; 7) the Coloured Mélange. The Deyader, Bajgan, and Durkan Complexes show variable extents of HP-LT metamorphic imprint.

The Ganj Complex consists of island arc tholeiitic (IAT) and calc-alkaline (CAB) volcanic sequences showing Turonian-Coniacian age (biostratigraphic data). This unit represents a Late Cretaceous volcanic arc that was likely forming at the southern margin of the Lut Block. Units of the Northern Ophiolites and the Bajgan metaophiolites show similar geochemistry and age. They are largely represented by mid-ocean ridge basalts (MORB) showing either normal (N-) and enriched (E-) compositions. Biostratigraphic and zircon U/Pb radiometric datings suggest Early Cretaceous and Late Jurassic-Early Cretaceous ages for the Northern ophiolites and the Bajgan Complex, respectively. The Durkan and Deyader Complexes are both Late Cretaceous in age. The Deyader metaophiolites range in composition from N-MORB to E-MORB and comparatively more enriched plume-type MORB (P-), whereas the Durkan metaophiolites show P-MORB and very enriched alkaline affinities and have been interpreted as remnants of a seamount chain. The Coloured Mélange includes volcanic arc basalt of both Early and Late Cretaceous age, as well as Late Cretaceous enriched oceanic plateau basalts and alkaline basalts (all ages based on biostratigraphic data).

This study indicates that the North Makran ophiolites and metaophiolites represent fragments of a unique Late Jurassic – Cretaceous oceanic basin, which was increasingly affected by mantle plume activity from Early to Late Cretaceous and experienced different extents of plume-ridge interaction in different times and areas. The different ophiolitic units represent distinct portions of the oceanic basin including plume proximal and plume distal mid-ocean ridges, seamounts. From Late Cretaceous, this basin subducted below the Lut Block forming the Ganj volcanic arc. 

How to cite: Saccani, E., Barbero, E., Pandolfi, L., Delavari, M., Dolati, A., Marroni, M., Catanzariti, R., and Chiari, M.: Nature and evolution of the Middle East Neotethys: New constraints from geochemistry and age of ophiolites and metaophiolites from the Makran Accretionary Prism (SE Iran), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5270, https://doi.org/10.5194/egusphere-egu23-5270, 2023.

X2.205
|
EGU23-12802
|
GD4.3
|
ECS
Emily R. Hinshaw, Daniel F. Stockli, and Konstantinos Soukis

Studies of exposed high pressure-low temperature (HP-LT) metamorphic complexes are critical for advancing our understanding of subduction processes, such as underplating, metamorphism, and exhumation. Exhumed blueschist-facies metasedimentary and volcanic rocks exposed on the Pelion peninsula (eastern Thessaly, Greece) represent one of the largest coherent exposures of subduction-complex rocks in the eastern Mediterranean and are key for understanding early Cenozoic Hellenic subduction processes. In this study, we present new detrital zircon and apatite U-Pb data to reconstruct the stratigraphic anatomy and provenance of these rocks and to understand their correlation with other Aegean (Cycladic) HP-LT rocks and the Pelagonian Zone of mainland Greece.

Detailed new U-Pb zircon and apatite data show two distinct, coherent, and stratigraphically upright structural slices, with (1) the South Pelion slice consisting of Permian-Late Cretaceous strata overlying Carboniferous basement and (2) the North Pelion slice comprising Triassic-Late Cretaceous strata overlying Neoproterozoic basement. Both slices exhibit Late Cretaceous strata at the top of the section characterized by cosmopolitan detrital zircon (DZ) signatures. Zircon U-Pb data of rim overgrowths suggest subduction-metamorphism occurred during the early Cenozoic with temperatures not reaching >450°C, as indicated by non-reset or -recrystallized apatite U-Pb ages and the absence of garnet.

Comparison of compiled DZ data from the CBU and our data from the Pelion blueschists supports a correlation in the pre-subduction paleogeography, with protolith deposition during Permo-Carboniferous intra-arc extension and early Mesozoic Adria-Pindos rifting. The data show that the Pelion blueschists, representing lateral equivalents of the CBU, are comprised of two coherently underplated upper-crustal slivers, separated by Late Cretaceous flysch, and metamorphosed during Cenozoic Hellenic subduction beneath the Pelagonian convergent margin.

