Ultrahigh-pressure Metamorphism Research Articles

A record of ultrahigh temperature metamorphism in the Dabie orogen during Triassic continental collision

Ultrahigh temperature (UHT) metamorphism is traditionally recognized by the development of characteristic mineral associations in Mg–Al-rich metapelitic rocks. However, recognition of UHT metamorphism in non-supracrustal rocks is more difficult. UHT metamorphic conditions are recorded by a migmatite from the North Dabie Terrane (NDT) of the Dabie orogen, east China. The migmatite is composed of intercalated layers of melanosome and K-feldspar-rich leucosome. Zircon grains in the migmatite have a core–rim structure comprising a metamorphic core and an anatectic rim. The metamorphic cores have low U contents (mainly <657 ppm) and low Th/U ratios (<0.2), and are depleted in heavy rare earth element (HREE). The metamorphic domains yield concordant 206Pb/238U ages ranging from 205.1 ± 4.8 Ma to 248.0 ± 4.1 Ma with a weighted mean of 217.7 ± 4.3 Ma (n = 20, MSWD = 4.2). They contain a granulite-facies inclusion assemblage of garnet + clinopyroxene + plagioclase + quartz + rutile. Conventional geobarometry and Ti-in-zircon thermometry constrain P–T conditions to approximately 11–12 kbar and 900–950 °C, suggesting UHT metamorphism. The discovery of Triassic UHT metamorphism in the Dabie orogen, which was previously best known for ultrahigh pressure metamorphism, provides new insights into the thermal structure and geodynamics of the orogeny during continental collision. The anatectic rims of zircon grains have relatively high U contents and low Th/U ratios (<0.14), and are enriched in HREE. They yield concordant 206Pb/238U ages of 133.6 ± 1.1 Ma to 156.4 ± 2.2 Ma, indicating that anatexis occurred during post-collisional collapse of the Dabie orogen.

Source-to-sink system reconstruction in the northern Jiaolai Basin, eastern China, by multiproxy provenance methods and implications for exhumation of the Sulu orogen

The Early Cretaceous sedimentary successions in the Jiaodong-Laiyang (Jiaolai) Basin provide valuable information about sediment provenances and tectonic processes. A clear understanding of the provenance development of the Jiaolai Basin along the northern margin of the Sulu orogen and would elucidate the exhumation history of the Sulu orogen during the post-orogenic period. However, the precise age and provenance evolution of the stratigraphy remain ambiguous. Here, we employ multiple provenance proxies – detailed paleocurrent, conglomerate clast counts, detrital composition point counts of sandstone, and detrital zircon U-Pb geochronology – to reconstruct the source-to-sink system of the northern part of the Jiaolai Basin. Visual and statistical analyses of the different proxy results and comparison with the signatures of potential source regions reveal that (1) Wawukuang Formation (Fm) (ca. 123 Ma) was mainly sourced from the Guocheng uplift in both the Haiyang and Laiyang regions; (2) Linsishan to Zhifengzhuang Fm (ca. 121 Ma), the Laiyang region was mainly fed by the Jiaobei terrane, while in the Haiyang region, the Guocheng uplift and Sulu orogen were primary sources; (3) Shuinan to Longwangzhuang Fm, the Haiyang and Laiyang regions were linked as a single basin fed by the Jiaobei terrane and Sulu orogen; and (4) Qugezhuang Fm (ca. 114 Ma) was mainly sourced from Early Cretaceous plutons in the Sulu orogen and Jiaobei terrane. Based on the inflection point in the proportion of ages of the ultrahigh-pressure metamorphic (UHP) rocks between the Wawukuang and Linsishan Fms, we conclude that the Sulu belt experienced a rapid uplift that resulted in the exposure and erosion of the Jurassic plutons and UHP rocks during ca. 123–121 Ma. In addition, our results show that the denudation processes of the Mesozoic plutons in the Sulu orogen have an obvious temporal sequence: Jurassic plutons first, followed by Cretaceous plutons.

Anatexis constraints on the post-collisional evolution of the Sulu Orogen, China: evidence from pegmatite veins in migmatite

