Yong-Fei Zheng - Academia.edu (original) (raw)

Papers by Yong-Fei Zheng

Professor Dr Jochen Hoefs on the occasion of his 75th birthday. Oxygen isotope fractionation fact... more Professor Dr Jochen Hoefs on the occasion of his 75th birthday. Oxygen isotope fractionation factors for phosphates were calculated by means of the increment method. The results suggest that Ag 3 PO 4 and BiPO 4 are enriched in 18 O relative to AgPO 4 , and the three phosphates are consistently depleted in 18 O relative to Ba 3 [PO 4 ] 2 ; fluorapatite and chlorapatite exhibit a similar behaviour of oxygen isotope fractionation with consistent enrichment of 18 O relative to hydroxya-patite. The valence, radii and coordination of metal cations play a quantitative role in dictating the 18 O/ 16 O partitioning in these phosphates of different compositions. The calculated fractionation factors for the Ag 3 PO 4 –H 2 O system are in agreement with experimental determinations derived from enzyme-catalysed isotope exchange between dissolved inorganic phosphate and water at the longest reaction durations at low temperatures. This demonstrates that the precipitated Ag 3 PO 4 has completely captured the oxygen isotope fractionation in the dissolved inorganic phosphate. The calculated fractionation factors for the F/Cl-apatite–water systems are in agreement with the enzyme-catalysed experimental fractionations for the dissolved phosphate–water system at the longest reaction durations but larger than fractionations derived from bacteria-facilitated exchange and inorganic precipitation experiments as well as natural observations. For the experimental calibrations of oxygen isotope fractionation involving the precipitation of dissolved phosphate species from aqueous solutions, the fractionation between precipitate and water is primarily dictated by the isotope equilibration between the dissolved complex anions and water prior to the precipitation. Therefore, the present results provide a quantitative means to interpret the temperature dependence of oxygen isotope fractionation in inorganic and biogenic phosphates.

Dedicated to Professor Dr Jochen Hoefs on the occasion of his 75th birthday Oxygen isotope fracti... more Dedicated to Professor Dr Jochen Hoefs on the occasion of his 75th birthday Oxygen isotope fractionations in double carbonates of different crystal structures were calculated by the increment method. Synthesis experiments were performed at 60 °C and 100 °C to determine oxygen and carbon isotope fractionations involving PbMg[CO 3 ] 2. The calculations suggest that the double carbon-ates of calcite structure are systematically enriched in 18 O relative to those of aragonite and mixture structures. Internally consistent oxygen isotope fractionation factors are obtained for these minerals with respect to quartz, calcite and water at a temperature range of 0–1200 °C. The calculated fractionation factors for double carbonate–water systems are generally consistent with the data available from laboratory experiments. The experimentally determined fractionation factors for PbMg[CO 3 ] 2 , BaMg[CO 3 ] 2 and CaMg[CO 3 ] 2 against H 2 O not only fall between fractionation factors involving pure carbonate end-members but are also close to the calculated fractionation factors. In contrast, experimentally determined carbon isotope fractionation factors between PbMg[CO 3 ] 2 and CO 2 are much closer to theoretical predictions for the cerussite–CO 2 system than for the magnesite–CO 2 system, similar to the fractionation behavior for BaMg[CO 3 ] 2. Therefore, the combined theoretical and experimental results provide insights into the effects of crystal structure and exchange kinetics on oxygen isotope partitioning in double carbonates.

