The Composition and Origin of the Connemara Dolomitic Marbles and Ophicalcites, Ireland

Abstract

The Connemara ophicalcites and associated marbles contain varying proportions of calcite, dolomite, tremolitic amphibole, serpentinized olivine, diopside, humite, phlogopite, clinochlore, and quartz. They formed from a chemically precipitated 'primary' dolomite with a small amount of mica and clay minerals in which much of the trace and minor element content of the rock, e.g. Al, Fe, Ti, Zn, Ni, Cr, Zr, K, Rb, and rare earths was concentrated. The rock was probably silicified after deposition, possibly during metamorphism, and was probably not deposited with major admixed detrital quartz or feldspar. The formation during metamorphism of complementary segregated layers rich in either olivine (Fo98±2) or calcite resulted from important chemical changes controlled by the composition of the stable metamorphic minerals, i.e. those for which solution concentrations exceeded the solubility product for the pore fluid. The mineralogy influenced the local concentrations of both major and trace elements and emphasizes the importance of solutions and the stable metamorphic mineralogy in manipulating the composition of some metamorphic rocks. Some criteria for recognizing segregated layers in metamorphic rocks are given. Serpentinization was probably by addition of water and silica and not by movement of Fe or Mg. Chemical analyses of forty-three samples each for twenty-six oxides and elements are given and the first occurrence of humite in Connemara is reported.