Zinc isotopic behavior of mafic rocks during continental deep subduction

Zinc isotopes may act as a new tool of tracking recycling of crustal materials that causes compositional heterogeneity of the mantle.This application relies on an investigation of Zn isotopic behaviors during slab subduction.In this study,we report Zn isotopic compositions for a suite of metabasalts(greenschists,amphibolites,and coesitebearing eclogites)from the Dabie Orogen(China),which were formed via the subduction of mafic rocks into different depths and up to>200 km.Three out of eight greenschists are characterized by lighterδ^(66)ZnJMC-Lyon(0.10‰-0.16‰)than those of global basalts(0.28‰±0.05‰),which may be caused by crustal assimilation of the protoliths by sedimentary rocks due to their extremely high ^(87)Sr/^(86)Sr(up to 0.7130)and lowεNd values(down to−12.3).The remaining greenschists have relatively low ^(87)Sr/^(86)Sr and theirδ66Zn values(0.21‰-0.38‰)overlap the ranges of amphibolites(0.18‰-0.32‰)and coesite-bearing eclogites(0.18‰-0.36‰).There is no correlation betweenδ^(66)Zn and sensitive indicators of dehydration(Rb/TiO_(2),Ba/Yb,and H_(2)O+),suggesting that no detectable Zn isotope fractionation has occurred during the deep subduction of mafic rocks even into>200 km,which is attributed to the limited loss of Zn during prograde metamorphism and dehydration.Thus,Zn isotopic compositions of the deeply subducted mafic rocks are inherited from their protoliths.Considering that these metamorphosed rocks have higherδ66Zn than that of the mantle value by up to 0.2‰,the recycled/subducted mafic crust can incorporate isotopically heavy Zn into the mantle.The subducted slabs may partially melt and generate a metasomatized mantle,resulting in changes of Zn isotopic composition of the hybridized mantle as have been observed in some mantle xenoliths and basaltic lavas.

Two Discrete UHP and HP Metamorphic Belts in the Central Orogenic Belt, China

An attempt is made to confirm the existence of the two discrete UHP and HP metamorphic belts in the central orogenic belt in China. Detailed geological mapping and structural and petrological analyses of the Kanfenggou （看丰沟 ） and Xiangfanggou （ 香坊沟 ） slices exposed in the eastern Qinling （秦岭） orogen indicate that they experienced ultrahigh pressure and high pressure metamorphism, respectively. The former, situated in northern Qinling, contains a large volume of fine-grained coesite and quartz pseudomorphs after coesite- and microdiamond-bearing eclogite lenses, whereas the latter, located in southern Qinling, preserves the relicts of a high pressure metamorphic mineral assemblage. Based on extensive fieldwork together with compilations at the scale of the orogenic belt, and a comparison of Pb isotopic compositions between the UHP metamorphic rocks from Kanfenggou slice and the Dabie UHP metamorphic belt, we propose that there are at least two discrete ultrahigh pressure metamorphic belts with different ages and tectonic evolution within the central orogenic belt in China. The first is the South AItun-North Qaidam-North Qinling ultrahigh pressure metamorphic belt of Early Paleozoic age （ -500 -400 Ma）. The Kanfenggou ultrahigh pressure slab is located at its eastern segment. The second is the well constrained Dabie （ 大别 ）-Sulu （苏鲁 ） ultrahigh/high pressure metamorphic belt of Triassic age （-250-220 Ma）. The Xiangfanggou high pressure metamorphic slab is a westward extension of the Dabie-Sulu ultrahigh/high pressure metamorphic belt. The Pb isotopic compositions of the UHP metamorphic rocks from Kanfenggou UHP fragment in East Qinling are different from those of the UHP rocks in Dabie UHP metamorphic belt, but are consistent with those of the rocks from the Qinling rock group and Erlangping （二郎坪） rock group. The East Qinling UHP metamorphic belt does not appear to link with the Dabie-Sulu UHP metamorphic belt. These two ultrahigh metamorphic belts are separated by the suture-like Shangnan （ 商南 ）-Danfeng （丹凤 ） fault system or Guishan （ 龟山 ）-Meishan （ 梅山） fault, and a series of fault-bounded tectonic slices with different ages and rock assemblages. The Early Paleozoic ultrahigh metamorphic belt towards the east cannot be linked to the Triassic Dabie-Sulu ultrahigh/high pressure metamorphic belt, and so does not form a huge ultrahigh pressure metamorphic belt extending more than 4 000 km in the central orogenic belt. In addition, the South Altun-North Qaidam- North Qinling ultrahigh metamorphic belt probably represents an intercontinental deep subduction/ collision belt between the Sino-Korean and Yangtze cratons, occurring during the Early Paleozoic. The Dabie-Sulu ultrahigh/high pressure belt may have been formed by an intracontinental deep subduction/ collision process occurring in the northern part of the Yangtze craton during the Triassic. It is suggested that the two phase crustal subduction/collision processes, with an interval of 4200--300 Ma, responsible for the formation of the two ultrahigh/high pressure metamorphic belts occurred along spatially different belts within the central orogenic belt, China.

