WERDINGITE, A BOROSILICATE NEW TO GRANITIC PEGMATITES

Werdingite, (Mg,Fe)(2)Al-12(Al,Fe)(2)Si4B2(B,Al)(2)O-37, occurs in gra nitic pegmatites cutting granulite-facies metapelitic rocks at Cap And rahomana, near Fort Dauphin: southeastern Madagascar (type locality of grandidierite), and at Almgjotheii, in the aureole of the Rogaland an orthosite complex, southwestern Norway. Pegmatitic werdingite is beryl lian (0.19-0.55 wt% BeO by ion microprobe). It varies in Al and Si con tents; a more general formula could be 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 sam ple from Andrahomana and in all samples from Almgjotheii; thus these s amples are the Fe-dominant analogues of werdingite. At Cap Andrahomana , the maximum-phase assemblage can be given as quartz + K-feldspar + g randidierite + garnet + sillimanite + werdingite, although werdingite replaces sillimanite. Niobian ilmenite, hercynite, late biotite and la te andalusite also are present. At Almgjotheii, the characteristic max imum-phase assemblage prior to extensive development of tourmaline app ears to have been quartz + K-feldspar + garnet + sillimanite (andalusi te)+ grandidierite + Ti-rich dumortierite; werdingite is limited to as semblages with boralsilite (Al16B6Si2O37), grandidierite and Ti-poor d umortierite, and formed later than grandidierite. Conditions of werdin gite formation at both localities are estimated to be T approximate to 600-700 degrees C, P = 3-4 kbar, P(H2O) < 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 granulit e-facies pelitic rocks, which are generally depleted in B, can be expl ained by a scenario involving remobilization of B present in the pelit ic precursors, incorporation of the remobilized boron in partial melts , and crystallization of the melts as pegmatite veins with borosilicat es. In terms of the model system FeO-MgO-Al2O3-B2O3-SiO2 open to H2O a nd with quartz in excess, werdingite is restricted to assemblages rich er in B2O3 than the tit-plane grandidierite - sillimanite and the plan e grandidierite - sillimanite - garnet, which are stable instead of th e join werdingite - cordierite, an assemblage not yet reported in natu re or experiments. At Almgjotheii, werdingite is further restricted by the tie-plane grandidierite - dumortierite, and thus is rare in rocks with garnet and sillimanite.