HP–UHP metamorphism and tectonic evolution of orogenic belts: introduction
Published:January 01, 2019
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Lifei Zhang, Zeming Zhang, Hans-Peter Schertl, Chunjing Wei, 2019. "HP–UHP metamorphism and tectonic evolution of orogenic belts: introduction", HP–UHP Metamorphism and Tectonic Evolution of Orogenic Belts, Lifei Zhang, Zeming Zhang, Hans-Peter Schertl, Chunjing Wei
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High pressure (HP) and ultrahigh pressure (UHP) metamorphic rocks play a key role for understanding the tectonic evolution of orogenic belts (Johnson, M.R.W. & Harley, S.L. 2012. Orogenesis: The Making of Mountains. Cambridge University Press, Cambridge). They have typically experienced complex changes during subduction and exhumation processes, arising from recrystallization, deformation, element redistribution, fluid–rock interactions and even partial melting, and may therefore carry a valuable record of evolving geodynamic systems in an orogenic belt. Until now, more than 20 UHP metamorphic belts, i.e. belts which contain rocks that experienced pressure–temperature (P–T) conditions exceeding the lower limit of the coesite stability field, have been identified all over the world (Carswell, D.A. & Compagnoni, R. 2003. Ultrahigh Pressure Metamorphism. Eotvos University Press, European Mineralogical Union, Notes in Mineralogy, Budapest; Liou, J.G., Ernst, W.G., Zhang, R.Y., Tsujimori, T. & Jahn, B.M. 2009. Ultrahigh-pressure minerals and metamorphic terranes – the view from China. Journal of Asian Earth Sciences, 35, 199–231; Zheng, Y.F., Zhang, L.F., McClelland, W.C. & Cuthbert, S. 2012. Processes in continental collision zones: preface. Lithos, 136, 1–9). New and innovative studies from different geoscience disciplines have been invaluable in developing a better understanding of the geodynamic evolution of orogenic belts. This special issue contains 15 papers, most of which were presented as part of the session ‘HP–UHP metamorphism and tectonic evolution of orogenic belts’, held at the 35th International Geological Congress in Cape Town, South Africa during 27 August to 4 September, 2016.
This special publication comprises of three parts: (I) new developments in the determination of metamorphic P–T conditions and their timing; (II) overview papers of well-known HP–UHP metamorphic belts; and (III) research papers for some newly discovered and/or less well understood HP–UHP belts.
Part I contains four papers. The first paper (P474-18-149C181O´Brien 2018) shows how the interpretation of even intensively investigated UHP rocks, in this case the Tso Morari (Himalaya) eclogite, can still be controversial when the results of thermodynamic pseudosection modelling are preferred over basic observations. Detailed petrography and microanalysis are shown to remain the main tools required for reliable P–T deductions, whereas modelling tools, especially in rocks with multistage reaction histories, may yield erroneous results unless extreme care is taken. The second paper focuses on mineralogical phase relations in metamorphic rocks, which is key to deciphering the metamorphic development during the tectonic evolution of an orogenic belt. Wei & Duan (2018) present an overview of the phase relations in metabasic rocks based on the results of phase modelling, experiments and ACF analysis. They propose a four-fold classification of metamorphism, including low-, medium-, high- and very high-P–T types, with apparent geothermal gradients at >110, 110–55, 55–28 and <28°C/kbar, respectively. This four-fold classification of metamorphism is matched to various tectonic settings, namely island arc, continental rift, oceanic range and contact aureoles for the low-P–T type, collisional orogenic belts for the medium-P–T type, warm subduction zones for the high-P–T type and cold subduction zones for the very high-P–T type. The third paper concerns garnet Lu–Hf and Sm–Nd isochron dating, which is important for establishing the P–T–t paths of garnet-bearing rocks. In this paper, Cheng (2018) provides a brief review of the development of the garnet Lu–Hf and Sm–Nd systems, the impact of the garnet Lu–Hf and Sm–Nd geochronology, and several critical issues related to the complexities of interpretation of the resulting radiometric ages. Garnet-bearing granitic gneisses from the Hong’an orogenic belt, in the central part of Qinling–Tongbai–Hong’an–Dabie–Sulu orogenic belt, China, are then discussed as a case study, using combined garnet Lu–Hf and Sm–Nd dating methods. The fourth paper focuses on the use of a Ti-in-zircon thermometer in eclogites. Lin et al. (2018) apply the Ti-in-zircon thermometer to low-temperature eclogites from North Qilian and Tianshan orogens, China, and demonstrate an overestimate of temperature, compared with results derived from conventional geothermometers. They also discuss the validity of the Ti-in-zircon thermometer to medium- and low-temperature eclogites. They propose that, although temperature is the dominant factor controlling the Ti content in zircon, other factors such as lattice defect, trace element substitutions in zircon, disequilibrium zircon growth and precipitation from metamorphic fluids may contribute to temperature overestimation; thus such knowledge is of considerable significance for the petrological study of eclogites.
