Continental Subduction-collision Research Articles

Compositional and metamorphic controls on tectonic erosion along a continental subduction-collision zone: Implications from mafic granulites in the northern Sulu orogen

Abstract Subduction erosion has been reported recently in continental subduction-collision zones, and the response of the upper plate remains enigmatic. The Dabie-Sulu orogen is considered to have formed by deep northward subduction of the Yangtze block (lower plate) beneath the North China block (upper plate). However, within the northern Sulu ultrahigh-pressure (UHP) belt, the Haiyangsuo complex has intriguingly been accepted as Neoarchean–Paleoproterozoic metamorphic basement from the North China block, implying subduction-erosion could have occurred. Here, we examined new petrographic, mineral chemical, and published geochronological data from the Haiyangsuo mafic granulites to decipher their multiphase metamorphic evolution. Pressure-temperature (P-T) estimates from pseudosection modeling and geothermobarometry show two major metamorphic events: a medium-pressure granulite-facies event (7.3–8.3 kbar and 830–895 °C) and a later high-pressure granulite-facies event (12.2–16.6 kbar and 800–875 °C). Geochronology and mineral geochemistry link these events to Paleoproterozoic and Triassic metamorphism, respectively. Peak Triassic high-pressure granulite-facies metamorphism was followed by near-isothermal decompression and then near-isobaric cooling. These rocks derive from the upper plate and were metamorphosed at the same time as UHP eclogites, but maximum pressures were substantially lower, and P-T paths did not converge until late-stage exhumation. Evidently, metamorphic basement from the North China block was dragged to lowercrustal to upper-mantle depths (50–60 km) while that from the Yangtze block was subducted deeply, and then they were tectonically juxtaposed during exhumation. These data corroborate thermal-mechanical models that predict tectonic erosion as a major process during continental subduction and collision.

From subduction channel to orogenic wedge: Exhumation recorded by orthogneiss microstructures in Erzgebirge, Bohemian Massif

The Erzgebirge dome, exposing Proterozoic - Early Palaeozoic metagranitoids and metasedimentary rocks of the Saxothuringian domain in the Bohemian Massif, is formed by a tectonic stack of several UHP-HP and HT units exhumed during Variscan continental subduction-collision process. A systematic study of orthogneiss deformation microstructures carried out on a regional scale has been done to evaluate the extent and relative structural position of these units in the subduction complex. Five different microstructural types have been distinguished, revealing two overprinting sequences with 1) coarse grained microstructures in the parautochthonous Kateřina-Reitzenhain unit and 2) finer grained microstructures in the allochthonous HP-HT Gneiss Eclogite Unit. P-T conditions of the individual overprinting microstructures calculated by conventional thermobarometry suggest continuous deformation during exhumation along two contrasting P-T paths. The warmer one in the parautochthonous unit showed exhumation from ~13.5 kbar and ~820 °C to ~3 kbar and ~500 °C while the colder one in the allochthonous unit showed exhumation from ~16 kbar and ~740 °C to ~3 kbar and ~400 °C. The overprinting microstructures are associated with vertical shortening interpreted as ductile thinning. This deformation started at depths of ~10 kbar and led to a temporal elevation of the crustal heat flow affecting the exhumation P-T paths of the structurally deeper units. We propose a model of continuous underplating of the former accretionary wedge by deeply subducted and exhumed continental material building up the orogenic wedge. Progressive heating of the orogenic wedge is explained by heat advection related to fast exhumation of the UHP/HP-HT units from the subduction channel followed by ductile thinning.

The genetic linkage between the Yuanjiang marble-hosted ruby deposit and Cenozoic tectonic evolution of the Ailao Shan-Red River shear zone (Southwest China)

The Yuanjiang marble-hosted ruby deposit in the middle Ailao Shan-Red River (ASRR) shear zone, is the only economic ruby deposit in China. This deposit is hosted in Paleoproterozoic sillimanite gneisses and the ores are found in metamorphosed platform carbonates. Rubies are unevenly distributed within calcite-rich marble layers and mainly associated with phlogopite, scapolite, graphite, pyrite. Rubies contain zircon, rutile, apatite, calcite inclusions. The genesis of the Yuanjiang ruby deposit and its genetic link to the ASRR shear zone remain poorly understood. In this contribution, its formation age has been indirectly constrained by the in-situ LA-ICP-MS U-Pb dating of zircon inclusions in rubies and 40Ar/39Ar dating of syngenetic phlogopite from the ruby-phlogopite bearing marble, which yielded the ages of 36.2 ± 1.1 Ma (MSWD = 1.4, 2σ, n = 4) and 22.2–22.5 Ma, respectively. The 36.2 Ma U-Pb age of metamorphic zircon inclusions falls within the range of local crustal thickening related subduction-collision and coupled with the spatial distribution in which ruby deposits were not restricted in the shear zone, we propose that ruby mineralization was triggered by the local continental subduction-collision rather than the large-scale left-lateral shearing. Due to the estimated closure temperature of the 40Ar-39Ar system is lower than the ore-forming temperature, the obtained phlogopite 40Ar/39Ar ages of 22.2–22.5 Ma represent the minimum ore-forming age of the Yuanjiang ruby deposit. These phlogopite ages reveal a slower cooling rate of ruby deposits within the shear zone (including the Yuanjiang and the Yen Bai ruby deposits) than outside of the shear zone (the Luc Yen deposit), which can be attributed to the additional heat from the left-lateral shearing movement. Therefore, we propose that the left-lateral shearing plays an important role in transporting marble-hosted ruby deposits in the Ailao Shan-Red River shear zone to the subsurface.

