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

S. Mitra ¹, F. Princivalle ², A. K. Samanta ¹, Hi-Soo Moon ³

Affiliations
1 Department of Geological Sciences, Jadavpur University, Calcutta - 700032, IN
2 Dipartimento di Scienze della Terra, Trieste University, Via Weiss 8, 34127 Trieste, IT
3 Department of Geology, Yonsei University, Seoul 120-749, KP

Abstract

Fe-Mg ordering as well as Fe²⁺ and Fe³⁺ distribution at different sites in coexisting orthopyroxene and clinopyroxene determined by, Mossbauer spectrpscopy and X-ray diffraction (sample 6a), have been used in deciphering the thermal history of two-pyroxene granulites of Sittampundi Complex (India). Orthopyroxenes and clinopyroxenes are compositionally in the granulite field of metamorphism. For orthopyroxenes, the Mossbauer spectra show that Fe²⁺ is present at M1 and two different M2 sites, while Fe³⁺ is localised at M1 site alone. For the coexisting clinopyroxenes, Fe²⁺ occurs at both M1 and M2. Fe³⁺ is present only at M1 site, but in Ca-rich clinopyroxenes, under high fO₂, conditions, it is seen to replace silicon in tetrahedral site.

From the distribution of Fe²⁺ - Mg, using Mossbauer data, the blocking temperature of intracrystalline ordering in orthopyroxene has been calculated to range between 427 and 699°C, in agreement with the estimated temperature obtained by means of X-ray data of about 500°C; in clinopyroxene it was 680°C as obtained from X-ray data. The recorded inter-crystalline ordering temperature was determined as between 661 and 785°C. The lowering of blocking temperature recorded by intracrystalline Fe²⁺ - Mg exchange suggests that the cation ordering in the M1 and M2 sites continued in the cooling path of the granulites below the temperature indicated by inter-crystalline distribution, and that the cooling (history) suffered by the host rocks was slow.

Keywords

Mineralogy, Geothermometry, Granulites, Pyroxenes, Sittampundi Complex, Tamil Nadu.

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