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2018 Vol. 29, No. 5

Special Issue on Metamorphic Processes and Geodynamics
Neng-Song Chen, Zeming Zhang, Roger Mason
2018, 29(5): Ⅰ-Ⅱ. doi: 10.1007/s12583-018-0890-5
2018, 29(5): .
P-T Conditions, Chemistry and Physics of Metamorphic Minerals
Metapelitic Garnet-Muscovite-Al2SiO5-Quartz (GMAQ) Geothermobarometry
Chun-Ming Wu
2018, 29(5): 977-988. doi: 10.1007/s12583-018-0851-z
The garnet-muscovite geothermometer and garnet-muscovite-Al2SiO5-quartz (GMAQ) geobarometer have been empirically calibrated under P-T conditions of 1-12 kbar and 460-760℃ using natural metapelitic rocks. The chemical compositions of the calibrant muscovite are in the ranges of Fe=0.03-0.21 atoms, Mg=0.02-0.32 atoms and Al=1.62-1.96 atoms, respectively, on the 11-oxygen basis per formula unit. The garnet-muscovite thermometer yields similar temperature estimates to the well calibrated garnet-biotite thermometer within error of ±55℃, and successfully discriminates the systematic temperature change of the different zones of either the prograde or inverted metamorphic terranes or thermal contact aureoles. The six formulations of GMAQ barometry yield similar pressure estimates to the well calibrated GASP barometer within error of ±1.2 kbar, and plot the Al2SiO5-bearing metapelite into the correct stability field of the Al2SiO5 polymorphs. Moreover, the GMAQ thermobarometers show that the pressure is almost constant for every thermal contact aureole within limited geographic region, which reflects geological condition. The random errors are estimated to be of ca. ±60℃ and ±1.4 kbar for the geothermometer and geobarometer, respectively. A spreadsheet for applying GMAQ geothermobarometry is supplied in the Electronic Supplementary Materials.
Natural End Member Samples of Pyrope and Grossular: A Cathodoluminescence-Microscopy and -Spectra Case Study
Hans-Peter Schertl, Joana Polednia, Rolf D. Neuser, Arne P. Willner
2018, 29(5): 989-1004. doi: 10.1007/s12583-018-0842-0
Garnet is one of the most significant minerals in metamorphic rocks, that provides key information on prograde, peak-metamorphic and retrograde parts of the pressure-temperature (PT) path. Such results require a detailed knowledge of its different growth domains. For iron-poor compositions, the cathodoluminescence (CL) microscopy is an important and often overlooked method and allows to identify the internal structures of all garnet grains in one thin section within only a few seconds. The advantage of the CL-microscope is to deliver low magnification images in true color, not only of garnet but also, for instance, of other rock forming silicates, carbonates, sulfates, etc., of metamorphic, but also of sedimentary and magmatic origin, using polished thin sections. Internal structures of grossular from Mexico and pyrope from the Italian Alps were characterized and visualized by CL-microscopy. The different growth domains were additionally studied using CL-spectra and electron microprobe (EMP) analysis. Grossular shows a patchy zonation in its core while in mantle and rim zones oscillatory zoning is observed. It contains zones of anomalous birefringence, zones of orange and bluish luminescence and zones lacking luminescence. Different but low amounts of the activator elements Mn2+ and Eu2+ are responsible for the orange and bluish luminescent domains. Pyrope is also characterized by oscillatory growth zones, shows a dull luminescent core with a change of crystal morphology during growth, and displays an increase of brightness from core towards rim-the outermost rim, however, is lacking luminescence. The different luminescent zones are characterized by different amounts of Dy3+, Tb3+, Sm3+ and Sm2+ as activator elements. Because of slow diffusion rates of activators such as the REEs Sm, Dy and Tb, it can be still possible to visualize possible prograde and/or peak pressure stage growth domains of garnet, even if later high temperature events may have homogenized the major element profiles. Such domains may help to identify respective assemblages of mineral inclusions, and hence these results can represent an integral part of a detailed PT path. Thus the CL-information can be used as an important pathfinder prior to supplementary investigations, as for instance EMP, ion probe, mineral or fluid inclusion studies.
Contact Metamorphic Processes and Rocks
The Origin of Spots in Contact Aureoles and Over-Heating of Country Rock Next to a Dyke
Roger Mason, Rong Liu
2018, 29(5): 1005-1009. doi: 10.1007/s12583-018-0882-5
Petrographic comparison of andalusite in the contact aureole of the Fangshan pluton, Zhoukoudian, Beijing, China, and hornfels in the aureole of the Markfield diorite, Leicestershire, England, shows that spots characteristic of the outer zones of contact metamorphic aureoles did not form during the progressive stage of contact metamorphism, but are pseudomorphs after earlier andalusite with characteristic chiastolite microstructure. The baked margin produced by contact metamorphism of syenite against a dyke in the Houshihushan ring complex, Shanhaiguan, Hebei Province, China, is an unusual feature caused by the dyke's role as a feeder to a family of cone-sheets.
