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Mineralogy, petrology, and phase relations of glaucophane-lawsonite zone blueschists from the Tavşanli Region, Northwest Turkey

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Abstract

Glaucophane-lawsonite facies blueschists representing a metamorphosed sequence of basic igneous rocks, cherts and shales have been investigated northeast of the district of Tavşanli in Northwest Turkey. Sodic amphiboles are rich in magnesium reflecting the generally high oxidation states of the blueschists. Lawsonite has a very uniform composition with up to 2.5 wt.% Fe2O3. Sodic pyroxenes show an extensive range of compositions with all the end-members represented. Chlorites are uniform in their Al/(Al+Fe+Mg) ratio but show variable Fe/ (Fe+Mg) ratios. Garnets from metacherts are rich in spessartine (>50%) whereas those from metabasites are largely almandine. Pistacite rich epidote is found in metacherts coexisting with lawsonite. Phengites are distinctly higher in their Fe, Mg and Si contents than those from greenschist facies. Hematites with low TiO2 are ubiquitous in metacherts.

Fe2+/Mg partitioning between chlorite and sodic amphibole is strongly controlled by the calcium content of the sodic amphibole and ranges from 1.1 for low calcium substitution to 0.8 for higher calcium substitution. The Al/Fe3+ partition coefficient between sodic amphibole and sodic pyroxene is 2.1.

A model system has been constructed involving projections from lawsonite, iron-oxide and quartz onto a tetrahedron with Na, Al, Fe2+ and Mg at its apices. Calcite is treated as an indifferent phase. The model system illustrates the incompatibility of the sodic pyroxene with chlorite in the glaucophanelawsonite facies; this assemblage is represented by sodic amphibole.

Sodic amphibole compositions are plotted in terms of coexisting ferromagnesian minerals. Five major areas on the sodic amphibole compositional field are delineated, each associated with one of the following minerals: chlorite, stilpnomelane, talc, almandine, deerite.

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  1. Department of Mineralogy and Petrology, Downing Place, CB2 3EW, Cambridge, England

    A. I. Okay

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Okay, A.I. Mineralogy, petrology, and phase relations of glaucophane-lawsonite zone blueschists from the Tavşanli Region, Northwest Turkey. Contr. Mineral. and Petrol. 72, 243–255 (1980). https://doi.org/10.1007/BF00376143

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