ABSTRACTThe widespread migmatites in the northwestern part of the Sulu Orogen, China, indicate regional anatexis that is of great significance when discussing the tectonic evolution of this continental orogenic belt. Cathodoluminescence (CL) images, U–Pb ages, and in situ trace element compositions of zircons from four pegmatite veins within these migmatites provide clear evidence for the nature of the post-collisional evolution of the Sulu Orogen. The inherited zircon cores reveal that the protoliths of the migmatites were middle Neoproterozoic magmatic rocks (810–620 Ma) of the South China Block. The protoliths underwent two partial melting events. The mantle domains of the inherited zircons record a Late Triassic (222.0–204.0 Ma) partial melting event that occurred during the exhumation and retrograde metamorphism, after ultrahigh-pressure (UHP) metamorphism. Subsequent newly grown zircons record a Middle–Late Jurassic to Early Cretaceous (164.1–125.5 Ma) anatexis event, indicating that the late Mesozoic anatexis started before ca. 164.1 Ma, reached a peak at ca. 152.1 Ma, and ceased at ca. 125.5 Ma. Combined with previous results of studies on the Sulu orogen, the late Mesozoic anatexis suggested that the thickened crust of the Sulu Orogen had started to become unstable before 164.1 Ma. The duration of ~164.1–137 Ma corresponds to a period of transition in the tectonic regime of the Sulu Orogen, enabling the early high-temperature ductile deformation. After ca. 137 Ma, the tectonic regime was fully transformed into extension and the Sulu Orogen underwent rapid thinning and collapse, thus leading to the late medium–low temperature ductile deformation (137–121 Ma) and laying the foundations for the large-scale magmatic emplacement during the late Early Cretaceous (127–115 Ma). These two partial melting events together promoted the rapid exhumation of the Sulu UHP rocks.

Correlation of lithospheric “de‐rooting” of the Sulu‐Dabie Orogen to tectonic‐sedimentary process of the Hefei Basin: Constraints from Mesozoic coupling of basin and orogen

The lithospheric “de‐rooting” of the Sulu‐Dabie Orogen (SDO) is an important tectonic mechanism for post‐orogenic tectono‐magmatism, which influenced the sedimentation, evolution, and development of oil–gas accumulation in the Hefei Basin (HB). Combined with seismic, geological, and sedimentary data of the HB and the SDO, this paper studies the coupling of the HB and the SDO since the Late Mesozoic. While the North China Block (NCB) indented south‐eastwards under the Greater South China Block (GSCB) since the Mesozoic, the SDO underwent three intense tectonic stages, i.e., subduction, exhumation, and uplift. The upwelling of deep mantle material causes the exhumation of the ultrahigh‐pressure metamorphic (UHPM) rocks to the upper crust or surface, forming the UHPM. Influenced by the indentation of the NCB, the “de‐rooting” of the SDO lithosphere, caused the delamination and thinning of the lithosphere, accompanied by the continuous flexural subsidence in the HB. The basin underwent at least four stages of tectonic evolution, involving sequentially: the flexural basin, the foreland basin, the strike‐slip‐related basin, and the faulted graben development periods. The tectonic evolution of the HB was closely coupled with the SDO orogeny. The “de‐rooting” of the SDO lithosphere resulted in a strong uplift and erosion of the orogen, providing abundant source of material for the HB and causing the successively northwards migration of basin depocenters at its early stage. Finally, influenced by the subduction retreat of the (Palaeo‐) Pacific Plate and the strike‐slip movement of the Tanlu Fault, the depocenters migrated to the east.

Metamorphic evolution of eclogites and associated metapelites from the Yuka terrane in the North Qaidam ultrahigh pressure metamorphic belt, NW China: Constraints from phase equilibrium modeling

As the products of early Paleozoic continental collision, widespread eclogite facies rocks crop out in the Yuka terrane, western North Qaidam Mountains, NW China. Eclogites commonly reach ultrahigh pressure (UHP) metamorphic conditions. However, whether the associated metapelitic rocks also experience UHP metamorphism has not been thoroughly elucidated to date. In this contribution, we chose two eclogite samples and one metapelite sample to investigate their metamorphic evolution. Combining with petrographic observation and P–T pseudosections, similar clockwise P–T paths with peak stages at 25–34 kbar, 580–633 °C were obtained for eclogites and metapelites. Eclogite ZM871 records a prograde metamorphic condition at 21.6 kbar/552 °C, defined by the assemblage garnet + omphacite + epidote + lawsonite + talc (+quartz + phengite + rutile) and a peak metamorphic condition at 34.0 kbar/633 °C by the assemblage garnet + omphacite + lawsonite + talc (+coesite + phengite + rutile). Eclogite ZM641 records a prograde metamorphic condition at 21.5 kbar/520 °C, defined by the assemblage garnet + omphacite + glaucophane + lawsonite (+quartz + phengite + rutile) and a peak condition at 30 kbar/616 °C by the assemblage garnet + omphacite + lawsonite + talc (+quartz + phengite + rutile). Metapelite ZM8–1 records a prograde metamorphic condition at 18.2 kbar/530 °C defined by the assemblage garnet + chloritoid + chlorite (+quartz + phengite + rutile), a peak condition at 25 kbar/580 °C by the assemblage garnet + kyanite + chloritoid (+quartz + phengite + rutile), and a retrograde condition at 16.2 kbar/595–598 °C by the assemblage garnet + kyanite + chloritoid + chlorite (+quartz + phengite + rutile). Combined with previous data from orthogneiss and impure marble in the Yuka terrane, which also recorded UHP metamorphic conditions, the Yuka eclogite–bearing unit commonly underwent UHP metamorphism, although further investigation is warranted to verify whether the Yuka unit represents a single coherent UHP metamorphic terrane or a collage of different UHP metamorphic slices.

