DISTRIBUTION, PETROGRAPHY AND CHEMISTRY OF RAPAKIVI FELDSPARS IN THE MT. WALDO PLUTON, COASTAL MAINE

The Mt. Waldo pluton crops out over an area of 150 km² at the northern end of Penobscot Bay, coastal Maine. Emplaced at 371 Ma (Stewart et al., 1995), it is one the younger granitoids of the Coastal Maine Magmatic Province. Recent field, petrographic and geochemical evidence suggests a definite role for magma mixing in the evolution of the Mt. Waldo magma chamber with abundant magmatic enclaves, mafic schlieren and a diverse feldspar population including rapakivi feldspars and boxy cellular plagioclase.

Rapakivi feldspars (plagioclase mantles on K feldspar) are common throughout the Mt. Waldo granite. They occur 1) as euhedral megacrysts (up to 5 cm in length) in the granite, 2) associated with all types of magmatic enclaves either bordering them, cutting across the host/enclave contact or as phenocrysts within the enclaves, 3) as part of equigranular, feldspar cumulate autoliths (?) which often enclose enclaves and 4) aligned parallel to planar mafic schlieren. Their distribution throughout the Mt Waldo pluton is random with modal abundances ranging from 10 – 80 rapakivis per m², irrespective of height within the intrusion or relative distance from its margins. This is in sharp contrast to the Deer Isle pluton (see Terrien and Hogan, this session) where there is a systematic variation of rapakivi feldspars with position in this intrusion. Thin section petrography reveals that the plagioclase rims are formed either by a continuous zone of plagioclase (some of which exhibit oscillatory zoning) or numerous small grains upon resorbed K feldspars. These are usually < An ₂₀, which is also consistent with the outer rims of normally zoned groundmass plagioclase.

The formation of rapakivi feldspars has been assigned to a number of processes such as 1) decompression during magma ascent, 2) syneusis 3) megacrystic growth and 4) magma mixing. The distribution of rapakivs in the Mt. Waldo pluton and their composition may suggest that they were formed as a result of repeated thermal inputs at lower levels in this magma chamber. An exposed example of a rapakivi forming environment is present in the co-mingled zones observed in the lower parts of the Vinalhaven pluton (Wiebe and Hawkins, 2000) where rapakivi feldspars are particularly abundant. Rapakivi feldspars (and enclaves) were subsequently distributed throughout the chamber by thermally induced convection.