How to cite: Hinshaw, E. R., Stockli, D. F., and Soukis, K.: Coherent subduction underplating of CBU-correlative blueschist-facies metasedimentary slices, Pelion, Greece, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12802, https://doi.org/10.5194/egusphere-egu23-12802, 2023.

X2.206
|
EGU23-8023
|
GD4.3
|
ECS
Pornchanit Sawasdee, John E. Booth, Etienne Skrzypek, and Christoph A. Hauzenberger

The Nan River mafic-ultramafic belt was identified when detailed geological mapping of NE Thailand began in the 1970s, and suspected to represent a suture zone. However, in the absence of an obvious ophiolite, its tectonic status was not confirmed until two short papers (Barr et al., 1985 and Barr & Macdonald, 1987) reported the discovery of associated  blueschists. Unfortunately military restrictions on access to detailed topographic maps meant they that they did not state an exact location and the outcrops were “lost” to Thai geologists and no further research was conducted. The two “lost” blueschist localities (south of Nan and west of Uttaradit) were recently re-discovered and related winchite – barroisite schist units identified. Additionally, garnet – glaucophane/riebeckite – white mica – quartz – magnetite – titanite – rutile ± albite ± stilpnomelane bearing gneisses were found among the bedload of a stream cutting through these schists. These gneisses are believed to be derived from “exotic” blocks in a mapped, but poorly exposed thrust sheet of tectonic melange, but to date no in place examples have been found. Similar blueschists/greenschists, gneisses and related garnet – white mica schists have been found further north as cobbles on point bars of the Wa river (west Nan), which cuts through a different section of the mafic – ultramafic unit in a mountainous and inaccessible national park.

At both in-place blueschist locations the schists have undergone two episodes of deformation, producing well developed schistosities and tight folding. The blue amphiboles are crossitic in composition. They do not contain garnet nor lawsonite, but abundant epidote and white mica with elevated phengite content. They are interbanded with winchite – barroisite bearing schists. The observed mineral assemblages are poorly suited to apply well established geothermobarometers. However, a PT window of the metramorphic overprint could be established with ca. 450 to 550 °C and 0.6 to 1.0 GPa. Geothermobarometry of the blue amphibole and garnet bearing exotic gneisses from the first blueschist locality (south Nan) indicates peak T conditions of ca. 550°C and a max. P of ca. 1 GPa. Comparable blue amphibole and garnet bearing gneisses from the second locality (Wa river) indicate similar peak PT conditions.

In-situ U-Pb zircon analyses from 6 blue amphibole – phengite bearing gneiss samples gave weighted mean 206Pb/238U dates ranging from 312 to 326 Ma, which is interpreted as the age of the protolith. Accessory phases within the blueschists and gneisses include variously zircon, titanite, rutile, allanite and monazite. Planned analysis of these phases should provide the age of HP/LT metamorphism.

How to cite: Sawasdee, P., Booth, J. E., Skrzypek, E., and Hauzenberger, C. A.: Blueschists and blue amphibole gneisses in the Nan suture zone, NE Thailand, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8023, https://doi.org/10.5194/egusphere-egu23-8023, 2023.

X2.207
|
EGU23-10648
|
GD4.3
|
Highlight
Multi–Collision Tectonic Evolution of the Tibetan–Himalayan Orogenic Belt, and Its Significance for the Mesozoic–Cenozoic Geodynamics of Neotethys North of East Gondwana
(withdrawn)
Yildirim Dilek and Yanxue Xie
X2.208
|
EGU23-9853
|
GD4.3
|
Highlight
Andrea Festa, Edoardo Barbero, Yildirim Dilek, Francesca Remitti, Kei Ogata, and Gian Andrea Pini

