Ren-Xu Chen

Geochronology and Stable Isotope Geochemistry of UHP Metamorphic Rocks at Taohang in the Sulu Orogen, East-Central China

Zircon U-Pb dating, mineral Sm-Nd isochron dating, and O and H isotope analyses were carried out for ultrahigh-pressure (UHP) eclogite and granitic gneiss from Taohang in the Sulu orogen. Besides heterogeneous 18O depletion on an outcrop scale, mineral-pair O isotope thermometry indicates that refractory garnet and zircon attained and preserved equilibrium fractionations at about 820 to 560°C under eclogite-facies conditions. Zircons from the UHP metamorphic rocks have low δ18O values of -1.3 to 4.2‰, variably lower than δ18O values of 5.3 ± 0.3‰ for normal mantle zircons. U-Pb discordia dating of 18O-depleted zircons yields a protolith age of 770 ± 23 Ma and a metamorphic age of 214 ± 9 Ma. Therefore, the 18O-depleted zircons crystallized from a mid-Neoproterozoic low-18O magma whose precursor experienced high-T meteoric-hydrothermal alteration prior to melting in an active rifting zone. Both H isotope composition and H2O concentration were measured by the TCEA-MS online technique. The results show δD values of -121 to -58‰ for nominally anhydrous minerals and -101 to -62‰ for hydroxyl-bearing minerals, consistent with incorporation of meteoric water into protoliths of UHP meta-igneous rocks by high-T alteration and remelting. Hundreds to thousands of ppm H2O were detected in the forms of both molecular water and structural hydroxyl to be present in the nominally anhydrous minerals, providing an important budget of water content (besides hydrous minerals) in deeply subducted continental crust. A Gt-Wr-Pl Sm-Nd isochron age of 214 ± 10 Ma was obtained, in agreement with the zircon U-Pb age and corresponding to the state of O isotope equilibrium between the isochron minerals. Thus both ages are interpreted to represent the time of high-pressure eclogite-facies recrystallization during the initial exhumation. A fluid-present process for zircon overgrowth and Nd-O isotopic reequilibration is evident for this episode of retrogression. On the other hand, a Gt-Kfs Sm-Nd isochron age of 164 ± 11 Ma was obtained, corresponding to the state of O isotope disequilibrium between garnet and K-feldspar. This age postdates the Triassic collision orogeny, and thus has no relevance to the processes of both continental subduction and exhumation, suggesting limited fluid activity in the post-collisional stage. Therefore, the state of O isotope equilibrium or disequilibrium between coexisting minerals in high-grade metamorphic rocks provides a direct test for the validity of the mineral Sm-Nd chronometer in either case.

Fluid-rock interaction and geochemical transport during protolith emplacement and continental collision: A tale from Qinglongshan ultrahigh-pressure metamorphic rocks in the Sulu orogen

Ultrahigh-pressure (UHP) metamorphic rocks from the Qinglongshan region of the Sulu orogen are comprehensively studied for their whole-rock geochemistry, mineral O isotopes and zirconology. The metamorphic minerals, which experienced eclogite- to amphibolite-facies metamorphism, exhibit low to negative δ18O values, suggesting that the 18O-depletion of UHP rocks was acquired from their igneous protolith due to high-T meteoric-hydrothermal alteration during the Neoproterozoic. The O isotope heterogeneity in the protolith was not homogenized during the Triassic UHP metamorphism, indicating very limited fluid flow during orogenesis. However, the fluid flow is locally significant during exhumation of the UHP rocks, resulting in the formation of quartz veins, symplectites and coronas. Geochemical transport due to fluid action is evident in whole-rock geochemistry and mineralogical composition. The UHP rocks exhibit unreasonably low 87Sr/86Sr ratios at t1 = 750 Ma but much radiogenic Sr isotopes at t2 = 230 Ma, suggesting the mobility of water-soluble elements due to hydrothermal alteration during protolith emplacement and metamorphic dehydration during continental collision. Fluid-rock interaction during exhumation would also have mobilized Al, Si, Ca and LREE, resulting in the formation of high-pressure veins in the UHP eclogites. The protolith zircon of magmatic origin underwent different types of metamorphic recrystallization in response to fluid-mineral interaction, leading to differential redistribution of trace elements and O-Hf isotopes. Newly grown zircons of metamorphic origin exhibit negative δ18O values, indicating precipitation from negative δ18O fluids that were likely generated by metamorphic dehydration of the hydrothermally altered negative δ18O rock-forming minerals during the Triassic. The metamorphic zircons exhibit relatively homogeneous Hf isotope compositions, suggesting that fluid Hf isotopes originated from the same Hf isotope composition of the protolith. Relict zircon domains of magmatic origin exhibit both positive εHf(t) and negative εHf(t) values, indicating that the protolith of UHP rocks formed by reworking of both juvenile and ancient crustal rocks.