The Dabie–Sulu orogenic belt of east-central China has long been a type location for the study of... more The Dabie–Sulu orogenic belt of east-central China has long been a type location for the study of geodynamic
processes associated with ultrahigh-pressure (UHP) tectonics. Much of our understanding of the world's most
enigmatic processes in continental deep-subduction zones has been deduced from various records in this belt. By
taking advantage of having depth profiles from core samples of the Chinese Continental Scientific Drilling (CCSD)
project in the Sulu orogen, a series of combined studieswere carried out for UHP metamorphic rocks from themain
hole (MH) at continuous depths of 100 to 5000m. The results provide newinsights into the chemical geodynamics
of continental subduction-zone metamorphism, especially on the issues that are not able to be resolved from the
surface outcrops. Available results from our geochemical studies of CCSD-MH core samples can be outlined as
follows. (1) An O isotope profile of 100 to 5000mis established for the UHP metamorphicminerals, with finding of
18O depletion as deep as 3300m. Alongwith areal 18O depletion of over 30,000 km2 along the Dabie–Sulu orogenic
belt, three-dimensional 18O depletion of over 100,000 km3 occurs along the northern margin of the South China
Block. (2) Changes in mineralOisotope,Hisotope andwater content occur in eclogite-gneiss transitions, concordant
with petrographic changes. The contact between different lithologies is thus themost favorable place forfluid action;
fluid for retrogression of the eclogites away fromthe eclogite-gneiss boundarywas derived fromthe decompression
exsolution. For the eclogites adjacent to gneiss, in contrast, the retrogrademetamorphismwas principally caused by
aqueous fluid from the gneiss that is relatively rich in water. Inspection of the relationship between the distance,
petrography and δ18O values of adjacent samples shows O isotope heterogeneities between the different and same
lithologies on scales of 20 to 50 cm, corresponding to the maximum scales of fluid mobility during the continental
collision. (3) Studies of major and trace elements in the two continuous core segments indicate highmobility of LILE
and LREE but immobility ofHFSE andHREE. Someeclogites have andesitic compositionswith high SiO2, alkalis, LREE
and LILE but low CaO, MgO and FeO contents. These features likely result from chemical exchange with gneisses,
possibly due to the metasomatism of felsic melt produced by partial melting of the associated gneisses during the
exhumation. On the other hand, some eclogites appear to have geochemical affinity to refractory rocks formed by
melt extraction as evidence by strong LREE and LILE depletion and the absence of hydrous minerals. These results
provide evidence for melt-induced elementmobility in the UHPmetamorphic rocks, and thus the possible presence
of supercritical fluid during exhumation. In particular, large variations in the abundance of such elements as SiO2,
LREE and LILE occur at the contact between eclogite and gneiss. This indicates theirmobility between different slab
components, although it only occurs on small scales and is thus limited in local open-systems. (4) Despite the
widespread retrogression, retrogradefluidwas internally buffered in stable isotope compositions, and the retrograde
fluid was of deuteric origin and thus was derived from the decompression exsolution of structural hydroxyl and
molecularwater in nominally anhydrous minerals. (5) A combined study of petrography and geochronology reveals
the episode of HP eclogite-facies recrystallization at 216±3Ma, with timescale of 1.9 to 9.3Myr or less. Collectively,
theDabie–SuluUHP terrenes underwent the protracted exhumation (2–3 mm/yr) in the HP-UHP regime. (6) Zircon
U–Pb ages and Hf isotopes indicate that mid-Neoproterozoic protoliths of bimodal UHPmetaigneous rocks formed
during supercontinental rifting along preexisting arc-continent collision orogen, corresponding to dual bimodal
magmatism in response to the attempted breakup of the supercontinent Rodinia at about 780Ma. The first type of
bimodalmagmatismwas formed by reworking of juvenile Late Mesoproterozoic crust, whereas the second type of
bimodal magmatism was principally generated by rifting anatexis of ancient Middle Paleoproterozoic crust. In
conclusion, the geochemical studies of CCSD-MHcore samples have placed important constraints on the nature and
scale of fluid action and element mobility during the continental subduction and UHP metamorphism.

The study of subduction-zone processes is a key to development of the plate tectonic theory. Plat... more The study of subduction-zone processes is a key to development of the plate tectonic theory. Plate interface interaction is a basic mechanism for the mass and energy exchange between Earth's surface and interior. By developing the subduction channel model into continental collision orogens, insights are provided into tectonic processes during continental subduction and its products. The continental crust, composed of felsic to mafic rocks, is detached at different depths from subducting continental lithosphere and then migrates into continental subduction channel. Part of the subcontinental lithospheric mantle wedge, composed of perido-tite, is offscrapped from its bottom. The crustal and mantle fragments of different sizes are transported downwards and upwards inside subduction channels by the corner flow, resulting in varying extents of metamorphism, with heterogeneous deformation and local anatexis. All these metamorphic rocks can be viewed as tectonic melanges due to mechanical mixing of crust-and mantle derived rocks in the subduction channels, resulting in different types of metamorphic rocks now exposed in the same orogens. The crust-mantle interaction in the continental subduction channel is realized by reaction of the overlying ancient subcontinental lithospheric mantle wedge peridotite with aqueous fluid and hydrous melt derived from partial melting of subducted continental basement granite and cover sediment. The nature of premetamorphic protoliths dictates the type of collisional orogens, the size of ultrahigh-pressure metamorphic terranes and the duration of ultrahigh-pressure metamorphism. continental collision, subduction channel, ultrahigh-pressure metamorphism, differential exhumation, tectonic melange Citation: Zheng Y F, Zhao Z F, Chen Y X. Continental subduction channel processes: Plate interface interaction during continental collision.

