Titre du document / Document title

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?

Auteur(s) / Author(s)

GREW E. S. ; RAO A. T. ; RAJU K. K. V. S. ; HEJNY C. ; MOORE J. M. ; WATERS D. J. ; YATES M. G. ; SHEARER C. K. ;

Résumé / Abstract

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. Hemo-ilmenite, 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.

Revue / Journal Title

Mineralogical Magazine   ISSN 0026-461X   CODEN MNLMBB 

Source / Source

2003, vol. 67, n^o5, pp. 1081-1098 [18 page(s) (article)]

Langue / Language

Anglais

Editeur / Publisher

Mineralogical Society, London, ROYAUME-UNI  (1969) (Revue)

Localisation / Location

INIST-CNRS, Cote INIST : 3052, 35400011610004.0150