Metamorphism, Magmatism and Tectonic Evolution of the Himalayan Orogen and Tianshan Orogen
High-Temperature Metamorphism, Anataxis and Tectonic Evolution of a Mafic Granulite from the Eastern Himalayan Orogen
Zeming Zhang, Huixia Ding, Xin Dong, Zuolin Tian, Dongyan Kang, Hongchen Mu, Shengkai Qin, Yuanyuan Jiang, Mengmei Li
2018, 29(5): 1010-1025. doi: 10.1007/s12583-018-0852-y
The Himalayan Orogen, resulting from the Tertiary collision of Indian and Asian continents, is a natural laboratory for studying metamorphism, partial melting and granite formation of collisional orogens. However, metamorphic and anatectic conditions and timescales of meta-mafic rocks in the Greater Himalayan Sequences (GHS) in the east-central Himalaya remain controversial. In this paper, we conduct a study of petrology and geochronology of mafic granulite from the Eastern Himalayan Syntaxis (EHS). The mafic granulite with abundant leucosome bands occurs as layers within felsic granulites and is well deformed. The granulite consists of garnet, plagioclase, amphibole and quartz with minor clinopyroxene, orthopyroxene, biotite, rutile, titanite and ilmenite. The garnet has growth compositional zoning and contains abundant mineral inclusions in its core. Peak metamorphic mineral assemblage of the granulite is garnet, amphibole, plagioclase, quartz, clinopyroxene and rutile, recording a high-pressure (HP) and high-temperature (HT) peak-metamorphism under conditions of 14-15.5 kbar and 780-790℃ in the presence of melt. The reconstructed clockwise P-T path is characterized by an early heating and burial prograde metamorphism, and late isothermal and cooling decompression retrogression. The granulite witnessed a long lasting HT metamorphism, partial melting and melt crystallization process which began at ca. 39 Ma and lasted to ca. 11 Ma. The present study shows that various high-grade rocks of the GHS in the EHS core experienced similar metamorphic conditions and P-T-t paths, indicating that they occurred as a coherent slab during the subduction and exhumation of Indian lithosphere. The significant melts generated during the prograde metamorphism of the GHS rocks not only contributed to the formation of the Himalayan leucogranite, but also resulted in the rheological weakening and ductile flow of the thickened lower crust of the Himalayan Orogen.
Metamorphic Characteristics and Tectonic Implications of the Kadui Blueschist in the Central Yarlung Zangbo Suture Zone, Southern Tibet
Guangming Sun, Xu-Ping Li, Wenyong Duan, Shuang Chen, Zeli Wang, Lingquan Zhao, Qingda Feng
2018, 29(5): 1026-1039. doi: 10.1007/s12583-018-0854-9
The Kadui blueschist is located in the central section of Yarlung Zangbo suture zone (YZSZ), southern Tibet, and has been subjected to the subduction of the Neo-Tethyan Ocean below the Asian Plate and provides important clues for better understanding the evolution of the India-Asia convergence zone. In this paper, the systematical petrographic and mineral chemical studies were carried out on the Kadui blueschist, which reveal a mineral assemblage of sodic amphibole, chlorite, epidote, albite and quartz with accessory minerals of titanite, calcite and zircon. Electron microprobe analyses demonstrate that amphibole shows zoned from actinolite core to ferrowinchite/riebeckite rim composition indicating that the sodic amphibole has formed during a prograde metamorphic event. The protolith of the blueschist is an intermediate-basic pyroclastic rock. The calculated pseudosection indicates a clockwise P-T path and constrains peak metamorphic conditions of about 5.9 kbar at 345℃. This condition is transitional between pumpellyite-actinolite, greenschist and blueschist facies with a burial depth of 20-22 km and a thermal gradient of 15-16℃/km. This thermal gradient belongs to high pressure intermediate P/T facies series and is possibly related to a warm subduction setting of young oceanic slabs. Our new findings indicate that the Kadui blueschist in the central part of YZSZ experienced a rapid subduction and exhumation process as a response to a northward subduction of the Neo-Tethyan oceanic lithosphere during the initial India-Asia collision stage.
Water in the Thickened Lower Crust of the Eastern Himalayan Orogen
Li Zhang, Yu Ye, Shan Qin, Zhenmin Jin
2018, 29(5): 1040-1048. doi: 10.1007/s12583-018-0880-7
Water content in nominally anhydrous minerals (NAMs) of the high-pressure (HP) metamorphic rocks controls the thermal structure, rheology and partial melting of orogenic belts. This paper conducts a systematic analysis of water in NAMs of the HP granulites from the Greater Himalayan Sequence (GHS), representing the thickened lower crust of the eastern Himalayan Orogen. The present result shows that the garnet, clinopyroxene, feldspar, quartz and kyanite contain 188 ppm-432 ppm, 193 ppm-547 ppm, 335 ppm-1 053 ppm, 125 ppm-185 ppm and 89 ppm H2O, respectively, and indicates that the thickened lower crust of the Himalayan Orogen is relatively wet rather than dry. The considerable concentrations of water in the HP granulites are expected to promote the rheological weakening of the metamorphic core of the Himalayan Orogen, providing a favorable evidence for the channel flow model of the exhumation of thickened lower crust.
