SAPPHIRINE-GROUP MINERALS IN AN ULTRA-MAGNESIAN LENS FROM KUHI-LAL, SW PAMIRS, TAJIKISTAN - ARE THESE ASSEMBLAGES FORBIDDEN(FORSTERITE AND SAPPHIRINE+HUMITE)

Sapphirine occurs with humite-group minerals and forsterite in Precamb rian amphibole-facies rocks at Kuhi-lal, SW Pamir Mountains, Tajikista n, a locality also for talc+kyanite+/-magnesiohornblende whiteschist. Most of these sapphirine-bearing rocks are graphitic and sulfidic (pyr ite and pyrrhotite) and contain enstatite, clinohumite or chondrodite, spinel, rutile, gedrite, and phlogopite. A phlogopite schist has the assemblage with X(Fe)=Fe/(Fe+Mg) increasing as follows: chlorite (0.00 3)<phlogopite (0.004-0.005)less than or equal to sapphirine (0.004-0.0 06)less than or equal to enstatite (0.006)less than or equal to forste rite (0.006-0.007)<spinel (0.014). This assemblage includes the incomp atible pair sapphirine+forsterite, but there is no textural evidence f or reaction. In one rock with clinohumite, X(Fe) increases as follows: clinohumite (0.002)<sapphirine (0.003)<enstatite (0.004-0.006)<spinel (0.010). Ion microprobe and wet-chemical analyses give 0.57-0.73 wt.% F in phlogopite and 0.27 wt.% F in chlorite in the phlogopite schist; 0.04, 1.5-1.9, and 4.4 wt.% F in forsterite, clinohumite, and chondro dite, respectively; and 0-0.09 wt.% BeO and 0.05-0.21 wt.% B2O3 in sap phirine. Stabilization of sapphirine+clinohumite or sapphirine+chondro dite instead of sapphirine+phlogopite is possible at high F contents i n K-poor rocks, but minor element contents appear to be too low to sta bilize sapphirine as an additional phase with forsterite+enstatite+spi nel. Although sapphirine+forsterite is metastable relative to spinel+e nstatite in experiments conducted at a(H2O)=1 in the MgO-Al2O3-SiO2-H2 O system, it might be stabilized at a(H2O)approximate to 0.5, P less t han or equal to 4 kbar, T approximate to 650-700 degrees C. Textures i n the Kuhi-lal whiteschists suggest a polymetamorphic evolution in whi ch the rocks were originally metamorphosed at T greater than or equal to 650 degrees C, P greater than or equal to 7 kbar, conditions under which sapphirine+clinohumite and sapphirine+chondrodite are inferred t o have formed, and subsequently affected by a later event at lower P, similar T, and lower a(H2O). The latter conditions were favorable for sapphirine+forsterite to form in a rock originally containing chlorite +forsterite+spinel+enstatite.