PLAGIOCLASE MANTLED ALKALI FELDSPARS – A RECORD OF THERMAL PERTURBATIONS IN FELSIC MAGMA CHAMBERS

Plagioclase mantled alkali feldspars (PMAF) or “rapakivi” are a distinctive characteristic of many granites. Their occurrence has attracted considerable attention leading to a plethora of models for their origin. Two processes applicable to Maine granites are: 1) Alkali feldspar instability due to decompression (e.g. Stewart and Roseboom, 1962; Nekvasil 1991) and 2) Alkali feldspar instability due to to compositional and thermal changes as a result of mixing between mafic and felsic magmas (Hibbard, 1981; Stimac & Wark, 1992). In the oxidized granites of the Deer Isle pluton PMAF are “sunny-side up” - salmon-red alkali-feldspar centers and cream colored plagioclase rims. PMAF are interspersed with euhedral non-mantled alkali feldspars requiring a process that selectively corrodes and mantles some grains and not others. Plagioclase mantles, continuous or multigrain, vary in thickness (typically 1-2mm) and are ~An_20-25. PMAF decrease in abundance, from the cumulate base (Flye Point Facies ~615 PMAF/m²)  towards felsic cap of the intrusion (Crotch Island Facies ~71 PMAF/m²). Scarce PMAF occur in felsic and mafic enclaves. Mafic enclaves and synplutonic mafic dikes also decrease in abundance towards the top of the pluton. Hbl-Plag geothermobarometry also records a decrease in P & T from the base (~2.5 kb & 710^oC) towards the top of the chamber where values < 1.0 kb & 650^oC suggest subsolidus reqeuilibration consistent with petrographic observations. Hbl-Plag within fine-grained enclaves from the upper portions of the pluton yields P-T values of 2.75 kb & 740^oC and preserves early magmatic conditions. If PMAF in the Deer Isle record decompression they were already present in great abundance in the rising magma and formed prior to hbl crystallization and prior to emplacement. Alternatively, PMAF formed in situ near the base of the chamber in response to local increases in T brought on by mafic replenishments. The scarcity of PMAF in mafic enclaves necessitates PMAF formation within felsic magma then entrainment in mafic magma. PMAF and mafic enclaves are subsequently mixed in with “naked” alkali feldspar grains prior to completion of crystallization. Thus, PMAF may record T increases associated with their traversing from cooler to hotter portions of the chamber or with mafic replenishments even if other evidence for mafic inputs in the granite is absent.