KORNERUPINE PARAGENESES IN WHITESCHISTS AND OTHER MAGNESIAN ROCKS - IS KORNERUPINE PLUS TALC A HIGH-PRESSURE ASSEMBLAGE EQUIVALENT TO TOURMALINE PLUS ORTHOAMPHIBOLE

Kornerupine, (square,Fe,Mg)(Mg,Fe,Al)(9)(Si,Al,B)(5) (O,OH,F)(22), has been reported with talc in rocks from six localities worldwide, but o nly at Chilapila Hill in the Lufilian Are, Zambia do textural relation ships imply that kornerupine (Krn) equilibrated with talc (Tlc) during a prograde metamorphic event at T approximate to 640 degrees C, P app roximate to 13 kbar: a prograde Km + Tlc assemblage has also been repo rted from Mautia Hill, Tanzania (P I 13 kbar). In order to estimate po ssible constraints on the stability range for the kornerupine + talc p aragenesis in nature, we constructed a P-T diagram in the model system MgO-Al2O3-SiO2-H2O (MASH) for seven phases quartz (Qtz), B-free korne rupine sensu stricto, anthophyllite (Ath), chlorite (Chl), cordierite (Crd), kyanite (Ky), and talc. The minimum pressure for Krn + Tlc + Ky stability in MASH is close to that for Ky + Tlc stability, i.e., 6-8 kbar, at T less than or equal to 780 degrees C. However, in the natura l system, B2O3 and Na2O are major constituents in Krn and orthoamphibo le (Oam), respectively, and dravitic tourmaline (Tur) is widespread. T he critical assemblage alternative to Krn + Tlc in nature is Tur + Oam . The upper pressure limit of Tur + Ath is determined by the upper pre ssure for anthophyllite: 7.7-10.5 kbar at 682-794 degrees C in the MgO -SiO2-H2O system (Chernosky et al. 1985, Am Mineral 70:223-236), and i s undoubtedly higher in the presence of Na2O, CaO, and Al2O3 At three of the six localities, talc is a retrograde phase; nonetheless, it pos sibly equilibrated with kornerupine on the retrograde path or during a later metamorphic event at P-T conditions appropriate for Ky + Tlc. A t the sixth locality (Mulvoj, southwestern Pamir Mountains, Tajikistan ), Krn is found in the same thin section as talc and kyanite and all t hree minerals formed during a prograde metamorphic event at T greater than or equal to 650 degrees C, P near 7 kbar. However, Krn is restric ted to a lens 4 to 6 mm thick of phlogopite + anthophyllite + Tur and it does not touch either talc or kyanite. A reaction relating the Mulv oj and Chilapila Hill (Krn + Tlc + Ky + Qtz + Tur) parageneses is calc ulated from compositions in the Mulvoj rock to be 0.40Tur + 2.55Ath 1.33H(2)O + 0.27F = Krn + 2.16Tlc + 0.36B(2)O(3) + 0.02Rutile + 0.19Na (2)O + 0.17CaO. Given the difference in metamorphic pressures estimate d for Mulvoj and Chilapila Hill, Krn + Tlc is inferred to be favored b y increasing pressure as well as by low Na2O and CaO contents. Some Fe O, F, Fe2O3, and BeO are present in measurable amounts in at least one of the phases in the Mulvoj and Chilapila Hill whiteschists (e.g., Km contains 0.24-0.67 wt% BeO), but the effect of these constituents is subordinate to that of Na2O, CaO and B2O3 The Km + Tlc could be a more important assemblage in B-bearing whiteschists than has been reported to date, particularly at pressures where orthoamphibole is no longer stable.