Felsic enclave swarms in the Gouldboro Granite, coastal Maine: a record of eruption through the roof of a silicic magma chamber

Wiebe, Robert; Adams, Steve

'Prominent zones of large globular, fine-grained, felsic enclaves occur at he 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 ôeruption trails.ö 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 and high-K slicic 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.' In the Comclusions of the paper the author mentions similar attributes occur in the roof of the Cadillac Mountian pluton; with 'the upper levels of the Cadillac Mountain granite suggest that highly fractionated high-SiO2 liquid occured at the top of the chamber, above a lower-SiO2 liquid.' /STUDY LOCATIONS:/ /SCHOODIC PENINSULA/Schoodic Point/MOUNT DESERT ISLAND/Cadillac Mountain/

Felsic enclave swarms in the Gouldboro Granite, coastal Maine: a record of eruption through the roof of a silicic magma chamber

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