HP–UHP metamorphism and tectonic evolution of orogenic belts: introduction

Abstract 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. &amp; 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. &amp; 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. &amp; 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. &amp; 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.

Tracking the timing and nature of protolith, metamorphism, and partial melting of tourmaline‐bearing migmatites by zircon U–Pb and Hf isotopic compositions in the Yuka terrane, North Qaidam UHP metamorphic belt

An integrated study using cathodoluminescence (CL) images, U–Pb geochronology, and Hf isotopic characterization of zircons was performed on migmatites from the Yuka terrane to provide insights into the timing and nature of the protoliths, metamorphism, and partial melting of these rocks. Zircon grains separated from the migmatites have three distinct domains. Residual zircon cores in samples YKT01 and YKT02 exhibit oscillatory zoning, high Th/U ratios (0.10–0.79), steep heavy rare earth element (HREE) patterns, and mean ages of 936 ± 5 Ma and 927 ± 11 Ma, respectively, indicating that they are igneous in origin and record Neoproterozoic magmatism during the Grenvillian Orogeny. CL‐grey cores record an ultrahigh‐pressure (UHP) peak metamorphism age of 435 ± 5 Ma and have low Th/U and 176Lu/177Hf ratios, weak negative Eu anomalies, and relatively flat HREE patterns. The CL‐bright rims have steep HREE patterns with lower middle rare earth element (MREE) concentrations but higher HREE concentrations compared with the metamorphic cores and a weighted average age of 422 ± 4 Ma, which records the timing of melt crystallization. The migmatites incorporate garnet relicts coexisting with fluid inclusions and hydrous minerals like peritectic phengite and peritectic large anhedral poikilitic tourmaline in the leucosomes. Peak temperatures are less than 750°C, suggesting that the anatectic melts may be derived from melting of felsic gneiss in the presence of water. The timing of melt crystallization is ca. 13 Ma later than UHP metamorphism, indicating that the partial melting took place during the exhumation stage. The metamorphic zircon cores have higher 176Hf/177Hf(t) ratios than the residual magmatic cores, which provides powerful evidence for the generation of the metamorphic zircons through the incorporation of internal high Hf minerals other than zircon in a closed system. Anatectic rims with lower 176Hf/177Hf(t) ratios but higher 176Lu/177Hf ratios than metamorphic cores suggest that the main formation mechanism is dissolution‐reprecipitation of pre‐existing zircons. In conclusion, the elevated 176Hf/177Hf(t), TDM2 age and decreasing 176Lu/177Hf(t) ratios in the metamorphic and anatectic domains imply that the new zircons in some migmatites and granites may not reflect the Hf isotope compositions of their source rocks, accounting for the variable Hf isotope compositions in S‐type granites.

Geology of the eastern part of the Tso Morari nappe, the Nidar Ophiolite and the surrounding tectonic units (NW Himalaya, India)

ABSTRACT The Tso Morari nappe is well known for having suffered ultrahigh-pressure metamorphism in the northwest Himalaya. Its pressure-temperature evolution has been widely studied and debated since the end of the 1990s. However, the lithostratigraphy, structures and metamorphism of the tectonic units around the Tso Morari nappe are poorly known, especially in the eastern part of the nappe. Here, a detailed geologic map based on recent field work is presented. The survey area is the eastern part of the ultrahigh-pressure Tso Morari nappe, the surrounding tectonic units and the Nidar Ophiolite. This map improves the knowledge of the geology and geometry of the North Indian continental plate and the ocean-derived units south of the Eurasian continent in eastern Ladakh. Knowledge of the present-day geometry will help to reconstruct the initial setting in the kinematic models of the formation and exhumation of the Tso Morari nappe in relation to its surrounding tectonic units.

Petrology of Chromitites in the Higashi-Akaishi Ultrahigh-Pressure (UHP) Peridotite Complex, Japan: Toward Understanding of General Features of the UHP Chromitites