Rutile in HP Rocks from the Western Tianshan, China: Mineralogy and Its Economic Implications
Wen Su, Jilei Li, Qian Mao, Jun Gao, Xin Liu, Fei Chen, Xiao-Mei Ge
2018, 29(5): 1049-1059. doi: 10.1007/s12583-018-0848-7
Rutile is a common Ti-bearing accessory mineral in high pressure (HP) metamorphic rocks of the western Tianshan. Distribution of rutile in the HP rocks varies from 0.5% in the greenschist to 30% in the rutile-bearing vein. Rutile can be subdivided into three groups based on the variation of trace elements:The first group has an averaged Zr content of 18 ppm-44 ppm and Hf content of 0.8 ppm-2.4 ppm, which correspond to occurrence of rutile from vein; the second group Zr of 59 ppm-63 ppm, Hf of 3.5 ppm-3.7 ppm; and the third group Zr of 150 ppm-160 ppm and Hf of 3.9 ppm, corresponding to rutile in the matrix of blueschist and eclogite, respectively. Rutile has been retrograded to ilmenite and titanite partly or completely, reducing the value of the ore. But rutile of HP rocks in the western Tianshan usually has the lowest content of uranium (< 1 ppm U), which might become an attractive raw material for the Ti industry. Therefore, rutile-bearing HP rocks in the western Tianshan as a mineral resource will be the focus of considerable attention.
Phase Equilibrium Modeling of Retrograded Eclogite at the Kekesu Valley, Eastern Segment of SW Tianshan Orogen and Tectonic Implications
Bin Xia, Qin Yang, Neng-Song Chen, Zhiqiang Zhou
2018, 29(5): 1060-1073. doi: 10.1007/s12583-018-0844-y
In this study, we report newly found strongly retrograded eclogite blocks in the Kekesu Valley, eastern segment of southwestern Tianshan, northwestern China. Based on detailed petrographical studies and mineral chemistry analyses, we constrain P-T evolution of two representative samples via phase equilibrium modeling method using THERMOCALC software. The peak stage is evidenced by the primary inclusions of omphacite+glaucophane+lawsonite (inferred)+chlorite+white mica in garnet. Using grossular and pyrope isopleth thermobarometry in garnet, the P peak conditions for this stage were constrained to be~23 kbar, 480-500℃ and the T peak conditions were constrained to be 19-20 kbar, 500-520℃. Subsequent exhumation led to the transformation of lawsonite to epidote. Using Si-in-phengite barometry, the P was constrained to be 16-20 kbar at T of 500-520℃ for this stage. Further retrograde stage was evidenced by the mineral assemblages of glaucophane+garnet+epidote+paragonite+ chlorite+white mica+quartz. Later stage retrogression is evidenced by glaucophane+epidote+chlorite+ quartz+white mica+albite+actinolite with P of 5-9 kbar when T < 490℃, indicating the eclogite may have been exhumed to the middle crust level and overprinted by blueschist-greenschist facies metamorphism. The results show that eclogite at the eastern segment of SW Tianshan shares similar P-T path to the HP rocks from the western segment, implying a similar P-T evolution for these rocks.
Metamorphism, Magmatism and Tectonic Evolution of Central China Orogenic Belts
Tectonic Boundary and Ceasing Time of Amalgamation be-tween the North China Craton and the North Qinling Belt
Jingbo Liu, Neng-Song Chen, Wen Su
2018, 29(5): 1074-1080. doi: 10.1007/s12583-018-0847-8
The Qinling orogenic belt is a composite orogen, consisting of the north and south Qinling belts with different collision ages. The North Qinling belt is composed of several petro-tectonic units, including the Kuanping, Erlangping, Qinling and Danfeng groups with numerous granitoid plutons from Neoproterzoic to Middle Devonian. To the north of the Kuanping Group, a mylonitized granite body in the Luanchuan Group was determined to have the magmatic age of 850 Ma, which is the northernmost Neoprotozoic granite body. The volcanic rocks of the Dahongkou Formation from the Luanchuan Group had the eruptive age of~1 600 Ma, obtained from zircon U-Pb LA-ICP-MS analysis. The detrital zircons in meta-sandstones from the Taowan Group have three populations of~920-1 400, ~1 600-2 100, and >2 200 Ma ages, whereas those from the Kuanping Group shows six populations with the peaks of~590, ~767, ~952, ~1 590, ~2 485, and~3 200 Ma. These data suggest that the boundary between the North China Craton and the North Qinling belt cannot simply be constrained by using the events determined from zircon U-Pb ages. The granitoid plutons with the magmatic age of 430 Ma were strongly deformed, while those with the magmatic age of~390 Ma show no or less deformation, indicating that the deformation of amalgamation between the North China Craton and the North Qinling belt ceased between 390 and 430 Ma.
