Abstract

Recent seafloor exploration has shown that volcanic-hosted massive sulfides (VHMS) occur in modern silicic calderas formed by highly explosive eruptions at substantial water depths. Sampling of these has so far been restricted to surficial deposits. Ancient analogs provide a time-integrated view of the structure and fill of deepwater calderas, but constraints on paleo-water depths have previously been lacking. The Ordovician Bald Mountain VHMS (northern Maine, USA) is unique in its degree of preservation, not only permitting detailed textural and structural analysis of the enclosing volcanic rocks, but also allowing fluid inclusion analysis to determine a paleo-water depth of eruption of greater than 1.45 km. The deposits that result from very deepwater explosive eruptions have distinctive textural characteristics recognized in other ancient examples where paleo-water depths are less well constrained. I propose that hydrostatic pressure suppresses the explosivity of the eruption sufficiently to retard the formation of fine ash, producing ignimbrite composed largely of pumice lapilli. It also inhibits exsolution of dissolved water in the magma, producing fluidal rhyolite eruptions and intrusions. These include silicic fire fountaining to produce proximal cones of hyalotuff or spatter accumulations, and intrusions of globular peperite.

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