Tianshan Gao

Multi-stage evolution of gneiss from North Dabie: evidence from zircon U-Pb chronology

TTG gneiss is a common rock to outcrop in the northern part of the Dabie orogen, a few of which are closely associated with eclogites that experienced the Triassic ultra-high pressure metamorphism. Although they were thermally metamorphosed by a large-scale magma activity in this region at the Early Cretaceous, it is unclear whether or not they are also affected by the Triassic metamorphism during continental subduction and exhumation. In order to resolve this issue, SHRIMP zircon U-Pb dating was carried out for the host gneiss of eclogites in North Dabie. The results show that cores from the gneiss have an age of 746±31 Ma, consistent with the protolith ages of granitic gneisses in the Dabie orogen. Zircon overgrowing with different U and Th concentrations give concordant ages of 212±21 and 120±11 Ma, respectively. Th/U ratios of overgrown zircons are both lower than 0.1, suggesting a metamorphic genesis. The present results suggest that the gneiss in North Dabie has the similar protolith ages of Neoproterozoic to those granitic gneisses elsewhere in the Dabie orogen, and experienced not only the Triassic metamorphism but also the thermal metamorphism due to the Early Cretaceous magmatism. This provides an important insight into the geodynamic evolution of gneissic rocks in the Dabie orogen.

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.

Re-Os dating of the Raobazhai ultra mafic massif in North Dabie

The ultramafic massif at Raobazhai in North Dabie is located in the suture zone between the Yangtze craton and North China craton. The Re-Os isotope compositions of the massif are used to decipher the origin and tectonics of the ultramafic rocks involved in continental subduction and exhumation. Fifteen samples were collected from five drill holes along the main SE-NW axis of the Raobazhai massif. Major and trace element compositions of the samples show linear correlations between MgO, Yb and Al2O3. This suggests that the massif experienced partial melting with variable degrees and is from fertile to deplete in basaltic compositions. Nine selected samples were analyzed for Re-Os isotope compositions. Re contents range from 0.004 to 0.376 ng/g, Os contents from 0.695 to 3.761 ng/g,187Re/188Os ratios from 0.022 to 2.564 and187Os/188Os ratios from 0.1165 to 0.1306. These indicate that the massif is a piece of continental lithospheric mantle with variable depletion. Using the positive correlations between187Os/188Os and Yb and Al2O3 respectively, a proxy isochron age of 1.8±0.1 Ga is obtained for the Raobazhai ultramafic massif, which is interpreted to represent a fragment of the ancient subcontinental lithospheric mantle. During Triassic subduction of the Yangtze craton beneath the North China craton, part of the wedge of subcontinental lithospheric mantle was intruded into the subduction belt, and then exhumed to crustal level together with the subducted crustal plate after ultrahigh pressure metamorphism at mantle depths. This ancient lithospheric mantle is now exposed as an orogenic peridotite massif.

Zircon SHRIMP U-Pb age of garnet olivine pyroxenite at Hujialin in the Sulu terrane and its geological significance

Garnet olivine pyroxenite at Hujialin is situated in the Sulu ultrahigh pressure (UHP) metamorphic belt, Shandong Province. Most of the zircon separated from the rock is well crystallized, prismatic and granular with a length to width ratio of 1:1.3-1:2.5. CL and BSE images show the magmatic oscillatory zoning in the zircon. Th/U ratio ranges from 0.99 to 2.81. These suggest a magmatic origin for the zircon studied. SHRIMP dating yields206pb/238U ages of 207–223 Ma, with a weighted average of 216±Ma. This age corresponds to zircon growth during exhumation of UHP slab and thus the timing of amphibolite-facies retrogression. The garnet olivine pyroxenite was wrapped and brought to the crust by the UHP slab during exhumation, and then suffered from metasomatism by fluid from the UHP slab itself. The zircon U-Pb age records the timing of the crystallization of metasomatic melt. Therefore, fluid that was released during exhumation of deeply subducted continental slab may be the important source for zircon growth.

The discovery of eclogite dikes in low-greenschist facies volcaniclastic rocks of the Dabie Mountains

Conclusions1.The eclogite dikes in low-greenschist facies epi-metamorphic volcaniclastic rocks in the Dabie Mountains areas show magma origin rather than tectonic melange or metamorphic origin.2.Coesite and quartz pseudomorphs in the eclogite show that they were formed at the pressure of over 2.8 GPa and belong to ultrahigh pressure type. Therefore, there were high-pressure eclogite magma at deep crust depth.3.From the country rock age of the low-grade metamorphic volcaniclastic rocks, we infer that the eclogite dikes were formed after the Sinian.