Early Paleozoic Granulite-Facies Metamorphism and Magmatism in the Northern Wulan Terrane of the Quanji Massif: Implications for the Evolution of the Proto-Tethys Ocean in Northwestern China
Qinyan Wang, Yanjun Dong, Yuanming Pan, Fanxi Liao, Xiaowei Guo
2018, 29(5): 1081-1101. doi: 10.1007/s12583-018-0881-6
The nature and evolution of the Proto-Tethys Ocean originated from the breakup of the supercontinent Rodinia remain controversial. Early Paleozoic magmatism and metamorphism can provide important constraints on the closure of the Proto-Tethys Ocean. This paper reports on a set of geological, petrographical, geochronological, mineralogical and geochemical data for Early Paleozoic granite, gabbro, granulite and granitic leucosome in the northern Wulan terrane of the Quanji Massif. Zircon LA-ICP-MS U-Pb dating reveals two episodes of magmatism, with the emplacement of a granitic pluton at 476.7±2.8 Ma and a gabbroic dike at 423±2 Ma. Whole-rock geochemistry suggests an arc affinity for the magma of the granitic pluton but a post-collisional extension setting for the gabbroic dike. Zircon LA-ICP-MS U-Pb dating also shows that the peak granulite-facies metamorphism and anatexis occurred at~475 Ma, coeval with the formation of the granitic pluton in the Quanji Massif as well as the early lawsonite-bearing eclogites in the North Qaidam high-pressure and ultrahigh-pressure (HP-UHP) metamorphic belt to the south. The granulite-facies metamorphism with peak P-T conditions at 718-729℃ and 0.46-0.53 GPa is characterized by an anticlockwise P-T path. Our data provide compelling evidence for Early Paleozoic paired metamorphic belts with HP-UHP metamorphism in the North Qaidam to the south and low P/T metamorphism in the Quanji Massif as a continental arc to the north, hence suggesting a northward subduction polarity for the Proto-Tethys oceanic plate. The intrusion of the post-collisional gabbroic dike supports for the closure of the Proto-Tethys Ocean in northwestern China before 423 Ma.
Petrology of Garnet Amphibolites from the Hualong Group: Implications for Metamorphic Evolution of the Qilian Orogen, NW China
Yilong Li, Limin Zhao, Zhuoyang Li, Biji Luo, Jianping Zheng, Fraukje M. Brouwer
2018, 29(5): 1102-1115. doi: 10.1007/s12583-018-0850-0
The Qilian Orogen marks the junction of the North China, South China and Tarim cratons. The mechanism of continental growth during the formation of the orogen remains unclear. Based on detailed fieldwork, we present a systematic study of petrography, mineral chemistry and phase equilibria of garnet amphibolites from the Hualong Group, which represents the Precambrian basement in the southern accretionary belt of the Qilian Orogen. The garnet amphibolites mainly consist of amphibole, plagioclase, garnet and quartz, with minor pyroxene, biotite and ilmenite. A peak stage of upper amphibolite facies to low-temperature granulite facies metamorphism and retrograde metamorphism in the amphibolite facies affected the samples. Garnet has a homogeneous composition of Alm66-71Grs14-17Prp9-12Sps3-5, amphibole is ferro-hornblende, biotite belongs to the ferro-biotite species and pyroxene is dominated by orthopyroxene with few clinopyroxene. Pseudosection modeling of the garnet amphibolite samples indicates clockwise P-T paths. The samples witness peak metamorphism at conditions of~4.9-6.3 kbar and~755-820℃ in the upper amphibolite facies to low-temperature granulite facies, and retrograde cooling and decompression at conditions of~2.5-3.1 kbar and~525-545℃. It is inferred that peak metamorphism with high temperature and low pressure occurred at ca. 450 Ma during northward subduction of the South Qilian oceanic crust beneath the central Qilian Block. When continental collision occurred between the central Qilian and the Qaidam blocks, the Hualong Block was accreted onto the South Qilian accretionary complex and experienced amphibolite facies retrograde metamorphism at ca. 440 Ma.
