Common Pb of UHP metamorphic rocks from the CCSD project (100–5000 m) suggesting decoupling between the slices within subducting continental crust and multiple thin slab exhumation

Abstract

In order to understand the vertical structure of the Dabie–Sulu ultrahigh-pressure metamorphic (UHPM) belt, common Pb isotopic compositions of omphacites in eclogites and feldspars in gneisses from the Chinese Continental Scientific Drilling (CCSD) project (100–5000 m) have been investigated in this study. Samples from 0 to 800 m (unit 1) in the drilling core have moderately high radiogenic Pb isotopes with small variations of 206Pb/ 204Pb (16.82–17.38), 207Pb/ 204Pb (15.37–15.49), and 208Pb/ 204Pb (37.21–37.72), indicating either high µ ( 238U/ 204Pb) or high initial Pb isotope ratios of their protoliths. In contrast, the samples from 1600 to 2040 m (unit 3) and most of samples from 3200 to 5000 m (unit 5) have moderately or very unradiogenic Pb (unit 3: 206Pb/ 204Pb from 16.05 to 16.46, 207Pb/ 204Pb from 15.22 to 15.29, and 208Pb/ 204Pb from 36.68 to 37.48; unit 5: 206Pb/ 204Pb from 15.52 to 15.69, 207Pb/ 204Pb from 15.15 to 15.27, and 208Pb/ 204Pb from 36.48 to 37.20), indicating either low µ or low initial Pb isotope ratios of their protoliths. Pb isotopes of samples from 800 to 1600 m (unit 2) and from 2040 to 3200 m (unit 4) in the drilling core with abundant ductile shear zones are intermediate between those of units 1 and 3 or 5 and display larger variations. Pb isotopes combined with the published oxygen isotope data of the CCSD samples reveal the original positions of the five units before the Triassic continental subduction. Units 1, 3, and 5 as three UHPM rock slabs could be derived from the subducted upper continental crust, upper–middle continental crust and lower–middle continental crust, respectively. The ductile shearing zones in units 2 and 4 could be the interfaces where the detachment and decoupling took place between the upper, upper–middle and lower–middle continental crusts. The detachment between the upper slab and subducting continental lithosphere probably occurred during continental subduction, and the upper slab (unit 1) was uplifted to a shallow depth along the detachment surface by thrusting. Units 3 and 5 may be detached later from the subducted middle and lower crust and uplifted to a shallow level underneath unit 1. The low δ 18O values (− 4.0 to − 7.4‰) [Xiao, Y.-L., Zhang, Z.-M., Hoefs, J., Kerkhof, A., 2006. Ultrahigh-pressure Metamorphic Rocks from the Chinese Continental Drilling Project-II Oxygen Isotope and Fluid Inclusion Distributions through Vertical Sections. Contribution Mineral Petrology 152, 443–458.; Zhang, Z.-M., Xiao, Y.-L., Zhao, X.-D., Shi, C., 2006. Fluid-rock interaction during the continental deep subduction: oxygen isotopic profile of the main hole of the CCSD project. Acta Petrologica Sinica 22 (7), 1941–1951.] in units 2 and 4 suggest that the detachment interfaces could be developed along an ancient fault zones which were the channels of meteoric water activity during the Neoproterozoic.