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Prismatine and ferrohögbomite-2N2S in granulite-facies Fe-oxide lenses in the Eastern Ghats Belt at Venugopalapuram, Vizianagaram district, Andhra Pradesh, India: do such lenses have a tourmaline-enriched lateritic precursor?

¹ Department of Earth Sciences, University of Maine, 5790 Bryand Center, Orono, Maine 04469, USA
² Department of Geology, Andhra University, Visakhapatnam 530 003, India
³ School of Physics, University of Edinburgh, The King’s Buildings, Edinburgh EH9 3JZ, UK
⁴ Department of Exploration Geology, Rhodes University, P O Box 94, Grahamstown, 6140 South Africa
⁵ Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK
⁶ Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico 87131, USA

^* E-mail: esgrew{at}maine.edu

Fluorine-rich prismatine, (,Fe,Mg)(Mg,Al,Fe)₅Al₄(Si,B,Al)₅O₂₁(OH,F), with F/(OH+F) = 0.36–0.40 and hercynite are major constituents of a Fe-Al-B-rich lens in ultrahigh-temperature granulite-facies quartz-sillimanite-hypersthene-cordierite gneisses of the Eastern Ghats belt, Andhra Pradesh, India. Hemoilmenite, sapphirine, magnetite, biotite and sillimanite are subordinate. Lithium, Be and B are concentrated in prismatine (140 ppm Li, 170 ppm Be, and 2.8–3.0 wt.% B₂O₃). Another Fe-rich lens is dominantly magnetite, which encloses fine-grained zincian ferrohögbomite-2N2S, (Fe²⁺,Mg,Zn,Al)₆ (Al,Fe³⁺,Ti)₁₆O₃₀(OH)₂, containing minor Ga₂O₃ (0.30–0.92 wt.%). Fe-Al-B-rich lenses with prismatine (or kornerupine) constitute a distinctive type of B-enrichment in granulite-facies rocks and have been reported from four other localities worldwide. A scenario involving a tourmaline-enriched lateritic precursor affected by dehydration melting is our preferred explanation for the origin of the Fe-Al-B-rich lenses at the five localities. Whole-rock analyses and field relationships at another of these localities, Bok se Puts, Namaqualand, South Africa, are consistent with this scenario. Under granulite-facies conditions, tourmaline would have broken down to give kornerupine-prismatine (±other borosilicates) plus a sodic melt containing H₂O and B. Removal of this melt depleted the rock in Na and B, but the formation of ferromagnesian borosilicate phases in the restite prevented total loss of B.

KEYWORDS: prismatine, ferrohögbomite, Eastern Ghats Belt, India, Namaqualand, South Africa

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