Abstract

Werdingite, (Mg,Fe) 2 Al 12 (Al,Fe) 2 Si 4 B 2 (B,Al) 2 O 37 , occurs in granitic pegmatites cutting granulite-facies metapelitic rocks at Cap Andrahomana, near Fort Dauphin, southeastern Madagascar (type locality of grandidierite), and at Almgjotheii, in the aureole of the Rogaland anorthosite complex, southwestern Norway. Pegmatitic werdingite is beryllian (0.19-0.55 wt% BeO by ion microprobe). It varies in Al and Si contents; a more general formula could be (Mg,Fe,Al) 2 Al 12 (Al,Fe) 2 (Si,Al) 4 B 2 (B,Al,Be,Si) 2 O 37 . The proportion of Fe exceeds that of Mg in one sample from Andrahomana and in all samples from Almgjotheii; thus these samples are the Fe-dominant analogues of werdingite. At Cap Andrahomana, the maximum-phase assemblage can be given as quartz+K-feldspar+grandidierite+garnet+sillimanite+werdingite, although werdingite replaces sillimanite. Niobian ilmenite, hercynite, late biotite and late andalusite also are present. At Almgjotheii, the characteristic maximum-phase assemblage prior to extensive development of tourmaline appears to have been quartz+K-feldspar+garnet+sillimanite (andalusite)+grandidierite+Ti-rich dumortierite; werdingite is limited to assemblages with boralsilite (Al 16 B 6 Si 2 O 37 ), grandidierite and Ti-poor dumortierite, and formed later than grandidierite. Conditions of werdingite formation at both localities are estimated to be T nearly equal 600-700 degrees C, P nearly equal 3-4 kbar, P(H 2 O)<P total , that is, at lower temperature than at the peak of metamorphism that affected the country rocks. The appearance of borosilicates in pegmatites cutting granulite-facies pelitic rocks, which are generally depleted in B, can be explained by a scenario involving remobilization of B present in the pelitic precursors, incorporation of the remobilized boron in partial melts, and crystallization of the melts as pegmatite veins with borosilicates. In terms of the model system FeO - MgO - Al 2 O 3 - B 2 O 3 - SiO 2 open to H 2 O and with quartz in excess, werdingite is restricted to assemblages richer in B 2 O 3 than the tie-plane grandidierite-sillimanite and the plane grandidierite-sillimanite-garnet, which are stable instead of the join werdingite-cordierite, an assemblage not yet reported in nature or experiments. At Almgjotheii, werdingite is further restricted by the tie-plane grandidierite-dumortierite, and thus is rare in rocks with garnet and sillimanite.

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