Zircon U-Pb dating,mineral inclusions,Lu-Hf isotopic data and their geological significance of garnet amphibolite from Songshugou,North Qinling

Abstract

Two discrete age groups were obtained through LA-ICP-MS zircon U-Pb dating of Songshugou garnet amphibolite:a residual core age of 729 ± 24Ma with Th / U = 0.70 ~ 1.01 and a metamorphic age of 496 ± 9.1Ma with Th / U = 0.02 ~ 0.11.Mineral inclusions of plagioclase,clinopyroxene,garnet,quartz,sphene and apatite were identified by laser Raman spectroscopy in metamorphic zircons from the garnet amphibolite.Combined with the garnet porphyroblasts and poikilitic intergrowth of clinopyroxene + plagioclase occurring in the rock,we infer that the metamorphic zircons of garnet amphibolite may grow under high-pressure granulitefacies metamorphism and we cannot rule out the possibility that there was an early eclogite-facies metamorphism experienced by the rock.The obtained metamorphic age of ~ 496Ma recorded a regional metamorphic event associated with continental deep subduction.Hf isotope compositions of recrystallized zircons were measured by LA-MC-ICP-MS.176Hf /177Hf = 0.282560 ~ 0.282663 and e Hf(t) = + 8.6 ~ + 12.2 when calculating with t = 730Ma.Hf model age is 867 ± 43Ma.Age of residual zircon cores and t DM(Hf) of recrystallized zircons from garnet amphibolite indicate that the protolith age of the rock should be older than 729Ma and the time for it fractionated from depleted mantle should not be later than 867 ± 43Ma.Take the previous research work into account,Songshugou garnet amphibolite was thought to be a product of an Early Paleozoic HP-UHP regional metamorphism on the early emplaced rock.