SILURIAN BIMODAL MAGMATISM IN THE COASTAL MAINE MAGMATIC PROVINCE: THE ROLE OF COMPOSITE DIKES IN THE TRANSPORT AND ERUPTION OF GRANITIC MAGMA

The transport of granitic magmas through the crust has been a long-standing topic of research and debate. Most models suggest that high viscosity silicic magmas cannot be transported significant distances through cold upper crustal rocks by conventional diking mechanisms unless the magma chamber is significantly (unrealistically?) overpressured.

The Swans Island region on the central Maine coast is relatively unique in preserving a record of both bimodal plutonism and volcanism, with exposures that provide a cross section through one of the many large Silurian magmatic centers of the CMMP. On Swans Island, a stratified plutonic complex is exposed with hornblende gabbros and diorites overlain by a biotite-hornblende granite, with localized zones of magma commingling and hybridization. The granite, in turn, intrudes and is overlain by a sequence of upper greenschist / lower amphibolite facies metavolcanic rocks dominated by mafic lava flows, lapilli and ash-tuffs, and rhyolitic lava flows. Over 300 diabase dikes have been observed cross-cutting the Silurian plutons and are interpreted to be the feeder dikes for the thick (>400 m) sequence of metabasalt lava flows. In contrast, only 11 silicic dikes have been observed, and only one intrudes the overlying metavolcanic sequence. Most are small and post-date the period of rhyolitic volcanism recorded in the thick (>450 m) sequence of silicic metavolcanic rocks.

The main feeders of silicic magmas to the volcanic sequence are 34 large, bimodal, composite dikes. Nearly all of the composite dikes have diabase margins in relatively sharp contact with felsite cores. A model is proposed whereby large mafic intrusions underplate a silicic magma chamber and diabase dikes intrude along the margins. These diabase dikes then become conduits for the increasing melt fraction in the silicic chamber, with relatively large volumes of rhyolitic melt now able to escape the chamber due to reduced viscous drag and reduced cooling during transport. This mechanism negates the need for unrealistic overpressures in the silicic magma chamber, and appears to be the primary way silicic magmas were transported to the surface. It also explains the abundant and nearly ubiquitous mafic enclaves found within the silicic volcanic sequence.