Metamorphic P-T-t Path of UHT Granulites from the North Tongbai Orogen, Central China
Hua Xiang, Zeming Zhang, Liming Zhao, Zengqiu Zhong, Hanwen Zhou
2018, 29(5): 1116-1131. doi: 10.1007/s12583-018-0855-8
Although zircon U-Pb geochronometer has been widely used for dating metamorphism in moderate-to high-grade metamorphic rocks, it is still difficult to link the zircon U-Pb age to pressure and temperature (P-T) conditions. In this study, zircon trace elements and Hf isotopes and REE partitioning between zircon and garnet are adopted to track the formation condition of zircon in the granulites from North Tongbai Orogen, Central China. Combined with previous metamorphic P-T path results, a quantitative integrated anticlockwise P-T-t path was established for Tongbai granulites. These granulites recorded an early low-P heating followed by a dramatic pressure increase. Evidence for the prograde history (M1) is provided by cordierite, orthopyroxene and biotite inclusions in garnet. The prograde metamorphism occurred at around 443±3 Ma, with P-T conditions of ca. 730-820℃ and < 6 kbar. The peak metamorphic (M2) condition is >920℃ and 8.5-10 kbar and the peak metamorphism age is ca. 432±4 Ma. At around 419 Ma, the granulites suffered an amphibolite-facies retrograde metamorphism (M3), represented by the replacement of garnet by biotite and plagioclase, and clinopyroxene by amphibole, with metamorphic condition of ca. 700℃ and ca. 7 kbar. The last retrograde metamorphism (M4) is a greenschist-facies overprint with an age of ca. 404 Ma. It is concluded that the metamorphism of Tongbai granulite lasted for more than 40 Ma, including a stage of more than 20 Ma granulite-facies metamorphism. The prolonged granulite-facies metamorphism resulted from the continuous northward subduction of the Shangdan oceanic crust beneath the North Qinling terrane.
Zircon and Monazite Ages Constraints on Devonian Magmatism and Granulite-Facies Metamorphism in the Southern Qaidam Block: Implications for Evolution of Proto-and Paleo-Tethys in East Asia
Jin Ba, Lu Zhang, Chuan He, Neng-Song Chen, Timothy M. Kusky, Qinyan Wang, Yusheng Wan, Xiaoming Liu
2018, 29(5): 1132-1150. doi: 10.1007/s12583-018-0853-x
High-temperature magma generation process and granulite-facies metamorphism can provide important information about mantle-crustal interaction and tectonic evolution. The strongly peraluminous monzonite pluton, the Jinshuikou cordierite granite on the southern margin of the Qaidam Block, can provide important information about the mantle-crustal interaction and constraints on tectonic transition from Proto-Tethys to Paleo-Tethys. This pluton develops enclaves of mafic granulite, amphibolite and quartzofeldspathic rocks, and is cut by massive monzonitic leuco-granite veins. Zircon and monazite U-Pb dating for the cordierite granite, the granulite enclaves and a massive monzonitic leuco-granite vein reveal that the cordierite granitic magma was generated from Mesoproterozoic continental crust with protolith derived from a provenance that was composed of >2.8 Ga old recycled crustal materials and recorded a~1.7 Ga magmatic event. The continental crust underwent low-pressure granulite-facies metamorphism at~380 Ma ago, whereas the cordierite granite magmas was generated and emplaced during 380 Ma, followed by intrusion of the massive monzonitic leuco-granite vein at circa 370-330 Ma. These data suggest that after the final closure of Proto-Tethys Ocean spreading along the southern Qaidam Block at~420 Ma, break-off of the subducted slab or delamination of the lower crustal base and upwelling of the asthenospheric mantle beneath the southern Qaidam Block occurred before the Mid-Devonian, and that the initiation of the Paleo-Tethys tectonics might initiate near the end of Early-Carboniferous in the East Kunlun-Qaidam region, East Asia.
Metamorphic P-T Path Differences between the Two UHP Terranes of Sulu Orogen, Eastern China: Petrologic Comparison between Eclogites from Donghai and Rongcheng
Zhuoyang Li, Yilong Li, Jan R. Wijbrans, Qijun Yang, Hua-Ning Qiu, Fraukje M. Brouwer
2018, 29(5): 1151-1166. doi: 10.1007/s12583-018-0845-x
The Sulu Orogen constitutes the eastern part of the Sulu-Dabie Orogen formed by Triassic collision between the Sino-Korean and Yangtze plates. An HP Slice Ⅰ and two UHP slices Ⅱ and Ⅲ with contrasting subduction and exhumation histories within the Sulu Orogen were postulated. This study presents the metamorphic P-T paths of eclogites from the two UHP belts constructed by petrography, mineral chemistry and Perple_X P-T pseudosection modeling in the MnC(K)NFMASHO system. Eclogites from Slice Ⅲ mainly consist of omphacite, garnet and quartz, with minor rutile, ilmenite, amphibole and phengite. Eclogites from Slice Ⅱ show a porphyroblastic texture with epidote porphyroblasts and garnet, omphacite, phengite, quartz and rutile in matrix. Pseudosection modeling reveals that eclogites from Slice Ⅱ witness a peak metamorphism of eclogite-facies under conditions of 3.1-3.3 GPa and 660-690℃, and a retrograde cooling decompression process. The eclogites from Slice Ⅲ record a heating decompressive P-T path with a peak-P stage of 3.2 GPa and 840℃ and a peak-T stage of 2.4 GPa and 950℃, suggesting an apparent granulite-facies metamorphism overprint during exhumation. Both eclogites recorded clockwise P-T paths with peak P-T conditions suggesting a subduction beneath the Sino-Korean Plate to~100-105 km depth. Combined with tectonic scenarios from previous studies, it is concluded that the two UHP crustal slices in the Sulu terrane have a similar geodynamic evolution, but the UHP rocks in Slice Ⅱ exhumed after the eclogitic peak-pressure conditions earlier than that of Slice Ⅲ. The existence of Slice Ⅱ diminished the buoyancy force on Slice Ⅲ, resulting in a granulite-facies overprint on Slice Ⅲ. The Sulu orogenic belt is made up of different crustal slices that underwent different subduction and exhumation histories, rather than a single unit.