Partial melting at continental lithosphere depths plays an important role in generating geochemic... more Partial melting at continental lithosphere depths plays an important role in generating geochemical variations in igneous rocks. In particular, dehydration melting of ultrahigh-pressure (UHP) metamorphic rocks during continental collision provides a petrological link to intracrustal differentiation with respect to the compositional evolution of continental crust. While island arc magmatism represents one end-member of fluid-induced large-scale melting in the mantle wedge during subduction of the oceanic crust, the partial melting of UHP rocks can be viewed as the other end-member of fluid-induced small-scale anatexis during exhumation of the deeply subducted continental crust. This latter type of melting is also triggered by metamorphic dehydration in response to P–T changes during the continental collision. It results in local occurrences of hydrous melts (even supercritical fluids) as felsic veinlets between boundaries of and multiphase solid inclusions in UHP metamorphic minerals as well as local accumulation of veinlet-like felsic leucosomes in foliated UHP metamorphic rocks and metamorphically grown zircons in orogenic peridotites. Thus, very low-degree melts of UHP rocks provide a window into magmatic processes that operated in continental subduction zones. This article presents a review on available results from experimental petrology concerning the possibility of partial melting under conditions of continental subduction-zone metamorphism, and petrological evidence for the occurrence of dehydration-driven in-situ partial melting in natural UHP rocks during the continental collision. Although the deeply subducted continental crust is characterized by a relative lack of aqueous fluids, the partial melting in UHP rocks commonly takes place during decompression exhumation to result in local in-situ occurrences of felsic melts at small scales. This is caused by the local accumulation of aqueous fluids due to the breakdown of hydrous minerals and the exsolution of structural hydroxyl and molecular water from nominally anhydrous minerals in UHP rocks during the exhumation. The dehydration melting of UHP rocks would not only have bearing on the formation of supercritical fluids during subduction-zone metamorphism, but also contribute to element mobility and ultrapotassic magmatism in continental collision orogens. Therefore, the study of dehydration melting and its effects on element transport in UHP slabs, rocks and minerals is a key to chemical geodynamics of continental subduction zones.

The composition of continental subduction-zone fluids varies dramatically from dilute aqueous sol... more The composition of continental subduction-zone fluids varies dramatically from dilute aqueous solutions at subsolidus conditions to hydrous silicate melts at supersolidus conditions, with variable concentrations of fluid-mobile incompatible trace elements. At ultrahigh-pressure (UHP) metamorphic conditions, supercritical fluids may occur with variable compositions. The water component of these fluids primarily derives from structural hydroxyl and molecular water in hydrous and nominally anhydrous minerals at UHP conditions. While the breakdown of hydrous minerals is the predominant water source for fluid activity in the subduction factory, water released from nominally anhydrous minerals provides an additional water source. These different sources of water may accumulate to induce partial melting of UHP metamorphic rocks on and above their wet solidii. Silica is the dominant solute in the deep fluids, followed by aluminum and alkalis. Trace element abundances are low in metamorphic fluids at subsolidus conditions, but become significantly elevated in anatectic melts at supersolidus conditions. The compositions of dissolved and residual minerals are a function of pressure-temperature and whole-rock composition, which exert a strong control on the trace element signature of liberated fluids. The trace element patterns of migmatic leucosomes in UHP rocks and multiphase solid inclusions in UHP minerals exhibit strong enrichment of large ion lithophile elements (LILE) and moderate enrichment of light rare earth elements (LREE) but depletion of high field strength elements (HFSE) and heavy rare earth elements (HREE), demonstrating their crystallization from anatectic melts of crustal protoliths. Interaction of the anatectic melts with the mantle wedge peridotite leads to modal metasomatism with the generation of new mineral phases as well as cryptic metasomatism that is only manifested by the enrichment of fluid-mobile incompatible trace elements in orogenic peridotites. Partial melting of the metasomatic mantle domains gives rise to a variety of mafic igneous rocks in collisional orogens and their adjacent active continental margins. The study of such metasomatic processes and products is of great importance to understanding of the mass transfer at the slab-mantle interface in subduction channels. Therefore, the property and behavior of subduction-zone fluids are a key for understanding of the crust-mantle interaction at convergent plate margins.

Plate subduction is an important mechanism for exchanging the mass and energy between the mantle ... more Plate subduction is an important mechanism for exchanging the mass and energy between the mantle and the crust, and the igneous rocks in subduction zones are the important carriers for studying the recycling of crustal materials and the crust-mantle interaction. This study presents a review of geochronology and geochemistry for postcollisional mafic igneous rocks from the Hong'an-Dabie-Sulu orogens and the southeastern edge of the North China Block. The available results indicate two types of the crust-mantle interaction in the continental subduction zone, which are represented by two types of mafic igneous rocks with distinct geochemical compositions. The first type of rocks exhibit arc-like trace element distribution patterns (i.e. enrichment of LILE, LREE and Pb, but depletion of HFSE) and enriched radiogenic Sr-Nd isotope compositions, whereas the second type of rocks show OIB-like trace element distribution patterns (i.e. enrichment of LILE and LREE, but no depletion of HFSE) and depleted radiogenic Sr-Nd isotope compositions. Both of them have variable zircon O isotope compositions, which are different from those of the normal mantle zircon, and contain residual crustal zircons. These geochemical features indicate that the two types of mafic igneous rocks were originated from the different natures of mantle sources. The mantle source for the second type of rocks would be generated by reaction of the overlying juvenile lithospheric mantle with felsic melts originated from previously subducted oceanic crust, whereas the mantle source for the first type of rocks would be generated by reaction of the overlying ancient lithospheric mantle of the North China Block with felsic melts from subsequently subducted continental crust of the South China Block. Therefore, there exist two types of the crust-mantle interaction in the continental subduction zone, and the postcollisional mafic igneous rocks provide petrological and geochemical records of the slab-mantle interactions in continental collision orogens.