Ultrahigh-pressure (UHP) chromitites containing UHP minerals such as coesite and diamond have been reported from some ophiolites in Tibet and the Polar Urals. Their nature, i.e., origin, P-T path and abundance, however, are still controversial and left unclear. Here we describe chromitites in the Higashi-akaishi (HA) ultramafic complex in the Cretaceous Sanbagawa metamorphic belt, Japan, which experienced UHP condition (up to 3.8 GPa) at the peak metamorphism via subduction, in order to understand the nature of UHP chromitites. The HA peridotites typically contain garnets and are associated with eclogites, and their associated chromitites are expected to have experienced the UHP metamorphism. The Higashi-akaishi (HA) chromitites show banded to massive structures and are concordant to foliation of the surrounding peridotite. Chromian spinels in the chromitite and surrounding peridotites were sometimes fractured by deformation, and contain various inclusions, i.e., blade- and needle-like diopside lamellae, and minute inclusions of pyroxenes, olivine, and pargasite. The peculiar UHP minerals, such as coesite and diamond, have not been found under the microscope and the Raman spectrometer. Spinels in the HA chromitites show high Cr#s (0.7 to 0.85), and low Ti contents (&lt;0.1 wt %), suggesting a genetic linkage to an arc magma. The HA chromitites share the basic petrographic and chemical features (i.e., diopside lamellae and arc-related spinel chemistry) with the UHP chromitites from Tibet and the Polar Urals. This suggests that some of the characteristics of the UHP chromitite can be obtained by compression, possibly via deep subduction, of low-P chromitite.

Linking high-pressure mafic granulite, TTG-like (tonalitic-trondhjemitic) leucosome and pluton, and crustal growth during continental collision

Research Article| November 09, 2018 Linking high-pressure mafic granulite, TTG-like (tonalitic-trondhjemitic) leucosome and pluton, and crustal growth during continental collision Shengyao Yu; Shengyao Yu † 1Key Laboratory of Submarine Geosciences and Prospecting Technique, MOE and College of Marine Geosciences, Ocean University of China, Qingdao 266100, China2Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266061, China †yushengyao@ouc.edu.cn Search for other works by this author on: GSW Google Scholar Sanzhong Li; Sanzhong Li 1Key Laboratory of Submarine Geosciences and Prospecting Technique, MOE and College of Marine Geosciences, Ocean University of China, Qingdao 266100, China2Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266061, China Search for other works by this author on: GSW Google Scholar Jianxin Zhang; Jianxin Zhang 3Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China Search for other works by this author on: GSW Google Scholar Deyou Sun; Deyou Sun 4College of Earth Sciences, Jilin University, Changchun 130061, China Search for other works by this author on: GSW Google Scholar Yinbiao Peng; Yinbiao Peng 1Key Laboratory of Submarine Geosciences and Prospecting Technique, MOE and College of Marine Geosciences, Ocean University of China, Qingdao 266100, China Search for other works by this author on: GSW Google Scholar Yunshuai Li Yunshuai Li 5Institute of Surface-Earth System Science, Tianjin University, Tianjin 300072, China Search for other works by this author on: GSW Google Scholar GSA Bulletin (2019) 131 (3-4): 572–586. https://doi.org/10.1130/B31822.1 Article history received: 06 Apr 2017 rev-recd: 10 Jan 2018 accepted: 14 Aug 2018 first online: 10 Nov 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Shengyao Yu, Sanzhong Li, Jianxin Zhang, Deyou Sun, Yinbiao Peng, Yunshuai Li; Linking high-pressure mafic granulite, TTG-like (tonalitic-trondhjemitic) leucosome and pluton, and crustal growth during continental collision. GSA Bulletin 2018;; 131 (3-4): 572–586. doi: https://doi.org/10.1130/B31822.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The early continental crust is composed predominantly of Archean tonalite-trondhjemite-granodiorite (TTG); however, the tectonic regime and melting conditions for TTG magmas have been debated. In this study, we report field and microscopic evidence for the partial melting of arc-related metagabbro and its products, including TTG-like melt, volumetrically significant plutons evolved from melt, and the associated granulitic residua during continental collision in the North Qaidam Mountains, China. Migmatite shows successive stages of initial intragranular or droplet-like melt along grain boundaries, which grew into a three-dimensional interconnected intergranular network, segregated, and accumulated in pressure shadow areas, and merged to form melt channels and sheets that finally combined to form a TTG-like tonalite pluton. Pressure-temperature (P-T) calculations indicate high-pressure granulite-facies metamorphism and the crystallization of leucosome at P = 15.5–18.5 kbar and T = 850–950 °C. Based on zircon U-Pb dating and petrological analyses, partial melting and magmatic crystallization occurred 438–430 m.y. ago, which is slightly younger or temporally overlaps with ultrahigh-pressure metamorphism in the region. The metagabbros exhibit a subduction-related arc signature with slightly positive εNd(t) values of 2.1–4.2. The felsic leucosomes and tonalite plutons are characterized by high Na, Sr, Sr/Y, and La/Yb values and low heavy rare earth element values, with εNd(t) values of 0.1–4.3, similar to typical TTGs. The geological context, geochemistry, and timing of the TTG-like melt formation observed in this study differ from the prevailing models; however, our observations and documentations demonstrate that melting of arc-like metagabbro under high-pressure granulite-facies conditions during continental collision may make important contributions to crustal growth and differentiation. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Ultrahigh pressure metamorphism and tectonic evolution of southwestern Tianshan orogenic belt, China: a comprehensive review