Part II contains overview papers of four HP–UHP metamorphic belts: the Dabie–Sulu, Tianshan, North Qaidam and Himalayan HP–UHP belts. The Dabie–Sulu UHP metamorphic belt is the largest in the world, and studies of this belt are of great relevance for our understanding of HP–UHP metamorphism and tectonic processes in continental subduction zones in general (Zheng et al. 2012). Y. F. Zheng et al. (2018) outline the petrographic features and geochemical compositions of UHP metamorphic rocks in the Dabie–Sulu belt, present an overview of the collisional modification and post-collisional reworking of continental crust, and discuss possible implications for subduction style, subduction polarity and exhumation mechanism. In contrast, UHP metamorphic belts that have witnessed the whole tectonic process of plate subduction/collision in an oceanic-type environment are very rare. The Tianshan UHP belt is the largest belt of oceanic type reported so far (Zhang et al. 2013). L. F. Zhang et al. (2018c) summarize recent progress in understanding mineralogical and petrological processes, P–T path estimation and the geochronology of UHP metamorphic rocks. On the basis of the metamorphic P–T conditions and in situ zircon dating, L. F. Zhang et al. (2018c) obtain a new P–T–t path characterized by the cold subduction to the Pmax, then the heating exhumation to the Tmax and a final slow uplift owing to the similar densities between UHP eclogites and country rocks. Combined with the studies of the coeval LP/HT metamorphic belt, L. F. Zhang et al. (2018c) propose a new tectonic model for the southwestern Tianshan orogenic belt. The North Qaidam–South Altun UHP belt associated with the North Qilian–North Altun HP metamorphic belt in north Tibet of China was formed in the early Paleozoic. In the third paper, J. Zhang et al. (2018b) propose that two distinct orogenic episodes, accretionary and collisional, developed in the Altun–Qilian–North Qaidam orogenic system. The North Qaidam–South Altun orogenic belt is characterized by UHP metamorphism, accompanied by Barrovian-type metamorphic overprinting and reworking of Precambrian continental blocks, and is associated with tectono-magmatic events that resulted from continental subduction and continent–continent collision. On the other hand, the North Qilian–North Altun orogenic belt consists of HP/LT metamorphic units associated with an ophiolitic mélange and arc metamorphic–magmatic rocks and formed by subduction–accretion processes. The young and very well-preserved metamorphic belt of the Himalayan orogen is a typical example of a continent–continent collision.
Part II also contains two papers about the Himalaya and Tibet. The first paper by Rehman (2018) discusses geochronological investigations of HP–UHP eclogites from the Himalayan orogeny. Updated petrological, geochronological and geological studies from the UHP metamorphic rocks (eclogites and host gneisses) in western Himalaya are presented; further studies focus on strongly retrogressed granulited eclogites from central and eastern Himalaya. The UHP event, interpreted as the result of the India–Asia collision, took place at c. 46 Ma in Kaghan and at 53–55 Ma in Tso Morari (western Himalaya), whereas the HP eclogite and subsequent granulite facies events in central and eastern Himalaya occurred between about 25 and 13 Ma. Rehman (2018) demonstrate that different metamorphic PT conditions and ages can be interpreted to reflect particular events within the peak metamorphic and exhumation history, i.e. that fast uplift in the west caused the preservation of UHP rocks and slow exhumation in the central Himalaya resulted in the granulite facies overprint. The second paper concerns the Sumdo eclogite terrane, which is a newly recognized HP–UHP metamorphic belt in Tibet (Yang et al. 2009). C. Zhang et al. (2018a) summarize progress in mineralogical, petrological, geochemical and geochronological studies of eclogites and related blueschist and garnet-bearing mica schists of the Sumdo terrane. On the basis of these studies, C. Zhang et al. (2018a) propose a new tectonic model involving the northward subduction of Paleo-Tethyan oceanic crust before 280 Ma and the collision between the South Lhasa and North Lhasa terrains during 266–225 Ma. These new studies are of vital importance since they represent important pieces of the puzzle that contribute to understanding of the formation and tectonic evolution of Tibet plateau.
