ORIGIN OF MAGMATIC ENCLAVES FROM THE "YOUNGER" GRANITES OF THE COASTAL MAINE MAGMATIC PROVINCE

Magmatic enclaves are common in granites of the coastal Maine magmatic province (CMMP). In the older (Silurian) granite/gabbro complexes, enclaves have been attributed to hybridization. This process occurred when mafic magma was episodically injected into the crystallizing felsic magma chamber forming Mafic and Silicic Layered Intrusions or MASLI (Wiebe, 1993). Enclaves are also common in the younger (Devonian) coarse-grained granites of the CMMP. However, their origin is problematic, as the spectacular mingling zones are not observed in the younger suite at their present erosion level.

Petrographic and geochemical studies of enclaves from examples of the younger Mt. Waldo and Deer Isle plutons reveal many similarities with enclaves from the Vinalhaven pluton, a classic MASLI. For example, similar enclave populations are observed within each pluton - small (<2.5cm) micro-enclaves, fine- to medium- grained diorites and fine–grained leucocratic granites and porphyries. The latter types are often porphyritic with phenocrysts similar to those of the host granite. Disequilibrium textures such as rapakivi feldspars, resorbed quartz and hornblende ocellar rims are also common in the enclaves. Likewise they have similar major and trace element compositions and display comparable trends on Harker diagrams. The diorites span the range of intermediate compositions with the leucogranites and porphyries overlapping with the least evolved granites of each pluton. These trends are consistently linear with very high correlations. Geochemical data for host/enclave paired samples will enable a critical assessment of the degree of interaction between the enclaves and their granitic hosts.

There is a distinct similarity in petrography and geochemistry between enclaves within the Vinalhaven granite and hybrid rocks developed at the co-mingled zones in the lower portions of this pluton. A similar origin for enclaves from the Mt Waldo and Deer Isle plutons is supported by the many intra-pluton similarities outlined here. The dissemination of the enclaves throughout these plutons may be due to convective circulation in response to influxes of mafic magma.