Chemical geodynamics is an integrated discipline that studies the geochemical structure and tecto... more Chemical geodynamics is an integrated discipline that studies the geochemical structure and tectonic evolution of geospheres with the aim of linking tectonic processes to geochemical products in the Earth system. It was primarily focused on mantle geochemistry, with an emphasis on geochemical recycling in oceanic subduction zones. It has been extended to geochemical reworking and recycling under high-pressure (HP) to ultrahigh-pressure (UHP) conditions in all convergent plate margins. In particular, UHP terranes, along with UHP metamorphic minerals and rocks in continental subduction zones, represent natural laboratories for investigating geochemical transport and fluid action during subduction and exhumation of continental crust. As a result of this extension, the study of UHP terranes has significantly advanced our understanding of tectonic processes in collisional orogens. This understanding has principally benefited from the deciphering of petrological and geochemical records in deeply subducted crustal rocks that occur in different petrotec-tonic settings. This review focuses on the following issues in continental subduction zones: the time and duration of UHP metamorphism, the origin and action of metamorphic fluid/melt inside UHP slices, the element and isotope mobilities under HP to UHP conditions during continental collision, the origin of preme-tamorphic protoliths and its bearing on continental collision types, and the crustal detachment and crust– mantle interaction in subduction channels. The synthesis presented herein suggests that the nature of preme-tamorphic protoliths is a key to the type of collisional orogens and the size of UHP terranes. The source mixing in subduction channels is a basic mechanism responsible for the geochemical diversity of continental and oceanic basaltic rocks. Therefore, the geochemical study of HP to UHP metamorphic rocks and their derivatives has greatly facilitated our understanding of the geodynamic processes that drive the tectonic evolution of convergent plate margins from oceanic subduction to continental collision. Consequently, the study of chemical geodynamics has been developed from oceanic subduction zones to continental collision zones, and it has enabled important contributions to development of plate tectonic theory.

The transport of water from subducting crust into the mantle is mainly dictated by the stability ... more The transport of water from subducting crust into the mantle is mainly dictated by the stability of hydrous minerals in subduction zones. The thermal structure of subduction zones is a key to dehydration of the subducting crust at different depths. Oceanic subduction zones show a large variation in the geotherm, but seismicity and arc volcanism are only prominent in cold subduction zones where geothermal gradients are low. In contrast, continental subduction zones have low geothermal gradients, resulting in metamorphism in cold subduction zones and the absence of arc volcanism during subduction. In very cold subduction zone where the geothermal gradient is very low (5C/km), lawsonite may carry water into great depths of 300 km. In the hot subduction zone where the geothermal gradient is high (>25C/km), the subducting crust dehydrates significantly at shallow depths and may partially melt at depths of <80 km to form felsic melts, into which water is highly dissolved. In this case, only a minor amount of water can be transported into great depths. A number of intermediate modes are present between these two end-member dehydration modes, making subduction-zone dehydration various. Low-T/low-P hy-drous minerals are not stable in warm subduction zones with increasing subduction depths and thus break down at forearc depths of 60–80 km to release large amounts of water. In contrast, the low-T/low-P hydrous minerals are replaced by low-T/high-P hydrous minerals in cold subduction zones with increasing subduction depths, allowing the water to be transported to subarc depths of 80–160 km. In either case, dehydration reactions not only trigger seismicity in the subducting crust but also cause hydration of the mantle wedge. Nevertheless, there are still minor amounts of water to be transported by ultra-high-pressure hydrous minerals and nominally anhydrous minerals into the deeper mantle. The mantle wedge overlying the subducting slab does not partially melt upon water influx for volcanic arc magmatism, but it is hydrated at first with the lowest temperature at the slab-mantle interface, several hundreds of degree lower than the wet solidus of hydrated peridotites. The hydrated peridotites may undergo partial melting upon heating at a later time. Therefore, the water flux from the subducting crust into the overlying mantle wedge does not trigger the volcanic arc magmatism immediately.

