First evidence for partly exhumed diamond-bearing ultra-high pressure (UHP) terrane, Bingara, NSW, Australia: garnet chemistry and exsolution microstructures

Abstract

In the last 100 years about two million (mostly eclogitic) diamonds were mined from Tertiary alluvial deposits at Bingara-Copeton, in the low metamorphic grade Phanerozoic New England Fold Belt, NSW. Even today their hard rock source remains unknown, so we targeted several basanitic igneous bodies to the south west of Bingara, and recovered more than 3000 garnets and five diamonds from samples of modern stream sediment and loam. Chemical classification of the garnets identified some from diamond eclogite and the Cr-poor megacryst suite indicating a mantle source, and formation near or within the diamond stability field. We defined an additional subgroup of UHP-crustal garnets with inherited continental crustal chemistry and unusual exsolution microstructures. These are similar to garnets from schist/orthogneiss, formed from deeply (>200km) subducted leucocratic continental crust, exhumed in the UHP terrane of Dabie Shan, China. They also compare with rare garnet inclusions in diamonds. Exsolution microstructures in garnets (world wide) are rare. They indicate a chemical change in UHP garnet due to exhumation of slab material and re-equilibration under lower pressure conditions. Such decompressed garnets from Bingara provide evidence for partial exhumation of deeply subducted diamond eclogite-grade oceanic crust and UHP-metamorphic continental crust to much shallower levels. This study predicts a buried diamond-bearing UHP terrane in the northern New England district, and a two-stage delivery of subduction diamonds and garnets to the surface; firstly by slab exhumation to relatively high levels, then by entrainment in shallow-sourced basanitic magma. Deep-sourced kimberlites and lamproites therefore are not necessary for diamond delivery here, and are unknown from eastern Australia.