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Some metasomatic calc-magnesium silicate rocks from Connemara, Western Ireland: mineralogical control of rock composition

Leake, Bernard Elgey 1980. Some metasomatic calc-magnesium silicate rocks from Connemara, Western Ireland: mineralogical control of rock composition. American Mineralogist 63 , pp. 26-36.

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Abstract

Chemical analyses for 33 major oxides and minor elements are reported for a series of 26 associated metasediments that range from staurolite-garnet schists through similar pelites with graphite to schists with varying contents of actinolitic amphibole and diopside-rich calcmagnesian silicate rocks. The calc-magnesian silicate rocks cannot be matched with any likely original sediment and have probably formed by metasomatism of pelites by processes similar to those that produce rodingites. The presence of graphite, gradation into pettes, and the positive correlation of Ti and Niggli al-alk support a part sedimentary parentage for the calc-magnesian rocks. ca, Mg, cr, and Ni (maxima l3.7vo CaO,24.2Vo MgO,247O ppm ct atd 620 ppm Ni) were added from solutions derived from nearby ultramafic rocks serpentinized during static regional metamorphism, which caused andalusite and cordierite growth in some of the associated pelites. Although the original sedimentary composition and the composition of the metasomattzitg fluids were important in determining the chemical composition of the metasomatic rocks, the stable amphibole and pyroxene mineralogy in the calc-magnesian silicate rocks was no less important. These rocks are an example of the partial mineralogical control of the composition of metamorphic rocks. It is proposed that a characteristic feature found in many sedimentary rocks, but not in igneous rocks, is a marked positive correlation of Ti and Niggli al-alk.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Ocean Sciences
Subjects: Q Science > QE Geology
Publisher: Mineralogical Society of America
ISSN: 0003-004X
Last Modified: 04 Jun 2017 08:16
URI: http://orca-mwe.cf.ac.uk/id/eprint/75228

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