Geostandards and Geoanalytical Research, 2004

Professor Dr Jochen Hoefs on the occasion of his 75th birthday. Oxygen isotope fractionation fact... more Professor Dr Jochen Hoefs on the occasion of his 75th birthday. Oxygen isotope fractionation factors for phosphates were calculated by means of the increment method. The results suggest that Ag 3 PO 4 and BiPO 4 are enriched in 18 O relative to AgPO 4 , and the three phosphates are consistently depleted in 18 O relative to Ba 3 [PO 4 ] 2 ; fluorapatite and chlorapatite exhibit a similar behaviour of oxygen isotope fractionation with consistent enrichment of 18 O relative to hydroxya-patite. The valence, radii and coordination of metal cations play a quantitative role in dictating the 18 O/ 16 O partitioning in these phosphates of different compositions. The calculated fractionation factors for the Ag 3 PO 4 –H 2 O system are in agreement with experimental determinations derived from enzyme-catalysed isotope exchange between dissolved inorganic phosphate and water at the longest reaction durations at low temperatures. This demonstrates that the precipitated Ag 3 PO 4 has completely captured the oxygen isotope fractionation in the dissolved inorganic phosphate. The calculated fractionation factors for the F/Cl-apatite–water systems are in agreement with the enzyme-catalysed experimental fractionations for the dissolved phosphate–water system at the longest reaction durations but larger than fractionations derived from bacteria-facilitated exchange and inorganic precipitation experiments as well as natural observations. For the experimental calibrations of oxygen isotope fractionation involving the precipitation of dissolved phosphate species from aqueous solutions, the fractionation between precipitate and water is primarily dictated by the isotope equilibration between the dissolved complex anions and water prior to the precipitation. Therefore, the present results provide a quantitative means to interpret the temperature dependence of oxygen isotope fractionation in inorganic and biogenic phosphates.

Dedicated to Professor Dr Jochen Hoefs on the occasion of his 75th birthday Oxygen isotope fracti... more Dedicated to Professor Dr Jochen Hoefs on the occasion of his 75th birthday Oxygen isotope fractionations in double carbonates of different crystal structures were calculated by the increment method. Synthesis experiments were performed at 60 °C and 100 °C to determine oxygen and carbon isotope fractionations involving PbMg[CO 3 ] 2. The calculations suggest that the double carbon-ates of calcite structure are systematically enriched in 18 O relative to those of aragonite and mixture structures. Internally consistent oxygen isotope fractionation factors are obtained for these minerals with respect to quartz, calcite and water at a temperature range of 0–1200 °C. The calculated fractionation factors for double carbonate–water systems are generally consistent with the data available from laboratory experiments. The experimentally determined fractionation factors for PbMg[CO 3 ] 2 , BaMg[CO 3 ] 2 and CaMg[CO 3 ] 2 against H 2 O not only fall between fractionation factors involving pure carbonate end-members but are also close to the calculated fractionation factors. In contrast, experimentally determined carbon isotope fractionation factors between PbMg[CO 3 ] 2 and CO 2 are much closer to theoretical predictions for the cerussite–CO 2 system than for the magnesite–CO 2 system, similar to the fractionation behavior for BaMg[CO 3 ] 2. Therefore, the combined theoretical and experimental results provide insights into the effects of crystal structure and exchange kinetics on oxygen isotope partitioning in double carbonates.