Abstract Recently, a huge ultrahigh-pressure (UHP) metamorphic belt of oceanic type has been recognized in SW Tianshan, China. Petrological studies show that the UHP metamorphic rocks of SW Tianshan orogenic belt include mafic eclogites and blueschists, pelitic garnet phengite schists, marbles and serpentinites. The well-preserved coesite inclusions are commonly found in eclogites, garnet phengite schists and marbles. Ti-clinohumite and Ti-chondrodite have been identified in UHP metamorphic serpentinites. Based on the P–T pseudosection calculation and combined U–Pb zircon dating, the P–T–t path has been outlined as four stages: cold subduction to UHP conditions before c. 320 Ma whose peak ultrahigh pressure is about 30 kbar at 500°C; heating decompression from the P max to the T max stage before 305 Ma whose peak temperature is about 600°C at 22 kbar; then the early cold exhumation from amphibolite eclogite facies to epidote-amphibolite facies metamorphism before 220 Ma; and the last tectonic exhumation from epidote amphibolite facies to greenschist facies metamorphism. Combining with the syn-subduction arc-like 333–326 Ma granitic rocks and 280–260 Ma S-type granites in the coeval low-pressure and high-temperature metamorphic belt, the tectonic evolution of Tianshan UHP metamorphic belt during late Cambrian to early Triassic has been proposed in this paper.

Timing of subduction initiation, arc formation, ophiolite obduction and India–Asia collision in the Himalaya

Abstract Reconstruction of the Western Himalaya requires three subduction systems operating beneath the Spong arc, Dras–Kohistan arc and the Asian continent during the Late Cretaceous–Paleocene. The timing of the closure of the Neo-Tethys Ocean along the Indus Suture Zone (ISZ) in Ladakh and south Tibet has been proposed to be as old as c. 65 Ma and as young as c. 37 Ma. The definition of the India–Asia collision can span &gt;15 myr from the first touching of Indian continental crust with Asian crust to the final marine sedimentation between the two plates. There is good geological evidence for a Late Cretaceous–Early Paleocene phase of folding, thrusting and crustal thickening of Indian Plate shelf carbonates associated with obduction of ophiolites. There is no geological evidence of any oceanic ‘Greater Indian Basin’ separating the northern Tethyan and Greater Himalaya from India. There is clear evidence to support final ending of marine sedimentation along the ISZ at 50 Ma (planktonic foraminifera zone P7–P8). There is no evidence for diachroneity of collision along the Pakistan–Ladakh–South Tibet Himalaya. The timing of ultrahigh-pressure metamorphism cannot be used to constrain India–Asia collision, and the timing of high-grade kyanite- and sillimanite-grade metamorphism along the Greater Himalaya can only give a minimum age of collision.

Petrology, geochemistry and geochronology of the meta-mafic rocks in the North Sulu ultrahigh-pressure belt: Implications for their petrogenetic diversity and complex tectonic evolution

The ultrahigh-pressure (UHP) eclogite and country rocks of the North Sulu UHP metamorphic belt have traditionally been considered the products of the deep subduction of the Yangtze Block (YB) beneath the North China Block (NCB). However, widely distributed non-UHP meta-mafic rocks, appearing as irregular lenses and thin layers together with UHP eclogite in the Haiyangsuo-Wuji-Weihai area along the northwestern margin of the North Sulu UHP belt, prompted investigation of the petrogenetic diversity and complex tectonic evolution of these rocks. The UHP index mineral assemblage of garnet + omphacite + rutile ± phengite ± coesite is common in the matrix of UHP and retrograde eclogites, or as inclusions preserved in metamorphic zircons indicating UHP metamorphism. In contrast with previous studies, herein high-pressure and medium-pressure (HP–MP) granulite facies index mineral assemblage of garnet + clinopyroxene + plagioclase + quartz ± orthopyroxene + ilmenite was commonly identified in the matrix of non-UHP mafic granulites and amphibolites, indicating that these rocks underwent HP–MP granulite facies metamorphism rather than UHP metamorphism. The whole-rock geochemical data revealed multiple origins for the UHP and retrograde eclogites with protoliths derived from heterogeneous mantle or lower–upper crust sources with weak fractional crystallization. In contrast, the protolith of the non-UHP meta-mafic rocks may have derived from a depleted or primitive mantle origin source with varying degrees of crustal contamination during emplacement. The UHP and retrograde eclogites exhibited Neoproterozoic (803–760 Ma) protolith ages and underwent UHP eclogite facies metamorphism in the Middle Triassic (233–225 Ma) with subsequent Late Triassic (219–210 Ma) amphibolite facies retrogression. In contrast, the non-UHP meta-mafic rocks exhibited older Paleoproterozoic (>1944 Ma) protolith ages. The two distinct metamorphic events that occurred in the Paleoproterozoic (1944–1825 Ma) and Middle–Late Triassic (226–201 Ma) are related to the Paleoproterozoic Jiao-Liao-Ji orogeny and the Triassic collisional event between the YB and NCB, respectively. A combined study of petrography, whole-rock geochemistry and zircon U–Pb ages revealed that the UHP and retrograde eclogites show a close affinity to the Neoproterozoic continental crust on the northern margin of YB, whereas the non-UHP meta-mafic rocks show a close affinity to the Paleoproterozoic Jiaobei Terrane. Both the UHP and non-UHP meta-mafic rocks, together with their country rocks form an approximately 200 km long mélange zone rather than a single UHP belt, which extends from the Haiyangsuo through the Wuji to the Weihai area along the northwestern margin of the North Sulu UHP belt.

