Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

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Petrogenetic Significance of Zn Distribution, and of Zn/Fe, Zn/Pb Ratios in Granitic Biotites


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1 Atomic Minerals Division, Department of Atomic Energy, New Delhi, India
     

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The zinc content of biotite is a potentially useful petrogenetic tracer to determine if a granitic pluton formed by magmatism (crystallization-differentiation), metasomatism, or anatexis. This conclusion stems from a geochemical study of Zn distribution, and of Zn/Fe, Zn/Pb ratios in granitic biotites from the 2.7 billion year old Giants Range batholith of northeastern Minnesota, U.S.A. Results show that the zinc contents of biotites from metasomatic granitoids (477 ppm), anatectic tonalites (498 ppm) and their parent metasedimentary rocks (498 ppm) are significantly lower than those characterizing magmatic granites (754 ppm) and magmatic leucogranites (943 ppm). Also, in a magmatically fractionated series of granit c rocks, the Zn contents and Zn/Fe ratios of biotires increase progressively in the sequence tonalite-granodiorite-adamellite-graniteleucogranite, whereas the Zn/Pb ratios decrease.
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  • Petrogenetic Significance of Zn Distribution, and of Zn/Fe, Zn/Pb Ratios in Granitic Biotites

Abstract Views: 182  |  PDF Views: 2

Authors

S. Viswanathan
Atomic Minerals Division, Department of Atomic Energy, New Delhi, India

Abstract


The zinc content of biotite is a potentially useful petrogenetic tracer to determine if a granitic pluton formed by magmatism (crystallization-differentiation), metasomatism, or anatexis. This conclusion stems from a geochemical study of Zn distribution, and of Zn/Fe, Zn/Pb ratios in granitic biotites from the 2.7 billion year old Giants Range batholith of northeastern Minnesota, U.S.A. Results show that the zinc contents of biotites from metasomatic granitoids (477 ppm), anatectic tonalites (498 ppm) and their parent metasedimentary rocks (498 ppm) are significantly lower than those characterizing magmatic granites (754 ppm) and magmatic leucogranites (943 ppm). Also, in a magmatically fractionated series of granit c rocks, the Zn contents and Zn/Fe ratios of biotires increase progressively in the sequence tonalite-granodiorite-adamellite-graniteleucogranite, whereas the Zn/Pb ratios decrease.