The Dabie–Sulu orogenic belt of east-central China has long been a type location for the study of... more The Dabie–Sulu orogenic belt of east-central China has long been a type location for the study of geodynamic
processes associated with ultrahigh-pressure (UHP) tectonics. Much of our understanding of the world's most
enigmatic processes in continental deep-subduction zones has been deduced from various records in this belt. By
taking advantage of having depth profiles from core samples of the Chinese Continental Scientific Drilling (CCSD)
project in the Sulu orogen, a series of combined studieswere carried out for UHP metamorphic rocks from themain
hole (MH) at continuous depths of 100 to 5000m. The results provide newinsights into the chemical geodynamics
of continental subduction-zone metamorphism, especially on the issues that are not able to be resolved from the
surface outcrops. Available results from our geochemical studies of CCSD-MH core samples can be outlined as
follows. (1) An O isotope profile of 100 to 5000mis established for the UHP metamorphicminerals, with finding of
18O depletion as deep as 3300m. Alongwith areal 18O depletion of over 30,000 km2 along the Dabie–Sulu orogenic
belt, three-dimensional 18O depletion of over 100,000 km3 occurs along the northern margin of the South China
Block. (2) Changes in mineralOisotope,Hisotope andwater content occur in eclogite-gneiss transitions, concordant
with petrographic changes. The contact between different lithologies is thus themost favorable place forfluid action;
fluid for retrogression of the eclogites away fromthe eclogite-gneiss boundarywas derived fromthe decompression
exsolution. For the eclogites adjacent to gneiss, in contrast, the retrogrademetamorphismwas principally caused by
aqueous fluid from the gneiss that is relatively rich in water. Inspection of the relationship between the distance,
petrography and δ18O values of adjacent samples shows O isotope heterogeneities between the different and same
lithologies on scales of 20 to 50 cm, corresponding to the maximum scales of fluid mobility during the continental
collision. (3) Studies of major and trace elements in the two continuous core segments indicate highmobility of LILE
and LREE but immobility ofHFSE andHREE. Someeclogites have andesitic compositionswith high SiO2, alkalis, LREE
and LILE but low CaO, MgO and FeO contents. These features likely result from chemical exchange with gneisses,
possibly due to the metasomatism of felsic melt produced by partial melting of the associated gneisses during the
exhumation. On the other hand, some eclogites appear to have geochemical affinity to refractory rocks formed by
melt extraction as evidence by strong LREE and LILE depletion and the absence of hydrous minerals. These results
provide evidence for melt-induced elementmobility in the UHPmetamorphic rocks, and thus the possible presence
of supercritical fluid during exhumation. In particular, large variations in the abundance of such elements as SiO2,
LREE and LILE occur at the contact between eclogite and gneiss. This indicates theirmobility between different slab
components, although it only occurs on small scales and is thus limited in local open-systems. (4) Despite the
widespread retrogression, retrogradefluidwas internally buffered in stable isotope compositions, and the retrograde
fluid was of deuteric origin and thus was derived from the decompression exsolution of structural hydroxyl and
molecularwater in nominally anhydrous minerals. (5) A combined study of petrography and geochronology reveals
the episode of HP eclogite-facies recrystallization at 216±3Ma, with timescale of 1.9 to 9.3Myr or less. Collectively,
theDabie–SuluUHP terrenes underwent the protracted exhumation (2–3 mm/yr) in the HP-UHP regime. (6) Zircon
U–Pb ages and Hf isotopes indicate that mid-Neoproterozoic protoliths of bimodal UHPmetaigneous rocks formed
during supercontinental rifting along preexisting arc-continent collision orogen, corresponding to dual bimodal
magmatism in response to the attempted breakup of the supercontinent Rodinia at about 780Ma. The first type of
bimodalmagmatismwas formed by reworking of juvenile Late Mesoproterozoic crust, whereas the second type of
bimodal magmatism was principally generated by rifting anatexis of ancient Middle Paleoproterozoic crust. In
conclusion, the geochemical studies of CCSD-MHcore samples have placed important constraints on the nature and
scale of fluid action and element mobility during the continental subduction and UHP metamorphism.

The study of subduction-zone processes is a key to development of the plate tectonic theory. Plat... more The study of subduction-zone processes is a key to development of the plate tectonic theory. Plate interface interaction is a basic mechanism for the mass and energy exchange between Earth's surface and interior. By developing the subduction channel model into continental collision orogens, insights are provided into tectonic processes during continental subduction and its products. The continental crust, composed of felsic to mafic rocks, is detached at different depths from subducting continental lithosphere and then migrates into continental subduction channel. Part of the subcontinental lithospheric mantle wedge, composed of perido-tite, is offscrapped from its bottom. The crustal and mantle fragments of different sizes are transported downwards and upwards inside subduction channels by the corner flow, resulting in varying extents of metamorphism, with heterogeneous deformation and local anatexis. All these metamorphic rocks can be viewed as tectonic melanges due to mechanical mixing of crust-and mantle derived rocks in the subduction channels, resulting in different types of metamorphic rocks now exposed in the same orogens. The crust-mantle interaction in the continental subduction channel is realized by reaction of the overlying ancient subcontinental lithospheric mantle wedge peridotite with aqueous fluid and hydrous melt derived from partial melting of subducted continental basement granite and cover sediment. The nature of premetamorphic protoliths dictates the type of collisional orogens, the size of ultrahigh-pressure metamorphic terranes and the duration of ultrahigh-pressure metamorphism. continental collision, subduction channel, ultrahigh-pressure metamorphism, differential exhumation, tectonic melange Citation: Zheng Y F, Zhao Z F, Chen Y X. Continental subduction channel processes: Plate interface interaction during continental collision.

