Fluid flow in regional metamorphism must, in general, be of the single-pass type; shallow-level contact phenomena may, on the other hand, involve circulation, provided depths are shallow enough for the strengths of the rocks to hold open pores and fractures. The fluid-rock ratios and fluid compositions observed in metacarbonates may readily be generated within the metamorphic pile if the limestones amount to <5-10% of the sequence by volume, a proportion not unusual for metamorphic terrains. Despite the low volumes of H2O generated during prograde metamorphism of pelitic rocks (approx 50 cm3/380 cm3 of rock), metapelites should record very high fluid-rock ratios; this is illustrated by the deposition of quartz in a single-pass fluid-flow system. Assuming equilibrium crystallization in a typical geothermal gradient and taking regionally-metamorphosed sequences 6-10 km thick, approx 1 vol.% of quartz should be deposited in an average pelitic rock by upward-flowing fluid; this is equivalent to a fluid-rock volume ratio of 6:1 calculated in the conventional manner.-R.A.H. Dept. of Geological Sciences, Northwestern Univ., Evanston, IL 60201, USA.
|Number of pages||20|
|Publication status||Published - 1986|