OH defects in coesite and stishovite during ultrahigh-pressure metamorphism of continental crust

The high-pressure silica polymorphs coesite and stishovite were synthesized under water-saturated conditions from a natural granitic composition doped with Li and B. Experiments were performed in a Multi-Anvil apparatus between 4 and 9.1 GPa and 900 and 950 °C, based on the conditions of a subducting continental crust as realistic for the ultrahigh-pressure metamorphic units Dora Maira and Kochetav massifs. Run products consisted of coesite/stishovite + kyanite ± phengite ± omphacite, and quench material. The synthesized silica polymorphs were successively analyzed by infrared spectroscopy, electron microprobe, and Secondary-Ion Mass Spectrometry (SIMS). No hydrous defects were observed in coesite synthesized at 4 GPa and 900 °C, whereas coesite grown at higher pressures revealed a triplet of infrared absorptions bands at 3575, 3523, and 3459 cm− 1, two minor bands at 3535 and 3502 cm− 1, and a small band at 3300 cm− 1 that was only visible at 7.7 GPa. The total amount of Al was charge-balanced by H and the other monovalent cations. However, the band triplet could not be associated with AlOH defects, while the band doublet was inferred to BOH defects and the small band probably corresponded to interstitial H. Stishovite displayed one dominant band at 3116 cm− 1 with a shoulder at 3170 cm− 1, and a minor band at 2665 cm− 1, probably all associated with AlOH defects. BOH defects were not observed in stishovite, and LiOH defects were neither observed in coesite nor stishovite, probably because of preferentially partition of Li in other phases such as omphacite. The total amount of defect protons increased with pressure and with metal impurity concentrations. The general increase in OH defects in silica polymorphs with increasing pressure (this study) contrasted the negative pressure trend of OH in quartz observed previously from the same starting material, and revealed an incorporation minimum of OH in silica polymorphs around the quartz/coesite phase transition.

Mottana, A., Sassi, F.P., Thompson, J.B., Jr. & Guggenheim, S. (Eds) Micas: Crystal Chemistry and Metamorphic Petrology.: Washington, D.C. (Mineralogical Society of America, Reviews in Mineralogy and Geochemistry, 46), 2002. xiii + 499 pp. Price US$32 for non-members, US$24 for members, ISBN 0 939950 58 8.

This book is another in the series of Reviews in Mineralogy and Geochemistry and is published by the Mineralogical Society of America in partnership with Accademia Nazionale dei Lincei. It is an update and expansion of the relevant sections of Reviews in Mineralogy, v. 13 , Micas, which was published in 1984 and quickly became the authoritative quick reference for micas and the platform for further research on these important rock-forming minerals. There has been a rapid expansion in the literature on micas in the eighteen-year interval between the publication of volumes 13 and 46 . Micas continue to have important roles in frontier areas of the earth sciences, particularly in quantification of metamorphic petrology, ultra-high pressure metamorphism, igneous processes in crustal and mantle tectonics, granite petrogenesis, redox equilibria, rock weathering, rock–fluid interaction, radiometric age dating of rocks and rock events, mechanical properties of rocks, and exotic element geochemistry. A recent landmark in mica mineralogy was the report and revised classification scheme …

Ultrahigh-pressure metamorphic rocks in the Dabie–Sulu orogenic belt: compositional inheritance and metamorphic modification