Partial melting at continental lithosphere depths plays an important role in generating geochemic... more Partial melting at continental lithosphere depths plays an important role in generating geochemical variations in igneous rocks. In particular, dehydration melting of ultrahigh-pressure (UHP) metamorphic rocks during continental collision provides a petrological link to intracrustal differentiation with respect to the compositional evolution of continental crust. While island arc magmatism represents one end-member of fluid-induced large-scale melting in the mantle wedge during subduction of the oceanic crust, the partial melting of UHP rocks can be viewed as the other end-member of fluid-induced small-scale anatexis during exhumation of the deeply subducted continental crust. This latter type of melting is also triggered by metamorphic dehydration in response to P–T changes during the continental collision. It results in local occurrences of hydrous melts (even supercritical fluids) as felsic veinlets between boundaries of and multiphase solid inclusions in UHP metamorphic minerals as well as local accumulation of veinlet-like felsic leucosomes in foliated UHP metamorphic rocks and metamorphically grown zircons in orogenic peridotites. Thus, very low-degree melts of UHP rocks provide a window into magmatic processes that operated in continental subduction zones. This article presents a review on available results from experimental petrology concerning the possibility of partial melting under conditions of continental subduction-zone metamorphism, and petrological evidence for the occurrence of dehydration-driven in-situ partial melting in natural UHP rocks during the continental collision. Although the deeply subducted continental crust is characterized by a relative lack of aqueous fluids, the partial melting in UHP rocks commonly takes place during decompression exhumation to result in local in-situ occurrences of felsic melts at small scales. This is caused by the local accumulation of aqueous fluids due to the breakdown of hydrous minerals and the exsolution of structural hydroxyl and molecular water from nominally anhydrous minerals in UHP rocks during the exhumation. The dehydration melting of UHP rocks would not only have bearing on the formation of supercritical fluids during subduction-zone metamorphism, but also contribute to element mobility and ultrapotassic magmatism in continental collision orogens. Therefore, the study of dehydration melting and its effects on element transport in UHP slabs, rocks and minerals is a key to chemical geodynamics of continental subduction zones.

The composition of continental subduction-zone fluids varies dramatically from dilute aqueous sol... more The composition of continental subduction-zone fluids varies dramatically from dilute aqueous solutions at subsolidus conditions to hydrous silicate melts at supersolidus conditions, with variable concentrations of fluid-mobile incompatible trace elements. At ultrahigh-pressure (UHP) metamorphic conditions, supercritical fluids may occur with variable compositions. The water component of these fluids primarily derives from structural hydroxyl and molecular water in hydrous and nominally anhydrous minerals at UHP conditions. While the breakdown of hydrous minerals is the predominant water source for fluid activity in the subduction factory, water released from nominally anhydrous minerals provides an additional water source. These different sources of water may accumulate to induce partial melting of UHP metamorphic rocks on and above their wet solidii. Silica is the dominant solute in the deep fluids, followed by aluminum and alkalis. Trace element abundances are low in metamorphic fluids at subsolidus conditions, but become significantly elevated in anatectic melts at supersolidus conditions. The compositions of dissolved and residual minerals are a function of pressure-temperature and whole-rock composition, which exert a strong control on the trace element signature of liberated fluids. The trace element patterns of migmatic leucosomes in UHP rocks and multiphase solid inclusions in UHP minerals exhibit strong enrichment of large ion lithophile elements (LILE) and moderate enrichment of light rare earth elements (LREE) but depletion of high field strength elements (HFSE) and heavy rare earth elements (HREE), demonstrating their crystallization from anatectic melts of crustal protoliths. Interaction of the anatectic melts with the mantle wedge peridotite leads to modal metasomatism with the generation of new mineral phases as well as cryptic metasomatism that is only manifested by the enrichment of fluid-mobile incompatible trace elements in orogenic peridotites. Partial melting of the metasomatic mantle domains gives rise to a variety of mafic igneous rocks in collisional orogens and their adjacent active continental margins. The study of such metasomatic processes and products is of great importance to understanding of the mass transfer at the slab-mantle interface in subduction channels. Therefore, the property and behavior of subduction-zone fluids are a key for understanding of the crust-mantle interaction at convergent plate margins.

Plate subduction is an important mechanism for exchanging the mass and energy between the mantle ... more Plate subduction is an important mechanism for exchanging the mass and energy between the mantle and the crust, and the igneous rocks in subduction zones are the important carriers for studying the recycling of crustal materials and the crust-mantle interaction. This study presents a review of geochronology and geochemistry for postcollisional mafic igneous rocks from the Hong'an-Dabie-Sulu orogens and the southeastern edge of the North China Block. The available results indicate two types of the crust-mantle interaction in the continental subduction zone, which are represented by two types of mafic igneous rocks with distinct geochemical compositions. The first type of rocks exhibit arc-like trace element distribution patterns (i.e. enrichment of LILE, LREE and Pb, but depletion of HFSE) and enriched radiogenic Sr-Nd isotope compositions, whereas the second type of rocks show OIB-like trace element distribution patterns (i.e. enrichment of LILE and LREE, but no depletion of HFSE) and depleted radiogenic Sr-Nd isotope compositions. Both of them have variable zircon O isotope compositions, which are different from those of the normal mantle zircon, and contain residual crustal zircons. These geochemical features indicate that the two types of mafic igneous rocks were originated from the different natures of mantle sources. The mantle source for the second type of rocks would be generated by reaction of the overlying juvenile lithospheric mantle with felsic melts originated from previously subducted oceanic crust, whereas the mantle source for the first type of rocks would be generated by reaction of the overlying ancient lithospheric mantle of the North China Block with felsic melts from subsequently subducted continental crust of the South China Block. Therefore, there exist two types of the crust-mantle interaction in the continental subduction zone, and the postcollisional mafic igneous rocks provide petrological and geochemical records of the slab-mantle interactions in continental collision orogens.

