A discussion of the Jahns–Burnham proposal for the formation of zoned granitic pegmatites using solid-liquid-vapour inclusions from the Tanco Pegmatite, S.E. Manitoba, Canada

Abstract

ABSTRACTJahns and Burnham (1969) proposed that the internal evolution of zoned granitic pegmatites could be explained by crystallisation from water-saturated melts which evolved to produce systems with a melt plus a separate aqueous fluid. Examination of microthermometric properties, chemical compositions and gas contents of solid-liquid-vapour inclusions from a number of the zones of the Tanco rare element granitic pegmatite places constraints on fluid evolution within the framework of the crystallisation history of the pegmatite, and contributes to an examination of the Jahns–Burnham proposal.Initial crystallisation at Tanco was from the wall rock inwards, producing the relatively unfractionated wall zone (potassium feldspar–quartz-albite-muscovite). Textural evidence, and an upward increase in the level of geochemical fractionation, indicate that much, but not all, of the subsequent crystallisation of the pegmatite was from the base upwards. Inclusions trapped by wall zone and metasomatic wall rock tourmaline indicate that the pegmatite was intruded as a 2 phase alumino-silicate melt/fluid mixture at ∼720°C, with an initial fluid composition of ∼98mol.% H2O (containing 2 equiv. mo1% NaCl) and <2mol% CO2 (containing <5 equiv. mo1% CH4). These observations indicate that both melt and fluid were present from the start of crystallisation (Jahns & Burnham 1969), but show that CO2 and dissolved salts were important additional components of the fluid phase. The bulk of the pegmatite then crystallised in the range 600-470°C from melts and fluids with continued low levels of CO2 (2-3mol.%) and approximately constant salinity (∼7 equiv. wt.% NaCl dissolved in the aqueous phase). Crystal-rich inclusions, which may represent trapped alumino-silicate melts, are present throughout pegmatite crystallisation down to temperatures as low as ∼262°C. The final stages of crystallisation resulted in the formation of the beryl fringe at 291 ± 33°C and the lower part of the quartz zone at 262 ± 29°C. By the later stages the fluid had cooled through an H2O-CO2– dissolved salt solvus resulting in H2O-CO2 phase separation. Gas chromatographic analysis of the fluid components in the vug quartz, beryl fringe and lower part of the quartz zone shows that the inclusions contain H2O, CO2, CH4, N2, CO, Ar, and trace C2H6 in the beryl fringe. Measured CH4:CO2 ratios of 0·0060 (±0·0015) for the beryl fringe (twenty crushes on five samples) and 0·0042 (±0.0021) for the quartz zone (thirty crushes on six samples) yield fO2 estimates of 1×10−36 and 2 × 10−38, respectively, which are just above QFM at these temperatures.