Abstract The Dabie–Sulu orogenic belt in east-central China contains one of the largest ultrahigh-pressure (UHP) metamorphic terranes in the world. The UHP eclogites are associated with gneiss, peridotite and marble. But all these rocks underwent in situ UHP metamorphism during the continental collision in the Triassic. Although fluid action is not significant during cold subduction, it becomes prominent during hot exhumation of UHP slices. Whereas the composition of UHP metamorphic rocks is primarily inherited from their protoliths, they were locally modified by partial melting to varying extents. The partial melting of UHP rocks is highly heterogeneous along the collisional orogen, and achieves a maximum during rifting orogeny at the post-collisional stage. This paper outlines the petrographical features and geochemical compositions of UHP metamorphic rocks, and presents an overview of collisional modification and post-collisional reworking of the subducted continental crust. Further discussions are devoted to continental subduction tectonics, as well as to subduction style, subduction polarity and exhumation mechanism. The results also provide insights into the initiation of continental rifting in a Wilson cycle. Consequently, studies of the Dabie–Sulu orogenic belt have contributed greatly to our understanding of tectonic processes, fluid regime and chemical geodynamics in continental subduction zones.

Origin of atoll garnets in ultra-high-pressure eclogites and implications for infiltration of external fluids

Atoll-shaped and normal garnets in a low-temperature (LT) and ultra-high-pressure (UHP) metamorphic eclogite of the Dabie orogen were studied by detailed petrographic observation and pseudosection modelling. The normal and atoll garnets exhibit pronounced chemical variations, characterized by the overall increase of pyrope and decrease of grossular content from core to rim. A compositional discontinuity between the inclusion-rich core and the inclusion-poor mantle was defined by a sharp decrease in grossular content and an increase in pyrope. Pseudosection modelling using different effective bulk compositions (EBCs) obtained using XRF and the point-counting method shows that the eclogite experienced two stages of eclogite-facies metamorphism from a prograde amphibole-eclogite-facies (∼500–700 °C, <2.2 GPa) to the peak UHP lawsonite-eclogite-facies (∼650 °C, ∼3.3 GPa), followed by an isothermal decompression (ITD) exhumation. Garnet essentially did not grow owing to the parallels between the prograde P–T path and the garnet mode isopleths after the formation of the garnet core. The stable field and the compositions of the peak mineral assemblage at LT-UHP conditions insignificantly depend on the scale of equilibrium. Partial lawsonite and omphacite should be subtracted from the EBCs to obtain reasonable P–T estimates/pseudosections that are consistent with the genesis of atoll-garnet-bearing eclogite. Pseudosection modelling indicates that the infiltration of external fluid into the “dry” eclogite weakened the rigid garnet and produced micro-cracks. These cracks served as permeable pathways, leading ultimately to the consumption of previously formed garnet core and the formation of atoll texture. Previous P–T estimates using conventional garnet-omphacite-phengite thermobarometry are less reliable because omphacite was highly altered owing to net transfer reactions involving omphacite. The atoll texture was developed from prograde to peak UHP metamorphism, and the atoll-garnet forming event in the Dabie eclogite predates the final stage of the UHP metamorphism. The previous Lu–Hf date of c. 221 Ma is thus a maximum estimate of the termination of UHP metamorphism.

Eclogite-facies metamorphism in impure marble from north Qaidam orogenic belt: Geodynamic implications for early Paleozoic continental-arc collision

In the North Qaidam ultrahigh-pressure (UHP) metamorphic belt, impure marble and interbedded eclogite represent a particular sedimentary provenance and tectonic setting, which have important implications for a controversial problem - the dynamic evolution of early Paleozoic subduction-collision complexes. In this contribution, detailed field work, mineral chemistry, and whole-rock geochemistry are presented for impure marble to provide the first direct evidence for the recycling of carbonate sediments under ultrahigh-pressures during subduction and collision in the Yuka terrane, in the North Qaidam UHP metamorphic belt. According to conventional geothermobarometry, pre-peak subduction to 0.8–1.3GPa/485–569°C was followed by peak UHP metamorphism at 2.5–3.3GPa/567–754°C and cooling to amphibolite facies conditions at 0.6–0.7GPa/571–589°C. U-Pb dating of zircons from impure marble reveals a large group with ages ranging from 441 to 458Ma (peak at ~450Ma), a smaller group ranging from 770 to 1000Ma (peak at 780Ma), and minor >1.8Ga zircon aged ca. 430Ma UHP metamorphism. The youngest detrital zircons suggest a maximum depositional age of ca. 442Ma and a burial rate of ca. 1.0–1.1cm/yr when combined with P–T conditions and UHP metamorphic age. The REE and trace element patterns of impure marble with positive Sr and U anomalies, negative high field strength elements (Nb, Ta, Zr, Hf, and Ti), and Ce anomalies imply that the marble had a marine limestone precursor. Impure marble intercalated with micaschist and eclogite was similar to limestone and siltstone protoliths deposited in continental fore-arc or arc setting with basic volcanic activity. Therefore, the Yuka terrane most likely evolved in a continental island arc setting during the Paleozoic. These data suggest that metasediments were derived from a mixture of Proterozoic continental crust and juvenile early Paleozoic oceanic and/or island arc crust. In addition, their protoliths were likely deposited in a terrigenous-dominated forearc marine basin rather than an intracontinental basin environment, further evidence that some continental arc volcanic rock may have been the source of eclogite in the North Qaidam. These sediments, formed in a forearc basin close to the Qaidam Block to the north, were transported in the subduction zone to 100–110km depth with UHP metamorphism prior to exhumation. Meanwhile, the new results suggest that subduction erosion occurred along the active continental margin during the Qaidam Block with north-dipping subduction, indicating that the North Qaidam UHP metamorphic belt may have formed during continental-arc collision.