Conjecture with water and rheological control for subducting slab in the mantle transition zone

Seismic observations have shown structural variation near the base of the mantle transition zone （MTZ） where subducted cold slabs, as visualized with high seismic speed anomalies （HSSAs）, flatten to form stagnant slabs or sink further into the lower mantle. The different slab behaviors were also accompanied by variation of the ＂660 kin＂ discontinuity depths and low viscosity layers （LVLs） beneath the MTZ that are suggested by geoid inversion studies. We address that deep water transport by subducted slabs and dehydration from hydrous slabs could affect the physical properties of mantle minerals and govern slab dynamics. A systematic series of three-dimensional numerical simulation has been conducted to examine the effects of viscosity reduction or contrast between slab materials on slab behaviors near the base of the MTZ. We found that the viscosity reduction of subducted crustal material leads to a sepa- ration of crustal material from the slab main body and its transient stagnation in the MTZ. The once trapped crustal materials in the MTZ eventually sink into the lower mantle within 20 30 My from the start of the plate subduction. The results suggest crustal material recycle in the whole mantle that is consistent with evidence from mantle geochemistry as opposed to a two-layer mantle convection model. Because of the smaller capacity of water content in lower mantle minerals than in MTZ minerals, dehydration should occur at the phase transformation depth, ~660 kin. The variation of the disconti- nuity depths and highly localized low seismic speed anomaly （LSSA） zones observed from seismic P waveforms in a relatively high frequency band （~ 1 Hz） support the hypothesis of dehydration from hydrous slabs at the phase boundary. The LSSAs which correspond to dehydration induced fluids are likely to be very local, given very small hydrogen （H＋） diffusivity associated with subducted slabs. The image of such local LSSA zones embedded in HSSAs may not be necessarily captured in tomography studies. The high electrical conductivity in the MTZ beneath the northwestern Pacific subduction zone does not necessarily require a broad range of high water content homogeneously.

Pre-seismic gravity anomalies before Linkou Ms6.4 earthquake by continuous gravity observation of Crustal Movement Observation Network of China

A Ms6.4 earthquake occurred at Linkou country, Heilongjiang Province （44.8°N, 129.9°E） on January 2, 2016 at a depth of 580 km. Pre-seismic graviW anomalies obtained at a 1 Hz sampling rate from Crustal Movement Observation Network of China （CMONOC） are analyzed after the earthquake. The results show that： （1） different from previous studies, both pre-seismic amplitude perturbation and co-seismic amplitude perturbation are not critical inversely proportional to epicentral distance; （2） unlike shallow earthquake, the pre-seismic and co-seismic amplitude perturbation of gravity illustrate syn- chronous spatial variation characters with decrease of epicentral distance for Linkou earthquake. This may because Linkou earthquake is a deep earthquake and occurred in Pacific Plate subduction zone; （3） compared to basement and semi-basement, cave can provide a better observation environment for gPhone gravimeter to detect pre-seismic gravity anomalies.