A Review of Ultrahigh Temperature Metamorphism
Hengcong Lei, Haijin Xu
2018, 29(5): 1167-1180. doi: 10.1007/s12583-018-0846-9
Ultrahigh-temperature (UHT) metamorphism represents extreme crustal metamorphism with peak metamorphic temperatures exceeding 900℃ and pressures ranging from 7 to 13 kbar with or without partial melting of crusts, which is usually identified in the granulite-facies rocks. UHT rocks are recognized in all major continents related to both extensional and compressive tectonic environments. UHT metamorphism spans different geological ages from Archean to Phanerozoic, providing information of the nature, petrofabric and thermal evolution of crusts. UHT metamorphism is traditionally identified by the presence of a diagnostic mineral assemblage with an appropriate bulk composition and oxidation state in Mg-Al-rich metapelite rocks. Unconventional geothermobarometers including Ti-in-zircon (TIZ) and Zr-in-rutile (ZIR) thermometers and phase equilibria modeling are increasingly being used to estimate UHT metamorphism. Concentrated on the issues about UHT metamorphism, this review presents the research history about UHT metamorphism, the global distribution of UHT rocks, the current methods for constraints on the UHT metamorphism, and the heat sources and tectonic settings of UHT metamorphism. Some key issues and prospects about the study of UHT metamorphism are discussed, e.g., identification of UHT metamorphism for non-supracrustal rocks, robustness of the unconventional geothermometers, tectonic affinity of UHT metamorphic rocks, and methods for the constraints of age and duration of UHT metamorphism. It is concluded that UHT metamorphism is of great importance to the understanding of thermal evolution of the lithosphere.
Petrogenesis and Tectonic Implications of Peralkaline A-Type Granites and Syenites from the Suizhou-Zaoyang Region, Central China
Hafizullah Abba Ahmed, Changqian Ma, Lianxun Wang, Ladislav A. Palinkaš, Musa Bala Girei, Yuxiang Zhu, Mukhtar Habib
2018, 29(5): 1181-1202. doi: 10.1007/s12583-018-0877-2
In this study, we present systematic petrological, geochemical, LA-ICP-MS zircon U-Pb ages and Nd isotopic data for the A-type granites and syenites from Suizhou-Zaoyang region. The results show that the peralkaline A-type granites and syenites were episodically emplaced in Suizhou-Zaoyang region between 450±3 and 441±7 Ma which corresponds to Late Ordovician and Early Silurian periods, respectively. Petrologically, the syenite-peralkaline granite association comprises of nepheline normative-syenite and alkaline granite in Guanzishan and quartz normative syenite and alkaline granite in Huangyangshan. The syenite-granite associations are ferroan to alkali in composition. They depict characteristics of typical OIB (oceanic island basalts) derived A-type granites in multi-elements primitive normalized diagram and Yb/Ta vs. Y/Nb as well as Ce/Nb vs. Y/Nb binary plots. Significant depletion in Ba, Sr, P, Ti and Eu indicates fractionation of feldspars, biotite, amphiboles and Ti-rich augite. The values of εNd(t) in Guanzishan nepheline syenite and alkaline granite are +1.81 and +2.26, respectively and the calculated two-stage model age for these rocks are 1 040 and 1 003 Ma, respectively. On the other hand, the Huangyangshan alkaline granite has εNd(t) values ranging from +2.61 to +3.46 and a relatively younger two-stage Nd model age values ranging from 906 to 975 Ma, respectively. Based on these data, we inferred that the Guanzishan nepheline syenites and granites were formed from fractional crystallization of OIB-like basic magmas derived from upwelling of metasomatized lithospheric mantle. The Huangyangshan quartz syenite and granite on the other hand, were formed from similar magmas through fractional crystallization with low input from the ancient crustal rocks. Typically, the rocks exhibit A1-type granite affinity and classified as within plate granites associated with the Ordovician crustal extension and the Silurian rifting.
