Diffusion-controlled metamorphic reaction textures in an ultrahigh-pressure impure calcite marble from Dabie Shan, China.

The overall metamorphic reaction in an impure calcite marble from the ultra-high pressure (UHP) zone at Changpu, eastern Dabie Shan, China, is partitioned into local reaction domains defined by isolated grains ofomphacite, garnet, epidote, quartz and ilmenite within the calcite matrix. These reaction domains witness different stages of the metamorphic evolution. Chemically homogeneous omphacite in some instances has partial or complete coronas of quartz which are interpreted as a prograde phenomenon, i.e. caused by replacement of a plagioclase precursor by omphacite with increasing pressure. Rare relics of talc in epidote indicate that the first breakdown reaction during exhumation was talcþomphaciteþgarnet¼amphiboleþepidotequartz (still within the eclogite facies). During this early retrogression stage, omphacite was partially or completely rimmed by chemically heterogeneous amphibole. Most garnets developed a thick corona of epidote plus some quartz and calcite during this early retrogression or are completely pseudomorphed. Where garnet occurs near or next to omphacite, this corona consists partly or entirely of amphibole. The second retrograde stage is commonly characterized by the breakdown of omphaciteþquartz to albiteþdiopsideþamphibole, often in the form of symplectite. Localized reactions become evident as omphacite at its site can decompose to three distinct types of symplectite (albite þ diopside, albite þ amphibole, albite þ diopside þ amphibole). Matrix quartz grains develop rims of individual diopside crystals at distances up to 2–3 millimetres from symplectites; additional outer rims of albite develop adjacent to garnet or corona epidote. Matrix quartz remote from omphacite sites is locally transformed into albite, where an Al-bearing mineral (mostly epidote) is found nearby. Epidote in contact with quartz develops albite coronas as it decomposes into albite þ minute grains of Cerich epidote (allanite). Primary epidote and corona epidote remote from any quartz develop a complex compositional zoning with enrichment of mainly Ce3þ at the outer rim, indicating exposure to a fluid undersaturated in alumina. Finally, large ilmenite grains in the matrix develop thick overgrowths of titanite, but in several instances an intermediate step of rutile formation is preserved. All these features indicate a direct correlation of textures with transport distances of dissolved chemical species in a non-pervasively infiltrating pore fluid. Simplified open-system reactions were derived for each site at each metamorphic stage and show that Al must have been quite mobile during prograde quartz corona formation, whereas both Si and Al were relatively immobile during the two retrograde stages – the relative sequence of mobility being Naþ . Mg2þ, Ca2þ, Fe2þ .. Si4þ . Al3þ, Ti4þ. Mass-balance between sites is consistent and no significant gain or loss of cations is indicated.