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

Abstract

Sapphirine occurs with humite-group minerals and forsterite in Precambrian amphibole-facies rocks at Kuhi-lal, SW Pamir Mountains, Tajikistan, a locality also for talc+kyanite ± magnesiohornblende whiteschist. Most of these sapphirine-bearing rocks are graphitic and sulfidic (pyrite 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 (0-003)<phlogopite (0.004-0.005) ≦sapphirine (0.004-0.006) ≦enstatite (0-006)≦forsterite (0-006-0-007) <spinel (0-014). This assemblage includes the incompatible pair sapphirine+forsterite, but there is no textural evidence for 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 chondrodite, respectively; and 0-0-09 wt.% BeO and 0-05-0-21 wt.% B 2O 3 in sapphirine. Stabilization of sapphirine+clinohumite or sapphirine+chondrodite instead of sapphirine+phlogopite is possible at high F contents in K-poor rocks, but minor element contents appear to be too low to stabilize sapphirine as an additional phase with forsterite+enstatite+spinel. Although sapphirine+forsterite is metastable relative to spinel+enstatite in experiments conducted at a H2O=1 in the MgO-Al 2O 3-SiO 2-H 2O system, it might be stabilized at a H2O≈0.5, P≦4 kbar, T≈650-700°C. Textures in the Kuhi-lal whiteschists suggest a polymetamorphic evolution in which the rocks were originally metamorphosed at T≦650°C, P≦ 7 kbar, conditions under which sapphirine+clinohumite and sapphirine+chondrodite are inferred to 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.