Geochronology, Geochemistry and Sr-Nd-Pb Isotopic Study of the Wulong Flower-Like Glomerophyric Diorite Porphyry (Central China): Implications for Tectonic Evolution of Eastern Qinling
Yuxiang Zhu, Lianxun Wang, Changqian Ma, Chao Zhang, Ke Wang
2018, 29(5): 1203-1218. doi: 10.1007/s12583-018-0878-1
The Wulong diorite porphyry displays a very unusual flower-like glomerophyric texture and is thus well-known as a gemstone type of "Luoyang peony stone" for non-geologists. Hereby, we present a comprehensive study in terms of petrography, zircon U-Pb geochronology, geochemistry and Sr-Nd-Pb isotope. The rock exposed as small-scale dykes in the field, intruding in the Archean migmatized biotite-plagioclase genesis at the southern margin of the North China Craton. It exhibits typical porphyrite texture with plagioclase and amphibole as dominant and subdominant phenocrysts, respectively. The diorite porphyry rocks contain 51 wt.%-59 wt.% SiO2, belonging to medium-K calc-alkaline rock series. They are characterized by enrichment of large ion lithophile elements (LILEs, e.g., Ba) and depletion of high field-strength elements (HFSEs, e.g., Nb, Ta, Zr and Hf), resembling island arc magmatic rocks. They show high initial Sr isotopic ratios (0.710 7-0.715 8), low εNd(t) values (-9.9 to -13.3) and variable initial Pb isotopic ratios (206Pb/204Pb vary from 17.3 to 19.3), similar to the coeval intermediate and mafic magmatic rocks of adjacent region. These geochemical features indicate that the Wulong diorite porphyry is likely to originate from an enriched mantle with contamination of crustal materials. Zircon U-Pb dating results reveal a magma crystallization age of 480±3 Ma, consistent with the summit of the magmatic events of the Qinling orogenic belt and reflecting the northernmost Paleozoic magmatic activity of the belt. This may indicate the influence of northward subduction of Paleotethyan oceanic slab can extend a distance of ca. 200 km to the north and reach Luoyang-Songshan region in the East Qinling Orogen.
Precambrian Metamorphism-Magmatism and Tectonics of the North China Craton and Erguna Massif
Metamorphic Evolution and Zircon U-Pb Ages of the Nanshankou Mafic High Pressure Granulites from the Jiaobei Terrane, North China Craton
Shuang Chen, Xu-Ping Li, Fanmei Kong, Qingda Feng
2018, 29(5): 1219-1235. doi: 10.1007/s12583-017-0956-9
Petrological analysis and LA-ICP-MS zircon U-Pb dating were conducted on high-pressure mafic granulites, which occured as xenolith within TTG gneisses, from the Nanshankou Village of the Jiaobei terrane, Shandong Peninsula in the north-eastern part of the North China Craton (NCC). The mafic HP granulite is composed of garnet, clinopyroxene, orthopyroxene, amphibole and symplectitic clinopyroxene, orthopyroxene, plagioclase, ilmente and magnetite which were formed after the decomposition of porphyroblastic garnet and clinopyroxene. Four stages of metamorphic mineral assemblages for the mafic HP granulites were constrained by detail petrological and mineralogical investigations. The early prograde assemblage is represented by the mineral inclusions within garnet and clinopyroxene porphyroblasts (Opx1+Pl1+Qtz1), recording the metamorphic conditions at~754-757℃, 0.63-0.71 GPa; peak metamorphic conditions were determined at~874-891℃, 1.32-1.35 GPa with the mineral assemblage of Grt2+Cpx2+Amp2+Pl2+Qtz2. Retrograde minerals derived from symplectitic assemblage Opx3+Cpx3+Amp3+Pl3+Qtz3+Ilm3±Mag3 were formed at 693-796℃, 0.60-0.84 GPa. A final greenschist to sub-greenschist facies event was recorded by the exsolution of actinolite and albite within a retrograded clinopyroxene, as well as the occurrence of prehnite, chlorite and calcite minerals. Accordingly, a clockwise P-T path was concluded on the basis of the different stages of mineral asseblage. Cathodoluminescence imaging, trace element and U-Pb dating of zircons from the mafic HP granulites recorded similar charactistics for three episodes of Paleo-Meso Proterozoic metamorphic events. These are the metamorphic events preserved in mafic and pelitic granulites in the Jiao-Liao-Ji belt (JLJB) with 207Pb/206Pb ages of 2.0-1.9 Ga for peak metamorphism and of 1.86-1.84 Ga for decomposing process, followed by a retrograde amphibolite facies metamorphic event related to the post-orogenic extension at the age of 1.76-1.74 Ga, resulting the exhumation of the granulite to the upper crust level.