Chemical geodynamics is an integrated discipline that studies the geochemical structure and tecto... more Chemical geodynamics is an integrated discipline that studies the geochemical structure and tectonic evolution of geospheres with the aim of linking tectonic processes to geochemical products in the Earth system. It was primarily focused on mantle geochemistry, with an emphasis on geochemical recycling in oceanic subduction zones. It has been extended to geochemical reworking and recycling under high-pressure (HP) to ultrahigh-pressure (UHP) conditions in all convergent plate margins. In particular, UHP terranes, along with UHP metamorphic minerals and rocks in continental subduction zones, represent natural laboratories for investigating geochemical transport and fluid action during subduction and exhumation of continental crust. As a result of this extension, the study of UHP terranes has significantly advanced our understanding of tectonic processes in collisional orogens. This understanding has principally benefited from the deciphering of petrological and geochemical records in deeply subducted crustal rocks that occur in different petrotec-tonic settings. This review focuses on the following issues in continental subduction zones: the time and duration of UHP metamorphism, the origin and action of metamorphic fluid/melt inside UHP slices, the element and isotope mobilities under HP to UHP conditions during continental collision, the origin of preme-tamorphic protoliths and its bearing on continental collision types, and the crustal detachment and crust– mantle interaction in subduction channels. The synthesis presented herein suggests that the nature of preme-tamorphic protoliths is a key to the type of collisional orogens and the size of UHP terranes. The source mixing in subduction channels is a basic mechanism responsible for the geochemical diversity of continental and oceanic basaltic rocks. Therefore, the geochemical study of HP to UHP metamorphic rocks and their derivatives has greatly facilitated our understanding of the geodynamic processes that drive the tectonic evolution of convergent plate margins from oceanic subduction to continental collision. Consequently, the study of chemical geodynamics has been developed from oceanic subduction zones to continental collision zones, and it has enabled important contributions to development of plate tectonic theory.

The transport of water from subducting crust into the mantle is mainly dictated by the stability ... more The transport of water from subducting crust into the mantle is mainly dictated by the stability of hydrous minerals in subduction zones. The thermal structure of subduction zones is a key to dehydration of the subducting crust at different depths. Oceanic subduction zones show a large variation in the geotherm, but seismicity and arc volcanism are only prominent in cold subduction zones where geothermal gradients are low. In contrast, continental subduction zones have low geothermal gradients, resulting in metamorphism in cold subduction zones and the absence of arc volcanism during subduction. In very cold subduction zone where the geothermal gradient is very low (5C/km), lawsonite may carry water into great depths of 300 km. In the hot subduction zone where the geothermal gradient is high (>25C/km), the subducting crust dehydrates significantly at shallow depths and may partially melt at depths of <80 km to form felsic melts, into which water is highly dissolved. In this case, only a minor amount of water can be transported into great depths. A number of intermediate modes are present between these two end-member dehydration modes, making subduction-zone dehydration various. Low-T/low-P hy-drous minerals are not stable in warm subduction zones with increasing subduction depths and thus break down at forearc depths of 60–80 km to release large amounts of water. In contrast, the low-T/low-P hydrous minerals are replaced by low-T/high-P hydrous minerals in cold subduction zones with increasing subduction depths, allowing the water to be transported to subarc depths of 80–160 km. In either case, dehydration reactions not only trigger seismicity in the subducting crust but also cause hydration of the mantle wedge. Nevertheless, there are still minor amounts of water to be transported by ultra-high-pressure hydrous minerals and nominally anhydrous minerals into the deeper mantle. The mantle wedge overlying the subducting slab does not partially melt upon water influx for volcanic arc magmatism, but it is hydrated at first with the lowest temperature at the slab-mantle interface, several hundreds of degree lower than the wet solidus of hydrated peridotites. The hydrated peridotites may undergo partial melting upon heating at a later time. Therefore, the water flux from the subducting crust into the overlying mantle wedge does not trigger the volcanic arc magmatism immediately.

Geostandards and Geoanalytical Research, 2004