RETROGRADE METAMORPHIC REACTIONS IN DEFORMING GRANITES AND THE ORIGINOF FLAME PERTHITE

Microprobe analyses of feldspars in granite mylonites containing flame perthite give compositions that invariably plot as three distinct clu sters on a ternary feldspar diagram: orthoclase (Or(92-97)), albite an d oligoclase-andesine. The albite occurs as grains in the matrix, as f lame-shaped lamellae in orthoclase, and in patches within plagioclase grains. We present a metamorphic model for albite flame growth in the K-feldspar in these rocks that is related to reactions in plagioclase, rather than alkali feldspar exsolution. Flame growth is attributed to replacement and results from a combination of two retrograde reaction s and one exchange reaction under greenschist facies conditions. React ion 1 is a continuous or discontinuous (across the peristerite solvus) reaction in plagioclase, in which the An component forms epidote or z oisite. Most of the albite component liberated by Reaction 1 stays to form albite in the host plagioclase, but some Na migrates to form the flames within the K-feldspar. Reaction 2 is the exchange of K for Na i n K-feldspar. Reaction 3 is the retrograde formation of muscovite (as 'sericite') and has all of the chemical components of a hydration reac tion of K-feldspar. The Si and Al made available in the plagioclase fr om Reaction 1 are combined with the K liberated from the K-feldspar, t o produce muscovite in Reaction 3. The muscovite forms in the plagiocl ase, rather than the K-feldspar, as a result of the greater mobility o f K relative to Al. The composition of the albite flames is controlled by both the peristerite and the alkali feldspar miscibility gaps and depends on the position of these solvi at the pressure and temperature that existed during the reaction. Using an initial plagioclase compos ition of An,,, the total reaction can be summarized as: 20 oligoclase + 1 K-feldspar + 2 H2O = 2 zoisite + muscovite + 2 quartz + 15 albite( plagioclase) + 1 albite(flame). This model does not require that any a dditional feldspar framework be accreted at replacement sites: Na and K are the only components that must migrate a significant distance (e. g. from one grain to the next), allowing Al to remain within the alter ing plagioclase grain. The resulting saussuritization is isovolumetric . The temperature and extent of replacement depends on when, and how m uch, water infiltrates the rock. The fugacity of the water, and theref ore the pressure of the fluid, may have been significantly lower than lithostatic during flame growth.