Trace element distribution in Central Dabie eclogites.

Coesite-bearing eclogites from Dabieshan (central China) have been studied by ion microprobe to provide information on trace element distributions in meta-basaltic mineral assemblages during high-pressure metamorphism. The primary mineralogy (eclogite facies) appears to have been garnet and omphacite, usually with coesite, phengite and dolomite, together with high-alu- mina titanite or rutile, or both titanite and rutile; kyanite also occurs occasionally as an apparently primary phase. It is probable that there was some development of quartz, epidote and apatite whilst the rock remained in the eclogite facies. A later amphibolite facies overprint led to partial replacement of some minerals and partic- ularly symplectitic development after omphacite. They vary from very ®ne-grained dusty-looking to coarser grained Am + Di + Pl symplectites. The eclogite facies minerals show consistent trace element compositions and partition coecients indicative of mutual equilibri- um. Titanite, epidote and apatite all show high con- centrations of REE relative to clinopyroxene. The compositions of secondary (amphibolite facies) minerals are clearly controlled by local rather than whole-rock equilibrium, with the composition of amphibole in par- ticular depending on whether it is replacing clinopy- roxene or garnet. REE partition coecients for Cpx/Grt show a dependence on the Ca content of the host phases, with DCpx=Grt REE decreasing with decreasing DCa. This behaviour is very similar to that seen in mantle eclogites, despite di€erences in estimated temperatures of forma- tion of 650±850 °C (Dabieshan) and 1000±1200 °C (mantle eclogites). With the exception of HREE in garnet, trace elements in the eclogites are strongly dis- tributed in favour of minor or accessory phases. In particular, titanite and rutile strongly concentrate Nb and Zr, whilst LREE±MREE go largely into epidote, titanite and apatite. If these minor/accessory minerals behave in a refractory manner during melting or fluid mobilisation events and do not contribute to the melt/ fuid, then the resultant melts and fuids will be strongly depleted in LREE±MREE.