Partial melting of ultrahigh-pressure metamorphic rocks at convergent continental margins: Evidences, melt compositions and physical effects

Ultrahigh-pressure (UHP) metamorphic rocks are distinctive products of crustal deep subduction, and are mainly exposed in continental subduction-collision terranes. UHP slices of continental crust are usually involved in multistage exhumation and partial melting, which has obvious influence on the rheological features of the rocks, and thus significantly affect the dynamic behavior of subducted slices. Moreover, partial melting of UHP rocks have significant influence on element mobility and related isotope behavior within continental subduction zones, which is in turn crucial to chemical differentiation of the continental crust and to crust-mantle interaction.Partial melting can occur before, during or after the peak metamorphism of UHP rocks. Post-peak decompression melting has been better constrained by remelting experiments; however, because of multiple stages of decompression, retrogression and deformation, evidence of former melts in UHP rocks is often erased. Field evidence is among the most reliable criteria to infer partial melting. Glass and nanogranitoid inclusions are generally considered conclusive petrographic evidence. The residual assemblages after melt extraction are also significant to indicate partial melting in some cases. Besides field and petrographic evidence, bulk-rock and zircon trace-element geochemical features are also effective tools for recognizing partial melting of UHP rocks. Phase equilibrium modeling is an important petrological tool that is becoming more and more popular in P-T estimation of the evolution of metamorphic rocks; by taking into account the activity model of silicate melt, it can predict when partial melting occurred if the P-T path of a given rock is provided.UHP silicate melt is commonly leucogranitic and peraluminous in composition with high SiO2, low MgO, FeO, MnO, TiO2 and CaO, and variable K2O and Na2O contents. Mineralogy of nanogranites found in UHP rocks mainly consists of plagioclase + K-feldspar + quartz, plagioclase being commonly albite-rich. Trace element pattern of the melt is characterized by significant enrichment of large ion lithophile elements (LILE), depletion of heavy rare earth elements (HREE) and high field strength elements (HFSE), indicating garnet and rutile stability in the residual assemblage. In eclogites, significant Mg-isotope fractionation occurs between garnet and phengite; therefore, Mg isotopes may become an effective indicator for partial melting of eclogites.

Zircon record of ocean-continent subduction transition process of dulan UHPM Belt, North Qaidam

The North Qaidam UHPM (ultra-high pressure metamorphism) belt is a typical continental subduction-collision belt containing continental crust deep subduction metamorphic products and oceanic crust relics, And it is an ideal region to study the ocean-continent transition and exhumation mechanism of oceanic UHP rocks during continental deep subduction process. In this paper, we report integrated in situ U-Pb, Lu-Hf and O isotope analyses of zircons from a serpentinized harzburgite as well as U-Pb dating for zircons from a kyanite eclogite from the North Qaidam Dulan UHPM terrane, and use these data to discuss the ocean-continent transition and exhumation mechanisms of oceanic UHP rocks during continental deep subduction. The serpentinized harzburgite was dated at 448±9 Ma, consistent with 455±5 Ma age for the kyanite eclogite within analytical errors. Zircons from the serpentinized harzburgite have uniform 176Hf/177Hf values ranging from 0.282 842 to 0.282 883 and e Hf(t) values from 11.6 to 13.3. Zircon δ18O values of the serpentinized harzburgite vary from 4.47‰ to 5.29‰, slightly lower than the value of 5.3‰±0.6‰ for the normal mantle zircon. These Hf-O isotopic features indicate that the protolith of the serpentinized harzburgite was derived from depleted-mantle source, and might have experienced high-temperature rock-water interaction. Therefore, the serpentinized harzburgite was possibly located in the lower part of an oceanic section. The serpentinized harzburgite and kyanite eclogite were both formed due to the subduction of oceanic crust. The UHP metamorphism occurred successively from the oceanic crust to continental crust rocks of the North Qaidam UHP terrane. Low-density serpentinized peridotite and continental rocks possibly have negative buoyancy and play a key effect on preservation and exhumation of high-density oceanic eclogite.

