EVIDENCE FOR MAFIC INPUTS IN DEVONIAN PLUTONS OF THE COASTAL MAINE MAGMATIC PROVINCE

The coastal Maine magmatic province (CMMP) consists of Middle Paleozoic age gabbro and granite plutons with few rocks of intermediate composition. Coarse-grained granites comprise the younger Devonian age suite. An older Silurian suite, consisting of granite/gabbro complexes, emplaced at low pressure, is associated with volcanic rocks. Intermediate enclaves and disequilibrium mineral textures such as rapakivi alkali feldspars, embayed/ocellar quartz, and complexly zoned plagioclase are found in many CMMP granites. Field relationships within some CMMP intrusions, Pleasant Bay, Cadillac Mt. and Vinalhaven, suggest mafic and silicic magmas interacted during their formation (Wiebe, 1993). Silicic magma chambers were repeatedly injected with mafic magmas forming Mafic and Silicic layered intrusions (MASLI).

Within the lower portions of the Vinalhaven pluton, porphyries of intermediate composition are found mingled with fine-grained mafic rocks. These hybrid rocks contain complexly zoned plagioclase, rapakivi alkali feldspar and resorbed quartz, some with hornblende ocellar rims. The grain size is similar to crystals in the coarse-grained Vinalhaven granite, however disequilibrium textures are more scarce in the granite. We suggest these hybrid rocks formed when mafic magmas partially dissolved silicic cumulates at the base of the chamber. Some enclaves within the coarse-grained Vinalhaven granite have textures that are remarkably similar to these hybrid porphyries. It is likely these are disrupted hybrids that were distributed throughout the chamber, perhaps during volcanic eruptions.

Intermediate enclaves along with rapakivi alkali feldspar and complexly zoned plagioclase are found within the younger coarse-grained Mt. Waldo and Deer Isle plutons. We suggest these intermediate enclaves and the disequilibrium textures also formed by the interaction of mafic magmas and silicic cumulates at the base of these chambers. Their presence is taken as (cryptic) evidence of interaction between mafic and silicic magmas in the younger granite suite.