Most ophiolitic mélanges and chaotic rock units in exhumed subduction zone complexes and orogenic belts are commonly interpreted as the products of tectonic processes (e.g., underplating and return flow) acting at intermediate to great depths (depth > 10–15 km, T > 250 °C) at convergent margins. Conversely, observations from modern and ancient, non- to poorly metamorphosed subduction–accretion complexes (recognized as mélanges and chaotic rock units) around the world show that these rock associations: (1) likely formed at shallow structural levels first, and (2) were later subducted and became tectonically reworked. As such, they mainly consist of broken formations (> 21.5%), and sedimentary (c. 20%), polygenetic (> 13.7%) and/or diapiric (c. 6.7%) mélanges. Tectonic mélanges are limited to <3.0% (in surface distribution), suggesting that tectonic processes do not make efficient mixing mechanisms at shallow structural levels. Subduction of structural inheritances (e.g., ocean-continent transition zones, and lithological and structural heterogeneities in ocean plate stratigraphy – OPS – assemblages) plays a more significant role in forming mélanges and chaotic rock units at shallow depths; it can also control the origin and location of plate interface and the dynamics of the wedge front (i.e., tectonic accretion vs. erosion). However, not all chaotic rock units that formed at shallow structural levels may become subducted; but, if subducted, their fate might be different depending on whether they become part of the plate interface or if they become part of the lower plate. Our global field observations, suggesting that most mélanges and chaotic rock units form at shallow depths, have significant implications for the tectonic evolution of subduction zone complexes and orogenic belts.

How to cite: Festa, A., Barbero, E., Dilek, Y., Remitti, F., Ogata, K., and Pini, G. A.: Significance of ophiolitic mélanges and chaotic rock units in the evolution of subduction complexes and orogenic belts, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9853, https://doi.org/10.5194/egusphere-egu23-9853, 2023.

Posters virtual: Tue, 25 Apr, 14:00–15:45 | vHall GMPV/G/GD/SM

Chairpersons: Yildirim Dilek, Andrea Festa, Edoardo Barbero
vGGGS.17
|
EGU23-14650
|
GD4.3
|
ECS
Qunye Qian, Bo Huang, Dong Fu, Man Liu, Timothy Kusky, and Lu Wang

Ophiolite is the remnants of ancient oceanic crust and mantle, which can reveal the tectonic evolution of paleo-oceanic basins. Podiform chromite deposit in ophiolites can retain the original information of petrogenesis and mineralization during the later deformation and metamorphism process, and it is a key object that can be used to decipher the origin and tectonic setting of ophiolites and the evolution of paleo-oceanic basins. Ophiolite suites are widely developed in Hegenshan and Solonker tectonic belts, the Inner Mongolia segment of the southern Central Asian Orogenic Belt. However, the genesis and tectonic environment of ophiolite and associated podiform chromitites remain debated, which restrict the understanding of the tectonic evolution and metallogenic background of the orogenic belt. Here, we conducted a detailed study of field, petrography, and mineral chemistry on the podiform chromitites in the Hegenshan and Solonker ophiolites in Inner Mongolia to explore their origin and tectonic environment. Petrographic results show that the Hegenshan chromites contain abundant high-pressure, hydrous mineral inclusions of sodic amphibole, white mica, and clinopyroxene, along with previously reported ultra-high pressure minerals (e.g., diamond); whereas the Solonker chromite contains minor white mica inclusions. Mineral chemical analysis shows that the Hegenshan ophiolite is dominated by high-Al type spinels with subordinate high-Cr type spinels; whereas the Solonker ophiolite mainly contains High-Cr type spinels. Accordingly, we suggest that the Hegenshan chromitites formed initially in a mid-ocean ridge (MOR) setting of a backarc ocean basin, then experienced modification in a suprasubduction zone (SSZ) setting, with deep mantle recycling and two stages of melt-peridotite interactions due to backarc subduction initiation; and the Solonker chromitites formed by boninitic melt-peridotite reaction in the SSZ forearc setting probably due to slab roll-back or subduction re-initiation following ridge subduction. These findings provide important constraints on the petrogenesis of chromites/ophiolites, regional tectonic evolution and mineralization background of chromitites in the Inner Mongolia segment of the Central Asian Orogenic belt.

How to cite: Qian, Q., Huang, B., Fu, D., Liu, M., Kusky, T., and Wang, L.: Genesis and tectonic setting of podiform chromitites in the Hegenshan and Solonker ophiolites, Inner Mongolia, southeastern Central Asian Orogenic Belt, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14650, https://doi.org/10.5194/egusphere-egu23-14650, 2023.