Felsic Enclave Swarms in the Gouldsboro Granite, Coastal Maine: A Record of Eruption Through the Roof of A Silicic Magma Chamber

Prominent zones of large globular, fine‐grained, felsic enclaves occur at the highest exposed levels of the Gouldsboro granite along the coast of Maine. The physical characteristics of these enclaves and their chemical relations to the host granite indicate that they represent globules of resident magma that were trapped, probably during eruption, in the crystal mush at the top of the magma chamber. These zones can appropriately be termed “eruption trails.” Enclave compositions provide a record of magma compositions within the chamber at the time of eruption. They fall into two groups (high‐K and low‐K with similar wt % SiO2), suggesting that magma in the chamber consisted of discrete compositional batches. The distribution of these two types of enclaves within the swarms is consistent with the low‐K source magma residing as a layer beneath the high‐K source magma. Chilled and vesiculated inclusions of basalt occur only in the low‐K enclaves. Their presence there suggests that basaltic magma was injected into the base of the chamber just prior to and may have triggered the eruption. The low‐K silicic magma probably developed by selective exchange of alkalies between normal high‐K silicic magma and periodic injections of basaltic magma. Evidence for these injections is seen in the chilled gabbroic sheets at the base of the Gouldsboro granite. Similar large felsic enclaves are known in other shallow‐level granites; many of these may represent centrally derived magma trapped during transport out of the chamber in an eruption rather than, as they have commonly been interpreted, portions of chilled margins engulfed and remobilized during granite emplacement.