SEQUENCE OF BOROSILICATE CRYSTALLIZATION OF A LATE STAGE PEGMATITE FROM THE LARSEMANN HILLS, PRYDZ BAY, EAST ANTARCTICA

The Larsemann Hills are underlain by medium-pressure granulite-facies rocks cut by several generations of anatectic pegmatites. The D₄ generation differs from previous generations in being planar and only tens of centimeters thick. The host metasedimentary rocks contain up to 10,000 ppm B. Using both electron microprobe analysis and optical petrography, we determined a sequence of crystallization of one pegmatite. The borosilicate minerals that are key to this investigation are tourmaline, boralsilite (Al₁₆B₆Si₂O₃₇), and dumortierite ((Al,,Ti)Al₆(BO₃)Si₃O₁₃(O,OH₂). Preliminary results show a wide range in composition in both tourmaline and dumortierite, for example, tourmaline ranges in composition from schorl-dravite, (Na_0.54,K_0.01,Ca_0.06,_0.39)(Fe_1.98,Mg_0.22,Al_0.66,Ti_0.03,_0.10) Al₆(Si_5.82,Al_0.18)O₁₈(BO₃)₃(OH_3.86,F_0.09,Cl_0.05), to foitite – olenite (Na_0.37,Ca_0.03,_0.59)(Fe_2.16,Mg_0.03,Al_0.72,_0.08) Al₆(Si_5.99,Al_0.01)O₁₈(BO₃)₃(OH_3.9,F_0.05,Cl_0.05). Primary dumortierite forms prisms and contains low amount of As + Sb + Nb (0.13 wt% average as oxide) whereas secondary dumortierite, commonly found in fractures, is more massive or fibrous, and contains higher concentrations of As + Sb + Nb (0.48 wt% average as oxide). Compositions of dumortierite and tourmaline vary with texture and distance from other minerals, notably K-feldspar. Dumortierite in close proximity to K-feldspar contains less TiO₂ (0.00 – 0.84 wt%) than dumortierite isolated from K-feldspar (2.45 – 5.45 wt%). Tourmaline is found with three distinct textures: large crystals with primary oscillatory zoning, in a graphic intergrowth with primary quartz and as a distinctly secondary phase replacing boralsilite. Both the primary and secondary tourmaline show a trend of increasing Al content with increasing proximity to K-feldspar. Boralsilite in splays, bundles or individual prisms surrounded by quartz is fractured and partially replaced by tourmaline, whereas boralsilite found in K-feldspar is mostly replaced by unidentified phyllosilicate. On the basis of microstructural and chemical analysis, we conclude that after primary mineral growth the pegmatite was altered by fracturing and fluids, resulting in secondary growth of tourmaline and dumortierite, together with replacement of boralsilite.