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

摘要

Zircon U-Pb dating, mineral Sm-Nd isochron dating, and 0 and H isotope analyses were carried out for ultrahigh-pressure (UHP) eclogite and granitic gneiss from Taohang in the Sulu orogen. Besides heterogeneous ~(18)O depletion on an outcrop scale, mineral-pair 0 isotope thermometry indicates that refractory garnet and zircon attained and preserved equilibrium fractionations at about 820 to 560 deg C under eclogite-facies conditions. Zircons from the UHP metamorphic rocks have low delta~(18)O values of -1.3 to 4.2 percent, variably lower than delta~(18)O values of 5.3 +- 0.3per thousand for normal mantle zircons. U-Pb disgordia dating of ~(18)O-depleted zircons yields a protolith age of 770 +- 23 Ma and a metamorphic agelof 214 +- 9 Ma. Therefore, the ~(18)O-depleted zircons crystallized from a mid-Neoproterozoic low~(18)O magma whose precursor experienced high-T meteoric-hydrothermal alteration prior to melting in an active rifting zone. Both H isotope composition and H_2O concentration were measured by the TCEA-MS online technique. The results show deltaD values of-121 to - 58 per thousand for nominally anhydrous minerals and -101 to -62 per thousand 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 H_2O 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 0 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-0 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 0 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 0 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.