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The Composition and Origin of the Connemara Dolomitic Marbles and Ophicalcites, Ireland |
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| Authors: | LEAKE B E; TANNER P W G; SENIOR A |
| Institution: | 1Department of Geology, University Glasgow GI2 8QQ 2Department of Earth Sciences, University Leeds LS2 9JT 3Geological and Mineralogical Institute Leiden, Netherlands |
| Abstract: | The Connemara ophicalcites and associated marbles contain varyingproportions of calcite, dolomite, tremolitic amphibole, serpentinizedolivine, diopside, humite, phlogopite, clinochlore, and quartz.They formed from a chemically precipitated ‘primary’dolomite with a small amount of mica and clay minerals in whichmuch 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, possiblyduring metamorphism, and was probably not deposited with majoradmixed detrital quartz or feldspar. The formation during metamorphismof complementary segregated layers rich in either olivine (Fo98?2)or calcite resulted from important chemical changes controlledby the composition of the stable metamorphic minerals, i.e.those for which solution concentrations exceeded the solubilityproduct for the pore fluid. The mineralogy influenced the localconcentrations of both major and trace elements and emphasizesthe importance of solutions and the stable metamorphic mineralogyin manipulating the composition of some metamorphic rocks. Somecriteria for recognizing segregated layers in metamorphic rocksare given. Serpentinization was probably by addition of water and silicaand not by movement of Fe or Mg. Chemical analyses of forty-threesamples each for twenty-six oxides and elements are given andthe first occurrence of humite in Connemara is reported. |
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