Phase Equilibria Modeling and P-T Evolution of the Mafic Lower-Crustal Xenoliths from the Southeastern Margin of the North China Craton
Jiazhen Nie, Yican Liu, Yang Yang
2018, 29(5): 1236-1253. doi: 10.1007/s12583-018-0849-6
The Precambrian lower crust rocks at the southeastern margin of the North China Craton (NCC) are mainly exposed as granulite xenoliths hosted by Mesozoic dioritic porphyry and metamorphic terrains in the Xuzhou-Suzhou area. Garnet amphibolites and garnet granulites are two kinds of typical lower-crustal xenoliths and were selected to reconstruct different stages of the metamorphic process. In this study, in view of multistage metamorphic evolution and reworking, phase equilibria modeling was used for the first time to better constrain peak P-T conditions of the xenoliths. Some porphyroblastic garnets have a weak zonal structure in composition with homogeneous cores and were surrounded by thin rims with an increase in XMg and a decrease in XCa (or XMg). Clinopyroxene contain varying amounts of Na2O and Al2O3 as well as amphibole of TiO2, while plagioclases are different in calcium contents. Peak metamorphic P-T conditions are calculated by the smallest garnet x(g) (Fe2+/(Fe2++Mg)) contours and the smallest plagioclase ca(pl) (Ca/(Ca+Na)) contours in NCFMASHTO (Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3) system, which are consistent with those estimated by conventional geothermobarometry. The new results show that the peak and decompressional P-T conditions for the rocks are 850-900℃/1.4-1.6 GPa and 820-850℃/0.9-1.3 GPa, respectively, suggestive of high and middle-low pressure granulite-facies metamorphism. Combined with previous zircon U-Pb dating and conventional geothermobarometry data, it is indicated that the xenoliths experienced a clockwise P-T-t evolution with near-isothermal decompressional process, suggestive of the Paleoproterozoic subduction-collision setting. In this regard, the studied region together with Jiao-Liao-Ji belt is further documented to make up a Paleoproterozoic collisional orogen in the eastern block of the NCC.
Tectonothermal Records in Migmatite-Like Rocks of the Guandi Complex in Zhoukoudian, Beijing: Implications for Late Neoarchean to Proterozoic Tectonics of the North China Craton
Yating Zhong, Chuan He, Neng-Song Chen, Bin Xia, Zhiqiang Zhou, Binghan Chen, Guoqing Wang
2018, 29(5): 1254-1275. doi: 10.1007/s12583-018-0856-7
Migmatite-like rocks transformed from strongly metamorphosed and deformed enclave-bearing felsic plutons usually make people confuse with the true migmatites and mistake in interpretation of their petrogenesis and tectonic implications. Here we report a suite of rocks that have long been called as migmatites from the Guandi complex in Zhoukoudian region, southwest of Beijing. The rocks are dominated by felsic gneisses with garnet-free amphibolites. Field occurrence, petrography and geochemistry indicate that the felsic gneisses and amphibolites were metamorphosed from protoliths of intermediate-acid and basic igneous rocks, respectively. New LA-ICP-MS zircon U-Pb dating and geothermobarometry study further reveal that precursor magmas of the two types of rocks were emplaced at 2.54-2.56 Ga and the rocks subsequently underwent medium P/T-type metamorphism with upper amphibolite facies conditions of 0.55-0.90 GPa and 670-730℃ at~2.48-2.50 Ga. Geochemically, precursor magmas of the amphibolites were suggested to be derived from an enriched lithospheric mantle source in continental arc setting, and those of the felsic gneisses are characterized by tonalitic to trondhjemitic magmas that are usually considered to be generated by partial melting of hydrated, thickened metamorphosed mafic crust with garnet as residues, suggesting that the rock associations are not of true migmatites but migmatite-like rocks. Our study reveal that protoliths of the migmatite-like rocks from the Guandi complex, were likely formed via magmatism in a continental arc setting, followed by accretion and collision of the continental arc as well as the intro-oceanic arc terranes to the Eastern Block of the North China Craton in the transition from the Late Neoarchean to Early Paleoproterozoic.
Phase Equilibria Modeling and Zircon Dating for Precambrian Metapelites from the Xinghuadukou Complex in the Lulin Forest of the Erguna Massif, Northeast China
Jiulei Xu, Changqing Zheng, Lucie Tajčmanová, Xin Zhong, Xiaomeng Han, Zhaoyuan Wang, Yan Yang
2018, 29(5): 1276-1290. doi: 10.1007/s12583-018-0843-z
The nature and formation time of the Xinghuadukou complex in Northeast China are important for determining the tectonic evolution of the Precambrian geological evolution of the Erguna massif. In this study, we present the results of zircon U-Pb dating of two metapelites from the complex. Detrital and metamorphic zircons from the metapelites yield a depositional age of~601 Ma and a metamorphic age of 496-509 Ma, indicating that the supracrustal rocks formed during the Neoproterozoic and recorded pan-African metamorphic events in the Erguna massif. Garnet porphyroblasts in Sil-Grt-Bt-Ms paragneiss show diffusion zoning, implying a decreasing P-T trend. Based on mineral transformation and P-T estimates using conventional geothermobarometers and phase equilibria modeling, three metamorphic stages were determined, including an early prograde metamorphic stage, a near peak upper amphibolite facies metamorphic stage, and a near-isobaric cooling stage with P-T conditions of 6.1 kb, 645℃, 5-6 kb, 710-740℃, and 4.4 kb, 625℃, respectively. The metamorphic history of the Xinghuadukou complex is thus defined by a clockwise P-T trajectory, which implies that the metamorphism of the metapelites documented the subduction, subsequent uplift and post collision process.