The geochemical characteristics of metabasites with a pseudo-pillow structure from Ganghe, Dabie Orogen, China

摘要

Several metabasite lenses in Ganghe, Central Dabie, that were previously described as pillow lavas are studied by elemental, Sr-Nd-Pb isotopic, and mineral oxygen isotopic analysis as well as zircon SHRIMP U-Pb dating. Zircon U-Pb geochronology results indicate that the protolith emplacement age of these metabasites is approximately 717 ± 38 Ma, consistent with the age of the volcanoclastic rocks in the same unit, and that they experienced the Triassic HP eclogite-facies retrograde metamorphism at 221 ± 2 Ma during exhumation after subduction to mantle depth and peak ultra-high pressure metamorphism. The low δ~(18)O values of -5.5% to -2.0% indicate that the protoliths underwent high temperature meteoric-hydrothermal alteration before subduction but had no seawater interaction. These metabasites had similar formation processes, water-rock interactions and metamorphisms as other eclogite-facies rocks cropped out in the Central Dabie terrain. They showed negative abnormalities in Nb, Sr, and Ti content and positive abnormalities in Ba, Th, and Pb content; they also showed LREE enrichment. The insusceptible Sm-Nd isotopes during metamorphism yielded δ_(nD) (t) = -12 to -10 and r_(DM) = 2.2-2.8 Ga for samples from lenses #1 to #3 and -7 to -6 and 2.1-2.2 Ga for lens #4; the samples also showed low radiogenic Pb isotope compositions of (~(206)Pb/~(204)Pb), = 15.34-16.50, (~(207)Pb/~(204)Pb), = 15.23-15.32, and (~(208)Pb/~(204)Pb)_i = 35.93-37.04. The data suggest that the protolith sources of the metabasites were contaminated to variable degrees by old crustal materials during formation. Unlike the Maowu layered intrusions, which were contaminated by upper crust, the magmas of the metabasites were contaminated by lower crust in the magma chamber and during eruption. It can be concluded that the protoliths of these metabasites were derived from old crustal-contaminated mantle sources and initially emplaced in the crust at the Neoproterozoic and that they were altered by meteoric water at high temperatures. In this respect, they might be similar to the Neoproterozoic mafic intrusions in the North Huaiyang terrain. However, the studied metabasites experienced the Permo-Triassic subduction and metamorphism, whereas the North Huaiyang Neoproterozoic mafic intrusions did not.