Two episodes of partial melting in ultrahigh-pressure migmatites from deeply subducted continental crust in the Sulu orogen, China

Partial melting plays an important role in the geodynamics of continental subduction zones. This is identified from a suite of ultrahigh-pressure (UHP) migmatites in the Sulu orogen through a combined study of zircon U-Pb ages, trace elements, and oxygen isotopes, as well as rock-forming mineral and inclusion compositions. The results indicate two episodes of partial melting in the subducted continental crust during continental collision, providing insights into subduction channel processes. The first episode of anatexis is indicated by the occurrence of nanogranites, not only in zircon, garnet, and monazite from diatexite, but also in zircon cores from leucosome. The anatectic zircon exhibits U-Pb ages of 230−227 Ma and flat rare earth element (REE) patterns with weak or no negative Eu anomalies, and it contains mineral inclusions of coesite and garnet + amphibole. Newly grown zircon grains in the diatexite and zircon cores in the leucosome exhibit high δ 18 O values of 8.3‰−17.3‰, indicating a metasedimentary protolith. The host rocks show high A/CNK (= molar ratio of Al 2 O 3 /[CaO + Na 2 O + K 2 O]) values and the occurrence of peritectic garnet in the diatexite. Thus, the diatexite was produced by partial melting of metasedimentary rocks. The Ti-in-zircon thermometry, the garnet-phengite Fe-Mg partition thermometry for mineral inclusions in the zircon, and the occurrence of coesite inclusions in zircon indicate partial melting at 650−800 °C and 2.5−3.0 GPa, corresponding to high-pressure (HP) to UHP conditions. On the other hand, the second episode of anatexis is recorded by newly grown zircon grains in metatexite and zircon rims in leucosome, which show U-Pb ages of 218−214 Ma, oscillatory zoning, steep heavy (H) REE patterns with negative Eu anomalies, low temperatures of 550−700 °C, and significant variations in Th, U, Nb, and Ta contents. The Zr-in-titanite thermometry for nanogranite-bearing titanite and the garnet-phengite Fe-Mg partition thermometry for mineral inclusions in the leucosome zircon rims indicate anatexis at 800−850 °C and 1.0−1.5 GPa. The zircon in the metatexite exhibits low δ 18 O values of −1.5‰−3.5‰ and Neoproterozoic U-Pb ages for relict magmatic cores, indicating the protolith of a low δ 18 O UHP metagranite. The two episodes of anatexis yield zircon domains with a series of differences not only in U-Pb age, but also in geochemical composition. Thus, protoliths with different origins were involved in the anatexis, with a possible difference in spatial positions. The UHP metasedimentary rocks atop the deeply subducted continental crust would have undergone the first episode of anatexis during the final subduction, whereas their underlying metagranite would have undergone the second episode of anatexis during the exhumation of deeply subducted crust. In either case, the breakdown of UHP hydrous minerals during exhumation is the key for the partial melting of UHP metamorphic rocks in the continental subduction channel.

Crust–Mantle Interaction in a Continental Subduction Channel: Evidence from Orogenic Peridotites in North Qaidam, Northern Tibet

A combined study of rock-forming mineral and O isotope compositions, zircon U–Pb ages, trace element compositions, mineral inclusions and Hf–O isotopes was made on orogenic peridotites from North Qaidam in northern Tibet to provide insights into crustal metasomatism at the slab–mantle interface in a continental subduction channel. Rock-forming mineral O isotope analyses yield olivine dO values of 5 0–5 6% for lherzolites and 3 5–5 3% for dunites; orthopyroxene has dO values of 4 5–5 3% for lherzolites, 4 5–5 3% for dunites, and 7 8% for pyroxenites. It appears that the protolith of these ultramafic rocks underwent high-temperature metasomatism by both lowand high-dO fluids. The occurrence of zircon in thin section and the presence of olivine inclusions in zircon from these rocks explicitly indicates the growth of metasomatic zircon during continental subduction zone processes. Two distinct groups of zircon domains are distinguished. Intermediateto highluminescent zircon domains have U–Pb ages of 417 6 4 to 426 6 3 Ma, variable Th/U ratios, low rare earth element (REE) contents, flat heavy REE (HREE) patterns with a relative lack of negative Eu anomalies, and inclusions of olivine, clinopyroxene, orthopyroxene and graphite. These features are interpreted as zircon growth through metasomatic reaction by addition of fluids in the early exhumation stage of deeply subducted continental crust. The Hf–O isotope compositions of these zircon domains suggest the same origin for these fluids. In contrast, low-luminescent zircon rims mostly exhibit younger U–Pb ages of 404 6 3 to 405 6 8 Ma, low Th/U ratios<0 1, high REE contents, and steep HREE patterns with negative Eu anomalies, and contain calcite inclusions as well as multiphase solid inclusions of CalþKfs and QtzþKfsþPl. Such observations indicate their growth through metasomatic reaction by infiltration of hydrous silicate melts during the late exhumation stage. All the zircon domains have variable Hf isotope compositions with a large difference between samples, but a narrow dO range from 5 4 to 6 6%. Their Hf–O isotope compositions are similar to those of UHP metamorphic rocks in North Qaidam, suggesting that the metasomatic fluids originated from deeply subducted continental crust. Whereas the high-dO fluids originated from deeply subducted continental crust, the low-dO fluids were derived from dehydration of the subducting oceanic crust that preceded the subduction of continental crust. The orogenic peridotites thus record the metasomatic reaction of mantle wedge peridotite with fluids of different origins.