Timing of metamorphism and exhumation in the Nordøyane ultra-high-pressure domain, Western Gneiss Region, Norway: New constraints from complementary CA-ID-TIMS and LA-MC-ICP-MS geochronology

We present the results of a combined CA-ID-TIMS and LA-MC-ICP-MS U–Pb geochronology study of zircon and associated rutile and titanite from the Nordøyane ultra-high-pressure (UHP) domain in the Western Gneiss Region (WGR) of Norway. The dated samples include 4 eclogite bodies, 2 host-rock migmatites, and 2 cross-cutting pegmatites and leucosomes, all from the island of Harøya. Zircon from a coesite eclogite yielded an age of ca. 413 Ma, interpreted as the time of UHP metamorphism in this sample. Zircon data from the other eclogite bodies yielded metamorphic ages of ca. 413 Ma, 407 Ma, and 406 Ma; zircon trace-element data associated with 413 Ma and 407 Ma ages are consistent with eclogite-facies crystallization. In all of the eclogites, U–Pb dates from zircon cores, interpreted as the times of protolith crystallization, range from ca. 1680–1586 Ma, consistent with Gothian ages from orthogneisses in Nordøyane and elsewhere in the WGR. A zircon core age of ca. 943 Ma from one sample agrees with Sveconorwegian ages of felsic gneisses and pegmatites in the western part of the area. Migmatites hosting the eclogite bodies yielded zircon core ages of ca. 1657–1591 Ma and rim ages of ca. 395–392 Ma, interpreted as the times of Gothian protolith formation and Scandian partial melt crystallization, respectively. Pegmatite in an eclogite boudin neck yielded a crystallization age of ca. 388 Ma, interpreted as the time of melt crystallization. Rutile and titanite from 3 samples (an eclogite and two migmatites) yielded concordant ID-TIMS ages of 378–376 Ma. The results are similar to existing U–Pb data from other Nordøyane eclogites (415–405 Ma). In combination with previous pressure-temperature data from the coesite eclogite, these ages indicate that peak metamorphic conditions of ~3 GPa/760 °C were reached ca. 413 Ma, followed by decompression to ~1 GPa/810 °C by ca. 397 Ma and cooling below ca. 600 °C by ca. 375 Ma. The results are compatible with protracted UHP metamorphism followed by relatively slow exhumation. The question of whether partial melting began at UHP conditions is not resolved by this study.

Two epochs of eclogite metamorphism link ‘cold’ oceanic subduction and ‘hot’ continental subduction, the North Qaidam UHP belt, NW China

Abstract Eclogites in the high-pressure (HP) and ultrahigh-pressure (UHP) belts record subduction-zone processes; exhumed eclogites of seafloor protoliths record low-temperature (mostly &lt;600°C), high-pressure and ‘wet’ environments: that is, relatively ‘cold’ subduction with highly hydrous minerals such as lawsonite. In contrast, eclogites formed by the continental subduction record relatively ‘hot’ ( T &gt; 650°C) and ‘dry’ ultrahigh-pressure metamorphic (UHPM) conditions with syncollisional magmatism. Here, we investigate some eclogites from two ophiolite sequences that intercalated in the North Qaidam UHPM belt, which is genetically associated with continental subduction/collision. The observations of lawsonite pseudomorphs in garnets, garnet compositional zoning, mineral and fluid inclusions in zircons, and zircons with distinct trace-element patterns and U–Pb ages all suggest that these eclogites represent two exhumation episodes of subduction-zone metamorphic rocks: the early ‘cold’ and ‘wet’ lawsonite eclogite and the late ‘hot’ and ‘dry’ UHP kyanite eclogite. The early lawsonite-bearing eclogite gives metamorphic ages of 470–445 Ma and the later kysnite-bearing eclogite gives metamorphic ages of 438–420 Ma, with a time gap of c. 7–10 myr. This gap may represent the timescale for transition from oceanic subduction and continental subduction to depths greater than 100 km. We conclude that evolution from oceanic subduction to continental collision and subduction was a continuous process. In addition, we find that titanium contents in zircons have a positive correlation with U contents. Ti-in-zircon thermometry is likely to be invalid or limited for low-temperature eclogites.