HYDROUS MINERAL INCLUSIONS IN RELICT PLAGIOCLASE OF CORONITIC, GRANULITE-FACIES METATROCTOLITES FROM THE BLUE RIDGE PROVINCE, WESTERN NORTH CAROLINA: ARE THEY LAWSONITE AND WHAT IS THEIR SIGNIFICANCE?

A few small grains and patches (10-50 µm in diameter) of a hydrous Ca-Al-silicate were detected in relict plagioclase from two samples of coronitic, granulite-facies metatroctolite from the Buck Creek ultramafic body in the Blue Ridge province of western North Carolina.  Layered coronas consisting of Opx, Cpx/Spl symplectite coexisting with Amp/Spl symplectite, and Cpx/sapphirine symplectite (in one sample) between olivine and plagioclase are well preserved in these samples. Although H₂O was involved in high P-T, granulite facies metamorphism, these samples were little affected by late, low P-T hydrous alteration. The hydrous Ca-Al-silicate contains 37-38 wt% SiO₂, 30-31.5 wt% Al₂O₃, 13-15.5 wt% CaO, 0.6-1.0 Na₂O and <0.1% other oxides (molar Ca:Na:Al:Si=0.9:0.1:2:2).  Although oxide totals (84%) of preliminary analyses are low and Na is higher than has previously been reported for lawsonite, lawsonite is the mineral that best fits the analyses. Optical properties are consistent with lawsonite, but not definitive.  Preliminary attempts to identify the small crystals by micro-XRD have so far been unsuccessful. 

Previous studies have determined that Buck Creek troctolites experienced granulite facies metamorphism at 825°C at pressure as high as 14 kilobars (Tenthorey, et al., 1996, J. Met. Geol., 14, 103-114; Emilio, 1998, M.S. thesis, U. South Florida), followed by hydration and cooling that began at relatively high pressure.  Available experimental data indicate that the minimum pressure at which lawsonite is stable in the CASH system is approximately 8 kilobars at 400°C, 14 kilobars at 500°C, 22 kilobars at 600°C and 32 kilobars at 700°C (Schmidt, 1995, Am. Min., 80, 1286-1292).  If these inclusions are confirmed to be lawsonite, they would require that the Buck Creek metatroctolites cooled nearly isobarically, with some H₂O present, following granulite facies metamorphism, before uplift exposed them at the surface. Retrograde amphibolite facies assemblages in the Buck Creek troctolites and surrounding amphibolites require that the P-T path also passed through the amphibolite facies.  Whatever the identity of the hydrous silicate inclusions, they provide additional constraints on the P-T path.