Identification and its geological significance of eclogite in Songshugou,the North Qinling

Abstract

The Songshugou garnet amphibolite occurs as lenses in amphibolite which is situated structurally next to the Songshugou ultramafic rock,and was considered to be formed by contact metamorphism or granulite-facies metamorphism so far. Detailed petrographic investigation and mineral elements analyses discovered relic omphacites both in garnet and zircon and in the matrix,and garnets in this rock preserved prograde metamorphic zonation both in major and trace elements,indicating that the garnet amphibolite is a retrograde eclogite and experienced at least a three stages clockwise metamorphism evolution from amphibolite facies to eclogite facies and then back to amphibolite facies. Zircon U-Pb dating obtained metamorphic age of 500 ± 8Ma and protolith age of 796 ± 16 Ma for the eclogite. These are consistent with the metamorphic and protolith ages of Guanpo UHP eclogite which lies at the north margin of the Qinling Group as well as other HP-UHP rocks spread widely in the Qinling Group. It suggests that the Songshugou eclogite together with existing HP-UHP rocks in the Qinling Group all were formed by continental deep subduction in Early Paleozoic,and the Songshugou ultramafic rock may be the metasomatic mantle rock in the subduction channel and was carried to the shallow crust by the subducted continental slab during exhumation.