Silica-undersaturated spinel granulites in the Daqingshan complex of the Khondalite Belt, North China Craton: Petrology and quantitative P–T–X constraints

The Da’nangou silica-undersaturated spinel-bearing granulites in the Daqingshan Complex of the Khondalite Belt, North China Craton, occurred within quartzo-feldspathic rocks or interlayered with garnet–cordierite–biotite gneiss. These granulites preserve a distinct mineral assemblage of spinel+garnet+biotite+K-feldspar+plagioclase±sillimanite with reaction textures that are useful for deciphering the metamorphic evolution of the Khondalite Belt. Quantitative pressure, temperature, and bulk composition constraints on the development and preservation of a characteristic granulite facies mineral assemblage of spinel+garnet+biotite+K-feldspar+plagioclase±sillimanite are assessed with reference to calculated phase diagrams. Combined studies of mineral assemblages, phase relations of spinel granulites in P–T–X pseudosections calculated for the NCKFMASH system, and the origin of the protoliths of these rocks reveal that spinel-bearing assemblages formed mainly in high-variance fields. Quantitative P–T–X pseudosections indicate that the spinel-bearing granulite facies assemblages are restricted only to conditions of extremely low Si with iron enrichment and high Al. A clockwise P–T evolution is inferred from the P–T pseudosection and the observed microtextures of the spinel granulites involving near-isothermal decompression and decompressional cooling stages, which is comparable to the metapelitic rocks of the Daqingshan Complex. The metamorphic history of the spinel-bearing granulites can be subdivided into the following successive stages. The peak assemblage of biotite+garnet+K-feldspar+plagioclase+sillimanite+liquid was stable at ∼830°C and ∼10.5kbar. During the post-peak decompressional stage at ∼870°C and ∼8kbar, partial melting introduced by the biotite dehydration events occurred, followed by melt extraction taking SiO2 and alkali elements, the restitic silica-undersaturated Al–Fe-rich composition remains. The coarse-grained spinel-bearing assemblage was therefore produced by the following biotite dehydration-melting reactions: biotite+garnet+sillimanite→spinel+K-feldspar+liquid and biotite+sillimanite±quartz→spinel+liquid. Symplectic spinel rimming the garnet was produced by the garnet breakdown reaction garnet+sillimanite+liquid→spinel+K-feldspar+plagioclase as the decompression and cooling. During the cooling stage after decompression at ∼800°C and ∼6.5kbar, garnet was further broken down by consumption of melt that had not been segregated, to give rise to the biotite+plagioclase symplectites related to the reactions garnet+spinel+liquid→biotite+plagioclase and garnet+liquid→biotite+plagioclase±quartz. Such silica-undersaturated spinel-bearing granulites were likely to occur in the exhumation and cooling processes after continental subduction–collision, as indicated by the P–T path.

Multistage exhumation and partial melting of high-T ultrahigh-pressure metamorphic rocks in continental subduction-collision zones

High-temperature (HT; >850°C) metamorphism in continental collision orogens, particularly for those ultrahigh-pressure (UHP) metamorphic rocks, has become one of the remarkable topics in Earth science. It has bearing on the element and isotope behaviors of UHP rocks, their partial melting and related geodynamic effects during exhumation. In this paper, five representative continental collision orogens with typical HT/UHP rocks, including the Dabie orogen in China, the Kokchetav in Kazakhstan, the Caledonides in Greenland, the Rhodope in Greece, and the Erzgebirge in Germany are introduced, and their HT/UHP metamorphism and evolution processes are summarized. In addition, metamorphic P-T-t paths, multistage exhumation processes, and partial melting and preservation and retrogression of UHP index minerals during exhumation and their possible mechanisms are discussed. On this basis, the forthcoming key fields and scientific subjects of HT/UHP rocks within continental subduction channel are proposed.

Famenno-Carboniferous (370-320 Ma) strike slip tectonics monitored by syn-kinematic plutons in the French Variscan belt (Massif Armoricain and French Massif Central)

AbstractThe Variscan continental collision has led to the development of large strike-slip shear zones in western Europe. Our study focuses on the regional deformation and shear zone patterns in the Massif Armoricain and the French Massif Central. The synthesis of granite emplacement ages associated to granite deformation fields, allow us to propose a geodynamic model for the tectonic evolution of this part of the Variscan belt between 370 Ma – 320 Ma (Late Devonian – Namurian).After the first steps of the continental subduction-collision, leading to high temperature and anatexis associated with N-S shortening at 380-370 Ma (Frasnian to Famennian), the southern part of the Massif Armoricain and western part of French Massif Central underwent large dextral shearing along N100-N130 trending shear zones up to early Visean time. These large-scale displacements progressively decreased at around 350-340 Ma, during the first emplacements of biotite bearing granites (Moulins-les Aubiers-Gourgé massif and Guéret massif intrusions).During middle Visean times, the shortening axis direction rotated towards a NNE-SSW direction implying changes in the regional deformation field. The occurrence of N070-N100 sinistral and N110-N130 dextral conjugate shear zones within leucogranites are related to that time. Finally, new N150-N160 dextral shear zones appeared in middle to late Visean times: as for examples, the Parthenay and the Pradines shear zones in the SE Massif Armoricain and the Millevaches massif, respectively. These shear zones were conjugated to the sinistral N020 Sillon Houiller in the French Massif Central. They reflect large scale brittle continental indentation in the French Variscan belt during the middle to late Visean.