Magmatism during continental collision, subduction, exhumation and mountain collapse in collisional orogenic belts and continental net growth: A perspective

Continental orogens on Earth can be classified into accretionary orogen and collisional orogen.Magmatism in orogens occurs in every periods of an orogenic cycle,from oceanic subduction,continental collision to orogenic collapse.Continental collision requires the existence of prior oceanic subduction zone.It is generally assumed that the prerequisite of continental deep subduction is oceanic subduction and its drag force to the connecting passive-margin continental lithosphere during continental collision.Continental subduction and collision lead to the thickening and uplift of crust,but the formation time of the related magmatism in orogens depends on the heating mechanism of lithosphere.The accretionary orogens,on the other hand,have no strong continental collision,deep subduction,no large scale of crustal thrusting,thickening and uplift,and no UHP eclogite-facies metamorphic rocks related to continental deep subduction.Even though arc crust could be significantly thickened during oceanic subduction,it is still doubtful that syn-or post-collisional magmatism would be generated.In collisional orogens,due to continental deep subduction and significant crustal thickening,the UHP metamorphosed oceanic and continental crusts will experience decompression melting during exhumation,generating syn-collisional magmatism.During the orogen unrooting and collapse,post-collisional magmatism develops in response to lithosphere extension and upwelling of asthenospheric mantle,marking the end of an orogenic cycle.Therefore,magmatism in orogens can occur during the continental deep subduction,exhumation and uplift after detachment of subducted oceanic crust from continental crust,and extensional collapse.The time span from continental collision to collapse and erosion of orogens(the end of orogenic cycle)is 50–85 Myr.Collisional orogens are the key sites for understanding continental deep subduction,exhumation,uplift and orogenic collapse.Magmatism in collisional orogens plays important roles in continental reworking and net growth.

Early Paleozoic granitic magmatism related to the processes from subduction to collision in South Altyn, NW China

Four episodes of granitic rocks at 517, 501-496, 462-451, and 426-385 Ma occurred in the South Altyn subduction-collision complex. The first episode of granite emplacement predates the formation of the ophiolite type mafic rock （〉500 Ma）, and the three subsequent episodes can be temporally correlated to high-pressure （HP） to ultrahigh-pressure （UHP） metamorphism at ca 500 Ma, retrograde granulite-facies metamorphism at ca. 450 Ma, and amphibolite-facies metamorphism at ca. 420 Ma, re- spectively. A comprehensive study of these granitic rocks, along with the regional geological background, mafic-ultramafic rocks, and HP-UHP metamorphism, indicates that the four episodes of granitic magmatism are sequentially derived from the partial melting of the earlier subducted oceanic crust at 517 Ma, the thickened continental crust due to continental subduction at ca. 500 Ma, the mid-upper crust in response to slab breakoff at ca. 450 Ma, and the tectonic transition from contraction to extension at ca. 420 Ma. The formation age of 517 Ma for oceanic adakite provides a direct constraint on the time of the oce- anic subduction in South Altyn. In addition, there is a ca. i0 Myr interval between the oceanic subduction to the continental deep subduction, suggesting that the Early Paleozoic tectonic evolution might have been a successive process in South Altyn. The four episodes of formation of granitic rocks, mafic-ultramafic rocks, and HP-UHP metamorphic rocks have fully recorded the tectonic evolution, beginning with the oceanic subduction, followed by continental subduction, and later exhumation dur- ing the Early Paleozoic in South Altyn.

