Yong-Fei Zheng
Phone: +86 551 63603554
Address: School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China
Address: School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China
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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.
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.