READING THE PLAGIOCLASE RECORD

Plagioclase crystals that exhibit a wide variety of zoning patterns are found in granite and mafic enclaves from the Deer Isle and Mt. Waldo plutons, coastal Maine. As the An content of plagioclase is a function of the chemical composition and the physical conditions (P, T, f_H2O) of the magma in which it grew, zoning may be used to assess the relative importance of magma chamber processes such as convection and magma recharge and mixing. Simple normal zoning, with superimposed oscillatory zones, is the rule for plagioclase from both plutons. Typical compositions vary between ~An_25-15. Oscillatory zones are narrow, with minor variations in An content (£5%) and are generally euhedral. However, some grains contain discordant zones. These are primarily either irregular, calcic (£An₅₈), embayed cores with patchy zoning, or discordant, calcic (£An₅₆), inclusion bearing zones that truncate oscillatory zones. Discordant zones are bounded by sharp discontinuities across which there is substantial change in An content. Normal zoning is produced as crystallization proceeds with falling temperature and when magma is relatively stagnant. Oscillatory zones develop in boundary layers around growing crystals under relatively static conditions. Rapid growth depletes the boundary layer in necessary components faster than they are supplied by diffusion through the melt, resulting in alternating periods of growth and no growth and/or dissolution. Discordant zones are the result of distinct dissolution events followed by renewed growth under different physical conditions. Dissolution may be accomplished either by decompression of rising magma, cycling through convections cells in a magma chamber, or by heating as more mafic magma is injected into the chamber. Synplutonic mafic dikes (SiO₂ <52%), composite dikes (mafic component SiO₂ <52%), and enclaves with calcic groundmass plagioclase (cores £An₅₆), provide clear evidence of mafic inputs that brought sufficient heat to cause plagioclase dissolution. Plagioclase crystals near mafic inputs would undergo dissolution, whereas crystals elsewhere in the chamber would not. Juxtaposition of crystals with markedly different zonation within the same rock indicates that these granites are cumulates.