Zircon Record of Ocean-Continent Subduction Transition Process of Dulan UHPM Belt, North Qaidam

The North Qaidam UHPM（ultra-high pressure metamorphism） belt is a typical continental subduction-collision belt containing continental crust deep subduction metamorphic products and oceanic crust relics, And it is an ideal region to study the ocean-continent transition and exhumation mechanism of oceanic UHP rocks during continental deep subduction process. In this paper, we report integrated in situ U-Pb, Lu-Hf and O isotope analyses of zircons from a serpentinized harzburgite as well as U-Pb dating for zircons from a kyanite eclogite from the North Qaidam Dulan UHPM terrane, and use these data to discuss the ocean-continent transition and exhumation mechanisms of oceanic UHP rocks during continental deep subduction. The serpentinized harzburgite was dated at 448±9 Ma, consistent with 455±5 Ma age for the kyanite eclogite within analytical errors. Zircons from the serpentinized harzburgite have uniform 176Hf/177 Hf values ranging from 0.282 842 to 0.282 883 and εHf（t） values from 11.6 to 13.3. Zircon δ^18O values of the serpentinized harzburgite vary from 4.47‰ to 5.29‰, slightly lower than the value of 5.3‰±0.6‰ for the normal mantle zircon. These Hf-O isotopic features indicate that the protolith of the serpentinized harzburgite was derived from depleted-mantle source, and might have experienced high-temperature rock-water interaction. Therefore, the serpentinized harzburgite was possibly located in the lower part of an oceanic section. The serpentinized harzburgite and kyanite eclogite were both formed due to the subduction of oceanic crust. The UHP metamorphism occurred successively from the oceanic crust to continental crust rocks of the North Qaidam UHP terrane. Low-density serpentinized peridotite and continental rocks possibly have negative buoyancy and play a key effect on preservation and exhumation of high-density oceanic eclogite.

Progress and Controversy in the Study of HP-UHP Metamorphic Terranes in the West and Middle Central China Orogen

During the past ten years, various types of HP-UHP metamorphic rocks have been discovered in the South Altyn Tagh, the North Qaidam and the North Qinling （秦岭） in the West and Middle Central China orogen. The UHP rocks, as lentoid bodies in regional gneisses, include eclogite （garnet-bearing pyroxenite）, garnet peridotite and various pelitic or felsic gneisses. There are many records of minerals and microstructures of exsolution indicate the UHP metamorphism, such as coesite （or its pseudomorph）, diamond, exsolution of clinopyroxene/amphibole/＋rutile or rutile＋quartz＋apatite in garnet, exsolution of quartz in omphacite and exsolution of kyanite＋spinel in precursor stishovite.The discovery of microstructure evidence for the presence of precursor stishovite in typical Alrich gneiss from the South Altyn Tagh reveals continental subduction and exhumation to and from a depth of more than 350 km. It is the petrological record of the deepest subduction and exhumation of continental rock in the world. The in situ zircon U-Pb dating using LA-ICP- MS or SHRIMP methods shows that the metamorphic ages of the HP-UHP rocks in the South Altyn Tagh, the North Qaidam and the North Qinling are 475-509, 420--457, and 485-514 Ma, respectively. The metamorphic ages of HP-UHP rocks in the North Qaidam are 20-80 Ma younger than those in the South Altyn Tagh and the North Qinling, and the metamorphic ages do not systematically increase or decrease from the South Altyn Tagh through the North Qaidam to the North Qinling. The absence of time transgressive variety of the metamorphism in the three regions does not support the hypothesis that the HP-UHP rocks in these re. gions form the same HP-UHP metamorphic zone. And the HP-UHP rocks in these regions can not be simply correlated to the collision between the North China plate and the South China plate. At present, the study of the HP-UHP rocks in the West and Middle Central China orogen faces several key issues or challenges, such as： （1） the continental subduction to the mantle depth of stishovite stability field （〉9 GPa） is occasional or universal; （2） the mechanism of exhumation for the continental rocks subducted to the depth of stishovite stability field （〉300 km）; （3） the tectonic setting and geodynamical mechanism of producing the HP-UHP metamorphic zones in the South Altyn Tagh, the North Qaidam and the North Qinling. Further studies aiming at these problems will make important progress not only in metamorphlsm of the HP-UHP rocks in the West and Middle Central China orogen, but also in continen. tal deep subduction and exhumation in solid earth science. It will also contribute to the establishment of the theory of continental deep subduction.