Part III consists of six papers which focus some new and lesser known HP–UHP belts. Schertl et al. (2018) describe the petrography, mineral chemistry and bulk-rock geochemistry of artefacts crafted from HP jadeitite, jadeite–lawsonite rock and garnet–lawsonite–omphacite–glaucophane schist found in an excavated archaeological settlement on the northern coast of the Dominican Republic. Seven of nine jadeite-rich artefacts are demonstrated to be jadeitite sensu stricto (>90 vol% jadeite), identical to material known from the HP-rock inventory of serpentinite mélanges in the nearby Río San Juan Complex (RSJC). Two jadeite–lawsonite rock and two blueschist artefacts show only minor differences from corresponding rocks of the RSJC source. Thus, the authors were able to demonstrate with considerable certainty that the raw material for the artefacts originated in the nearby occurrences of the RSJC and not necessarily from other more distant localities in the Caribbean area such as Guatemala or Cuba, a theory held by a number of archeologists. Y. Y. Liu et al. (2018b) present new petrological data obtained from eclogites from Marun–Keu complex, Polars, Russia. In this paper, they present a clockwise P–T path for eclogites using multi-equilibrium geothermobarometry, and suggest that they and related peridotites and non-metamorphic gneisses of the Marun–Keu complex represent a coherent crustal block that experienced the same P–T evolution but provide a metamorphic record of different P–T parameters because of an uneven introduction of aqueous-bearing fluids during subduction and exhumation. As a result, very dry rocks, like the Marun–Keu complex, failed to reach a thermodynamic equilibrium state, and thus, preserve their original fabrics. Song et al. (2018) report two episodes of eclogite-facies metamorphism, recorded in eclogites from the North Qaidam UHP metamorphic belt. They are able to distinguish an early lawsonite–eclogite from a late kyanite–eclogite stage. In this paper, they present metamorphic ages of 470–445 Ma for a lawsonite-bearing eclogite and ages of 438–420 Ma for a kyanite-bearing eclogite and conclude that the tectonic transition from oceanic subduction to continental subduction and collision was a continuous process. Ding & Zhang (2018) report petrological, geochronological and geochemical data from eight Oligocene (32–24 Ma) granitoids with protolith ages of 57–49 and 27 Ma from the eastern Gangdese arc. They propose that these Oligocene granitoids were formed by the melting of Early Eocene and Late Oligocene magmatic rocks which were buried to lower crust depths during thickening and reworking within the Gangdese arc. J. Liu et al. (2018a) present data from more than 70 gneiss samples collected along an ∼35 km long north–south-striking transect in the Dabie eclogite-bearing terrane and estimate values of metamorphic temperatures using Ti-in-zircon thermometry. Combining these results with data on P–T conditions of eclogites determined from previous studies, they have deduced large temperature differences from 50 to 200°C between neighboring units or samples within the distances of 1–4 km, and propose that, along the studied profile, the Dabie eclogite-bearing terrane is not a coherent unit but consists of a stack of seven or eight tectonometamorphic slices. Tian et al. (2018) provide a geochronology case study of HP metamorphic paragneisses from the Eastern Himalayan Syntaxis. Using phase equilibrium modelling of garnet growth combined with partitioning of rare earth elements between zircon and garnet, the authors were able to link different stages of zircon growth to garnet growth and to date prograde (32.6–25.2 Ma), peak pressure (25–24 Ma) and retrograde (23.5–13.1 Ma) stages of metamorphism, and to obtain an unified scenario of a PT–t evolution, which is important for the understanding of the tectonic development of the Eastern Himalayan Syntaxis.
We are grateful to all the presenters of the session ‘HP–UHP metamorphism and tectonic evolution of orogenic belts’ at the 35th International Geological Congress in Cape Town, South Africa. Our sincere thanks go to all the authors and reviewers for their great contributions to make this special volume of ‘HP–UHP metamorphism and tectonic evolution of orogenic belts’ possible. Special thanks go to the members of the editorial office who accompanied and supported our work.
This paper was supported by funding from the National Natural Science Foundation of China awarded to LFZ.
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HP–UHP Metamorphism and Tectonic Evolution of Orogenic Belts
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High pressure (HP) and ultrahigh pressure (UHP) metamorphic rocks play a key role in understanding the tectonic evolution of orogenic belts. They have typically experienced complex changes during subduction and exhumation processes arising from recrystallization, deformation, fluid–rock interactions and even partial melting, and may therefore carry valuable records of evolving geodynamic systems in an orogenic belt. This special publication addresses the current work on HP–UHP metamorphism and its relation to the tectonic evolution of orogenic belts.
This special publication contains fifteen papers covering the important orogenic belts of the Himalaya, Dabie–Sulu, Tian Shan, North Qaidam and others that have been grouped into three parts: (I) new developments in the determination of metamorphic pressure–temperature (PT) conditions and their timing, (II) overview papers of well-known HP–UHP metamorphic belts and (III) research papers for some newly discovered HP–UHP belts.